Electron spin resonance and electron spin echo spectroscopic studies of paramagnetic rhodium species produced in RhCa-X zeolite during ethylene dimerization: evidence for a .sigma.-bonded intermediate

Abstract

Electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies were used to characterize paramagnetic active sites formed in RhCa-X zeolite during ethylene dimerization and the structure of a key reaction intermediate for the first time. Adsorption of ethylene at 195 K onto RhCa-X activated in flowing O{sub 2} at {approx} 673 or {approx} 773 K produced two new Rh(II) ESR signals, E1 and E2, which decay as a function of time. The rate of decay in samples activated at {approx} 673 K is faster compared to the decay in samples activated at {approx} 773 K, and this correlates with the percent ethylene conversion observed after 18-20 h at 296 K under static reaction conditions. Species E2 formed with deuterated ethylene could be analyzed by ESEM to give the number and distance of the nearest deuteriums. On the basis of the available data, a suggested reaction mechanism is proposed in which the reaction proceeds by a mechanism similar to that for dimerization by a homogeneous RhCl{sub 3} catalyst.