Silicoaluminophosphate molecular sieves SAPO-11, SAPO-31 and SAPO-41: synthesis, characterization and alkylation of toluene with methanol

MICROPOROUS MATERIALS ELSEVIER

Microporous Materials 6 (1996) 89-97

Silicoaluminophosphate molecular sieves SAPO- 11, SAPO-31 and SAPO-41" synthesis, characterization and alkylation of toluene with methanol A.M. Prakash ", Satyanarayana V.V. Chilukuri b R.P. Bagwe a, S. Ashtekar a D.K. Chakrabarty a,, a Solid State Laboratory, Department of Chemistry Indian Institute of Technology, Bombay 400076. India b Regional Sophisticated Instrumentation Centre. Indian Institute of Technology, Bombay 400076, India

Received 17 July 1995; accepted 23 October 1995

Abstract Medium pore molecular sieves SAPO-11, SAPO-31 and SAPO-41 have been synthesized using di-n-propylamine as the organic template. They have been characterized by various methods such as X-ray powder diffraction, solid state MAS NMR spectroscopy, thermal analysis, chemical analysis and temperature programmed desorption (TPD) of ammonia. Catalytic activities of these materials in the alkylation of toluene with methanol have been studied. While SAPO-I1 and SAPO-31 showed moderate activity, very high catalytic activity was shown by SAPO-41. All the catalysts showed varying degrees of shape selectivity towards p-xylene. SAPO-41 gave a large amount of 1,2,4-trimethylbenzene in addition to the xylenes. Ke3,words: Molecular sieves; Medium pore silicoaluminophosphates; Silicoaluminophosphates; MAS NMR spectroscopy; Alkylation; Toluene

1. Introduction Since their first synthesis in 1984, crystalline silicoaluminophosphate (SAPO) molecular sieves have generated considerable interest as solid acid catalysts [ 1]. These new materials consist of tetrahedral oxides of silicon, aluminium and phosphorus arranged in a manner that can be regarded as silicon substitution into a hypothetical aluminophosphate (AIPO4) framework. SAPO materials are prepared under mild hydrothermal conditions

* Corresponding author. 0927-6513/96/$15.00 © 1996Elsevier Science B.V. All rights reserved SSDI 0927-6513(95)00091-7

from gels containing reactive sources of aluminium, phosphorus and silicon and a structure directing template. The catalytic properties of SAPO materials are strongly related to the nature of acid sites in the framework. Bronsted acidity of SAPO materials is attributed to silicon incorporation into a hypothetical phosphorus T site of the A1PO4 framework. Formation of silica-rich regions also generates some Bronsted acidity, except for those silicon atoms that are linked to four other silicon atoms through oxygen. Besides, if some of the silicon atoms in the silica-rich regions are substituted by A13÷, this will generate Bronsted acidity as in aluminosilicate zeolites [2].

