Collaborative Research: Crossing the percolation threshold for selective gas transport using interconnected crystals of metal–organic frameworks in polymer-based hybrid membranes

甲烷、乙烷和乙烯等轻质气体在工业应用中发挥着重要作用，包括用作气体和液体燃料以及作为聚合物制造的前体。对于这些需要工业气体分离过程的应用，通常需要获得高纯度气体。还需要气体分离技术来降低大气中二氧化碳和其他温室气体的浓度。因此，开发低能耗、低成本的混合气体分离器对于满足工业需求、解决环境问题和提高生活水平至关重要。从混合物中分离气体分子需要材料(分子筛)含有与要分离的小气体分子大小相当的具有统一尺寸的孔。然而，合适的分子筛颗粒通常很难形成与工业应用或环境修复相关的规模所需的几何形状。该项目将推进将分子筛颗粒形成连接良好的网络的基础科学，这将使大规模分离技术的发展成为可能。在网络中，颗粒将通过聚合物界面结合在一起，旨在将对颗粒筛分功能的任何不利影响降至最低，同时保持材料的结构完整性。这种网络的分离性能、气体传输特性和结构特性将在所有相关的长度尺度上进行量化。该项目的成果将为合理设计分子筛形态奠定基础，分子筛形态可以优化以实现所需的气体分离。调查人员还将启动一项新的研究指导计划，目的是在STEM教授和培训来自代表性不足群体的学生。将利用现有的机构计划，并将强调使用在线工具来提高计划的有效性。金属-有机骨架(MOF)是一种高孔率的分子筛，应用于膜基气体分离时具有非凡的性能集。然而，这些材料不容易形成无缺陷的膜几何形状。因此，利用MOF固有的传输优势进行膜分离是一项具有挑战性的工作。将MOF晶体与聚合物混合制成混合基质膜(MMM)是一种众所周知的形成基于MOF的膜的策略。然而，MMM的典型分离性能通常低于相应的MOF，因为MMM的传输性质受到聚合物相的不利影响，在某些情况下，还受到MOF-聚合物界面缺陷的影响。提高MMM性能的一个明显途径是增加MOF浓度，甚至达到渗流阈值，以实现主要通过MOF相的扩散，即当MMM中的气体扩散可以主要在相互连接的MOF晶体上进行的情况下。研究人员将开发跨越MOF-聚合物MMM相的气体传输的逾渗阈值的基础科学，以使分离性能与纯MOF膜的分离性能相当。膜制备策略将基于MOF晶体外表面的一种新的功能化，并结合对膜内气体传输的基本了解的发展。先进的核磁共振将被用来研究MMM中所有相关类型的微观气体传输随着MOF负载和扩散长度尺度的增加而发生的变化。在提高膜的机械性能方面，MOF的表面功能化也将得到优化。该项目将促进与复合材料渗流理论相关的基本化学分离知识的发展。如果成功，这一概念将使纯MOF类传输特性能够在膜中获得，而不需要形成纯MOF膜。通过这种方式，可以实现MMM的新的性能限制，包括对不易形成结晶薄膜的MOF的渗流传输。作为一种设计平台，这种方法可以用来提高膜的化学分离的生产率和效率。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Light gases such as methane, ethane, and ethylene play an important role in industrial applications, including their use as gas and liquid fuels and as precursors in polymer manufacturing. Access to high purity gases is typically required for these applications necessitating an industrial gas separation process. Gas separation technology is also required to reduce the concentration of carbon dioxide and other greenhouse gases in the atmosphere. Accordingly, the development of low-energy and low-cost separations of gas mixtures is critically important to meet industrial demand, address environmental concerns, and improve standards of living. Separating gas molecules from a mixture requires materials (molecular sieves) containing holes with uniform dimensions that are comparable with the sizes of the small gas molecules to be separated. However, suitable molecular sieve particles are usually difficult to form into the geometries necessary for scales relevant to industrial applications or environmental remediation. This project will advance the fundamental science of forming molecular sieve particles into well-connected networks, which will enable the development of large-scale separation technologies. In the networks, the particles will be held together by polymer interfaces designed to minimize any adverse effects on the particle sieving function and, at the same time, preserve the structural integrity of the material. The separation performance, gas transport properties, and structural properties of such networks will be quantified on all relevant length scales. The outcomes of this project will lay the foundation for rationally designed molecular sieve morphologies that can be optimized for the desired gas separation. The investigators will also initiate a new research mentoring program with the objective of teaching and training students from underrepresented groups in STEM. Existing institutional programs will be leveraged, and the use of online tools will be emphasized to enhance the program's effectiveness.Metal-organic frameworks (MOFs) are high porosity molecular sieves exhibiting extraordinary property sets when applied in membrane-based gas separations. However, these materials cannot be easily formed into defect-free membrane geometries. Hence, it is challenging to leverage the intrinsic transport benefits of MOFs for membrane separations. Mixing MOF crystals with polymers to make mixed-matrix membranes (MMMs) is a well-known strategy to form MOF-based membranes. However, the typical separation performance of MMMs is usually lower than that of the corresponding MOFs because MMM transport properties are unfavorably affected by the polymer phase and, in some cases, by interfacial MOF–polymer defects. A clear route to improve MMM performance is to increase MOF concentrations and even reach a percolation threshold to enable diffusion predominantly through the MOF phase, i.e., a situation when gas diffusion in MMMs can proceed mostly over interconnected MOF crystals. The investigators will develop the fundamental science of crossing the percolation threshold for gas transport in the MOF phase of MOF–polymer MMMs to enable separation performance comparable with that of pure MOF membranes. Membrane fabrication strategies will be developed based on a novel functionalization of the external surface of MOF crystals in combination with the development of fundamental understanding of intramembrane gas transport. Advanced nuclear magnetic resonance will be used to investigate changes of all relevant types of microscopic gas transport in MMMs as a function of increasing MOF loading and the diffusion length scale. MOF surface functionalization will also be optimized with respect to enhancing the mechanical properties of the membranes. This project will lead to the development of fundamental chemical separations knowledge related to percolation theory in composites. If successful, this concept will enable pure MOF-like transport properties to be accessed in membranes without the requirement of forming pure MOF films. In this way, new performance limits may be achieved for MMMs, including percolated transport for MOFs that are not easily formed into crystalline films. As a design platform, this approach could be used to improve the productivity and efficiency of chemical separations for membranes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.