Valorization of cellulosic biomass requires efficient technologies for the selective conversion of glucose carbohydrates to platform chemicals such as 5-hydroxymethylfurfural (HMF). To achieve high HMF yields, glucose has to be selectively isomerized to its more reactive isomer – fructose – before the dehydration starts. Sn-containing zeolites are the current benchmark  Lewis acid glucose isomerization catalysts. When combined with Brønsted acids, they form highly efficient catalytic systems for glucose to HMF conversion. Similar two step isomerization-dehydration path is commonly postulated for other systems able to convert glucose to HMF including doped WO<sub>3</sub> and enzyme-based systems. An apparent exception from the general trend are TiO<sub>2</sub>-based catalysts modified by phosphoric acid, which catalyze the direct glucose dehydration to HMF via the 3-deoxyglucosone intermediate. It is not clear yet what are the fundamental properties of the catalyst surface that define the preference for the particular reaction path. In this study we aimed at addressing this question by carrying out a detailed theoretical study of glucose activation by a range of oxide and zeolite-based systems.Periodic density functional theory calculations employing realistic models of heterogeneous catalysts reveal the detailed mechanisms of glucose conversion to HMF. The analysis of different reaction paths provided by the oxide and zeolite-based catalysts points to the general importance of the synergetic action of different surface active sites to establish a low-energy path for the selective glucose transformations. The cooperation between Lewis acid and Bronsted acid surface sites is necessary for the efficient glucose to fructose isomerization reaction, whereas the synergy between the Lewis acid and neighboring base sites open an avenue towards a direct glucose to HMF dehydration. The fundamental concepts formulated in this study pave the way towards a generic reactivity concept for chemocatalytic conversion of carbohydrates.

Active site cooperativity in selective sugar transformations by oxide and zeolite-based catalysts

253rd National Meeting (2017)

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