Journal of Catalysis 219 (2003) 310–319 www.elsevier.com/locate/jcat On the enhanced para-selectivity of HZSM-5 modiﬁed by antimony oxide Shourong Zheng, Andreas Jentys, ∗ and Johannes A. Lercher Institut für Technische Chemie II, Technische Universität München, Lichtenbergstrasse 4, 85747 Garching, Germany Received 10 January 2003; revised 12 May 2003; accepted 15 May 2003 Abstract HZSM-5 was modiﬁed with antimony oxide by solid-state reaction at 773 K to enhance diffusional constraints. Crystalline phases of antimony oxide were not detected at a Sb 2 O 3 loading up to 10.2 wt%. Antimony oxide was found to strongly interact with the hydroxyl groups of the zeolite, i.e., silanol and bridging hydroxyl groups, and with extra-framework aluminum oxide. Only a small amount of Sb 2 O 3 penetrated the pores of the zeolite, while the main fraction of Sb 2 O 3 was deposited on the external surface of the zeolite crystals. With increasing antimony oxide loading a pronounced pore narrowing of the zeolite was observed, which suppressed the further penetration of antimony oxide into the pores of the zeolite. The dispersion of antimony oxide on the surface of the zeolite led to complete removal of unselective Brønsted acid sites and to a narrowing/blocking of the pore mouth openings of the zeolite. Both effects induced enhanced para-selectivity for toluene disproportionation.  2003 Elsevier Inc. All rights reserved. Keywords: Antimony oxide modiﬁcation; HZSM-5; Passivation of acid sites; Diffusivity; para-Selectivity; Toluene disproportionation 1. Introduction Due to their unique structure HZSM-5 zeolites are para- selective catalysts in toluene disproportionation, methylation of toluene, and xylene isomerization [1,2]. Further enhance- ment of the shape selectivity can be achieved by postsyn- thetic modiﬁcation, which leads to removal of unselective acid sites located in the pore mouth regions and/or to a nar- rowing of the pore openings. This modiﬁcation of HZSM-5 effectively suppresses the secondary isomerization to unde- sired o- and m-xylenes and enlarges the differences in the diffusivities between o- or m-isomers and p-isomer [3–6]. The aim of the postsynthetic modiﬁcation is to deposit (non)metal oxides on the external surface of zeolite crystals. Niwa et al. [7,8] systematically studied the modiﬁcation of zeolites by chemical vapor deposition (CVD) and found that amorphous SiO 2 layers were formed on the external surface of the zeolite only if bulky molecules, such as Si(OCH 3 ) 4 and Si(OC 2 H 5 ) 4 , were used as silylation agents. Tynjala et al. [9] found that the deposition of SiO 2 and Ge 2 O 3 by CVD on the surface of zeolites effectively enhanced the selectivity to small oleﬁns and linear aliphatic products in the conver- sion of methanol to hydrocarbons. Kaeding et al. [10,11] * Corresponding author. E-mail address: andreas.jentys@ch.tum.de (A. Jentys). observed an enhanced para-selectivity for the alkylation of toluene with methanol on phosphorus-modiﬁed HZSM-5 ze- olites and suggested that the phosphorus reagent interacted with the acid sites of the zeolite and attached to the zeo- lite via framework oxygen atoms, which partially blocked the pore openings and, therefore, restricted the diffusion of m- and o-xylenes. Kaeding et al. [12,13] further studied the modiﬁcation of HZSM-5 with Ca, Mg, B, and Mn oxides and achieved enhanced para-selectivity for toluene methy- lation and toluene disproportionation. The deposition of oxides onto the surface of zeolites can be carried out by impregnation [14], ion exchange [15], chemical deposition [7,8,16,17], and solid-state reactions [18,19]. Karge et al. [18] used solid-state reactions to achieve a markedly higher exchange degree of La ions in zeolite Y compared with ion exchange in liquid phase. The ion exchange of Ni and Co into HZSM-5 and zeolite Y was studied by Jentys and co-workers [20,21], who also achieved a signiﬁcantly higher degree of exchange degree with a solid-state reaction than with ion exchange in liq- uid phase due to the absence of hydration of the cations in the solid reaction. MoO 3 -modiﬁed HZSM-5 zeolite obtained by solid-state reaction showed benzene selectivity for the conversion of methane to aromatics similar to that prepared by the impregnation method [22,23]. Xiao et al. [24] ob- served enhanced conversion over CuCl 2 /HZSM-5 catalysts 0021-9517/$ – see front matter  2003 Elsevier Inc. All rights reserved. doi:10.1016/S0021-9517(03)00234-3