Theoretical study on ethene metathesis proceeding on Mo-VI and Mo-IV methylidene centres of heterogeneous molybdena-alumina catalyst

Theoretical studies on ethene metathesis reaction proceeding on molybdename thylidene centres of molybdena-alumina catalyst are reported. Calculations were carried out with the Gaussian 94 program, using density functional the ory. Models of active sites containing Mo-VI and Mo-IV (both singlet and tr iplet state) have been developed. Molybdenum in these centres is attached t o a cluster of formula Al-2(OH)(6), which represents alumina. The thermodyn amics of the reactions of ethene with Mo-VI and Mo-IV carbene centres were calculated. The reaction pathways of ethene addition to Mo-VI and Mo-IV met hylidene complexes were also investigated. In these calculations simpler st ructures of the carbene and molybdacyclobutane complexes have been applied, in which the bonds between molybdenum and the carrier are replaced by hydr oxyl groups. It was concluded that active centres of ethene metathesis do n ot contain Mo-IV because of high energy barriers of some elementary steps o f the process. (C) 2001 Elsevier Science B.V. All rights reserved.