280
of lignin-derived molecules such as phenol, cresol, guaiacol, syringol, etc. to
produce arenes. In contrast, monometallic catalysts such as Ru and Pd before
catalyzing the HDO reaction tend to hydrogenate the aromatic ring. High selectivity
for arenes on these bimetallic catalysts is ascribed either to the oxophilicity or to the
perpendicular adsorption of phenolics which enhances the C–O bond cleavage and
inhibits hydrogenation of aromatic ring. Similarly, PdZn bimetallic catalyst showed
higher selectivity for the HDO reaction of 5-HMF to 2,5-dimethylfuran (2,5-DMF)
[54]. Experimental studies by Saha et al. showed that Pd alone exhibited low
conversion and selectivity for the reaction [57]. However, on addition of Zn, both
conversion (~99%) and selectivity (~85%) increased. By utilizing DFT simulations,
Gupta et al. showed that when oxophilic Zn is present on the steps of Pd catalyst,
the activation barriers for the HDO steps decrease which in turn helps in improving
the conversion and selectivity [54] (Fig. 4b). For the HDO of biomass-derived
platform molecules, Jalid et al. by utilizing micro kinetic model (MKM) studied the
reactivity trends for the C–O bond hydrogenolysis in ethanol on the steps of
transition metal catalyst [58]. The study showed that reaction conditions affected
the bimetallic catalyst design for carrying out HDO reaction. At 523 K, the metal
catalyst followed the order Co > Ru > Ir > Rh > Ni > Fe > Pt > Pd > Cu > Re > Ag
> Au, whereas at 373 K, Cu, Pt, Rh, Ir, Ru, Ni, and Co were found to be more
selective. More specifically, three bimetallic catalysts (Co 3 Ni, Co 3 Fe, and Ni 3 Fe)
exhibited maximum turnover for the HDO product.
2.4 RO and Decarboxylation Reactions of Biomass-Derived
Lactones and Cyclic Esters
Apart from hydrogenation and HDO reactions, RO and decarboxylation reactions
have been found to be effective for the deoxygenation of biomass-derived platform
molecules to produce value-added fuels and chemicals [59]. Several studies have
reported the RO and decarboxylation of the biomass-derived platform molecule
γ-valerolactone (GVL) to produce renewable fuels and chemicals [60]. In the
presence of Brønsted acid catalyst such as SiO 2 /Al 2 O 3 at 648 K temperature and
under 35 bar H 2 pressure, GVL underwent RO to produce pentenoic acid which
upon undergoing decarboxylation formed isomers of butene (~92%) [59]. However,
in the presence of Lewis acid catalyst (γ-Al 2 O 3 ), selectivity toward α-butene, a
commercially relevant polymer precursor, was found to be higher but in lieu of
dropped overall yield (~43%) [61]. The overall yield of butene was retrieved back
(~80%) on adding tungsten oxide to γ-Al 2 O 3 which was attributed to the presence of
Brønsted acidity. Thus, the presence of both Lewis and Brønsted acid catalysts was
found to be essential for the production of linear alpha olefins from lactones. By
utilizing DFT simulations, Gupta et al. showed that in the presence of Brønsted acid
environment, oxocarbenium ions are formed via protonation of carbonyl oxygen of
GVL [62] (Fig. 5a). These oxocarbenium ions then led to GVL RO to form
S. Gupta
of lignin-derived molecules such as phenol, cresol, guaiacol, syringol, etc. to
produce arenes. In contrast, monometallic catalysts such as Ru and Pd before
catalyzing the HDO reaction tend to hydrogenate the aromatic ring. High selectivity
for arenes on these bimetallic catalysts is ascribed either to the oxophilicity or to the
perpendicular adsorption of phenolics which enhances the C–O bond cleavage and
inhibits hydrogenation of aromatic ring. Similarly, PdZn bimetallic catalyst showed
higher selectivity for the HDO reaction of 5-HMF to 2,5-dimethylfuran (2,5-DMF)
[54]. Experimental studies by Saha et al. showed that Pd alone exhibited low
conversion and selectivity for the reaction [57]. However, on addition of Zn, both
conversion (~99%) and selectivity (~85%) increased. By utilizing DFT simulations,
Gupta et al. showed that when oxophilic Zn is present on the steps of Pd catalyst,
the activation barriers for the HDO steps decrease which in turn helps in improving
the conversion and selectivity [54] (Fig. 4b). For the HDO of biomass-derived
platform molecules, Jalid et al. by utilizing micro kinetic model (MKM) studied the
reactivity trends for the C–O bond hydrogenolysis in ethanol on the steps of
transition metal catalyst [58]. The study showed that reaction conditions affected
the bimetallic catalyst design for carrying out HDO reaction. At 523 K, the metal
catalyst followed the order Co > Ru > Ir > Rh > Ni > Fe > Pt > Pd > Cu > Re > Ag
> Au, whereas at 373 K, Cu, Pt, Rh, Ir, Ru, Ni, and Co were found to be more
selective. More specifically, three bimetallic catalysts (Co 3 Ni, Co 3 Fe, and Ni 3 Fe)
exhibited maximum turnover for the HDO product.
2.4 RO and Decarboxylation Reactions of Biomass-Derived
Lactones and Cyclic Esters
Apart from hydrogenation and HDO reactions, RO and decarboxylation reactions
have been found to be effective for the deoxygenation of biomass-derived platform
molecules to produce value-added fuels and chemicals [59]. Several studies have
reported the RO and decarboxylation of the biomass-derived platform molecule
γ-valerolactone (GVL) to produce renewable fuels and chemicals [60]. In the
presence of Brønsted acid catalyst such as SiO 2 /Al 2 O 3 at 648 K temperature and
under 35 bar H 2 pressure, GVL underwent RO to produce pentenoic acid which
upon undergoing decarboxylation formed isomers of butene (~92%) [59]. However,
in the presence of Lewis acid catalyst (γ-Al 2 O 3 ), selectivity toward α-butene, a
commercially relevant polymer precursor, was found to be higher but in lieu of
dropped overall yield (~43%) [61]. The overall yield of butene was retrieved back
(~80%) on adding tungsten oxide to γ-Al 2 O 3 which was attributed to the presence of
Brønsted acidity. Thus, the presence of both Lewis and Brønsted acid catalysts was
found to be essential for the production of linear alpha olefins from lactones. By
utilizing DFT simulations, Gupta et al. showed that in the presence of Brønsted acid
environment, oxocarbenium ions are formed via protonation of carbonyl oxygen of
GVL [62] (Fig. 5a). These oxocarbenium ions then led to GVL RO to form
S. Gupta