394
I. Cano and P. W. N. M. van Leeuwen
was speculated that Co
++ would coordinate the aldehyde and Co (0) was needed for
the H 2 dissociation which then could spillover to the oxide for the reduction.
Shi, Wei and co-workers took a different approach in their effort to reduce alkene
hydrogenation and favour aldehyde hydrogenation with citral (the mixture of geranial
and neral) as the substrate [122]. They calculated by DFT that both the alkene and
the aldehyde group coordinate to a Co surface in a η
2 –μ 2 fashion. They argued that
separating Co atoms by Sn should decrease alkene coordination and increase C=O
coordination. Thus four samples Co, Co 2.9 Sn 2 , CoSn and CoSn 2 (18–20 nm size)
were obtained on support prepared from double-layered hydroxides of Co, Zn and
Al by urea decomposition that after heating maintained the original platelet structure
(20 μm). The four catalysts were studied in the hydrogenation of citral (isopropanol,
160 °C, 40 bar H 2 ). All Sn-containing catalysts are more than an order of magnitude
slower than Co. The latter gives under these conditions up to 90% of citronellol,
which diminishes by further exposure as the last double bond is also hydrogenated.
For the Sn catalysts, the maximum selectivities to geraniol/nerol were 60% Co 2.9 Sn 2 ,
68% CoSn and 62% CoSn 2 , proving the authors’ hypothesis qualitatively.
Related to Co NPs, we turn to Co silicides on N-doped carbon (Co x Si@N-C) made
and used by Peng, Liang et al. for the hydrogenation of CAL [123]. A high-surface
area MOF containing Co (ZIF-67, based on 2-methylimidazolate) was carbonized
to give Co NPs (6–10 nm) inside the porous N-carbon. This was treated with an
organosilane to obtain Co 2 Si and CoSi NPs of 12–15 nm in size. The Co content
is around 9%. Three catalysts were considered, Co, a Co 2 Si rich one, and one rich
in CoSi. Typical results in ethanol are shown in Table 11.35. The silicide improves
both the activity of Co and the selectivity to COL, but the selectivity to HCOL would
further increase unfortunately if higher conversions are desired.
The favourable effect of intermetallic compounds of Co immobilized on layered
double hydroxides in the conversion of α,β-unsaturated aldehydes was further
exploited recently by Zhang, Wei and co-workers [125]. Here they made CoIn 3
and CoGa 3 nanoparticles with a size of 20 nm on top of the discs stemming from
the hydroxides (4 μm). The CoGa 3 catalyst showed a selectivity of 96% (entry 7,
Table 11.35) in the hydrogenation from cinnamaldehyde (CAL) to cinnamyl alcohol
(COL), significantly higher than CoIn 3 (80%) and the monometallic Co catalyst
(42%). Measurements and calculations substantiate once more that the electropositive element serves as an active site and facilitates the adsorption of the polarized
C=O group, while C=C bond adsorption on the Co site is depressed.
Yang and co-workers developed a support of biomass-derived carbon from
pyrolyzed bamboo shoots which was derivatized with cobalt oxide NPs and used
as catalysts for the hydrogenation α,β-unsaturated aromatic and aliphatic ketones
[126]. The Co NPs (20 nm) have a core of Co and a shell of Co oxides. The catalysts, denoted as CoO x @NC, were used in water for the hydrogenation of a broad
spectrum of unsaturated ketones, of which several are important pharmaceutical
hydrogenated chalcone intermediates. Reactions were carried out in the presence of
tetrabutylammonium iodide as a surfactant as otherwise the educt would react very
sluggishly (water, 110 °C, 20 bar H 2 ). The products were the saturated ketones with
>99% selectivity, a little surprising in the context of the results in this chapter as we
I. Cano and P. W. N. M. van Leeuwen
was speculated that Co
++ would coordinate the aldehyde and Co (0) was needed for
the H 2 dissociation which then could spillover to the oxide for the reduction.
Shi, Wei and co-workers took a different approach in their effort to reduce alkene
hydrogenation and favour aldehyde hydrogenation with citral (the mixture of geranial
and neral) as the substrate [122]. They calculated by DFT that both the alkene and
the aldehyde group coordinate to a Co surface in a η
2 –μ 2 fashion. They argued that
separating Co atoms by Sn should decrease alkene coordination and increase C=O
coordination. Thus four samples Co, Co 2.9 Sn 2 , CoSn and CoSn 2 (18–20 nm size)
were obtained on support prepared from double-layered hydroxides of Co, Zn and
Al by urea decomposition that after heating maintained the original platelet structure
(20 μm). The four catalysts were studied in the hydrogenation of citral (isopropanol,
160 °C, 40 bar H 2 ). All Sn-containing catalysts are more than an order of magnitude
slower than Co. The latter gives under these conditions up to 90% of citronellol,
which diminishes by further exposure as the last double bond is also hydrogenated.
For the Sn catalysts, the maximum selectivities to geraniol/nerol were 60% Co 2.9 Sn 2 ,
68% CoSn and 62% CoSn 2 , proving the authors’ hypothesis qualitatively.
Related to Co NPs, we turn to Co silicides on N-doped carbon (Co x Si@N-C) made
and used by Peng, Liang et al. for the hydrogenation of CAL [123]. A high-surface
area MOF containing Co (ZIF-67, based on 2-methylimidazolate) was carbonized
to give Co NPs (6–10 nm) inside the porous N-carbon. This was treated with an
organosilane to obtain Co 2 Si and CoSi NPs of 12–15 nm in size. The Co content
is around 9%. Three catalysts were considered, Co, a Co 2 Si rich one, and one rich
in CoSi. Typical results in ethanol are shown in Table 11.35. The silicide improves
both the activity of Co and the selectivity to COL, but the selectivity to HCOL would
further increase unfortunately if higher conversions are desired.
The favourable effect of intermetallic compounds of Co immobilized on layered
double hydroxides in the conversion of α,β-unsaturated aldehydes was further
exploited recently by Zhang, Wei and co-workers [125]. Here they made CoIn 3
and CoGa 3 nanoparticles with a size of 20 nm on top of the discs stemming from
the hydroxides (4 μm). The CoGa 3 catalyst showed a selectivity of 96% (entry 7,
Table 11.35) in the hydrogenation from cinnamaldehyde (CAL) to cinnamyl alcohol
(COL), significantly higher than CoIn 3 (80%) and the monometallic Co catalyst
(42%). Measurements and calculations substantiate once more that the electropositive element serves as an active site and facilitates the adsorption of the polarized
C=O group, while C=C bond adsorption on the Co site is depressed.
Yang and co-workers developed a support of biomass-derived carbon from
pyrolyzed bamboo shoots which was derivatized with cobalt oxide NPs and used
as catalysts for the hydrogenation α,β-unsaturated aromatic and aliphatic ketones
[126]. The Co NPs (20 nm) have a core of Co and a shell of Co oxides. The catalysts, denoted as CoO x @NC, were used in water for the hydrogenation of a broad
spectrum of unsaturated ketones, of which several are important pharmaceutical
hydrogenated chalcone intermediates. Reactions were carried out in the presence of
tetrabutylammonium iodide as a surfactant as otherwise the educt would react very
sluggishly (water, 110 °C, 20 bar H 2 ). The products were the saturated ketones with
>99% selectivity, a little surprising in the context of the results in this chapter as we