Master of Engineering Science
Chemical and Biochemical Engineering
Boutilier, Michael S.H.
Membranes made from atomically thin materials promise hundreds of times higher production rates than conventional polymer membranes for separation applications. Graphene is impermeable to gases but becomes selectively permeable once pores are introduced into it but creating trillions of nanopores over large areas is difficult. By instead choosing an inherently porous two-dimensional material with naturally identical pores repeated at high density, we may circumvent this challenge. In this work, we explore the potential of two candidate materials, 2D polyphenylene and graphdiyne. We synthesize cyclohexane-m-phenylene, a monomer of 2D polyphenylene. We then develop an atomic force microscopy technique for measuring the permeance of nanoscopic areas of materials and perform the first gas permeance measurements of graphdiyne and demonstrate molecular sieving. Efforts to scale-up employ continuum transport equations for simple modeling so we develop analytical approximations for the rate of mass transfer rate by advection-diffusion in creeping flow through an orifice plate.
Summary for Lay Audience
Gas separation membranes are widely used in industrial applications for hydrogen separation, helium separation/recovery, oxygen enrichment, and much more. Current commercially available membranes require high temperatures or pressures to operate at their optimal conditions, which increases the cost of production. These membranes also suffer from a tradeoff between permeance (easy flow of the desired species through the membrane) and selectivity (ability to reject undesired species), which means that when a membrane is highly selective, the flow through the membrane is slow, and vice versa. In this work, we explore atomically thin two-dimensional materials as gas separation membranes that can not only reduce energy consumption and cost but also provide maximum achievable permeance with high selectivity. This is possible as materials like graphene are a single atom thick, and their thinness imposes minimal resistance to fluid flow enabling high production rates and resolving the major performance limitation of conventional polymer membranes. As they can support holes the size of smaller molecules, they act as a molecular sieve which allows smaller molecules to pass through while completely blocking larger molecules.
While there is a strong interest in graphene as a separation membrane material, the lattice structure is very densely packed and does not allow the passage of gases or liquids. Engineers need to use special methods to generate nanometer-scale pores in graphene sheets to make them selectively permeable. However, creating trillions of sub-nanometer holes for large-scale application has proven difficult. My project aims to develop atomically thin membranes that solve this problem by replacing the graphene layer with an intrinsically porous 2D polymer. Such materials naturally have a high density of sub-nanometer pores repeated exactly over their entire surface area, circumventing the pore creation challenge in graphene. In this work, we successfully synthesized a building block for porous graphene. We also developed a measurement setup that allowed the first experimental measurements of fluid flow through sub-nanometer pores of Graphdiyne (2D polymer). Furthermore, we developed correlations for simple transport modeling that did not exist before. This project is a major step towards developing macroscopic, selectively permeable 2D polyphenylene membranes for industrial applications.
Atwal, Harpreet, "Inherently Porous Atomically Thin Membranes for Gas Separation" (2021). Electronic Thesis and Dissertation Repository. 8050.