FULL PAPER DOI: 10.1002/ejic.200501140 High-Zirconium-Content Nano-Sized Bimodal Mesoporous Silicas David Ortiz de Zárate, [a] Andrés Gómez-Moratalla, [a] Carmen Guillem, [a] Aurelio Beltrán, [a] Julio Latorre, [a] Daniel Beltrán, [a] and Pedro Amorós* [a] Keywords: Mesoporous materials / Zirconia / Surfactants / Nanoparticles / Bimodal porous materials Silica-based nanoparticulated bimodal mesoporous materials with high Zr content (43  Si/Zr  4) have been synthesized by a one-pot surfactant-assisted procedure from a hydroalco- holic medium using a cationic surfactant (CTMABr = cetyltri- methylammonium bromide) as structure-directing agent, and starting from molecular atrane complexes of Zr and Si as hy- drolytic inorganic precursors. This preparative technique al- lows optimization of the dispersion of the Zr guest species in the silica walls. The bimodal mesoporous nature of the final high surface area nano-sized materials is confirmed by XRD, TEM, and N 2 adsorption–desorption isotherms. The small in- traparticle mesopore system (with pore sizes around 2–3 nm) is due to the supramolecular templating effect of the surfac- tant, while the large mesopores (around 12–24 nm) have their Introduction Silicon/zirconium mixed oxides, (SiO 2 ) 1–x (ZrO 2 ) x , have attracted considerable attention for a long time because of their mechanical (low thermal-expansion coefficient) and chemical (resistance to alkaline attack) properties. [1–7] Moreover, they are also interesting materials in catalysis. Indeed, they exhibit catalytic activity due to both their moderate acidity and oxidizing capability, and also can be used as catalyst supports. [8] However, the possibility for in- dustrial applications has been limited because these materi- als generally show low surface area and no size selectiv- ity. [9,10] Hence, the search for improved-performance (high surface area) catalysts stimulated the preparation of (SiO 2 ) 1–x (ZrO 2 ) x solids in the form of particulated xerogels, which was approached through a diversity of sol-gel routes starting from alkoxide precursors. [11–13] The structure of these xerogels was studied some time ago by SAXS and WAXS techniques, [14] which revealed a very open architec- ture (involving interconnected 3D pore systems) that has been recently referred to as optimal for catalytic applica- tions. [15] In practice, the (SiO 2 ) 1–x (ZrO 2 ) x particulated xero- gels are high surface area catalysts displaying strong acidity and showing satisfactory activity in a diversity of organic [a] Institut de Ciència dels Materials (ICMUV), Universitat de València, P. O. Box 2085, 46071 València, Spain Fax: +34-96-3543633 E-mail: pedro.amoros@uv.es © 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2006, 2572–2581 2572 origin in the packing voids generated by aggregation of the primary nanometric mesoporous particles. The basicity of the reaction medium seems to be a key parameter in the defini- tion of this last pore system. The effects induced by the pro- gressive incorporation of Zr atoms on the mesostructure have been examined, and the local environment of the Zr sites in the framework has been investigated by UV/Vis spec- troscopy. Observations based on the consequences of post- treatments of the as-synthesized materials with HCl/ethanol mixtures corroborate that the atrane method leads to Zr-rich materials showing enhanced site accessibility and high chemical homogeneity throughout the pore walls. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) reactions (cumene dealkylation, alcohol dehydration, alkene isomerization). [16–18] However, the use of these (SiO 2 ) 1–x (ZrO 2 ) x xerogels as catalysts presents a certain limitation derived from their lack of size selectivity. [9,10] Until now, the most promising strategies to overcome this problem have been those con- ceived to incorporate Zr atoms into the frameworks of mo- lecular sieves or high surface area silica-based materi- als. [19–21] Thus, there are reports on Zr-containing materials derived from both microporous solids (such as zeolites and also ALPOS) [22–25] and mesoporous silicas (Zr-MCM-41, Zr-MCM-48, Zr-HMS, Zr-MSU, Zr-SBA-15), [26–39] which simultaneously display high surface area and size selectivity. Hence, a diversity of both small and large substrates could be processed with the help of such a variety of Zr-contain- ing molecular sieves. In short, the preparative procedures implemented for incorporating zirconium in silica-based porous matrices can be referred to as one-pot (cohydrolysis) methods [26,32,34–36,38,39] or postsynthesis (grafting) treat- ments. [33,37] Independent of the synthesis strategy used, an important condition for having very active supported zirco- nia catalysts (high-Zr-content materials) lies in site isola- tion: the Zr atoms must be isolated and well dispersed throughout the silica walls, thus avoiding the formation of ZrO 2 domains. [40] In fact, while the preparation of doped mesoporous silicas is straightforward and usually leads to a relatively homogeneous distribution of cations, the situation changes when high dopant loads are required, and we must