Oxydehydrogenation of propane over Mg-V-Sb-oxide catalysts. I. Reaction network

Magnesium vanadates have been shown by various groups to be active oxydehydrogenation catalysts for the conversion of light paraffins to the corresponding olefins. The olefins produced have significant commercial value in petroleum and petrochemical industry. Recently, we reported that doping of the magnesium vanadates with antimony, antimony-phosphorus, or boron, produces catalysts with significantly better selectivities to olefins than those of the parent undoped catalysts. Among these, the composition Mg₄V₂SbO_x was selected for further study of propane oxydehydrogenation and its reaction network. At 500°C and atmospheric pressure, the selectivity to propylene decreases monotonically from 75% to 5% as propane conversion is increased from 2% to 68%. An analysis of the reaction network reveals, that propylene is the only useful first formed product, that CO_x is produced largely by sequential oxidation of the in situ formed propylene, but also to a lesser extent direct from propane by a deep oxidation route. The presence of two parallel pathways for CO_x formation is of interest, as it suggests that partial and deep oxidations may occur at different surface sites or involve different forms of reactive oxygens. Both of these might be amenable to electronic modification by substitution or doping to achieve higher propylene selectivities and yields at higher propane conversions, or their catalytic behavior might be advantageously alterable through site isolation of the paraffin activation centers.     

On the Role of the Vanadium Distribution in MoVTeNbO x Mixed Oxides for the Selective Catalytic Oxidation of Propane

The selective oxidation of propane to acrylic acid is studied over a series of nearly pure M1-phase MoVTeNbO _x catalysts. Quantitative analysis of the reaction network shows that the ratio of the rate constants for propane oxidative dehydrogenation to propene and for the further oxidation of propene is constant. The rates towards acrolein and acetone, however, vary subtly with the concentration of vanadium and the location of its substitution. The reaction of acrolein to acetic acid and carbon oxides, associated with accessible metal cations, contributes two-thirds towards the non-selective pathway. The other third is associated with acetone formation. Vanadium is first substituted selectively at sites that are inactive for propane activation. Depending on the selectivity of this substitution two groups of materials have been identified, which show a distinctly different dependence on the concentration of vanadium. Statistic distribution of vanadium in the M1 phase appears to be the most promising strategy to improve the performance of MoVTeNbO _x catalysts for a given vanadium concentration.     

Catalytic (amm)oxidation of propane with molecular oxygen over complex metal oxides: Involvement of homogeneous reaction in gas phase

Partial oxidation of propane to acrolein and ammoxidafion to acrylonitrile with molecular oxygen proceed over complex metal oxide catalysts under the restricted conditions of the high partial pressures of both propane and oxygen. The selective formations of acrolein and acrylonitrile also required high reaction temperature around 500°C. Effective catalysts for the selective (amm)oxidation were mostly made up of bismuth oxide and molybdenum oxide and were further modified with another metal oxide. From the studies of the volume effect of pre-catalyst zone on the conversion and the selectivities and of the reactions in the absence of the catalyst, it is suggested that the reactions involve homogeneous reactions in the gas phase where thermally activated propane converts into propene, followed by catalytic oxidation of propene over the metal oxide surface.     

Surface-Initiated Gas-Phase Epoxidation of Propylene with Molecular Oxygen by Silica-Supported Molybdenum Oxide: Effects of Addition of C3H8 or NO and Reactor Design

The gas-phase epoxidation of propylene was studied over MoO _x /SiO₂ catalysts in a reaction system with a post-catalytic bed volume. In the reaction of a mixture of propylene and propane with oxygen below 578 K, propylene oxide (PO) was mainly formed from the oxidation of propylene. It was found that the oxidation reaction was very sensitive to the temperature of the post-catalytic space more than the temperature of the catalyst bed, strongly indicating that radical reactions occurring in the post-catalytic bed free space were responsible for the PO formation. The addition of NO increased propylene conversions and PO selectivity at low conversions, confirming that radical reactions were involved in the propylene reactions.     

