Wet Grafted Amine Silicas for CO2 Capture and Natural Gas Desulfurization

Abstract

The introductory chapter gives an in-depth overview of gas separation technologies. It provides insight into the problems associated with liquid absorption processes and why adsorption technologies would be better suited for gas separation and purification. The second chapter utilizes a novel wet grafting approach on small-pore and large-pore SBA-15. To further develop superior adsorbents, it is important to understand how pore properties affect the adsorption performance. We investigated the CO2 and H2S capture performance and the results indicated that pore-expanded as opposed to small-pore SBA-15 displayed the best performance. Pore-expanded SBA-15 displayed better adsorptive performance due to enhanced accessibility of amines. H2S is another acid gas that needs to be removed. H2S adsorption measurements were performed and pore-expanded and small-pore SBA-15 achieved similar H2S capacities, suggesting that diffusional resistance does not play a role. These findings allowed for the identification of commercially available silicas. With previous findings demonstrating that large-pore silica greatly enhances adsorption performance, it was imperative to identify commercially available large-pore silica to further promote the use of amine-grafted silicas in industrial settings. In chapter three, several commercially available low-cost silica gels were identified and grafted. We assessed CO2 capture performance, textural properties and the results indicate that the silica gel with the largest pores and lowest surface area performed better than the other silica gels. This suggests that for wet grafted adsorbents, surface area does not play a significant role and accessibility of the amine is the dominant factor during adsorption. This was further confirmed by the direct air capture capacity. To the best of our knowledge, all studies on amine-grafted silicas used unimodal silicas. We hypothesized that bimodal silica would achieve superior adsorption performance. Bimodal silica was synthesized and amine-grafted. Pore-expanded SBA-15, which possesses a unimodal structure was employed for comparison and CO2 and H2S adsorption measurements were taken. The results indicate that bimodal as opposed to unimodal silicas display the best adsorption performance. Bimodal silicas permit greater amine dispersion and consequently, greater amine accessibility. The CO2 capacity and the direct air capture capacity were the highest in literature for amine-grafted silicas. In the fifth chapter, SBA-15 was used to study the influence of water during amine grafting. We also carried out amine grafting in the presence of alcohol to determine if it is a more effective agent for influencing amine loading. We found that alcohol did not result in an increase in the amine loading but continuous additions of water increased the amine loading. Furthermore, the time required to graft amines was reduced from 12 hours to 30 minutes and results confirmed the cyclic stability of wet grafted materials. In the final chapter, we identified that amine-grafted adsorbents are not easily synthesized. To address this, we conducted low temperature (30 °C) amine grafting of SBA-15 in anhydrous and hydrous conditions. Comparative studies at amine grafting reaction temperature of 85 °C were performed. The results show that under dry grafting conditions, 85 and 30 °C grafted adsorbents displayed similar amine loadings and under wet grafting conditions, adsorbents grafted at 30 °C exhibited higher amine loadings. CO2 measurements revealed that adsorbents grafted at 30 °C achieved higher CO2 adsorption capacities at 75 °C. The results showed that the observed effects were universal for other aminosilanes and mesoporous silicas.