J. CHEM. SOC. FARADAY TRANS., 1991, 87(19), 3309-3313 3309

Synthesis and Catalytic Properties of MAPO-36 Molecular Sieve

Katsumi Nakashiro and Yoshio Ono* Department of Chemical Engineering, Tokyo Institute of Technology, Ookayama , Meguro-ku, Tokyo 152, Japan

The effects of t h e conditions of hydrothermal synthesis and the composition of t h e starting gels on MAPO-36 synthesis have been examined. Pure MAPO-36 was obtained from t h e gel with Mg/P = 0.08-0.25 after hydro- thermal synthesis at 373 K for 44 h and t h e n at 423 K for 22 h .

The infrared spectrum of adsorbed pyridine confirmed that MAPO-36 has Brsnsted acid sites in conformity with t h e expected substitution of Mg2+ for A13+ in the framework. The strength of t h e acid sites on MAPO-36, as estimated by temperature-programmed desorption of ammonia, is as high as t h a t of the sites on H-ZSM-5, though t h e number is smaller.

The order of t h e catalytic activities for dehydration of ethanol and cracking of butane and hexane is as follows: H-ZSM-5 > MAPO-36 3 HY. The higher activity of MAPO-36 compared to that of HY may be caused by the greater strength of acid sites, as indicated by the temperature-programmed desorption of ammonia.

The cracking of isobutane and 3-methylpentane is in t h e decreasing order: MAPO-36 3 HY > H-ZSM-5. T h i s order is a result not only of the acidic nature of the catalysts, but also of t h e pore sizes. The lower activity of H-ZSM-5, in spite of the stronger acid sites, can be explained in terms of spatial constraints on t h e reactions. On the other hand, the high activity of MAPO-36 confirms that t h i s material is a large-pore molecular sieve.

Aluminophosphate molecular sieves (AlP0,-n) were first syn- thesized by Wilson et al.' in 1982. The structures of AlP0,-n consist of alternating tetrahedra of aluminium and phos- phorus connected by oxygen. It is known that various ele- ments can be incorporated into the framework of AlP0,-n. Lok et a!.' reported the synthesis of silico-alumino- phosphates (SAPO-n), which contain Si4+ in the framework. Flanigen et aL3 reported the incorporation of metal cations such as Mg", Co2+, Fez+ and Fe3+, into the AlP0,-n framework. These metal(Me)-containing alumino-phosphates are denoted by MeAPO-n. Since Me2'- is substituted for A13 + hypothetically, MeAPO-n are negatively charged on the framework. This negative charge must be balanced by protons or metal cations, resulting in Brsnsted acidity or ion- exchange capacity. The structures of AlP0,-n-type molecular sieves have been studied by various techniques including MAS NMR., The catalytic properties of aluminophosphate molecular sieves have been investigated for reactions of butane ~ rack ing ,~ formation of alkenes from methan01,~ aro- matic alkylation6 and alkene oligomerization.6

MAPO-36 is a magnesium-containing aluminophosphate molecular sieve. Although the exact structure has not been determined, it has been shown to have large pores of ca. 0.8 nm in diameter.3 MAPO-36 can be synthesized hydrother- mally in gels containing tri-n-propylamine as an organic base. It is, however, difficult to obtain the pure phase of MAPO-36 because MAPO-36 and MAPO-5 are crystallized competi- tively in this ~ y s t e m . ~ It has also been reported that in the synthesis of CoAPO-36 (isostructural with MAPO-36, con- taining Co instead of Mg), CoAPO-5 is co-crystallized easily.8 It is important to establish the synthesis conditions of MAPO-36 in order not only to obtain the pure phase of MAPO-36, but also to characterize the chemical properties of the material.

Few studies on the catalytic properties of MAPO-36 have been performed. The catalytic activities of MAPO-36 for butane cracking3 and ethanol dehydration7 have been shown to be relatively high compared with those of other alumino- phosphates, and close to that of HY ~ e o l i t e . ~

The aim of this work is to optimize the synthesis condi- tions of MAPO-36 and to characterize the acidic properties

by means of infrared spectroscopy of adsorbed pyridine and temperature-programmed desorption of ammonia. The cata- lytic properties of MAPO-36 for dehydration of ethanol and cracking of butane and hexane were also studied.

