preparation and characterization of the magnesium aluminophosphate mapo-39

Preparation and characterization of the magnesium aluminophosphate MAPO-39

Deepak B. Akolekar and Serge K. Kaliaguine Dkpartement de GEnie Chimique et CERPIC, Universite’ Laval, Que’bec, Canada

MAPO-39, a small-pore magnesium aluminophosphate molecular sieve (ATN topology), has a pore opening of 4.0 A and tetragonal symmetry. The preparation of MAPO-39 was studied using a one-gel composition and different crystallization periods. Pure MAPO-39 was obtained by hydrothermal crystallization of the gel (molar composition: 1 .l n-PrzNH . 0.40 MgO . 0.80 A&O3 1.02 PzOs .41 HzO) at 423 K for 96 h. Investigations on the crystal structure, morphology, thermal conversion, and surface properties of the crystalline product were carried out. The crystallinity of MAPO-39 increases with increase in the crystallization period up to 96 h and decreases beyond the period of 96 h. As compared with MAPO-43 (another 8-O ring molecular sieve), MAPO-39 could be calcined at a higher temperature (up to 793 K) without any loss of the crystal structure. MAPO-39 possesses HzO-sorption capacity of 0.226 cm3 g-‘. SEM investigations reveal that the crystals of MAPO-39 are irregular platelike in shape and 2.5 x 3.5 pm in size. X.p.s. analysis indicates that the concentration of aluminum is significantly higher on the surface than in the bulk and the observed binding energy of Mg2, (50.3 eV) corresponds to that of tetrahedrally coordinated magnesium.

Keywords: MAPO-39; ATN topology; preparation; X.p.s.; thermal analysis, i.r.

INTRODUCTION

Magnesium aluminophosphates (MAPO-n) have been reported as a new class of molecular sieves.’ In the aluminophosphate molecular sieve, the substitution of some of aluminum atoms by magnesium atoms produces acidic hydroxyl groups that affect the cat- alytic and adsorptive properties.1*2-5 The small pore MAPO-39 molecular sieve has ATN topology.6 The structure determination with Reitveld refinement of MAPO-39 was reported by McCusker et a1.7 The X-ray powder pattern of MAPO-39 was indexed on a body centered tetragonal unit cell with a = 13.09 A and c = 5.18 A.7 A sigma contraction of the MER structure type can be described as the framework structure of MAPO-39. The channel system is l- dimensional with an 8 oxygen ring pore opening (pore diameter 0.4 nm).7 The framework density of MAPO-39 is 18.0 T/1000 A3.6 The information avail- able on MAPO-39 is scarce.2’6*7 This work on the preparation and characterization of MAPO-39 was thus undertaken as the first phase of a program the objectives of which were to assess the suitability of this material as an adsorbent or as a catalyst. The present study encompasses the synthesis and investigations on the morphology, thermal decomposition, and sorp- tive and surface properties of MAPO-39.

Address reprint requests to Dr. Kaliaguine at the Department de Genie Chimique et CERPIC, Universite Laval, Quebec, Canada GlK 7P4. Received 18 August 1993; revised 2 May 1994; accepted 8 May 1994

0 1994 Butterworth-Heinemann

620 ZEOLITES, 1994, Vol 14, November/December

EXPERIMENTAL

The synthesis of the type 39 magnesium aluminophosphate molecular sieve was carried out using one-gel composition and different crystallization conditions. Pure magnesium aluminophosphate of a type 39 molecular sieve was synthesized by crystallizing a gel of the composition 1.1 n-Pr2NH . 0.40 MgO * 0.80 Al203 * 1.02 I’205 . 41 Hz0 hydrothermally in a Teflon-coated autoclave at 423 K for 96 h. The sources of AlzO3, PsO5, and MgO were pseudo- boehmite (courtesy: Vista Chemical Co. USA), orthophosphoric acid (85%, Aldrich), and magnesium acetate (puriss, Fluka), respectively. The 99% pure di-n-propylamine was supplied by Aldrich (USA).

