Supported Chromium Oxide Catalysts Research Articles

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.

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.

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.

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.

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.

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.

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.

Effect of modifiers on the activity of a Cr2O3/Al2O3 catalyst in the dehydrogenation of ethylbenzene with CO2

Chromium oxide supported on alumina was modified with various transition metal oxides by an incipient wetness method. The effect of modifiers on the activity of 20% Cr2O3/Al2O3 catalyst in the dehydrogenation of EB with CO2 was investigated. The activity is enhanced for ceria and vanadia modified supported chromium oxide catalysts, whereas the activity is decreased for catalysts modified with cobalt, manganese, molybdenum and zinc oxides. Carbon monoxide was detected in the course of the reaction, suggesting that carbon dioxide, which is one of the most important greenhouse gases, could be utilized as a mild oxidant in the oxidative dehydrogenation of ethylbenzene to styrene over unmodified and modified Cr2O3/Al2O3 catalysts.

Towards real-time spectroscopic process control for the dehydrogenation of propane over supported chromium oxide catalysts

UV–Vis and Raman spectroscopy are applied under realistic reaction conditions to investigate the changes occurring to an industrial-like supported chromium oxide catalyst during the dehydrogenation of propane. Linking the catalytic activity to the simultaneously collected spectroscopic data has shown that coke is a key parameter to the activity of the catalyst. A small amount of coke improves its activity, most likely by an improved adsorption of propane on the catalyst, but the activity of the catalyst drops when the amount of coke exceeds a certain value. This point is visualized by a sharp increase in the overall absorbance of the sample as can be observed by UV–Vis spectroscopy and this information can be used as a signal for starting a regeneration cycle.

Propane oxidative dehydrogenation over chromium oxide-based catalysts

Supported chromium oxide catalysts have been tested and found to be active in propane oxidative dehydrogenation (POD). The effects of various supports (γ-Al 2O 3, TiO 2, SiO 2, and MgO) and feed compositions were investigated. On the 10 wt.% chromium oxide on alumina, the propane degree of conversion was 16.7% and the selectivity to C 3H 6 was 54.1% at 450 °C. A sample of the catalysts was characterized using BET, XPS, and temperature-programmed reduction (TPR) techniques. A kinetics study was also performed on the catalyst. The rates of propane consumption and propylene productions were found to depend strongly on the oxygen partial pressures. Langmuir and redial-type rate expression were tested; the latter was found to give a better fit for the rate data. A model for the reaction was proposed.

Modeling of Ethylene Polymerization Kinetics over Supported Chromium Oxide Catalysts

AbstractSummary: Silica supported chromium oxide catalysts have been used for many years to manufacture polyethylene and they still account for more than 50% of world production of high‐density polyethylene. Along with its commercial success, the catalytic mechanism and polymerization kinetics of silica supported chromium oxide catalysts have been the subject of intense research. However, there is a lack of modeling effort for the quantitative prediction of polymerization rate and polymer molecular weight properties. The chromium oxide catalyzed ethylene polymerization is often characterized by the presence of an induction period followed by a steady increase in polymerization rate. The molecular weight distribution is also quite broad. In this paper, a two‐site kinetic model is developed for the modeling of ethylene polymerization over supported chromium oxide catalyst. To model the induction period, it is proposed that divalent chromium sites are deactivated by catalyst poison and the reactivation of the deactivated chromium sites is slow and rate controlling. To model the molecular weight distribution broadening, each active chromium site is assumed to have different monomer chain transfer ability. The experimental data of semibatch liquid slurry polymerization of ethylene is compared with the model simulations and a quite satisfactory agreement has been obtained for the polymerization conditions employed.Polymerization rates at different reaction temperatures: symbols – data, lines – model simulations.imagePolymerization rates at different reaction temperatures: symbols – data, lines – model simulations.

Polymerization rate modeling of ethylene polymerization with supported chromium oxide catalysts

AbstractSupported chromium oxide catalysts have long been used for the polymerization of ethylene to high‐density polyethylene. Unlike Ziegler–Natta catalysts, chromium oxide catalyst systems do not require cocatalysts to obtain high polymerization activity. One of the most distinctive characteristics of the chromium oxide catalyst is the presence of an induction period during the initial period of polymerization. The duration of induction period is dependent on many factors such as catalyst preparation, activation procedures, and polymerization conditions. After the induction period, the polymerization rate increases steadily with reaction time until it reaches a stationary value. Although chromium oxide catalysts have been used for years in industrial processes, there is still a dearth of literature concerning the kinetic modeling of chromium oxide catalyzed polymerization processes. In this article, a kinetic model is developed for the chromium oxide catalyzed ethylene polymerization in gas and slurry phases. In this model, it is proposed that the reaction byproduct generated by the reduction of hexavalent chromium species to Cr2+ by ethylene poisons the active sites. With the desorption of the poison species, the deactivated sites are transformed to active sites to polymerize ethylene. The validity of the proposed model is illustrated by comparing the model calculations with the experimental data obtained from gas phase and liquid slurry polymerization experiments. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2923–2927, 2004

Oxidation of methylene chloride on alumina supported chromium oxide catalysts

Chromium oxides of loading ranges from 5 to 15 wt. % on γ-alumina were tested. The optimum chromium oxide loading for the catalytic oxidation is 10 wt. %. Catalysts were investigated by BET, Raman spectroscopy and XPS. The formation of crystalline Cr2O3 has a detrimental effect on catalysts in CH2Cl2 oxidation.

