Studies On Supported Silver Catalysts Toward Oxidant-free Dehydrogenation Of Benzyl Alcohol

Benzaldehyde,as an important organic intermediate,has been widely used in medicine,resins,pesticides,spices and other industries.Liquid phase oxidation of benzyl alcohol to Benzaldehyde has been regarded as a "green"synthesis technology,with advantages of mild reaction conditions,atomic economy,less by-products and facile separation.Liquid phase oxidation of benzyl alcohol involves two formulations:aerobic conditions and oxidant-free conditions.The former,in the presence of O2,suffers from deep oxidation and poor selectivity to benzaldehyde.However,these issues are avoided in the latter one with a high utility value of alcohols.In recent years therefore,liquid phase dehydrogenation of alcohols under oxidant-free conditions has attracted much attention.Supported Ag catalysts serving as selective oxidation catalysts of alcohols have been used in the industrialization of methanol oxidation to formaldehyde.In the oxidant-free dehydrogenation of benzyl alcohol,supported Ag catalysts theoretically have great research potential.On the one hand,supported Ag catalysts show a huge cost advantage over the platinum-group noble metal-supported catalysts.On the other hand,rational design and preparation of supported Ag catalysts would provide opportunities for enhancement of catalytic performance.However,several issues remain unresolved in the study of high performance supported silver catalysts:1)How to effectively regulate the active structure of the supported silver catalysts;2)How to understand synergistic catalytic mechanism and reveal the structure-property correlation.In view of the above problems,in this dissertation,mixed metal oxides from the topological transformation of Mg-Al hydrotalcite precursors were prepared and used as support for the immobilization of metallic silver nanoparticles.The loading content of silver,calcination temperature,and acidity and basicity of the support were tuned so as to obtain the optimal supported silver catalyst.In addition,through a series of characterization methods,the structure of catalyst and the active sites were deeply understood,and the structure-property relationship was revealed.A synergistic catalysis between Ag active site and acid/basic sites of support occurs in the oxidant-free dehydrogenation of benzyl alcohol.The main research results are as follows:1.Influence of Ag active site on the catalytic performance of oxidant-free dehydrogenation of benzyl alcoholBy using impregnation method,silver nanoparticles were supported on a Mg-Al MMO support formed by topological conversion of a Mg-Al LDH precursor.By controlling the Ag loading content(1%,2%,3%,4%),which were then used in the oxidant-free dehydrogenation of benzyl alcohol.The catalytic evaluation results showed the catalyst sample with 2%Ag loading(2Ag/MgAl MMO(350))gives the best catalytic performance.At 100?,the conversion is 95.89%,the selectivity to benzaldehyde is 93.28%,and the benzaldehyde yield reaches to 89.45%.Structural characterizations confirm that Ag0 and Ag?+ coexist in these catalyst samples,in which Ag?+ species originates from specific dissociative adsorption of Ag and O.Catalytic evaluations show that the rate of benzaldehyde formation is positively correlated with the concentration of Ag?+ in the catalyst samples,indicating Ag?+ serves as intrinsic metal active site of the reaction.The sample of 2Ag/MgAl MMO(350)possesses the largest concentration of Ag?+ species,accounting for the best catalytic performance.2.Influence of acid-base structure of support on the catalytic performance of oxidant-free dehydrogenation of benzyl alcohol.Based on the above work,the influence of acid-base active site structure of support on catalytic performance was further studied,with the same loading of Ag nanoparticles.Three catalyst samples were synthesized with the same Ag loading conditions at activation temperature of 350?,400? and 450?,respectively(denoted as 2Age/MgAl-MMO(350),2Age/MgAl-MMO(400),and 2Age/MgAl-MMO(450)).Structural characterizations show that for these three samples,the morphology,particle size,crystal structure of Ag,and the geometry structure of MMO support remain close.Subsequently,the acid-base sites,acid-base concentration and strength of the support were studied by CO2-TPD and NH3-TPD characterization,and the active site structure of support toward the oxidant-free dehydrogenation of benzyl alcohol was explored.These three catalyst samples were found to give rather different acid/base distribution,acid/base concentration and acid/base strength.Catalytic evaluation results show the rate of benzaldehyde formation is positively correlated with the concentration of medium-strong acid/base sites,which are attributed to the structure of Mg?+-O?-is the support.Based on the results of this work and previous study,a synergistic catalysis between metal active site and acid/base site of support was revealed.