Final Report for Project Titled Nanoporous Materials: Experimental Studies and Molecular Simulations

[摘要] In recent years significant research has been carried out aimed at developing a fundamental understanding of the phenomena involved in the transport of mixtures in nanoporous systems, such as adsorbents and membranes, which are crucial to many industrial separation processes. Carbon molecular-sieve membranes (CMSM) were the key focus early on in our DOE/BES-supported investigations. They are thought to be more stable and versatile than polymeric membranes, and capable of operating at higher temperatures, up to 300 C. In our research the emphasis was on understanding the factors determining the ability of the CMSM to separate mixtures based on differences in molecular mobility, and in affinity to the pore surface. Our study involved: (1) the preparation and characterization of the CMSM; (2) the computational modeling of their structure, and (3) the measurement and computer simulations of sorption and transport of mixtures through the membranes. The membranes developed are currently undergoing field-testing by Media & Process Technology (M & PT), our industrial collaborators in the project. In this research project we adopted the methodology and tools developed with the nanoporous CMSM to the preparation of novel membranes and films made of SiC. Our efforts were motivated here by the growing interest in the hydrogen economy, which has necessitated the development of robust nanoporous films that can be used as membranes and sensors in high-temperature and pressure processes related to H{sub 2} production. SiC is a promising material for these applications due to its many unique properties, such as high thermal conductivity, thermal shock resistance, biocompatibility, resistance in acidic and alkali environments, chemical inertness (e.g., towards steam, H{sub 2}S, NH{sub 3}, and HCl, of particular concern for H{sub 2} production from biomass and coal), and high mechanical strength. Though the CMSM exhibit many similar good properties, they are themselves unstable in the presence of O{sub 2} and steam at temperatures higher than 300 C (conditions typically encountered in reactive separations for H{sub 2} production). Other inorganic membranes, like ceramic (e.g., alumina, silica, and zeolite) and metal (Pd, Ag, and their alloys) membranes have, so far, also proven unstable, in such high-temperature applications in the presence of steam and H{sub 2}S. The preparation of SiC nanoporous membranes involves two important steps. First, the preparation of appropriate SiC porous supports, and second the deposition on these supports of crack- and pinhole-free, thin nanoporous SiC films. Our early research, in collaboration with M & PT, focused on the preparation of quality porous SiC substrates. Our recent efforts involved the deposition of thin nanoporous films on these substrates by the pyrolysis of pre-ceramic polymeric precursors. We have made substantial strides in this area (as discussed further in Section II) preparing hydrogen-selective membranes and films. The objective of the project was not only to advance the 'state-of-the-art' of preparing the SiC membranes and films, but also to significantly broaden our understanding of factors that determine the ability of the SiC materials to separate gas mixtures, based on differences in molecular mobility and molecule-pore surface interactions. It is only such an improved fundamental understanding that will lead to further substantial improvements in the techniques for preparing such materials. In our studies we proceeded along two paths: (1) the preparation and characterization of SiC membranes, and the computational modeling of their molecular structure, and (2) the measurement and simultaneous computer simulation of sorption and transport of mixtures through the membranes. Coupling experiments and simulations facilitated our efforts to relate the membrane's structure with its transport properties, and separation efficacy. This, in turn, enabled progress towards the long-term goal of first-principle molecular engineering and design of improved materials for adsorption and separation. Understanding the transport characteristics of gas mixtures in SiC membranes is a problem of technological significance, as well as challenging technical complexity. A number of important issues that our study addressed, which are generic to the area of transport of gas mixtures through nanoporous systems are given.

[发布日期] 2008-12-31 [发布机构] UNIVERSITY OF SOUTHERN CALIFORNIA, UNIVERSITY PARK, LOS ANGELES, CA 90089-1211

[关键词] Alloys;36 Materials Science;Deposition;Carbon;Sic Membranes