synthesis and characterization of mapo-11 molecular sieves

Synthesis and characterization of MAPO-11 molecular sieves

Romilda Fernandez a, Marcus V. Giotto a,1, Heloise O. Pastore b,Dilson Cardoso a,*

a Department of Chemical Engineering, Federal University of S~aao Carlos, P.O. Box 676, 13565-905, S. Carlos, SP, Brazilb Instituto de Qu�ıımica, Universidade Estadual de Campinas, P.O. Box 6154, 13083-970 Campinas, SP, Brazil

Received 7 January 2000; received in revised form 19 March 2000; accepted 11 February 2002

Abstract

In this work MAPO-11 molecular sieve was prepared with several degrees of magnesium substitution. The syntheses

were carried by using phosphoric acid, pseudo-boehmite and magnesium acetate as sources of structural elements and

diisopropylamine as the structure-directing agent. The samples were characterized by powder X-ray diffraction, ther-

mogravimetry, infrared spectroscopy, solid-state nuclear magnetic resonance, scanning electron microscopy and ele-

mental analysis. The X-ray diffractograms indicate that the AEL structure is obtained free of contaminating phases up

to a magnesium molar ratio of 0.3 in the reaction mixture. Higher levels of magnesium cause the formation of other

phases. Thermogravimetry, MAS nuclear magnetic resonance of 31P and infrared spectroscopy confirm the magnesium

isomorphous substitution for aluminum up to this magnesium content in the reaction mixture. � 2002 Published by

Elsevier Science Inc.

Keywords: Aluminophosphate; Synthesis; AEL; MAPO-11; Magnesium incorporation; NMR

1. Introduction

Aluminophosphates (AlPO) are a class of mo-lecular sieves with a variety of new structures. Thisfamily has raised a lot of attention due to its po-tential application in heterogeneous catalysis. Al-POs present strict alternance of AlO�

4 and POþ4

tetrahedra, and therefore have a neutral frame-work, which limits their application to selectiveadsorbents. One possibility to generate Br€oonsted

acidity is the isomorphous substitution of Al3þ byM2þ ions. The magnesium substituted MAPO-11,which has the AEL structure, is one member ofthat family whose structure comprises medium sizepores and therefore, might exhibit shape selectivity[1]. Its structure is orthorhombic, formed bystacking four- and six-membered rings, arrangedin columns and forming monodimensional chan-nels with elliptic cross-sections [2]. Studies oncatalytic systems using this magnesoalumino-phosphate indicate that it is resistant to coke for-mation, due probably, to its shape selectivity,contrary to large pore aluminophosphates whosedeactivation is fast [3].Despite being known since 1986, few works

have dealt systematically with the synthesis and

Microporous and Mesoporous Materials 53 (2002) 135–144

www.elsevier.com/locate/micromeso

*Corresponding author. Tel.: +55-16-2608264; fax: +55-16-

2608266.

E-mail address: [email protected] (D. Cardoso).1 Present address: Carlson School of Chemistry, Clark

University, Worcester, Massachusetts 01610.

1387-1811/02/$ - see front matter � 2002 Published by Elsevier Science Inc.

PII: S1387-1811 (02 )00333-5

properties of MAPO-11 [1,4]. Therefore, in thepresent work, the synthesis was performed and theinfluence of parameters such as magnesium con-tent and crystallization times upon the propertiesof the products was evaluated. Toward that ob-jective, powder X-ray diffraction, thermogravime-try, infrared spectroscopy, 27Al and 31P solid-statenuclear magnetic resonance and elemental ana-lyses of products were used.

2. Experimental

2.1. Syntheses

In a typical procedure, pseudo-boehmite (Con-dea) was dispersed in a half of the total waterneeded in the synthesis. A solution of phosphoricacid (85 wt.%, Merck) dissolved in half of theremaining quantity of water was added to thepseudo-boehmite suspension and stirred for 1 h atroom temperature. After that, magnesium acetate(Mg(OAc)2 � 4H2O, Vetec) dissolved in the lastremaining part of distilled water was added andthe reaction mixture kept under magnetic stirringfor another 12 h period at room temperature. Fi-nally, diisopropylamine (Riedel–de-H€aaen) wasadded under stirring.The reaction mixture had the following com-

position:

xgMgO : ð1� xg=2ÞAl2O3 : P2O5 : i-Pr2NH : 50H2O

ð1Þwhere xg is the magnesium content varying from 0to 0.5.The suspension was transferred to Teflon-lined

autoclaves and placed in a pre-heated oven at 363K for 24 h [4] for aging. The autoclaves were thentransferred to another pre-heated oven at 473 Kfor various periods of time. The products werecentrifuged, washed with distilled water untilneutral pH and dried in an oven at 373 K for 4 h.

