Potassium and Zeolitic Structure Modified Ultra-microporous Adsorbent Materials from a Renewable Feedstock with Favorable Surface Chemistry for CO₂ Capture

Novel hierarchically structured microporous biocarbons with exceptionally high capacities for CO₂ capture have been synthesized from the abundant agricultural waste of rice husk (RH), using a facile methodology that effectively integrated carbonization, activation, and potassium intercalation into a one-step process. Textural characterization demonstrates that the synthesized biocarbons exhibit exceedingly high ultra-microporosity accounting for up to 95% of total porosity mainly as a result of the naturally occurring silicon compounds within the RH molecular framework structures. With a modest surface area of up to 1035 m²/g and a total pore volume of 0.43 cm³/g, the best performing RH carbon has shown exceptionally high and fully reversible CO₂ uptake capacity of 2.0 mmol/g at 25 °C and a CO₂ partial pressure of 0.15 bar, which represents one of the highest uptakes ever reported for both carbon and MOF materials usually prepared from using cost-prohibitive precursor materials with cumbersome methodologies. It has been found that up to 50% of the total CO₂ uptake is attributable to the unique surface chemistry of the RH carbons, which appears to be dominated by the enhanced formation of extra-framework potassium cations owing to the exceedingly high levels of ultra-microporosity and the presence of zeolitic structures incorporated within the carbon matrices. Characterizations by EDX element mapping, XPS, and heat of adsorption measurements confirm the existence of a range of zeolitic structures, which essentially transforms the RH carbons into a kind of zeolite-carbon nanocomposite material with strong surface affinity for CO₂.