Selective Oxidative Dehydrogenation Research Articles

Nature, Density, and Catalytic Role of Exposed Species on Dispersed VOx/CrOx/Al2O3 Catalysts

The structure and surface composition of binary oxides consisting of CrO(x) and VO(x) dispersed on alumina and their effects on the rate and selectivity of oxidative dehydrogenation (ODH) of propane were examined and compared with those for CrO(x) and VO(x) dispersed on alumina. VO(x) deposition on an equivalent CrO(x) monolayer on alumina and deposition of CrO(x) on an equivalent monolayer of VO(x) deposited on alumina led to CrVO(4) species during thermal treatment with concomitant reduction of Cr(6+) to Cr(3+). Autoreduction of Cr(6+) to Cr(3+) is also detected for CrO(x), even without the presence of VO(x). Infrared spectroscopy of NO adsorbed at 153 K probes the relative abundance of alumina and of V(5+), Cr(3+), and Cr(6+) at surfaces. This technique detects differences in the surface composition of VO(x)/CrO(x)()/Al(2)O(3) and CrO(x)/VO(x)/Al(2)O(3). The first of these samples is enriched in VO(x) relative to CrO(x) compared with the second sample. Consistent with this finding, VO(x)/CrO(x)/Al(2)O(3) and CrO(x)/VO(x)/Al(2)O(3) are distinguishable in their ODH activities and propene selectivities. The highest ODH activity and propene selectivity is observed for VO(x)/CrO(x)/Al(2)O(3), which exhibits a surface enriched in VO(x) and having a low surface concentration of Cr(6+).

Reaction pathways of ethane oxidative and non-oxidative dehydrogenation on γ-Al 2O 3 studied by temperature-programmed reaction (TP-reaction)

In this work, the reactions of ethane and ethene in an oxidizing and non-oxidizing atmosphere over γ-alumina were investigated under temperature-programmed conditions, in an attempt to estimate the possible contribution and functionality of the support in the reaction pathway of ethane ODH over MoO 3/Al 2O 3 catalysts. The results indicate that alumina contributes to the primary deep oxidation and dehydrogenation routes of ethane to CO x and coke respectively, which proceed effectively over the acidic OH groups and the Al 3+–O 2− acidic centers. On the contrary, the formation of ethylene seems to be coupled to the presence of redox sites on the catalytic surface and requires the presence of the molybdena phase. Moreover, the redox sites of the MoO x species were found to unselectively activate the further overoxidation of the olefin to carbon oxides. Therefore, Al 2O 3 catalyzes the unselective primary oxidation of ethane to carbon oxides, whereas the molybdena phase is involved in the selective oxidative dehydrogenation (ODH) of ethane to ethene and the secondary overoxidation of ethene to CO x .

Selective oxidative dehydrogenation of ethane over MoVSbO mixed oxide catalysts

Mo–V–Sb–O mixed metal oxides were prepared, characterized and tested for the oxidative dehydrogenation of ethane (ODH). These catalysts are active and selective in the oxidative dehydrogenation of ethane to ethylene, although their catalytic performance depends strongly on the calcination temperature. The best catalytic results were obtained on samples heat-treated at 600 °C in N 2 (with selectivities to ethylene higher than 80% at ethane conversions about 65%). Moreover, a loss in the catalyst activity is initially observed on these catalysts as a consequence of modifications in the nature of the crystalline phases. Changes on the oxidation state of antimony ions on the surface of the used catalyst have also been observed, with the appearance of Sb 5+ on the surface of used catalysts. The good catalytic performance of these catalysts is related to the presence of (SbO) 2M 20O 56 crystalline phase in a similar way to that proposed for selective Mo–V–Te–Nb–O mixed metal oxide catalysts.

Preparation of MoVTe(Sb)Nb mixed oxide catalysts using a slurry method for selective oxidative dehydrogenation of ethane

The preparation of bulk MoVTe(Sb)Nb mixed oxide catalysts using a traditional slurry method, results in highly active catalysts for oxidative dehydrogenation of ethane to ethene. Several major phases including orthorhombic M1, hexagonal M2 or Mo x M 1− x O 2.8 (M = V or Nb) have been detected in the catalysts from characterization results such as X-ray diffraction (XRD), SEM and EDX analyses. Ethane conversion and yield to ethene increase with increasing content of the M1 phase in the catalysts. The maximum yield of ethene (ca. 87% selectivity and ca. 90% conversion, ST Y C 2 H 4 of 176 g k g cat − 1 h −1) has been obtained with a MoV 0.31Te 0.2Nb 0.14 mixed oxide catalyst, calcined at 873 K under nitrogen, containing almost pure orthorhombic M1 phase and small amounts of unidentified impurity phases, operating at a relatively low reaction temperature of 673 K. The orthorhombic M1 phase has been shown to be the most active in ethane activation and the most selective for ethene formation. The hexagonal M2 phase is relatively inactive in ethane activation and less selective for ethene formation. The Te-free phases such as Sb 4Mo 10O 31 and Mo x M 1− x O 2.8 (M = V or Nb) show the lowest selectivity to ethene.

