Chromium-based Catalyst Research Articles

CrOx/Ce1−xZrxO2 for chemical looping propane oxidative dehydrogenation: The redox interaction between CrOx and the support

Propane dehydrogenation is an attractive alternative for propylene production to petroleum-based routes. However, overcoming deactivation and toxicity of catalysts in propane direct dehydrogenation (PDH), and over oxidation in oxidative dehydrogenation (ODHP) remain challenging. Here, a chromium-based catalyst supported on a Ce-Zr solid solution is constructed for chemical looping oxidative propane dehydrogenation (CL-ODHP). A dual modulation of oxygen species by Zr doping was unraveled by characterizations and DFT calculation: the oxygen of surface CrOx was constrained while the oxygen mobility of the Ce-Zr support was promoted. Based on these properties, the interaction between surface CrOx and the support further enables an oxygen “donor–acceptor”, which potentially functions as in situ activation and regeneration of redox CrOx species as well as blockage of non-selective oxygen released from the Ce-Zr support. The optimized material (7.5Cr/Ce0.8Zr0.2O2) achieves propane conversion up to 50% and propylene yield of 31% at 600 °C, and has the lowest Cr content among previously reported Cr-based catalysts that have comparable performance. These results provide insights for the design of highly active and selective redox catalysts for chemical looping as a reaction engineering strategy for the upgrading of propane dehydrogenation processes.

Catalytic tri- and tetramerization of ethylene: a mechanistic overview

ABSTRACT On purpose, selective oligomerization of ethylene to its linear trimer and tetramer such as 1-hexene and 1-octene has assumed significant commercial interest. Catalytic systems based on chromium and titanium have shown remarkable selectivity and productivity for the tri and tetramerization of ethylene to 1-hexene or 1-octene. Chromium-based catalysts are the most selective and active and show the highest structural diversity. This article discusses the most recent mechanistic approaches regarding active catalytic species that determine the selectivity to either hexene-1 or octene-1 and reaction parameters that control selectivity of byproducts. Isotopic labeling protocols, in-situ spectroscopic investigations and DFT studies on selected chromium-based catalyst systems are reviewed.

Chromium-based catalyst for HFC-125 synthesis: promoters effect

A variation of promoters, including rare-earth elements, allows to control of the specific surface and, perhaps, oxidation state of chromium and, consequently, catalyst activity and selectivity of chromium-based catalyst for HFC-125. To improve the catalytic properties of the 15% Cr2O3 γ - 85% -Al2O3 oxide system were added promoters (Ni or Cu or Co) in an amount of 5 wt% in terms of oxides. It was found that additional promotion Cr-Al samples by nickel and copper allow to increase the specific surface area of about 25-40% and the activity increased about 2 times. Modification of cobalt resulted in a decrease of the surface by 20% and the activity decreased by about 2 times.

Heterolytic Activation of C-H Bonds on Cr(III)-O Surface Sites Is a Key Step in Catalytic Polymerization of Ethylene and Dehydrogenation of Propane.

We describe the reactivity of well-defined chromium silicates toward ethylene and propane. The initial motivation for this study was to obtain a molecular understanding of the Phillips polymerization catalyst. The Phillips catalyst contains reduced chromium sites on silica and catalyzes the polymerization of ethylene without activators or a preformed Cr-C bond. Cr(II) sites are commonly proposed active sites in this catalyst. We synthesized and characterized well-defined chromium(II) silicates and found that these materials, slightly contaminated with a minor amount of Cr(III) sites, have poor polymerization activity and few active sites. In contrast, chromium(III) silicates have 1 order of magnitude higher activity. The chromium(III) silicates initiate polymerization by the activation of a C-H bond of ethylene. Density functional theory analysis of this process showed that the C-H bond activation step is heterolytic and corresponds to a σ-bond metathesis type process. The same well-defined chromium(III) silicate catalyzes the dehydrogenation of propane at elevated temperatures with activities similar to those of a related industrial chromium-based catalyst. This reaction also involves a key heterolytic C-H bond activation step similar to that described for ethylene but with a significantly higher energy barrier. The higher energy barrier is consistent with the higher pKa of the C-H bond in propane compared to the C-H bond in ethylene. In both cases, the rate-determining step is the heterolytic C-H bond activation.

