Fuel, Vol.239, 484-490, 2019
Insight into cross-linking reactions induced by carboxylates in direct coal liquefaction using coal-related model compounds and hydrogen transfer calculation
Low-rank coals, preferred feedstocks for direct coal liquefaction (DCL), are rich in ion-exchange metal species which are present in the form of carboxylates. In this work, to better understand the negative effects and mechanisms of ion-exchange metal species on oil yield in DCL, cross-linking reactions which were possibly induced by ion-exchange metal species were investigated by hydrogenation of coal-related model compounds and hydrogen transfer calculation for the first time. The results show that CO and CO2 are main products from thermal decomposition of benzoates, implying that decarboxylation of low-rank coals is active during DCL. Compared with benzoic acid, an obvious change of product distribution during hydrogenation of benzoates is that the total yields of biphenyl, diphenylmethane, and benzophenone (products of cross-linking reactions) increase significantly. Besides, the content of CO2 released from hydrogenation of benzoates is much higher than that from benzoic acid. These phenomena indicate that ion-exchange metal species can accelerate decarboxylation reactions and promote formation of free radicals. In addition, hydrogen transfer calculation further proves that ion-exchange sodium species can affect the formation of free radicals. With the increase of the content of ion-exchange sodium species, more free radicals are generated by decarboxylation reactions and combination among free radicals is enhanced. Eventually, less hydrogen is consumed during DCL, which leads to decrease of oil yield. To conclude, cross-linking reactions can be promoted by ion-exchange metal species through enhancing formation of free radicals.