Experimental Synthesis of MAPO-36

MAPO-36 was synthesized based on the method reported in the l i t e r a t ~ r e . ~ * ~ * ~ The general procedures of gel preparation are as follows: magnesium oxide (1.86 g) is added to a solu- tion (39.0 cm3) of phosphoric acid (85 wt.%, Merck) in 199 cm3 of water. To the mixture, pseudo-boehmite alumina (40.5 g, Cataloid AP from Catalysts & Chemicals Ind. Co.) was added, and then tri-n-propylamine (62-77 cm3) as an organic base was also added to adjust the pH of the mixture to a value of 6 to 7. The compositions of the final mixture in molar ratio were Mg/P = 0.08, Al/P = 0.92, base/P = 0.84- 1.0 and H 2 0 / P = 21.0 [see runs (aHe) in Table 13. The mixture was stirred until homogeneous and then transferred into a Teflon vessel placed in a stainless-steel autoclave with a capacity of ca. 300 cm3. After hydrothermal synthesis, the product was washed and dried at 493 K overnight. The struc- ture of the sample was identified by its X-ray diffraction pattern. The product was calcined at 823 K in dry air for 20 h. The compositions of the products were determined by X-ray fluorescence analysis for magnesium, aluminium and phosphorus, or atomic absorption analysis for magnesium and aluminium. The X-ray fluorescence analysis was carried out on Philips PW- 1400 instrument using the glass-beads method.

Source of other Materials

SAPO-11 (Si/Al = 0.04, P/Al = 1.0) and Na-Beta were syn- thesized as described previously,"." and calcined at 823 K and 773 K, respectively. Na-Beta was converted into H-Beta by ion-exchange with aqueous ammonium chloride. HY was obtained from NaY (Si/Al = 2.88) of Tosoh Co. Ind. also by ion-exchange.

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3310 J. CHEM. SOC. FARADAY TRANS., 1991, VOL. 87

Infrared Spectra

Infrared spectra of the samples (6.4 mg cm-2) of self- supported wafers were recorded on a Shimadzu IR-460 spec- trometer at room temperature. After the samples were heated in dry air at 773 K for 1 h and evacuated at 473 K for 1.5 h, pyridine (1.3 kPa) was adsorbed on the samples at 473 K for 1 h and then desorbed under vacuum at the same tem- perature for 1 h.

Temperature-programmed Desorption of Ammonia

After the samples (10 mg) were heated at 773 K for 1 h and evacuated at 773 K for 1 h, ammonia (5.3 kPa) was adsorbed at 423 K for 0.5 h and then desorbed under vacuum at 353 K for 0.5 h. Temperature-programmed desorption of ammonia was carried out at the linear heating rate of 5 K min-'. The rate of ammonia desorption was recorded with a quadrupole mass spectrometer.

Catalytic Reactions

The reactions were carried out in a fixed-bed continuous flow reactor. Catalysts (0.3-0.5 g) were crushed, sieved into grains of 16-32 mesh, and placed in a quartz reactor with a dia- meter of ca. 10 mm. The catalysts were heated at 773 K for 1.5 h in a flow of air before the reactions. Products were analysed by gas chromatography. Product distributions of hydrocarbons were expressed in terms of carbon number. The selectivity for hydrogen was expressed in units of mol of molecular hydrogen formed from 100 mol of butane or iso- butane converted.

