High-Surface-Area Alumina–Silica Nanocatalysts Prepared by a Hybrid Sol–Gel Route Using a Boehmite Precursor Padmaja Parameswaran Nampi, w,z,ww Padmanabhan Moothetty, y Wilfried Wunderlich, z Frank John Berry, J Michael Mortimer, J Neil John Creamer, J and Krishna Gopakumar Warrier z z Materials and Minerals Division, National Institute for Interdisciplinary Science and Technology (NIIST) [Formerly Regional Research Laboratory] (CSIR), Trivandrum 695019, Kerala, India y School of Chemical Sciences, Mahatma Gandhi University, Athirampuzha, Kottayam 685650, Kerala, India z Department of Materials Science, Faculty of Engineering, Tokai University, Kanagawa-Ken 259-1292, Japan J Department of Chemistry and Analytical Sciences, The Open University, Milton Keynes, Buckinghamshire, UK High-surface-area alumina–silica mixed oxide (Al 2 O 3 :SiO 2 ) nanocatalysts have been prepared by a hybrid sol–gel method using boehmite (synthesized from aluminum nitrate) as the source of alumina and tetraethyl orthosilicate as the source of silica. The gels, after calcination at 4001C, result in mixed ox- ides with specific surface areas of 287 m 2 /g (Al 2 O 3 :SiO 2 5 3:1) and 262 m 2 /g (Al 2 O 3 :SiO 2 5 3:4). Further heating to 6001C produces materials with specific surface areas of 237 and 205 m 2 /g, respectively. The larger specific surface areas character- istic of the 3Al 2 O 3 :SiO 2 samples are attributed, via transmission electron micrograph investigations, to the presence of B10 nm size, needle-like particles having an aspect ratio of 1:50. Further addition of silica leads to the formation of larger needles of 20– 75 nm size. Calcination at 6001C induced an approximately 5% decrease in the total pore volume for the 3Al 2 O 3 :SiO 2 sample. In contrast, the material with Al 2 O 3 :SiO 2 5 3:4 showed an ap- proximately 12% increase in pore volume when heated at 6001C. The pore-size distribution was in the range 1–3.5 nm with r max at B2 and B2.5 nm at 6001 and 8001C, respectively. Adsorption isotherms and pore-size distribution analyses are discussed in some detail for the aluminosilicates at different ca- lcination temperatures. This discussion is supported by struc- tural information determined from FTIR and 27 Al MAS NMR studies. Relatively high acidity values (0.234 mmol/g for Al 2 O 3 : SiO 2 5 3:4) are observed for silica-rich compositions consistent with their application as efficient acid catalysts. I. Introduction T HE alumina–silica mixed oxide system is of considerable interest because of its various applications in catalysis, its use as a support in automobile exhaust systems, its role in func- tional coatings, and its applications as a precursor for high-tem- perature ceramics. 1–4 A variety of methods have been used to prepare aluminosilicates, where the sol–gel route is reported to be promising because it can produce homogeneous powders with high purity and specificity. 5 The effect of precursors and preparative conditions on the structure and properties of alum- inosilicate gels has received detailed attention. 6–15 In particular, in recent work, we have developed an interest in synthesizing nanosize, biphasic, alumina–silica mixed oxides through sol–gel processes and monitoring their porosity features. Instead of us- ing aluminum alkoxides, which are the usual precursors used by previous workers, we have used boehmite (derived from alumi- num nitrate) as the alumina source, which is cost effective in the synthesis of ceramic materials with a wide variety of applica- tions. 16–21 In the present study, the temperature- and composi- tion-dependent formation features of nanosize, alumina–silica mixed oxide catalysts prepared by a biphasic sol–gel route using boehmite as a precursor are investigated. We report adsorption isotherms and pore-size distributions for the synthesized mate- rials with a view to developing strategies for the synthesis of mixed catalysts with tailor-made porosities and catalytic prop- erties. We also report on structural aspects of the samples, ob- tained from FTIR and 27 Al MAS NMR investigations that correlate with the surface and porosity features. In addition, temperature-programmed desorption (TPD) studies were car- ried out to obtain information on surface acidity. The transmis- sion electron micrograph (TEM) investigations on the compositions synthesized provide adequate information on the nanoparticulate nature. II. Experimental Procedure Boehmite sol prepared from aluminum nitrate, Al(NO 3 ) 3  9H 2 O (98% pure, SD Fine Chemicals Ltd., Mumbai, India), by a method reported by us earlier 22 and tetraethyl orthosilicate (TEOS) (Fluka Chemicals, Switzerland) were used as precur- sors. The pH of the stable sol was maintained at 3.5 and the concentration of the sol was 1.6  10 2 g/cm 3 Al 2 O 3 . In the present study, a hybrid sol–gel approach was used for the syn- thesis of high-surface-area alumina–silica nanoparticulates. For example, 20 g of aluminosilicate having a mullite composition (3Al 2 O 3  2SiO 2 ) was prepared by mixing 865 mL of boehmite sol and 18.03 mL of TEOS, followed by stirring for 5 h, floc- culating at pH 8, and final drying in an oven at 601C. A similar synthetic approach was used to prepare aluminosilicate samples with Al 2 O 3 :SiO 2 molar ratios 3:1, 1:1, and 3:4. The materials of different compositions were calcined at 4001, 6001, 8001, and 10001C, at a heating rate of 101C/min under air atmosphere. BET-specific surface areas of the calcined samples were de- termined by nitrogen adsorption at 77 K (Micromeritics, Gem- ini Model No: 2360, Norcross) after previously degassing the samples in flowing nitrogen at 3001C for 5 h in a separate degas- sing unit attached to the instrument. Values of total pore vol- umes were obtained by applying a relative pressure between 0.05 and 0.90. A procedure involving only the wall area, and based J. Blendell—contributing editor w Author to whom correspondence should be addressed. e-mails: padmavasudev@ gmail.com; padmaja@sctimst.ac.in ww Present address: Bioceramics Laboratory, Biomedical Technology Wing, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, Kerala, India Manuscript No. 27040. Received October 30, 2009; approved June 22, 2010. J ournal J. Am. Ceram. Soc., 93 [12] 4047–4052 (2010) DOI: 10.1111/j.1551-2916.2010.03997.x r 2010 The American Ceramic Society 4047