substitution of transition metal ions into aluminophosphates and silicoaluminophosphates:...

Res. Chem. Intermed., Vol. 28, No. 7–9, pp. 625–695 (2002)Ó VSP 2002.Also available online - www.vsppub.com

Substitution of transition metal ions intoaluminophosphates and silicoaluminophosphates:characterization and relation to catalysis

MARTIN HARTMANN1;¤ and LARRY KEVAN 2

1 Department of Chemistry, Chemical Technology, P.O. Box 3049, University of Kaiserslautern,D-67653 Kaiserslautern,Germany

2 Department of Chemistry, University of Houston, Houston, TX, 77204-5003, USA

Abstract—The literature related to the incorporationof transitionmetal ions (TMI) into aluminophos-phate (AlPO) and silicoaluminophosphate (SAPO) molecular sieves is reviewed. Microporous crys-talline metal aluminophosphates (MeAPOs) and metal silicoaluminophosphates (MeAPSOs) repre-sent an important group of inorganic materials because of their high potential as adsorbents and cata-lysts. This review focuses mainly on the spectroscopic characterizationof TMI containing MeAPOsand MeAPSOs covering the literature through 2000. The characterization of these materials is sum-marized and discussed in the light of possible isomorphous substitution of transition metal ions intothe aluminophosphate framework. Moreover, the literature devoted to the use of these materials incatalysis and adsorption is reviewed.

ABBREVIATIONS

DRS diffuse re� ectance spectroscopy (UV-VIS)ENDOR electron nuclear double resonanceESEM electron spin echo modulationESR electron spin resonanceEXAFS extended X-ray absorption � ne structureIR infraredMAS NMR magic angle spinning nuclear magnetic resonanceSAS small angle scattering

¤To whom correspondence should be addressed. E-mail: [email protected]

626 M. Hartmann and L. Kevan

TMI transition metal ionTPD temperature programmed desorptionXANES X-ray absorption near edge structureXPS X-ray photoelectron spectroscopyXRD X-ray diffraction

1. INTRODUCTION

Aluminophosphate (AlPO-n), where n denotes a particular structure type, molecularsieves form a new class of microporous crystalline materials comparable to the wellknown zeolites, which are aluminosilicate molecular sieves. Zeolites have poresor channels formed by alumina and silica tetrahedra linked by oxygen bridges.Substitution of other elements for Al and/or Si in the molecular sieve frameworkcan yield various kinds of new materials. Wilson et al. reported in 1982 thesynthesis of microporous aluminophosphate (AlPO) molecular sieves [1]. Thestructures of AlPO molecular sieves cover a wide range of different structure types;some are analogous to certain zeolites such as AlPO4-42 (zeolite A structure),AlPO4-34 (chabazite structure) or AlPO4-37 (faujasite structure). But there is alsoa large number of aluminophosphates like AlPO4-11, AlPO4-41 or VPI-5, whichare unique structures with no zeolite analog. An exciting property of the AlPO4-nmaterials is that Al and/or P can be partially replaced by silicon (SAPO-n) and/orother metals (MeAPO-n, MeAPSO-n) [2–7]. It has been claimed that a variety ofmetals (Me D Co, Zn, Ni, Mn, etc.) can be incorporated into the aluminophosphatestructure (see Table 1). The numbering of structure types of SAPO-n (MeAPO-n,MeAPSO-n) follows that of AlPO4-n, so that SAPO-5 denotes the same structuretype as AlPO4-5. The structures crystallized in the class of MeAPO and MeAPSOmolecular sieves are shown in Table 1. Beside more than twenty novel structures,at least seven structures with framework topologies related to aluminosilicates(zeolites), chabazite (34, 44, 47) erionite (17), faujasite (37), gismondine (43),levynite (35), Linde Type A (42) and sodalite (20) were discovered. In some caseslike the AFI structure, the synthesis of the corresponding zeolite SSZ-24 has beenachieved after the synthesis of the aluminophosphate AlPO4-5. Most studies dealwith the structure types AFI (AlPO4-5), AEL (AlPO4-11) or CHA (AlPO4-34), butnumerous other structure types (ATS, AFO, VFI, etc.) have also been investigated.

Transition metal ions (TMI) can be incorporated by three different methods:impregnation, ion-exchange or isomorphous substitution. In the latter method, thetransition metal ion salt is directly introduced into the synthesis mixture. Theincorporation of transition metal ions into framework sites of AlPO and SAPOmolecular sieves yielding MeAPO and MeAPSO-n materials is of particular interestfor the design of novel catalysts.

Information on isomorphous substitution of TMIs into silicoaluminophosphates istypically hard to obtain since the metal concentration is very low. Although some ef-

Substitution of TMI into AlPO and SAPO 627

Table 1.Incorporation of selected transition metal ions into different aluminophosphate frameworks reportedin the literature [£ D incorporation reported]

Material Structure code Ring opening Ti V Cr Mn Fe Co Ni Cu Zn CdMeAPO-5 AFI 12 £ £ £ £ £ £ £ £ £ £MeAPSO-5 £ £ £ £ £MeAPO-11 AEL 10 £ £ £ £ £ £ £ £MeAPSO-11 £ £ £ £ £ £MeAPO-14 AFN 8 £MeAPO-16 AST 6 £MeAPO-17 ERI 8 £MeAPO-18 AEI 8 £ £ £ £MeAPSO-18 £ £MeAPO-20 SOD 6 £ £ £MeAPO-25 ATV 8 £MeAPO-31 ATO 12 £ £ £ £ £MeAPSO-31 £MeAPO-34 CHA 8 £ £ £ £MeAPSO-34 £ £ £ £MeAPO-35 LEV 8MeAPO-36 ATS 12 £ £ £ £MeAPSO-36 £MeAPO-37 FAU 12 £ £MeAPSO-37 £ £ £ £ £ £MeAPO-39 ATN 8 £MeAPO-40 AFR 12 £ £MeAPSO-40 £ £MeAPO-41 AFO 10 £ £MeAPSO-41 £ £ £MeAPO-42 LTA 8 £ £MeAPO-44 CHA 8 £ £MeAPSO-44 £ £ £ £MeAPO-46 AFS 12 £ £ £ £MeAPO-47 CHA 8 £MeAPSO-47 £MeAPO-50 AFY 12 £ £ £MeAPSO-50 £ £MeVPI-5/APO-8 VFI /AET 18/14 £ £ £MeSiVPI-5 £ £ £ £DAF-1 DFO 12 £STA-6 SAS 8 £ £ £ £STA-7 SAV 8 £ £ £ £

fort has been devoted to the synthesis of materials with high transition metal concen-tration aiming at the ultimate goal of preparing porous transition metal phosphatephases [8], the transition metal concentration in the vast majority of materials is stillvery low. Therefore, a large number of different characterization methods has beused to collect information on the status of the transition metal ion in the molecular


sieve. Nevertheless, conclusive judgments can only be made by using different tech-niques for the characterization of the same samples. Among the methods that havebeen used to characterize MeAPO-n and MeAPSO-n materials are IR and UV-Visspectroscopy, 27Al, 29Si and 31P MAS NMR, electron spin resonance (ESR), elec-tron spin echo modulation (ESEM) spectroscopy, X-ray absorption (EXAFS andXANES), scanning electron microscopy, Mössbauer spectroscopy, X-ray photoelec-tron spectroscopy (XPS), chemical analysis, temperature programmed desorption ofbases (ammonia, pyridine), test of sorption and ion-exchange capacity, catalytic testreactions and powder and single crystal X-ray diffraction techniques. The variousdata published so far, show that under controlled synthesis procedures, molecularsieves with highly dispersed metal ions in their frameworks can be reproduciblyprepared.

In the present paper, we review the main results obtained in the characterizationof transition metal ion substitution into microporous aluminophosphate and silicoa-luminophosphate molecular sieves through the end of 2000. The main emphasisis placed on reviewing the evidence for true isomorphous substitution of transitionmetal ions into these molecular sieves obtained by spectroscopic characterization.Ion-exchanged and impregnated Me-AlPOs and Me-SAPOs, where the hyphen in-dicates extraframework metal ion sites, are also discussed when their properties aredistinctly different from MeAPOs and MeAPSOs. Such differences support the oc-currence of true isomorphous substitution in MeAPOs and MeAPSOs. The presentreview updates and extends previously published reviews [6, 7] by also discussingthe utilization of these materials for catalysis and adsorption. Although not a tran-sition metal ion, magnesium-containing AlPOs and SAPOs are also included in thisreview, because of certain analogies and similarities to transition metal ion MeAPOsand MeAPSOs with respect to their catalytic properties.

2. ISOMORPHOUS SUBSTITUTION

Isomorphous substitution in dense oxides is de� ned as the replacement of an ele-ment in the crystalline framework by another element in the crystalline frameworkwith similar cation radius and coordination requirements [4]. However, many el-ements incorporated in the aluminophosphate framework have radius ratios withoxygen, and T O distances that are inconsistent with the accepted crystal chemicalconcept for tetrahedral coordination. Their successful incorporation may be due tothe � exibility of the microporous aluminophosphate framework and to speci� c in-teractions with the organic template, coupled with the mildly acidic gel chemistryused in their synthesis.

Different substitution mechanisms can be envisaged for transition metal ions [9].This is schematically illustrated in Fig. 1. An aluminum atom can be replaced witha TMI with valence C1, C2 and C3, while P can be replaced with elements withvalence from C1 to C5 without generating positively charged frameworks whichare unlikely. However, the replacement of phosphorus by transition metal ions with


Figure 1. Proposed models for framework substitution (adapted from Ref. [9]).

low valences would produce a high negative charge density which has also beenconsidered unlikely. Nevertheless, evidence is found that support the isomorphoussubstitution of low-valent transition metal ions into phosphorus sites [10]. It is alsoclear from Fig. 1 that by isomorphous substitutions neutral or negatively chargedframeworks are generated which must be balanced by an equivalent number ofpositively charged extraframework species such as protons or transition metal ions.


Several indirect tools have been proposed to con� rm the incorporation of tran-sition metal ions into framework sites of AlPOs and SAPOs including the follow-ing [7].

Expansion of the unit cell. The replacement of Al3C or P5C by TMIs leads to anincrease of the unit cell volume of the crystal structure assuming that the symmetryremains constant. Typically the radii of Al(III) or P(V) are smaller compared to theTMI and the resulting expansion is directly accessible by powder X-ray diffraction(XRD) employing equation (1) [7], where

4=3[.h2 C hk C k2/=a2] C l2=c2 D .3 sin2 µ/=¸2/; (1)

a and c represent the unit cell parameters of a hexagonal system, µ the diffractionangle, ¸ the X-ray wavelength and h, k, l are the Miller indices. However, suchlattice expansions are not always observed and even lattice contractions are reportedin the literature [7]. It is plausible that the incorporation of a small amount ofhetreoelements is not always suf� cient to change the unit cell dimensions withinthe experimental error.

Chemical analysis. Initially, isomorphous substitution was solely veri� ed bychemical analysis. It is expected that the molar ratio of (nTMI C nAl/=nP (for a TMIreplacing Al) or (nTMI C nP/=nAl (for a TMI replacing P) is equal to one. Althoughobeyed in several systems, this method is not direct and can be inaccurate especiallywhen extraframework transition metal ions are present or different substitutionmechanisms occur simultaneously.

Weight loss. Some evidence for isomorphous substitution is obtained fromanalysis of the weight loss of the material due to template decomposition bythermogravimetric analysis. The negative charges introduced by replacement ofAl(III) or P(V) by a lower valent TMI are typically balanced by the positivelycharged template. It is often observed that decomposition of the positively chargedframework occurs at a higher temperature than that of a neutral template. Therefore,from the thermogravimetric curves the amount of charged template and, hence, theextent of isomorphous substitution is estimated.

Formation of Brønsted acid sites. Replacement of Al(III) or P(V) by metal ionsMenC results in the formation of Brønsted acid sites [11]. In the case of Mn-containing samples a model has been proposed by Gielgens et al. [12], whichrationalizes the surface acidity observed for this solid. According to this model(Fig. 2), a divalent metal incorporated into the framework generates one P OH

Figure 2. Model explaining the way isomorphous substitution generates surface acidity. A Brønsted–Lewis interaction is apparent (adapted from Ref. [13]).


group for each metal ion. Metal ions act as Lewis acid sites interacting withBrønsted P OH entities [13]. The number of acid sites formed can be measuredby temperature programmed desorption of amines, viz. ammonia, pyridine orisopropylamine. A major problem with this method is to distinguish between Lewisand Brønsted acid sites. The formation of acid sites is also frequently assessed bycatalytic test reactions.

Other indirect criteria to verify isomorphous substitution of transition metal ionsinto aluminophosphate frameworks include NMR line broadening and determina-tion of the void space in the AlPO crystals. However, most of these criteria are notable to distinguish between isomorphous substitution or the mere presence of thetransition metal ion.

It is therefore clear that more direct evidence for isomorphous substitution isneeded. Several spectroscopic techniques have been used to distinguish betweenframework and extraframework transition metal ions. Because each spectroscopicmethod has its own sensivity and application domain, a range of complementarytechniques has been applied to obtain detailed information of the coordination en-vironment of the transition metal ion, such as UV-VIS (DRS), ESR and ESEM,EXAFS and XANES. A major disadvantage is the occurrence of complex and over-lapping spectra and therefore comparison with samples with clear extraframeworksites which have been obtained by ion-exchange or impregnation is mandatory.

Transition metal ions, which are often paramagnetic, are introduced into molec-ular sieves to generate a catalytic reactive species or site. Various pretreatment oractivation procedures are typically used to generate intermediate, reactive metal ionvalence states, which are often paramagnetic. Therefore, ESR spectroscopy is usedin a large number of studies as a tool for characterization of these species. Thelocation and structure of the reactive metal ion site is of considerable importancefor understanding the chemistry of such sites. In particular, the interaction of suchactive metal ion sites with different adsorbates and reactants helps to understandcatalysis on a molecular level. A suf� ciently good understanding of the local ad-sorbate structure of catalytically active metal species in microporous materials canpotentially enable optimization and control of the catalytic activity of such systems.

Finally, many catalytic reactions have been employed to probe the acid or re-dox properties of transition metal ion containing aluminophosphates and silicoalu-minophosphates. Some of them are test reactions (ethylene dimerization, 1-buteneisomerization, ethylbenzene disproportionation) and others are used to investigateenvisaged applications.

3. CHARACTERIZATION AND LOCATION OF TRANSITION METAL IONS INALUMINOPHOSPHATES

The � exibility of the aluminophosphate system to substitute other elements into itsframework structure appears to be greater than that of the silicate system. Suchsubstitutions are presumably limited on the basis of structure, however. A wealth


Figure 3. Partial periodic table with the transition elements marked which have been introduced intoaluminophosphatesand silicoaluminophosphates.

of aluminophosphate molecular sieves have been reported to contain as many as 19additional elements (the number is still growing) in their structure (Table 1) [14].These elements include Li, B, Be, Mg, Ga, Ge, As and the transition metal ions Ti,V, Cr, Mn, Co, Ni, Fe, Zn, Cu (see Fig. 3). The introduction of some combinationsof these ions has also been reported and will be discussed in a separate section.

3.1. Titanium

After the discovery of titanium substituted silicalite-1 (TS-1) with its remarkablecatalytic properties, particularly in oxidation reactions under mild conditions,titanium incorporation has been attempted in other materials as well. Titaniumsubstitution has been claimed for aluminophosphates with AFI [15, 16], AEL [17],ATS [18, 19], CHA [20] and FAU [21] topology. While titanium replaces siliconin large silica patches of SAPO-5 [15] Ti substitution in pure AlPO4-n, leading toTi O P bonds [22], is expected to be more dif� cult [17].

UV-VIS spectra of titanium-containing samples showed a single line centerednear 230 nm indicating tetrahedral Ti(IV). The absence of a shoulder near 300 nmsuggests that anatase like phases were not formed during the synthesis of titaniumcontaining silicoaluminophosphates [19].

Samples with incorporated titanium also showed an IR band at 960 cm¡1. Thisband has been taken as evidence for the presence of Ti(IV) ions in tetrahedralcoordination and can be attributed to SiO¡ defects in silica patches indicating thesubstitution of Ti(IV) for Si(IV) in the SiO2 domains [16, 23]. However, this bandis also frequently observed in Ti-free materials and therefore should be presentedas evidence for Ti4C only along with other supporting evidence. A shoulder at1040 cm¡1 in the IR spectrum of calcined TAPO-n is assigned to the presence oftitanium in the framework [15].

More direct evidence for the isomorphous substitution of titanium into alu-minophosphates with AFI, AEL, ATO or ATS topology is reported by combineduse of ESR and ESEM spectroscopy [24]. Some Ti(IV) was reduced to Ti(III) by° -irradiation or CO reduction to serve as a spin probe. The ESR parameters con� rmTi(III) stabilization, but do not give clear evidence for tetrahedral coordination. 31Pand 27Al electron spin echo modulation of Ti(III) provide evidence based on phos-


phorus distances for titanium substitution into tetrahedral phosphorus frameworksites in all four materials [24, 25].

The acid properties of TAPO molecular sieves with AFI and ATS structure wereprobed by FTIR of chemisorbed pyridine and proton MAS NMR [26]. Thesetechniques con� rmed the presence of Brønsted acid sites in TAPO-5 and TAPO-36. The bridging Ti(OH)Al species showed a Brønsted acid strength which is onlyslightly weaker compared to the acid sites in SAPO-5.

3.2. Vanadium

There is a large interest in vanadium analogs of molecular sieves based on zeolitesand aluminophosphates mainly based on their potential to catalyze oxidation reac-tions. The incorporation of vanadium into silicalite-1 [27, 28] and AlPO4-n hasbeen reported. However, successful incorporation of vanadium into the frameworkof aluminophosphates is still controversial. Therefore, several spectroscopic tech-niques have been used to investigate vanadium species in AlPOs and SAPOs.

