Zeolite Composite Membranes with a Nanoporous Fluorinated Carbonaceous Sheath for Organic Solvent Filtration

Inorganic membranes for organic solvent nanofiltration (OSN) inherently rely on the chemical affinity of pores, in particular, hydrophobicity of the pores and pore functionalities, to reject small solutes with a viable permeance level. This work reports a carbon-based composite membrane (F-ZCM) composed of zeolite grains amalgamating with fluorinated carbonaceous scaffolds in the form of a continuous and nanoporous sheath. The membrane is fabricated by pyrolysis of a compressed composite blend of zeolite with poly(ethyl methacrylate) (PEMA) and given amounts of Nafion-117 and poly(vinylidene fluoride) (PVDF). Both Nafion and PVDF leave behind a fluorinated carbonaceous scaffold sheath offering discrete dipoles at the sub-nanoscale (F-Cn, ≤35 wt % F or 1F/3C atom ratio ↔ sub-nanosplits of C–F dipoles), besides playing other assisting roles. F-ZCM presents a torturous foam matrix, in which micron-scaled voids aid in reducing the length of percolation barriers. The F-Cn sheath surrounding the micron voids plays a critical role in solute retention by deploying polarized patches for anchoring solute molecules, which weakens the solute–solvent association, as well as offering nanopores for scrubbing permeation streams. The membrane shows high rejection (>94%) for 50 mg/L methylene blue (MB, 320 Da) in isopropanol (IPA) and 50 mg/L brilliant blue G (BBG, 830 Da) in a mixture of ethyl acetate (EA)–IPA (50% v/v) with stable solvent permeance in the range of 2–4 L/h·m2·bar. This OSN capability, in contrast to the suspension adsorption control, is sustained by a mechanism analogous to the dynamic separation mechanism.