Understanding and Prevention of Carbon Membrane Physical Aging

University Of Texas At Dallas, Richardson TX

Investigators

Abstract

Complex and energy-intensive technologies are required to separate the molecular components of gas in commercial, energy, or environmental applications. Replacing these technologies with membranes, a sort of molecular filter, could significantly simplify gas separations by reducing energy consumption and operating costs. Carbon membranes, obtained from the carbonization of polymer precursors at high temperatures, have demonstrated exceptional separation properties. However, the performance of carbon membranes declines over time because the material is subject to physical aging, wherein the pores in the membrane collapse. The goal of this research project is to understand the underlying mechanisms of physical aging in carbon membranes in order to implement appropriate methods of stabilization. Two distinct methods will be examined. The first involves increasing the rigidity of the pore structure in the carbon membrane. The second method embeds small metal pillars to prop open the pores of the carbon membrane preventing collapse. This project provides effective strategies to understand and minimize aging in carbon membranes, addressing a long-standing challenge. Robust carbon membranes can be immediately deployed in gas separation processes with minimal modifications, benefiting society through reduced environmental impact, lower energy consumption, and reduced energy production costs. Students at the high school, undergraduate, and graduate levels, including women and students from underrepresented groups, will be engaged in the research project, contributing to the advancement of science. This research is expected to have a positive impact on the education of students and the engineering community. Gas separations such as carbon dioxide/methane, oxygen/nitrogen, or methane/nitrogen could significantly benefit from the implementation of aging-resistant membranes. Carbon membranes have the desired properties of high selectivity, high permeability, chemical resistance, and thermal stability to perform these separations under industrial conditions over other types of membrane materials. One major concern for carbon membranes is their tendency to age over time. Before the potential of carbon membranes for gas separations can be fully realized, a fundamental understanding of the aging process is required. The governing hypothesis of the project is that physical aging is driven by two factors that can be predicted by the intensity ratio of the disordered and graphitic bands from Raman spectroscopy. The first derives from non-covalent pi-pi interactions in large graphitic domains (intensity ratio <0.7), which provide a thermodynamically-driven force to restack, leading to pore collapse. The second originates from the carbonization of high free volume polymer precursors, which yields smaller graphitic domains (intensity ratio >0.7) with a higher density of sp3 carbons. The open structures of these carbons are not fully connected and collapse over time. The degree and mechanism of aging can be matched with the proposed methods of stabilization, that is crosslinking for highly graphitic carbons or pillaring for high free volume carbons. Thermal crosslinking of the polymer precursor will be utilized to rigidify the subsequent carbon membrane, thereby reducing physical aging. Pillaring will be accomplished by evenly dispersing metal-organic polyhedras (MOPs) into the polymer matrix. Upon pyrolysis, the MOPs produce metal nanoparticles between the graphitic layers of the carbon membranes, stabilizing the pore structure. Success of this project will eliminate a major barrier currently preventing the utilization of carbon membranes. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.