“Ideal” bifunctional catalysis over Pt-acid zeolites

The conditions of “ideal” bifunctional catalysis were specified on the example of two reactions involved in major refining and petrochemical processes, the hydroisomerisation of n-alkanes (n-Cx) and of ethylbenzene (EB) over PtHzeolites. In both cases, the balance between the hydrogenating and acid functions (CPt/CH+) was shown to affect positively the activity (actually the turnover frequency of the acid sites), the stability and the selectivity of the bifunctional catalysts. Optimal catalytic characteristics were obtained for CPt/CH+ values high enough so that the acid catalysed steps be rate-limiting, provided however, an additional condition is satisfied: low number of acid sites between two Pt sites in order that only one skeletal transformation of n-Cx intermediates could occur or distance between zeolite protonic sites sufficient for limiting the demanding reaction of ethylbenzene disproportionation. The location of the active acid sites which determines their access by the olefinic intermediates plays also an essential role; the highest selectivity values were obtained with sites located at the micropore mouth of the zeolites, after however blockage by carbonaceous deposits of the accessibility to the non-selective inner sites. The only limitation of the corresponding catalysts is their low activity related to the low concentration of zeolite outer sites, which should be easily overcome by preparing zeolite samples with larger outer surface. The generalisation of this analysis to other reactions should facilitate the design of the “ideal” bifunctional catalysts, opening the route to cleaner commercial processes.