JOURNAL OF CATALYSIS 159, 368–374 (1996) ARTICLE NO. 0099 Inactivation of External Surface of ZSM-5: Zeolite Morphology, Crystal Size, and Catalytic Activity Y. S. Bhat, J. Das, K. V. Rao, and A. B. Halgeri 1 Research Centre, Indian Petrochemicals Corporation Limited, Baroda, 391 346 India Received April 14, 1995; revised October 4, 1995; accepted November 20, 1995 The enhancement of the para selectivity feature of the zeolite by inactivation of the external surface was studied. The morphol- ogy and crystal size play a major role in deciding the extent of modification required to achieve a predesired para selectivity. The extent of modification was higher for the zeolite batches contain- ing smaller, spheroidal shaped crystals, while it was lower for the batches having longer, oblong shaped crystals. Any covering of the oblong crystals by foreign particles such as silica enhances the ex- tent of inactivation required to achieve a specific para-compound selectivity. c  1996 Academic Press, Inc. 1. INTRODUCTION One of the unique features of ZSM-5 zeolite is its shape selectivity. In zeolite catalyzed reactions, selective catalysis that occurs inside the intracrystalline voids can be affected by nonselective activity taking place on the external surface sites. Paparatto et al. (1) have reported that during aromatic alkylation, the para isomer formed selectively inside ZSM-5 channels, while isomerization proceeded just on the exter- nal surface, decreasing the desired product selectivity. The roles of external surfaces in reactions with ZSM-5 catalysts have been dealt with earlier (2–4). Several methods have been reported in the literature to deactivate or almost neutralize the catalytic activity on ex- ternal surface sites of the zeolites. Yashima et al. (2) have observed that the selectivity for para-xylene was increased up to 65% with the addition of 4-methylquinoline. Para se- lectivity was increased by coking with mesitylene because of the shape selectivity enhancement produced by the deposi- tion of coke species on the external surface of ZSM-5 zeolite (5). Rodewald (6) has discussed the use of silicon polymers to enhance zeolite shape selectivity. The effect of different silicon compounds as modifier agents on the catalytic activ- ity of ZSM-5 zeolite during toluene ethylation (7), n-hexane cracking (8), and selective toluene disproportionation (9) have been reported. Likewise Bhat and Halgeri (10) have 1 To whom correspondence should be addressed. modified ZSM-5 zeolite by chemical vapor deposition (CVD) of tetraethyl orthosilicate and used it for ethylben- zene alkylation with ethanol. The CVD method changes neither the channel size nor the acidity. The deposited silica narrows the pore opening size and at the same time deacti- vates the external surface, due to the coating of inert silica on the external surface (11). As a result, the shape selective property of the zeolite is enhanced to a great extent. The CVD modifies the pore opening size and the external surface area; this area of the zeolite depends on the mor- phology and crystal size. The smaller the crystal size, the larger the specific external surface area and the larger the crystal size, the smaller the specific external surface area. External surface area has a direct bearing on the extent of chemical vapor deposition required to achieve a particular selectivity. In this context the present work was addressed to establish the relation between morphology, crystal size, and catalytic activity of the external surface inactivated ze- olite. Toluene alkylation with ethanol was used to test the catalytic activity of the zeolites. 2. EXPERIMENTAL ZSM-5 zeolite batches (A1–A5) used in this study have been synthesized by following a patented procedure (12). Two different silica sources, viz. sodium silicate and Ludox, and two different templating agents, viz. tetrapropyl am- monium bromide and triethyl n-butyl ammonium bro- mide, were used in the zeolite synthesis. In order to make the gel of batch A1, the following three solutions were prepared: S1 = 45.697 g sodium sulfate + 56.6 g water; S2 = 3.965 g TEBA + 20 g water; S3 = 0.662 g Al 2 (SO 4 ) 3 · 16H 2 O + 2.5 g sulfuric acid + 26 g water. Solutions S2 and S3 were added to S1 drop by drop with stirring. After the complete addition the gel was stirred for 2 h and its pH was adjusted to 10.5 by adding dropwise dilute sulfuric acid. 0021-9517/96 $18.00 Copyright c  1996 by Academic Press, Inc. All rights of reproduction in any form reserved. 368