Alumina Xerogel Research Articles

Sol–gel derived structures for optical design and photocatalytic application

In this work, we synthesised xerogels embedded in honeycomb porous anodic alumina films of 1–150μm thickness, with pore diameters of 40–180nm and cell sizes of 120–250nm, and investigated their luminescence and photocatalytic properties. Sols with compositions corresponding to alumina, garnet, willemite and titania in porous anodic alumina films were generated. It addition to the photoluminescence properties reported previously, the structures comprising titania xerogels in porous anodic alumina reveal efficient photocatalytic activity resulting in decomposition of an organic dye in an aqueous medium. Development of the xerogel/porous anodic alumina structure towards a matrix X-ray converter is discussed. Terbium luminescence under X-ray excitation, with the most intensive emission band at 542nm, is observed for the first time under excitation with X-rays and cathode rays for a matrix convertor consisting of a sequence of Tb-doped alumina xerogel layers confined in porous anodic alumina of 1μm thickness grown on a silicon substrate.

Hydrogenation of succinic acid to γ-butyrolactone (GBL) over palladium catalyst supported on alumina xerogel: Effect of acid density of the catalyst

Mesoporous alumina xerogel (AX) supports prepared by a sol–gel method were calcined at various temperatures to control their acid property. Palladium catalysts supported on mesoporous alumina xerogel (Pd/AX) were then prepared by an impregnation method. The Pd/AX catalysts were characterized by XRD, BET, NH 3-TPD, N 2 adsorption–desorption isotherm, and H 2 chemisorption analyses. Liquid-phase hydrogenation of succinic acid to γ-butyrolactone (GBL) over Pd/AX catalyst was carried out in a batch reactor. The effect of acid property of Pd/AX catalyst on the catalytic performance was examined. In the hydrogenation of succinic acid, conversion of succinic acid and yield for GBL showed volcano-shaped curves with respect to calcination temperature of AX support. Selectivity for succinic anhydride (an intermediate product formed by acid catalysis) increased with increasing acid density of Pd/AX catalyst. Correlations between acid density of Pd/AX catalyst and catalytic performance also revealed that conversion of succinic acid and yield for GBL increased with increasing acid density of Pd/AX catalyst. Thus, acid density served as an important factor determining the catalytic performance of Pd/AX in the hydrogenation of succinic acid.

Methane production from carbon monoxide and hydrogen over nickel–alumina xerogel catalyst: Effect of nickel content

Nickel–alumina xerogel catalysts (XNiAl) with different nickel contents (X, wt%) were prepared by a single-step sol–gel method for use in the methane production from carbon monoxide and hydrogen. The effect of nickel content on the catalytic performance of XNiAl catalysts was investigated. Conversion of CO and yield for CH 4 over XNiAl catalysts drastically increased with increasing nickel content from 20 to 40 wt%, but they were almost constant at nickel content above 40 wt%. This indicates that XNiAl catalysts with nickel content above 40 wt% served as efficient catalysts in the methane production from carbon monoxide and hydrogen. The enhanced catalytic performance of nickel–alumina xerogel catalysts with nickel content above 40 wt% was attributed to the abundant active surface nickel species caused by well-developed framework mesopores and large pore size of the catalysts. When considering the amount of nickel used for the preparation of catalyst, it is reasonable to conclude that the optimal nickel content of nickel–alumina xerogel catalyst for methanation reaction was 40 wt%.

Hydrogenation of Succinic Acid to γ-Butyrolactone over Palladium Catalyst Supported on Mesoporous Alumina Xerogel

Mesoporous alumina xerogel (AX) was prepared by a sol–gel method for use as a support for palladium catalyst. Palladium catalyst supported on mesoporous alumina xerogel (Pd/AX) was then prepared by an impregnation method. For comparison, commercial alumina (AC) was also used to prepare Pd/AC catalyst by an impregnation method. Liquid-phase hydrogenation of succinic acid to γ-butyrolactone (GBL) was carried out using supported palladium catalysts (Pd/AC and Pd/AX). The supported palladium catalysts (Pd/AC and Pd/AX) were characterized by XRD, BET, HR-TEM, and hydrogen chemisorption analyses. In the hydrogenation of succinic acid to GBL, Pd/AX catalyst showed a better catalytic performance than Pd/AC catalyst. Fine dispersion of palladium of Pd/AX catalyst was responsible for its high catalytic activity. In the hydrogenation of succinic acid to γ-butyrolactone (GBL), Pd/AX catalyst (palladium catalyst supported on alumina xerogel) showed a better catalytic performance than Pd/AC catalyst (palladium catalyst supported on commercial alumina). GBL could be efficiently produced from a bio-based platform chemical (succinic acid) using Pd/AX catalyst.

