CATALYTIC AND STRUCTURAL-PROPERTIES OF RUTHENIUM BIMETALLIC CATALYSTS- KINETICS OF 2,2,3,3-TETRAMETHYLBUTANE HYDROGENOLYSIS

The kinetics of 2,2,3,3-tetramethylbutane (TeMB) hydrogenolysis has be en investigated over alumina-supported mono- and bimetallic ruthenium catalysts. The bimetallic catalysts were prepared from the monometalli c ones by controlled surface modification with Sn, Pb and Ge organomet allics. As reported earlier, large Ru particles (> 3 nm) favour the cl eavage between the two quaternary carbon atoms leading to isobutane (a lpha delta process); moreover, deep hydrogenolysis of the adsorbed alk yl fragments occurs before desorption. At the opposite, on small Ru pa rticles (approximate to 1 nm) demethylation to 2,2,3-trimethylbutane i s the main reaction (alpha gamma process) and deep hydrogenolysis is s uppressed. 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 unch anged upon Ge addition, The addition of either Sn or Ge to large Ru pa rticles 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 alpha gamma and alpha delta processes go through a maximum as a function of hydrogen pressure, the maximun for the alpha delta process being shifted to higher H-2 pressures. Ass uming (i) Langmuir adsorption isotherms, (ii) multisite competitive ad sorption between hydrogen and TeMB, (iii) hydrogenation of the dehydro genated adsorbed alkane (most abundant surface intermediate, MASI) by a surface hydrogen atom as the rate determining step, a rate law was p roposed for both alpha gamma and alpha delta processes. The kinetic an alysis suggests that highly dehydrogenated intermediates (alpha delta process, -5H) are formed on large Ru particles. These species need a l arge ensemble of Ru atoms to be formed. At variance, on small Ru parti cles hydrogenolysis could proceed through less dehydrogenated adsorbed species: metallacyclobutane for the alpha gamma process (-2H) and met allacyclopentane for the alpha delta process (-3H), the formation of t he latter being inhibited at high H-2 pressure. These metallacycles ca n be formed on small ensembles of Ru atoms. The addition of Sn or Ge t o small Ru particles does not change very much the positions of the ra te maxima for alpha gamma and alpha delta processes, or the hydrogen d ependence of the rate. It emerges that the main effect of Sn and Ge ad dition is to decrease the number of sites active for alpha gamma and a lpha delta processes respectively, the former being the sites of low c oordination, the latter the sites of high coordination.