There are some reports on vanadium containing aluminophosphate molecularsieves using ESR spectroscopy for the detection of V(IV) in VAPSO- and VAPO-5, -11, -17, -31, -37, -40 and 41 and VPI-5 [29–40]. ESR of these samples hasshown well resolved hyper� ne splittings corresponding to V(IV) cations bondedstrongly to oxygen ions in the matrix. The gjj values are 2.0221 and 2.0046 and g?is 2.0440 and 2.0380 for VAPSO and VAPO catalysts, respectively [41]. Accordingto another study the V(IV) ESR parameters for as-synthesized samples of VAPO-5are gjj D 1:94, Ajj D 193 G, g? D 2:00 and A? D 73 G [29]. Whittington etal. [31] and Montes et al. [32] � nd two different V(IV) species with very closeg-tensor parameters. One of these species is also found in a V2O5 impregnatedAlPO4-5 sample, in which a single line with g D 1:988 is detected, which is alsofound in pure V2O5. The one compound with seven well resolved hyper� ne lines isascribed to isolated vanadyl-like V(IV) (I D 7=2) species. After calcination of thesample in � owing oxygen at 500±C the signal intensity decreases by 40-fold [29],which is ascribed to the oxidation of the vanadyl-like species toV(V) according toFig. 4.

The signal intensity can be increased by hydrogen reduction or high-temperatureexposure to organic molecules like toluene or p-xylene. The reported resultsmainly underline the redox properties of vanadium in aluminophosphates anddo not interpret the ESR spectra in terms of possible framework incorporation

Figure 4. Proposed vanadium species under oxidative and reductive conditions.


of V(IV). More recent ESR results by Weckhuysen et al. [42, 43] and Lohseet al. [44] found no unambiguous evidence for true incorporation of V(V) intotetrahedrally coordinated framework positions in aluminophosphates with AEL,AFI, ATO and CHA topology. In as-synthesized samples two signals with verysimilar g-tensor parameters are ascribed by Weckhuysen et al. [42, 43] to isolated(pseudo-) octahedral V(IV). Very similar g-tensor parameters are ascribed by Lohseet al. to isolated immobile VO2C species in square-pyramidal symmetry [44]. Uponcalcination, V(IV) ions are oxidized to tetrahedral V(V) which can only be partiallyreduced to V(IV). However, the ESR detectable vanadium content is below 20%.Furthermore, there are different views on whether vanadium substitutes for Al(III)[30] or P(V) [32, 36, 42, 44, 45]. The replacement of P(V) by vanadyl VO2C

ions would generate negative charges to be neutralized by either HC (H3OC/ or apositively charged template molecule. Very recently, this model has been supportedby the combined use of ESR and ESEM spectroscopy, which suggests that vanadiumis located at sites closer to aluminum than to phosphorus [10, 45]. X-ray absorptionstudies (XAS) also claim the presence of isolated monomeric VO2C complex unitscoordinated to two framework oxygens with or without either three water moleculesor one hydroxyl group and two water molecules in as-synthesized and calcinedVAPO-5 [46]. Upon dehydration, the coordination symmetry was found to shiftgradually from either square pyramidal or octahedral to tetrahedral. This modelwas further re� ned by density functional theory (DFT) calculations and comparedto experimental XAS and Raman data [47, 48]. The results from DFT calculationshave indicated that the substitutions of either phosphorus or aluminum by vanadiumare, in general, not feasible. The isolated vanadium was found to exist as amono-oxo (V4C O) (Of)4 species for reduced, dehydrated VAPO-5 and as a di-oxo V5CO4 species after calcination (see Fig. 5). The coordination geometry ofthe mono-oxo V(IV) species is sensitive to moisture and, hence, water moleculesmay exchange with the framework oxygens coordinated to the V4C center. Thecomputed structures of the vanadium oxide species have also been con� rmed byRaman spectroscopy [48].

The diffuse re� ectance UV-VIS spectra (DRS) of reference compounds such asV2O5, NH4VO3 and vanadium supported on Mg were compared with VAPO-5samples of different vanadium content [49]. A band around 270 to 290 nmis indicative of isolated tetrahedral V(V) sites supporting the incorporation ofvanadium into the framework. A broad band is observed in the 350 to 450 nmregion, suggesting that some polymeric vanadium species, probably V2O5-like, arepresent. AlPO4-5 samples impregnated with vanadium show different DR spectra.These spectra also give peaks at 290 nm, suggesting a tetrahedral environment forV(V), and around 410 nm, which is ascribed by Blasco et al. [49] to polymericforms of vanadium probably like in V2O5.

Solid-state 51V NMR studies on V-containing catalysts [30] have shown that it ispossible to obtain information on the symmetry environment of 51V by comparisonwith model compounds. The 51V NMR spectra have been taken under static and


Figure 5. The proposed scheme of reduction of V(V) species to V(IV) species in the AFI framework(after Ref. [47]).

magic angle spinning (MAS) conditions [30]. The 51V isotope has a moderatelylarge electric quadrupole moment and, hence, the line shape of the wide linespectra is dominated by chemical shift anisotropy rather than by second-orderquadrupolar effects. The wideline 51V NMR spectra are mainly formed by a broadsignal with a maximum slightly up� eld from ¡300 ppm (reference VOCl3), whichis characteristic of V5C in an octahedral or square pyramidal environment [50].Besides, often a second band at ¡550 ppm is observed indicating the presence oftetrahedral V(IV). The presence of tetrahedrally coordinated metal cations is oftentaken as evidence for isomorphous substitution. However, the bonding to extramolecules (as observed for V(IV)) to increase the oxidation state is not unusualand, therefore, the spectroscopic evidence is not unambiguous [50]. Recent XPSresults by Blasco et al. [51] suggest a homogeneous distribution of the vanadiumspecies in as-synthesized and calcined VAPO-5. The Al/P surface atomic ratio isclose to 1 (like in pure AlPO4-5), which con� rms a low vanadium concentration onthe catalyst surface. 51V MAS NMR spectra of calcined VAPSO-11 show a narrowchemical shift distribution with an isotropic shift ¾ iso D ¡540 ppm. The 51V MASNMR pattern is consistent with previously published literature of predicted squarepyramidal or distorted octahedral vanadyl(V) species in VAPO-5. Similar 51V MASNMR spectra are obtained for VAPO-11 [52].

Low-temperature CO adsorption on VAPO-5 has been studied by IR spectroscopy.No carbonyl species were formed with V(V) in oxidized VAPO-5 [53]. Reductionresults in the formation of V(IV) and V(III), which form stable CO complexes at85 K. ESR data also indicate a high concentration of V(IV) in samples reduced at673 K in hydrogen [49].


3.3. Chromium

In the patent literature [33], the synthesis of CrAPO-5 has been reported, but theremaining question concerns the possibility and the extent of tetrahedral Cr(III)substitution for Al(III) in the framework. Isomorphous substitution requirestetrahedral coordination of Cr(III), which is very rare in complexes or inorganicstructures and can be understood in terms of the crystal � eld stabilization energy(CFSE) of octahedral Cr(III) (224.5 kJ/mol) versus tetrahedral Cr(III) (66.9 kJ/mol)[54, 55]. Therefore, tetrahedral coordination of Cr(III) in aluminophosphates seemsdif� cult to obtain. This is also concluded in several papers reporting synthesisand characterization of large crystals of chromium-containing AlPO-5 [56, 57]and SAPO-5 [58, 59]. The latter materials have been used for quenching of thebackground luminescence in Raman spectroscopy [59] and have been investigatedas potential laser media [58]. Also for CrAPO-14, an aluminophosphate withoctahedral framework sites, it is shown by single crystal XRD that up 5% of theoctahedral sites are occupied by Cr(III) [60].

In papers by Weckhuysen and Schoonheydt [61–63] CrAPO-5, where Cr2O3 orCr(NO3/3 ¢ 9H2O was added to the synthesis mixture, was compared to AlPO4-5impregnated with Cr(III). The ESR signals of synthesized and impregnated AlPO4-5containing Cr(III) are reported to be comparable. A redox change of Cr(III) toCr(V) is reported on the basis of DRS and ESR data. During and after hydrothermalsynthesis, Cr(III) is in pseudooctahedral coordination with oxygen in CrAPO-5 andCrAPSO-11 As-synthesized CrAPSO-11 and CrAPO-5 show a broad ESR signalaround g D 2:0 with a positive lobe between g D 5 and g D 4. This signal hassome similarities with the ±-signal obtained for chromium species on amorphousoxide supports and can be simulated using high zero � eld parameters D and E[64]. After calcination and rehydration during cooling, CrAPSO-11 shows a muchnarrower ESR spectrum, which is assigned to Cr(V), possibly in square-pyramidalcoordination [65]. A three-pulse 31P ESEM spectrum was obtained for CrAPSO-11 evacuated at room temperature for 12 h and simulated with 11–12 phosphorusinteracting with Cr(V) at a distance 0.58 nm. The three-pulse ESE for the solid-state ion-exchanged Cr-SAPO-11 does not show any 31P modulation. Althoughthere are some additional hints for isomorphous substitution of Cr(III) in SAPO-11, there are still some unclear aspects, which are beyond the scope of this paper.Weckhuysen et al. [63] concluded based on their X-band, Q-band and DRS datathat Cr(III) is not incorporated into the framework, but is present as octahedralions at the surface of the AlPO4-5 crystals. Analogous results were obtained byRajic et al. [66] in the Cr/SAPO-34-system, where Cr is also believed to be anextraframework species. Absorption maxima for as-synthesized Cr/SAPO-34 arefound at 420, 575 and 666 nm, while for calcined Cr/SAPO-34 a maximum appearsat 370 nm. The spectrum of as-synthesized Cr/SAPO-34 indicates the presenceof octahedrally coordinated Cr(III), which is oxidized to Cr(VI) and the UV-VISspectrum of the calcined material corresponds to the chromate ion CrO2¡

4 .


Zhu et al. reported the synthesis of CrAPSO-5 using tripropylamine as a structure-directing agent [67]. ESR shows Cr(III) with g1 D 5:20, g2 D 2:00 and g3 D 0:98,which is assigned to Cr(III) in distorted octahedral coordination. After dehydration,Cr(III) shows sharper ESR lines with g? D 4:00 and gjj D 2:00, which areassigned to Cr(III) in tetrahedral coordination. After calcination and dehydration,Cr(V) is found to be tetrahedrally coordinated in CrAPSO-5. These conclusions aresupported by the respective UV-VIS spectra. 31P ESEM data indicate that Cr(V)substitutes for phosphorus in a framework site of calcined CrAPSO-5. A detailedESR study of chromium in CrAPO-5 was very recently reported by Kornatowskiet al. [68, 69]. These authors conclude in agreement with Zhu et al. [67] thatafter synthesis Cr(III) is mainly present in an octahedral form in the framework. Acertain amount of Cr(VI) and/or Cr(V) detected by UV-VIS or ESR spectroscopyis believed to be either a framework species substituted for P or, more likely, atype of surface species anchored to the framework. After dehydration in vacuum,Cr remains mostly Cr(III) and two forms of Cr(V) are found: square pyramidal(� ve coordinated) chromyl groups and tetrahedrally coordinated Cr(V) centers,which may be extraframework species. Oxidative and reductive treatments revealreproducible redox properties of Cr(V) and a high stability for Cr(III), which istaken as evidence for framework incorporation of Cr(III).

Large CrAPO-5 crystals have been synthesized [56] and characterized by singlecrystal UV-VIS spectroscopy and a polarization dependent absorption has beenobserved [57]. The 4T1 band has its maximum at 454 nm for polarization parallel tothe pores and at 445 nm for polarization perpendicular to them. The 4T2 band is near625 §15 nm for both polarizations. This indicates an aniostropic coordination ofCr(III) distorted by the molecular sieve structure. However, the band positions pointto a 6-fold coordination, which is in line with reports by Zhu [67] and Kornatowski[68]. Miyake et al. [70] also studied the synthesis of CrAPO-5 using differenttemplates and succeeded in the synthesis of large crystals of Cr(III)-doped AlPO4-5.They also concluded from UV-VIS data that Cr(III) ions in the framework areoctahedrally surrounded by four framework oxygens and two water molecules.

Isomorphous substitution of Cr(III) into framework sites of CrAPO-11 was re-ported by Eswaramoorthy et al. [71]. Distinct differences between the CrAPSO-11system and the supported Cr /SAPO-11 catalyst were observed by the combined useof UV-VIS, XPS and IR spectroscopy [54] and linked to the catalytic performanceof these systems in the transformation of 1-butene. On oxidized CrAPSO-11 a largefraction of Cr(VI) was found by UV-VIS and XPS, which showed that almost 70%of the chromium existed as Cr(VI). For oxidized supported material 82% of the Crwas found as Cr(III). These results are supported by IR spectroscopy employingNO as a probe. The NO chemisorption experiments showed the presence of highchromium oxidation states on oxidized CrAPSO-11, while no evidence of thesestates was observed for the supported catalyst. After reduction the distribution ofoxidation states on CrAPSO-11 was about 40% Cr(VI) and 60% Cr(IV). In contrast,no differences between oxidized and reduced Cr/SAPO-11 were observed.


Figure 6. Location of chromium in as-synthesized and calcined CrAPO-5.

Incorporation of chromium(III) into the SAPO-37 framework was investigated bySpinacé et al. [72]. The UV-VIS spectrum shows two bands at 420 and 620 nm,which are assigned to d-d transitions of Cr(III) in octahedral coordination indicativeof extraframework species. The ESR spectrum of as-synthesized CrAPSO-37 isanalogous to the spectra of CrAPO-5 and CrAPSO-11 and shows a broad peak atg D 1:98 and another peak at g D 4.

In the light of their data on CrAPO-5, Chen and Sheldon [73] put forward a model,which explains the increase in acidity in their system (Fig. 6). They assumed thatin as-synthesized CrAPO-5, Cr(III) is octahedrally coordinated (there are two extrawater ligands) within the framework. During calcination Cr(VI) is formed, which isstill bound to the internal aluminophosphate structure (Fig. 6). In order to balancethe charges Cr(VI) AlPO must contain an acidic P OH group in its environment.By using acetonitrile, the authors con� rmed the presence of strong Lewis acid sitesand acidic P OH groups in CrAPO-5.

Peeters et al. [74] reported a quantitative 31P MAS NMR study of CrAPO-11.They found that substitution of Cr had a signi� cant effect, in which phosphorusin the � rst and third coordination spheres around Cr became NMR invisible. Incontrast, extraframework chromium did not have such an effect.

Although some data suggest the incorporation of chromium into the molecularsieve framework of AlPO4-11 or AlPO4-5 or into the analogous silicoaluminophos-phates, the controversial issue is, by no means, settled. Some authors have foundfor Cr-SAPO-34 [66] and CrAPO-5 [61–63] that Cr(III) is not incorporated into themolecular sieve framework during synthesis. Other studies have suggested the in-corporation of tetrahedral Cr(VI) [54, 65, 73] or framework [67] or extra-frameworkCr(V) [68] while another has stressed the signi� cant effect that the substitution ofAl by Cr has on the framework [74].

3.4. Manganese

Manganese is commonly accepted as a possible candidate for isomorphous substi-tution into microporous aluminophosphates [75, 76]. Mn(II) incorporation into theframework of AlPO4-n and SAPO-n (n D 5; 11) was studied by ESR methods in


several papers [77–85]. Goldfarb et al. [77, 78] concluded that the majority ofMn(II) in MnAPO-5 does not occupy a framework site, while Brouet et al. [79, 82]reported direct evidence by ESEM for Mn substitution into a framework position ofMnAPO-11 and MnAPSO-11. Pluth et al. [84] also showed that Mn(II) is tetrahe-drally coordinated in framework positions of as-synthesized MnAPO-11 by singlecrystal X-ray diffraction.

The ESR spectrum of MnAPO-5 with a low manganese content shows sixresolved hyper� ne lines with splitting characteristics of octahedral Mn(II) (I D5=2), supporting the existence of extraframework manganese(II) [78, 79]. Ondehydration, the sextet of MnAPO-5 coalesces into a single absorption line. Thiscan also be seen in fresh samples having a higher manganese concentration. Thecoalescence is attributed to an increase in the spin-exchange interaction due tomigration of extraframework Mn(II) within the framework [79]. X-band andQ-band ESR spectra reveal that the sextet seems to have a superimposed broadsignal. Levi et al. [78] assign the origin of this broad signal to transitions other than¡1=2 ! 1=2 of Mn(II) with I D 5=2 or to a possible additional Mn(II) species.Removal of water at increasing temperatures up to 400±C signi� cantly broadens thesextet signal and � nally at 400±C the resolution is completely lost and only a singletis detected. This loss of the hyper� ne structure is not detected in samples with muchlower Mn(II) contents. Rehydration usually completely restores the sextet spectrumof the hydrated sample.

At high manganese concentrations (1 mol%) the ESR spectra of MnAPO-11, likethose of MnAPO-5, are broad with unresolved hyper� ne structure and are assignedto predominantly extraframework Mn [77, 78, 82]. This was also tested by ion-exchange with CaCl2, which removed about 30% of the Mn(II). Lowering theconcentration to 0.1 mol% Mn gives ESR spectra of hydrated and calcined sampleswith well resolved sextet lines. The ESR signal of calcined and hydrated MnAPO-11 is better resolved than that of an as-synthesized sample. In addition to thisresolution enhancement, an increase of the hyper� ne splitting from 87.7 to 93.7 Gis measured upon calcination and hydration. Dehydration at 330±C leads againto a decrease to 87.6 G. From experiments with samples of different manganesecontent it is concluded that similar concentrations in synthesized and impregnatedsamples exhibit different ESR spectra. The spectra of impregnated samples are lesswell resolved in comparison to those of as-synthesized MnAPO-11. This indicatesthat the spin-spin interaction is much stronger in impregnated samples and suggestssome aggregation of Mn(II) in impregnated samples. Obviously the manganesespecies environments in MnAPO-11 and Mn-AlPO4-11 are different, suggestingthat Mn(II) is framework substituted in MnAPO-11 as also shown directly by31P ESEM [82], since it occupies an extraframework position in Mn-AlPO4-11.This framework incorporation is further con� rmed by 2D ESEM showing differentgeometrics for coordinated D2O and partially deuterated methanol with Mn(II) inMnAPO-11 versus Mn-AlPO4-11 [82].


