Alumina Aerogel Research Articles

Fabrication of hydrophobic alumina aerogel monoliths by surface modification and ambient pressure drying

Hydrophobic crack-free alumina aerogel monoliths were fabricated by –Si(CH 3) 3 (trimethylsilyl substituent) modification of alcogels followed by an ambient pressure drying procedure. One-step solvent exchange and surface modification were simultaneously progressed by immersing alumina alcogels in trimethylmethoxysilane (TMMOS)/hexane solution. It is found that the hydrophobic property of alumina aerogels is affected by the contents of TMMOS from the measurements of contact angle and Fourier transform infrared spectrometry. Thermogravimetry/differential scanning calorimetry analyses reveal that the modified aerogels maintain their hydrophobic behavior up to a temperature of 260 °C. The structure and morphology of the obtained hydrophobic alumina aerogels were characterized by the measurements of N 2 physical adsorption and scanning electron microscopy, which showed that they were highly porous materials with narrow slit-like pore geometry and a high degree of pore size uniformity.

Alumina aerogels prepared via rapid supercritical extraction

Alumina aerogels with surface areas from 460 to 840 m2/g and bulk densities from 0.025 to 0.079 g/cm3 were successfully fabricated using variations of an aluminum isopropoxide-based recipe developed by Armor and Carlson and the rapid supercritical extraction (RSCE) process developed at Union College. By utilizing the Union College RSCE method, it is possible to convert an alumina aerogel precursor mixture into aerogel monoliths in as little as 7.5 h. This process is safer than methanol extraction in an autoclave and faster and simpler than liquid CO2 solvent exchange and extraction. By increasing the concentration of aqueous HNO3 used in the precursor mixture, we were able to fabricate aerogels with significantly increased surface area, decreased bulk density, and altered microstructure. We attribute the observed variation in these aerogel properties at a given HNO3 concentration to environmental factors such as humidity. The ability to more easily fabricate alumina aerogels with desirable properties will assist in making them a viable option for catalytic and other applications.

Characterization of nickel–alumina aerogels with high thermal stability

The characterization of NiO–Al2O3 aerogels prepared from nickel nitrate and aluminum iso-propoxide through a sol–gel processing and subsequent supercritical drying was performed. The UV–visible, XPS and FT–IR investigations revealed that nickel ions were incorporated into alumina spinel structure as nickel aluminate form, not as nickel oxide, after calcination. TEM observations after the subsequent reduction exhibited uniform and fine nickel particles less than 6 nm diameter throughout the alumina aerogel support with high dispersion, by which not only high thermal stability of the metal at elevated temperatures but also high reforming activity and stability should be brought about. The large surface area and pore volume also provided the catalyst stability through the improvement of the thermal stability of alumina support.

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.

Evaluating and understanding the hydrothermal stability of alumina aerogel doped with yttrium oxide and used as a catalyst support for the thermo-catalytic cracking (TCC) process

The current work presents results concerning the hydrothermal stability of yttria-doped alumina aerogel, which was prepared by the sol–gel method using a supercritical drying technique. The influence of atmospheric steam at 750 and 1000 °C on the rate of sintering of yttria-doped alumina aerogel with various yttria contents was investigated. BET N2 adsorption measurements, X-ray diffraction, thermogravimetric analysis, FTIR of adsorbed pyridine, and 1H and 27Al MAS NMR were used for characterization of solid samples. It was found that the hydrothermal stability of yttria-doped alumina aerogel was greatly improved compared with alumina aerogel and conventional alumina. This was evident from the decrease in the loss of surface area and pore volume, and the minimization of structural transformations. These significant improvements were attributed to the combined effect of sol–gel method and the presence of yttrium ions. Yttria-doped alumina aerogel with various yttria loadings, when compared with undoped alumina aerogel, exhibited lower concentrations of surface –OH groups, higher amounts of bridged –OH groups, and lower degrees of crystallinity with smaller particle sizes. It was also observed that the thermal and hydrothermal stabilities of both yttria-impregnated alumina aerogel and conventional alumina were improved but to a much lower extent. These improvements were attributed to the presence of yttria, and subsequently the formation of surface yttrium aluminate. This structure was found to be more thermally stable than bare alumina owing to the presence of less surface defects and –OH groups. Therefore, it was concluded that to observe the most effective role of yttria, it is quite essential to incorporate it into the alumina structure via the sol–gel technique.Key words: thermo-catalytic cracking (TCC), yttria-doped alumina aerogel, yttria-impregnated alumina, hydrothermal stability, surface –OH groups, Lewis sites.

