Catalytic behavior of vanadium substituted mesoporous molecular sieves

Mesoporous catalysts, supports and catalytic membranes based on MCM-41. Final report for the period January 15,2000 - January 14, 2001

“…The catalytic activities exhibit a volcano pattern of activities whch increase with increasing pore size to a maximum and then decrease. A similar pore size effect has been observed on the V-MCM-41 catalysts also (see below) and was assumed to be related to the change of local bond angles of Si-0-V [21]. While the ring size (of silica units in the wall structure) is varied in these MCM-41 catalysts with the same Ti loading and structure type, the most likely changes that may be induced are the changes of the local Si-0-Ti bond angles.…”

“…Several laboratories have contributed to the recent literature on V-MCM-41 [37,38,39] in addition to our own work on this catalyst [21,33] (also, see references therein). In all studies there is general agreement that the calcined catalyst contains V5+ incorporated in the pore wall in a tetrahedral environment (in the dehydrated state).…”

Structural and Electronic Heterogeneity of Two Dimensional Amorphous Silica Layers

“…[ 2,3 ] In terms of chemical properties, the reactivity of silicates is sensitive to the T-O-T bond angle, where T refers to Si or substitutional dopants; in framework silicates, including zeolite and mesoporous catalysts, this angle varies substantially from the favored 145 o . [4][5][6] The opening of the bond angle causes a rehybridization of the orbitals surrounding the oxygen which has been related to the changes in reactivity. [ 5,7 ] Understanding the connection between local disorder and physical and chemical properties would appear to be an ideal problem for scanning probe microscopy (SPM); however, until recently diffi culties in deconvoluting topographic variations from true atomic scale features have precluded imaging disordered silica surfaces.…”

“…Alternatively, it has long been suggested that distortions from ideal SiO 4 tetrahedra linked at optimal T-O-T bond angles can play an important role in determining the stability of silicates and the reactivity of catalytically important doped zeolites and mesoporous silicas. [ 5,6,16 ] The tetrahedral bond angles around the Si atom are largely maintained and so this effect is associated primarily with a rehybridization of the oxygen atom from sp 2 towards sp as the T-O-T angle increases. [ 5,7 ] As the hybridization decreases, the electron density around the oxygen also decreases and becomes more diffuse.…”

Radius of Curvature Effect on the Selective Oxidation of Cyclohexene Over Highly Ordered V-MCM-41

“…Vanadium-containing MCM-41 has received more attention since many oxidation catalysts have vanadium as an active component [25,[28][29][30][31][32][33][34][35][36]. Epoxidation of cyclohexene over vanadium catalysts is one of the well-studied reactions [37,38].…”