A new route for preparation of sodium-silicate-based hydrophobic silica aerogels via ambient-pressure drying

Abstract

An in-depth investigation into the synthesis of hydrophobic silica aerogels prepared by the surface derivatization of wet gels followed by subsequent drying at ambient pressure is reported. The following sol–gel parameters were examined for their effect on the physical properties of the derived aerogels: number of gel washings with water, percentage of hexane or methanol in silylating mixture, molar ratio of tartaric acid: Na₂SiO₃, gel aging period, weight% of silica, trimethylchlorosilane (TMCS) percentage, and silylation period. These parameters were varied from 1 to 4, 0 to 100%, 0.27 to 1.2, 0 to 4 h, 1.5 to 8 wt.%, 20 to 40% and 6 to 24 h, respectively. The properties of hydrophobic silica aerogels synthesized by this new route were investigated in terms of bulk density, percentage volume shrinkage, percentage porosity, thermal conductivity and contact angle with water, and by Fourier transform infrared spectroscopy (FTIR). The as-prepared hydrophobic silica aerogels exhibited high temperature stability (up to approximately 435 °C) as measured by thermogravimetric/differential thermal analysis (TGA-DTA). The optimal sol-gel parameters were found to be a molar ratio of Na₂SiO₃:H₂O : tartaric acid : TMCS of 1 : 146.67 : 0.86 : 9.46, an aging period of 3 h, four washings with water in 24 h and the use of a 50% hexane- or methanol-based silylating mixture. Aerogels prepared with these optimal parameters were found to exhibit 50% optical transparency in the visible range, 84 kg m^?3 density, 0.090 W mK^?1 thermal conductivity, 95% porosity and a contact angle of 146° with water.     

常压干燥制备疏水性SiO_2气凝胶

以正硅酸四乙酯(TEOS)为前驱体,盐酸和氨水作为催化剂,通过溶胶-凝胶法制备了SiO2醇凝胶,再利用三甲基氯硅烷(TMCS)和正己烷(Hexane)对醇凝胶进行表面改性,最后在常压条件下干燥后制备了疏水性SiO2气凝胶。采用X射线衍射、扫描电镜(SEM)、比表面积测试(BET)、傅里叶转换红外光谱(FT-IR)等测试方法对所得样品的形貌、结构及化学组成等进行了分析。结果表明,所得SiO2气凝胶比表面积可达900m2/g以上,密度200kg/m3,并且具有优异的疏水性。     

疏水性二氧化硅气凝胶性能表征

目前SiO2气凝胶是世界上最轻、隔热性最好、孔隙率较高且声传播速率较低的固体材料,由于其特殊的网络结构使其具有很多独特的性能,而制备疏水性气凝胶是解决常压干燥过程中体积收缩和开裂的关键.采用溶胶-凝胶法以三甲基氯硅烷(TMCS)为化学表面修饰剂通过衍生法制备了疏水性二氧化硅气凝胶,并利用X射线衍射光谱、扫描电镜、能量色散谱、透射电镜、红外光谱、差热-热重分析等测试方法对其结构、形貌及化学组成进行了分析.研究表明:该样品是表面连有疏水基团-CH3的疏水性SiO2气凝胶,且组成其连续网络结构的球状纳米粒子纯度较高、粒径均匀,是热稳定性较高的非晶、多孔、轻质纳米材料.     

疏水SiO2气凝胶的制备及表征

以聚二乙氧基硅氧烷(PDEOS)为原料,通过六甲基二硅氮烷(HMDZ)改性剂对制备的凝胶进行表面改性,并常压干燥得到疏水的SiO2气凝胶.研究了改性剂用量对气凝胶疏水性和结构的影响.疏水气凝胶与水的接触角达120°,常压干燥制备的气凝胶具有典型的气凝胶结构特征,气凝胶颗粒尺寸＜100nm,密度和比表面积分别在140～255kg/m3和480～605m2/g范围内.     

