Silica Alumina Aerogel Research Articles

Robust and exceptional thermal insulating alumina-silica aerogel composites reinforced by ultra IR-opacified ZrO2 nanofibers

Fiber-reinforced alumina-silica aerogel composites (FASA), renowned for their high-temperature resistance, low thermal conductivity, and robust mechanical properties, have emerged as a promising candidate for aerospace thermal insulation, especially in harsh environments characterized by intense vibration and oxidation. However, a significant challenge persists in homogeneously doping opacifiers and minimizing thermal conductivity of FASA at elevated temperature, which remains a thorny issue. Herein, we propose a novel fiber-reinforced and opacified (FRO) strategy to fabricate the ZrO2 fiber-reinforced alumina-silica aerogel composites (ZFASA) that boast ultra-low thermal conductivity (0.031 W·m−1·K−1 at 1100 °C) and excellent mechanical strength (0.44 MPa for compressive strength at 10 % strain and 2.1 MPa for bending strength, maintaining intact even after being rolled by a 1.7 t car). Furthermore, ZFASA exhibits remarkable thermal stability, enduring quartz lamp and butane torch tests at 1300 °C without any obvious change. This work provides a simple yet effective FRO strategy, paving a new path for the preparation of thermal insulating aerogel composites. Consequently, the highly durable and exceptional thermally insulating ZFASA we have developed is poised to better satisfy the demands of the aircraft thermal protection systems in extreme heat environments.

Transformation of Fly Ash-Based Oxide Particles into a Functional Silica-Alumina Aerogel and Its Potential Application as an Anti-icing Surface.

Lightweight, surface hydrophobic, highly insulating, and long-lasting aerogels are required for energy conservation and ice-repellent applications. Here, we present the conversion of fly ash to a silica-alumina aerogel (SAA) by utilizing its high silica content. The extracted silica component replaces expensive precursors typically used in conventional aerogel production. Ice adhesion performance was compared to that of polypropylene (PP), an insulating commodity polymer. First, we removed some salt impurities and heavy metals via water and alkaline washing protocols. Then, we produced SAA via the ambient pressure drying method by using trimethylchlorosilane (TMCS) as an adhesion promoter. The newly produced SAA has a surface area of 810 m2 g-1 and shows hydrophobic properties with a contact angle of 140 ± 5°. The thermal conductivity of SAA is 0.0238 W m-1 K-1 with C P = 1.1922 MJ m-3 K-1. The ice adhesion strength of the PP substrate was calculated as 188.30 ± 51.24 kPa, while the ice adhesion strength of the SAA was measured as 1.21 ± 0.40 kPa, which was about 150 times lower than that of PP. This indicated that SAA had icephobic properties since ice adhesion strength was less than 10 kPa. This study demonstrates that fly ash-based SAA can be utilized as an economical material with a large surface area and exceptional thermal insulation capacity and is free of harmful compounds (heavy metals), making it potentially suitable as an anti-ice thermal insulation material.

Production of high-carbon-number bio-aviation fuels from furans using sulfated zirconia and silica-alumina aerogel catalysts

