Al Oxide Catalysts Research Articles

Template-induced mesoporous Ni–Al oxide catalysts with tuned physico–chemical properties for the oxidative dehydrogenation of ethane

Oxidative dehydrogenation of ethane is a potential industrial process to produce ethylene. In this work, the template-induced mesoporous Ni–Al mixed oxide catalysts synthesized by the template-assisted self-assembly method were investigated for the oxidative dehydrogenation of ethane to ethylene using O2 as an oxidant. The template-induced structure evolution is an effective means to modulate physico-chemical properties of the Ni–Al mixed oxide, thus, it further improves the catalytic performance. All the mesoporous Ni–Al–template catalysts present higher C2H6 conversions and C2H4 selectivities than bulk NiO and non-porous Ni–Al–O catalysts in the temperature range of 350–500 °C. Meanwhile, up to 49.6% C2H6 conversion and 75.3% C2H4 selectivity (37.3% of C2H4 yield) can be obtained over Ni–Al–F127 catalyst at 500 °C. And the mesoporous Ni–Al–F127 catalyst exhibits a superior stability after a long run. The structure–performance correlation indicates that the remarkable catalytic performance is ascribed to the synthesis driven structural effect. The strong interaction between Ni and Al cations caused by the addition of template promotes the formation of small NiO particles and the better dispersion of NiO, which decreases the content of non-selective oxygen species and increases the availability of lattice oxygen.

Preparation of Pt supported on mesoporous Mg–Al oxide catalysts for efficient dehydrogenation of methylcyclohexane

Hydrogen energy, characterizing by high-energy density, non-pollution and renewability, is regarded as an ideal clean green energy, and the chemical hydrogen storage is an optimal strategy to realize its large-scale utilization. In this study, to enhance the hydrogen evolution rate in the dehydrogenation of methylcyclohexane (MCH), Pt supported on Mg–Al oxide catalysts were prepared and the effects of the co-precipitation reaction time during the preparation of Mg–Al hydrotalcite on their structural properties were studied in detail. The results showed that both the pore diameter and Pt dispersion were increased after prolonging the precipitation reaction time. During the dehydrogenation of MCH, these resultant catalysts presented high activity and good stability: hydrogen evolution rate reached up to 1892 mmol·gPt−1 min−1 at 623 K and the conversion was still held at 92% after 218 h. Of course, a slight decrease on the conversion during the dehydrogenation reaction was also observed, which was mainly attributed to the aggregation of Pt particles at high temperature.

In Situ X-Ray Absorption Spectroscopy Studies of Carbon Monoxide Oxidation in the Presence of Nanocomposite Cu–Fe–Al Oxide Catalysts

Nanocomposite Fe–Al and Cu–Fe–Al oxide catalysts are studied in the CO oxidation reaction. It is shown that the introduction of copper leads to a more than two orders of magnitude increase in the reaction rate. The most active catalyst is a nanocomposite containing 5 wt % CuO, 78 wt % Fe2O3, and 17 wt % Al2O3. According to X-ray diffraction analysis, the catalysts consist of hematite nanoparticles, α-Fe2O3, and amorphous alumina. Copper is in a highly dispersed state to form CuO and CuFeOx clusters. Results of in situ X‑ray absorption spectroscopy studies suggest that the reduction of copper from Cu2+ to Cu1+ and Cu0 in a CO stream begins at a temperature of 200–250°C; at 400–600°C, copper is mostly in the metallic state; the Fe3O4 → FeO → Fe reduction begins at 400–500°С. In a stoichiometric mixture of CO and O2, Fe3+ does not undergo reduction; the reduction of copper oxide to metal occurs at temperatures above 400°C. In the presence of excess oxygen in a temperature range of 100–600°C, no changes in the chemical state of the catalysts are detected. The mechanism of CO oxidation in the presence of Cu–Fe–Al nanocomposites is proposed.

