Effect of coal rank on structure of coal-based solid acids and their catalytic performance in cellulose hydrolysis

Coal-based solid acids (CSAs) were successfully prepared from coals with different ranks at various carbonization temperatures. XRD, FT-IR and ¹³C NMR were employed to characterize structure of the catalysts. The influence of coal rank and carbonization temperature on the heterogeneous catalytic hydrolysis of cellulose was investigated by comparing the yield of reducing sugar and glucose. The results show that coal has a structure advantage over the other traditional carbon sources for solid acids. In contrast to the traditional carbon-based catalysts, the unit structure of CSA is fused aromatic rings linked by bridge bonds (-O-, -CH₂-) and the fused aromatic rings are bearing side chains (-CH₃, -OCH₃, -CH₂CH₃) besides phenolic -OH, -COOH and -SO₃H groups. In addition to the sulfonic groups, the others are derived from the raw coal structure. With rising carbonization temperature, the species and amounts of functional groups and the density of sulfonic acid groups are found to decrease gradually in CSAs, while the aromaticity of CSAs is opposite. The adjustability of CSAs structure and required carbonization temperatures are reduced with increasing coal rank. The CSAs show a higher activity in the cellulose hydrolysis process, wherein the Huolinhe CSA has the highest catalytic activity. The hydrolytic activity is influenced by the size and the stack height of the aromatic sheet, the bridge bond and the sulfonic acid group density, which is a result of the synergistic effect of a number of reactive groups.