Hierarchical zeolites for improved separation performance: synthesis, characterization, growth mechanism and applications
thesisposted on 17.02.2017 by Hasan, Fatin
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Zeolites are highly crystalline materials with well-defined micropores that have the ability to strongly adsorb certain molecules and completely exclude others. Zeolites are widely used as adsorbents in gas separation by adsorption processes. Gas separation by zeolites might occur via three different routes; namely, equilibrium separation, molecular sieving and kinetic selectivity. However, the small size of zeolite pores (0.2 – 1.3 nm), hinders the large molecules from accessing the zeolite structure cavities and even the small molecules may exhibit very limited diffusion speeds within these pores. Accordingly, there has been considerable interest in synthesizing zeolite with larger pores for better adsorption characteristics. In this thesis, a new strategy described as slow crystallization and growth rate at room temperature in the presence of surfactant micelles was used to synthesize hierarchical Na-A zeolites. The surfactant used in this work is cetyltrimethylammonium bromide – CTAB which is cheap and commercially available. The sluggish formation and growth of the zeolite crystals in this system due to the moderate synthesis conditions was responsible for retaining the surfactant micelles within the crystallization domain. After the removal of CTAB template, the products showed large mesopores which were attributed to the interstitial voids between the aggregated zeolite nanocrystallites. The size of the mesopores can be further expanded by using linear hydrocarbon (n-dodecane) as a swelling agent. Increasing the amount of n-dodecane in the synthesis mixture resulted in increasing the size of the mesopores. Conversely, the size of the mesopores decreased when a swelling agent with shorter hydrocarbon chain than n-dodecane, such as n-hexane was used. The effects of the aging period and the concentration of CTAB in the synthesis mixture on the pore size distribution were also investigated. Additionally, the effect of combining two swelling agents in one system on the textural properties of the synthesized mesoporous Na-A zeolite was also examined. Different calcination conditions and the hydrothermal stability of the resultant material were also studied. The colloidal suspension of the synthesized zeolite showed negative zeta potential in the entire range of pH. The mesoporous Na-A zeolite synthesized in this work showed higher ethylene adsorption capacity as compared to the conventional microporous Na-A zeolite. For fundamental understanding of the formation of the resultant micro-mesoporous Na-A zeolite, a detailed study of the formation and growth of the zeolite particles was necessary. In this study, the concentration of n-dodecane was raised in the system until high internal phase emulsion (HIPE) was formed. The characterization showed that Na-A zeolite crystals consisting of thin crystalline polyhedral shell and multi-hollow polycrystalline core were formed following a reverse crystallization mechanism. A systematic investigation of the crystal growth over different crystallization stages indicates that these extraordinary crystals were formed via two step crystallization. Initially, n-dodecane droplets stabilized with a thin layer of cetyltrimethylammonium bromide enhance the formation of aggregates of amorphous nanoparticle which later transform into polycrystalline aggregates by local crystallization of each nanoparticle into a crystallite. Crystallized islands are built on the surface of the polycrystalline aggregates which are then extended and merged resulting in typical cubic morphology of LTA zeolite. Surface recrystallization continues towards the core via Oswald ripening process increasing the thickness of the shell. During the entire growth process, the n-dodecane droplets are retained inside the polycrystalline aggregates, probably due to slow crystallization rate at room temperature. The removal of the n-dodecane droplets and cetyltrimethylammonium bromide molecules results in Na-A zeolite particles with core-shell structure. The shell is highly crystalline Na-A zeolite and the core comprises of multi-hollow polycrystalline aggregates. The product shows mesoporosity with large size distribution attributed to polycrystalline aggregates that are free of the crystallized shell. This part of our work demonstrates two points; the synthesis of new kind of mesoporous Na-A with multi-hollow polycrystalline morphology and also demonstrates, for the first time, formation of LTA zeolite crystal via reverse crystallization route while retaining the identity of LTA zeolite. The resultant micro-mesoporous zeolite particles were ion exchanged to 5A zeolite and used as primary building units for construction of walls of 5A monolithic structures with novel channeling design that mimics a tree’s vascular system. The performance of these monoliths on CO2 separation was examined and compared to conventional 5A beads. Three different sizes of channels were introduced into the monoliths body; large parallel channels (500 μm) which are interconnected via their walls to medium and small scale channels. The medium (85 − 20 μm) and small (9 − 1 μm) scale channels are abundantly distributed throughout the walls of the monoliths. The effects of the number of the parallel channels (cell density per square inch), the concentration of medium scale channels and the concentration of the binder in the monoliths were investigated. Monoliths with high cell density (209 cpsi) showed sharper CO2 breakthrough front compared to the low cell density monolith (88 cpsi). Additionally, increasing the concentration of the medium scale channels in the walls of the monolith resulted in a significant reduction in the width of CO2 breakthrough curve suggesting even gas flow distribution and lower mass transfer resistance. The pressure drop per unit length measured across bed loaded with the prepared monoliths was 1.3 times less than 5A beads packed bed. The experimental data were simulated and the results showed that the effective diffusivity of CO2 molecules in beds of monoliths was six times faster than in 5A packed bed. On the other hand, the concentration of the binder concentration in the monoliths body did not show a noticeable influence on the trend of the breakthrough front or the pressure drop measurement. The high effective diffusivity and low pressure drop in addition to a considerable adsorption capacity in our monoliths indicates that they are competitive alternatives for conventional 5A beads and promising adsorbents for CO2 separation by adsorption processes.