Supported Chromium Oxide Research Articles

Active and selective nature of supported CrOx for the oxidative dehydrogenation of propane with carbon dioxide

Although the supported chromium oxides are important industrial catalysts for a number of reactions including the propane dehydrogenation, the rational catalyst design is still challenged by the deficient understanding on the active and selective nature of the catalysts. In this work, CrOx/silicalite-1 with a Cr loading of 0.5–5 wt.% prepared by the impregnation method is probed for the oxidative dehydrogenation of propane with carbon dioxide (CO2-ODP) under industrially relevant conditions. Characterization results reveal that CrOx is highly dispersed over silicalite-1, and crystal Cr2O3 is still very limited even over the catalyst with the highest Cr loading. Moreover, the isolated and polymeric Cr6+ oxides coexist over all of CrOx/silicalite-1 catalysts, and the amount and the extent of the polymerization of the Cr6+ oxides are continuously increased with increasing the Cr loading. Reaction results indicate that the CrOx/silicalite-1 is superior for CO2-ODP, and the highest space-time yield of 16.7 kg(C3H6) h−1 kgCr)−1 is obtained over the catalyst with a Cr loading of 2 wt.%. By correlating with the reaction results, the polymeric Cr6+ oxides with a high degree of the polymerization are determined to be the most active but least selective sites for CO2-ODP, which is originated from the varied adsorption strengths of C3H8 and the different desorption behaviors of C3H6 on the specifically structured Cr6+ oxides over CrOx/silicalite-1. As a result of the strong adsorption of the C3H8 reactant and the produced C3H6 over the most active polymeric Cr6+ oxides, the deposited coke is found to play dual roles for both decreasing the C3H8 conversion and increasing the C3H6 selectivity. The catalytic results of CrOx/silicalite-1 for CO2-ODP are well explained with these understandings, which are important for guiding the development of the high-performance CrOx-based catalysts for dehydrogenation reactions, especially CO2-ODP.

Mesoporous Organo-Silica Supported Chromium Oxide Catalyst for Oxidative Dehydrogenation of Ethane to Ethylene with CO2

Chromium oxide supported on mesoporous organo-silica (MOS) was synthesized with different Cr loading by an incipient method. The catalytic performance of a Cr(x)/MOS catalyst for CO2-based ethane dehydrogenation was investigated. The synthesized catalysts were characterized by XRD, BET, TEM, SEM, XPS, FTIR, and UV–Vis DR measurements. The textural properties of the prepared samples showed that the mesoporous nature of MOS sample was not disturbed by chromium impregnation. Among the prepared samples, Cr(8)/MOS catalyst exhibited good distribution of chromium species along with superior concentration of Cr6+ and the highest recorded Cr6+/Cr3+ ratio. The results revealed that the superior catalytic performance was reached at Cr(8)/MOS, with 50.4% and 90.1% of ethane conversion and ethylene selectivity, respectively. The catalytic activity decreased slowly over reaction time; it declined approximately 22% after 10 h of stream operation. The roles of CO2-based ethane dehydrogenation were also studied, where carbon dioxide can be a source of lattice oxygen and as a hydrogen consumer in reverse water–gas shift (RWGS) reaction. The effect of various catalytic factors, such as catalytic temperature, reaction time, space gas velocity, and CO2 partial pressure on the conversion of ethane, yield, and selectivity to ethylene, were investigated as well.

Revealing nature of active site and reaction mechanism of supported chromium oxide catalyst in propane direct dehydrogenation