The analysis of the 31P modulation should provide more evidence for frameworkincorporation of manganese. If Mn(II) substitutes for 27Al into the framework, thenit should be surrounded by a tetrahedral arrangement of four 31P nuclei in a dis-tance at about 0.31 nm. In MnAPO-5, the 31P ESEM spectrum is best � t with onephosphorus at a distance of 0.42 nm and two more phosphorus at a distance of0.58 nm. Furthermore, the strong aluminum modulation found indicates the closerproximity of Mn(II) to aluminum than to phosphorus. Framework incorporation ofa small amount of manganese can be also achieved in MnAPO-11 and MnAPSO-11 [79–82]. The incorporation of manganese into aluminophosphate MnAPO-20(SOD) has been investigated by Arieli et al. [86] employing ENDOR and ESEMspectroscopies. Evidence for Mn in an Al framework site was deduced from thepresence of a pulsed ENDOR doublet separated by 8 MHz symmetrically � ankingthe Larmor frequency of 31P at W-band at 34 kG, while such a doublet is missingin the region around the Lamor frequency of 27Al. This doublet was assigned to anisotropic hyper� ne interaction between Mn(II) and phosphorus and, hence, replace-ment of aluminum by manganese was suggested in MnAPO-20. This conclusion,however, is not consistent with ESEM data presented for MnAPO-5, where 27Aland 31P ESEM data suggest a replacement of phosphorus by manganese [10].

The redox behavior of manganese cations in MnAPO-5 has been investigated byKatzmarzyk et al. [83]. Upon calcination in � owing dry oxygen the color of thematerial changes from white to violet, while the ESR intensity decreases to 1/3 ofthe intensity of the as- synthesized MnAPO-5. The decrease in signal intensity isattributed by the authors to a partial oxidation of Mn(II) to non-detectable Mn(III).With no other signal appearing even at 93 K, an oxidation to Mn(IV) was ruledout. Reduction of the sample in � owing dry hydrogen at temperatures below450 K does not change the ESR spectrum, while after reduction at 475 K the ESRsignal becomes narrower and more intense. A possible explanation is that reductionof Mn(III) to Mn(II) under these conditions leads to an intense ESR signal. Thematerial behaves reversibly in redox cycles near 500 K and can undergo reversiblehydration/ dehydration cycles. Both framework and extraframework Mn(II) specieswere suggested to be present. XPS results also indicated that manganese was betterdispersed in MnAPSO-11 than in Mn/SAPO-11 material prepared by impregnation[87]. Redox cycles showed a strong difference in H2 or O2 consumed by each solid.Mn/SAPO-11 consumed nearly three times as much H2 (O2/ per manganese atomthan MnAPSO-11. Moreover, a larger number of moderate and strong Brønstedacid sites were detected in MnAPSO-11, which also supports the idea of frameworkincorporation of manganese.

MnAPSO-44 and MnAPO-44 (CHA topology) were synthesized with cyclohexy-lamine as a template [88] and characterized by NMR and ESR spectroscopies[89, 90]. The 31P, 29Si and 27Al MAS NMR spectra are similar to those of thecorresponding SAPO-44 and AlPO4-44 samples except for signi� cant broaden-ing due to dipolar interaction of part of the framework nuclei with the paramag-netic Mn(II). The ESR spectra measured at X- and Q-band, exhibit Mn(II) in as-


synthesized and calcined samples with hyper� ne splittings of 85 and 95 G, respec-tively. The latter value is characteristic of Mn(II) in an octahedral environment.While calcination does not change the ESR spectra, dehydration at 400±C reducedthe hyper� ne constant to 65 G, indicating a change to tetrahedral coordination uponwater removal. Both 31P and 27Al modulation were observed in ESEM measure-ments. The ESR and ESEM results are interpreted in terms of Mn incorporationinto tetrahedral framework sites at least for samples with low manganese content(P=Mn D 0:026). Furthermore, ‘2 C 1’ ESE experiments indicate that only 15%of the Mn(II) ions are homogeneously distributed and contribute to the echo signalin as-synthesized and calcined samples. The amount of homogeneously distributedspecies in MnAPSO-44 is rather close to that observed in MnAPO-5 [78] with sim-ilar Mn content. Microwave synthesis of MnAPO-5, MnAPO-44 and MnAPSO-44has been reported by Lohse et al. [91]. Their ESR measurements reveal a cluster-like arrangement of Mn(II) ions (Mn(II) O P O Mn(II)) and suggest that man-ganese replaces aluminum in the framework. Infrared spectroscopy gives evidencefor P OH groups and Lewis acid sites. It was shown that 16% of the aluminumatoms can be replaced by Mn(II). However, calcination in vacuum results in par-tial release of Mn(II) from the framework resulting in partial collapse of the struc-ture. This process is even more pronounced in the presence of moisture [91, 92]. Arecent study by Sinha et al. [93] reports that the fraction of manganese incorporatedis larger in MnAPO-11 than in MnAPO-41. At least four different Mn(II) species lo-cated in both framework and extraframework positions could be identi� ed by theirESR studies. Isolated Mn(II) in framework positions with Oh or Td symmetry aswell as patches of Mn(II) exhibiting Mn-Mn interaction were found in addition toextraframework Mn(II) and MnOx species [93].

A high degree of manganese incorporation in the MnAPO-50 framework wasreported by Tusar et al. [94]. Their single-crystal X-ray structure analysis servedas direct evidence of a very high degree of isomorphous substitution of aluminumby manganese. According to their analysis, 35% of the aluminum framework siteswere occupied by manganese, which was in reasonable agreement with chemicalanalysis.

3.5. Iron

The question whether iron can be incorporated into the framework of AlPO4-5 hasbeen addressed using different techniques such as powder X-ray diffraction, solidstate NMR spectroscopy, ESR and Mössbauer spectroscopy [95, 96]. The amountof iron reported to be incorporated into FAPO-5 is rather low Therefore changes inthe XRD pattern are not observed. Typical ESR spectra of as-synthesized FAPO-5show � ve distinct signals at g D 2:0, g D 2:2–2:4, g D 4:3, g D 5:2 andg D 8:4 [97]. Bands at g D 2:0, g D 4:3 and g D 6:0 have earlier beenreported by Park and Chon [98]. While the band at g D 2:0 has been attributedto octrahedrally coordinated iron in extraframework oxides, the signal at g D 4:3 istypically assigned to Fe(III) tetrahedrally coordinated to oxygen [99, 100]. Catana


et al. [97] suggest that the g D 2:0 signal is due to Fe(III) in a tetrahedral site andthe g D 4:3 line can be ascribed to Fe(III) in a distorted tetrahedral (defect) site. Thetotal intensity of the two lines increase linearly with the iron concentration, whenthey are beyond the saturation level [97]. The ESR intensities linearly increase with1=T (Curie–Weiss behavior) in the temperature range of 120 to 350 K. Similarresults were obtained for the FAPO-11 molecular sieve [101].

Mössbauer spectra of iron-substituted AlPO4-5 are � tted with a singlet and twodoublets suggesting that there are two types of Fe(III) and one type of Fe(II) presentin the sample. The singlet (isomer shift ± D 0:188) is assigned to tetrahedralFe(III) in the framework, while one doublet .± D 0:3/ is assigned to Fe(III) in anamorphous phase [102]. The second doublet .± > 1/ is assigned by Das et al. [102]to divalent iron, which sums up to between 10 and 20% in the FAPO-5 sample. InFAPO-11 the amount of divalent iron is very low. Cardile et al. [103] also foundFe(II) and Fe(III) in their FAPO-5 samples. Considering that a Fe(III) salt was addedto the synthesis gel, some reduction of Fe(III) takes place during the preparationof the samples. If a Fe(II) salt is used as a starting material, a large amount ofextraframework iron (II) was found along with Fe(III) in the framework [104]. TheMössbauer spectra of iron impregnated AlPO4-5 exhibit a symmetric quadrupolarsplitting doublet and an isomer shift of about 0.3 mm/s, [105] which is attributed tosuperparamagnetic Fe(III).

Hoppe et al. synthesized FAPO-5 in the presence of methylene blue to obtaininnovative materials with special optical properties [99]. Fe (II) .± D 1:4/ was foundas well as Fe(II) .± D 0:8/ in the Mössbauer spectra. The addition of methyleneblue has no in� uence on the Fe2C/Fe3C ratio as long as FeSO4 is used for thesynthesis. The use Fe(III) leads to an irreversible decomposition of the complex.

Among other methods used for the characterization of FAPO materials, photoa-coustic (PA) spectroscopy can give information on the incorporation of iron intothe framework of aluminophosphates [98]. All calcined FAPO-5 samples show PAspectra of four well resolved bands at 380, 410, 435 and 480 nm. These bands arealso found in as-synthesized Fe-silicalite and were assigned to Fe(III) surroundedtetrahedrally by oxygen [106, 107].

Recently, the incorporation of iron(III) into the framework of SAPO-37 (FAU)replacing tetrahedrally coordinated aluminum has been reported by Spinacé etal. [72]. Crystallization of FAPSO-37 has been achieved by slight variations ingel composition and synthesis temperature compared to SAPO-37. The UV-VISdiffuse re� ectance spectrum of FAPSO-37 shows two bands at 215 and 250 nm,which are usually assigned to charge transfer from oxygen to an isolated frameworkiron(III). Also a very weak band at 370 nm is observed, which might be assigned toforbidden d–d transitions of iron(III) in tetrahedral coordination, hence, con� rmingthe incorporation of iron into the framework. Two ESR signals at g D 4:3 andg D 2:0 have been observed and assigned to framework incorporated iron(III) andhydrated iron(III) in a symmetric tetrahedral framework site.


Using microwave synthesis, Brückner et al. [108] prepared FeAPO-5 samplescontaining up to 3 mol% Fe. Mössbauer spectroscopy revealed that duringcrystallization in air more than half of the iron ions are oxidized for Fe(II) containinggels and that only slight reduction occurs for gels containing Fe(III) ions. Theauthors concluded that in as-synthesized and hydrated samples all iron ions areoctahedrally coordinated residing either in defect sites (ESR signal at g D 4:3) orin intact framework sites (ESR signal at g D 2). Octahedral coordination is realizedby coordination to two ligands located in the pores [108].

57Fe Mössbauer spectroscopic studies were carried out on ferrocene guest mole-cules in AlPO4-5 and AlPO4-8 at 300 and 20 K [109]. The room temperature spectrashow complete orientational (including � ipping between parallel and perpendicularorientations) freedom which is still rapid on the Mössbauer timescale, whereas thelow temperature spectrum shows either � xed molecules or rotations which are slowon the Mössbauer timescale.

The incorporation of Fe(III) into the AFI framework was studied by computa-tional calculations using energy minimization techniques [110]. This study showsthat the substitution of Fe(III) for Al(III) in the AlPO4-5 framework is energeticallyunfavorable. The substitution produces strong distortion around the Fe atom. Ac-cording to this study, Fe(III) in tetrahedral sites destabilizes, but does not disrupt,the structure.

3.6. Cobalt

The isomorphous substitution of cobalt into AlPO4-n and SAPO-n has been studiedextensively with many different techniques and in numerous structures including -5,-11, -14, -18, -25, -34, -36, -41, -44 and -50 [39, 111–133]. Also the preparationof large CoAPO-5 crystals has been achieved [134, 135]. In-situ time resolvedsynchroton powder XRD studies of the hydrothermal synthesis of CoAPO-5 havebeen reported [136]. Most studies agree that Co(II) is substituted, at least partially,for Al(III) [137]. Owing to their acidic and redox properties, cobalt containingaluminophosphates have been employed in a large number of catalytic studies.Nevertheless, the nature, coordination and oxidation state are still clearly underdebate [138–140]. Initial ESR studies by Iton et al. [141] in CoAPO-5, CoAPO-34and CoAPSO-34 at 4.2 K showed a signal at g D 5–6, which was ascribed to Co(II).Calcination of the as-synthesized materials led to a signi� cantly reduced intensityof Co(II) ESR signal strength. This reduced intensity (»23%) was ascribed tooxidation of Co(II) to Co(III) during the calcination procedure. Similar conclusionswere drawn by Montes et al. [128] and others, [141–143] who studied CoAPO-5.

Kurshev et al. [143, 144] observed an unusual temperature dependence for theCo(II) ESR signal of CoAPO-5. The absorption signal of Co(II) in calcined CoAPO-5 does not follow the Curie law and therefore the ESR intensity of calcined samplesis not directly associated with the amount of paramagnetic cobalt. While thetemperature dependence of the intensity follows the Curie law in as-synthesizedsamples, the calcined samples exhibit an abnormal temperature behavior, which


is well explained by distortion of the tetrahedral symmetry of Co(II) to a lowerdihedral symmetry by interaction of two oxygen molecules with cobalt. Theanomalous temperature dependence of the ESR intensity of Co(II) in calcinedCoAPO-5 can then be explained by the existence of negative zero-� eld splittingbetween the Ms D §1=2 and Ms D §3=2 Kramer doublets. When the temperaturedecreases, the Co(II) ions tend to populate the Ms D §3=2 level, which isnot ESR active, and the intensity of the transition between Ms D C1=2 andMs D ¡1=2 decreases. Therefore, changes of the local symmetry near the cobaltion are responsible for the intensity loss of the ESR signal without a changein the oxidation state. These conclusions were also drawn in two independentstudies [145, 146] using IR and UV-VIS spectroscopy (vide infra). Temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA) of amineprobe molecules show that Co(II) at low concentrations is present in frameworkpositions and generates discrete Brønsted sites in a concentration equal to thecobalt content in both yellow-green and blue samples consistent with no oxidationassociated with the color change [142].

Quantifying the number of Brønsted acid sites created by the bridging OHgroups between Co(II) and a pentavalent phosphorus has also been utilized toinvestigate the redox properties of CoAPO materials based on the assumption thatthe number of bridging OH groups correspond to that of divalent cobalt specieslocated in the framework. Following the intensity change of IR bands of adsorbedpyridine, Kraushaar-Czarnetzki et al. [163] concluded that oxidation of CoAPO-5and CoAPO-11 results in partial oxidation of the framework cobalt centers. Similar� ndings were reported by Marchese et al. [127] for CoAPO-18. In contrast, Lohseet al. [145] noted that oxidation of framework Co(II) to Co(III) can not be con� rmedon the basis of the concentration of bridging OH groups.

The use of NMR spectroscopy in characterizing cobalt-containing AlPOs hasgained some momentum recently following the work of Canesson et al. [147] andVan Breukelen et al. [148]. The presence of cobalt in the samples typically results insignal broadening or the appearance of multiple peaks. Montes et al. [128] observedthat compared with AlPO4-5 in the spectrum of CoAPO-5 multiple and intense sidebands were present. They ascribed the observed increase in anisotropy to strongdipolar coupling of phosphorus with paramagnetic cobalt and, hence, as evidencefor framework incorporation of cobalt replacing aluminum. The observation ofintense sidebands is reported by several authors, [149, 150] while in other studies noincrease of the anisotropy was reported at all [148, 151]. However, measurements ofimpregnated Co/AlPO4-5 and Co/AlPO4-11 [74, 149] also showed in increase ofthe anisotropy and, therefore, chemical shift anisotropy can not be used as evidencefor isomorphous substitution of cobalt into aluminophosphates.

Prakash et al. [150] suggest that the broad peak for CoAPSO-46 is causedby the presence of two NMR signals, namely P(3Al, 1Co) and P(4Al). Recentresults suggest that the P(3Al, 1Co) signal is broadened and/or displaced beyonddetection in the spectral window used for routine NMR measurements. Due to


dipolar interaction of the phosphorus with nearby paramagnetic cobalt, these signalsare moved over very large distances and cannot be excited when the irradiationfrequency is centered at ± D 0 ppm. Canesson and Tuel showed for CoAPOsthat a signal for P(3Al, 1Co) is found at about ± D 2500 ppm [147]. Theysuggest that for every cobalt atom which is a direct neighbor to the phosphorusatom, the resonance of the phosphorous atom under investigation is shifted up� eldby about 2500 ppm. Application of the usual inversion-recovery sequence doesdistinguish between relaxation processes of 31P nuclei in synthesized CoAPO-n andimpregnated Co/AlPO4-n molecular sieves [152]. The values of time (¿ ) at zerointensity for 31P NMR signals in the inversion-recovery sequence are much reducedin CoAPO-5 in comparison to Co/AlPO4-5. Mali et al. [153] later showed that spinlattice relaxation rates are independent of the nature of cobalt bonding and dependprimarily on the effective cobalt concentration. Therefore, this method can onlyindicate whether or not paramagnetic centers are distributed uniformly throughoutthe sample.

The clustering of cobalt in CoAPO-5 has been studied by Van Breukelen et al.[148] employing quantitative 31P NMR spectroscopy. They found that with increas-ing cobalt content in CoAPO-5 from 0.001 wt% to 3.15 wt% the percentage ofNMR visible 31P decreases from 97 to 42%. If the existence of extraframeworkcobalt can be ruled out, then around 10 NMR-invisible phosphorus atoms per cobaltatom are needed to explain the observed decrease in signal intensity. The amount ofNMR-invisible phosphorus can not be explained by assuming that only 31P signalsof phosphorus in the � rst coordination sphere (four) become invisible. On the con-trary, if one assumes that the phosphorus atoms in the � rst and the third coordinationsphere become invisible, then the extent of NMR-invisible phosphorus becomes toohigh (25). A reasonable explanation for these observations is formation of cobaltclusters of � ve or more cobalt atoms, which leads to about 10 invisible phosphorusper cobalt atom.

