RATIONAL DESIGN, SYNTHESIS, CHARACTERIZATION AND OPTIMIZATION OF FACILITATED TRANSPORT MATERIALS IN NATURAL GAS AND PARAFFIN/OLEFIN SEPARATIONS

Abstract

Facilitated transport membranes (FTMs) have demonstrated the ability to simultaneously enhance permeability and selectivity, overcoming the traditional perm-selectivity trade-off observed in conventional polymer membranes. Despite showing high performance, metal ion carriers experience photo/chemical aging, which in turn, leads to a rapid decay in performance. Additionally, when embedded into polymer materials, structural defects may form at the metal-polymer interface that, despite accelerating small molecule transport, cause a parallel loss in selectivity. To address the issues mentioned above, we fabricated silver nanoparticles (NPs) stabilized using a series of poly(amic-acid)s (PAAs) derived from 4,4-oxydiphthalic anhydride and a polyetheramine, Jeffamine. Overall, this approach attempts to simultaneously i) achieve defect-free mixed matrix membranes (MMMs), ii) target CO2 and ethylene (C2H4) selective transport, and iii) reduce photo/chemical-induced degradation.As part of the particle design procedure, dynamic light scattering (DLS) is proposed as a more efficient alternative to traditional methods, such as viscometry measurements, to estimate the polymer solubility parameter (δ). In determining these values for the series of PAAs used throughout this work, justification and development of a liquid-liquid extraction process for producing silver NPs was established. The results show that DLS achieved similar δ values for an array of polymers, within 8%, to those found using viscometry measurements. The DLS method was further established by making a comparison to the group contribution method, for which we provide updated group contribution parameters, along with their uncertainty, according to the technique recently reported by Smith et al. These updated group contribution parameters result in a mean absolute relative error of 9.0% in predicting the solubility parameter on a test set of 40 polymers, which is on par with the average 10% error reported previously. The synthesized silver-PAA NPs, which were thoroughly characterized via TEM/EDS analysis, showed near spherical morphology (~5 nm) and lack of aggregation. The occurrence of the chelating reaction between silver and the PAA showed that favorable carbonyl-silver coordination interactions were achieved. The effect of the PAA length and ether functional group concentration on the structure and transport properties of the NPs were systematically investigated. The silver-PAA NPs were added into a commercial polymer, Pebax 1657, to fabricate defect-free MMMs for CO2/CH4 and C2H4/C2H6 separations, whose structure and transport properties were examined at various NP loadings (0-4 wt%). Remarkably, the inclusion of only 2.5 wt% NPs in Pebax enhanced CO2 and C2H4 permeability by 50% and 100%, respectively, while increasing both CO2/CH4 and C2H4/C2H6 selectivity between 80-100% relative to the neat polymer. A detailed analysis of the transport mechanism and underlying energetics, as well as changes in thermo-mechanical properties, was performed to elucidate the molecular origin of the observed membrane performance. This information was subsequently used to shorten the length of the PAA, in effort to minimize suppression of the glass transition temperature, i.e., increases in polymer chain mobility, as well as to prevent decreases of crystallinity at higher loadings of NPs, both of which negatively impact overall gas selectivity. Finally, the synthesized materials were exposed to hydrogen over 1 week to assess the long-term chemical stability. Each material exhibited drops between 10-30% in both CO2 and C2H4 permeability, as well as CO2/CH4 and C2H4/C2H6 selectivity. However, despite the observed decline in performance, each MMM retains properties superior to that of neat Pebax. In summary, this dissertation explores the design, synthesis, characterization, and optimization of novel facilitated transport membranes (FTMs) specifically tailored for natural gas and paraffin/olefin separations. The overarching goal of this work is to elucidate the fundamental mechanisms governing gas diffusivity, sorption, selectivity, and stability in these materials, providing insight into their potential use in advanced gas separation technologies.