Silica Aerogel-Synthesis and Applications
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KH550的水解工艺及其对Si02表面改性的研究第39卷第2期2012正北京化工大学(自然科学版) JournalofBeijingUniversityofChemicalTechnology(NaturalScience)V oI.39.No.22O12KH550的水解工艺及其对SiO2表面改性的研究高正楠江小波郭锴(北京化工大学教育部超重力工程研究中心,北京100029)摘要:用电导率在线测量法和红外光谱法研究了硅烷偶联剂3一氨丙基三乙氧基硅烷(KH550)的水解工艺.采用共沸蒸馏和溶剂置换方式置换出湿凝胶中物理吸附水,并用KH550水解液对SiO 湿凝胶进行了改性.通过红外光谱(FT—IR),邻苯二甲酸二丁酯(DBP)吸油值,粒度分析仪和接触角测定仪等方式对改性效果进行了表征.结果表明,采用KH550对SiO湿凝胶进行改性后,产品的接触角显着提高,吸油值增大70%以上,孑L容为未改性样品的2倍,有机相中的分散性显着提高.同时,对比共沸蒸馏和溶剂置换两种方式,共沸蒸馏得到疏水性更好的超细SiO,,改性后样品的接触角可以达到140.以上.共沸蒸馏过程中,当改性剂KH550用量为超细SiO绝干粉重的17.5%(质量分数)时,改性效果最好.关键词:KH550;超细SiO,;共沸蒸馏;溶剂置换中图分类号:TQ127.2引言超细SiO:作为一种重要的无机化工产品,可应用于橡胶,吸附剂,涂料,化妆品,药物,医学诊断,功能材料等许多领域.但是由于SiO,表面羟基的存在,使其表现亲水性,在有机介质中难以润湿和分散,与有机基体之间结合力差,使复合材料性能降低,限制了产品的实际应用.因此需要对其进行改性,减弱SiO表面的极性,提高粉体与有机分子的相容性和结合力.目前大多数文献报道的有关液相沉淀法制备SiO的改性,都是对已制备粉体的改性.而SiO:湿凝胶在干燥过程中,伴随水分的脱除,凝胶网络结构出现坍塌,很容易造成硬团聚.由此制备的改性产品,分散性较差,在一定程度上影响改性效果..,同时由于对粉体进行了重复处理,也会造成资源的浪费.克服凝胶表面结构坍塌可以采用表面张力较小的溶剂代替湿凝胶网络孔道中的水,避免团聚现象¨.3-氨丙基三乙氧基硅烷(KH550)是一种较好的无机粒子表面改性剂,可用于SiO表面修饰.经KH550改性后的颗粒均取得了较好的效收稿日期:2011一l1—21第一作者:女,1988年生,硕士生通讯联系人E—mail:**************果,粉体与有机分子的相容性提高.本文采用电导率在线测量方法研究了硅烷偶联剂KH550的水解工艺,同时采用溶剂置换和共沸蒸馏置换凝胶孔道中的水分,并使用KH550水解液对产品进行湿法改性.通过对比改性前后样品性能, 分析KH550对SiO的改性效果.1实验部分1.1主要仪器及药品恒温水浴锅,上海树立仪器仪表公司;DZG-403型真空干燥箱,天津天宇技术实业公司;FW一100型高速粉碎机,天津泰斯特仪器有限公司;MP521型pH/电导率仪,上海三信仪表厂.SiO湿凝胶,自制,含水量92%~93%(质量分数),pH7~8;KH550,电导率0.05IxS/cm,国药集团化学试剂有限公司;无水乙醇,电导率2.0itS/ cm,北京化工厂;去离子水,电导率0.4I~S/cm;正己烷,正丁醇,分析纯,北京化工厂.1.2KH550水解实验按照KH550与去离子水体积比为1:1,加入乙醇溶液,配制不同体积分数的KH550水解液,磁力搅拌进行混合,室温条件下进行水解,通过电导率仪在线测量KH550水解过程,讨论水解液,水解浓度对水解状况的影响.1.3超细Sio:改性称取适量硫酸沉淀法制备的SiO:湿凝胶,直接北京化工大学(自然科学版)进行干燥,得到未改性产品.取适量SiO,湿凝胶,采用乙醇置换3次,然后使用正己烷置换,置换后的凝胶加入KH550水解液进行改性,得到的产品洗涤干燥后得到溶剂置换产品;另称取一定量的湿凝胶, 加入正丁醇,搅拌加热蒸馏,脱除滤饼中大部分水,持续加热至体系温度升至正丁醇沸点,此时产品已变成粉末,然后加入KH550水解液进行改性,洗涤, 抽滤,干燥得到共沸改性产品.1.4分析表征用美国Nicolet60一SXB型FT—IR光谱仪进行红外分析;用美国康塔公司QuadrasorbSI型全自动比表面和孔隙度分析仪测定比表面积和孔分布;用英国Malven公司ZETASIZER-3000HS型粒度分析仪, 测定改性前后样品在有机相(乙醇)中的粒度分布;邻苯二甲酸二丁酯(DBP)测定吸油值;德国Kruss公司K100C型全自动表面界面张力仪测定水滴在SiO,粉体压片上的接触角.采用滴定法测定SiO表面羟基数.称取样品2.0g于400mL烧杯中,加入250mL20%的NaC1 溶液,搅拌均匀后,用0.1mol/L的HC1标准液调节试液pH为4,这一步耗用的酸碱量不计.然后用0.1mol/L的NaOH标准液以每S2~3滴速度对上述试液进行滴定,到试液的pH=9,保持5min不变后即为终点.计算每nmSiO:表面积上的羟基个数n n:(×了V)胁1010(1)I—了J(1)式(1)中:S为样品的比表面积,m/g;V为0.1mol/L NaOH滴定体积,mL.2结果与讨论2.1KH550水解条件的确定采用电导率(y)测定法对硅烷偶联剂水解程度进行检测.由于硅烷偶联剂和去离子水的电导率较低,而水解产物硅醇和醇的电导率相对较高.因此,在KH550水解过程中,伴随硅醇的产生,电导率将逐渐增大,一定时间后水解反应达到平衡时,相应的电导率会稳定在某一值.图1是KH550水解和醇解过程中电导率随时间的变化规律.实验发现KH550在无水乙醇溶液中很快形成均匀透明的溶液.观察电导率随时间变化规律,反应初期电导率有些许升高,随后电导率变化非常小,这表明反应初期KH550与无水乙醇中的水分发生了反应,水分消耗完,电导率基本保持不变.按照水解平衡原理,水解过程产生醇,醇的加入应该会抑制硅烷偶联剂的水解,不利于生成硅醇. 因此可知,醇解过程中乙醇只是起到溶解的作用….同时,观察KH550在水溶液中的水解过程,可以发现,水溶液中KH550水解完全的时间极短, 而且硅醇之间很容易发生缩聚,使改性效果变差. 为增大完全水解的时间,根据反应动力学,可以在体系中加入醇,抑制其水解速度.因此,KH550水解溶剂选择一定配比的水醇混合溶剂.图1KH550水解和醇解过程电导率变化Fig.1ConductivitychangesofKH550during hydrolysisandalcoholysis图2显示了不同体积分数()的KH550水解完全时电导率变化.随着溶液中KH550体积分数的增加,达到的最大电导率先增加后减小.KH550 体积分数较小时,KH550用量的增加使体系中水解产生的硅醇增多,电导率增加;当KH550含量达到一定值时,水解生成的硅醇过多,硅醇缩合生成硅氧烷的几率增大,电导率降低.因此,最佳KH550体积分数应选择在15.75%附近.图2不同体积分数的KH550水解液电导率变化Fig.2Conductivitychangesfordifferent volumefractionsofKH550用傅里叶变换红外光谱仪检测水解前后特征基团的变化,结果如图3所示.第2期高正楠等:KH550的水解工艺及其对SiO:表面改性的研究40003O002o00lo000O'/cm一'1一KH550样品;2--水解0.5h后的KH550浴掖图3KH550和水解0.5h的红外光谱图Fig.3IRSpectraofpristineKH550andKH550afterhydrolysisfor0.5h图3中2974cm~,2887cm~,1457cm~,1378cm~,1079emI1处的谱带为si一0一cHCH基团的特征峰.3400cm和1616cm代表了N—H的伸缩和弯曲振动.水解0.5h后,3400cm处吸收峰变宽,可能是改性剂中N—H的伸缩振动和水解后Si—OH基团的伸缩振动发生重叠,KH550水解过程出现硅羟基.2.2改性前后SiO:粉体表征2.2.1粉体结构样品各官能团可以通过文献上的FT—IR数据得到.图4给出了改性前后样品的红外光谱图.40o03O002o0ol0oO0ocma一未改性样品;b--共沸改性;c一溶剂置换改性图4改性前后样品红外光谱图Fig.4IRspectraofunmodifiedandmodifiedsilica图4中3450cm~,1100cm和950cm处的吸收带分别代表了SiO:表面Si—OH伸缩振动,反对称伸缩振动及弯曲振动.1200~1100cm和467m处的密集带代表si—O—si反对称伸缩振动和弯曲振动.3437ClXl和1630cm附近的吸收峰分别代表水分子(包括表面的吸附水和结构水)的O—H和H一0H的伸缩振动和弯曲振动.经共沸蒸馏和溶剂置换后,采用改性剂KH550对产品进行改性,2850~2950cm区域内出现较弱的光谱带,代表了改性基团一cH的c—H伸缩振动峰.2.2.2粉体性能在相同改性介质中,采用溶剂置换和共沸蒸馏两种方式对湿凝胶进行处理,利用水解后的硅烷偶联剂KH550对样品进行改性,改性后粉体性能如表1.从表1数据可以看出经改性处理后的超细SiO粉体的比表面积比未改性样品有所减小.SiO:是一种具有一定微孑L结构的物质,氮吸附法测定的比表面积包括粒子外表面和内微孔的表面积.由于改性过程中表面改性剂在外表面和孔内部的吸附,造成改性后比表面积的减少.表1KH550不同改性方式样品数据Table1PropertiesofKH550afterdifferentmodification processes通过动态氮气吸附容量法测定3种样品的孔容,结果如表1所示.改性后样品孔容较改性前变大.观察图5共沸改性前后样品孔径分布,未改性样品a(3~10rim)孔径分布范围较窄,平均孔径为6.63nm,相比之下,采用共沸蒸馏改性样品b的孔径分布主要集中在3~50nm之间.这主要是由于共沸蒸馏和溶剂置换两种方式置换出了滤饼中大部分水分,减少了干燥过程中水分蒸发造成的孔道坍塌,使大部分中孔和一些大孔得以保留.a一未改性样品Ib一共沸蒸馏改性样品图5改性前后SiO孔径分布Fig.5Poredistributionsofunmodifiedandmodifiedsilica第2期高正楠等:KH550的水解工艺及其对SiO表面改性的研究?l1? 但是溶剂置换过程中消耗的有机溶剂量较大,置换时间较长.综合来看,共沸蒸馏改性效果要优于溶剂置换改性.图8KH550不同改性方式沉降体积变化趋势Fig.8SedimentationvolumechangeforKH550 modifiedbydifferentmethodsT口'鲁宕料蚓世图9KH550不同改性方式沉降速率变化趋势Fig.9SedimentationratechangeforKH550 modifiedbydifferentmethods2.4改性剂用量对改性效果的影响图l0研究了采用共沸蒸馏水浴加热65℃,反应2h,改性剂KH550用量对表面羟基数的影响.由图1O可以看出随着改性剂用量的增加,表面羟基数逐渐减少,改性剂KH550用量为17.5%(质量分数)左右时,表面羟基数最小,改性效果较好.随着改性剂用量的继续增大,KH550水解产生硅醇的数量相对较多,硅醇缩合为硅氧烷的几率增大,不利于改性,出现了改性剂用量为23%左右时表面羟基数增大的情况.3结论(1)KH550适宜的水解条件为:采用水/乙醇混合溶剂,KH550水解体积分数为l5.75%.(2)KH550改性SiO后得到了大孔径,疏水性能良好的改性产品,分散性提高.对比不同的改性w(KH550)/%图10共沸KH550不同改性剂用量对表面羟基数的影响Fig.10Effectoftheamountofmodificationagentonthe hydroxylnumberofthesilicaSurface方式,共沸蒸馏改性效果要明显优于溶剂置换改性.