276
Under Brønsted acidic environment, fructose has been shown to undergo dehydration to produce 5-HMF. Yang et al. analyzed 100 reaction pathways by utilizing
DFT simulations to identify the main reaction channel for obtaining 5-HMF from
glucose [24]. The simulations showed that the dehydration of fructose is initiated by
the protonation of O2H as shown in Fig. 1(ii) with subsequent removal of water
resulting in the formation of carbocationic intermediate (Fig. 1(ii) (2a–2b)).
Dehydration is followed by deprotonation which results in a C–C double bond
(Fig. 1(ii) (2b–2c)). Subsequently, two more water molecules are removed (Fig. 1(ii)
(2d–2h) which led to the formation of 5-HMF [24].
Similarly, the hemicellulosic part of biomass upon hydrolysis produces C5 sugars such as xylose which can be converted to furfural, another class of platform
molecule that can be upgraded to C4 to C5 molecules which find applications as
fuels (2-methylfuran, 2-methyltetrahydrofuran, etc.), fuel additives, and valueadded chemicals (butanediol, gamma valerolactone, etc.) [28]. Heterogeneous
catalyst such as H-zeolites [29], modified mesoporous silicas [30], sulfonated
graphene oxides [31], ion exchange resins [32], etc. have been employed for the
production of furfural from xylose. Similar to glucose conversion to 5-HMF, it has
been observed that in the presence of a Lewis acid catalyst, xylose first gets
isomerized to produce xylulose [33]. A six-membered transition state was formed
during the formation of xylulose with a barrier of 150.8 kJ/mol in water [34] in
which hydrogen from O2H migrated to O of the aldehyde group and another H from
C2 migrates to C1. On undergoing hydrogenation-cyclization and dehydrations,
xylulose can be converted to furfural [34].
2.2 Hydrogenation Reactions
Hydrogenation is the most common reaction carried out for biomass upgradation
and is generally carried out at moderate pressure (10–30 bar) and temperature
(370–420 K) conditions in the presence of metal catalyst (Pd, Ni, Pt, Ru, or Cu) to
either saturate the C=C or C=O bonds present in the multifunctional biomassderived platform molecules [35]. The type of bond hydrogenated depends upon the
choice of the catalyst. Selective hydrogenation of C=O bond present outside the
furan ring in furfural and 5-HMF produces furfuryl alcohol (FA) and
2,5-bishydroxymethylfuran (BHMF) in the presence of noble metal (Ru/MSN-Zr
[36], Pt/MCM-41 [37], Ir/TiO 2 [38], etc.) or non-noble metal catalyst such as
Cu-ZnO at mild reaction conditions (T = 25–120 °C, P H2 = 8–60 bar) in polar
solvents [39]. Addition of oxophilic promoters such as Fe [40], Re [41], etc. in these
metal catalysts provided better selectivity at low temperature (~50 °C) and pressure
conditions (8 bar). DFT simulations have also been used to explain the observed
selectivity trends for C=O or C=C bond hydrogenation. Using DFT simulations, it
was showed that Cu has less affinity for C=C bond, and as a result, furfural prefers
to bind through η1(O)-aldehyde due to which carbonyl in furan ring forms a direct
bond with the surface of Cu via the lone pairs present on the oxygen and favors C=O
S. Gupta
Under Brønsted acidic environment, fructose has been shown to undergo dehydration to produce 5-HMF. Yang et al. analyzed 100 reaction pathways by utilizing
DFT simulations to identify the main reaction channel for obtaining 5-HMF from
glucose [24]. The simulations showed that the dehydration of fructose is initiated by
the protonation of O2H as shown in Fig. 1(ii) with subsequent removal of water
resulting in the formation of carbocationic intermediate (Fig. 1(ii) (2a–2b)).
Dehydration is followed by deprotonation which results in a C–C double bond
(Fig. 1(ii) (2b–2c)). Subsequently, two more water molecules are removed (Fig. 1(ii)
(2d–2h) which led to the formation of 5-HMF [24].
Similarly, the hemicellulosic part of biomass upon hydrolysis produces C5 sugars such as xylose which can be converted to furfural, another class of platform
molecule that can be upgraded to C4 to C5 molecules which find applications as
fuels (2-methylfuran, 2-methyltetrahydrofuran, etc.), fuel additives, and valueadded chemicals (butanediol, gamma valerolactone, etc.) [28]. Heterogeneous
catalyst such as H-zeolites [29], modified mesoporous silicas [30], sulfonated
graphene oxides [31], ion exchange resins [32], etc. have been employed for the
production of furfural from xylose. Similar to glucose conversion to 5-HMF, it has
been observed that in the presence of a Lewis acid catalyst, xylose first gets
isomerized to produce xylulose [33]. A six-membered transition state was formed
during the formation of xylulose with a barrier of 150.8 kJ/mol in water [34] in
which hydrogen from O2H migrated to O of the aldehyde group and another H from
C2 migrates to C1. On undergoing hydrogenation-cyclization and dehydrations,
xylulose can be converted to furfural [34].
2.2 Hydrogenation Reactions
Hydrogenation is the most common reaction carried out for biomass upgradation
and is generally carried out at moderate pressure (10–30 bar) and temperature
(370–420 K) conditions in the presence of metal catalyst (Pd, Ni, Pt, Ru, or Cu) to
either saturate the C=C or C=O bonds present in the multifunctional biomassderived platform molecules [35]. The type of bond hydrogenated depends upon the
choice of the catalyst. Selective hydrogenation of C=O bond present outside the
furan ring in furfural and 5-HMF produces furfuryl alcohol (FA) and
2,5-bishydroxymethylfuran (BHMF) in the presence of noble metal (Ru/MSN-Zr
[36], Pt/MCM-41 [37], Ir/TiO 2 [38], etc.) or non-noble metal catalyst such as
Cu-ZnO at mild reaction conditions (T = 25–120 °C, P H2 = 8–60 bar) in polar
solvents [39]. Addition of oxophilic promoters such as Fe [40], Re [41], etc. in these
metal catalysts provided better selectivity at low temperature (~50 °C) and pressure
conditions (8 bar). DFT simulations have also been used to explain the observed
selectivity trends for C=O or C=C bond hydrogenation. Using DFT simulations, it
was showed that Cu has less affinity for C=C bond, and as a result, furfural prefers
to bind through η1(O)-aldehyde due to which carbonyl in furan ring forms a direct
bond with the surface of Cu via the lone pairs present on the oxygen and favors C=O
S. Gupta