Alumina Carrier Research Articles

Evaluation of Cryotechniques for Tem Observation of Sols - Application to Boehmite Sols Used in Catalysts Forming

The use of suspensions in the heterogeneous catalysis field being a big issue, it appears important to be able to control this step as much as possible. Size of the objects in the suspension, aggregation state, homogeneity or heterogeneity (all called texture or structure here) are some important parameters which are sometimes difficult to evaluate with traditionalmethods such as X-ray scattering or rheology. The cryotechniques applied to transmission electron microscopy is then an interesting alternative. Three different methods of cryofixation have been tested on low solid content suspensions of boehmite commonly used during the dispersion step of alumina carrier forming. Two different cryoprotectants were tested as well. It is shown that liquid propane immersion cooling (PL) and slam freezing (SF) methods require the use of a cryoprotectant whereas high pressure technique (HP) avoids ice crystal formation without any addition of cryoprotectant in the suspension. It is shown also that DMSO is efficient with PL and SF techniques only when added at a high concentration (50%) to the suspension. Glycerol is preferred because water is vitrified since that 30% of glycerol is added. Comparisons of high pressure preparations with and without glycerol show very little differences of texture on the replica. Explanations of the origin of the small differences of density are tentatively given. More analyses are now under progress to clarify the role of glycerol. Cryomicroscopy is conclude to be a very good technique when the texture of a suspension must be studied for example in the elaboration or in the improvement of a peptisation or a colloid formation process.

Preparation of the Mn/γ‐Al2O3 acceptor for high temperature regenerative H2S removal

AbstractAn alumina supported manganese acceptor can be used for high temperature regenerative removal of hydrogen sulfide. Such an acceptor was prepared by a wet impregnation method on a 10 kg scale. A porous alumina carrier with a size of 50 to 70 microns was impregnated with a manganese acetate solution. The impregnation slurry was centrifuged and dried in a microwave oven. The impregnation parameters and drying conditions were investigated. It is shown that this impregnation process is dominated by the solution penetrating the pores. The manganese loading of the acceptor was estimated based on the concentration of the impregnating solution. This showed a good agreement with the results of AAS (Atom Absorption Spectrometer) measurements. The microwave drying time needed was calculated by a heat balance over the drying system. The preparation exhibited a good reproducibility and the acceptors prepared have better dispersion and less initial deactivation.

Effect of manganese content on the properties of high temperature regenerative H 2S acceptor

In this paper a high manganese content Mn/γ-Al 2O 3 acceptor for high temperature, regenerative H 2S removal was prepared by repeated impregnation. The effect of manganese content on the crystal structure, pore structure and reduction properties of acceptors was investigated. The results showed that repeated impregnation induced a high dispersion of manganese on alumina carrier surface. The sulfur capacity linearly increased with the increase of manganese content and the S/Mn molar ratios were maintained to be larger than unity. When the manganese content was raised to 35 wt% a high sulfur capacity of 22 wt% S was achieved. In 11 sulfidation–regeneration operation cycles no deactivation was observed.

Group 4 metal halides/alumina solid superacids Relation between electronegativity, acid strength and catalytic properties

The catalytic properties of solid superacids obtained by the action of the vapour of Group 4 metal halides on alumina have been related to the electronegativities of the halides used. It was found that the acid strength of the Lewis superacid centres and the reaction pathways of low-temperature pentane isomerization depend on the electron acceptor ability of the metal halide species bonded to the alumina carrier.

Combustion of non-halogenated volatile organic compounds over group VIII metal catalysts

Catalytic combustion of volatile organic compounds (VOCs), present in low concentrations (10–1000 ppm) in industrial effluent streams, is a promising air abatement technology. The oxidation of benzene, butanol and ethyl acetate over group VII metal catalysts supported on alumina carriers has been investigated. Pt, Pd and Co were found to be the most active among group VIII metals, while ethyl acetate was found to be the most-difficult-to-oxidize compound. Benzene and ethyl acetate oxidations over Pt/Al 2O 3 were found to be structure sensitive reactions with the turnover frequency (TOF) increasing with increasing mean metal particle size. The presence of chloride on the catalyst surface, originating from chloride-containing metal precursor compounds was found to exert an inhibiting effect on the activity of Pt. Apparent activation energies of the reactions over Pt and Pd catalysts were found to be in the 70–120 kJ/mol range while the reaction order with respect to the VOC was positive in all cases. During oxidation of benzene-butanol mixtures, benzene oxidation was completely suppressed as long as butanol was present in the reaction mixture.

