VMgO Catalysts Research Articles

Characterization and reactivity of VMgO catalysts prepared by wet impregnation and sol–gel methods

ABSTRACTThis paper reports the study of physicochemical, surface, and catalytic properties of two series of VMgO catalysts prepared by two different methods: wet impregnation and sol–gel. The characterizations of the elaborated materials were performed using N2-sorption (Brunauer, Emmett and Teller (BET)), X-ray diffraction, Raman, transmission electron microscopy–energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy analyses. The catalytic properties of the elaborated materials were investigated in the isopropanol decomposition reaction to determine their acid–base character and in the selective oxidation of n-butane to evaluate their dehydrogenation properties. The preparation method and vanadium content strongly affected the properties of our materials. The sol–gel method leads to smaller crystallite size, higher specific surface area, and uniform particle distribution compared to the impregnation one. Both impregnation and SG solids promote the formation of acetone, which is related to the presence of strong basic sites (O2− species) on the catalytic exposed surface. The more pronounced basic character was obtained through the SG samples. The sol–gel samples exhibited the highest catalytic activity and C4-olefin selectivity in the partial oxidation of n-butane. Whatever the preparation procedure, the nature of surface oxygen species plays an important role in the orientation of catalytic performances.

Oxidative dehydrogenation of propane over three-dimensionally ordered macroporous VMgO catalysts with different vanadium doping

A series of three-dimensionally ordered macroporous (3DOM) VMgO catalysts were prepared by the colloidal crystal templating method. The obtained catalysts were characterized by means of BET, SEM, TEM, FT-IR, XRD and H2-TPR. The results show that the amount of vanadium species can have an impact on the morphology of the 3DOM VMgO catalysts, i.e. particle size and porous structure. The catalyst with 32 wt% V2O5 shows the highest catalytic performance with the yield of propene amounting to 11.3%. Compared with the amorphous conventional VMgO, the 3DOM structured catalysts exhibited higher propene selectivity due to that the efficient diffusion of intermediates and inhibition of consecutive propene oxidation. The FT-IR, XRD and H2-TPR data confirm that isolated VO4 tetrahedral groups are the main promoting factor in the catalysis. Further, the 3DOM VMgO catalysts show high stability in the catalysis.

Oxidative dehydrogenation of n-butane to butenes on Mo-doped VMgO catalysts

A small amount of Mo-doping in a VMgO catalyst significantly results in an increase in the total butene selectivity.

Effects of boron and barium dopants on VMgO catalysts employed in the oxidative dehydrogenation of n-octane

Boron and barium were employed as dopants for the VMgO system. The catalysts were characterized by ICP-OES, BET, IR, powder XRD, EDX, TPR-H2, TPD-NH3, XPS, and 51V MAS NMR. These catalysts were used for the oxidative dehydrogenation of n-octane in a continuous flow fixed bed reactor using air as an oxidant. Catalytic tests were carried out at a GHSV of 8000 h−1 and an n-octane to oxygen ratio of 0.8. The dopants induced some changes in the catalysts’ properties, e.g. the acid-base character, the reducibility, and the symmetry and the heterogeneity of the VO4 tetrahedra. Changes in the catalytic performance were also observed. Doping with barium enhanced styrene selectivity and yield, while boron adversely affected the activity and the ODH selectivity of the VMgO system. The catalytic activities may be explained by the reducibility of the vanadium species.

Effects of Boron and Barium Dopants on VMgO Catalysts Employed in the Oxidative Dehydrogenation ofn-octane

Effects of Boron and Barium Dopants on VMgO Catalysts Employed in the Oxidative Dehydrogenation of<i>n</i>-octane

Oxidative dehydrogenation and aromatization of n-octane over VMgO catalysts obtained by using different MgO precursors and different precursor treatments

VMgO catalysts with a V2O5 loading of 15% were prepared by the wet impregnation method. Catalysts synthesis was carried out by employing different MgO precursors (MgC2O4 and Mg(OH)2) and different precursors treatments (under vacuum and under static air). The synthesized catalysts were characterized by ICP-OES, BET, powder XRD, SEM, EDX, 51V NMR, H2-TPR, and NH3-TPD. The catalysts were tested for the oxidative dehydrogenation of n-octane using a continuous flow fixed bed reactor at GHSV of 8000h−1 and n-octane/O2 molar ratio of 0.8. Changing the above synthesis parameters led to the formation of catalysts with different morphologies, textural properties, and acidity. As a consequence, the results of the catalytic testing were also different, and treating the MgO precursors under vacuum induced a general positive effect on the catalytic performance. The aromatization of n-octane was predominantly via C1–C6 bonding.

