Design and processing of metal-organic frameworks for greenhouse gas capture

Abstract: Anthropogenic emission of greenhouse gases has long been suspected to contribute to global warming and climate change. Most greenhouse gases are emitted in a mixture, so efficient methods and materials to separate and capture the gases are in demand in order to reduce emissions. A promising material group for this purpose is metal-organic frameworks (MOFs). This class of material have the ability to selectively adsorb green house gases due to its high porosity and high surface area. Zeolitic imidazolate frameworks (ZIFs) are a subclass of MOFs that are topologically similar to zeolites and are known for their good chemical and thermal stability.   The aim of this project was to investigate if the greenhouse gas (i.e. CO2 and SF6) capture performance of ZIFs could be improved and tuned using a mixed-linker approach with seven different imidazolate-based organic linkers of different sizes or with various functional groups. As well as to investigate the processability of MOFs using 3D printing. ZIFs composed of different ratios of 2-methylimidazolate as base linker and a second linker of imidazolate, benzimidazolate, 2-aminobenzimidazolate, 5,6-dimethylbenzimidazolate, and 4,5-dichloroimidazolate were succesfully made. The materials were all found to be crystalline, however, mixed-linker ZIFs containing 2-aminobenzimidazole, 5,6-dimethylbenzimidazole and dichloroimidazole were observed to contain more than a single phase. All samples showed to be somewhat porous towards CO2 and SF6, and there seem to be a trend where a low % of a bulkier linker (eg. bIm, ambIm) resulted in a higher uptake of SF6 whereas a high % resulted in a higher uptake of CO2. For dcIm it was the other way around, a low % showed a higher uptake for CO2  whereas a high % showed a higher uptake for SF6. For CO2, the ZIF containing 80% benzimidazolate showed the highest uptake of 9.81 wt%. For SF6, the 25% 4,5-dichloroimidazolate showed the highest uptake of 17.73 wt%. Furthermore, direct ink writing (DIW) 3D printing was also successfully utilized to process and structure a Mn-based MOF using carbopol as binder. The printed structure was found to have similar properties to the pristine MOF in regards to crystallinity and porosity.