Selectivity improvement in xylene isomerization

To passivate the external surface of nano-crystalline Pt/HZSM-5 the modification techniques of pre-coking and liquid phase deposition of organosilane have been applied. The nature and properties of parent and modified samples were characterized by sorption measurements and NMR spectroscopic techniques. The effects of surface modifications on catalyst selectivity have been investigated during xylene isomerization. Silanization treatment of the catalyst via water-induced decomposition of tetraethoxysilane (TEOS) of approximate to4 wt.% coverage yielded higher product selectivity, i.e., a decrease of the undesired disproportionation products (toluene and trimethylbenzenes). A more significant selectivation effect was achieved for the pre-coked sample containing; approximate to0.3 wt.% carbonaceous deposits. This was found due to the efficient inactivation of the extracrystalline strong acid sites by the pre-coking treatment. Whereas silanization treatment tends to result in insufficient inactivation of external acidity and substantial pore-narrowing of the zeolite. (C) 2004 Elsevier Inc. All rights reserved.