Tuesday, May 05, 2015

Novel catalyst used to make styrene in one step


(Phys.org)—Styrene is an important industrial chemical. It is the precursor to polystyrene which is used in various every day plastic products, like disposable cups, packaging, and insulation. Over 18.5m tons of styrene is produced industrially around the world. Typically, styrene synthesis begins with benzene and ethylene, and involves a multi-step process under harsh reaction conditions and often leads to unwanted by-products.

In research that aims to streamline and optimize styrene synthesis Benjamin A. Vaughan, Michael S. Webster-Gardiner, Thomas R. Cundari, and T. Brent Gunnoe from the Department of Chemistry at the University of Virginia and the Center for Advanced Modeling in the Department of Chemistry at the University of North Texas have devised a single-step synthesis of styrene using a novel rhodium catalyst. Their work appears in Science.
The production of styrene, or vinyl arenes in general, involves benzene alkylation. This is typically done under harsh conditions involving high temperatures and either a Friedel-Crafts or zeolite catalyst. This process typically involves poly-alkylated bi-products. Additionally, the alkyl substituent will then need to be oxidized to form the vinyl group, which is usually accomplished with a metal oxide at high temperatures. The products are the target vinyl arene and hydrogen gas, as well as any bi-products from poly-alkylation. Products from poly-alkylation are converted to ethylbenzene in a trans-alkylation step, which follows the alkylation reaction.

Prior research from this group, found that this synthetic process can be streamlined using a 
platinum catalyst for the alkylation of benzene, and the alkyl group will subsequently undergo beta-hydride elimination, forming free styrene. However, this process degrades the platinum catalyst, likely because the platinum ion further reduces to platinum metal during the beta-elimination phase of the reaction.
To make this one-step process industrially feasible, they need to find an optimal catalyst. Ideally, this catalyst would directly vinylate the benzene ring rather than initial alkylation followed by oxidation of the alkyl group. Additionally this catalyst would not lead to multiple side reactions and have a high turnover number even in harsh oxidative conditions and in the presence of highly reactive metal hydrides. Furthermore, industrially favorable mechanisms would involve recovering and recycling the oxidant using air or oxygen.
For this paper, Vaughan et al. designed a catalyst with rhodium rather than platinum in hopes that the less favorable reduction of Rh(I) to elemental rhodium compared to the reduction of Pt(II) would maintain the integrity of the catalyst.

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