Study On The Mechanism Of Catalytic Hydrogenation Promoted By The Microscopic Behavior Of Water Medium On Palladium Surface

Selective hydrogenation of aromatic carbonyl compounds is an important reaction in industrial catalysis.The selective hydrogenation of carbonyl compounds on Pd catalysts commonly used at present has low selectivity,which leads to low yield of target products and serious consumption of precious metals.In this paper,Pd catalyzed hydrogenation pathway of aromatic carbonyl compounds were regulated by introducing adsorbed water.The microscopic behavior of adsorbed water on the surface of Pd was investigated,and the mechanism of adsorbed water in promoting catalytic hydrogenation of carbonyl was revealed.1.Effect of adsorbed water amount on catalytic hydrogenation of aromatic carbonyl compounds by PdThe traditional supported Pd catalysts were prepared in saturated impregnation method,used in catalytic hydrogenation aromatic carbonyl compounds.The experimental results showed that the hydrogenation activity of all the catalysts increased firstly and then decreased with the increase of adsorption water,while the selectivity of carbonyl hydrogenation increased gradually with the increase of adsorption water.Based on the similar molecular structure of furfural and benzaldehyde,and the degree of hydrogenation promoted by water was significantly different,therefore,furfural hydrogenation and benzaldehyde hydrogenation were selected to study the kinetics,kinetics analysis shows that The addition of water significantly reduced the activation energy of furfural hydrogenation(36%).At the same time,the rate limiting step of the hydrogenation of benzaldehyde is the second step and the reaction rate constant increases from 0.18 mol-1·min-1 to 0.35mol-1·min-12.Study on the mechanism of catalytic hydrogenation of Pd promoted by adsorption of waterAtomic adsorption fluorescence spectroscopy of 4,4-Difluoro-4-bora-1,3,5,7-tetrame thyl-2,6-diethyl-8-(3’-(N,N-dimethyl)benznyl)methyl-3a,4a-diaza-s-indacene(BODIPY)a nd pyridine adsorption showed that in the presence of H2 and water,hydrated ions f ormed near normal pressure XPS characterization analysis on the surface of Pd and i n situ infrared characterization of CO adsorption showed that with the introduction of water,the electron density on the surface of Pd increased.The formation mechanis m of hydronium ions on Pd surface was studied by using density functional theory.I t was found that the hydronium ions generated by the reaction of adsorbed water mo lecules and H atoms on Pd surface could be stabilized by hydrogen bonding with ne ighboring water molecules.With the increase of hydrogen bonding number,the energ y barrier of hydronium ions formation decreased from 0.52 eV to 0.35 eV.At the sa me time,the formation of hydronium ion is accompanied by the transfer of electrons to the Pd surface.3.Study on the mechanism of catalytic hydrogenation of Pd promoted by adsorption of waterThe in-situ attenuation total reflection infrared spectrum showed that the presence of adsorbed water did not affect the flat adsorption of the reactants on the Pd surface.Density functional theory calculations show that,adsorption water acts as a bridge for H transfer between Pd surface H and carbonyl oxygen through hydrogen bond,which can significantly reduce the energy barrier of hydrogenation of carbonyl oxygen.At the same time,the resulting hydroxyl intermediates can be stabilized by hydrogen bonds bridged by water molecules.When the hydrogen bond of the bridging water molecule reaches saturation,the hydrogen transfer ability and stability ability of the bridging water molecule no longer increase with the increase of the adsorbed water amount.On the other hand,the increase of Pd surface electron density leads to the decrease of hydrogenation energy barrier of a-C.Finally,kinetic isotopic effect experiments confirm that the rate-controlling step is α-C hydrogenation in the water-promoted Pd-catalyzed furfural hydrogenation reaction,which is consistent with the results of kinetic analysis and density functional theory.Pd catalyst treated with ammonia water can significantly inhibit the catalytic hydrogenation effect of adsorbed water,which is because the alkaline environment is not conducive to the existence of hydronium ions,which further proves that hydronium ions play an important role in promoting the catalytic hydrogenation of aromatic carbonyl compounds.