采用共沸蒸馏改性,KH550质量分数为17.5%时,改性效果最好.参考文献:[1]NozawaK,GailhanouH,RaisonL,eta1.Smartcontrol ofmonodispersest/~bersilicaparticles:effectofreactant additionrateongrowthprocess[J].Langmuir,2005,21:1516—1523.[2]RahmanlA,JafarzadehM,SipautCS.Synthesisofor- gano?functionalizednanosilicaviaCO??condensationmodifi-? cationusing-aminopropytriethoxysilicane(APTES)[J].CeramicsInternational,2009,35:1883—1888.[3]刘琪,崔海信,顾微,等.硅烷偶联剂KH一570对纳米二氧化硅的表面改性研究[J].纳米科技,2009,6(3):15—18.LiuQ,CuiHX,GuW,eta1.Surfacemodificationof nano—silicabysilaneeouplingagentKH一570[J].Nano—science&Nanotechn01ogy,2009,6(3):15—18.(in Chinese)[4]解小玲,郭睿劫,贾虎生,等.KH一550改性纳米二氧化硅的研究[J].太原理工大学,2008,39(1):26—28.XieXL,GuoRJ,JiaHS,eta1.Studyonnano—scale silicamodificationbyKH?550[J].JournalofTaiyuan UniversityofTechnology,2008,39(1):26—28.(in Chinese)[5]吴海艳,周莉,臧树良.纳米二氧化硅表面改性的研究[J].矿冶,2010,19(4):49-52.WuHY,ZhouL,ZangSL.Surfacemodificationofnano?silica[J].Mining&Metallurgy,2010,19(4):49-52.(inChinese)[6]林金辉,王美平,魏双凤.超细SiO:的化学沉淀法制备及其原位改性[J].硅酸盐通报,2007,26(4):12?北京化工大学(自然科学版)2012在[7][8][9]842—844.LinJH,WangMP,WeiSF.Preparationandin—situ modificationofuhrafineSiO2bychemicalprecipitation method[J].BulletinoftheChineseCeramicSociety, 2007,26(4):842—844.(inChinese)姚明明,姚欣.疏水性二氧化硅气凝胶的常压制备与表征[J].广东化工,2010,37(1):5-8.Y aoMM,YaoX.Preparationandcharacterizationcfhy—drophobicsilicaaerogelatambientpressure[J].Guang dongChemicalIndustry,2010,37(1):5—8.(inChi—nese)WuZJ,XiangH,KimT,eta1.Surfacepropertiesof submicrometersilicaspheresmodifiedwithaminopropyl—triethoxysilaneandphenyltrieth0xysilane[J].Journalof ColloidandInterfaceScience,2006,304:119—124.赵光磊,郭锴,王宝玉,等.超重力硫酸沉淀法白炭黑的连续化生产研究[J].无机盐工业,2009,41(9):34—36.ZhaoGL,GuoK,WangBY,eta1.Studyc11continu- OUSproductionofsilicabyhypergravitysulfuricacidpre—cipitationmethod[J].InorganicChemicalsIndustry,2009,41(9):34—36.(inChinese)[10]潘懋.滴定法测定气相法白炭黑比表面积的讨论[J].[12]化学世界,1993(8):380—383.PanM.Discussionontitrationmethodforsurfacearea determinationoffumedsilica[J].ChemicalWorld,1993(8):380—383.(inChinese)王斌,霍瑞亭.硅烷偶联剂水解工艺的研究[J].济南纺织化纤科技,2008(2):25—27.WangB.HuoRT.Studyonhydrolysisofsilanecouplingagem[J].JinanTextileTechnology,2008(2):25—27.(inChinese)ZhuravlevLT.Thesurfacechemistryofamorphoussili—ca.Zhuravlevmodel[J].ColloidsandSurfacesA:Phys—icochemicalandEngineeringAspects,2000,173:1—4.Studyofthehydrolysisof3-aminopropyltriethoxysilane(KH550) andthesurfacemodifiicationofsilica GAOZhengNanJIANGXiaoBoGUOKai fResearchCenteroftheMinistryofEducationforHighGravityEngineeringandTechnology BeijingUniversityofChemicalTechnology,Beijing100029,China)Abstract:Throughmonitoringthechangeinconductivityduringthehydrolysiscf3一aminopropyltriethoxysilane(KH550)andusingFT—IRspectroscopy,theoptimumconditionsf0rthehydrolysisofKH550wereinvestigated. Wetsilicagelfromwhichthephysisorbedwaterwasremovedbyazeotropicdistillationorrapi dsolventreplacementwastreatedwithKH550.TheproductswerecharacterizedbyFouriertransforminfraredspect roscopy(FT-IR),di—n—butylphthalate(DBP)oilabsorption,laserparticlesizeanalysisandcontactanglemeasureme ntsinorderloin—vestigatetheeffectofmodification.Theresultsshowedthatthecontactanglecfthemodifiedsi licaincreased,and theDBPabsorptionvaluesignificantlyincreasedbymo/ethan70%comparedtotheunmodifi edproducts.Theporevolumewastwicethatoftheunmodifiedsilica.Theamountofproductsintheorganicphaseals oincreasedsignifi—cantly.Theazeotropicdistillationmethodforwetgelmodificationaffordedmorehydrophobi csilicathanlhesolventreplacementmethod.andthecontactanglebetweenmodifiedsilicaandwaterreachedashigh as140..Theoptimal conditionsforsilicamodificationinvolvedamodifiermassfractionof17.5%oftheweightofs ilicaandtheuseofazeotropicdistillation.Keywords:3-aminopropytriethoxysilane;ultrafinesihca;azeotropicdistillation;solventre placement。
正己烷对溶胶-凝胶过程及常压干燥工艺制备SiO 2气凝胶的影响卢斌;张丁日;卢孟磊【摘要】以正硅酸乙酯(TEOS)为硅源,三甲基氯硅烷(TMCS)为表面修饰剂,采用酸碱两步催化溶胶−凝胶法和常压干燥法,通过在凝胶中填充适量正己烷(N-hexane)控制溶胶−凝胶过程,使凝胶孔洞趋于均匀,提高凝胶溶剂置换和表面改性效率,制备高性能SiO2气凝胶,制备工艺周期为30 h。
采用BET,SEM和FT-IR等对样品进行表征。
研究结果表明:正己烷填充量为0.2(TEOS与N-hexane物质的量比为1:0.2),制备周期最短,制备出的样品具有最大比表面积(972.5 m2/g)、最大孔容(2.9 cm3/g)和最小密度(0.08 g/cm3),疏水性最佳。
%Silica aerogels were prepared with TEOS as raw material by sol-gel method, surface modification of TMCS and ambient pressure drying within 30 h. Appropriate amount of N-hexane was filled into silica gel to improve efficiency of sol-gel and surface modification process. The structures of samples were characterized by means of BET, SEM and FT-IR etc. The results show that when filler content of N-hexane is 0.2(molar ratio of TEOS to N-hexane is 1:0.2), hydrophobic silica aerogels has low apparent density (0.08 g/cm3), high surface area (972.5 m2/g) and high pore volume (2.9 cm3 /g).【期刊名称】《中南大学学报(自然科学版)》【年(卷),期】2015(000)006【总页数】7页(P2020-2026)【关键词】SiO 2气凝胶;溶胶-凝胶法;常压干燥法;正己烷;胶粒双电层结构【作者】卢斌;张丁日;卢孟磊【作者单位】中南大学材料科学与工程学院,湖南长沙,410083;中南大学材料科学与工程学院,湖南长沙,410083;中南大学材料科学与工程学院,湖南长沙,410083【正文语种】中文【中图分类】O648二氧化硅气凝胶具有极高的比表面积(800~1 500 m2/g)、极大的孔洞率(85%~99%)、极低的热传导率(5 mW/(m·K))和独特的声学性能等,因此,在很多领域发挥重要作用,如用作高性能催化剂、保温涂料、超绝热材料、窗体材料等[1−4]。
