Hybrid Mesoporous Materials with Functionalized Monolayers** By Jun Liu,* Xiangdong Feng, Glen E. Fryxell, Li-Qiong Wang, Anthony Y.Kim, and Meiling Gong Mesoporous materials have great potential for environmental and industrial processes, but many applications require th terials to exhibit specific surface chemistry and binding sites. A new approach has been developed so that organized func- tional monolayers are covalently bound to mesoporous supports. The functionalized hybrid materials show exceptional tivity and capacity for removing heavy metals from waste streams. Tailored hybrid materials have also shown potential to selectively bind anions and radionuclides. Rational design of the surface properties of mesoporous materials will lead to sophisticated functional composites. 1. Introduction In 1992, scientistsat Mobil Oil Research successfully synthesized ordered mesoporous materials using surfactant micellar structures as templates. [1] These materials have very high surface area (> 1000 m 2 /g), ordered pore structure (mostly hexagonal packed cylindrical pore channels), and extremely narrow pore size distribution. The pore diameter can be adjusted from 2 to 15 nm. The preparation methods involve mixing ceramic precursors (such as sodium alumi- nate, tetramethyl ammonium silicate, and silica) in a surfac- tant (cetyltrimethylammonium chloride, CTAC, or cetyltri- methylammonium bromide, CTAB) solution and reacting the agents at temperatures below 150 C. In principle, surfac- tants form ordered micellar phases. The most common phase consists of rod-like micelles packed in hexagonal arrays. The ceramics precursors bind to the head groups of the surfac- tant molecules, and finally condense together, forming a continuous ceramic phase. Subsequently, the surfactant mol- ecules can be removed by thermal or chemical treatment. Since 1992, mesoporous materials research has become a very active area because of the great potential for applica- tions in environmental and industrial processes. Numerous papers have been published on the preparation of mesopor- ous materials of novel chemical compositions and on the fun- damental understanding of the reaction processes. [2] A wide range of mesoporous materials have been prepared, including alumina, zirconia, titania, niobia, tantalum oxide, and manga- nese oxide. With a few exceptions (for example, manganese oxide [3] ), the pore structure of non-silica-based mesoporous materials is not as well-defined as silica-based materials, and is not stable at elevated temperatures. Mesoporous silica has also been doped with elements possessing catalytic proper- ties [4] and with conducting polymers. [5] Recently, progress has been made in the fabrication of oriented mesoporous films on various substrates, [6] in making free-standingfilms, [7] spheres, [8] and single-crystalline mesoporous materialsin which all the pore channels are aligned. [9] Although the potential of mesoporous materials has been widely recognized, progress on the practical use of these novelmaterials has been slow. Many applications, such as adsorption, ion exchange, catalysis, and sensing, require the materials to have specific binding sites, stereochemical con- figuration or charge density, and acidity. [10] Most mesopor- ous materials do not themselves have the appropriate sur- face properties.For example,mercuryand heavy-metal contamination is a serious problem at waste-contaminated sites of the Department of Energy. [11] Industrial and civilian sources also deposit a large amount of mercury into the envi- ronment every year. [12] It is necessary to develop a method to systematically modify the surface chemistry and tailor the molecular recognition process of mesoporous materials to- ward these targets. 2. Approach Recently,a class of hybrid mesoporous materialshave been developed, based on organized monolayers of func- tionalmolecules covalently bound to the mesoporous sup- port. [13] The functional molecules are attached to the meso- porous support similarly to the preparationof self- assembled monolayers (SAMs) on flat substrates (Fig. 1). This approach provides a unique opportunity to rationally engineer the surface properties. The hybrid mesoporous ma- Chem. Eng. Technol., 21 (1998) 1, Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998 0930-7516/98/0101-0097 $ 17.50+.50/0 97 ± [*] Dr. J. Liu, Dr. X. Feng, Dr.G. E.Fryxell, Dr.L.-Q. Wang, A. Y. Kim, Dr. M. Gong Pacific Northwest National Laboratory PO Box 999, Richland, WA 99352 (USA) [**] Pacific Northwest National Laboratory is operated by Battelle Memori- al Institute for the US Departmentof Energy underContractDE- AC06-76RL01830. The EXAFS study was conducted by Dr. K. Kemner from Argonne National Laboratory. 0930-7516/98/0201-0097 $ 17.50+.50/0 Research News