Designed Synthesis Of Hierarchical Porous Carbon Monoliths For CO₂Capture

Due to the dual roles of CO2as a greenhouse gas and a renewable C1source, its selective capture and separation has attracted tremendous attention. Adsorption is an effective separation technique because of its easy regeneration and superior cycling capability, and therein the key factor is the design and synthesis of high-performance sorbents. However, presently available sorbents show varying degrees of limitations regarding capacity, selectivity, stability, sorption kinetics and so on. It is also of great importance to disclose the interrelationship between sorbents’structural features and CO2sorption performances. Considering the characteristics of porous carbons such as developed porosity, good stability as well as the advantages of monolith materials in term of low pressure drop and easy operation, herein, we have designed and synthesized a series of hierarchically porous carbon monoliths with tailored pore architecture and surface chemistry through solution synthesis strategy. Furthermore, we have fundamentally investigated the sorption behavior, and their relations with the structure and surface features of sorbents. Specifically, the work includes the following parts:(1) Aiming to address the issues of low capacity and selectivity, we have designed and synthesized nitrogen-doped macro-/microporous carbon monoliths. In this synthesis, the basic amino acid, lysine was verified as nitrogen-containing active precursor, co-polymerizing with resorcinol and formaldehyde through condensation. Remarkably, the effective polymerization leads to a very fast solidification within5min at room temperature, ending with crack-free polymer monolith. After pyrolysis at targeted temperature, the new type nitrogen-doped carbon monolith was obtained. CO2sorption tests show that the sample exhibits very high CO2adsorption capacity of3.13mmol g-1at room temperature and can be easily regenerated and cycles stably. The solution synthesis strategy delivers a high purity of products as well as the molecular level dispersion of active functional groups in carbon skeletons and pore surfaces, which in turn enhances the CO2sorption capacity and selectivity.(2) In order to facilitate molecules transport in porosity, on the basis of (1), we tried to impart the mesoporosity between microporosity and macroporosity, and thus a rapid and scalable synthesis of hierarchical carbon monoliths with ordered mesostructure and fully interconnected macropores has been demonstrated. In this synthesis, resorcinol and formaldehyde were used as the carbon precursor, F127as soft templates, organic base lysine as both the nitrogen source and mesostructure assembly promoter. The remarkable result is that in the presence of lysine, homogeneous and crack-free polymer monoliths can be achieved through a rapid gelation within15min at90℃. The obtained polymer monoliths have robust framework, which can be directly dried in air and carbonized at high temperature under nitrogen. The products are crack-free and have ordered mesostructure with fully interconnected macropores. The surface area and macropore volume are high up to600m2g-and3.52cm3g-1. Further steam activation of such carbon monolith can significantly improve its surface area to2422m2g-1,and the ordered mesostructure remains well.(3) Considering of the high velocity, strong impact force and humid features of gas mixture flows, on the basis of (1) and (2), herein, porous carbon monoliths with defined multi-length scale pore structures, nitrogen-containing framework and high mechanical strength were synthesized through the designed sequential reaction and self-assembly of poly(benzoxazine-co-resol), and a carbonization process. By controlling the reaction conditions, porous carbon monoliths exhibited fully interconnected macroporosity and mesoporosity with cubic Im3m symmetry and can withstand a compressive pressure of up to15.6MPa, which is80times better than the reported materials with similar structures. The carbon monoliths show outstanding CO2sorption and separation capacities, high selectivity. At104KPa, the equilibrium capacities of the monoliths are in the range of3.3-4.9mmol g-1at0℃, and of2.6-3.3mmol g-1at25℃; while the dynamic capacities are in the range of2.7-4.1wt%at25℃using14%(v/v) CO2in N2. The carbon monoliths show the separation factor of CO2/N2ranging from13to28. Meanwhile, they undergo a facile CO2release in Ar purge at25℃.(4) In order to enhance the transport kinetics as well as raise the utilization rate of microporosity, we have designed and synthesized a series of2D layered building blocks with significantly shortened thickness, which were further assembled into hierarchical carbon monoliths. In this strategy, we have fully employed the structural and surface properties of graphene oxide (GO), on both surfaces of which the co-polymerization of resorcinol, formaldehyde and amino acids occurred, obtaining functional polymer homogeneously coated GO hybrids. This novel layered carbon macro-assemblies show the following features:the precisely tunable coating thickness ranging from5to140nm; the widely tunable pore volume from0.3to1.0cm3g-1and surface area from300to1500m2g-1; the superior high compressive strength with values up to28.9MPa. At110KPa, the equilibrium capacities of the monoliths are in the range of2.8-4.3mmol g-1at0℃, and of2.1-3.0mmol g-1at25℃. More importantly, rate of adsorption tests show that CO2sorption can reach equilibrium within10s. Dynamic breakthrough tests show that, over such series of samples, CO2/H2O/N2 streams with concentration of4/3/93v%can be fully separated, indicating its potential application in the case of dilute wet gas, such as in enclosed spaces (e.g., space ships, submarines).