SiO2气凝胶骨架增强研究进展李明; 陈跃【期刊名称】《《湖北理工学院学报》》【年(卷),期】2019(035)005【总页数】6页(P59-64)【关键词】SiO2气凝胶; 增强; 纤维; 复合【作者】李明; 陈跃【作者单位】湖北理工学院材料科学与工程学院湖北黄石435003【正文语种】中文【中图分类】TB321SiO2气凝胶(Silica Aerogel)是一种由纳米级基元颗粒构成,具有三维多孔网络骨架的非晶态固体材料[1]。
SiO2气凝胶三维网络骨架由一次粒子和二次粒子构成,其典型结构如图1所示,二次粒子之间以Si-O键相互连接,并以点接触的方式形成“项颈”结构,粒子之间通过连续、无序地延伸构成三维多孔网络骨架主体[2-3]。
SiO2气凝胶骨架间充斥着大量介孔孔隙(2~50 nm),气孔率高达99.8%,因而具有众多特殊性能,比如低密度(0.03~0.35 g/cm3)、低热导率(常温下0.02~0.09 W/(m·K))、高比表面积(600~1000 m2/g)、低折射率(1.02~1.08)等性质[4-5]。
图1 SiO2气凝胶典型结构图1 SiO2气凝胶的应用领域SiO2气凝胶在保温隔热、催化吸附等领域具有广泛的应用价值。
比如,SiO2气凝胶可作为隔热保护衬板,抵挡宇宙飞船或者航天飞机在穿越大气层时产生的白炽高温,阻隔大型舰艇的蒸发器、锅炉等高温系统向舱内散热,以及用于液化天然气(Liquefied Natural Gas, LNG)工程或其他低温建设项目的防“漏热”处理等。
SiO2气凝胶在日用保温领域也已有一些应用,比如气凝胶保温杯套、气凝胶防寒服、高山靴保暖层。
在催化吸附领域,SiO2气凝胶高比表面积及多孔结构的特点可将其作为吸附剂、催化剂及催化剂载体应用于工业或生物医药行业。
例如,利用SiO2气凝胶作为Fe2O3或TiO2的载体制备工业催化剂[6-7],作为药物的搭载工具[8]等。
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200985.Li X;Bai M G;Tao X M Carbon dioxide reforming of methane to synthesis gas by an atmospheric pressure plasma jet[期刊论文]-Journal of Fuel Chemistry and Technology 2010(02)86.Li X;Tao X M;Yin Y X An atmospheric-pressure glowdischarge plasma jet and its application 2009(06)87.李祥;白玫瑰;陶旭梅大气压等离子体射流重整CH4-CO2制合成气[期刊论文]-燃料化学学报 2010(02)88.钟犁;严建华;薄拯滑动弧放电等离子体重整甲烷制取合成气[期刊论文]-浙江大学学报(工学版) 2010(03)89.Bradford M C J;Vannice M A CO2 reforming of CH4[外文期刊] 1999(01)1.徐军科.任克威.王晓蕾.周伟.潘相敏.马建新.XU Jun-ke.REN Ke-wei.WANG Xiao-lei.ZHOU Wei.PAN Xiang-min.MA Jian-xin甲烷干重整制氢研究进展[期刊论文]-天然气化工2008,33(6)2.徐海燕.雷廷宙.任素霞.何晓峰.朱金陵.Xu Haiyan.Lei Tingzhou.Ren Suxia.He Xiaofeng.Zhu Jinling生物质合成气一步法合成二甲醚CuZnFeZr/HZSM-5催化剂的研究[期刊论文]-河南科学2012,30(2)3.Takaku Yamamoto.Hirotaka Sato.Yoshinori Matsukura.Yutaka Ujisawa利用钢铁生产吹氧法的垃圾气化与熔融系统[期刊论文]-钢铁2003,38(z1)4.丁荣刚.闫子峰.宋林花.刘欣梅.Ding Ronggang.YAN Zifeng.Song Linhua.LIU Xinmei甲烷干气重整反应进展[期刊论文]-天然气化学(英文版)2001,10(3)5.王铁军.常杰.吕鹏梅.张喜通.鲁皓沼气重整生物质富氢燃气合成醇醚燃料[会议论文]-20046.汪家铭干重整技术利用甲烷和二氧化碳制合成气[期刊论文]-大氮肥2011,34(4)7.王铁军.常杰.吕鹏梅.祝京旭生物质间接液化一步法合成燃料二甲醚[期刊论文]-煤炭转化2003,26(4)8.薛冰.冯杰.李文英双气头多联产中醇醚燃料合成流程的经济评价[会议论文]-20099.刘晓凤.刘莉.尹小波.徐恒.张辉.邓宇.胡国全.张敏.LIU Xiao-feng.LIU Li.YIN Xiao-bo.XU Heng.ZHANG Hui.DENG Yu. 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黄冈市人民政府关于颁授黄冈市第十届自然科学优秀学术论文的通报正文:----------------------------------------------------------------------------------------------------------------------------------------------------黄冈市人民政府关于颁授黄冈市第十届自然科学优秀学术论文的通报各县、市、区人民政府,龙感湖管理区、黄冈高新区管委会、黄冈白潭湖片区筹委会、白莲河示范区管委会,市直各单位:近年来,全市广大科技工作者潜心钻研,大胆创新,取得了一批自然科学成果及优秀学术论文。
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Hindawi Publishing CorporationJournal of NanomaterialsVolume2010,Article ID409310,11pagesdoi:10.1155/2010/409310Review ArticleSilica Aerogel:Synthesis and ApplicationsJyoti L.Gurav,1In-Keun Jung,1Hyung-Ho Park,1Eul Son Kang,2and Digambar Y.Nadargi3 1Department of Materials Science and Engineering,Yonsei University,120-749Seoul,Republic of Korea2Agency for Defense Development,Daejeon305-600,Republic of Korea3Empa,Swiss Federal Laboratories for Materials Science and Technology,Laboratory for Building Technologies,8600Dubendorf,SwitzerlandCorrespondence should be addressed to Hyung-Ho Park,hhpark@yonsei.ac.krReceived26January2010;Revised12May2010;Accepted30June2010Academic Editor:Ping XiaoCopyright©2010Jyoti L.Gurav et al.This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use,distribution,and reproduction in any medium,provided the original work is properly cited.Silica aerogels have drawn a lot of interest both in science and technology because of their low bulk density(up to95%of their volume is air),hydrophobicity,low thermal conductivity,high surface area,and optical transparency.Aerogels are synthesized from molecular precursors by sol-gel processing.Special drying techniques must be applied to replace the pore liquid with air while maintaining the solid network.Supercritical drying is most common;however,recently developed methods allow removal of the liquid at atmospheric pressure after chemical modification of the inner surface of the gels,leaving only a porous silica networkfilled with air.Therefore,by considering the surprising properties of aerogels,the present review addresses synthesis of silica aerogels by the sol-gel method,as well as drying techniques and applications in current industrial development and scientific research.1.IntroductionThe rapid development of sol-gel techniques during the past two decades has led to fast progress in the deliberate synthesis of porous materials.These techniques complement conventional procedures used for the preparation of amor-phous solids or glasses,such as precipitation or impregnation methods followed by high-temperature treatments.Porous materials are of immense importance in various applications such as adsorption,sensing,and catalysis,owing to their high surface area,porosity,adjustable framework,and surface properties.Out of all known solid porous materials,aerogels are particularly known for their small pore size,large specific surface area,and best optical transmission.Further, among all aerogels,silica aerogels have become quite popular because they possess a wide variety of exceptional properties such as low thermal conductivity(∼0.01W/m.K),high porosity(∼99%),high