Selective Oxidation Of 1-Propanol To Propionic Acid Catalyzed By Heteropolyoxometalates

Propionic acid,a very valuable chemical,is widely used as a preservative in the feed,food,and pharmaceutical industries and incorporated in the perfume,herbicide,and polymer industries.Propionic acid is industrially produced via the oxo-synthesis of ethylene or ethyl alcohol and via the oxidation of propionaldehyde with oxygen.However,these methods have some problems,such as high requirements on equipment,harsh reaction conditions and large pollution.With the increasing amount of propionic acid and the increasing awareness of environmental protection in China,the development of a green and efficient method to synthesize propionic acid has become an important research direction for researchers.Due to the abundance and easy availability of 1-propanol,oxidation of 1-propanol to propionic acid with green oxidation is a valuable method.In this paper,economic and eco-friendly hydrogen peroxide is chosen as a green oxidant instead of the traditional highly polluting oxidant,and a series of heteropolyoxometalates catalysts have been synthesized for the preparation of propionic acid from the oxidation of 1-propanol.The specific research contents of the three systems are as follows:（1）Oxidation system of 1-propanol catalyzed by quaternary ammonium heteropolyacid salts:A series of quaternary ammonium heteropolyacid salts were synthesized by using different types of heteropoly acid anions combined with quaternary ammonium cations with different chain lengths,and they were used to catalyze the oxidation of 1-propanol to propionic acid.The best catalyst[（CH₃）₃N(n-C₈H₁₇)]₃PW₁₂O₄₀ was selected and characterized by FT-IR,¹H NMR and TG.After optimizing the reaction conditions one by one,the optimal process conditions were obtained as follows:n（catalyst）:n（1-propanol）=3%:1,n（H₂O₂）:n（1-propanol）=2:1,T=60℃,t=6 h,no solvent.The conversion of 1-propanol reached 56.77%,and the selectivity of propionic acid reached 65.21%.After the reaction,the catalyst could be reused after centrifugation,separation and drying.Under the optimal reaction conditions,the catalyst was recycled for 4 times and still maintained good catalytic activity.1-Propanol conversion and propionic acid selectivity decreased slightly.The mass loss of the catalyst was about 16%after four cycles.When the equal amount of catalyst was replenished,the conversion of1-propanol and the selectivity of propionic acid could be maintained at 55.26%and61.67%,respectively.At the same time,the infrared spectra of the newly prepared catalyst and the recovered catalyst were compared,and it was found that the infrared characteristic peaks before and after the recovery were basically the same,indicating that the structure of the catalyst was not destroyed.The catalytic effect of[（CH₃）₃N(n-C₈H₁₇)]₃PW₁₂O₄₀ on other straight-chain alcohols and aromatic alcohols was investigated under the optimal reaction conditions.The results showed that the catalytic activity of[（CH₃）₃N(n-C₈H₁₇)]₃PW₁₂O₄₀ on other alcohols was moderate.（2）Oxidation system of 1-propanol catalyzed by amino acid heteropolyacid salts:A series of amino acid heteropolyacid salts were synthesized by using the reaction of different types of heteropoly acids and amino acid,and these catalysts were used to catalyze the oxidation of 1-propanol to propionic acid.The best catalyst[Pro H]₃PW₁₂O₄₀ was selected and characterized by FT-IR,¹H NMR,XRD and TG.After optimizing the reaction conditions one by one,the optimal process conditions were obtained as follows:n（catalyst）:n（1-propanol）=3%:1,n（H₂O₂）:n（1-propanol）=3:1,T=60℃,t=8 h,no solvent.The conversion of 1-propanol reached 87.96%,and the selectivity of propionic acid reached 74.61%.After the reaction,the catalyst could be reused after centrifugation,separation and drying.The mass loss of the catalyst was about 20%after four cycles.When the equal amount of catalyst was replenished,the conversion of 1-propanol and the selectivity of propionic acid maintained at 86.37%and 72.84%,respectively.At the same time,the infrared spectra of the newly prepared catalyst and the recovered catalyst were compared,and it was found that the infrared characteristic peaks before and after the recovery were basically the same,indicating that the structure of the catalyst was not destroyed.Under the optimum conditions,the catalytic effect of[Pro H]₃PW₁₂O₄₀ on other straight-chain alcohols and aromatic alcohols was investigated.The results showed that the catalyst had good catalytic activity on other alcohols.（3）Oxidation system of 1-propanol catalyzed by lacunary heteropolyacid salts:Sodium bicarbonate was added to the aqueous phosphotungstic acid solution to adjust the p H of the system to form a monolacunary heteropolyacid sodium salt(Na₇PW₁₁O₃₉).Some quaternary ammonium cations with different chain lengths were used to replace the sodium ions,and a series of substituted monolacunary heteropolyacid salts were synthesized.They were used to catalyze the oxidation of1-propanol to propionic acid.The best catalyst[（CH₃）₃NC₁₂H₂₅]₂Na₅PW₁₁O₃₉ was selected and characterized by FT-IR,¹H NMR,XRD and TG.After optimizing the reaction conditions one by one,the optimal process conditions were obtained as follows:n（catalyst）:n（1-propanol）=2%:1,n（H₂O₂）:n（1-propanol）=3:1,T=60℃,t=6 h,no solvent.The conversion of 1-propanol reached 80.23%%,and the selectivity of propionic acid reached 72.63%.After the reaction,the catalyst could be reused after centrifugation,separation and drying.The mass loss of the catalyst was about 17%after four cycles.When the equal amount of catalyst was replenished,the conversion of 1-propanol and the selectivity of propionic acid maintained at 77.53%and 68.42%,respectively.At the same time,the infrared spectra of the newly prepared catalyst and the recovered catalyst were compared,and it was found that the infrared characteristic peaks before and after the recovery were basically the same,indicating that the structure of the catalyst was not destroyed.Under the optimum conditions,the catalytic effect of[（CH₃）₃NC₁₂H₂₅]₂Na₅PW₁₁O₃₉ on other straight-chain alcohols and aromatic alcohols was investigated.The results showed that the catalyst had general adaptability to different alcohols.In this paper,three kinds of heteropolyoxometalates catalysts were prepared for the oxidation of 1-propanol to propionic acid with hydrogen peroxide.It was found that the lacunary heteropolyacid salts catalysts not only have higher catalytic activity,but also have better recycling performance.In addition,the catalytic system has the advantages of environmental protection,mild reaction conditions and easy operation.Therefore,this study has laid an important theoretical foundation for the application of heteropoly acid catalyst in the industrial application of n-propanol oxidation to propionic acid.