March 28, 2011
Produce “green” epichlorohydrin from biodiesel glycerol. Converting renewable feedstocks to chemicals is gaining momentum as oil prices continue to rise. For example, we have seen the development of “green” polyethylene made from ethylene that comes from from bioethanol dehydration and the hydrogenolysis of biodiesel-derived glycerol to propylene glycol. Both of these examples use low-cost feedstocks and benefit from the huge economies of scale of the transportation fuels business. A similar idea involves converting biodiesel glycerol to epichlorohydrin, the key intermediate in epoxy resin production.
Previous attempts to carry out this conversion focused on adding anhydrous HCl to glycerol to give mixtures of dichlorohydrins, which can be converted to epichlorohydrin by caustic treatment. The disadvantages of the first reaction are the need for large excesses of HCl, low glycerol conversions, and relatively large amounts of chlorinated byproducts. These problems stem from the notion that it is necessary to remove water as it is formed to drive the reaction equilibrium toward the desired dichlorohydrins. Thus, many inventions prescribe azeotropic removal of byproduct water, which requires the use of large excesses of HCl to push glycerol conversion forward because low pressures must be used to facilitate water removal.
W. J. Kruper, Jr., and co-inventors found that removing water is not necessary to achieve high glycerol conversion. This allowed them to use superatmospheric pressures and a minimum of HCl to achieve good yields with few chlorinated byproducts. The process uses a simple reactor design that requires minimum capital investment in a commercial plant.
Whereas many examples are provided in the patent, the example in which the feedstock is crude glycerol from biodiesel production most closely resembles the conditions to be expected in a commercial process. To a 100-mL pressure vessel is added 30 g crude glycerol and 0.6 g HOAc catalyst. The reactor is sealed and pressurized to 120 psig with anhydrous HCl gas and heated to 120 °C for 90 min. After this time, the contents of the reactor contain 97.9% dichlorohydrins. The glycerol conversion is 100%. The only byproducts were various acetoxylated derivatives.
To demonstrate the conversion of the dichlorohydrins to epichlorohydrin, the inventors placed the crude reaction mixture in a reactive distillation column with 10% caustic [most likely NaOH—Ed.]. The reaction conditions were 25 mm Hg pressure, a kettle temperature of 75–77 °C, and an overhead temperature of 65–67 °C. Analysis of the crude epichlorohydrin overhead product showed that it is 99% pure. (Dow Global Technologies [Midland, MI]. US Patent 7,906,690, March 15, 2011; Jeffrey S. Plotkin)