Novel insights into the catalytic effect of hydrogen bonds on aldehyde groups generation during direct coal liquefaction

Abstract

Direct coal liquefaction (DCL) is considered to be the effective method for production of aviation fuel and valuable chemicals, while low rank coal is appropriate feedstock because of high thermal reactivity. Original hydrogen bonds could promote low-temperature crosslinking reactions. liquefaction solvents could partially break original hydrogen bonds by forming new species. Impacts of hydrogen bonds formed by solvents on DCL reactions have rarely been studied. In order to figure out role of hydrogen bonds in DCL, a conjoint study of DCL experiments and density functional theory (DFT) calculations was carried out. More carboxyl groups were promoted to decompose. Meanwhile more aldehyde groups were found in solid products with increase of liquefaction temperature. As aldehyde groups are generated, hydrogen consumption increases. In terms of mechanism, effect of stronger hydrogen bonds on cleavage of covalent bonds were investigated by DFT-based potential energy surface scan. It turns out that cleavage of CO bonds in carboxyl groups could be enhanced, so that stabilization of free radicals results in consumption of extra hydrogen, which is provided by tetralin. Results of experimental methods are consistent with DFT calculations. The catalytic mechanism of hydrogen bonds upon conversion of carboxyl groups during DCL is proposed. Hydrogen bonds between coal and liquefaction solvents promote decomposition of carboxyl groups and consequently improve hydrogen transfer and thermal reactivity of solid products. Consequently, hydrogen bonds modulation should be taken into account during preparation of DCL solvents, for suppression of crosslinking reactions.