Solid-state crosslinking of thin film composite membranes for organic solvent reverse osmosis separations

Organic solvent separations are energy intensive yet ubiquitous unit operations in the chemical, petrochemical, and pharmaceutical industries. Membrane technology can significantly reduce the demands for energy, carbon, and water in these operations. However, the separation of similar molecules is often required, which is a difficult challenge for membranes. Some polymers used in organic solvent reverse osmosis (OSRO) have been known to suffer from plasticization and dilation, making them inefficient for these molecular separations. Here we describe a method to address this problem by uniformly crosslinking a selective layer of a thin film composite (TFC) membrane to improve stability in organic solvents and tune separation performance. Crosslinking agents hexamethylene diamine, p-xylylene diamine, and 4,4′-biphenyldicarbontrile diamine, protected as N-tert-butyloxycarbonyl (Boc) carbamates, were introduced to the commercial polymer Matrimid® 5218. Boc protection rendered them sufficiently soluble in the polymer matrix to obviate costly liquid-phase crosslinker infusion steps. After casting, the amines within the membrane were thermally deprotected to induce a solid-state crosslinking reaction. The resulting crosslinked TFC membranes showed excellent stability in organic solvents, separation performance with binary (toluene/triisopropylbenzene) and 9-component organic solvent mixtures, and tunable separation characteristics based on the nature and concentration of the crosslinking agent.