An improved method for preparing monolithic aerogels based on methyltrimethoxysilane at ambient pressure Part I: Process development and macrostructures of the aerogels

In this paper, we introduced an improved method for producing methyltrimethoxysilane (MTMS) based aerogels at ambient pressure. The method developed is able to produce monolithic aerogels with low bulk densities over a range of the precursor concentrations at ambient pressure without the need for any solvent exchange and surface modification. This has been achieved through specifically tailored processing conditions according to the MeOH/MTMS molar ratios used. The effect of the key processing parameters on alcogel formation and macrostructures of the aerogels has been systematically investigated for achieving minimum volume shrinkage and bulk density. The specific process developed for MTMS based aerogel has not been reported in previous literature, and it is of significance in improving the practical feasibility of the aerogel fabrication.     

Methyltrimethoxysilane based monolithic silica aerogels via ambient pressure drying

We have developed a novel route to monolithic silica aerogels via ambient pressure drying by the acid–base sol–gel polymerization of methyltrimethoxysilane (MTMS) precursor. An extent of silica polymerization in the alcogels plays a crucial role in obtaining the monolithic aerogels which could be optimized by a proper control over the MeOH/MTMS molar ratio (S) during the sol–gel synthesis. The alcogel undergoes the distinct “spring-back effect” at the critical stage of the drying and thereby preserving the highly porous silica network without collapse. The process yields silica aerogels exhibiting very low bulk density and high specific surface area of 0.062 g/cm³ and 520 m²/g, respectively. The average pore diameter and the cumulative pore volume varied from 4.5 to 12.1 nm and 0.58 to 1.58 cc/g, respectively. In addition, the aerogels are superhydrophobic with contact angle as high as 152°. We anticipate that the new route of the monolithic silica aerogel production will greatly expand the commercial exploitation of these materials.     

Preparation of low-density carbon aerogels by ambient pressure drying

With the addition of hexamethylenetetramine (HMTA) and alcohol as solvent, an ambient pressure drying technique was developed for the fabrication of low-density organic aerogels and related carbon aerogels. When a suitable ratio of resorcinol to HMTA (R/H ratio) and ratio of resorcinol to solvents (R/S ratio) are selected, the low-density alco-gels obtained can be dried under ambient pressure conditions without observable shrinkage. The addition of HMTA increases the size of carbon nano-particles and the pore size of the aerogels that are produced. The carbon aerogels prepared in this work have similar nano-particle structures typical of the aerogels prepared with CO₂ or by the isopropanol supercritical drying technique.     

The role of rheological premonitory of hydrogels based on cellulose nanofibers and polymethylsilsesquioxane on the physical properties of corresponding aerogels

Cellulose nanofibers (CNFs) and polymethylsilsesquioxane (PMSQ)-based aerogels (CNF/PMSQ aerogels) have been prepared by the sol–gel method. The extraction of CNFs from pine wood and formation of composite aerogel with PMSQ were confirmed by Fourier transform infrared spectroscopic and field emission scanning electron microscopic analyses. The rheological premonitory of hydrogel has been performed to predict the physical properties (i.e., density, mechanical properties) of aerogels. The variation in the precursor (methyltrimethoxysilane [MTMS]) and urea content has shown a considerable effect on the storage modulus and phase angle of the hydrogel. The variation in urea content showed a 4–8% increment in the density with a pronounced difference in aerogel's morphology. The increment in the MTMS concentration demonstrated a 10–20% enhancement in density with a minor change in morphology. The van Gurp–Palmen plot of hydrogel has represented a relationship between complex modulus and phase angle. This study establishes that hydrogel's premonitory analysis could compare aerogel's physical properties without going through drying and further analysis.     

Physico-chemical properties of silica aerogels prepared from TMOS/MTMS mixtures

In the present work, results on the physico-chemical properties of the silica aerogels prepared by sol–gel process using mixtures of TMOS and MTMS as precursor are reported. The wide range of precursor mixture was studied with ratio of MTMS/TMOS in precursor mixtures as 0:100, 25:75, 50:50, 75:25, and 100:0 by volume. The gels with these precursor mixtures were successfully prepared using two step acid–base catalysis for gelation. Acetic acid (0.001 M) and NH₄OH (1.5 M) were used for catalysis and resulting alcogels were subsequently dried by supercritical solvent extraction method. FTIR spectroscopy revealed that the aerogels show more intense peak at 1,260 and 790 cm⁻¹ attributed to Si–CH₃ resulting in more hydrophobic nature and these results were concurrent with adsorbed water content measurements made using Karl Fischer’s titration technique. The resulted aerogels were characterized using differential thermal analysis, thermo gravimetric analysis and surface area measurements. The surface area measurements showed an interesting trend that the surface area increased from 395 to 1,037 m²/g with increase in MTMS content in the precursor mixture from 0 to 50% and then again decreased to 512 m²/g for further increase in MTMS content from 50 to 100% in the precursor mixture. It was observed from our studies that silica aerogels prepared using a starting mixture of 50% TMOS and 50% MTMS resulted in high moisture resistance (adsorbed water content of 0.721% w/w), low density of 90 kg/m³ and the highest surface area of 1,037 m²/g, which has great potential for catalysis support applications.     

