Mixed Matrix Membranes

CTS-0107488

Eva Marand

Virginia Polytechnic Institute and State University

The Development of Mixed-Matrix Membranes

Abstract

Membrane separations represent a growing technological area with potentially high economic reward due to low energy requirements and facile scale-up of membrane modular design. Advances in membrane technology, especially in novel membrane materials, will make this technology even more competitive with traditional, highly energy-intensive, and costly processes such as low-temperature distillation and adsorption. In particular, there is need for large-scale gas-separation-membrane systems that can accomplish processes such as nitrogen and oxygen enrichment, hydrogen recovery, acid gas (CO2, H2S) removal from natural gas, and capture of greenhouse gases as well as separation of various hydrocarbon mixtures. Materials employed in these applications must offer durability, productivity, and highly selective separation performance if they are to be economically viable. Currently, polymers and certain inorganic membranes are the only candidates, and each of these two classes has limitations.

The goal of this research is to develop structured composite zeolite/polyimide thin-film membranes that will exhibit gas-separation performance superior to that of existing polymer-based membranes and which will retain their processing versatility and ruggedness compared to inorganic membranes. This work incorporates anisotropic ETS-4, ZSM-2, LTL and MFI plate-like molecular sieves in mixed-matrix membranes. These zeolites can be produced with controlled nanometer-scale size distribution and surface functionalization. The success of the mixed-matrix materials lies in the elimination of defects at the molecular sieve/polymer interface and in the control of the film's microstructure at the sub-nanometer level. This can be achieved by employing zeolite nanoparticles with functionalized surfaces to promote bonding with the polymer matrix. A series of new, well-characterized polyimides has been developed with pendant carboxylic functional groups to serve as the membrane matrix. These polyimides already have excellent separation properties for various gas mixtures and are thermally stable above 400oC in air. A defect-free polymer-zeolite interface is achieved by forming hydrogen bonds or direct covalent linkages between the polyimide chains and the functionalized zeolite nano-particles.