The hydroformylation of alkenes is a major commercial process used for the production of oxygenated organic compounds. 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. Development of a selective heterogeneous catalyst would allow simplification of the process design in an integrated system that minimizes waste generation. Recent studies have shown that supercritical carbon dioxide (scCO2) as a reaction solvent offers optimal environmental performance and presents advantages for ease of product separation. In particular, we have considered the conversion of 1-hexene to heptanal using rhodium- and platinum-phosphine catalysts tethered to supports insoluble in scCO2 to demonstrate the advantages and to understand the limitations of a solid-catalyzed process. One of the historical limitations of supported catalysts is the inability to control product regioselectivity. To address this concern, we have developed tethered catalysts with phosphinated silica and controlled pore size MCM-41 and MCM-20 supports that provide improved regioselectivity and conversion relative to their nonporous equivalents. Platinum catalysts supported on MCM-type supports were the most regioselective whereas the analogous rhodium catalysts were the most active for hydroformylation of 1-hexene in scCO2.
Homogeneously catalyzed reactions are often among the most selective reactions that can be accomplished, yet they are often unsuitable for industrial applications because recovery of the expensive catalyst can be difficult. Many schemes have been developed to circumvent this problem. We have attempted to simplify catalyst recovery by grafting a homogeneous catalyst onto a solid support and utilizing this supported catalyst in combination with a supercritical fluid to reduce the phase-transfer effects that often inhibit the performance of supported catalysts. When used for the hydroformylation of 1-hexene, we demonstrate that the performance of the catalyst can be varied using traditional methods from homogeneous catalysis, heterogeneous catalysis, and reactions in supercritical fluids. Catalyst performance is compared over several sets of conditions, and the surface mechanism is probed using high-pressure diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The results suggest that the homogeneous mechanism is effectively transferred to the supported material.
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