Efficient And Selective Hydrogenation Of Benzonitrile To Benzylamine: Improvement On Catalytic Performance And Stability In Trickle-bed Reactor

Benzylamine?BA?is a versatile chemical compound that has found widespread applications as pharmaceutical intermediates,paints,solvents,herbicides and synthetic textiles.In this paper,efficient and selective hydrogenation of benzonitrile to benzylamine in trickle-bed reactor was investigated systematically.The hydrogenation of benzonitrile?BN?was compared in the trickle bed reactor?TBR?and the batch reactor?BR?.Excellent performance of Pd/?-Al₂O₃ catalyst in TBR and several meaningful results were obtained:?1?Thermodynamic analysis was carried out which indicates that toluene is the most stable product thermodynamically and benzylamine,regarded as the intermediate product,prefer to generate toluene?TOL?through deamination reaction.?2?In the conventional batch reactor,the catalysts were optimized and the excellent performance was obtained on both Raney Ni and Pd/Al₂O₃ catalyst.However,they exhibited different performance in the byproduct selectivity:Raney Ni preferred to generate dibenzylamine?DBA?while the Pd/Al₂O₃ preferred to obtain TOL and rare formation of DBA.?3?Various phase alumina supports were prepared including Gamma???,Alpha???,Eta???and Theta???Al₂O₃.These supports were then impregnated and calcined to prepare 0.5 wt%Pd catalysts.The prepared catalysts showed different active sites,dispersion and adsorption strength.Their catalytic performances were evaluated for selective hydrogenation of benzonitrile.The benzonitrile conversion showed limited relationship with the alumina type.The dibenzylamine formation was considered to be a result of competitive reactions and performed as a function of the metal dispersion.?4?The Pd/Al₂O₃ catalyst exhibited a higher catalytic performance in TBR than that in BR.Further investigation showed the excellent mass transmission and substantial back-mixing restraint in TBR,which leads to extremely lower selectivity of DBA with high activity simultaneously.Moreover,a conversion rate(96 mol/mol_pd h)with 93.2%selectivity to benzylamine on Pd/?-Al₂O₃,are significantly higher than that reported(ca.7.5 mol/mol_pd h)for the same reaction over Pd/?-Al₂O₃ with the highest reported selectivity of benzylamine?90%?in batch reactor.?5?The catalysts were optimized in TBR and the Pd/Al₂O₃ catalyst delivered a excellent performance than that in document with benzonitrile conversion of 96.7%and benzylamine selectivity of 88.9%.?6?The reaction parameters of Pd/Al₂O₃ catalyst were studied in TBR and the results were showed as follows:The optimum temperature range for catalyst reduction is 290 ^oC to 325°C,the solvent was ethanol,the catalyst particles were 30-50 mesh,the reducing pressure was atmospheric pressure,the reaction temperature was 70 ^oC,the reaction pressure was 1.5 MPa,the ratio of ethanol to benzonitrile was 4,the ratio of hydrogen to benzonitrile was 20,and the liquid hourly space velocity was 0.5 h^-1.?7?The Pd/Al₂O₃ catalyst was investigated the deactivation and mechanism of coke formation on metal catalysts for the selective hydrogenation of benzonitrile in TBR.The Pd/Al₂O₃ catalyst exhibited the initial benzonitrile conversion of¹00%with a high benzylamine selectivity of⁸6%,and continuous deactivation during the long-term reaction period of¹500 h.The comparative investigation of the Pd/Al₂O₃catalysts before and after the benzonitrile hydrogenation demonstrated that the deactivation of the Pd/Al₂O₃ was mainly attributed to carbonaceous compounds deposited on the catalyst.The carbonaceous species contained the aromatic ring conjugated with C=N double bond and tertiary amine groups.Moreover,the carbon deposition also resulted in partial blockage of the support pores.