A major thrust within the CGS is to design new adsorbents tuned to efficiently carry out a given gas separation. Research in the Center focuses heavily on metal-organic framework (MOF) adsorbents, which are a relatively new class of porous materials composed of metal ions connected by organic linkers in three dimensions. An essentially infinite library of possible metal and ligand combinations and their porous nature has rendered MOFs promising for specifically tailored gas storage and separation applications, as these materials can be designed with widely varying pore shapes, sizes, and internal surfaces. The CGS is focusing on the design of MOFs with open metal cations for separation of N2 from O2 and CH4, pore shapes that are selective for various hydrocarbons, and appended amines for selective adsorption of CO2. Some of the materials being investigated include a particularly promising family of frameworks exhibiting open metal sites, namely M2(dobdc) (dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate, M = Zn, Mg) and its more recently discovered expanded pore analogue, M2(dobpdc) (dobpdc4− = 4,4'-dioxido-3,3'-biphenyldicarboxylate, M = Mg, Zn). The bare M2(dobdc) exhibits efficient separation of various gases, while rapid synthesis of M2(dobdc) nanocrystals as recently demonstrated by researchers within the CGS potentially opens new applications for this material. Most notably, appending N,N’-dimethylenediamine to the open metal sites in the expanded Mg2(dobpdc) results in a material with remarkably high selectivity and capacity for CO2 over N2, even in the presence of water. Importantly, this framework exhibits a sharp step in its CO2 adsorption isotherm that shifts dramatically to higher pressures with increasing temperature (Figure 1, top left). This step arises from an unprecedented cooperative CO2 adsorption mechanism (top right and bottom) that was recently elucidated through the collaborative effort of several groups within the CGS.
Membranes are another promising class of materials that exhibit selective chemical transport, and may be organic, inorganic, or hybrid in nature. Membranes are of great interest to replace energy-intensive distillation processes used in the petrochemical industry, such as in the separation of olefin / paraffin mixtures, and may offer a more energetically favorable means of carrying out certain gas separations. However, wide-spread implementation of membranes is currently limited by factors such as poor processability, selectivity, and permeability. The CGS is seeking to capitalize on the selectivity for various gases exhibited by MOFs and the transport properties of polymer membranes to produce composite, or mixed matrix, membranes with exceptional separation performance. Researchers in the CGS have demonstrated that polymer membranes incorporating the well-known M2(dobdc) exhibit enhanced selectivity for ethylene over ethane and improved resistance to plasticization compared to the pure polymers (Figure 2). Progress is also being made in the development of pure MOF membranes through templated growth mechanisms.
To understand the separation performance of new materials, it is essential to develop experimental techniques that can provide insight into their relevant molecular properties and how these influence gas adsorption behavior. The CGS is currently pioneering new methods for characterizing gas adsorption in situ, including gas cells for single-crystal X-ray diffraction (Figure 3) and X-ray absorption spectroscopy, which can give valuable information regarding material structure and electronic state upon gas binding, respectively. Solid state Nuclear Magnetic Resonance spectroscopy is also a valuable tool for studying adsorbate behavior, and research within the Center has given elucidated unique phase dynamics within MOFs wherein the critical point of a liquid can be decreased simply by changing framework structure.
Computational guidance is an invaluable asset given the overwhelming number of possible materials that may be synthesized and evaluated for gas separations. In addition to serving as a valuable tool to corroborate experimental results, computational efforts within the Center are geared toward developing a detailed microscopic understanding of the structure, electronic, and magnetic properties of MOFs, and in particular how these properties give rise to gaseous adsorption. Center researchers use novel density functional theory (DFT), quantum mechanical, and statistical mechanical methods in order to predict the performance of materials in various separation and capture processes[11, 12] and to carry out large-scale screening calculations, for instance to determine the best materials to separate xenon from krypton gas. For example, a new DFT-based approach has been used to compute binding energies for several small molecules within the M2(dobdc) family. The gases investigated include some of the major and minor components of power plant exhaust streams (flue gas), and the results corroborated experimental results while also identifying new framework/gas pairings for advantageous separations.
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