Silica–titania aerogel monoliths with large pore volume and surface area by ambient pressure drying

Ambient pressure drying has been carried out for the synthesis of silica–titania aerogel monoliths. The prepared aerogels show densities in the range 0.34–0.38 g/cm³. The surface area and pore volume of these mixed oxide aerogels are comparable to those of the supercritically dried ones. The surface area for 5wt% titania aerogel has been found to be as high as 685 m²/g with a pore volume of 2.34 cm³/g and the 10wt% titania aerogel has a surface area of 620 m²/g with a pore volume of 2.36 cm³/g. Some gels were also made hydrophobic by a surface treatment with methyltrimethoxysilane and trimethylchlorosilane. The surface modified aerogels possess high surface areas in the range of 540–640 m²/g, and are thermally stable in terms of retaining hydrophobicity up to a temperature of 520 °C. The pore size distribution of the aerogels clearly indicates the preservation of the aerogel structure. High Resolution Transmission Electron microscopy has been employed to characterise the aerogels and Fourier Transform infrared spectroscopy to study the effect of titania addition to silica and the surface modification. X-ray diffraction patterns were recorded to verify the molecular homogeneity of the aerogel.     

Critical micelle concentrations of cetyltrimethylammonium chloride and their influence on the periodic structure of mesoporous silica

The influence of micelle morphology transformation on the structure of mesoporous materials is fundamental in designing optimal, well-ordered, mesoporous materials. Firstly, the steady-state fluorescence technique was adopted to determine the first and second critical micelle concentrations of cetyltrimethylammonium chloride (CTAC) as 125 and 210 mmol L⁻¹ in an equimolar mixture of water and ethanol at 25°C. Using tetraethylorthosilicate (TEOS) as the precursor, mesoporous silicas (with a surface area of 545.7–1210.5 m 545.7 ∼ 1210.5 m² g⁻¹ and a pore volume of 0.26 ∼ 0.80 cm³ g⁻¹) were synthesized with the CTAC templates in the equimolar mixture of water and ethanol. Characterization by small-angle X-ray diffraction and N 2 adsorption/desorption techniques revealed that the surface area and pore volume of silica increased with the content of CTAC, whereas there existed a transition point at the CTAC/TEOS ratio of 0.15 corresponding to the fade-away of well-ordered mesoporous structure. The negative effect of the much higher content CTAC on the periodic structure of mesoporous silica is attributed to micelle transformation from spheres to cylinders related to the second critical micelle concentration of CTAC. The text was submitted by the authors in English.     

Sol–gel synthesis of Na<Subscript>2</Subscript>CaSiO<Subscript>4</Subscript> and its in vitro biological behaviors

In this work we prepared the hybrid material (SG) by the sol–gel method through the reaction between tetraethylortosilicate (TEOS) and acetylacetonatepropyltrimethoxysilane (ACACSIL). We also immobilized the acetylacetonate on silica surface (GR) by the grafting method through the reaction between a commercial silica and ACACSIL. Infrared thermal analysis showed that these materials were thermally stable until 200 °C. SG is a microporous material and has surface area of 500 m² g⁻¹, average porous volume of 0.09 cm³ g⁻¹ and organic content of 1 mmol g⁻¹. GR is a mesoporous material and has surface area of 300 m² g⁻¹, average porous volume of 0.7 cm³ g⁻¹ and organic content of 0.4 mmol g⁻¹. Iron(III) was coordinated to SG and GR resulting in the SG–Fe and GR–Fe silicas which were tested as catalysts on the aerobic epoxidation of cis-cyclooctene. SG–Fe yielded 100% of conversion and 94% of selectivity in epoxide whereas GR–Fe silica led to a maximum conversion of 50% and 100% of selectivity.     

Thermal evolution of mesoporous titania prepared by CO2 supercritical extraction

Porous anatase is attractive because of its notable photo-electronic properties. Titania wet gel prepared by hydrolysis of Ti-alkoxide was immersed in the flow of supercritical CO₂ at 60°C and the solvent was extracted (aerogel). Mesoporous TiO₂ consisting of anatase nano-particles, about 5 nm in diameter, have been obtained. Thermal evolution of the microstructure of the aerogel was evaluated by TGA-DTA, N₂ adsorption, TEM and XRD, and discussed in comparison with that of the corresponding xerogel. The diffraction peaks of anatase were found for the as-extracted gel while the xerogel dried at 90°C was amorphous. After calcination at 600°C, the average pore size of the aerogel, about 20 nm in diameter, was 4 times larger than that of the xerogel, and the pore volume, about 0.35 cm³ g⁻¹, and the specific surface area, about 60 m² g⁻¹, were 2 times larger than those of the xerogel. XRD peaks of rutile have been found after calcination at 600°C. The particle sizes of anatase and rutile are about 13 and 25 nm in diameter, respectively. The surface morphology of TiO₂ nano-particles has been discussed in terms of their surface fractal dimensions estimated from the N₂ gas adsorption isotherms.     

