How significant the charge density of olefins to their epoxidation reactions over M~(4+)-substituted zeolitic catalysts: A DFT investigation

摘要

Density functional calculations were performed to study the active sites of M~(4+)-substituted zeolites (M=Ti, Zr, Ge, Sn, Pb) and find a descriptor to correlate the olefin structures and epoxidation activities. The active-site formation is the most thermodynamically favorable for M=Ti. Transition elements (Ti and Zr) form the bidentate M(η~2-OOH) active sites whereas main group elements (Ge, Sn and Pb) are apt to form the monodentate M(η~1-OOH) species. The charge density of the CC double bond (Q_(CC)) was found to be closely correlated with the activation free energy (δG~≠), and the linear functions were fitted as δG~≠=32.5*Q_(CC)+54.4 (R~2=0.89) and δG~≠=41.2*Q_(CC)+55.5 (R~2=0.95) for the ethylene- and styrene-related systems, respectively. Several exceptions to these functions have been detected suggesting the presence of other influencing factors. Nonetheless, the Q_(CC) descriptor proves to be applicable not only for the common electrophilic reactions, but also for those that may fall outside the scope of "electrophilic" as discussed in this work.