11 Selective Hydrogenation of Aldehydes and Ketones
387
Table 11.29 Hydrogenation of crotonaldehyde catalyzed by 3Ir/SiO 2 and 3Ir/0.1Fe/SiO 2 catalysts
Catalyst
Ir:Fe content (%)
TOF (h −1 )
Crotyl alcohol selectivity (%)
3Ir/SiO 2
3:0
13.3
78.3
3Ir/0.1Fe/SiO 2
3:0.1
64.8
90.8
Reaction conditions: Catalyst (200 mg), H 2 (26 mL/min), crotonaldehyde (0.0106 bar), 80 °C. The
Ir:Fe content refers to the weight percentage in the catalyst
Fig. 11.8 Proposed adsorption mechanism of crotonaldehyde catalyzed by Ir–FeO x /SiO 2 systems
carbonyl oxygen (σ 2 ). As a result, the activation of the C=O functionality on Ir
0 –
FeO x sites by this di-σ CO adsorption mode is favoured over that produced on Ir
0 –Ir
δ+
sites, leading to enhanced activity and selectivity.
In the same way, the catalytic behaviour of this Ir/SiO 2 system is also improved
by the appropriate promotion with CrO x and combined CrO x –FeO x oxides [110],
although the reported activities and selectivities are lower than those observed for
the Ir–FeO x /SiO 2 catalysts previously described. The catalytic performance is very
dependent on CrO x and FeO x content, since high ratios of additives strengthen the
σ 1 and σ 2 bonds (Fig. 11.8), which hinders the desorption of products and block the
active sites, thus deactivating the catalyst.
The use of FeO x as additive may also have a beneficial effect on the catalytic
activity of IrNPs immobilized on other types of support, such as boron nitride (BN)
[111]. In fact, 91.2% selectivity towards allylic alcohol product was achieved in the
hydrogenation of crotonaldehyde mediated by FeO x -promoted Ir/BN catalysts, as
compared to 59.5% selectivity obtained with bare Ir/BN systems (Table 11.30).
Kinetic experiments showed higher intrinsic rate and crotonaldehyde adsorption
equilibrium constants for Ir
0 –FeO x sites than those for Ir
0 –Ir
δ+ , which proves that as
in Ir–FeO x /SiO 2 systems [109], the activation of the C=O group is favoured in the
former. In addition, these studies suggest a heterolytic cleavage of H 2 on Ir
0 sites.
However, high FeO x ratios partially cover the iridium surface and, as commented
above, hinder the desorption of products, which thus leads to a decreased activity
(Table 11.30).
387
Table 11.29 Hydrogenation of crotonaldehyde catalyzed by 3Ir/SiO 2 and 3Ir/0.1Fe/SiO 2 catalysts
Catalyst
Ir:Fe content (%)
TOF (h −1 )
Crotyl alcohol selectivity (%)
3Ir/SiO 2
3:0
13.3
78.3
3Ir/0.1Fe/SiO 2
3:0.1
64.8
90.8
Reaction conditions: Catalyst (200 mg), H 2 (26 mL/min), crotonaldehyde (0.0106 bar), 80 °C. The
Ir:Fe content refers to the weight percentage in the catalyst
Fig. 11.8 Proposed adsorption mechanism of crotonaldehyde catalyzed by Ir–FeO x /SiO 2 systems
carbonyl oxygen (σ 2 ). As a result, the activation of the C=O functionality on Ir
0 –
FeO x sites by this di-σ CO adsorption mode is favoured over that produced on Ir
0 –Ir
δ+
sites, leading to enhanced activity and selectivity.
In the same way, the catalytic behaviour of this Ir/SiO 2 system is also improved
by the appropriate promotion with CrO x and combined CrO x –FeO x oxides [110],
although the reported activities and selectivities are lower than those observed for
the Ir–FeO x /SiO 2 catalysts previously described. The catalytic performance is very
dependent on CrO x and FeO x content, since high ratios of additives strengthen the
σ 1 and σ 2 bonds (Fig. 11.8), which hinders the desorption of products and block the
active sites, thus deactivating the catalyst.
The use of FeO x as additive may also have a beneficial effect on the catalytic
activity of IrNPs immobilized on other types of support, such as boron nitride (BN)
[111]. In fact, 91.2% selectivity towards allylic alcohol product was achieved in the
hydrogenation of crotonaldehyde mediated by FeO x -promoted Ir/BN catalysts, as
compared to 59.5% selectivity obtained with bare Ir/BN systems (Table 11.30).
Kinetic experiments showed higher intrinsic rate and crotonaldehyde adsorption
equilibrium constants for Ir
0 –FeO x sites than those for Ir
0 –Ir
δ+ , which proves that as
in Ir–FeO x /SiO 2 systems [109], the activation of the C=O group is favoured in the
former. In addition, these studies suggest a heterolytic cleavage of H 2 on Ir
0 sites.
However, high FeO x ratios partially cover the iridium surface and, as commented
above, hinder the desorption of products, which thus leads to a decreased activity
(Table 11.30).