Ethanol/water separation characteristics of thermally rearranged polymers

This study examines the development of thermally rearranged (TR) polymers as membranes for ethanol dehydration. These TR polymers consist of polybenzoxazoles (PBOs) synthesized from aromatic polyimides with ortho positioned alcohols which are thermally rearranged in the solid state. This rearrangement leads to the development of a unique microstructure that gives the membrane excellent separation characteristics. The membrane separation performance can be tailorable depending on polymer chemistry and fabrication procedure. In general, these TR polymers have sufficiently high selectivity for water/ethanol mixture. They are also much more permeable towards water than polyimide membranes such as BPDA-ODA, which can lead to significant cost-saving in actual application. In addition, they can withstand the harsh environments of separation for extended periods of time because of the very stable PBO structure. No change in separation performance was observed after exposure to a gaseous mixture of 50:50 (w/w) water and ethanol, at 120°C and 3 bar for 2 weeks. In comparison, polyimide membranes show a significant decrease in water/ethanol selectivity within a week following the same exposure. It was discovered that the polyimides are subject to hydrolysis at the operation conditions during pervaporation and vapor permeation.