Processing and performance of zeolites for efficient carbon dioxide separation
Sammanfattning: We have structured zeolites from powders of zeolite 13X and 4A into hierarchically porous monoliths for efficient carbon dioxide capture by tailoring the pore dimensions to facilitate rapid gas uptake and release. Freeze-casting was used for the first time to shape adsorbents into a lamellar structure. Lamellar walls with thicknesses and spacing in the range of 10 µm were found to be the best combination between rapid gas transport and a short diffusion distance in the zeolite-containing walls for rapid carbon dioxide uptake and release.Compressive strength measurements of the freeze-cast zeolite-based monoliths showed that monoliths with small pores, thin walls and a lot of interconnectivity between the walls were stronger than monoliths with large pores and thick walls. Image analysis of the structures together with modelling of the deformation behavior suggests that the failure mechanism of freeze-cast monoliths is dominated by buckling.Binder-free zeolite Y and ZSM-5 -based monoliths were produced by pulsed current processing (PCP) into strong, hierarchically porous monoliths with minimal loss of crystallinity. Ranges for the maximum PCP processing temperatures for the zeolites with different aluminium contents were identified by powder x-ray diffraction (PXRD) with full-profile fitting analysis.Matching the thermal expansion behavior of the supports with the zeolite film is important to minimize the risk of thermally induced cracking of zeolite membranes. Zeolite supports with a macroporous structure was prepared by PCP and the thermal expansion coefficient was determined by PXRD and compared to traditional alumina substrates. It was found that the slightly negative thermal expansion coefficient of the zeolite supports matched the thermal expansion of the zeolite films very well, whereas the alumina support would induce large stresses upon fluctuating temperatures.Methylcellulose-directed synthesis of zeolite 4A produced nano-sized crystals with a narrow size distribution, which could be tuned by adjusting the methylcellulose content. The crystallinity of the synthesized 4A was controlled by PXRD and found to be very high, and the gas uptake capability performed well in comparison with available micron-sized zeolites.
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