Results and Discussion Synthesis of MAPO-36

Hydrothermal-synthesis conditions were optimized in order to obtain the pure phase of MAPO-36. Table 1 shows the effect of the conditions of hydrothermal synthesis on the product phases. The compositions of the starting gels were same. The product phases are listed in the order of majority. When the gel was heated at 373 K for 44 h [run (a)] , MAPO-36 was obtained as a single phase. When the gel was heated at 373 K for 44 h and then at 423 K for 22 h [run (b)], MAPO-36 was obtained again as a single phase with higher crystallinity than that obtained in run (a). This indicates that the nuclei of MAPO-36 have already been produced at the first stage of heating at 373 K before the second stage of heating at 423 K. The gels of runs (c), (d) and (e) were heated at 393 K for 48 h and then at 423 K for 12 h, 24 h, and 72 h, respectively. As synthesis time at 423 K was extended, the main product phase was changed from MAPO-36 to MAPO-5. This suggests that at the second stage a growth of crystal size in MAPO-36 occurs, but further heating causes the conversion of MAPO-36 to the more stable phase of MAPO-5. MAPO-36 was synthesized using the method of run (b) in the following synthesis.

Table 1 product phases

Effect of the hydrothermal-synthesis conditions on the

run hydrothermal synthesis product phasesP

(4 373 K/44 h MAPO-36 MAPO-36

(b) (c ) 393 K/48 h; 423 K/12 h MAPO-36, MAPO-5 MAPO-5, MAPO-36

(d ) (4 393 K/48 h ; 423 K/72 h MAPO-5

373 K/44 h ; 423 K/22 h

393 K/48 h; 423 K/24 h

Gel compositions: Mg/P = 0.08, AI/P = 0.92. a Major product cited first.

The effect of the magnesium content of the gels on MAPO-36 synthesis was examined. Comparison between the compositions of the gels and the crystalline products is given in Table 2. Pure MAPO-36 was obtained only from the gels with Mg/P ratios of 0.08 to 0.25; MAPO-5 (or A1P04-5) was co-crystallized from the gels with other compositions. When the Mg/P ratio in the gels increased from 0.08 to 0.25, the Mg/P ratio in the crystalline products increased from 0.11 to 0.31, while the Al/P ratio decreased from 1.06 to 0.74. The AlPO, structure requires Al/P ratios of unity. The hypotheti- cal substitution of Mg2+ for A13+ in the AlPO, framework means that the sum of the molar ratios of Mg/P and Al/P should be unity. The sums in the samples of runs 3-6 are close to but always larger than unity. The 27Al NMR spectra of all samples of runs 3-6 showed no evidence for the pres- ence of extra-framework aluminium species.12 The origin of the deviation from unity of the sum is not clear at this moment. The crystallinities of MAPO-36 in runs 3 and 4 were slightly reduced by calcination at 823 K. The samples in runs 5 and 6 turned grey after calcination at 823 K, and even at the higher temperature of 923 K, indicating that the carbon residue remains in the pore systems.

MAPO-36 with Mg/P = 0.11 or 0.19 (runs 3 and 4) was used for further studies.

Acidic Properties

Acidic properties of MAPO-36 (Mg/P = 0.19, sample 4) were studied using IR absorption of adsorbed pyridine and temperature-programmed desorption (TPD) of ammonia.

I R Absorption

The IR spectrum of pyridine adsorbed on MAPO-36 (sample 4) was examined. The spectrum measured after evacuation at 473 K for 1 h showed two absorption bands at 3592 and 3690 cm-', which are ascribed to acidic OH and P-OH, re~pectively.'~ The spectrum measured after adsorption of pyridine (1.3 kPa) at 473 K for 1 h and then evacuation at 473 K for 1 h showed that adsorption of pyridine gave rise to new absorption bands in the region 14W1700 cm-'. Fig. 1 shows the spectrum (14W1700 cm-l) resulting from the sub- traction of the spectrum obtained before adsorption of pyri- dine from that after adsorption. The bands at 1449, 1490 and 1545 cm-' were ascribed to pyridine adsorbed on Lewis, Lewis and/or Brsnsted, and Brsnsted acid sites, respec- tively.', Thus, it is confirmed that MAPO-36 has Brernsted acidity, as expected from the hypothetical substitution of Mg2+ for A13+ in the framework.

