Carbon Aerogels Synthesizd with Cetyltrimethyl Ammonium Bromide (CTAB) as a Catalyst and its Application for CO2 Capture

A series of carbon aerogels were synthesized by polycondensation of resorcinol and formaldehyde using cetyltrimethyl ammonium bromide (CTAB) as a catalyst. The structure and properties of carbon aerogels were characterized by X‐ray diffraction (XRD), Raman, scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT‐IR), and N₂ adsorption‐desorption technologies. Besides, the CO₂ capture behavior of carbon aerogels was also investigated. It was found that the amount of CTAB affected the structure and morphology of carbon aerogels, thus influenced the CO₂ adsorption behavior. The sample CA‐125 (the ratio of resorcinol and CTAB is 125) had the highest CO₂ adsorption capacity (63.71 cm³ · g^–1 at 1 bar and 24.14 cm³ · g^–1 at 0.15 bar) at 25 °C. In addition, the higher CO₂ adsorption capacity was ascribed to the higher surface area, pore volume and appropriate pore size, as well as the more defects over carbon aerogels.     

Hydrophobic Silica Aerogels Strengthened with Nonwoven Fibers

Hydrophobic silica aerogels were prepared via a sol‐gel process by surface modification at ambient pressure. Nonwoven fibers were distributed inside the silica aerogels as a composite to act as a supporting skeleton which increased the mechanical property of the silica aerogels. The morphology and pore structure of the composites were characterized by scanning electron microscopy (SEM) and N₂ adsorption analyzer. The contact angle and the adsorption capacities of the composites were also determined. The results show that silica aerogels dispersed uniformly and maintained high porosity in the aerogel‐fiber composites. They have excellent hydrophobic properties and are excellent adsorptive materials.     

Synthesis of TiO2–SiO2 aerogel via ambient pressure drying: effects of sol pre-modification on the microstructure and pore characteristics

TiO₂–SiO₂ composite aerogels were prepared via ambient pressure drying by sol–gel and surface modification for both the sol and gel samples. The organosilane reagents of decamethyltetrasiloxane (DMTSO)/trimethylchlorosilane (TMCS) and hexamethyldisiloxane (HMDSO)/TMCS were introduced into the TiO₂–SiO₂ composite sol for pre-modification respectively, and subsequently the TMCS/hexane solution was used for surface modification of the obtained TiO₂–SiO₂ composite gel. The effects of sol pre-modification on the microstructure and pore characteristics of TiO₂–SiO₂ composite aerogels were investigated. The results indicate that HMDSO/TMCS coupling reagents is more appropriate for the pre-modification of TiO₂–SiO₂ composite sol than the DMTSO/TMCS reagents. The best volume ratio of HMDSO/TMCS/composite sol for preparing mesoporous TiO₂–SiO₂ composite aerogels is in the range of 1:0.33:10–1:1.0:10, with which the specific surface area and pore volume of the obtained TiO₂–SiO₂ composite aerogels are 492–645 m²/g and 2.63–2.85 m³/g, respectively. The results of adsorption and photocatalytic degradation of rhodamine B show that the as-prepared TiO₂–SiO₂ composite aerogels have higher adsorption/photocatalysis. Particularly, the as-prepared TiO₂–SiO₂ composite aerogels with HMDSO/TMCS showed prominent adsorption capability with the adsorption rate attaining to 89.4 % within 60 min.     

Influence of supercritical drying fluids on structures and properties of low-density Cu-doped SiO2 composite aerogels

The Cu-doped SiO₂ composite aerogels were successfully prepared by sol–gel process and subsequently supercritical drying with ethanol and CO₂. The Cu-doped SiO₂ composite aerogels had porous texture, low density (<100 mg cm^?3) and high specific surface area (>800 m² g^?1), which were investigated by FESEM and nitrogen adsorption desorption porosimetry. The FTIR spectra of the aerogels showed that the ethanol-dried aerogels had been modified by ethyl while the corresponding CO₂-dried aerogels had more Si–OH groups. The phase structure and thermal stability were investigated by XRD and TGA, respectively. Due to the reducibility of ethanol, the copper was crystalline in ethanol-dried sample. The Cu-doped SiO₂ composite aerogels dried with supercritical ethanol had larger pore diameter and better thermal stability under 400 °C in comparison with CO₂-dried composite aerogels. The structures and properties of Cu-doped SiO₂ composite aerogels are obviously affected by supercritical drying conditions. The effect research could instruct the synthesis of different state of Cu in composite aerogels.     

