VPI-5 and related aluminophosphates: Preparation and thermal stability

VPI-5 and related aluminophosphates: Preparation and thermal stability W. Schmidt, F. Schiith, H. Reichert and K. Unger

Institut fi~r Anorganische Chemie und Analytische Chemie,Johannes Gutenberg-Universit~it, Mainz, Germany B. Zibrowius

Zentralinstitut fi~r Physikalische Chemie, Akademie der Wissenschaften, Berlin, Germany The results of investigations on the crystallization conditions of VPI-5 and other AIPO4 phases are described. Four different aluminophosphates occurred and have been characterized by XRD, SEM, and t.g.a. The thermal stability of VPI-5 has been studied by n.m.r, and F'/3.r. techniques and its micropore volume has been evaluated by nitrogen adsorption. Keywords: VPI-5; AIPO4-H3; AIPO4-11; AIPO4-39; AIPO4-8; synthesis; thermal stability

INTRODUCTION

EXPERIMENTAL

VPI-5 was synthesized for the first time in 1988 by M.E. Davis et al. 1'2 Since then, it has become the subject of numerous investigations. This is because it was the first aluminophosphate with pore diameters greater than 0.8 nm. The unidirectional channels of VPI-5 are formed by 18 membered T-rings with an internal diameter of 1.2-1.3 nm. Aluminophosphates are usually synthesized by hydrothermal reactions, using an organic amine or ammonium salt as a "template". However, since one template can usually lead to several aluminophosphates, one has to carefully choose the correct template and reaction conditions to build a specific aluminophosphate. It is known that several different templates can be used to "build" VPI-5, for example, tetrabutylammonium hydroxide (TBAOH) or ndipropylamine (DPA). Following the Davis procedure exactly 1'2 did not lead to phase pure VPI-5, but to a mixture of AIPO,t-I 1 and a little VPI-5. From this we checked the synthesis with DPA by examining the effects of different gel compositions, reaction temperatures, reaction times, gel aging procedures, and autoclave dimensions. The syntheses products were characterized by XRD, SEM, and simultaneous t.g.a./d.t.a. Further investigations on VPI-5 were carried out by using 27A1 a~ d 31P MAS n.m.r, spectroscopy, i.r. spectroscopy, and N2 adsorption techniques.

The hydrothermal reactions were carried out in Teflon beakers with Teflon lids, enclosed by stainlesssteel vessels. Beakers of two different sizes were used: 50 and 250 ml. Preparation of the reaction gels was according to the instructions given by Davis et al. 2 First, the pseudoboehmite was slurried in 2/3 of the water available. Phosphoric acid was diluted in the rest of the water and then added to the slurry. Before and after the addition of the template, the gel was aged, typically for 2 h each time. T h e pseudoboehmite used was Pural SB (CONDEA) with a water content of 23.4%, whereas the 85% phosphoric acid and the DPA were obtained from Merck and Aldrich, respectively. The reaction gel was transferred into the autoclaves and placed in a preheated oven. Termination of the reaction was achieved by quenching the autoclaves in cold water. The reaction product was washed with water after the amorphous particles were removed by decanting them with water. The crystalline product was obtained by filtration and drying under reduced pressure at room temperature. For XRD, we used a Philips powder diffractometer with CuK~ radiation. XRD was used for phase analysis and to check the crystallinity of the AIPO4 phases. SEM photographs obtained by a Philips PSEM 500 scanning electron microscope documented the morphology of the crystals and gave information about the agglomeration and the size of the single crystals. A Linseis L81 thermobalance was employed for t.g.a. to examine the water and template content of the different phases. N.m.r. spectra were taken with a Bruker 400 MAS n.m.r, spectrometer, and i.r. spectra were obtained with a Nicolet 5SXB FTi.r. spectrometer. To prove the microporosity of some samples,

Address reprint requests to W. Schmidt at the Institut for Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universit~t, W-6500 Mainz, Germany. Received 18 February 1991, accepted 3 June 1991 © 1992 Butterworth-Heinemann

2 ZEOLITES, 1992, Vol 12, January

VPI-5 and related aluminophosphates: W. Schmidt et al.

