Conversion of Cellulose Hydrothermal depolymerization of cellulose involves
several reaction pathways where linear bulk chain of molecules is fragmented into
small fractions by hydrolysis and Lobry de Bruyn-Alberda van Ekenstein transformation. It was also observed that the molecular interaction was different in alkaline
and acidic environments during the hydrothermal treatment. The alkaline environments led the products from hydrolysis to further degrade by retro-aldol transformation, followed by rearrangement of aldehydes, hydration, and dehydration into
simpler acid and alcohols, while acidic environment resulted in the production of
mostly 5-hydroxmethylfurfural (5-HMF) and its acid derivatives [26].
Conversion of Hemicellulose Hemicelluloses constitute about 20–40% of the total
plant biomass, where during high pressure hot compressed water uncatalysed solvolysis on various wood and herbaceous biomass materials showed that 100%
hemicellulose was hydrolyzed and selectively decomposed into saccharides at
230
C, 34.5 MPa at 200 reaction time of 2 min [27]. Sasaki et al. [28] experimented
on D-xylose, which is a substitute compound for hemicellulose under subcritical and
supercritical water extraction, which confirmed the same reaction pathway at lower
temperatures. At higher temperatures and pressures of 360–420
C and 25–40 MPa
with a time of 0.02–1 s, retro-aldol condensation and dehydration resulting in
glycolaldehyde, glyceraldehyde, and dihydroxyacetone followed.
Conversion of Lignin Lignin consists of p-coumaryl alcohol, coniferyl alcohol,
and sinapyl alcohol units of p-hydroxy-phenylpropanoids held together by C-C or CO-C bonds. Compared to cellulose and hemicellulose, lignin is relatively resistant to
chemical or enzymatic degradation during hydrothermal conversion. In alkaline
hydrolysis conditions of lignin, the C-O-C bonds result in phenols and
methoxyphenols, where the condensation of these phenolic products resulted in
significant amounts of solid residue.
Fig. 2 Hydrothermal conversion reaction pathways
208
C. D. Venkatachalam et al.
several reaction pathways where linear bulk chain of molecules is fragmented into
small fractions by hydrolysis and Lobry de Bruyn-Alberda van Ekenstein transformation. It was also observed that the molecular interaction was different in alkaline
and acidic environments during the hydrothermal treatment. The alkaline environments led the products from hydrolysis to further degrade by retro-aldol transformation, followed by rearrangement of aldehydes, hydration, and dehydration into
simpler acid and alcohols, while acidic environment resulted in the production of
mostly 5-hydroxmethylfurfural (5-HMF) and its acid derivatives [26].
Conversion of Hemicellulose Hemicelluloses constitute about 20–40% of the total
plant biomass, where during high pressure hot compressed water uncatalysed solvolysis on various wood and herbaceous biomass materials showed that 100%
hemicellulose was hydrolyzed and selectively decomposed into saccharides at
230
C, 34.5 MPa at 200 reaction time of 2 min [27]. Sasaki et al. [28] experimented
on D-xylose, which is a substitute compound for hemicellulose under subcritical and
supercritical water extraction, which confirmed the same reaction pathway at lower
temperatures. At higher temperatures and pressures of 360–420
C and 25–40 MPa
with a time of 0.02–1 s, retro-aldol condensation and dehydration resulting in
glycolaldehyde, glyceraldehyde, and dihydroxyacetone followed.
Conversion of Lignin Lignin consists of p-coumaryl alcohol, coniferyl alcohol,
and sinapyl alcohol units of p-hydroxy-phenylpropanoids held together by C-C or CO-C bonds. Compared to cellulose and hemicellulose, lignin is relatively resistant to
chemical or enzymatic degradation during hydrothermal conversion. In alkaline
hydrolysis conditions of lignin, the C-O-C bonds result in phenols and
methoxyphenols, where the condensation of these phenolic products resulted in
significant amounts of solid residue.
Fig. 2 Hydrothermal conversion reaction pathways
208
C. D. Venkatachalam et al.