The effect of heat treatment conditions on the textural and catalytic properties of nickel–titania composite aerogel catalysts

Highly dispersed nickel–titania composite aerogel catalysts have been prepared by CO₂ supercritical drying of alcogels obtained by the sol–gel process. The effect of heat treatment conditions on the textural and structural properties of the resulting aerogels was investigated by nitrogen adsorption–desorption, TG/DTA, XRD and TEM. The nickel–titania composite aerogel calcined and reduced under carefully controlled conditions exhibited an excellent catalytic performance for the liquid-phase hydrogenation of benzophenone to benzhydrol.     

Preparation of carbon aerogels with different pore structures and their fixed bed adsorption properties for dye removal

Controlled porosity carbon aerogels are prepared by a sol–gel polymerization method and the prepared materials are used as fixed bed adsorbents for dye removal. The influences of operating conditions and preparation factors on the adsorption of C. I. Reactive Red 2, as a model compound, from aqueous solution were investigated. Many column parameters were estimated at different stages and the Bed-Depth-Service-Time model was used to analyze the experimental data. The results showed that the adsorption bed capacity increased with increasing bed height, decreasing liquid flow rate and decreasing initial dye concentration. Moreover, the work indicated that the adsorption ability of the carbon aerogel could be controlled by adjusting the molar ratio of resorcinol to surfactant and carbonization conditions. In addition, the adsorption capacity could be improved when the carbon aerogel was activated by CO₂. The reasons for the difference in adsorption ability could be related to the different pore structure characteristics of various samples.     

Hydrothermal-Freeze-Casting of Poly(amidoamine)-Modified Graphene Aerogels towards CO2 Adsorption

This article presents novel poly(amidoamine) (PAMAM) dendrimer-modified with partially-reduced graphene oxide (rGO) aerogels, obtained using the combined solvothermal synthesis-freeze-casting approach. The properties of modified aerogels are investigated with varying synthesis conditions, such as dendrimer generation (G), GO:PAMAM wt. ratio, solvothermal temperature, and freeze-casting rate. Scanning electron microscopy, Fourier Transform Infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy are employed to characterize the aerogels. The results indicate a strong correlation of the synthesis conditions with N content, N/C ratio, and nitrogen contributions in the modified aerogels. Our results show that the best CO₂ adsorption performance was exhibited by the aerogels modified with higher generation (G7) dendrimer at low GO:PAMAM ratio as 2:0.1 mg mL⁻¹ and obtained at higher solvothermal temperature and freeze-casting in liquid nitrogen. The enclosed results are indicative of a viable approach to modify graphene aerogels towards improving the CO₂ capture.     

Porosity and surface area of monolithic carbon aerogels prepared using alkaline carbonates and organic acids as polymerization catalysts

Carbon aerogels were prepared by polymerization of a resorcinol-formaldehyde mixture using different polymerization catalysts such as: sodium or potassium carbonates, oxalic acid or para-toluenesulfonic acid. The carbon aerogel obtained with this last acid was further CO₂-activated to 8.5% and 22% burn-off. All samples were characterized by N₂ and CO₂ adsorption at −196 and 0 °C, respectively, and by mercury porosimetry, scanning electron microscopy, and thermogravimetric analysis. Samples prepared using Na₂CO₃ were denser than those prepared using K₂CO₃. In addition, the density of samples prepared under acidic conditions was greater than that of samples prepared using alkaline carbonates as catalysts. Most of the carbon aerogels prepared were mesoporous with narrow pore size distributions. Results obtained showed that the nature of the acid used in the preparation of these aerogels only affected the gelation process. Finally, it is noteworthy that CO₂ activation of the carbon aerogel prepared with para-toluenesulfonic acid as catalyst only increased and widened the microporosity and had virtually no effect on the mesoporosity.     

Catalysis of NiO–Al2O3 aerogels for the CO2-reforming of CHF4

Uniform and monolithic NiO–Al₂O₃ aerogels were prepared from cyclic nickel glycoxide, (CH₂O)₂Ni, and boehmite sol, AlOOH, and the catalyst performance of the aerogels for the CO₂-reforming of methane was investigated. The NiO–Al₂O₃ aerogels showed higher activity than impregnation NiO/Al₂O₃ catalysts, while the aerogels exhibited much less activity for coking than the impregnation catalysts. Less deactivation was also observed on the aerogel catalysts than on the impregnation catalysts in the continuous-flow reaction. The Ni was uniformly incorporated throughout alumina where both the metal and the support exist in the aerogel form, i.e., Ni–O–Al bond was considered to be formed in the aerogels. As a result, fine Ni particles appeared after H₂ reduction throughout the alumina support with high dispersion, which brought about not only higher activity but also much less activity for coking on the aerogels. Retardation of catalyst deactivation was ascribed to the suppression of both coking and sintering of Ni particles on the aerogels. This revised version was published online in July 2006 with corrections to the Cover Date.     

