Physicochemical and textural characterization of vanadium–magnesium mixed oxides (original) (raw)
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2009
Abstract. V-Mg-Al mixed-oxide catalysts for oxidative dehydro-genation of propane were prepared by thermal decomposition of Mg-Al-layered double hydroxides with vanadium interlayer doping. The obtained catalysts were tested for the oxidative dehydrogenation of propane, obtaining good results in catalytic activity (conversion 16.55 % and selectivity 99.97 %). Results indicated that catalytic per-formance of these materials depends on how vanadium is integrated in the layered structure, which is determined by the Mg/Al ratio. Vanadium interlayer doping modifies the oxidation state of vanadium and consequently catalytic properties. Surface properties were studied by X-ray photoelectron spectroscopic and diffuse reflectance, UV-vis-ible spectroscopy, and temperature programmed reduction (TPR). The analyses provided information about the oxidation state, before and after the reaction. From these results, it is suggested that selectivity to propylene and catalytic activity depend mainly o...
Journal of the Mexican Chemical Society, 2019
V-Mg-Al mixed-oxide catalysts for oxidative dehydrogenation of propane were prepared by thermal decomposition of Mg-Al-layered double hydroxides with vanadium interlayer doping. The obtained catalysts were tested for the oxidative dehydrogenation of propane, obtaining good results in catalytic activity (conversion 16.55 % and selectivity 99.97 %). Results indicated that catalytic performance of these materials depends on how vanadium is integrated in the layered structure, which is determined by the Mg/Al ratio. Vanadium interlayer doping modifies the oxidation state of vanadium and consequently catalytic properties. Surface properties were studied by X-ray photoelectron spectroscopic and diffuse reflectance, UV-visible spectroscopy, and temperature programmed reduction (TPR). The analyses provided information about the oxidation state, before and after the reaction. From these results, it is suggested that selectivity to propylene and catalytic activity depend mainly on vanadium ox...
Kinetics and Catalysis, 2017
⎯The physicochemical properties of V 2 O 5 /Al 2 O 3 and MgO-V 2 O 5 /Al 2 O 3 supported catalysts (Mg : V = 1 : 1, 2 : 1, and 3 : 2) obtained by consecutive impregnation of the support with solutions of vanadium and magnesium precursors are studied using a complex of mutually complementary methods (XRD, Raman spectroscopy, UV-Vis spectrometry, and TPR-H 2). The effect of the formation of surface magnesium vanadates of various composition and structure on the catalytic properties of the supported vanadium oxide catalysts in the oxidative dehydrogenation of propane is studied. The introduction of magnesium in the samples and an increase in its content, accompanied by a change in the structure of the surface vanadium oxide phases from polymeric VO 6 /VO 5 species to surface metavanadate species, magnesium metavanadate, and further to magnesium divanadate, significantly affects their catalytic properties in the reaction of the oxidative dehydrogenation of propane to propylene.
Applied Catalysis A: General, 2010
In this study, vanadium-magnesium oxide (VMgO) catalysts with different vanadium loadings were synthesized by the impregnation method and characterized by BET, ICP-OES, IR, powder XRD, TEM, SEM, TPR-H 2 , and TPD of ammonia. The catalysts were tested for the oxidative dehydrogenation (ODH) of noctane in a continuous flow fixed bed reactor at GHSV of 6000 h À1 and a temperature range of 350-550 8C using air as an oxidant and nitrogen as a make-up gas to give 9% n-octane in the gaseous mixture. The results showed that the vanadium concentration affects both textural and chemical properties of the catalysts and consequently their catalytic activity and selectivity towards the desired products (octenes and aromatics). The catalyst with a V 2 O 5 concentration of 15% showed the best catalytic activity and productivity for the ODH products; especially for 1-octene and styrene. ß
Catalysis Letters, 1997
Different magnesium vanadate phases, V^Mg^O phases (-Mg 2 V 2 O 7 , Mg 3 V 2 O 8 and -MgV 2 O 6 ), MgO and V 2 O 5 oxides have been compared with respect to their surface properties and their oxygen exchange capacities with C 18 O 2 in the gas phase. By temperature-programmed desorption of carbon dioxide, the absence of any basic impurities (i.e., MgO or residual oxidised K impurities resulting from the preparation) has been evidenced on the surface of magnesium vanadate phases. This demonstrates that the catalytic properties of the magnesium vanadate phases for oxidative dehydrogenation of propane as previously studied cannot be explained by synergetic effects due to the presence of any basic component impurities since they are absent in this case. While on MgO an important surface exchange process occurs with C 18 O 2 of the gas phase, this exchange is very low on V 2 O 5 and pure V^MgÔ phases. A comparison of the different magnesium vanadate phases in the same experimental conditions indicates that the -Mg 2 V 2 O 7 phase (which exhibited the highest selectivity for oxidative dehydrogenation of propane to propene) shows the lowest lattice oxygen exchange with C 18 O 2 of the gas phase. This is another specificity of this phase.
Effect of crystal size on the oxidative dehydrogenation of butane on V/MgO catalysts
Journal of Catalysis, 2004
Producing metal oxide catalysts with nanometer crystal sizes offers an attractive means of controlling catalytic behavior in selective oxidation reactions. In this work, the catalytic behavior of MgO nanocrystal-supported vanadium was compared to that of vanadium supported on conventionally prepared MgO. It was found that nanocrystals gave higher butene selectivity for weight loadings of 5, 15, and 25%, producing less CO, ethylene, and propylene. Both types of catalysts were characterized using nitrogen adsorption, Raman spectroscopy, XRD, TEM, and XANES to determine the structure of vanadium on the magnesium oxide surface. The catalyst surface structure appears to be similar on both types of catalysts, with a magnesium orthovanadate phase supported on a MgO phase. It is hypothesized that the chemical nature of the support, specifically acid/base properties, was responsible for the differences noted in catalytic behavior, though differences in the domain size of Mg 3 (VO 4 ) 2 could also play a role.