optical transmission(99%)in the visible region,high specific surface area(1000m2/g),low dielectric constant(∼1.0–2.0),low refractive index(∼1.05), and low sound velocity(100m/s)[1–4].Before exploring more details regarding silica aerogels, wefirst provide an overview of the history of aerogel inven-tion and its development.In the1930s,Samuel Stephens Kistlerfirst produced silica aerogels by formulating the idea of replacing the liquid phase by a gas with only a slight shrinkage of the gel.He prepared aerogels from many other materials,including alumina,tungsten oxide,ferric oxide, tin oxide,nickel tartarate,cellulose,cellulose nitrate,gelatin, agar,egg albumen,and rubber,which are out of scope of the discussion.Kistler’s method involves tedious and time-consuming procedures,and as such there was no follow-up interest in thefield of aerogels until1968when rediscovery of aerogels took place by a team of researchers headed by Professor S.J.Teichner at the University Claude,Bernard, Lyon,France.They substantially simplified the procedure by carrying out the sol-gel transition in a solvent,which was then removed at supercritical conditions.Thefirst Cerenkov radiation detector based on silica aerogels was developed in1974by Cantin et al.Since then,aerogels have been used or considered for use in laser experiments, sensors,thermal insulation,waste management,for molten metals,for optics and light guides,electronic devices, capacitors,imaging devices,catalysts,pesticides,and cosmic dust collection.More recently,several groups around theworld began working in thefield of silica aerogels for the various applications mentioned above.Strictly speaking,to understand silica aerogels,it is necessary tofirst understand sol-gel chemistry and related physicochemical aspects.In the following,we shall discuss sol-gel chemistry,synthetic strategy of silica aerogels,and some recent developments in applications of aerogels.2.Synthesis of Aerogel by Sol-Gel ProcessSol-gel processing is a very popular and reliable methodology for the synthesis of materials,especially metal oxides with uniform,small particle sizes,and varied morphologies[5–10].It involves the transition of a system from a liquid“sol”into a solid“gel”phase.The sol-gel process can ordinarily be divided into the following steps:forming a solution, gelation,aging,drying,and densification.A few of the important advantages of the sol-gel process are its simplicity, and the fact that it is an economic and effective means of producing high-quality materials.Sol-gel processing has found application in the production of high-quality glasses for optical components andfibers,thinfilm coatings,and fine oxide powders[11–15].2.1.Sol-Gel Process.Sol-gel processing implies the synthesis of an inorganic network by a chemical reaction in solution at low temperatures or the formation of an amorphous network in opposition to crystallization from the solution.The most obvious feature of this reaction is the transition from a colloidal solution(liquid)into a di-or multiphase gel(solid) that led to the expression“sol-gel process”.The formation of uniform suspensions of colloidal par-ticles can be understood by calculation of the sedimentation rates assuming that the particles are spherical so that Stokes’law can be applied.Sedimentation rate isdx/dt=4πr3/3ρ −ρg/6πrη=2r2ρ −ρg/9η,(1)whereη=viscosity of surrounding medium,r=radius of colloid particle,ρ =density of colloid particle material,and ρ=density of surrounding material.More clearly,a sol is a colloidal suspension of the solid particles in a liquid in which the dispersed phase is small(1–1000nm).Therefore,the gravitational force is negligible and short-range forces,such as van der Waals attraction and surface charges,dominate interactions.The inertia of the dispersed phase is small enough such that it exhibits Brownian motion,a random walk driven by momentum imparted by collision with molecules of the suspending medium.Sol can be prepared by two techniques, condensation and dispersion of particles[16].Condensation proceeds by nucleation growth of particles to the appropriate size,whereas dispersion involves the reduction of large particles down to the colloidal dimensions.The size and properties of the resulting particles depend on the relative rates of these two processes.Sol formation is favored when the rate of nucleation is high and the rate of crystal growth is low.Depending on the degree of crosslinking and the growth process by which they are formed,the inorganic clusters can be either colloidal or polymeric in nature and can range from10to200˚A in diameter.Generation of inorganic sols also requires controlled conditions,such that the resulting sol is stable with respect to agglomeration and precipitation. Several factors,such as polarity of the solvent,ionic strength of the reaction medium,and temperature,can be used to manipulate the formation of the sol.Gelation is the process whereby a freeflowing sol is converted into a3D solid network enclosing the solvent medium.A gel is a semisolid rich in liquid.It is interesting