| The emissions of chlorophenols (CPs) into the environment from widespread chemical industries are strictly regulated due the severe influence on health and ecological damage. Therefore, various disposal techniques such as catalytic hydrodechlorination (HDC), thermal degradation, chemical oxidation, ozonation and biodegradation have been developed to eliminate poorly biodegradable CPs from industrial effluents and polluted groundwater. Among these techniques, HDC has been remarkably considered as one of the promising technologies for only removing the toxic chlorine atoms from the objective pollutants with relatively low energy consumption and generating less toxic products that can be more easily biodegraded or even of commercial importance. Supported Pd catalysts are receiving more and more attention for their catalytic activity in treatment of wastewaters containing chlorinated organic pollutants, especially in the HDC reactions. Hence, the development of high activity of Pd catalysts for the HDC of CPs has scientific and practical value.In this paper, a series of supported Pd catalysts were prepared by impregnation method and used for catalytic HDC. The results achieved are listed as follows:1. Catalytic HDC of chlorophenols over Pd/SBA-15and Pd/PS catalystsPd/PS catalysts with various loadings of phosphoric acid were prepared by incipient impregnation of HaPdCl4and then reduced by NaBH4. Characterization of catalysts by means of N2adsorption/desorption, XRD, H2-TPR, H2-TPD and XPS measurement indicates that the modification of phosphoric acid changed the texture of catalysts such as the-decreasing in surface area and pore volume. The mesoporous structure of carrier remains stable after the reducing by NaBH4under the modification of a little phosphoric acid. The mean size of Pd nanoparticles were about6nm whether modified by phosphoric. The ratio of Si-OH/P were50,25,10,7and5, respectively. The corresponding ratio of Pd0/Pdn+were0.424,0.589,0.961,1.264and0.807, respectively. The ratio of Pd0/Pdn+was0.537over Pd/SBA-15.In a typical reaction procedure,1.2wt.%Pd/SBA-15or Pd/PS (nPd:nCl=0.005) was added to a stirred solution of50ml methanol contain of4-CP (0.1mol/L) and Et3N (nN:ncl=1:1) in a100ml three-necked flask. The results show that the activity of Pd/PS catalysts increases with the increase of Pd0/Pdn+ratios. Pd/PS-7catalyst showed the highest activity with the largest ratio of Pd0/Pdn+. And the conversions were50%,68%and98%after480minutes, at the temperature of268,278, and298K, respectively. The only product was phenol. In other conditions, under the temperature of298K, for the hydrodechlorination of2,4-DCP, Pd/PS catalysts showed different dependency of Pd0/Pdn+ratios compared with4-CP. The Pd/PS-50catalyst showed the highest activity with the largest amount of Pdn+. And the conversions were>92%after480minutes. And the products were consists of phenol,2-CP and4-CP. No biphenyl compounds were detected, indicating the mechanism involved ion mechanism.2. Catalytic HDC of chlorophenols over Pd/Al2O3and Pd/PA catalystsPd/PA catalysts were prepared by the method as Pd/PS catalysts. And the same characterizations are applied in these catalysts. The results reveals that the modification of phosphoric acid also changed the texture of catalysts such as decreasing in surface area and pore volume and forming AlPO4. The average size of Pd nanoparticles was about3nm on Al2O3or PA supports. The loading of P was4.85%,2.85%,1.32%and0.57%, respectively. The corresponding ratio of Pdn+/(Pd0+Pdn+) was0.182,0.112,0.103and0.164, respectively. The ratio of Pd0/Pdn+was0.233over Pd/Al2O3.In a typical reaction procedure,0.5wt.%Pd/Al2O3or Pd/PA (nPd:nCl=0.03) was added to a stirred solution of50ml aqueous liquid contain of2-CP,4-CP and2,4-DCP (CCI=0.78mmol/L) in a100ml three-necked flask at298K. The results show that the activity of Pd/PA catalysts increases with the decrease of Pdn+/(Pd0+Pdn+) ratios basically. Pd/PA-0.57%catalyst showed the highest activity for4-CP hydrodechlorination with the ratio of Pdn+/(Pd0+Pdn+) of0.164. The initial rate is84μmol/gcat min-1. Under the same condition for2,4-DCP hydrodechlorination over Pd/PA-1.32%catalyst showed the highest activity with lowest ratio of Pdn+/(Pd0+Pdn+). The initial rate was20.6μmol/gcat min-1. In other conditions, under the temperature of273K, for2,4-DCP hydrodechlorination, the activity of Pd/PA catalysts increases with the decrease of Pdn+/(Pd0+Pdn+) ratios. Pd/Al2O3catalyst showed the highest activity and the initial rate was3.25μmol/gcat min-1. The products were phenol and cyclohexanone. No biphenyl compounds were detected by FID, indicating the mechanism involved ion mechanism.3. Catalytic selectivity hydrogenation of phenol to cyclohexanone over Pd catalystsCyclohexanone (CHONE) is an important intermediate in many petrochemical industries. Traditional preparation of CHONE through direct oxidation of cyclohexane requires high temperature and generates byproducts such as cyclohexanol and organic acids that complicate purification, and the yield of CHONE is usually low. To our best knowledge, a method of liquid phase catalytic selectivity hydrogenation of phenol, which has a high selectivity, mild operating condition and less secondary pollution in simple process installation, shows a much potential in the industry. So the study of selectivity hydrogenation of phenol is still necessarySupported Pd catalysts were used in this work, and traditional SiO2and Al2O3were employed as the supports. Pd/SiO2and Pd/Al2O3were prepared by incipient impregnation of H2PdCl4and then reduced by NaBH4. Characterization of catalysts by means of N2adsorption/desorption, XRD, H2-TPR, H2-TPD and XPS measurement indicates that Pd particles size decreased from13nm to5nm with Pd loading decreasing from1wt%to0.5wt%and from10nm to5nm over Pd/Al2O3and Pd/SiO2, respectively. If the reduction of temperature decreases from298K to273K, the Pd particles sizes reduced to3nm. All the Pd species were metallic state.In a typical reaction procedure, Pd/Al2O3or Pd/SiO2(nPd:nphenol=0.02) was added to a stirred solution of50ml cyclohexane contain of phenol(CPhenol=0.212mol/L) in a100ml three-necked flask at333K. Pd/Al2O3-298and Pd/SiO2-298catalysts with various of Pd particle sizes showed different activity.0.5%Pd/Al2O3-298with5nm Pd particles showed the highest activity and the TOF was304mol mol surPd-h-1. The complete conversion of phenol achieved after180minutes and the yield of CHONE is higher than94%. Pd/SiO2catalysts reached the highest yield of CHONE (>98%) after480minutes. In other conditions, using water as the solvent? the activity of all catalysts decreased especially for Pd/Al2O3-298catalysts.1%Pd/Al2O3-298with Pd particles of13nm showed the highest activity and the TOF was115.2mol mol surPd-1h-1. The complete conversion of phenol achieved after300minutes and the yield of CHONE is higher than95%. The rate of phenol converting to CHONE is higher than the rate of CHONE converting to cyclohexanol in both organic phase and aqueous liquid because of the stronger absorption of phenol. All the reaction steps were speeded up with the increase of hydrogen pressure, which is not favor to high yield of CHONE. The relationship of hydrogenation activity of CHONE to Pd particle sizes and yield of CHONE was studied by changing the concentration of phenol in solutions. For1%Pd/Al2O3-298catalyst, the best condition was that the concentration of phenol is higher than0.128mol/L. Moreover, for the selectivity hydrogenation of phenol derivative to corresponding ketones,1%Pd/Al2O3-273catalyst showed the good activity. |
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