Table 2 crystalline products

Comparison between the compositions of the gels and the

gels crystalline products

run Mg/P AI/P Mg/P Al/P phases"

1.17b A1P04-5 - 1 0 1 .o 2 0.04 0.96 0.07 1.20 MAPO-5, MAPO-36 3 0.08 0.92 0.11 1.06 MAPO-36 4 0.13 0.87 0.19 0.94 MAPO-36 5 0.20 0.80 0.27 0.92 MAPO-36 6 0.25 0.75 0.31' 0.74' MAPO-36 7 0.30 0.70 O.3Ob 0.64b MAPO-5, MAPO-36

Hydrothermal-synthesis conditions: 373 K for 44 h and then 423 K for 22 h. Determination of compositions of crystalline materials by X-ray fluorescence analysis. Major product cited first. Determined by atomic absorption analysis.

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I T *OI-----l I zl I 0

m

1700 1400 wavenumber/cm -'

Fig. 1 IR spectrum of pyridine adsorbed on MAPO-36 (sample 4). The spectrum is that resulting from the subtraction of the spectrum obtained before adsorption of pyridine from that obtained after adsorption.

T P D of Ammonia The TPD spectra of ammonia on MAPO-36 (sample 4), H-ZSM-5, HY and SAPO-11 are given in Fig. 2. The total number of acid sites on MAPO-36 is smaller than that on HY and similar to that on H-ZSM-5. MAPO-36 has three kinds of acid site from which ammonia desorbs at 395, 490 and 590 K. The strongest acid sites on MAPO-36 are as strong as the stronger acid sites on H-ZSM-5, from which ammonia desorbs at 590 K, though the number of these was smaller on MAPO-36. HY does not have such strong acid sites. MAPO-36 has stronger and a greater number of acid sites than SAPO-11. The orders of the number of acid sites and the acid strength are as follows: HY % MAPO-36, H-ZSM-5 > SAPO-11 and H-ZSM-5 > MAPO-36 > HY $ SAPO-11, respectively.

Catalytic Activities

Dehydration of Ethanol Dehydration of ethanol was carried out at 473 K using MAPO-36 (Mg/P = 0.11, 0.19), HY and H-ZSM-5 as cata-

Ti K

Fig. 2 Temperature-programmed desorption spectra of ammonia over MAPO-36 (sample 4), HY, H-ZSM-5 and SAPO-11; heating rate, 5 K min- '

lysts. The results are given in Table 3. The main product over MAPO-36 and HY was diethyl ether. Both the activities and the selectivities for ethene over MAPO-36 with Mg/P = 0.1 1 and 0.19 were close to those over HY, though MAPO-36 (Mg/P = 0.19) has a smaller number of acid sites than HY. This suggests that MAPO-36 has stronger acid sites than HY in conformity with the TPD spectra of ammonia (Fig. 2). H-ZSM-5 showed much higher activity than MAPO-36 and gave ethene as the main product, in conformity with the pre- sence of stronger acid sites.

Cracking of Butane Cracking of butane was performed over MAPO-36 (Mg/ P = 0.19), HY and H-ZSM-5 at 773 K. The results are shown in Table 4. The activity of MAPO-36 was close to that of HY, and lower than that of H-ZSM-5. This tendency agrees with that found for ethanol dehydration. As for the product distributions, propane was one of the predominant products over each of the three catalysts. The main difference between MAPO-36 (or HY) and H-ZSM-5 was found in the selectivity for isobutane. The yield of isobutane was large over MAPO-36 and HY, and quite small over H-ZSM-5. The

Table 3 Dehydration of ethanol

selectivity (Yo)

catalyst molar ratio, Mg/P conversion ("%) C,H, hydrocarbons diethyl ether

80.4 19.4 0.1 80.5 84.0 28.7 0.2 71.1 84.1 29.1 0.9 70.0 HY

H-ZSM-5 -. 94.2 91.0 8.6 0.4

MAPO-36 -

Reaction conditions: 473 K; ethanol pressure, 10 kPa; W/F = 11.5 g h mol-'.