Copper Aluminum Spinels Doped with Cerium as Catalysts for NO Removal

Cu-Ce(Mn)-Al oxide catalysts to NO removal in the broad temperature range were synthesized and tested. The precursor of copper aluminium spinel was obtained with the coprecipitation method. By this method, Cu–Al spinels with various amounts of manganese and cerium were synthesized as well. These oxides crystallized in the structure of inverse spinel; however, Ce doping caused the appearance of additional CeO2 phase as determined by XRD. The samples were mesoporous solids with moderate surface area and porosity measured by low temperature sorption of nitrogen. The addition of another metal to Cu–Al spinel caused an increase of activity in selective catalytic reduction of nitrogen oxide with ammonia. The presence of manganese caused the formation of a higher amount of N2O by-product. The catalytic activity increased with the cerium concentration. For the sample with the atomic ratio Ce0.15Cu0.18, ca. 90% of NO conversion was registered between 200 and 350 °C. As examined with XPS spectroscopy, such conversion was attained due to the good dispersion of copper on the catalyst surface. This copper was placed mainly in spinel octahedral positions which enable its easier reduction. The spinel structure causes the presence of cerium as the trivalent cation important in redox cycles with the participation of copper.

Effect of the cerium modification on acid–base properties of Mg–Al hydrotalcite-derived oxide system and catalytic performance in ethanol conversion

The prospects of using cerium as a modifying additive of Mg–Al hydrotalcite-derived oxide system have been estimated by low-temperature nitrogen ad(de)sorption, X-ray diffraction, scanning electron microscopy, 1H and 27Al solid-state nuclear magnetic resonance (NMR), X-ray photoelectron and UV–Vis diffuse reflectance spectroscopy techniques. The effect of such tuning of structural-textural characteristics and acid–base properties of the surface on its catalytic performance in ethanol conversion has been studied. Solid-state NMR spectroscopic study of the probe molecule adsorption and temperature-programmed desorption of NH3 and CO2 with mass-spectrometry control are used to determine the nature and strength distribution of acidic and basic sites of Mg–Al(–Ce) oxide systems. It has been found that the selectivity towards 1-butanol up to 68.1% (548 K) is achieved in ethanol conversion over Mg–Al–Ce sample at time-on-stream ≤ 2 h. At higher time-on-stream, the specific rates of the target product (1-butanol) formation are comparable for both modified and un-modified catalysts, but the products distribution differs. Cerium modification of Mg–Al oxide catalyst leads to increase in specific rate of acetaldehyde formation due to increase in the number of basic sites and their surface density. Wherein, the rate of ethanol dehydration products (ethylene and diethyl ether) is reduced, apparently, due to decrease in a number of Lewis acidic sites formed by Al3+ cations octahedrally coordinated to oxygen.

Preparation of bimetal Co–Ni supported on Mg–Al oxide for chemocatalytic upgrading of tailored fermentation products to energy intensive fuels

The great challenge in the aldol condensation of tailored fermentation products (acetone–butanol–ethanol, ABE) into energy intensive fuels is to develop a suitable catalyst with high activity and low–cost. In this study, Co, Ni, and Co–Ni supported on Mg–Al oxide catalysts were prepared and their pore diameters were enlarged via adding active carbon as a hard template into Mg–Al hydrotalcite. During the aldol condensation reaction, the catalyst activity was enhanced after enlarging the pore diameter and Co–Ni bimetal supported catalyst presented the highest activity, which was resulted from that the electron transfer between Co and Ni in Co–Ni alloy enhanced the dehydrogenation activity and large pore lowered the mass transfer resistance. After optimizing the reaction conditions, acetone conversion and the total selectivity of C5–C11 desired products in the aldol condensation of ABE reached up to 76% and 90%, respectively. The stability study showed that the activity was decreased with the increase of reaction number because of the oxidation of metallic Co and Ni, but this could be solved via a simple hydrogen reduction method.

