Liquid versus gas-phase operation in heterogeneously catalyzed hydroformylation

Abstract

Heterogeneously catalyzed hydroformylation over a commercial 5 % rhodium nanoparticle catalyst has been performed within the intrinsic kinetics regime in a high-throughput kinetics setup. Owing to the use of a paraffinic solvent, the reaction was carried out either in the gas or the liquid phase. An ethylene conversion of around 4 % mol/mol was obtained in the gas-phase, whereas liquid-phase operation allowed achieving around 9 % mol/mol conversion under comparable reaction conditions. The presence of the paraffinic solvent is supposed to better tune the reactant concentration to which the catalyst is exposed. Propanal and ethane were the main products observed, the highest propanal selectivity, 75 % mol/mol, being obtained at the lowest temperature, 120 °C. Apparent activation energies for both hydroformylation (59 kJ/mol (l) / 68 kJ/mol (g)) and hydrogenation (87 kJ/mol (l) / 94 kJ/mol (g)) were found to be lower at liquid compared to gas-phase conditions, suggesting a lower overall surface coverage at liquid-phase conditions. Ethylene and hydrogen were found to exhibit a positive impact on gas-phase hydroformylation and hydrogenation, resulting in higher ethylene conversion when increasing their molar reactant ratios. Based on the positive impact of ethylene observed in the gas-phase operation, it is likely that in the liquid phase, where ethylene solubility is higher, the altered molar ratio distribution at the catalyst surface further enhances yields toward propanal.