The question whether or not it is possible to switch between Co(II) and Co(III)in CoAPOs is of paramount importance for oxidation reactions. It is believedthat the blue color of as-synthesized materials is characteristic of tetrahedralCo(II)O4 environments and is therefore consistent with cobalt occupying tetrahedralframework sites. Tetrahedral coordination of cobalt has been identi� ed by their d–delectron transitions in the UV-VIS spectra [137]. Calcining of the blue materialsat 400 to 500±C causes the color to change to green, green-yellow or beige forhigh, medium and low cobalt contents respectively. It has also been suggested thatthese color changes re� ect the varying degrees of oxdiation to cobalt(III) [141, 150,154–156], which might well be dependent on the structure type under investigation[157]. Barrett et al. [156] have used X-ray absorption spectroscopy (XAS) toinvestigate different cobalt-containing materials after synthesis, calcination andreduction. They found that while in as-synthesized materials Co(II) is in a regularfour-coordinated site, the situation is more complex in calcined materials. InCoAPO-18 essentially complete oxidation of Co(II) to Co(III) is observed, the


local coordination of high-spin Co(III) being distorted. In contrast, incompleteoxidation is observed for CoAPO-5 and CoAPO-36. Chao et al. [158] investigatedthe redox behavior in CoAPO-5 using ESR and IR spectroscopy. They explainedthe behavior of cobalt in terms of a species that they characterized as a dioxygencomplex, O2-CoAPSO-5, in which the framework tetrahedral Co(II)O4 increases itscoordination and valence state by attaching a OC

2 molecular ion to form a complex.Recently, Wu et al. [159] proposed a dioxygen-cobalt species in CoAPO-11 basedon their combined analysis of Raman and ESR spectroscopy. Calcination at 550±Cyields the diamagnetic hydroperoxo Co O O H species, which shows a Ramanband a 1019 cm¡1. After subsequent hydrogen reduction followed by O2 adsorption,a superoxo-like cobalt species (Co O O?) is generated (Fig. 7), which has aº(O O) Raman band at 1048 cm¡1. The O O and Co O stretching modes havebeen further con� rmed by 16O2/18O2 isotope substitution.

Diffuse re� ectance UV-VIS spectroscopy has been frequently employed forthe characterization of cobalt-containing aluminophosphates [160– 162]. As-syn-thesized CoAPO-5 has been characterized by a triplet at 16 000, 17 300 and18 500 cm¡1, which corresponds to 625, 578 and 541 nm [149, 154, 160, 161, 163,164]. These absorptions are interpreted as due to Co(II) in a tetrahedral environmentof oxygen atoms in the framework of the aluminophosphates. After calcinationthe intensities of these bands are reduced and the band positions are shifted to15 500 cm¡1 (645 nm), 17 300 cm¡1 (578 nm) and 19 500 cm¡1 (513 nm). Intensebands arise around 25 200 cm¡1 (400 nm) and 31 500 cm¡1 (317 nm). The spectrumis ascribed to Co(II) in a tetrahedrally distorted environment and to Co(III). Thisstructural Co(III) is then reported to be reducible with hydrogen, carbon monoxide,NO, methanol, acetone, toluene and water [146]. Spectroscopic criteria for thedistinction of framework or extraframework Co(II) in CoAPO-5 have been recentlypresented by Verberckmoes et al. [162]. Tetrahedral Co(II) in the frameworkis characterized after reduction by bands at 5500 and 15 150 cm¡1. TetrahedralCo(II) in extraframework positions exhibits bands at 5500 and 16 850 cm¡1,while octahedral Co(II) in extraframework positions shows bands at 8100 and19550 cm¡1. Upon calcination, (pseudo)-tetrahedral Co(III) in the framework,(pseudo)-octahedral Co(II) in extraframework sites and (pseudo)-tetrahedral Co(II)in framework and in extraframework sites are formed depending on the qualityof the sample. Upon reduction Co(III) is transformed to tetrahedral Co(II) in theframework with formation of an OH bond with a typical stretching frequency of3530 cm¡1. However, the mechanism of the redox process is still under discussionand there is disagreement as to the existence of trivalent cobalt in the calcinedmaterials. Instead, it has been suggested that the color changes are the result ofdistortions of the symmetry of the original tetrahedral environment. The redoxproperties of CoAPO-5 and CoAPO-44 have been investigated by Berndt et al. [146]by temperature-programmed reduction, oxidation and desorption. Furthermore, theacidity of the samples after oxidation or reduction treatments was investigated bytemperature-programmed desorption of ammonia and FTIR using pyridine as a


Figure 7. Proposed model for cobalt species in CoAPO-n under redox treatment according to(a) Verberckmoes et al. [162] and (b) Wu et al. [159].

probe molecule. Berndt et al. [146] found no evidence for a valence state change ofa signi� cant portion of the framework cobalt ions nor adsorption of oxygen on suchsites. The formation of framework Co(III) from Co(II) was also not supported byLohse et al. [165] who conducted IR investigations of basic probe molecules (NH3,pyridine and acetonitrile) adsorbed on CoAPO-5 and CoAPO-44. The insertion ofone cobalt ion into a molecular sieve framework creates one acid site. Concerningthe nature of the acid site, Marchese et al. [166] proposed an equilibrium betweenbridging hydroxy groups, paired centers of Lewis sites and POH groups (Fig. 8). Itis of note that the presence of Lewis acid sites, such as an unsaturated cobaltin structure B of Fig. 8, is revealed by adsorbing water [166], ammonia [145]acetonitrile [167] or carbon monoxide [145, 168]. The correlation between cobaltframework substitution and acidity has been investigated in a series of papers by


Figure 8. Proposed equilibrium between bridging hydroxyl groups, paired centers of Lewis acid sitesand P OH groups.

Thomas and co-workers [166, 169]. The resulting solid acid catalysts were alsotested for methanol conversion to light ole� ns to study the in� uence of acidityon the catalytic activity. To overcome the dif� culties in revealing the Brønstedacidity of CoAPO catalysts, Thomas et al. [170] proposed the use of nitrogenas a molecular probe, which is considered to be advantageous because nitrogenis a weaker base and is unlikely to affect the catalytic centers. The bridged OHstretching mode is shifted 100 cm¡1 downward by adsorbing N2 at 77 K becauseof the formation of a weak H-bond. This shift is about 10–20 cm¡1 smaller thanthat of the OH in HZSM-5, which suggests a lower Brønsted acidity for CoAPO-18 compared to HZSM-5. The downward shift of the stretching and the upwardshift .1±OH D C14 cm¡1/ of the bending of the P OH groups upon adsorption ofnitrogen is evidence for the formation of H-bond complexes between N2 and OH(Fig. 8) [170]. The adsorption of more basic probe molecules can greatly affect theCo O bond in the POHCo groups in the equilibrium in Fig. 8. Structure B shouldthen be favorable and the cobalt ions act as Lewis acid sites which is evident fromthe adsorption of Lewis bases such as NH3, CO or acetonitrile.

The local environments of cobalt atoms in as-synthesized, calcined and hydrogen-reduced CoAPSO-34 have been studied by X-ray absorption spectroscopy [171].Cobalt is shown to be sited as Co(II) in the framework and calcination of CoAPSO-34 (4.4 wt% Co) leads to two-thirds of the Co(II) being oxidized to Co(III).Reduction of the sample leads to materials containing only Co(II). The change ofthe local environment of Co in CoAPO-18 has been investigated by combined useof EXAFS and XANES [169]. The oxidation of Co(II) to Co(III) during calcinationresults in a displacement of the K-edge of 1.2 eV, an increase of the structure beforethe edge at 7710 eV and a reduction of the frequency of the EXAFS vibrations. Thisreduction re� ects a decrease of the Co O-bond length from 0.195 nm in as-synthesized material to 0.183 nm in a calcined sample. Upon reduction, the bondlength increases to 0.19 nm. Detailed EXAFS investigations showed that tetrahedralCo coordination is slightly disordered, which is due to one Co O bond beingsubstantially longer (0.204 nm). A preliminary explanation is the protonation ofthe respective oxygen [169].

A photoacoustic spectroscopic (PAS) technique was applied to obtain depth-resolved information on CoAPO-44 samples [172]. PA spectra showed the presenceof cobalt species with a lower symmetry and of tetrahedral framework cobalt(II)


ions. The species with lower symmetry are more concentrated in the inner partof the crystallite of the as-synthesized CoAPO-44, while the cobalt(II) ions aredistributed rather uniformly in a calcined sample. PA spectra of CoAPO-5 andCoAPO-11 have shown that, in agreement with results by Schoonheydt et al. [154],the amount of cobalt that can be incorporated is limited to about 0.5 mol% [173].ESR spectra reveal that allyl or cation type radicals are formed via interaction withframework cobalt ions upon adsorption of alkenes like cyclohexene, cyclopentene,and tetramethylethene.

The interaction of NO with Co(II)/Co(III) redox centers in CoAPO-5 and CoAPO-18 has been studied by FT-IR and UV-VIS spectroscopy at 298 and 85 K [174]. Twodifferent Co(II) sites were found in these materials. Co(II) sites located in theframework [Co(II)(OH)P] adsorb NO to form dinitrosyl complexes, which arereactive and form Co(III) and N2O, and Co(II) ions in structural defect sites whichact as Lewis acid sites.

In the view of the confusion still existing about the presence (or absence) and thepossible nature of the redox centers in CoAPOs, new detailed spectroscopic studieshave been recently undertaken for CoAPO-5, CoAPO-11, CoAPO-41 and CoAPO-46 employing ESR, UV-VIS and X-ray absorption spectroscopy [137–139]. Thesethree independent studies report that oxidation of isomorphously substituted frame-work Co(II) to Co(III) is not (or only to a small extent) observed. The observedchanges in the ESR spectra and the EXAFS data are best explained by the modelproposed by Barrett et al. [156]. Charge compensation in the calcined materials isachieved through the formation of anion vacancies following the loss of protonatedtemplates (one vacancy for every two Co ions). Such vacancy formation will dis-tort the framework in the proximity of Co(II) causing the observed effects on theESR line width and EXAFS. UV-VIS spectra also con� rm the formation of oxygenvacancies in the close vicinity of cobalt sites. It can be assumed that such anionvacancies are also responsible for the acid sites reported by some authors and theformation of radical cations. The formation of oxygen adducts as reported by severalauthors was ruled out.

3.7. Nickel

Because of their catalytic importance in particular for the methanol to ole� nsprocess, the synthesis and characterization of nickel-containing aluminophosphateshas been extensively studied [175–177]. Spectroscopic characterization of thenickel ions in these materials is, however, often limited to ESR spectroscopy. Itis, therefore, of particular interest that the intermediate paramagnetic valence statenickel(I) can be stabilized in these materials [178–182]. Gamma-irradiation at77 K can be used to generate nickel(I), but this method is rather inef� cient andis therefore not used very much [183]. A better way to reduce nickel(II) is bythermal reduction at 473–573 K under 100 to 500 torr of hydrogen. Thermalreduction is complicated by the formation of metallic nickel in addition to nickel(I), so that particular care is needed to establish the conditions of temperature,


Table 2.31P ESEM parameters for Ni(I) in NiH-SAPO-11 and NiH-SAPO-5 [181, 182]

Sample Dehydration Shell N R (nm) Aiso (MHz) Sitetemperature

NiH-SAPO-11 473 1 3:6 0.33 0.59 near SII2 7:2 0.60 0.18

773 1 5:3 0.34 0.50 SI2 12:3 0.70 0.12

NiH-SAPO-11(5) 573 1 3:5 0.28 0.42 near SII2 4:5 0.60 0.10

773 1 5:2 0.33 0.56 SI2 10:5 0.72 0.01

hydrogen pressure and reduction time for generation of a suitable concentration ofnickel(I) species [178, 180–185]. A third possibility is to utilize the autoreductionprocess which takes place when the hydrated molecular sieve is dehydrated undervacuum at temperatures above 573 K. Particular care is needed to avoid formation ofsuperferromagnetic nickel(0) (g D 2:25) which is thermodynamically more stablethan Ni(I) [183, 186].

ESR studies of nickel(I) in SAPOs must be carried out carefully since Ni(I) isreadily oxidized by water or oxygen. Isolated Ni(I) species are characterized byan axially symmetric g-tensor (gjj D 2:519, g? D 2:111/. The location of theactive species is of great importance for catalytic applications. The location ofnickel(I) has been determined at various stages of dehydration [181, 182]. But theESR spectra alone do not reveal such information, since the changing g-values arenot discriminatory. A powerful tool is 31P ESEM spectroscopy, where a signi� cantdifference in the spectra recorded after dehydration at 473 K and 773 K can bedetected. It is deduced that Ni(I) is located in the center of a hexagonal prism(SI sites) with 5.3 nearby phosphorus atoms after dehydration at 773 K (Table 2)[181, 182].

Polar adsorbates such as water, methanol or ammonia are shown to interactreadily with Ni(I) in different silicoaluminophosphates [182, 187]. However, forless polar adsorbates such as ethylene the development of new g parameters takesa longer time and even a higher temperature (353 K) in the case of NiH-SAPO-11.It has been concluded that Ni(I) has to migrate from site I to a site in a largechannel, most likely SII*, to interact with the adsorbate molecules. The number ofadsorbate molecules interacting with the Ni(I) center can be determined by ESEMspectroscopy [185, 187, 188].

The kinetics of nitric oxide adsorption on NiH-SAPO-17 and NiH-SAPO-35were followed by ESR spectroscopy and different Ni(I)-NO species have beenidenti� ed. After a time of 24 h, NO2 is also observed in NiH-SAPO-17, but inNiH-SAPO-35 a Ni(I)-(NO)C complex is still observed after a few days [189].


The interesting question of whether Ni(I) can be incorporated into the (silico)-aluminophosphate framework has prompted several studies in different structuretypes. Among these are NiAP(S)O-5 [184, 185, 190–192] and –11 [181, 185, 193],NiAP(S)O-34 [194– 197], NiAPO-20 [198] and NiAPSO-41 [199]. The synthesisis usually attempted by adding a nickel salt (NiCl2 ¢ 6H2O, Ni(CH3COO)2) to thesynthesis gel. But actual incorporation is dif� cult to prove. After synthesis andsubsequent thorough washing, nickel is found by chemical analysis. A detectedincrease of acidity gives only limited evidence for nickel being in a frameworksite [200]. All investigations have the similar conclusion that the amount of nickel,if any, which can be incorporated into the framework is relatively low.

ESR and ESEM spectroscopy are useful tools for evaluation of possible incorpora-tion of nickel into SAPO frameworks [180, 184, 185]. A study on nickel in AlPO4-5and AlPO4-11 reveals a room temperature ESR spectrum, which exhibits two tran-sitions with parameters gjj D 2:09 and g? D 2:07 for NiAPO-5 and gjj D 2:08 andg? D 2:07 for NiAPO-11 [193]. These ESR spectra were recorded directly aftercalcination and ascribed to nickel in framework positions.

A better approach to the ESR spectroscopic detection is the reduction of nickel(II)to nickel(I) by thermal or hydrogen reduction and subsequent ESR and ESEMmeasurements at 77 K and 4 K, respectively. While the ESR spectra of Ni(I) in ion-exchanged or synthesized Ni-materials are not very different, 2D and 31P ESEM dataprovide some evidence for framework incorporation of nickel. There is a signi� cant

Table 3.31P ESEM data of nickel(I) in ion exchange versus framework positions [185, 197, 199]

Sample Shell N R (nm)NiAPSO-5 1 8:8 0.51

2 24:5 0.97NiAPSO-11 1 10:5 0.52

2 30:1 0.95theory: P-site 1 10–12 0.5–0.6

2 28 0.9–1.0NiH-SAPO-5 1 5:2 0.33

2 10:5 0.72NiH-SAPO-11 1 5:3 0.34

2 12:3 0.70theory: SI position 1 5–6 0.3–0.4

2 10–12 0.65–0.75NiH-SAPO-34 1 2:2 0.39

2 3:0 0.65NiAPSO-34 1 3:2 0.42

2 2:4 0.53theory: P site 1 7 0.42–0.55NiH-SAPO-41 1 2 0.35

2 3 0.59NiAPSO-41 1 12 0.54

2 10 0.78


difference in the 31P modulation pattern between Ni(I) in ion-exchange sites andNi(I) in framework positions (Table 3). The ESEM data for Ni(I) in NiAPSO-5, NiAPSO-11, NiAPSO-34 and NiAPSO-41 are consistent with nickel replacingphosphorus in the original alternating Al-P framework (see Table 3) [184, 185, 199].Furthermore, the unique adsorbate coordination geometries found for the interactionof methanol and ethylene support that Ni(I) occupies framework positions inNiAPSO-5, NiAPSO-11, NiAPSO-34 and NiAPSO-41 [180, 182, 185, 197, 199].

3.8. Copper

Cupric ions have proved to be very fruitful spin probes in molecular sieves[201–203]. Of particular interest is the fact that the cupric ion relocates uponpartial or complete dehydration of the silicoaluminophosphate. Also the adsorptionof different adsorbates, like water, methanol, benzene, ethylene and DMSO, affectsthe location and coordination of the Cu(II) cation in silicoaluminophosphates withAFI [204–207], AEI [206, 208], AEL [205, 209–211], CHA [212–214], FAU[215, 216], ERI [217] and LTA [218] topologies, respectively.

Cu(II) was also ion-exchanged into SiVPI-5 and SAPO-8 which were synthesizedby a new method and which are large pore materials with 18-ring and 14-ringchannels, respectively [219]. The reaction of copper salts with AlPO4-5 or VAPO-5 has been investigated by Whittington and Anderson [220]. Ion incorporation isproposed to occur initially at defect sides, which gives rise to a number of uniqueenvironments upon Cu(II) incorporation as detected by ESR spectroscopy.

Some attempts have been made to incorporate copper(II) into the frameworkof the silicoaluminophosphates [210, 221, 222]. To attempt this, a copper salt isadded to the synthesis gel. While Rajic et al. [221] report that copper additionto the gel actually inhibits the synthesis, Lee et al. [210] and Moen et al. [222]were able to synthesize H-SAPO-5 and H-SAPO-11. Nevertheless both groupsconclude that Cu is not incorporated into the framework of these structures underthe conditions prevailing in their synthesis. In the synthesis of SAPO-5 reported byMoen and Nicholson [222] copper(II) is believed to be reduced by the templatingamine to copper (I), which subsequently disproportionates to metallic copper andcopper(II). Calcination of this material produces a mixture of CuO and SAPO-5.Lee et al. [210] used ESR and ESEM measurements to conclude that Cu(II) insynthesized nominal CuSAPO-11 is located in ion-exchange positions, which werepreviously also detected in CuH-SAPO-11 prepared by conventional liquid stateion-exchange. In nominal CuAPO-11, Cu(II) is located in extraframework positionsof AlPO4-11. It was also concluded that the complexation of the cupric ions withthe template during the synthesis may prevent successful incorporation of copper(II) into the framework [220].

