Solid catalysts based on mesoporous carbon Sibunit for hydrothermal hydrolysis of cellulose and hemicelluloses
Nowadays, a very high interest to biomass is aroused by the possibility to substitute exhaustible fossil resources by renewable ones . Cellulose and hemicellulose biomass seems to be the most promising carbon-containing feedstocks not only for power and fuels but also for producing chemicals such as sugars (glucose, galactose, xylose etc.) and its derivatives like 5-hydroxymethylfurfural (5-HMF), furfural, levulinic, 2,5-furandicarbonic acids and some other chemicals so-called “platform molecules” because of their relevance and applicability in the modern industry [2-4].
The current work is devoted to the development of the solid acid carbon catalysts which have to be used for processing cellulose and hemicelluloses into sugars and their derivatives in aqueous media under mild conditions.
The catalysts were prepared on basis of mesoporous graphite-like carbon material of Sibunit series using the methods of sulfonation with the fuming sulfuric acid at different temperatures (80, 150, 200 and 250 oC) and oxidation with nitric acid and wet air. The catalysts were characterized by the N2 low-temperature absorption and acid-base titration by sodium hydroxide. Catalytic properties were tested in the hydrolytic one-pot transformation of mechanically activated microcrystalline cellulose at 180 oC and 1 MPa (in Ar) and hemicellulose (xylan) at 100 oC.
The treatment of Sibunit with sulfuric acid at the temperatures above 200 oC is found to lead to a considerable decrease of carbon material surface area while at the temperatures less than 200 oC the structural properties are not changed. The amount of acidic species on the surface of carbon increases with increasing the sulfonation temperature. The oxidation leads to formation of carboxylic species.
The carbon samples oxidized and sulfonated at 200-250 oC appear to be highly active in cellulose hydrolysis. Pure carbon does not show noticeable catalytic activity.
HPLC analysis of reaction solutions has revealed the main products of the process, which are glucose and 5-HMF. The small amounts of cellobiose, mannose and fructose have also been detected. The formation of levulinic and formic acids are not recorded. This observation indicates the stability of 5-HMF towards the decomposition into levulinic and formic acids under the chosen reaction conditions. The maximal glucose and 5-HMF yields (46.0% and 21.5%, respectively) have been reached over the Sibunit sulfonated at 200 oC.
Based on these results we can conclude that the oxidized carbon catalysts treated by fuming sulfuric acid at 200 oC seem to be very promising due to the stability of structure, high concentration of sulfonate species on the surface and high yields of the platform molecules (glucose and 5-hydroxymethylfurfural).
Main products in the case of xylan treatment were xylose and furfural.