Preparation of activated carbons and their adsorption properties for greenhouse gases: CH4 and CO2

Three kinds of activated carbons were prepared using coconut-shells as carbon precursors and characterized by XRD, FT-IR and texture property test. The results indicate that the prepared activated carbons were mainly amorphous and only a few impurity groups were adsorbed on their surfaces. The texture property test reveals that the activated carbons displayed different texture properties, especially the micropore size distribution. The adsorption capacities of the activated carbons were investigated by adsorbing CH4, CO2, N2 and O2 at 25 ?C in the pressure range of 0-200 kPa. The results reveal that all the activated carbons had high CO2 adsorption capacity, one of which had the highest CO2 adsorption value of 2.55 mmol/g at 200 kPa. And the highest adsorption capacity for CH4 of the activated carbons can reach 1.93 mmol/g at 200 kPa. In the pressure range of 0-200 kPa, the adsorption capacities for N2 and O2 were increased linearly with the change of pressure and K-AC is an excellent adsorbent towards the adsorption separation of greenhouse gases.     

Optically transparent silica aerogels based on sodium silicate by a two step sol–gel process and ambient pressure drying

Interest in improving the optical transmission of sodium silicate-based aerogels by ambient pressure drying led to the synthesis of aerogels using a two-step sol–gel process. To produce optically transparent silica aerogel granules, NH₄F (1 M) and HCl (4 M) were used as hydrolyzing and condensation catalysts, respectively. The silica aerogels were characterized by their bulk density, porosity (%), contact angle and thermal conductivity. Optical transmission of as synthesized aerogels was studied by comparing the photos of aerogel granules. Scanning electron microscopic study showed the presence of fractal structures in these aerogels. The degree of transparency in two step sol–gel process-based aerogels is higher than the conventional single step aerogels. The N₂ adsorption–desorption analysis depicts that the two step sol–gel based aerogels have large surface areas. Optically transparent silica aerogels with a low density of ∼0.125 g/cc, low thermal conductivity of ∼0.128 W/mK and higher Brunauer, Emmett, and Teller surface area of ∼425 m²/g were obtained by using NH₄F (1 M), HCl (4 M), and a molar ratio of Na₂SiO₃::H₂O::trimethylchlorosilane of 1::146.67::9.46. The aerogels retained their hydrophobicity up to 500 °C.     

Physico-chemical properties of ambiently dried sodium silicate based aerogels catalyzed with various acids

Experimental results on the physico-chemical properties of ambiently dried sodium silicate based aerogels catalyzed with various acids are reported. The aerogels were prepared by hydrolysis and polycondensation of sodium silicate followed by subsequent washings, surface chemical modification and ambient pressure drying using 10 various acid catalysts consisting of strong and weak acids. The strength and concentration of acids have the major effect on the gelation of sol and hence the physico-chemical properties of the silica aerogels. Strong acids such as HCl, HNO₃ and H₂SO₄ resulted in shrunk (70–95%) aerogels whereas weak acids such as citric and tartaric acids resulted in less shrunk (34–50%) aerogels. The physical properties of silica aerogels were studied by measuring bulk density, volume shrinkage (%), porosity (%), pore volume, thermal conductivity, contact angle with water, Transmission Electron Microscopy (TEM), Atomic Absorption Spectroscopy (AAS), Fourier Transform Infrared Spectroscopy (FTIR), Thermo Gravimetric-Differential Thermal (TG-DT) analyses and N₂ adsorption–desorption BET surface analyzer. The best quality silica aerogels in terms of low density (0.086 g/cm³), low volume shrinkage (34%), high porosity (95%), low thermal conductivity (0.09 W/m K) and hydrophobic (148°) were obtained for molar ratio of Na₂SiO₃:H₂O:citric acid:TMCS at 1:146.67:0.72:9.46 with 20 min gelation time. The resulting aerogels exhibited the thermal stability up to around 420 °C.     

