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Dilute sulfuric acid hydrolysis of cellulose is the oldest and a well-studied technology for converting biomass to ethanol. Despite continuous technology improvements, the process still gives only a moderate selectivity due to glucose decomposition in the cellulose hydrolysis conditions. According to the recent literature, carboxylic acids have been recognized as more selective acid hydrolysis catalysts than sulfuric acid in low hydrogen ion concentrations. In this paper, formic acid was compared to sulfuric acid as a glucose decomposition catalyst at values below pH 2.2 at 180-220 °C. It was found that glucose decomposition depends only on the hydrogen ion concentration in the prevailing reaction conditions and is independent of the hydrogen ion source. Earlier contradictory findings can be the consequence of ignoring the temperature function of the dissociation constants of the acids and setting hydrogen ion concentrations at room temperature.

Acid-catalyzed hydrolysis of cellulose is a starting point for producing glucose-based fuels and chemicals. According to the mechanism of cellulose hydrolysis, hydrogen ions are responsible for the reaction. This study aims to elucidate the phenomena in cellulose hydrolysis at 180–220 °C using 5–20% (w/w) formic acid or 0.09–0.50% (w/w) sulfuric acid as catalyst. A significant difference between the reaction rates in formic and sulfuric acids was found despite the same hydrogen ion concentration. The findings could not be explained unambiguously through changes in the crystallinity of cellulose. It is suggested that, in addition to hydrogen ions, cellulose hydrolysis is affected by other species found in reaction media. A kinetic model based on specific acid catalysis combined with the effect of bisulfate ions was developed for cellulose hydrolysis in sulfuric acid.

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