11 Selective Hydrogenation of Aldehydes and Ketones
397
activity. For the untreated material, at 50% conversion the HCAL selectivity was 90%
and that for COL only 10%. The selectivity towards COL could be improved to 60
at 50% conversion by heat treatment of the material at the cost of activity (ten-fold).
In the next publication, Ungureanu, Royer, et al. described Cu/SBA-15 which
contained residual polyethers from the hybrid materials in their pores [137]. Cu
contents were 5, 10 and 20% wt treated at various calcination temperatures, or
the organic material was extracted. A mean crystallite size of 25 nm for CuO was
observed, larger than the pores of SBA-15 and thus it was thought that considerable
amounts of CuO were sitting on the outer surface. Extraction of the organic material
gave Cu NPs after reduction of which the size varied strongly with the loading (1.7,
9.1, 12.3 nm). The sintered, calcined material had by far the lowest activity in CAL
hydrogenation (isopropanol, 10 bar H 2 , 130 °C). The other well-dispersed catalysts
showed good activity, but the highest selectivity towards COL was 28%, the main
products being HCAL and HCOL. Earlier the authors had reported on the favourable
effect of adding Cr 2 O 3 to Cu/SBA-15 systems, which enhanced the selectivity for
COL to 52% at low conversions (propylene carbonate, 1 bar H 2 flow, 150 °C) [138].
The improved chemoselectivity was explained by the existence of dual Cu
0 –Cr
3+
sites to which C=O can coordinate.
The last example concerns a hydrogen transfer reaction with 12 nm Cu NPs
supported on MgO particles of 30–50 nm size obtained by reduction of CuO with
H 2 at 350 °C [139]. Cyclohexanol was a convenient donor for CAL hydrogenation
to >99% COL at 180 °C. Aliphatic substrates such as crotonaldehyde and prenal
showed lower selectivities towards the allyl alcohols (78 and 85%, respectively) at
full conversion. The nanostructure is important for the activity as conventional Cu
catalysts give no reaction. TOFs reported were 10–20 h
−1 . It was suggested that both
Cu and Mg
2+ participate in a cyclic Meerwein–Ponndorf–Verley like mechanism.
11.9 Concluding Remarks
In the last decade, the enormous progress in metallic nanoparticle-based catalysts has
opened new possibilities into the chemoselective hydrogenation of α, β-unsaturated
carbonyl compounds. However, the selective formation of allyl alcohol, the product
with the most industrial interest, is still fraught with difficulties. Some metals
show higher intrinsic selectivity towards the carbonyl functionality than others, but
improvements are necessary to obtain the target product with acceptable yields and
rates. In this context, several modifications have shown to be successful to increase
the selectivity. Among them, the most common strategy consists in the use of Lewis
acid species (addition of metal cations or electropositive metal species generated
in situ in the catalytic system) that favour the adsorption and subsequent activation
of the polar C=O group. Along this line, acidic and/or oxide supports can produce
an analogous effect. Similarly, the use of bimetallic nanoparticles has been revealed
as an efficient strategy to improve the selectivity. In most cases, this enhancement
is achieved by an electron transfer between the two metals which makes one of
397
activity. For the untreated material, at 50% conversion the HCAL selectivity was 90%
and that for COL only 10%. The selectivity towards COL could be improved to 60
at 50% conversion by heat treatment of the material at the cost of activity (ten-fold).
In the next publication, Ungureanu, Royer, et al. described Cu/SBA-15 which
contained residual polyethers from the hybrid materials in their pores [137]. Cu
contents were 5, 10 and 20% wt treated at various calcination temperatures, or
the organic material was extracted. A mean crystallite size of 25 nm for CuO was
observed, larger than the pores of SBA-15 and thus it was thought that considerable
amounts of CuO were sitting on the outer surface. Extraction of the organic material
gave Cu NPs after reduction of which the size varied strongly with the loading (1.7,
9.1, 12.3 nm). The sintered, calcined material had by far the lowest activity in CAL
hydrogenation (isopropanol, 10 bar H 2 , 130 °C). The other well-dispersed catalysts
showed good activity, but the highest selectivity towards COL was 28%, the main
products being HCAL and HCOL. Earlier the authors had reported on the favourable
effect of adding Cr 2 O 3 to Cu/SBA-15 systems, which enhanced the selectivity for
COL to 52% at low conversions (propylene carbonate, 1 bar H 2 flow, 150 °C) [138].
The improved chemoselectivity was explained by the existence of dual Cu
0 –Cr
3+
sites to which C=O can coordinate.
The last example concerns a hydrogen transfer reaction with 12 nm Cu NPs
supported on MgO particles of 30–50 nm size obtained by reduction of CuO with
H 2 at 350 °C [139]. Cyclohexanol was a convenient donor for CAL hydrogenation
to >99% COL at 180 °C. Aliphatic substrates such as crotonaldehyde and prenal
showed lower selectivities towards the allyl alcohols (78 and 85%, respectively) at
full conversion. The nanostructure is important for the activity as conventional Cu
catalysts give no reaction. TOFs reported were 10–20 h
−1 . It was suggested that both
Cu and Mg
2+ participate in a cyclic Meerwein–Ponndorf–Verley like mechanism.
11.9 Concluding Remarks
In the last decade, the enormous progress in metallic nanoparticle-based catalysts has
opened new possibilities into the chemoselective hydrogenation of α, β-unsaturated
carbonyl compounds. However, the selective formation of allyl alcohol, the product
with the most industrial interest, is still fraught with difficulties. Some metals
show higher intrinsic selectivity towards the carbonyl functionality than others, but
improvements are necessary to obtain the target product with acceptable yields and
rates. In this context, several modifications have shown to be successful to increase
the selectivity. Among them, the most common strategy consists in the use of Lewis
acid species (addition of metal cations or electropositive metal species generated
in situ in the catalytic system) that favour the adsorption and subsequent activation
of the polar C=O group. Along this line, acidic and/or oxide supports can produce
an analogous effect. Similarly, the use of bimetallic nanoparticles has been revealed
as an efficient strategy to improve the selectivity. In most cases, this enhancement
is achieved by an electron transfer between the two metals which makes one of