Recent experimental work addressed C-C coupling vs. C-H bond formation on zeolite-supported Rh(ethene)2 complexes1 and small well-defined Rh clusters.2 The complex catalyzes predominantly C–C coupling of ethene to butene under ethene-rich conditions.1 It was speculated2 that the zeolite walls may participate actively in that catalysis by activating ethene moieties. Using DFT calculations at the PBE-D level, we explored computationally such chemistry at Rh centers, using a plane-wave based supercell method as well as a QM/MM embedded cluster method. Both approaches yield a comparable energetics. At the Rh(ethene)2 center, we examined (i) a direct condensation of two ethene units forming a metallocyclopentane that subsequently transforms into a butene species, and (ii) insertion of an ethene into the Rh–C bond of an ethyl moiety. For the latter variant we found one conformation with a rather low activation barrier of 90 kJ mol–1, while other variants, including those involving the zeolite wall, had substantially higher barriers of at least 120 kJ mol–1. In addition, we determined that during constant supply of molecular hydrogen, the hydrogenation of ethene to ethane is kinetically preferred with a rate-limiting activation barrier of only 34 kJ mol–1. Small Rh clusters lead preferentially to ethane formation, with similar barriers as the complex Rh(ethene)2.3,4 These computational findings put into question the structure and the composition of the active centers involved in C-C coupling reactions to form butene. In consequence, the latter reaction may be more complex than thus far hypothesized.

Rh-based single-site catalysis in faujasite – a surprisingly complex chemistry

253rd National Meeting (2017)

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