Comparative Study of Methanol Activation by Different Small Mixed Silicon Clusters Si(2)M with M = H, Li, Na, Cu, and Ag

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