Thomas S. Zemanian

Supercritical Processing of Functionalized Size Selective Microporous Materials

Abstract A supercritical fluid silanation process is applied to effectively functionalize commercial microporous materials without blocking the pore channels. In this approach, supercritical carbon dioxide (SCCO 2 ) is used to deposit mercaptopropyl silane into the microporosity of zeolite beta. The thiol group was subsequently oxidized into sulfonic acid. The size selective catalytic properties of the SCCO 2 modified zeolite compare well with similar materials prepared by an in situ silanation process reported by Jones et al. The active functional groups were delivered to the internal pore surface of the microporous material and remain accessible to molecules that can enter the pore channels. The supercritical process provides an alternative approach to functionalize microporous materials because of the enhanced diffusivity of the functional molecules in the micropore channels and accelerated reaction kinetics. Furthermore, the supercritical process simplifies the materials preparation and may open up new opportunities for commercial zeolites.

Synthesis and Applications of Functionalized Nanoporous Materials for Specific Adsorption

Abstract Surface chemistry is one of the most important properties of mesoporous materials for many applications. There are several approaches to functionalize mesoporous materials in order to tailor the surface chemistry: one step synthesis by co-condensation, direct silanation of partially hydroxylated surface, controlled hydrolysis and condensation, and silanation using supercritical fluid as the reaction medium. Varying the amount chemically and physically adsorbed water can systematically tailor the quality and the population density of functional groups. Furthermore, using supercritical fluid as the reaction medium, organic molecules can be effectively delivered and attached to the internal surfaces of pores less than 1 nm in diameter. The ability to construct high quality functional monolayers allows rational design of molecular recognition and binding sites in mesoporous materials, and has led to the development of very efficient adsorbing materials. One approach to form a host structure that matches the size and shape of the target species is to take advantage of the coordinate chemistry between the functional molecules and metal ions. Highly selective bindings of the target species have been observed against competing species of similar sizes and shapes. More sophisticated surface sites can be constructed for the recognition of complicated molecules and species using large pore mesoporous materials.

Strategies for the Design and Synthesis of Hybrid Multifunctional Nanoporous Materials

This chapter discusses the design and synthesis of multifunctional active sites in ordered nanoporous materials. First, the formation of homogeneous molecular monolayer structures is described. Hybrid nanoporous materials modified with functional molecules and groups are widely investigated for many applications. The molecular chain conformations depend on the surface roughness of the pore channels. A step-wise growth model has been proposed to account for the step-wise pore dimension change. This paper also discusses the use of supercritical fluids as delivery media to improve the effectiveness of surface functionalization. This technique has been used successfully to synthesize size-exclusive microporous acid catalysts. Finally, the formation of architectured monolayer molecular structures is discussed. The use of imprinting or lithograph techniques allows the synthesis of hierarchical porous materials with tunable size-and-shape selective microporosity.

Chemical functionalization of nanostructured materials using supercritical reaction media

There exists a need for durable and thin functional coatings to utilize the afforded surface area of highly porous ceramic materials. Deposition of silane-based Self Assembled Monolayers (SAMs) has thus far been limited to maximum coverages of 4-5 molecules/nm/sup 2/ and long processing times (up to 2 weeks), due to the restricted internal geometry of the substrates. Results are presented for SAMs deposited on high surface area silica from supercritical fluids (SCFs). The SAMs so produced display unprecedented coverages, high monolayer integrity, and extremely low surface defect density. Moreover, the depositions and subsequent removal of reaction byproducts are complete in a matter of minutes rather than days. Nuclear Magnetic Resonance (NMR) spectra of the surface modified silica are presented, demonstrating the SAM integrity and evolution over time. Sorption of aqueous metal ions is demonstrated, and results are given demonstrating the broad pH stability of the deposited SAMs. A chemical explanation for the enhanced deposition is posited, and the kinetics of mass transport into and out of the nanostructured spaces are discussed. Related experiments using zeolite substrates show deposition of thiol-terminated silanes to internal surfaces of 6 /spl Aring/ microporous material. After oxidation of the thiol functional group size selective chemistry was demonstrated using the produced catalyst, proving the efficacy of the supercritical reaction medium for installing functional coatings inside pores of similar diameters to the chain length of the deposited molecule. Comparisons are made between the response of the different substrates to the supercritical fluid-based processing, and remarks on the utility of SCF based processing of nanostructured materials are presented.