Supercritical fluids and gas-expanded liquids as tunable media for multiphase catalytic reactions

Solvents play a vital role in multiphase catalysis. They are selected to perform several functions during liquid phase catalytic transformations such as solubilizing reactants, facilitating product/catalyst separation, increasing reaction rates, enhancing solubilities of gaseous reactants (such as O-2, CO, H-2) in the liquid phase, and providing heat capacity to effectively manage the heat of reaction. This review discusses such roles of conventional solvents in multiphase catalytic processes such as hydroformylation, carbonylation, hydrogenation and oxidation. For each of these chemistries, industrially relevant examples are presented, highlighting the advantages and limitations of the conventional solvents used therein. Further, it is shown how the pressure-tunable properties of supercritical fluids (SCFs) and gas-expanded liquids (GXLs) have been exploited in such systems to develop novel multiphase catalytic technology concepts. GXLs in particular provide advantages such as rate intensification, efficient feedstock utilization, enhanced process safety, waste minimization and reduced use of volatile organic solvents, all at relatively mild pressures and temperatures. Conventional reactors and reaction engineering tools that integrate phase behavior (for reactions and separations), catalytic kinetics and multiphase reactor modeling may be applied for the rational development of multiphase reactors using GXLs. Quantitative economic and environmental impact analyses during early stages of process development provide valuable research and process engineering guidance for developing practically viable GXL processes. The Rh catalyzed hydroformylation in CO2-expanded liquids and methyltrioxorhenium-based ethylene epoxidation are highlighted as exemplars of GXL-based technology concepts. Emerging feedstocks, such as plant-based biomass, shale gas and sequestered CO2, that require new catalytic conversion technologies to produce chemical intermediates, offer excellent opportunities for using tunable solvents. (C) 2014 Elsevier Ltd. All rights reserved