Preparation of molded biomass carbon from coffee grounds and its CH4/N2 separation performance

Abstract

Coffee grounds are promising precursors for excellent porous carbon adsorbents. During the preparation of the porous carbons, sodium silicate was used as a binder and pore-forming agent, and extrusion molding technology was used to prepare them in a columnar form. After carbonization, steam activation and silica removal by alkaline washing, high-strength columnar porous carbon adsorbents (CGCs) were obtained. Their CH4/N2 separation performance was studied by multicomponent breakthrough experiments. The Brunauer-Emmett-Teller (BET) surface area of CGC-1.5 (where 1.5 is the mass ratio of a 9 wt% sodium silicate aqueous solution to the coffee grounds) is 527 m2·g-1. Both the N2 and CO2 adsorption isotherms show that the CGCs are rich in micropores and mesopores, with the micropores size mainly centered at about 0.48 nm. FT-IR results show that CGC-1.5 has abundant oxygen-containing functional groups. At 298 K and 0.1 MPa, its equilibrium adsorption capacity for CH4 is 0.87 mmol·g-1, and the separation selectivity for a CH4/N2 mixture (3/7, vol/vol) is 10.3, which is better than most biomass-based porous carbon adsorbents and crystalline materials. Dynamic breakthrough tests show that CGC-1.5 has an excellent CH4/N2 separation performance at both high and atmospheric pressures. The dynamic selectivities at 298 K, 0.11 and 0.5 MPa are 10.4 and 17.9, respectively. The adsorption capacity is unchanged after 10 adsorption-desorption cycles. The mechanical strength of CGC-1.5 is as high as 123 N·cm-1, which meets the criteria of industrial applications.