The hydrothermal synthesis of CuAPO-5 has been attempted by using CuO inthe presence of tetraethylammonium hydroxide as a structure directing agent [223].ESR and ESEM results indicate that Cu(II) is located as octahedral Cu(II) in theframework of the as-synthesized material. Upon dehydration, which is shown to be


reversible, tetrahedrally coordinated copper is observed in the molecular sieve. Itis evident that the location and coordination geometry of Cu(II) in CuAPO-5 isdifferent from CuH-SAPO-5, where Cu(II) is located in an ion-exchange site. Morerecently, the syntheses of CuAPO-5 and CuAPO-31 with copper sulfate added tothe synthesis gel have been reported [224, 225]. The XRD pattern of CuAPO-31indicates a transition from triclinic to a rhombohedral structure symmetry, whichwas taken as a hint for successful framework incorporation. However, the questionwhether true isomorphous substitution of copper into AlPOs is possible is or not isstill not decided yet.

3.9. Zinc

Zinc is generally considered a good candidate for isomorphous substitution intothe framework of (silico)aluminophosphates. Framework incorporation has beenclaimed for several structure types, including AEI [226], AEL, AFI [227– 229],AFO [119], AFR [230], AFY [231], CHA [232– 235], ATS [236] and AlPO4-44 [237]. The incorporation of zinc into the aluminophosphate frameworks isgenerally dif� cult to prove and usually requires EXAFS data and/or Rietveldre� nements of powder X-ray diffraction data. The parameters extracted from theZn K-edge EXAFS spectra measured on powdered MnZnAPO-34 show a zincatom coordinated to four oxygen neighbors at a distance of 0.194 nm in the� rst shell. According to these data, zinc atoms are isomorphously substituted foraluminum in the molecular sieve framework [232]. Tusar et al. [233] have reportedthe synthesis of a phase pure zinc rich ZnAPO-34 where 20% of the aluminumatoms have been replaced by zinc. EXAFS analysis shows that zinc is coordinatedto four oxygens at a distance of 0.194 nm in an as-synthesized sample and to threeoxygens at a distance of 0.196 nm in calcined material. This decrease in the numberof oxygen atoms coordinated to zinc is explained by (2).

.O/2Zn.OH/2 ¡! .O/2Zn O C H2O (2)

The 31P MAS NMR spectra of ZnAPO-34 show up to four signals, while the 31PNMR spectrum of SAPO-34 shows only one line at about ¡30 ppm resulting fromtetrahedral phosphorus (P(4Al)). The various lines are attributed to different P(nAl,(4 ¡ n)Me) units, although a band at ¡20 ppm can also be attributed to the presenceof P OH defect groups. Moreover, the calculation of the fraction of metal inthe sample after deconvolution of the spectra is in good agreement with chemicalanalysis. Therefore, this has been taken as evidence for framework incorporation ofZn into the AlPO4-34 framework [233, 238].

Elangovan et al. [239] found a considerable increase of the overall unit cell vol-ume with introduction of ZnSO4 into the synthesis gel for AlPO4-5 and AlPO4-11.Together with a shoulder in the 31P MAS NMR spectra these hints are taken asevidence for the isomorphous substitution of zinc into the aluminophosphate frame-work. An increase in acidity of ZnAPO-11 and ZnAPO-5 compared to the parent


materials is shown by temperature programmed desorption of pyridine and is re-� ected in the catalytic transformation of camphene (see below) [239].

Large pore zinc-incorporated ZAPO-36 (ATS topology) was prepared and charac-terized by various techniques [240]. The surface composition determination by XPSanalysis revealed that the concentrations of zinc and phosphorus are higher on thesurface than in the bulk. The binding energy of Zn 2p3=2 corresponds to that of Zn2p3=2 in ZnO, but the site energy distribution of ZAPO-36 has been found to be verybroad. The larger number of strong acid sites compared with AlPO4-5, SAPO-5 andMAPO-5 is re� ected in a high catalytic activity in acid catalyzed reactions.

The synthesis of zinc-containing aluminophosphates with FAU and AFR struc-tures has been reported [230, 241]. Several characterization techniques indicate thattrue framework incorporation of zinc has occurred for the latter material. The iso-merization of m-xylene was used as a model reaction to probe the catalytic activityof the samples and con� rm the presence of Brønsted acid sites (see below). Thecatalytic tests showed that the acid sites generated upon framework incorporationof Zn are stronger than those resulting from incorporation of Si in the AFR frame-work [230].

3.10. Cadmium

The syntheses of CdAPO-31 and CdAPO-5 have been claimed recently [224, 225,242, 243]. The obvious question, whether isomorphous substitution of cadmiuminto AlPO and SAPO frameworks occurs, was so far not addressed by spectroscopictechniques and still remains undecided.

3.11. Palladium

Paramagnetic palladium species have been studied by ESR and ESEM spectroscopyin SAPO-5, SAPO-11 [244–248] and SAPO-34 [249]. It has been found that incontrast to zeolite X and Y, SAPO materials are not able to stabilize paramagneticPd(III) species; therefore only paramagnetic Pd(I) is found. The formation of Pd(I)during this treatment is not fully understood [250].

Regardless of the mechanism of the formation of the Pd(I) species, the mainquestion concerns the location of these species. As evident from the ESR spectrum,two different Pd(I) species A and B are formed. Different locations are determinedfrom 31P modulation [248]. Simulation of the nuclear modulation in terms of thegeometry of nearby phosphorus nuclei identi� es Pd(I) species A as located in ahexagonal prism site (site I) and Pd(I) species B as located at site II* near a 6-ringwindow. A change of this location is observed if different adsorbates are introducedinto the system. Signal A usually disappears after adsorption of e.g. methanol,which indicates a migration of Pd(I) from the inaccessible site I to a site close to themain channel (site II, or II*).

ESR and ESEM spectroscopic methods have been used to deduce several adsor-bate interactions of Pd(I) in PdH-SAPO-11 and PdH-SAPO-5 molecular sieves. The


adsorption of water, ammonia, carbon monoxide, hydrazine and benzene was inves-tigated after activation to form Pd(I). The ESR parameters are very sensitive to theinteraction of Pd(I) with different adsorbates. Pd(I) is coordinated to two moleculesof ammonia and CO, which is derived from the 14N and 13C superhyper� ne split-tings, respectively. Only one molecule of benzene and pyridine is coordinated toPd(I). Analysis of three pulse ESEM spectra of PdH-SAPO-11 indicates interac-tion with four deuteriums (two water molecules) in a distance of 0.35 nm. In bothstructure types the Pd-H2O complex is suggested to be located in the SII* sites[245, 247].

The introduction of Pd into the framework of SAPO-5 and SAPO-11 was alsoattempted. ESR and ESEM results reveal no signi� cant differences between ion-exchanged and synthesized palladium-containing SAPO-11. It was found that inthe range 0.5 to 1.0 mol% Pd in PdSAPO-11, it is most probable that the palladiumions are not incorporated into framework sites [244].

Palladium ion-exchanged SAPO-5 has been used as a catalyst for low temperaturecombustion of methane [251]. A high dispersion of Pd was attained and a CH4

conversion of 90% was achieved at 753 K, which is 200 K lower than that of aconventional Pd/Al2O3 catalyst.

3.12. Platinum

Platinum incorporation into the framework of AlPOs or SAPOs has not beenreported so far, but some studies on the characterization and catalytic testing ofplatinum clusters in silicoaluminophosphates prepared by impregnation or ion-exchange have been published (vide infra).

3.13. Rhodium

The adsorption of CO on Rh/SAPO-5, which was prepared by ion-exchange ofSAPO-5 with Rh(Cl)3 and subsequent hydrogen reduction, has been monitoredby infrared spectroscopy [252]. The formation of a high-frequency dicarbonylspecies was observed, as has been noted before for molecular sieve supportedrhodium. This species was related to the presence of oxygen rather than water ashas been previously suggested. The authors propose that this dicarbonyl species actsas a neutralizing cation for the anionic SAPO-5 structure. ESR spectroscopy wasused to monitor the behavior of paramagnetic rhodium species in the AEL structure[253, 254]. The introduction of Rh(III) into the SAPO-11 synthesis mixture givesRhAPSO-11 in which a paramagnetic Rh(0) species is more stable than in solid-state ion-exchanged Rh-SAPO-11. The different behaviors of these two synthesizedand ion-exchanged materials with respect to ethylene interaction supports theconclusion that rhodium species are in different sites in these two materials.


3.14. Molybdenum

Molybdenum can be introduced into molecular sieves via a solid state reactionusing MoO3. ESR experiments reveal that after solid state reaction and subsequentdehydration at 298 K no signal due to Mo(V) is obtained [255]. Dehydration atelevated temperatures produces a weak ESR spectrum which is composed of twosignals. The signals are ascribed to Mo6c(V) (compressed octahedral) and Mo5c(V)(square pyramidal) species where the subscripts denote coordination numbers. Afterdehydration, the interaction with different adsorbates was investigated [256]. Uponadsorption of polar molecules (D2O, methanol) a partial oxidation to molybdenumblue (a mixed valent Mo(V)–Mo(VI) oxide) is detected. Nonpolar moleculeslike ethylene interact only with one of the two Mo species showing that onespecies has a more recessed location [256]. From ESEM data and adsorbateinteractions it is shown that Mo(V) exists as oxomolydenum ions as either (MoO)3C

or (MoO2/C. The [Mo(V)O2]C species is more likely formed because of the inabilityof SAPO materials to stabilize highly charged clusters. Oxidation of the sample maylead to oxidation of Mo(V) to Mo(VI). This is indicated by an intensity loss of theMo(V) ESR signal and the appearance of an O¡

2 signal in the spectrum. Hydrogenreduction and subsequent ammonia treatment produces a strong signal, which isascribed to a Mo4c(V) species of distorted tetrahedral coordination.

3.15. Zirconium

The addition of zirconium to the synthesis gel of SAPO-11 has been reportedby Mériaudeau et al. [257]. Zirconium is not incorporated into the SAPO-11framework but is localized, at least partially, in the pores of the material. When usedin n-butene isomerization, Zr-SAPO-11 exhibited a higher isobutene selectivitycompared with SAPO-11. The improved isobutene selectivity is attributed byMériaudeau et al. to the presence of zirconium in the pores inducing a partialblocking and, therefore, causing an increased shape selectivity. In addition, a verysmall number of external sites of Zr-SAPO-11 might explain the improvement inisobutene selectivity and stability with time-on-stream found in that study. Dongareet al. [258] reported the synthesis of ZrAPO-5. An increase in Brønsted and Lewisacidity is presented as evidence for isomorphous substitution of zirconium into theAFI framework.

3.16. Magnesium

Although not a transition metal ion, there are good reasons to include magnesiumin this review. Magnesium-containing AlPOs and SAPOs are also often tested incatalytic reactions and compared to transition metal ion containing counterparts.Magnesium has been incorporated into several topologies including AEL [259],ATS [236, 260–263], AFY [264], AFI [259, 265–267], AFS [268, 269], ATN[270–273], CHA [90, 259, 265, 266], DFO [274], GIS [270, 275, 276], SAO [277],


SAS [278], SAT [279] and SAV [280]. Magnesium incorporation in AlPO-20(sodalite) has been studied by Barrie and Klinowski [281]. They presentedevidence by means of 31P MAS NMR spectra for the isomorphous substitution ofaluminum by magnesium. Their concept has been expanded to a variety of differentstructures [282]. The multiple lines observed for the as-synthesized samples inthe 31P MAS NMR spectra are typically assigned to different P(4 ¡ n Al, n Mg)(n D 0–4) environments. With an increasing number of Mg neighbors a down� eldshift of the 31P resonance is observed. The magnesium content and the nP=nAl

ratio can be determined from the line intensities and are typically found to be ingood agreement with the results of chemical analysis [281, 282]. Moreover, therelative populations of the different phosphorus sites have been calculated from theintegrated intensities and compared with those calculated from a binomial theoremassuming a random distribution of the substituting element. While random orderingwas observed for the AEL, ATS, AFI and CHA topologies, the existence of orderingin the Mg distribution (e.g. Mg O P O Mg units) has been suggested forother structures like MAPO-20, MAPO-34, MAPO-39, MAPO-43 and MAPO-46.Another possible explanation of the observed peak intensities is the effect of organicmolecules like the template, water or P OH groups on the observed line intensitiesas suggested by several authors. [266, 283]. After calcination, typically broad andnot well resolved 31P MAS NMR spectra are obtained which show that the lineassignment of the spectra for an as-synthesized sample is not de� nitely settled. Thechemical and electronic nature of the various phosphorus environments in MgAPO-22 have also been investigated by 31P magic-angle-turning NMR (MAT NMR)[284]. Although the P(4Al) unit is highly symmetric (isotropic lineshape), Mgincorporation into the framework distorts the symmetry by altering the three 31P

Figure 9. Cation hydrolysis in MAPO-n frameworks (after Ref. [285]).


shielding tensor components unevenly and producing chemical shift anisotropy(CSA). The dramatic changes observed in the electronic environment of P by Mgsubstitution is taken as evidence for magnesium framework incorporation.

Calcination of magnesium-containing aluminophosphates is often accompaniedby distortion of the AlPO framework, which results in some cases in partial orcomplete structural breakdown. Prasad et al. [285] concluded from 1H NMR studiesthat hydrolysis of extraframework Mg(H2O)2C results in the formation of MgOHC

and a P OH group (Fig. 9), which accounts for the observed Brønsted acidity. Thenumber and the strength of the acid sites in Mg-containing AlPOs and SAPOs hasbeen assessed by temperature-programmed desorption of ammonia and pyridine andby IR spectroscopy. The presence of weak basic sites in MAPO-5 and MAPO-36has been detected by IR spectroscopy of adsorbed pyrrole [286].

3.17. Multimetal-substituted AlPOs and SAPOs

Only a few publications so far deal with the simultaneous incorporation of more thanone metal into the AlPO-framework [287, 288]. Akolekar studied the incorporationof up to � ve metals (Mg, Co, Mn, V, Zn) into the ATS topology [288]. The extentof framework incorporation was predominately assessed by chemical analysis andthe formation of acid sites, which were characterized by pyridine TPD and catalytictest reactions. An important result from XPS analysis in comparison to chemicalanalysis is that the distributions of the metals are different on the surface andin the bulk of the materials indicating that multimetal-incorporated AlPOs arenot homogeneous in composition. The synthesis of aluminophosphates with CHAtopology containing two metals (combinations of Zn, Mn and Co) was reported byTuel et al. [238] and Novak Tusar et al. [232]. It is suggested that both metalscan occupy framework positions in the chabazite structure. However, no clearadvantage of bimetal substitution is reported. In contrast, magnesium incorporationinto AlPO4-5 seems to prevent the formation of vanadyl cation (as observed forVAPO-5) and V(IV) occurs preferentially outside the framework as aggregated andseparated species [50, 51]. Based on the results by Blasco et al. [51], V ionsmainly occupy framework sites in calcined VAPO-5, while they are mainly locatedin extraframework positions in VMgAPO-5. It is suggested that the Mg ions inducea lower stabilization for vanadium species in the framework. A higher temperatureis required to reduce V ions in VMgAPO-5, which also explains the lower catalyticactivity of this catalyst in oxidative dehydrogenation reactions. However, moreresearch is needed to understand the isomorphous substitution of two or more metalsinto AlPO frameworks.

4. ADSORPTION

The adsorption of organics into aluminophosphates has been investigated [289–292],but studies on the sorption properties of MeAPOs and MeAPSOs are rare. Nitrogen


Table 4.Benzene adsorption capacities of molecular sieves with AFI (AlPO4-5) and ATO (AlPO4-31)structures at 298 K and p=p0 D 0:1 [242, 297]

Sample Metal content (atom%) Amount adsorbed (mmol g¡1/

AlPO4-5 — 0.642MgAPO-5 4.4 1.32CrAPO-5 0.32 1.10CoAPO-5 2.1 1.073VAPO-5 1.3 0.944CdAPO-5 0.3 0.395ZrAPO-5 1.8 0.343CuAPO-5 0.8 0.106NiAPO-5 0.1 0.085FeAPO-5 0.9 0.060AlPO4-31 — 0.684CdAPO-31 0.05 0.996CuAPO-31 0.04 0.727CoAPO-31 0.18 0.691MnAPO-31 0.24 0.608ZnAPO-31 0.59 0.595MgAPO-31 0.37 0.494

adsorption is sometimes used to prove that no extraframework metal oxide speciesare deposited in the channels of the molecular sieves and that isomorphous sub-stitution of the metal was successful. The acid properties of these materials are fre-quently addressed by IR spectroscopy of adsorbed bases like ammonia and pyridine.Less often, other probe molecules such as benzene [293, 294] and acetaldehyde[295] are used.

The adsorption isotherms of water and benzene for a series of MeAPO-5 andMeAPO-31 materials were reported recently by Kornatowski et al. [224, 242,243, 296, 297]. While water adsorption exhibits an isotherm of type V (IUPACclassi� cation), benzene adsorption isotherms are generally of type I. These materialsrevealed a high similarity for the sorption of water, which is rationalized in terms ofcomplete pore � lling by H2O molecules. In contrast, large differences are observedfor benzene adsorption (Table 4), which con� rms that complete pore � lling isfrequently not achieved The effect of incorporation of transition metal ions into AFIand ATO structures on benzene adsorption involves several aspects. The presence ofextraframework species in the pores may lead to reduced adsorption capacity; this isobserved for Cu and other metals. The amount of framework substitution and metaldistribution throughout the material greatly in� uences the adsorption capacity. Thismight be explained by an interaction between the metal centers and the defect siteswith the ¼ electrons of the benzene molecules. The type of the metal, however, isconsidered to play a secondary role. Adsorption potential calculations based on thePolany-Dubinin potential theory support these qualitative conclusions [243]. Theadsorption potentials of substituted MeAPOs with small metal content are lower


than those of the parent AlPO4-n, which is tentatively explained by strong effectsof localized sorption. An increase of substitution leads to both an increase ofadsorption potential and suppression of the effect of localized adsorption on isolatedmetal centers.

