Strategy for optimizing the synthesis and characterization of activated carbons obtained by chemical activation of coffee husk

Abstract

The present work aims on the one hand to optimize the main influencing factors in the synthesis of activated carbon obtained from coffee husk as a widely available lignocellulosic precursor, and on the other hand to study its textural and chemical characteristics. The preparation methodology is done by chemical activation using phosphoric acid (H₃PO₄) and potassium hydroxide (KOH) in an atmosphere of N₂. The optimization of the main factors obtained by screening was carried out by the response surface methodology (RSM). The effects of the concentration of the activating agent, the residence time in the furnace and the carbonization temperature on the iodine number and the yield were evaluated by the ANOVA test, the Pareto diagram and the t-test. The optimal conditions for producing activated carbon have been recognized as being: an activation temperature of 300 °C associated with a residence time of 120 min with a concentration of 0.1 mol L⁻¹ of KOH and an activation temperature of 600 °C associated with a residence time of 120 min with a concentration of 0.1 mol L⁻¹ of H₃PO₄, which gave for the iodine number a value of 603.45 and 520.65 mg g⁻¹ and the yield of 59.054% and 67.236% respectively for activation with KOH and H₃PO₄. The various carbons with optimal properties have undergone various physicochemical analyzes such as pH at the point of zero charge (pH_ZPC), Bohem titration, Fourier transform infrared spectroscopy (FTIR), X-Ray Diffraction (XRD), Brunauer–Emmett–Teller (BET) analysis, Thermogravimetric analysis (TGA) and Scanning Electron Microscopy (SEM). The analysis results obtained under these optimal conditions revealed a predominantly amorphous structure, the presence of various surface functional groups (carboxylic, lactone and phenol), a dominant acid character (1.89 and 2.28 meq g⁻¹) and an advanced development of the porosity of the activated carbon produced with a specific surface of 520.55 and 560.65 m² g⁻¹ respectively for carbon activated with base and carbon activated with acid, indicating good candidates for the production of porous materials. The activated carbons obtained by impregnation using 0.1 M of activated KOH at a temperature of 300 °C for 120 minutes have a good affinity with respect to the diode molecules which provide information on the microporosity of the activated carbons.