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Start with the transition state
Zeolites catalyze a wide range of industrial reactions, but identifying the reactions that they catalyze well tends to be done by trial and error. Gallego et al. propose a way to rationalize the process by using structure-directing agents that act as scaffolds for creating the zeolite pores and channels (see the Perspective by Millini). They picked molecules that mimic the transition states for reactions such as the isomerization of ethylbenzene into xylene, and they successfully identified catalysts with improved reactant conversion and product selectivity.
Abstract
Unlike homogeneous catalysts that are often designed for particular reactions, zeolites are heterogeneous catalysts that are explored and optimized in a heuristic fashion. We present a methodology for synthesizing active and selective zeolites by using organic structure-directing agents that mimic the transition state (TS) of preestablished reactions to be catalyzed. In these zeolites, the pores and cavities could be generated approaching a molecular-recognition pattern. For disproportionation of toluene and isomerization of ethylbenzene into xylenes, the TSs are larger than the reaction products. Zeolite ITQ-27 showed high disproportionation activity, and ITQ-64 showed high selectivity for the desired para and ortho isomers. For the case of a product and TS of similar size, we synthesized a catalyst, MIT-1, for the isomerization of endo-dicyclopentane into adamantane.