Nanocarbons in selective oxidative dehydrogenation reaction

Nanocarbons (multiwalled carbon nanotubes (CNTs) and onion-like carbon) are tested for the oxidative dehydrogenation (ODH) of ethylbenzene to styrene. The catalytic performances are compared with these using carbon black and graphite as catalysts. The tested nanocarbons are catalytic active in the reaction. The highest yield is achieved by using onion-like carbon as catalyst. The combustion stability of graphite, carbon nanotubes and onion-like carbons gives stable catalytic performances on stream. The comparison of onion-like carbons as a model catalyst with other carbon materials gives the criteria for efficient carbon catalyst. A reaction model for oxidative dehydrogenation on carbon catalysts is proposed too. Our findings provide a new perspective for the application of nanostructured carbon materials.

Spectroscopic and Structural Characterization of Chlorine Loading Effects on Mo/Si:Ti Catalysts in Oxidative Dehydrogenation of Ethane

The structural changes induced in a silica-titania mixed-oxide support (1:1 molar ratio) by chlorine addition at different loading levels, their relation to the structural characteristics of supported MoOx species over the support, and their correlation with ethane oxidative dehydrogenation (ODH) activity have been examined. The molybdenum and chlorine precursors are incorporated into the Si/Ti support network as it forms during gelation by using a "one-pot" modified sol-gel/coprecipitation technique. In situ X-ray diffraction during calcination shows the Si/Ti 1:1 mixed-oxide support is in a state of nanodispersed anatase titania over amorphous silica. With the addition of molybdenum and chlorine modifier, this anatase feature becomes more pronounced, indicating a decreased dispersion of titania. The effective titania surface area on the chlorine-doped Si:Ti support obtained from 2-propanol temperature-programmed reaction supports this observation. Raman spectra of dehydrated samples point to an enhanced interaction of MoOx species with silica at the expense of titania. X-ray photoelectron spectroscopic results show that, without forming a molybdenum chloride, the presence of chlorine significantly alters the relative surface concentration of Si vs Ti, the electronic structure of the surface MoOx species, and the oxygen environment around supported MoOx species in the Si/Ti network. Secondary ion mass spectrometry detected the existence of SiCl fragments from the mass spectra, which provides molecular insight into the location of chlorine in Mo/Si:Ti catalysts. The observed increase in ethane ODH selectivity with chlorine modification may be ascribed to the MoOx species sharing more complex ligands with silica and titania with the indirect participation of chlorine. Steady-state isotopic transient kinetic analysis (SSITKA) is used to to examine the oxygen insertion and exchange mechanisms. The catalysts show very little oxygen exchange with the gas phase in the absence of a reaction medium. During the steady-state ODH reaction, lattice oxygen appears to be the primary source of oxygen in the formation of water and CO2.

Mesoporous tin silicate: an efficient liquid phase oxidative dehydrogenation catalyst

Mesoporous tin silicate materials have been synthesized at room temperature using a self-assembly of cationic surfactant cetyltrimethylammonium bromide (CTAB) and tetraethylorthosilicate (TEOS) and SnCl 4 as Si and Sn precursors, respectively, under mild acidic (pH 4.5–6.5) condition. Si/Sn mol ratios in the product for different mesoporous tin silicate samples were 12.4, 27.2, 48.5 and 74.7 vis-à-vis the ratios 10, 25, 50 and 75, respectively, in the hydrothermal synthesis gels. These mesoporous samples were characterized using powder X-ray diffraction (XRD), N 2 sorption, UV–vis diffuse reflectance (UV DRS), FT-IR spectroscopy, and scanning electron microscopy (SEM) on an instrument with an energy dispersive X-ray (EDS) attachment. Characterization data demonstrated that the samples were moderately ordered, with pore dimension of 2.1–2.4 nm. UV DRS and IR spectral bands suggested the incorporation of tin in the tetrahedral lattice sites of the mesoporous silica network. After calcination, these tin silicate samples retained their mesophase and showed excellent catalytic activity in one-pot liquid phase selective oxidative dehydrogenation of cyclohexane to 1,4-cyclohexadiene using dilute aqueous hydrogen peroxide as oxidant. A very high selectivity for 1,4-cyclohexadiene (83–98%) was observed using different mesoporous tin-silicate samples and acetonitrile solvent. Very little to almost no further oxidation product of the parent and product olefins, i.e. epoxides or cyclohexanone/cyclohexanol was observed.

