Acidity of titania-supported tungsten or niobium oxide catalystsCorrelation with catalytic activity

Abstract

The acidity of catalytic systems based on tungsten oxide or niobium oxide supported on titania was compared. Two series with metal contents up to 3.6 atom nm À2 were prepared by incipient wetness impregnation of the titania support with ammonium metatungstate or niobium oxalate solutions. Characterization of both systems by X-ray diffraction and Raman spectroscopy studies did not show evidence of bulk metal oxide formation. The acidity was monitored by adsorption of 2,6-dimethylpyridine (2,6-lutidine) followed by infrared spectroscopy. The catalytic activity was tested for the reaction of isopropanol dehydration.At a reaction temperature of 403 K, WO x /TiO 2 catalysts were inactive for a surface density of W 1.2 W atom nm À2 . Above this loading, the activity increased progressively with increasing W content. Similar evolution was observed for the abundance of relatively strong Brønsted acid sites (i.e. able to retain lutidine at 573 K). In contrast, NbO x /TiO 2 catalysts were essentially inactive at this reaction temperature and a higher reaction temperature (473 K) was required to reach a comparable catalytic activity. No threshold of Nb loading for the development of catalytic activity was observed. Similar behavior was evidenced for the abundance of medium strength Brønsted acid sites (able to retain lutidine at 523 K). For both systems, a direct correlation between the catalytic activity and the abundance of Brønsted acid sites was observed. #

Supported and inserted monomeric niobium oxide species on/in silica: a molecular picture

“…An increase in temperature or a decrease in hydration stabilizes the di-grafted ONb(OH)(O-Si) 2 complex. High temperatures favor the formation of the pyramidal ONb(O-Si) 3 arrangement. Nevertheless it should be noted that the pentacoordinated tri-grafted Nb species (C) with two hydroxyl groups is only 0.13 eV less stable than the di-grafted species.…”

Vapour phase dehydration of glycerol to acrolein over NbOPO4catalysts

“…Characterization of the NbP catalysts suggests that the active species correspond to O=P=O (Fig. ) species, which generates Brønsted acid sites which are active and selective for the dehydration reaction . The phosphate based niobium catalysts are found to be more active and stable than the bulk niobium catalysts in acid catalyzed reactions …”

“…3) species, which generates Brønsted acid sites which are active and selective for the dehydration reaction. 23,24 The phosphate based niobium catalysts are found to be more active and stable than the bulk niobium catalysts in acid catalyzed reactions. 14 As discussed earlier the XRD results suggest the amorphous nature of the catalysts prepared by calcination at 350-650 • C, and also exhibits high surface area and pore volume.…”

Kinetics of the dehydration of 2-propanol on modified activated charcoal containing acid sites

“…This is especially true for the preparation of systems with chemically-attached surface groups. As shown in our previous work [7], samples of activated bone charcoal (ABC) with supported acid sites are extremely active catalysts for the dehydration of 2-propanol in comparison with oxide systems [2][3][4]: the reaction onset temperature (T o ) and temperature for 100% conversion of 2-propanol to propylene (T 100% ) are 100-110°C and 150-170°C, respectively (Table 1). Samples of ABC oxidized with H 2 O 2 or HNO 3 as well as samples containing polymethylstyrene (PMS) on the surface were used as the matrices for the deposition of acid sites.…”

“…In contrast to typical acid-base catalysts such as metal oxides and cation exchangers [2][3][4][5][6], a functional coating not only containing chemically-attached highly acid sites but also possessing considerable thermal and hydrolytic stability in a broad pH range may be produced on the AC surface. The insignificant use of acid AC presently in catalysis is a function of the paucity of approaches and methods for the preparation of materials derived from AC with a given structure of the surface layer.…”