A chemical titration method for quantification of carbenes in Mo- or W-containing catalysts for metathesis of ethylene with 2-butenes: verification and application potential

Abstract

We introduce and experimentally validate a simple titration method for quantifying the number of carbenes acting as active sites in the metathesis of ethylene with 2-butenes over Mo- or W-containing catalysts. This method is based on the experimentally proven mechanistic sequence of carbene formation through the reaction of ethylene with oxidized metal oxide species: (i) reduction of M(VI) to M(IV) by ethylene, (ii) addition of ethylene to M(IV) to yield MC₂H₄ carbene and (iii) transformation of the latter to MCH₂ upon reaction with C₂H₄ yielding C₃H₆. As MCH₂ cannot produce C₃H₆ when reacting with C₂H₄, the amount of C₃H₆ formed in step (iii) corresponds to the number of catalytically active carbenes. The applicability of this method was supported by establishing a correlation between this number and the steady-state rate of propene formation in the metathesis of ethylene with 2-butenes.