Study of the Mechanism
of Hydrolysis of Hemicellulose
from Lignocellulose during Alkali Thermal Pretreatment by Density
Functional Theory and Experiment
The
covalent bond fracture of hemicellulose leads to hemicellulose
hydrolysis during lignocellulosic alkali thermal pretreatment, which
has not previously been reported. Density functional theory was used
to study the mechanism of hydrolysis of the hemicellulose model compounds
under alkali conditions. There are four reaction paths for xylose
formation, among which the reaction path with the lowest energy barrier
is that in which the nucleophile captures H30 to generate water. The
deprotonated hydroxyl group attacks the carbon on the glycoside bond,
resulting in the cleavage of the glycoside bond and the formation
of a new carbon–oxygen covalent bond, with an energy barrier
of 154.2 kJ/mol. The nucleophile further attacks the glycosidic bond
to form a new xylose residue with an energy barrier of 111.9 kJ/mol.
When the glycosidic bond breaks, the orbital interaction with the
largest proportion causes the transfer of ∼0.511 electron from
the glycosidic bond oxygen to the deprotonated hydroxy oxygen. In
situ Fourier transform infrared spectroscopy is used for the identification
of functional groups during the alkali thermal pretreatment. As the
temperature increases, the feasibility of the reaction increases.
This study lays a theoretical foundation for the development of the
alkali thermal pretreatment of lignocellulose.