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

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 O2 at ∼673 or ∼773 K produces two new Rh(II) ESR signals, E1 and E2, which decay as a function of time. The rate of decay in samples activated at ∼673 K is faster compared to the decay in samples activated at ∼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. The structural result for this reaction intermediate indicates a Rh(II) σ-bonded to an ethylene carbon that has rehybridized toward sp3 in contrast to π-bonded intermediates observed for Pd(I). Species E2 is suggested to result from a reductive coordination of Rh(III) with ethylene to produce Rh(II). The subsequent decay results from a further reductive coordination to Rh(I), which is suggested to be the active species for the ethylene dimerization. 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 RhCl3 catalyst. © 1990 American Chemical Society.