to note that liquid does not allow the solid network to collapse, and the solid network does not allow the liquid toflow out[17].The point of gelation is typically identified by an abrupt rise in viscosity and an elastic response to stress.For preparation of aerogels,the gelation is most conveniently induced through a change in the pH of the reaction solution. The mechanical state of the gel depends very much upon the number of cross-links in the network.It is obvious that the greater the degree of cross-linking,the more rigid the structure formed.2.2.Chemistry of Sol-Gel Process.There are several parame-ters which influence the hydrolysis and condensation reac-tions(sol-gel process),including the activity of the metal alkoxide,the water/alkoxide ratio,solution pH,temperature, nature of the solvent,and additive used.Another consid-eration is that catalysts are frequently added to control the rate and the extent of hydrolysis and condensation reactions.By varying these processing parameters,materials with different microstructures and surface chemistry can be obtained.Further processing of the“sol”enables the fabrication ceramic materials in different forms.Thinfilms can be produced on a piece of substrate by spin coating or dip-coating.When the“sol”is cast into a mold,a wet“gel”will form.With further drying and heat-treatment,the“gel”is converted into dense ceramic or glass particles.If the liquid in a wet“gel”is removed under supercritical conditions, a highly porous and extremely low-density material called an“aerogel”is obtained.The evidence of silicate hydrolysis and condensation to form polysilicate gel and particles is seen in many natural systems like opals and agates[18].The first metal alkoxide was prepared from SiCl4and alcohol by Ebelmen,who found that the compound gelled on exposure to the atmosphere and Si-(OC2H5)4can therefore be regarded as thefirst“Precursor”for glassy materials[19].2.3.Precursors for Sol-Gel Processing.The precursor is noth-ing but the starting materials for the sol-gel process.(1)Precursors should be soluble in the reaction media.(2)They should be reactive enough to participate in thegel forming process[20].Some salts,oxides,hydroxides,complexes,alkoxides,acy-lates,and amines are used as precursors if soluble in proper solvents[21,22].Alkoxides are the most common sol-gel precursor,since they are commonly available.Bradley et al.have well explained the basic chemistry of the precursor [23].It is very di fficult to predict the type of precursor to be used for a given purpose.The reactivity of precursor does not depend only on its chemical nature but also on the applied reaction condition [24].Compared to the precursors of other element,the network forming power of Si is more to build up a gel [18].That is why other expensive alkoxide precursors can be substituted by cheaper ones like silicon alkoxide such as TEOS,TMOS,and water-soluble precursor such as Na 2SiO 3for sol-gel processing.Hydrophobic aerogels obtained from the precursor with-out surface chemical modification are called hydrophobic precursors,and that of hydrophilic are called hydrophilic precursor as provided in Table 1.2.4.Reaction Mechanism.2.4.1.For Silicon Alkoxide.Silicate gels are synthesized by hydrolyzing monomeric tetrafunctional and trifunctional silicon alkoxide precursors employing a mineral acid (e.g.,HCl,C 2O 4H 4)or a base (e.g.,NH 3,NH 4OH)as a catalyst.The following sol-gel reactions occur during silica network formation [25,26].Hydrolysis.Si(OR)4+4H 2OSi(OH)4+4ROH Silicic acid,(2)where R =Vinyl,Alkyl,or Aryl groups.Condensation.(a)Water condensation:Si(OH)+OSi +H 2O(3)(b)Alcohol condensation:Si(OH)+(OCH 3+CH 3OH .(4)2.4.2.Water Soluble Silicates and Minerals.Sodium silicate (Na 2SiO 3)has been and probably will always be the cheapest source of relatively pure silicic acid from which silica gel can be made.Sodium silicate reacts with water to give silicic acidand then the silicic acid polymerizes and forms silica gel as shown in the following reactions:Na 2SiO 3+H 2O +2HCl −→Si(OH)4+2NaCl .(5)The silicic acid condenses to form small silica particles,chains and consequently forms a network resulting in a silica gel asshown below.2n H 2O .n +n [Si(OH)4+(6)Further,there are some reports available on preparation of silica gels using aluminosilicates,calcium silicates,wolle-stonite,and so forth [27–29].It is evident from reactions (2)–(5)that the structure of sol-gel glasses evolves sequentially as the product of suc-cessive hydrolysis and condensation reactions (and reverse reactions,i.e.,esterification and alcoholic or hydrolytic depolymerization)[15].Thus,knowledge of mechanisms and kinetics of these reactions will provide insight into the gels and gel-derived glasses.The hydrolysis reaction is catalyzed by the addition of an acid or a base [29].In fact,the final form of hydrolyzed silica depends on the pH of the solution.At low pH levels (highly acidic),the silica particle tends to form a linear chain with low crosslink density.This leads to a soft gel,which is reversible and can be redispersed