Kinetic and mechanistic study of CO oxidation over nanocomposite Cu−Fe−Al oxide catalysts

AbstractThe oxidation of CO has been studied over Fe−Al and Cu−Fe−Al oxide nanocomposite catalysts prepared by melting of copper, iron, and aluminum nitrates. It was shown that the addition of copper significantly increases the catalytic activity of the Fe−Al nanocomposites. The catalysts were characterized by low‐temperature nitrogen adsorption, X‐ray diffraction (XRD), and X‐ray photoelectron spectroscopy (XPS). It was found that the catalysts contain Fe2O3 with the hematite structure modified by aluminum. Copper in the three‐component catalyst is in the Cu2+ state, forming CuO and CuFeOx clusters on the catalyst surface. An increase in the copper content leads to the formation of a CuxAlyFe3‐x‐yO4 spinel phase. In situ XPS study showed that a treatment of the catalysts in a CO flow leads to the reduction of both copper and iron cations into the metallic state. In contrast, a treatment in a CO/O2 flow leads only to partial reduction of Cu2+ to Cu1+, while Fe3+ are not reduced. The tests of catalytic activity performed in a flow fixed bed reactor using a CO pulse technique showed that the light‐off temperature in the oxidation of CO over the Cu−Fe−Al nanocomposite catalysts depends on the copper content. The minimal light‐off temperature was achieved over the catalyst containing 5 wt% CuO. In addition, we performed kinetic measurements in a differential reactor and obtained the activation energy and the reaction orders with respect to the reactants. The reaction mechanism of the catalytic oxidation of CO and the origin of active species are discussed.

Impact of Incorporation of Active Nanoporous Components or Their Precursors in a CuAlO/CuAl Ceramometal Skeleton on the Properties in the Low-Temperature Water-Gas Shift Reaction

Enhanced activity in low-temperature water-gas shift (LT-WGS) reaction of some ceramometal catalysts compared to conventional Cu–Zn–Al oxide catalyst was demonstrated. Porous ceramometals were synthesized from powdered CuAl alloys prepared by mechanical alloying with the addition of either CuAlexp powders produced by current spark explosion of Cu+Al wires or CuZnAl oxide obtained by coprecipitation. Their structural, microstructural, and textural characteristics were examined by means of X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray spectrometry, NMR, and adsorption methods, and catalytic properties were studied in the LT-WGS reaction. CuAlO/CuAl ceramometals were found to have mostly the egg-shell microstructure with the metallic cores (AlxCu1–x, Al2Cu, and Al4Cu9) and the oxide shell containing copper oxides and/or mixed oxides of copper and aluminum and, at same time, CuAlO/CuAl ceramometal with incorporated additives was found to create a more complicated microstructure. A large amount of X-ray amorphous oxides of copper and aluminum is typical for all composites. CuAl ceramometal was shown to be more active than the CuZnAl oxide catalyst in spite of a much lower specific surface area. The CuAl+CuZnAl catalyst consisting of prismatic granules showed a higher activity in comparison with CuZnAl oxide consisting of cylindrical granules. The activity of the composite granulated catalyst referred to its unit weight was more than 6-fold higher as compared to the oxide catalyst, while the activity per the surface area was found to be more than an order of magnitude higher due to much higher specific activity of small fraction and additively much lower diffusion limitation of granules.

Effect of Calcination Temperature on Activity of Fe2O3–Al2O3 Nanocomposite Catalysts in CO Oxidation

Nanocomposite Fe–Al oxide catalysts were prepared by the melting of iron and aluminum nitrates with the subsequent calcination in air at different temperatures. It was found that the catalysts calcined at 450 °C are more active in the oxidation of CO than the catalysts calcined at 700 °C. X-ray diffraction and X-ray photoelectron spectroscopy showed that all the catalysts consist of hematite, α-Fe2O3 nanoparticles, and Al2O3 in an amorphous state. Iron oxide is the active component, which provides the oxidation of CO, while alumina is a texture promoter. The increase in the calcination temperature leads to a minor increase in the average size of hematite nanoparticles and an insignificant decrease in the specific surface area. Kinetic measurements showed that the oxidation of CO over the Fe–Al catalysts calcined at 450 and 700 °C proceeds with the activation energy of 61–69 and 91 kJ/mol, respectively. This means that the low-temperature and high-temperature catalysts contain different active species. Temperature-programmed reduction with CO indicated that the decrease in the calcination temperature improves the reducibility of the Fe-Al nanocomposites. According to 57Fe Mossbauer spectroscopy, the low-temperature catalysts contain hydrated iron oxides (acagenite and ferrihydrite) and a significant amount of highly defective hematite, which is absent in the high-temperature catalyst. These species can provide the enhanced activity of the low-temperature catalysts in the oxidation of CO.

