Valorization of coffee grounds to biochar-derived adsorbents for CO₂ adsorption

Global warming is mainly caused by the continuous increase of CO₂ concentration from the enormous utilization of fossil fuels in the environment. To circumvent this problem, it is crucial to develop an efficient and economic method to capture CO₂ from large stationary sources, such as coal-fired power plants, cement, steel factories, and so on. In this study, the CO₂ adsorbents are prepared with biomass wastes (coffee grounds) by combining ammoxidation process (via three different methods) and potassium hydroxide activation. Various spectroscopic and analytical techniques, such as high resolution scanning electron microscope, nitrogen adsorption-desorption isotherms, elemental analysis, Fourier-transformed infrared spectroscopy and X-ray photoelectron spectroscopy are used to study the physicochemical properties of various adsorbents. Carbon dioxide adsorption capacities of prepared adsorbents are measured by using a thermogravimetric analyzer at atmospheric pressure. Furthermore, we test the stability and selectivity of adsorbents by measuring ten CO₂ adsorption-desorption cycles and selectivity of CO₂ and N₂. Results obtained from this study reveal that the adsorbents which are nitrogen-doped by melamine from ammoxidation treatments, followed by potassium hydroxide chemical activation, have highly developed microporosity, high amounts of N-doping and more pyrrolic nitrogen (active adsorption sites), resulting in the highest CO₂ uptake of 2.67 mmol CO₂ g⁻¹_sorbent at 35 °C. Most importantly, these biochar-derived adsorbents have excellent selectivity ([CO₂/N₂] = 74.2) for CO₂–N₂ separation and good cyclic stability of CO₂ adsorption-desorption process, which may be suitable for practical applications in capturing CO₂ emitted from coal-fired power plant.