(410f) Preparation of Beta Zeolite Catalysts for Dehydration of Sugars

Effective utilization of renewable biomass resources has attracted much attention because biomass resources have potential for providing alternatives to crude oil. Monosaccharides including glucose and xylose are the most abundant biomass resources available. Furfural, which is of high commercial interest for the production of a wide range of furan derivatives, has been industrially produced through dehydration of xylose in the presence of homogeneous mineral acid catalyst, such as sulfuric acid. However, this process is not environmentally benign and high furfural yield cannot be achieved. Herein, we report the dehydration of sugars to furfurals over Beta zeolite catalysts and the development of the heterogeneous reaction system for the dehydration of monosaccharides to corresponding furfurals by using Beta zeolites.

Beta zeolites with different Al contents were hydrothermally synthesized in an aqueous media containing fluoride anion. Modification of the prepared Beta zeolites was performed by steam-treatment. Acid properties were investigated by NH₃-TPD and in-situ IR spectroscopy. The dehydration of monosaccharides was conducted in water or dimethylsulfoxide (DMSO) as solvent. The reaction mixtures after the reaction were analyzed by HPLC equipped with an ion-exclusion column (Phenomenex).

Dehydration of xylose was conducted as a model reaction. The calcined Beta zeolite, Beta(F), gave the xylose conversion of 12 % with the furfural yield of 5 % at the reaction of 2 h. Beta(F)-ST, which was treated with 50 % steam, gave much better reaction result; the yield reached to 31 % in the reaction time of 1.5 h with the conversion of 77 %. The turn over frequency (TOF) of Beta(F)-ST was 50 times as high as that of Beta(F), though the acid amount was decreased after the steam-treatment. The product distribution as a function of the reaction time suggested that xylulose was an intermediate species between xylose and furfural. The reaction proceeded through two sequential steps; the isomerization of xylose to xylulose, and the dehydration of xylulose to furfural.

To clarify the difference in the catalytic performances between Beta(F) and Beta(F)-ST, the acid properties of these samples were investigated in detail by in-situ IR spectroscopy. The IR spectra of these samples after CO adsorption revealed that both samples had Brønsted and Lewis acid sites. Note that the proportion of strong Lewis acid sites, which can be ascribed to the IR band appeared at 2233 cm^-1, to total acid sites in Beta(F)-ST sample was higher than that in Beta(F) sample. It can be assumed that the strong Lewis acid sites were generated by the steam-treatment and thus formed acid sites can activate xylose molecule.

In conclusion, Beta zeolite catalysts were found to be promising catalysts for dehydration of sugars to furfurals. Both types of acid sites are indispensable for the production of furfural with a high yield.