Chromia Alumina Research Articles

Preliminary results from a study of luminescent materials—For application in the beam imaging system at the ESS

As part of the development of the beam imaging system at the European Spallation Source, luminescent screens have been fabricated by the flame spraying of scintillating materials onto stainless steel backings. A total of seven screens were produced, three of chromia alumina (Al2O3:Cr), two of YAG (Y3Al5O12:Ce) and two of a 50/50 mix of these. The properties of these screens under proton irradiation were evaluated using a 2.55MeV proton beam at currents of up to 10 μA. Irradiation times were up to 25 h per sample, during which luminescence-, spectrographic-, thermal- and current-data was sampled at a rate of 1Hz. Preliminary results of these measurements are reported here; with a quantitative analysis presented for one of the chromia alumina screens and a qualitative comparison of all three material types. The luminescent yield for chromia alumina was determined to be around 2000photons/MeV for a virgin screen, and was found to drop to 1.5% after 167mC of proton irradiation. A recovery of the luminescence of chromia alumina to >60% was observed after beam current was reduced for an 8 h period. Observations indicate that the YAG and mixed composition screens retain higher luminescence than the chromia alumina even at temperatures of over 200 °C. It is indicated that the luminescence from YAG feeds the R-lines of chromia alumina in the mixed composition screens.

Influence of the granular component type and the ratio of batch components on the properties of pressed alumina-chromia-zirconia-silica refractories with a mass fraction of 60 % Cr2O3

Investigations on the influence of granular component type and ratio of batch components on the properties of pressed alumina-chromia-zirconiasilica refractories with a content of 60 % Cr2O3 have been carried out.
 The optimal composition of batch, which includes scrap bakor products, scrap chromia products, alumina and metallurgical chromium oxide, was established.
 This composition provides, after firing in an oxidizing atmosphere, obtaining refractory samples with an open porosity of 23.1 %, an apparent density of 3.52 g/cm3, cold crushing strength 120 N/mm2, thermal shockresistance 950 °C — water 5 thermal cycles.It was shown that, the introduction of granular tabular alumina intothe batch composition (instead of granular fused white alumina) leads to adecrease in the open porosity of samples, although to a lesser extent thanwith the introduction of scrap bakor products.It was established that, for samples containing 60 % metallurgicalchromium oxide, the introduction of scrap bakor products instead of fusedwhite alumina does not lead to a significant decrease in their open porosity.
 It was shown that, firing in a reducing atmosphere and titanium oxide addition do not significantly affect the increase in apparent density and cold crushing strength of the studied samples.
 Comparative studies of the corrosion resistance of alumina-chromiazirconia-silica refractories containing 60 % Cr2O3 of different composition to melts of E glass and basalt have found that, E glass and basalt impregnate refractories, and under the influence of glass, some samples were slightly corroded. The samples of developed optimal composition were less impregnated than samples of other compositions and were not corroded by E glass and basalt, which makes it possible to predict an increase in the stability of these refractories in service in glass furnaces for the production of glass fiber from E glass and basalt.
 As a result of the carried out research, the technology of aluminachromia-zirconia-silica refractories with a content of 60 % Cr2O3 has been improved.

Screening of Granulated Catalysts for the Dehydrogenation of Light C4 Paraffins

Methodological aspects of the screening of granular chromia–alumina dehydrogenation catalysts are discussed, along with ways of estimating their lifetime by treating catalysts in a steam–air medium. The efficiency of using small reactors (with diameters of less than four effective granule sizes) to compare the catalytic activities of catalysts at the screening stage is demonstrated by the example of Mg- and Zr-promoted chromia–alumina catalysts.

Influence of Method of Introduction of Cu- and Zn-Based Modifiers on the Properties of Chromia–Alumina Catalysts

Three methods of introduction of modifiers based on Cu and Zn compounds into the CrOx/Al2O3 catalysts for dehydrogenation of light paraffin hydrocarbons are considered: Introduction from sol, introduction using successive impregnation technique and introduction of modifiers by impregnation along with precursor of chromium oxide. The obtained samples are studied by a complex of physical-chemical methods (XRD, UV-Vis spectroscopy, temperature-programmed reduction with hydrogen (TPR-H2), X-ray fluorescent (XRF) spectrometry, low-temperature N2 sorption). The catalytic properties of the samples are studied in kinetic mode in isobutane dehydrogenation. Cu- and Zn-modifiers are shown to influence on the peculiarities of reduction of Cr6+ and, hence, specify the state of active surface of CrOx/Al2O3 catalysts formed in the reductive reaction medium. Not only do the states of modifiers influence on the initial activity of the catalyst, but also on its activity after oxidative regeneration. Introduction of modifiers by successive impregnation method results in formation of copper and zinc aluminates or defective spinels on the Al2O3 surface. When the active component is introduced, the modified surface of the support promotes formation and stabilization of Cr6+ sites that can undergo reversible reduction–oxidation and provide high activity and selectivity towards formation of isobutylene (>98%).

