Hydroisomerization of long-chain n-alkanes on bifunctional Pt/zeolite catalysts:: Effect of the zeolite structure on the product selectivity and on the reaction mechanism

The transformation of n-decane, n-tetradecane and n-hexadecane was carried out in a fixed-bed reactor at 220 degrees C under a 30 bar total pressure on bifunctional Pt-exchanged HBEA, HMCM-22 and HZSM-5 zeolite catalysts. The activities of the catalysts and especially the reaction scheme depended strongly on the zeolite structure. Whatever the reactant, monobranched isomers were the only primary reaction products on PtHBEA, while cracking was the main reaction observed on HMCM-22 and HZSM-5 with n-tetradecane and n-hexadecane. This was explained in terms of diffusion of the reaction intermediates inside the zeolites porosities: rapid diffusion inside the large channels of the HBEA zeolite, improved by the small size of the crystallites, blocking of the reaction intermediates inside the porosity of HMCM-22 and HZSM-5 yielding extensive cracking. (c) 2007 Elsevier B.V. All rights reserved.