Chromium Oxide Catalysts Research Articles

Ammoxidation of 3-picoline to nicotinonitrile using silica-supported VCrO catalysts

A series of vanadium–chromium oxide (VCrO) catalysts supported on silica was prepared by wetness impregnation method with different Cr/V molar ratios from 0.2 to 1.0. These catalysts were characterized by XRD, TG, temperature-programmed desorption of ammonia (NH3-TPD), X-ray photoelectron spectra (XPS), and Raman spectroscopy, and their catalytic activity was evaluated in the ammoxidation of 3-picoline (3-PIC) to nicotinonitrile (NN). The results of XRD, TG, Raman, and XPS confirmed that the active components on the silica surface were mainly amorphous V2O5 and CrVO4. The results of NH3-TPD showed that acidity of the catalysts decreased with the increase of Cr/V ratio. Catalytic results revealed that acidity of the catalysts was closely related to the catalytic performance. Low acidity gave low conversion of 3-PIC and high NN selectivity. Furthermore, the conversion of 3-PIC increased with rise in reaction temperature, and the selectivity of NN was slightly influenced by the 3-PIC conversion. Therefore, among the catalysts (Cr/V ratio was 0.2, 0.4, 0.6) tested, Cr/V-0.6 catalyst retained the lowest acidity and exhibited the highest selectivity and yield of NN in the ammoxidation of 3-PIC.

Preparation and characterization of SBA-1–supported chromium oxide catalysts for CO2 assisted dehydrogenation of propane

A series of Crx/SBA-1 cubic mesoporous catalysts with 1, 3, 5, 7, 10, and 15wt.% content of Cr (Crtotal) were obtained by incipient wetness impregnation of SBA-1. The obtained catalysts were characterized with various physicochemical techniques (chemical composition, low-temperature adsorption of N2, H2–TPR, XRD, UV–vis DRS, Raman, and EPR) and tested in dehydrogenation of propane to propene in the presence of CO2. Various chromium species including Cr6+, dispersed Cr5+, and crystalline Cr2O3 were found in the calcined Crx/SBA-1 samples. Cr6+ species were present mainly in the form of mono- and dichromates, despite the Cr loading. The presence of dispersed Cr5+ species and crystalline α-Cr2O3 in the calcined catalysts depended on the total Crtotal content. Cr5+ species were found in the samples containing below 7wt.% of Crtotal, while particles of crystalline α-Cr2O3 were detected in the catalyst with Crtotal content above 5wt.%. All of the Crx/SBA-1 catalysts exhibited excellent catalytic activity and high selectivity in the dehydrogenation of propane to propene in the presence of CO2 with the maximum propane conversion (37.7% at 550°C) for ∼7wt.% of Crtotal. For higher Cr contents the conversion of propane remained almost constant which was connected with the formation of crystalline Cr2O3 which was inactive in the test reaction. Operando UV–vis DRS measurements performed during the dehydrogenation of propane, both in the presence and absence of CO2 at 550°C, indicated that the Cr6+ species (main redox sites) were reduced rapidly to Cr2+/Cr3+ species already at the beginning of the process. The dispersed Cr2+/Cr3+ ions were the main sites available to the reactants under the dehydrogenation conditions in all of the studied feed compositions.

Effect of the introduction of alkaline promoters into chromium oxide catalysts for propane dehydrogenation in the presence of CO2

The influence of the introduction of alkaline promoters into chromium oxide catalysts for the dehydrogenation of propane in the presence of CO2 was investigated. The positive role of potassium, which manifested itself in an increase in the selectivity and stability of catalysts, was shown. It was established by IR spectroscopy that, in the chromium oxide catalysts where a substantial part of protons is replaced by an alkaline ion, a decrease in their activity and stability in the dehydrogenation of propane in the presence of CO2 was observed.

Chromium oxide supported on ZSM-5 as a novel efficient catalyst for dehydrogenation of propane with CO 2

ZSM-5-supported chromium oxide catalysts prepared by an incipient wetness method have been tested for the dehydrogenation of propane to propylene in the presence of carbon dioxide. Several techniques including XRD, N 2 adsorption, SEM, 27Al MAS NMR, XPS, Raman spectroscopy, diffuse reflectance UV–Vis spectroscopy and H 2-TPR were applied to characterize the physicochemical properties of the as-synthesized materials. Catalytic tests show that the chromia catalyst supported on Na-type ZSM-5 with smaller crystal size (ca. 400 nm) exhibits a good catalytic performance, which is much more active than that supported on ZSM-5 with larger crystal size (ca. 2 μm). A high number of Cr(VI) species in the calcined catalyst is crucial for its high catalytic activity in the dehydrogenation of propane to propylene with CO 2.

