Role of potassium on the structure and activity of alumina-supported vanadium oxide catalysts for propane oxidative dehydrogenation

The influence of potassium on the structure and properties of alumina-supported vanadium oxide catalysts has been studied by in situ Raman spectroscopy, temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), a probe reaction of acid/base–redox sites (methanol chemisorption) and tested in oxidative dehydrogenation (ODH) of propane. Potassium coordinates to surface vanadium oxide species altering its structure but does not form bulk compounds, possibly because the total V+K coverage does not reach the monolayer coverage on alumina. The interaction of K with V weakens the terminal V=O bond. K-doped alumina (KAl)-supported vanadia catalysts show lower acidity, a decrease of reducibility and a decrease of propane conversion values. These trends do not correspond with the changes in the terminal V=O bond energy. Thus, it appears that the terminal V=O bond of surface vanadium oxide species is not the active site for propane ODH, oxidation of methanol to formaldehyde and for the reduction of surface vanadium oxide species by hydrogen. Potassium also changes the acid–base characteristic of the system and decreases the acidic character of surface vanadia. This shift in the acid–base character to a more basic system must also account for the better selectivity in propane ODH due to a variation in the interaction between the intermediates and the surface.     

Characterization and FT-IR study of nanostructured alumina-supported V-Mo-W-O catalysts

The effect of tungsten incorporation in the surface composition and its catalytic performance is evaluated for alumina supported Mo-V-O, Mo-W-O, V-W-O and Mo-V-W-O nanostructurated-oxide catalysts. The characterization results reveal that the surface of Mo-V-W-O catalysts is further different from the binary counterparts, due to the presence of stabilized and reduced structures dispersed on the support. Such species are not present in the Mo-V-O catalysts; indicative that tungsten acts as a structural–chemical promoter. The in situ FT-IR study of these catalysts under propane + oxygen atmosphere showed that Mo-V-W-O catalyst is able to thermally activate the propane oxotransformation and ODH-products are registered in the gas-phase whereas oxygenate-compounds are detected on the surface of catalysts.     

Effect of the oxide support on the propane ammoxidation with Sb–V–O-based catalysts

The role of Nb₂O₅ and γ-Al₂O₃ oxide supports on the ammoxidation of propane on supported mixed Sb–V oxide at different Sb+V surface coverages is studied. Sb and V oxide species on alumina and on niobia support show different structural features that reflect in different performance during the ammoxidation of propane to acrylonitrile. Niobia-supported catalysts are much more selective to acrylonitrile than alumina-supported ones. Alumina interacts weakly with the supported oxides while niobia forms new phases through solid state reactions with the supported oxides during catalytic operation that must account for its higher selectivity values towards acrylonitrile and higher specific rate of acrylonitrile formation per vanadium site.     

V–Al–O catalysts prepared by flame pyrolysis for the oxidative dehydrogenation of propane to propylene

A flame pyrolysis (FP) procedure has been set up for the preparation of V/Al/O catalysts to be employed for the oxidative dehydrogenation of propane to propylene. The samples have been characterised by means of various techniques (FT-IR, Raman, EPR, ICP-MS, TGA, XRD, SEM) and their catalytic activity has been evaluated in two different operating modes, i.e. under anaerobic conditions and by co-feeding oxygen. The particle size distribution became progressively more homogeneous with increasing V concentration, due to the catalytic effect of the V ions during the FP synthesis. Some V₂O₅ segregation was observed even at low V loading. However, higher V dispersion was attained with respect to a reference sample prepared by impregnation of the FP-prepared alumina support.The increase of V concentration always led to an improvement of propane conversion, though selectivity showed different trends depending on the operating conditions. The comparison with the sample prepared by impregnation showed similar catalytic activity, with a bit higher selectivity for the FP-prepared sample under anaerobic conditions.     

Support effects on structure and activity of molybdenum oxide catalysts for the oxidative dehydrogenation of ethane

The structural and catalytic properties of MoO₃ catalysts supported on ZrO₂, Al₂O₃, TiO₂ and SiO₂ with Mo surface densities, n_s, in the range of 0.5–18.5 Mo/nm² were studied for the oxidative dehydrogenation (ODH) of ethane by in situ Raman spectroscopy and catalytic activity measurements at temperatures of 400–540 °C. The molecular structure of the dispersed surface species evolves from isolated monomolybdates (MoO₄ and MoO₅, depending on the support) at low loadings to associated MoO_x units in polymolybdate chains at high loadings and ultimately to bulk crystalline phases for loadings exceeding the monolayer coverage of the supports used. The nature of the oxide support material and of the Mo–O–support bond has a significant influence on the catalytic behaviour of the molybdena catalysts with monolayer coverage. The dependence of reactivity on the support follows the order ZrO₂ > Al₂O₃ > TiO₂ > SiO₂. The oxygen site involved in the anchoring Mo–O–support is of relevance for the catalytic activity.     

