Screening On Hydrothermally Stable Materials And Their Application In Aqueous Phase Catalytic Reactions

With the deepening of the perception in environmental protection,clean energy production and resource recovery from pollutants have become a very pursuing direction in the development of chemical technologies,including resourceful treatment of high concentration organic wastewaters from coal chemical industry,Fischer-Tropsch synthesis,biomass conversion processes,and hydrogen production using aqueous-phase reforming,etc.These processes are all facing harsh hydrothermal reaction conditions and stability problems of the employed catalysts.The activeγ-Al₂O₃ has advantages of low cost,high mechanical strength and high specific surface area,and is widely used as supporting material for solid catalysts.However,it will be easily hydrated under hydrothermal conditions,which changes its structure and properties,reduces its mechanical strength,leads to an irreversible deactivation and a reduced catalyst life.Therefore,improving the hydrothermal stability of catalyst support is the key to increase the catalysts’lifetime.Developing hydrothermally stable supporting material thus becomes a prerequisite in the synthesis of hydrothermally stable catalysts for aqueous phase,high efficient,and environmental benign processes.The hydrolysis and broken of Al-O bond on the surface of alumina in aqueous phase is the key step in the hydration of alumina.It can be summarized from the literature results that the methods for improving the hydrothermal stability of materials for catalyst support include heteroatoms doping,surface modification,and water-resisting film coating onto the surface.In this research work,to develop a catalyst support with high hydrothermal stability,various kinds of metal and non-metal elements were introduced into alumina,and the influence of material composition,crystalization state,treatment temperature and preparation methods,etc.on the hydrothermal stability of the material were investigated.The finally obtained material was used to prepare catalyst,and the hydrothermal stability and catalytic behavior of the prepared catalyst in aqueous phase reactions were further investigated under different reaction conditions.The main results obtained are as follows:（1）Different elements show different effects on improving the hydrothermal stability of alumina,Fe,Mg,Znshowed the strongest effects on it.For the elements having significant effect,increasing their addition amount improves the hydrothermal stability ofγ-Al₂O₃ to a certain extent,but no substantial improvement on the hydrothermal stability was observed when the calcination temperature was increased,and it only improves the crystallinity of the sample.（2）ZnO@Al₂O₃ and ZnAl₂O₄were found to exhibit excellent hydrothermal stability.After24 h hydrothermal treatment at 200°C,no new hydrates phase can be detected and the pore structure of the sample is also maintained.The reason for the higher hydrothermal stability of the ZnO-modified alumina is discussed regarding to the material properties characterized by XRD,XPS,FTIR,SEM,nitrogen physical adsorption,and contact angle test.（3）The contact angles ofγ-Al₂O₃,ZnO@Al₂O₃,ZnAl₂O₄ with water are 9.9°,14.8°and 7.9°,respectively,which all show hydrophilic surfaces,and indicate no direct correlation between the higher hydrothermal stability of the ZnO-modified alumina and its surface hydrophilicity/hydrophobicity.The surface energy ofγ-Al₂O₃,ZnO@Al₂O₃,and ZnAl₂O₄calculated using the derived formula of Young’s equation is 71.7 m J/m²,70.5m J/m² and72.1m J/m²,respectively.Theoretical calculation shows that OH^-ions prefer to adsorb with or attach on Al atoms instead of Znatoms,OH^-is more likely to react with Al₂O₃ than with ZnO.The XPS data shows that the addition of Znto alumina increases the O1s binding energy of the sample,which increases the difficulty of breaking Al-O bond under hydrothermal environment.Znshows greater effect than Mg,which explains the better hydrothermal stability of ZnAl₂O₄.（4）Pd/ZnAl₂O₄ catalyst was prepared by impregnation method,using ZnAl₂O₄ as the support.Its catalytic performance is compared with that of Pd/γ-Al₂O₃.In the aqueous phase hydrodechlorination of 2-chlorophenol and the aqueous phase reforming of methanol and acetic acid,Pd/ZnAl₂O₄ demonstrated similiar catalytic activities but better hydrothermal stability than Pd/γ-Al₂O₃.（5）The effects of reaction temperature,the formic acid concentration in water and the catalyst amount used for the reaction were investigated on the reaction rate of hydrodechlorination2-chlorophenol in aqueous phase using formic acid as the hydrogen donor.The apparent rate constant of the reaction was determined,and the kinetic equation of this hydrodechlorination reaction was established.The calculated apparent activation energy E_a is 30.859 k J/mol,and the pre-exponential factor A is 8.764×10⁷ mol/（L·min）,and the relationship between the rate constant and temperature is determined as lnk=-3711.665/T+11.381.The hydrodechlorination of 2-chlorophenol on Pd/ZnAl₂O₄ was determined to be a pseudo-first-order reaction.The optimal reaction temperature was 40℃,and the optimal amount of the catalyst was 0.5 g/L,respectively.99.9%conversion of 2-chlorophenol on Pd/ZnAl₂O₄ can be obtained,and after 100 h of reaction,the catalyst can still maintain a relatively stable catalytic performance.