Acidic mesostructured silica-carbon nanocomposite catalysts for biofuels and chemicals synthesis from sugars in alcoholic solutions

- One-pot conversion of fructose in ethanol with a series of sulfonated silica-carbon nanocomposites was performed.
- Kinetic modeling was performed to calculate the rate constants for each reaction step in the proposed reaction pathway.
- Reaction constants were correlated with physicochemical properties to provide insights into the ethanolysis reaction.
- Variation of reaction temperature, solvent and substrate has been conducted for a systematic study of the reaction.
- The role of H2O in the formation of humins by-products is emphasized.

Sulfonated mesostructured silica-carbon nanocomposites with varying carbon content, acidic site density and porosity, obtained via the one-pot evaporation induced self-assembly (EISA) synthesis, were used here to convert sugars into useful chemicals and biofuel components in alcoholic solvent. The nanocomposites show a remarkable catalytic performance in ethanol, yielding up to 80%, predominantly ethyl levulinate, 5-ethoxymethylfurfural and 2-(diethoxymethyl)-5-(ethoxymethyl)furan. Fructose is the sugar substrate of choice, but the sulfonated composites are also able to convert di- and polymeric forms of fructose. Due to a lack of a glucose-to-fructose isomerization ability, the composites are unable to form the above products from the glucose resources (glucose and cellulose), ethyl glucoside being the dominant product from these feedstocks. The composite has a peculiar hierarchical pore architecture, which is stable on shelf in ambient for at least six years. While the mesoporosity facilitates entrance and fast transport (even of soluble poly-carbohydrates like inulin and cellulose polymers), the presence of microporosity is beneficial to attain fast sugar catalysis. Since the microporosity is associated with voids in the carbon phase (preferably pyrolyzed at 400 °C), composites with high carbon contents are preferred. Due to the fast transport, reactions with fructose in ethanol run in the chemical regime in the applied thermal conditions. Kinetic inspection of the reaction further clarifies the complex network of consequent and parallel reactions, demonstrating that HMF is the main precursor of humins, while the formation of EL directly from HMF should also be considered. While this observation corroborates the protective role of alcohols like ethanol, this work also concludes based on a series of reactions in different alcohols and water, that the presence of water plays a crucial role in the HMF-to-humins formation. While alcohols are known to stabilize HMF, the unprotected HMF is fairly stable in non-aqueous reaction circumstances like in tert-butanol solvent.

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