Alumina supported Mo–V–Te–O catalysts for the ammoxidation of propane to acrylonitrile

The effect of Te addition over Mo–V–O catalysts supported on alumina is discussed for the ammoxidation of propane to acrylonitrile. Catalyst composition and atmosphere of activation are evaluated. Catalysts are characterized before and after catalytic reaction by XPS, XRD and in situ Raman spectroscopies. The absence of Te in catalysts formulation and the presence of a high amount of vanadium induce the presence of V⁵⁺ species and the formation of V₂O₅ oxide; associated with a decrease in acrylonitrile selectivity. The presence of Mo-based polyacids structures decreases the selectivity to acrylonitrile. V⁵⁺ sites are responsible of propane activation and of the subsequent -H abstraction to form the intermediate propylene. Then, a Mo–V rutile-like structure in which vanadium species are reduced as V⁴⁺, is responsible for nitrogen insertion and acrylonitrile formation. The formation of such structure is favoured when Te is added to catalysts and is promoted during propane ammoxidation.     

Nanoparticles of vanadia–zirconia catalysts synthesized by polyol-mediated route: Enhanced selectivity for the oxidative dehydrogenation of propane to propene

A polyol-mediated route was employed to obtain nanoparticles of vanadia–zirconia (10 nm) and Ti⁴⁺-modified zirconia catalyst for the selective oxidative dehydrogenation reaction of propane to propene. The catalytic activity and selectivity of samples thus prepared were compared with the values for the sample synthesized by the conventional impregnation method. More dispersed amorphous vanadia species on zirconia support could be obtained by polyol method compared to those obtained by conventional impregnation route, as discerned from transmission electron microscopy (TEM) and Raman studies. Raman spectra of samples prepared by polyol method indicated the presence of monovanadate and polyvanadate species on the zirconia support surface, whereas the impregnated sample showed the existence of aggregated vanadia besides mono and polyvanadate species though the vanadia loading was the same on all samples. XPS studies revealed that vanadia existed as both V⁵⁺ and V⁴⁺ in the samples prepared by the polyol method, whereas only V⁵⁺ state was seen in the impregnated sample. The catalysts prepared by polyol method exhibited enhanced selectivity for propene formation compared to the sample prepared by impregnation method. The enhanced selectivity is attributed to the presence of dispersed vanadia species with lower valence state of vanadium. The present results demonstrate that the polyol-mediated synthesis is an efficient method for the preparation of supported vanadia catalysts containing such active species.     

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: VWTiVTi>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.     

High performance of V–Ga–O catalysts for oxidehydrogenation of propane

The catalytic performance in the oxidehydrogenation (ODH) of propane of vanadium oxide catalysts supported on gallium oxide, VO_x/Ga₂O₃, with vanadium coverages lower or near the theoretical monolayer has been studied as a function of the vanadium content and compared with those of other known effective V–M–O (M=Mg, Ca) catalysts. Catalyst activity was very high and increased with the increase of vanadium loading in the range studied, while the selectivity trend was similar for the studied catalysts, excepting that with the lower V content. FT-Raman and ⁵¹V solid state NMR spectroscopies show that for coverages below the theoretical monolayer vanadium atoms are in tetrahedral co-ordination either in isolated or polymeric species, while the onset of vanadia formation is detected above that coverage. Interestingly, these catalysts show an one order of magnitude higher area-specific rate, similar initial olefin selectivity and slightly higher selectivity decrease with the increase of conversion than the best VMgO catalyst. This is due to the high intrinsic activity of isolated tetrahedral vanadium species. The combination of these factors produces an enhanced olefin productivity of V–Ga–O catalysts.     