90

A.M. Prakash et al./Microporous Materials 6 (1996) 89-97

Several studies have been carried out to determine the type of silicon substitution as well as the acidity in these materials [2-5]. Substitution of silicon into the AIPO4 framework can be visualised in terms of silicon substituting only aluminium (mechanism 1), only phosphorus (mechanism 2) or a phosphorus-aluminium pair (mechanism 3). It has been generally accepted that mechanisms 2 and 3 are responsible for the formation of SAPO structures. This is based on the fact that all known SAPO compositions can be rationalized by taking into account mechanisms 2 and 3. Furthermore, mechanism 1 would lead to an anionic framework for which there is no evidence as yet. Medium pore zeolites (e.g., ZSM-5, ZSM-11, etc.) are well known for their catalytic activity and shape selectivity in acid catalysed reactions such as alkylation of aromatics [6], isomerization of alkylaromatics [7], methanol conversion [8] etc. However, studies on these reactions using medium pore SAPO structures are scarce. Most of the reported work on medium pore SAPO materials has been carried out on SAPO-11 [9-11]. Very few reports have appeared on the catalytic behaviour over the other known medium pore structures such as SAPO-31 and SAPO-41 [12]. Alkylation of toluene with methanol was studied by us on the large-pore SAPO-46 [13]. In this paper, we report on the synthesis and properties of three medium-pore SAPOs, namely SAPO-11, SAPO-31 and SAPO-41 and their catalytic behaviour towards alkylation of toluene with methanol. SAPO-11 has one-dimensional 10-membered ring channels with an elliptical pore opening of 6.3 x 3.9 A. The structure possesses orthorhombic symmetry with lattice parameters a = 8.4, b= 18.5 and c= 13.5 ,~ [14]. SAPO-31 exhibits hexagonal symmetry with lattice parameters a=20.8 and c= 5.0 A, [15]. Although SAPO-31 has 12-membered ring channels in the structure, the nearly circular free opening with an effective diameter of 5.4 A brings it under the medium-pore category. SAPO-41 has one-dimensional 10-membered ellipotical channels with a free aperture of 7.0 × 4.3 A and belongs to the orthorhombic symmetry with lattice parameters a=9.7, b=25.5 and c=8.4,~ [16]. The free pore openings of both SAPO-11

and SAPO-41 are more elliptical than that of ZSM-5 with 10-ring apertures [17].

2. Experimental The silicoaluminophosphates SAPO- 11, SAPO-31 and SAPO-41 were prepared hydrothermally using di-n-propylamine as the organic template. The starting materials were fumed silica (Aerosil-200, Degussa), pseudoboehmite (Catapal-B, Vista), orthophosphoric acid (85%, Merck) and di-n-propylamine (99%, Merck). Syntheses were carried out in (500 cm 3 or 100 cm 3) stainless steel reactors lined with PTFE at autogenous pressure without agitation. The molar composition of the reaction mixtures and the synthesis conditions for the preparation of the pure SAPO materials are given in Table 1. In a typical synthesis, a 1:1 molar mixture of A1203 and P205 was prepared by slowly adding the pseudoboehmite to dilute phosphoric acid, and the mixture was stirred well for 6 h to form a uniform gel. Fumed silica was mixed with water which was then added dropwise to this mixture followed by dropwise addition of di-n-propylamine. After adding the remaining part of water, the mixture was stirred for about 4 h. In the preparation of SAPO-31 and SAPO-41, the pH of the gel was adjusted to 4.0 and 7.7, respectively, by adding dilute phosphoric acid. The gel was then charged into the autoclave and kept at the required temperature for the specified duration. After crystallization the product was separated from the mother liquor, washed with water and dried at 373 K for 12 h. X-ray powder diffraction patterns were recorded on a PW1710 diffractometer using CuK~ radiation with a nickel filter. Thermogravimetric (TG) analyses were carried out in air on a Du Pont 9900 thermal analyser at a heating rate of 10 K/min. Solid state MAS NMR spectra were recorded on a Varian VXR 300S spectrometer with a Doty scientific CP-MAS probe. The frequencies were 78.15, 121.41 and 59.59 MHz for 27A1, 31p and 29Si, respectively. Fortyfive-degree pulses were used for all measurements with repetition times 3 s for 27A1 and 10 s for 31p and 29Si. Data were acquired at a MAS speed of 4.5 kHz. Aluminium nitrate in

A.M. Prakash et al./Microporous Materials 6 (1996) 89-97

91

Table 1 Molar composition of the synthesis gels and crystallization conditions for the preparation of various SAPO structures from systems containing di-n-propylamine Structure

SAPO-11 SAPO-31 SAPO-41 SAPO-46"

Gel composition A1203

PzO3

SiO2

Pr2NH

H20

1.0 1.0 1.0 1.0

1.0 1.1 1.2 1.2

0.6 0.4 0.1 0.4

1.0 1.5 4.0 4.0

60 60 55 55

pH

Temp. tK)

Time (hi

4.5 4.0 7.7 7.7

473 483 453 453

24 24 252 120

a The gel was initially heated at 423 K for 96 h.

