Catalytic and structural properties of ruthenium bimetallic catalysts: kinetics of 2,2,3,3-tetramethylbutane hydrogenolysis

Abstract

The kinetics of 2,2,3,3-tetramethylbutane (TeMB) hydrogenolysis has been investigated over alumina-supported mono- and bimetallic ruthenium catalysts. The bimetallic catalysts were prepared from the monometallic ones by controlled surface modification with Sn, Pb and Ge organometallics. As reported earlier, large Ru particles (> 3 nm) favour the cleavage between the two quaternary carbon atoms leading to isobutane (αδ process); moreover, deep hydrogenolysis of the adsorbed alkyl fragments occurs before desorption. At the opposite, on small Ru particles (≈ 1 nm) demethylation to 2,2,3-trimethylbutane is the main reaction (αγ process) and deep hydrogenolysis is suppressed. The addition of Sn or Pb onto small Ru particles has only a small effect on the reaction rate but shifts the selectivity pattern towards that of large Ru particles, while the selectivity remains unchanged upon Ge addition. The addition of either Sn or Ge to large Ru particles decreases the rate by two orders of magnitude. With respect to the rate, hydrogen has a stronger inhibiting effect on the sample of lower dispersion. Both rates for αγ and αδ processes go through a maximum as a function of hydrogen pressure, the maximum for the αδ process being shifted to higher H 2 pressures. Assuming (i) Langmuir adsorption isotherms, (ii) multisite competitive adsorption between hydrogen and TeMB, (iii) hydrogenation of the dehydrogenated adsorbed alkane (most abundant surface intermediate, MASI) by a surface hydrogen atom as the rate determining step, a rate law was proposed for both αγ and αδ processes. The kinetic analysis suggests that highly dehydrogenated intermediates (αδ process, −5H) are formed on large Ru particles. These species need a large ensemble of Ru atoms to be formed. At variance, on small Ru particles hydrogenolysis could proceed through less dehydrogenated adsorbed species: metallacyclobutane for the αγ process (−2H) and metallacyclopentane for the αδ process (−3H), the formation of the latter being inhibited at high H 2 pressure. These metallacycles can be formed on small ensembles of Ru atoms. The addition of Sn or Ge to small Ru particles does not change very much the positions of the rate maxima for αγ and αδ processes, or the hydrogen dependence of the rate. It emerges that the main effect of Sn and Ge addition is to decrease the number of sites active for αγ and αδ processes respectively, the former being the sites of low coordination, the latter the sites of high coordination.