Recently, the use of compositions comprising tetrafluoropropenes have been disclosed as a refrigerant and as a blowing agent, among other uses, with advantageous characteristics, such as low global warming potential, low toxicity, and low ozone depletion potential.
One process for producing tetrafluoropropenes involves the use of tetrachloropropenes as a reactant. (See, e.g., US 2007-0197842 A1). As a result, applicants have come to recognize a need for improved methods for producing certain tetrachloropropenes.
Several processes for making tetrachloropropenes such as CCl3CCl═CH2 and CCl2═CClCH2Cl are known. Conventional starting materials include allyl chloride and 1,2,3-trichloropropane, the latter being formed upon chlorination of the former. Starting from 1,2,3-trichloropropane, a repetitive series of dehydrochlorinations and chlorinations with elemental chlorine follow until the desired number of chlorine atoms have been added (Shavanov, S. S.; Tolstikov, G. A.; Shutenkova, T. V.; Ryabova, N. A.; Shurupov, E. V. USSR. Khimicheskaya Promyshlennost (Moscow, Russian Federation) (1987), (2), 79-81).
Dehydrochlorinations are usually conducted with an aqueous base, such as aqueous NaOH, in the presence of a phase transfer catalyst, such as quaternary ammonium salts. The phase transfer catalysts not only improve the rate compared to aqueous NaOH alone, but also help maintain high selectivity at a high conversion. Nonetheless, such processes are not entirely ideal due to the potential environmental hazards associated with quaternary ammonium salts. In addition, after each of the reaction steps involving aqueous base, a drying step and/or waste treatment of the aqueous solution may be required, which adds to the cost and time required to conduct the manufacturing operation. Also, the current invention allows for recovery of the hydrochloric acid co-product, if so desired.
Another general method is the chlorination of chlorinated alkanes having fewer chlorine atoms than desired in the final product. This process, which involves hydrogen substitution by chlorine, can reduce the number of process steps required to achieve the desired number of chlorine substitutions. However, these processes frequently suffer from a lack of selectivity. As a consequence, both un-chlorinated and over-chlorinated materials are produced in addition to an array of isomers.
With respect to the tetrachloropropenes, the isomers CCl3CCl═CH2 and CCl2═CClCH2Cl are in some instances equivalent in terms of chemical transformations, but the former is thermodynamically unstable relative to the latter. Since the isomerization of the former is exothermic to the latter, storage and shipping of the CCl3CCl═CH2 presents a potential hazard. Thus there is a need for a process of making CCl2═CClCH2Cl which has high selectivity for this particular isomer and which does not suffer from the other limitations already noted.
All the steps of the process described in the current invention can be run in a continuous mode which is economically advantages over its predecessors which appear to be a combination of continuous and batch mode steps.