After the reduction treatment, the MWCNT-supported Ir NPs (Ir/MWCNT973)
showed a higher selectivity to cinnamyl alcohol than the same catalyst before
(Ir/MWCNT). Specifically, at 50% conversion, a selectivity towards the unsaturated
alcohol of 68% was observed. The higher selectivity was explained by the stronger
interaction between the Ir and the carbon nanotubes after removal of the oxygenated
surface groups. An identical support effect, but even more pronounced, was
observed for Pt NPs supported on the same MWCNTs (Fig. 5). However, Mayer
et al. observed that the oxidative treatment of CNTs only had a small influence on the
catalytic activity of supported Ir NPs. Here, the CNT-supported Ir NPs were
prepared through a hydrogenolytic approach and tested in the hydrogenation of
quinaldine [94]. It was observed that the temperature of the hydrogenolysis had a
substantial effect on the catalytic activity of the supported NPs. When low temperatures were used (À30
C), large and almost inactive particles were obtained. On the
contrary, higher hydrogenolysis temperatures (60 or 150
C) assist the formation of
smaller NPs (1–20 nm), which exhibit a higher catalytic activity in the hydrogenation of N-heterocycles. Shortly after, Du/Fan et al. employed Ir NPs supported on
CNTs for the selective hydrogenation of biomass-derived levulinic acid (LA) into
γ-valerolactone (GVL) [95]. Apart from carrying out the hydrogenation under mild
aqueous conditions (50
C; 2 MPa of H2), these CNT-supported Ir NPs can operate
in the presence of formic acid (FA), which is present in the LA obtained from the
acid hydrolysis of lignocellulosic biomass. In 2016, Ma et al. applied Ir NPs
immobilised in MWCNTs as catalyst for the selective hydrogenation of
nitroaromatics into the corresponding anilines [96]. This Ir catalyst not only showed
an excellent activity and selectivity for a wide range of nitroaromatics but also
presented a remarkable stability, the catalyst being recycled and reused up to five
times without any signal of deactivation. Ir NPs, but this time supported on CNFs,
were also used by Motoyama et al. for the selective hydrogenation of functionalised
nitroarenes and imines [97]. Here the Ir NPs were generated in three types of CNFs
(platelet, CNFs-P; tubular, CNF-T; and herringbone, CNF-H) and activated carbon
(AC) by thermal decomposition of an iridium carbonyl clusters, Ir 4 (CO) 12 . Among
them, Ir/CNF-T demonstrated to be the most efficient catalyst for the hydrogenation
of nitroarenes, being highly active and selective even in the presence of other
functional groups such as nitriles, ketones or esters.
Murzin et al. used a mesoporous carbon material, namely, Sibunit, which combine the benefits of graphite (conductivity and stability) and active carbons (high
adsorption capacity and surface area), to support Ir NPs [98]. The resulting catalysts,
with different metal loadings (0.5, 1 and 3 wt%), were used for the hydrogenation of
citral under atmospheric hydrogen pressure at 70
C. These catalysts were not
selective at all, obtaining a mixture of geraniol, nerol, the isopropyl ethers of geraniol
and nerol (catalysis was carried out in 2-propanol), citronellal and citronellal
diisopropyl acetal (Fig. 6). Ir/Sibunit (1 wt%), which present the larger NPs
(6 nm), produces less geraniol and nerol than the other catalysts, whereas Ir/Sibunit
(3 wt%) forms mostly the ether products.
Ir NPs intercalated between graphite layers (Ir-GIC) were presented by Shirai
et al. for the selective hydrogenation of cinnamaldehyde into cinnamyl alcohol in
422
L. M. Martínez-Prieto et al.
showed a higher selectivity to cinnamyl alcohol than the same catalyst before
(Ir/MWCNT). Specifically, at 50% conversion, a selectivity towards the unsaturated
alcohol of 68% was observed. The higher selectivity was explained by the stronger
interaction between the Ir and the carbon nanotubes after removal of the oxygenated
surface groups. An identical support effect, but even more pronounced, was
observed for Pt NPs supported on the same MWCNTs (Fig. 5). However, Mayer
et al. observed that the oxidative treatment of CNTs only had a small influence on the
catalytic activity of supported Ir NPs. Here, the CNT-supported Ir NPs were
prepared through a hydrogenolytic approach and tested in the hydrogenation of
quinaldine [94]. It was observed that the temperature of the hydrogenolysis had a
substantial effect on the catalytic activity of the supported NPs. When low temperatures were used (À30
C), large and almost inactive particles were obtained. On the
contrary, higher hydrogenolysis temperatures (60 or 150
C) assist the formation of
smaller NPs (1–20 nm), which exhibit a higher catalytic activity in the hydrogenation of N-heterocycles. Shortly after, Du/Fan et al. employed Ir NPs supported on
CNTs for the selective hydrogenation of biomass-derived levulinic acid (LA) into
γ-valerolactone (GVL) [95]. Apart from carrying out the hydrogenation under mild
aqueous conditions (50
C; 2 MPa of H2), these CNT-supported Ir NPs can operate
in the presence of formic acid (FA), which is present in the LA obtained from the
acid hydrolysis of lignocellulosic biomass. In 2016, Ma et al. applied Ir NPs
immobilised in MWCNTs as catalyst for the selective hydrogenation of
nitroaromatics into the corresponding anilines [96]. This Ir catalyst not only showed
an excellent activity and selectivity for a wide range of nitroaromatics but also
presented a remarkable stability, the catalyst being recycled and reused up to five
times without any signal of deactivation. Ir NPs, but this time supported on CNFs,
were also used by Motoyama et al. for the selective hydrogenation of functionalised
nitroarenes and imines [97]. Here the Ir NPs were generated in three types of CNFs
(platelet, CNFs-P; tubular, CNF-T; and herringbone, CNF-H) and activated carbon
(AC) by thermal decomposition of an iridium carbonyl clusters, Ir 4 (CO) 12 . Among
them, Ir/CNF-T demonstrated to be the most efficient catalyst for the hydrogenation
of nitroarenes, being highly active and selective even in the presence of other
functional groups such as nitriles, ketones or esters.
Murzin et al. used a mesoporous carbon material, namely, Sibunit, which combine the benefits of graphite (conductivity and stability) and active carbons (high
adsorption capacity and surface area), to support Ir NPs [98]. The resulting catalysts,
with different metal loadings (0.5, 1 and 3 wt%), were used for the hydrogenation of
citral under atmospheric hydrogen pressure at 70
C. These catalysts were not
selective at all, obtaining a mixture of geraniol, nerol, the isopropyl ethers of geraniol
and nerol (catalysis was carried out in 2-propanol), citronellal and citronellal
diisopropyl acetal (Fig. 6). Ir/Sibunit (1 wt%), which present the larger NPs
(6 nm), produces less geraniol and nerol than the other catalysts, whereas Ir/Sibunit
(3 wt%) forms mostly the ether products.
Ir NPs intercalated between graphite layers (Ir-GIC) were presented by Shirai
et al. for the selective hydrogenation of cinnamaldehyde into cinnamyl alcohol in
422
L. M. Martínez-Prieto et al.