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Hydrogenation. Hydrogenation is a chemical reaction between molecular hydrogen (H 2 ) and an unsaturated hydrocarbon (e.g., alkene) in the presence of Ni, Pd, or
Pt catalyst to saturate (remove double bond) the hydrocarbon. The 3 moles of fatty
acids derived from the hydrolysis of lipids consist of unsaturated C-O double bonds.
These double bonds and other C-C double bonds and double bonds in aromatic
compounds can react with hydrogen to transform these compounds into saturated
hydrocarbons or saturated aromatics via catalytic hydrogenation. Metal catalysts
such as Ni, Ru, Pt, and Pd can be used. Hydrogenation enhances the quality of the
biocrude oil by removing the heteroatoms. One example is the hydrogenation of
phenol in the presence of Ni catalyst to cyclohexenol (1-, 2-, or 3-cyclohexenol) and
the subsequent hydrogenation to cyclohexanol [93]. Another example is the conversion of five-member rings with one heteroatom from unsaturated to saturated: (1)
oxygen with H 2 and Raney nickel, oxole (furan) → oxacyclopentane (tetrahydrofuran); (2) sulfur with H 2 and Pd/C, thiol (thiophene) → thiacyclopentane (tetrahydrothiophene); and (3) nitrogen with H 2 and Pt, azole (pyrrole) → azacyclopentane
(pyrrolidine) [29, 93, 94].
Hydrogenation is initialized when H 2 is cleaved and absorbed on the surface of
the catalyst. Alkene (e.g., C-H) is then absorbed to the catalyst surface via the weak
π bonds. A hydrogen atom is transferred to the alkene, forming a C-H bond. A second hydrogen atom is transferred to the alkene, forming a second C-H bond. The
two hydrogens are finally added to the same face of the double bond (syn addition)
to produce saturated alkane.
Deoxygenation. The produced biocrude oil has high viscosity and low energy
content due to its high oxygen content of about 6–15 wt% [72, 95]. Improvement of
biocrude oil quality entails upgrading or the removal of oxygen to produce CO 2 ,
H 2 O, and CO via catalytic deoxygenation. Deoxygenation types are (1) generation
of CO 2 and R-H from the decomposition of carboxylic acid (decarboxylation), (2)
generation of H 2 O and R-H from the reaction of ketone with H 2 (hydrodeoxygenation), and (3) generation of H 2 O and ester from the reaction of carboxylic acid and
alcohol (dehydration). The conversion of phenol to cyclohexane is a hydrodeoxygenation process with the following steps: hydrogenation of phenol using Pd catalyst, dehydration of cyclohexanol using acid catalyst, and hydrogenation of
cyclohexene using Pd catalyst [82]. The hydrodeoxygenation of phenol is accomplished using other metal catalysts such as Ni/ZrO2, Ru/C, Ni/Al 2 O 3 , Pd/C, and
Pt/C [93]. Ni-supported catalyst is found to be more effective than metal-supported
catalyst in the hydrodeoxygenation of phenol because Ni catalyst requires metal
oxides which facilitate dissociation of the O-H bond [93]. The choice of a suitable
catalyst ensures the success of hydrodeoxygenation of complex aromatic compounds. For instance, the hydrogenation of methoxyphenol is actively mediated by
the sulfide form of cobalt-molybdenum, nickel-molybdenum, and nickeltungsten [96].
Decarboxylation. Decarboxylation is a type of catalytic deoxygenation that
removes CO 2 from the carboxyl group and produces straight-chain hydrocarbons
(alkanes, alkenes). It improves biocrude quality by increasing the hydrocarbon content of HTL-produced biocrude oil in the presence or absence of a catalyst. A study
E. P. Resurreccion and S. Kumar
Hydrogenation. Hydrogenation is a chemical reaction between molecular hydrogen (H 2 ) and an unsaturated hydrocarbon (e.g., alkene) in the presence of Ni, Pd, or
Pt catalyst to saturate (remove double bond) the hydrocarbon. The 3 moles of fatty
acids derived from the hydrolysis of lipids consist of unsaturated C-O double bonds.
These double bonds and other C-C double bonds and double bonds in aromatic
compounds can react with hydrogen to transform these compounds into saturated
hydrocarbons or saturated aromatics via catalytic hydrogenation. Metal catalysts
such as Ni, Ru, Pt, and Pd can be used. Hydrogenation enhances the quality of the
biocrude oil by removing the heteroatoms. One example is the hydrogenation of
phenol in the presence of Ni catalyst to cyclohexenol (1-, 2-, or 3-cyclohexenol) and
the subsequent hydrogenation to cyclohexanol [93]. Another example is the conversion of five-member rings with one heteroatom from unsaturated to saturated: (1)
oxygen with H 2 and Raney nickel, oxole (furan) → oxacyclopentane (tetrahydrofuran); (2) sulfur with H 2 and Pd/C, thiol (thiophene) → thiacyclopentane (tetrahydrothiophene); and (3) nitrogen with H 2 and Pt, azole (pyrrole) → azacyclopentane
(pyrrolidine) [29, 93, 94].
Hydrogenation is initialized when H 2 is cleaved and absorbed on the surface of
the catalyst. Alkene (e.g., C-H) is then absorbed to the catalyst surface via the weak
π bonds. A hydrogen atom is transferred to the alkene, forming a C-H bond. A second hydrogen atom is transferred to the alkene, forming a second C-H bond. The
two hydrogens are finally added to the same face of the double bond (syn addition)
to produce saturated alkane.
Deoxygenation. The produced biocrude oil has high viscosity and low energy
content due to its high oxygen content of about 6–15 wt% [72, 95]. Improvement of
biocrude oil quality entails upgrading or the removal of oxygen to produce CO 2 ,
H 2 O, and CO via catalytic deoxygenation. Deoxygenation types are (1) generation
of CO 2 and R-H from the decomposition of carboxylic acid (decarboxylation), (2)
generation of H 2 O and R-H from the reaction of ketone with H 2 (hydrodeoxygenation), and (3) generation of H 2 O and ester from the reaction of carboxylic acid and
alcohol (dehydration). The conversion of phenol to cyclohexane is a hydrodeoxygenation process with the following steps: hydrogenation of phenol using Pd catalyst, dehydration of cyclohexanol using acid catalyst, and hydrogenation of
cyclohexene using Pd catalyst [82]. The hydrodeoxygenation of phenol is accomplished using other metal catalysts such as Ni/ZrO2, Ru/C, Ni/Al 2 O 3 , Pd/C, and
Pt/C [93]. Ni-supported catalyst is found to be more effective than metal-supported
catalyst in the hydrodeoxygenation of phenol because Ni catalyst requires metal
oxides which facilitate dissociation of the O-H bond [93]. The choice of a suitable
catalyst ensures the success of hydrodeoxygenation of complex aromatic compounds. For instance, the hydrogenation of methoxyphenol is actively mediated by
the sulfide form of cobalt-molybdenum, nickel-molybdenum, and nickeltungsten [96].
Decarboxylation. Decarboxylation is a type of catalytic deoxygenation that
removes CO 2 from the carboxyl group and produces straight-chain hydrocarbons
(alkanes, alkenes). It improves biocrude quality by increasing the hydrocarbon content of HTL-produced biocrude oil in the presence or absence of a catalyst. A study
E. P. Resurreccion and S. Kumar