Selective oxidation of propane to oxygenates over mesoporous SBA-15-supported potassium catalysts

As a novel catalyst system for the selective oxidation of low alkanes, mesoporous SBA-15-supported potassium catalysts were firstly employed for the selective oxidation of propane to oxygenates by using molecular oxygen as oxidant. It was found, compared with bare mesoporous SBA-15, that the selectivities to the oxygenates including formaldehyde, acetaldehyde, acrolein and acetone were remarkably enhanced over K _x /SBA-15(K:Si = x:100, mol) catalysts, and the main products were acrolein and acetone. At 500 °C, the yield of the oxygenates can reach 464% over K_3.0/SBA-15, which is the highest value over SBA-15–supported potassium catalysts. The catalysts were characterized by XRD and BET techniques. The results demonstrated that the catalytic performance was strongly dependent on the potassium content of the catalysts. Furthermore, the highly dispersed potassium on the catalyst surface was shown to be important to orientate the reaction toward the production of oxygenates. The obtained results showed that mesoporous structure, uniform pore sizes and appropriate pore surface area were favorable for the selective oxidation of propane. The samples with moderate amount of potassium promoted the selectivity to the oxygenates.     

Mechanistic aspects of the oxidative dehydrogenation of propane over an alumina-supported VCrMnWOx mixed oxide catalyst

Mechanistic and kinetic aspects of the catalytic oxidative dehydrogenation of propane (ODP) were studied within a wide range of temperatures (673–773 K), partial pressures of oxygen (0–20 kPa), propane (0–40 kPa) and propene (0–4 kPa) under both steady-state ambient-pressure and transient, vacuum conditions in the temporal analysis of products (TAP) reactor. A Mn_0.18V_0.3Cr_0.23W_0.26O_x–Al₂O₃ catalyst was identified as a selective catalyst for ODP by high-throughput experiments. For comprehensive catalyst characterization, XRD, BET, and in situ UV–visible techniques were applied. The results from transient experiments in combination with UV–visible tests reveal that selective and non-selective propane oxidation occurs on the same active surface sites, i.e., lattice oxygen. CO_x formation takes place almost exclusively via consecutive propene oxidation, which involves both lattice and adsorbed oxygen species, with the latter being active in CO formation. However, the adsorbed species play a minor role. CO₂ formation was found to increase in the presence of propene in the reaction feed. Optimized operating conditions for selective propane oxidation were derived and discussed based on the experimental observations with respect to the influence of temperature and partial pressures of O₂, C₃H₆ and C₃H₈ on the reaction. In co-feed mode with a propane to oxygen ratio of 2, optimal catalytic performance is achieved at low partial pressures of oxygen and high temperature. Propene selectivity can be also improved by carrying out the ODP reaction in a periodic mode; that is an alternate feed of propane and air.     

Selective conversion of propane to propene by the catalytic oxidative dehydrogenation over cobalt and magnesium molybdates

Catalytic performances of various metal molybdates were tested in the oxidative dehydrogenation of propane to propene with molecular oxygen under an atmospheric pressure. Most of the molybdates tested promoted the selective oxidative conversion of propane to propene and among them cobalt and magnesium molybdates were found highest in the activity and selectivity. It was also found that their catalytic activities were highly sensitive to the catalyst composition, and it turned out that Co_0.95MoO _x and Mg_0.95MoO _x catalysts which have slightly excess molybdenum showed the highest activity in the oxidative dehydrogenation of propane. Under the optimized reaction conditions, higher reaction temperatures and lower partial pressures of oxygen, these catalysts gave 60% selectivity to propene at 20% conversion of propane. Since the molybdates having the surface enriched with molybdenum oxide tended to show high activity for the propane oxidation, surface molybdenum oxide clusters supported on metal molybdate matrix seem to be the active sites for the selective oxidative dehydrogenation of propane.     

Partial oxidation of propane to acrolein in a membrane reactor – Experimental data and computer simulation

Acrolein is synthesized in a tubular membrane reactor with a porous membrane acting as oxygen distributor at 450–550 °C with the catalyst Ag_0.01Bi_0.85V_0.54Mo_0.45O₄ by direct partial oxidation of propane. The reaction in the membrane reactor is compared with the reaction in the classical co-feed reactor. If the oxygen is dosed through the porous reactor wall to the tube side of the reactor, higher acrolein yields and selectivities are obtained. The catalyst is located inside the tube, where the propane streams through. For the simulation – based on a kinetic model – the membrane reactor was partitioned into 14 separate sections through which the oxygen supply takes place. In accordance with the experiment the simulations show that the acrolein selectivities will increase if the oxygen is dosed through the reactor wall acting as membrane oxygen distributor.     