Table 4 Cracking of butane

product distributions (YO) molar ratio,

catalyst Mg/P conversion (Yo) CH, C,H, C,H, iso-C,H,, C,H, C,H, C,H, C,+ aromatics H, selectivity" ~~~~~~~~~~

MAPO-36 0.19 16.8 2.9 3.9 38.7 28.6 7.0 5.9 5.6 6.1 1.3 4.5 HY - 13.8 4.2 5.6 27.5 39.5 6.4 4.3 4.3 5.5 2.7 8.1 H-ZSM-5 - 44.7 4.3 9.9 44.5 4.9 9.3 9.5 7.4 3.0 7.2 10.7

Reaction conditions: 773 K; butane pressure, 34.2 kPa; W/F = 4.2 g h mol- '. In mol per 100 mol butane converted.

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3312 J. CHEM. SOC. FARADAY TRANS., 1991, VOL. 87

most probable route to the isobutane skeleton from a butane molecule may be as follows :

+ -H+ C-C-C-C + R+ - C-C-C-C + RH

C-C-C-C + RH

c c I I +

(1) c-c-&-c + c-c=c-c -+ c-c-c-c-c-c -+

C C 1 + 1

(2) C

I I + I c c

c-c-c-c-c-c

-+ c-c-c-c-q-c 4 c-c-c-c + c=c-c

Propane may be formed uia intermediates 1 and 2 as follows:

7 c c I I +

1 + 2 -b c-c-y-c-c-c -+ c-c-c=c + c-c-c

c-6-c + c-c-c-c + c-c-c + c-c-6-c

The suppression of isobutane formation over H-ZSM-5 in comparison with its formation over MAPO-36 or HY may be due to the difficulty of formation of a dibranched interme- diate (1) in the narrow pore systems of the zeolite. Over ZSM-5, dimerization and cracking may occur as follows :

C I c-c-6-c + c=c-c-c + c-c-c-c-8-c-c

F <i + -+ C-C-~-C-C-C-C + c-c-c=c + c-c-c

Further repetition of dimerization and cracking gives propane as a major product.

Cracking of Isobutane Cracking of isobutane was carried out over MAPO-36 (Mg/ P = 0.1 1 and 0.19), HY and H-ZSM-5 at 773 K. The results are shown in Table 5. MAPO-36 with Mg/P of 0.19 was more active than that with Mg/P of 0.11. The activities of both MAPO-36 catalysts were close to that of HY, while H-ZSM-5 gave a lower conversion than did MAPO-36 or HY, in contrast to ethanol dehydration and butane cracking. A difference between MAPO-36 (or HY) and H-ZSM-5 was also observed in the product distributions. The selectivity for butane was high over MAPO-36 and HY, while it was quite low over H-ZSM-5. In contrast, the selectivities for lower

alkenes and hydrogen were high over H-ZSM-5 compared with those over MAPO-36 and HY.

Over large-pore catalysts, isobutane molecules are acti- vated by a hydride-transfer mechanism :

C C C I I - n + I

C-C-C + R' + C-q-C + RH C-C-C + RH

Butane molecules may be formed uia interaction of (CH,),C+ and isobutene:

C C C C C I I I I I +

I C

c-~-c+c-c-c + c-c-c-$ -c=c-c-c-c-c-c I C

(3) C C I + I I C

~c-c-c-c-c-c -+ c-y-c + c-c-c-c

C I c-c-c-c + c-5-c

On the other hand, the formation of dibranched dimers (3) or the hydride-transfer reaction involving isobutane may be sterically hindered in the pores of H-ZSM-5. This explains the lower activity of ZSM-5 and a lower yield of butane. Over H-ZSM-5 isobutane molecules may be activated by direct attack of protons to form methane, propene, hydrogen and isobutene:

C I +

C-C-C + H + -+ CH, + C-C-C- CH, + C=C-C

C C C I I

C-C-C + H + -+ H, + C - k - C z H, + C=C-C

The higher selectivities for methane, hydrogen and alkenes over H-ZSM-5 are in accord with this mechanism. Ono and Kanae showed that isobutane is exclusively activated over H-ZSM-5 by the two reactions, giving methane and hydro- gen with the ratio of 2 : 1 at low conversion level."

