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

据估计，美国能源消耗的10-15%用于工业化学分离。石油精炼将原油分馏成汽油、柴油、燃料油以及作为大多数消费品原料的化学前体。在这些石油衍生的化学原料中，有一种称为对二甲苯的化学中间体，主要用于生产聚对苯二甲酸乙二醇酯（PET）塑料纤维、薄膜、瓶子和包装材料。随着PET的广泛使用，到2022年对二甲苯的需求量将超过6000万吨，市场价值超过600亿美元。在将其转化为纺织品和包装塑料之前，对二甲苯必须分离出其他非常相似的分子，称为异构体，其差异仅在于分子内几个原子的位置。由于它们的化学结构几乎相同，通过蒸馏将对二甲苯及其异构体彼此分离是不切实际的，因为它们的挥发性非常相似。纯化对二甲苯的现有技术是利用吸附和/或结晶，但这两种方法都是能量密集型的。该研究项目正在开发具有高渗透性和选择性的新型膜，可以连续分离对二甲苯及其异构体，与当前最先进的技术相比，能源效率得到提高。沸石是水合铝硅酸盐，通常用作阳离子交换树脂、催化剂，并且由于其高度控制的孔径大小，分子筛。沸石在高温高压下在有机液体和蒸气中是稳定的，但直到最近，人们才发现它只是三维晶体，不适合制作薄膜。目前的沸石膜仍然太厚，不能允许目标分子的高通量，因此与其他技术相比没有成本竞争力。该研究项目正在探索沸石纳米片合成的基础研究，以使沸石晶体超薄，这将使膜厚度减少10倍，并相应地使通过膜的渗透性增加10倍。同时，该研究正在寻求减少膜缺陷，预计这将使对二甲苯相对于其异构体的分离因子增加20倍。通过与GOALI工业合作伙伴埃克森美孚公司的合作，首次在工业相关条件下的高温和高二甲苯压力下进行了系统的渗透测量，并与膜微观结构分析和定量渗透建模相结合。这项工作使用电子结构计算和分子模拟来辅助膜的表征，并预测准确的吸附-扩散特性，这将有助于进一步的材料设计和工艺优化。这项关于替代分离技术的研究正在作为例子纳入明尼苏达大学的本科生和研究生课程，并作为项目纳入工艺和产品设计高级本科生课程。

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.