Alternative Method of Crosslinking Poly(vinyl alcohol) for Hydrophilic Pervaporation Membrane Formation: Free Radical Initiation with Potassium Persulfate

Pervaporation with hydrophilic polymer membranes for ethanol dehydration is a promising alternative to conventional methods such as dehydration with molecular sieves. Poly(vinyl alcohol) (PVA) is the benchmark solvent dehydration membrane material. Standard methods of crosslinking PVA involve the use of di-aldehydes or di-acids, such as glutaraldehyde (GA), and thermal curing. The bonds formed during traditional PVA crosslinking typically include hemi-acetals, ethers, and esters, which are hydrolytically unstable, limiting the operating temperature and water concentration ranges of the resulting membranes. In order to minimize membrane area for pervaporation systems or to operate in vapor permeation mode, high temperature operation is necessary.

Crosslinking of PVA via carbon-carbon bond formation would greatly enhance the hydrolytic stability of PVA membranes. Potassium persulfate (KPS) has been shown to initiate free radical reactions in a variety of materials, including PVA. In this work, KPS was investigated as a crosslinking initiator in PVA pervaporation membrane formation. The pervaporation performance and stability of KPS-crosslinked membranes was compared to that of GA-crosslinked membranes. The KPS-crosslinked PVA membranes showed network formation with high thermal and chemical stability with desirable performance. Water molar permeability (P_H2O) was over 12×10⁻¹² kmol-m/m²-s-kPa (36,000 Barrer) and water-ethanol permselectivity α_H2O/EtOH was over 100. Continuous pervaporation operation for up to 40 days at 50 °C with a 20/80 w/w water/ethanol feed demonstrated the stability of the membrane under moderate conditions. A modest increase in water permeability and decrease in permselectivity was observed, likely due to the dissolution of potassium sulfate salt from the membrane. Hydrolytic stability was demonstrated by autoclaving the membranes in the 20/80 w/w water/ethanol feed mixture at 150 °C, followed by pervaporation testing. After autoclaving for 2 days, a KPS-crosslinked membrane (10/100 w/w KPS/PVA) exhibited a doubling of water permeability and 5-fold drop in permselectivity. By comparison, a GA-crosslinked membrane (10/100 w/w GA/PVA) exhibited a 27-fold increase in water permeability and a 55-fold decrease in water permselectivity after the same 2 day autoclave treatment.