Synthesis of Porous Silica Foams via a Novel Vacuum-Induced Sol–Gel Method Paul Delaney, w,z John P. Hanrahan, z Mark P. Copley, y Justin O’Byrne, y Justin D. Holmes, y and Michael A. Morris y z Environmental Research Institute (ERI), Cork, Ireland y Department of Chemistry, Supercritical Fluid Centre and Materials Section, University College Cork, Cork, Ireland Mesoporous silica foams having a macroporous skeleton and well-ordered internal mesoporous structure were synthesized by a novel vacuum-induced sol–gel process. This contrasts with the usual route involving expensive sacrificial microspheres. The re- sulting materials had a tri-modal pore size distribution and were characterized by nitrogen physisorption, X-ray diffraction, transmission electron microscopy. I. Introduction I N many cases the advantages of very high surface area and aligned pore structure of mesoporous powders cannot be re- alized because of poor mass transport through the pore system. 1 Much research has thus been focused on the synthesis of meso- porous foams as they combine the advantages of high surface areas associated with mesoporous solids with the accessible diffusion pathways associated with macroporous solids and in turn provide additional functionality compared with strictly or- dered counterparts, e.g. SBA-15 or MCM-41. 1,2 These foam structures resemble xerogels and aerogels; however, the silica foams have the benefit of large (420 nm) randomly ordered macropores, highly ordered mesopores (with interconnecting micropores), and a large specific surface area. Mesoporous sil- ica foams (MPFs) are also more mechanically robust, hydro- thermally, thermally, and chemically stable compared with their xerogels and aerogels analogues, thereby allowing them to be utilized in industrial processes such as separations, catalysis, and sorption. 3,4 MPFs have been utilized in adsorption process such as vol- atile organic compound absorption, 5,6 gas–liquid phase reac- tions where bulky constituents are of concern and in the area of photocatalytic oxidation. 7 The catalytic activity of the foams can be further increased by doping with transition metals 3,4 or functionalization by organic moieties. 8,9 These modified materials have found applications in areas such as catalysis, 4 oxidation of bulky molecules, 1 and protein separations. 10 There has been a variety of MPF-like material prepared and the conventional process uses polymer microspheres as a sacri- ficial template 11,12 or the use of an organic swelling agent. 13 The incorporation of transition metals into the framework of the MPF’s has also been widely investigated. Su et al. 4 reported the synthesis of a tungsten-containing MPF as a recyclable cat- alyst for liquid phase reactions. Carn et al. 14 also demonstrated the synthesis of open cell mesoporous crystalline titanium macrocellular foams that demonstrated an increased efficiency in the ability to control the titanium dioxides mesoporosity. This paper reports the synthesis of MPF and Ti-MPFs with a hexagonally ordered mesoporous structure (with interconnect- ing micropores) and a macroporous foam-like bulk structure. Titania-doped silica are also of interest due to their environ- mental applications in areas such as photo-catalysis of polluted waste water. 15 A unique vacuum-induced sol–gel synthesis method was utilized. These foams are shown to have a high specific surface (4600 m 2 /g), a random ordered macroporous structure and an ordered mesoporous structure (mean pore diameter 5.9 nm). II. Experimental Procedure The MPFs were synthesized based on a variation of the method described by Copley and colleagues 16 and Attard et al. 17 Pluro- nic (P123-EO 20 PO 70 EO 20 ) was used as the structure-directing agent. Tetraethyl orthosilicate (TEOS) was used as the silica source. Briefly, TEOS (9.00 g, 0.043 mol) was added to the tri- block copolymer (5.00 g, 0.0008 mol). A homogenous solution was obtained by stirring at room temperature for 30 min. Water (4.0 mL) was added to this and the solution heated to 313 K in a water bath. Aqueous hydrochloric acid (1 mL, 2.5 mol) was then added over a 2-min period. The Ti-MPF was synthesized in a similar fashion to the pure MPF with titanium tetra-isopropox- ide (0.33 g, 0.011 mol) utilized as the titania precursor. In order to ensure homogeneity and titania placement in the pore wall of the material, the precursor was reduced by addition of ace- tylacenoate as detailed in supporting information. The proce- dure was carried out as per the undoped material with the Ti precursor added to the initial surfactant solution. The synthe- sized gels were placed under vacuum to remove ethanol gener- ated from the silica condensation reaction; conventionally this step is carried out using a rotary evaporator (250 mbar, 187.5 torr 16,18 ) here high vacuum (8 mbar, 6 torr, measured using a Vacuubrand Pirani VAP-5 fine vacuum gauge, Vacuubrand gmbH Co Kg, Brackley, Northamptonshire, U.K.) rotary pumping was utilized to examine the role of solvent evapora- tion on the product formed. The gel was seen to foam on vac- uum application, which resulted in the vacuum fluctuating between 8 and 9.5 mbar until the bulk of the solvent was evap- orated at which point it was stabilized at 8 mbar. The solid product was aged under vacuum with constant pumping (8 mbar) at 313 K for 24 h. Calcination of the resultant gel was preformed in air at 723 K for 24 h. III. Results and Discussion In a recent publication, 19 we showed hexagonal ordering was present in the as-formed gels but long-range ordering was im- K. Ziegler—contributing editor This work was financially supported by the EPA (2006-PhD-ET-12). w Author to whom correspondence should be addressed. e-mail: pauld135@gmail.com Manuscript No. 25725. Received April 1, 2009; approved June 16, 2009. J ournal J. Am. Ceram. Soc., 92 [11] 2798–2800 (2009) DOI: 10.1111/j.1551-2916.2009.03282.x r 2009 The American Ceramic Society 2798