Selective hydrogenation of unsaturated aldehydes over Pt nanoparticles promoted by the cooperation of steric and electronic effects

Article abstract of DOI:10.1039/c7cc08942b

The selective hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohols can reach high selectivity and activity at room temperature using Pt nanoparticles immobilized on a non-porous Al₂O₃ support stabilized by aspartic acid. Aspartic acid molecules had a significant steric effect on CC hydrogenation and could modify the electronic state of metal particles.

Full text of DOI:10.1039/c7cc08942b

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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Selective Hydrogenation of Unsaturated Aldehydes over Pt
Nanoparticles Promoted by Cooperation of Steric and Electronic
Effects
Received 00th January 20xx,
Accepted 00th January 20xx
Hangyu Liu,a,b Qingqing Mei,a Shaopeng Li,a,b Youdi Yang,a,b YanYan Wang,a,b Huizhen Liu,*a,b
LiRong Zheng,c Pengfei An,c Jing Zhangc and Buxing Han*a,b
DOI: 10.1039/x0xx00000x
www.rsc.org/
Selective hydrogenation of α,β-unsaturated aldehydes to selective hydrogenation of unsaturated aldehydes at room
unsaturated alcohols can reach high selectivity and activity at temperature is highly desirable.
room temperature using Pt nanoparticles immobilized on non-
Improving the performance of heterogeneous metal
porous Al2O3 support stability by aspartic acid. Aspartic acid catalysts by the steric and electronic effects using inorganic
molecules had significant steric effect for C=C hydrogenation and and organic modifiers has been studied widely.27-33 For
could modify the electronic state of metal particles.
example,
ethylenediamine-coated
ultrathin
platinum
nanowires exhibited excellent performance for the selective
hydrogenation of nitroaromatics to N-hydroxylanilines.27
Polyvinyl pyrrolidone has been used as electronic and
geometric modifier of palladium nanoparticles.28 Phosphine
oxide ligands also affect the performance of gold nanoparticles
for the chemoselective hydrogenation of substituted
aldehydes.29-30 Self-assembled monolayers (SAMs) have also
been used to modify catalysts that thiol molecules covalently
Producing chemicals using feedstocks from natural
resources has attracted much attention because the
feedstocks can be obtained sustainably. Many aldehydes can
be obtained from biomass or biomass derivatives, such as
cinnamaldehyde,
crotonaldehyde,
furfural
and
5-
hydroxymethylfurfural. Selective hydrogenation of α,β-
unsaturated aldehydes can produce unsaturated alcohols
which are widely used in flavor, perfume and pharmaceutical
industries, etc.1-13 However, it is a challenge to develop
effective catalysts for this kind of transformation because
hydrogenation of the C=C double bond is thermodynamically
more favourable than the C=O group,14-21 especially at room
temperature.22-26 Tang and his co-workers reported that MIL-
101(Fe)@Pt@MIL-101(Fe) can catalyse the selective
hydrogenation of unsaturated aldehydes at room temperature,
however, the synthesis procedure of the catalyst was
complex.22 Amine-capped platinum-cobalt nanocrystals were
also used to catalyse the selective hydrogenation of
unsaturated aldehydes under mild conditions, however, the
stability of the catalyst were not checked.24 Development of
simple, efficient, recyclable heterogeneous metal catalysts for
bind to the metal through
a sulfur attachment group
producing ordered and stable monolayer films.31-32
Herein, we develop an active, selective, and stable catalyst
for the selective hydrogenation of unsaturated aldehydes to
unsaturated alcohols by immobilizing Pt nanoparticles on
commercial non-porous Al2O3 (n-Al2O3 hereafter) support
using aspartic acid (1a hereafter). The reaction could proceed
effectively at room temperature and the selectivity to