High-performance adsorptive desulfurization by ternary hybrid boron carbon nitride aerogel

Separation of aromatic organosulfur compounds is vital for upgrading the feed to clean fuel products, however, it remains a formidable challenge for the present adsorbents. Herein, a series of novel aerogel-like boron carbon nitride (BCN) with three-dimensional interconnected porous networks were fabricated via a hydrogel template and freeze-casting strategy. The as-prepared aerogel-like BCN had a variable carbon content and a flexible size of graphene domain on its porous structure. The optimal BCN aerogel represents the top-level of adsorptive desulfurization (ADS) capacity for aromatic organosulfur compounds (30.8 mg S/g adsorbent), strikingly higher than the state-of-the-art aerogels adsorbents (17.1 mg S/g adsorbent) reported under similar conditions. A selective adsorption for dibenzothiophene in the presence of aromatic hydrocarbons, nitrogen compounds were also achieved on BCN aerogel. The adsorption isotherm and XPS measurement reveal heterogeneous adsorption on the BCN aerogel via π–π and Lewis acid–base interactions.     

An environment‐friendly thermal insulation material from cellulose and plasma modification

Cellulose aerogels were prepared by combining the NaOH/thiourea/H₂O solvent system and the freeze‐drying technology. Hydrophobic aerogels were obtained with the cold plasma modification technology. The results showed that cellulose aerogel had good heat insulation performance, while the main factors affecting thermal conductivity were density and porosity. Thermal conductivity decreased with the decrease of density and the increase of porosity. It could be as low as 0.029 W/(m K). Cellulose aerogel adsorbed moisture easily. The moisture adsorption had a significant influence on the heat insulation performance of aerogel. After conducting hydrophobic modification using CCl₄ as plasma, cellulose aerogel was changed from hydrophilic to hydrophobic and water contact angle was as high as 102°. Hydrophobic modification did not affect the heat insulation performance of aerogel. This work provided a foundation for the possibility of applying cellulose aerogels in the insulating material field. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3652–3658, 2013     

The aminosilane functionalization of cellulose nanocrystal aerogel via vapor-phase reaction and its CO2 adsorption characteristics

The cellulose nanocrystal (CNC) aerogel was functionalized using aminosilane via vapor-phase reaction and then the modified CNC aerogel was characterized by several techniques such as Fourier transform infrared spectroscopy, FE-SEM, and elemental analysis. Finally, the CO₂ adsorption on the aminosilane-grafted CNC aerogel was featured using a dual-site Langmuir adsorption model. The result showed triethylamine significantly enhanced the amine loading, being 7.06 mmol g⁻¹ at 120°C. The primary amine groups in the aminosilane survived the vapor-phase reaction. The amine groups were homogeneously distributed inside the aerogel due to its porous structure. The dual-site Langmuir model could well describe its CO₂ sorption characteristics. The adsorption capacity was up to 2.57 mmolCO₂ g⁻¹ at 25°C and 101.33 kPa, of which the chemisorption entirely dominated, and it decreased only 3%–4% after six runs. Therefore, these features positively suggested the vapor-phase reaction provided a new and feasible method to functionalize CNC aerogel for the capture of CO₂.     

Preparation of amine-modified SiO<Subscript>2</Subscript> aerogel from rice husk ash for CO<Subscript>2</Subscript> adsorption

Amine-modified SiO₂ aerogel was prepared using 3-(aminopropyl)triethoxysilane (APTES) as the modification agent and rice husk ash as silicon source, its CO₂ adsorption performance was investigated. The amine-modified SiO₂ aerogel remains porous, the specific surface area is 654.24 m²/g, the pore volume is 2.72 cm³/g and the pore diameter is 12.38 nm. The amine-modified aerogel, whose N content is up to 3.02 mmol/g, can stay stable below the temperature of 300 °C. In the static adsorption experiment, amine-modified SiO₂ aerogel (AMSA) showed the highest CO₂ adsorption capacity of 52.40 cm³/g. A simulation was promoted to distinguish the adsorption between the physical process and chemical process. It is observed that the chemical adsorption mainly occurs at the beginning, while the physical adsorption affects the entire adsorption process. Meanwhile, AMSA also exhibits excellent CO₂ adsorption–desorption performance. The CO₂ adsorption capacity dropped less than 10 % after ten times of adsorption–desorption cycles. As a result, AMSA with rice husk ash as raw material is a promising CO₂ sorbent with high adsorption capacity and stable recycle performance and will have a broad application prospect for exhaust emission in higher temperature.     