The reactive magnesium aluminophosphate gel and di-n-propylamine mixture was prepared as follows: Finely ground reactants were used for the gel preparation. In the first step, the specific amount of boehmite was stirred in deionized water and a solu- tion of magnesium acetate (rate of addition 1 cm3 min-‘) added with continuous stirring for 15 min. A heavy-duty overhead stirrer (motorized) with a high- density polyethylene stirrer shaft and blades was used for mixing. In the second step, a particular amount of orthophosphoric acid was added to the magnesium- aluminum slurry at the addition rate of 1 cm3 min-’ and stirred for 25 min. In the third step, a required amount of di-n-propylamine was added at the rate of 1.0 cm3 min-’ to the aluminophosphate-based gel with continuous stirring for 20 min at room tempera-

-L ii I

10 20 30 40 50

-29-

Figure 1 X-ray powder diffractogram of the as-synthesized magnesium aluminophosphate of type 39.

ture. The gel was transferred into the stainless-steel autoclave coated with Teflon with a capacity of ca. 150 cm3 and heated under static conditions at 423 K for 96 h. The solid product obtained was processed by washing with deionized water and dried in an air oven at 373 K for 24 h. The organic template (di-n- propylamine) was removed from the material by calcination in the presence of air (flow: 100cm3 min-‘) at 773 K for 12 h.

The X-ray powder diffraction spectrum was obtained using a Holland Philips PW/1730 X-ray generator with a Ni-filtered CuKor radiation source and a scintillation counter. The atomic absorption spectroscopy (using a Perkin-Elmer 11 OOB spectro- meter) and gravimetric analysis were used for the determination of composition of the synthesized material. X.p.s. measurements were conducted for determining the surface concentration and binding energy of C( Is), N(ls), O(Is), Mg(2p), A1(2p), and P(2p). A V.G. Scientific Escalab Mark II system with a hemispherical analyzer operated in the constant pass energy mode (20 eV) was employed. An MgKor X-ray source (hu = 1253.6 eV) was operated at 20 mA and 15 kV. The intensity of the X.p.s. bands was deter- mined using linear background subtraction and in- tegration of peak areas. The accuracy of the B.E. determination was within +O.lO eV.

The HzO-sorption capacity of MAPO-39 was deter- mined gravimetrically. * The size and morphology of the crystals of MAPO-39 were studied using a JEOL JSM-840A scanning electron microscope. The mid- FTi.r. spectrum was recorded for a self-supporting wafer prepared by pressing 1.5 mg of the MAPO-39 sample in 100 mg of KBr. The in situ i.r. measurements in the hydroxyl stretching vibration region were performed on self-supported wafers (8 mg). The wafer was mounted in a Pyrex vacuum cell with calcium fluoride windows. The cell was then con- nected to a vacuum line and the sample was degassed at 673 K overnight (p = 1 Om5 Torr). The spectra were recorded in the range of 4000 - 400 cm-’ with 2 cm-’ resolution using a Digilab FTS-60 spec- trometer. Thermal analysis of the n-PrzNH-MAPO- 39 dried at 373 K in air for 16 h was performed.

Preparation of MAPO-39: D.B. Akolekar and S.K. Kaliaguine

Thermal analysis (t.g./d.t.g./d.t.a) data of the TZ- PrzNH-MAPO-39 were obtained under inert and oxidizing atmospheres, using a fully automated and computer-assisted t.g./d.t.a. 220 simultaneous thermogravimetric/differential thermal analyzer (Seiko Instruments Inc., Japan) under the following conditions: sample size, 20 mg; reference compound, OL alumina; sample holder, platinum crucible; temperature range, min-‘;

303 - 1203 K; heating rate, 10 deg atmosphere, flowing air or nitrogen (flow rate

100 cm3 min-‘).

RESULTS AND DISCUSSION Different crystallization conditions were tested to find optimum conditions for preparing the pure magnesium aluminophosphate of type 39. In all the batches, the gel composition [ 1.1 n-PrzNH . 0.40 MgO * 0.80 AlzOs . 1.02 PzOs * 41 Hz01 and the crystallization temperature (423 K) were the same; only the crystallization period was varied from 24 to 144 h with incre- ments of 24 h. The crystallinity results show that by increasing the crystallization period from 24 to 96 h the crystallinity of MAPO-39 increases. Further, beyond the crystallization period of 96 h, the crystallinity decreases. Highly crystalline MAPO-39 is thus obtained by crystallizing the gel at 423 K for 96 h.

