Silica gel impregnated with amino acid-based deep eutectic solvents (AADES) for CO2 capture

Abstract

Carbon dioxide (CO2), the major greenhouse gas that causes global warming, has become a major concern as its shows an increasing emission trend. Amino acids (AA) are gaining more attention as an alkanolamines alternative for CO2 capture because they are naturally non-toxic and contain an amine functional group. The primary goal of this research is to develop a new amino acid-based deep eutectic solvent (AADES) supported silica gel (SG) for the adsorption-desorption study of CO2 in post-combustion conditions. The effect of L-Arginine (Arg)’s addition in the urea (U), ethylene glycol (EG) and glycerol (Gly) based deep eutectic solvent (DES) mixture was investigated for screening purposes. The effectiveness of using the wet impregnation method to immobilize AADES on SG has been studied using an elemental analyser and attenuated total reflection-fourier transform infrared (ATR-FTIR) while thermal stability and porosity were characterized using a thermogravimetric analyser (TGA), and an automated gas sorption analyser (Autosorb iQ2). The new AADES/SG adsorbents were then evaluated for CO2 sorption performance from CO2 isotherm using an Autosorb iQ2 analyser at 100% CO2 loading and TGA kinetics at flue gas conditions (15% CO2/85% N2). Results show that different hydrogen bond donors (U, EG, Gly) gave different effects on appearance and solubility. The basicity of AADES increased as Arg was added and this shows a good sign for CO2 capture analysis. The AADES has successfully impregnated in SG as the value of C, H and N contents increased and an appearance of hydroxyl, amino, and alkyl group peaks was observed in the FTIR spectrum. Utilization of Arg as hydrogen bond acceptor (HBA) and addition in ChCl- EG gives a slightly higher thermal stability and N content. Amongst others, Arg-EG/SG showed the best CO2 capture capacities of 18.1 mg/g at 25 °C under 100% CO2 inlet concentration conditions. The trend of adsorption capacities is similar to the reducing trend of AADES/SG’s surface area and the CO2 uptakes of all sorbents were increased almost linearly with increasing pressure. The Arg-EG/SG also shows a higher CO2 adsorption performance of 4.70 mg/g for a 1-hour loading of 15% CO2 at room temperature (25 °C). The Arg-EG(1:8)/SG is found more selective toward CO2 than ChCl-EG(1:2)/SG and ChCl-EG-Arg(1:2:0.1)/SG sorbents. Due to the good performance of Arg-EG/SG on CO2 capture, the effect of using different molar ratios of Arg-EG from 1:5 to 1:8 was further analysed. By using Autosorb iQ2 analyser at 25 °C and 1 atm, the order of CO2 capture capacity’s at 100% CO2 loading for prepared sorbents was Arg-EG(1:6)/SG > SG > Arg-EG(1:7)/SG > Arg-EG(1:8)/SG > Arg- EG(1:5)/SG. However, at low pressure (<0.2 atm), the Arg-EG(1:5)/SG shows the highest capacity with the help of a chemical reaction between CO2 and the primary amine groups. Besides, the Arg-EG(1:5)/SG shows better selectivity of CO2 adsorption (6.60 mg/g) at flue gas conditions (15% CO2/85% N2) than other sorbents using a TGA analyser. The N content increases as the molar ratio of EG decreases and this may be the cause of the higher selectivity for Arg-EG (1:5)/SG. The CO2 cyclic adsorptiondesorption studies were conducted for Arg-EG(1:5)/SG at 25 °C and the finding shows that the CO2 capture capacity remained substantially constant within four cycles. Overall, the Arg-EG sorbents can be utilised as a potential green sorbent for CO2 capture as it shows good CO2 adsorption and desorption capacity at flue gas composition.