Precipitated K-Promoted Co–Mn–Al Mixed Oxides for Direct NO Decomposition: Preparation and Properties

Direct decomposition of nitric oxide (NO) proceeds over Co–Mn–Al mixed oxides promoted by potassium. In this study, answers to the following questions have been searched: Do the properties of the K-promoted Co–Mn–Al catalysts prepared by different methods differ from each other? The K-precipitated Co–Mn–Al oxide catalysts were prepared by the precipitation of metal nitrates with a solution of K2CO3/KOH, followed by the washing of the precipitate to different degrees of residual K amounts, and by cthe alcination of the precursors at 500 °C. The properties of the prepared catalysts were compared with those of the best catalyst prepared by the K-impregnation of a wet cake of Co–Mn–Al oxide precursors. The solids were characterized by chemical analysis, DTG, XRD, N2 physisorption, FTIR, temperature programmed reduction (H2-TPR), temperature programmed CO2 desorption (CO2-TPD), X-ray photoelectron spectrometry (XPS), and the species-resolved thermal alkali desorption method (SR-TAD). The washing of the K-precipitated cake resulted in decreasing the K amount in the solid, which affected the basicity, reducibility, and non-linearly catalytic activity in NO decomposition. The highest activity was found at ca 8 wt.% of K, while that of the best K-impregnated wet cake catalyst was at about 2 wt.% of K. The optimization of the cake washing conditions led to a higher catalytic activity.

Facile one-step preparation of ordered mesoporous Ni–M–Al (M = K, Mg, Y, and Ce) oxide catalysts for methane dry reforming

Excellent DRM performances of a Ni–Y–Al oxide catalyst is related to the comprehensively advantageous textural properties, such as high Ni dispersion, large surface area, and small Ni nanoparticles inlaid in the honeycomb-like mesoporous skeleton.

Structure and Magnetic Properties of Fe,Ni(Zr)/Al Oxide Catalysts Under the Conditions of Methanation of Carbon Dioxide

It is shown that under the conditions of carbon dioxide methanation, the Fe,Ni(Zr)/Al oxide catalyst is a nanostructured system with a particle size of 6-8 nm and is a superparamagnetic. An in situ study of iron–nickel catalysts showed that the formation of highly dispersed Fe-Ni oxide structures depends on the Fe content. A scheme for the CO2 methanation mechanism, which in the case of Zr-containing catalysts includes a step with the formation of dioxymethylene groups and their subsequent hydrogenation to formaldehyde, is considered.

Dimethyl carbonate synthesis over solid base catalysts derived from Ca–Al layered double hydroxides

The Ca–Al oxide catalysts are prepared via thermal decomposition of Ca–Al layered double hydroxide precursors and characterized by XRD, TG, SEM, BET, CO2-TPD, Hammett indicators, XPS and ICP techniques. The obtained catalysts are then tested for transesterification of methanol with propylene carbonate (PC), highest dimethyl carbonate (DMC) yield of 59.9% has been reached with methanol: PC molar ratio of 12, catalysts weight of 2 wt% total reactants, and reaction time of 2 h. Research results show that though the transesterification occurs over various basic sites, the strong basic site has a leading role on transesterification, which is consistent with the activity results. The catalysts can be recycled and the reason for the decrease in catalytic activity is further discussed in detail. The catalytic performance of Ca-Al solid base catalysts derived from Ca-Al layered double hydroxides for dimethyl carbonate production under heterogeneous conditions.

