Hydrogenation Of Methyl Benzoate To Benzaldehyde And Benzyl Alcohol Over MnO/γ-Al₂O₃ And Cu/SiO₂ Catalysts

Recently, the chemical production process and technology is rapidly renovated in environmental friendly manufacture as the green chemical developing. Nowadays, benzaldehyde is mainly manufactured by the method of toluene chloride hydrolysis, the product manufactured by this way is more and more limited in medication, food additives and flavors industry in order to avoid chloride residue in benzaldehyde. Our research works are focus on developing hydrogenation of methyl benzoate (MB) to benzaldehyde and benzyl alcohol over MnO/γ-Al₂O₃ and Cu/SiO₂ catalysts. The main results are as follow:1．Hydrogenation of MB was investigated in a continuous fixed-bed reactor at atmospheric pressure. The results showed that MnO/γ-Al₂O₃ catalysts prepared by co-precipitation method exhibited high activity and selectivity. The optimum reaction temperature is about 410-420℃. As raw material is methyl benzoate instead of benzoic acid, the optimum molar ratio of hydrogen to methyl benzoate decreased from 80 to 20, LHSV of methyl benzoate increased significantly. The catalyst with manganese loading weight of 20 wt% gave 97% conversion and 86．6% selectivity of benzaldehyde.2．The MnO/γ-Al₂O₃ catalyst was modified by several metal elements such as Li, Na, K, Mg, Ca, Ce, Ni and Zr. The results showed that formation of toluene and benzyl methyl ether (BME) was remarkbly inhibited over MnO/γ-Al₂O₃ catalyst modified by alkali metals and alkali earth metals. The conversion of MB and the selectivity of benzaldehyde reached 92．1% and 92．2%, respectively, over the catalyst with 5 wt% loading of potassium at reaction temperature of 400℃and molar ratio of H₂: MB of 20 : 1．3．The continuous hydrogenation of methyl benzoate to benzyl alcohol was carried out over K-MnO/γ-Al₂O₃ and M-Cu/SiO₂ catalysts in a consecutive fixed-bed stainless-steel reactor. K-MnO/γ-Al₂O₃ and M-Cu/SiO₂-CX were used as catalysts in the first and second part of the reactor at reaction temperatures of 400-430℃and 175-250℃, respectively, with the mole ratio of hydrogen to MB was 30 : 1．The conversion of MB was 89．2% and the selectivity of benzyl alcohol is 84．1%. It was found that increasing mole ratio of hydrogen to benzaldehyde in the second reaction step was beneficial to inhibiting formation of toluene.4．Cu/SiO₂ catalyst modified by 1．0 wt%Na exhibited a high activity and selectivity. The FT-IR of pyridine TPD and TPR results revealed that the Na species reduce the intensity and amounts of the acidic centers, which effectively inhibited the formation of toluene and improved the selectivity of benzyl alcohol.5．The structure and properties of catalysts were characterized by XRD, XPS, SEM, TPD, TPR, FT-IR, TG, etc. The FT-IR and TG characterization results revealed that the optimal calcination temperature was about 500℃. XRD and XPS results indicated that MnO₂/γ-Al₂O₃ is the main species before reduction and MnO is the main species after reduction. Monolayer dispersion capacity of MnO/γ-Al₂O₃ obtained by XPS method is about 11．3 wt%. Pyridine TPD FT-IR results revealed that there are only Lewis acidic centers on the catalytic surface. Characterization results verified that the catalyst with a certain number of moderate strength acidic sites was advantageous to hydrogenation of methyl benzoate to benzaldehyde.6．IR pyridine TPD and NH₃-TPD results indicated that there were two types of Lewis acidic centers (strong acid center and weak acid center) existed on the catalytic surface. Acidic sites were neutralized after modification by alkali metals, strong acid centers exhibited the priority tendency of neutralized. The strong acidic centers contributed to formation of over hydrogenation products, such as toluene and BME. Toluene and BME was remarkably inhibited over M-MnO/Al₂O₃ (M = Li, Na, K) catalysts.