二氧化硅气凝胶常压干燥工艺的研究进展

二氧化硅气凝胶因具有低密度、高比表面积、稳定的物理化学性质等特性在吸附分离、隔热保温等领域表现出巨大的应用潜力。但长耗时、高成本的制备工艺限制了它的发展,尤其是湿凝胶向气凝胶转变的干燥工艺。本文介绍了二氧化硅气凝胶在常压干燥的过程中面临的主要难点及解决方法,虽然常压干燥方法工艺简单、过程安全、对设备要求低且可连续制备,成为近年来的研究热点,但也存在制备周期长、体积收缩大、需要消耗大量有机溶剂和改性剂等不足。文中从凝胶基体增强与优化、降低毛细管力与减少不可逆收缩两种角度,介绍了二氧化硅气凝胶常压干燥的改进方法及其发展现状,分析归纳了不同改进方法的优缺点,总结了二氧化硅气凝胶常压干燥目前面临的技术挑战。并且,立足于目前二氧化硅气凝胶基体增强和表面改性技术发展的趋势,对今后二氧化硅气凝胶常压干燥过程中结构可控、成本降低以及产品多功能化的发展路线进行了展望。     

Effect of methyltrimethoxysilane as a synthesis component on the hydrophobicity and some physical properties of silica aerogels

The effects of adding methyltrimethoxysilane (MTMS) to the synthesis formulation on the hydrophobicity and physical properties of silica aerogels are reported. The molar ratio of the methanol (MeOH) solvent, water (H₂O), and the ammonia (NH₄OH) catalyst to tetramethoxysilane (TMOS) precursor was fixed at 1TMOS:12MeOH:4H₂O:3.6×10⁻³NH₄OH throughout the experiment and the MTMS/TMOS molar ratio M was varied from 0 to 1.55. After gelation, the alcogels were dried supercritically by high-temperature solvent extraction. The hydrophobicity of the resulting aerogels was tested by measuring the water uptake by the aerogel as a function of time, after putting them directly on the surface of water. It was found that for M<0.26 the aerogels were less hydrophobic but more transparent (>90% in the visible range), whereas for M>1.03 the aerogels were more hydrophobic but semi-transparent to opaque. Aerogels that possessed good hydrophobicity and transparency (85% in the visible range) were obtained with an M≈0.70. An increase in the MTMS content in the gels shifted the pore size distribution towards larger pore radii with a broad distribution. In order to determine the thermal stability of the hydrophobic nature of the aerogels, they were heat-treated in air in the temperature range between 25 and 350°C. It was found that below 280°C the aerogel samples showed hydrophobic properties, whereas above 280°C the hydrophobicity vanished. This is due to the disappearance of the CH₃ groups in the aerogels. The aerogels were characterized by optical transmittance, pore size distribution, BET surface area and infrared spectroscopy measurements.     

Mercury porosimeter as a means to measure mechanical properties of aerogels

It is ascertained that the mercury does not enter pores of some aerogels in experiments with mercury intrusion porosimeter. The curves obtained from mercury porosimeter represent the compaction of aerogels under isostatic pressure conditions. Thus, a novel area of research for the application of the mercury porosimeter emerges. The mercury porosimeter is proposed as a convenient tool to study the mechanical properties of aerogels based on the studies of the volume decrease/expansion of an aerogel sample under isostatic pressure deformation to selected pressures.     

通过控制醇凝胶强度常压制备低密度疏水SiO2气凝胶

常压干燥制备低密度气凝胶是促进高性能气凝胶发展应用的重要途径。以正硅酸乙酯为硅源,采用溶胶-凝胶和常压干燥工艺制备出低密度(<100 kg·m^-3)的疏水SiO₂气凝胶,通过工艺参数的控制制备出不同压缩模量的醇凝胶,探讨了反应物配比对醇凝胶压缩模量和气凝胶密度间的影响关系,获得了通过控制醇凝胶压缩模量制备低密度疏水SiO₂气凝胶的方法;发现将醇凝胶压缩模量控制在0.25~2.5 kPa范围内,可制备出密度小于100 kg·m^-3的疏水SiO₂气凝胶,该研究可以为低密度疏水SiO₂气凝胶的低成本常压制备及其控制方法提供指导。     

