Photocatalytic Performances Of MgAl(ZnTi)-LDH And Their Composite Materials For Selective Oxidation Of Benzyl Alcohol Under Visible Light

In recent years,photocatalytic selective oxidation of alcohols to aldehydes has attracted the attentions and researches of a large number of scientists.The reaction condition is mild,efficient and pollution-free.It is a green synthesis method.But the key challenge for this technology is to develop new,highly efficient and inexpensive photocatalysts.As an inorganic layered anion material,layered double hydroxides(LDHs)have been widely used in photocatalysis researches because of their unique structural characteristics,but their researchs in photocatalytic organic synthesis are few.In addition,as a typical alkaline material,LDHs show great advantages in the field of thermal catalytic oxidation of alcohols.The surface basic sites can promotes the cleavage of the alcoholic hydroxyls and improve the reaction efficiency.Therefore,in this thesis,the LDHs with high visible light catalytic performance for the selective oxidation of benzyl alcohol were synthesized by noble metal modification and metal cations tunability.The influence of LDHs surface basic sites,lamellar elements and surface-supported noble metals on photocatalytic reactions were revealed.The application potential of LDHs in the field of photocatalytic organic synthesis was demonstrated.The main research contents and results are as follows:(1)The hydrothermal method was used to synthesize MgAl-LDH.Au-Pd alloy was successfully supported on the surface of MgAl-LDH by impregnation-reduction method to gain a series of Au-Pd/MgAl-LDH composite photocatalysts that are used for the selective oxidation of benzyl alcohol to benzaldehyde under visible light irradiation.The experimental results showed that Au9-Pd1/LDH had the best catalytic activity with the 91.1%conversion and 99%selectivity.Its conversion was much higher than that of corresponding single-metal supported catalysts and was four times higher than that of Au9-Pd1/TiO2.It has been found that the synergistic electron effect between Au-Pd alloys enhances the interaction between Pd atoms and O2 and promotes the formation of superoxide radicals.At the same time,the surface base sites of MgAl-LDH can activate benzyl alcohol molecules to form metal alkoxide intermediates.Finally,the possible photocatalytic mechanism was proposed.(2)ZnTi-LDH was synthesized by distributing TiO6 and ZnO6 units into theLDHs matrix and was firstly applied to photocatalytic organic synthesis.Under UV light,ZnTi-LDH showed better catalytic activity than TiO2.This is due to the high dispersion of the active components and the large specific surface area,which effectively inhibits the photoelectron-hole recombination.In addition,ZnTi-LDH,which did not have visible light absorption,also exhibited excellent catalytic activity under visible light,with conversion and selectivity of 38%and 99%,respectively.The surface complex formed by ZnTi-LDH and benzyl alcohol molecules can be excited by visible light to generate photo-generated carriers,which induced the visible light catalytic reaction.The surface basic sites on ZnTi-LDH have an important influence on the complexation.The noble metals were modified on ZnTi-LDH by impregnation-reduction method,and the M/ZnTi-LDH composite photocatalysts(M=Au,Pd,Au-Pd)were constructed,and their visible light catalytic performances were studied.The results showed that the noble metals will cover the base site of ZnTi-LDH and inhibited the complexation of ZnTi-LDH with benzyl alcohol molecules.