11 Selective Hydrogenation of Aldehydes and Ketones
379
water) [85]. Pt was deposited as 2.6 nm NPs on MWCNTs by reduction in two-phase
water/toluene in the presence of oleylamine as stabilizer. Next magnetic Fe 3 O 4 was
also deposited on the material, which enabled them to separate catalyst and product
mixture magnetically. At conversions of 60–80%, selectivities to 3-MeCol of 99–
97% were obtained, the remainder being mostly 3-MeBol. Thus, the triply substituted alkene of 3-MeCal gives better results than the doubly substituted one in CAL,
containing also the phenyl group in conjugation with the alkene.
Dai and co-workers reported on the influence of water on the CAL hydrogenation
by Pt–Fe catalysts on CNTs [86]. The MNPs were prepared from the corresponding
acetylacetonates by heating at 180 °C under CO in benzyl alcohol in the presence of
CNTs. XPS showed that Pt was mainly Pt(0) (6.7 nm in Pt 3 Fe) and Fe consisted of
mixtures of di- and trivalent metal. In the absence of Fe smaller Pt NPs were formed
(2.7 nm size). The latter gave poor selectivity for CAL to COL (50%) at a low rate
in isopropanol, but for Pt 3 Fe this increased to over 90% with a ten times higher rate
(isopropanol, 60 °C, 20 bar H 2 ). When the reaction was carried out in water, the
selectivity increased for both catalysts (97% for Pt 3 Fe), while for Pt 3 Fe in addition
the rate tripled. When the reaction was carried out in D 2 O, deuterium was found
to be incorporated in the hydrogenated alkene, but this was mainly the case for Fe
containing catalysts. The water molecule was proposed to act as a bridge to facilitate
hydrogen exchange between the CAL species and the Pt sites.
Hydrogenation of citral (Scheme 11.3) on Pt or Pd on activated carbon gave
complicated mixtures of products and this will not be discussed [87].
SnO 2 NPs (4.8–5.8 nm size) were first coated onto the surface of reduced graphene
oxide (rGO), and then very small Pt 3 Sn NPs (0.6–1.2 nm size) were deposited on top
of the SnO 2 NPs by the microwave-assisted decomposition of H 2 PtCl 6 in ethylene
glycol [88]. This catalyst exhibits a high selectivity for the CAL to COL hydrogenation, 92.5% (ethanol, 70 °C, 20 bar H 2 ). Crotonaldehyde (90%), citral (82%), furfural
(>99%) and perillyl aldehyde (83%) also gave excellent results. According to XPS,
the PtSn catalyst contains less Pt(II) than the tin-free catalyst.
11.5 Palladium Nanoparticles
11.5.1 Pd NPs in Solution
Pd NPs have been much less studied than their Pt counterparts, perhaps because the
‘natural’ preference for Pd catalysts seems to be the hydrogenation of the C=C bond
in enones and enals. The remarkable success of many polar modifications of the Pt
environment that led to very high selectivity to unsaturated alcohols might stimulate
more work on Pd as well.
Pd NPs were made water-soluble by sticking cyclodextrins (CD) to their surface
[89]. β-CDs can act as hosts for the targeted substrate molecules and thus the Pd
surface would be accessible for the substrates. Alcohol groups in CDs were replaced
379
water) [85]. Pt was deposited as 2.6 nm NPs on MWCNTs by reduction in two-phase
water/toluene in the presence of oleylamine as stabilizer. Next magnetic Fe 3 O 4 was
also deposited on the material, which enabled them to separate catalyst and product
mixture magnetically. At conversions of 60–80%, selectivities to 3-MeCol of 99–
97% were obtained, the remainder being mostly 3-MeBol. Thus, the triply substituted alkene of 3-MeCal gives better results than the doubly substituted one in CAL,
containing also the phenyl group in conjugation with the alkene.
Dai and co-workers reported on the influence of water on the CAL hydrogenation
by Pt–Fe catalysts on CNTs [86]. The MNPs were prepared from the corresponding
acetylacetonates by heating at 180 °C under CO in benzyl alcohol in the presence of
CNTs. XPS showed that Pt was mainly Pt(0) (6.7 nm in Pt 3 Fe) and Fe consisted of
mixtures of di- and trivalent metal. In the absence of Fe smaller Pt NPs were formed
(2.7 nm size). The latter gave poor selectivity for CAL to COL (50%) at a low rate
in isopropanol, but for Pt 3 Fe this increased to over 90% with a ten times higher rate
(isopropanol, 60 °C, 20 bar H 2 ). When the reaction was carried out in water, the
selectivity increased for both catalysts (97% for Pt 3 Fe), while for Pt 3 Fe in addition
the rate tripled. When the reaction was carried out in D 2 O, deuterium was found
to be incorporated in the hydrogenated alkene, but this was mainly the case for Fe
containing catalysts. The water molecule was proposed to act as a bridge to facilitate
hydrogen exchange between the CAL species and the Pt sites.
Hydrogenation of citral (Scheme 11.3) on Pt or Pd on activated carbon gave
complicated mixtures of products and this will not be discussed [87].
SnO 2 NPs (4.8–5.8 nm size) were first coated onto the surface of reduced graphene
oxide (rGO), and then very small Pt 3 Sn NPs (0.6–1.2 nm size) were deposited on top
of the SnO 2 NPs by the microwave-assisted decomposition of H 2 PtCl 6 in ethylene
glycol [88]. This catalyst exhibits a high selectivity for the CAL to COL hydrogenation, 92.5% (ethanol, 70 °C, 20 bar H 2 ). Crotonaldehyde (90%), citral (82%), furfural
(>99%) and perillyl aldehyde (83%) also gave excellent results. According to XPS,
the PtSn catalyst contains less Pt(II) than the tin-free catalyst.
11.5 Palladium Nanoparticles
11.5.1 Pd NPs in Solution
Pd NPs have been much less studied than their Pt counterparts, perhaps because the
‘natural’ preference for Pd catalysts seems to be the hydrogenation of the C=C bond
in enones and enals. The remarkable success of many polar modifications of the Pt
environment that led to very high selectivity to unsaturated alcohols might stimulate
more work on Pd as well.
Pd NPs were made water-soluble by sticking cyclodextrins (CD) to their surface
[89]. β-CDs can act as hosts for the targeted substrate molecules and thus the Pd
surface would be accessible for the substrates. Alcohol groups in CDs were replaced