The effect of the Ce content on the oxidative dehydrogenation of propane over CrOy-CeO2/γ-Al2O3 catalysts

A series of CrO_y (17.5 wt%)-CeO₂ (X wt%)/γ-Al₂O₃ catalysts (X = 0, 0.5, 2, 5, 8) with various Ce contents were prepared by a wetness impregnation method and were applied to the dehydrogenation of propane to propylene at 550 °C and 0.1 MPa. The prepared catalysts were characterized by BET, H₂-TPR, O₂-TPD, XPS, XRD, SEM-EDS and Raman spectroscopy. Among the prepared catalysts, the 17.5Cr-2Ce/Al catalyst with the largest amount of lattice oxygen exhibited the best catalytic performance for the dehydrogenation of propane to propylene with lattice oxygen. The decreased presence of oxygen defects and reducibility were the factors responsible for the improved dehydrogenation activity of the catalysts. The CeO₂ layer could inhibit the evolution of lattice oxygen (O²⁻) to electrophilic oxygen species (O₂⁻), and the oxygen defects on the catalyst surface were reduced. The inhibited lattice oxygen evolution prevented the deep oxidation of propane or propylene, the average CO_x selectivity decreased from 24.41% (17.5Cr/Al) to 5.71% (17.5Cr-2Ce/Al), and the average propylene selectivity increased from 60.15% (17.5Cr/Al) to 85.05% (17.5Cr-2Ce/Al).     

Vanadium Oxide Based Nanostructured Materials: Novel Oxidative Dehydrogenation Catalysts

Novel vanadium oxide based catalyst derived from the open-framework solid, [Co₃V₁₈O₄₂(H₂O)₁₂(XO₄)]·24 H₂O (X = V, S) (1) catalyses oxidative dehydrogenation of propane to propylene. Catalyst activity was evaluated in the temperature range 250–400 °C with varying gas hourly space velocity (GHSV). At 350 °C and GHSV of 9786 h^?1 and at 1.3% propane conversion the selectivity to propylene was 36.8%. The major products obtained were propylene and CO _x (CO₂ and CO). The ratio of the propylene to CO _x depended directly on the catalytic sites present. Thus, as the amount of the catalyst was decreased, the conversion decreased with an increase in the propylene selectivity and a decrease in the selectivity to carbon oxides—CO _x . The catalyst has been characterized by temperature programmed reduction and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).     

Mesoporous silica-supported chromium catalyst: Characterization and excellent performance in dehydrogenation of propane to propylene with carbon dioxide

Nonordered mesoporous molecular sieves MSU-x supported chromium catalysts (Cr/MSU-x) were prepared and characterized with X-ray diffraction, diffuse reflectance UV–vis, and H₂-temperature programmed reduction techniques. Excellent results in dehydrogenation of propane to propylene with carbon dioxide (CO₂) over Cr/MSU-x, 36.8% of propane conversion with 89.1% of propylene selectivity, were obtained. Lower Cr loading results in formation of Cr species with higher oxidation state, whereas higher Cr loading leads to bulk chromium oxide (Cr₂O₃) crystal on catalyst surface. The active sites of the catalysts and the promoting effect of mesoporous MSU-x as support were also discussed.     

Crystalline Mo-V–W-mixed Oxide with Orthorhombic and Trigonal Structures as Highly Efficient Oxidation Catalysts of Acrolein to Acrylic Acid

We succeeded in introducing W in Mo₃VO_x with keeping the orthorhombic, trigonal, and amorphous structures. Synthesized crystalline Mo(W)₃VO_x with orthorhombic and trigonal structures, both of which possess heptagonal channels, showed catalytic activity for gas-phase selective acrolein oxidation to acrylic acid superior to amorphous Mo(W)₃VO_x and to tetragonal Mo₃VO_x. The results strongly suggest that the crystalline Mo(W)₃VO_x with orthorhombic and trigonal structures are real active phase of industrial acrolein oxidation catalysts based on Mo, W, and V oxides. Furthermore, we found that the resulting W-containing catalyst showed less-dependency of water partial pressure in the reactant feed on the acrolein conversion.     