Cracking of Hexane and 3-Methylpentane Cracking of hexane and 3-methylpentane was carried out at 523 K over MAPO-36 (Mg/P = 0.19), HY, H-Beta and H-ZSM-5. In the case of HY, the reactions were also carried out at 573 K, since the conversion of hexane was too low over HY. The order of activity depends considerably on the reactants. The order of the activities for 3-methylpentane cracking was HY > H-Beta > MAPO-36 > H-ZSM-5, while the order for hexane cracking was H-ZSM-5 > MAPO- 36 > H-Beta > HY. There are two factors affecting the

Table 5 Cracking of isobutane

product distributions ( O h )

molar ratio, conversion H2 catalyst M g P (Yo) CH, C2H, C,H, n-C,H,, C2H4 C,H, C4Hs C,+ aromatics selectivity"

30.7 2.2 0.5 31.4 31.0 3.7 6.7 5.9 18.4 0.2 8.1 44.2 1.7 0.6 38.3 26.8 4.2 6.5 5.4 15.9 0.6 6.0 MAPO-36 {:::;

HY - 36.0 1.4 0.5 35.8 33.4 1.5 0.8 4.5 19.8 2.3 2.8 26.4 7.1 2.5 30.0 6.1 12.8 22.3 10.5 4.7 4.0 23.2 H -ZS M - 5 -

Reaction conditions: 773 K ; isobutane pressure, 34.2 kPa; W/F = 4.2 g h mol-I. In mol per 1 0 0 mol isobutane converted.

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dependence of the order of activity on the reactant. The first is the acidic properties. As shown by the TPD spectra of ammonia, MAPO-36 has stronger acid sites than HY and this may explain the higher activity of MAPO-36 for hexane cracking over that of HY. H-Beta has stronger acid sites compared with HY because of the higher Si/Al ratio. H-ZSM-5 has a large number of strong acid sites and thus exhibits a higher activity. Thus, the order of activity for hexane cracking can be explained. in terms of the acidic properties of the catalysts.

The other factor affecting the dependence of the order of activity on the reactant is the spatial constraint. Therefore, the constraint index (CI) was determined for the four cata- lysts. CI is defined by the following equation.16

log(fraction of n-C, remaining) log(fraction of 3-Me-C, remaining)

CI =

CI for H-ZSM-5 is 3.3, while MAPO-36, HY and H-Beta give much smaller constraint indices than H-ZSM-5. It has been reported that the reaction of 3-methylpentane over H-ZSM-5 is affected by a shape-selective ~ons t ra in t . '~ This explains why highly acidic H-ZSM-5 shows a low activity for 3-methylpentane. Furthermore, MAPO-36, HY and H-Beta are known to be large-pore zeolites. HY has the largest amount of acid sites and therefore is most active for 3- methylpentane cracking, which does not require stronger acid sites than hexane cracking.

Conclusion Pure MAPO-36 can be obtained in the gel with Mg/ P = 0.08-0.25 after hydrothermal synthesis at 373 K for 44 h and then at 423 K for 22 h. The nuclei of MAPO-36 are believed to be formed at the first stage of heating at 373 K and grow at the second stage of heating at 423 K. Further heating at 423 K causes conversion of the MAPO-36 phase into the more stable MAPO-5 phase.

MAPO-36 has Br~lnsted acid sites resulting from the sub- stitution of Mg2 + for A13 + in the framework, and also Lewis acid sites. The mechanism of generation of the Lewis acid sites is not clear. The TPD spectra of ammonia show that the total number of acid sites on MAPO-36 is similar to that on

Table 6 Cracking of hexanes ~~

conversions (%)

catalyst n-C, 3-Me-C, CI

MAPO-36 17.7 39.2 0.39 HY trace 60.9 - HY" 7.3 67.7 0.067 Beta 12.6 57.1 0.16 H-ZSM-5 48.3 18.3 3.3

H-ZSM-5 and smaller than that on HY. The acid sites of MAPO-36 are as strong as those of H-ZSM-5, though their number is smaller.