Catalysts for the thermo-catalytic cracking (TCC) process: Interactions between the yttria in yttria-doped alumina aerogel and the mono-oxide MoO3, CeO2, and bi-oxide MoO3–CeO2 species

The influence of yttria used as a doping agent for catalyst support, on the dispersion and the resistance to sintering of MoO3, CeO2 and bi-oxide MoO3–CeO2 species in the TCC catalysts has been investigated. Characterization techniques used included N2 adsorption, X-ray diffraction, Raman, thermogravimetric analysis, and iso-electric point (IEP). It was explicitly found that yttria-doped alumina aerogel is by far more capable of homogenously dispersing the active molybdenum species and significantly retarding their sintering at quite high-calcination temperature than conventional alumina and alumina aerogel. Characterization results of supported mono-oxide MoO3 catalysts have clearly indicated that the fraction of (surface) tetrahedral monomolybdate species increases in the detriment of crystalline MoO3 as we varied the support from conventional alumina to yttria-doped alumina aerogel with increasing amounts of the yttria loading. This can be attributed to the incorporation of yttria into alumina aerogel network, which led to a change in the support surface charge (IEP) and subsequently the stability of surface molybdate species. The dispersion degree of ceria (substituted ceria) on supported mono-oxide CeO2 was improved as well, upon using yttria-doped alumina aerogel. Such improvement was attributed to the formation of stronger Ce–Y–O bond, and/or the formation of CeO2–Y2O3 solid solution. In the bi-oxide MoO3–CeO2, characterization results have indicated the presence of surface interaction between Mo and Ce, probably through the formation of surface “Mo–O–Ce” type phase between the dispersed ceria and the molybdate monolayer. This interaction, which was highly favourable over yttria-doped alumina surface, contributed significantly to the overall surface stability of Mo–Ce catalysts.

Thermal stability of alumina aerogel doped with yttrium oxide, used as a catalyst support for the thermocatalytic cracking (TCC) process: An investigation of its textural and structural properties

Abstract Undoped and yttria doped alumina aerogel are prepared by the (solution) sol–gel method using a supercritical drying technique. The doping concentrations are in the range of 2.5–20.0 wt.% Y2O3, using yttrium(III) nitrate as a precursor. The effect of yttria doping on the structural and textural properties of alumina aerogel is investigated by BET surface area measurement, X-ray diffraction (XRD), TEM and 27Al MAS NMR. This method of preparation of alumina aerogel provides the most important increases in surface area at 750 °C and up to 1000 °C, when compared to conventional alumina that is obtained via thermal dehydration of boehmite. These results show that the much smaller size of crystallites of alumina aerogel is quite efficient to retard the sintering occurring at these high temperatures. Further improvements in thermal stability are obtained by incorporation of yttrium species during the aerogel preparation. The stabilizing effect goes through a maximum with a yttrium content of ca. 10 wt.%. The presence of yttrium significantly delays the lattice rearrangement into new phases, as normally experienced by the conventional alumina when the temperature increases. These yttrium species induce the formation of distorted tetrahedral Al sites. Such binding effect of Y to Al is clearly detected by 27Al MAS NMR already at 750 °C. However, at 1200 °C, even the doped aerogel cannot withstand the combined effect of sintering and α-phase transformation, so that extremely low surface areas corresponding to those of alumina α-phase and segregated Y-Al2O3 are obtained.

Processing and Property Evaluation of Alumina Aerogel Based Ceramic Nanocomposites

Aerogels are sol-gel derived mesoporous materials with low apparent density and large specific surface areas. The mechanical property evaluation of pulsed electric current sintered (PECS) alumina and its composite aerogels with nanosized metallic dispersions is an area of interest. In the present work alumina, alumina-copper, alumina-nickel, alumina-cobalt and alumina-tungsten aerogels have been synthesized by super critical drying of respective the sols. The samples were pre-calcined under controlled sintering conditions and pulsed electric current sintered at two different temperatures: (1) at a lower sintering temperature with short residence time (1250°C for 5min) and (2) at a high temperature with prolonged soaking time (1450°C for 30 min) and their mechanical properties were evaluated.