Recycling of Wastes from the Production of Alumina-Based Catalyst Carriers

Preparation of microspherical chromia–alumina KDM-type catalysts for alkane dehydrogenation is based on utilization of an amorphous nanostructured hydroxide–oxide Al2O3–x(OH)2·nH2O (where x = 0–0.28, n = 0.03–1.8) prepared via the centrifugal thermal activation (CTA) of gibbsite. The fine fraction of this carrier is considered as industrial waste, requiring efficient recycling options, which are explored in this work. A range of physicochemical methods was applied to analyze the main properties (phase composition, morphology, reactivity, etc.) of pilot and industrial batches of gibbsite CTA products. It has been clearly shown that the properties of CTA products are dependent on the nature of gibbsite used. The largest amount of the active amorphous phase of alumina is formed using gibbsite made from concentrated nepheline. Regularities of rehydration of the CTA-products fine fraction in acidic/alkaline media and dissolution in sulfuric acid of the fine fraction of gibbsite and the industrial CTA product w...

Effect of the design of a feedstock injection device in a fluidized-bed reactor on the efficiency of the reaction using the dehydrogenation of iso-paraffins in a fluidized chromia–alumina catalyst bed as an example

Mathematical modeling is performed for the operation of two units of industrial chemical fluidized-bed reactors with different gas feedstock injection devices, i.e, three toroidal rings with nozzles in unit 1 and a false bottom with nozzles distributed over it in unit 2. Efficiency is analyzed (using the target product (iso-butylene) yield) for the operation of the two units over 4 months under industrial conditions and revealed the higher efficiency of unit 2. To dedetrmine the reasons for different product yields in the two units, a numerical solution is found by mathematical modeling to obtain characteristic pictures of catalyst particle concentrations and temperature fields in these units. It is concluded that unit 2 is characterized by a more uniform and dense distribution of the catalyst along with more uniform heating of the reactor. Pictures of the principal catalyst circulation flows are plotted to explain the considerable difference between the catalyst concentrations and gas temperature fields. Based on the numerical solution, the operational efficiency of the two units is subjected to comparative analysis, which showed good agreement with the results from an analysis of industrial reactors. The approach used in this work could be used in designing new units and optimizing existing units.

Isobutane dehydrogenation over chromia alumina catalysts prepared from MIL-101: Insight into chromium species on activity and selectivity

Various mesoporous chromia alumina catalysts were prepared by five different methods based on a metal-organic framework MIL-101 and their catalytic performances over isobutane dehydrogenation were investigated. The highly dispersed chromium species were produced on catalyst KCrAl-I1 with largest specific surface area of 198 m2·g−1 prepared with aluminium isopropoxide (Al(i-OC3H7)3) by ultrasonic impregnation method. However, the catalyst KCrAl-I2 synthesized by stirring impregnation possessed crystalline α-Cr2O3 phase, which was poorly dispersed. Two types of Cr-rich and Al-rich CrxAl2–xO3 solid solutions, designated as CrAl-I and CrAl-II phase, were formed over the catalysts KCrAl-I3 (prepared by Al(i-OC3H7)3 with nitric acid regulation), KCrAl-C4 (prepared by aluminium chloride hexahydrate) and KCrAl-N5 (prepared by aluminium nitrate nonahydrate). Catalytic evaluation results revealed that KCrAl-I1 exhibited the high isobutane conversion due to its highly dispersed chromium species. However, KCrAl-I3, KCrAl-C4 and KCrAl-N5 showed the higher isobutene selectivity (95.2 %–96.4%) on account of the formation of chromia alumina solid solutions in the catalysts. Moreover, the solid solution over the chromia alumina catalysts could greatly suppress the coke formation.

Synthesis and catalytic application in isobutane dehydrogenation of the mesoporous chromia/alumina catalysts based on a metal–organic framework

The mesoporous chromia/alumina (Cr2O3/Al2O3) catalysts were successfully synthesized using a porous metal–organic framework MIL-101 (Cr3F(H2O)2O(BDC)3·nH2O, BDC=1,4-benzenedicarboxylate) as a molecular host and chromium precursor, inorganic aluminium salt as the aluminium precursor. The aluminium sources had the significant effects on the structure of the products. The formation of α-Cr2O3 phase was observed in the mesoporous catalyst (Cr2O3/Al2O3–C) prepared by AlCl3·6H2O, whereas additional chromia alumina solid solution CrxAl2−xO3 phase was produced in the catalyst (Cr2O3/Al2O3–N) using Al(NO3)3·9H2O as the aluminium precursor. The surface Cr species existed in the Cr6+ and Cr3+ state over the mesoporous catalysts. The Cr species had a strong interaction with the alumina support. Preliminary catalytic studies showed that the Cr2O3/Al2O3–N catalyst exhibited much higher isobutene selectivity and higher stability than the reference catalyst in the isobutane dehydrogenation. The maintainable dehydrogenation activity during the five dehydrogenation-regeneration cycles indicated high regenerative ability of the catalyst Cr2O3/Al2O3–N. Consequently, this study represents a feasible way toward the facile synthesis of the mesoporous chromia/alumina catalyst. Moreover, this work proposes a novel application of metal–organic framework.