Synthesis of silica aerogels from waterglass via new modified ambient drying

A new modified ambient drying process for synthesizing silica aerogels cost-effectively from waterglass has been developed. Crack-free silica aerogels were obtained via solvent exchange/surface modification of wet gels using IPA/TMCS/n-Hexane solution. Silica aerogels were heated at different temperatures. The effects of heating temperature on chemical bonding state of aerogels were investigated by means of DTA and FTIR. The surface characteristic of the aerogel was hydrophobic when heat-treated under 350°C. The porosities, densities, and specific surface areas of the silica aerogels were in the range of 93–94%, 0.12–0.15 g/cm³, and 630 m²/g, respectively. Distinct spring back phenomena were observed in surface modified wet gels during drying.     

疏水型SiO2气凝胶薄膜的制备

二氧化硅气凝胶薄膜是一种应用广泛的功能材料.本文采用sol-gel技术和dip-coating镀膜工艺,在玻璃表面上形成了SiO2气凝胶薄膜,对SiO2膜层进行表面修饰处理,得到了疏水型SiO2气凝胶薄膜.研究了催化剂与老化过程对薄膜物性的影响.结果表明该疏水膜能使玻璃表面与水的接触角由50°提高至125°,有明显的疏水效果.     

常压制备玻璃基疏水SiO2气凝胶

以水玻璃为原料,通过三甲基氯硅烷(TMCS)和六甲基二硅氧烷(HMDSO)的混合改性剂对制备的水凝胶直接进行表面疏水改性,经乙醇洗涤后可在常压条件下干燥得到疏水SiO2气凝胶,避免了超临界干燥和大量的溶剂交换,大幅度降低了成本,并研究了溶胶pH值对凝胶过程和改性剂组成对气凝胶性质的影响。常压干燥制备的气凝胶密度和比表面积分别在100~160Kg/m3和500~720m2/g范围内。     

Synthesis and characterization of silica aerogels by a novel fast ambient pressure drying process

Using cheap waterglass as silica source, silica aerogels were synthesized via a novel fast ambient drying by using an ethanol/trimethylchlorosilane (TMCS)/Heptane solution for modification of the wet gel. One-step solvent exchange and surface modification were simultaneously progressed by immersing the hydrogel in EtOH/TMCS/Heptane solution, in which TMCS reacting with pore water and Si–OH group on the surface of the gel, with ethanol and heptane helping to decrease the rate of TMCS reacting with pore water and extrude water from gel pores. The synthesized silica aerogel was a light and crack-free solid, with the density of 0.128–0.136 g/cm³ and 93.8–94.2% porosity. The microstructure, morphology and properties of the aerogels were studied by FTIR, SEM, TEM and BET measurement. The results indicate that silica aerogels exhibit a sponge structure with uniform nano-particle and pores size distribution. The specific surface areas of silica aerogels are 559–618 m²/g. And there is an obvious Si–CH₃ group on the surface of the silica aerogel.     

十七氟癸基修饰的疏水二氧化硅膜材料的制备及氢气分离性能

通过溶胶-凝胶法,以正硅酸乙酯(TEOS)和十七氟癸基三乙氧基硅烷(PFDTES)为前驱体,在酸性和洁净室条件下制备了十七氟癸基修饰的SiO2膜材料,分别通过动态光散射技术、接触角测量、红外光谱以及热重分析等测试手段对溶胶的粒径分布及膜材料的疏水性能进行了表征,并深入研究了十七氟癸基修饰后膜材料的氢气渗透和分离性能.结果表明,当摩尔比n(PFDTES)/n(TEOS)=0.2时,溶胶的粒径狭窄分布在3.9nm.十七氟癸基已成功修饰到SiO2膜材料中,十七氟癸基的修饰使得膜材料从亲水性变为疏水性,在上述摩尔比例下,膜材料对水的接触角达到112.0°±0.6°.H2的单组分渗透率随温度的升高而增大,300℃时达到10.00×10-7 mol/(m2·s·Pa),H2/CO2的单组分理想分离系数及双组分分离系数分别达到6.10和13.30,均高于其Knudsen扩散分离因子(4.69),H2在膜材料中的输运主要遵循活化扩散机理.     