in solution.As the pH value increases,the number of cross-links between the polymer chains also increases.At high pH (highly basic),the polymers become more branched and the number of cross-links increases.At low pH,hydrolysis occurs by electrophilic attack on the oxygen atom of the alkoxide group,whereas at higher pH,hydrolysis and polymerization occur by nucleophilic attack on the Si ion (Si 4+).Atoms,ions,or groups which have a strong a ffinity for electrons are termed electrophiles,while positive ions are termed nucleophiles.In general,all electrophiles are oxidizing agents and all nucleophiles are reducing agents.It is generally found that the process of gelation proceeds with smaller segments dissolving and redepositing onto the larger chains so that the smaller molecules decrease in number but assist the larger molecules to grow until they form fractal aggregates.This process is called Ostwald ripening [30].2.4.3.Hydrophilic and Hydrophobic Surfaces.The names hydrophobic and hydrophilic arise from the combination of “hydro”,meaning water in Greek means,“phobos”meaning “hating”in Greek,and “philic”meaning “loving”in Greek.These terms describe the apparent repulsion and attraction between water and surfaces.As shown in Figure 1,hydrophilicity or hydrophobicity is distinguished from the value of contact angle:smaller or larger than 90◦.Table1:Precursors for silica aerogel synthesis.Hydrophilic precursors Hydrophobic precursors Tetramethoxysilane(TMOS)Methyltrimethoxysilane Tetraethoxysilane(TEOS)Methyltriethoxysilane(MTES) Sodium Silicate(Na2SiO3)(i)Aerogels-high optical transmission(>90%).(i)Aerogels-opaque.(ii)Density<0.1g/cm3.(ii)Density>0.1g/cm3. (iii)Hard&Brittle.(iii)Soft andflexible.When the surface energy of the solid is low it repels the water from its surface and vice versa,showing hydrophobicity or hydrophilicity.Currently,hydrophobic surfaces are used in industry for a variety of applications including hydrophobic coatings for rust prevention,oil removal from water,man-agement of oil spills,and chemical separation processes to separate non-polar and polar compounds.For synthesis of hydrophobic and hydrophilic aerogels, two main steps are involved:(a)synthesis of alcogel by sol-gel process and(b)drying of the alcogel by various techniques.3.Drying of AlcogelAfter gel formation by hydrolysis and condensation reac-tions,an Si–O–Si network is formed.The term aging refers to the strengthening of the gel network;it may involve further condensation,dissolution,and reprecipitation of the sol particles or phase transformations within the solid or liquid phases.This results in a porous solid in which the solvent is trapped.The process of removing the majority solvent from the gel(which in the case of an alkoxide-derived gel is mainly alcohol and water)is called drying.During the drying process,cracking of the gel network occurs due to capillary forces that set up in thefine pores by the liquid-vapour interfaces.The Laplace equation applies in this case, as the smaller the capillary radius is,the higher the liquid will rise,or the higher the hydrostatic pressure will be.Since this surface energy is responsible for the rise of a column of liquid in a capillary,the magnitude of an interfacial pressure within a capillary can be calculated by balancing the static forces that is,2πrγcosθ=πr2hρg,hρg=p r=2γcosθr.(7)The diameters of the pores in the gel are on the order of nanometers,so that the gel liquid must exert high-hydrostatic pressure.The meniscus in the pores and the surface tension forces try to pull the particles together as the liquid in the pores evaporates.These forces can act in such a way that they try to collapse the pores,and hence the structure.Thus,the gels with veryfine pores have a tendency to crack and shrink during drying.To avoid this drying stress,Kistler described thefirst synthesis of an aerogel by supercritical drying in the early1930s[31],andvariousγγγθContact angleFigure1:Hydrophobic and hydrophilic surfaces. aerogel synthesis processes have been reported since.In the 1970s,silica aerogels were synthesized by high temperature supercritical drying of a wet gel produced by the hydrolysis of TMOS in methanol[32].In the1980s,researchers gained a new understanding of the potential of aerogels,and TEOS-based silica aerogels,whose synthesis was less expensive and used fewer toxic sources compared to TMOS-based aerogels, were developed.The low-temperature supercritical drying technique,which uses liquid carbon dioxide,was introduced at the same time[33].3.1.Supercritical Drying of the Alcogel.In supercritical drying methods,gels are dried at a critical point to eliminate the capillary forces,as described below.As soon as the liquid begins to evaporate from the gel, surface tension creates concave menisci in the pores of the gel.As the evaporation of the liquid continues,compressive forces build up around the perimeter of the pore and it contracts.Eventually,surface tension causes the collapse of the gel body[34].In order to prevent the surface tension from building up,the gel