Electron paramagnetic resonance and gas chromatographic study of ethene dimerization catalysed by Rh/SiO2: effect of silica pore size

Abstract

Electron paramagnetic resonance (EPR) was used to characterize paramagnetic rhodium species on Rh/SiO₂ after activation in vacuum and after subsequent adsorption and dimerization of ethene. Activation above 430 K produces two major Rh^II species which do not appear to be active for dimerization. Adsorption of ethene onto Rh/SiO₂ at 77 K followed by warming to 195 K produces another EPR signal, E, which does not change at 195 K. Warming to 296 K results in the decay of signal E in ca. 6 min. The intensity of EPR signal E increases with decreasing pore size and decreasing activation temperature to 470 K. The intensity of signal E is also found to correlate with the dimerization activity in both static and flow reactors. Species E is suggested to result from reductive coordination of ethene with Rh^III to produce a σ-bonded Rh^II–ethene complex which then decays by a subsequent reduction with another ethene to a Rh^I–butene species. This scheme is analogous to that suggested for RhCa-X zeolite. In a flow reactor the dimerization activity increases with decreasing average pore diameter from 14.0 to 2.5 nm. This is attributed to a decrease in the time required for a successful encounter of ethene with Rh^III in the smaller pores. In a static reactor the effect of pore size is decreased.