Improving the flame retardance of hydrophobic silica aerogels through a facile post-doping of magnesium hydroxide

A facile post-doping of magnesium hydroxide (MH) is proposed to improve the flame retardancy of hydrophobic silica aerogels (SA). The lower tap density (0.12 g/cm³) and thermal conductivity (26 mW/m/K) are still maintained at the largest MH content of 20%. Meanwhile, the gross calorific value of the SA/MH is reduced by 15% and the thermal stability is improved to over ~545 °C. The SA/MH pyrolysis process can be divided into four stages, while the hydrophobic SA is a two-stage pyrolysis process. Furthermore, the improved flame retardancy of SA/MH is mainly attributed to the thermal decomposition of MH, the pyrolysis products (MgO, SiO₂ and H₂O) and the reaction between MgO and SiO₂. The research outcomes demonstrate the validity of using MH as dopant to improve the flame retardancy of hydrophobic SA by a facile post-doping, which is beneficial to the expansion of hydrophobic SA in their practical thermal insulation application.     

以甲基三乙氧基硅烷为原料制备疏水性整体气凝胶的研究

以甲基三乙氧基硅烷(MTES)为原料,通过两步水解以及超临界干燥制备疏水性整体气凝胶,通过改变凝胶形成过程中的酸碱浓度、原料比例等因素,制备孔结构特性不同的气凝胶。通过对气凝胶样品进行BET分析仪、压汞仪、热重分析仪和透射电镜等分析,考察各种因素对气凝胶孔结构的影响。     

Effect of solvent exchanging process on the preparation of the hydrophobic silica aerogels by ambient pressure drying method using sodium silicate precursor

Experimental results obtained on the preparation of hydrophobic silica aerogels by ambient pressure drying method using the sodium silicate precursor with the variation of solvent exchanging process, are reported. The silica hydrogel was prepared by passing the 1.12 specific gravity sodium silicate through the Amberlite (TM) 120 Na⁺ resin and addition of 1 M ammonium hydroxide to silicic acid. The gel was kept in an oven for 3 h to strengthen the gel. Solvent exchange was carried out with ethanol and hexane for 36 h each followed by 24 h silylation using 20% hexamethyldisilazane (HMDZ) in hexane. Unreacted HMDZ was washed with hexane by keeping the gel in hexane for 24 h. Solvent was decanted and the gel was dried for 24 h by keeping the gel at 50 °C for 6 h, at 150 °C for 12 h and at 200 °C for 6 h. The low density (0.06 g/cm³), highly porous (96.9%), highly hydrophobic (contact angle of 160°), low thermal conductivity (0.07 W/m K) aerogels were obtained for the process of three times exchange with ethanol and three times exchange with hexane in 36 h each, followed by silylation with 20% HMDZ in hexane and two times washing with hexane in 24 h. FTIR studies showed the increase in the intensity of the Si–H and C–H bands of the aerogels with the increase of solvent exchanging times because of increase in silylation for more times of solvent exchange processes. It was found from the TG–DTA studies that the hydrophobicity of the aerogels retained up to the temperature of 325 °C. Water absorption studies show that the aerogels were remained hydrophobic up to 4 months when the aerogels were placed over the water as well as for up to 60 h in a 90% humid atmosphere. SEMs of the aerogels reveal that the pore sizes of the silica network increased, so the percentage of optical transparency decreased with the increase in exchange times with ethanol and hexane.     

块状纯氧化铜气凝胶的制备研究

以无机铜盐CuCl2的醇溶液为前驱体，采用聚丙烯酸（PAA）为分散剂，环氧丙烷为凝胶促进剂，通过溶胶-凝胶工艺（sol-gel process）制备了铜基醇凝胶，再经CO2超临界流体干燥工艺得到浅绿色块状初始铜基气凝胶，最后经过热处理获得黑色块状纯氧化铜气凝胶样品，其平均密度约为300mg／cm^3。用扫描电子显微镜（SEM）对气凝胶的微结构进行分析可知，最终获得的气凝胶样品主要由百纳米级的矩体颗粒堆砌而成。X射线衍射（XRD）分析表明初始样品具有典型的无定形结构，其主要的结晶成份为斜方晶Cu^2＋ 2 Cl（OH）3，而X射线荧光光谱（XRF）测试结果表明其Cu与Cl的元素比为3．55：1，高于Cu^2 ＋2 Cl（OH）3中两元素的化学计量比，结合化学原理分析可知，其未结晶成分主要为水合氢氧化铜。而XRD的测试结果表明，经过500℃热处理后的样品成分主要为单斜晶氧化铜。     

Eco-friendly approach for preparation of hybrid silica aerogel via freeze drying method

Journal of Materials Science - In this paper, an eco-friendly method of producing hybrid silica aerogels by freeze-drying method (FD) is proposed. In the freeze-drying system, deionized water was...     