water, 85% phosphoric acid and tetramethylsilane were employed as references. Chemical analysis of the samples was carried out by atomic emission spectrometry with an ICP source (Labtam Plasma Lab 8440) following the method described earlier [13]. The as-synthesized samples were first calcined by raising the temperature at 2 K/min up to 773 K, and the sample was held at this temperature for 12 h for complete removal of the organic template. Acidities of the calcined samples were determined by temperature programmed desorption of ammonia at a heating rate of 10 K/rain. Alkylation of toluene with methanol was carried out in an all-glass flow reactor. 1 g of the calcined SAPO ( 180-300 mesh size) was used for each run. The catalyst was first calcined by raising the temperature at 2 K/rain up to 773 K in nitrogen and was held at this temperature for 12 h in air. The liquid reactants fed by a peristaltic pump were vapourized in a preheater and passed through the catalyst bed. Nitrogen was used as diluent. The liquid products were condensed and analysed with a gas chromatograph using a flame ionization detector. A 4 m long stainless steel column packed with 5% diisodecylphthalate (DIDP)+5% Bentone-34 on Chromosorb (100-200 mesh) was used for the separation of products.

3. Results and discussion

Since di-n-propylamine is a commonly used template in the synthesis of A1PO4-based molecular sieves giving at least six structure types, it is extremely important to control the synthesis

parameters in order to obtain crystals of a given structure type in pure form. Initial composition of the gel, pH, temperature and duration of heating influenced the crystallinity and purity of SAPO-11, SAPO-31 and SAPO-41. The concentrations of silica and the template in the synthesis gel are important factors in determining the structure type to be formed (see Table 1 ). At relatively low template concentration (1.0-1.5 moles per mole A1203) SAPO-11 and SAPO-31 are the competing structures. SAPO-31 is preferred at low silica concentration and high temperature. On the other hand, at high template concentration (2-4 moles per mole A1203) and low temperature, SAPO-41 and SAPO-46 crystallized depending on the amount of silica in the synthesis gel. Whereas SAPO-41 crystallized from gels having very low concentration of silica (0.1 moles per mole A1203), SAPO-46 was found to crystallize when the gel had a high silica concentration (0.3-0.4 moles per mole A1203). The X-ray powder diffraction pattern confirmed the structure types SAPO-ll, SAPO-31 and SAPO-41 (Fig. 1). The patterns, both in intensity and line positions, match the patterns reported for these structures [ 18]. Practically no loss in crystallinity was observed when the as-synthesised samples were heated at 773 K for 12 h in order to remove the organic molecules. From chemical analysis and TGA, the composition of the as-synthesised samples were found to be SAPO-11:0.05Pr~zNH(Sio.o6Alo.51Po.43)O2

' xH20

SAPO-31: 0.05Pr~NH(Sio.osAlo.48Po.44)O2.x H 2 0 SAPO-41: 0.03Pr~NH(Sio.o3Alo.sxPo.46)O2 •xH20

92

A.M. Prakash et al./Microporous Mater&& 6 (1996) 89-97

"N:~............................ 5

b

£

20 273

I

I

373

1,73

I

i

I

I

573

673

773

873

973

Temperature ( K )

Fig. 2. TG curves of (a) SAPO-11, (b) SAPO-31 and (c) SAPO-41.

5

10

30

20

t,O

50

20

Fig. 1. X-ray powder diffraction patterns of as-synthesized (a) SAPO-11, (b) SAPO-31 and (c) SAPO-41,