• The decisive role played by the oxidation state of Cr. • The comparison of reactivity between various oxygen species on CrO X catalysts. • The complete reaction pathway including regeneration. • The role of OH group on SiO 2 support. Chromium oxide is the conventional catalyst for propane direct dehydrogenation. However, the active site and reaction mechanism are still remaining unresolved due to the complicated valence of chromium and co-exists of several active sites. In this work, DFT calculations are performed to reveal the nature of active center and reaction mechanism for the supported chromium catalyst on silica. Three valences of chromium including Cr 3+ , Cr 5+ , and Cr 6+ are examined to give a unbiased survey of reactivity. The reaction pathway and C–H activation are carefully examined. Moreover, a detailed analysis of active site regeneration is performed. The reactivities of different oxygen groups attached to Cr including terminal, bridge, and OH group are evaluated at the same footing. Furthermore, the calculations also reveal the intrinsic catalytic properties of chromium which show comparable abilities with oxygen groups. The influence from OH group on support is also evaluated. The comparisons of different reaction pathway give useful indication on the possible reaction mechanism. In the end, the most favorable active site is identified. The current work not only deepen the understanding of catalytic properties of supported chromium catalyst but also supply the doable optimization methods for further improvements.

Oxidative dehydrogenation of ethane with carbon dioxide over silica molecular sieves supported chromium oxides: Pore size effect

SBA-15 with varied pore size from 4 to 8 nm were synthesized by tuning the temperature of hydrothermal treatment, the supports were then used to load the active phase CrO x through a conventional impregnation method. The resulting catalysts were characterized by small/wide angle XRD, N 2 adsorption/desorption, FT-IR, TEM-EDX, XPS, TPR and CO 2 -TPD to study the feature of structure, surface chemical state, redox and basicity. It was found from these results that the metal species could be well dispersed on catalysts with larger pore size. Cr 6+ species could enter into the framework by substituting the Si atoms of SBA-15, and Cr 3+ mainly exist on extra framework. Pore size had profound effects on reducibility, surface composition and basicity. Cr 6+ species were necessary to activate the C-H bonds of alkanes, while the basicity played an important role in activating C-O bonds of CO 2 . The best performances were achieved over the sample Cr supported on SBA-15 with a pore diameter of 7 nm in oxidative dehydrogenation of ethane in the presence of CO 2 . • Pore size of SBA-15 was tuned by changing hydrothermal temperature. • Cr 6+ could enter the framework of SBA-15 by partially replacing Si atoms. • Cr 6+ and Cr 3+ species co-existed on extra framework of SBA-15. • Cr 6+ species were important for activating alkane. • Activation of CO 2 was related to basicity of catalysts.

Investigation of Isobutane Dehydrogenation on CrOx/MCM-41 Catalyst

The syntheses of MCM-41 (Mobil Composition of Matter No. 41) supported chromium oxide cat-alysts at different chromium concentrations (4–10 % by mass) were carried out hydrothermally. The aim of this study was to determine the effect of chromium concentration in the catalyst structure on the chro-mate types and chromium oxidation states, as well as the activity of the catalyst in the isobutane dehydro-genation reaction. Inactive α-Cr2O3 crystals for isobutane dehydrogenation were shown to increase in the catalyst structure as the chromium loading increased. The highest amount of Cr6+ on the catalyst surface was detected in the catalyst (H4-MCM-41) with 4 % chromium by mass. Catalytic tests (T = 600 °C, P = atmospheric pressure, WHSV = 26 h–1) were performed under fixed bed reactor conditions. The high-est isobutane conversion (~60 %) and selectivity (~80 %) were observed on the H4-MCM-41 catalyst, which had the highest amount of Cr6+ and monochromate structures. Catalyst deactivation was not due to coke deposition but, rather, was caused by the formation of inactive α-Cr2O3 crystal structures.

Characterization and catalytic property of nano Cr-based catalysts for ethane dehydrogenation in CO2

ABSTRACTA series of CeO2 promoted nano Na-ZSM-5 supported chromium oxide catalysts were prepared using impregnation method and characterized by XRD, FE-SEM, DTA-TG, and CO2-TPD techniques. Their catalytic performance of ethane dehydrogenation to ethylene in the presence of CO2 was investigated. The addition of Ce species obviously improve the dispersion of active component Cr species on Na-ZSM-5 catalysts, leading to increase of the activity and stability of the catalysts, and reduction of the amount of carbon deposition on the catalyst surface at the same time. The nano 7.5Ce-7.5Cr/Na-ZSM-5 catalyst exhibits both high activity and stability, with an ethane conversion of ∼20% and ethylene yield of ∼19% without any obvious trend of deactivation in 360 min. Characterization results show that the excellent catalytic performance comes from well dispersion of CrOx species on nano ZSM-5 surface, proper acidic-basic property, and less carbon deposition.

