Biomass utilization has been considered a rather attractive topic when it comes to green chemistry and renewable energy. One way to utilize non-edible biomass is to transform it into aqueous oxygenates solution through fermentation, however the concentration of the oxygenates are pretty low and need to be enriched through various methods such as pervaporation, gas stripping or adsorption, which could be pretty costly. Our aim in this study is to convert the aqueous solution straightly into water-immiscible value-added chemicals and thus achieve easy separation of the products.

We selected ABE solution(acetone-butanol-ethanol-water) as the feedstock in our study, which is a common product in the fermentation industry. At first we discovered that ceria could effectively convert feedstock to products in oil phase which contains 78% of 4-HPO and 15% 2-pentanone. Surprisingly, we observed a drastic decrease in both conversion and selectivity of 4-HPO in absence of water, which suggested that water is a key factor to achieve ideal performance of the catalyst. Then we proposed a seven-step reaction route from n-butanol to 4-HPO involving water as the agent for producing active oxygen and hydrogen on ceria. Isotope labeling experiments were carried out to verify the role of water in the reaction, and ceria catalyzed reactions with various mixed oxygenate substrates were also conducted to confirm the complicated reaction network.

After the investigation of reaction route, we noticed that aldehydes generated through dehydrogenation of ethanol and butanol are critical for 4-HPO production. We tested various metal oxides in n-butanol dehydrogenation and found that SnO₂ is the best candidate for the reaction. And thus different approaches were employed to synthesize Sn-modified ceria catalysts, and they were tested in the conversion of ABE solution. Sn-ceria catalyst prepared through co-precipitation method showed highest 4-HPO selectivity and stability. It is proposed that ceria provides oxygen vacancies for condensation, esterification and Guerbet reactions while Sn promotes dehydrogenation. Results of characterization shows that for 2wt% Sn-ceria which possesses the best catalytic performance, Sn species are highly dispersed on ceria throughout the reaction, which might promote dehydrogenation process. Besides, Sn-doped ceria contains more oxygen vacancies and promotes the condensation process. These two factors may contribute to the superior performance of Sn-ceria.

To fit our results well to practical fermentation process, ABE solutions with different concentration of water were tested for conversion, and 4-HPO selectivity are higher than 80% in all cases. Impact of factors such as reactant ratios and addition of organic acid were also investigated.

The Sn-ceria catalyst we have developed can convert aqueous ABE solutions to easily separable 4-HPO with high selectivity and conversion, and this work contributes to the more effective biomass utilization in the near future. 

Our paper published in Nature Communications (2018) can be read from https://www.nature.com/articles/s41467-018-07593-0.