Study On Catalytic Activity Of Polyxometales In Oxidation Of Alcohol Into Food Perfume Aldehyde(Ketone)

The selective oxidation of alcohol into the corresponding carbonyl compound is an important organic reactionm,whose products aldehydes or ketones are the raw materials to produce fine chemicals such as spices,drugs and vitamins.Benzaldehyde（BzH）is one of food additives in the national food safety standard GB 28320-2012,which is widely used in food flavoring industry due to its unique sweet,fragrant and bitter almond.At the same time it is a kind of important fine chemical intermediates which can be used in the synthesis of other spices and seasonings such as cinnamaldehyde,cinnamic acid,benzene acetaldehyde,bitter almond acid and so on.The traditional production of benzaldehyde in industry includes hydrolysis of benzyl chloride and oxidation of toluene,but they have the shortcoming of environmental pollution,resource waste,low yield and so on.From the perspective of atom economy and environmental protection,liquid phase oxidation of benzyl alcohol with H₂O₂ as oxidant liquid is feasible.However,in the reaction process not only benzaldehyde can be produced,but also side reaction such as oxidation of benzaldehyde and esterification can occur.Thus,it attracts much attention to use highly effective,stable and less polluted catalyst with high selectivity taking place of traditional catalysts.Heteropoly acid is recognized as environmentally friendly acid,it has little pollution,little corrosivity and simple post-processing.In our work,based on the principle that cation and metal oxide carrier can verify acidic property or redox property of heteropoly acid and thus improve catalytic activity and recyclability of heteropoly acid,precious metal or rare earth metal modified heteropolyacid,amino acids modified heteropolyacids and supporting heteropolyacids were synthesized,characterized by many methods such as FT-IR,TGA,XRD,BET,XPS and 31P NMR,and used as catalysts in the oxidation of benzyl alcohol.The influence of amount of catalyst,alcohol/hydrogen molar ratio,reaction time,reaction temperature and amount of water were investigated on the oxidation of benzyl alcohol and the reaction parameters were optimized by response surface method（RSM）.Besides,the kinetic model of the oxidation reaction was established under the optimal conditions.The following conditions were obtained:（1）Precious metal ion Ag⁺ modified heteropoly acid H₃PW₁₂O₄₀ especially AgH₂PW₁₂O₄₀ showed excellent catalytic activity in the oxidation of benzyl alcohol into benzaldehyde with small amount.Under the optimal conditions:benzyl alcohol/hydrogen molar ratio of 1:2,reaction time of 4.4 h,amount of catalyst of 5 wt%and amount of water of 10.5 mL,the yield of benzaldehyde was 96.1%catalyzed by AgH₂PW₁₂O₄₀,in good agreement with the predicted model,and the activation energy is 25.9 kJ/mol and the kinetic equation is as follows:（2）A series of Bronsted-Lewis acidic catalysts,denoted MyH₃-3yPW₁₂O₄₀（M = La,Ce,Pr,Nd,Sm;y = 1/3,2/3,1）and prepared by incorporating rare earth metal cations as Lewis centers onto the superacidic tungstophosphoric acid（HPW）,among which Ce1/3H₂PW₁₂O₄₀ showed the best catalytic activity,accounting for Bronsted-Lewis acid synergy.In the oxidation of benzyl alcohol（BzOH）with hydrogen peroxide,Ce1/3H₂PW₁₂O₄₀ was utilized to optimize the reaction conditions while nearly full BzOH conversion,and excellent bezaldehyde yield（98.2%）and selectivity（98.4%）could reach by Response Surface Methodology（RSM）.Additional recyclability and kinetic studies revealed that the catalyst could be reused for 6 times without obvious activity loss,an apparent order was 2.5 and an active energy was 37.3 kJ/mol.（3）These organic-inorganic hybrid catalysts,amino acid including phenylalanine（Phe）,alanine（Ala）and glycine（Gly）modified phosphotungstic acid（HPW）,namely（MH）xH₃-xPW₁₂O₄₀（x = 1,2,3）were synthesized and explored in selective oxidation of benzyl alcohol with hydrogen dioxide.The[PheH]H₂PW₁₂O₄₀ catalyst exhibited the best catalytic activity with benzaldehyde yield of 98.6%and revealed reaction order of 2 and activation energy of 56.7 kJ·mol^-1 under the optimized experimental variables as follows:benzyl alcohol/hydrogen peroxide molar ratio of 1:1.5,catalyst loading of 6.1 wt%,reaction time of 3.8 h and amount of water of 30.2 mL.（4）To improve the recycling characteristic and catalytic activity of heteropolyacids,heteropolyacids could be supported on carriers.Heteropolyacids such as H4SiW₁₂O₄₀（HSiW）,H₃PW₁₂O₄₀（HPW）and H₃PM012O40（HPMo）supported on metal oxides including CeO₂,ZrO₂,TiO₂ were characterized by BET,XRD,FT-IR and 31P NMR and used as catalyst for selective oxidation of benzyl alcohol（BzOH）to benzaldehyde（BzH）.HPW/CeO₂ with 20 wt%HPW loading showed the best catalytic performance owing to moderate to strong acidity of the catalyst and better synergy effects between acid and support.Response surface methodology（RSM）based on Box—Behnken design（BBD）model was used to optimize the operation conditions.94.2%yield of benzaldehyde（95.2%conversion of benzyl alcohol and 98.9%selectivity of benzaldehyde）was obtained and the yield of benzaldehyde was 92.1%after 6 times under the optimal conditions.Moreover,benzaldehyde oxidation could be modeled as irreversible parallel reactions and the activation energy Ea was 44.73 kJ/mol with successfully fitting to a power-rate law model.