二氧化硅气凝胶的气相热导率模型分析

气凝胶是一种超级隔热材料,具有极低的整体热导率。气凝胶的纳米多孔网络结构极大限制了气体分子热运动,使得气凝胶中的气相热导率低于自由气体的气相热导率。本文介绍并讨论了气凝胶气相热导率的基本理论和模型,考察了孔径尺度和气凝胶固相骨架对气相热导率的影响。结果表明,气凝胶气相热导率随气压和孔径的减小而迅速降低,随气凝胶密度的增大而降低。当压力极低时,气凝胶的气相热导率远低于常压下大空间的静止空气。气凝胶纳米固体网格对气相热导率存在重要影响,在(0.01～100)×10⁵ Pa的压力范围内影响尤其显著。     

常压制备疏水性二氧化硅气凝胶

以正硅酸乙酯(TEOS)为硅源,氢氟酸作催化剂,采用溶胶-凝胶法常压下制备二氧化硅气凝胶,并研究催化剂、乙醇、水等因素对凝胶过程的影响。采用傅里叶变换红外分析(FT-IR)、电子扫描探针(SPM)等对二氧化硅气凝胶的结构和性能进行研究。结果表明,经三甲基氯硅烷(TMCS)表面改性处理后的气凝胶表现出了很好的疏水性能。该气凝胶密度为200~400 kg/m³,与水的接触角大于120°。当n(TEOS)∶n(乙醇)∶n(H₂O)∶n(HF)=1∶6∶4∶0.25时,得到的气凝胶各方面的综合性能最好。凝胶时间随着水和氢氟酸用量增加而缩短,随乙醇用量的增加而增加。     

常压制备疏水型二氧化硅气凝胶及透光率分析

采用溶胶-凝胶法,通过常压干燥制备了疏水型二氧化硅气凝胶。研究了pH、水解时间等因素对二氧化硅气凝胶透光率的影响。以正硅酸乙酯为原料,通过酸（草酸）-碱（氨水）两步催化,采用溶胶-凝胶法常压干燥制备了疏水型介孔二氧化硅气凝胶。正硅酸四乙酯、乙醇、草酸、氨水物质的量比为1∶4∶5∶0.2,草酸和氨水的浓度分别为0.008、0.05 mol/L时,采用二甲基二氯硅烷为改性剂常压制备了二氧化硅气凝胶。透射电镜、扫描电镜测试表明：二氧化硅气凝胶具有纳米介孔结构。接触角测定表明：二氧化硅气凝胶与水的接触角为148°,表现出疏水性。     

MTMS基SiO_2气凝胶的制备及其影响因素

以甲基三甲氧基硅烷(MTMS)为前驱体,通过酸碱两步溶胶凝胶法,常压干燥制备了SiO2气凝胶,综合研究了反应条件对气凝胶导热系数,堆积密度,比表面积和孔径的影响,利用FT-IR分析了SiO2气凝胶的化学结构。     

A novel preparation of superhydrophobic silica aerogels via the combustion drying method

Silica aerogels with prominent physical, thermal, optical, and acoustic properties are considered to be highly promising materials. However, owing to the high cost and the complex production processes associated with existing drying technologies, the application of silica aerogels is limited in many fields. In this study, a novel combustion drying method (CDM) was successfully used in the synthesis of superhydrophobic silica aerogels for the first time. It was confirmed that silica aerogel prepared by CDM has a typical aerogel structure with low density, high porosity, high specific surface area, high total pore volume, superhydrophobicity and high thermal stability. Compared with supercritical fluid drying, freeze drying and ambient pressure drying, CDM possesses significant advantages in the drying efficiency and low-cost production due to its active drying mode. Finally, the mechanism of the combustion drying method is proposed based on the combustion of organic solvents. The results will be meaningful for the design and production of aerogel materials in the future.     

Hydrophobic nanocellulose aerogels with high loading of metal-organic framework particles as floating and reusable oil absorbents

In this paper, we employed a facile approach to prepare flexible and porous metal-organic frameworks (MOFs) containing cellulose nanofiber (CNF) aerogels (MNCAs) through freeze-drying MOF-containing cellulose nanofiber suspensions. After coating with methyltrimethoxysilane (MTMS) by chemical vapor deposition, recycled and hydrophobic MTMS-coated MNCAs (MMNCAs) were obtained. Due to the low density (0.009 g/cm³), high porosity (97%) and good mechanical properties of the aerogel, the adsorption capacity of MMNCAs reached up to 210 g/g, which was nearly 3–5 times that of pure CNF aerogels. These prepared aerogels showed excellent oil/water selectivity and high capacity to adsorb oil and organic solvents. This kind of cellulose-based aerogel may be applicable in the field of environmental protection.     