Nanoengineering mechanically robust aerogels via control of foam morphology

Potential of aerogels for technological applications is often limited by their poor mechanical properties. Here, we demonstrate that alumina aerogel monoliths with excellent mechanical properties can be made by controlling the crystallographic phase, shape, and size of nanoligaments. In particular, we show that thermal processing of aerogels with a morphology of interconnected nanoleaflets causes dehydration and associated curling of the nanoleaflets, resulting in a dramatic improvement of mechanical properties. This study shows an effective way to control mechanical properties of the nanoporous solids that can be synthesized with ligaments having a quasi-two-dimensional shape, such as platelets, ribbons, or leaflets.

Mesoporous silica–alumina aerogels with high thermal pore stability through hybrid sol–gel route followed by subcritical drying

Silica–alumina mixed oxide aerogels have been synthesised for the first time by subcritical method, having compositions of 5, 10, 15, 20 and 25 wt.% of alumina in silica through a hybrid sol–gel technique. Boehmite was used as the precursor for alumina and TEOS for silica. Subcritical drying method involving repeated solvent exchange and gel wall strengthening is followed in place of supercritical method. The aerogels are calcined at 500, 700, 900 and 1200 °C. Nitrogen sorption studies indicate that the mixed oxide aerogels are mesoporous in nature with high surface areas comparable to supercritically dried aerogels and also possess excellent thermal pore stability. FT-IR spectroscopy has been used to study the effect of an alumina addition to silica. The linear shrinkage of aerogels upon heat treatment is also reported. The acid strength distribution has been measured by temperature programmed desorption of ammonia (TPD). The present method involving hybrid sol–gel route followed by subcritical drying could be further developed for synthesis of mixed oxide mesoporous hybrids for catalyst and gas adsorption.

Synthesis of Alumina Aerogels by Ambient Drying Method and Control of Their Structures

A new method to synthesize alumina aerogels was introduced in this article. Alumina aerogels were firstly prepared by sol-gel process, using Al(β) inorganic compound as a precursor, 1,2-epoxy propane as a gelation inducing agent (GIA) and methane amide as drying control chemical additive (DCCA), followed by ambient drying method (ADM). The bulk alumina aerogels were examined by TEM, SEM and BET method, which have similar qualities to those prepared by supercritical fluid drying method (SCFM). Propane oxide was introduced to accelerate the gelation rate of Al(β) ions and improve the homogeneity of pore distributions of aerogels, formamide as drying control chemical additive (DCCA) to improve the homogeneity of oxide particles in the gel texture. Alumina gels were aged and reinforced through a solvent exchanging of alcohol, the immersing of tetraethyl orthosilicate (TEOS)/alcohol and absolute alcohol at a certain temperature. The aged gels were dried at the ambient pressure and the bulk alumina aerogels with high surface areas and higher porosity were obtained. On the base of the relationship of the variation of alumina aerogels, structures with molar ratio of propylene oxide/Al, the structure of alumina aerogels can be controlled by adjusting molar ratio of propylene oxide/Al.

Synthesis of High-Surface-Area Alumina Aerogels without the Use of Alkoxide Precursors

Alumina aerogels were prepared through the addition of propylene oxide to aqueous or ethanolic solutions of hydrated aluminum salts, AlCl3·6H2O or Al(NO3)3·9H2O, followed by drying with supercritical CO2. This technique affords low-density (60−130 kg/m3), high-surface-area (600−700 m2/g) alumina aerogel monoliths without the use of alkoxide precursors. The dried alumina aerogels were characterized using elemental analysis, high-resolution transmission electron microscopy, powder X-ray diffraction, solid-state NMR, acoustic measurements, and nitrogen adsorption/desorption analysis. Powder X-ray diffraction and TEM analysis indicated that the aerogel prepared from hydrated AlCl3 in water or ethanol possessed microstructures containing highly reticulated networks of pseudoboehmite fibers, 2−5 nm in diameter and of varying lengths, whereas the aerogels prepared from hydrated Al(NO3)3 in ethanol were amorphous with microstructures comprised of interconnected spherical particles with diameters in the 5−15 nm ra...

