Membrane development for organic solvent nanofiltration: A sustainable solution to chemical separation and purification

Abstract

Chemical separation is an integral part of the pharmaceuticals, food, petrochemicals, and fine chemicals industries for purifying and producing high-value products, extracting catalysts, and recycling organic solvents. These industries depend on conventional distillation processes for chemical separation, which are energy-intensive, costly, and generate millions of tons of CO2 annually. Membrane-based separation, particularly organic solvent nanofiltration (OSN), has emerged as an alternative low-energy separation process for separating solutes in organic media. However, current OSN membranes face challenges such as limited solvent stability, poor rejection of low molecular-weight solutes, and trade-offs between permeability and selectivity. This research was designed to address these issues through systematic design, fabrication, and modification of polybenzimidazole (PBI)-based membranes, utilizing chemical crosslinking, interfacial engineering, and supramolecular functionalization. PBI was selected as the membrane-fabrication polymer because of its exceptional thermal stability and mechanical strength, making it an ideal candidate for large-scale fabrication and deployment of OSN technology. Pristine PBI membranes could be used for separation in both polar and non-polar solvents, but they are not resistant to polar aprotic solvents, which hinder its large-scale application. Therefore, proper post-treatment modifications are required to make the membrane chemically resistant for a wide range of OSN applications.

A series of PBI membranes was fabricated and crosslinked using covalent and non-covalent techniques to improve their chemical stability and selective transport behavior. In Chapter 3, interfacial oxidative polymerization of polydopamine (PDA) combined with covalent crosslinking using α,α′-dibromo-p-xylene (DBX) yielded composite membranes with excellent solvent stability. The membrane showed excellent rejection efficiency and solute fractionation while maintaining competitive permeance to organic solvents. Following the successful completion of the first study, Chapter 4 investigated the impact of chemical crosslinker chemistry on membrane performance. Two different crosslinkers with aromatic (dichloro-p-xylene (DCX)) and aliphatic (1,4-dibromobutane (DBB)) structures were used to control the interchain spacing of the PBI polymer and enhance its solvent stability. It was revealed that aromatic crosslinkers led to higher degrees of crosslinking and improved chemical resistance to strong aprotic solvents (e.g., NMP and DMAc), while aliphatic crosslinkers resulted in membranes with larger intersegmental spacing and lower chemical resistance. Subsequently, integrating with sulfocalix[4]arene (SCA4) host molecules created a framework of dual crosslinking and ionic interactions, significantly tightening the polymer network while maintaining a sharp pore size cut-off without compromising the membrane’s solvent permeance.

Building upon these insights, in Chapter 5, a chemically stable PBI membrane was reported by covalent crosslinking with a novel crosslinker, triglycidyl isocyanurate (TGIC). The results revealed that the epoxide groups in TGIC underwent nucleophilic ring-opening reactions with the secondary amine groups of PBI, forming a three-dimensional polymer network with exceptional chemical resistance to both organic solvents and extreme pH conditions (pH 1 & pH 14). The membranes showed higher rejection of organic dyes with robust permeance to organic solvents and water, surpassing the available membranes. The reported membrane also demonstrated selective fractionation of mixed solutes, underscoring its potential for industrial separation processes.

Overall, this thesis provides the structure-property-performance relationships of chemically modified PBI membranes involving a combination of covalent crosslinking, supramolecular assembly, and interfacial modification. This approach improves the chemical stability of PBI membranes and finely tunes selectivity and permeability at the molecular level. The findings demonstrate that PBI-based membranes can achieve the stability and performance required for industrial organic solvent separations, enabling their use for solvent recovery and fine chemical purification in large-scale industrial applications.