(217a) Conversion of Hemicellulose to Furfural Using Solid Acid Catalysts in Gamma-Valerolactone, a Biomass-Derived Solvent

Catalytic conversion
strategies for the hemicellulose portion are of particular importance, because
biological conversion of C₅ sugars is not as efficient as the
conversion of C₆ sugars.  In addition, C₅
sugars/oligomers are produced as a side stream in the pulp and paper industry.
Among the products that can be obtained from C₅ sugars, furfural is
highly valuable. However, current methods for production of furfural from
hemicellulose suffer from drawbacks that do not comply with the principles of
green and sustainable biorefining, such as use of corrosive and hazardous
mineral acids and/or extractive solvents (in biphasic systems) that cannot be
derived effectively from biomass. In addition, the use of solid acids in
aqueous environments is problematic in view of catalyst degradation and/or
leaching of catalyst components into aqueous solution at elevated temperatures
(e.g., 430 K).

            These difficulties
associated with the conversion of xylose to furfural can be alleviated using
γ-valerolactone (GVL) as a solvent in a monophasic system and using solid
acid catalysts. Importantly, GVL is a green solvent that can be
produced from lignocellulose.  Furthermore, by minimizing the
amount of water employed in the process, it is possible to use
solid catalysts for the conversion of xylose (and xylose oligomers) to
furfural, with minimal degradation of the catalyst and without leaching of acid
sites into solution, as opposed to the case of using solid acid catalysts in
aqueous solutions at elevated temperatures. High yields of furfural (>70%,
with the exception of H-ZSM-5 and sulfated-zirconia) can be achieved in GVL at
448 K over a wide range of acid catalysts, including sulfonic acid
functionalized catalysts (Amberlyst-70, Nafion SAC-13, sulfonated SBA-15 and
carbon catalysts), zeolites (H-ZSM-5, H-mordenite and H-Beta), sulfated
inorganic metal oxides (sulfated zirconia), and a homogenous mineral acid (0.02
M H₂SO₄). H-mordenite was chosen
in this work as the solid acid catalyst for more detailed studies, due to high
(ca. 80%) furfural yields obtained and its low cost and potential for
regeneration with a calcination treatment following deactivation upon
deposition of carbonaceous deposits (e.g., humins). The effect of water on
xylose dehydration over H-mordenite was investigated by changing the water
concentration (0-20 wt%) in the xylose-GVL feed mixture and it was found that
as the water concentration was increased, the rate of furfural production
decreased, and the maximum yield of furfural began to decrease at water
concentrations higher than 10 wt%.  The stability of the H-mordenite was investigated by employing this catalyst in a
series of xylose dehydration reactions in GVL solvent containing 10 wt% H₂O.
Importantly, the recycled catalysts showed activities that were similar to that
of the fresh catalyst, and the maximum yield of furfural achieved for was still
~80% even after the third recycle. Finally, using GVL
as a solvent in the presence of 10 wt% water and H-mordenite, a mixture of
monomeric and oligomeric C₅ sugars obtained from the hot water treatment
of biomass (i.e., poplar wood chips) could also be converted to furfural with
high yields (ca. 75%), showing the applicability of this monophasic solvent-solid
catalyst system to process the hemicellulose stream obtained at a pulp and
paper facility.