2.2. Characterization

The samples were analyzed by powder X-raydiffraction (XRD) with CuKa radiation using aSiemens Diffractometer (30 kV, 10 mA) at a rate of

2� min�1 2h. Thermogravimetric analysis (TGA)was accomplished in a TA Instruments modelThermalAnalyst 2100 at a rate of 10Kmin�1, undernitrogen or oxygen with a 100 mlmin�1 gas flow.Diluted samples in KBr (50:50) were analyzed

by diffuse reflectance infrared spectroscopy(DRIFTS) in a Nicolet 520 FTIR. The spectrawere accumulated by using 128 scans at a resolu-tion of 4 cm�1. Elemental Analysis was performedby inductively coupled plasma-atomic emissionspectroscopy, in a Thermo-Jarrel model Atom-Scan 25. Scanning electron microscopy (SEM) wasperformed by using gold sputtered samples ob-served in a Zeiss DSM 960 with an acceleratingvoltage of 25 kV.All MAS NMR spectra were acquired by using

a Varian Unity Plus 400 MHz spectrometer oper-ating at x ð31PÞ ¼ 161:90 MHz and x ð27AlÞ ¼104:21 MHz. The spinning frequency of 7.5 kHzwith silicon nitride (Si3N4) rotors (7 mm) was usedat room temperature. Chemical shift referenceswere obtained from 85% H3PO4 and 1 MAl(NO3)3 solution to

31P and 27Al, respectively. Inorder to obtain quantitative phosphorus NMRspectra, a pulse length of 4 ls ðp=4Þ was used withhigh power heteronuclear decoupling, 60 s delaytime, 100 kHz sweep width and accumulation of 50transients. The 1H–31P CP MAS NMR spectrawere recorded using 4.5 ls 1H (90�) pulse length, acontact time tc ¼ 5 ms, delay time of 5 ms andaccumulation of 1000 transients. For aluminumNMR spectra, a selective excitation was performedwith a short pulse ðp=20Þ of 1 ls, delay time of 2 s,100 kHz sweep width and accumulation of 200transients. When AlPO4 sites are crystallographi-cally equivalent, the P/Al molar ratio might becalculated in much the same manner as Si/Almolar ratio for zeolites [5] by using Eq. (2), whereIPðnAlÞ is the NMR signal intensity correspondingto the PðnAlÞ group and n is the number of Alneighbors to that phosphorus atom.

Mg

MgþAlþ P

� �network

¼

P4n¼0

ð4� nÞIPðnAlÞ

8P4n¼0

IPðnAlÞ

� 1 ð2Þ

However, AlPO4-11 has three crystallographicallynon-equivalent phosphorus atoms and two of

136 R. Fernandez et al. / Microporous and Mesoporous Materials 53 (2002) 135–144

those are overlapped in the NMR spectra (peak P1contain two non-equivalent overlapped phospho-rus signals, sample M000 in Fig. 12), therefore Eq.(2) has to be applied independently to both signals(P1 and P2 in Fig. 12) and averaged the result toobtain the overall P/Al molar ratio.