1 DPA" 1 AI203 " 1 P205" 100-

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RESULTS AND DISCUSSION

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Figure 1 The dependence of the various AIPO4 phases upon the reaction temperature for different molar ratios (using 50 ml autoclaves): (a) 1 DPA:I AI203:1 PzOs:40 H20, 100-200°C; (b) 0.9 DPA:I AI203:1 P205:30 H20, 100-200°C; (c) 1 DPA:I AIzO3:l PzOs:40 H20, 120-130°C

We examined the reaction temperature in the range of 100-200°C using two different gel compositions in order to find the optimized reaction conditions for the crystallization of VPI-5. For this, 50 ml autoclaves were used, and the reaction time for all these syntheses was 20 h. The amounts of the different phases in the crystalline products were estimated from XRD data. s The results are shown in Figure la-c. One can see that at a molar ratio of 1 DPA:I A120~:l P20~:40 H20 at 120°C mainly A1PO4-H3 with very little VPI-5 is obtained. At 130°C, VPI-5 is the main phase and no AIPO4-H3 could be found, but A1PO4-11 appears as a co-phase, while AIPO4-11 becomes the main phase at 140°C. It was ascertained that temperatures higher than 140°C led only to the formation of A1PO4-11. Reaction gels with a molar ratio of 0.9 DPA:I A12Os:l P20~:30 H20 led to similar results, but VPI-5 appears in a much broader temperature range, with a maximum of the VPI-5 amount between 120 and 130°C. At temperaturs below 120°C, VPI-5 is observed only as the minor phase beside AIPO4-H3, and at temperatures above 130°12, beside AIPO4-11. From reactions at temperatures higher than 160°C, only AIPO4-11 is obtained. Since at 132°C AIPO4-H3 is still observed as a co-phase beside AIPO4-11 and VPI-5, one can therefore expect it to appear in the whole range between 120 and 130°C. This implies that there is no possibility of obtaining phase-pure VPI-5 by variation of the synthesis temperature only at this molar ratio. In the case of the lower concentrated gel, the crystallization fields of the aluminophosphates seem to be much smaller, related to the reaction temperature; however, the temperatures between 120 and 130°C were next examined, using the molar ratio 1 DPA:I A12Os:l P205"40 H20. This investigation showed that there must be an optimum temperature around 124°C (as shown in Figure lc). At this temperature, only a little AIPO4-H3 is found as a co-phase beside VPI-5. The amount of AIPO4-H3 varies from 0 to 5% when using 50 ml autoclaves. It can be reduced to zero by using bigger autoclaves. This effect might be explained by slower heating rates in 250 ml autoclaves and/or different temperature gradients in these beakers. The next parameter we looked at was the reaction time. Using the above molar ratio, 50 ml autoclaves, and a reaction temperature of 124°C, we varied the synthesis time from 3 h to several days (Figure 2a). After 3 h recation time, only VPI-5 was crystallized, but the yield of crystalline product was about 20% of that produced after a longer reaction time (as shown by Figure 2b). Six hours of reaction lead to a product that contains small amounts of A1PO4-H3. This amount keeps to a constant up to reaction times of 21 h, after which A1PO4-11 also appears as a co-phase

ZEOLITES, 1992, Vol 12, January 3

VPI-5 and related aluminophosphates: W. Schmidt et al.

