This invention relates to an improvement in the process for dehydrohalogenating a halogenated hydrocarbon to an ethylenically unsaturated product in the presence of a phase-transfer catalyst.
The term "phase-transfer catalysis" describes reactions between reactants located in different phases brought about by the use of small quantities of an agent which transfers one reactant across the interface into the other phase so that the reaction can proceed. The phase-transfer agent is not consumed but performs the transport function repeatedly. See Starks, J. Am. Chem. Soc. 93:1, 195 (1971) and Starks et al. J. Am. Chem. Soc. 95:11, 3613 (1973). Also see U.S. Pat. No. 3,992,432 (to Napier et al.) and a book by Starks et al, entitled "Phase-Transfer Catalysis", Academic Press, New York, N.Y. 1978. The first paper in the above series considers organic-soluble quaternary ammonium and phosphonium ions to be excellent agents for the transport of anions from aqueous phase to an organic phase. Such quaternary ions thus are effective phase-transfer catalysts in reactions in which anions participate, for example, in displacement reactions.
A typical catalytic dehydrohalogenation process is described in U.S. Pat. No. 3,981,937 to Campbell et al., wherein 3,4-dichlorobutene-1 (sometimes hereafter abbreviated to DCB) is dehydrochlorinated with aqueous alkali to 2-chlorobutadiene-1,3 (also known as chloroprene). The catalyst is a quaternary ammonium chloride but can also be another phase-transfer catalyst; see, for example, U.S. Pat. Nos. 3,639,492, 3,639,493, and 3,876,716 (all to Campbell).
In the industrial practice of this process, the dehydrochlorination is often carried out in a series of continuous stirred-tank reactors, the initial stages being cooled to remove the heat of the reaction. DCB, the catalyst, and an excess of aqueous NaOH are fed into the first reactor, where much of the reaction takes place. As the reactants become depleted, the reaction rate decreases, so that a large proportion of the total reactor volume is required to complete the last few percent of the reaction. Crude chloroprene is recovered from the effluent of the last stage; aqueous and organic phases are separated, and waste organics and waste brine are disposed of.
It is desirable to improve the reactor utilization in the chloroprene manufacturing process and in similar dehydrohalogenation processes so that better process economy can be achieved.