It is interesting to note that MAPO-11, MAPO-39, MAPO-43, MAPO-46, and MAPO-50 could be readily synthesized hydrothermally using the same template (di-n-propylamine). The template-structure specificity is high for each of these materials, and the relative concentrations of Mg and the template is critical in order to obtain one particular structure.2’9-”

The powder XRD pattern of the as-synthesized MAPO-39 shown in Figure 1 is identical to that reported earlier for n-PrzNH-MAPO-39.* The XRD data for the n-Pr2NH-MAPO-39 is presented in Table I. The unit cell composition of the calcined MAPO-39 calculated from elemental analysis (expressed in wt%, 6.31 MgO 34.32 A1203 59.37 P205) is Hi.s[Mgi.s A16,4 Ps Os2]. The framework charge calculated from the chemical composition is -0.09 electron/T atom. Figure 2 shows the particles and the close up of the particle surface of the type 39 magnesium aluminophosphate. Morphological investigations (Figure 2 and Table 2) reveal that MAPO-39 crystals are irregular platelike in shape and 2.5 X 3.5 pm in size.

The characteristics of MAPO-39 are presented in

Table 1 X-ray powder diffraction data for the tnagneSiUm aluminophosphate of type 39 molecular sieve

d (A) I (%I d (A) I (%I

9.34 24 2.96 37 6.61 48 2.74 17 4.91 40 2.64 10 4.19 83 2.45 3 3.95 100 2.36 12 3.30 6 1.86 5 3.12 12 1.78 7 3.01 16

ZEOLITES, 1994, Vol 14, November/December 621

Preparation of MAPO-39: D. B. Akolekar and S. K. Kaliaguine

Figure 2 Scanning electron micrograph of the as-synthesized magnesium aluminophosphate of type 39.

Table 2. The HTO-sorption capacity of MAPO-39 is 0.226 cm3 g-‘. Earlier investigations’V”*8’0 on the HPO-sorption capacity of A1P04-5, MAPO-36, and MAPO-46 materials indicated a higher HpO-sorption capacity than the one of MAPO-39. As compared with the magnesium aluminophosphate of type 43 (a small-pore molecular sieve with GIS topology), the MAPO-39 possesses 2 1% lower HZO-sorption capacity. It is important to note that MAPO-39 has a higher thermal stability than that of MAPO-43 (both materials have approximately similar chemical com-

Table 2 Characteristics of the magnesium aluminophosphate of type 39 molecular sieve

Product molar composition (0.094 Mg .0.404 Al .0.502 P]OZ HzO-sorption capacity (cm3 g-r) 0.226 Crystal shape Irregular platelike Crystal size (pm) 2.5 x 3.5 Frame charge (electrofl atom) -0.09

DT

T(

DTI

273 523 773 1023 1273

TEMPERATURE 6)

Figure 3 T.g., d.t.g., and d.t.a. curves for the decomposition of the as-synthesized magnesium aluminophosphate of type 39 in nitrogen.

position and pore size). The crystal structure of MAPO-43 starts to collapse above the calcination temperature of 653 K.” In the case of MAPO-39, no change in the crystal structure was observed up to the calcination temperature of 798 K.

Figures 3 and 4 show the t.g., d.t.g., and d.t.a. curves for the decomposition of n-PrzNH-MAPO-39 in inert and oxidizing atmospheres, respectively. The thermal analysis data of n-PrzNH-MAPO-39 are presented in Table 3. From the t.g. and d.t.g. data, it follows that there are three and four distinct zones of weight loss from MAPO-39 containing di-n- propylamine in inert and oxidizing atmospheres, respectively.

