Study on carboxyl groups in direct liquefaction of lignite: Conjoint analysis of theoretical calculations and experimental methods

In order to investigate interactions between carboxyl groups and aromatic hydrocarbons during direct coal liquefaction (DCL), tetralin (THN) and benzene (BZ) were selected to represent hydrogenated and nonhydrogenated aromatic compounds in DCL solvents, respectively. 3-phenylpropionic acid (PA) was used as model compound of fragments with carboxyl groups in lignite. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) coupled with density functional theory (DFT) calculations were employed to evaluate noncovalent interactions between carboxyl groups and aromatic compounds (THN and BZ). Results of DFT calculations were verified by ATR-FTIR. Moreover, influence of hydrogen bonds on reaction behaviors of carboxyl groups was investigated by DCL experiments of demineralized Yunnan lignite (DeYN) in THN, with or without addition of BZ. Results show that stronger hydrogen bonds exist between carboxyl groups and BZ, and consequently C-O bonds in carboxyl groups are significantly weakened, compared to THN. Presence of BZ results in decomposition of more carboxyl groups and production of less CO2, but has little effects on aromatic structure. Conclusions drawn from DCL experiments can be explained by ATR-FTIR and DFT calculations. Hydrogen bonds play crucial roles in reaction behaviors of carboxyl groups in coal-oil slurry during DCL. C-O bonds in carboxyl groups of lignite can be obviously weakened by formation of stronger hydrogen bonds. Consequently, decomposition of carboxyl groups is enhanced but generation of CO2 is suppressed.