Production of low-density sodium silicate-based hydrophobic silica aerogel beads by a novel fast gelation process and ambient pressure drying process

We report a method to synthesize low-density transparent mesoporous silica aerogel beads by ambient pressure drying (APD). The beads were prepared by acid–base sol–gel polymerization of sodium silicate in aqueous ammonia solution via the ball dropping method (BDM). To minimize shrinkage during drying, wet silica beads were initially prepared; their surfaces were then modified using trimethylchlorosilane (TMCS) via simultaneous solvent exchange and surface modification. The effects of the volume percentage (%V) of TMCS on the physical and textural properties of the beads were investigated. The specific surface area and cumulative pore volume of the silica aerogel beads increased with an increase in the %V of TMCS. Silica aerogel beads with low packing bed density (0.081 g/cm³), high surface area (917 m²/g), and large cumulative pore volume (2.8 cm³/g) was obtained when 10%V TMCS was used. Properties of the final product were examined by FE-SEM, TEM, BET, and TG–DT analyses. Surface chemical modifications were confirmed by FTIR spectroscopy. The hydrophobic silica aerogel beads were thermally stable up to 411 °C. We discuss our results and compare our findings for modified versus unmodified silica beads.     

Synthesis of freestanding silica and titania-silica aerogels with ordered and disordered mesopores

Freestanding blocks of silica and titania-silica aerogels were prepared by the sol-gel method. It is possible to prepare crack-free, titania-silica aerogels with high titanium content by a careful control of the synthesis conditions. Prehydrolysis, complexation and polymer addition were used to adjust the hydrolysis and condensation rates of the silicon and titanium alkoxide precursors. Photoactive anatase TiO₂ nanocrystals with a large surface area (i.e., up to 300m²g⁻¹) were crystallized from the gel network by the high-temperature ethanol supercritical drying, and the resulting aerogel blocks were gas permeable and display a transition-regime diffusion behavior. Pore and volume shrinkages were observed in samples prepared by ethanol supercritical drying when the titanium content was increased resulting in a lower flux. Adding Pluronic P123 creates ordered mesopore domains and produces large pore aerogels even at high titanium contents. The photocatalytic oxidation reaction of trichloroethylene was performed by flowing the reactant gas mixture through the UV-irradiated aerogel blocks with excellent results.     

Influence of thermal process on microstructural and physical properties of ambient pressure dried hydrophobic silica aerogel monoliths

The experimental results of thermal process on the microstructural and physical properties of ambient pressure dried hydrophobic silica aerogel monoliths are reported and discussed. With sodium silicate as precursor, ethanol/hexamethyldisiloxane/hydrochloric acid as surface modification agent, the crack-free and high hydrophobic silica aerogel monoliths was obtained possessing the properties as low density (0.096 g/cm³), high surface area (651 m²/g), high hydrophobicity (~147°) and low thermal conductivity (0.0217 Wm/K). Silica aerogels maintained hydrophobic behavior up to 430 °C. After a thermal process changing from room temperature to 300 °C, the hydrophobicity remained unchanged (~128°), of which the porosity was 95.69% and specific density about 0.094 g/cm³. After high temperature treatment (300–500 °C), the density of final product decreased from 0.094 to 0.089 g/cm³ and porosity increased to 96.33%. With surface area of 466 m²/g, porosity of 91.21% and density about 0.113 g/cm³, silica aerogels were at a good state at 800 °C. Thermal conductivities at desired temperatures were analyzed by the transient plane heat source method. Thermal conductivity coefficients of silica aerogel monoliths changed from 0.0217 to 0.0981 Wm/K as temperature increased to 800 °C, revealed an excellent heat insulation effect during thermal process.     