Influence of Ph2PN(R)P(Ph)N(R)H (R=c-Hex,iPr) Ligand Substituents, Modifier, and Process Parameters on Reaction Kinetics of a Chromium-based Catalyst for the Selective Trimerization of Ethylene

The influence of ligand substituents on a homogeneous, highly selective catalyst system for the ethylene trimerization to 1-hexene, consisting of chromium(III)acetylacetonate as a chromium source, tetraphenylphosphonium chloride ([Ph4P]Cl) or dodecyltrimethylammonium chloride ([CH3(CH2)11N(CH3)3]Cl) as a modifier and chlorine source, Ph2PN(iPr)P(Ph)N(iPr)H (L0) as a ligand (Ph=phenyl, iPr=isopropyl), and triethylaluminum as an activator, was investigated. It was shown that the phosphonium or ammonium salts act as both chlorine source and as modifier for the activator triethylaluminum. Additionally, a systematic parameter study using Ph2PN(c-Hex)P(Ph)N(c-Hex)H (L1) as the ligand (c-Hex=cyclohexyl) was performed and important performance figures for catalyst systems using L0 and L1 ligands, respectively, were compared under different reaction conditions. As a result, both catalyst systems demonstrate very similar kinetics at identical ethylene pressures, temperatures, and ligand concentrations. Additionally, the influence of the modifiers on the performance of the catalyst systems at different ethylene pressures and temperatures was studied in detail. The results suggest that the modifier has a greater influence on the reaction mechanism than expected. Moreover, the influence of the modifier on the catalytic performance was found to be more significant than the impact of the ligand’s chemical nature.

The Effect of Substituents in PNPNH Ligands on Activity and Selectivity of a Chromium-Based Catalyst System for Highly Selective Ethylene Trimerization

The influence of ligand substituents on a highly selective homogeneous catalyst system for the trimerization of ethylene to 1-hexene consisting of chromium(III) acetylacetonate, a ligand with a PNPNH backbone, tetraphenylphosphonium chloride as chlorine source, and triethyl aluminum as activator was investigated. These N-modified and P-modified ligands, which differ strongly in their electronic and steric properties of the substituents, were tested in the ethylene trimerization at three temperatures (50, 70, and 90 °C). Interestingly, the substituents at the N atoms of the PNPNH-backbone had only little effect on the catalytic activity, on 1-hexene selectivity, and on thermal stability of the catalyst system, whereas the substituents at the P atoms significantly influenced the efficiency of the system. This is even more surprising as the PNP ligand from SASOL for ethylene tetramerization is much more sensitive against substituent variation at its N atom.

Laboratory- and bench-scale studies of a sweet water–gas-shift catalyst for H2 and CO2 production in pre-combustion CO2 capture

In the first section of this paper, the activity of an iron–chromium-based catalyst for the water–gas-shift reaction is studied at laboratory scale. The influence on the activity of the sweet shift catalyst of the main operating parameters-temperature, space velocity, excess steam, pressure, and gas composition-is evaluated. Base case gas composition M1 was set according to that of the IGCC Power Plant of Puertollano, Spain (ELCOGAS): N2 13.4vol%, CO 60.7vol%, H2 22.3vol%, and CO2 3.6vol%. The effect of gas hourly space velocity GHSV (2000–5000h−1), steam to CO ratio R, (1–3molmol−1), pressure P (0.1–2MPa), and temperature (523–653K) has been determined. In order to study the effect of gas composition on the activity of the WGS catalyst, the following gas mixtures have been evaluated: M2 (CO 60.7vol%, H2 22.3vol%, and CO2 17vol%); M3 (CO 60vol%, N2 40vol%); and M4 (CO 60vol%, and CO2 40vol%).The second part of this paper deals with subsequent WGS testing at bench-scale level, based on the results obtained at laboratory scale. Tests were performed using a gas composition according to that of ELCOGAS IGCC plant. The operating temperature was 583K, pressure was set at 1MPa, and gas space velocity was 4715h−1. The effect of steam to CO ratio was evaluated.

