Title : Competitive reactions in hydrogenation of aqueous acetic acid by flow-type reactor with Ni-Sn/TiO2 catalyst for bioethanol production
A novel bioethanol production process from lignocellulosics via acetic acid fermentation has been proposed in our laboratory as a “third-generation” technology for minimizing the by-production of CO2 . This paper focuses on the last step of this process, which includes three consecutive steps, that is, hot-compressed water treatment to hydrolyze lignocellulosics, acetic acid fermentation, and hydrogenation of acetic acid into ethanol. As the effective catalysts, we exploited TiO2 (Lewis acid)-supported Ru-Sn and Ni-Sn for direct hydrogenation of aqueous acetic acid into ethanol. A high yield (88mol%) production of ethanol from aqueous acetic acid was also successfully demonstrated with 8wt%Ni-8wt%Sn/TiO2 catalyst by using a flow-type reactor [liquid hourly space velocity (LHSV): 1.31 h-1]. However, the hydrogenation reaction into ethanol was slowed down when the reaction temperature reached 280-300°C under the reaction pressure of 10 MPa, and acetaldehyde that is the intermediate to ethanol started to be detected. In the present paper, the slowdown mechanism is studied by investigating the reactivity of acetaldehyde as the intermediate. Aqueous acetaldehyde solution was treated with the flow-type reactor in the temperature range of 200 to 380°C at 10 MPa under the catalytic (4wt%Ni-4wt%Sn/TiO2 ) or non-catalytic conditions. Cannizzaro-type reaction giving acetic acid and ethanol, oxidation into acetic acid, and gasification were found to occur as the competitive reactions to hydrogenation into ethanol. These three side-reactions occurred without the addition of any hydrogen but required the presence of the catalyst. Because acetaldehyde did not react at all on TiO2 , all these reactions were suggested to be catalyzed by 4wt%Ni-4wt%Sn/TiO2 . The selectivities of these side reactions and hydrogenation into ethanol were determined in the temperature range of 200-380°C, which suggests that the slow-down mechanism is related to the oxidation of acetaldehyde to acetic acid that becomes important at temperatures > 300°C. The slow-down temperature in acetic acid hydrogenation shifted to the lower temperature region when the reaction pressure decreased from 10MPa. In addition, the slow-down temperature was very close to the boiling point of water at each reaction pressure. Based on these results, the slow-down mechanism is discussed focusing on the influence of boiling of solvent water on hydrogenation and three side reactions.