Evolution of the catalytic activity in Pt sulfated zirconia catalysts: Structure, composition, and catalytic properties of the catalyst precursor and the calcined catalyst

摘要

A 3% Pt/sulfated zirconia catalyst was prepared and characterized before and after calcination at 900 K by XRD, XPS, EM, and in the catalytic hydroisomerization of n-hexane. The "fresh" sample exhibited small but definite catalytic properties. Calcination brought about a dramatic increase of the activity with practically constant high (90-100%) selectivity for hydroisomerization versus cracking. This increased activity was accompanied by the transformation of the predominantly amorphous support to predominantly tetragonal crystals and the wrapping up of most parts of surface Pt atoms into the bulk, as shown by the physical characterization methods. Hence metallic Pt particles exhibited mainly Pt-O rather than Pt-S interactions. S was present as sulfate. Pt-sulfated zirconia was different from traditional bifunctional metal catalysts on acidic supports. We attributed its higher catalytic activity and favorable isomerization selectivity to a few but very active centers, formed by interaction of Pt sites with sulfate groups on the high Miller-index surfaces of ZrO2, Calcination must be essential to create these active sites. H-2 dissociating on Pt sites would provide the hydride species that are necessary for isomerization occurring on the acidic (sulfate-zirconia) part of that ensemble. We proposed the name "compressed bifunctional sites" for these centers of acid-metal cooperation. The assumption of such active sites, the maximum activity as a function of the hydrogen pressure, can also be explained in a consistent way. (C) 1998 Academic Press. [References: 62]