In SAPO-11, the sum of the mole fractions of Si and P is marginally lower than that of A1 which may suggest the presence of a small amount of extra-framework aluminium in the sample. Chemical compositions of the samples indicate that silicon mostly goes to isolated phosphorus sites (mechanism 2) in SAPO-41. The SAPO-31 sample has Si+ P >A1 suggesting the presence of some silica-rich regions. Hence, silicon substitution in this sample possibly occurs via both mechanisms 2 and 3. Since the sum of the mole fractions of silicon and phosphorus is nearly equal to that of aluminium in SAPO-11 and SAPO-41, the extent of formation of the silica-rich regions in them will depend on how much of the aluminium is outside the framework. 2VA1 MAS NMR shows the presence of some octahedral aluminium in SAPO-11, but not in SAPO-41. This suggests that some silicarich regions may be present in SAPO-11. Fig. 2 shows the thermogravimetric curves of the as-synthesised samples of the various SAPO types used in the present study. They show multiple step mass loss. The initial small mass loss below 470 K was due to the desorption of water. The

mass loss due to the oxidative decomposition of Pr~NH occurred in multiple steps over the temperature range 470 to 870 K from the various SAPOs. The decomposition of the template took place in SAPO-41 at a lower temperature compared to SAPO- 11 and SAPO-31. When compared to other large pore SAPO molecular sieves these medium pore structures had a lower amount of template occluded in their channels. Approximately 2 template molecules were present per unit cell of SAPO-11 and SAPO-31 as seen from the mass loss. In the case of SAPO-41, 1.3 template molecules were present per unit cell. These values are comparable with those reported in the literature for these structure types [19]. It is likely that only the protonated form of the amine is retained after washing and calcination. In this context, it is interesting to note that the amount of the template matches exactly that of silicon in SAPO-41 in which substitution possibly follows mechanism 2. The amount of the protonated amine exactly compensates the framework charge generated by silicon. In SAPO-11 and SAPO-31 the amine content is slightly lower than the mole fraction of silicon which will be expected if they have some silicarich regions. Solid state MAS NMR spectroscopy gives valuable information on the ordering of framework elements and the nature of substitution of silicon into the framework. The 31p and 27A1 MAS NMR spectra of the three SAPOs studied are given in Fig. 3. Except for small variations in the exact line positions, the spectra are quite similar. In the 31p NMR, a sharp resonance around - 3 0 ppm char-

93

A. M. Prakash et aL / Mieroporous Materials" 6 ( 1996J 89-97

27A!

31 p

I

Q

,~0' go ' -io ;,~'

;o ~;-,;-3;-~'o P'~'

Fig. 3. 2~AIand 31p MAS NMR spectra of (a) SAPO-ll, (b) SAPO-31 and (c) SAPO-41. acteristic of tetrahedral phosphorus [20] was observed. In the particular case of SAPO-11, two small signals at - 1 2 and - 1 8 ppm were also observed. These resonances are probably due to the interaction of template or water molecules with the framework phosphorus. Similar peaks were already reported in several A1PO4-based molecular sieves [21]. In 2VA1 N M R spectra, a single resonance at around 37 ppm characteristic of tetrahedral aluminium in the framework [20] was observed. The weak signal at 5.5 ppm observed in the case of SAPO- 11 is possibly due to the presence of a small amount of extra-framework aluminium as amorphous alumina. This assumption is based on the fact that amorphous alumina such as in pseudoboehmite is known to show a resonance around 6 ppm in its 27A1 MAS N M R spectrum [22]. The presence of a small amount of extraframework aluminium in this sample is also supported by its chemical composition. In molecular sieve structures where non-equivalent T sites exist, the 31p andZTA1 MAS N M R are expected to give multiple signals (e.g., A1PO4-21, VPI-5, etc.) [23]. Structural analyses by X-ray diffraction pointed to non-equivalent T sites in SAPO-I1 [14], and SAPO-31 [15]. The single resonance in the 31p and 27A1 N M R spectra indicates that the non-equivalent T sites are obviously not resolved in these spectra.