3. Results and discussion

Table 1 presents the peak positions (2h) andrelative intensity for the AEL structure [6] and forthe sample considered 100% crystalline in thepresent work (MAPO-11, xg ¼ 0:05, 60 h crystal-lization).Fig. 1 displays the percentage degree of crys-

tallinity as a function of time of crystallization upto a magnesium molar ratio of 0.2. From thecrystallization curves in Fig. 1 it can be observed

that the AEL phase is obtained with a reasonablecrystallinity between 30 and 60 h of crystallization.Samples containing Mg and smaller crystallizationperiods are contaminated with AFI structure in-dicating that, under these conditions, probablythis structure is a precursor for MAPO-11. A slightdecrease in the degree of crystallinity is observedfor higher crystallization time but no other phasewas present in the XRD patterns. When 0:3 <xg 6 0:5, parallel to the AEL the GIS phase is alsoformed.For further analysis of the influence of Mg in

the properties of the solid, some samples were se-lected which showed only patterns of pure AELstructure (Fig. 2). The elemental analyses of thesesamples (Fig. 3) show that �20–22% of the mag-nesium in the reaction mixture is present in thesolid, which agrees with the results reported by

Table 1

Positions (2h) and relative intensity in the AEL structure and ofthe standard sample used in the present work (MAPO-11,

xg ¼ 0:05, 60 h crystallization)2ha 100 I=I0a 2hstandard 100I=I0ðstandardÞ

20.320 36.21 20.430 69.11

21.027 100.00 20.984 100.00

22.074 44.41 22.178 83.99

22.692 41.01 22.663 95.83

23.129 60.39 23.169 78.60

aRef. [6].

Fig. 1. Crystallization curves of AEL structure as a function of

crystallization time and magnesium contents.

Fig. 2. X-ray diffraction of selected samples containing differ-

ent Mg amounts Crystallization time: 48, 60, 48, 60 and 30 h for

xg ¼ 0:00, 0.05, 0.10, 0.20 and 0.30, respectively. The asteriskindicates a peak probably from the AFI structure.

R. Fernandez et al. / Microporous and Mesoporous Materials 53 (2002) 135–144 137

other authors [1,7]. Rios et al. performed a simi-lar study for MAPO-5 [8] and the authors foundan 80% efficiency of magnesium incorporation.This higher value is probably due to the fact thatMAPO-5 is a large pore molecular sieve that mightaccommodate protonated directing agent mole-cules better than MAPO-11, which is a mediumpore molecular sieve.The SEM shows that AlPO4-11 (Fig. 4a) crys-

tals are prismatic ranging from 5 to 15 lm, quite

similar to those reported by Ojo and McCusker [9]when pseudo-boehmite was used as aluminumsource. With the addition of xg ¼ 0:05 magnesiumin the reaction mixture, the morphology of thecrystals changes to needles (Fig. 4b). As theamount of magnesium in the reaction mixture in-creases further, the crystals form agglomeratesranging from 15 to 30 lm (Fig. 5a and b).Thermogravimetry of AlPO4-11 and MAPO-11

samples under nitrogen (Fig. 6) or oxygen (Fig. 8)as well as the derivative thermograms (Figs. 7 and9, respectively) show four steps of mass loss, whichare in the same region as found by Singh and co-workers [10] for SAPO-11:

(I) 303–383 K: desorption of water,(II) 383–623 K: desorption of physisorbed amine,(III) 623–823 K: Hoffman degradation on proto-

nated amine, and(IV) 823–1073 K: coke combustion (under an

oxygen atmosphere, Figs. 8 and 9).

Regardless of the gas used under TGA, AlPO4-11 presents the highest mass loss of the samplesprepared in this work: �10 wt.% (Figs. 6 and 8).For MAPOs however, the mass loss under nitro-gen is always lower than the one under oxygen andthe difference increases as the magnesium contentsin the sample is increased. This is due to the factthat under an oxidizing atmosphere, coke, formed

Fig. 3. Magnesium molar fraction in the solids with AEL

structure as a function of the molar magnesium fraction in the

reaction mixture. Crystallization time: the same as in Fig. 2.