Fluidized-Bed Isobutane Dehydrogenation over Alumina-Supported Ga2O3 and Ga2O3–Cr2O3 Catalysts

Dehydrogenation of isobutane to isobutene over supported gallium oxide microspherical catalysts was investigated in a fluidized-bed reactor. A partially crystallized nanostructured aluminum hydroxide-oxide, which is a product of gibbsite centrifugal thermal activation (CTA) obtained using a CEFLAR technology, was used as a catalyst support. The structural and textural properties of Ga2O3/Al2O3 catalysts were characterized by a range of techniques including XRD, N2-physisorption, TPD of NH3 and CO2, IRS of adsorbed pyridine, and selective adsorption of a series of acid–base indicators. A Ga–Al oxide catalyst exhibited a stable performance close to activity of Cr–Al oxide catalysts not containing soluble hexavalent chromium. Upon addition of Cr2O3 (6 wt % of Cr), in amounts lower than in an industrial chromia/alumina catalyst (10.9% Cr), and 1% ZrO2 to 6%Ga/Al2O3 catalytic activity in isobutane dehydrogenation reaches the performance of the industrial KDM catalyst (“Sintez”, Russia).

Effect of Zr addition on catalytic performance of Cu-Zn-Al oxides for CO2 hydrogenation to methanol

A series of 50%CuO-25%ZnO-25%Al(Zr) oxide catalysts was synthesized by a reverse co-precipitation method and characterized by XRD, N2 adsorption-desorption, H2-TPR, H2-TPD and CO2-TPD, and used as catalyst for CO2 hydrogenation to methanol under mild conditions(200―260 °C and 1―3 MPa). Consequently, H2-TPR ex-hibited that the dispersed CuO species increased with the zirconium content, while the H2-TPD showed that small amount of zirconia can increase the H2 desorption amount. As a result, Cat-2 and Cat-3 exhibited an enhanced H2-spillover effect, the most H2 desorption amount, which showed an optimum activity of about 7% methanol yield.

Promotional effects of Ce on the activity of Mn[sbnd]Al oxide catalysts derived from hydrotalcites for low temperature benzene oxidation

Ce/MnAl and MnAl mixed metal oxides catalysts have been obtained by calcination of layered double hydroxides precursors. The composite oxides catalysts were studied in total oxidation of benzene. Physicochemical properties of all the catalysts were characterized by using a series of specific analytical techniques. The results revealed that the 0.2Ce/MnAl catalyst exhibited the highest catalytic performance with T90 about 210°C at a high space velocity (SV=60,000mLg−1h−1), ascribed to its lower-temperature reducibility, the abundant surface lattice oxygen (Olatt), and synergetic effect between Mn4+ and Ce3+/Ce.

Preparation of Zn–Al oxide catalyst and its catalytic performance in propylene carbonate synthesis from urea and propylene glycol on a fixed-bed reactor

The synthesis of propylene carbonate (PC) from urea and propylene glycol (PG) catalyzed by Zn–Al oxide catalyst was successfully performed on a fixed-bed reactor. The appropriate conditions for the preparation of Zn–Al oxide were obtained as follows: Zn(NO3)2 and Al(NO3)3 as the precursors of Zn–Al oxide, NaOH and Na2CO3 aqueous solution as the precipitant, using parallel-flow precipitation method, pH=9.5, aging at 40°C for 24h, and calcination at 500°C for 4h. The structures, compositions, and acidities of the Zn–Al oxide catalysts were characterized by means of XRD, N2 adsorption–desorption, ICP-AES and NH3-TPD. Characterization and activity evaluation results indicate that Zn–Al oxide has an excellent catalytic performance for PC synthesis and specific surface area is the main factor influencing the catalytic performance of Zn–Al oxide. Under the suitable reaction conditions of n(PG):n(urea)=6:1, Zn–Al oxide volume of 3mL, reaction temperature of 140°C and LHSV=0.8h−1, the yield and selectivity of PC could reach 87.4% and 96.8%, respectively. Furthermore, the yield of PC was no less than 85% during the 60h stability test, demonstrating that Zn–Al oxide catalyst exhibits a pretty stable activity. Additionally, React-IR was employed to investigate the reaction pathway for PC synthesis from urea and PG. The results show that the reaction of PC synthesis proceeds in two steps: 2-hydroxypropyl carbamate (HPC) is formed from urea and PG first and then PC is produced by elimination of an ammonia from HPC molecule.

