Towards a Benign and Viable Rhodium Catalyzed Hydroformylation of Higher Olefins: Economic and Environmental Impact Analyses, Solvent Effects and Membrane-based Catalyst Separation

Abstract

Researchers at the Center for Environmentally Beneficial Catalysis (CEBC) had previously reported a novel rhodium-based hydroformylation process concept based on the use of CO2-expanded liquids (CXLs) to intensify rates and obtain higher linear/branched aldehydes selectivity at relatively mild temperatures (30-60 °C) and pressures (~4 MPa). This dissertation continues investigations aimed at addressing the fundamental and practical issues associated with this concept. ReactIR studies of Rh/triphenylphosphine-catalyzed 1-octene hydroformylation, complemented by microkinetic and reactor modeling investigations revealed that the intrinsic kinetic rate constants are of similar magnitude with or without CO2 addition to the reaction mixture. This implies that the enhanced reaction rate observed in CXL is due to the increased hydrogen solubility in that medium. Environmental impact analysis revealed that the overall toxicity index for the CEBC process is approximately 40 times less than the Exxon process against which the CEBC process was benchmarked. Economic analysis of the CXL concept revealed that at an aldehyde production rate of 19,900 kg / (kg Rh h), 99.8% rhodium has to be recovered per pass for the CEBC process to be competitive with the Exxon process. Assuming a similar hydroformylation turnover frequency, rhodium recovery levels that exceed this criterion for economic viability were successfully demonstrated in a membrane-based nano/ultra-filtration reactor system using polymer supported phosphorus ligands, synthesized and provided by researchers from the Department of Chemistry. During continuous filtration of a toluene-based solution containing polymer-supported Rh complexes, the Rh and P concentrations in the permeate, quantified using ICP analysis, were on the order of a few tens of ppb. During continuous 1-octene hydroformylation studies in the membrane reactor at a syngas pressure of 0.6 MPa and 60 °C, the 1-octene conversion and product (mostly aldehydes) concentrations reached a steady state with the Rh concentrations in the permeate stream being lower than 120 ppb. However, the conversions and product concentrations during the continuous run are lower than those obtained in a batch ReactIR under identical operating conditions. This is attributed to syngas starvation in the membrane reactor that might be caused by inadequate mixing. In complementary investigations, it was found that the dissolution of CO2 in the organic phase (to create CO2-expanded liquids) decreases the viscosities of the mixtures with increasing CO2 pressure. This offers an opportunity to enhance mixing and also tune the membrane flux so as to increase the throughput of the membrane filter. The demonstrated technology concept, when fully optimized, should find applications in a variety of other applications in homogeneous catalysis, including hydrogenation and carbonylation of conventional and biomass-based substrates.