UC Berkeley

The dramatic rise in atmospheric carbon concentrations and consequences thereof has spurred a renewed effort to develop strategies to mitigate consumption of fossil fuels and emission of carbon dioxide. Membranes occupy a small niche within the larger strategy of carbon capture and storage as an effective medium to curb carbon dioxide emissions. However, to become competitive with incumbent technologies such as amine scrubbing, the performance of membranes needs to be greatly improved. Separation membranes are engineered to selectively permeate one molecular species over others through a combination of chemical and physical interactions between the molecule and gas. The performance limitations of current commercial polymeric membranes are due to an intrinsic trade-off between its permeability and selectivity. A novel and emerging approach to bypass traditional polymeric transport limitations is through the addition of inorganic nanomaterials. Inorganic materials often have inherently higher selective chemical and size sieving properties than polymers, but their material brittleness limits their large-scale adoption. However, we can harvest the selective properties of inorganics in hybrid polymer/inorganic composites to design membranes whose performance exceeds that of conventional polymers.

While promising, the primary obstacle towards developing high-performing hybrid membranes is to understand the role of polymer/inorganic interactions on molecular transport properties. These interactions can be highly complex and more often than not, the combination of a polymer and inorganic creates a non-selective membrane due to poor adhesion between phases. As a result, only a few systematic studies aimed to understand the complex nature of the interface have been reported in hybrid systems. Here, I describe the development of a model system using silica nanoparticles to examine contributions from the interface on transport properties. Utilizing this model system, I develop design rules for hybrid membranes and provide a scientific foundation on the role of inorganic size and surface chemistry on molecular transport properties. Polymer dynamics and structure, features that largely govern molecular transport properties, sensitively depend on three key variables (size, surface functionalization, and total volume loading) that are all linked by the total available interfacial surface area of the nanomaterials. From insights gained in these model studies, I explore the design of a completely new class of dual transport membranes using metal-organic frameworks (MOFs) as the inorganic material. By pushing the boundaries of previously achievable MOF loadings, I demonstrate the ability to create a secondary transport pathway through a percolating MOF network, which bypasses transport inefficiencies of the polymer. The new transport channel through the MOF greatly enhances the total CO2 permeability without sacrificing selectivity. Finally, I demonstrate the ability to design a flexible MOF membrane on a polymer support, leading to a new paradigm in inorganic membranes.

The findings presented in this dissertation offer a stronger understanding of fundamental transport properties in hybrid membranes and strategies to design membranes to overcome traditional limitations of polymer membranes. The utilization of dual transport membranes can lead to marked improvement in separation performance, which extends beyond carbon capture applications.