Effect of metal-acid site balance on hydroconversion of decalin over Pt/Beta zeolite bifunctional catalysts

摘要

The main objective of this work is contributing to the understanding of the ring opening of naphthenic model molecules, an important reaction to improve the cetane number of diesel fuels. Accordingly, the hydroconversion of decalin was studied over a series of Pt/Beta catalysts with different metal contents in order to evaluate the effect of changing the proportion of hydrogenation/acid functions of the catalyst on conversion and products distribution. The overall reaction rate did not change by increasing the hydrogenation activity (metal content) of the catalyst. However, ring contraction and ring opening product yields increased with metal content up to 1.0 wt% and then became constant for higher metal contents, while cracking product yields followed an opposite trend, decreasing with increasing metal content up to 1.0 wt%. This behavior clearly provides evidence of a bifunctional mechanism in the hydroconversion of decalin over Pt/BEA catalysts, since the selectivity depends on the metal/acid function balance. Although decalin is a saturated molecule, its naphthenic character and probably the presence of a tertiary carbon in its structure facilitate activation of the molecule directly on the strong acid sites of zeolite, even at relatively moderate temperatures. Thus, it is not necessary to form an olefin on a metal site, which would undergo subsequent protonation, as in a classical bifunctional mechanism, such as in hydroisomerization and hydrocracking of n-paraffins. However, the hydrogenation component of the catalyst influences the selectivity of the reaction, controlling the process of desorption/adsorption of any intermediate olefins formed. Greater hydrogenation activity implies lower availability of adsorbed carbocations that can be converted to cracking products. This proposition is consistent with the results obtained by varying the content of metal and poisoning of the hydrogenation sites in the catalysts. (C) 2015 Elsevier Ltd. All rights reserved.