Membranes with improved performance in solvent resistant nanofiltration

Solvent resistant nanofiltration (SRNF) involves the separation of an organic mixture down to a molecular level by simply applying a pressure gradient over a membrane. Since this process is energy- and waste-efficient, it has the potential to compete with other less environmentally friendly processes such as solvent extraction and distillation, which are now abundantly used in the (petro)chemical, food and pharmaceutical industries. However, to turn SRNF into a viable industrial process, excellent membranes should become available, combining chemical, mechanical and thermal stability with good rejections and sufficiently high fluxes.This work focuses on the improvement of SRNF membranes and membrane processes, more specifically aiming at enhanced membrane fluxes. The research is based on three different concepts: (i) chemical crosslinking to increase the membrane stability, (ii) MMMs containing hollow selective fillers to increase both fluxes and selectivities and (iii) membranes that can be activated via a photo-thermal process to increase fluxes.Polyimide-based membranes are resistant to a broad range of solvents, but dissolve in aprotic solvents such as N,N -dimethylformamide and N-methylpyrrolidon, which are common polymer solvents. The chemical cross-linking of asymmetric polyimide membranes by a post-synthesis reaction with p-xylylenediamine in a methanol environment is studied, aiming at an improved membrane stability in these solvents. The influence of this straightforward post-treatment on membrane stability, morphology and performance in aprotic solvent based feeds is investigated. The crosslinking treatment is also performed on nano-zeolite filled polyimide membranes. Next to crosslinking as a post-treatment, another method is explored wherein the crosslinking reaction is combined with the phase inversion process by adding the crosslinker to the aqueous coagulation bath. This eliminates the need of an extra methanol-based production step. Membranes prepared by these two methods are thoroughly characterized and the crosslinking efficiency of the methods is compared for reactions between different types of diamines and polyimides. The main parameters influencing the rate of crosslinking for each method are also determined.In addition to increasing the chemical stability of SRNF membranes, enhancing their permeability is another critical issue. The crosslinked polyimide membranes are further tested as support for zeolite-filled polydimethylsiloxane (PDMS) membranes to be used in solvents such as toluene en ethyl acetate. While PDMS usually swells in these solvents, drastically reducing the membrane selectivity, the zeolites reinforce the polymer matrix and reduce the swelling. They also introduce a higher selectivity due to their molecular sieving abilities. As a new concept, aiming at further increasing solvent fluxes, micron-sized hollow spheres with zeolitic shell are used as inorganic fillers in the PDMS mixed matrix membranes. The membranes are characterized and tested in SRNF and their performance iscompared to membranes filled with more conventional zeolite crystals.Finally, a completely new strategy is presented to achieve local heating of a membrane during a separation process, in order to realize a flux increase without affecting the membrane selectivity. By incorporating gold nanoparticles into the membrane structure, it is possible to heat the membrane during the separation process by light irradiation, applying the principle of photo-thermal heating. To prove this concept, gold nanoparticles are generated inside a pre-made cellulose acetate membrane by chemical reduction of gold salts. The effect of the photo-thermal heating by the gold nanoparticles on the membrane temperature, flux and selectivity is investigated by irradiation with a continuous wave laser in a specially adapted filtration cell. As for polyimide membranes, good selectivities for very small compounds are often combined with very low fluxes, the photo-thermal heating method may specifically be a solution for such membranes. To prepare polyimide membranes containing gold nanoparticles, two different synthesis methods are explored: one via in-situ reduction of a gold salt in the membrane matrix and another via the use of preformed polymer-protected gold nanoparticles. The influence of the synthesis method on the nanoparticle size, dispersion and surface plasmon behavior is investigated, as well as the influence on the membrane structure and performance. The effect of the concentration of the gold nanoparticles and the laser intensity on the photo-thermal heating effect, the temperature and the permeance behavior of the membrane are investigated for both methods.status: publishe