Development of additive manufacturing systems for 3D structured carbon dioxide adsorbents

• Main Author: Lee, Junghyun
• Other Authors: Song Juha
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Chemical engineering
• Online Access: https://hdl.handle.net/10356/164404
• Institution: Nanyang Technological University

In the last several decades, global warming has been accelerating due to increased carbon dioxide (CO2) emissions with remarkable population growth and industrial development, leading unprecedented environmental issues. Because the adsorptive CO2 capture processes using solid adsorbents have been known to be more cost-effective than traditional amine scrubbing approaches, studies on development of solid adsorbents such as active carbons, microporous/mesoporous silica or zeolite and MOFs have actively conducted. These solid adsorbents, however, could not be well-adapted for practical use, due to their reduced performance under dynamic operational environments. To improve dynamic performance of solid adsorbents, 3D-structured CO2 adsorbents have been proposed by a few research groups, utilizing emerging 3D printing technology. Nevertheless, due to low printability, reduced absorption capability by using non-CO2 adsorptive binder, and little structural optimization of 3D printed adsorbent systems, this approach still has required significant advances in material development and structural design optimization for improved CO2 capacity, in parallel to resolving scalability issues of printing platforms. Therefore, I proposed a new approach to highly processible and applicable CO2 adsorbents for scalable 3D printing platforms. First, the study has begun with the development of solution-processible 3D printed adsorbents. The printable starting material for CO2 adsorbents was chosen and the pre-formed precursors with desirable 3D structures was fabricated. Then, through a stepwise chemical process, those precursors were transformed into CO2 adsorbents. As a proof-of-concept, I first chose brominated poly (2,6-dimethyl-1,4-diphenyl oxide) (PPO-Br) as a starting material, and successfully formed beads or 3D-printed monoliths using PPO-Br. The PPO-Br could eliminate the usage of carcinogenic chemicals for amination and easily be dissolved in either dichloromethane or chloroform with toluene solvent for preparation of the feed solution for 3D printing. Then, through the subsequent hypercrosslinking and amine-grafting processes, the CO2 uptake of PPO-Br based adsorbents was increased due to increased micropores as well as the existence of amine functional groups that had strong affinity to CO2. More importantly, those 3D-structured CO2 adsorbents maintained their adsorption capability regardless of various environment conditions (e.g., humidity, low-flow rate), implying their improved applicability. Subsequently, further improvement on printable materials was attempted through development of composite systems using this solution-processible 3D printing platform. The composites were fabricated by mixing the PPO-Br polymers with the filler materials (typically in solid phase). Three filler candidates such as zeolite 13X (Z13X), porous polymer (PP) and/or DETA-grafted porous polymer (PP-DETA), and polyethyleneimine-impregnated silica (PEI/SiO2) were examined for those composite systems, and Z13X was chosen due to its high CO2 adsorption capability and good chemical stability in dope solution. Once the 3D structures were formed via 3D printing technique with the composite inks, further post-treatment processes were applied to impose CO2 adsorption capability on the PPO-Br matrix, which was novelty of this composite system compared with conventional filler-polymer binder adsorbents. The significant improvement on intrinsic CO2 adsorption capability of Z13X/PPO-Br adsorbents was clearly observed, whereas their CO2 adsorption performance in dynamic flow condition didn’t outperform the pure PPO-Br system due to low compatibility between Z13X and PPO-Br during the post-treatment and activation processes. Development of composite systems using the solution-processible 3D printing platform was meaningful for high performing 3D-structured CO2 adsorbents, but still new filler materials with better compatibility to the PPO-Br matrix should be developed. Another strategy to increase the CO2 adsorption capability was optimizing starting material and adsorbent structure. Despite structural flexibility of 3D printing methods, the most popular structure of the adsorbent for 3D printing was a simple monolith. Therefore, I hypothesized that variation of 3D porous network structures would bring macro-structural advantages to CO2 adsorbent systems. First, through increasing degree of bromination, the PPO-Br polymer matrix obtained higher number of substituted amines through the post-treatment processes after 3D printing, resulting in significantly enhanced CO2 adsorption capability. Additionally, the cylinder-type adsorbents with various pore structures, which fully occupied the target volume of the measurement tubing cell, allowed fully utilized CO2 adsorption capability of the loaded adsorbents per target volume, maximizing their performance. It was noteworthy that the structural complexity and modularity for CO2 adsorbents could only be realized by the 3D printing technology, resolving scalability issues of 3D printed CO2 adsorbents. Moreover, each modular unit could be functionalized with different chemical groups, for multi-gas adsorbents. In this thesis, I successfully developed new 3D-structured CO2 adsorbents utilizing printable starting materials with relatively low CO2 adsorption capability, but applicability for various chemical modifications, through a 3D printing method and following post-treatment processes. The proposed systems were also further improved through material and structural approaches, maximizing the advantage of 3D printing processes. I envisage that my approaches will open a new avenue for high-performing and structurally versatile gas adsorbents.