Fig. 4. Scanning electron micrographs of (a) AlPO4-11 and (b) MAPO-11, xg ¼ 0:05 (48 and 60 h of crystallization, respectively).


by the Hoffman degradation of the protonatedamine, is also eliminated.For the AlPO4-11, calcined under nitrogen or

oxygen, there were mass losses only in regions Iand II (Figs. 7 and 9) that are due, respectively, todesorption of water and physisorbed amine.One of the clear evidences of magnesium iso-

morphous substitution in the AEL structure is thatincreasing the magnesium contents in the solidcauses an increased mass loss in region III of thethermogram which is attributed to the decompo-

sition of protonated template. This hypothesis issupported by the thermal analyses under oxygen(Figs. 8 and 9) where it is observed the increase inpeak IV (the coke combustion) as the magnesiumcontents increases. This increase in coke forma-tion is probably caused by polymerization of theproducts formed through the Hoffman degrada-tion of the protonated amine.In order to verify if there is magnesium compen-

sating framework charge as stated by Prasad et al.[11], Fig. 10 shows the weight loss of as-synthesized

Fig. 6. TGA of AlPO4 and MAPO samples under inert atmo-

sphere (same crystallization time as in Fig. 2).

Fig. 7. Derivative TGA of AlPO4 and MAPO samples under

inert atmosphere.

Fig. 5. Scanning electron micrographs of MAPO-11 samples (a) xg ¼ 0:10 and (b) xg ¼ 0:20 (48 and 60 h of crystallization, respec-tively).


samples as a function of framework magnesiumamount estimated from NMR measurements. Theweight loss data were obtained under an oxygenatmosphere at temperatures higher than 623 K (seeFig. 8), which, according to Singh et al. [10], corre-sponds to decomposition of protonated amine. Theresults show that there is an augmentation of weight

loss as the framework magnesium contents in-creases, indicating that the protonated amine isthe responsible for the charge compensation. Fig.10 shows also the expected weigh loss, estimatedconsidering that framework magnesium is exclu-sively compensated by protonated amine. As can beobserved, the experimental values are somewhathigher probably due to dehydroxylation of POHgroupsoriginated fromdefects.Asaconsequence, inthe case of in MAPO-11, apparently there is nocationic magnesium compensating frameworkcharge.Fig. 11 displays part of the infrared diffuse re-

flectance spectra of the as-synthesized AlPO4-11and of MAPO-11 with increasing magnesiumsubstitution and the spectrum of diisopropylam-monium hydrochloride as well (11d). Comparisonof the spectra presented in Fig. 11d and a (AlPO4-11) reveals that around 1589–1585 cm�1 bothspectra show a band normally attributed to N–Hbending. Marchese et al. [12] stated that in the caseof the as synthesized AlPO4-34, the band in thisregion corresponds to the deformation mode ofthe NHþ

2 , possibly formed by association of mor-pholine to P–OH groups. In the case of AlPO4-11,this could also occur, despite the formation ofP–OH � � �NH-(i-prop)2 bridges. In the MAPO-11spectra (Fig. 11b and c), besides the 1589–1585cm�1 band, a new absorption appears at 1608 cm�1

Fig. 8. TGA of AlPO4 and MAPO samples under oxygen.

Fig. 9. Derivative TGA of AlPO4 and MAPO samples under

oxygen.

Fig. 10. Weight loss of as-synthesized MAPO-11 samples as a

function of framework magnesium content (T > 630 K underoxidizing atmosphere).


and becomes more intense as the amount ofmagnesium increases from xg ¼ 0:05 (Fig. 11b) to0.3 (Fig. 11c). This band probably corresponds tothe deformation mode of NHþ

2 from diisopropyl-amine interacting with Br€oonsted acid sites [12]created by magnesium substitution in the frame-work, as already shown through DTG, at regionIII of thermograms (Figs. 7 and 9).Fig. 12 shows 31P MAS NMR spectra of as-

synthesized samples with different magnesiumcontents. The sample without magnesium (M000)produces two main signals assigned to three crys-tallographically non-equivalent sites [13] present inAlPO4-11, with chemical shifts P1 ¼ �31 ppm andP2 at �26.1 ppm. Tapp et al. [13] attribute the�30.0 ppm signal (P1) to the resonance of phos-

phorous located at two crystallographic non-equivalent sites, which are found at the junctionbetween the six- and four-membered rings. Theauthors claim also that the signal at �23.8 ppm(P2) belongs to phosphorus sites located at thethird crystallographic non-equivalent site, foundat the junction between two six-membered rings.Table 2 reports the values of the chemical shifts forthese peaks P1 and P2 as a function of magnesiumcontents. The chemical shift of the signal corre-sponding to P1 is constant while P2 shifts to lowerfields as the magnesium contents increases, whichimproves the overall resolution of the spectra(for example P2 ¼ �24:3 ppm for MAPO-11 in-dicated as M030). The difference of chemical shifts

Fig. 11. DRIFTS profiles for (a) AlPO4-11, and MAPO-11, (b)

xg ¼ 0:05, (c) xg ¼ 0:30 (same crystallization time as in Fig. 2)and (d) (i-prop)2NH

þ2 Cl

�.