壳聚糖基多孔碳材料的制备及其超级电容性能

以胶态SiO₂纳米粒子为模板,壳聚糖为碳源,ZnCl₂为活化剂,制备了具有不同比表面积和孔体积的氮掺杂介孔碳。采用多种表征手段对碳材料的微观形貌、比表面积和孔道结构进行了表征,探究了壳聚糖与SiO₂纳米粒子的比例以及ZnCl₂活化剂对碳材料孔体积和比表面积的影响。结果表明,在未使用活化剂时碳材料(CSi-1.75)的孔体积高达4.53 cm³·g^-1,但其比表面积最小(729 m²·g^-1);使用ZnCl₂作为活化剂制备的碳材料(CSi-1.75-Zn)比表面积为1 032 m²·g^-1,但其孔体积下降到1.99 cm³·g^-1,且具有最多的吡啶氮和吡咯氮。在以6.0 mol·L^-1KOH为电解液的三电极体系中,当电流密度为0.5 A·g^-1时,CSi-1.75...     

Gels dried under supercritical and ambient conditions: a comparative study and their subsequent conversion to silica–carbon composite aerogels

In present work, we have prepared gels with various compositions of methyltrimethoxysilane—3-(2,3-epoxypropoxy) propyltrimethoxysilane (MTMS-GPTMS) using a two-step acid base sol–gel process. To make a comparative study between the two common drying routes, we prepared gels under supercritical and also under ambient conditions. The density of the supercritically dried hybrid aerogels lies between 0.18 and 0.31 gcm^?3, while the density of the ambient dried ones ranges between 0.35 and 0.42 gcm^?3. The surface area of MTMS-0.25 GPTMS aerogel dried under supercritical conditions, has been found to be 464 m² g^?1 with a pore volume and average pore diameter of 1.24 cm³ g^?1 and 11 nm respectively. The same composition dried under ambient conditions is found to have similar properties i.e. a BET surface area of 439 m² g^?1, pore volume of 1.22 cm³ g^?1 and average pore diameter of 11 nm. The aerogels were later pyrolyzed yielding silica/carbon composite aerogels. The pyrolized aerogels possessed a surface area as high as 207 m² g^?1 with a total pore volume of 0.98 cm³ g^?1. The pyrolysed aerogels were also calcined to yield carbon free materials.     

Preparation of amino-functionalized mesoporous silica thin films with highly ordered large pore structures

Highly ordered amino-functionalized mesoporous silica thin films have been directly synthesized by co-condensation of tetraethoxysilane (TEOS) and 3-aminopropyltriethoxysilane (APTES) in the presence of triblock copolymer Pluronic P123 surfactant species under acidic conditions by sol-gel dip-coating. The effect of the sol aging on thin films organization is systematically studied, and the optimal sol aging time is obtained. The amino-functionalized mesoporous silica thin films exhibit a long-range ordering of 2D hexagonal (p6mm) mesostructure with a large pore size of 8.3 nm, a large Brunauer–Emmett–Teller (BET) specific surface area of 680 m² g⁻¹ and a large pore volume of 1.06 cm³ g⁻¹ following surfactant extraction as demonstrated by X-ray diffraction (XRD), Transmission electron microscope (TEM), and physical adsorption techniques. Based on BET surface area and weight loss, the surface coverage of amino-groups for the amino-functionalized mesoporous silica thin films is calculated to be 3.2 amino-groups per nm². Moreover, the functionalized thin films display improved properties for immobilization of cytochrome c in comparison with pure-silica mesoporous thin films.     

壳聚糖基多孔碳材料的制备及其超级电容性能

以胶态SiO₂纳米粒子为模板,壳聚糖为碳源,ZnCl2为活化剂,制备了具有不同比表面积和孔体积的氮掺杂介孔碳。采用多种表征手段对碳材料的微观形貌、比表面积和孔道结构进行了表征,探究了壳聚糖与SiO₂纳米粒子的比例以及ZnCl2活化剂对碳材料孔体积和比表面积的影响。结果表明,在未使用活化剂时碳材料(CSi-1.75)的孔体积高达4.53 cm³·g^-1,但其比表面积最小(729 m²·g^-1);使用ZnCl₂作为活化剂制备的碳材料(CSi-1.75-Zn)比表面积为1032 m²·g^-1,但其孔体积下降到1.99 cm³·g^-1,且具有最多的吡啶氮和吡咯氮。在以6.0 mol·L^-1 KOH为电解液的三电极体系中,当电流密度为0.5 A·g^-1时,CSi-1.75-Zn的比电容为344 F·g^-1,而CSi-1.75的比电容仅为255 F·g^-1。这表明碳材料的比表面积对超级电容性能影响最大,而孔体积影响较小。电容贡献分析结果表明,相对于CSi-1.75,CSi-1.75-Zn的双电层电容和赝电容都得到了提高,这表明更大的比表面积和更多的吡啶氮和吡咯氮有利于提高碳材料的超级电容性能。     

