K. Wcisło

Effect of alkaline promoters on catalytic activity of V2O5/TiO2 and MoO3/TiO2 catalysts in oxidative dehydrogenation of propane and in isopropanol decomposition

Oxidative dehydrogenation of propane has been studied on V2O5/TiO2 and MoO3/TiO2 catalysts, consisting of 1 and 5 monolayers of the deposited oxides, promoted with Li, K and Rb cations. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS) and by isopropanol decomposition, a probe for acid-base properties. For both types of catalysts, irrespective of the vanadium or molybdenum content, the total activity in oxidative dehydrogenation of propane decreased for the promoted catalysts in the sequence: non-promoted ≥ Li > K > Rb-promoted catalyst. The propene yields and selectivities at equal conversion increased in the same order. The same sequence was also observed for the increase in the rate of acetone formation and the decrease in the rate of propene formation in the isopropanol decomposition on the promoted catalysts. It is proposed that the alkaline promoters decrease the acidity and increase the basicity of the catalysts, and hence facilitate desorption of propene from the catalyst surface, preventing it from further oxidation to carbon oxides. The XPS data suggest that the alkaline promoters increase the dispersion of the vanadia or molybdena phase in the high vanadium or molybdenum content catalysts.

Oxidative dehydrogenation of isobutane on supported chromia catalysts

Abstract Oxidative dehydrogenation, ODH, of isobutane has been performed on chromia supported on SiO 2 , Al 2 O 3 , TiO 2 , ZrO 2 and MgO. The potassium promoted (K/Cr= 0.1) catalysts have also been examined. The supported chromia has been found active in the ODH of isobutane at relatively low temperatures 200–400°C, the total activity and selectivity to isobutene depending on the nature of the support and the potassium presence. The highest selectivities to isobutene (ca. 70% at 5% conversion) and the isobutene yields of ca. 9% have been obtained for Cr/O x TiO 2 and K-promoted CrO x /Al 2 O 3 preparations.

Effect of doping of TiO2 support with altervalent ions on physicochemical and catalytic properties in oxidative dehydrogenation of propane of vanadia-titania catalysts

Vanadia phase (one monolayer) was deposited on TiO2 anatase doped with Ca2+, Al3+, Fe3+ and W6+ ions and the catalysts thus obtained (VMeTi) were characterized by XPS, work function technique, decomposition of isopropanol (a probe reaction for acido–basic properties) and tested in oxidative dehydrogenation of propane. The doping of the TiO2 support modifies physicochemical and catalytic properties of the active vanadia phase with respect to the undoped TiO2. The specific activity in the propane oxydehydrogenation decreases in the order: VFeTi>VWTi>VTi>VAlTi>VCaTi (3), whereas the selectivity to propene follows the sequence: VWTi VTi>VFeTi>VAlTi>VCaTi. This implies that the lower is the surface energy barrier for transfer of electrons from the catalyst to the reacting molecules the higher is the selectivity to the partial oxidation product. It is argued that owing to the decrease in this energy barrier the reoxidation step in the catalytic reaction, involving such a transfer: O2+4e→2O2− is fast, thus, preventing the presence of intermediate non-selective electrophilic oxygen species on the surface.

Effect of alkali metals additives to V2O5/TiO2 catalyst on physicochemical properties and catalytic performance in oxidative dehydrogenation of propane

The effect of alkali metal additives Li, K, and Rb to V2O5/TiO2 catalyst on the rate of catalyst reduction with propane and reoxidation with oxygen, sorption of propene, and the electron work function has been examined. The results have been correlated with the catalytic performance in oxidative dehydrogenation, ODH, of propane. It has been found that the rates of reduction, reoxidation and the ODH of propane decrease in the order: VTi>LiVTi>KVTi>RbVTi. The activation energies of the reduction and reoxidation are not, however, affected by the presence of the alkali metals. The same sequence has been observed for the work function values of the catalysts. It is argued that alkali metal poisons the centres of the hydrocarbon activation. The yield and selectivity to propene in the ODH of propane increase, however, for the promoted catalysts, following the above sequence. This effect is ascribed to the decrease in the heat of the propene adsorption, which is due to the increase in the basicity and decrease in acidity on the promoted catalysts.