Selective oxidative dehydrogenation of ethane on MoVTeNbO mixed metal oxide catalysts

MoVTeNbO catalysts, prepared by hydrothermal synthesis, are active and highly selective in the ODH of ethane, especially those with a MoVTeNb molar ratio of 1–0.15–0.16–0.17 and heat-treated at 600–650 °C. On the best catalyst, selectivities higher than 80% at ethane conversion levels higher than 80% have been obtained operating at relatively low reaction temperatures (340–400 °C). Thus, yields of ethylene of ca. 75% have been obtained, which exceeds the best yield reported in the literature. Te 2 M 20O 57 ( M=Mo, V, Nb) and (V,Nb)-substituted θ-Mo 5O 14, in addition to small amounts of the Te 0.33 MO 3.33 ( M=Mo, V, Nb) phase, can be proposed in the most selective catalysts from XRD, SAED, and HREM results. However, the catalytic performance in ethane oxidation can mainly be related to the presence of the multifunctional Te 2 M 20O 57 orthorhombic phase in cooperation with the Mo 5O 14-type phase. The nature of active and selective sites is discussed.

Effect of additives on properties of vanadia-based catalysts for oxidative dehydrogenation of propane

Abstract Effect of additives (A) comprising main group elements (K, P) and transition metals (Ni, Cr, Mo) to VOx/SiO2 catalysts on their catalytic behavior in oxidative dehydrogenation (ODH) of propane are studied. The experimental findings are compared with results of quantum chemical calculations performed for a model O = V  O  V ∗ cluster in which V∗ atom is replaced by an atom of A. The selectivities to propene found in experiments change on introducing the additives, decreasing in the sequence VK(64)>VCr(38)>VMo=VNi(32)>VP(28)∼V0(27). Quantum chemical calculations indicate the change in the electron density on the atoms of the active O=VO groups for the clusters with the additives’ atoms. The extent of the electron transfer from A towards the active group (the increase in the centre basicity) follows the sequence of the decreasing selectivity to propene, confirming the role of acid–base properties in controlling the selectivity in ODH of propane. Results of calculations show also an exceptional behavior of the potassium (the most effective additive for the increase in the selectivity in experiments); in contrast to all other additivies, K transfers electrons towards both V and O atoms in the active group, and does not form a definite OK bond, but is rather “adsorbed” on the cluster.

Effects of O2 Concentration on the Rate and Selectivity in Oxidative Dehydrogenation of Ethane Catalyzed by Vanadium Oxide: Implications for O2 Staging and Membrane Reactors

Staged-O2 introduction and the effects of O2 concentration on primary and secondary reactions were examined during oxidative dehydrogenation on V2O5/γ-Al2O3 containing predominately isolated monovanadates. Cofeed and staged-O2 introduction modes led to similar ethane dehydrogenation and combustion rates, despite significant differences in the average O2 concentrations, as expected from their zero-order O2 dependences. The rate of ethene conversion to COx, however, was lower when O2 coreactants were introduced gradually as ethane conversion increased. These effects reflect inhibition of homogeneous ethene combustion pathways, which in contrast with their heterogeneous counterparts show a positive dependence in O2. Axial O2 distribution using multiple injectors or membranes will therefore influence alkene yields only by decreasing homogeneous alkene oxidation rates. Homogeneous contributions are much smaller in large reactors, because catalyst-to-volume ratios are greater than those in laboratory reactors. ...

