Investigation of vacuum-assisted preparation methods of inorganic membranes

Abstract

This research project systematically investigated the pore size tailoring of inorganic derived membranes by a novel vacuum-assisted preparation method. In particular, the effect of vacuum exposure time on the morphological features of the as-prepared membranes was investigated. Phenolic resin and titanium (IV) propoxide (TTP) precursors were chosen for the synthesis of carbon and titania membranes, respectively. The innovation in this study is that the preparation method is based on a combined dip-coating and vacuum-assisted method which has not been explored in literature. The pore size of the membranes was tailored by varying the vacuum times during preparation (0-1200s). The membranes were characterised by measuring the flux for a variety of organic substances, each having a different molecular weight cut-off. The first major contribution of this work is the effective preparation of carbon and titania membranes via the vacuum-assisted method. This has not been achieved before in the literature. The significant structural changes that occurred under different vacuum time exposures correlated very well with water permeation results. Water permeation through the carbon membranes increased proportionally to vacuum time, with the highest flux of 169 L m-2 h-1 observed at a transmembrane pressure of 5 bar and a vacuum time of 1200s. This was 12 times larger than the carbon membrane without vacuum exposure. Similar trends were also observed for titania membranes, with the water flux 37% higher for the membranes prepared for 1200 s as compared to no vacuum exposure. The membranes were also able to achieve rejections close to 100% for large MW PVP-360000 organics. However, no significant separation was observed for the lower MW glucose and sucrose. Separation values were measured for PVP-40000, and the carbon membranes delivered higher rejection than the titania membranes. The results suggest that the effect of vacuum time on pore size was stronger for the carbon membranes compared to the titania membranes. The second contribution of this thesis is related to the proposed mechanism of formation of the carbon membranes derived from the phenolic resins. It was found that the vacuum-assisted method imparted different morphologies and even influenced the chemistry of the carbon membranes. Specifically, resin crosslinking reactions were promoted as a function of the vacuum time. Further, pore volumes and surface areas increased from 0.81 cm3 g-1 and 834 m2 g-1 to 2.2 cm3 g-1 and 1910 m2 g-1 as the respective vacuum exposure time increased from 0 to 1200 s. This work postulates that a cluster to cluster aggregation mechanism brought the phenolic oligomers in close proximity, initiating polycondensation and the formation of micro porous regions. Concomitantly, the distances between the clusters increased creating meso and macroporous regions. This results in increased water fluxes due to a reduction of mass transfer resistance as pore volume increased as a function of the vacuum time. The third contribution of this thesis is related to the structural formation of the titania membranes by the vacuum-assisted method. The aggregation of Ti-O-Ti networks to form titania nanoparticles increased in proportion to vacuum exposure time. Further, the thesis proposes that upon calcination the titania crystallite size increased as a function of the vacuum time, accompanied by an increase in particle size. Therefore, porosity was controlled by the inter-particle void, which is a function of the particle size. As the particle size of titania increased, the water flux of the titania membranes also increased. These results strongly suggest that the vacuum exposure time played an important role in changing the particle size of the titania. This in turn is theorised to alter the inter-particle voids, thus affecting the membrane mass resistance to water transport. These carbon and titania membranes were also studied for oil-water separation containing a high concentration of oil (3000 ppm). The discharge of oily wastewaters to the environment is a serious global concern. Similar to the MW cut-off study of organic substances, the pore size of carbon or titania membranes played an important role in terms of water flux and oil rejection by the vacuum-assisted method. Carbon membranes delivered superior oil rejection up to 99.9%, whilst titania membranes rejection values were lower at 93%. The results in this thesis showed that both membranes were seriously fouled by oil, particularly the carbon membranes. Chemical cleaning of the surface of both carbon and titania membranes were carried out over 9 cleaning cycles using sodium hydroxide and citric acid. It was found that chemical cleaning allowed the recovery of water fluxes to some extent, though the structure of the carbon membranes seemed to be affected by chemical cleaning due to changes in both water fluxes and oil rejection.