Hydrogen Transfer Mechanism Of Solvent In Coal Hydrogenation Reaction

In the process of preparing liquid oil by coal hydrogenation reaction,the composition and structure of hydrogen-donor solvents have a decisive influence on the conversion rate and oil yield.The most important component of hydrogen-donor solvents is hydrogenated aromatics whose hydrogen supplying capacity is closely related to the dissociation energy of C–H bonds(BDE_C–H)present in their molecular structure.Despite the adequate knowledge about the composition of solvents is so far attained,the complexity in the composition of liquefied solvents still leads to obscurity in the direct relationship between composition and molecular structure.This impedes the process of improving the yield of coal liquefaction by optimizing the solvent system.In this paper,the functional groups and aromatic structures of Hydrogen-donor solvents procured from Shenhua Shanghai Research Institute Pilot Plant（RS-S）and Ordos Industrial Plant（RS-E）were initially analyzed by infrared spectroscopy,ultraviolet-visible spectroscopy and simultaneous fluorescence spectroscopy to have the knowledge about molecular structure.The liquefied solvents were qualitatively and quantitatively analyzed based on the group-selective two-dimensional gas chromatography-mass spectrometry/hydrogen flame ionization detector method.Finally,the suitable solvent model compounds were selected and the C–H bonding energy of hydrogenated aromatics in the solvent molecules and the influencing factors,such as substituents,number of benzene rings and degree of hydrogenation,were calculated using density functional theory.The reaction mechanism between the Hydrogen-donor solvents and coal model compounds was further investigated based on the above analysis.The following main results and conclusions were obtained:1.The composition and structure of RS-S and RS-E.The spectral data corresponding to C–H stretching and bending vibrations,and C=C vibrations indicate that the Hydrogen-donor solvents contain the aromatic rings as well as alkyl groups.Both the Hydrogen-donor solvents obviously have the condensed aromatic rings,mainly the bi-cyclic or tri-cyclic aromatic systems.The content of hydrogenated aromatics was the highest in the analyzed two Hydrogen-donor solvents.The content of hydrogenated aromatics in RS-S was 15.79%higher than that in RS-E,and the content of cycloalkanes was 17.11%lower than that in RS-E,indicating that the liquefied solvent of RS-E was over-hydrogenated,and the hydrogen supplying capacity of RS-S was higher than that of RS-E.2.BDE_C–H of cycloalkane ring in hydrogenated aromatics.Density functional theory calculations show that the substitution of different functional groups at theα（1）position of tetralin promoted the cleavage of C1–H and had no favorable effect on the cleavage of other C–Hs.The effect of electron-withdrawing substituents that promotes the cleavage of C1–H is significantly higher than that of electron-donating substituents.The BDE_C1–Hof C–H bond located at theα（1）position of substituted tetralin was correlated with the spin density value on C1 after the dissociation of H atom.If the spin density value on C1 after the dissociation of H atom is small,BDE_C1–H will be lower and the C1–H bond will be easily cleaved.The solvent polarity has a less effect on the C–H cleavage.The increase in the number of benzene rings decreases the BDE,i.e.,C–H bonds are easily cleaved and relatively,C–H bonds of linearly aligned molecules are more easily cleaved than C–H bonds of angularly aligned molecules.The solvent with a higher degree of hydrogenation increases the BDE_C–H,which is not favorable for the bond cleavage.3.Hydrogen transfer mechanism of Hydrogen-donor solvent.Density functional theory calculations also indicate that the simultaneous transfer ofβ-H andα-H from tetralin to ipso-C and meta-C of diphenylmethane,respectively,is a possible pathway in the direct coal liquefaction process,i.e.,a synergistic bi-hydrogen transfer mechanism.This mechanism is in good agreement with the hydrogen supplying capacity of different solvents in the direct coal liquefaction experiments.According to the proposed mechanism,it is found that the weakest bonds（BDE<270kJ/mol）are first thermally dissociated into radicals and then stabilized by the hydrogen donor.The bonds with a higher BDE are cleaved and stabilized by the synergistic bi-hydrogen transfer mechanism.