Hydrogen Transfer Rate Constants Of H-donor Solvents In Direct Coal Liquefaction

In the direct coal liquefaction process,the molecular structure of the hydrogen-donor solvents（H-donors）affects its hydrogen-donating activity,which in turn affects the liquefied oil yield and products distribution.However,the hydrogen-donating activity of H-donors is mainly evaluated by liquefaction experiments.There is no effective method to evaluate the hydrogen-donating activity of H-donors based on theoretical analysis.In addition,the structure-activity relationship between the molecular structure and hydrogen-donating activity of H-donors is not clear.It is impossible to determine which structure of H-donors has higher hydrogen-donating activity.In this paper,based on the hydrogen transfer mechanism,the hydrogen transfer rate constants of H-donors were calculated using the density functional theory（DFT）calculations and transition state theory（TST）.Rate constants of hydrogen transfer from H-donors to coal radicals were calculated based on the pyrolytic radical mechanism;rate constants of hydrogen transfer from H-donors radicals to coal model compounds were calculated based on the solvent-mediated hydrogenolysis mechanism.Then the hydrogen-donating activity of H-donors was quantitatively analyzed by their rate constants.After that,the structure-activity relationship of H-donors was investigated based on electronic effect and frontier orbital theory to reveal the chemical nature affecting the hydrogen-donating activity of H-donors.In addition,the effects of different coal radicals and coal model compounds on hydrogen transfer in the liquefaction process were also investigated.The main results and conclusions were obtained as follows:（1）Hyperconjugation and ring strain play essential roles in the hydrogen-donating activity of H-donors.Compared with aromatics and cycloalkanes,hydrogenated aromatics have higher hydrogen-donating activity due to hyperconjugation.The hydrogen-donating activity of different hydrogenated aromatics is related to their hydrogenation depth of aromatics.The lower the depth of hydrogenation,the higher the hydrogen-donating activity.Therefore,controlling hydrogenation depth is an effective measurement of testing the hydrogen-donating activity of H-donors.Different positions,chain lengths and isomerization of alkyl substituents also affect hyperconjugation,resulting in various hydrogen-donating activity.The hydrogen-donating activity is enhanced when the methyl group is substituted at theαposition of tetralin.With the increase of alkyl substituents’chain length,the hydrogen-donating activity of 1-ethyltetralin,1-propyltetralin,and 1-butyltetralin are close but lower than that of 1-methyltetralin.The alkyl isomerization has a significant influence on the hydrogen-donating activity of H-donors.Due to a higher ring strain,the hydrogen-donating activity of H-donors containing five-membered saturated rings is lower than that of the hydrogenated aromatics with similar structures.（2）Different coal radicals have different abilities to extract hydrogen from H-donors.In the polycyclic aromatic radicals with more benzene rings,the stronger p-πconjugation makes these radicals more stable.As a result,the hydrogen transfer from tetralin to the polycyclic aromatic radicals with more benzene rings is slow.It may lead to the condensation between these radicals,reducing the liquefied oil yield.（3）In addition to stabilizing coal radicals,H-donors can also transfer hydrogen to ipso-C of the coal model compounds and promote the breakage of the C_aryl–C_alkylbonds in the model compounds.Hyperconjugation in 9,10-dihydrophenanthrene,4,5-dihydropyrene and4,5,9,10-tetrahydropyrene radicals are stronger,causing more effective delocalization ofσelectrons on C_α–H.Therefore,it is easier for them to transfer hydrogen to ipso-C of the coal model compounds,which can promote the breakage of the C_aryl–C_alkylbonds in model compounds more effectively.Hence,when evaluating the hydrogen-donating activity of9,10-dihydrophenanthrene,4,5-dihydropyrene and 4,5,9,10-tetrahydropyrene,in addition to their ability to stabilize coal radicals,their hydrogenolysis ability should also be considered.Rate constants of hydrogen transfer from H-donors radicals to ipso-C of different coal model compounds did not correlate with the dissociation enthalpies of C_aryl–C_alkylbond.Among the coal model compounds containing the same functional group,the coal model compounds with functional groups directly attached to benzene rings are more easily hydrolyzed by H-donors radicals.