Comparative Study of Methanol Activation by Different Small Mixed Silicon Clusters Si<sub>2</sub>M with M = H, Li, Na, Cu, and Ag

High-accuracy quantum chemical calculations were carried out to study the mechanisms and catalytic abilities of various mixed silicon species Si<sub>2</sub>M 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 Si<sub>2</sub>Cu and Si<sub>2</sub>Ag. The most stable complexes generated by the interaction of methanol with the mixed clusters Si<sub>2</sub>M possess low-spin states and mainly stem from an M–O connection in preference to Si–O interaction, except for the Si<sub>2</sub>H 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 Si<sub>2</sub>M considered, the dissociation pathways of both O–H and C–H bonds with Si<sub>2</sub>Li 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 Si<sub>2</sub>Li. The breaking of O–H and C–H bonds with the assistance of Si<sub>2</sub>H, Si<sub>2</sub>Li, and Si<sub>2</sub>Na is kinetically preferred with respect to the Si<sub>2</sub>Cu and Si<sub>2</sub>Ag cases, apart from the case of Si<sub>2</sub>Na for O–H cleavage. In comparison with other transition-metal clusters with the same size, such as Cu<sub>3</sub>, Pt<sub>3</sub>, and PtAu<sub>2</sub>, the energy barriers for the O–H bond activation in the presence of small Si species, especially Si<sub>2</sub>H and Si<sub>2</sub>Li, 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.