The sorption properties of CoAPO-11 and MnAPO-11 were studied by Singh et al.using water, ammonia, cyclohexane and n-hexane as probe molecules [298]. Theadsorption of cyclohexane and n-hexane is reduced in CoAPO-11 and MnAPO-11 compared to the parent material AlPO4-11. The uptake of ammonia is in thefollowing order: MnAPO-11 < CoAPO-11 < AlPO4-11. Although the sorption up-take is highest in AlPO4-11, the amount of retained ammonia (viz. the amount ofammonia molecules held irreversibly upon evacuation) is higher in the metal substi-tuted analogs due to the formation of Brønsted acid sites upon metal incorporation[298]. The acidity of CoAPO molecular sieves with AEL, AFI and CHA topolo-gies has been studied by adsorption of ammonia and acetonitrile [299]. At constantequilibrium pressure, the loading of acteone increases with the Co content of thesamples, which is attributed to the formation of strong acid sites by isomorphoussubstitution of Co into the aluminophosphate framework.

5. ACID CATALYSIS

5.1. Isomerization of ole� ns

The isomorphous substitution of transition metal ions into the framework of mediumpore aluminophosphates like AlPO4-11 enhances the selectivity of ole� n skeletalisomerization even beyond that observed with SAPOs [300]. SAPO-11 gives 42%conversion of hexene to skeletal isomers with 3% cracked products, while MnAPO-11 gives 64% conversion to skeletal isomers and only 2% cracked products [301].FAPO-11 is even more selective, giving yields of 71% and 1.5% for isomerizationand cracked products, respectively. Similar trends are observed for MnAPO-31 andFAPO-31 [301].

MnAPO-31 and MnAPSO-31 have been tested for catalytic isomerization of1-butene, which proceeds by either double bond or skeletal isomerization.MnAPSO-31 (MnO/P2O5 D 0:01) yields the highest percentage of isobutene,whereas the parent AlPO4-31 leads almost completely to a double bond shift withonly minor skeletal isomerization [302]. However, compared to SAPO-31, the in-crease in selectivity is limited. It is therefore likely that better catalytic performancecorresponds to a higher concentration of acid sites and presumably to higher acidstrength of these sites [302].

Metal-containing MeAPO-11 (Me D Mg, Mn, Co, Cr, Fe) molecular sieves areactive, selective and stable catalysts for the selective isomerization of n-butene toisobutene [12, 13, 303]. The isobutene yield at 400±C increases from 6% for AlPO4-11 to around 40% for the framework substituted analogs. At a reaction temperatureof 80±C, however, dimerization occurs to a large extent, which is also observed


Figure 10. Reaction scheme in camphene conversion of acid catalysts.


with MeAPO-5 catalysts, which are not very selective for isobutene formation(S < 5%). It is argued that the active site is a weakly acidic OH-group, possiblyin the neighborhood of or in combination with a Lewis acid site (Fig. 1) [12]. Thesynergism between Brønsted and Lewis acids sites is further supported by a studyof Arias et al. [13] showing that the skeletal isomerization ef� ciency during thetransformation of 1-butene correlates well with the number of both Lewis andBrønsted acid sites of medium and high acid strength. It is known that Lewis sitesalone can not act as active centers for skeletal isomerization.

The catalytic transformation of 1-butene was studied by Escalante et al. [304]over ZnAPSO-11, impregnated Zn/SAPO-11 and unpromoted SAPO-11. The dataindicate a higher skeletal isomerization selectivity for ZnAPSO-11, which is inagreement with a higher density of acid sites for this catalyst. The results wereexplained in terms of the model of Gielgens et al. [12], where the substitution ofAl(III) by Zn(II) ions leads to partially unsaturated Zn(II) ions in the vicinity ofstructural P OH groups.

MeAPOs are also found to be selective catalysts for the isomerization of 1-pentene,yielding around 60% of the three pentene isomers, viz. 3-methyl-1-butene, 2-me-thyl-1-butene and 2-methyl-2-butene. The favored formation of the latter isomercan be rationalized by a reaction mechanism via a protonated cyclopropane ringwith subsequent C C bond cleavage [305].

The catalytic transformation of camphene over a series of MeAPO (Me D V,Co, Ni, Zn) catalysts with AEL and AFI topologies has been studied by Elangovanet al. [306, 307]. The products formed are tricyclene, bornylene and monocyclicterpenes (Fig. 10). The product distribution is in� uenced by number and strengthof acid sites and the inverse weight hourly space velocity (WHSV)¡1. Tricyclene isformed over weak acid sites with low (WHSV)¡1, while monocyclics like dipentene,terpinolene, ®- and ° -terpinenes, p-menthene and p-cymene are formed overstrong acid sites. With increasing reaction temperature, the camphene conversionincreases, which is accompanied by a decrease in tricyclene selectivity.

Hydration of ethylene in the gas phase over nickel and zinc containing alu-minophosphates with AEL and AFI structures yields isobutanol, ethanol andn-butanol [308, 309]. With increasing partial pressure of water, selectivity forethanol and n-butanol increases at the expense of isobutanol selectivity. The au-thors conclude that Brønsted acid sites favor the formation of ethanol, which reactsfurther on Lewis acid sites to form isobutanol. With increasing water pressure, theseLewis sites are blocked, which prevents the conversion of ethanol to isobutanol.

5.2. Methanol and ethanol conversion

The demand for ole� nic feedstocks has increased rapidly in the past few years dueto increased needs for synthetic � bers, plastics and petrochemicals. The increase indemand for small ole� ns has periodically caused a shortage of basic raw materialseither because of limited ole� nic feedstock of suitable quality or a limitationin the present ole� nic production capacity. Thus, alternative sources of ethylene


production from non-petroleum sources are required to keep pace with the demandfor ethylene and other ole� ns. The progress in the conversion of methanol togasoline (MTG process) and ole� ns (MTO process) has been recently reviewed byStöcker [310].

H-SAPO-34 is acknowledged to be a powerful catalyst, with attractive perfor-mance in converting methanol to light alkenes, principally ethene, propene andbutene (MTO process) [311]. However, the distribution among C2, C3 and C4

alkenes is roughly comparable. SAPO-18 (AEI) is a chabazite-related structureand in activity and selectivity is comparable to SAPO-34 [312]. The conversion ofmethanol to hydrocarbons can reach 100% and the optimum distribution of etheneand propene in the hydrocarbon product can reach 80%. The catalyic activity ofSAPO-34, which has roughly twice as many Brønsted acid sites as good qual-ity SAPO-18, drops off more rapidly than the latter [312]. For further improve-ment, transition metal substituted aluminophosphates with the chabazite structure(MeAPSO-34 and MeAPO-18) have been tested by several groups [313–318].

Inui et al. [313] and Thomas et al. [314] reported independently that nickel-containing SAPO-34 (NiAPSO-34) is one of best catalysts for methanol-to-ole� nconversion yielding close to 90% ethylene at almost 100% conversion at a reactiontemperature of 450±C. The catalytic performance was maintained at this tempera-ture for 13 h; during this time no signi� cant change in methanol conversion or eth-ylene selectivity was observed [313]. For this catalyst, it was established by X-rayabsorption (EXAFS and XANES) that Ni was exclusively located in tetrahedrallycoordinated framework sites [313]. Loosely bound extraframework protons werethought to be key components of these catalysts. In contrast, a clear effect of reactiontemperature on the product distribution is reported by Thomas et al. [314] for theNiAPSO-34 catalysts after 2 h on stream; a temperature increase from 250 to 450±Cresults in an increase of methanol conversion from 90.5 to 95.5%, whereas the ethyl-ene selectivity decreases from 94.5 to 60.5%. The conversion of methanol to ole� nsover MeAPSO-34 (Me D Co, Ni) has been studied by van Niekerk et al. [319]. Thecatalytic performance of the catalyst was found to be closely related to the numberof strong acid sites determined by TPD of ammonia. The almost complete absenceof C5 and larger ole� ns was ascribed to the cage effect, which imposes a restric-tion on the formation of those compounds. Further treatments such as steaming,silanization and poisoning of the strong acid sites all reduced the number of strongacid sites and, hence, the catalytic activity.

By controlling the uniformity of the gel mixture, Inui et al. synthesized NiAPSO-34 crystals with Si/Ni ratios of 100 and 40 and a sharp crystal size distributionaround 0.85 ¹m [320, 321]. With increasing nickel concentration, the acid strengthmeasured by ammonia TPD decreases not only for the strong acid sites but alsofor the weak acid sites, which in turn results in increased ethylene selectivityin the order SAPO-34 < NiAPSO-34 (100) < NiAPSO-34 (40) at high methanolconversion [322]. In order to improve ethylene selectivity and mitigate cokeformation, the acid sites located on the external surface of NiAPSO-34 crystals


were selectively neutralized mechanochemically with basic oxides supported onmicrospherical non-porous silica [323]. For methanol conversion, the modi� edNiAPSO-34 catalyst exhibited a higher selectivity to ethylene and a longer catalystlifetime than the parent material. This was rationalized with a decrease of cokeformation on acid sites located on the external surface of the crystals.

In order to obtain NiAPSO-34 crystal containing a larger concentration of nickel,nickel formate was used as the nickel source [324]. However, not only theamount of nickel incorporated in the framework but also the concentration ofextraframework nickel was enhanced. As a consequence, the selectivity for methaneformation increased. Sul� dation of extraframework nickel by H2S suppressesmethane formation and restores high ethylene selectivity [324]. Steam addition tothe methanol feed also results in a higher ethylene selectivity and higher catalyticactivity for methanol conversion with time-on-stream [325].

Other small-pore aluminophosphates like SAPO-17 (ERI), SAPO-18 and SAPO-35 (AEI), as well as SAPO-11 doped with Ni or Cr have also been testedfor methanol conversion in comparison to SAPO-34 based catalysts [336, 327].NiAPSO-34 proved to have the best performance when both ethylene selectivityand catalyst lifetime are considered. MeAPSO-n samples appeared to be generallybetter catalysts than Me-SAPO-n materials. Incorporation of Cr into SAPO-34 alsoincreases the ethylene selectivity and the catalyst lifetime, most probably due to areduction of the number of strong and moderate acid sites. In contrast, NiAPO-18,which has essentially the same framework structure is found to be one of the poorestcatalysts for the MTO reaction [316]. ZnAPO-18, CoAPO-18 and MgAPO-18 areall very reactive at temperatures above 350±C [317]. Lower ethene selectivity andhigher propene and butene selectivity of the Mg-, Co- and ZnAPO-18 catalysts isalso obtained with materials with the CHA structure [317]. The authors, therefore,suggest that the nature of the substituting element is more important than the dif-ference in pore geometry between the AEI and CHA structures. Several authors in-vestigated a correlation between Brønsted acidity and methanol conversion activity[328, 329]. Besides selecting a molecular sieve with a different topology, the moststraightforward way to change the acid strength and concentration of acid sites is byisomorphous substitution of elements having different atomic electronegativities,concentration, and distribution in the framework of the molecular sieve. Hocevaret al. [315, 330, 331] investigated MeAPSO-34 and MeAPSO-44 molecular sieveswith Me D Co, Mn, Cr. They showed that introduction of transition metal ions in� u-ences the acidity strength of MeAPSO-34 and -44 and consequently the selectivitytowards ethylene formation, which follows the stability order of transition metal-ligand complexes suggested by Irving and Williams. This order is the consequenceof the extra stability of complexes due to contributions of the crystal � eld stabi-lization energy (CFSE) of atomic d-orbitals to the ligation energy. If the ligationenergy of ethene .¼ -complex) with a transition metal is higher, it will be less proneto subsequent oligmerization and, consequently, such a catalyst is more selective forethylene. If this model holds true, then Ni-containing samples should be the most


selective ones for ethylene, since Ni-complexes have the highest CFSE. This was atleast observed for NiAPSO-34 [311, 314].

Kaiser tested MeAPOs and MeAPSOs in order to form light ole� ns from a 70 : 30wt% water-methanol mixture [332]. MgAPO-34, CoAPSO-34 and MnAPO-34can completely convert methanol, but methane and carbon dioxide selectivities arehigh. For CoAPO-34, the following product distribution at a reaction temperatureof 425±C has been reported: ethylene 45.3 mol%, ethane 0.8%, propylene 27.1%,butenes 8.3%, methane 6.2%, CO2 10.0%.

Some larger topologies such as AFI and ATS have been tested by Lischke et al.[333] and Akolekar [240]. The best catalytic results with respect to the formationof lower ole� ns are achieved with NiAPO-5, which contains a suf� ciently highdensity of Brønsted sites of moderate strength and a comparatively small portionof strong Lewis sites. However, in molecular sieves with 12-ring pores such as

Figure 11. Reaction mechanism of methanol to ole� n conversion.


NiAPO-5 and ZnAPO-36, where aromatics formation takes place to a considerableextent, ethylene and propene selectivity is low compared to materials with the CHAtopology [240, 333].

Methanol conversion has been proposed as a test reaction for the elucidation offramework cobalt in CoAPO-5 and CoAPSO-5 after pretreatment in air or hydrogenat different temperatures [334]. Several studies on the kinetics [335] and mechanism[336, 337] of the methanol transformation have been published. Fig. 11 exhibits theproposed mechanism of methanol conversion to ole� ns [338]. The Al and the Siatoms of the framework can also be replaced by Me and P.

The conversion of ethanol was studied over MAPO-5, MnAPO-11, MAPO-36 andMAPO-46. At conversions around 50%, the selectivity for aromatics is relativelylow (<4.5%) and predominately C2 aliphatics are formed [190, 263, 268]. Similarresults were obtained for aluminophosphates with ATS topology containing Zn, Coand Mn [236].

5.3. Conversions of paraf� ns

The catalytic properties of MeAPO-n molecular sieves were assessed with n-butanecracking as a probe for Brønsted acidity [339]. The materials exhibit activitiesthat are both structure and metal dependent. Metal incorporation into the AFIframework imparts low activity for n-butane cracking, whereas the incorporation ofmagnesium into the CHA and ATS topology leads to moderate to high activity [4].The results of n-butane and n-hexane cracking on MgAPO-5 and CoAPO-5 revealthe presence of isolated strong acid sites in these materials [126]. For n-hexane,primary cracking, dehydrogenation followed by cracking as well as secondarycracking were observed. The catalytic activity found was three times higher forMgAPO-5 and CoAPO-5 compared to SAPO-5. The higher activity is ascribedto a higher contribution of the bimolecular cracking mechanism and not to ahigher acid site strength. The cracking of isobutane has been investigated overMgDAF-1, MAPO-5 and MAPO-36 [274]. The catalytic activity increases in theorder MAPO-5 < MgDAF-1 < MAPO-36. In contrast to MAPO-36, the productselectivities vary little with temperature and mainly butenes and propene are formed(Y D 50 to 70%). The n-butane yield, however, never exceeds 11%; these productselectivities con� rm the presence of mild acid sites.

The conversion of a series of aliphatic hydrocarbons like cyclohexane, cyclo-heptane, cyclooctane, n-octane, n-hexane, 3-methylpentane and isooctane, overMeAPOs has been reported [132, 190, 263, 268, 288]. The observed order of cat-alytic activity and aromatic selectivity is the same as the order found, when thenumber of strong acid sites was determined. Moreover, the catalytic activity de-creases in the order MAPO-n > CoAPO-n > ZnAPO-n > MnAPO-n. This orderis also observed for other acid-catalyzed reactions. A detailed description of the ob-served activities and observed selectivities for aromatics formation is beyond thescope of this review.


5.4. Reactions of aromatics

Aniline methylation has been studied by Elangovan et al. [340, 341] over a series oftransition metal ion containing AlPOs with AEL topology. Alkylation of anilinewith methanol (methanol/ aniline D 2) gives mainly N-methylaniline and to asmaller extent N,N-dimethylaniline over AlPO4-11 and AlPO4-5 at yields (Y) of 9and 12%, respectively. On CoAPO-11 and ZAPO-11, the yields increase to 35 and40%, respectively, again with predominant formation of N-methylaniline (Y D 24and 26%). On CoAPO-5 and ZAPO-5, the conversion increases an additional 10%compared to CoAPO-11 and ZnAPO-11, which is accompanied by the formationof N-methyltoluidine. A similar trend is observed for VAPO-5 and VAPO-11. Thenumber of acid sites correlates nicely with the increase in aniline conversion, butwhether the larger channel diameter or the higher number of strong acid sites ofCoAPO-5, ZAPO-5 and VAPO-5 promote the formation of N-methyltoluidine isstill unclear.

Pinacol rearrangement mainly yielding pinacolone and 2,3-dimethyl-1,4-buta-diene was found to proceed at relatively mild temperatures over MeAPOs [342].However, a clear correlation between metal content, the number and/or strength ofacid sites and the observed conversions and selectivities was not found.

The isomerization of m- or o-xylene is often used to characterize the acid sitesin transition metal ion containing AlPOs and SAPOs [190, 258, 263, 268, 343].Over large pore molecular sieves, besides the formation of other xylene isomers,transalkylation forming toluene and trimethylbenzenes also occurs. The xylene loss(which re� ects the selectivity for transalkylation) decreases in the order MAPO-46(40%) > MAPO-36 (19.7%) > MAPO-5 (10.3%) > MAPO-11 (5%), which probablyre� ects the differences in acidity rather than different channel geometries. This isfurther con� rmed by the observed decrease in o-xylene conversion: MAPO-36 >CoAPO-36 > ZAPO-36, which resembles closely the number of strong acid sitesderived from TPD of pyridine [132].

Consequently, the same materials have also been tested for toluene disproportion-ation yielding benzene and xylenes [268, 288, 344]. Toluene conversion at 673 Kincreases in the order MAPO-5 < MAPO-46 < MAPO-36. The acidity of thecatalyst was also modi� ed by substituting different metals resulting in the follow-ing conversion order MAPO-36 (53%) > CoAPO-36 (44%) > ZnAPO-36 (37%) >MnAPO-36 (10%) [288].