Surface Characterization of Zirconia-Coated Alumina and Silica Carriers

Silica- and alumina-supported zirconia samples have been prepared by impregnation of the supports with a solution of zirconium alkoxide (n-propoxide) inn-propanol containing ZrO2in the ranges 1.2–28.6 and 2.2–23.2 wt%, respectively. The samples were characterized by theSBETmethod, XRD, XPS, and FTIR. The x-ray diffraction showed that zirconia on silica was amorphous for all concentrations of ZrO2. Zirconia on alumina was amorphous up to 17.1 wt% ZrO2; beyond this value crystallites were formed. The increase in the XPS IZr 3d/ISi 2pindicates that ZrO2appears as a monolayer on silica near the theoretical monolayer coverage (about 28.6 wt%), whereas for alumina-supported zirconia samples the monolayer is formed at lower ZrO2content (between 12.9 and 17.1 wt%). It was observed by pyridine adsorption that the strong Lewis acid sites on alumina decreased after deposition of zirconia. However, the number of Lewis acid sites on silica-supported zirconia samples, evoked by an increase of the positive charge on Zr atoms compared to pure zirconia, increases with increasing ZrO2content. Some Brønsted acid sites were detected on ZrO2/SiO2samples due to the slightly higher electron density on the oxygen associated with Si atoms detected by XPS.

Method for the hydroisomerization processing of feedstocks from the Fischer-Tropsch process

A method for the hydroisomerisation processing of feedstocks from the Fischer-Tropsch process. The catalyst essentially consists of 0.05-10 wt % of a noble metal and a silica(5-70 %)/alumina carrier having a specific surface area of 100-500 m2/g. The catalyst has an average pore diameter of 1-12 nm, the pore volume of pores with a diameter equal to the average diameter ± 3 nm is greater than 40 % of the overall pore volume, the dispersion of the noble metal is 20-100 % and the noble metal distribution coefficient is higher than 0.1. The method is performed at 200-450 °C at a partial hydrogen pressure of 2-25 MPa and with an hourly space velocity of 0.1-10 h-1 and a hydrogen/hydrocarbon volume rate of 100-2000.

Redispersion of Sintered Pt/Al2O3 Naphtha Reforming Catalysts: An in Situ Study Monitored by X-ray Absorption Spectroscopy

Redispersion of sintered Pt (0.6−1.0%)/Al2O3 catalysts at 773 K under HCl/H2O/O2/N2 atmospheres was studied by hydrogen chemisorption, temperature-programmed reduction (TPR), transmission electron microscopy (TEM) and in situ X-ray absorption spectroscopy (EXAFS) experiments. TPR and H2 chemisorption results showed that the concentration of hydroxychlorided Pt species on the alumina carrier and, as a consequence, the metallic dispersion constantly augmented during the 8 h redispersion treatment. In situ EXAFS experiments indicated that Pt(OH)4Cl22- species, which are formed by the attack of gaseous/surface chlorided species to partially oxidized metallic Pt particles, are responsible for Pt redispersion. EXAFS studies also suggested that once the Pt−Cl/Pt−O coordination number ratio was stable, the Pt atoms were present on the alumina carrier either in the form of small rafts containing Cl and O atoms in a slighly distorted octahedral environment or in relatively large metallic particles covered by Pt(OH)4Cl22- species. This suggestion was supported by the bimodal distribution of metal platinum particles observed by TEM when the redispersed samples where examined after its reduction with H2.