Fig. 12. 31P MAS NMR of AlPO4-11 (M000) and MAPO-11

with different magnesium contents: xg ¼ 0:1 (M010), xg ¼ 0:3(M030) and same crystallization time as in Fig. 2. The inset

shows the deconvoluted P(3Al,1Mg) species at �22.7 ppm.Asterisks denote spinning sidebands.


between P2 and P1 in the samples are indicated byD ¼ d2 � d1 in Table 2. It is possible to explain thisshift effect, considering the hypothesis that mag-nesium is substituting aluminum located in sitessurrounding the phosphorus atoms correspondingto P2 signal (P between the two six-rings), but notthat surrounding the P atoms corresponding to P1signal (two P sites between six and four rings).Besides these two 31P signals, the spectrum ofAlPO4-11 (sample M000 in Fig. 12) shows twoother minor peaks at �11 and �17 ppm, whichwill be discussed further.Thus in this work, the intensity of the P1 signal

for MAPO-11 samples was assigned only to P(4Al)sites while the intensity of the P2 signal is a resultof superposition of P(4Al) and P(3Al,1Mg) speciesand the intensity of this latter depends on mag-nesium framework contents. The inset in Fig. 12shows the deconvolution of spectrum of sampleM010 and the contribution of this specie. In orderto verify this hypothesis, the spectrum of samplesshowed at Fig. 12 were recorded under cross-polarization conditions and the result obtained inthe case of sample M010 is shown in Fig. 13. It canbe observed that the intensity of signal from site P2at �25.5 was enhanced by 1H–31P CP MAS ex-periment indicating the presence of hydrogen sur-

rounding this site, which can be a consequence ofP(3Al,1Mg) formation.The population of phosphorus on sites P(4Al)

and P(3Al,1Mg) were estimate by spectral decon-volution using Gaussian line shapes as shown inthe inset of Fig 12. In order to accomplish thedeconvolution, the signal corresponding to theP(3Al,1Mg) specie, from phosphorous located inP2 site, was simulated at a fixed value of �22.7ppm. From deconvolution, the Mg molar ratio onAEL framework was estimated using Eq. (2). Theresults displayed in Table 3 indicate that theamount of framework magnesium, Mgframe, in-creases, as the magnesium molar ratio in reactionmixture is higher. However, although the globalmagnesium content in the solid increases linearly

Table 2

Magnesium contents in reaction mixture and chemical shifts for

the as-synthesized samples

Sample xg d2(P2)(ppm)

d1(P1)(ppm)

D ¼ d2 � d1(ppm)

M000 0.00 �26.1 �31.0 5.0

M005 0.05 �25.6 �30.9 5.3

M010 0.10 �25.5 �30.8 5.5

M020 0.20 �24.5 �31.0 6.5

M030 0.30 �24.3 �30.8 6.5

Fig. 13. 31P MAS NMR spectra of sample M030 (a) with and

(b) without 1H–31P CP MAS (tc ¼ 5 ms).

Table 3

Framework and extraframework magnesium contents based on 31P NMR spectra deconvolution and estimated by Eq. (2)

Sample xg tc(h) Mgsolida Mgframe

a Mgextraa %Mgframe

b

M000 0.00 48 0 0 0 –

M005 0.05 60 0.010 0.007 0.003 70

M010 0.10 48 0.022 0.016 0.006 72

M020 0.20 60 0.048 0.022 0.026 45

M030 0.30 30 0.068 0.023 0.045 34

aMg/(MgþAlþ P).b 100�Mgframe=ðMgsolidÞ.