Structural characterization of hierarchically porous alumina aerogel and xerogel monoliths

Detailed nanostructures have been investigated for hierarchically porous alumina aerogels and xerogels prepared from ionic precursors via sol–gel reaction. Starting from AlCl 3·6H 2O and poly(ethylene oxide) (PEO) dissolved in a H 2O/EtOH mixed solvent, monolithic wet gels were synthesized using propylene oxide (PO) as a gelation initiator. Hierarchically porous alumina xerogels and aerogels were obtained after evaporative drying and supercritical drying, respectively. Macroporous structures are formed as a result of phase separation, while interstices between the secondary particles in the micrometer-sized gel skeletons work as mesoporous structures. Alumina xerogels exhibit considerable shrinkage during the evaporative drying process, resulting in relatively small mesopores (from 5.4 to 6.2 nm) regardless of the starting composition. For shrinkage-free alumina aerogels, on the other hand, the median mesopore size changes from 13.9 to 33.1 nm depending on the starting composition; the increases in PEO content and H 2O/EtOH volume ratio both contribute to producing smaller mesopores. Small-angle X-ray scattering (SAXS) analysis reveals that variation of median mesopore size can be ascribed to the change in agglomeration state of primary particles. As PEO content and H 2O/EtOH ratio increase, secondary particles become small, which results in relatively small mesopores. The results indicate that the agglomeration state of alumina primary particles is influenced by the presence of weakly interacting phase separation inducers such as PEO.

Effect of calcination temperature of mesoporous alumina xerogel (AX) supports on hydrogen production by steam reforming of liquefied natural gas (LNG) over Ni/AX catalysts

Mesoporous alumina xerogel (AX) supports prepared by a sol–gel method were calcined at various temperatures. Ni/mesoporous alumina xerogel (Ni/AX) catalysts were then prepared by an impregnation method, and were applied to the hydrogen production by steam reforming of liquefied natural gas (LNG). The effect of calcination temperature of AX supports on the catalytic performance of Ni/AX catalysts in the steam reforming of LNG was investigated. Physical and chemical properties of AX supports and Ni/AX catalysts were strongly influenced by the calcination temperature of AX supports. Crystalline structure of AX supports was transformed in the sequence of γ-alumina → (γ + θ)-alumina → θ-alumina → (θ + α)-alumina with increasing calcination temperature from 700 to 1000 °C. Nickel species were strongly bonded to the divalent vacancy of γ-alumina, (γ + θ)-alumina, and θ-alumina through the formation of nickel aluminate phase. In the steam reforming of LNG, both LNG conversion and hydrogen composition in dry gas showed volcano-shaped curves with respect to calcination temperature of AX supports. Among the catalysts tested, Ni/AX-900 (nickel catalyst supported on AX that had been calcined at 900 °C) showed the best catalytic performance. The smallest nickel crystalline size and the strongest nickel–alumina interaction were responsible for high catalytic performance of Ni/AX-900 catalyst in the steam reforming of LNG.

Effect of calcination temperature of alumina supports on the wax hydrocracking performance of Pd-loaded mesoporous alumina xerogel catalysts for the production of middle distillate

Abstract Selective production of middle distillate (C10–C20) from synthesis gas (CO + H2) through wax hydrocracking was carried out in a dual-bed reactor. Co/TiO2 catalyst was used in the first-bed reactor to produce wax (C21+) from synthesis gas, and Pd-loaded mesoporous alumina xerogel (denoted as Pd/XA) catalysts were used in the second-bed reactor to produce middle distillate through wax hydrocracking. The effect of calcination temperature of mesoporous alumina xerogel supports on the wax hydrocracking performance of Pd/XA catalysts was investigated. The increment of middle distillate selectivity increased with increasing medium acidity of Pd/XA catalyst. Among the Pd/XA catalysts, the Pd catalyst loaded on mesoporous alumina xerogel support calcined at 800 °C retained the highest medium acidity, and at the same time, showed the best catalytic performance in the hydrocracking of wax.