SiO 2-supported vanadium magnesium mixed oxides as selective catalysts for the oxydehydrogenation of short chain alkanes

Unsupported and SiO 2-supported VMgO mixed oxides catalysts have been prepared, characterized and tested in the oxidation of propane, n-butane and propylene. Mg-vanadates are observed by XRD, IR and Raman spectroscopy in both pure and supported VMgO catalysts, although the effective Mg/V ratio, in the VMgO phases formed, depends on the silica content. Since SiO 2 reacts partially with MgO during the calcination step, forming Mg 2SiO 4, the effective Mg/V ratio depends on the silica content. In this sense, pyro-Mg 2V 2O 7 is mainly formed on supported catalysts with Mg/V atomic ratios lower than 4, while ortho-Mg 3V 2O 8 is observed on supported catalysts with Mg/V atomic ratios higher than 4. For this reason, it can be concluded that the Mg/V atomic ratio of crystalline VMgO phases observed on supported catalysts is lower than those observed on unsupported ones. The catalytic tests for the oxidation of propane and n-butane indicate that the catalytic behavior of supported catalysts depends on both the SiO 2 content and the Mg/V atomic ratio, although a different behavior during the oxidation of n-butane or propane is observed: the most selective catalysts for the oxydehydrogenation (OXDH) of propane were those with ortho-Mg 3V 2O 8/ pyro-Mg 2V 2O 7/MgO or ortho-Mg 3V 2O 8/MgO mixed phases while the most selective catalysts for the OXDH of n-butane are formed by ortho-Mg 3V 2O 8/MgO mixed phases. On the other hand, V-containing compounds with VO double bonds are required in order to obtain selective catalysts for the oxidation of propylene to acrolein. In the selective oxidation of propylene, acrolein is not formed on catalysts in which ortho-Mg 3V 2O 8/MgO mixed phases are mainly detected. However, acrolein was obtained on catalysts in which VO double bonds are present.

Short contact time oxidative dehydrogenation of propane

Short contact time oxidative dehydrogenation of propane has been investigated over a Pt/10%Rh gauze catalyst and a catalyst consisting of VMgO supported on a monolithic substrate. The main focus has been on obtaining information about the roles of homogeneous and heterogeneous reactions by comparing the behavior of the two catalysts with blank runs with no active catalyst in the reactor. The reaction was studied at temperatures ranging from 500 to 950°C and with propane/oxygen ratios between 1.1 and 2.3 at atmospheric pressure. It was observed that ethene, methane and synthesis gas (with a hydrogen/carbon monoxide ratio close to 1.0) were the main products at high conversions of propane. Propene becomes increasingly more important at lower conversions, especially in the presence of an active VMgO catalyst. Very low selectivities to ethene and high selectivities to carbon oxides were obtained with the VMgO catalyst at low conversions of propane (<20%). At high temperatures and high conversions no significant effect of the presence of neither the noble metal catalyst nor the VMgO catalyst was found. Small amounts of ethyne, propyne and benzene were always observed in the product gas. The conversion of oxygen and the selectivity to carbon oxides were higher over the Pt/10%Rh gauze and the VMgO catalyst than in the empty reactor, particularly at the lower conversions of propane. One important role of both catalysts seems to be to ignite and to facilitate the internal production of heat for the gas phase formation of alkenes. The effect of varying the flow rate on the performance of the VMgO catalysts indicates that the heterogeneous reactions are subject to transport limitations as would be expected at these conditions.

Oxidative Dehydrogenation of Butane over VMgO Catalysts

A VMgO catalyst has been characterized using a battery of both in situ and ex situ techniques in order to gain insight into the relationship between the state of the catalyst surface and the catalytic performance observed during the oxidative dehydrogenation of n-butane. It has been found that treating the catalyst under oxygen-rich or oxygen-lean atmospheres markedly changes the activity, selectivity, and rate of catalyst deactivation. The performance of the catalyst under both types of atmospheres is also notably different. A certain degree of catalyst reduction is desirable for this reaction, because it increases the selectivity to dehydrogenation products by limiting the amount of oxygen available at the catalyst surface. However, the performance of the catalyst under oxygen-depleted reaction atmospheres is not stable, the main cause for catalyst deactivation being the formation of coke. An overall analysis of the observed effects is proposed on the basis of a redox mechanism.

Monte-Carlo methods for simulating the catalytic oxidative dehydrogenation of propane over VMgO catalyst

The oxidative dehydrogenation of propane (ODHP) has been extensively studied by non-steady-state kinetic experiments and by various experimental characterization techniques under conditions as close as possible to reaction conditions. It can be described in terms of the Mars–Van Krevelen mechanism. This work reports on Monte-Carlo simulations of the ODHP over VMgO catalysts, aimed at relating the main experimental insights obtained previously on the catalytic reaction to a theoretical description of the working catalyst. The “theoretical field of catalysis” is explored by handling new Monte-Carlo models with a catalytic reaction graph, 2D pattern recognition of adsorbed molecules, 3D oxygen bulk diffusion and finally the description of the selective, non-selective routes of the ODHP process.

Biphasic synergy catalysis and active sites of VMgO catalysts for oxidative dehydrogenation of propane

Biphasic synergy catalysis and active sites of VMgO catalysts for oxidative dehydrogenation of propane

Role of acid—base, redox and structural properties of VMgO catalysts in the oxidative dehydrogenation of propane

The oxidative dehydrogenation of propane was studied over VMgO catalysts in a wide range of V contents (5–45 wt%). The highest propene yields were observed for samples with low-to-medium V content. Electrical conductivity measurements demonstrated the presence of anionic vacancies, which participated to the redox catalytic cycle. The reoxidation step was shown to be rate limiting. Microcalorimetry measurements and DRIFT spectroscopy showed that an acid/base balance was equally required. Thus, the highest propene yields corresponded to adjusted redox potential combined with a strong Lewis acidity and a mild basicity. These observations, completed by the structural properties of the VMgO solids, led to the description of the active sites for the ODHP reaction.