Computational study of the catalytic synthesis of 5‐nitro‐1,2,4‐triazol‐3‐one

AbstractIn this study, 5‐nitro‐1,2,4‐triazol‐3‐one (NTO) was theoretically synthesized from urea via chlorination followed by amination, formylation, and nitration under aqueous and gaseous environments based on experience of experimental methods, and metal chlorides and metal oxides were used as catalysts to promote reaction. Reaction routes closely related to experimental processes were successfully constructed, and the corresponding energy barriers were estimated for each elementary reaction. Reaction conditions distinct from those reported in the literature (including the adoption of aluminum chloride, ferric chloride, aluminum oxide, ferrous oxide, and chromium oxide catalysts, the use of nitric acid and dinitrogen pentoxide as nitration agents, and adjustment of the reaction temperature) were used in corresponding reaction systems, and the modeling results suggested that ferric chloride is a good catalyst for the chlorination reaction, ferrous oxide is suitable for catalyzing amination, formylation, and nitration, and nitric acid is the better agent for nitration. Estimates of the comparable energy barriers for each reaction stage were considered to imply more feasible pathways for NTO synthesis. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

In situ Raman spectroscopy studies on chromium oxide catalyst in an anhydrous hydrogen fluoride atmosphere

AbstractChanges of different chromium oxide species [Cr(VI), Cr(V) and Cr(III)] were investigated using in situ Raman spectroscopy in an anhydrous hydrogen fluoride (AHF) atmosphere. It was found that the Cr(VI) (CrO3) and Cr(V) (YCrO4) species were easy to fluorinate, and generated CrO2F2 or/and CrOF3, while the Cr(III) (Cr2O3) species was difficult to fluorinate. Moreover, the CrO2F2 and CrOF3 species were stable in the AHF atmosphere, but they easily transform into CrO3 in N2 atmosphere and further decomposed to Cr2O3 at 400 °C. Moreover, Cl/F exchange reaction of CF3CH2Cl to CF3CH2F over these Cr species indicated that the high‐oxidation‐state CrOxFy (CrO2F2 and CrOF3) species exhibited much higher reactivities than Cr2O3, suggesting that these species (CrO2F2 and CrOF3) may be the active phases of the reaction. Copyright © 2010 John Wiley & Sons, Ltd.

Partially hydrated iron–chromium oxide catalyst for the Fischer-Tropsch synthesis

Due to their well-developed surface, partially hydrated iron–chromium oxides with the ferrihydrite structure exhibit much higher activity in the Fischer-Tropsch synthesis than those that have the hematite structure.

Chromium, vanadium, molybdenum, tungsten, magnesium, and aluminum hydrotalcite hydroxo salts and oxide catalysts on their base

A method is developed for the synthesis of chromium-containing catalysts for the oxidative dehydrogenation (ODH) of alkanes, comprising the precipitation, from nitrate solutions, of mixtures of isomorphic hydrotalcite-type magnesium aluminum and magnesium chromium hydroxocarbonates and the incorporation of oxovanadate, oxomolybdate, and oxotungstate ions by means of anion exchange and subsequent heat treatment. A series of oxide catalyst samples with progressively more complex compositions were prepared: Mg-Al, V-Mg-Al, V-Mo-Mg-Al, Cr-V-Mg-Al, Cr-V-Mo-Mg-Al, and Cr-V-Mo-W-Mg-Al. The catalytic properties of these complex oxides in the ODH of ethane and propane are improved with progressively complex compositions. Chromium-containing catalysts have higher selectivities and provide higher conversions compared to state-of-the-art iron- and nickel-containing oxides.

