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For example, vapor-phase furfural hydrogenation when carried out on Pt nanoparticles of different shapes (cubes, spheres, octahedral) and sizes (1.5–7.1 nm) exhibited structure sensitivity [46]. Product selectivity toward furfuryl alcohol increased
from 1 to 66% on increasing the size of the Pt nanoparticles. Under similar reaction
conditions, smaller nanoparticles showed higher selectivity for decarbonylation
reaction, thus favoring furan formation. Additionally, octahedral particles favored
furfuryl alcohol production, whereas cubes produced equal amount of furan and
FA. Similar structure sensitivity was reported in lignin-derived phenol hydrogenation over Pd catalyst where Pd cubes (100) exhibited higher selectivity toward
cyclohexanone (~90%) irrespective of the reaction time (~40 h) [47]. In contrast, Pd
octahedra (111) and spherical particles having both (111) and (100) exhibited higher
selectivity toward cyclohexanol on increased reaction time (>20 h). The selectivity
trends were explained using DFT simulations which showed that the activation barriers for cyclohexanone hydrogenation on Pd (100) (Fig. 3, E a = 97 kJ/mol (2a–2b)
and 101 kJ/mol (2b–2c)) were higher as compared to Pd (111) (Fig. 3, E a = 77
(1a–1b) and 57 kJ/mol (1b–1c)) [47].
2.3 Hydrodeoxygenation Reactions
Hydrodeoxygenation (HDO) reactions form an important class of reaction in biomass upgradation to produce hydrocarbons because of their ability to selectively
remove the oxygen content present in biomass which in turn improves the effective
H/C ratio. Noble (Pd, Pt, Ru, etc.) or non-noble (Fe, Ni, or Cu) metal catalysts show
lower selectivity toward HDO products as they promote other side reactions such as
decarboxylation or decarbonylation [48]. Apart from metal catalysts, sulfided
transition state metal catalysts (Mo, Co, and Ni) have been studied for the HDO of
bio-oil. Sulfided Ni-Mo and Co-Mo catalyst supported on Al 2 O 3 exhibited higher
yield for deoxygenated products for different HDO reactions [49]. However, these
sulfided catalysts are prone to deactivation due to the presence of oxygenated
molecules and aqueous environment [50]. On the other hand, bifunctional catalysts
having both metal and acid sites and bimetallic catalysts which contain a combination
of reducing (Ni, Pt, Pd, Rh, etc.) and oxophilic metals (Fe, Co, Mo, Re, W, Cr, etc.)
have shown higher selectivity for HDO reactions [51]. Bimetallic catalysts such as
Pt-WO x /C, Rh-ReO x /SiO 2 , Ir-ReO x /SiO 2 , etc. have shown high selectivity to produce
terminal diols from biomass-derived cyclic ethers [52]. Chia et al. by utilizing DFT
simulations showed that under aqueous environment when Ir is present near the
vicinity of Re, the hydroxyl group is adsorbed on the oxophilic Re atom and is
responsible for donating a proton to the cyclic ethers for the formation of carbenium
ion which leads to RO via the sterically hindered C–O bond [53] hydrogenolysis
(Fig. 4a). The ring-opened intermediate upon undergoing hydride transfer formed
oxocarbenium ion which on further addition of hydrogen led to terminal diol
formation (Fig. 4a). Similarly, bimetallic catalysts such as PdFe/C [55], NiFe/SiO 2
[5], Pd/Fe 2 O 3 [56], etc. have been seen to be highly efficient for the HDO reaction
S. Gupta
For example, vapor-phase furfural hydrogenation when carried out on Pt nanoparticles of different shapes (cubes, spheres, octahedral) and sizes (1.5–7.1 nm) exhibited structure sensitivity [46]. Product selectivity toward furfuryl alcohol increased
from 1 to 66% on increasing the size of the Pt nanoparticles. Under similar reaction
conditions, smaller nanoparticles showed higher selectivity for decarbonylation
reaction, thus favoring furan formation. Additionally, octahedral particles favored
furfuryl alcohol production, whereas cubes produced equal amount of furan and
FA. Similar structure sensitivity was reported in lignin-derived phenol hydrogenation over Pd catalyst where Pd cubes (100) exhibited higher selectivity toward
cyclohexanone (~90%) irrespective of the reaction time (~40 h) [47]. In contrast, Pd
octahedra (111) and spherical particles having both (111) and (100) exhibited higher
selectivity toward cyclohexanol on increased reaction time (>20 h). The selectivity
trends were explained using DFT simulations which showed that the activation barriers for cyclohexanone hydrogenation on Pd (100) (Fig. 3, E a = 97 kJ/mol (2a–2b)
and 101 kJ/mol (2b–2c)) were higher as compared to Pd (111) (Fig. 3, E a = 77
(1a–1b) and 57 kJ/mol (1b–1c)) [47].
2.3 Hydrodeoxygenation Reactions
Hydrodeoxygenation (HDO) reactions form an important class of reaction in biomass upgradation to produce hydrocarbons because of their ability to selectively
remove the oxygen content present in biomass which in turn improves the effective
H/C ratio. Noble (Pd, Pt, Ru, etc.) or non-noble (Fe, Ni, or Cu) metal catalysts show
lower selectivity toward HDO products as they promote other side reactions such as
decarboxylation or decarbonylation [48]. Apart from metal catalysts, sulfided
transition state metal catalysts (Mo, Co, and Ni) have been studied for the HDO of
bio-oil. Sulfided Ni-Mo and Co-Mo catalyst supported on Al 2 O 3 exhibited higher
yield for deoxygenated products for different HDO reactions [49]. However, these
sulfided catalysts are prone to deactivation due to the presence of oxygenated
molecules and aqueous environment [50]. On the other hand, bifunctional catalysts
having both metal and acid sites and bimetallic catalysts which contain a combination
of reducing (Ni, Pt, Pd, Rh, etc.) and oxophilic metals (Fe, Co, Mo, Re, W, Cr, etc.)
have shown higher selectivity for HDO reactions [51]. Bimetallic catalysts such as
Pt-WO x /C, Rh-ReO x /SiO 2 , Ir-ReO x /SiO 2 , etc. have shown high selectivity to produce
terminal diols from biomass-derived cyclic ethers [52]. Chia et al. by utilizing DFT
simulations showed that under aqueous environment when Ir is present near the
vicinity of Re, the hydroxyl group is adsorbed on the oxophilic Re atom and is
responsible for donating a proton to the cyclic ethers for the formation of carbenium
ion which leads to RO via the sterically hindered C–O bond [53] hydrogenolysis
(Fig. 4a). The ring-opened intermediate upon undergoing hydride transfer formed
oxocarbenium ion which on further addition of hydrogen led to terminal diol
formation (Fig. 4a). Similarly, bimetallic catalysts such as PdFe/C [55], NiFe/SiO 2
[5], Pd/Fe 2 O 3 [56], etc. have been seen to be highly efficient for the HDO reaction
S. Gupta