1 D P A " 1 A I 2 0 3 " 1 P 2 0 5 " 40 H 2 0 100-

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stirrer while aging. For the mixing process, a stainless-steel stirrer, which covers the bottom of the beakers, was used. No stirring, i.e., just homogenization of the gel before the aging process, leads to the co-formation of VPI-5 and AIPO4-11. After stirring at medium speed, VPI-5 is formed. Rigid stirring supports the formation of A1PO4-11 and VPI-5, but with VPI/5 as the minor component in the product. The duration of the gel aging is also important for the crystallization of different aluminophosphates. Without gel aging, all three aluminophosphates (AIPO4-11 and H-3, and VPI-5) are found in the reaction product. We observed the pH during the gel aging process and found it in accordance with that reported by Davis et al. I in that it stabilizes after approximately 2 h during both aging periods. Extension of the aging period of a few hours did not affect the results. After very long aging periods, the formation of a small amount of A1PO4-11 was observed. As mentioned previously, the 250 ml autoclaves gave better results (more phase-pure VPI-5), but using these, we found a further A1PO4 phase: It

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reaction time [h] Om

Figure 2 (a) The dependence of the various AIPO4 phases upon the reaction time at 124°C for the m o l a r ratio: 1 DPA:I AIzO3:l P2Os:40 H20 (using 50 ml autoclaves); (b) the dependence of the total yield of the crystalline products upon the reaction time

alongside VPI-5 and A1PO4-H3. If the duration of the synthesis is very long, VPI-5 and AIPO4-11 are obtained but no AIPO4-H3 is detected. From the various investigations, it seems as if VPI-5 crystallizes first, then A1PO4-H3, and lastly AIPO4-11. During the formation of A1PO4-11, it was found that the pH of the gel rises up to values of around 10. It is also known that aluminophosphates are dissolved in a basic medium. This could be the explanation for the disappearance of AIPO4-H3 and a lower VPI-5 amount in the product, which are observed after very long reaction times. Fi~.re 2b shows the total yield of all phases v. the reaction time. The yield increases up to 12 h reaction time and keeps constant for the next 9 h; it then decreases with increasing reaction times. The formation of the different aluminophosphate phases is also affected by the gel aging procedure. We also found an effect from the stirring speed of the

4 ZEOLITES, 1992, Vol 12, January

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VPI-5 and related aluminophosphates: W. Schmidt et al.

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sediments very slowly, during several hours, out of the decanted washing water and can totally be removed from the VPI-5 phase. The XRD pattern of this phase does not fit to any pattern of known aluminophosphates, but was found to be similar to the pattern of MeAPO-39, which is described by Wilson and Flanigen. 4 Therefore, we suggest that this

I

a

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phase could be the corresponding aluminophosphate, AIPO4-39. The amount increases when the gel is more concentrated, e.g., 1 DPA: 1 AI2Os: 1 P205:30 H20. The XRD patterns of all observed phases are shown in Figure 3. Calcination of these aluminophosphates leads to weight losses; these were determined by thermogravimetric analysis. Some typical t.g. curves are shown in Figure 4. Most of the weight loss for A1PO4-11 and -39 occurs in two major steps. The first step is due to the loss of the water, and the second is due to the loss of the template contained in the pores of the structure. A1PO4-11 contains 4.3% of water and 7.6% of DPA. For A1PO4-39, a weight loss of 9.5% for water was observed and a 9.3% weight loss for template occurred at temperaturs above 230°C. Both structures are stable to 500°C under ambient pressure. As can be seen, VPI-5 and AIPO4-H3 do not exhibit a second weight loss at temperatures above 200°C. This implies that VPI-5 crystals, as well as A1POa-H3 crystals, contain no or very little DPA. To prove this hypothesis, we recorded an FTi.r. spectrum of an "as-synthesized" sample of VPI-5 and compared it with a spectrum of a sample calcined under reduced pressure and then rehydrated. Both spectra were

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Figure 5 (a) 31p M A S n.m.r, spectrum of a VPI-5 sample; (b) 31p M A S n.m.r, spectrum of the sample after thermal treatment; (c) 27AI M A S n.m.r, spectrum of a VPI-5 sample; (d) 27AI M A S n.m.r, spectrum of the same sample after thermal treatment under ambient pressure

Z E O L I T E S , 1992, V o l 12, J a n u a r y

5

VPI-5 and related aluminophosphates: W. Schmidt et al.

