Development of Zeolitic Imidazolate Frameworks for Enhancing Post-combustion Co2 Capture

Author

Dustin Lee, California Polytechnic State University, San Luis ObispoFollow

DOI: https://doi.org/10.15368/theses.2020.126
Available at: https://digitalcommons.calpoly.edu/theses/2238

Date of Award

9-2020

Degree Name

MS in Civil and Environmental Engineering

Department/Program

Civil and Environmental Engineering

College

College of Engineering

Advisor

Amro El Badawy

Advisor Department

Civil and Environmental Engineering

Advisor College

College of Engineering

Abstract

Post-combustion CO₂ capture is a promising approach for complementing other strategies to mitigate climate change. Liquid absorption is currently used to capture CO₂ from post-combustion flue gases. However, the high energy cost required to regenerate the liquid absorbents is a major drawback for this process. As a result, solid sorbents have been investigated extensively in recent years as alternative media to capture CO₂ from flue gases. For example, metal organic frameworks (MOFs) are nanoporous materials that have high surface areas, large pore volumes, and flexible designs. A large number of MOFs, however, suffer from 1) low CO₂ adsorption capacity at low pressure, which is the typical condition for flue gases, 2) degradation upon exposure to water present in flue gases, and 3) low selectivity of CO₂ when present in a mixture of gases. Zeolitic Imidazolate Frameworks (ZIFs) are heavily investigated MOFs for CO₂ sorption applications because they have better selectivity for CO₂ compared to other MOFs and are resistant to degradation in water due to their hydrophobic nature. However, ZIFs (e.g., ZIF-8) investigated for CO₂ sorption applications are typically produced using toxic solvents and their CO₂ sorption capacity is drastically lower than other types of MOFs. Post-synthesis modifications with amine functional groups have been known to increase CO₂ sorption capacity and selectivity within nanoporous materials. For ZIFs, previous research showed that sufficient loading with linear polyethyleneimine increased their CO₂ sorption capacity. Therefore, the objectives of this research were to a) investigate the CO₂ sorption capacity of ZIF-8 synthesized by solvothermal methods that use more eco-friendly solvents (e.g., methanol and water) and b) introduce post-synthetic modifications to ZIF-8 using branched polyethyleneimine (bPEI) to enhance its sorption capacity. A custom quartz crystal microbalance (QCM) system was assembled and used to measure the CO₂ sorption capacity of unmodified and bPEI-modified ZIF-8 sorbent. The tests were conducted at 0.3 - 1 bar. The results showed that the unmodified ZIF-8 synthesized in methanol (ZIF-8-MeOH) had comparable crystal structure, thermal stability, surface area, and chemical properties to that of literature (Ta et.al 2018). ZIF-8-MeOH had a surface area of 1300 m²/g and a CO₂ sorption capacity of 0.85 mmol CO₂/g ZIF-8 @ 1 bar. This surface area and sorption capacity are comparable to those of ZIF-8 made in dimethylformamide (DMF). Therefore, ZIF-8-MeOH proved to be a worthy candidate MOF for replacing the ZIF-8 made in DMF for CO₂ capture research. Water-based ZIF-8 was also synthesized in this study; however, its CO₂ sorption capacity was not tested because it exhibited a significantly lower surface area (732 m²/g) compared to that of ZIF-8-MeOH. Modification of the ZIF-8-MeOH with bPEI resulted in a decrease in its CO₂ sorption capacity. This undesired outcome is likely a result of insufficient bPEI load (mass attached), on ZIF-8-MeOH (~ 10% w/w) combined with the surface area lost (~ 770 m²/g) due to bPEI blocking some of the ZIF-8-MeOH pores. Therefore, the bPEI load attained in this study was not enough to compensate for the loss of surface area of the modified ZIF-8 and thus, the CO₂ sorption capacity decreased. Future investigations should enhance the post-synthetic modification by increasing the loading of amine functional groups onto the eco-friendlier ZIF-8-MeOH used in this study.