Ensemble and ligand effects in selective alkane hydrogenolysis catalysed on well characterised RhIr and RhFe bimetallic clusters inside NaY zeolite

Abstract

A series of metal/bimetal catalysts has been prepared from molecular carbonyl clusters such as Rh_6–x Ir_x(CO)₁₆(x= 0–6) and Rh₄Fe₂(CO)₁₆^2– which were synthesized inside NaY zeolite supercages. Na Y-entrapped Rh₆, Ir₆, Rh₄Ir₂, Rh₃Ir₃ and Rh₂Ir₄ crystallites derived from the precursor carbonyl clusters have been well characterized by f.t.i.r., EXAFS, ¹²⁹Xe n.m.r. and CO/H chemisorption on their metallic structures and electronic states. The results suggested that the reduced crystallites inside NaY zeolite consist of cluster ensembles < 10 Å in size with controlled metal compositions. These ensembles are fairly stable through several cycles of oxidation, reduction and alkane hydrogenolysis. As the probing reactions, hydrogenolysis of n-butane and ethane, and benzene hydrogenation were conducted. There is a dramatic decrease of hydrogenolysis activity by four orders of magnitude across the series of clusters on increasing the number of Ir atoms in the active Rh ensembles, while a slight enhancement of activity is observed for benzene hydrogenation. The remarkable activity suppression of hydrogenolysis has been interpreted not simply in terms of the geometric Rh ensemble-size effect with a small amount of Ir atoms but of the different electronic states of the RhIr bimetallic clusters in decreasing the electron deficiency through the series clusters. This was demonstrated by the unusually large chemical shifts of n.m.r. signals of ¹²⁹Xe adsorbed on NaY-Rh-rich clusters in reflecting the higher electron deficiency. For butane hydrogenolysis, the smaller Rh ensembles in bimetallic RhIr clusters gave the maximum selectivities towards the central C—C bond scission to give C₂H₆, higher than those on the Rh₆ and Ir₆ inside zeolites. In contrast, RhFe/NaY derived from [Rh₄Fe₂(CO)₁₆]^2–/NaY exhibited conversely a high selectivity towards terminal C—C bond scission to give CH₄+ C₃H₈. The Fe promotion for the terminal C—C bond scission is suggested to be associated with the electronic ligand effect on the heteronuclear sites consisting of Rh–Fe³⁺ located on the internal zeolite cages.