Physical Properties of Silica Aerogels Prepared with Polyethoxydisiloxanes

Silica aerogels were made by sol-gel techniques using industrial silicon derivatives (polyethoxydisiloxanes, E-40), followed by supercritical drying with ethanol. The morphology and microstructure of the silica aerogels were investigated by using specific surface area, S_BET, SEM, TEM and the pore size distribution techniques. The thermal conductivity was also measured as a function of air pressure. The results show that the diameter of the silica particles is about 13 nm and the pore size of the silica aerogels is 20–80 nm. The specific surface area of the silica aerogel is about 470 m²/g and the thermal conductivity of the silica aerogel prepared with E-40 is 0.014 w m^–1 K^–1 at room temperature and 1 atm.     

Effective preparation of crack-free silica aerogels via ambient drying

Effective ambient-drying techniques for synthesizing crack-free silica aerogel bulks from the industrial waterglass have been developed. Silica wet gels were obtained from aqueous colloidal silica sols prepared by ion-exchange of waterglass solution (4–10 wt% SiO₂). Crack-free monolithic silica aerogel disks (diameter of 22 mm and thickness of 7 mm) were produced via solvent exchange/surface modification of the wet gels using isopropanol/trimethylchlorosilane/n-Hexane solution, followed by ambient drying. The effects of the silica content in sol and the molar ratio of trimethylchlorosilane/pore water on the morphology and property of final aerogel products were also investigated. The porosity, density, and specific surface area of silica aerogels were in the range of 92–94%, 0.13–0.16 g/cm³, and ∼675 m²/g, respectively. The degree of springback during the ambient drying processing of modified silica gels was 94%.     

疏水介孔二氧化硅膜的制备与表征

用甲基三乙氧基硅烷(MTES)代替部分正硅酸乙酯(TEOS)作为前驱体，以聚乙烯醚-聚丙烯醚-聚乙烯醚三嵌段共聚物(P123)作有机模板剂，通过共水解缩聚反应制备了甲基修饰的介孔SiO₂膜。利用N₂吸附、FTIR、²⁹Si MAS NMR以及接触角测量仪对膜的孔结构和疏水性进行了表征。结果表明，修饰后的膜材料具有良好的介孔结构，最可几孔径为4.65 nm，孔体积为0.69 cm³·g^-1，比表面积为938.4 m²·g^-1；同时疏水性明显提高，当n_MTES/n_TEOS达到1.0时，其对水的接触角达到109°± 1.1°。气体渗透实验表明气体通过膜孔的扩散由努森机制所控制。     

Zirconium oxide aerogel for effective enrichment of phosphopeptides with high binding capacity

In this study, zirconium oxide (ZrO₂) aerogel was synthesized via a green sol–gel approach, with zirconium oxychloride, instead of the commonly used alkoxide with high toxicity, as the precursor. With such material, phosphopeptides from the digests of 4 pmol of β-casein with the coexistence of 100 times (mol ratio) BSA could be selectively captured, and identified by MALDI-TOF MS. Due to the large surface area (416.0 m² g⁻¹) and the mesoporous structure (the average pore size of 10.2 nm) of ZrO₂ aerogel, a 20-fold higher loading capacity for phosphopeptide, YKVPQLEIVPN[pS]AEER (MW 1952.12), was obtained compared to that of commercial ZrO₂ microspheres (341.5 vs. 17.87 mg g⁻¹). The metal oxide aerogel was further applied in the enrichment of phosphopeptides from 100 ng nonfat milk, and 17 phosphopeptides were positively identified, with a 1.5-fold improvement in phosphopeptide detection compared with previously reported results. These results demonstrate that ZrO₂ aerogel can be a powerful enrichment material for phosphoproteome study.     

Preparation and characterization of alumina-zirconia composite material with different acid ratios by the sol-gel method

Alumina-zirconia composite materials were produced with different acid ratios by the sol-gel method using aluminum isopropoxide and zirconium chloride. The composites were produced by changing acid/alkoxside ratio in alumina. The composite materials were calcinated at 600°C, 900°C and 1300°C. The effects of acid concentration and calcination temperature on the surface area and pore radius were determined from the nitrogen adsorption isotherm at 77 K. The density of the composites was also measured. The minimum density of produced material was recorded as 1.35 g cm⁻³ at an acid/alkoxside ratio of 0.2. The highest specific surface area and pore diameter of the lightest material are 191.86 m² g⁻¹ and 18.4 Ǻ, respectively. Although pore diameter and specific surface area are not changed at any of the experimental temperatures which were tested by decreasing acid/alkoxside ratio, the density is slightly increased. However, it was observed that the calcination temperature significantly affects the surface area and density of the material.      

