Improving gas separation properties of polymeric membranes based on glassy polymers by gas phase fluorination

The application area of existing gas separation membranes is limited by commercially available polymers for their preparation. In many cases the separation selectivity of these polymers is not sufficient for effective separation processes. One of the ways to improve the separation effectivity of existing membranes from glassy polymers is gas phase fluorination. It is very important to note that this method can be successfully used for modification of polymeric films, different types of membranes (flat sheet and hollow fiber) and membrane modules. The hollow fibers produced from Matrimid 5218 in Twente University and a flat-sheet membrane from polyvinylthrimethilsilane (PVTMS) produced in Russia were investigated. The treatment of samples by fluorination was carried out in closed reactor at 20–22°C with gaseous mixtures of F2/He of different composition. A gas mixture of different concentrations (2–10 vol.% of F2) at 1 atm was used The time of fluorination was from 2 min to 265 min. The dependence of the structure of the PVTMS and Matrimid 5218 on gas phase fluorination by IR-spectroscopy and the influence of modification on membrane structure by SEM were studied. The productivity of He, CO2, N2 and CH4 through the membranes modified at different conditions of fluorination was measured, and it was shown that it is possible to achieve a significant increase of He/CH4, CO2/CH4 and He/N2 selectivity at high level He and CO2 productivity by this type of modification. Thus, modified membranes and membrane modules can be successfully used for separation of components of biogas in membrane contactors and selective membrane valves for extraction of He/Ne fraction from waste in the metallurgy industry and separation of natural gas components.

The Matrimid 5218 hollow-fiber module and PVTMS flat-sheet asymmetric and composite membranes were modified by gas phase fluorination using He/F2 gas mixtures. The fluorination mechanism of polymeric matrixes was studied by IR-spectroscopy. It was shown for Matrimid 5218 that there is substitution of hydrogen on fluorine, preferably in normal- and tret-positions, then in the sec-position. As to PVTMS, it was shown that the substitution process occurs practically simultaneously for hydrogen in the normal-, secand tret-positions. The influence of fluorination conditions on gas separation properties was studied and it was found that this way modification significantly increases selectivity of separation of CO2/CH4, He/CH4 and He/N2 gas mixture at a negligible decrease of permeability for He and CO2, i.e., improves the gas separation properties for hollow fibers from the Matrimid 5218 as for the flatsheet PVTMS composite membranes. PVTMS membranes can be successfully used for biogas separation, particularly in membrane contactors [10]. The results obtained allow significant improvement of the selectivity of PVTMS mem- branes which can be effectively used in the development of new active membrane systems.