Effect of potassium on alumina supported chromium oxide catalysts for methylene chloride oxidation

Alumina supported chromium oxide catalysts added potassium were prepared and tested for methylene chloride oxidation. They were investigated by XRD, BET, XPS, chemisorption. Chromium oxide catalyst added potassium of 1 wt.% has a large beneficial effect on activity.

Determining the active site in a catalytic process: Operando spectroscopy is more than a buzzword

Determining the active site in a catalyst material and elucidating the related reaction mechanism remain intellectual challenges and are of paramount importance for the rational development of new or better catalysts. Identifying active sites should in principle be possible by studying the catalyst at its working place under real reaction conditions. This is the field of operando spectroscopy, a research area, which is under intense development and of high interest to the catalysis community. This feature article presents an attempt of a roadmap for future operando spectroscopy studies. Two case studies will illustrate the potential and limitations of the operando approach. A first study deals with operando vibrational spectroscopy to obtain mechanistic insight in the destruction of chlorinated hydrocarbons with steam over lanthanide oxide-based catalysts. The second case study elaborates on the use of operando electronic spectroscopy to elucidate the active alkane dehydrogenation site in supported chromium oxide catalysts.

Catalytic decomposition of methylene chloride on oxidative carbon supported metal oxide catalysts II. Chromium oxide catalyst

Several carbon supported chromium oxide catalysts were prepared by varying textural and surface properties of support and tested for methylene chloride oxidation. They were investigated by TGA and XPS. The difference in acidity and high valence of chromium led to difference in activity.

Effect of modifiers on the reactivity of Cr2O3/Al2O3 and Cr2O3/TiO2 catalysts for the oxidative dehydrogenation of propane

Chromium oxide supported on alumina and titania supports was modified with oxides of sodium, vanadium and molybdenum. The modified and unmodified chromium oxide catalysts were characterized by several techniques. The presence of surface chromium oxide and surface molybdenum and vanadium oxide species was detected in the unmodified and molybdenum and vanadium oxide modified supported chromium oxide catalysts. The reducibility (Tmax and H/Cr ratio) of the surface chromium species was not affected for the vanadium and molybdenum oxide modified catalysts; however, the reducibility changed noticeably for sodium modified supported chromium oxide catalysts. Studies of the reactivity of the ODH of propane revealed the effect of modifiers on the reactivity properties of the surface chromium oxide species. The activity and propene selectivity decreased for sodium modified supported chromium oxide catalysts. However, the activity increased for vanadium oxide modified catalysts and was similar for molybdenum oxide modified catalysts irrespective of the support. The propene selectivity was higher for molybdenum oxide modified chromium oxide catalysts. However, the propene selectivity for vanadium oxide modified catalysts depends on the support since it appears that the inherent selectivity of the surface vanadium oxide species is reflected.

Oxidative Dehydrogenation of Propane over Supported Chromia Catalysts: Influence of Oxide Supports and Chromia Loading

Several loadings of chromium oxide ranging from sub- to above monolayer coverages were prepared on various oxide supports and characterized by various techniques. From a combination of Raman, XPS, and DRS characterization studies the monolayer surface coverages were determined to be ∼12, ∼4, ∼1, and ∼3% Cr 2 O 3 on the Al 2 O 3 , TiO 2 , SiO 2 , and SiO 2 –Al 2 O 3 supports, respectively. In situ Raman and DRS characterization studies under dehydrated conditions revealed the presence of dispersed surface Cr 6+ oxide species for samples below monolayer loadings. Additional Cr 5+ and δ-Cr 3+ species were detected by EPR. From the TPR studies, the reducibility of the surface chromium oxides ( T max ) was decreasing in the order TiO 2 <SiO 2 –Al 2 O 3 ∼Al 2 O 3 <SiO 2 . The reactivity studies using the oxidative dehydrogenation of propane revealed that the activity and propene selectivity of the supported chromia catalysts depend on the loading and the oxide support used. The highest activity was obtained on the Cr 2 O 3 /Al 2 O 3 sample, since the largest amount of chromium oxide could be dispersed on the supports. The selectivity to propene at monolayer coverages was similar for the CrAl and CrTi samples, since at these coverages exposed support sites are unavailable for degrading propene. For the CrSi and CrSiAl samples, however, a significant amount of support is exposed for degrading propene. The intrinsic activity of the surface chromium oxide species, the TOF values, follows the trend CrTi≅CrSi>CrSiAl≥CrAl. There is a difference between the variation in TOF values and the reducibility of the supported chromium oxide catalysts. Thus, it appears that the reducibility of the catalyst is not the only factor that is important in determining the activity of the surface chromium oxide species. Furthermore, based on the dispersion amount of chromium oxide on the different oxide supports, the CrTi catalysts appear to be the most suitable among the catalysts studied for the ODH of propane.