Polymer Composite Membranes and Metal–Organic Framework Adsorbents for Industrial Gas Separations
- Author(s): Bachman, Jonathan Enoch
- Advisor(s): Long, Jeffrey R
- et al.
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.