The catalytic activities for dehydration of ethanol and cracking of butane and hexane are in the order: H-ZSM- 5 > MAPO-36 b HY. This order of activities can be explained in terms of the differences in the acidic properties of these materials.

The order of the catalytic activities for cracking of iso- butane and 3-methylpentane is MAPO-36 2 HY > H-ZSM- 5. The lower activity of H-ZSM-5 can be explained by con- sidering the spatial constraint for these reactions. The high activity of MAPO-36 similar to that of HY for the reactions confirms that MAPO-36 is a large-pore molecular sieve.

We thank Dr. Shin-ichi Nakata of Chiyoda Chemical Engin- eering & Construction Co. for the X-ray fluorescence analysis of the compositions of the materials.

References 1

2

3

4 5

6

7 8

10

11

12 13

14 15

16

17

S. T. Wilson, B. M. Lok, C. A. Messina, T. R. Cannan and E. M. Flanigen, J. Am. Chem. SOC., 1982, 104, 1146. B. M. Lok, C. A. Messina, R. L. Patton, R. T. Gajek, T. R. Cannan and E. M. Flanigen, J. Am. Chem. SOC., 1984,106,6092. E. M. Flanigen, B. M. Lok, R. L. Patton and S. T. Wilson, Pro- ceedings of 7th International Zeolite Conference, Kodansha- Elsevier, Tokyo-Amsterdam, 1986, p. 103; E. M. Flanigen, R. L. Patton and S. T. Wilson, in Studies in Surface Science and Catalysis, vol. 37, Innovation in Zeolite Materials Science, Else- vier, Amsterdam, 1988, p. 13. C. S. Blackwell and R. L. Patton, J. Phys. Chem., 1984,88, 6135. R. Khouzami, G. Coudurier, B. F. Mentzen and J. C. Vedrine, in Studies in Surface Sciences and Catalysis, vol. 37, Innovation in Zeolite Materials Science, Elsevier, Amsterdam, 1988, p. 355. J. A. Rabo, R. J. Pellet, P. K. Coughlin and E. S. Shamshoum, in Studies in Surface Science and Catalysis, vol. 46, Zeolites as Cata- lysts, Sorbents and Detergent Builders, Elsevier, Amsterdam, 1989, p. 1. Y. Ono and H. Adachi, Nippon Kagaku Kaishi, 1989,555. S. Ernst, L. Puppe and J. Weitkamp, in Studies in Surface Science and Catalysis, vol. 49, Zeolites : Facts, Figures, Future, Elsevier, Amsterdam, 1988, p. 447. B. M. Lok, C. A. Messina, R. L. Patton, R. T. Gajek, T. R. Cannan and E. M. Flanigen, U.S.P.4 440 871/1984. R. B. Calvert, E. W. Valyocsik and S. S. F. Wong, E.P.164 939/ 1985. To be published. G. Dworezkov, G. Rumplmay, H. Mayer and J. A. Lercher, in Studies in Surface Science and Catalysis, vol. 21, Adsorption and Catalysis on Oxide Surfaces, Elsevier, Amsterdam, 1985, p. 163. T. R. Hughes and H. M. White, J. Phys. Chem., 1967,71,2192. Y. Ono and K. Kanae, J. Chem. SOC., Faraday Trans., 1991, 87, 663. V. J. Frillette, W. 0. Haag and R. M. Lago, J. Catal., 1981, 67, 218. W. 0. Haag, R. M. Lago and P. B. Weisz, Discuss. Faraday SOC., 1982,72, 317.

Reaction conditions: 523 K ; hexane pressure, 5.0 kPa; W/F = 13.0 g h mol-'. a 573 K. Paper 1/01212F; Received 14th March, 1991

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