Oxidation of C2C4 alkanes on chromium oxide/alumina and on Cr2O3: catalytic and TPD studies

Abstract Oxidation of ethane, propane and n-butane on chromium oxide supported on alumina and on unsupported chromia yields carbon oxides as the main reaction products. The selectivity to olefins, the oxidative dehydrogenation products does not exceed about 8% at 10% conversion for propane and n-butane and 1% for the ethane reaction. Oxidative dehydrogenation is the main reaction for the isobutane reaction, the latter hydrocarbon showing, moreover, the highest overall activity. The TPD experiments show that the adsorbed alkane species desorb in the form of CO2 and H2O, only traces of olefins being observed.

Layered double hydroxide-derived vanadium catalysts for oxidative dehydrogenation of propane: Influence of interlayer-doping versus layer-doping

Abstract Mixed-oxide vanadium catalysts for oxidative dehydrogenation (ODH) of propane have been prepared by thermal decomposition of Mg, Al-layered double hydroxides (LDHs) containing vanadium either in the brucite layer or in the interlayer. The materials have been characterised by XRD, ICP-AES chemical analysis, XPS, BET and ESR. The catalytic performance of the samples depended on the manner of incorporation of the vanadium into the LDH structure. The sample obtained from interlayer-doped precursor was more active and more selective than mixed oxides obtained from layer-doped LDHs. The difference in the catalytic properties was attributed to the different magnesium vanadates nucleating in the calcined samples, the pyrovanadate formed from the interlayer-doped LDH giving better performance than ortho-vanadate crystallising from the layer-doped precursor. It has been suggested that one of the factors contributing to the difference in the behaviour of both types of catalysts might be the difference in the covalency of VO in-plane bonds around the reduced V centres.

Oxygen adsorption and catalytic performance in oxidative dehydrogenation of isobutane on chromium oxide-based catalysts

Oxygen adsorption on γ-Al2O3-, TiO2 (anatase)-supported chromium oxide and on unsupported chromia in amorphic and crystalline form, has been studied with a gravimetric method in the temperature range 523–623 K. The equilibrium oxygen uptake at p→∞ was the same for all the studied catalysts, the rate of the adsorption was, however, different, increasing in the order: CrOx/Al2O3<CrOx/TiO2<Cr2O3 amorph.<Cr2O3 cryst. Measurements of oxidative dehyrogenation of isobutane on the catalysts given above have shown that the selectivity to isobutene decreases in the same sequence. The higher selectivity values and the change in the rate constant of the isobutane oxidative dehydrogenation with conversion, observed for the Al2O3- and TiO2-supported chromium oxide catalysts, have been ascribed to the lower coverage of the surface of the catalyst with oxygen in the stationary state of the isobutane oxidative dehydrogenation, as compared with unsupported chromia.

Synthesis and physicochemical properties of vanadium-doped zirconia-pillared montmorillonites in relation to oxidative dehydrogenation of propane

V-doped zirconia-pillared montmorillonites have been obtained by different methods and characterised with XRD, ESR, nitrogen adsorption and chemical analyses. The samples are microporous, their structure, texture and location of V species depending on the method of preparation. Catalytic results in the reaction of the oxidative dehydrogenation of propane to propene are discussed in terms of the physicochemical properties of the materials.

Oxidative dehydrogenation of propane on TiO2-supported chromium and zinc oxides

Oxidative dehydrogenation of propane has been studied on chromium, zinc and zinc-chromium mixed oxides supported on anatase titania. The order of activity and selectivity to propene was CrTi>ZnCrTi>ZnTi. The activity and selectivity to propene did not change markedly with increase in the reaction temperature.

Vanadium-doped titama-pillared montmorillonites as catalysts for the oxidative dehydrogenation of propane

V-containing titania pillared montmorillomtes have been prepared and characterised with XRD, ESR, BET and chemical analysis. Their performance in the oxidative dehydrogenation of propane depends on the manner of doping. Co-pillarmg gives catalysts of little V content which are less active but more selective than those containing more vanadium and prepared by means of cation exchange. The latter ensure highest yields of propene. The increased selectivity of the co-pillared sample is tentatively assigned to the presence of V centres of higher π-covalency within bonds between vanadium and bridging oxygens.