This NSF award by the Chemical and Biological Separations program supports the work by Professor John P. Ferraris, Kenneth J. Balkus, Jr., and Inga H. Musselman at the University of Texas at Dallas to design, fabricate and test mixed matrix membranes (MMMs) for gas separations. This project will utilize novel materials that simultaneously target selected separations and provide improved interfacial contact with the polymer matrix. We have discovered that high loadings of nanoporous metal-organic frameworks (MOFs) in polymers can provide the long sought after breakthrough technology. MOFs offer some of the highest surface areas ever reported as well as the selective adsorption of gases involved in industrially important separations including CO2, CH4, O2, and N2. Many of these metal-organic frameworks have exceptional thermal and chemical stability such that MOFs could be competitive with zeolites for commercial separations. This novel class of materials has enabled us to fabricate MMMs with unprecedented loadings [>80% (w/w)], which we believe is the key to finally realizing the promise of mixed-matrix membranes for gas separations.
The broader impacts of this project on energy and the environment include numerous tasks that will lead to the integration of research and multilevel education in the area of membrane science and novel nanomaterials. The success of this potentially high impact research effort will lead to significant benefits to society including replacement of energy-intensive separations with membranes resulting in both energy and economic savings. A strong educational component will coincide with the research activities that will engage students at both the graduate and undergraduate levels as well as students from underrepresented groups and women. The skills acquired by students during this project will enhance their preparation for careers in membrane engineering, nanotechnology, energy, and materials science. In addition to seminars and course development on membranes and their applications, we seek to engage students at all levels in the study of membranes and nanomaterials. We are also committed to high school student research experiences through a number of summer programs and anticipate that this project will also impact the community at large by educating our high school teachers and students.
Project Outcomes 1. Introduction Membrane technology has become a promising alternative for the separation of industrially relevant gas mixtures, over the currently used energy intensive methods such as cryogenic distillation. In fabrication of membranes, polymers have become attractive due to the better membrane forming properties and low cost. However, the selectivity/permeability tradeoff of polymer membranes in gas separations, shown by Robeson, has motivated researchers to develop membranes with both high gas permeability and selectivity. It has been two decades since this upper bound was established, and only a few polymers have emerged that exceed it, and those that do are not always stable at industrial operating conditions. On the other hand, porous materials such as metal-organic frameworks (MOFs) have demonstrated superior gas separation performances, but are limited by the lack of mechanical strength to form free-standing membranes. In this research project we have developed novel mixed-matrix membranes (MMMs) by combining the superior gas separation properties of MOFs, with the membrane-forming properties of polymers. These novel materials provide the additional advantages of easy handling and low fabrication costs. The use of MOFs provides high surface areas, leading to unprecedented adsorption properties. MOFs also offer the capability of performing gas separation through molecular sieving or by enhancing the solubility of gases in the membrane. MOFs of different pore sizes can easily be synthesized by changing the composition of the organic linkers. In addition, the presence of organic linkers in MOFs, improves the compatibility between additives and polymers. This helps to overcome the formation of nonselective voids, a major limitation of MMMs. 2. Major Findings. We have synthesized and characterized a series of MOFs with controlled pore sizes to serve as porous additives for MMM fabrication. As the matrix, we chose polymers with high glass transition temperatures above 300 °C, which are suitable for actual industrial gas separation conditions. Polymers such as Matrimid®, PBI, and VTEC were purchased, but other high performance polymers such as polymers of intrinsic microporosity-1 and polyimides were synthesized in our labs. Synthesized polymers were characterized using several spectroscopic, chromatographic, and thermal analysis methods. A membrane fabrication protocol was developed including mixing of MOFs and polymers under acoustic sonication for a uniform distribution of MOFs in the polymer matrix. An automated membrane casting table equipped with a doctor blade was used for the membrane fabrication. After annealing under vacuum, resulting membranes were characterized with a variety of analytical techniques including scanning electron microscopy, X-ray diffraction, infrared spectroscopy, and thermal analysis, prior to determination of the gas permeability properties. These characterizations revealed information about membrane morphology, crystallinity of the MOFs, chemical interactions between MOFs and polymers and thermal stability of the membranes. Other strategies to improve gas separation performance of these MMMs were also investigated such as the cross-linking of polymer membranes with ethylenediamine. Also, by combining highly gas permeable polymers with highly gas selective polymers, immiscible polymer blend based membranes were developed using MOFs as compatibilizers. The gas permeability properties of these novel MMMs have been very promising compared to parent polymers and some have reached commercially attractive Robeson upper bound for industrially relevant gas mixtures such as hydrogen /carbon dioxide mixtures. 3. Importance to the General Public This NSF funded project provided numerous opportunities for student training and professional development. Students (high school, undergraduate and graduate) participated in the synthesis and characterization of MOF materials and polymers, and MMMs from the synthesized materials. Students gained hands-on experience with major instrumentation including spectroscopy, chromatography, microscopy, thermal gravimetric analysis, and X-ray diffractometry. Also, senior graduate students developed teaching skills by supervising undergraduate students from various universities and high school students under several programs (Plano Independent School District/High Technology Education Coalition of Collin County (PISD/Hi-TECCC), NanoExplorers, Clark Summer Scholarship). Furthermore, all students learned skills in areas of inorganic and organic synthesis, analytical chemistry, and nanotechnology. Underrepresented parties also benefitted from this project which is reflected in the number of women participants in these projects, and two of whom obtained their Ph.Ds with the support of this funding. The students got the opportunity to present their research in national (NAMS, ACS) and regional (DFW ACS meetings) conferences and won awards in both graduate and undergraduate levels. Seven peer reviewed journal articles and one book chapter were published with the support of this funding and more manuscripts are in preparation. The PIs have also participated in numerous professional society activities, advisory boards and committees. In addition, they have served as reviewers for numerous journals and funding agencies.
