Sustainable coffee‐based CO₂ adsorbents: toward a greener production via hydrothermal carbonization

Activated carbons for adsorption of CO 2 under flue gas conditions were prepared by means of hydrothermal carbonization (HTC) and subsequent CO 2 activation of spent coffee grounds. The HTC of the samples consisted of their heating at moderate temperature with a high water content in autoclave. A preliminary screening concluded that 1:2 biomass/water ratio (spent coffee grounds as received) and no chemicals added during HTC with further activation in CO 2 at 800°C for 1 h are suitable conditions to produce the CO 2 adsorbents. In addition, the response surface methodology (RSM) successfully evaluated the combined effect of HTC temperature and dwell time, to maximize the CO 2 capture capacity within the experimental region. Both the lowest temperature and dwell time (120°C, 3 h) resulted in the maximum CO 2 capture capacity (2.95 wt.%). Two activated carbons (ACs) were then produced: one via hydrothermal carbonization optimized by means of response surface methodology (RSM) followed by CO 2 activation (HC‐Co) and the other one by single‐step CO 2 activation as described in the Group's patent ES2526259 (AC‐Co). Analysis of the features and performances of the two ACs revealed superior chemical and textural characteristics on HC‐Co for CO 2 adsorption under post‐combustion capture conditions; HTC process is the sole responsible of this enhancement. Moreover, the proposed methodology to produce CO 2 adsorbents from spent coffee grounds represents a more energy‐efficient approach.