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Environmental needs associated with green chemistry require a reduction in the use of reaction solvents and separation materials. Heterogenization of homogeneous catalysts can be used to simplify the recovery steps in a reaction process. The use of a supercritical fluid for reaction and separation can further reduce the need for organic solvents throughout the process. The current paper describes the development of a tethered homogeneous catalyst for the hydroformylation of 1-hexene in supercritical carbon dioxide that is shown to be active, stable, and selective for the reaction. A reaction rate model that shows similarities to both the homogeneous and heterogeneous models is developed based on the assumption of a tethered catalyst and provides rate constants that are consistent with previous estimates.

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Green chemistry and engineering is the design of chemical manufacturing systems to minimize their adverse affects on the environment. Thus, a primary goal of green chemistry and engineering is to reduce the environmental impact of chemical processes and chemical manufacturing while simultaneously enhancing the overall process performance. Although it is beneficial to simply reduce the use of organic solvents in chemical processes, green chemistry and engineering goes further, in that it evaluates the entire manufacturing operation to identify techniques that can be applied to minimize the overall process hazard, while maintaining economic practicality. Evaluation of the environmental impacts of the manufacturing process requires a systems approach and appropriate metrics that permit quantitative assessment of environmental hazards. Thus, this chapter begins with a discussion of the drivers for green engineering and the metrics through which processes can be evaluated. Then, the hydroformylation process is used as a case study to illustrate the way in which green chemistry principles can be applied to real processes. Two elements are specifically highlighted: (a) the use of catalysts to facilitate active and selective chemistry and the immobilization of said catalysts within the reactor system, and (b) the development of processes based on benign reaction solvents, and the benefits that can accrue from simplified separations operations.

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.