298i MAS N M R spectra of the samples are shown in Fig. 4. SAPO-11 and SAPO-31 show a broad band covering - 85 to - 115 ppm indicating multiple tetrahedral environments of the silicon atoms. This suggests that some of the silicon atoms entered the framework by mechanism 3 forming silica rich regions. Unlike substitution by mechanism 2 that will generate one Bronsted acid site per silicon atom, substitution by mechanism 3 will lead to a much lower acidity. The presence of amorphous silica can be ruled out due to high crystallinity of the materials as seen from their XRD. SAPO-41 showed a sharp N M R peak at - 9 3 ppm that can be assigned to Si(4A1) environment in SAPO, although some minor peaks associated with zeolitic and silica rich regions were also present. In accordance with this, SAPO-41 should have more acid sites as compared to SAPO-11 and SAPO-31, although the former had the lowest amount of silicon. Formation of silica rich and zeolitic domains in a SAPO composition has been reported by several authors [ 2 , 3 , 5 , 2 4 , 2 5 ] . 3.1. Acidio"

Temperature programmed desorption of ammonia for the SAPO samples have been studied. The results are shown in Fig. 5. All of them showed a low temperature peak between 440-460 K due to surface hydroxyls. In SAPO-31 and SAPO-41, a peak at 570 K appeared due to structural acidity. This peak for SAPO-11 appeared at 520 K and was very weak. As predicted from 298i MAS N M R 29Sf

(3

b

2% '-6'o '-16o '-do p#. Fig. 4. 29Si MAS NMR spectra of (a) SAPO-11, (b) SAPO-3I and (c) SAPO-41.

94

A.M. Prakash et al./Microporous Materials 6 (1996) 89-97

c

o

,g

373

I 473

| 573

Temperature (K)

1 673

Fig. 5. Temperature programmed desorption of ammonia curves of (a) SAPO-11, (b) SAPO-31 and (c) SAPO-41.

results, SAPO-41 showed the highest number of acid sites as a result of higher silicon substitution through mechanism 2. The results, however, showed that the acid strength in these SAPO materials was much lower as compared to that of ZSM-5 that showed an ammonia desorption peak at 690 K [10].

3.2. Alkylation of toluene with methanol All three catalysts showed toluene alkylation activity that decreased in the order SAPO-41 > SAPO-31 > SAPO-1 I. This is in accordance with the decreased acidity as shown by the ammonia desorption patterns (Fig. 5). In addition to the three xylenes, the product contained a fair to large amount of trimethylbenzenes (TMB). Fig. 6 shows the variation in toluene conversion, p-xylene selectivity meaning (p-xylene/total xylene)x 100 and YxM~ defined as (gram of TMB/gram of toluene converted) x 100 as a function of time on stream (TOS). Product distribution after 3 h of TOS are presented in Tables 2 and 3. All three catalysts showed some para selectivity

but this followed the opposite order of their catalytic activity. This can be accounted for by the pore size of the SAPOs. SAPO- 11 that showed the highest para selectivity has the smallest pore dimension and hence should cause the greatest hinderance in the diffusion of the m- and particularly that of o-xylene. Thus, selectivity seems to be a consequence of product shape selectivity. The activity of the SAPO-41 catalyst for toluene methylation is very high, it is even higher than that of HZSM-5 under comparable conditions [6]. The mechanism of toluene alkylation with methanol over zeolite HZSM-5 has been discussed by Kaeding et al. [6]. It is assumed that methanol reacts with the zeolitic proton forming an oxonium ion that is followed by the transfer of the methyl group to the aromatic ring and transfer of the proton back to the catalyst site. One can assume the same mechanism to be prevalent for the SAPO catalysts. It can be seen that all the catalysts gave some trimethylbenzenes, mainly 1,2,4-TMB (pseudocumene) and it was formed in large quantity on SAPO-41. TMB can be formed either by (i) alkylation or (ii) disproportionation of xylene. By reacting xylenes in the absence of methanol on SAPO-41, it was found that isomerization of xylene was the predominating reaction and there was hardly any disproportionation. The same observation was made with HZSM-5 as catalyst [6]. Hence, it can be assumed that TMB is formed by alkylation of the xylenes. If we assume that methylation of the xylenes follows a mechanism similar to that of alkylation of toluene, it can be written as shown in Fig. 7 involving p-xylene. Alkylation of p-xylene will lead to only 1,2,4-TMB, whereas o-xylene would give both 1,2,4- as well as 1,2,3-trimethylbenzenes. m-Xylene would give all three TMBs. Table 3 shows that the 1,2,4-TMB selectivity (in total TMB) is more than 98%. The transition state in the reaction of p-xylene would occupy the minimum space as compared to those formed by m- and o-xylenes. Hence the former can be preferentially formed in the restricted space inside the SAPO channels. This may be the reason why pseudocumene predominates among the TMBs. The kinetic diameter of the transition state involving p-xylene will be about 7.6 .A, which is