海藻酸盐基纳米复合气凝胶的制备与性能研究

以海藻酸铵(ALG)、纳米氢氧化铝(ATH)和纳米蒙脱土(MMT)为原料,采用溶液共混法和真空冷冻干燥法制备了海藻酸盐基纳米复合气凝胶.采用扫描电子显微镜、傅里叶红外光谱仪、电子万能实验机、X射线衍射仪、热重分析仪对气凝胶进行了形貌、结构表征,并分析了其力学性能和热稳定性能.结果表明:采用ALG和MMT制备的气凝胶具有...     

Preparation and characterization of silica aerogels from oil shale ash

The silica aerogels were successfully synthesized using oil shale ash which is a by-product of oil shale processing via ambient pressure drying. The physical and textural properties of the silica aerogels have been investigated and discussed. The results showed that the organic modification of hydrogels was a crucial step during the processing which preserved mesopores in ambient pressure drying. The unmodified hydrogel underwent tremendous shrinkage during the drying and yielded microporous silica aerogel. Using this novel route, it could produce silica aerogel with low tapping density of 0.074 g/cm³, high specific surface (909 m²/g) and cumulative pore volume of 2.54 cm³/g. From the industrial point of view, the present process is quite suitable for a large scale production of powdered silica aerogel. Furthermore, it provides a new way to solve the problem of oil shale ash pollution.     

On-chip encapsulation of lipase using mesoporous silica: A new route to enzyme microreactors

A polydimethylsiloxane/glass microfluidic reactor containing lipase-mesoporous silica (FSM-22) conjugates has been successfully constructed without chemical cross-linking between enzyme and support. A direct visualization of conjugates of lipase and FSM-22 immobilized in the microreactor by optical microscopy revealed that the enzymes were uniformly dispersed throughout the particles of the FSM-22, because of the successful immobilization of the enzyme. Moreover, the lipase-FSM-22 conjugates contained in the microreactor indicated higher enzymatic activity in hydrolysis of triglyceride, as compared with a batch experiment. These results demonstrate that the microreactor using mesoporous silica performs not only the reagent-less enzyme immobilization but also the high reactivity of the enzyme.     

A Prussian blue route to nitrogen-doped graphene aerogels as efficient electrocatalysts for oxygen reduction with enhanced active site accessibility

Developing high-performance nonpredous-metal electrocatalysts for the oxygen reduction reaction (ORR) is crudal for a variety of renewable energy conversion and storage systems.Toward that end,rational catalyst design principles that lead to highly active catalytic centers and enhanced active site accessibility are undoubtedly of paramount importance.Here,we used Prussian blue nanoparticles to anchor Fe/Fe3C species to nitrogen-doped reduced graphene oxide aerogels as ORR catalysts.The strong interaction between nanosized Fe3C and the graphitic carbon shell led to synergistic effects in the ORR,and the protection of the carbon shell guaranteed stability of the catalyst.As a result,the aerogel electrocatalyst displayed outstanding activity in the ORR on par with the state-of-the-art Pt/C catalyst at the same mass loading in alkaline media,good performance in acidic media,and excellent stability and crossover tolerance that rivaled that of the best nonprecious-metal ORR electrocatalysts reported to date.     

Synthesis of paramagnetic iron incorporated silica aerogels by ambient pressure drying

The iron incorporated silica aerogels were prepared under ambient pressure drying through a two-step sol-gel route by using tetraethoxysilane (TEOS), methyltriethoxysilane (MTES) and ferric nitrate as co-precursors, and hexamethyldisilazane (HMDZ) as a surface modification reagent. Compared with the silica aerogels modified by trimethylchlorosilane (TMCS) or HMDZ, the iron incorporated silica aerogels exhibited good integrity, paramagnetic behavior, and highly hydrophobicity which could be maintained up to 650 °C. However, the introduction of iron into silica aerogels resulted in more apparent volume shrinkage, higher density, lower porosity, and smaller pore diameter. It was found that a large amount of water was retained in the silica network, accordingly, the water containing iron oxides (Fe–OH) were more readily formed than water free iron oxides (Fe–O), which adversely affected the physical properties of iron incorporated silica aerogels.