GOALI: Ultra-selective Molecular Sieve Membranes: Novel Synthesis and Performance at Refinery Conditions

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An estimated 10-15% of the U.S. energy consumption is devoted to industrial chemical separations. Petroleum refining fractionates crude oil to make gasoline, diesel, fuel oil, as well as chemical precursors that serve as the feedstocks for the majority of consumer goods. Among these petroleum-derived chemical feedstocks is a chemical intermediate, called para-xylene, that is mainly used for the production of poly(ethylene terephthalate) (PET) plastic fibers, films, bottles and packaging materials. With the widespread use of PET, the demand for para-xylene will exceed 60 million tons, with a market value exceeding 60 billion US dollars, by 2022. Prior to its conversion to textiles and packaging plastics, para-xylene must be separated other very similar molecules, called isomers, that differ only by the positioning of a few atoms within the molecule. Given their near-identical chemical structure, it is impractical to separate para-xylene and its isomers from each other by distillation, as their volatilities are very similar. The current state of the art to purify para-xylene is to utilize adsorption and/or crystallization, but both processes are energy intensive. This research project is developing new membranes with high permeance and selectivity that allow continuous separation of para-xylene from its isomers with improved energy efficiency compared with the current state-of-the-art. Zeolites are hydrated aluminosilicates that are commonly used as cation exchange resins, catalysts, and due to their highly controlled pore size, molecular sieves. Zeolites are stable in organic liquids and vapors at high temperatures and pressures, but until recently, were found only as three-dimensional crystals, which are unsuitable for making thin membrane films. Current zeolite membranes remain too thick to allow high flux of a target molecule and are thus not cost-competitive with other technologies. This research projects is exploring fundamental research on zeolite nanosheet synthesis to make the zeolite crystal ultra thin, which will enable a 10-fold reduction in membrane thickness and a corresponding 10-fold increase in permeance through the membrane. Simultaneously, the research is seeking to reduce membrane defects which is anticipated to increase the separation-factor of para-xylene relative to its isomers 20-fold. Through collaboration with the GOALI industrial partner, Exxon-Mobil, the first-ever systematic permeation measurements are being conducted at the high temperatures and high xylene pressures found at industrially relevant conditions and being coupled with membrane microstructure analysis and quantitative permeation modeling. The work is using electronic structure calculations and molecular simulations to aid in the characterization of the membranes and to predict accurate adsorption-diffusion properties, which will aid in further material design and process optimization. This research on alternative separation technologies is being incorporated as examples in the undergraduate and graduate curriculum and as projects in the process and product design senior undergraduate courses at the University of Minnesota.