Understanding and Prevention of Carbon Membrane Physical Aging

在商业、能源或环境应用中，需要复杂的能源密集型技术来分离气体的分子组分。用膜（一种分子过滤器）取代这些技术，可以通过降低能耗和运营成本来显著简化气体分离。由聚合物前体在高温下碳化得到的碳膜已显示出优异的分离性能。然而，碳膜的性能随着时间的推移而下降，因为材料经受物理老化，其中膜中的孔塌陷。该研究项目的目标是了解碳膜物理老化的潜在机制，以便实施适当的稳定方法。将研究两种不同的方法。第一个涉及增加碳膜中孔结构的刚性。第二种方法是嵌入小金属柱，以撑开碳膜的孔隙，防止塌陷。该项目提供了有效的策略来了解和最大限度地减少碳膜的老化，解决了一个长期存在的挑战。坚固的碳膜可以立即部署在气体分离过程中，只需进行最小的修改，通过减少对环境的影响，降低能源消耗和降低能源生产成本来造福社会。高中、本科和研究生阶段的学生，包括妇女和代表性不足群体的学生，将参与该研究项目，为科学进步做出贡献。该研究预计将对学生和工程界的教育产生积极的影响。气体分离，如二氧化碳/甲烷，氧气/氮气，或甲烷/氮气可以显着受益于抗老化膜的实施。碳膜具有高选择性、高渗透性、耐化学性和热稳定性的所需性质，以在工业条件下进行这些分离，优于其他类型的膜材料。碳膜的一个主要问题是它们随着时间的推移而老化的趋势。在完全实现碳膜用于气体分离的潜力之前，需要对老化过程有基本的了解。该项目的主导假设是，物理老化是由两个因素驱动的，这两个因素可以通过拉曼光谱的无序带和石墨带的强度比来预测。第一个来源于大石墨域（强度比0.7）中的非共价π-π相互作用，其提供了重新堆叠的热驱动力，导致孔塌陷。第二种起源于高自由体积聚合物前体的碳化，这产生了较小的石墨结构域（强度比0.7）和较高的sp3碳密度。这些碳的开放结构并不完全连接，并随着时间的推移而崩溃。老化的程度和机制可以与所提出的稳定化方法相匹配，即对于高石墨化碳的交联或对于高自由体积碳的柱撑。将利用聚合物前体的热交联来硬化随后的碳膜，从而减少物理老化。柱撑将通过将金属有机多面体（MOPs）均匀分散到聚合物基质中来实现。在热解时，MOPs在碳膜的石墨层之间产生金属纳米颗粒，从而稳定孔结构。该项目的成功将消除目前阻碍碳膜使用的主要障碍。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.