The result of the thermal analysis indicates that in an inert atmosphere all the processes occurring in the removal of occluded materials from the metal- aluminophosphate are endothermic. The total weight loss at 1203 K amounts then to 9.9 wt%. In the first zone, the weight loss is 3.8 wt% at 303-570 K, which

15 c 273 523 773 1023 12

TEMPERATURE,(K)

3

Figure 4 T.g., d.t.g., and d.t.a. curves for the decomposition of the as-synthesized magnesium aluminophosphate of type 39 in air.



Table 3 T.g. d.t.g., and d.t.a. data of the as-synthesized magnesium aluminophosphate of type 39 molecular sieve

Nitrogen Air

Decomposition Temp. wt. loss Total wt. Peak temp. (K) Temp. range wt. loss Total wt. Peak temp. (K)

zone range (K) (%I loss (%) D.t.g. D.t.a. (K) (%) loss (%I D.t.g. D.t.a.

II 303-570 570-922 3.8 5.3 3.8 9.1 412 801 372’ 801* 303-579 578-904 3.9 5.4 3.9 9.3 412 808 788b 426’

Ill 922-l 203 0.8 9.9 - - 904-1017 1.7 11.0 1014 10146 1017-1203 0.5 11.5 - -

d endothermic b exothermic

is due to the desorption of physisorbed water and possibly di-n-propylamine from the metal- aluminophosphate. The major weight loss (5.3 wt%) occurs in the second zone at 570-922 K. The second zone corresponds to the desorption of di-n- propylamine and possibly to cracking of the occluded di-n-propylamine molecules to smaller hydrocarbons and ammonia. In the third zone, the weight loss of 0.8 wt% occurs at 922-1203 K. This weight loss is due to the very slow desorption of ammonia and/or small hydrocarbon fragments adsorbed strongly on MAPO-39. In the oxidizing atmosphere, the thermal analysis results indicate that the decomposition is a very complex process that occurs in four distinct zones with a total weight loss of 11.5 wt%. The first zone, slightly endothermic, corresponds to the desorption of physisorbed water and di-n-propylamine from the n-PrsNH-MAPO-39 occurring at lower temperatures (301-659 K). The weight loss is 1.4 wt% in the first zone. In the second zone, a weight loss of 5.4 wt% occurs at 579-904 K. In this zone, the removal of di-n-propylamine is expected to be due mostly to its oxidative decomposition, which is an exothermic process. The third weight loss (1.7 wt%) and fourth (0.5 wt%) weight losses occurs at 904- 1017 and 1017-1203 K, respectively.

The oxidative decomposition is thought to involve thermal cracking of the occluded di-n-propylamine, followed by oxidation of the cracking products in the intra- and intercrystalline spaces. The nature of the exothermic phenomena occurring at high temperature is probably more complex. The comparison of the thermal analysis results of the n-PrzNH-MAPO- 39 and n-PrsNH-MAPW3 (Ref. 9) revealed that the removal of the organic template (n-PrsNH) in the

Table 4 Bulk and surface composition of the as-synthesized and calcined magnesium aluminophosphate of type 39 molecular sieve

Atomic ratio

AI/P MglP [Mg + All/P] C/N C/P --

Sample BSBSB SSS

Pr,NH-MAPO-39 0.81 1.35 0.19 0.23 0.99 1.57 8.23 0.70 MAPO-39 0.81 1.45 0.19 0.25 0.99 1.70 - -

B, bulk; S, surface

inert and oxidizing atmospheres from MAPO-39 is more complex than for MAPO-43.

The bulk and surface composition of the as- synthesized MAPO-39 and the calcined MAPO-39 samples are presented in Table 4. The estimation of surface atomic ratios is obtained from the ratios of the Mg+, A12,, PQ, Cur and N1, X.p.s. band intensities as reported elsewhere. i2 The surface concentrations of the elements on the as-synthesized and the calcined magnesium aluminophosphate of type 39 are as follows: n-PrzNH-MAPO-39: Ci, 8.5%; Ni, 1.0%; Mgsp 2.8%; A1zp and of calcined

16.4%; Psp 12.2%; and 01, 59.1%, MAPO-39: Mgpp 3.4%; Al+ 19.8%;

P2p 13.6%; and Oi, 63.2%. In the uncalcined and calcined MAPO-39 samples, the Al/P ratio on the surface is higher than in the bulk, which indicates that the concentration of aluminum is higher (67-70%) on the surface. Comparisons of bulk and surface Mg/P ratios for n-Pr2NH-MAPO-39 and MAPO-39 indicate that the surface concentration of Mg is also higher than in the bulk. The interpretation of these results is not straightforward, because both XRD and HsO-sorption capacity indicate the absence of any significant amount of amorphous phase in the pores. It may thus be that a minor amount of Al-rich material is segregated on the external surface of the particles, specifically affecting the X.p.s. intensity of Al and, to a lesser extent, Mg.

