No abstract

It is known that both acids and salts have a positive catalytic effect on the dehydration of pentoses to form furfural, a potentially attractive platform chemical. In this study the effects of the combined usage of an organic acid, instead of stronger mineral acids, and a saline catalyst is investigated. In order to assess these effects, the kinetics of pentose dehydration to furfural are studied using oxalic acid as the primary catalyst and NaCl or seawater as the secondary saline catalyst. The interactions between these two types of catalysts are complex and are, therefore, also assessed thermodynamically. The addition of salts lowers the activity coefficient of the hydronium ions, but simultaneously favours the dissociation of the organic acid. It turned out that these two effects are of similar magnitude, resulting in a fairly constant hydronium ion activity. Because nonetheless higher furfural yields are obtained using the salts as a secondary catalyst, it is concluded that the salts influence the pentose dehydration mechanism directly. The final furfural yields obtained using oxalic acid as the primary catalyst were only slightly lower than those for similar experiments using HCl. The most distinctive difference between the two acids is the lower reaction rate (and thus longer reaction times) when using oxalic acid. Finally, it was observed that if no acidic catalyst is used, the salts tend to catalyse a loss reaction, which is suppressed when an acid is present.

Furfural promises to be a very important product of the lignocellulosic feedstock biorefinery. In this study the kinetics of both xylose and arabinose dehydration toward furfural is investigated in a dilute acidic medium under three different salt conditions. These comprise no salts, a 500 mM NaCl solution, and seawater. The results demonstrate that the salts catalyze all disappearance reactions of the pentoses, both toward furfural and toward loss products. Especially at higher temperatures, the increase in the reaction rate toward furfural is larger than toward loss products. The values of the reaction rate constants at different salt conditions and temperatures indicate that different ions catalyze specific (temperature dependent) reactions in the dehydration mechanism of a pentose. Furthermore, the increase in the reaction rates is more pronounced with the combined salts naturally present in seawater compared to only NaCl, even at the same salinity and ionic strength. It is shown in additional experiments with no acid added that the observed salt effects are independent of the acidic environment. Furthermore, the effects of the salts are larger for the dehydration of xylose compared to arabinose. Moreover, for all temperatures the molar furfural yield was improved by the addition of the salts. Finally, it is shown that the salts inhibit furfural loss reactions toward formic acid. The presented results further contribute to the understanding of the effects of saline catalysis on the mechanism of pentose dehydration.