Transparent and flame retardant cellulose/aluminum hydroxide nanocomposite aerogels

Cellulose-based nanocomposite aerogels were prepared by incorporation of aluminum hydroxide (AH) nanoparticles into cellulose gels via in-situ sol-gel synthesis and following supercritical CO₂ drying. The structure and properties of cellulose/AH nanocomposite aerogels were investigated by Fourier transform infrared spectroscopy, scanning electron microscopy, ultraviolet-visible spectrometry, N₂ adsorption, thermogravimetric analysis, and micro-scale combustion calorimetry. The results indicated that the AH nanoparticles were homogeneously distributed within matrix, and the presence of AH nanoparticles did not affect the homogeneous nanoporous structure and morphology of regenerated cellulose aerogels prepared from 1-allyl-3-methylimidazolium chloride solution. The resultant nanocomposite aerogels exhibited good transparency and excellent mechanical properties. Moreover, the incorporation of AH was found to significantly decrease the flammability of cellulose aerogels. Therefore, this work provides a facile method to prepare transparent and flame retardant cellulose-based nanocomposite aerogels, which may have great potential in the application of building materials.     

Amino functionalised Silica-Aerogels for CO2-adsorption at low partial pressure

Effective adsorption of CO₂ at low partial pressures is required for many technical processes, such as gas purification or CO₂ removal in closed loop environmental control systems. Since the concentration of CO₂ in such applications is rather low, a high adsorption capacity is a required property for the adsorbent. Silica aerogels possessing an open pore structure, a high porosity and a high surface area, have a great potential for utilisation as CO₂ adsorbents. Nonetheless in order to reach high adsorption capacities, silica aerogels should be functionalised, for instance by amino functionalisation. In this work, two different functionalisation methods were applied for the generation of amino functionalised aerogels: co-condensation during the sol-gel process and post-treatment of the gel. The co-condensation functionalisation allows the introduction of up to 1.44 wt.% nitrogen into the aerogel structure with minor reductions in surface area, leading however only to minor increases in the adsorption capacity at low partial pressures. The post functionalisation of the gel causes a greater loss in surface area, but the CO₂ adsorption capacity increases, due to the introduction of higher amounts of amino groups into the aerogel structure (up to 5.2 wt.% nitrogen). Respectively, 0.523 mmol CO₂/g aerogel could be adsorbed at 250 Pa. This value is comparable with the adsorption capacity at this pressure of a standard commercially available adsorbent, Zeolite 13X.     

Doped-ZrO2 Aerogels: Catalysts of Controlled Structure and Properties

Doped ZrO₂ aerogels (characterised by TEM, DTA and N₂ adsorption) have been prepared and catalytically tested in CO/CO₂ hydrogenation [1] and CH₄ oxidation [2]. The primary aerogels showed cross-linked clusters of (X-ray) amorphous particles smaller than 5 nm which led to well-developed mesoporous solids with an average pore size of about 10 nm and high surface area (up to 250 m²g⁻¹) [1]. Cu/ZrO₂ aerogels (known to be very active and selective towards methanol synthesis in CO hydrogenation without predominant formation of alkanes even at higher temperatures [1]) are now seen to show these effects even more clearly in CO₂ hydrogenation. In methane oxidation, both Rh/ZrO₂ and Y₂O₃/ZrO₂ were very active. Consideration is given to the nature of the active sites, the role of CO₂ and metal/oxide interfaces and how an understanding of this reactivity can lead to better dispersed ZrO₂.     

Study of zirconia microporous structure

The microporous structure of zirconium dioxide obtained by precipitation at different pH and different times of gel aging is studied by the physical adsorption of nitrogen, oxygen, and molecular hydrogen. It is shown that the ratio between the supermicropore surface areas measured by the adsorption of O₂ and N₂ as a function of the supermicropore volume previously revealed for SnO₂ is also retained for zirconia (the surface area of supermicropores measured by O₂ adsorption is larger than that measured by N₂ adsorption). Possible reasons for this phenomenon are discussed. The conclusions on the factors responsible for the formation of the zirconia microporous structure are drawn     

Improvement of the bioactivity of organic–inorganic hybrid aerogels/wollastonite composites with TiO2