Isolated Chromium(VI) Oxide Species Supported on Al-Modified Silica: A Molecular Description

Supported chromium oxide systems are widely used catalysts. Experimental data indicate that modification of the silica support with Al changes the properties of the surface Cr species. In this computational work, extensive density functional theory investigations of the monomeric Cr(VI) oxo species variously located on the Al-modified silica surface have been done using a large number of systematically generated periodic and cluster models. It is predicted that the surface modification can influence both the geometry and the relative stabilities of the Cr species. Monooxo Cr(VI) species located in the vicinity of two Al sites usually adopts a pseudotetrahedral geometry, in contrast to square pyramidal monooxo Cr(VI) species on silica. Under dehydrated conditions, the relative stabilities of the tetrahedral monooxo and dioxo Cr(VI) species are comparable, whereas the dioxo Cr(VI) species is thermodynamically preferred on silica. The obtained results explain the experimentally estimated higher ratio of the ...

Reduced graphene oxide supported chromium oxide hybrid as high efficient catalyst for oxygen reduction reaction

Reduced graphene oxide (rGO) supported nano-chromium oxide (Cr2O3/rGO) catalyst for oxygen reduction reaction (ORR) has been successfully synthesized by the pyrolysis of chromium-urea coordination compound. The structure and morphology of the hybrid are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) tests. XRD tests reveal that the Cr2O3 with hexagonal structure is obtained. SEM and TEM tests show that the nano-Cr2O3 is supported by rGO sheet. The cyclic voltammetry, tafel, linear scanning voltammetry and current-time chronoamperometric tests prove that the obtained Cr2O3/rGO hybrid has a remarkable catalytic activity and good stability for oxygen reduction. Both the rotating disc electrode and rotating ring disc electrode tests approve that the ORR major happens through 4-electron reaction style. The Cr2O3/rGO hybrid is a promising low cost and high performances catalyst for ORR of alkaline electrolyte.

Sono-dispersion of Cr over nanostructured LaAPSO-34 used in CO2 assisted dehydrogenation of ethane: Effects of Si/Al ratio and La incorporation

A series of SAPO-34 supported chromium oxide catalysts varying in the Si content and presence/absence of La3+ ions were prepared by sonochemically impregnation method and tested in the dehydrogenation of ethane with CO2. SAPO-34 supports with Si/Al ratios of 0.1, 0.2, 0.3 and 0.4 were synthesized via hydrothermal method. The samples were characterized by XRD, BET, FESEM, TEM, ICP, FTIR, EDX and TPD-NH3 techniques. The variation of silicon content indicates that the relative crystallinity, surface area and effective acidity of samples increased with the Si/Al ratio up to 0.2. However, a decreasing trend was observed with further increase of the silicon content. This can be justified by the appearance of irregular particles. It was found that there is the optimum silicon content in the SAPO-34(0.2) support for the best catalytic activity, attributable to higher relative crystallinity, surface area and appropriate surface acidity. The incorporation of La3+ into the SAPO-34(0.2) framework generated in LaAPSO-34 sample was proved by XRD and TEM techniques. The La incorporation not only increases the surface area but also neutralizes mostly strong acid sites, resulted in higher dispersion of Cr species and more effective surface acidity/basicity as evidenced by EDX and TPD-NH3 analysis, respectively. This reflects in higher content of redox Cr species stabilized in comparison with parent SAPO-34(0.2), which alongside homogenous and fine Cr nanoparticles account for the superior catalytic performance and stability of Cr/LaAPSO-34(0.2). It was found that this catalyst effectively dehydrogenated ethane to ethylene in the presence of CO2 at 700 °C even after 10 h on-stream operation, giving 42.3% ethylene yield. Following the obtained results, a possible reaction mechanism was also proposed.