A novel SiO2-supported inorganic and organic hybrid chromium-based catalyst for ethylene polymerization

In this work, a novel SiO2-supported hybrid chromium-based catalyst for ethylene and ethylene/1-hexene polymerization was prepared by using the residual surface hydroxyl groups in Phillips catalyst to further support bis(triphenysilyl) chromate (BC) in order to get the merits from two important chromium-based catalysts namely inorganic Phillips and organic S-2 catalysts. By characterization of the catalysts, investigation of the polymerization behavior and the microstructures of polymers, several vital factors such as cocatalyst, the addition amount of BC, calcination temperature, total chromium loading, polymerization temperature, pressure and hydrogen were systematically investigated. The proper addition weight amount of BC was CrBC≤0.25wt%. The suitable calcination temperature and total chromium loading were ≤600°C and ≤0.5wt%, respectively. With increasing the relative addition amount of CrBC from 0 to 20, 50, 80 and 100wt% (corresponding from Phillips to HCat-1, HCat-2, HCat-3 and S-2 catalysts), the copolymerization abilities of the above catalysts showed a decreasing tendency. By comparing different chromium catalysts, HCat-2 catalyst showed higher activity than S-2 catalyst and its polymer had higher average molecular weight (MW) than that obtained from Phillips catalyst. HCat-2 catalyst got well-balanced properties mainly including activity, MW of polymers and copolymerization ability.

Chromium-Based Catalyst for Ethylene Tetramerization to 1-Octene

Novel diphoshinoamine(PNRNP) ligands have been used in ethylene oligomerization with Cr(III) compounds and cocatalyst. The results indicate that the catalyst system can yield 1-octene in selectivities up to 80%. Another novel diphoshinoamine ligands (((Ar2P)2N)2R)(R=cyclopropylamine) also were synthesized and characterized. Factors which affect the result of ethylene tetramerisation, such as cocatalyst, temperature, pressure and the molar ratio of Al/Cr were examined. Steric effects and electric effects were also considered in this report.

Kinetics study of the perchloroethylene hydrofluorination into pentafluoroethane (Freon 125) over chromium-based catalyst

The gas-phase hydrofluorination of perchloroethylene (PCE) into pentafluoroethane (non ozone-depleting Freon 125) at T = 330–390 °C and P = 0.4 MPa in the presence of a chromium–magnesium catalyst has been studied. A hydrofluorination scheme including main product and by-products formation routes is deduced from the observed dependences of particular products selectivity. The kinetic model of the PCE hydrofluorination has been developed. The rate parameters and the activation energy of the reactions were determined. The model satisfactorily describes the experimental data in the temperature range 330–390 °C.

Synthesis and characterization of Cr-MSU-1 and its catalytic application for oxidation of styrene

Chromium-containing mesoporous silica material Cr-MSU-1 was synthesized using lauryl alcohol–polyoxyethylene (23) ether as templating agent under the neutral pH condition by two-step method. The sample was characterized by XRD, TEM, FT-IR, UV–Vis, ESR, ICP-AES and N 2 adsorption. Its catalytic performance for oxidation of styrene was studied. Effects of the solvent used, the styrene/H 2O 2 mole ratio and the reaction temperature and time on the oxidation of styrene over the Cr-MSU-1 catalyst were examined. The results indicate that Cr ions have been successfully incorporated into the framework of MSU-1 and the Cr-MSU-1 material has a uniform worm-like holes mesoporous structure. After Cr-MSU-1 is calcined, most of Cr 3+ is oxidized to Cr 5+ and Cr 6+ in tetrahedral coordination and no extra-framework Cr 2O 3 is formed. The Cr-MSU-1 catalyst is highly active for the selective oxidation of styrene and the main reaction products over Cr-MSU-1 are benzaldehyde and phenylacetaldehyde. Its catalytic performance remains stable within five repeated runs and no leaching is noticed for this chromium-based catalyst.

