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。石油精炼将原油进行分馏，以制造汽油、柴油、燃料油以及用作大多数消费品原料的化学前体。在这些石油衍生的化工原料中，有一种名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酯(PET)塑料纤维、薄膜、瓶子和包装材料。随着聚酯的广泛使用，到2022年，对二甲苯的需求量将超过6000万吨，市场价值将超过600亿美元。在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，这些分子的不同之处只是分子中几个原子的位置不同。考虑到它们的化学结构几乎相同，用蒸馏分离对二甲苯及其异构体是不切实际的，因为它们的挥发性非常相似。目前提纯对二甲苯的技术状态是利用吸附和/或结晶，但这两个过程都是能源密集型的。该研究项目正在开发具有高渗透率和选择性的新膜，与目前最先进的膜相比，这种膜可以连续分离对二甲苯及其异构体，并提高能源效率。沸石是水合铝硅酸盐，通常用作阳离子交换树脂、催化剂，并由于其高度可控的孔径而被用作分子筛。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，人们还发现沸石只是三维晶体，不适合制作薄膜。目前的沸石膜仍然太厚，不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。本研究项目正在探索沸石纳米片合成的基础研究，使沸石晶体超薄，使膜厚减少10倍，膜透过率相应增加10倍。同时，这项研究正在寻求减少膜缺陷，预计膜缺陷将使对二甲苯的分离系数相对于其异构体增加20倍。通过与Goali工业合作伙伴埃克森美孚的合作，首次在工业相关条件下发现的高温和高二甲苯压力下进行了系统的渗透测量，并结合膜微结构分析和定量渗透建模。这项工作正在使用电子结构计算和分子模拟来帮助表征膜的特性，并预测准确的吸附-扩散性能，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被纳入明尼苏达大学本科生和研究生课程的范例，以及作为工艺和产品设计高级本科课程的项目。

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.