Lignocellulose can help produce fuels in biorefineries through its fractionated components such as cellulose, hemicelluloses, and lignin. For instance, hemicellulose-derived xylose has been studied for obtaining petroleum-substituting fuels. Therefore, in this study, catalytic hydroalkylation/alkylation (HAA) of xylose-derived 2-methylfuran (2-MF) with butanal was targeted to selectively produce high-carbon-number diesel fuels. To that end, silica–alumina (SiAl) aerogels with different Si/Al ratios and a series of sulfated zirconia (SZ) compounds calcined at various temperatures were prepared as catalysts to overcome the limitations of conventional hazardous homogeneous acid catalysts. The activity of the SiAl catalysts in forming the desired HAA product, 5,5-bissylvylbutane (denoted as BB), was found to be directly related to the number of acid sites and acid density. Moreover, increasing the Si/Al molar ratio, which reduced the acid density and number of acid sites, led to lower 2-MF conversions and BB yields. The optimal 2-MF conversion and BB yield with the SiAl catalysts (67 % and 45 %, respectively) were achieved using a Si/Al ratio of 8:2 mol/mol. Furthermore, SZ calcined at 700 °C (denoted as SZ-700) exhibited the highest activity among the SZ catalysts (72 % 2-MF conversion, 63 % BB yield), despite having the fewest acid sites and lowest acid density. This was presumably because of the physical structure of SZ-700, which featured mixed monoclinic and tetragonal phases, many oxygen vacancies, and a large accessible external surface area, which facilitated the HAA reactions. Hydrogenation and hydrodeoxygenation (HDO) of BB were performed thereafter using Pd/C and Ru/SiO2–Al2O3 catalysts, respectively. The resulting deoxygenated hydrocarbons comprised 15.1 % light hydrocarbons (C4–C8) and 83.1 % heavier hydrocarbons (C9–C20). Simulated distillation analysis of the post-HDO product distribution indicated that the liquid-phase product contained 70 % high-carbon-number bio-aviation fuels.

Thermally insulating, fiber-reinforced alumina–silica aerogel composites with ultra-low shrinkage up to 1500 °C

Highly porous, heat resisting and thermal insulating alumina-based aerogel materials are considered as promising high-temperature thermal insulations. The major obstacles for their applications are poor mechanical strengths, transparency to infrared radiation and complex preparation processes. In this study, a direct sol-immersion-gel (SIG) and supercritical fluid drying (SCFD) strategy for synthesizing mullite fiber reinforced alumina–silica aerogel composites (MFASs) was developed to overcome these problems. No tedious operation such as solvent exchange or gel modification was needed. The MFASs, with bulk density less than 0.4 g/cm3, experienced no deformation and ultra-low dimensional shrinkage even under 1500 °C exposure at air atmosphere, also exhibit good compressive strength and toughness. Thanks to cooperation of mullite fibers and the alumina–silica aerogel (ASA), the MFASs show quite low thermal conductivity (0.082 W/(m·K) at 1200 °C) and proved to be extremely thermal insulating during the quartz lamp heating test at 1500 °C. The light-weight, strong MFASs with superior heat resistance and thermal insulating performances can satisfy the urgent need for high-temperature thermal insulations such as thermal protection system (TPS) for space vehicles.

Experiment and kinetic model study on modified potassium-based CO2 adsorbent

In this paper, aluminium chloride crystal was used as precursor to prepare silica-alumina aerogel support by sol-gel method, and the active component K2CO3 was supported on the aerogel support by incipient-wetness impregnation method to obtain a modified potassium-based adsorbent. CO2 adsorption experiment was performed on the adsorbent under different reaction conditions using a small fixed-bed reactor self designed. Based on the characterization results of low temperature N2 adsorption-desorption experiments and scanning electron microscopy, CO2 adsorption characteristics of modified potassium-based adsorbents were studied. The prepared silica-alumina aerogel support has good microstructure and uniform pores with specific surface area and pore volume are 445.7517 m2/g and 2.3270 cm3/g, respectively, and the mesopores account for more than 99%. After loading, the active component K2CO3 is uniformly dispersed. Through model comparison, it was determined that the shrinking core model and Avrami fractional kinetic model were used to fit the experimental results in order to study the reaction kinetics and adsorption kinetics. The results show that as the temperature rises, the amount of CO2 adsorbed by the adsorbent increases first and then decreases. The increase in temperature can improve the activity of the active site of the adsorbent and make the reversible exothermic reaction occur while the adsorbent adsorbs CO2 cannot proceed in the forward direction at the same time. According to the kinetic analysis, the control period of the surface chemical reaction is too short due to the high temperature, which caused low carbonation conversion. The optimal designed loading of modified potassium-based adsorbent, reaction temperature, water vapor pretreatment time, CO2 concentration and total gas volume were 30%, 60 °C, 30 min, 12.5%, and 500 mL/min, respectively.

