Nitrogen-doped micro- and mesoporous carbons for CO2 capture [electronic resource]
Keywords: Energy Resources Engineering.

Global annual energy-related CO2 emissions reached a record high of 32.3 gigatonnes (Gt) in 2014, and is expected to continuously rise with our ever growing energy demands despite a finite remaining fossil fuel-dependent energy infrastructure. In specific, mitigation of CO2 emissions has been recognized as a crucial necessity, since CO2 is a major contributing greenhouse gas towards global warming and associated consequences. Today's state-of-the-art technology for CO2 capture, namely aqueous amine scrubbing, has not yet been proven cost-effective at scale due to considerable energy penalties because of the high heat capacity of water for regeneration. A number of sorbent technologies are currently under investigation such as zeolites, metal-organic frameworks (MOFs), covalent organic frameworks (COFs), surface modified porous silica, porous polymer networks and porous carbon. Among them, carbon sorbents possess a number of advantages, such as relatively low regeneration energy, high surface area, high thermal conductivity, chemical stability, tunability over pore geometries, pore dimensions, as well as flexibility for heteroatom doping or surface functionalization. This dissertation is focused on the material design for different CO2 capture processes. Two specific applications are of interest, including post-combustion capture and natural gas sweetening. According to the unique conditions and constrains of each application, two different types of solid carbon sorbents have been developed. The property-performance relationships have been investigated in detail in each case, which will provide useful insights into future material developments. In post-combustion processes, sorbents with both high capacity and selectivity are required for reducing the cost of carbon capture. Although physisorbents have the advantage of low energy consumption for regeneration, it remains a challenge to obtain both high capacity and sufficient CO2/N2 selectivity at the same time. A novel N-doped hierarchical carbon has been developed, which exhibits record-high Henry's law CO2/N2 selectivity among physisorptive carbons while having a high CO2 adsorption capacity. Specifically, the synthesis involves the rational design of a modified pyrrole molecule that can co-assemble with the soft Pluronic template via hydrogen bonding and electrostatic interactions to give rise to mesopores followed by carbonization. The low-temperature carbonization and activation processes allow for the development of ultra-small pores (d < 0.5 nm) and the preservation of nitrogen moieties, essential for enhanced CO2 affinity. Furthermore, the described work provides a strategy to initiate the development of rationally-designed porous conjugated polymer structures and carbon-based materials for various potential applications. In addition to post-combustion capture, natural gas sweetening is another topic of interest. Natural gas, having the lowest carbon intensity compared to coal and petroleum, is projected to increase in production and consumption in the coming decades. However, a drawback associated with natural gas is that it contains considerable amounts of CO2 at the recovery well, making on-site CO2 capture necessary. Solid sorbents are advantageous over traditional amine scrubbing due to their relatively low regeneration energies and non-corrosive nature. However, it remains a challenge to improve the sorbent's CO2 capacity at elevated pressures relevant to natural gas purification. A series of porous carbons have been developed, which were derived from an intrinsic 3D hierarchical nanostructured polymer hydrogel, with simple and effective tunability over the pore size distribution. The optimized surface area reached a record-high of 4196 m2 g-1 among carbon-based materials. This high surface area along with the abundant micro/narrow mesopores (1.94 cm3 g-1 with d < 4 nm) results in a record-high CO2 capacity (28.3 mmol g-1 at 25 °C and 30 bar) among carbons. This carbon also showed good CO2/CH4 selectivity and excellent cyclability. In addition, this work for the first time combines experimental studies with first-principle molecular simulations for high-pressure CO2 adsorption on porous sorbents. It was found that at elevated pressures, the CO2 density in the adsorbed phase is significantly enhanced in the micro- and narrow mesopores with quantitative values provided for CO2 density. Furthermore, surface nitrogen functionalities have a trivial contribution to the CO2 uptake at high pressures. These findings emphasize the importance of being able to tune a sorbent's pore size to achieve high CO2 uptake. Thus, the simulation studies help in our understanding of our sorbent's superior performance as well as provides useful insight into future sorbent development.