Characterization of HTS and LTS Reactions in Microstructured Reactors

The production of hydrogen from synthesis gas with a high CO content is investigated with microstructured reactors. The main advantages of using microstructured reactors are the excellent heat and mass transfer characteristics inside of the reactor and the possibility to include heat exchangers to reduce the system size and heat losses. High- and low-temperature water gas shift reactions are examined for the ability to decrease the CO fraction of the synthesis gas down to a level tolerable for proton exchange membrane fuel cells. These gas purification methods have the advantage of a higher hydrogen yield being achievable by converting CO into H 2 , thus leading to a higher overall efficiency of the fuel cell system. A next step in further reducing the CO content could be-if necessary-preferential oxidation. The high-temperature shift reaction takes place at a temperature of approximately 350 °C in the presence of an iron- or chromium-based catalyst. The low-temperature shift reaction is operated at approximately 250 °C with copper catalysts. The performance of microstructured reactors fitted with different catalysts is discussed within this paper as a function of the temperature, gas flow rate, and gas composition.

Experimental study of the gas-phase hydrofluorination of perchloroethylene over a chromium-based catalyst

Gas-phase hydrofluorination of perchloroethylene to pentafluoroethane in the presence of a chromium-magnesium catalyst at 0.4 MPa and 330-390°C has been studied. А reaction scheme taking into account the formation of by-products is suggested.

Experimental study of the gas-phase hydrofluorination of perchloroethylene over a chromium-based catalyst

Gas-phase hydrofluorination of perchloroethylene to pentafluoroethane in the presence of a chromium-magnesium catalyst at 0.4 MPa and 330-390°C has been studied. А reaction scheme taking into account the formation of by-products is suggested.

Effect of structural parameters on rapid crack propagation and slow crack growth in high density polyethylene pipeline materials

The influence of molecular structure and microstructure on the fracture of high density polyethylene materials has been studied. By changing the chromium-based catalyst composition and optimising polymerisation conditions, polymers having differing levels of branching, comonomer unit placement and breadth of molecular weight (MW) distribution have been prepared. Structural analyses obtained by gel permeation chromatography (GPC), thermal analysis and elution fractionation were correlated with mechanical characteristics obtained from S4 tests, notch pipe tests and modified plane strain impact Charpy tests. The structural factors underlying this correlation are discussed. Specifically, the ability to place short chain branches into high MW fractions appears to provide good resistance to slow crack growth, whereas the degree of crystallinity and lamellar thickness play a decisive role in the rapid crack propagation process. Polymer grades with improved resistance to both rapid crack propagation and slow crack growth have been successfully produced.

Ethylene trimerisation and tetramerisation catalysts with polar-substituted diphosphinoamine ligands

Chromium-based catalyst systems with polar-substituted diphosphinoamine ligands are selective for either trimerisation or tetramerisation of ethylene, depending on the position of the polar groups on the aryl rings.

The effect of halide and the coordination geometry of chromium center in homogeneous catalyst system for ethylene trimerization

In homogeneous chromium-based catalyst system [Cr(III) tris(2-ethylhexanoate)/2,5-dimethylpyrrole/AlEt3/halide], the effect of halide compound to 1-hexene formation selectivity and catalytic activity was investigated. The comparative studies showed that the promotion effect of a chloride was much better than that of a corresponding fluoride or bromide and the best kind of halides is multi-chloro compound which shows significant promotion effect to assist chromium center to achieve high 1-hexene formation selectivity and highly catalytic activity. In catalyst system, facial coordination geometry of chromium center is much more favorable for ethylene trimerization than meridional coordination geometry. The effect of halide could be interpreted well from the stabilization on facial geometry of chromium complex through the coordination of halo group with chromium center in some specific ways. Under the guide of the roles proposed on halide component, a new catalyst system [Cr(2-ethylhexanoate)3/2,5-dimethylpyrrole/AlEt3/2-fluoro-6-chloro-α, α, α-trichlorotoluene] was developed successfully, demonstrating both high 1-hexene formation selectivity and highly catalytic activity.