Chromium oxide catalysts for CO x-free hydrogen generation via catalytic ammonia decomposition

Chromium oxide nanocrystallites have been synthesized by thermal decomposition of solid precursors and evaluated as catalysts for ammonia decomposition. The physical and chemical properties of Cr 2O 3 catalysts are characterized by various techniques such as N 2 adsorption–desorption isotherms, XRD, TEM, and XPS. The relationship between catalytic activities and physicochemical and electronic properties of Cr 2O 3 catalysts is investigated. The Cr 2O 3 catalysts are active for ammonia decomposition and the activity is structure-sensitive with a close relation to particle size. During the ammonia decomposition reaction, N atoms are inserted into the Cr 2O 3 catalyst thus enhancing the activity of catalyst, due to the variation in the physical, chemical and electronic structures of Cr 2O 3 catalysts.

Ethane dehydrogenation over pore-expanded mesoporous silica-supported chromium oxide: 2. Catalytic properties and nature of active sites

Pore-expanded mesoporous MCM-41 silica-supported chromium oxide catalysts with different chromia loadings were synthesized using three different methods. The catalysts were thoroughly characterized and used for ethane dehydrogenation (DH). Reaction studies revealed that the ethane conversion and coke formation on the catalyst depend on the chromia loading and the catalyst preparation method. All catalysts were highly selective toward ethylene. However, among the three catalysts containing 5 wt% chromia, the 5 wt% Cr/PE-MCM-41 catalyst exhibited the best performance with an ethane conversion of 23% and ∼99% ethylene selectivity at 650 °C (C 2H 6/N 2 = 0.176, total contact time = 0.0032 g min/mL). The chromia catalysts prepared using as-synthesized pore-expanded mesoporous silica (Cr/PE-MCM-41) also exhibited the highest stability. XPS analysis revealed the presence of three different chromium, namely Cr(II), Cr(III) and Cr(VI), whose overall surface concentration as well as relative content varied with the reaction time-on-stream (TOS). Furthermore, the nature and relative amounts of surface chromium species were monitored by in situ XPS measurements as a function of TOS. Correlation of the XPS findings with ethane DH data showed clearly that the Cr(III) species are the main active sites.

Synthesis of LPPE by gas-phase copolymerization of ethylene with 1-butene: A highly efficient chromium oxide supported catalyst

A modified chromium oxide supported catalyst has been developed and applied in industry for the manufacture of LPPE via the gas-phase (co)polymerization of ethylene. The catalyst contains surface chromium oxide in the oxidation number Cr2+, two modifiers (aluminum oxide and fluorine surface compounds), and silicon dioxide as a support. The activity of the new chromium oxide catalyst in the gas-phase copolymerization of ethylene with 1-butene is higher by a factor of 4–5 than that of the traditional commercial catalytic system based on the supported bis(triphenylsilylchromate) catalyst. An increased reactivity of 1-butene in its copolymerization with ethylene in the presence of the chromium oxide catalyst makes it possible to reduce the consumption of 1-butene in the synthesis of a linear medium-density PE (0.937–0.938 g/cm3). Gas pipes made of PE prepared with the new catalyst are characterized by improved resistance to crack propagation.

Chemistry of olefin polymerization reactions with chromium‐based catalysts

AbstractDetailed GC analysis of oligomers formed in ethylene homopolymerization reactions, ethylene/1‐hexene copolymerization reactions, and homo‐oligomerization reactions of 1‐hexene and 1‐octene in the presence of a chromium oxide and an organochromium catalyst is carried out. A combination of these data with the analysis of 13C NMR and IR spectra of the respective high molecular weight polymerization products indicates that the standard olefin polymerization mechanism, according to which the starting chain end of each polymer molecule is saturated and the terminal chain end is a CC bond (in the absence of hydrogen in the polymerization reactions), is also applicable to olefin polymerization reactions with both types of chromium‐based catalysts. The mechanism of active center formation and polymerization is proposed for the reactions. Two additional features of the polymerization reactions, co‐trimerization of olefins over chromium oxide catalysts and formation of methyl branches in polyethylene chains in the presence of organochromium catalysts, also find confirmation in the GC analysis. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5330–5347, 2008

