(475d) Carbohydrate Stabilization Extends the Kinetic Limits of Chemical Polysaccharide Depolymerization

Carbohydrate stabilization extends the kinetic limits
of chemical polysaccharide depolymerization

Ydna
M. Questell-Santiago, Raquel Zambrano-Valera, Masoud Talebi Amiri
and Jeremy S. Luterbacher

Ecole Polytechnique Fédérale de
Lausanne (EPFL)

ydna.questell@epfl.ch

Biomass-derived carbohydrates are important
platform molecules for the production of renewable fuels and chemicals. The production
of carbohydrates from lignocellulosic biomass requires the extraction of lignin
and the cleavage of ether bonds in hemicellulose (mostly xylan) and cellulose
chains while minimizing further degradation of the resulting carbohydrates.(1) Current methods lead to incomplete
biomass depolymerization (producing only polysaccharides) and high process
costs due to mineral acid recovery and enzyme production.(2) In inexpensive systems like pure water or dilute acid mixtures,
carbohydrate monomer degradation rates exceed hemicellulose and especially
cellulose depolymerization rates at most easily accessible temperatures,
limiting sugar yields.

Here, we use a reversible stabilization of
xylose and glucose by acetal formation with
formaldehyde to alter this kinetic paradigm, preventing sugar dehydration to
furans and their subsequent degradation (Figure 1). During a harsh organosolv pretreatment in the presence of formaldehyde and
low water content, over 90% of xylan in beech wood
was recovered as diformylxylose (compared to 16% xylose recovery without
formaldehyde). The subsequent depolymerization of cellulose led to
carbohydrates yields over 70% and a final concentration of ~5 wt%, whereas the same conditions without formaldehyde led
to a yield of 28%.

Figure 1 | Carbohydrates stabilization
using formaldehyde. Prevention of polysaccharide degradation by reversibly
forming (a) diformylxylose and (b) diformylglucose by the addition of
formaldehyde during acid-catalyzed biomass pretreatment and cellulose
depolymerization, respectively. DX refers to diformylxylose and DGs refers to
the two diformylglucose isomers.

This approach could lead to new processes
for depolymerizing and valorizing biomass derived-carbohydrates or their stabilized
equivalents. For example, when diformylxylose was used as a starting reactant,
similar furfural yields (65%) to those obtained with xylose via hydrogen transfer (1,2-hydride
shift) were achieved without Lewis acid addition. Interestingly, no difference
in the reaction kinetics was observed when a Lewis acid was added, which
suggested that diformylxylose proceeded to furfural through a new mechanism. In
light of these results, current efforts are focused on the utilization of these
protected carbohydrates as new platform molecules to produce important building
blocks such as furans and polyols. These new catalytic processes could help us understand
in more depth the reactivity of stabilized carbohydrates and their potential
applications within biorefineries.     

References:

1.         J.
S. Luterbacher et al., Nonenzymatic
Sugar Production from Biomass Using Biomass-Derived γ-Valerolactone.
Science. 343, 277–280 (2014).

2.         L.
Shuai, Y. M. Questell-Santiago,
J. S. Luterbacher, A mild
biomass pretreatment using γ-valerolactone for
concentrated sugar production. Green Chem. 18, 937–943 (2016).