Theoretical Study For Solvent Effect On Configuration And Hydrogenation Of Nitrile Butadiene Rubber

The research of solvent effect on heterogeneous hydrogenation of nitrile butadiene rubber（NBR）is significantly important for the production of HNBR.The characteristics for different types of solvents have different effects on the dissolution and hydrogenation processes of NBR.In this paper,the effects of different solvents on the configuration（bond length,bond angle）,dipole moment,and frontier orbit of NBR molecules were investigated from the theoretical calculation using density functional theory（DFT）.At the same time,the influence of solvent effects on the dissociation process of H₂ on Pd clusters was systematically investigated,and the influence of solvent effect on the hydrogenation of NBR was explored.This study determined the selection principle for solvents and constructed a solvent system suitable for heterogeneous hydrogenation of NBR.Based on the results of NMR characterization,we first obtained the optimized and stable structure of NBR configuration model i,and investigated the influence of different types of solvents with increasing polarity（dioxane,benzene,ethyl acetate,tetrahydrofuran,butanone,acetone and dimethylformamide）on the configuration of NBR molecules.The bond length of carbon-carbon double bonds（C=C）and carbon-nitrogen triple bonds（C≡N）,bond angle,dipole moment,frontier orbit,and solvation energy were statistically analyzed both in gas phase and in different solvents.The calculation results showed that compared with the gas phase environment,the C=C and C≡N bond lengths of NBR molecules in the solvent became longer,the dipole distance became larger,and which increased with increasing solvent polarity but when the solvent polarity was higher than 20,its influence gradually became less.The bond angle had a certain change in different solvents,but the change was small and the trend was not obvious.The orbital distribution of the highest occupied molecular orbital（HOMO）of NBR molecules gradually shifted into the internal molecular chain with the increase of solvent polarity,making the active center of the chemical reaction move to the inside the NBR molecular chain,which had a positive influence on the NBR hydrogenation.The particle sizes of NBR molecules in different solvents were characterized by dynamic light scattering（DLS）.The calculation results showed that the particle size of NBR was related to the solvation energy.The larger the solvation energy,the greater the interaction between solvent and NBR,and the more fully dissolved of NBR molecules,leading to the small particle size of NBR.While,when the solvation energy was small,the interaction between NBR molecules was large,and the molecular chains were easy to agglomerateto form a large particle size.The second part of the dissertation systematically studied the mechanism of the adsorption,dissociation and migration of H₂ molecules on Pd₆ clusters（as the theoretical model of Pd catalysts）in different solvents.The dissociation energy barriers of H₂ on Pd₆ clusters in different solvents were also calculated to explore the dissociation process of H₂ on Pd₆ clusters under different enviorment.The calculation results showed that due to the solvent effects on the surface electrostatic potential of Pd₆ clusters,the initial adsorption states（IS）of H₂ on Pd₆ clusters were different,and thus the dissociation of H₂ on Pd₆ followed different pathways.The dissociation pathway in gas phase,dioxane and benzene was the same,and when the solvents were ethyl acetate,tetrahydrofuran,butanone,acetone,and dimethylformamide,it follows another H₂ dissociation parthway.Due to the dissociation pathway of H₂ on Pd clusters were different in different solvents,the dissociation energy barriers of H₂were different.The energy barriers were higher in the less polar dioxane and benzene solvents（34.60 and 34.63 kJ/mol）,which were not conducive to the dissociation of H₂molecules.While energy barriers in the more polar solvents were lower such as ketones（butanone:15.16 kJ/mol,acetone:14.99 kJ/mol）and dimethylformamide（14.37 kJ/mol）,favoring for the H₂ dissociation.Finally,a polymerized unit combining a acrylonitrile and a 1,3-butadiene molecule was chosen as a theoretical model for the calculation of catalytic hydrogenation process of NBR molecules on Pd₆ clusters.The solvent effect on NBR adsorption,co-adsorption of H₂ and NBR and the catalytic hydrogenation reaction mechanism were systematically studied.The calculated results showed that the adsorption energy of NBR on Pd₆ clusters and the bond lengths of C=C and C≡N bonds increased with the increase of solvent polarity,and when the solvent polarity was higher than 20,the increase trend is not obvious any more.The solvent with a higher polararityhad a greater degree of activation of the C=C double bond in the NBR molecule,which was more conducive to the hydrogenation reaction of NBR on the Pd catalyst.Compared to the dioxane and benzene less polar solvents,the formation of intermediate state and transition state were easy,and the hydrogenation barrier were lower in the middle of higher polar ketones and dimethylformamide.However,the intermediate,transition state energy and hydrogenation energy barrier were relatively higher in the low-polarity solvents,which was not conducive to the progress of the hydrogenation reaction.