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A MoO x /SiO 2 system is an effective catalyst for alkene metathesis; however, the mechanism of the transformation of the surface metal oxide species into active alkylidene sites is not well recognized. In this work, comprehensive density functional theory studies of the initiation mechanisms for alkene metathesis on the MoO x /SiO 2 catalyst have been performed. It is shown that surface silanol groups interacting with Mo species and constituting Brønsted acid sites can play a key role in reduction of the dioxo Mo(VI) species to the mono-oxo Mo(IV) species by alkene, through Mo(VI) alkoxy species, and in subsequent formation of the Mo(VI) alkylidene species. An alternative activation pathway avoiding the reduction step is also possible. The proposed mechanisms of silanol-assisted reduction/initiation with propene are predicted to be more kinetically and thermodynamically accessible than the often assumed pseudo-Wittig mechanism. The silanol-assisted activation of the mono-oxo Mo(IV) species by propene is kinetically preferred over non-silanol−assisted initiation mechanisms, that is, 1,2-hydrogen shift mechanism, allyl mechanism, and oxidative coupling mechanism involving molybdacyclopentane species. The reactivity of the Mo sites is significantly affected by their geometry and the local structure of silica. Our results suggest that only a small fraction of the Mo oxide species with a suitable geometry and neighborhood can be effectively activated by alkenes.

The amino acid condensation reaction on a heterogeneous mineral surface has been regarded as one of the important pathways for peptide bond formation. In this work, the mechanism of peptide bond formation over a silica surface in an aqueous environment is studied using ab initio molecular dynamics calculations coupled with enhanced sampling methods such as metadynamics and umbrella sampling. The model includes a periodically repeated slab of amorphous SiO2 forming an interface with explicit liquid water. The adopted simulation method allowed reconstruction of a prejudice-free reaction mechanism of glycine dimerization and quantification of the corresponding free energy profile, with a detailed characterization of transition states and of the role of water. The resulting three-step mechanism features an overall free energy barrier of 155 kJ/mol at 300 K. In comparison to the bulk liquid phase, our results indicate that the interface has a strong catalytic effect on the condensation reaction, which we trace back to the capability of the silica–water interface in promoting an addition reaction by a transition state stabilization. The silica–water interface is found to behave as a less-polar reaction medium with respect to bulk water, promoting addition reactions and disfavoring elimination reactions.