Novel SiO2‐Supported Chromium Oxide(Cr)/Vanadium Oxide(V) Bimetallic Catalysts for Production of Bimodal Polyethylene

Novel SiO2‐supported Cr–V bimetallic oxides catalysts, which were capable of one‐pot synthesizing polyethylene with bimodal molecular weight distribution, had been successfully developed. These catalysts were prepared with a simple introduction of vanadium oxide onto the traditional Phillips CrOx/SiO2 catalyst. The polymerization activity over the Cr–V bimetallic catalysts was significantly improved compared with the V‐free Phillips CrOx/SiO2 catalyst. Moreover, the bimodal ethylene/1‐hexene copolymer prepared by the novel catalyst showed a better short chain branch distribution with more 1‐hexene comonomer inserted into the PE chains with high molecular weight, which would be favorable for the long term mechanical properties of the bimodal PE as potential high grade pipe materials.

Catalytic combustion of benzene over γ-alumina supported chromium oxide catalysts

The γ-alumina supported chromium oxide catalysts with Cr loadings varying from 1.7 to 13.6wt.% were prepared by the impregnation method and characterized using N2 adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV–vis spectroscopy and Raman spectroscopy. Benzene combustion catalyzed by the catalysts was investigated. The results showed that chromia was well dispersed on the surface of γ-Al2O3 with a monolayer dispersion capacity of 7.5wt.%. For the supported chromia catalyst with a Cr loading of 1.7wt.%, only Cr6+ species were observed. Increasing Cr loading to 5.1wt.% led to the presence of both Cr3+ and Cr6+ species. Catalytic benzene combustion over the catalysts was found to be strongly dependent on Cr loading amount. For supported chromia catalyst with a Cr loading of 8.5wt.%, complete benzene combustion was achieved at 350°C which was substantially lower than the temperature for other catalysts, indicating that supported chromia catalyst at a near monolayer dispersion capacity exhibited the optimum catalytic behavior for benzene combustion.

Microspherical chromium oxide/alumina catalyst KDM for fluidized-bed isobutane dehydrogenation: Development and industrial application experience

A microspherical chromium oxide/alumina catalyst (KDM) has been developed for fluidized-bed isobutane dehydrogenation. This catalyst shows a high catalytic activity, selectivity, and thermal stability. The KDM preparation technology involves an alumina support produced by centrifugal thermal activation (CTA) of gibbsite-CEFLAR™ technology. This technology makes it possible to regulate the phase composition of the CTA product in a wide range and to synthesize, from this product, supports with preset structural and textural properties. The support and catalyst production technologies have been commercialized by Altailyuminofor Co. Since 2008, the KDM catalyst has been employed by SIBUR Holding Co. and Ekooil Co. (Omsk). Industrial-scale application of the catalyst has demonstrated that isobutane dehydrogenation over an IM-2201 + KDM mixture reduces the catalyst consumption (kg/(t isobutylene)) from 22–23 for pure IM-2201 to 8-9.5 for the IM-2201 + KDM mixture. For further improvement of the KDM catalyst, fundamental studies of the nature of the active component of this catalyst have been carried out: the role of different states of supported chromium oxide species in dehydrogenation has been elucidated and approaches to controlling the proportion of active and selective chromium oxide species have been devised. These studies have culminated in the development of a catalyst modification whose activity and selectivity exceeds the same characteristics of the original KDM catalyst by 2–4 wt %.

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.

Photonic Activity for MB Solution of Metal Oxide/CNT Catalysts Derived from Different Organometallic Compounds

Multi-walled carbon nanotube (MWCNT) supported chromium oxide (Cr2O3), magnesium oxide (MgO), vanadous oxide (V2O3) catalysts were prepared from MWCNT with different organometallic compounds chromium acetylacetonate {Cr(C5H7O2)3}, magnesium acetate tetrahydrate {Mg(CH2COO)2·4H2O}, and vanadyl acetylacetonate {VO(C5H7O2)2}. The obtained catalysts were characterized by BET surface area, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray analysis (EDX). In addition, we used methylene blue (MB) in aqueous solution under condition of visible light irradiation to determine their photocatalytic degradation efficiency. In conclusion, the sample V2O3/CNT had excellent photocatalytic degradation for MB solution, which showed much higher photocatalytic degradation efficiency than samples Cr2O3/CNT and MgO/CNT.

