Recyclability of water-soluble ruthenium–phosphine complex catalysts in multiphase selective hydrogenation of cinnamaldehyde using toluene and pressurized carbon dioxide

The recyclability of water-soluble ruthenium–phosphine complex catalysts was investigated in water–toluene and in water–pressurized carbon dioxide systems for selective hydrogenation of trans-cinnamaldehyde (CAL). For the first hydrogenation run, the selectivity for cinnamyl alcohol (COL) is high for both toluene and dense CO2, because of interfacial catalysis in which the reaction mainly occurs at the interface between the aqueous phase and the other toluene or dense CO2 phase. The total CAL conversion and the COL selectivity decrease on the second run, more significantly with dense CO2 than toluene. On the subsequent runs, however, less significant changes were observed. During the first run, the active metal complexes should change to much less active ones such as Ru(H)2Ln(TPPTS)m (L = COL) by accumulation of the main product of COL. This structural change may occur more easily in multiphase hydrogenation with dense CO2 than that with toluene, probably because the solubility in the dense CO2 gas phase is even smaller than that in toluene. For homogeneous reaction of COL in aqueous phase, Ru(H)2Ln(TPPTS)m catalyzes the isomerization to HCAL compared with the hydrogenation to hydrocinnamyl alcohol. With those complexes, however, the selectivity for COL is still comparable to that for HCAL for multiphase hydrogenation reactions because the hydrogenation of an ampholytic substrate of CAL occurs mainly at interface between water and toluene or dense CO2 gas phase. Interactions of CO2 molecules with CAL would also increase the reactivity of carbonyl group of the substrate.

The recycling performance of water-soluble Ru/TPPTS complex catalysts was studied in multiphase hydrogenation of trans-cinnamaldehyde (CAL) using either toluene or carbon dioxide at low and high pressures. Figure optionsDownload as PowerPoint slide