A.M. Prakash et al./Microporous Materials 6 (1996) 89 97 GO ¸

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120

610

t

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180

6LO

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~20

180

Time on stream (rain.)

Fig. 6. Alkylation of toluene with methanol over (a) SAPO-1 l, (b) SAPO-31 and (c) SAPO-41: conversion and selectivity as a function of time on stream: (a) and (b) at 673 K and WHSV 3 h ~; (c) at 623 K and WHSV 4.5 h ~. (2~) Toluene conversion; individual xylene isomers selectivity in total xylene: (&)p-xylene; (II) m-xylene; ( • ) o-xylene and (O) YT~R= (gram of TMB/gram of toluene converted ) x 100. Table 2 Alkylation of toluene with methanol at various temperatures over SAPOd I and SAPO-31 SAPO-11 temperature, K

Products (wt. %) benzene toluene p-xylene m-xylene o-xylene ethyltoluene 1,3,5-TMB 1,2,4-TMB 1,2,3-TM B C9+

623

648

0.4 89.5 6.1 2.3 0.8

86.0 7.3 3.2 1.3

SAPO-31 temperature, K 673

0.3

.

.

673

74.8 7.0 6.1 4.6 0.2

73.0 7.2 6.6 4.6 0.4

67.9 8.3 8.2 5.8 0.2

6.4

6.5

7.3

0.8

0.1 1.5

O.2 2.0

.

.

.

1.5

2.4

--

0.1 0.6

0.4

648

.

80.9 9.3 4.6 2.0

-

-1.1 -0.3

623

.

.

O.I

Conversion

toluene methanol Xylene composition (%) para meta ortho YTMI3"

10.5 100

19.1 100

25.2

27.0

32.1

1 O0

14.0

1 O0

1 O0

100

66.0 25.0 9.0 10.6

61.9 27.3 10.8 10.7

58.1 29.1 12.8 12.7

39.3 34.6 26.1 25.5

39.0 35.9 25.1 24.4

37.3 36.7 25.9 235

Conditions: WHSV, 3 h - l ; MeOH:toluene, 1:2; time on stream, 3 h. " Y,~, =(gram of TMB/gram of toluene converted) x 100. equal to the kinetic diameter 1,2,4-TMB molecule. Although the crystallographically determined pore dimensions of SAPO-31 and SAPO-41 (that too

produce a significant amount of 1,2,4-TMB) are s m a l l e r , it s h o u l d b e r e m e m b e r e d t h a t t h e s e p o r e sizes a r e n o t rigid. O n e m i g h t u s e C s i l e r y ' s d i m e n -

A.M. Prakash et al./Microporous Materials 6 (1996) 89-97

96

Table 3 Alkylation of toluene with methanol at various temperatures and weight hourly space velocities (WHSV) over SAPO-41 Temperature a