Investigation of MoS 2 on γ-Al 2O 3 by HREM with atomic resolution

High resolution electron microscopy and image calculations are performed on MoS 2 γ- Al 2O 3 based catalysts. It is shown that HREM allows atomic imaging of the MoS 2 particles on the alumina carrier. Restrictions with respect to atomic imaging are found from image simulations. The orientation of the slabs with respect to the electron beam is important. Whereas atomic imaging can be obtained in 〈100〉 and 〈110〉 directions, 〈001〉 oriented crystals will be invisible by HREM because of the way the slabs are mounted on the alumina support. The slabs are basal bounded. Since essentially single slabs are observed, it is suggested that a strong interaction exists between the slabs and the alumina support. EDX analysis shows that the MoS 2 particles are sulfur deficient. We suggest that sulfur deficiency is present in the first sulfur layer on the γ-Al 2O 3 support. In specimens sulfided below 673 K an intermediate phase is present, which is presumably an oxy-sulfide of molybdenum.

Surface Characterization of Zirconia-Coated Alumina and Silica Carriers

Zirconia catalysts supported on alumina or silica have been prepared by means of the controlled hydrolysis of a zirconium alcoxide by the hydroxyl groups on the surface of the support, followed by calcination in air at 820 K. The samples were characterized by XRD, XPS, and IR of surface hydroxyl groups and of CO 2 adsorption, and by isopropanol decomposition reaction. The dispersion of zirconia obtained by this technique is very high, but it is sensitive to some parameters, such as surface properties of the support, solvent of the alcoxide, and addition of acetyl acetone to the preparation mixture. The amorphous zirconia dispersed on alumina or silica displays surface properties different from those of the bulk zirconia.

Deactivation and Regeneration of Alumina Catalysts for the Rearrangement of Cyclohexanone Oxime into Caprolactam

Abstract Modified alumina catalysts have been studied for the Beckmann rearrangement of cyclohexanone oxime to caprolactam in the temperature range 250-35°C. Boron oxide supported on alumina was the most successful catalyst preparation used. Good initial activities and high selectivities to the lactam were observed, other products observed included cyclohexanone, 5-cyanopent-1-ene and aniline. Coke formation was noted from the commencement of reaction which eventually led to loss in activity, followed by a lessening in selectivity to caprolactam and a lowering of surface areas. The loss in activity was related to the cumulative amount of cyclohexanone oxime converted in the reactor, and selectivity to caprolactam diminished as the oxime partial pressure increased. There was a direct relationship also between the amount of coke which formed on the catalysts and the decline in catalytic activity. Coke formed readily on the more basic catalysts. It is postulated that two reactions which occur on surface basic sites are responsible for coke formation. The first of these is polymerization of caprolactam. The second is the formation of aniline; this by-product can condense with cyclohexanone to form a Schiff base and further condensation of this base with cyclohexanone, via the Mannich reaction, represents a pcssible starting point for coke formation. Evidence is presented that surface acidic sites were responsible for caprolactam formation. Treating the deactivated alumina supported boron oxide catalysts in air at 500°C did not regenerate the original catalytic activity. although the surface area was fully regenerated. X-ray diffraction, infrared, and X-ray photoelectron spectroscopies as well as chemical analysis indicated that a proportion of the boron had entered into the alumina carrier to form an amorphous phase during the deactivation process, and temperature-programmed desorption of ammonia indicated that this process was accompanied by an irreversible decline in the surface concentration of acidic sites.

Catalytic hydroformylation of 1-hexene over Co 2Rh 2(CO) 12 supported on inorganic carrier materials

Co 2Rh 2(CO) 12 supported on alumina, silica, magnesium silicate or zeolites catalyses 1-hexene hydroformylation to C 7-aldehydes with good yields. The effects of different amines on the 1-hexene hydroformylation were studied with Co 2Rh 2(CO) 12 strongly bound on an alumina carrier. This catalyst converted 85% of 1-hexene to C 7-aldehydes. When Et 3N is added to the hydroformylation solution, the yield of C 7-alcohols was maximum at 97%. With other cocatalysts Et 2NH, aniline, pyridine or NH 3 the yields Of C 7-alcohols were 62%, 24%, 17% and 0%, respectively. When studying the influence of the amount of Et 3N, it was found that less amine was needed for good alcohol conversion when the catalyst was supported on alumina than on silica. On the whole, the Co 2Rh 2(CO) 12 catalyst supported either on silica, magnesium silicate or zeolite in the same reaction conditions in the presence of Et 3N gave very similar C 7-oxygenate yields (99%) as Co 2Rh 2(CO) 12 on alumina. However, the alcohol contents of the above C 7-oxygenate yields were much lower than in the case Of Co 2Rh 2(CO) 12 on alumina. A 4–5 wt.% metal content of a Co 2Rh 2(CO) 12—alumina—Et 3N or Et 2NH catalyst was optimum in 1-hexene hydroformylation for 17 h. A synthesis gas pressure of 50 bar and a temperature of 100 °C provided the optimum reaction conditions.

