Scalable Synthesis of Ultrathin 2D Covalent Organic Framework Membranes with Sub-1 nm Pores for Molecular Separations

Arizona State University, Scottsdale AZ

Investigators

Abstract

Industrial processes for producing fuels, chemicals, and clean water rely upon separations technologies to isolate one or more chemical species from another. The ability to separate chemicals using energy-efficient membranes reduces the environmental burden of these industrial processes. However, robust, high-performance membranes must be developed for many relevant applications. Covalent organic frameworks (COFs) are stable, crystalline polymers with highly ordered porous structures that can provide fast and selective transport pathways for small molecules. These characteristics make COFs ideal separation materials from which to construct next-generation membranes. Previous studies on COF-based membranes have been limited to using large-pore COFs (mostly >1 nm) obtained by synthesis methods that are difficult to scale up. This research project will enable the rational design and scalable synthesis of ultrathin microporous COF membranes with sub-1 nm pores that are suitable for the molecular separation of various gas/vapor mixtures such as carbon dioxide/nitrogen and xylene isomers. The fundamental knowledge gained from this research will accelerate the deployment of two-dimensional (2D) COF membranes in applications including chemical separations, carbon capture, desalination, catalysis, and sensing, thus addressing societal challenges ranging from energy availability, to global warming, to freshwater scarcity. The project also entails research-related education and outreach efforts, including the development of education-oriented online videos on membrane separations and the creation of a new undergraduate-level laboratory course module on membrane synthesis. The goal of this project is to study the synthesis of 2D COF membranes with sub-1 nm pores by a more easily scalable method, i.e., filtration coating of exfoliated 2D COF nanosheets, and understand the molecular transport in the synthesized microporous 2D COF membranes. High-quality (i.e., large size and molecularly thin) exfoliated microporous 2D COF nanosheets will be synthesized using two complementary approaches: modulated solvothermal growth and post-synthesis ionic functionalization. Vacuum-assisted filtration coating of these fully exfoliated 2D COF nanosheets on commercial macroporous/mesoporous supports will be systematically conducted to obtain defect-free ultrathin (~100 nm) microporous 2D COF membranes with sub-1 nm pores. The interlayer interactions between the exfoliated 2D COF nanosheets will be precisely controlled to modulate their stacking geometry and d spacing, which in turn dictates the pore topology and crystallinity of the resulting COF membranes. Finally, molecular transport and separation measurements will be conducted using small gas and hydrocarbon molecules <1 nm in size (e.g., carbon dioxide/nitrogen, carbon dioxide/methane, xylene isomers, and propylene/propane) to establish the fundamental pore structure−molecular transport−separation performance relations in these microporous 2D COF membranes. The team will create education-oriented TikTok/YouTube content on membranes and their applications to introduce membrane technologies to the broader community and promote student recruiting. The research results will be integrated into a Modern Separations undergraduate/graduate course, and a new laboratory module on polymer membrane synthesis will be developed to train hundreds of students in the next-generation STEM workforce. Projects such as dye rejection will be designed through the SCience and ENgineering Experience (SCENE) program to expose local K-12 students to a research environment and stimulate their interest in separation science. 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.