Kinetics of ethylene polymerization reactions with chromium oxide catalysts

Principal kinetic data are presented for ethylene homopolymerization and ethylene/1-hexene copolymerization reactions with two types of chromium oxide catalyst. The reaction rate of the homopolymerization reaction is first order with respect to ethylene concentration (both for gas-phase and slurry reactions); its effective activation energy is 10.2 kcal/mol (42.8 kJ/mol). The r1 value for ethylene/1-hexene copolymerization reactions with the catalysts is ∼30, which places these catalysts in terms of efficiency of α-olefin copolymerization with ethylene between metallocene catalysts (r1 ∼ 20) and Ti-based Ziegler-Natta catalysts (r1 in the 80–120 range). GPC, DSC, and Crystaf data for ethylene/1-hexene copolymers of different compositions produced with the catalysts show that the reaction products have broad molecular weight and compositional distributions. A combination of kinetic data and structural data for the copolymers provided detailed information about the frequency of chain transfer reactions for several types of active centers present in the catalysts, their copolymerization efficiency, and stability. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5315–5329, 2008

Oxidative Dehydrogenation of Ethane over Chromium−Vanadium Mixed Oxide and Chromium Oxide Catalysts

Oxidative dehydrogenation of ethane to ethylene was investigated at temperatures below 500 °C over Cr−O and Cr−V−O oxide catalysts prepared by the complexation method. The catalysts were characterized by XRD, H2 TPR, and XPS techniques. The Cr−O catalyst was more active than Cr−V−O mixed oxide in the selective oxidation of ethane using O2 or CO2 as the oxidant. Cr−O showed activity at temperatures as low as 250 °C. TPR results showing easier reduction of Cr−O at much lower temperatures than Cr−V−O support a correlation between the reducibility and the activity of the catalysts in this reaction. XPS results indicating the reduction of chromium and vanadium species from Cr6+ to Cr3+ and from V5+ to V4+ (and V3+) support a redox mechanism for this oxidative dehydrogenation reaction. Over Cr−O catalyst, total conversion and ethylene selectivity values of 0.11 and 0.91 were obtained, respectively, at 250 °C for an O2/C2H6 feed ratio of about 0.33 and at a space time of 0.24 g s/cm3. With this catalyst, the eth...

Carbon deposition during propane dehydrogenation in a fuel cell

Carbon deposition in a high temperature proton-conducting fuel cell for selective propane dehydrogenation to propylene with co-generation of electrical power was investigated. Comparison of carbon deposition was made for catalytic propane dehydrogenation under an open circuit condition and electro-catalytic conversion of propane to propylene on the anode catalyst of synthesized chromium(III) oxide during fuel cell operation conditions with current flow. Carbon deposition under the fuel cell operating conditions was much less than that under the open circuit conditions. Chromium(III) oxide catalyst modified by potassium showed a better resistance to carbon formation under the open circuit conditions but was similar to the unmodified catalyst under current flow conditions.

DRIFTS and DRS studies of Phillips ethylene polymerization catalysts

Formation of the CrO 3/SiO 2 catalyst prepared by the reaction of CrO 3 vapor with silica pre-dehydroxylated at 250–800 °C was studied by the DRIFT and DRS methods. It was found that CrO 3 reacts with the silica OH-groups at 250 °C to produce various chromate species and water. The latter is removed from the catalyst surface by vacuumation at 250 °C. It was found the nature of CrO 3 interaction with silica and the structure and composition of the surface species depend on silica dehydroxylation temperature. The reactions of CrO 3 with the silica dehydroxylated at 250, 400 and 800 °C yielded, respectively, monochromates; mono- and dichromates; polychromates. The catalyst activity at ethylene polymerization strongly increases with increasing dehydroxylation temperature, especially in the range of 250–400 °C. The results obtained prove di- and polychromates to be the active components of the Phillips type chromium-oxide catalysts from which the active centers are formed at interaction of the active component with co-catalyst (AlEt 3) and ethylene. Active centers contain the surface organo-chromium compounds with chromium ions in low oxidation states (lower than Cr(VI)).

Dehydrogenation of Propane in the Presence and Absence of CO2 Over β-Ga2O3 Supported Chromium Oxide Catalysts

The dehydrogenation of propane to propene in the presence and absence of CO2 over β-Ga2O3 supported chromium oxide catalysts has been studied. The effect of chromium content and feed composition on the dehydrogenation of propane has been investigated. It has been found that deposition of chromium oxide enhances the dehydrogenation activity and resistance to coke deposition. Moreover, it has been shown that CO2 promotes the dehydrogenation of propane above 848 K. At that temperature in the presence of CO2 both the yield of propene and conversion of propane were higher than in the inert gas atmosphere.