Alkylation of toluene with ethanol has been studied with different MeAPOswith AEL, AFI and AFS topologies [344–347]. The products formed are ben-zene, styrene, diethylether and m-ethyltoluene, which is the commercially desiredproduct. In a subsequent step, m-ethyltoluene is dehydrogenated to methylstyrene,which is the monomer for the production of polymethylstyrene. Maximum conver-sion is achieved at a reaction temperature of 350±C and a toluene/ ethylene ratioof 2. Toluene conversion and m-ethyltoluene selectivity increase with catalyst acid-ity, which depends on the structure and the nature of the isomorphously substitutedmetal ion (Table 5).


Table 5.Ethylation of toluene over different catalysts (TR D 350±C, WHSV = 5 h¡1,ntoluene=nethylene D 1 : 2) [345–347]

Catalyst Conversion (%) m-ethyltoluene yield (%)NiAPO-11 10 —ZnAPO-11 11 —NiAPO-5 20 4.1ZnAPO-5 28 4.2AlPO4-31 2.2 1.0MnAPO-31 27.2 23.2NiAPO-31 10.7 9.1ZnAPO-31 21.7 18.7MnAPO-46 55 19.3NiAPO-46 26 10.0ZnAPO-46 47 18.3

6. BIFUNCTIONAL CATALYSIS

Isomerization of n-paraf� ns plays an important role in the petroleum industry andseveral processes have been implemented using bifunctional catalysts like the TIPprocess and isodewaxing (Chevron). The branching of n-paraf� ns is needed to im-prove the octane number of gasoline and to increase the low temperature perfor-mance of diesel. Bifunctional zeolite based catalysts containing a group VIII metalas a hydrogenation function and an acid function (Brønsted acid sites) have shownhigh ef� ciency in isomerizing n-paraf� ns [348]. A major problem is that the isomer-ization reaction is always accompanied by hydrocracking which lowers the isomer-ization yield and produces unwanted (less valuable) side products. The pathwaysof isomerization and hydrocracking over bifunctional molecular sieve based cata-lysts are well known (Fig. 12) [349]. It is well established that isomerization ofn-alkanes occurs � rst and that cracking is a consecutive reaction which is fasterfor multibranched alkanes. Monobranched paraf� ns are less susceptible to crackingthan multibranched paraf� ns [349]. Palladium or platinum containing aluminophos-phates with AEL, AFI, ATO and AFO topologies have been found to have highselectivities for the isomerization of wax or model feedstocks such as n-octane, n-heptane and n-hexane (Table 6) [350]. As the hydrogenation function palladium,platinum metal or bimetallic Pd–Pt clusters are introduced. Acid sites are typi-cally generated by isomorphous substitution of silicon or cobalt [294] into the alu-minophosphate framework. Remarkably, the medium pore SAPO-11 and SAPO-41molecular sieves exhibit a high n-octane isomerization selectivity while retainingsome transition state selectivities as evident from the low yield of 3-methylheptane(3M-Hep) and of 2,6-dimethylhexane (2,6DM-Hex) [351].

Criteria have been claimed which are considered important for high performancecatalytic isomerization and dewaxing catalysts. These include (1) intermediatepore size molecular sieve, (2) crystallite size below 0.5 ¹m (to minimize productdiffusion rate limitations), (3) oval-shaped pores with a minimum diameter of


(a)

(b)

Figure 12. (a) Isomerization and hydrocracking reaction path and (b) reaction network for isomer-ization and hydrocracking of alkanes on bifuntional catalysts.

at least 0.48 nm and a maximum diameter of 0.71 nm, (4) suf� cient acidityand suf� ciently high isomerization selectivity and (5) a group VIII metal as ahydrogenation function.

The isomerization of n-heptane was reported over a series of bifunctional SAPO-based catalysts, which contain 0.1 wt% palladium [352]. Best activities andselectivities for branched heptane isomers are achieved with SAPO-11 and SAPO-31. SAPO-17 and SAPO-5 show substantially lower activities. With SAPO-5, ahigh cracking activity is observed, which is assumed to be caused by the reducedaccessibility of parts of the bridged hydroxyl groups within the molecular sieveframework. Different locations of these acid sites are evidenced by IR spectroscopyrecorded after the adsorption of n-heptane [352].

A series of papers was published by Campelo et al. on hydrocracking and hy-droisomerization of n-alkanes on bifunctional Pt /SAPO-5 and Pt/SAPO-11 cata-lysts [353–356]. These materials were prepared by impregnation of the silicoalu-minophosphates with Pt(NH3/4Cl2 and subsequent reduction. The platinum loadingwas adjusted to 0.5 wt%. In studies with n-hexane [354], n-heptane [355, 357] andn-octane [353] it was found that the size of the pores of the silicoaluminophosphate


Table 6.Survey of catalysts tested for the isomerization of alkanes

Feed Catalyst support Metal Metal content (wt%) Ref.n-butane SAPO-11 Pt 0.5 [365]n-butane MnAPSO-11 Pt 0.5 [365]n-butane MnAPO-11 Pt 0.5 [364]n-hexane SAPO-5 Pt 0.5 [354]n-hexane SAPO-11 Pt 0.5 [354]n-hexane SAPO-11 Pt 1.0 [359]n-hexane SAPO-11 Pd 1.0 [359]n-heptane SAPO-5 Pt 0.5 [355]n-heptane SAPO-11 Pt 0.5 [355]n-heptane SAPO-11 Pt 1.0 [357]n-heptane SAPO-11 Pd 1.0 [357]n-heptane SiVPI-5 Pt 0.5 [350]n-heptane MgVPI-5 Pt 0.5 [350]n-heptane SAPO-11 Pd 0.1 [352]n-heptane SAPO-17 Pd 0.1 [352]n-heptane SAPO-31 Pd 0.1 [352]n-heptane SAPO-5 Pd 0.1 [352]n-heptane CoAPO-11 Pd 1.0 [294]n-heptane CoAPO-5 Pd 1.0 [294]isooctane SAPO-5 Pt 0.25–1.0 [358]isooctane SAPO-11 Pt 0.25–1.0 [358]n-octane SAPO-5 Pt 0.5 [353]n-octane SAPO-11 Pt 0.5 [353]n-octane SAPO-5 Pt 1.0 [360]n-octane SAPO-11 Pt 1.0 [360]n-octane SAPO-31 Pt 1.0 [360]n-octane SAPO-41 Pt 1.0 [360]n-octane SAPO-11 Pt 1.0 [363]n-octane SAPO-11 Pt–Pd 1.0 [361]n-octane SAPO-11 Pt–Pd 1.0 [351]2,2,4TMP SAPO-11 Pt 1.0 [361]2-Me-Hp SAPO-11 Pt 1.0 [351]3-Me-Hp SAPO-11 Pt 1.0 [351]4-Me-Hp SAPO-11 Pt 1.0 [351]2,5DM-Hx SAPO-11 Pt 1.0 [351]2,2,4TMP SAPO-11 Pt 1.0 [351]n-octane / 2,2,4TMP (50 : 50) SAPO-11 Pt 1.0 [363]n-hexadecane SAPO-11 Pt 1.0 [363]

support can determine, to a large part, the selectivity for hydroconversion. The dif-ferences in selectivity between Pt/SAPO-5 and Pt/SAPO-11 have been explainedby the slower migration of the alkanenic intermediates in the smaller channels ofSAPO-11 and by steric constraints at the pore mouths. The catalytic activity of then-alkanes increase with chain length because of the involvement of more stable car-


benium ion intermediates. The selectivity for isomerization decreases with chainlength for n-alkenes in Pt /SAPO-5 and increases in Pt /SAPO-11. In a subsequentstudy by the same authors n-octane and isooctane hydroconversion has been inves-tigated to determine the in� uence of the pore structure on the activity and selectiv-ity [358]. They have found that the reaction schemes for n-octane and isooctanetransformation are not the same due to the different pore structure of the SAPOsused. The size of the pores as well as the shape of the space available near the acidcenters can determine the selectivity of n-octane and isooctane isomerization andcracking on the Pt containing bifunctional catalysts [358].

The conversion of n-heptane and n-hexane over Pt /SAPO-11 was investigated byButt et al. [357, 359]. The reaction pathways were found to be similar on SAPO-11based catalysts. With incorporation of sodium a pronounced formation of aromaticsis detected. The bifunctional nature of the catalyst is re� ected in the differencein activity decay versus constant selectivity for cracking and isomerization. Forisomerization over palladium-impregnated catalysts with AEL and AFI topologies,it was observed that the higher acidity of CoAPO compared with SAPO materialis re� ected by a higher activity and a slightly higher selectivity for dibranchedisomers [294].

High selectivity for n-octane isomerization has been observed with medium porePt/SAPO-11, -31 and -41, while preferential hydrocracking has been found for largepore Pt/SAPO-5 [360]. Isomerization products consist of monobranched isomerswith a negligible amount of dibranched isomers, which also accounts for the lowyield of cracked products (see Fig. 12). The isomerization selectivity decreasesin the order SAPO-41 > SAPO-31 > SAPO-11. The differences in selectivityare explained by diffusional restrictions and steric constraints. In subsequentpapers from the same group, n-octane isomerization was studied over bifunctionalPt-Pd/SAPO-11 and Pt-Pd/SAPO-41 catalysts [351, 361, 362]. A 90% selectivitytowards isomerization (mainly monobranched isomers) was observed even at highconversion. SAPO-41 was found to be slightly more selective than SAPO-11.Furthermore, small grains (0.1–0.2 ¹m) were more active than large grains(1–1.5 ¹m), but no signi� cant effect on the product distribution was observed. Theauthors conclude that restricted transition state selectivity is responsible for theproduct distribution observed. The preferential formation of the monobranched2-methylheptane can be explained via a PCP (protonated cyclopropane) transitionstate. The terminal PCP n-octane transition state should be almost linear and thuswould accommodate more easily in the SAPO-n (n D 11; 41) pores than wouldan internal C8 transition state. This hypothesis is nicely supported by the largeamounts of 2-methylheptane and n-octane formed from 3-methylheptane. Whetherthe concept of ‘pore-mouth’ catalysis is necessary to explain the observed results isnot decided yet.

A new molecular sieve process using Pt/SAPO-11 for lube dewaxing by wax iso-merization was described by Miller [363]. Pt /SAPO-11 shows both a high selectiv-ity for wax isomerization and a low selectivity for secondary hydrocracking. Mod-


erate acid activity and the one-dimensional nature of the 10-ring pores appear tocontribute to its performance. However, much of the catalysis occurs at or near theexternal surface of the molecular sieve.

The direct transformation of n-butane into isobutene (dehydroisomerization) wasperformed over bifunctional Pt /SAPO-11, Pt /MnAPSO-11 and Pt/MnAPO-11 cat-alysts [364, 365]. The research was conducted to explore the possibility of produc-ing isobutene from n-butane in a single process, thus avoiding the costs of separateindependent reactors (dehydrogenation and acid skeletal isomerization), which arepresently used for the production of isobutene. At a reaction temperature of 500±C,Pt/SAPO-11 was the most active catalyst, mainly producing cracked products viahydrogenolysis (S>C4 D 65%). The selectivity to isobutane is about 20%. Isobutaneis also a valuable feedstock used in the production of isooctane (formed by reactionwith n-butenes). However, the isobutene selectivity is only 7%. Over Pt /MnAPSO-11 and Pt/MnAPO-11, hydrogenolysis is largely suppressed resulting in higher se-lectivities for isobutene (18%) and isobutane (42% over Pt /MnAPO-11; 30% overPt /MnAPO-11). The low selectivities for cracked products (about 11.5% at 43%conversion of n-butane over Pt /MnAPSO-11 and 6.5% at 31% conversion of n-butane over Pt /MnAPO-11) con� rm that on these catalysts the acidic and metalfunctions are balanced and bifunctional catalysis occurs [364, 365].

7. REDOX CATALYSIS

In a continual effort to transform � ne chemical production into more environmen-tally benign technologies, metal-containing AlPOs and SAPOs offer tremendouspotential as catalysts for heterogeneous oxidation for the production of these chem-icals. This follows from the signi� cant advancement in the � eld of heterogeneouscatalysis by molecular sieves following the success of titanium-containing silicalite(TS-1). Substitution of metal cations with redox properties such as Fe, Co, Mn andV into aluminophosphates is expected to afford novel catalysts for heterogeneouslycatalyzed oxidation reactions.

7.1. Oxidation of hydrocarbons with oxygen and air

Hydrocarbons that are oxidized at the terminal or the ®- or 1-position are impor-tant feedstocks for the chemical and pharmaceutical industries. Nevertheless, se-lective oxidation of alkanes at their terminal methyl group is still a challenge inmodern catalysis research. It is well known that some enzymes are capable ofperforming selective terminal oxidations. In principle, selective partial oxidationis easier to control when hydrogen peroxide or organic hydroperoxides like tert-butylhydroperoxide (TBHP) are used as oxygen donors, although from an economicpoint of view, the use of molecular oxygen, air or O2, is preferred [366, 367].

The selective oxidation of linear paraf� ns (n-pentane, n-hexane and n-octane)over Co- and Mn-containing catalysts with AEI or ATS topologies in the liquid


phase using air as an oxidant has been reported by Thomas et al. [368]. Theregioselectivity observed for the oxidation of n-hexane in CoAPO-18 and MnAPO-18 is remarkable. After a reaction time of 24 h, 60% of the oxidation products areoxyfunctionalized at the terminal carbon atom and an additional 36% are oxidizedin the 2-position (hexan-2-ol, hexan-2-one). Interestingly, the corresponding acid isformed with a high selectivity of 54%. Consequently, the oxidation of n-hexaneto adipic acid has been attempted over CoAPO-18 and CoAPO-34 [369]. OverCoAPO-18 after a reaction time of 24 h (conversion 9.3%) a selectivity towardsadipic acid of 35% is obtained. A reduced selectivity to adipic acid of 20% wasachieved in CoAPO-34, while the formation of adipic acid was not observed withCoAPO-36 as a catalyst. The terminal oxidation of dodecane with air was foundto be highly selective at C1 and C2 with MnAPO-18, while with larger molecularsieves such as MnAPO-5, MnAPO-11 and MnAPO-36, oxidation is favored at thecarbon atoms in 3- or 5-positions [370].

Cobalt-containing aluminophosphates with AFI, AEL, AEI, ATS and VFI topolo-gies have been used for the oxidation of cyclohexane with oxygen or air aiming atpreferential formation of cyclohexanone and cyclohexanol [371–377]. For CoAPO-5, cyclohexylhydroperoxide is the main product (Fig. 13), which is easily con-verted into monofunctional oxidation products. According to this study, CoAPO-11is catalytically more active than CoAPO-5 despite the lower channel diameter ofCoAPO-11 which should hinder diffusion of the products. These catalytic resultshave been ascribed to a higher dispersion of cobalt in CoAPO-11. Over CoAPO-36, also minor quantities of adipic acid are formed [371, 372]. Cyclohexane canbe directly oxidized to adipic acid by air over FeAPO-5 and FeAPO-31 catalysts[378]. Over FAPO-5 mainly cyclohexanol and cyclohexanone are formed besides ayield of roughly 15% adipic acid (24 h, 373 K), while over FAPO-31 predominately

Figure 13. Main products and byproducts of cyclohexane ( 1 ) and cyclohexene ( 2 ) oxidation:cyclohexanol ( 3 ), cyclohexanone ( 4 ), cyclohexenoxide ( 5 ), adipic acid ( 6 ), glutaric acid ( 7 ) andsuccinic acid ( 8 ).


adipic acid (S D 65%) is formed. CrAPO-5 also predominately produces cyclo-hexanone [379]. Selectivities to adipic acid of around 8% are found for CoAPO-5,MgVAPO-5, and CoVAPO-5 while for VAPO-5 the selectivity to adipic acid is only3% [380]. The conversion after 2.5 h increases in the order MgVAPO-5 (2.8%) <CoVAPO-5 (5.8%) D VAPO-5 (5.7%) < CoAPO-5 (14.7%).

Among the products of liquid phase oxidation of cyclohexane, adipic acid is themost important. Industrially, adipic acid is produced via a two-step liquid phaseoxidation of cyclohexane with air and nitric acid as oxidants. To increase theyield of adipic acid in a environmentally friendly one-step process, most studiesseek reaction conditions that might favor the oxidation of cyclohexane and itsintermediates [375]. CoAPO-5 has been found to be a potential catalyst for a one-step liquid phase oxidation of cyclohexane to produce mainly adipic acid withoutany promoter being added and with no induction period [375]. This study showsthat high cyclohexane concentration, a high reaction temperature and high oxygenpressure will result in a 45% yield of adipic acid after a reaction time of 3 h(cyclohexane conversion 50%) [381]. However under these reaction conditions inthe presence of acetic acid, leaching of Co has been found, [382] which has not beenreported for solvent-free oxidations of cyclohexane on CoAPOs [374, 375, 383].

Meanwhile, it has been shown that TMI-containing catalysts of ATS and AEItopologies are also suitable for the epoxidation of alkenes with air [367, 384] and theBaeyer-Villinger oxidation of ketones to lactones [385]. However, many questions(leaching, long-term stability, pore blocking by products, structure and uniformityof the active sites towards activity and selectivity) require further investigationsbefore a commercial process can be realized [366, 383].

Sheldon and co-workers found that CoAPO-11 and CoAPO-5 are both effec-tive, stable and recyclable solid catalysts for the facile oxidation of p-cresol top-hydroxybenzaldehyde at 50±C in NaOH/MeOH with conversion and selectivityboth reaching 90% (Fig. 14) [386, 387]. The superior performance of these cata-lysts over cobalt salts used as homogeneous catalysts was attributed to the fact thatunlike cobalt salts, CoAPOs can not form ¹-hydroxo-bridged cobalt dimers. How-ever, the catalyst is not stable under these conditions and dissolves slowly form-ing soluable cobalt compounds which serve at least partially as catalytically activespecies [375, 383].

Figure 14. Reaction of p-cresol to o-hydroxy-benzaldehyde.