Organic–inorganic hybrid aerogels containing P and Ti have been synthesized by supercritical drying of alkogels prepared by hydrolysis and poly-condensation of metalo-organic precursors under high-power ultrasound. These materials become bioactive when doped with Ca. Wollastonite particles (CaSiO₃) were added as an active phase, instead of incorporating Ca into the aerogel atomic network. These particles had previously been precipitated and were then added to the sol. The aerogels were studied by Fourier transform infrared analysis, scanning electron microscopy coupled with energy dispersive spectroscopy and X-ray diffraction and N₂ adsorption. The stress–strain behaviours were evaluated under compression to obtain the Young’s modulus. It was found that the incorporation of TiO₂ into wollastonite-P₂O₅ hybrid aerogels increased their capacity to form apatite and, consequently, improving their bioactive response.     

Effects of Ti Addition on Alumina/Titania Composite Aerogels Synthesized by Sol–Gel Process and Supercritical Ethanol Drying

Alumina/titania composite aerogels with different titania contents were synthesized by the sol–gel process and supercritical ethanol drying. The structures and morphologies of synthesized aerogels were analyzed by X‐ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetry, and N₂ adsorption–desorption tests. Supercritical ethanol drying induced the crystallization of titania, which prompted the transformation of the structure from pseudoboehmite to γ‐Al₂O₃ . Reversely, alumina retarded the anatase‐to‐rutile transformation of titania. The content of titania significantly affected the structure and morphology of alumina/titania composite aerogels. A high content of titania (≥40%) resulted in the phase separation of titania particles, which grew to form the anatase phase octahedral particles with well‐developed facets. When the titania content was low, titania particles could be homogeneously dispersed in alumina particles to form spherical clusters with the poor crystallinity. Titania particles were in the anatase phase, and no rutile phase was formed until the temperature rose to 1000°C. In addition, titania addition resulted in a decrease in the specific surface area (SSA) of alumina aerogels because the SSA of titania was lower than that of alumina aerogels.     

Fractality of materials based on silica gel

Based on the analysis of the isotherms of N₂ adsorption, the fractal dimension and other texture geometric parameters were determined for silica gel, modified silica gel, and relevant composite materials. The correlations between the texture parameters were found. A criterion for the estimation of the efficiency of using the composite adsorbent volume was proposed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 781–785, May, 2000.     

Fine pore mouth structure of molecular sieve carbon with GCMC-assisted supercritical gas adsorption analysis

N₂ adsorption isotherms of molecular sieve carbon were measured at 77 K and 303 K. The Ar adsorption isotherms of molecular sieve carbon samples were also measured at 303 K. The grand canonical Monte Carlo (GCMC) simulation technique was applied to calculate the N₂ and Ar adsorption isotherms at 303 K using the ultramicropore volume determined by H₂O adsorption. The comparative method of experimental and simulated isotherms of supercritical N₂ and Ar at 303 K gave the width of the micropore mouth of the molecular sieve carbon, which can be applied to the ultramicropore width determination for other noncrystalline porous solids.     

Bioactivity and Mechanical Properties of Polydimethylsiloxane (PDMS)-CaO-SiO2 Hybrids with Different PDMS Contents

Silica-titania and titania aerogels were prepared by supercritical drying using different solvents such as low temperature CO₂ (353 K), high temperature CO₂ (553 K), ethanol (553 K) and ethanol with zeolite (553 K) and their efficiencies for the removal of benzene from a synthetic air mixture were investigated. The aerogels obtained showed both large capacities for benzene adsorption and high photocatalytic activity for its decomposition in the adsorbed state. The degree of benzene removal by silica-titania aerogel seemed to depend on the crystallinity of the titania and was in the order low temperature CO₂ < high temperature CO₂ < ethanol = ethanol with zeolite. The amount of CO₂ liberated by the photocatalytic decomposition of benzene also followed the same sequence. Titania aerogels showed the greatest efficiency in the decomposition of benzene, while the amount of CO₂ evolved was lower than those of the silica-titania systems investigated. The adsorption capacity of benzene basically depended on the silica matrix and was not greatly influenced by the difference in the crystallinity of titania.     