A chitosan-assisted co-assembly synthetic route to low-shrinkage Al2O3–SiO2 aerogel via ambient pressure drying

Abstract A straightforward chitosan/inorganic salts co-assembly sol-gel strategy was explored to modify the alumina gel at a molecular level, based on polymerization and covalent interaction of chitosan polysaccharide. Serving as a 3D soft template, the chitosan could chelate with hydrolyzed aluminum species to form an enhanced interdigitated network. A monolithic chitosan/alumina-silica (CAS) composite aerogel with fairly low shrinkage of about 17% was contrived through a follow-up surface modification by tetraethoxysilane and a simple ambient pressure drying (APD) process. It was found that the morphology and nanostructure of the aerogel could be readily tuned by changing the amount of acetic acid during the sol-gel process. Compared with the conventional alumina-silica aerogel, CAS aerogel exhibited analogous high temperature thermal stability and refined microstructure, making it as a favorable candidate in adsorption and catalysis arenas at higher temperature. Moreover, the monolithic CAS composite aerogel is expected to be beneficial in modifying membranes for baromembrane separation due to its high porosity as well as hydrophilic property.

Thermal Gravimetric Analysis of Unsaturated Polyesters Filled with Alumina Trihydrate and Silica Aerogel

Thermal degradation of the composite blend consisting unsaturated polyester resin, alumina trihydrate and silica aerogel was studied using thermal gravimetric analysis. Composite filled with silica aerogel show lower density and slightly improve the thermal stability of the pure polymer. The addition of alumina trihydrate slows down the degradation of the polymer due to the release of bond water while the combination of silica aerogel and alumina trihydrate in polyester matrix does not interrupt the function of alumina trihydrate due to inert properties of silica aerogel.

Heat-resistant, strong alumina-modified silica aerogel fabricated by impregnating silicon oxycarbide aerogel with boehmite sol

In order to overcome the fragility and sintering behavior of silica aerogel at high temperature, heat-resistant and strong alumina-modified silica aerogel was fabricated by impregnating silicon oxycarbide (SiOC) aerogel with industrial boehmite sol. The partial sintering of SiOC aerogel during its pyrolysis endowed alumina-silica aerogel its high strength. Meanwhile, the carbon in SiOC can also act as a pore former due to oxidation with O2 during the subsequent calcination in air. This approach is straightforward and economical. The as-prepared composite aerogel exhibited high surface area (278m2/g) and compressive strength of up to 2.17MPa. Compared to SiO2 aerogel prepared using the same method, the composite aerogel was found to be more heat resistant. After being heated to 1000°C and 1200°C for 2h, the surface area of composite aerogel reached up to 183 and 22m2/g, respectively. On the other hand, the corresponding values for linear shrinkage were as low as 6.93% (for 1000°C) and 18.90% (for 1200°C). The XRD patterns indicated that amorphous SiO2 and γ-Al2O3 remained in the composite aerogel after heat-treatment at 1200°C.

Synthesis of fibre reinforced Al2O3−SiO2 aerogel composite with high density uniformity via a facile high-pressure impregnation approach

Alumina-silica (Al2O3?SiO2) aerogel composite, with low density, low thermal conductivity and hightemperature stability, is attracting increased interest in the field of thermal insulation application. In this paper, a novel way to fabricate fibre reinforced Al2O3?SiO2 aerogel composite via a facile high-pressure impregnation approach was reported. Two Al2O3?SiO2 aerogel composites, HPe and LPe, were synthesized via high-pressure and low-pressure impregnation approach, respectively. The effects of the impregnation approach on the aerogel composites performance were studied, and the impregnation model was established. The results showed that the as-prepared HPe exhibited higher density uniformity, better high-temperature stability, higher specific surface area and lower thermal conductivity. It was also shown that the effect of impregnation approach on the mean density and morphology of the aerogel composites is negligible. However, the standard deviation of density (0.00857) and mean thickness shrinkage (9.98%) of HPe were 37.8% and 15.4% lower than that of LPe, respectively. The specific surface area (884.1m2/g) of HPe was 43.5% higher than that of LPe. The thermal conductivity of HPe at 1100?C was 2.74% lower than that of LPe. The impregnation model of the aerogel composites presented that the density uniformity and thermal conductivity of HPe were improved obviously, because there were less large pores in HPe than in the LPe.

