Density Functional Theory (DFT) calculations were performed to optimize the Mo active sites in HBeta zeolite catalysts as well as to locate the reaction pathways to form the Mo-methylidene species. Two different Mo active sites. i.e., the oxidized (MoO2)-O-VI and its reduced form (MoO)-O-V(OH), were developed and incorporated into HBeta zeolites by replacing a pair of Bronsted acidic sites. The Mo-methylidene species were found to be produced through two elementary reaction steps, and the Mo-oxametallacyclobutanes were identified as the intermediates. The activation barriers of the decompositions of the oxametallacyclobutane intermediates (Step 2) were estimated to be higher than those of the ethene addition on the Mo active sites (Step 1). The oxidation states of the Mo centers exerted marked influences on the stabilities of the intermediates as well as oil the activation barriers and reaction heats of Steps 1 and 2, which were elucidated by the electronic properties of the O-b-ligands directly bonded to the Mo centers. Both free energy barriers and reaction heats have indicated that the whole processes of generating the Mo-methylidene species were preferred over the Mo(VI) rather than Mo(V) active site. Accordingly. the Mo(VI) active site was more efficient in catalyzing the formation of Mo-methylidene species in the heterogeneous Mo/HBeta catalytic systems. (C) 2008 Elsevier B.V. All rights reserved.