is dried supercritically in an autoclave,as shown in Figure2.When the temperature and the pressure in the autoclave are increased above a critical point(for methanol the critical temperature and the critical pressure values are243◦C and7.9MPa,resp.),the liquid is transformed into a“supercritical”fluid in which every molecule can move about freely and the surface tension ceases.Without surface tension,menisci do not form.The vapours are then slowly released from the autoclave,until the pressure in the autoclave reaches atmospheric pressure. Finally,the autoclave isflushed with dry nitrogen(∼3bar) in order to remove the trapped solvent molecules from theFigure 2:Schematic representation of supercritical drying auto-clave.Temperature (T )cP P r e s s u r e (P )Figure 3:Pressure-temperature correlation for solid-liquid-vapour phase equilibrium phase diagram.dried gel.This method of drying of the alcogels is referred to as “supercritical drying”.Figure 3shows the pressure-temperature cycles followed during the supercritical drying of the alcogels.3.2.Ambient Pressure Drying and Surface Chemical Modi-fication.Traditionally,silica aerogels have been synthesized using supercritical drying methods,but this has certain limitations in terms of its cost e fficiency,process continuity,and safety because a high-temperature and pressure are needed to approach the critical point.If liquid carbon dioxide were used as a solvent in the low-temperature supercritical drying process,the chemical durability of the aerogels in the atmosphere would be gradually decreased,since the aerogel particles are hydrophilic.To overcome these problems,Brinker introduced a commercially attractive ambient pressure drying method for the production of silica aerogel [35].In this process,the surface of the wet gel is chemically modified by substituting hydrophobic functional groups by replacement of H from hydroxyl groups followed by ambient pressure drying.Surface silanol groups (Si–OH)on the adjacent silica cluster undergo condensation reactions resulting in an irreversible shrinkage of the gel network during drying,as shown in Figure 4.This process can create surfaces with extremely low energies,which dramatically reduce surface tension.Therefore,it is necessary to modify alcogel surfaces with appropriate modifying agents,so that the surface of the aerogel is rendered hydrophobic.There are several substances capable of altering the wet-ting properties of the surface,that is,hydrophobic reagents.These include methyltrimethoxysilane (MTMS),hexameth-yldisilazane (HMDZ),dimethylchlorosilane (DMCS),dime-thyldichlorosilane (DMDC),trimethylchlorosilane (TMCS),trimethylethoxysilane (TMES),and hexadecyltrimethoxysi-lane (HDTMS).Surface modification of the gels through the replacement of H from Si–OH by non-polar alkyl or aryl groups is a crucial step in the ambient pressure drying method.That prevents condensation reactions of silica clusters,and,by extension,prevents shrinkage of the gel during drying.Since ambient pressure drying can reduce the production cost of the aerogels,their importance has changed from an area of purely scientific interest in to one of practical usage.3.3.Freeze Drying.Another possibility to avoid phase boundaries between the liquid and the gas phase during drying is freeze drying.The pore liquid is frozen and then sublimed in vacuo.There were some attempts to use this method for the production of aerogels [36–38].However,the aging period must be extended to stabilize the gel network,the solvent must be replaced by one with a low expansion coe fficient and a high sublimation pressure,and low freezing temperatures are attained by addition of salts.Another disadvantage is that the network may be destroyed by crystallization of the solvent in the pores.Cryogels are therefore only obtained as powders.4.Aerogel Properties and ApplicationsAerogels have some unique properties which makes them attractive in science and technology,as given in Table 2.Due to these unique properties,aerogels are used for various applications as mentioned in Table 3,and some recent applications are as briefly discussed below.4.1.Aerogel as a Composite.As silicon alkoxide precursor is reactive enough to form gel networks with other metal oxides,several studies were carried out to synthesized silica aerogel composites for various applications.Structural and magnetic properties of silica aerogel-iron oxide nanocom-posites were studied by Casas et al.[39,40].Figure 5shows a silica-titania aerogel composite synthesized viaOH 0<θ<90◦90◦<θ<180◦ROSi(CH 3)3Figure 4:Surface chemical modification of the gel.Table 2:Typical properties of silica aerogels.PropertyValueApparent density 0.03–0.35g/cm 3Internal surface area600–1000m 2/g %solids0.13–15%Mean pore diameter∼20nm Primary particle diameter 2–5nm Refractive index1.0–1.08Coe fficient of thermal expansion2.0–4.0×10−6Dielectric constant ∼1.1Sound velocity100m/sFigure 5:Photographs of silica–titania aerogels (from left 5wt%and Four 10wt%.In front 2wt%samples).ambient pressure drying.There are several reports which describe synthesis of silica-titania[41],silica-carbon,silica and alumina microfibers,[42]or activated carbon powder [43]composite aerogels.4.2.Aerogel as an Absorbent.Synthesis of flexible and super hydrophobic aerogels and their use in absorption of organic solvents and oils were studied by A.Venkateshwara Rao et al.