High resolution Auger electron spectroscopy and microscopy of a supported metal catalyst

High spatial resolution Auger electron spectra and scanning Auger microscope (SAM) images of supported metal catalysts have been obtained in a UHV scanning transmission electron microscope. Ag/α-Al 2O 3 was used as a model catalyst system, where silver was evaporated, in situ, onto polycrystalline alumina carriers. Silver particles, as small as 2 nm in diameter, were clearly revealed in SAM images with high contrast. On large islands, an edge resolution < 3 nm was achieved. Information about surface and bulk properties of supported catalysts can be extracted from images formed with different signals generated from the same area which are obtained simultaneously.

Surface analysis of lanthanum-modified cobalt catalysts. Invited lecture

The effect of the addition of La on the structure of 3% m/m Co/Al2O3 catalysts has been studied by the combined use of X-ray photoelectron spectroscopy, Raman spectroscopy, X-ray diffraction and gravimetric analysis. It was shown that La exists as a highly dispersed La—Al2O3 interaction species for alumina carriers impregnated with La and calcined at 600 °C. For 3% m/m Co/Al2O3 catalysts containing La, little effect is observed for La/Al atomic ratios below 0.0075. For higher La loadings, an increase in Co dispersion is observed owing to suppression of Co3O4 formation. Tetrahedral Co2+ and a small amount of an La—Co surface phase are formed. Formation of tetrahedral Co2+ is caused by migration of Co from the La—Co surface phase into the alumina support during calcination at 600 °C.

Deactivation in catalytic hydrodemetallation II. Catalyst characterization

This paper details the characterization of metal-sulfide deposits on catalysts aged in model-compound hydrodemetallation studies. Transmission electron microscopy provides evidence that nickel and vanadium sulfides were present on the catalyst as large and spatially dispersed crystallites. A study of a series of samples with different nickel loadings on a CoMo/Al 2O 3 catalyst shows that the diameter of the nickel-sulfide crystallites increased with increasing nickel loading, whereas the number of crystallites per unit volume remained approximately constant. X-ray photoelectron spectroscopy of these samples yielded nickel to aluminum signal ratios significantly lower than the bulk composition. This result is consistent with the nickel-sulfide deposits having a crystallite form, and inconsistent with them being uniform on the catalyst surface. Vanadium-sulfide crystallites on an aged CoMo/Al 2O 3 catalyst were similar in size and number-density to nickel-sulfide crystallites on the same catalyst. In contrast, nickel-sulfide crystallites on a low-promoter alumina carrier were significantly larger, and therefore had a smaller number-density, than those on an equivalently loaded CoMo/Al 2O 3 catalyst.

