Polymer Composite Membranes and Metal–Organic Framework Adsorbents for Industrial Gas Separations

The work presented in this thesis includes the design, synthesis, and characterization/nof polymer composite membranes and metal–organic framework adsorbents for/napplication in industrially relevant gas separations. Specifically, these materials were/ndesigned for the purification of olefins in various processes, and for the removal of CO2/nfrom natural gas. A wide variety of techniques and characterization methods are covered,/nincluding gas permeation, gas adsorption, powder X-ray diffraction, transient/nbreakthrough, electron microscopy and dynamic light scattering./nChapter 1 provides a brief introduction to membranes and metal–organic frameworks/nfor gas separations, including their limitations and opportunities. The potential for metal–/norganic framework/polymer composites in ethylene/ethane and CO2/methane separations/nis discussed. Additionally, the promise of using metal–organic frameworks in adsorptive/nseparations for purifying ethylene and propylene, and the importance of these gas/nseparations, is highlighted./nChapter 2 describes a method for improving the gas separation performance for/nmembranes in a broadly applicable and synergistic way. Specifically, it addresses two/nfundamental problems with conducting gas separations in neat polymer materials, namely/nselectivity/permeability tradeoff as well as membrane plasticization. These two/nphenomena limit the application of membrane materials in numerous applications, and/none severe case is the separation of ethylene from ethane. As the condensability and/nkinetic diameter of these gases are similar, there is no good chemical handle to conduct a/nseparation. Introducing adsorption-based selectivity by compositing nanocrystals of the/nmetal–organic frameworks M2(dobdc) (M = Mg, Mn, Co, Ni; dobdc4– = 2,5-dioxido-1,4-/nbenzenedicarboxylate) with a the polyimide 6FDA-DAM (6FDA = 4,4ʹ-/n(hexaflouroisopropylidene)diphthalic anhydride; DAM = 2,4,6-trimethyl-1,3-/nphenylenediamine), a unique chemical handle is used to improve the permeability of/nethylene and the selectivity for ethylene over ethane. Additionally, we found that the/nmetal–organic frameworks physically crosslink the polymer, which addresses/nplasticization, the second major issue with polymers for this gas separation./nChapter 3 expands upon the work highlighted in the previous chapter, by applying the/ncrosslinking effects of the metal–organic frameworks to the purification of CO2 from/nnatural gas, another industrially relevant gas separation. In this case, nanocrystals of the/nmetal–organic framework Ni2(dobdc) is incorporated into six different polymers that can/nbe used for this gas separation. This resulted in significantly improved plasticization/nresponse in the majority of materials, as measured by CO2 permeation hysteresis and/nbinary gas permeation measurements. Notably, high selectivity is retained under/npressures of CO2 in excess of 20 bar. These pressures represent some of the most/naggressive feed environments that might be encountered in the field./nChapter 4 mainly focuses on adsorptive-based separations, using metal–organic/nframeworks to purify olefins from paraffins. Using single-component and/nmulticomponent equilibrium gas adsorption measurements, we have shown that M2(mdobdc)/nhas superior performance for the physisorptive olefin/paraffin separation relative/nto any adsorbent, including its para-functionalized structural isomer, M2(p-dobdc) (pdobdc4–/n= 2,5-dioxido-1,4-benzenedicarboxylate). Notably, M2(m-dobdc) exhibits/nincreased affinity for olefins over paraffins relative to their corresponding structural/nisomers, with the Fe, Co, and Ni variants showing more than double the selectivity./nAmong these frameworks, Fe2(m-dobdc) displays the highest ethylene/ethane (> 25) and/npropylene/propane (> 55) selectivity under relevant conditions with olefin capacities/nexceeding 7 mmol/g. Through this work, we show that the excellent olefin/paraffin/nselectivity, high olefin capacity, rapid adsorption kinetics, and low raw materials cost/nmake M2(m-dobdc) the material of choice for adsorptive olefin/paraffin separation./nIn Chapter 5, the metal–organic frameworks M2(m-dobdc) are employed to purify/nethylene from the broad mixture of products encountered in the effluent from an/noxidative coupling of methane process, including ethane, CO, CO2, CH4, and H2./nThrough variable temperature, single-component equilibrium adsorption experiments, we/nshow that each metal can accomplish ethylene purification at varying effectiveness./nWhile all variants show the ability to purify ethylene from ethane, CO2, and CH4, only/nMn2(m-dobdc) and Fe2(m-dobdc) can effectively conduct an ethylene/CO separation,/nwhile Mn2(m-dobdc) is most selective by an order of magnitude.

Advisor: Long, Jeffrey R