Chitosan Aerogels Exhibiting High Surface Area for Biomedical Application: Preparation,Characterization, and Antibacterial Study

The objective of the present work is to improve the surface area of aerogel via supercritical carbon dioxide (sc · CO₂) treatment and thus to obtain the chitosan derivative. The resulting mesoporous material exhibits the typical characteristics of aerogels such as high porosity and high surface area. The aerogels were characterized using FTIR, SEM, TEM, and thermal analysis. The specific surface areas and porosities of aerogels were determined using N₂ adsorption. The antibacterial assays were done using E. coli. The prepared chitosan aerogels show important properties such as biocompatibility, non-toxicity, and antibacterial activity, making them suitable for biomedical applications.     

High-performance heterocyclic para-aramid aerogels for selective dye adsorption and thermal insulation applications

The high-performance polymer para-aramid (PPTA) is discovered to gel too soon during the polymerization process, resulting in poor processing performance. In this work, a homogeneous polymer solution containing heterocyclic para-aramid (HPPTA) was successfully synthesized by introducing 2,4-aminophenyl-5-aminobenzimidazole groups into the molecular chains of PPTA, and then HPPTA aerogel was prepared using a supercritical drying technique that took advantage of the HPPTA solution's excellent property of slow gelation. When the HPPTA polymer mass fraction was 1 wt%, the aerogel had the lowest density of 0.086 g cm⁻³ with a BET specific surface area of 376.59 m² g⁻¹. The HPPTA-2 aerogel had better adsorption performance for anionic dye methyl orange, with a maximum adsorption capacity of 319.47 mol g⁻¹; however, its adsorption capacity for cationic dye methylene blue and neutral dye dimethyl yellow was very low, at only 19.68 and 0 mol g⁻¹, respectively. The selective adsorption ability of HPPTA aerogel made it a simple and scalable platform for removing anionic dyes from water solutions. Furthermore, the HPPTA aerogel has outstanding thermal properties for thermal insulation applications in severe environments due to the synergistic effect of the 3D porous structure inside the aerogel and the exceptional thermal stability of the HPPTA.     

Carbon aerogels from gallic acid-resorcinol mixtures as adsorbents of benzene, toluene and xylenes from dry and wet air under dynamic conditions

Carbon aerogels were prepared by carbonization of organic aerogels synthesized by the polycondensation reaction of gallic acid-resorcinol mixtures with formaldehyde. One carbon aerogel was further CO₂-activated. They were characterized by gas adsorption, mercury porosimetry, immersion calorimetry, and temperature programmed desorption. Pore texture of carbon aerogels was constituted by a well-developed, monomodal mesoporous network superimposed over a microporous network with different mean micropore widths according to the degree of activation. In some cases, the BET surface area was lower than that determined by immersion enthalpy into benzene. Dynamic adsorption of benzene, toluene, and xylenes, in dry or wet air flow, was carried out on carbon columns to obtain the breakthrough curves. Adsorption from dry air was governed by the total microporosity of the adsorbent. Amounts adsorbed at the breakthrough point were close to or higher than W₀(CO₂) and lower than W₀(N₂) and increased in the order benzene < toluene < xylenes, in agreement with the variation in relative pressure of the hydrocarbons. Results obtained with wet air were qualitatively similar to those obtained with dry air. However, slightly lower amounts were adsorbed at the breakthrough from the wet versus dry air because of competition between water and hydrocarbon molecules.     

Facile fabrication of multifunctional monolithic polyamide aerogels

The polyamide (PA) aerogels with good-formability via a sol–gel technology were facilely fabricated by using melamine and aroyl chloride followed by CO₂ supercritical drying. The synthesis procedure was straightforward and simple, relying on no nitrogen-based protective atmosphere. The influences of aroyl chloride monomer on the gelation time and aerogel structure were discussed. The structural properties of PA aerogels were characterized by the scanning electron microscopy (SEM) and Brunauer–Emmett–Teller methods (BET). The results indicated that the PA aerogels had a typical three-dimensional porous structure. The PA aerogels exhibited well multifunctional properties, such as flame resistance, thermal insulation, dielectric characteristics and mechanical properties. Due to well multifunctional properties, the PA aerogels had potential for the use in construction and building materials.     

