A DFT Study of Tungsten-Methylidene Formation on a W/ZSM-5 Zeolite: The Metathesis Active Site

Tungsten-methylidene formation from ethene on either the W-IV, W-V, or W-VI active sites of a W/ZSM-5 zeolite is investigated by using the M06-L functional. The reaction is assumed to proceed in two steps; the first step is the [2+2] cycloaddition between ethene and the W-O active site to form an oxametallacycle intermediate. The intermediate is then decomposed to produce the W-methylidene active site from the metathesis reaction. The overall activation barrier of the reaction on W-VI (27.3kcalmol(-1)) is considerably lower than the ones for W-IV and W-V (69.4 and 37.1kcalmol(-1), respectively). Moreover, the reaction involving the W-VI site also stabilizes intermediates and products to a larger extent than the ones on the W-IV and W-V sites. As a result, we have demonstrated that the reaction of the W-methylidene metathesis active site is both kinetically and thermodynamically favored to occur on the W-VI active site of the zeolite.