Influences of acidic solvents on changes in microstructure and adsorptive properties of lean coal

Abstract

To study changes in the microstructures and adsorptive properties of lean coal due to acidization of different acid solutions, XRD, 13C NMR, XPS, FTIR, and LTGA-N2 were conducted on lean coal samples before and after acidization by HF, HNO3, HCl, and CH3COOH. The changes and influence mechanisms of microstructures and adsorptive properties of the coal were obtained. The results show that:HF and HNO3 increase the C molecular structures by 5.29 % and 3.17 % while HCl and CH3COOH decrease them by 15.92 % and 5.55 %; the four acid solutions reduce the N molecular structures by 53 %, 88 %, 45.9 %, and 46.3 %, respectively. The acidized coal samples have a decreased hydrogen index I1 and oxygen index I2 while showing an increased aromaticity I3;The mass and horizontal diameter La of acidized coal samples show linear negative correlations with the adsorption capacity; the inter-crystal spacing d002 and packing height Lc are linearly positively correlated with the adsorption capacity.After acidization, changes in the CO-C and Si-O contents control the relationship between the content of oxygen-containing functional groups and N2 adsorption capacity. After HF acidization, non-protonized aromatic carbon faN controls the relationship between aromatic structure f'a and adsorption capacity; after HCl, HNO3, and CH3COOH acidization, protonized aromatic carbon faH controls the relationship between aromatic structure f'a and N2 adsorption capacity.