Layered MWW zeolite-supported Rh catalysts for the hydrodeoxygenation of lignin model compounds

The improved catalytic activity of Rh nanoparticles deposited on the swollen and pillared zeolites was observed in the hydrodeoxygenation (HDO) reactions of 1,3,5-trimethoxybenzene (1,3,5-TMB), a bulky lignin model compound, and guaiacol, one of the most frequently used lignin model compounds. As the high dispersion of metal nanoparticles increases HDO activity, the swelling/calcination and pillaring of crystalline MCM-22 zeolites increased the dispersion of Rh metal nanoparticles on the external surface area, and thus the corresponding HDO activity with respect to 1,3,5-TMB. On the contrary, although the mesopores in the amorphous MCM-41 and silica-alumina aerogel (SAA) supports accommodated higher Rh dispersion, overall, the resulting catalysts suffered from mass transfer limitation, and thus showed poor HDO reaction activity for 1,3,5-TMB. Finally, the Rh nanoparticles supported on the pillared zeolite showed the highest HDO activity for guaiacol, mainly due to the higher Rh dispersion and acid sites on the external surface among the zeolite-supported Rh catalysts.

Production of high-energy-density fuels by catalytic β-pinene dimerization: Effects of the catalyst surface acidity and pore width on selective dimer production

High carbon number hydrocarbon fuels were produced by catalytic β-pinene dimerization. These fuels can be used as high-energy-density fuels, including diesel and jet fuels. To suppress the production of oligomers composed of more than two pinene units, which are undesirable in liquid fuels because of their high boiling temperatures and viscosities, the pore width of the silica–alumina aerogel catalysts was modified and their surfaces were treated chemically. The chemical treatment of the catalyst surface improved the selectivity of fuel-grade dimer production and suppressed oligomer formation. The reaction pathways for dimer production from pinenes were also studied. It was found that Brønsted acids are responsible for pinene condensation, although earlier works reported that Lewis acids are responsible for condensate formation.

Production of renewable p-xylene from 2,5-dimethylfuran via Diels–Alder cycloaddition and dehydrative aromatization reactions over silica−alumina aerogel catalysts

We report the selective conversion of biomass-derived 2,5-dimethylfuran (DMF) to p-xylene (~70% selectivity) through Diels–Alder cycloaddition and subsequent dehydration with silica−alumina aerogel (SAA) catalysts. The high activity of SAA can be attributed to its high surface area, large mesoporous volume, and high acid site concentrations. The conversion of DMF and the yield of p-xylene were strongly dependent on the silica alumina ratio of SAA. A higher aluminum content in SAA led to a progressive increase in the concentration of Brønsted acid sites and a corresponding increase in the p-xylene production rate. The effect of solvent on the production of p-xylene was examined, and it was found that the p-xylene production rate increases significantly in polar aprotic solvents (i.e. 1,4-dioxane).

Keggin-type phosphotungstic acid supported on mesoporous SiO2–Al2O3 aerogel like beads and their application in the isopropylation of naphthalene

Spherical mesoporous silica–alumina aerogel like beads based on sol–gel technology and the drop wise addition have been synthesized and used as catalyst support for phosphotungstic acid (PWA). Their catalytic performances in the isopropylation of naphthalene with isopropanol were investigated in a batch reactor. It was found that PWA was highly dispersed on the silica–alumina support and their Keggin structure can be retained. In addition, PWA/SiO2–Al2O3 catalyst showed high surface area, both of Lewis acid sites and Bronsted acid sites. Because of having more Bronsted acid sites, silica–alumina supported acid catalysts showed much higher conversion (87.97 %) and selectivity to diisopropylnaphthalenes (41.41 %) and β,β-products (59.82 %) than pure acid and reactive supports in the isopropylation of naphthalene. The catalytic behavior has been discussed in relation with the physical chemical properties of catalysts, reaction and activation temperature and reaction time.

