Carbon molecular sieve membranes with integrally skinned asymmetric structure for organic solvent nanofiltration (OSN) and organic solvent reverse osmosis (OSRO)

Organic solvent nanofiltration is an energy-efficient separation process for solutes and solvents. Robust membranes that show stability in harsh environments are highly desirable. In this study, we developed free-standing integrally skinned asymmetric carbon molecular sieve membranes that synergize the advantages of stable carbon materials and porous polymer membranes. The membranes were prepared using a polyimide of intrinsic microporosity (PIM), known as 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)-3,3′-dimethylnaphthidine (DMN), via a phase inversion technique. Carbonization preserved the surface porosity and finger-like porous morphology of the membranes after structural rearrangement. The effects of pyrolysis temperature, membrane thickness, dope solution concentration, and polymer porosity on separation performance were investigated. The molecular sieving performance of the membranes was investigated using five solvents with different polarities. The membranes showed no swelling and high stability in strong acids, bases, and organic solvents, as well as an excellent rejection profile and reasonable permeance. The membrane pore size, molecular-weight cutoff, and performance were fine-tuned by controlling the pyrolysis temperature, dope solution concentration, and polymer porosity. The polymer porosity and the asymmetric structure of the membrane strongly affected the molecular sieving performance. Carbonizing porous 6FDA-DMN afforded a 10-fold higher permeance than that obtained by carbonizing nonporous 6FDA-m-phenylenediamine (mPDA). To the best of our knowledge, the developed membrane fabrication platform yielded one of the tightest, most robust, and highly solvent-resistant nanofiltration membranes reported thus far.