MOF-derived AgCo Alloy Nanoparticle Protected By Porous Carbon As Chemoselective Catalyst For Hydrogenation Of Citral

Citral is a ?,?-unsaturated aldehyde,mainly origined from Litsea cubeba,lemongrass,penicillium,calyx and other plants.Citral can be used as an important raw material for the production of vitamins,carotenoids and other spices in the industry.Nerol and geraniol,the hydrogenation products of citral,have higher economic value and have been widely used in food,Spices and pharmaceutical intermediates,etc.Therefore,the selective hydrogenation of citral to produce nerol and geraniol is of great significance.However,the selective hydrogenation of citral to produce nerol and geraniol is difficult,since it has three unsaturated double bonds:conjugated C=C and C=O double bonds,and isolated C=C double bond in the citral molecule.From a thermodynamic point of view,the C=C double bond is easier to hydrogenate among the conjugated double bonds of citral molecular due to that the C=C bond has lower bond energy and shorter bond length.In order to achieve high conversion,precious metal based catalysts such as Pt,Ru,Ir are used to facilitate the hydrogenation of citral to unsaturated alcohols,but their expensive price limit the large-scale industrial application.The transition metal Co has attracted people's attention due to the low price and excellent selective hydrogenation performance for C=O double bond.However,the excellent effect of the citral catalytic hydrogenation is difficult to achieve with single metal cobalt.The composite material produced by alloying the transition metal Co and the precious metal can solve the problem.The transition-noble metal composite materials not only can reduce the cost of the precious metal catalyst by reducing the amount of precious metal,but also enhance the selectivity and activity of the catalyst by changing the electronic structure via the interaction with the noble metal and transition metal.Metal-organic framework(MOFs)is a new type of crystalline porous material which is constructed by inorganic metal center and organic functional group through covalent bond or ion-covalent bond.Due to the unique characteristics such as periodic porous structure,high surface area,tailored composition,and adjustable functionality,MOFs have achieved rapid development and have a wide range of applications in separation and purification,energy storage,optical sensing,medical imaging,and catalysis.Moreover,employing MOFs as self-sacrificed precursors or templates,a series of nano-structural materials such as porous carbon,metal oxides,metal/carbon composite materials and metal oxide/carbon composite materials could be prepared facilely.Compared with the traditional synthetic methods,the MOF-derived nano-structural materials can not only inherit the structure property of corresponding MOFs,but also allow precise control of the composition and integration of certain functionalities.The metal nanoparticles encapsulated in the porous carbon constructed from MOF templates/precursors were considered as good candidates as the catalytic materials.The content of this work can be mainly divided into the following two parts:1.The xAg/Co-MOF(x represents the mass percentage of silver)precursors were synthesized by one-pot method.The pyrolysis temperature of the precursor was test by thermogravimetric experiments,and the precursor was pyrolyzed under a nitrogen atmosphere to prepare a series of 6AgCo@C-T(T represents the pyrolysis temperature)catalysts.According to the XRD and SEM characterization,the structure of Co-MOF is not affected by the addition of Ag ions,and XPS,TEM and HAADF-STEM showed the formation of the alloy between Co and Ag metals.2.The selective hydrogenation of citral was employed to evaluate the catalytic activities of the as-prepared AgCo alloy.The reaction result shows that the catalyst of 6AgCo@C-510 delivered remarkably selective hydrogenation activity of citral with full conversion and 70%selectivity for hydrogenation of citral to nerol and geraniol under 80?,60 min and 1 MPa H2,which is comparable to the precious metal catalysts,and the catalyst exhibited extremely high catalytic recyclability without any decrease of conversion and selectivity after ten cycles of reuse.In addition,the 6AgCo@C-510 catalyst shows lower activation energy compared with Co@C-510 catalyst for the reaction.