Development of Dehydrogenation Catalysts and Processes

Catalytic dehydrogenation plays an important role in production of light (C3–C4 carbon range), detergent range (C10–C13 carbon range) olefins and for ethylbenzene dehydrogenation to styrene. During the World War II, catalytic dehydrogenation of butane over a chromia–alumina catalyst was practiced for the production of butenes that were dimerized to octenes and hydrogenated to octanes to yield high-octane aviation fuels. The earlier catalyst development employed chromia–alumina catalyst and more recent catalytic developments use platinum or modified platinum catalysts. Dehydrogenation is a highly endothermic process and as such is an equilibrium limited reaction. Thus important aspects in dehydrogenation entail approaching equilibrium or near-equilibrium conversion while minimizing side reactions and coke formation.

Dehydrogenations in fluidized bed: Catalysis and reactor engineering

Dehydrogenation is a widely exploited route for large scale production of pure olefins. Its industrial application is however complicated by clashing kinetic and thermodynamic constraints. A commercial bubbling fluidized bed technology for isobutylene production from isobutane has been developed to solve these constraints. An economically attractive implementation was achieved by defining an optimal synergy between chromia alumina catalyst design and reactor engineering. The fluidized bed know-how developed has also led to the development of a revolutionary technology for the production of styrene from ethylbenzene using a riser reactor and a gallium based catalyst.

Application of a carbon membrane reactor for dehydrogenation reactions

This research tests a membrane reactor, equipped with a molecular-sieve carbon membrane, using isobutane dehydrogenation on a chromia alumina catalyst as a model reaction. Most pores of the carbon membrane employed are 6– 8 A in size and previous independent transport studies show that the permeability ratio of hydrogen to isobutene is larger than 100. These features make the membrane an excellent highly selective low-cost candidate for application in a membrane reactor. The novelty of this study is in the proposed application at relatively high temperatures (450°C and 500°C); only a few studies have tested carbon membrane reactors. Two types of operation modes were studied, using either nitrogen as a sweeping gas in counter current flow or using vacuum as a driving force for membrane transport. As expected, higher conversions were achieved with decreasing feed flow rate. The conversion achieved in the counter-current flow operation method was higher than in all other modes achieving a maximum of 85% at 500°C. While this result is much higher than in the corresponding PFR, the obtained improvement is a result of nitrogen transport and dilution. The conversions obtained in the vacuum mode show modest gains above the ones received in the PFR (40% vs. 30% at 500°C). These results were compared with simulations that used the experimentally determined transport parameters.

Axial effective thermal conductivities of packed beds

Experimental investigations have been carried out to measure axial effective thermal conductivities of packed beds for a number of particles and catalyst pellets. Measurements were made for three gases (air, nitrogen and carbon dioxide) in beds packed with ball bearings, copper chromite, chromia alumina, alumina hollow cylinders and alumina spheres. A glass vacuum vessel was employed for most measurements, but a thin wall stainless steel vessel was used in a few experiments. Empirical correlations to predict the axial effective thermal conductivity of packed bed reactors have been derived from the experimental results.

Catalytic nitroxidation of toluene into benzonitrile on chromia–alumina aerogel catalyst

The Cr 2O 3–Al 2O 3 catalyst aerogel with atomic percentage in chromium equals 5% presents a high selectivity in the reaction between toluene and nitrogen monoxide to benzonitrile in the high temperature range 400–460°C. Kinetic and EPR studies show that alumina stabilizes the chromium species which produce benzonitrile and the catalyst becomes more stable at the steady state. At the lower temperature range 250–350°C, conditions under which partial oxidation of toluene to benzaldehyde predominates, the Cr 2O 3–Al 2O 3 catalyst is inactive in the toluene nitroxidation. This result seemed to indicate that nitrogen monoxide dissociation requires higher energy than dioxygen. The mechanistic study shows that the reaction proceeds by a redox process in which the chromium species swing between two types Cr 3+ and Cr 5+ and these ions are interconverted by oxidizing and reducing atmosphere.

