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) (27.3 kcal mol(-1) ) is considerably lower than the ones for W(IV) and W(V) (69.4 and 37.1 kcal mol(-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.
Department of Chemistry, Faculty of Liberal Arts and Science, Kasetsart University, Kamphaeng Saen Campus, Nakhon Pathom, 73140, Thailand), Fax. faastnm@ku.ac.th.|Department of Chemistry, NANOTEC Center for Nanoscale Materials Design for Green Nanotechnology, Kasetsart University, Bangkok, 10900, Thailand. faastnm@ku.ac.th.
Thana
T
Maihom
Institute of Ion Physics and Applied Physics, University of Innsbruck, 6020, Innsbruck, Austria.
Michael
M
Probst
Department of Materials Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong, 21210, Thailand.
Jumras
J
Limtrakul
density functional calculations; metathesis; methylidene; tungsten oxide; zeolites