Experiments were performed on the products of glucose decomposition at short residence times
to elucidate the reaction pathways and evaluate kinetics of glucose and fructose decomposition
in sub- and supercritical water. The conditions were a temperature of 300−400 °C and pressure
of 25−40 MPa for extremely short residence times between 0.02 and 2 s. The products of glucose
decomposition were fructose, a product of isomerization, 1,6-anhydroglucose, a product of
dehydration, and erythrose and glyceraldehyde, products of C−C bond cleavage. Fructose
underwent reactions similar to glucose except that it did not form 1,6-anhydroglucose and
isomerization to glucose is negligible. The mechanism for the products formed from C−C bond
cleavage could be explained by reverse aldol condensation and the double-bond rule of the
respective enediols formed during the Lobry de Bruyn Alberda van Ekenstein transformation.
The differential equations resulting from the proposed pathways were fit to experimental results
to obtain the kinetic rate constants.
Glucose decomposition kinetics in subcritical and supercritical water were studied for the temperatures 573, 623, and 673 K, pressures between 25 and 40 MPa, and residence times between 0.02 and 2 s. Glucose decomposition products were fructose, saccharinic acids, erythrose, glyceraldehyde, 1,6-anhydroglucose, dihydroxyacetone, pyruvaldehyde, and small amounts of 5-hydroxymethylfurfural. Fructose was also studied and found to decompose to products similar to those of glucose, except that its epimerization to glucose was negligibly low and no formation of 1,6-anhydroglucose was detected. We concluded that only the forward epimerization of glucose to fructose was important. The glucose decomposition pathway could be described in terms of a forward epimerization rate, r gf , a fructose to decomposition products rate, r f , and a glucose to decomposition products rate, r g . A kinetic model based on this pathway gave good correlation of the experimental data. In the subcritical region, r g , r f , and r gf showed only small changes with pressure at a given temperature. In the supercritical region, the rate of glucose decomposition decreased with pressure at a given temperature. The reason for this decrease was mainly due to the decrease in r gf . The pressure effect in the supercritical region shows that there is a shift among the kinetic rates, which can lead to higher selectivity for glucose when decomposing cellulosic materials.
Cellobiose decomposition kinetics and products in sub- and
supercritical water were studied
with a flow apparatus at temperatures from 300 to 400 °C at pressures
from 25 to 40 MPa, and
at short residence times (0.04−2 s). Cellobiose was found to
decompose via hydrolysis of the
glycosidic bond and via pyrolysis of the reducing end. Pyrolysis
products were glycosylerythrose
(GE) and glycosylglycolaldehyde (GG) which were confirmed by FAB-MS.
Hydrolysis products
were glucose, erythrose, and glycolaldehyde from cellobiose, GE, and
GG, respectively, as well
as glucose decomposition products. The kinetics from glucose
decomposition were used to fit
the experimental results and evaluate rate constants of hydrolysis
(k
H) and pyrolysis rate
constants (k
1 and k
2).
The activation energy for the hydrolysis of cellobiose and
pyrolysis products
GG and GE was found to be 108.6, 110.5, and 106.1 kJ/mol, respectively.
In the supercritical
region, there was a decrease in the pyrolysis rates
k
1 and k
2 and a
corresponding increase in
hydrolysis selectivity from 85% to 95% as the pressure increased from
30 to 40 MPa.
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