identical, showing that no template molecules are present in these crystals. The water contents of VPI-5 and AIPO4-39 were evaluated as 24.0 and 19.0%, respectively. Measuring the same sample of VPI-5 again after some weeks leads to a weight loss of 27.2%. VPI-5 seems to adsorb water and/or other molecules from the air. Since the water in "as-synthesized" VPI-5 is located only in the large channels, which are built from 18-membered rings, this further adsorption might, therefore, take place in the smaller channels (4- and 6-ring channels). Both VPI-5 and AIPO4-H3 undergo a phase transition while being calcined. As a first step, AIPO4-H3, an aluminophosphate hydrate, is reversibly dehydrated to A1PO4-C. At higher temperatures, it changes irreversibly to A1PO4-D, which can, in turn, be hydrated reversibly to A1PO4-H6. 5'6 Heating a wet VPI-5 sample to 130°C under ambient pressure leads to a total phase transition to AIPO4-8. To avoid this transition, the sample must be heated slowly under reduced pressure (10 -5 Torr). By utilizing this method, it can be heated up to 450-500°C without a change in structure. The transition to AIPO.r8 has been documented by XRD and MAS n.m.r. (Figures 3 and 5a-d). The 31p n.m.r, spectrum of VPI-5 exhibits three typical bands at -23.7, -27.4, and -33.1 ppm. 7's Upon heating the sample for 3 h at 130°C, the spectrum was found

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6 ZEOLITES, 1992, Vol 12, January

obtained

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VPI-5 and related aluminophosphates: W. Schmidt et al.

r

e Figure 8 SEM photos of (a) a spheric VPI-5 agglomerate (bar = 10 ~tm); (b) a segment of a spheric VPI-5 agglomerate (bar = 10 I~m); (c) AIPO4-H3 agglomerates (bar = 10 p,m); (d) AIPO4-11 aggregates (bar = 1 ~tm); (e) ALP04-39 aggregates (bar = 10 t~m); (f) ALP04-39 aggregates (bar = 10 I~m)

to be completely c h a n g e d . T h e new s p e c t r u m shows o n e large p e a k at - 3 0 . 3 p p m and t h r e e smaller ones at - 1 5 . 3 , - 1 9 . 6 , and - 2 3 . 1 p p m . T h e smaller peaks could be d u e to i n t e r m e d i a t e stages between VPI-5 a n d AIPO.t-8. VPI-5 is an a l u m i n o p h o s p h a t e hydrate, which is 97 c o n f i r m e d by the - A1 n.m.r, s p e c t r u m , w h e r e two b a n d s at 40.1 a n d - 19,9 p p m are found. 7'x T h e b a n d at 40.1 p p m is d u e to t e t r a h e d r a l - c o o r d i n a t e d alumi-

n u m , w h e r e a s the o n e at - 1 9 . 9 p p m indicates o c t a h e d r a l - c o o r d i n a t e d a l u m i n u m . T h e two additional o x y g e n a t o m s belong to water molecules that are c o o r d i n a t e d to these a l u m i n u m positions, After thermal t r e a t m e n t , this b a n d d i s a p p e a r s and only one b a n d at 36.1 p p m is observed, which belongs to t e t r a h e d r a l - c o o r d i n a t e d a l u m i n u m . U s i n g FTi.r. spectroscopy, e m p l o y i n g a heatable D R I F T cell, we were able to observe this c h a n g e f r o m VPI-5 to

ZEOLITE& 1992, Vol 12, January

7

VPI-5 and related aluminophosphates: W. Schmidt et al.