Table 5 shows the X.p.s. binding energy data for n-PrsNH-MAPO-39. The main carbon signals observed on the MAPO-39 sample did not result from the adventitious contamination but from the organic template (di-n-propylamine). The binding energy measured for Mg+, in n-PrsNH-MAPO-39 is 50.3 eV. This value is identical with the one reported for tetrahedrally coordinated Mg in the spine1 structure MgAlsO+ i3*i4 It is therefore concluded that the observed Mgnp line in MAPO-39 is for the tetra-

Table 5 X.p.s. data for the as-synthesized magnesium aluminophosphate of type 39 molecular sieve

Binding energy (eV)/f.w.h.m. (eV)”

N IS Mgzp 4 P, 0 IS

400.713.1 50.312.3 74.612.4 1342.5 531.8l2.5

o Referenced to C,, = 285 eV

ZEOLITES, 1994, Vol 14, November/December 623


hedrally coordinated Mg in this lattice. The observed binding energies of Al+ P2 the values that we reporte B

, and Oi, are very close to earlier for Al,,, Pzp, and

Oi, in the MAPO-43 (Ref. 9) and MAPO-46 (Ref. 10) molecular sieves.

Figure 5 shows the mid-FTi.r. spectrum of the as-synthesized MAPO-39 in the lattice vibration range (1400-400 cm-‘). The strongest vibration in the 1000-250 cm-’ region is assigned to the asymmetric stretch mode of TO4 tetrahedra.15 The next strongest band in the region 430-500 cm-’ is assigned to a T-O bending mode. The symmetric stretch modes are assigned to the bands in the region of 650-800 cm-‘.

The hydroxyl region infrared spectrum of MAPO- 39 after vacuum activation is shown in Figure 6. Numerous very weak absorption bands are observed between 3530 and 3631 cm-‘. The line at 3745 cm-’ is associated with a nonacidic terminal OH associated with surface Al or Mg atoms. The strong band at 3678 cm-’ is thought to belong to a terminal OH linked to a surface P atom and is slightly acidic. It is interesting to note that the intensity of P-OH band (at 3678 cm-‘) decreases after pyridine adsorption at 473 K, indicating its weakly acidic nature. The acidic hydroxyl group is assigned to the band at 363 1 cm-‘. The values of the terminal OH group, the lattice termination P-OH group, and acidic hydroxyl rou are close to the usual values reported earlier. x16-w

In the aluminophosphate molecular sieve, the incor- poration of Mg atoms into some of Al sites in the AlP04 framework generates the anionic framework charge and leads to Bronsted acidity.

SUMMARY The present study reports the procedures for the synthesis and the characterization of the highly crystalline small-pore magnesium aluminophosphate of type 39 molecular sieve. MAPO-39 possesses a lower

WAVENUMBER. cm-l

Figure 5 Mid-FTi.r. spectrum of the as-synthesized magnesium aluminophosphate of type 39.

cm-’

Figure 6 Hydroxyl region infrared spectrum of the MAPO-39 after 673 K vacuum activation.

HzO-sorption capacity than that of MAPO-36, MAPO-43, and MAPO-46. MAPO-39 has a higher thermal stability than that of MAPO-43. Owing to its higher thermal stability, MAPO-39 can be employed as a practical adsorbent or as a catalyst.

ACKNOWLEDGEMENT One of the authors (D.B.A.) is thankful to the Natural Sciences and Engineering Research Council of Cana- da, Ottawa, Canada, for the award of an international postdoctoral fellowship.

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preparation and characterization of the magnesium aluminophosphate mapo-39

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