Advanced gas separation via combined use of carbon nanomaterials and microporous materials

• Main Author: Li, Wen
• Other Authors: Bae Tae Hyun
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: Engineering::Chemical engineering
• Online Access: https://hdl.handle.net/10356/105593; http://hdl.handle.net/10220/50153
• Institution: Nanyang Technological University

Extensive research on gas adsorption and separation has been widely developed in view of the requirement of effective CO2 and H2 capture as well as biogas upgrading (CO2/CH4 separation) so as to ensure a sustainable environment for future generation. Thus, with regards to this behavior, microporous materials and carbon nanomaterials are potential candidates to be developed and utilized as adsorbents and membranes, due to their excellent capabilities for effective gas separation, lower capital cost and energy penalty, together with feasibility of scale-up into industrial operations. Herein, the objectives of this thesis are: (1) Tailoring gas uptake capacities for a three-dimensional nanocomposite that leads to an increase of overall gas adsorption sites; (2) Optimizing mixed-matrix membranes (MMMs) with the combined use of carbon nanomaterials and microporous materials to achieve improved gas separation performance; (3) Developing carbon molecular sieve membranes (CMSMs) with functionalized microporous materials to fulfill excellent gas permeation. The investigation began with the adjustment of surface area and porosity in NiDOBDC/GO nanocomposites with well-designed 3-D architecture. NiDOBDC (MOF) which possessed unsaturated open metal sites exhibited outstanding CO2 adsorption capability which was attributed to its high accessible surface area. In this case, the incorporation of GO can be served as a structural agent that can be assisted in the formation of MOF/GO 3-D hybrid. Such configuration was feasible in reducing the aggregation effects that were present in MOF nanocrystals as well as creating additional adsorption sites for gas molecules, thus resulting in significant improvement in gas uptake capacity. Interestingly, the nanocomposites exhibited remarkable CO2 and H2 uptake, especially for the case of nanocomposite with 10 wt% of GO, which showed an increase in adsorption amount of 24% (at 25 oC and 1 bar) and 50% (at 25 oC and 20 bar) for CO2 and H2, respectively. Results proved that the novel methodology for improving gas adsorption via incorporating GO was highly effective, which indicated promising applications in gas storage. Then, the utilization of NiDOBDC/GO nanocomposite in Matrimid® polymer has been conducted to fabricate MMMs. Attributing to the excellent CO2 adsorption performance of nanocomposite with 10 wt% loading of GO, NiDOBDC/GO-10% composite was chosen as the filler in Matrimid® so as to realize desirable CO2/CH4 separation ability without the trade-off effect. It was reported that CO2/CH4 selectivity in Matrimid®/Composite-20% membrane has been enhanced up to 60% as compared to nascent Matrimid® membrane (at 25 oC and 1 bar), without any noticeable loss in CO2 permeability. Furthermore, the NiDOBDC/GO nanocomposite did not weaken the mechanical strength of MMMs in this work, as compared to the incorporation of pristine NiDOBDC in Matrmid® membrane where substantial decrease in mechanical strength has been observed. On the other hand, apart from the utilization of MOF/GO nanocomposite, the effectiveness of MOF or GO that was separately incorporated into polymer matrix has also been investigated for CO2/CH4 separation. It is reported that ZIF-8 is feasible to improve CO2 permeability due to its high surface area together with good thermal, mechanical and chemical stabilities, which could be prepared via an easy synthesis procedure. Besides, ODPA-TMPDA polyimide, which has greater permeability than commercial polymers and is also readily synthesized in large quantities, was used as the polymer matrix. Results exhibited that both CO2 permeability and CO2/CH4 selectivity were improved by 60% and 28% (at 25 oC and 1 bar), respectively, when ZIF-8 and GO were introduced together. Importantly, the mechanical strength of mixed-matrix membrane with the incorporation of both ZIF-8 and GO was improved as compared to the nascent membrane, whereas the incorporation of ZIF-8 alone decreased the mechanical strength. Subsequently, CO2/CH4 separation performance has been further improved via the carbonization process, where high temperature and inert atmosphere were applied. Carbonization process is considered to be a promising approach to optimize the structure of the polymer chain or disrupt polymer’s rigidity, which is effective to alleviate the inherent limitations in polymeric membranes. It is well-known that zeolite 5A is facile to tune the pore structure, leading to a hierarchical structure with both mesopores and micropores. Also, Matrimid® has been applied as the matrix by taking its cheap cost, strong chemical resistance as well as high solubility towards organic solvents into consideration. As a whole, the mixed-matrix CMSM (Matrimid/H-zeolite 5A–30%) was feasible in surpassing the 2008 Robeson upper bound limit, which was attributed to its huge enhancement of CO2 permeability. In a nutshell, the utilization of microporous materials (NiDOBDC, ZIF-8 and zeolite 5A) as well as carbon materials (GO, CMSMs) has proven to be promising candidates in advanced CO2 separation, which can be conducted in an energy-effective manner. Particularly, in each work, CO2 adsorption or separation (CO2 permeability and/or CO2/CH4 selectivity) has been significantly improved via the combined use of microporous materials and carbon materials, namely NiDOBDC/GO nanocomposite, ZIF/GO binary fillers and zeolite 5A/carbon membranes. Considering the potential of microporous and carbon materials in other gas pairs as well as particle size effect, future work is mainly focused on the application of such strategies (combination of microporous and carbon materials) in O2/N2 separation in view of tunable particle size and pore structure.