Tuning of Microenvironment in Covalent Organic Framework via Fluorination Strategy promotes Selective CO2 Capture

Herein, we have designed and synthesized two heteroatom (N, O) rich covalent organic frameworks (COF), PD-COF and TF-COF , respectively, to demonstrate their relative effect on CO₂ adsorption capacity and also CO₂/N₂ selectivity. Compared to the non-fluorinated PD-COF (BET surface area 805 m² g⁻¹, total pore volume 0.3647 ccg⁻¹), a decrease in BET surface area and also pore volume have been observed for fluorinated TF-COF due to the incorporation of fluorine to the porous framework (BET surface area 451 m² g⁻¹, total pore volume 0.2978 ccg⁻¹). This fact leads to an enormous decrease in the CO₂ adsorption capacity and CO₂/N₂ selectivity of TF-COF , though it shows stronger affinity towards CO₂ with a Qst of 37.76 KJ/mol. The more CO₂ adsorption capacity by PD-COF can be attributed to the large specific surface area with considerable amount of micropore volume compared to the TF-COF . Further, PD-COF exhibited CO₂/N₂ selectivity of 16.8, higher than that of TF-COF (CO₂/N₂ selectivity 13.4).     

Characterization of Alumina Gel Catalysts by Emanation Thermal Analysis (ETA)

Alumina gels made from the metal alkoxide is known to have high catalyst activity for the selective reduction of NO _x by hydrocarbons. It is also reported that the fine structure of the gels effects the activity. In this study, the effect of the preparation method on the fine structure and catalyst activity of the gels was investigated. Monolithic gels were obtained by hydrolysis of Al(sec-C₄H₉O)₃. The wet gels were dried at 90°C (xerogels), supercritically dried (aerogels), or dried after immersion in an ethanol solution of methyltrimethoxysilane (modified xerogels). The changes in the microstructure during heating were discussed using the results of TG-DTA, ETA and N₂ adsorption. The ETA curves show the ²²⁰Rn-release rate, E, of the samples, previously labelled with ²²⁸Th and ²²⁴Ra, during heating. The decrease in E of the xerogel at temperatures higher than 400°C indicates a gradual decrease in the surface area and porosity. A remarkable decrease in the BET surface area of the xerogel was found after heat-treating at 500°C. On the other hand, constant E of the aerogels and modified xerogels above 450°C suggests high thermal stability. The pore radii, estimated by BJH method, and the catalyst activities at 500°C of the aerogels and the modified xerogels were higher than those of the xerogels. The temperature range in which the alumina gels are applicable as catalysts was determined.     

Predicting Low-Pressure O2 Adsorption in Nanoporous Framework Materials for Sensing Applications

A set of 98 nanoporous framework material (NFM) structures was investigated by classical Grand canonical Monte Carlo simulations for low-pressure O₂ adsorption properties (Henry’s constant and isosteric heat of adsorption). The set of materials includes those that have shown high O₂ uptake experimentally as well as a subset of more than 2000 structures previously screened for noble-gas uptake. While use of the general force field UFF is fruitful for noble-gas adsorption studies, its use is shown to be limited for the case of O₂ adsorption—one distinct limitation is a lack of sufficient O₂–metal interactions to be able to describe O₂ interaction with open metal sites. Nonetheless, those structures without open metal sites that have very small pores (<2.5 Å) show increased O₂/N₂ selectivity. Additionally, O₂/N₂ mixture simulations show that in some cases, H₂O or N₂ can hinder O₂ uptake for NFMs with small pores due to competitive adsorption.     

Effect of surface chemistry on the adsorption of basic dyes on carbon aerogels

This paper focuses on how the surface chemical groups of carbon aerogels (CAs) affect their adsorption properties of basic dyes. First of all, the surface structures of CAs were tailored by controlling carbonization temperatures and oxidation treatments including HNO₃ oxidation, H₂O₂ oxidation and hot air oxidation. The pore structures of samples were investigated by N₂ adsorption–desorption analysis. In addition, their surface chemistries were characterized by Boehm titration, elemental analysis and XPS analysis. Subsequently, the adsorption capacities of the typical basic dyes on them were determined by UV Spectroscopy. The results show that the surface chemistry of CAs plays a key role in dye adsorption performance. HNO₃ oxidation can produce much acidic groups on the surface of sample, such as carboxyl, lactone and phenolic groups, resulting in an increase in the adsorption amounts of basic dyes.