Time-dependent growth of ceramic supported NaA membranes : a morphological and permeation based study
Based on its ideal aperture size (4.1 A) and hydrophilic framework, the NaA membrane possesses significant potential in the separation of many industrially important gaseous and liquid mixtures. In the local South African context, the foreseeable production of affordable, high-purity ethanol in the alternative fuel market exemplifies one such a possibility. However, there are still certain aspects to the composite NaA membrane that are not clearly understood. These include the time-dependent morphological and compositional development of the polycrystalline zeolite layer during its direct synthesis from a clear solution, and the subsequent relation between the intrinsically different layers thus obtained, and their (selective) permeation properties. In addition, the surface-chemical and structural influence of the support on membrane integrity and permeation resistance, respectively, requires further elucidation for the optimisation of selectivity and flux parameters. This project envisaged addressing these needs. Using a standard clear solution synthesis regime (Na2O:AI2O3:SiO2:H2O = 49:1:5:980; 85oC) and high-integrity a-AI2O3 supports, the aim was to improve the fundamental understanding of the composite NaA membrane as a whole, including structural and permeation related aspects, under the auspices of optimising and broadening the application potential of supported zeolite membranes in general. Layer development: Membrane growth proceeded along two distinct morphological pathways over the duration of synthesis (1-4 h): an initial layer of semicrystalline, hemisphere-shaped grains (after 2 h) transforming into a fully crystalline layer with cubic morphology at the end of the growth process (4 h). A two-step growth rate trend was observed and could be correlated to the respective growth phases within the two underlying morphology types. The development of the hemisphere-shaped grains was associated with a period of accelerated growth during the first 2.5 h of synthesis (3.3 x 10-10 m.s-1), followed by a period of slower growth for the formation of the cubic morphology (1.9 x 10-10 m.s-1). Localised changes in supersaturation, combined with the possible effects of grain crowding, were offered as feasible explanations for the observed morphology and growth rate tendencies. Single gas permeatron: Single gas permeation of H2, N2 and SF6 were measured at two temperatures (23 and 107oC), specifically related to the semicrystalline (70 %; 2 h synthesis) and fully crystalline layers (100 %; 4 h synthesis). By comparing the permselectivity values with the respective Knudsen factors, it was shown that diffusion through the semicrystalline layer, at lower temperature, was predominantly based on molecular sieving (PSH2/SF6 = 63.8) which was much higher than the traditional membrane under the same conditions (PS H2/SF6 = 11.4). However, the opposite was observed at higher temperature - the H2lSFa permselectivity of the crystalline layer (5.7) was somewhat higher than the first (5.2). Based on theoretical considerations, it was concluded that the crystallamorphous interface in the semicrystalline membrane constituted a denser closure of the boundary interface, which could be attributed to a lower charge barrier presented by the amorphous phase (Si/Al > l), but this integrity was lost at higher temperature due to thermal instability of the amorphous component. The results therefore suggested that interventions in the charge loading in the boundary phase, during synthesis, could provide a means for stricter control over the intercrystalline porosity in NaA membranes in general. Pervaporation: Based on the outcomes of both the layer development and gas permeation studies, a comprehensive series of compositionally different NaA layers (tc, 2.0, 2.5, 3.0, 3.5 and 4.0 h) were tested in the pervaporative dehydration of water. Selected layers were also synthesised on two structurally different supports, to investigate the role of the support microstructure and its resistance to mass flow. The separation performance of the layers on the first support (Φpore = 163 nm) were compared using a 95 wt.% EtOH feed at 45oC. The selectivity (aWE) depended strongly on the relative degree of crystallinity and the amount of amorphous material occluded in the intercrystalline pore regions. The highest selectivities were obtained with either low crystallinity combined with significant amorphous content (aWE = 9 000 for the 2.0 h layer), or high crystallinity combined with a small amount of amorphous content (aWE = 12 500 for the 3.5 h layer). This general trend was also observed for the respective layers synthesised on the second support (Φpore = 101 nm), but the a,, values were much lower, ranging between 340 (for the 2.0 h layer) and 3 000 (for the 3.5 h layer). The difference was attributed to the increased dissolution of the second support, retarding the intergrowth of the zeolite layers. Despite the selectivity differences. the fluxes through each series of membranes on a specific support remained constant, showing that the support resistance to permeation was significantly high for both support types. The relative contributions to the total transmembrane resistance were calculated at -60 % and -70 % for the first and second support types respectively. The fugacity values at the zeoliteisupport interface of a given membrane (3.5 h synthesis on the second support) showed that the support resistance can limit the driving force achievable across the zeolite layer, even if the driving force across the composite membrane is increased. Support surface chemistry: A supplementary study examined the influence of ultraviolet (UV) radiation on the a-Al20, support surface prior to synthesis, specifically in terms of the resultant effects on membrane integrity. Using pervaporation under similar conditions (95 wt.% EtOH; 45oC) the selectivity values indicated a significant improvement in pervaporation performance of a given NaA layer (tc 3.5 h, second support) after pre-exposing the support to UV radiation (aWE = 25 500 for the pretreated membrane versus aWE = 3 000 for the control). A simple hypothesis for the selectivity enhancement was described in terms of the UV-induced increase in the number of OH-groups on the a-Al2O3 surface, which improves the wettability of the support, particularly in the macroscopic defect sites. As a result, the initially formed precursor gel is spread uniformly over the surface, leading to a high integrity zeolite layer with reduced intercrystalline porosity. In essence, this investigation showed that UV radiation provides a simple, yet highly effective tool for optimising the physicochemical interaction between zeolite and support during synthesis, thereby increasing the selectivity performance of the ensuing NaA layer. The aim of the project was met successfully by gaining new insights into the workings of the composite NaA membrane as a whole, including different structural and permeation related aspects. Future advances for the NaA membrane should be possible by finding condition-specific applications for the intrinsically different, time-dependent layers developed here, due to their high selectivity and permeance attributes under given conditions, or by applying the fundamental principles gained from their synthesis and permeation behaviour, to better suit existing applications. The generated data should also contribute to the further optimisation of supported zeolite membranes in general, both in terms of selectivity and permeance considerations.
- ETD@PUK