Investigation of the doping effect of Ti on Supported Chromium Oxide Species on Silica

Investigation of the doping effect of Ti on Supported Chromium Oxide Species on Silica

Ethane dehydrogenation over pore-expanded mesoporous silica supported chromium oxide: 1. Catalysts preparation and characterization

Pore-expanded MCM-41 silica was prepared in two steps, namely synthesis of MCM-41 in the presence of cetyltrimethylammonium bromide (CTAB) at 100 °C followed by a hydrothermal pore expansion treatment in the presence of dimethyldecylamine (DMDA) at 120 °C for 72 h. The as-synthesized material (PE-MCM-41), the material obtained after selective ethanol extraction of DMDA (PE-MCM-41E) and the organic-free material obtained after calcination of PE-MCM-41 (PE-MCM-41C) were used as supports for chromium oxide up to a chromium loading of 7 wt.%. All catalysts were thoroughly characterized by X-ray diffraction, N 2 adsorption, temperature programmed reduction (TPR), Raman and X-ray photoelectron spectroscopy (XPS), and used for ethane dehydrogenation (DH). N 2 adsorption measurements showed that the pore and surface characteristics strongly depend on the catalyst preparation method and relatively independent of chromia loading. The structural properties of the chromium-loaded materials were associated with the effect of water on the support, which is dependent on how well the silica framework is protected by the occluded surfactant(s) against direct contact with water. XPS analysis showed that in most cases, the surface chromium contained significant amounts of Cr(III) and Cr(VI) and minor amounts of Cr(II). Furthermore, H 2-TPR results revealed that the extent of reduction of the catalysts depend on the chromia loading and the preparation method.

The charge percolation mechanism and simulation of Ziegler-Natta polymerizations, Part VII: Effects of the distribution of chromium active centers on silica on the polymerization of ethylene

The charge percolation mechanism (CPM) of olefin polymerization in the presence of transition metal compounds has been applied to explain the polymerization of ethylene by silica supported chromium oxide. In the previous work of this series, the fundamental issues and mechanism of this polymeri?zation were presented. In this work the compatibility of the CPM with the em?pirical findings is confirmed. The CPM has been applied to explain: the appea?rance of an induction period; the deactivation of active centers and the forma?tion of oligomers; the effects of chromium concentration on the silica surface, the silica surface discontinuity and the pore size of silica on polymerization and the formation of the structure of polyethylene. A mathematical model has been derived to explain the effects of the CrOx/SiO2 ratio on the productivity of Phil?lips catalysts in the polymerization of ethylene. The empirical findings have also been confirmed by computer simulations.

The charge percolation mechanism and simulation of Ziegler-Natta polymerizations, part VI: Mechanism of ethylene polymerization by supported chromium oxide

Despite intensive research over the last 50 years, many questions concerning ethylene polymerization by supported chromium oxide are still unanswered. Hence, the very fundamental issues of this polymerization are discussed in this paper. It is shown that a charge percolation mechanism (CPM) of olefin polymerization by Ziegler-Natta transition metal complexes, recently proposed by us, can give the answers in this case, too.

Studies on CrO x /La2O3/ZrO2 catalysts modified by Mg

Lanthana–zirconia supported chromium oxide and magnesium chromium mixed oxide catalysts were studied in the dehydrocyclization of n-octane and characterized by temperature-programmed desorption of NH3, temperature-programmed reduction, XPS and DRIFTS. The Mg-free catalyst shows the highest activity, but suffers from rapid deactivation due to coke formation. The addition of Mg decreases the initial activity of the supported chromium oxide and retards its deactivation. The characterization results reveal that the deactivation retarding effect of Mg species not only consists in the deletion of strong acid sites but also in the decoration and/or dilution of Cr3+ oxide cluster, supposedly due to the formation of Mg–Cr surface compounds and, thus, in preventing the formation of coke.

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.