Preferential oxidation of carbon monoxide in hydrogen-rich gas over platinum–cobalt–alumina aerogel catalysts

The performance of the platinum–cobalt catalysts in a carbon monoxide preferential oxidation (PROX) reactor was investigated for polymer electrolyte fuel cell systems. First, the PROX reaction was analyzed based on two major reactions involved in it, i.e., oxidation of carbon monoxide and the H 2–O 2 reaction. Both reactions were affected by the other reaction depending on the reaction temperature regions, but the extent of the influence is not so large. The platinum–cobalt catalysts were found to exhibit high performance in PROX of carbon monoxide in rich hydrogen as a result of catalytic synergy effect between platinum and cobalt. In order to improve such synergy effect, the catalysts were prepared by sol–gel method and subsequent supercritical drying. The platinum–cobalt catalysts prepared by the single step sol–gel procedure exhibited higher activity for PROX than the catalysts prepared by the conventional impregnation. Supercritical drying preserved the active species of platinum–cobalt phase, therefore, the platinum–cobalt composite aerogel catalyst exhibited excellent ability for the carbon monoxide removal.

Fluorescent impact cavities in a titanium-doped Al2O3–SiO2 aerogel: implications for the velocity resolution of calorimetric aerogels

Silica aerogels have been shown to be superior at capturing hypervelocity projectiles with minimal alteration. On the basis of what is known about the abundance of natural projectiles in low Earth orbit (LEO), aerogel collectors flown in LEO should collect numerous scientifically interesting extraterrestrial dust particles, including interstellar dust grains. However, to date, only a few extraterrestrial dust grains have been found and not a single contemporary interstellar dust grain has ever been identified or analyzed in the laboratory. This lack of success is due to the fact that when using conventional aerogels it is very difficult, if not impossible, to reconstruct the velocity of captured natural projectiles. To address this problem, we are currently developing aerogel collectors/detectors that passively record the kinetic energy of an impacting projectile. In our previous work, we demonstrated that (Gd, Tb)-doped alumina aerogels may transform into a fluorescent phase(s) as a result of the rapid shock heating experienced by the capture of hypervelocity projectiles and that the amount of fluorescence excited, using a UV light source, is an increasing function of the projectile’s kinetic energy. However, our previous work did not demonstrate the accuracy of this ‘calorimetric’ technique in reconstructing the impact velocity of projectiles. In this paper, we report on the production of fluorescent impact cavities in a Ti-doped (5%) SiO2–Al2O3 aerogel monolith that resulted from the capture of 4.37kms−1 20μm glass beads. In addition we demonstrate that the dispersion of the fluorescent response of this aerogel implies that we should be able to reconstruct the velocity of a captured projectile with a resolution of 20% or better.

Preparation of WO3 aerogel catalysts using supercritical CO2 drying

Single tungsten oxide aerogels (WO3), binary oxide aerogels (WO3–Al2O3) and ternary oxide aerogels (WO3–SiO2–Al2O3) were prepared using standard sol–gel route. Tungsten oxide tetraethoxide (WO(OCH2CH3)4) was used as the sol–gel precursor. The excellent properties of the gels obtained by the sol–gel synthesis were preserved upon supercritical drying with CO2. After supercritical drying at 40°C and 100bar, all aerogels were calcined to 800°C. The influence of the synthesis parameters on the catalytic activity of WO3 as supported on silica and/or alumina aerogels was investigated through the transformation of N-(phosphonomethyl)iminodiacetic acid to N-(phosphonomethyl)glycine. Despite including WO3 into single and mixed silica and alumina aerogels, high specific surface areas (284–653m2g−1) were preserved. Higher conversion was obtained for catalysts with higher ratios of WO3 in the mixed silica–alumina aerogels that were calcined at 800°C.

Preparation and characterization of mesoporous titania–alumina ceramic by modified sol–gel method

Mesoporous titania–alumina ceramics are interesting as catalysts and catalyst supports. Monolithic wet-gels of 0.2TiO2–0.8Al2O3, prepared by hydrolysis of metal alkoxides in alcoholic solutions, were dried at 90°C (xerogel), immersed into a surfactant solution before drying (surfactant-modified gel), or the solvent in wet gels was supercritically extracted in one step from CO2 at 60°C and 24Mpa for 2h (aerogel). Thermal evolution of the microstructure of the gels was evaluated by thermal analyses (TG-DTA), N2 adsorption, scanning electron microscopy, and X-ray diffraction. After calcination at 500°C, the specific surface area of the gels was >400m2g−1. The average pore radius for the supercritically extracted titania–alumina gel, about 8nm (∼50% of that for alumina aerogels), was 2–4 times larger than that of the surfactant-modified gels and of the xerogel. The specific surface area of the surfactant-modified gels and of the supercritically extracted gels was more than 150m2g−1 after calcination at 800°C. The pore size and pore volume of these gels minimally decreased after calcination at 800°C, while those values of the xerogel markedly decreased. The thermal stability of the microstructure of mesoporous titania–alumina is improved by surfactant immersion or supercritical extraction.