A simple template-free sol–gel synthesis of spherical nanosilica from agricultural biomass

Mesoporous silica nanoparticles with a spherical morphology have been synthesized from rice husk (agricultural biomass) by a simple, template-free synthetic approach, which was carried out via sol–gel technique at ambient condition. Transmission electron micrographs revealed the formation of spherical silica nanoparticles with an average diameter of 50.9 nm. From the nitrogen adsorption–desorption analysis, the rice husk silica shows a high specific BET surface area of 245 m² g⁻¹. The silica nanoparticles have a narrow pore size distribution of 5.6–9.6 nm.     

Hydrophobic and Physical Properties of the Two Step Processed Ambient Pressure Dried Silica Aerogels with Various Exchanging Solvents

The experimental results by using various exchanging solvents in the preparation of two step (acid and base) processed ambient pressure dried hydrophobic silica aerogels, are reported. Silica alcogels were prepared by hydrolysis with oxalic acid and condensation with NH₄OH of ethanol diluted tetraethylorthosilicate (TEOS) precursor and hexamethyldisilazane(HMDZ) methylating agent. The exchanging solvents used were: hexane, cyclohexane, heptane, benzene, toluene and xylene. The physical properties such as % of volume shrinkage, density, pore volume, % of porosity, thermal conductivity, % of optical transmission, surface area, pore size distribution and contact angle (θ) of the silica aerogels with water, were measured as a function of EtOH/TEOS molar ratios (R) for all the exchanging solvents. It was found that the physical and hydrophobic properties of the silica aerogels strongly depend on the nature of the solvent and R. Heptane solvent resulted in highly transparent (≈90% optical transmission at 700 nm for 1 cm thick sample), low density (≈0.060 g/cm³), low thermal conductive (≈0.070 W/m·K), high % of porosity (97%), high surface area (750 m²/g), uniform porosity and hydrophobic (θ ≈ 160°) aerogels compared to other solvents. On the otherhand, xylene resulted in aerogels with higher hydrophobicity (θ ≈ 172°) among other solvents.     

The molar formation enthalpy of nano-SiO<Subscript>2</Subscript> with different surface area

Three samples of silicon dioxide were syhthesized and their surface areas were measured. A thermo-chemical cycle was designed to calculate the molar formation enthalpy. The molar formation enthalpy, Δ_f H _m^Φ, for three amorphous silica with the Langmuir surface area 198.0854, 25.1108 and 11.9821 m² g⁻¹ gave −895.52, −910.86 and −915.67 kJ mol⁻¹, respectively. With the increasing surface area, the values of Δ_f H _m^Φ increased accordingly. The results suggest that the silica with larger surface area is more unstable. The wetting heat was also measured by adding the silica powder into water. With the rehydration of the more SiOH groups on the surface, the larger surface areas of silica lead to the more wetting heat. A smaller particle has the more unstable hydroxyl groups and surface energy.     

MOR/SBA-15复合分子筛的合成、表征及其催化性能评价

采用MOR纳米晶和正硅酸四乙酯为硅源,P123三嵌段共聚物为模板剂水热合成MOR/SBA-15复合分子筛催化剂。采用XRD、SEM、TEM和EDX等手段对催化剂进行了表征,在固定床反应器中评价二甲醚制乙醇催化性能。结果表明,通过控制合适的MOR纳米晶种及MOR纳米晶种在SBA-15水热合成体系中的添加量,可以成功地将MOR纳米晶作为SBA-15的结构单元嫁接到SBA-15的介孔骨架中,水热合成的MOR/SBA-15复合分子筛催化剂同时具有MOR和SBA-15的XRD特征衍射峰,相比于SBA-15,其比表面积和总孔体积由756 m2·g~(-1),1.07 cm3·g~(-1)降低至628 m2·g~(-1),0.85 cm3·g~(-1),平均孔径由8.1 nm提高到9.3nm,Cu修饰的MOR/SBA-15复合分子筛催化剂同时具有Cu MOR羰基化和Cu SBA-15加氢的双功能催化性能,其催化剂评价结果显示二甲醚转化率为43.6%左右,乙醇选择性为95.3%,Cu MOR/SBA-15复合分子筛催化剂实现了二甲醚到乙醇的一步转化。