Production of high carbon number hydrocarbon fuels from a lignin-derived α-O-4 phenolic dimer, benzyl phenyl ether, via isomerization of ether to alcohols on high-surface-area silica-alumina aerogel catalysts

Two-step hydrodeoxygenation of benzyl phenyl ether (BPE), a lignin-derived phenolic dimer containing an α-O-4 linkage, was performed to produce high carbon number saturated hydrocarbons. The ether linkage of BPE was first isomerized to alcohols of benzylphenols on the solid acid catalysts of silica (SA), alumina (AA), and silica-alumina aerogels (SAAs), which were further hydrodeoxygenated to saturated cyclic hydrocarbons on a silica-alumina-supported Ru catalyst. During the isomerization of BPE, noble-metal-free catalysts suppressed the formation of phenyl monomers but produced the phenolic dimers. SA, AA, and SAA-73 (Al/(Si+Al)=0.73) exhibited negligible activity. However, SAA-38 and SAA-57 containing Al/(Si+Al) contents of 0.38 and 0.57, respectively, exhibited high catalytic activity among the prepared aerogel catalysts. The BPE conversion on SAA-38 reached 100% at a temperature range of 100-150°C. The Brönsted acid sites appear to be catalytic active sites. On the basis of the predominant isomerization of phenyl ether to phenols over ether decomposition on the SAAs, the following second step of hydrodeoxygenation (HDO) after the first step of isomerization of BPE produced deoxygenated C13–19 cyclic hydrocarbons, as opposed to the saturated deoxygenated cyclic hydrocarbons produced trhough a one-step reaction process with silica-alumina-supported Ru catalysts, demonstrating this to be a promising process for producing high carbon number hydrocarbons from lignin dimers and oligomers.

Preparation and Characterization of Silica–Titania and Silica–Alumina Aerogels

Silica–titania and silica–alumina aerogels were prepared by the two-step hydrolysis of metal alkoxides, the sol–gel polymerization and the supercritical drying with carbon dioxide. As a result of characterization by nitrogen adsorption, the aerogels were mesoporous materials with high surface areas and had few micropores. The experimental results suggested that the surface area, the mesopore volume, and the pore radius of the aerogel depended on the titanium (Ti) or aluminum (Al) content. Adsorption isotherms of ethane, ethylene, propane, and propylene were measured on the aerogels prepared. It was found that the adsorption capacities for these hydrocarbons were changed by mixing Ti or Al to the silica aerogel. In the ethane–ethylene or propane–propylene system, the adsorption selectivity of ethylene or propylene on the silica–titania aerogel was lower than that on the silica aerogel. On the other hand, the selectivity on the silica–alumina aerogel was higher than that on the silica aerogel. Hence, the silica–alumina aerogel was useful for separating ethylene from ethane or propylene from propane.

Alumina-silica aerogel catalysts prepared by two supercritical drying methods for methane combustion

Palladium-supported alumina-silica (10 mol%) aerogels were prepared by two different supercritical drying methods. In one method, an alumina-silica wet gel was dried under supercritical conditions of ethanol in an autoclave at a temperature of 270‡C and pressure of 26.5 MPa. In the other, ethanol was supercritically extracted by carbon dioxide in an extractor at 80‡C and 15.7 MPa. Their catalytic properties for methane combustion were measured after being fired at 1200‡C for 5 h. The palladium-supported alumina-silica aerogels prepared in the autoclave showed higher activity (methane conversion) than the one prepared in the extractor. This was attributed to the autoclave-dried aerogel having a larger pore size and better palladium dispersion.