Effect of Ag promoter on redox properties and catalytic performance of Ag-Mo-P-O catalysts for oxidative dehydrogenation of propane

The influence of addition of sliver ions on the redox properties and catalytic performance of molybdenum-phosphate catalyst in oxidative dehydrogenation (ODH) of propane has been investigated. MoO 3 and AgMoO 2PO 4 have been detected in Ag-doped Mo-P-O catalysts. The redox properties have been characterized by H 2-temperature-programmed reduction (TPR), electronic paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). The catalytic performance of MoO 3, Ag 2O, AgMoO 2PO 4 and Ag-doped Mo-P-O catalysts has also been examined. The incorporation of Ag improved the number of Mo 5+ species and the reducibility of the catalysts. The Ag-doped catalysts favor activity and selectivity in ODH of propane higher than the undoped catalyst. The best catalyst was obtained with Ag/Mo ratio of 0.3 and MoO 3/AgMoO 2PO 4 ratio of 1.1. The influence of promoter Ag + on redox properties and catalytic performance is due to forming redox couple “Ag 0+Mo 6+⇔Ag ++Mo 5+” and the synergetic effect originating from “ coherent interface” between MoO 3 and AgMoO 2PO 4.

Oxidative dehydrogenation of butane over nanocrystalline MgO, Al 2O 3, and VO x/MgO catalysts in the presence of small amounts of iodine

High surface area nanocrystalline MgO, Al 2O 3, MgO·Al 2O 3, commercial MgO, and a series of 10% V/MgO samples were used as catalysts in one-step selective oxidative dehydrogenation of butane to butadiene in the presence of oxygen and iodine. Molecular iodine shifts the equilibrium of the dehydrogenation reactions to the right and makes it possible to achieve high butane conversion with high selectivity to butadiene. When excess oxygen is present in the feed, iodine is successfully regenerated and can be recycled. Butadiene selectivity as high as 64% has been achieved in the presence of small amounts of iodine (0.25 vol%) over a vanadia–magnesia catalyst at 82% butane conversion. The best performance was observed over a catalyst containing the magnesium orthovanadate phase.

Oxidative Dehydrogenation of Propane over Vanadia–Magnesia Catalysts Prepared by Thermolysis of OV(OtBu)3 in the Presence of Nanocrystalline MgO

The influence of vanadium content on the performance of V–Mg–O catalysts for the oxidative dehydrogenation (ODH) of propane was investigated. High-surface-area (380 m2/g) MgO was prepared by hydrolysis of Mg(OCH3)2 followed by hypercritical drying. Vanadia was deposited on this support by thermolysis of OV(OtBu)3. Catalysts prepared by this means have BET surface areas of 187–299 m2/g and apparent surface densities of V2O5 of 1.1–10.3 VOx/nm2. All of the catalysts were characterized by X-ray diffraction, temperature-programmed reduction, and Raman, UV–visible, and nuclear magnetic resonance spectroscopy. The environment of the V atoms depends strongly on the apparent surface density of vanadia. Isolated VO42− units are present at very low apparent surface densities (∼1 VOx/nm2). As the vanadia density increases, magnesium vanadate structures are formed and above a surface density of 3.5 VOx/nm2 well-dispersed magnesium orthovanadate domains become evident. The rate of ODH per V atom increases with increasing VOx surface density and reaches a maximum value at 3.5 VOx/nm2. Above this surface density, the rate of ODH per V atom decreases because an increasing fraction of the V atoms lie below the catalyst surface and, hence, are inaccessible. Consistent with this interpretation, the ODH activity per unit surface area reaches a plateau at a VOx surface density of about 4 VOx/nm2. The propane ODH selectivity of the catalysts increases with increasing VOx surface density and reaches a plateau of 80% for an apparent surface density of about 4 VOx/nm2. Rate coefficients for propane ODH (k1), propane combustion (k2), and propene combustion (k3) were calculated for each catalyst. The value of k1 increases with increasing VOx surface density, reaching a maximum at about 4 VOx/nm2. By contrast, the ratios (k2/k1) and (k3/k1) decrease monotonically with increasing VOx surface density. The observed trends in k1, (k2/k1), and (k3/k1) are discussed in terms of the surface structure of the catalyst.

Continuous oxygen ion transfer medium as a catalyst for high selective oxidative dehydrogenation of ethane

An oxygen permeable mixed ion and electron conducting membrane (OPMIECM) was used as an oxygen transfer medium as well as a catalyst for the oxidative dehydrogenation of ethane to produce ethylene. O2- species transported through the membrane reacted with ethane to produce ethylene before it recombined to gaseous O2, so that the deep oxidation of the products was greatly suppressed. As a result, 80% selectivity of ethylene at 84% ethane conversion was achieved, whereas 53.7% ethylene selectivity was obtained using a conventional fixed-bed reactor under the same reaction conditions with the same catalyst at 800 °C. A 100 h continuous operation of this process was carried out and the result indicates the feasibility for practical applications.