Solid state chemistry of bulk mixed metal oxide catalysts for the selective oxidation of propane to acrylic acid

Syntheses of Mo–V–Sb–Nb–O bulk materials, which are candidate catalyst systems for the selective oxidation of propane to acrolein and acrylic acid, were made using soluble precursor materials. The products were characterized by X-ray powder diffraction and Raman spectroscopic studies. The objectives of this work were to explore the utility of liquid phase automated synthesis for the preparation of bulk mixed metal oxides, and the identification of the oxide phases present in the system. This is the first published study of the phase composition for these materials. After calcination of these bulk oxides under flowing nitrogen at 600°C, and using stoichiometric ratios of Mo–V–Sb–Nb (1:1:0.4:0.4) and Mo–V–Sb–Nb (3.3:1:0.4:0.4) it was demonstrated that a mixture of phases were obtained for the syntheses. X-ray powder diffraction studies distinguished SbVO₄, Mo₆V₉O₄₀, MoO₃, and a niobium-stabilized defect phase of a vanadium-rich molybdate, Mo_0.61–0.77V_0.31–0.19Nb_0.08–0.04O_x, as the major phases present. Complementary data were provided by the Raman spectroscopic studies, which illustrated the heterogeneity of the phases present in the mixture. Raman also indicated bands attributable to the presence of phases containing terminal M=O bonds as well as M–O–M polycrystalline phases. Previous studies on this system have identified SbVO₄ and niobium-stabilized vanadium molybdate species as the active phases necessary for the selective oxidation of alkanes.     

MoOx-based catalysts for the oxidative dehydrogenation (ODH) of ethane to ethylene: Influence of vanadium and phosphorus on physicochemical and catalytic properties

The influence of vanadium and phosphorus on the physicochemical properties of the MoO_x oxide and on its catalytic properties in the oxidation of ethane to ethylene is examined. A series of MoO_x, MoVO_x (Mo/V = 11) and MoVPO_x (Mo/V = 11, V/P = 1) catalysts were prepared, characterized by several techniques (BET, XRD, XPS, LRS and ATG) and studied in the oxidative dehydrogenation of ethane to ethene at atmospheric pressure and at the temperature of reaction of 550 °C. Their structural properties, during reduction and re-oxidation, were examined by in situ X-ray diffraction and by X-ray photoelectron spectroscopy after pre-treatment. The sample containing phosphorus is the most active (conversion 14%) and selective to ethylene (SC₂H₄ = 67%). The formation of [PMo₁₁VO₄₀]⁴⁻ is assumed during preparation, and its decomposition during calcination leads to well dispersed phosphate groups and improved interactions between Mo and V species. During the catalytic reaction Mo^VI is stabilised by means of solid solutions of V in Mo₅O₁₄ and in MoO₃ (V_xMo_1−xO_3−x/2). A synergetic effect between these two phases could be responsible for the best performance of Mo₁₁VPO_x as compared to those of MoO_x and Mo₁₁VO_x.     

V–Mg–O catalysts for oxidative dehydrogenation of alkylpyridines and alkylthiophenes

Vanadium appears to be the element that is most frequent (along with molybdenum) used in the catalyst formulations for oxidative dehydrogenation (ODH) of hydrocarbons and alcohols. In the present work the employment of ODH reaction in the presence of air has been extended for the preparation of vinyl substituted pyridines and thiophenes using vanadium (and for comparison molybdenum) oxide catalysts.The efficiency of vanadium–magnesium oxide catalysts in the production of vinylpyridines and vinylthiophenes has been evaluated. A strong dependence of the yield and selectivity of the latter upon the vanadium (molybdenum) oxide loading and the conditions of heat treatment were observed. In optimized reaction conditions V–Mg–O catalysts at the temperature approximate 450 °C ensured the formation of vinylpyridines and vinylthiophenes with the yield of 40–60% at the selectivity of 90%. In prolonged runs no visible changes in the performance of the catalyst were observed. DTA–DTG, XRD, IR ESR, NMR methods have been used detecting the formation of species of V–Mg–O catalysts that appear to be responsible for the catalyst efficiency in the reactions under consideration.     

Alumina-supported catalysts for propane oxidative dehydrogenation from mixed VXM(6−X)O19 (M=W, Mo) hexametalate precursors

γ-Al₂O₃ supported vanadium oxides were modified by tungsten and molybdenum oxides in order to improve dispersion and selectivity towards olefins in propane oxidative dehydrogenation (ODH). Both vanadium–tungsten and vanadium–molybdenum catalysts were obtained by adsorption of mixed isopolyanions (VW₅O₁₉⁵⁻, V₂W₄O₁₉⁴⁻, VMo₅O₁₉⁵⁻ and V₂Mo₄O₁₉⁴⁻) from aqueous solutions. The isopolyanion solutions were characterized by UV-Vis and ⁵¹V NMR spectroscopy. Vanadium, vanadium–tungsten and vanadium–molybdenum precursors and catalysts were also characterized by UV-Vis (diffuse reflectance) and solid state ⁵¹V NMR spectroscopy. An improved selectivity to propene in the presence of tungsten and molybdenum in VO_x/γ-Al₂O₃ was observed and attributed to dilution of vanadium by tungsten or molybdenum oxides on the γ-Al₂O₃ surface.     

