ABSTRACT
Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, KS 66047
bsubramaniam@ku.edu, and busch@ku.edu
Abstract Chemists have long recognized the need to replace the Chlorohydrin and Halcon industrial processes for propylene oxide production because they pose environmental pollution concerns and also yield large amounts of undesirable by-products. Although O2 and H2O2 have been suggested as clean oxidants, their use in propylene oxide production is still confronted by many problems. In this work, “green” methods for the epoxidation of propylene were explored by studying systems based on CO2 expanded liquid solvents (CXLs). The phase behavior of several CXLs involving C3H6, CH3CN, H2O2 and H2O were evaluated to systematically apply these media to the epoxidation of propylene under mild reaction conditions in the presence of a variety of catalysts. These catalyst systems give good conversion and selectivity of propylene oxide at moderate operating temperatures and pressures, paving the way for novel routes for the production of propylene oxide and propylene glycol using environmentally benign solvents such as CO2 and H2O. Introduction Propylene oxide is mainly used for the production of polyesters, polyols, and propylene glycol; the latter is the starting material for polyurethane and unsaturated resins, and other products. Industrially over six million tons of propylene oxide (PO) is produced each year using either the Chlorohydrin or the Halcon process. In the Chlorohydrin process, propylene reacts with Cl2 and H2O to produce 1-chloro-2propanol and HCl, which is then treated with base to generate propylene oxide and salt. There are two pounds of salt waste for each pound of PO produced. Both present methods consume large amounts of chlorine and lime which are finally converted to useless and environmentally polluting waste (1). Minimizing waste in the selective oxidation of propylene to PO has long been an important objective of industrial chemistry. Direct oxidation of propylene to PO with O2 would be highly desirable, but the presence of propylene’s highly combustible allylic hydrogens renders this quite difficult. In pursuit of a more environmentally friendly process, CO2 has been investigated as a medium (2). CO2 is considered environmentally acceptable, non-toxic, relatively cheap (~5 cents/lb), non-flammable, inert toward oxidation and readily available (3).
Supercritical media, in general, have the potential to increase reaction rates, to enhance the selectivity of chemical reactions and to facilitate relatively easy separations of reactants, products, and catalysts after reaction (3). However reactions involving CO2 and water are typically conducted as biphasic processes, with the organic substrate dissolved mostly in the CO2-rich phase and the water-soluble catalysts and/or oxidant dissolved in the aqueous phase. Such systems suffer from inter-phase mass-transfer limitations (4). In this project, a ‘green” method for the epoxidation of propylene is developed by employing CO2 expanded liquid solvents (CXLs) in the absence of a catalyst. Ternary CXLs are homogeneous mixtures of dense CO2, an organic solvent such as acetonitrile or methanol, and water. The oxidant is formed in situ through the reaction of carbon dioxide and hydrogen peroxide to produce peroxycarbonic acid as shown in Figure 1. A base will be used to control the acidity of the solution during reaction between the peroxycarbonic acid and substrate propylene. By tuning the solvent properties of the CXL containing CH3CN/H2O2/H2O with dense-phase CO2 both substrate and oxidant will be present in a homogeneous reaction mixture inside the view cell reactor.