The selective oxidative dehydrogenation of ethane over hydrothermally synthesised MoVTeNb catalysts.

Mo-V-Te-Nb metal oxide catalysts prepared by hydrothermal synthesis and heat-treated in N2 at high temperatures (600-700 degrees C) show high activity and selectivity for the oxidative dehydrogenation of ethane to ethene. Yields of ethene of 75% have been obtained at 400 degrees C on the best catalysts.

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

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 approach (773 K, ambient pressure, a feed composition of C3H8 : O2 : N2=30 : 10 : 60 and 40 : 20 : 40). For further improvement of catalytic materials, fundamental insights were derived from oxygen transient experiments and XRD as well as XPS studies. Fitting transient experiments showed that irreversible and dissociative adsorption on two active centers provides a good description of the transient oxygen responses over all the catalytic materials studied. The composition of these materials influenced strongly the apparent first-order rate constant of oxygen activation. Based on the characterisation data it was found that an optimal bulk concentration (2.6–8.3 wt%) of VOxspecies on the support and their distribution on the surface are an essential requirement for the selective oxidative dehydrogenation of propane to propene.

On the role of heterogeneous and homogeneous processes in oxidative dehydrogenation of C 3-C 4 alkanes

The effects of reactor arrangement, catalyst particle size and void volume on the oxidative dehydrogenation (ODH) of propane and iso-butane are studied. In an empty quartz reactor, a high conversion of alkane (up to 36% with a 40% ODH selectivity) can be achieved. Packing of the reactor with different solids (quartz, γ-Al 2O 3, complex V-containing oxide catalysts) leads to a drastic change in reaction parameters, which is strongly dependent on the chemical nature of solids and their particle size. The analysis of observed phenomena indicates that: (a) the termination of gas-phase chain reaction takes place on the surface of any solid material; (b) certain solids behave as active generators of free radicals, thus initiating the new reaction pathway which develops both in the gas-phase and on the solid surface.

ChemInform Abstract: FePO Catalysts for the Selective Oxidative Dehydrogenation of Isobutyric Acid into Methacrylic Acid

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Selective oxidative dehydrogenation of propane over surface molybdenum-enriched MgMoO 4 catalyst

Drastic activity increases were observed by the treatments of the magnesium-rich MgMo 0.99O y catalysts, which are poorly active for the oxidative dehydrogenation of propane, with inorganic or organic acid to remove excess magnesium on the surface. MoO 3 loading on magnesium-rich MgMo 0.99O y catalysts also resulted in drastic activity increases. The activity increases followed non-effective loadings of MoO 3 in the range 0–2 wt%, because it is necessary to neutralize the surface magnesium with MoO 3 before the formation of molybdenum-rich surface. The pH of the aqueous (NH 4) 6Mo 7O 24 solution for the MoO 3 loading apparently influenced the activity. Under the acidic conditions the MoO 3 loading resulted in the drastic activity increase but under the basic conditions the effect of the MoO 3 loading was poor, suggesting that a cluster-type MoO 3 on MgMoO 4 surface is responsible for the activity of propane oxidative dehydrogenation.

Oxidative dehydrogenation of iso-butane to iso-butene II. Rare earth phosphate catalysts

Among various rare earth phosphates, CePO 4 and LaPO 4 were found to catalyze the oxidative dehydrogenation of iso-butane to iso-butene at 450–550°C using a feed gas of 75 mol% iso-butane and 5 mol% O 2. As for the phosphates of Y, Pr, Nd, Gd, Er, and Yb, iso-butene was formed with a limited selectivity of 20–40% selectivity, and C 3H 6 formation and deep oxidation into CO and CO 2 were promoted over the same temperature range. Temperature-programmed-desorption spectra of NH 3 for CePO 4 and LaPO 4 consisted of two large desorption peaks with maxima at 100°C and 270°C suggesting that these phosphates have strong acid sites. But the rest of the phosphates have only weak acid sites. Rare earth phosphates have no adsorbed oxygen which can be desorbed up to 550°C. The lattice oxygen of CePO 4 and PrPO 4 can be reduced at 200°C and that of the rest of the phosphates at about 400°C. The amount of oxygen removed up to 550°C in TPR was smaller than the amount corresponding to a monolayer for all phosphates. Oxygen isotope equilibration experiments over CePO 4 and LaPO 4 revealed that only surface lattice oxygen can participate in the equilibration at 500°C and 550°C. It can be concluded that acidic character is a key factor in the selective oxidative dehydrogenation of iso-butane over the rare earth phosphate catalysts.