Ethene Protonation Over Silica-Grafted Metal (Cr, Mo, and W) Oxide Catalysts: A Comparative Nanocluster Modeling Study

Fundamental insights into the ethene protonation reaction was obtained over different cluster models of acidic CrOx/SiO2, MoOx/SiO2, and WOx/SiO2 catalysts at the M06/Def2-TZVP level of theory. The clusters varied from MSiO4H3 structures (all-fixed, H-optimized, and all-relaxed) to MSi4O4H9 (saturated with H atoms) and further to MSi4O13H9 (saturated with OH atoms). The formation of the ethene protonation adducts followed the order of WOx/SiO2 < MoOx/SiO2 < CrOx/SiO2 in terms of the thermodynamic favorability which agreed well with the partial charges and the global softness data. The natural bond orbital analysis revealed a partial flow of electrons from the bridging O atom to the hydrocarbon fragment than to the metal during the initiation. Although the interbond angles were comparably different in the largest cluster, the bond lengths and orbital energy levels did not change significantly from a cluster to another. Concerning the thermochemical properties, any of the cluster models would be utilized within a 2 kcal/mol confidence limit.