Hydrogen production by steam reforming of liquefied natural gas (LNG) over mesoporous nickel–alumina xerogel catalysts: Effect of nickel content

Abstract Mesoporous nickel–alumina xerogel (XNiAl) catalysts with various nickel contents were prepared by a single-step sol–gel method for use in hydrogen production by steam reforming of liquefied natural gas (LNG). The effect of nickel content on the catalytic performance of XNiAl catalysts was investigated. Nickel species were finely dispersed in the XNiAl catalysts through the formation of Ni–O–Al composite structure. The XNiAl catalysts served as efficient catalysts in the hydrogen production by steam reforming of LNG. Both LNG conversion and H2 composition in dry gas showed volcano-shaped curves with respect to nickel content. Thus, optimal nickel content was required for maximum catalytic performance. The performance of XNiAl catalysts in the steam reforming of LNG increased with increasing reducibility of the catalyst. Among the catalysts examined, the 30NiAl (30 wt% Ni) catalyst with the highest reducibility showed the best catalytic performance. The highest surface area and the largest pore volume of the 30NiAl (30 wt% Ni) catalyst were also partly responsible for its superior catalytic performance.

Hydrogen production by steam reforming of liquefied natural gas over a nickel catalyst supported on mesoporous alumina xerogel

Mesoporous alumina xerogel (A-SG) is prepared by a sol–gel method for use as a support for a nickel catalyst. The Ni/A-SG catalyst is then prepared by an impregnation method, and is applied to hydrogen production by steam reforming of liquefied natural gas (LNG). The effect of the mesoporous alumina xerogel support on the catalytic performance of Ni/A-SG catalyst is investigated. For the purpose of comparison, a nickel catalyst supported on commercial alumina (A-C) is also prepared by an impregnation method (Ni/A-C). Both the hydroxyl-rich surface and the electron-deficient sites of the A-SG support enhance the dispersion of the nickel species on the support during the calcination step. The formation of the surface nickel aluminate phase in the Ni/A-SG catalyst remarkably increases the reducibility and stability of the catalyst. Furthermore, the high-surface area and the well-developed mesoporosity of the Ni/A-SG catalyst enhance the gasification of surface hydrocarbons that are adsorbed in the reaction. In the steam reforming of LNG, the Ni/A-SG catalyst exhibits a better catalytic performance than the Ni/A-C catalyst in terms of LNG conversion and hydrogen production. Moreover, the Ni/A-SG catalyst shows strong resistance toward catalyst deactivation.

Synthesis, characterization and electrochemical properties of the V 2O 5· nH 2O/AlO(OH)· nH 2O xerogel composite

In this work, we report the synthesis, characterization and electrochemical properties of a new multicomponent material obtained from the polymerization of vanadium pentoxide in an inorganic matrix (alumina xerogel), forming a xerogel composite. The material has been characterized by X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, electron microscopy, energy dispersive X-ray spectrometry, cyclic voltammetry and impedance spectroscopy. It was found that the V 2O 5 xerogel is dispersed in the alumina matrix, but its lamellar structure is not strongly affected, thus, its conductivity properties are maintained. Moreover, the electrochemical behaviour of the V 2O 5 xerogel dispersed in the alumina matrix is quite similar to that found for the V 2O 5 xerogel alone and the inorganic matrix leads to stabilization of V 2O 5 xerogel structure.