Effect of acidity upon activity of oxide catalysts. Part II. Dehydration of isopropyl alcohol

Many pure and multicomponent inorganic aerogels have been prepared using the autoclave method which has been recently largely developed in this laboratory. The first step of the process consists in the preparation of a mixed alcogel (in the case of a nickel oxide-alumina aerogel for example). Organic derivatives of aluminium and nickel, both dissolved in methanol, are coprecipitated as alcogels by addition of water. In the second step the solvent is evacuated under hypercritical conditions in an autoclave, in order to avoid the collapse of the texture. If the autoclave is then flushed out by hydrogen instead of nitrogen, metallic nickel-on alumina aerogel is directly obtained, rather than nickel oxide-on alumina aerogel.This procedure gives solids with a large pore volume and high surface areas. These aerogels are amorphous provided some conditions during hydrolysis are satisfied and in particular the proportion of water and the pH.The NiO-Al2O3 aerogels exhibit good catalytic activities and selectivities in the partial oxidation of olefins (isobutylene). When a nickel oxide on silica-alumina aerogel, prepared by the autoclave process, is used as a catalyst for the partial oxidation of an olefin (isobutylene), dimerization of this reactant occurs showing in this case the effect of the silica-alumina aerogel support, whereas in the case of NiO-alumina aerogel partial oxidation without dimerization of the olefin is recorded.Metal on oxide aerogels are good hydrogenation-dehydrogenation catalysts. Thus, nickel on alumina aerogel is very selective in the hydrogenolysis of ethylbenzene into benzene, whereas copper on alumina aerogel gives 100% selectivity in the hydrogenation of cyclopentadiene into cyclopentene.Nickel on alumina aerogel provides also a good mean of testing the hydrogen spillover phenomena in an olefin hydrogenation catalyzed reaction. Infra-red investigations of the spilt-over hydrogen species show that this species is linked to the alumina as a surface OH group.Another kind of multicomponent catalyst which can be prepared by the autoclave process is a nickel on molybdenum oxide aerogel. Pure molybdenum oxide aerogel shows a very good electrical conductivity so it may be expected that Ni dispersed on MoO2 aerogel may be used as a fuel cell electrode component. The effect of the amount of Ni on the textural and electrical properties of the Ni-MoO2 aerogel is described.

On theory of chemisorption

Many pure and multicomponent inorganic aerogels have been prepared using the autoclave method which has been recently largely developed in this laboratory. The first step of the process consists in the preparation of a mixed alcogel (in the case of a nickel oxide-alumina aerogel for example). Organic derivatives of aluminium and nickel, both dissolved in methanol, are coprecipitated as alcogels by addition of water. In the second step the solvent is evacuated under hypercritical conditions in an autoclave, in order to avoid the collapse of the texture. If the autoclave is then flushed out by hydrogen instead of nitrogen, metallic nickel-on alumina aerogel is directly obtained, rather than nickel oxide-on alumina aerogel.This procedure gives solids with a large pore volume and high surface areas. These aerogels are amorphous provided some conditions during hydrolysis are satisfied and in particular the proportion of water and the pH.The NiO-Al2O3 aerogels exhibit good catalytic activities and selectivities in the partial oxidation of olefins (isobutylene). When a nickel oxide on silica-alumina aerogel, prepared by the autoclave process, is used as a catalyst for the partial oxidation of an olefin (isobutylene), dimerization of this reactant occurs showing in this case the effect of the silica-alumina aerogel support, whereas in the case of NiO-alumina aerogel partial oxidation without dimerization of the olefin is recorded.Metal on oxide aerogels are good hydrogenation-dehydrogenation catalysts. Thus, nickel on alumina aerogel is very selective in the hydrogenolysis of ethylbenzene into benzene, whereas copper on alumina aerogel gives 100% selectivity in the hydrogenation of cyclopentadiene into cyclopentene.Nickel on alumina aerogel provides also a good mean of testing the hydrogen spillover phenomena in an olefin hydrogenation catalyzed reaction. Infra-red investigations of the spilt-over hydrogen species show that this species is linked to the alumina as a surface OH group.Another kind of multicomponent catalyst which can be prepared by the autoclave process is a nickel on molybdenum oxide aerogel. Pure molybdenum oxide aerogel shows a very good electrical conductivity so it may be expected that Ni dispersed on MoO2 aerogel may be used as a fuel cell electrode component. The effect of the amount of Ni on the textural and electrical properties of the Ni-MoO2 aerogel is described.