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STYRENE BREAKTHROUGH Novel ENGINEERED CATALYST opens up alternative route to commodity polymer feedstock STEPHEN K. RITTER, C&EN WASHINGTON
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IT'S NOT EVERY DAY in the chemical industry that a potential new process for making a commodity chemical comes along and is called a breakthrough. But that's what some chemists are saying about a recently unveiled method for producing styrene monomer. Researchers at Exelus Inc., in Livingston, N.J., have demonstrated a lab-scale process to make styrene from toluene and methanol in a single step. The route, when implemented on a production scale, could result in "dramatic reductions" in feedstock and energy costs compared with the traditional two-step route that employs benzene and ethylene as the reactants, says chemical engineer Mitrajit Mukherjee, the company's founder and president. Styrene is an important monomer with a global demand of more than 25 million metric tons per year, Mukherjee notes. It is used to make plastics and synthetic rubbers for numerous applications. Dow Chemical, in the U.S., and I. G. Farben, in Germany, introduced the twostep styrene process some 70 years ago, he says. In the first step, benzene and ethylene react via a solid-acid catalyst to form ethylbenzene. In the subsequent step, the ethylbenzene, mixed with high-temperature steam, passes over an iron oxide catalyst at 625 °C. The catalyst dehydrogenates the ethyl group to yield styrene. Styrene also is commercially available as a by-product of industrial propylene oxide production from ethylbenzene and propylene. Industrial chemists have been eager for some time to find an alternative route to styrene to solve a few unsavory problems. For one thing, benzene from crude oil and
ethylene from natural gas are high-priced starting materials for producing a staple like styrene monomer. In addition, the high-temperature dehydrogenation is an energy-intensive process, which is a curse for chemical companies these days because of high energy prices. The opportunity to use less toxic toluene in place of benzene, a known carcinogen, is another plus. A number of researchers have attempted to synthesize styrene via the toluenemethanol route during the past 30 years, Mukherjee says. But up to now, no one has been able to devise a catalyst with the type of yield and selectivity needed for a commercially viable process. Decomposition of methanol to hydrogen and carbon monoxide during the reaction was one limitation. Another shortcoming was that the hydrogen could react with styrene to re-form ethylbenzene. According to Mukherjee, the key to the Exelus process, called ExSyM, is a novel engineered catalyst that facilitates styrene synthesis in a single step at 425 °C and atmospheric pressure. The catalyst is a proprietary zeolitic material containing basic active sites in a highly optimized pore structure that can be used in a standard fixed-bed reactor, he explains. The active sites selectively adsorb toluene over methanol to limit methanol decomposition, and the pore structure facilitates the diffusion and residence time of the reactants to enhance toluene alkylation, he says. Styrene and ethylbenzene are produced in a 9:1 mixture in the lab-scale process, with nearly complete conversion of toluene and methanol. At production scale, the ethylbenzene by-product could be separated and sold to a conventional styrene producer, or
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