Omniphobic, Bilayer Carbon Nanotube-Immobilized Fluorinated (FAS) Membranes for Bioethanol Recovery at High Concentrations via Membrane Distillation

Thermal distillation is the traditional method of choice for recovering ethanol from the diluted fermentation broth. However, this conventional approach incurs significant energy and capital expenses. While membrane distillation (MD) has shown promise in reducing costs, traditional MD membranes can only handle relatively low feed ethanol concentrations, which limits the range of ethanol purification. In this paper, we present the development of novel omniphobic membranes with antiwetting properties for separating relatively high concentrations of bioethanol via MD, which has not been achievable using conventional membranes. The bilayer membrane structure consists of a coating of carbon nanotubes (CNTs) with poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP) and a second layer comprising a fluorinated alkyl silane (FAS), namely, 1H,1H,2H,2H-Perfluorooctyltriethoxysilane. The FAS-coated carbon nanotube-immobilized membranes (FAS-CNIM) exhibited excellent omniphobic characteristics, with contact angles of 155° for water and 125° for 50% aqueous ethanol, which were 29 and 92.3% higher than those of a conventional poly(tetrafluoroethylene) (PTFE) membrane, respectively. The study evaluated ethanol vapor flux, separation factor, mass transfer coefficient, and permeate separation index, all of which were superior for FAS-CNIM. Ethanol flux reached as high as 5.53 kg/m²·h, and the separation factor reached 10.78. The FAS-CNIM successfully prevented membrane wetting up to 40% ethanol concentration with activated diffusion on the CNT surface playing a key role. The results show a remarkable 173.37% increase in ethanol enrichment and a 70% improvement in the separation factor compared to that of the control FAS membrane without the CNT coating.