Porous structure and liquid‐phase adsorption properties of activated carbon aerogels

In this article, activated carbon aerogels (ACAs) were prepared by CO₂ activation. Their pore structures were investigated by N₂ adsorption–desorption analysis. ACAs have excellent microporosity (e.g. 0.36 cm³/g) and mesoporosity (e.g. 1.72 cm³/g). Adsorption characteristics of phenol, methylene blue, I₂, and VB₁₂ on ACAs in the liquid phase were studied by static adsorption experiments. Results showed that CO₂ activation process is an effective way to introduce micropores in carbon aerogels, which is enhanced with the increase of activation time. As a result, the adsorption capacities of the four mentioned adsorbates on ACAs were improved gradually with the increase of activation time. However, mesopore volume is also a factor on improving adsorption properties for the relatively giant molecules methylene blue and VB₁₂. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Effect of reduction temperature on the preparation of zero-valent iron aerogels for trichloroethylene dechlorination

Zero-valent iron (ZVI) aerogels have been synthesized by sol-gel method and supercritical CO₂ drying, followed by H₂ reduction in the temperature range of 350–500 °C. When applied to trichloroethylene (TCE) dechlorination, the ZVI aerogel reduced at 370 °C showed the highest performance in the conditions employed in this study. Thus, the effect of reduction temperature in preparing ZVI aerogels has been investigated by several characterizations such as BET, XRD, TPR, and TEM analyses. As the reduction temperature decreased from 500 to 350 °C, the BET surface area of the resulting aerogels increased from 6 to 30 m²/g, whereas their Fe⁰ content decreased up to 64%. It was also found that H₂ reduction at low temperatures such as 350 and 370 °C leads to the formation of ZVI aerogel particles consisting of both Fe⁰ and FeO _x in the particle cores with a different amount ratio, where FeO _x is a mixture of maghemite and magnetite. It is, therefore, suggested that reduction at 370 °C for ZVI aerogel preparation yielded particles homogeneously composed of Fe⁰ and FeO _x in the amount ratio of 87/13, resulting in high TCE dechlorination rate. On the other hand, when Pd- and Ni-ZVI aerogels were prepared via cogellation and then applied for TCE dechlorination, we also observed a similar effect of reduction temperature. However, the reduction at 350 or 370 °C produced Pd- or Ni-ZVI aerogel particles in which Fe⁰ and Fe₃O₄ co-exist homogeneously. Since both Fe⁰ and Fe₃O₄ are advantageous in TCE dechlorination, the activities of Pd- and Ni-ZVI aerogels reduced at 350 °C were comparable to those of both aerogels reduced at 370 °C, although the former aerogels have less Fe⁰ content.     

Facile preparation for gelatin/hydroxyethyl cellulose-SiO2 composite aerogel with good mechanical strength,heat insulation,and water resistance

The advanced thermal insulation materials with low cost and high mechanical properties play an important role in transport packaging and thermal protection fields. An inorganic/organic composite aerogel was prepared through hydrogen bonds and chemical crosslinking among silica aerogel particles, gelatin (GA), and hydroxyethyl cellulose (HEC). The as-prepared GA/HEC-SiO₂ composite aerogels were characterized by compression tests, scanning electron microscopy, Fourier transform infrared, thermogravimetric analyzer, and contact angle tests to investigate the chemical composition and physical structure. The GA/HEC-SiO₂ composite aerogels exhibited a strong mechanical strength (0.53–4.01 MPa), a high compression modulus (1.33–11.52 MPa), a lower volume density (0.035–0.081 g/cm³), thermal conductivity as low as 0.035 W/[m K]), a porosity of more than 93%, and hydrophobic angle as high as 150.01° after hydrophobic modification. These results indicate that biopolymer composite aerogels embedded with SiO₂ aerogel particles display a bright future in thermal insulation.     

