Studies On Conversion Of Vanillyl Alcohol To Vanillin Catalyzed By Supported Noble Metal Nanoparticles

Vanillin is a white or yellow powder with a delicate,soft,creamy aroma.Vanillin is one of the earliest spices discovered and used by people,it is widely used in cosmetics,tobacco,food and many other industries.In order to meet the market demand,people have invented a variety of artificial vanillin synthesis methods.However,there are many defects in the current industrial synthesis methods,so we need to develop a new method of artificial vanillin catalysis synthesis.The dissertation focuses on the reaction of vanillyl alcohol to vanillin on supported noble metal nanoparticles catalysts.The research content mainly includes two parts:the aerobic oxidation reaction of vanillyl alcohol over Pd nanoparticles on activated carbon catalysts and the oxidant-free dehydrogenation reaction over gold nanoparticles on copper-cerium composite oxide catalysts.In this paper,the structure-activity correlation of vanillyl alcohol to vanillin reaction on supported noble metal nanocatalyst was established by testing the two kinds of reactions and characterization of corresponding catalysts.For the aerobic oxidation reaction,we used activated carbon as the support and adopted the adsorption method to change the aging temperature of Pd/C catalyst and prepared the catalysts with the size of Pd nanoparticles ranging from 2.2 nm to 6.2 nm.We found that the lower aging temperature is conducive to the formation of smaller size Pd nanoparticles.The dispersion of Pd（62%-20%）and the intrinsic conversion frequency(iTOF:266 h^-1-193 h^-1)decrease with the increase of aging temperature,which proves that this reaction is structurally sensitive;at the same time,the strong interaction between Pd-Sn was found by adding the auxiliary agent Sn,which formed the alloy between Pd-Sn.The addition of Sn changed the electron cloud density of Pd atoms and promoted the generation of Pd nanoparticles with smaller particle size.Under the conditions of 0℃ ice bath,the activated carbon was loaded with 1 wt.%Pd and 0.1 wt.%Sn,we can prepare Pd nanoparticles with the size of 1.8 nm.the reaction was conducted for 8 hours at 3 bar O₂,120℃ in a high-pressure reactor.The yield of vanillin for the 15 mg 0.1Sn-Pd/C catalyst was（29）99%,and the iTOF was also increased to 458 h^-1.The catalyst also maintained high stability after five cycles.For the oxidant-free dehydrogenation reaction,we used copper-cerium composite oxide as the support and Au/Cu_XCe _YO_Z catalyst was prepared by the adsorption method,we changed the stoichiometric ratios of Cu,Ce in the support and calcining temperature.For Au/Cu_XCe₁O_Z-500 catalysts with different Cu,Ce stoichiometric ratios,the catalytic activity of the catalysts increased with the increase of the specific surface area of the support and the adsorbed oxygen on the surface.The content of adsorbed oxygen on the surface of Au/Cu₂Ce₁O_Z-500 catalysts with better catalytic activity（Conv.=73%）was52%,and the specific surface area of the support was 71 m²/g.For Au/Cu₂Ce₁O_Z-T catalysts with different calcining temperature（200℃-600℃）,the specific surface area of the support linearly decreased（144 m²/g-44 m²/g）,and the adsorbed oxygen species on the catalyst surface also decreased（65-35%）.The content of Ce³⁺（18%-25%）on the catalyst surface increased with the increase of catalytic activity（67-95%）.In an open reaction vessel containing 0.5 mmol vanillin,the best yield of vanillin was obtained with50 mg Au/Cu₂Ce₁O_Z-300 catalyst at 120℃ for 24 hours under argon purge condition,reached 95%,and the maximum surface Ce³⁺content on the catalyst surface was 25%.The catalyst was also stable after five cycles.