WHSV b

623

648

673

698

3

7

0.1 56.8 13.8 11.7 5.2 9.7 2.7

0.2 52.0 14.3 12.8 5.6 11.9 0.1 3.1

42.4

30.2

0.3 34.5

59.7

13.5 15.5 6.6 0.2 0.1 16.8 0.3 4.6

14.2 17.8 8.2 24.5 0.2 4.9

16.9 17.1 7.9 0.13 18.6 0.2 4.2

13.1 11.3 5.1 8.5 2.3

43.2 1O0

48.0 1O0

57.6 1O0

69.8 1O0

65.5 1O0

40.3 1O0

44.9 38.1 16.9 22.4

43.7 39.1 17.1 25.0

37.9 43.5 18.5 29.7

35.3 44.3 20.4 35.4

40.3 40.8 18,9 28.7

44.4 38.3 17.4 21.0

Products (wt. %)

benzene toluene ethylbenzene p-xylene

m-xylene o-xylene ethyltoluene 1,3,5-TMB 1,2,4-TMB 1,2,3-TMB C9+

O. 1

-

-

-

Conversion toluene methanol

Xylene composition (%) para meta ortho

YTMB¢

Conditions: MeOH:toluene, 1:2; time on stream, 3 h. WHSV =4.5 h -1 . b Temperature = 673 K. YTMRsame as in Table 2. CH3OH 4- HSAPO ~

CH30H2 SAPO"

÷

~

CH3

H3C~H'~"CH3 CH3

~CH3 .~t+J H3C

H->~.CH3

s~o-

--

~'~3 c

+

HS~PO

CH3

Fig. 7. Mechanism of toluene alkylation on SAPO molecular sieve. sions wh i ch are o b t a i n e d by a d d i n g 1.5 ,~ to the c r y s t a l l o g r a p h i c d i m e n s i o n s [26]. In o r d e r to verify t h a t p-xylene is the m a i n i n t e r m e d i a t e in the f o r m a t i o n o f T M B s , we carried out a l k y l a t i o n o f a n e a r - e q u i l i b r i u m m i x t u r e o f the three xylenes. It was f o u n d t h a t p - x y l e n e was preferentially c o n s u m e d , whereas the o-xylene conc e n t r a t i o n h a r d l y changed. E v e n if a s i m u l t a n e o u s

i s o m e r i z a t i o n o f the xylenes is envisaged to occur, p - x y l e n e is m o s t easily alkylated with m e t h a n o l on SA PO - 4 1 a n d the ease o f f o r m a t i o n o f the less b u l k y t r a n s i t i o n state is the m a i n cause o f 1,2,4-TMB selectivity. Once f o r m ed , this isomer can diffuse o u t easily because o f its l o w er kinetic d i a m e t e r as c o m p a r e d to the o t h e r t w o isomers. Table 3 shows the effect o f space velocity on the p r o d u c t d i s t r i b u t i o n d u r i n g alkylation o f toluene o n S A P O - 4 1 . As expected, an increase in space velocity brings a b o u t a decrease in t o l u en e co n v er sion, b u t it is the f o r m a t i o n o f T M B that is affected m o r e severely t h a n that o f the xylenes. This is an expected result, because the f o r m a t i o n o f T M B is a co n secu t i v e reaction.

4.

Conclusions

T h e m e d i u m p o r e S A P O m o l e c u l a r sieves S A P O - 1 1 , SAPO-31 an d SAPO-41 were synthe-

A.M. Prakash et al./Microporous Materiel& 6 (1996) 89 97

sized using di-n-propylamine as the organic template, and materials were characterized by various physico-chemical methods. Chemical analyses and MAS NMR studies suggests a combination of mechanism 2 and mechanism 3 for the substitution of silicon into the framework of SAPO-11 and SAPO-31. In SAPO-41 mechanism 2 was the prominent substitution mechanism. Relatively high concentrations of strong acid sites were observed in SAPO-41 in comparison to SAPO-11 and SAPO-31. The alkylation of toluene with methanol showed very high activity on SAPO-41, while moderate activities were observed on SAPO-11 and SAPO-31. All the three SAPO's showed paraselectivity among the various xylene isomers in the product. Secondary alkylation of xylene isomers with methanol yield 1,2,4-TMB selectively. SAPO-41 gave the highest amount of 1,2,4-TMB among the three SAPO's studied.

Acknowledgement This work has been funded by a research grant from CSIR, New Delhi. AMP and SA are grateful to the CSIR for the award of research fellowships.

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