Influence of the Preparation Method and Metal Precursor Compound on Alumina-Supported Pd Catalysts

Supported palladium catalysts were prepared by two different methods: wet impregnation and incipient-wetness impregnation. Commercial gamma-Al2O3 and sol-gel derived alumina were used as support. The synthesis of alumina xerogel was carried out by hydrolysis of aluminum isopropoxide (AIP) in 2-propanol solution. Organometallic -Pd(C5H7O2)2- and inorganic precursor -Pd(NH3)4Cl2.H2O- were used for the synthesis of the catalytic systems.Samples were characterized by BET surface area measurement, atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), temperature-programmed reduction (TPR), and hydrogen chemisorption. The activity of these catalysts was studied for the stoichiometric reduction of NO by methane. The decomposition of NO and methane oxidation were also used as a test reaction. Catalyst prepared by wet impregnation method using Pd(C5H7O2)2 and alumina xerogel presented the best performance for the studied reaction. The activity of Pd/alumina catalysts for NO-CH4 reaction is related to the facility for reduction/oxidation of the metal. This property is affected by the nature of the support as well as the metal precursor.

Enhancement of Green Terbium-Related Photoluminescence from Highly Doped Microporous Alumina Xerogels in Mesoporous Anodic Alumina

Strong enhancement of green photoluminescence (PL) from microporous alumina xerogels, highly doped with terbium (from 30 to 60 wt % as is reported. The regular structure of a 30 μm thick, mesoporous anodic alumina layer was exploited for spin-on deposition of the alumina xerogel in a single step. The green PL, associated with predominant transitions, along with 4, 5, 6 transitions of was found to increase with terbium concentration in the alumina xerogel. This effect is attributed to cross-relaxation. The thermal quenching of the green terbium-related emission does not exceed a factor of two within a temperature range from 10 to 300 K for any of the alumina xerogels confined in anodic alumina. Further, such quenching is much reduced with the rise of temperature compared with Tb-doped titania xerogel, (ii) Tb-implanted thermally grown silicon dioxide film, and (iii) Tb-doped alumina xerogels fabricated onto monocrystalline silicon. Thus, the terbium-doped structure comprising alumina xerogel/anodic alumina is proposed as a basis for green room-temperature luminescent images. © 2001 The Electrochemical Society. All rights reserved.

Modification of non-hydrolytic sol-gel derived alumina by solvent treatments

The effect of wetting non-hydrolytic derived alumina xerogels with water and organic solvents in the 20–70°C range on the alumina's properties was investigated. Wetting with organic solvents does not affect the alumina. However, contact with water was found to change the sharp crystallization at ∼800°C to a continuous crystallization starting at ∼450°C. Water treatment for a day at room temperature (RT) followed by second calcination decreased the surface area by 10%. This decrease in surface area is less pronounced with increasing wetting periods. On the other hand water treatment at 50–70°C followed by a second calcination resulted in a surface area increase of up to 15%. Upon water treatment the total pore volume has decreased from 0.65 (cm3/gr) to 0.48 (cm3/gr) and the average pore size decreased from 6.8 nm to 4.1 nm. The Cl content was found to be uneffected by the water treatment, remaining at ∼2.5% wt. Wetting with water at elevated temperature (70°C) accelerated the morphological changes, eliminating the crystallization peak at 800°C in one hour. A dissolution-reprecipitation mechanism is suggested to explain the results. In addition, Mass-Spectroscopy of the effluent gas during heat treatment revealed the emission of CO2 and water upon phase transition into α-Al2O3, at 1150–1300°C.

The Evolution of Microstructure in Nonhydrolytic Alumina Xerogels

The effect of drying, aging and thermal treatment of alumina xerogels prepared by the nonhydrolytic route was investigated using SAXS, BET and HR-SEM techniques. The microstructure of the fresh xerogels prepared under different procedures varied drastically, ranging from aerogel-like mass fractals to narrow pore size distribution materials. By variation of the drying conditions the N2-BET surface area was varied from an immeasurable low level up to 600 m2/g. The initial microstructure has a significant influence on the xerogel behaviour during the post-drying heating stage. The ability to produce aerogel-like mass fractal materials from the nonhydrolytic systems is discussed. Finally, a brief theoretical treatment of the drying process of mass fractals is presented as well.

Porosity and Surface Acidity of SiO2–Al2O3Xerogels

The structural and chemical properties of silica–alumina xerogels obtained using the sol–gel process were investigated. As methods of characterization, sorption of nitrogen, thermal analysis, and potentiometric titration were chosen. The results obtained showed a high contribution of micropores in the structure of the silica–alumina xerogels, which decreased with increasing content of aluminum. An investigation of the acidic–basic properties of the surfaces showed the presence of centers which have not been detected in commercial catalysts obtained in a more traditional way. These centers may be the result of synergy between our components and/or different coordinations between the cations.

