Development of poly (ether ether ketone) nanofiltration membranes for organic solvent nanofiltration in continuous flow systems
Creators: name: Da Silva Burgal, Joao Porfirio
Publisher: name: Imperial College London
Year: 2016

description: Organic solvent nanofiltration (OSN) is an energy saving technology that can replace more energy demanding separation technologies, such as evaporation and distillation. Nevertheless, OSN membranes that can withstand high temperature conditions as well as acidic or basic conditions are lacking on the market. In this thesis a poly(ether ether ketone) (PEEK) membrane is investigated for its suitability for OSN applications using polar aprotic solvents, such as DMF and THF, high temperatures, and basic/acidic conditions. By studying four grades of PEEK polymer powder from two different brands (VESTAKEEP® and VICTREX®), the VESTAKEEP® 4000P was selected for the subsequent studies. The post-phase inversion drying process of membrane fabrication was also studied and the drying step was shown to be crucial in obtaining separation performance in the nanofiltration (NF) range. The degree of sulphonation (DS) was also important and had to be maintained at low levels in order to retain the chemical and thermal stability of PEEK membranes. Subsequently, the scaling-up of PEEK membranes to spiral-wound modules was successfully achieved. In order to further manipulate the performance of PEEK NF membranes, two ways of controlling the molecular weight cut-off (MWCO) of PEEK membranes prepared via phase inversion and subsequent drying were studied. The two methods explored were the change of polymer concentration in the dope solution – 8 wt. %, 10 wt. % and 12 wt. % - and the variation of solvent filling the pores prior to drying – e.g. water, methanol, acetone, tetrahydrofuran and n-heptane. For each solvent, the drying temperature was proved to have an effect on the membrane performance - the higher the drying temperature, the higher the rejection and the lower the permeance. Following the drying treatment results, the negligible aging of PEEK membranes was demonstrated; a comparison with crosslinked polybenzimidazole (PBI) and polyimide (PI) membranes was also performed. The results showed a structural change for PBI and PI membranes due to a non-equilibrium glassy state, in contrast with PEEK membranes which were in quasi-equilibrium glassy state. High temperature filtrations were also performed in DMF up to 140 °C for the three polymeric membranes. PEEK was the most robust membrane with a stable performance after 4 filtration cycles whereas PBI and PI were stable for 2 and 1 cycles respectively. Due to their stability at high temperatures, and also their compatibility with catalysts, PEEK membranes were used in two different continuous Heck coupling reactions combined with OSN separation of the catalyst in situ. Two reactor configurations were investigated: a continuous single stirred tank reactor/membrane separator (m-CSTR); and a plug flow reactor (PFR) followed by m-CSTR (PFR-m-CSTR). It was possible to decrease the catalyst leaching to the product stream and to increase the overall turnover number (TON) of the Heck reactions.