Development of dual-layer membrane and metal organic framework for gas separation to improve indoor air quality

Chong, Kok Chung (2023) Development of dual-layer membrane and metal organic framework for gas separation to improve indoor air quality. PhD thesis, UTAR.

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Abstract

Humans spend up to 80% of their lifetime either at their workplaces or homes. The World Health Organization (WHO) report indicated that higher proportions of individuals associated with indoor environmental sickness (such as asthma and sick building syndrome) are children, the elderly, and people with chronic diseases. Enhancing indoor oxygen (O2) levels and decreasing indoor carbon dioxide (CO2) levels are ways to effectively decrease indoor air pollution and maintain optimal indoor air quality (IAQ). Traditional approaches for oxygen gas production, including pressure swing adsorption (PSA) and cryogenic distillation, are known for their high energy consumption. In contrast, membrane technology offers a more cost�effective alternative with reduced energy demands. However, the gas separation efficiency in terms of permeance and selectivity is still constrained by Robeson's upper limits. The conventional methods of CO2 removal from ambient air involve solvent adsorption, physical adsorption, and cryogenic fractionation. A promising recent development in this regard is the use of MOF due to their exceptional material properties and extensive surface area. By exploring the synergy between membrane technology and MOF, there is a potential to improve indoor air quality. This study aimed to enhance indoor air quality by separating O2 and CO2 from the ambient air using membrane technology with the following objectives. In this study, PSU or PEI membranes were fabricated by phase inversion methods. The membranes were dip coated by either PDMS or PEBAX to evaluate its performance in O2/N2 gas separation. The study results revealed that the pristine polysulfone (PSU-3) and polyetherimide (PEI-3) hollow fiber membranes exhibited superior permeance performance than the same materials at varying bore fluid flow rates. The polydimethylsiloxane (PDMS) coating layer in the dual-layer membrane indicated the coating could improve surface defects on the membrane. Moreover, it had a superior affinity with O2 and N2 gases, enabling better diffusion of these gases across the membrane. In this study, the most superior O2/N2 gas separation membrane was PSU membrane with 3wt% coating of PDMS (PSU-3PDMS). Two types of MOF were investigated, namely chromium (Cr) and copper (Cu) based MOF with either terephthalic acid (BDC) or trimesic acid (BTC) as organic ligand. The BET surface analysis revealed Cu-BTC MOFs generally showed a greater surface area (516 to 1,039 m2 /g) than other MOFs in this study. The physiochemical findings were coherent with the CO2 gas adsorption results. The Cu-BTC MOFs typically yielded a better CO2 adsorption performance relative to other MOFs, whereby Cu-BTC-1:1 recorded the most significant impact at 1.31 mmol/g. In summation, the PSU-3PDMS dual-layer hollow fiber membrane was found to be able to increase the O2 concentration up to 0.2% at 12 hours sampling time where the performance of the gas separation decreased attributed to the accumulation of particles and dust on the membrane surface. In the meantime, the Cu-BTC-1:1 MOF successfully reduced the CO2 concentration to 16 ppm. The results from this study suggested the possibility of integrating the dual-layer membrane with MOF to improve indoor air quality for the betterment of human life.

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