Comparative Study of Methanol Activation by Different Small Mixed Silicon Clusters Si₂M with M = H, Li, Na, Cu, and...

High-accuracy quantum chemical calculations were carried out to study the mechanisms and catalytic abilities of various mixed silicon species Si₂M with M = H, Li, Na, Cu, and Ag toward the first step of methanol activation reaction. Standard heats of formation of these small triatomic Si clusters were determined. Potential-energy profiles were constructed using the coupled-cluster theory with extrapolation to complete basis set CCSD(T)/CBS, and CCSD(T)/aug-cc-pVTZ-PP for Si₂Cu and Si₂Ag. The most stable complexes generated by the interaction of methanol with the mixed clusters Si₂M possess low-spin states and mainly stem from an M-O connection in preference to Si-O interaction, except for the Si₂H case. In two competitive pathways including O-H and C-H bond breakings, the cleavage of the O-H bond in the presence of all clusters studied becomes predominant. Of the mixed clusters Si₂M considered, the dissociation pathways of both O-H and C-H bonds with Si₂Li turns out to have the lowest energy barriers. The most remarkable finding is the absence of the overall energy barrier for the O-H cleavage with the assistance of Si₂Li. The breaking of O-H and C-H bonds with the assistance of Si₂H, Si₂Li, and Si₂Na is kinetically preferred with respect to the Si₂Cu and Si₂Ag cases, apart from the case of Si₂Na for O-H cleavage. In comparison with other transition-metal clusters with the same size, such as Cu₃, Pt₃, and PtAu₂, the energy barriers for the O-H bond activation in the presence of small Si species, especially Si₂H and Si₂Li, are found to be lower. Consequently, these small mixed silicon clusters can be regarded as promising alternatives for the expensive metal-based catalysts currently used for methanol activation particularly and other dehydrogenation processes of organic compounds. The present study also suggests a further extensive search for other doped silicon clusters as efficient and more realistic gas-phase catalysts for important dehydrogenation processes in such a way that they can be experimentally prepared and implemented.