The origin of rhodium promotion of Fe 3O 4–Cr 2O 3 catalysts for the high-temperature water–gas shift reaction

Rhodium promotion of iron–chromium oxide (Fe–Cr) catalysts for the water–gas shift (WGS) reaction was investigated through measurement of the rates of individual reduction and reoxidation steps. The temperature-programmed reduction of the starting Fe 2O 3 form by H 2 is accelerated by rhodium, with the extent of oxygen removal at 450 °C corresponding to Fe 3O 4 formation. The corresponding reduction by CO proceeds further, in agreement with bulk thermodynamic expectations. However, rhodium has no effect on the oxygen removal rate, although it leads to substantially increased carbon deposition. The behavior with respect to reoxidation of reduced catalysts is somewhat similar. Rhodium greatly enhances H 2 release during reoxidation by water, presumably by recombining hydrogen atoms transferred from oxide to metal by reverse spillover. The rate of H 2 evolution from the Rh/Fe–Cr system at 280 °C is greater than that from Fe–Cr at 380 °C. Reoxidation by CO 2 is not promoted by Rh and is much slower than that by H 2O for the Rh-containing catalyst. Likewise, reoxidation by H 2O/CO 2 mixtures results in a much higher yield of H 2 relative to CO for the promoted catalyst compared with the unpromoted one. The extent to which catalysts previously equilibrated in WGS feeds at 350 °C can then be reduced in pulses of CO or H 2 depends on the composition of the WGS feed. Fe–Cr catalysts do not reduce in H 2 after exposure to H 2O/CO (2:1) or a reformate (H 2O/CO/H 2/CO 2) feed. Hydrogen reduction is possible with Rh/Fe–Cr after exposure to H 2O/CO but not after exposure to reformate, for which thermodynamic considerations suggest that the working oxidation state may be lower. The greater reducing power of CO leads to continuing reduction after treatment with both prior feeds. Overall, it is concluded that of the two steps that may restrict the rate of the WGS reaction over iron–chromium oxide catalysts (reduction by CO and H 2 generation through reoxidation by water), rhodium acts primarily by accelerating the second.

Dealing with a local heating effect when measuring catalytic solids in a reactor with Raman spectroscopy

In continuation of our previous work on the applicability of the G(R(infinity)) correction factor for the quantification of Raman spectra of coke during propane dehydrogenation experiments (Phys. Chem. Chem. Phys., 2005, 7, 211), research has been carried out on the potential of this correction factor for the quantification of supported metal oxides during reduction experiments. For this purpose, supported chromium oxide catalysts have been studied by combined in situ Raman and UV-Vis spectroscopy during temperature programmed reduction experiments with hydrogen as reducing agent. The goal was to quantify on-line the amount of Cr(6+) in a reactor based on the measured in situ Raman spectra. During these experiments, a significant temperature effect was observed, which has been investigated in more detail with a thermal imaging technique. The results revealed a temperature 'on the spot' that can exceed 100 degrees C. It implies that Raman spectroscopy can have a considerable effect on the local reaction conditions and explains observed inconsistencies between the in situ UV-Vis and Raman data. In order to minimize this heating effect, reduction of the laser power, mathematical matching of the spectroscopic data, a different cell design and a change in reaction conditions has been evaluated. It is demonstrated that increasing the reactor temperature is the most feasible method to solve the heating problem. Next, it allows the application of in situ Raman spectroscopy in a reliable quantitative way without the need of an internal standard.

Kinetics of the water–gas shift reaction over a rhodium-promoted iron–chromium oxide catalyst

Abstract The addition of a small amount of rhodium to a high temperature iron–chromia water–gas shift catalyst has been found to accelerate significantly the approach to equilibrium. The kinetics of the reaction have been studied in terms of the separate forward and backward reactions, and using a feed composition typical of those produced by steam reforming. It was found impossible to avoid the influence of mass transfer under relevant high-temperature operating conditions. A power rate law incorporating thermodynamic limitations best described the kinetic data for the reforming mixture. An equation based on forward and backward expressions was less satisfactory. Comparison of the observed kinetics with those reported in the literature for undoped catalysts indicates that the promoter has the greatest effect on the reactivity of carbon monoxide and hydrogen.