A1PO4-8. VPI-5 has typical bands in the range of 1000-1400 cm -l. One sharp band at 1265 cm -1 is significam for'VPI-5. Between 1200 and 1000 cm-1, some overlapping bands can be found. 7 The sample was heated in 10°C steps from 40 to 100°C and then up to 150°C (Figure 6). The phase transition begins at 70°C, which is detected by the size reduction of the peak at 1265 cm -I. The AIPO4-8 spectrum, observed after heating directly to 150°C, shows a shoulder at 1229 c m - 1 and a broad peak with a maximum at 1114 cm -I. Heating the sample at 150°C for 18 h did not lead to a further change in the spectrum. To check whether this is a reversible phase transition, we cooled the cell immediately after recognizing the first indications of the transition. Since we did not see a reappearance of the VPI-5 structure, the phase transition seems to be irreversible. The microporosity of the VPI-5 sample was confirmed by nitrogen adsorption measurements at 77 K under dynamic conditions, after activating the samples at 10 -5 mbar at 200°C. Figure 7 shows the obtained type I isotherm, which is typical for microporous materials. The micropore volume of VPI-5 was calculated from the adsorbed nitrogen volume of 118.8 cm 3 gaseous N2 per gram VPI-5 at P/Po = 0.4. From this value, the volume of fluid N2 at 77 K, i.e., the micropore volume, was calculated as 0.184 cm3/g. The SEM photos of the AIPO4 phases are shown in Figures 8a-f. VPI-5 crystallizes into long needles that agglomerate as spheres of 300 to 500 ~tm diameter. A segment of such an agglomerate shows that the crystals grow from the center to the outer surface of these spheres. Smaller spheres of 20 p.m diameter can be found beside these big ones, but since no other phase is detected by XRD, it must be also VPI-5. The shape of A1PO4-H3 agglomerates is also spherical, but much smaller than VPI-5 and built from lamellae. In contrast, A1PO4-11 forms long prisms of varying sizes aggregated irregularly. In spite of crystallizing

8

ZEOLITES, 1992, Vol 12, January

also as spheres, the morphology of AIPO4-39 is different to the upper ones. Very small crystals are aggregated together, forming ball-shaped structures, some of which exhibit holes in the spheres reminiscent of pearls.

CONCLUSIONS Optimum synthesis conditions for the crystallization of VPI-5 were found as a gel composition of 1 DPA: 1 AI20~: 1 P205:40 H20, gel aging times of 2 h each, a synthesis temperature of 124°C, and reaction times in the range of 12-16 h. The synthesis gel must be stirred, but rigid stirring should be avoided. To decrease the crystallization of co-phases, larger autoclaves should be preferred. Variation of these conditions can lead to the formation of three further aluminophosphates: A1PO4-H3, AIPO4-11, and A1PO4-39. VPI-5 undergoes an irreversible phase transition into AIPO4-8 while heated at temperatures over 60°C under ambient pressure. In case of calcination under reduced pressure, the transition can be prevented and the VPI-5 structure with the micropore volume of 0.184 cm~/g is preserved.

REFERENCES 1 M.E. Davis, C. Montes and J.M. Garces Zeolite Synthesis (Eds. M.L. Occelli and H.E. Robson) ACS Symp. Ser. 398, Am. Chem. Soc., Washington, DC, 1989, p. 291 2 M.E. Davis, C. Montes, P.E. Hathaway and J.M. Garces Zeolites: Facts, Figures, Future (Eds. P.A. Jacobs and R.A. van Santen) Elsevier, Amsterdam, 1989, p. 199 3 H.P. Klug and L.E. Alexandeer, X-Ray Diffraction Procedures, Wiley, NY, 1974 4 S.T. Wilson and E.M. Flanigen, ACS Symp. Set. 398, Am. Chem, Soc., Washington, DC, 1989, p. 329 5 F. d'Yvoire Bull. Soc. Chim. Fr. 1961, 1762 6 E.B. Keller,.PhD Thesis, ETH Z0rich, 1987 7 M.E. Davis, C. Montes, P.E. Hathaway, J.P. Arhancet, D.L. Hasha and J.M. Garces J. Am. Chem. Soc. 1989, 111, 3919 8 P.J. Grobet, H. Geerts, J.A. Martens and P.A. Jacobs, Recent Advances in Zeolite Science (Eds. J. Klinowski and P.J. Barrie) Elsevier, Amsterdam, 1989, p. 193