Dehydration of fructose over thiol– and sulfonic– modified alumina in a continuous reactor for 5–HMF production: Study of catalyst stability by NMR

Abstract

In the present study, the synthesis of an organic group–modified alumina by the sol–gel method is proposed. This material has shown to have an enhanced catalytic performance with grafted organic groups and showed an improved stability. The prepared material has shown to have several OH groups and an enhanced surface acidity. The alumina acidity was improved by incorporating thiol groups by grafting method, which promotes the tautomerization of fructose to its furanose form. Furthermore, the grafting of sulfonic groups catalyzes its dehydration. The modified alumina was thermally treated up to 200 °C to improve the functional groups stability. After, this modified material was packed into a continuous reactor system, designed and built by this group, to obtain 5–hydroxymethylfurfural (5–HMF) from fructose dissolved in a single–phase solution of tetrahydrofuran (THF) and H2O (4:1 w/w). The catalytic activity of this material was evaluated by the reaction of fructose dehydration at different reaction temperatures (60, 70, 80 and 90 °C). Fructose conversion and selectivity towards 5–HMF were determined by high performance liquid chromatography (HPLC), obtaining 95% and 73% respectively for the highest temperature. The catalyst showed an efficient stability after 24 h in continuous flow at 70 °C. The loss of sulfur content was 15%, but the fructose conversion yield and the selectivity to 5–HMF after 24 h of continuous reaction did not undergo significant changes (less than 5%). The nuclear magnetic resonance (NMR) tests confirmed the presence of the thiol and sulfonic groups before and after 24 h of reaction, as well as the conservation of the same structure, demonstrating the efficient catalytic performance of the material. The catalysts were characterized by nitrogen adsorption/desorption, X–ray diffraction and infrared (IR) spectroscopy. Also, before and after use by utilizing elemental analysis and 1H–13C cross–polarization magic–angle spinning (CPMAS) and dynamic–nuclear polarization (DNP)–enhanced 1H–13C and 1H–29Si CPMAS as well as directly excited 29Si magic–angle spinning (MAS) NMR methods in solid–state.