364
I. Cano and P. W. N. M. van Leeuwen
Table 11.18 Influence of particle size in the chemoselective hydrogenation of crotonaldehyde and
cinnamaldehyde to allyl alcohol catalyzed by AgNPs/SiO 2
Size
Crotonaldehyde a
Cinnamaldehyde b
Conversion (%)
Selectivity (%)
Conversion (%)
Selectivity (%)
4.8 ± 1.1
38
59
19
70
11.3 ± 1.8
30
49
13
60
n.d. c
27
44
9
55
Reaction conditions: catalyst (0.6 g), isopropanol (80 mL), 30 bar H 2
a Crotonaldehyde (5 mL), 140 °C, 7 h
b CAL (1 mL), 140 °C, 7 h
c Not determined due to NP agglomeration
A less frequently used oxide support, CeO 2 , was employed to immobilize a core–
shell AgNPs@CeO 2 nanocomposite containing AgNPs (10 nm size) in the core and
spherical CeO 2 nanoparticles (3 nm diameter) in the shell (Fig. 11.4) [41]. The gaps
between CeO 2 NPs in the shell allow the access of substrate molecules to the Ag
sites in the core, and a cooperative effect between AgNPs and basic sites of CeO 2
promotes the heterolytic cleavage of H 2 and therefore the selective reduction of the
C=O group. This system provided the highest chemoselectivities reported to date
for the hydrogenation of unsaturated aldehydes catalyzed by AgNPs (THF, 150 °C,
15 bar H 2 ). In addition, the catalyst can be easily recovered by filtration without loss
of efficiency.
Metal–organic frameworks (MOFs) can also be employed as an interesting alternative to typical oxide supports. AgNPs immobilized on a zirconium-based MOF,
UIO-66, showed complete conversion and 66% selectivity to the unsaturated alcohol
COL in the hydrogenation of CAL [42]. AgNPs/SiO 2 used as reference displays an
increased selectivity (71%), but a characterization of both systems after catalytic
reactions by powder XRD indicates that the UIO-66 MOF provide a better stabilization. Indeed, the catalytic system AgNPs/UIO-66 was reused up to 5 runs with only
a slight decrease in the selectivity, showing good recycling behaviour.
The use of Pd–Ag alloys is another innovative strategy to improve the catalytic
performance of AgNPs in the selective hydrogenation of unsaturated aldehydes [43].
AgNPs/SiO 2 (8% Ag) doped with 0.01% Pd displayed higher activity and selectivity
Fig. 11.4 Representative scheme of AgNPs@CeO 2 nanocomposite
I. Cano and P. W. N. M. van Leeuwen
Table 11.18 Influence of particle size in the chemoselective hydrogenation of crotonaldehyde and
cinnamaldehyde to allyl alcohol catalyzed by AgNPs/SiO 2
Size
Crotonaldehyde a
Cinnamaldehyde b
Conversion (%)
Selectivity (%)
Conversion (%)
Selectivity (%)
4.8 ± 1.1
38
59
19
70
11.3 ± 1.8
30
49
13
60
n.d. c
27
44
9
55
Reaction conditions: catalyst (0.6 g), isopropanol (80 mL), 30 bar H 2
a Crotonaldehyde (5 mL), 140 °C, 7 h
b CAL (1 mL), 140 °C, 7 h
c Not determined due to NP agglomeration
A less frequently used oxide support, CeO 2 , was employed to immobilize a core–
shell AgNPs@CeO 2 nanocomposite containing AgNPs (10 nm size) in the core and
spherical CeO 2 nanoparticles (3 nm diameter) in the shell (Fig. 11.4) [41]. The gaps
between CeO 2 NPs in the shell allow the access of substrate molecules to the Ag
sites in the core, and a cooperative effect between AgNPs and basic sites of CeO 2
promotes the heterolytic cleavage of H 2 and therefore the selective reduction of the
C=O group. This system provided the highest chemoselectivities reported to date
for the hydrogenation of unsaturated aldehydes catalyzed by AgNPs (THF, 150 °C,
15 bar H 2 ). In addition, the catalyst can be easily recovered by filtration without loss
of efficiency.
Metal–organic frameworks (MOFs) can also be employed as an interesting alternative to typical oxide supports. AgNPs immobilized on a zirconium-based MOF,
UIO-66, showed complete conversion and 66% selectivity to the unsaturated alcohol
COL in the hydrogenation of CAL [42]. AgNPs/SiO 2 used as reference displays an
increased selectivity (71%), but a characterization of both systems after catalytic
reactions by powder XRD indicates that the UIO-66 MOF provide a better stabilization. Indeed, the catalytic system AgNPs/UIO-66 was reused up to 5 runs with only
a slight decrease in the selectivity, showing good recycling behaviour.
The use of Pd–Ag alloys is another innovative strategy to improve the catalytic
performance of AgNPs in the selective hydrogenation of unsaturated aldehydes [43].
AgNPs/SiO 2 (8% Ag) doped with 0.01% Pd displayed higher activity and selectivity
Fig. 11.4 Representative scheme of AgNPs@CeO 2 nanocomposite