Effect of preparation condition on the performance of silica-supported MoVTeO catalysts for selective oxidation of propane to acrolein

The effects of chemical composition and preparation conditions, especially calcination atmosphere on the catalytic performances of silica-supported MoVTeO catalysts for the selective oxidation of propane to acrolein have been studied. Among the catalysts studied, the MoV_0.2Te_0.1/SiO₂ catalyst calcined in inert atmosphere showed the best performance in terms of propane conversion and selectivity to acrolein. The results of XRD, XPS and Raman revealed that the calcination atmosphere affected greatly the catalysts in many ways including structure, chemical composition and Mo oxidation state, which are related to their catalytic performances.     

Preparation, Characterisation and Catalytic Behaviour of a New TeVMoO Crystalline Phase

TeM_xMo_1.7O mixed oxides (M = V and/or Nb; x = 0-1.7) have been prepared by calcination of the corresponding salts at 600 °C in an atmosphere of N₂. A new crystalline phase, with a Te/V/Mo atomic ratio of 1/0.2-1.5/1.7, has been isolated and characterised by XRD and IR spectroscopy. This phase is observed in the TeVMo or TeVNbMo mixed oxide but not in the TeNbMo mixed oxide. The new crystalline phase shows an XRD pattern similar to Sb₄Mo₁₀O₃₁ and probably corresponds to the M1 phase recently proposed by Aouine et al. (Chem. Commun. 1180, 2001) to be present in the active and selective MoVTeNbO catalysts. Although these catalysts present a very low activity in the propane oxidation, they are active and selective in the oxidation of propene to acrolein and/or acrylic acid. However, the product distribution depends on the catalyst composition. Acrolein or acrylic acid can be selectively obtained from propene on Nb-free or Nb-containing TeVMo catalysts, respectively. The presence of both V and Nb, in addition to Mo and Te, appears to be important in the formation of acrylic acid from propene.     

Effect of vanadium dispersion and of support properties on the catalytic activity of V-containing silicas

Two V-SBA-15 and V-MCF materials (containing about 2.5 wt.% vanadium) were prepared by direct synthesis and tested as catalysts in the decomposition of the most stable chlorinated-alkane, dichloromethane (a total oxidation reaction) and in the oxidative dehydrogenation (ODH) of propane (a partial oxidation reaction). Comparison was made with: (i) two V-SBA-15 and V-MCF materials prepared by “traditional” impregnation method and (ii) a non-porous V-SiO₂ catalyst prepared by flame pyrolysis. All catalysts tested had a vanadium content of about 2.5 wt.%. Samples properties were investigated by means of complementary techniques (TEM, IR and DR UV-vis spectroscopies, N₂ sorption at −196 °C) in order to find possible correlations between catalytic properties of the studied materials and their different physico-chemical features. It is shown that direct synthesis allows a better vanadium dispersion to be achieved, a feature that positively affects catalytic performances in both total and partial oxidations. The different porous networks of the SBA-15 and MCF supports also play an important role on catalytic activity: both V-SBA-15 samples gave better results in dichloromethane decomposition, whereas both V-MCF samples were more selective in propane ODH. The latter findings are ascribed to different molecules diffusion and residence time inside the channels of either SBA-15 or MCF networks.     

Surface active sites present in the orthorhombic M1 phases: low energy ion scattering study of methanol and allyl alcohol chemisorption over Mo–V–Te–Nb–O and Mo–V–O catalysts

The outermost surface compositions and chemical nature of active surface sites present on the orthorhombic (M1) Mo–V–O and Mo–V–Te–Nb–O phases were determined employing methanol and allyl alcohol chemisorption and surface reaction in combination with low energy ion scattering (LEIS). These orthorhombic phases exhibited vastly different behavior in propane (amm)oxidation reactions and, therefore, represented highly promising model systems for the study of the surface active sites. The LEIS data for the Mo–V–Te–Nb–O catalyst indicated surface depletion for V (−23%) and Mo (−27%), and enrichments for Nb (+55%) and Te (+165%) with respect to its bulk composition. Only minor changes in the topmost surface composition were observed for this catalyst under the conditions of the LEIS experiments at 400 °C, which is a typical temperature employed in these propane transformation reactions. These findings strongly suggested that the bulk orthorhombic Mo–V–Te–Nb–O structure is terminated by a unique active and selective surface layer in propane (amm)oxidation. Moreover, direct evidence was obtained that the topmost surface VO _x sites in the orthorhombic Mo–V–Te–Nb–O catalyst were preferentially covered by chemisorbed allyloxy species, whereas methanol was a significantly less discriminating probe molecule. The surface TeO _x and NbO _x sites on the Mo–V–Te–Nb–O catalyst were unable to chemisorb these probe molecules to the same extent as the VO _x and MoO _x sites. These findings suggested that vastly different catalytic behavior exhibited by the Mo–V–O and Mo–V–Te–Nb–O phases is related to different surface locations of V⁵⁺ ions in the orthorhombic Mo–V–O and Mo–V–Te–Nb–O catalysts. Although the proposed isolated V⁵⁺ pentagonal bipyramidal sites in the orthorhombic Mo–V–O phase may be capable of converting propane to propylene with modest selectivity, the selective 8-electron transformation of propane to acrylic acid and acrylonitrile may require the presence of several surface VO _x redox sites lining the entrances to the hexagonal and heptagonal channels of the orthorhombic Mo–V–Te–Nb–O phase. Finally, the present study strongly indicated that chemical probe chemisorption combined with low energy ion scattering (LEIS) is a novel and highly promising surface characterization technique for the investigation of the active surface sites present in the bulk mixed metal oxides.     

