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


The work presented in this thesis includes the design, synthesis, and characterization

of polymer composite membranes and metal–organic framework adsorbents for

application in industrially relevant gas separations. Specifically, these materials were

designed for the purification of olefins in various processes, and for the removal of CO2

from natural gas. A wide variety of techniques and characterization methods are covered,

including gas permeation, gas adsorption, powder X-ray diffraction, transient

breakthrough, electron microscopy and dynamic light scattering.

Chapter 1 provides a brief introduction to membranes and metal–organic frameworks

for gas separations, including their limitations and opportunities. The potential for metal–

organic framework/polymer composites in ethylene/ethane and CO2/methane separations

is discussed. Additionally, the promise of using metal–organic frameworks in adsorptive

separations for purifying ethylene and propylene, and the importance of these gas

separations, is highlighted.

Chapter 2 describes a method for improving the gas separation performance for

membranes in a broadly applicable and synergistic way. Specifically, it addresses two

fundamental problems with conducting gas separations in neat polymer materials, namely

selectivity/permeability tradeoff as well as membrane plasticization. These two

phenomena limit the application of membrane materials in numerous applications, and

one severe case is the separation of ethylene from ethane. As the condensability and

kinetic diameter of these gases are similar, there is no good chemical handle to conduct a

separation. Introducing adsorption-based selectivity by compositing nanocrystals of the

metal–organic frameworks M2(dobdc) (M = Mg, Mn, Co, Ni; dobdc4– = 2,5-dioxido-1,4-

benzenedicarboxylate) with a the polyimide 6FDA-DAM (6FDA = 4,4ʹ-

(hexaflouroisopropylidene)diphthalic anhydride; DAM = 2,4,6-trimethyl-1,3-

phenylenediamine), a unique chemical handle is used to improve the permeability of

ethylene and the selectivity for ethylene over ethane. Additionally, we found that the

metal–organic frameworks physically crosslink the polymer, which addresses

plasticization, the second major issue with polymers for this gas separation.

Chapter 3 expands upon the work highlighted in the previous chapter, by applying the

crosslinking effects of the metal–organic frameworks to the purification of CO2 from

natural gas, another industrially relevant gas separation. In this case, nanocrystals of the

metal–organic framework Ni2(dobdc) is incorporated into six different polymers that can

be used for this gas separation. This resulted in significantly improved plasticization

response in the majority of materials, as measured by CO2 permeation hysteresis and

binary gas permeation measurements. Notably, high selectivity is retained under

pressures of CO2 in excess of 20 bar. These pressures represent some of the most

aggressive feed environments that might be encountered in the field.

Chapter 4 mainly focuses on adsorptive-based separations, using metal–organic

frameworks to purify olefins from paraffins. Using single-component and

multicomponent equilibrium gas adsorption measurements, we have shown that M2(mdobdc)

has superior performance for the physisorptive olefin/paraffin separation relative

to any adsorbent, including its para-functionalized structural isomer, M2(p-dobdc) (pdobdc4–

= 2,5-dioxido-1,4-benzenedicarboxylate). Notably, M2(m-dobdc) exhibits

increased affinity for olefins over paraffins relative to their corresponding structural

isomers, with the Fe, Co, and Ni variants showing more than double the selectivity.

Among these frameworks, Fe2(m-dobdc) displays the highest ethylene/ethane (> 25) and

propylene/propane (> 55) selectivity under relevant conditions with olefin capacities

exceeding 7 mmol/g. Through this work, we show that the excellent olefin/paraffin

selectivity, high olefin capacity, rapid adsorption kinetics, and low raw materials cost

make M2(m-dobdc) the material of choice for adsorptive olefin/paraffin separation.

In Chapter 5, the metal–organic frameworks M2(m-dobdc) are employed to purify

ethylene from the broad mixture of products encountered in the effluent from an

oxidative coupling of methane process, including ethane, CO, CO2, CH4, and H2.

Through variable temperature, single-component equilibrium adsorption experiments, we

show that each metal can accomplish ethylene purification at varying effectiveness.

While all variants show the ability to purify ethylene from ethane, CO2, and CH4, only

Mn2(m-dobdc) and Fe2(m-dobdc) can effectively conduct an ethylene/CO separation,

while Mn2(m-dobdc) is most selective by an order of magnitude.

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