Development of Heterogeneous Catalysts for Hydroformylation of 1-hexene in Supercritical Carbon Dioxide

The hydroformylation of alkenes is a major commercial process used for the production of oxygenated organic compounds. Commercial hydroformylation processes may produce significant quantities of waste material. When the hydroformylation reaction is performed using a homogeneous catalyst, an organic or aqueous solvent is employed and a significant effort must be expended to recover the catalyst so it can be recycled. The hydroformylation of long-chain alkenes using homogeneous catalysts in aqueous solution is compromised because of the lowsolubility of C5 alkenes and above. Development of a selective heterogeneous catalyst would allow simplification of the process design in an integrated system that minimizes waste generation. Supported catalysts are well-known to have minimal capacity for product selectivity. To remedy this problem, we have developed tethered rhodium-phosphine catalysts with modified silica and compared them with catalysts prepared on MCM-41 and MCM-20 supports that provide improved selectivity and conversion relative to their nonporous equivalents. Platinum and palladium catalysts analogous to those of rhodium were also investigated. Our synthesis and characterization of the rhodium, platinum and palladium complexes and evaluation of their catalytic activity and selectivity for hydroformylation in supercritical carbon dioxide will be described. Recent studies have shown that a supercritical fluid may be used as a solvent for hydroformylation reactions. The use of carbon dioxide as a reaction solvent offers optimal environmental performance because it is non-toxic, non-flammable and renewable, and simplifies product separation. In particular, we have considered the conversion of 1-hexene to heptanal using rhodium-phosphine catalysts tethered to supports insoluble in supercritical carbon dioxide to demonstrate the advantages and understand the limitations of a solid-catalyzed process.