Selective oxidation of propane over alkali-doped Mo–V–Sb–O catalysts

Alkali metal-doped MoVSbO catalysts have been prepared by impregnation of a MoVSbO-mixed oxide (prepared previously by a hydrothermal synthesis) and finally activated at 500 or 600 °C in N₂. The catalysts have been characterized and tested for the selective oxidation of propane and propylene. Alkali-doped catalysts improved in general the catalytic performance of MoVSbO, resulting more selective to acrylic acid and less selective to acetic acid than the corresponding alkali-free MoVSbO catalysts. However, the specific behaviour strongly depends on both the alkali metal added and/or the final activation temperature. At isoconversion conditions, catalysts activated at 600 °C present selectivity to acrylic acid higher than that achieved on those activated at 500 °C, both K-doped catalysts presenting the highest yield to acrylic acid. The changes in the number of acid sites as well as the nature of crystalline phases can explain the catalytic behaviour of alkali-doped MoVSbO catalysts.     

Characterisation of vanadium-oxide-based catalysts for the oxidative dehydrogenation of propane to propene

Topics in Catalysis - The study is based on the previous development of α-Al2O3-supported multi-metal-oxide materials for oxidative dehydrogenation of propane by applying a combinatorial...     

Operando Raman–GC studies of alumina-supported Sb-V-O catalysts and role of the preparation method

The interactions between Sb and V are studied by operando Raman–GC methodology during propane ammoxidation in order to understand the effect of the preparation method and reaction conditions on the structure and activity/selectivity of alumina-supported Sb-V-O catalysts. Dispersed V(V) species react with antimony species during propane ammoxidation to form VSbO₄; partially reversible transformations towards surface vanadium (V) species may account for the catalytic redox cycle. The catalytic performance is determined by the interaction between Sb and V, which is affected by the preparation method and the reaction conditions.     

Fundamental and combinatorial approaches in the search for and optimisation of catalytic materials for the oxidative dehydrogenation of propane to propene

An evolutionary approach was applied to create five generations of -Al₂O₃-supported multi-metal-oxides to be used as catalytic materials for the oxidative dehydrogenation of propane at 773 K. Each generation consisted of 56 differently composed materials, i.e., a total amount of 280 materials. These catalytic materials were tested in parallel. For the best materials propene yields from 7% (1st generation) to 9% (5th generation) were achieved. Some of these superior catalysts were characterised by XRD, XPS and EPR. A correlation between catalytic performance and the Mg/V ratio on the surface was found. Based on the structural knowledge obtained, from which the requirement of isolated or at least weakly interacting vanadium sites was derived, VO_x (2.8 wt.%)/MCM-48 and VO_x (2.8 wt.%)/MCM-41 catalysts with a high dispersion of vanadia were used as reference giving a maximal propene yield of 17 and 15%, respectively.     

From Structure–Activity to Structure–Selectivity Relationships: Quantitative Assessment,Selectivity Cliffs,and Key Compounds

The exploration of structure–activity relationships (SARs) in chemical lead optimization is mostly focused on activity against single targets. Because many active compounds have the potential to act against multiple targets, achieving a sufficient degree of target selectivity often becomes a major issue during optimization. Herein we report a data analysis approach to explore compound selectivity in a systematic and quantitative manner. Sets of compounds that are active against multiple targets provide a basis for exploring structure–selectivity relationships (SSRs). Compound similarity and selectivity data are analyzed with the aid of network‐like similarity graphs (NSGs), which organize molecular networks on the basis of similarity relationships and SAR index (SARI) values. For this purpose, the SARI framework has been adapted to quantify SSRs. Using sets of compounds with differential activity against four cathepsin thiol proteases, we show that SSRs can be quantitatively described and categorized. Furthermore, local SSR environments are identified, the analysis of which provides insight into compound selectivity determinants at the molecular level. These environments often contain “selectivity cliffs” formed by pairs or groups of similar compounds with significantly different selectivity. Moreover, key compounds are identified that determine characteristic features of single‐target SARs and dual‐target SSRs. The comparison of compounds involved in the formation of selectivity cliffs often reveals chemical modifications that render compounds target selective.