On the mechanism of the selective oxidation of n-butane, but-1-ene and but-1,3-diene to maleic anhydride over a vanadyl pyrophosphate catalyst

The mechanism of the selective partial oxidation of n-butane, but-1-ene and but-1,3-diene over a vanadyl phosphate catalyst has been investigated by temperature-programmed desorption (TPD) and by anaerobic temperature-programmed oxidation (TPO). TPD showed lattice oxygen to be desorbed in two states at 998 and 1023 K. The anaerobic TPO of n-butane produced butene and butadiene at 1020 K; anaerobic TPO of but-1-ene produced butadiene and furan at 990 K and dehydrofuran at 965 K, while anaerobic TPO of but-1,3-diene produced dehydrofuran at 970 K, furan at 1002 K and maleic anhydride at 1148 K. The total amount of oxygen removed from the lattice in these anaerobic selective partial oxidations was the same as that evolved from the vanadyl phosphate catalyst by TPD. This, and the fact that the selective oxidation reactions occurred at the same temperature at which the oxygen evolves from the lattice, suggests that the lattice oxygen is uniquely selective when it appears at the surface of the catalyst. (Under identical conditions of flow rate, weight of catalyst, heating rate etc., the reaction of n-butane or of but-1,3-diene in air produced only CO₂ and H₂O.) This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of gel composition in the synthesis of MoVTeNb catalysts over their catalytic performance in partial propane and propylene oxidation

MoVTeNb mixed oxides catalysts have been prepared by a slurry method with different molar compositions (Mo/Te ratio from 2 to 6 and Nb/(V + Nb) ratio from 0 to 0.7) in the synthesis gel leading to different crystalline phases distribution and catalytic behaviour in the partial oxidation of both propane and propylene to acrylic acid. Chemical analysis indicates that the composition of samples before and after the heat-treatment changes, especially the Te-content, since a significant amount of Te is lost during the heat-treatment step when the amount of oxalate (from niobium oxalate) increases in the synthesis gel. Thus, the nature of the crystalline phases and the catalytic performance of heat-treated materials will be related to the final chemical composition. On the other hand, only the catalysts presenting Te₂M₂₀O₅₇ (M = Mo, V, Nb) crystalline structure, the so-called M1 phase, were active and selective in the partial oxidation of propane to acrylic acid. Moreover, all catalysts were active and relatively selective to the formation of O-containing products, i.e. acrolein and/or acrylic acid, during the partial propylene oxidation although the more active ones were those presenting M1 phase.     

Ammoxidation of propene over antimony-vanadium-oxide catalysts

Antimony-vanadium-oxide catalysts were prepared with various Sb/V ratios and were used for propene ammoxidation. It was observed that antimony in excess of the amount required for forming SbVO₄ was required to have a catalyst that is selective to acrylonitrile formation. Characterisation of catalysts with FTIR revealed partial reduction of the oxidised phase Sb_0.92V_0.92O₄ upon use to form Sb_0.95V_1.05O₄. XPS data showed the surfaces of most selective catalysts to be further enriched with antimony in course of the catalytic reaction, thereby creating a surface structure that is selective. For acrylonitrile formation a yield of 55% was obtained at 90% of conversion. In propene oxidation, on the other hand, the yield for acrolein formation was limited to 20% due to consecutive combustion of the aldehyde.