A department research group is combining experimental testing with computational chemistry in an effort to develop methods that reduce the need for environmentally hazardous chlorinated aromatics in various industrial processes.
Heterogeneous catalysis, chemical kinetics, reaction engineering
Carl R.F. Lund, Professor
In Search of an Environmentally Benign Process for the Chlorination of Aromatics
The synthesis of many engineering plastics, specialty polymers, pharmaceuticals, dyes, fragrances and pesticides relies upon the use of chlorinated aromatics as intermediates. Because of the demonstrated toxicity of many chlorinated aromatics (for example, PCBs), these chemicals are slowly being removed from the marketplace as end-use products. Nonetheless, chlorinated aromatics will likely continue to be used as intermediates in the production of the value-added materials because in such cases the end product does not contain chlorinated aromatics and because their use is often essential to producing the final product efficiently and economically. The production of chlorinated aromatics by current methods generates hazardous or toxic wastes, providing the impetus for an on-going research program within the Chemical Engineering (CE) Department to develop processes that are more environmentally benign.
In a representative commercial process an aromatic hydrocarbon is chlorinated to produce the intermediate. Research in CE has used toluene as a prototypical aromatic hydrocarbon. Chlorination processes begin with the addition of a soluble Lewis acid like iron chloride to the toluene. The Lewis acid serves to catalyze the chlorination reaction. Chlorine is then admitted and the chlorination reaction occurs. Once the reaction is complete, it is necessary to remove the Lewis acid from the products. Usually this is done by adding excess water to the system, leading to a two-phase mixture. The Lewis acid is thereby extracted into the water phase which can be separated from the chlorinated aromatic phase. The water that is removed is the first waste generated by the process; it contains dilute acid that can't be economically recovered, and it probably also contains very low levels of chlorinated aromatics. There is a second waste product from the process. Continuing with toluene chlorination as an example, there are two chlorinated aromatics produced in the reaction: ortho-chlorotoluene and para-chlorotoluene. These are produced in roughly equal amounts, but only the para-chlorotoluene is useful for making value-added products. The ortho-chlorotoluene is a second waste stream, amounting to approximately 50% of the product.
The strategy being followed in Dr. Lund's research group in CE for making the chlorination process “greener” involves efforts to develop heterogeneous catalysts to replace the soluble Lewis acids currently used. Such catalysts, being solids, can be very simply separated from the products without the use of any water. Hence, the generation of an aqueous waste stream is completely eliminated. The scientific and engineering challenge lies in also eliminating the second waste stream, namely the undesired chlorinated aromatic products. The research group is using a combination of experimental testing and computational chemistry to find materials that are more selective and thereby produce more para-chlorotoluene and less ortho-chlorotoluene.
The group has found that certain zeolites, also known as molecular sieves, can be very selective when used as chlorination catalysts. For example, by using a zeolite known as zeolite-L as the catalyst and by selecting appropriate reaction conditions, the group has been able to chlorinate toluene so that only 10% of the chlorinated product is the undesired ortho-chlorotoluene with 90% selectivity for the desired para-chlorotoluene product. There is another challenge yet to be met, however. The lifetime of the zeolites is still too short. That is, they lose their catalytic activity very rapidly, and so are not yet suitable for commercial use. Thus experimental research continues, seeking to build upon the promising results to date.
At the same time, computational chemistry is being applied to generate an understanding of the molecular processes involved in the catalytic process. Using quantum chemistry, new insights into the reaction pathways have been developed. For example, most textbooks state that aromatic chlorination proceeds through a molecular pathway that involves an arenium cation. The computational studies indicate that this is most likely true if a solvent is employed, but it does not appear that ionic species are involved in the absence of a solvent (such as in a commercial process). Similarly, one Lewis acid is often identified as AlCl3, but quantum chemical calculations show that the formation of this species is energetically unfavorable in the absence of solvent, and this again has implications upon the reaction pathway.
Presently the research group is using the molecular understanding afforded by the computational chemistry to guide the development of new classes of chlorination catalysts. The goal is to design materials at the molecular level so that they display the same kind of selectivity as the L zeolites, but also sustain their catalytic activity indefinitely. These efforts include polymer-based catalyst materials as well as templated inorganic materials. In each case, it is desired to situate the catalytically active site in a geometrically constrained location such that the desired reaction occurs readily, but the undesired reaction is precluded by steric constraints. Such materials will be essential for the development of environmentally benign chlorination processes.
