As the energy costs rise, membrane technology for separation will play an increasingly important role in reducing costs of industrial process and the environmental impact. Carbon nanotubes (CNTs) membranes have attracted much attention due to the unique properties of CNTs and the high gas permeance in aligned carbon nanotube membranes and mixed matrix membranes. In addition, the thermal stability and mechanical strength of the synthesized membranes can also be promoted by CNTs. Though various CNT based membranes have been fabricated and studied, further work is still highly desired to further improve the membrane performance.
This thesis is focused on developing novel carbon nanotube membranes for gas separation with high permeability and selectivity, as well as good thermal and chemical stability. Aligned diameter-controlled CNTs will be synthesized, the CNT/polymer mixed matrix membrane and vertically-aligned CNT membranes will be fabricated, and the permeation performance will be assessed. It aims to establish the relationship between the characteristics of the CNTs (diameter, length, doped metals and surface functional groups) and the gas separation of the membranes, and to greatly increase the membrane permeability and selectivity.
The first part of the experimental chapters focuses on the CNT/polymer mixed matrix membranes. Well-dispersed CNTs/polyethersulfone (PES) polymer matrix membranes are successfully obtained with carboxyl functionalized CNT through phase inversion method. Compared with pure polymer membrane, the gas permeation fluxes of the derived nanocomposite membranes increased by ~67 % without sacrificing the selectivity when 5 wt.% MWCNTs is introduced. Ru (Fe) Metals modified CNTs are also embedded into the PES matrix to study the gas permeability of the nanocomposite membranes. Those nanocomposite membranes derived from Ru (Fe) modified CNTs also show uniform CNT dispersion in the polymer matrix and improved gas permeation fluxes at low CNT concentration (<5 wt.%). Nevertheless, the membranes exhibit different trends in gas selectivity when modifying the CNTs component by Ru, Fe or carboxyl groups. The membranes containing Ru-modified CNTs show higher gas selectivity, while those with Fe-modified CNTs present lower selectivity, and those with carboxyl CNTs have similar gas selectivity to pure PES membrane. By controlling Ru modification location on the CNT channels, better gas selectivity of the corresponding membranes can be observed. We also conduct density functional theory calculation to investigate the adsorption difference. Combining with both the experimental and simulation results, we can deduce that the different gas adsorption behaviours are introduced by doping metals or carboxyl functional groups, which then influence the gas permeability. Our experimental and theoretical results also indicate that the gas diffusion passes through the interface between polymer chains and carbon nanotubes, instead of the CNT channels, in this nanocomposite system. Therefore, tailoring modification site on the external surface of carbon nanotubes can be more effective to improve the gas separation performance of CNT mixed matrix membranes.
The second part of the experimental chapters deals with the synthesis of vertically aligned carbon nanotube (VACNTs) and the fabrication of membranes based on VACNTs. VACNTs with high purity have been grown on quartz substrate via the gas phase catalytic chemical vapour deposition (CVD) method by using ferrocene as the catalyst source and camphor as the carbon source. The effects of catalyst concentration, flow rate and water assistance on the morphology and structure of VACNTs are investigated.
For membrane investigation, VACNTs with different tube inner diameters and areal densities are tuned by the catalyst precursor content. The gas permeantion of the VACNTs membranes is much higher than the Knudsen predicated permeability. Moreover, the enhancement factors (how the factor defined) of the smaller-sized VACNT membranes are higher (30~80, 20 oC) than the membrane with larger tubes (30~60, 20 oC). In the other hand, the hydrogen selectivities are all in the Knudsen range. The differences in the permeance increment and enhancement factor for various gases may be attributed to the surface adsorption diffusion.
To improve the gas selectivity, molecular sieved zeolite imidazolate frameworks (ZIF), as the gas selective layer, are grown on the VACNT membranes by secondary seeded growth. The ZIF layer shows good contact with VACNT membrane surface with a thickness of 5~6 µm. Comparing to the Knudsen selectivity of VACNT membranes, much higher ideal selectivity of H2 to Ar, O2, N2 or CH4 at 7.0, 13.6, 15.1 and 9.8, respectively, are observed in the ZIF modified VACNT membrane. This work opens up an alternative way to improve the gas selectivity of VACNT membrane, which could contribute to our existing knowledge on the VACNTs membrane system and achieve a membrane system that could make full use of the superflux of CNTs and molecular sieving of MOFs.