Comparative Study of Methanol Activation by Different Small Mixed Silicon Clusters SiM 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 SiM 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 SiCu and SiAg. The most stable complexes generated by the interaction of methanol with the mixed clusters SiM possess low-spin states and mainly stem from an M-O connection in preference to Si-O interaction, except for the SiH 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 SiM considered, the dissociation pathways of both O-H and C-H bonds with SiLi 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 SiLi. The breaking of O-H and C-H bonds with the assistance of SiH, SiLi, and SiNa is kinetically preferred with respect to the SiCu and SiAg cases, apart from the case of SiNa for O-H cleavage. In comparison with other transition-metal clusters with the same size, such as Cu, Pt, and PtAu, the energy barriers for the O-H bond activation in the presence of small Si species, especially SiH and SiLi, 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.