Acidic functionalities of zeolites are often used for catalyzing reactions of C–C bond formation and breaking [1]. Experiments showed that Rh(C2H4)2 complexes, anchored at acidic centers of faujasite, predominantly lead to C–C coupling of ethene to butene under ethylene-rich conditions, while under hydrogen-rich conditions ethane was found as the main product [2]. To understand these variations in selectivity we carried out plane-wave based PAW calculations at the PBE-D level as well as QM/MM calculations at the PBE level. Both approaches yield a comparable energetics [3]. Surprisingly, a straightforward use of the suggested active center Rh(C2H4)2+ preferentially leads to hydrogenation, with very low activation barriers, below 40 kJ mol–1. For this model, barriers on the way to C–C coupling were calculated much higher, more than 90 kJ mol–1. Thus, rationalizing the mechanism of the experimentally determined C–C coupling on a single Rh center becomes a challenging puzzle. We present an exhaustive computational search for the active catalyst including acidic centers and Rh complexes near acidic centers, as suggested in the literature [2,4]. We will show that C–C coupling comprises a modified Rh complex and synergistic reaction steps between zeolite and metal center. With the suggested mechanism, we were able to rationalize available experimental results, e.g., those of deuteration experiments [2]. Corma, A. Chem. Rev. 95, 559 (1995). Serna, P., Gates, B.C. Angew. Chem. Int. Ed. 50, 5528 (2011). Dinda S., Govindasamy A., Genest A., Rösch N. J. Phys. Chem. C, 118, 25077 (2014). Namuangruk, S., Pantu, P., Limtrakul, J. Chem. Phys. Phys. Chem., 6, 1333 (2005).

C-H vs. C-C bond formation in a faujasite. Modeling a subtle balance for anchored Rh centers

251st National Meeting (2016)

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