Propylene oxide (PO) is an important building block for a number of consumer products such as rigid foam insulation, surfactants, and flame retardants. The current fossil-based routes to PO from propylene, the chlorohydrin and peroxidation processes, both produce stoichiometric quantities of undesirable byproducts. As global production of PO is expected to increase, the ability to manufacture PO from greener routes becomes important. Propylene glycol (PG) is now commercially produced from glycerol and sorbitol, both easily available as side products of biodiesel production and cellulose fermentation respectively, and presents an opportunity to develop a cleaner, renewable process to PO. The conversion of PG to its acetates (PGA) was found to significantly suppress the production of ethers of PG, thereby increasing selectivity to PO.
In this work, the conversion of PGA to PO over potassium salts on silica has been investigated. High salt loadings, temperatures of 380-420oC, and short contact times give molar selectivities up to 88%. Post-reaction characterization of several potassium salt catalysts using FTIR and XPS show that the stable chemical form of the catalyst depends on potassium surface coverage. At greater than monolayer potassium loading on silica, potassium salts undergo a sequence of reactions to form stable potassium carbonate, whereas at sub monolayer coverages the primary active component was found to be potassium silicate. Silica gel support essentially collapses in the presence of alkali salts, leading to low specific reaction rates. The use of chemically resistant silica as a support gave significantly higher conversions at sub monolayer potassium coverages, without sacrificing selectivity.
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