Molecular sorption of carbon-based porous structures: a study on water harvesting and carbon dioxide capture

Carbon (CO2) capture using regenerable sorbents is an effective means of mitigating green-house gas emission to the environment. The first-generation sorbents, based on liquid amines, suffer from instability, toxicity and high-energy penalty for regeneration. Solid sorbents, e.g. based on porous silica or active carbon, offer the potential of long cyclability and low cost. However, the sorption capacity is limited due to low surface area and pore volume, particularly if only physisorption mechanism dominates. The challenge and main aim of this study is to identify an effective porous carbon-based solid sorbent that can possess high capacity and low regeneration energy (hence cost) penalty. The structures should offer enhanced physisorption (e.g. van der Waals binding at slit pores, with a binding energy ~-10-20 kJ mol-1) and moderate chemisorption (e.g. binding at graphene edges, point defects or carbon-supported amine-groups, with a binding energy ~-20-50 kJ mol-1) so that adsorption and desorption “window” for CO2 can be narrow and at relatively low temperature. The porous structure must show high specific surface area and well-connected pores, so that the capacity can be maximised. To achieve such goals, the project first studied graphene oxide (GO) with various degrees of oxidation, ranging from 30 at% oxygen content, to enhance surface area and defect density; secondly, highly hierarchical porous graphene networks were derived through GO via moderate temperature thermal shock (300 °C), thermal annealing (600 °C) and/or KOH activation, to promote micro-pores and porosity hierarchy; and finally, for comparison and porosity improvement, another type of porous carbon structures were derived from carbonised metal-organic frameworks (MOFs), namely MOF-5 and MOF-74. The chemical and structural properties of synthesized materials were characterised by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption-desorption isotherms (the BET method) and Raman spectroscopy. Sorption capacity and kinetics were assessed by CO2 adsorption isotherms, thermogravimetric-differential scanning calorimetry (TG-DSC) and in-house water saturation apparatus. GO demonstrated a stacked layered structure with oxygenated functional groups such as hydroxyl, epoxy and carbonyl groups on its basal planes and edges, resulting in a hybrid structure comprising a mixture of sp2 and sp3 hybridized carbon atoms. GO is hydrophilic due to the presence of the oxygenated functional groups and the laminated structure can allow slow water diffusion into the layers. As water exist in practical cases of CO2 capture, the sorption of water was studied separately and together with CO2. From the study of highly porous GO derived exfoliated GO (exfGO), it was identified that the resulting materials possessed ultrahigh surface area and total pore volume up to 853 m2 g-1 and 6.68 cm3 g-1. The structures were applied as solid sorbents with chemical modification by TEPA and PEHA polyamines wet impregnation to incorporate amine-based sorption sites. The solid-amine system exhibited ultrahigh selective flue-gas CO2 capture of 6.16 mmol g-1 at 75 °C. The desorption occurred at 100 °C, giving a desirable narrow temperature-swing window. Further testing showed the cycling stability under simulated flue-gas stream conditions had moderate decay of ~7 wt% over 40 adsorption-desorption cycles and demonstrated stable CO2 uptake ~25 wt%. From the study on MOF carbons, it was found that the carbonisation process of MOF precursors led to loss of local metal centres and produced defective carbon structures with mainly sp2 bonding. By varying the synthesis conditions and solvents, micrometre to millimetre-sized MOF-5 crystals can be synthesized. Carbonisation process retained both meso- and macro-pores and yielded MOF carbons with high surface area up to 2237 m2 g-1 and total pore volume up to 4.6 cm3 g-1. The resulting amine-impregnated MOF carbons achieved CO2 adsorption of 4.37 mmol g-1. In summary, the project has developed highly porous carbon-based solid sorbents, which are stable and environmentally benign, with high specific surface area to offer a CO2 adsorption capacity > 6 mmol g-1. The binding energy is typically controlled at -50 kJ mol-1, which allows CO2 adsorption and desorption to be carried out between 25 °C and 100 °C. The developed sorbents have met the identified challenges of flue-gas conditions CO2 capture (>50 °C, humid), high thermal stability, chemical resistance and potential for large-scale production at low-cost, as well as offering great potential for practical applications in industry. The results also show great potential for the development of high capacity carbon-based sorbents for effective pre-combustion CO2 capture and energy storage applications