The fractal and percolation analysis of a polymeric Al 2O 3 gel

Fractal and percolation analysis have been performed on nitrogen adsorption isotherms of polymeric Al 2O 3 xerogel, prepared from a low-water non-aqueous solution of aluminum alkoxide. These xerogels showed a distinctively low pore connectivity compared to that reported in the literature on more conventional alumina gels. The alumina xerogel exhibited a surface fractal behavior in the scale range from nitrogen size to a few nanometers. The mean pore coordination number and the surface fractal dimension of the xerogel were rather stable upon calcination, until the appearance of the α-alumina crystalline phase, despite of a large decrease in BET surface area and pore volume.

Synthesis of Alumina and Silica-Containing Alumina Xerogel Hosts

Using the sol-gel process, we prepared two types of xerogels. Thin, transparent, crack-free alumina disks were prepared from aluminum tri-secondary butoxide (ASB). Addition of partially hydrolyzed tetraethylortho-silicate (TEOS) to such alumina sols produced translucent, warped silica-containing alumina xerogels. While the pure alumina xerogels were water soluble, addition of silica improved their durability. To understand the change in water solubility, we investigated the chemical and structural changes that occurred when silica was added. Characterization was performed using X-ray diffraction analysis, thermal analysis, nitrogen sorption analysis and 27Al NMR spectroscopy.

Effect of Aging on Nonhydrolytic Alumina Xerogels

Nonhydrolytic sol-gel route is a relatively recent process which enables production of complex, multicomponent oxide materials. This process has some advantages over the conventional hydrolytic sol-gel route due to the ability to produce low-shrinkage, homogeneous, multicomponent gels. The objective of this work was to determine the effects of aging of nonhydrolytic gels on the composition, yield, phase transformations and morphology. Xerogels were prepared from aluminum chloride and isopropyl ether. Properties were studied using AgNO3 titrations, TGA/DTA, XRD, and BET analysis. We have found that the gels contain significant amount of chlorine where the Cl/Al atomic ratio ranges from 1.1–0.6 depending on the aging time. The crystallization temperature and enthalpy of crystallization decreased with aging time. The decrease of the surface area near the crystallization temperature correlates well with the decrease of the enthalpy of crystallization as a function of aging time. A closed pore phenomenon has been observed in the nonhydrolytic alumina system. Finally, analysis of the condensation degree (CD) yielding Al–O–Al bonds suggests that the rate determining step before the gel point is the alkoxy groups formation. However, during aging of the gels, the CD remains constant since the condensation of chloride with isopropoxy groups is stericly inhibited. Surface areas in the 300–650 m2/g range were obtained depending on the aging time.

Fluorinated ?-alumina aerogels and xerogels: characterisation and catalytic behaviour

The porosity of γ-alumina-based materials is an important parameter affecting the extent of fluorination (aerogels > commercial γ-Al2O3 > xerogels) and, consequently, also the textural, acidic and catalytic properties of the final fluorinated materials. Only the highly fiuorinated aluminas having strong Lewis acidic sites catalyse the isomerisation of CHF2CHF2 to CF3CH2F.

Preparation of alumina by sol-gel process, its structures and properties

Alumina monolithic gel has been prepared by ammonia method from anhydrous aluminium chloride and n-butanol using formamide as the solvent. The gel has been made by hydrochloric acid catalysis and using large quantity of water for hydrolysis (molar ratio of water to aluminium-n-butoxide, R>99). Transparent alumina xerogel of size 37.5×20×5 mm approximately has been prepared. The alumina gel has been dried and sintered at 400°C, 1000°C and 1200°C respectively and the powders formed thereby have been examined by XRD, FTIR spectroscopy, SEM and particle size analysis. These studies have confirmed the formation of α-alumina at 1200°C having particle size as low as 0.2 μm or less along with agglomerates. The density of the powder has increased gradually, where as its particle size has decreased, with the increase of the sintering temperature.