Carbon aerogels for catalysis applications: An overview

Carbon aerogels are nanostructured carbons obtained from the carbonization of organic aerogels, which are prepared from the sol-gel polycondensation of certain organic monomers. These materials have a great versatility both at the nanoscopic level in terms of their pore texture and at the macroscopic level in terms of their form. Thus, the surface area, pore volume, and pore size distribution are tuneable surface properties related to the synthesis and processing conditions, which can produce a wide spectrum of materials with unique properties. In addition, carbon aerogels can be obtained in the form of monoliths, beads, powders or thin films. All these properties make them promising materials for application in adsorption and catalysis. Metal-doped monolithic organic aerogels can be easily prepared by following three main strategies: by addition of the metal precursor to the initial mixture, by ion-exchange or by deposition of the metal precursor on the organic or the carbon aerogel by one of various methods. These metal-doped carbon aerogels have been used as catalysts and as electrodes for electrical double-layer capacitors. This article shows the preparation of metal-doped carbon aerogels, their physico-chemical surface properties and their applications as catalysts in various reactions.     

溶胶凝胶一步溶剂交换与表面改性制备疏水硅气凝胶

以正硅酸乙酯为硅源,采用酸碱两步催化的溶胶-凝胶法,以无水乙醇/六甲基二硅氮烷/正己烷为溶剂交换与表面改性试剂,通过一步溶剂交换与表面改性和常压干燥工艺制备疏水SiO2气凝胶。用傅立叶变换红外光谱(FTIR)、X射线衍射(XRD)、热重-差示(TG-DSC)、场发射扫描电镜(F-SEM)和比表面积(BET)等检测手段对样品的结构、形貌和性能进行了表征。结果表明,将传统的溶剂交换与表面改性由多个步骤改为一步完成,使制备周期从原来的6d缩减到3d,不仅缩短了周期,而且使所得的SiO2气凝胶样品具有842.63m2/g的高比表面积、130kg/m3的低密度、15nm的超细颗粒、2～170nm的孔洞结构和良好的疏水性能。     

Effect of aluminium and copper acetylacetonate on physico-chemical properties of tetraethoxysilane based silica aerogels

Silica aerogel granules have been synthesized by doping aluminium acetylacetonate (Al(acac)₃) and Copper acetylacetonate (Cu(acac)₂), in the sol of tetraethoxysilane followed by ambient pressure drying process. The well established silica network provides effective confinement for metal particles which resists the collapse of silica network during ambient pressure drying of wet gel. In this paper, the effect of Al(acac)₃ and Cu(acac)₂ on physico-chemical properties of the aerogels have been studied by varying the concentration of both from 0.0005 to 0.05 M. The chemical composition, hydrophobicity and elemental analysis of Al(acac)₃ and Cu(acac)₂ doped silica aerogels were studied by using X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, contact angle measurement, X-ray diffraction, atomic absorption spectroscopy. The morphological study was carried by using transmission electron microscopy and surface area and pore diameter was measured by BET analysis. The thermal stability of hydrophobic nature was analyzed by simultaneous thermogravimetry and differential scanning calorimetry. The Al(acac)₃ added silica aerogels are found to be superhydrophobic, low dense, high surface area and optically transparent as compared with Cu(acac)₂ doped silica aerogels.     

Preparation and characterization of antibacterial silver-dispersed activated carbon aerogels

Silver-dispersed carbon aerogels (CAs) were obtained by direct immersion of organic aerogels prepared by ambient pressure drying technique in AgNO₃ aqueous solution and then carbonization. The effect of preparation conditions such as the resorcinol/catalyst ratio, the feed AgNO₃ concentration, the ratio of aerogel mass/solution volume, immersion time and carbonization temperature on the bulk density and silver content as well as the BET surface area of the dispersed CAs was studied. The dispersion and structure of silver nanoparticles in obtained materials were investigated by means of scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The Ag-dispersed CAs prepared exhibit strong and long-term antibacterial activity.     

Influence of aging conditions on textural properties of water-glass-based silica aerogels prepared at ambient pressure

The experimental results of aging time and temperature on the textural properties of water-glass (sodium silicate)-based silica aerogels are reported and discussed. Aging of the hydrogel for different times and temperatures led to an ability to increase the stiffness and strength of the networks. These improvements enabled the gel to withstand ambient pressure drying (APD) and, consequently, preserve the highly porous silica network without collapse. The pore size and volume increased with increasing aging temperature and time, while the specific surface area decreased. Monolithic aerogels with extremely low bulk density (～0.069 g/cm³), high specific surface area (820 m²g^?1), large cumulative pore volume (3.8 cm³g^?1), and high porosity (～96%) were obtained by aging at 60 °C for 18 hours. Therefore, easy synthesis of monolithic silica aerogels at ambient pressure is achievable using a relatively inexpensive silica precursor (sodium silicate).