11 Selective Hydrogenation of Aldehydes and Ketones
355
AuNPs present a diameter of about 31 nm. The AuNPs immobilized on APTMSSBA-15 (Au/APTMS-SBA-15) are more active and selective than Au/SBA-15 for
the hydrogenation of crotonaldehyde (Table 11.7). A transfer of electron density from
APTMS to gold is proposed as a cause of this improved selectivity, in such a way that
these electron-enriched AuNPs favour the activation of the C=O group. Other authors
invoke Lewis acidic Au
3+ sites for higher selectivity to the alcohol, vide infra. In
addition, the alloyed Au-In NPs/APTMS-SBA-15 display higher selectivity towards
the crotyl alcohol product than Au/APTMS-SBA-15. The authors suggest that the
C=O group is preferentially activated through a synergistic effect of electron-deficient
indium and electron-rich gold sites.
A similar effect due to different phenomena was observed for the vapourphase hydrogenation of crotonaldehyde catalyzed by Ag-doped AuNPs supported
on SBA-15 [23]. The Ag-decorated Au/SBA-15 catalysts (8AuxAg/SBA-15, with
8% Au and x% Ag) provided higher TOF and selectivity values than those shown
by monometallic 8Au/SBA-15 systems (Table 11.8), whereas Ag/SBA-15 is only
slightly active. An enhancement in the chemisorption of crotonaldehyde was noted
by in situ FTIR analyses, which is associated with a stronger binding of the C=O group
on Ag sites. Additional experiments suggested that dihydrogen and C=C bonds are
preferentially activated on Au sites. It was proposed that the active hydrogen species
are generated on Au sites and then react with crotonaldehyde through a spillover
mechanism (Fig. 11.2).
Table 11.7 Liquid-phase hydrogenation of crotonaldehyde catalyzed by Au and Au-In APTMSSBA-15 systems
Catalyst
In/Au wt% Time (h) Conversion (%) Selectivity (%) a
Au 5.0 /SBA-15
0
4
8
3
Au 5.0 /APTMS-SBA-15
0
2
97
55
Au 5.0 In 0.75 APTMS-SBA-15 0.15
5
94
75
Reaction conditions: Catalyst (1 mg, 4.5–5% Au), crotonaldehyde (1 mL), H 2 (20 bar), hexane
(49 mL), 120 °C
a Crotyl alcohol selectivity
Table 11.8 Vapour-phase continuous hydrogenation of crotonaldehyde catalyzed by Au/SBA-15
and AuAg/SBA-15 systems
Catalyst
Au wt%
Ag wt%
Conversion (%)
Selectivity (%) a
8Au/SBA-15
8
0
13.1
55.3
8Au0.5Ag/SBA-15
8
0.5
68.8
72.3
8Au0.75Ag/SBA-15
8
0.75
71.7
70.4
Reaction conditions: Catalyst (200 mg), H 2 /crotonaldehyde = 50, reaction pressure: 20 bar, 304 min
on stream, 120 °C
a Crotyl alcohol selectivity
355
AuNPs present a diameter of about 31 nm. The AuNPs immobilized on APTMSSBA-15 (Au/APTMS-SBA-15) are more active and selective than Au/SBA-15 for
the hydrogenation of crotonaldehyde (Table 11.7). A transfer of electron density from
APTMS to gold is proposed as a cause of this improved selectivity, in such a way that
these electron-enriched AuNPs favour the activation of the C=O group. Other authors
invoke Lewis acidic Au
3+ sites for higher selectivity to the alcohol, vide infra. In
addition, the alloyed Au-In NPs/APTMS-SBA-15 display higher selectivity towards
the crotyl alcohol product than Au/APTMS-SBA-15. The authors suggest that the
C=O group is preferentially activated through a synergistic effect of electron-deficient
indium and electron-rich gold sites.
A similar effect due to different phenomena was observed for the vapourphase hydrogenation of crotonaldehyde catalyzed by Ag-doped AuNPs supported
on SBA-15 [23]. The Ag-decorated Au/SBA-15 catalysts (8AuxAg/SBA-15, with
8% Au and x% Ag) provided higher TOF and selectivity values than those shown
by monometallic 8Au/SBA-15 systems (Table 11.8), whereas Ag/SBA-15 is only
slightly active. An enhancement in the chemisorption of crotonaldehyde was noted
by in situ FTIR analyses, which is associated with a stronger binding of the C=O group
on Ag sites. Additional experiments suggested that dihydrogen and C=C bonds are
preferentially activated on Au sites. It was proposed that the active hydrogen species
are generated on Au sites and then react with crotonaldehyde through a spillover
mechanism (Fig. 11.2).
Table 11.7 Liquid-phase hydrogenation of crotonaldehyde catalyzed by Au and Au-In APTMSSBA-15 systems
Catalyst
In/Au wt% Time (h) Conversion (%) Selectivity (%) a
Au 5.0 /SBA-15
0
4
8
3
Au 5.0 /APTMS-SBA-15
0
2
97
55
Au 5.0 In 0.75 APTMS-SBA-15 0.15
5
94
75
Reaction conditions: Catalyst (1 mg, 4.5–5% Au), crotonaldehyde (1 mL), H 2 (20 bar), hexane
(49 mL), 120 °C
a Crotyl alcohol selectivity
Table 11.8 Vapour-phase continuous hydrogenation of crotonaldehyde catalyzed by Au/SBA-15
and AuAg/SBA-15 systems
Catalyst
Au wt%
Ag wt%
Conversion (%)
Selectivity (%) a
8Au/SBA-15
8
0
13.1
55.3
8Au0.5Ag/SBA-15
8
0.5
68.8
72.3
8Au0.75Ag/SBA-15
8
0.75
71.7
70.4
Reaction conditions: Catalyst (200 mg), H 2 /crotonaldehyde = 50, reaction pressure: 20 bar, 304 min
on stream, 120 °C
a Crotyl alcohol selectivity