High performance polymer membranes for CO₂ removal and water treatment

Ahmad A. Shamsabadi

doi:10.17918/pswz-0m90

To decrease the current level of CO₂ emissions appreciably, more efficient CO₂-capture technologies are needed. Absorption is currently used widely to lower CO₂ level in gases. However, it is relatively expensive (in terms of both operating and capital costs), bulky, and challenging to operate (foaming, entrainment and flooding of the absorber). Compared to absorption, the membrane technology is more energy-efficient, has lower operating costs, is compact and modular, and has a simple process. Among membranes, polymer of intrinsic microporosity (PIM) membranes with high free-volume created by rigid moieties have superior gas-transport properties. However, presently these membranes suffer from two fundamental and industrially-important problems of low-to-modest selectivity and fast physical aging. Among PIMs, pentiptycene-based membranes exhibit satisfactory selectivity, and excellent resistance against plasticization and physical aging. However, the presence of two ethereal oxygens in the backbone of these membranes causes efficient chain packing, resulting in a decrease in free volume and consequently gas permeability. Pentiptycene is a highly rigid moiety from iptycene family consisting of five benzene rings forming an H-shaped core block. A fractional free volume (FFV) of less than 0.2 has been reported for existing pentiptycene-based membranes. To enhance the FFV of pentiptycene-based membranes, in this work: for the first time a pentiptycene-based dianhydride containing two etheric oxygens was synthesized and characterized; and a high molecular-weight pentiptycene-based polyimide was synthesized and characterized. The polyimide showed remarkable thermal stability, excellent solubility in common organic solvents, and the highest CO₂ permeability (812 barrer) among all reported pentiptycene-containing polymers (about 6 times higher than the most permeable one) without sacrificing the selectivity. Currently polymer membranes suffer from the trade-off relationship between the permeability and selectivity. The fabrication of membranes made from polymers embedded with nanomaterials can lead to mixed matrix membranes (MMMs) with high gas permeability and excellent sieving ability. However, the agglomeration of nanomaterials, the formation of interfacial defects, and the incompatibility between the polymer and nanomaterials are still challenges in the fabrication of MMMs. In this research, these fundamental problems were studied through engineering the dispersion of the nanoparticles in polymer matrices via nanoparticles functionalization and considering polymer chemistry. TiO₂ nanoparticles were grafted using 3-aminopropyl-diethoxymethylsilane [a silane coupling agent (AS)], their surface was then modified using carboxymethyl chitosan (CMC), and, the modified TiO₂ nanoparticles were incorporated into the Pebax as the polymer matrix. The Pebax-modified-TiO₂ nanocomposite membranes showed improved CO₂ permeability and CO₂/N₂ selectivity, compared to the pristine Pebax. Characterization techniques showed good compatibility of the nanoparticles and the polymer and satisfactory dispersion of the nanofillers into the polymer. The influence of cyanuric chloride (CC) nanoparticles and its derivatives, melamine (M) and 2, 4, 6-trihydazino-1, 3, 5-triazine (THDT), on the performance and characteristics of a polyurethane (PU)-based nanocomposite membranes was investigated. Characterization results showed that the CC nanoparticles were mostly distributed in the PU hard segments. Also, M and THDT nanoparticles were scattered in the PU soft segments. Incorporating organic nanoparticles with many amine groups (i.e., M and THDT) showed to be an appealing strategy to fabricate highly selective membranes. This work indicated how robust interfacial interactions between nanoparticles and a polymer improve the polymer/nanoparticles compatibility and lower the level of defects in CO₂-separating membranes. Increasing daily demand for clean water and limited fresh water resources has motivated studies on sustainable water furification. The membrane technology is suitable for water and wastewater treatment owning to its compactness, simplicity, small footprints, and sustainability. Among membrane-based processes, forward osmosis (FO) is one of the most promising methods having high water recovery, hydraulic pressure independency, and low-cost. However, fouling and biofouling of thin-film composite (TFC) FO membranes are still major challenges. An attractive strategy to improve fouling-resistance of FO membranes is the addition of antibacterial nanomaterials to the active layer of a TFC membrane. We studied the primary antibacterial mode-of-action (MoA) of Ti₃C₂T_x MXene nanosheets against both Gram-negative and Gram-positive bacteria. To characterize the antibacterial properties of Ti₃C₂T_x MXene in a well-quantitative manner, we used the flow cytometry (FC) and fluorescence imaging (FI) techniques. Our results indicated that the antibacterial activity of the MXene nanosheets are both size- and time-dependent; that is, smaller sizes of the nanosheets result in the release of bacteria cytoplasmic DNA and eventually dispersion of the bacteria envelope. In another work, we synthesized and characterized a silver-based metal-organic framework (Ag-MOF) embellished with graphene oxide (GO) [GO-Ag-MOF] with potent antibacterial activity. A thin-film nanocomposite (TFN) membrane with superior anti-biofouling and good antifouling properties was fabricated the GO-Ag-MOF nanocomposite to the selective layer of a TFC membrane. In addition to membrane characterization, fouling and antifouling resistance of the membrane was assessed through FO experiments in the presence of sodium alginate and E.coli and in the feed solution, respectively.

High performance polymer membranes for CO₂ removal and water treatment

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High performance polymer membranes for CO₂ removal and water treatment

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