Preparation of freestanding and crack-free titania–silica aerogels and their performance for gas phase,photocatalytic oxidation of VOCs

Freestanding and crack-free titania–silica aerogels with high titanium content (i.e., Ti/Si = 1) were successfully prepared by adjusting the hydrolysis of the two alkoxide precursors to a comparable rate during the sol–gel processing. Two titania–silica aerogels were prepared by ethanol and CO₂ supercritical drying methods. Well-dispersed, nanometer-sized anatase crystal domains (ca. 10 nm) were crystallized by high temperature, ethanol supercritical drying. The crystalline domains were solidly anchored to the aerogel network by Ti–O–Si bonds. Titania–silica aerogels prepared by CO₂ supercritical drying method were devoid of TiO₂ crystals. A molecular-level mixing was achieved and anatase TiO₂ was only crystallized with difficulty by high temperature calcination (1073 K). Both aerogels were mesoporous and displayed similar open pore structure that is readily accessible to reactant molecules. However, only the titania–silica aerogel with anatase TiO₂ prepared by ethanol supercritical drying was active for the gas phase, photocatalytic oxidation of volatile organic compounds (i.e., isopropanol and trichloroethylene). Catalysts prepared from Degussa P25 TiO₂ displayed lower activity under similar reaction conditions.     

Monolithic zinc oxide aerogel with the building block of nanoscale crystalline particle

In the present paper, cost-effective zinc chloride has been utilized to synthesize zinc-based aerogel using the epoxide addition sol–gel process. After supercritical drying of wet gels with CO₂, zinc-based aerogels have been prepared. With the subsequently thermal treatment in selected atmospheres and designed heating program, zinc oxide aerogel of high porosity, large specific area (125 m² g^?1), narrow particle size distribution and excellent hexagonal structure was obtained. The microstructures and compositions of zinc oxide aerogels obtained in different atmospheres were characterized. The results showed that decreasing the ratio between oxygen and nitrogen contributed to the maintenance of monolithic aerogels and desirable microstructure. FTIR, XRD, and TG were applied to analyze the chemical and physical changes of the thermal treatment process. Those analyses are able to provide a reference for preparing other metal oxide aerogels. It should be a promising method to design new metal oxide aerogels with attractive nanoarchitectures.     

Graphene-based sorbents for iodine-129 capture and sequestration

The capture and sequestration of iodine-129 (¹²⁹I), a long-lived byproduct of nuclear fission, is essential to the implementation of advanced nuclear fuel cycles and effective nuclear waste management. Current state-of-the-art technologies inherently require silver to bind iodine, e.g., silver-loaded silica aerogels or silver-loaded zeolite (AgZ), which are very expensive and an environmental concern. It is highly desirable to develop alternative cost-effective adsorbents for iodine capture and sequestration. Herein, we report graphene-based nanomaterials including graphene powder and graphene aerogel as novel iodine sorbents showing exceptional adsorption capability and kinetics. By measuring iodine sorption capacities and uptake rates in an I_2(g) saturated environment, graphene sorbents display impressive iodine sorption capacities with powdered samples achieving mass gains in excess of 85 mass%, and aerogels exceeding 100% mass gains. A direct correlation among specific surface area, defect concentration, and maximum sorption capacity has been established, and the sorption kinetics of the graphene for iodine capture was determined.     

Preparation of dye sensitized solar cell by using supercritical carbon dioxide drying

Dye-sensitized solar cells (DSSC) derived from TiO₂ aerogel film electrodes were fabricated. TiO₂ aerogels were obtained by using sol–gel method and supercritical carbon dioxide (sc-CO₂) drying. First, TiO₂ wet gels were obtained by sol-gel method. Then, the solvents in the TiO₂ wet gels were replaced by acetone. The TiO₂ aerogels were obtained by using sc-CO₂ drying from the TiO₂ wet gels. The conditions of sc-CO₂ drying were at 313, 323 K and 7.8–15.5 MPa. The electrodes with TiO₂ aerogel films were obtained by deposition of the aerogels on glass substrates. The electrodes with TiO₂ aerogel films and a commercial particle film of various thickness were obtained by repetitive coatings and calcinations. The amount of dye adsorbed on the TiO₂ films with sc-CO₂ drying was higher than that of commercial particle film. The amount of dye adsorbed on the TiO₂ films increased with increasing surface area of the TiO₂ film. DSSCs were assembled by using the TiO₂ aerogel film electrodes and their current–voltage performance was measured. The power performance of DSSC made by supercritical drying was higher than that of commercial particles. The DSSC with the film electrode made at 313 K and 15.5 MPa showed the best power performance (J_sc = 7.30 mA/cm², V_oc = 772 mV, η = 3.28%).