Evolution of the Structure and Magnetic Properties of FeCo Nanoparticles in an Alumina Aerogel Matrix

Nanocrystalline γ-Al2O3 and FeCo−Al2O3 nanocomposite aerogels with high surface areas and pore volumes were prepared by high-temperature supercritical drying of alcogels obtained by a fast sol−gel procedure. The formation of γ-Al2O3 occurs via a sequence of stages: in the parent aerogel an alkyl derivative of boehmite is observed whose calcination gives rise to a disordered phase and finally to γ-Al2O3 which is stable up to 1000 °C. In the presence of iron and cobalt, calcination of the aerogel gives rise to a spinel phase similar to γ-Al2O3 where metal ions partially fill the vacancies. Nanocomposites constituted of FeCo alloy nanoparticles dispersed into γ-Al2O3 matrix are obtained via reduction in hydrogen flow of the aerogels containing iron and cobalt. The amount and average size of the nanoparticles depends both on the temperature and time of the reduction treatment and affects the resulting magnetic properties. All the reduced aerogels show superparamagnetic behavior, but the collective magnetic p...

Chemistry of Preparation of Alumina Aerogels in Presence of a Complexing Agent

Transition alumina with both high specific surface area (SSA) and quite important pore volumes deserve some interest in the field of heterogeneous catalysis as such alumina can be used as supports for different active phases, namely those used in hydroprocessing reactions. Alumina has been synthesised by a sol-gel method for which a key parameter, examined in this study, is the introduction of chelating agents such as butan-1,3-diol or acetylacetone (Hacac) in the aluminium-tri-sec butylate (ASB, the chosen Al precursor) solution. The study of the complexation chemistry for a complexing agent/Al alkoxide molar ratio (C) equal to 1, 2 and 3 was carried out. Some species in initial solutions were identified by IR and 27Al NMR spectroscopies, and the dried solids were characterised by the combined use of elemental analysis, XRD and IR spectroscopy. After hydrolysis of the precursor, the dried solids exhibited the boehmitestructure, more or less crystallised whatever the C ratio when butan-1,3-diol was used whereas a formulation Al(acac)(OH)2 was obtained when Hacac was introduced with a ratio C = 1. It has been shown that the increase in the C ratio up to 2 or 3 induces the precipitation of well-defined Al(acac)3 particles before hydrolysis. This compound was also present in small quantities in the dried gel for C = 1. After calcination at 500°C, more or less well crystallised γ-alumina was obtained. Compared to those of the reference alumina, the textural parameters are strongly modified by the use of a complexing agent. Moreover, Hacac has a stronger effect than butane-1,3-diol on the textural characteristics of the support.

Catalytic reactions of chlorofluoroethanes at fluorinated alumina and chromia aerogels and xerogels: A comparison of reaction pathways in alumina- and chromia-based catalysts

The differences in the dynamic behaviour of 1,1,2-trichlorotrifluoroethane over fluorinated alumina or chromia catalysts at moderate temperatures are significant. Alumina-based catalysts favour the isomerization of CCl 2FCClF 2 followed by the dismutation of CCl 3CF 3 so formed. Fluorinated chromias are less selective since halogen exchange and isomerization reactions both occur. An explanation in terms of the differences in AlX, X=Cl or F, bond energies compared with their CrX counterparts is suggested. The identity of the catalyst precursor has little effect in the chromium case and no effect in the case of aluminium.

Synthesis of high porosity, monolithic alumina aerogels

Many non-silica aerogels are notably weak and fragile in monolithic form. In particular, few monolithic aerogels with densities less than 50 kg/m 3 have any significant strength. It is especially difficult to prepare uncracked monoliths of pure alumina aerogels that are robust and moisture stable. In this paper, we discuss the synthesis of unusually strong, stable, monolithic, high porosity (>98% porous) alumina aerogels, using a two-step sol–gel process. The alumina aerogels have a polycrystalline morphology that results in enhanced physical properties. For an alumina aerogel with a density of 37 kg/m 3 , the specific surface area is 376 m 2/g, the elastic modulus is 550 kPa, and the thermal conductivities at 30°C, 400°C and 800°C, are 29, 98 and 298 mW/mK, respectively. All of the measured physical properties of the alumina aerogels except the specific surface area are superior to those for silica aerogels for equivalent densities.