CO2 hydrogenation for C2+ hydrocarbon synthesis over composite catalyst using surface modified HB zeolite

Synthesis of C2+ hydrocarbons such as LPG (liquefied petroleum gas) from CO2 is expected to reduce CO2 emission by the consumption of fossil fuels. In this study, CO2 hydrogenation over composite catalysts comprised of Cu–Zn–Al oxide catalyst and HB zeolite was examined for the synthesis of C2+ hydrocarbons by the combination of methanol synthesis over Cu–Zn–Al oxide and concurrent conversion of methanol over HB zeolite. When a non-modified zeolite was used for the composite catalyst, the yield of C2+ hydrocarbons was poor (<0.5C-mol%), which was lower than that of oxygenated compounds (methanol and dimethyl ether). The strong acid sites of zeolite for the transformation of dimethyl ether to C2+ hydrocarbons were seriously deactivated. The employment of zeolites modified with 1,4-bis(hydroxydimethylsilyl) benzene remarkably improved the catalytic activity of the corresponding composite catalysts to produce C2+ hydrocarbons in yields of more than 7C-mol%. The best yield of C2+ hydrocarbons became approximately 12.6C-mol% under a pressure of 0.98MPa. The disilane modification produced hydrophobic zeolites showing water contact angles more than 130°. The disilane compound was converted to some condensed aromatics during CO2 hydrogenation at 300°C, and the hydrophobicity was maintained even after the reaction. The enhanced catalytic activity is caused by suppressing the deactivation of the strong acid sites of HB zeolite with the hydrophobic surface. This improved composite catalyst will promote the production of C2+ hydrocarbons from CO2 even under low pressure conditions.

Synthesis of C2+ hydrocarbons by CO2 hydrogenation over the composite catalyst of Cu–Zn–Al oxide and HB zeolite using two-stage reactor system under low pressure

The combination of methanol synthesis and methanol-to-gasoline (MTG) reaction is a beneficial process to produce particular C2+ hydrocarbons from CO2 and H2, because MTG reaction can produce C2+ hydrocarbons selectively without the formation of methane. However, the composite catalysts consisting of methanol synthesis catalyst and zeolite previously reported have produced C2+ hydrocarbons in poor yields especially under low pressure conditions. This paper reports the high yield production of C2+ hydrocarbons by CO2 hydrogenation using a two-stage reaction system consisting of a Cu–Zn–Al oxide catalyst in the first reactor and a composite catalyst of the Cu–Zn–Al oxide and HB (zeolite beta) in the second reactor. The key points of this system are reversed water-gas-shift (RWGS) reaction in the first reactor and the following H2O removal before the second reactor. The gas mixture supplied to the second reactor containing much amount of CO and little H2O enhanced the methanol synthesis and suppressed the methanol decomposition to CO over the Cu–Zn–Al oxide catalyst in the composite catalyst, promoting the MTG reaction to produce C2+ hydrocarbons over HB zeolite. The highest yield of C2+ hydrocarbons reached to approximately 15C-mol% under 0.98MPa.

Effect of Composition and Pretreatment Parameters on Activity and Stability of Cu–Al Catalysts for Water–Gas Shift Reaction

AbstractWe investigated various Cu species responsible for highly efficient Cu–Al oxide catalyst for the water–gas shift reaction (WGSR). The formation of various Cu species was achieved by systematically varying the Cu–Al composition in the coprecipitated mixed Cu–Al oxides. The Cu–Al composition of 70:30 (Cu–Al‐7) was the best for WGSR using the reformate gas composition. In addition, the Cu–Al‐7 catalyst reduced under 100 % H2, was relatively stable with time on stream of 100 h, at higher gas hourly space velocity of 36 201 h−1.The structural investigation of our coprecipitated catalysts with varying Cu–Al compositions revealed the formation of nonzero oxidation state copper and metallic Cu to be essential for the observed WGSR activity. In addition, the highest activity and stability of Cu–Al‐7 catalysts reduced under 100 % H2 at lower temperature was attributed to particle‐size stabilization and a lower extent of Cu aggregation by Cu2O and boehmite phases, respectively, along with the formation of various Cu species during the activation protocol for 12 h. Complete CO2 selectivity without methanation was observed for all the Cu–Al compositions irrespective of their pretreatment conditions.