Catalysis Letters, 1997
Different magnesium vanadate phases, V^Mg^O phases (-Mg 2 V 2 O 7 , Mg 3 V 2 O 8 and -MgV 2 O 6 ), MgO and V 2 O 5 oxides have been compared with respect to their surface properties and their oxygen exchange capacities with C 18 O 2 in the gas phase. By temperature-programmed desorption of carbon dioxide, the absence of any basic impurities (i.e., MgO or residual oxidised K impurities resulting from the preparation) has been evidenced on the surface of magnesium vanadate phases. This demonstrates that the catalytic properties of the magnesium vanadate phases for oxidative dehydrogenation of propane as previously studied cannot be explained by synergetic effects due to the presence of any basic component impurities since they are absent in this case. While on MgO an important surface exchange process occurs with C 18 O 2 of the gas phase, this exchange is very low on V 2 O 5 and pure V^MgÔ phases. A comparison of the different magnesium vanadate phases in the same experimental conditions indicates that the -Mg 2 V 2 O 7 phase (which exhibited the highest selectivity for oxidative dehydrogenation of propane to propene) shows the lowest lattice oxygen exchange with C 18 O 2 of the gas phase. This is another specificity of this phase.
Journal of Non-crystalline Solids, 2010
Substoichiometric magnesium vanadate catalysts, (Mg-Vi with i = 1, 2, and 3) with different V/Mg atomic ratios and stoichiometric (Mg 3 V 2 O 8 , Mg 2 V 2 O 7 and MgV 2 O 6), were prepared by citric acid technique. Characterization showed that substoichiometric samples correspond to well-dispersed VO 4 units in the MgO matrix. Moreover, taking into account the Raman data, the coexistence of ortho, pyro and meta vanadate phases cannot be discarded. The Mg 3 V 2 O 8 sample was more active for the oxidative dehydrogenation of nbutane, but less selective to C 4 alkenes than the Mg 2 V 2 O 7 sample. Stoichiometric samples produce mainly 1butene, whereas for substoichiometric samples butadiene was the major reaction product for reaction temperatures higher than 748 K. The easily reducible VO 4 species, and eventually the coexistence of different vanadates phases, appear to be responsible for the catalytic behaviour displayed by the substoichiometric catalysts prepared by citrate technique.
Catalysis Letters, 2014
Vanadium oxide-based catalyst obtained by grafting VOCl 3 on Florisil (MgO:SiO 2 ) with the molar ratio of 15:85 have been studied for the selective oxidation of cyclohexane in order to obtain cyclohexyl hydroperoxide, cyclohexanol and cyclohexanone. The performances obtained have been compared with those of other catalysts in which vanadium oxide was supported on the same support by impregnation with ammonium oxalate. All the prepared catalysts have been characterized using XRD, FTIR, TEM, SEM, EDX, DRS and TGA in order to rationalize the differences in performance observed. The presence of magnesium oxide species (15 % MgO) on the surface of silica significantly modifies the molecular structure of the surface vanadium oxide species and changes their molecular structure from hydrated VO 5 /VO 6 polymers to less polymerized VO 4 species and/or isolated VO 2 (OH) 2 species. The catalytic activities indicate that the VO x /Florisil catalysts show high conversions and TONs for those types containing isolated VO 4 . Compared to previously studied VO x /SiO 2 catalysts, the Florisil based systems show significantly improved leaching behavior.
Journal of Molecular Catalysis A-chemical, 2010
MgO-supported vanadium oxide catalysts with different vanadium loadings have been prepared by wet impregnation and grafting using ammonium metavanadate or VOCl 3 as precursor, respectively. The prepared catalysts have been characterized by several techniques such as XRD, FT-IR, DR-spectroscopy, BET, 51 V solid-state NMR, SEM and EDX. For comparison, three different phases of magnesium vanadate (meta--MgV 2 O 6 , pyro-␣-Mg 2 V 2 O 7 , and ortho-Mg 3 V 2 O 8 ) have been synthesized as reference materials for the structural characterization. The catalytic activity of the prepared catalysts was evaluated for the liquidphase oxidation of cyclohexane as a model reaction to obtain cyclohexylhydroperoxide, cyclohexanol and cyclohexanone with PCA as co-catalyst. The results show that VO x /MgO catalysts containing isolated VO 4 species either impregnated or grafted do not show considerable activity in the oxidation process. In contrary, catalysts with higher vanadium loading, which contain ortho-Mg 3 V 2 O 8 , pyro-Mg 2 V 2 O 7 and V 2 O 5 phases as surface species show conversions up to 70% and TONs up to 34,700. Compared to previously studied support materials for this catalytic system the leaching behavior is significantly improved. (R. Pietschnig). the course of our investigations it occurred to us that the basicity of the support material has a substantial influence on the catalytic performance and moreover on the leaching rate which can be a substantial problem for supported vanadium oxide catalysts. These findings prompted us to consider rather basic support materials like MgO as support materials for vanadium oxide species, which to our knowledge have not been used for this purpose so far.