with its concentration in the reaction mixture (Fig.3), Table 3 shows that the incorporation efficiencyinto AEL framework, %Mgframe, diminishes. As aconsequence, the percentage of extraframeworkmagnesium increases and can be explained con-sidering that there is formation of magnesiumphosphate species that are not part of the frame-work. Indeed, the 31P MAS NMR spectrum ofMAPO-11 with the highest magnesium contentspresents a wide signal at �14.2 ppm (sampleM030, indicated by an arrow in Fig. 12) not pre-sent in the samples with lower contents. Analo-gously, Prasad and Haw [14] observed a minorresonance of 31P at �16.7 ppm in MAPO-20 andour results for MAPO-36 [8] show a signal at�13.2 ppm, which were attributed to phosphatespecies that are not part of the framework.Therefore, the signal observed in sample M030 of

MAPO-11 might be assigned to dense phosphates,indicating that the AEL framework had alreadyachieved a saturation limit for Mg incorporationwhen xg > 0:2.The 31P MAS NMR spectrum of AlPO4-11

(sample M00, Fig. 12) shows, besides the twoframework peaks, P1 and P2, two smaller signals at�11 and �17 ppm. These signals were assigned byAkolekar and Howe [7] to magnesium substitutionon AEL framework. However, this assignmentshould be looked at with care since these signalswere not formed in the MAPO-11 samples of thispresent work (M010 in Fig. 12 and M005 in Fig.14). Furthermore, the intensity of these signalsenhances as the crystallization time of AlPO4-11 isincreased from 24 to 60 h (samples M000 and

Fig. 14. 31P MAS NMR spectra of one sample of MAPO-11

and of AlPO4-11 with two crystallization times: xg ¼ 0:05,tc ¼ 60 h (M005), xg ¼ 0:00, tc ¼ 24 h (M000), xg ¼ 0:00,tc ¼ 60 h (M000a).

Fig. 15. 27Al MAS NMR spectra of samples with varying

magnesium substitution (same sample as in Fig. 12).


M000a in Fig. 14). Although the X-ray patterns ofthese two AlPO4-11 samples are very similar, theseresults suggest that the 31P NMR signals at �11and �17 ppm probably belong to another alu-minophosphate structure formed as impurity par-allel to AEL.The 27Al MAS NMR spectra of some samples

are displayed in Fig. 15. It can be observed thepresence of two signals: one of higher intensity at35.7 ppm assigned to tetrahedral aluminum atomsin the AEL structure and the second, of smallerintensity, at 14.0 ppm assigned to octahedral alu-minum belonging to unreacted pseudo-boehmite[12]. The intensity of this latter signal diminishes asthe magnesium contents in the reaction mixtureincreases, probably due to the smaller amount ofthe pseudo-boehmite needed for the synthesis ofMAPO-11 samples (see Eq. (1)).

4. Conclusions

The results show that the AEL structure has alower capacity to incorporate magnesium com-pared to other AlPOs. Whereas about 80% ofmagnesium existing in reaction mixture incorpo-rates in MAPO-5, in the case of MAPO-11 only20% of the magnesium is present in the solidphase. As a consequence, the maximum levelachieved of framework isomorphous magnesiumsubstitution for aluminum was of about 5%. Theattempt to prepare this molecular sieve with ahigher amount of substitution resulted in thecontamination with dense phases.The magnesium isomorphous substitution was

confirmed by thermogravimetry considering amass loss at high temperature (between 623 and823 K), which is absent in the thermograms ofAlPO4-11.This mass loss increased as the magne-sium contents increased in the solid and is relatedto the formation of protonated amine. Addition-

ally, the diffuse reflectance infrared spectroscopyshowed a band at 1608 cm�1 related to thepresence of diisopropylamine protonated by theBr€oonsted acid sites generated by isomorphousmagnesium substitution. Finally, the incorpora-tion of magnesium was corroborated by the de-convolution of the 31P MAS NMR, whichevidenced the presence of a peak at �22.7 ppm,assigned to P(3Al, 1Mg) groups in the framework.

Acknowledgements

The Fundac�~aao de Amparo �aa Pesquisa no Es-tado de S~aao Paulo (FAPESP) is acknowledged forthe financial support for this work.

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synthesis and characterization of mapo-11 molecular sieves

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