Mesoporous (Cr)MCM-41: A Mild and Efficient Heterogeneous Catalyst for Selective Oxidation of Cyclohexane

Liquid-phase oxidation of cyclohexane was carried out at moderate reaction conditions over mesoporous (Cr)MCM-41 molecular sieves using acetic acid as solvent, hydrogen peroxide as oxidant, and methyl ethyl ketone as initiator. Under these conditions, the catalysts showed high substrate conversion and excellent product selectivity. Although the activity of the catalyst decreased after the first recycle, owing to leaching of nonframework chromium ions, it, however, remained nearly the same thereafter. This observation was further confirmed by washing experiments where the nonframework chromium ions were removed upon ammonium acetate treatment or under reaction conditions. Further, the washed catalyst also showed a activity similar to that of the recycled catalyst. The recycled or washed (Cr)MCM-41, therefore, behaves truly as a heterogeneous catalyst, and thus it forms the first example of a nonleaching chromium-based catalyst reported so far. The use of a stronger oxidizing agent, e.g., tertiary butyl hydroperoxide, resulted in the formation of cyclohexanone as the major product. On the other hand, the use of solvents like methanol, dioxan, and acetone showed lower conversion. Likewise, the use of initiators such as acetone, cyclohexanone, and acetaldehyde also resulted in lower activity.

Mesoporous (Cr)MCM-41: A Mild and Efficient Heterogeneous Catalyst for Selective Oxidation of Cyclohexane

Liquid-phase oxidation of cyclohexane was carried out at moderate reaction conditions over mesoporous (Cr)MCM-41 molecular sieves using acetic acid as solvent, hydrogen peroxide as oxidant, and methyl ethyl ketone as initiator. Under these conditions, the catalysts showed high substrate conversion and excellent product selectivity. Although the activity of the catalyst decreased after the first recycle, owing to leaching of nonframework chromium ions, it, however, remained nearly the same thereafter. This observation was further confirmed by washing experiments where the nonframework chromium ions were removed upon ammonium acetate treatment or under reaction conditions. Further, the washed catalyst also showed a activity similar to that of the recycled catalyst. The recycled or washed (Cr)MCM-41, therefore, behaves truly as a heterogeneous catalyst, and thus it forms the first example of a nonleaching chromium-based catalyst reported so far. The use of a stronger oxidizing agent, e.g., tertiary butyl hydroperoxide, resulted in the formation of cyclohexanone as the major product. On the other hand, the use of solvents like methanol, dioxan, and acetone showed lower conversion. Likewise, the use of initiators such as acetone, cyclohexanone, and acetaldehyde also resulted in lower activity.

A new chromium-based catalyst coated with paraffin for ethylene oligomerization and the effect of chromium state on oligomerization selectivity

A new method is proposed in which a chromium-based catalyst system containing chromium(III) tris (2-ethylhexanoate) (Cr(EH) 3), 2,5-dimethylpyrrole (2,5-DMP), GeCl 4 and triethylaluminum (TEA) coated with solid paraffin is used for oligomerization of ethylene. GC-MS and 1 H -NMR analyses show that the coated catalyst system prepared via this approach is air-insensitive so it can be stored under ambient conditions. The selectivity for 1-hexene is up to 97.91 wt.%, and only slightly decreases when the catalyst system is exposed in air for 24 h; in contrast, the selectivity is only 35.6% for its non-coated counterpart. XPS reveals that an attachment of atmospheric oxygen to catalyst makes the chemical state of chromium change from Cr 3+ to Cr 6+. Such a change, in turn, reduces remarkably its selectivity for 1-hexene.