Promotion effect of mixed halides on homogeneous ruthenium catalysis in direct synthesis of ethanol from hydrogen and carbon monoxide

Abstract

In the synthesis gas reaction using homogeneous ruthenium catalysts combined with an onium halide, the nature of halogen anions affected characteristically the yields of oxygenated products. Of the ruthenium-onium catalysts examined, the Ru-PPNCl system was best for the formation of methanol in non-polar aprotic solvents. An appreciable synergistic effect was observed when bis(triphenylphosphine)iminium chloride (PPNCl) was combined with other halide additives. HX (X = Cl, Br or I) was almost inactive when added alone to the ruthenium catalyst, but accelerated both the rate of methanol formation and the conversion of the intermediate methanol to ethanol when coexistent with PPNCl. Some mechanistic studies, including 13C tracer experiments, were carried out to elucidate these effects.