| Summary: | 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. === Thesis (Ph.D. (Pharmaceutical Chemistry))--North-West University, Potchefstroom Campus, 2006
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