VAPOs with AEL, ATO, AFI and ERI topologies were found to be active in thevapor phase oxidation of toluene with molecular oxygen [40]. VAPO-11 and VAPO-31 exhibited high catalytic activity and high selectivity for benzaldehyde. Theactivity decreases in the order VAPO-11 > VAPO-31 > VAPO-5 D VAPO-17, whichis ascribed to the structural properties of these catalysts [40].

The oxidative dehydrogenation (ODH) of alkanes, which is one of the importantroutes to obtain alkenes, has been tested by some researchers using vandium-containing aluminophosphates [49, 388–390]. VAPO-5 samples are found to beactive and selective for the oxidative dehydrogenation of propane. The catalyticproperties of VAPO-5 are better than catalysts of supported vanadium oxide onAlPO4-5. Isolated tetrahedral V(V) species in the AFI framework structure areproposed to be active and selective sites for this reaction [49, 390]. The manganesesubstituted molecular sieve MnAPO-5 was found to be an active catalyst forethane dehydrogenation to produce ethene [391]. Good crystallinity was foundto be essential for the MnAPO-5 catalyst to have the best properties, becausethe occlusion of manganese oxide particles in the channels or a collapse of thestructure upon calcination decrease the ethene selectivity [391]. The presence ofMg(II) and V(V) species in MgVAPO-5 results in a more selective catalyst forODH of ethane [390]. The authors relate the catalytic properties of MgVAPO-5to the presence of acid sites (Mg(II)) near the (V(V)) redox sites in the molecularsieve framework. CoVAPO-5 has been shown to be even more active for ODHof ethane than MgVAPO-5 (44% conversion compared to 8%), suggesting that theredox properties of cobalt are responsible for the observed activity. Recent resultsfrom the same group show that the activity in the oxidative dehydrogenation ofethane decreases in the order CoVAPO-5 > VAPO-5 > CoAPO-5 > MgVAPO-5.

The ammoxidation of toluene and benzyl alcohol with ammonia and air yieldingbenzonitrile was reported on vanadium-containing aluminophosphates and silicoa-luminophosphates by Kulkarni et al. [392]. Vanadium-containing SAPOs are alsohighly active and selective in the one-step ammoxidation of ethanol to acetonitrile[393]. The ammoxidation of 3- and 4-picolines over SAPOs with an AFI topologycontaining V and combinations of V with Mn or V with Mo was studied by Srinivaset al. [394] in comparison to modi� ed SAPOs with an AEL topology. These novelmaterials have been found to be very active and selective and are believed to havepotential to replace amorphous catalysts (V2O5 or ®-VOPO4 supported on Al2O3,silica-alumina or Cr2O3/ used in the existing commercial process.

7.2. Oxidation with peroxides

Direct hydroxylation of phenol yielding catechol and hydroquinone is observedover TAPO-5 and TAPO-11 catalysts [17]. Hydroxylation occurs to the extentof 32.2 and 17.7% for TAPO-5 and TAPO-11, respectively. In a more recentstudy, TAPSO-5 and TiSiVPI-5 are found to catalyze the hydroxylation of phenolef� ciently, especially under slow addition of H2O2 [395]. The conversion ofphenol and the selectivities towards catechol and hydroquinone were found to be


Figure 15. Oxidation of carveol and 1-phenyl-1,2 ethanediole.

affected by the polarity of the solvent, the Ti content and the crystallinity of thecatalyst. Solvents of medium polarity such as acetone are preferred because they canef� ciently transfer the reactants to and products away from the catalyst surfaces. Thecatalytic activity of TAP(S)O-n molecular sieves is comparable to that of TS-1,while the catechol/ hydroquinone ratios are always higher than with TS-1. Someexperimental results, however, suggest that the hydroxylation reaction proceedsmainly on the external surface of the catalysts due to complete � lling of the catalystpores by polar molecules from the reaction mixture [17, 395].

The catalytic activities of MeAPO-11 (Me D Fe, Co, Mn) for the hydroxylationof phenol with hydrogen peroxide have been examined by Dai et al. [396]. Theydemonstrated that MeAPOs have signi� cant catalytic activities for this reactionand that introduction of these metals signi� cantly improves the level of conver-sion. FeAPO-11 shows comparable performance to TS-1. The activity depends uponthe Al/Fe ratio and the level of acidity of the molecular sieve. Medium pore MeA-POs are more active than their larger pore counterparts. However, the external sur-face of the catalyst was also found to play a signi� cant role in their catalytic activity.

Chromium-containing catalysts with the AFI topology are found to catalyze alco-hols to ketones with TBHP [383, 397, 398]. Carveol and 1-phenyl-1,2-ethanediolare oxidized chemoselectively (Fig. 15). These reactions presumably proceed viaoxidation of the alcohol via an oxochromium(VI) species and subsequent reductionof the chromium(VI) species by TBHP.

Vanadium-containing aluminophosphates with AFI and AEL topologies havebeen studied to a lesser extent as catalysts for the epoxidation of allylic alcoholssuch as geraniol and cinnamon alcohol using TBHP as an oxidant [30, 399]. The sta-bility of such catalysts against leaching, however, seems to be a problem. Haanepenet al. studied the stability of VAPO-5 as catalysts for the epoxidation of 3-phenyl-2-


Figure 16. Oxidation of ®-pinene to verbenon.

Figure 17. Oxidations catalyzed by CrAPO-5 (after Ref. [404]).

propene-1-ol and the oxidation of 3-octanol by TBHP [400, 401]. They concludedthat VAPOs are not suitable catalysts for oxidation reactions in the liquid state whereit is desired to improve the selectivity of the reaction by means of the pore struc-ture. In the early stage of the reaction, small amounts of vanadium are extractedfrom the framework. The amount depends on the substrate and the reaction con-ditions. The vanadium ions in solution contribute largely to the observed activ-ity. Thereafter, the leaching of vanadium stops due to strong adsorption of polarmolecules in the micropores of VAPO-5. This makes most vanadium sites inacces-sible for reaction and leaching [402, 403].

CrAPO-5 and CrAPO-11 catalyze a variety of oxidations using TBHP or O2 asan oxidant (Fig. 16) [404]. CrAPO-5 catalyzes the oxidation of the side chainsof aromatic substrates like ethylbenzene, p-alkyltoluene (alkyl D ethyl, propyl,butyl) and p-ethylanisole with TBHP forming the corresponding ketones with highselectivity [398, 405–407]. CrAPO-5 catalyzes the alkyloxidation of ole� ns withTBHP at 80±C to the corresponding enones like ®-pinene to verbenon (Fig. 17)[408]. Recent studies, however, have demonstrated that in the presence of TBHPa small amount of chromium ions leaches into the solution which might beresponsible for the observed catalytic activity [405, 409, 410].


TAPSO-5 has been used as a catalyst for the liquid-phase epoxidation of cyclo-hexene [15, 16]. When H2O2 is used as an oxidant, the formation of 1,2-cyclo-hexanediol is observed, which is a result of ring opening that may be catalyzedby the acid sites of the catalyst. When TBHP is used as an oxidant, the epoxideis formed very selectively. Only traces of other products such as cyclohexanone,the diol and bicyclohenenyl are detected. Such high selectivity for the epoxide hasalready been reported by Rigutto and van Bekkum for the epoxidation of cyclo-hexene over VAPO-5 catalysts [27]. Titanium-containing AlPOs with AEL, AFIand ATS topologies were found to be active for cyclohexane oxyfunctionaliza-tion, and cyclohexanol and 2-hexanol oxidation in the presence of dilute H2O2

[411, 412]. According to these studies, the oxidation activity is associated withframework-incorporated Ti atoms. The selectivity and H2O2 activity decrease inthe order TAPO-5 > TAPO-11 > TAPO-11 with TAPO-5 being only slightly lessactive and less selective than TS-1 with similar titanium content.

The oxidation of hydrocarbons like benzene and cyclohexane with H2O2 andTBHP was investigated employing FeAPSO-37 and CrAPSO-37 as catalysts[72, 413]. Only chromium(III) is active as an oxidation catalyst, while overFeASPO-37 no conversion was detected. In spite of the framework stability un-der the reaction conditions, leaching of small quantities of chromium occurs andthe observed catalytic reaction is mainly due to chromium in solution.

The oxidation of glycerol with H2O2 using a range of aluminophosphate catalystshas been investigated by McMorn et al. [414]. Unfortunately, the desired productsof glyceric acid and glycerol aldehyde were not formed; instead formic acid andmonoformate esters of glycol were observed as major products together with acomplex mixture of acetals.

8. OTHER CATALYTIC APPLICATIONS

One of the important goals of catalysis research is to relate the structural propertiesof the catalysts to their catalytic ef� ciency and guide the development of new cat-alysts. Initial studies of ethylene dimerization, which is catalyzed by nickel(I) andpalladium(I) have been carried out to demonstrate this approach [415– 417]. Thereaction proceeds via two steps, [418] involving the dimerization of ethylene to1-butene and the subsequent isomerization of 1-butene to predominantly but-2-eneon acidic catalysts [419]. Studies demonstrate that the process is indeed catalyzedby nickel(I) or palladium(I) which have been detected by ESR spectroscopy under–conditions showing enhancement of catalytic ef� ciency [415–417].

ESR and ESEM spectroscopy enable a detailed investigation of the reaction ona molecular level. After the adsorption of ethylene, a ¼ -bonded Ni(I)- or Pd(I)-ethylene complex is obtained. In most cases the active center is coordinated to oneethylene molecule at a distance of about 0.35 nm. Under reaction conditions theESR spectra change and new species are detected, which can be attributed to Ni(I)-butene complexes. The same complexes are obtained by n-butene adsorption on


activated samples and therefore their assignment is unambiguous. Additionally,an ESEM analysis of the Ni(I)-butene complex shows interaction with eightdeuteriums at different distances which is consistent with eight deuteriums onbutene [416]. Butene complexes are not obtained in PdH-SAPO-34and NiH-SAPO-34, which shows that due to the smaller channel size no reaction at temperaturesbelow 100±C occurs [416].

Fe /AlPO4-5 catalysts prepared by impregnation in aqueous and organic mediaand subsequently activated by a reductive treatment were tested for CO hydrogena-tion in a stainless steel microreactor (p D 1:1 MPa, T D 598 K, WHSV D 1200/h,H2 /CO D 1). It was shown that in aqueous solution impregnated Fe/AlPO4-5shows no activity, while the catalysts prepared by impregnation in organic mediashow conversions between 6 and 23% depending on the type of organic solventused [105]. MeAPO-5 (Me D Co, Fe) exhibits a low activity (ca. 3%) in syn-thesis gas (CO and H2/ conversion, while Co/AlPO4-5 gives 42% CO conversion[420]. Signi� cant differences in selectivity were observed for the different cata-lysts. The mixture of an iron-based Fischer-Tropsch catalysts with FAPO-n andFAPSO-n (n D 5; 11) molecular sieves, which add acidic properties to the catalyst,results in enhanced syngas conversion and enhanced selectivity for light hydrocar-bons (C2 to C4/. It is anticipated that iron species associated with the molecularsieve framework participate in the reaction.

A one-step synthesis of methylisobutylketone (MIBK), which is the most im-portant product derived from acetone, over palladium supported on AlPO4-11 andSAPO-11 has been reported by Yang et al. [421]. Their results show that a balancebetween condensation and hydrogenation is necessary to achieve a high selectivityfor MIBK. The highest MIBK yield is obtained over 0.3 Pd/SAPO-11 at a reactiontemperature of 200±C, a H2 / acetone ratio of 1 and a WHSV of 0.7 h¡1. Basic sitesare found to promote the activity of the catalyst in this reaction.

The interaction of nitric oxide with carbon monoxide on the surface of coppercontaining aluminophosphates and silicoaluminophosphates of the AFI structurehas been studied by the transient response technique [422]. In the temperature rangestudied (60–300±C) the catalysts start to interact � rst with the carbon monoxide.Cu/AlPO4-5 and Cu/SAPO-5 exhibit activity towards the conversion of nitric oxideto nitrogen above 100±C, which is comparable to a CuO/Al2O3 catalyst.

The catalytic activity of copper ion exchanged SAPO-5, -11 and -34 in theselective reduction of NO with C3H6 under an oxidizing atmosphere has been testedby Ishihara et al. [423]. Under the experimental conditions of their study, Cu-SAPO-34 exhibits higher activity for NO reduction than Cu-ZSM-5 and sustainsits catalytic activity up to 600±C. While the catalytic activity of Cu-SAPO-11is comparable to that of Cu-ZSM-5, the activity of Cu-SAPO-5 is signi� cantlylower. CoAPSO-34 has been evaluated as a catalyst for NO conversion to N2 inthe presence of various hydrocarbons like CH4, C3H6, C3H8, C8H18 and C16H34

[118]. The NO conversion increases with the Co content of the sample up to


nSi=nCo D 5. It was concluded that the cobalt is mainly located in tetrahedralframework positions and causes an increase in the number of acid sites.

Cu-exchanged MgAPO-11 and ZnAPO-11 molecular sieves have been tested forNO decomposition by Dedecek et al. [424]. Both Cu-MgAPO-11 and Cu-ZnAPO-11 exhibited constant conversion for NO decomposition with time-on-stream. Theturn-over-frequencies of NO per Cu atom at 770 K are comparable to those of Cuions in cationic sites of ZSM-5.

9. MISCELLANEOUS APPLICATIONS

Photoionization of methylphenothiazine, which is of interest in solar energy utiliza-tion, was studied in microporous SAPOs with different pore sizes of CHA, AFI,AEL and VFI topologies [425]. The photoyield of methylphenothiazine cation rad-icals was enhanced by an order of magnitude or more by the incorporation of thetransition metal ions Ni, Co and Cu as electron acceptors in framework or in ion-exchange sites of the microporous material. Ni(II) is the most ef� cient acceptor,whereas the addition of Co(II) into either ion exchange or framework sites doesnot produce any signi� cant effect on the photoyield. For Cu(II) as an electron ac-ceptor, there is a distinct difference between copper located in framework versusion-exchange sites. It was also found that the photoyield increases with the SAPOpore size. Chromium-containing SAPO materials exhibited the highest photoyieldamong the Cr, Fe and Mn transition metal ions studied [426]. The presence ofchromium in the framework further increases the photoyield and the stability ofthe methylphenothiazine cation radical. It follows from these studies that silicoalu-minophosphates provide appropriate steric and electrostatic environments to retardback electron transfer and increase the lifetime of the photogenerated radical ionsfor many days or even months at room temperature.

A new and growing � eld is the use of TMI-containing SAPOs and AlPOs as ad-vanced materials such as the dielectric phase in capacitance-type chemical sensorsas demonstrated by Balkus et al. [427, 428]. Aluminophosphate-based molecu-lar sieves represent suitable host systems for the accommodation of polarizablemolecules. The applicability of dye-loaded AlPO4-5 for second harmonic gener-ation based on the organized arrangement of molecular dipoles and for persistentspectral hole burning has been summarized by Schulz-Ekloff [429]. More researchwork is expected to expand this area.

Increasing attempts have been made to develop aligned molecular sieve crystals,especially in a membrane form for sensors and nonlinear optical devices. A mem-brane of vertically-aligned MeAPO-5 (Me D Co, V) crystals on anodic aluminaas a support has been reported by Chao et al. [430]. The MeAPO crystals werefound to be preferentially oriented along the c-axis. Thin � lms of MAPO-39 havebeen prepared via pulsed laser deposition (PLD) [431]. The MAPO-39 crystals growwith pores oriented primarily normal to the porous metal substrate. The separation


of water/ alcohol mixtures were also studied showing that selective permeation ofwater occurs.

10. CONCLUSIONS AND OUTLOOK

It has been shown that transition-metal-ion containing AlPO4s and SAPOs are inter-esting catalysts for a variety of heterogeneously catalyzed reactions including oxi-dations, acid catalyzed reaction and isomerizations over bifunctional catalysts. Al-though improvements have been made for some catalytic systems, only two areclose to being realized commercially. UOP and Norsk Hydro have built a pilot plantfor the conversion of methanol into lower ole� ns using a SAPO-34 based catalystand have announced the planning of large plant (250 000 tons/year). Chevron isworking on the realization of an isodewaxing unit using Pt/SAPO-11. With thesetwo large scale applications on the horizon, new processes might emerge, if fur-ther improvements of the catalyst systems based on transition-metal-ion containing(silico)aluminophosphates are made.

The introduction of transition metal ions into silicoaluminophosphates can beachieved by liquid phase or solid-state ion-exchange, by impregnation or by addi-tion to the synthesis gel. The interesting question of whether several metal ions canbe incorporated into the framework of SAPO-n or AlPO4-n materials can be suc-cessfully addressed by the use of complementary techniques like UV-VIS, IR, ESRand ESEM, NMR, Mössbauer spectroscopy etc. It is also possible to characterizeredox processes involving paramagnetic and nonparamagnetic oxidation states of atransition metal ion in aluminophosphate materials. The future use of in-situ tech-niques such as NMR and IR spectroscopy might lead to advanced understanding of(silico)aluminophosphate based catalysts under ‘working conditions’. In particular,the implications of isomorphous substitution on the catalytic properties of MeAPOsare still not understood in many cases.

Although the isomorphous substitution of transition metal ions into aluminophos-phates is considered to yield powerful liquid phase oxidation catalysts, the initialeuphoria has been tempered by the realization that many of these materials are notstable under oxidizing conditions in the liquid phase. Moreover, pore blocking ofpolar products severely reduces the lifetime of these catalysts, which at presentprevents commercial realization. Many of the liquid phase oxidations are quite se-lective. One ultimate goal is the design of chiral redox molecular sieves, whichcatalyze asymmetric oxidations yielding a high enantiomeric excess.

Finally, the MeAPOs employed in acid-catalyzed reactions typically contain acidsites ranging from mild to strong in the same catalyst. Therefore, the design ofcatalysts with uniform sites representing a narrow distribution of acid site strengthis another challenge in modern catalysis research.


Acknowledgements

This work has been generously supported by the US National Science Foundation,the US Department of Energy and the Robert A. Welch Foundation. We also havebene� ted from support of the Texas Advanced Research Program. M. H. wishes tothank the Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industriefor support.

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substitution of transition metal ions into aluminophosphates and silicoaluminophosphates:...

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