Modulating the porosity of N-doped carbon materials for enhanced CO2 capture and methane uptake

Abstract

N-doped carbons with modulated porosity have been prepared via the addition of melamine or urea as a N source to an activation mixture containing biomass-derived carbonaceous matter of low O/C ratio (air-carbonised date seed, Phoenix dactylifera, ACDS), and potassium hydroxide (KOH) as an activating agent. To access a broad range of surface area and mix of porosity characteristics, a series of carbons were prepared by varying the following: (i) the amount of melamine or urea (at melamine or urea/ACDS ratio of 1 or 2), (ii) the KOH/ACDS ratio (2 or 4), and (iii) activation temperature (600, 700, or 800 °C). We found that the N added to the activation mix acts both as an N-dopant and porogen, with the latter effect enabling formation of larger pores, which extended the pore size distribution of resulting porous carbons into the mesopore region. Furthermore, the presence of N acts to increase the surface area and provides carbons with tuneable porosity (with respect to the mix of microporosity and mesoporosity) and variable packing density, all of which may be tailored towards suitability for enhanced uptake of CO₂ and/or methane. The activated carbons have significant N content of up to 18 wt%, surface area of up to 3600 m² g⁻¹, and pore volume that reaches 2.1 cm³ g⁻¹. Depending on the preparation conditions and resulting mix of micro/mesoporosity, the carbons show excellent low pressure CO₂ capture at 25 °C of 1.7 mmol g⁻¹ at 0.15 bar and 4.7 mmol g⁻¹ at 1 bar, and uptake of up to 25 mmol g⁻¹ at 20 bar. The porosity and packing density may also be directed towards excellent methane storage with gravimetric uptake of up to 0.42 g g⁻¹ at 25 °C and 100 bar, volumetric storage capacity of up to 266 cm³ (STP) cm⁻³ at 25 °C and 100 bar, and a working capacity (for 100 to 5 bar pressure swing) of 196 cm³ (STP) cm⁻³.