Deactivation in catalytic hydrodemetallation I. Model compound kinetic studies

Model compound hydrodemetallation (HDM) studies are used to examine the effect of accumulating metal-sulfide deposits on catalyst deactivation in a clean reactant system. Experiments are carried out in a catalyst-slurry batch reactor, using a sulfided CoMo/Al 2O 3 catalyst and a low-promoter alumina carrier. The model compounds are nickel etioporphyrin (Ni-EP) and vanadyl etioporphyrin (VO-EP). Standard experimental conditions are 320°C, 4.8 MPa total pressure, and 14 kPa hydrogen sulfide. Ni-EP HDM studies carried out with the sulfided CoMo/Al 2O 3 catalyst operating in the kinetic regime show that, contrary to expectations, the catalyst does not deactivate significantly as it accumulates nickel-sulfide deposits to levels as high as 100 wt% Ni. Ni-EP HDM studies carried out with a low-promoter alumina carrier show that the carrier has a low level of catalytic activity, and neither deactivates nor acquires catalytic activity as nickel-sulfide deposits accumulate to 25 wt% Ni. The relative activity for the two catalysts matches the relative promoter loadings. VO-EP HDM studies carried out under diffusion-limited conditions with the CoMo/Al 2O 3 catalyst show that it deactivates steadily as vanadium-sulfide deposits accumulate. Both the reactivity and the apparent effective diffusivity of VO-EP on the sulfided catalyst are significantly lower than those of Ni-EP.

Titania-modified hydrodesulfurization catalysts: II. Dispersion state and catalytic activity of molybdena supported on titania-alumina carrier

A series of molybdena catalysts supported on titania-modified alumina carrier was characterized by X-ray diffraction (XRD), analytical electron microscopy (AEM), X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR). The results show that the type of Mo6+ species depends upon the coverage of titania on alumina. The molybdenum atomic concentration was improved when most of the surface of alumina was occupied by titania. TPR profiles of molybdena/titania-alumina exhibit two peaks at 680-710 K and 1090-1100 K. The amount of hydrogen consumption in the TPR process increases with a rise in titania content, which suggests that titania promotes the reduction of molybdena to much lower valency. The XPS data is in agreement with the TPR results. The measurements of the activity of the catalyst in hydrodesulphurization (HDS) of thiophene and hydrogenation (HYD) of cyclohexene indicate that titania-modified alumina supports can improve the HDS primary activity of the catalyst.

The effects of the environment on the growth of nickel metal particles supported on a range of Y zeolites prepared by ion exchange and impregnation

The nature of the nickel metal phase supported on a range of alkali metal cation-exchanged Y zeolite catalysts was characterized by X-ray diffraction line broadening. The sintering process is shown to occur via a crystallite migration mechanism. Nickel crystallite size was measured and correlated with such diverse factors as reduction time, reduction temperature, heating rate, sample precalcination, and ammonia adsorption. The nature of the alkali metal co-cation (Li+, Na+, K+, Rb+, or Cs+) is shown to strongly influence the dimensions of the supported metal, yielding the following order of decreasing crystallite size: NiLiY > NiNaY > NiKY > NiRbNaY > NiCsNaY. In the case of the CeNiNaY and CeNiKY samples, the introduction of an additional level of acidity, due to the presence of the highly polarizing Ce3+ ions, during thermal activation, serves to suppress crystallite growth. For comparative purposes, data on the sizes of nickel metal crystallites formed on the reduction of nickel-impregnated Y zeolites and silica and alumina supports are also presented; the nature of the alkali metal co-cation has a much greater influence on the dispersion of nickel supported on the impregnated zeolites. Under identical reduction conditions, the amorphous alumina and silica carriers exhibit the smallest supported crystallite sizes.

Catalytic properties of copper oxide supported on zinc aluminate in methane combustion

Copper oxide was deposited on a zinc aluminate prepared from high specific area alumina and zinc nitrate. The activity of the solid obtained has been measured in the catalytic combustion of methane before and after an aging test performed at 1340 K. The environment of the copper species was followed by UV–VIS reflectance spectroscopy whereas the state of copper was deduced from temperature-programmed reduction experiments. By comparison with a series of CuO/Al2O3 catalysts, the CuO/ZnAl2O4 sample is slightly less active in the complete oxidation of methane. This decrease in catalytic activity is connected with the presence of bulk CuO particles on the zinc aluminate support instead of isolated copper ions in the case of the alumina carrier.

Industrial scale selective high-temperature conversion of 2-butene to 1-butene: I. Development of catalysts

In order to convert 2-butene to 1-butene with a productivity and selectivity high enough for industrial application the commercially available catalysts have to be modified. In this paper we describe the preparation and optimization of two different alumina-based systems. One is silicated alumina modified by rare earth oxides, developed by Snamprogetti; the other is sodium aluminate on alumina carrier, developed by SZKFI.