Preparation And Characterization Of Pd-Fe/AC Bimetallic Catalyst And Its Application In The Process Of Hydrogenation And Dechlorination Of Bleached Shella

Shellac is a natural resin secreted by shellac insects after eating the juice of parasitic tree species.After being bleached,it is widely used in medicine and food industries.Because NaClO is often used as bleach agent in the bleaching process,chlorine will be introduced and combined with shellac in the bleaching operation to form binding chlorine which is difficult to be removed.In recent years,Pd-Fe bimetallic catalysts have great application prospects in the removal of chlorinated organics due to mild reaction conditions,high catalytic activity and good selectivity.However,the traditional liquid phase reduction process has the disadvantage that the active metal components are easy to agglomerate,which leads to the decrease of catalytic activity.In this paper,a new method of preparation of Pd-Fe/AC bimetallic catalyst by ultrasonic impregnation and ultrasonic liquid phase reduction was proposed.The technological conditions of preparation of Pd-Fe/AC bimetallic catalyst were studied and the best preparation technology was obtained.The structure and properties of the prepared catalyst were characterized by FTIR,XRD,XPS,HRTEM and electrochemical tests.Meanwhile,the catalytic removal of chlorine from bleached shellac was studied.Firstly,the modification of activated carbon(AC)was studied.The results showed that the number of polar groups on the AC surface of the carrier increased by nearly40% compared with the traditional H2O2 modification when the mass fraction of HNO3 was 15% as the modifier and the temperature was 50℃ for 60 min.The modified AC was an ideal catalyst carrier.Secondly,the preparation of Pd-Fe/AC bimetallic catalyst using modified AC as support by ultrasonic impregnation-ultrasonic liquid reduction process was studied.The effects of ultrasonic intensity,ultrasonic time,reduction time,reduction temperature and sodium borohydride dosage on Pd-Fe bimetallic catalysts were investigated.The results showed that the optimal preparation process was as follows: ultrasonic intensity 720 W,ultrasonic time 30 min,reduction time 120 min,reduction temperature 50℃,sodium borohydride dosage(4mg-m L-1)100m L.Under these conditions,the active components were evenly distributed on the activated carbon structure and showed high catalytic activity of Pd-Fe bimetallic alloy.The electrochemical tests show that the activity of the catalyst obtained is 14% higher than that prepared by the traditional liquid reduction method,and the electrochemical stability is higher.Finally,the effects of catalyst dose,reaction time,reaction temperature and stirring rate on the removal of chlorine from bleached shellac were studied.The results show that: The Pd-Fe/AC bimetallic catalyst with the molar ratio of Fe at 1:2 was prepared by modifying AC with 15% HNO3 at 75℃ for 1 hour.The catalyst dosage was 0.8g,the reduction time was 90 min,and the reduction temperature was 75℃.The chlorine removal rate of bleach shellac could reach 92.62%.The kinetics of dechlorination process of bleaching lac showed that the activation energy of dechlorination reaction was Ea=27.57KJ/mol,which was lower than common chemical reactions.The reaction rates of 50℃,60℃ and 70℃ were 0.02456min-1,0.03496min-1 and 0.04464min-1,respectively.