The effect of pre-sulfiding of catalysts for the vapour phase catalytic synthesis of thiophenes

A study of catalyst pre-sulfiding for the synthesis of 3-methylthiophene from 2-methylbutanol and carbon disulfide over potassium-promoted chromia–alumina has been performed. Pretreatment with CS2 results in enhancements in 3MT yield and catalyst lifetime. The benefits observed are ascribed to a combination of enhanced Cr2S3 formation and poisoning of sites responsible for side reactions which limit selectivity and result in the formation of deactivating by-products.

The Effect of the Method of Preparation on the Textural, Acidic and Catalytic Properties of Chromia–Alumina Catalysts

Pure alumina 'A' and chromia–alumina catalysts containing 25 mol% Cr2O3 (IAC, IIAC and IIIAC) were prepared by impregnation, coprecipitation and coating techniques, respectively. The adsorption of nitrogen at 77 K and of benzene and water vapour at 308 K was determined. The chemisorption of pyridine at 400 K was measured and the surface acidities calculated. The conversion of 2-propanol and cracking of cumene were investigated using the pulse microcatalytic technique. Coating alumina with chromia causes considerable changes in the textural and adsorption properties, impregnation of alumina with chromia leads to smaller variations while coprecipitation gives intermediate changes in textural and adsorption properties. The same was found true for the surface acidities as determined by the chemisorption of pyridine. The dehydration of 2-propanol and cracking of cumene follow first-order kinetics. The dehydration of propanol on alumina and chromia–alumina catalysts is a structure-insensitive reaction independent of the surface area but related to the surface acid density. The cracking of cumene is a structure-sensitive reaction depending on both the extent of the surface and the number of acid sites.

Dismutation of CFC-12 to CFC-13 over chromia–alumina catalyst

Selective transformation of CCl2F2(CFC-12) to fluorine-rich CClF3(CFC-13) in the absence of HF over a chromia–alumina catalyst has been achieved.

Characterization of some catalysts using vacuum balances

A vacuum quartz-spring balance was used to determine the nitrogen adsorption isotherms at −196°C of silica—titania, silica—alumina, silica—magnesia and chromia—alumina catalysts. The surface acidities of these acid catalysts were determined by following the desorption of pyridine using an electronic vacuum balance. Good agreement was found between the nitrogen adsorption isotherms determined by the vacuum quartz-spring balance and those followed by conventional volumetric methods. The adsorption of pyridine at 35°C was found to be of the physical type where the sorbed pyridine was completely expelled by prolonged out-gassing at the adsorption temperature. The adsorption of pyridine at 150°C on the catalysts investigated is mainly chemical and the amount of pyridine chemisorbed determines the number of acid sites per unit weight of the catalyst or per unit surface area. The thermal desorption of pyridine was found to be related to the strength of the surface acid sites. A fairly good relationship was found between the surface acid density (number of acid sites per cm 2) and the catalytic activity towards isopropanol dehydration.

Regeneration of fixed beds of coked chromia—alumina catalyst

Experimental measurements and modelling studies have been made for the regeneration of fixed beds of coked chromia—alumina particles. The catalyst was coked initially by the dehydrogenation of butene. Temperature profiles obtained during regeneration were found to exhibit steep gradients and sharp maxima unlike those observed during regeneration of other coked catalysts such as silica—alumina or zeolite-cracking catalyst. A mathematical model has been developed and solved using the method of orthogonal collocation on finite elements. This method has enabled the steep temperature gradients in the reactor to be simulated whereas the global collocation technique either fails or requires a very large number of collocation points The modelling results gave excellent agreement with the experimentally obtained temperature and coke profiles.

Characterization of a chromia–alumina catalyst using small-angle neutron scattering

Neutron scattering has been employed to study the pore structure of a chromia–alumina catalyst. The results indicated the existence of three distinct scattering regimes with distinct slopes corresponding to the values of the wavevector, Q. The analysis of the scattering in these regions suggested that the large-Q region corresponds to Porod's law scattering and provides information on the specific surface area. The intermediate Q region arises from scattering from hollow capillaries and provides information on the dimension of the capillaries. The small-Q behaviour arises from the relative spatial arrangement of capillaries, which appear to form a `fractal' network.

ChemInform Abstract: Aromatization of Isophorone to 3,5‐Xylenol.

Abstract Aromatization of isophorone to 3,5-xylenol was studied in a continuous-flow fixed-bed reactor at temperatures of 450–580°C and space velocities of 0.5–4.0 h−1 at atmospheric pressure using γ-alumina and magnesia (prepared in the laboratory) and commercially available chromia—alumina as catalysts. With γ-alumina, decomposition of isophorone was the predominant reaction, resulting in low conversions to 3,5-xylenol. With magnesia and commercial chromia—alumina catalysts, the reaction path was found to be preferably the transformation of isophorone to 3,5-xylenol, conversion of isophorone to 3,5-xylenol increasing in the order γ-Al2O3