[44,45].They investigated the absorption and desorption capacity of super hydrophobic silica aerogels using eleven solvents and three oils.Figure 6shows various stages of absorption and desorption of organic liquids from the aerogel.The mass (m)of a liquid that rises into the capillaries (aerogel pores)is given by the following formula:2πr γcos θ=m g.(8)For liquids that completely wet the surface,the contact angle θis zero and for such surfaces:2πr γ=m g(9)orγ=km,(10)orγ=kV ρ(11)orγ=kV,(12)where r is the radius of the aerogel pores,V is the volume of the liquid absorbed,ρis the density of the liquid and k =g /2πr is a constant for the given aerogel sample.Therefore,it follows from (10)that the mass of the liquid absorbed increases linearly with an increase in the surface tension of the liquid.Elastic superhydrophobic MTMS aerogels were found to be e ffective and e fficient absorbents of oils and organic liquids.4.3.Aerogel as a Sensor.Aerogels have high overall porosity,good pore accessibility,and high surface active sites.They are therefore potential candidates for use as sensors.A study by Wang et al.[46]on silica nanoparticle aerogel thin films showed that their electrical resistance markedly decreases with increasing humidity.They are highly sensitive to 40%RH and greater and operate with a 3.3%hysteresis,which is attributed to their pore structure.Xerogels of the same material,on the other hand,show very low sensitivity.Surface modified aerogels are less a ffected by humidity as compared to hydrophilic aerogels and can be used as anticorrosive,hydrophobic agents,as shown in Figure 7[47].Wub and Chen-yang [48]studied aerogels for biosensor applications.In this study,mesoporous aerogels were pre-pared at room temperature by sol-gel polymerization with an ionic liquid as the solvent and pore-forming agent.The as-prepared aerogel was characterized by di fferent instruments(a)Beforeabsorption(b)Immediately after ab-sorption,t=0min(c)At t=20min(d)At t=30min.(e)After desorption,t=40min.Figure6:Picture showing various stages of absorption and desorption of organic liquid from the aerogel.and was found to have high porosity and large internal networking surface area.The as-prepared aerogel was further arrayed onto slides and successfully recognized a short human gene ATP5O by an immobilized oligonucleotide probe on the aerogel surface,as shown in Figure8.The large capturing capacity of the porous structure was also demonstrated by comparing with a planar surface at high target concentrations.The results indicate that the as-prepared aerogel can function as a recognition substrate for nucleotide acids.This report proposes a preparation technique to synthesize mesoporous aerogel using the sol-gel process and utilize the aerogel’s high surface area and large internal porous volume for molecular recognition of nucleotide acids.4.4.Aerogel as Material with Low-Dielectric Constant.SiO2 aerogel thinfilms have received a significant attention in IC applications because of their unique properties such as their ultralow dielectric constants,high porosity,and high thermal stability.Park et al.investigated silica aerogel thin films for interlayer dielectrics,and the dielectric constant was measured to be approximately1.9[48–52].They produced ultra low dielectric constant aerogelfilms for intermetal dielectric(IMD)materials.The SiO2aerogelfilms having a thickness of9500˚A,a high porosity of79.5%,and a low dielectric constant of2.0were obtained by a new ambient drying process using n-heptane as a drying solvent.4.5.Aerogel as Catalysts.The high surface area of aerogels leads to many applications,such as a chemical absorber for cleaning up spills.This feature also gives it a great potential as a catalyst or a catalyst carrier.Aerogels aid in heterogeneous catalysis,when the reactants are either in gas or liquid phase.They are characterized by very high surface area per unit mass,high porosity which makes them a very attractive option for catalysis.Some of the reactions catalyzed by aerogels are listed below.Some Examples of Aerogels in Catalysis.(1)Synthesis of nitrile from hydrocarbons using nitricoxide(NO)[53].(2)Isobutene can be converted into methacrylonitrile byreacting it with NO on zinc oxide aerogel[54].102030405060708090100Weightgain(%)Figure7:Effect of humidity on surface modified and unmodifiedaerogel.DNA-target concentration(nM)Figure8:Nonspecific molecular recognition test on the aerogel biochips.Human gene PSMA5(432b)served as the target and was tested by10lMATP5Oc probe immobilized on the aerogel surface.(3)Synthesis of methanol from CO using copper zirconiaaerogel[55].4.6.Aerogel as a Storage Media.The high porosity and very large surface area of silica aerogels can also be utilized for。