This paper summarizes the main data appeared in the last years on the oxidative dehydrogenation (ODH) of short chain alkanes on supported vanadium oxide catalysts. The acid-base character of metal oxide support influences the dispersion of vanadium on the surface of the support, as well as the nature of the vanadium species. The reducibility and structure of surface vanadium oxide species and the acid-base character of catalysts, in addition to their catalytic properties in the ODH of C2–C4 alkanes, strongly depend on the metal oxide used as support and the vanadium loading. In this way, it appears that tetrahedral V5+-species are active and selective sites in the ODH of C2–C4 alkanes. The effect of the coordination number and aggregation state of surface vanadium oxide species, and the presence of acid/base sites on the catalytic behavior of supported vanadium oxide catalysts are discussed. It is concluded that these are important factors that must be considered to develop selective catalysts in ODH reactions.

Oxidative dyhydrogenation of short chain alkanes on supported vanadium oxide catalysts

This review summarizes published data on catalytic performances of different vanadium-containing oxides, including unsupported and supported V2O5, vanadates, solid solutions and mixed phases, in the oxidative dehydrogenation of C2–C5 alkanes. The analysis of the structure-activity relationships shows that various species characterized by a different reactivity exist on the surface of these catalysts. There are some indications that tetrahedral vanadium species are the most favourable for alkane oxidehydrogenation. Its intrinsic activity seems to be largely dependent on the structure of the nearest surrounding and, therefore, can be modified by changing the nature and number of the neighbouring ions. Although the reaction mechanism is generally accepted to be through the redox cycle between V5+ and V4+, opposite relationships between catalyst activity/selectivity and reducibility have been established, even for the same catalytic systems. No clear interpretation regarding the mode of alkane activation can be derived from a survey of the literature. This and some other aspects of the reaction mechanism need further studies at a molecular level.

Oxidative dehydrogenation of lower alkanes on vanadium oxide-based catalysts. The present state of the art and outlooks

The surface chemical composition and physicochemical properties (hydrophobicity and zeta potential) of two lactic acid bacteria, Lactococcus lactis subsp. lactis bv. diacetilactis and Lactobacillus helveticus, have been investigated using cells harvested in exponential or stationary growth phase. The surface composition determined by X-ray photoelectron spectroscopy (XPS) was converted into a molecular composition in terms of proteins, polysaccharides, and hydrocarbonlike compounds. The concentration of the last was always below 15% (wt/wt), which is related to the hydrophilic character revealed by water contact angles of less than 30 degrees. The surfaces of L. lactis cells had a polysaccharide concentration about twice that of proteins. The S-layer of L. helveticus was either interrupted or crossed by polysaccharide-rich compounds; the concentration of the latter was higher in the stationary growth phase than in the exponential growth phase. Further progress was made in the interpretation of XPS data in terms of chemical functions by showing that the oxygen component at 531.2 eV contains a contribution of phosphate in addition to the main contribution of the peptide link. The isoelectric points were around 2 and 3, and the electrophoretic mobilities above pH 5 (ionic strength, 1 mM) were about -3.0 x 10(-8) and -0.6 x 10(-8) m(2) s(-1) V(-1) for L. lactis and L. helveticus, respectively. The electrokinetic properties of the latter reveal the influence of carboxyl groups, while the difference between the two strains is related to a difference between N/P surface concentration ratios, reflecting the relative exposure of proteins and phosphate groups at the surface.

Surface of lactic acid bacteria: relationships between chemical composition and physicochemical properties.

In the present work, a systematic study on microwave properties of Ca1-xBixMo1-xVxO4 (0.2 ≤ x ≤ 0.5) solid solution ceramics synthesized by using the traditional solid-state reaction method was conducted. A scheelite structured solid solution was formed in the composition range 0.2 ≤ x ≤ 0.5. We successfully prepared a microwave dielectric ceramic Ca0.66Bi0.34Mo0.66V0.34O4 with a temperature coefficient of resonant frequency (TCF) near to zero and a low sintering temperature by using (Bi, V) substituted (Ca, Mo) in CaMoO4 to form a solid solution. The Ca0.66Bi0.34Mo0.66V0.34O4 ceramic can be well sintered at only 870 °C and exhibits good microwave dielectric properties with a permittivity (er) ˜21.9, a Qf ˜18,150 GHz (at 7.2 GHz) (Q = quality factor = 1/dielectric loss; f = resonant frequency), a TCF ˜ + 0.1 ppm/°C. The chemical compatibility with silver indicated that the Ca0.66Bi0.34Mo0.66V0.34O4 ceramic might be a good candidate for the LTCC applications.

Microwave dielectric properties of low firing temperature stable scheelite structured (Ca,Bi)(Mo,V)O4 solid solution ceramics for LTCC applications

51V Solid state NMR studies of vanadia based catalysts

Vanadium oxide supported on amorphous aluminium phosphate (0.2–15 wt.-% V) has been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), surface area, Fourier transform (FT) Raman and FT-IR spectroscopy, and used for catalytic oxidation of propane, n-butane and toluene. At low vanadium concentrations, ⩽ 2 wt.-% V, formation of highly dispersed vanadyl species is indicated by Raman and XPS measurements. The vanadyl species is after dehydration/calcination characterized by a Raman band at 1031 cm−1, At 6 wt.-% V the amount of highly dispersed vanadyl species is lower and tetrahedral vanadium species and crystalline V2O5 are formed. The tetrahedral vanadium species is characterized by an infrared band at 929 cm−1 , and the amount appears to increase somewhat up to 15 wt.-% V. Both Raman and XRD indicate a strong increase in V2O5 formation at 10 and 15 wt.-% V. XRD also reveals partial crystallization of A1PO4 affected by high vanadium concentrations. FT-IR studies of carbon monoxide adsorbed on pretreated catalysts (oxidation (O2) at 500°C/ reduction (CO) at 500°C) indicated that vanadium species had reacted with aluminium and phosphorus surface hydroxyl groups, forming Al-O-bonded and P-O-bonded species. The latter is abundant at low vanadium loading, but after a more severe pretreatment involving oxidation at 800°C followed by reduction at 500°C, the Al-O bonded species is dominating. Further evidence for formation of Al-O-bonded and P-O-bonded vanadium species is the decrease of P-OH and Al-OH IR band intensities. The activities for oxidation of propane, n-butane and toluene increase with the vanadium loading. Acidic support sites show activity for dealkylation of both toluene and propane. Oxidative dehydrogenation of propane is best performed at 6 wt.-% V, which is associated with the presence of tetrahedral vanadium species. A lower selectivity at higher loading is associated with more active and less selective sites on V2O5.

Zhi-guang Yan

Papers

Characterization of vanadia supported on amorphous AlPO4 and its properties for oxidative dehydrogenation of propane

Catalytic oxidation of toluene over V2O5-WO3 catalysts