unsaturated alcohols was >90% for different substrates. The
TOF value can reach 3746. The non-porous Al2O3 exhibited
better performance than mesoporous Al2O3. The catalyst can
be recycled at least 5 times without notable loss of activity.
Detailed study indicated that aspartic acid and the non-porous
property of the support played key roles for the excellent
performance of the catalyst.
The catalysts prepared using n-Al2O3 as support without and
with 1a-1c as modifier are denoted as Pt/n-Al2O3, Pt-1a/n-
Al2O3, Pt-1b/n-Al2O3 and Pt-1c/n-Al2O3 respectively. Pd-1a/n-
Al2O3, Ni-1a/n-Al2O3, Au-1a/n-Al2O3, and Ru-1a/n-Al2O3 have
similar meanings, i.e., the Pt was replaced by the
corresponding metals. The catalysts prepared using
34
mesoporous Al2O3 as the support without and with aspartic
acid as the modifier are named as Pt/mesoporous-Al2O3 and
Pt-1a/mesoporous-Al2O3, respectively.
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The performance of the catalysts for selective
It has been reported that size of metal particles could
hydrogenation of cinnamaldehyde
because cinnamyl alcohol (3a) is an important perfume and Al2O3,
(2a)
is first discussed influence the selectivity to cinnamyl alcohol.39-41 The Pt-1a/n-
Pt/n-Al2O3,
Pt-1a/mesoporous-Al2O3,
and
organic synthesis intermediate.35-38 The activity of the Pt/mesoporous-Al2O3 were characterized using transmission
Pt/mesoporous-Al2O3 was higher than Pt/n-Al2O3, while the electron microscopy (TEM). Figures 1a, 1b and 1e show that
selectivity of cinnamyl alcohol was higher over Pt/n-Al2O3 than size of Pt particles in the Pt-1a/n-Al2O3 was about 4 nm. Figure
that over Pt/mesoporous-Al2O3 (Table 1, entries 1-2). Both the 1c demonstrates that the Pt particles were larger and obvious
conversion of cinnamaldehyde and the selectivity of cinnamyl aggregation was observed in the Pt/n-Al2O3 catalyst. This
alcohol were improved greatly when aspartic acid was used as indicates that aspartic acid was necessary for the uniform
the modifier and n-Al2O3 as the support (Table 1, entries 3-5), dispersion of the Pt nanocatalysts. In EDS mapping of the Pt-
while the effect of aspartic acid on the catalytic performance 1a/n-Al2O3 catalyst, Pt, O, N and Al were detected, suggesting
of Pt-1a/mesoporous-Al2O3 was much smaller (Table 1, entries that aspartic acid was introduced into the catalyst (Figure 1f-
6-8).
1i). The surface active metal atoms in Pt-1a/n-Al2O3 and Pt/n-
We also performed the reaction using Pd-1a/Al2O3, Au-1a/n- Al2O3 were determined by chemisorption method42 in this
Al2O3, Ru-1a/n-Al2O3 and Ni-1a/n-Al2O3 catalysts for the work were 9.69% and 2.12%, respectively. Figures S2a and S2b
reaction. The conversion of cinnamaldehyde was only 20% show the nanostructure of Pt-1a/mesoporous-Al2O3 and
over Pd-1a/n-Al2O3 and no cinnamyl alcohol was generated Pt/mesoporous-Al2O3. The size of Pt particle in the Pt-
(Table S2, entry 7). Au-1a/n-Al2O3, Ru-1a/n-Al2O3 and Ni-1a/n- 1a/mesoporous-Al2O3 was also about 4 nm, and obvious
Al2O3 exhibited no activity for the hydrogenation of aggregation of Pt particles was observed in Pt/mesoporous-
cinnamaldehyde (Table S2, entries 8-10). The effect of succinic Al2O3.
acid (1b) and ethanediamine (1c) were also checked. The
conversion of cinnamaldehyde was 56% and the selectivity of
cinnamic alcohol was 41% over Pt-1b/Al2O3 (Table S2, entry
11). These results indicate that the functional group -COOH
can't increase the selectivity of the reaction. The conversion of
cinnamaldehyde was 49% and the selectivity of cinnamic
alcohol was 65% over Pt-1c/Al2O3 (Table S2, entry 12). It makes
clear that only -NH2 group also can't improve the performance
of the catalyst. Aspartic acid also improved the performance of
Pt/SiO2. The conversion of cinnamaldehyde and the selectivity
of cinnamyl alcohol over Pt/SiO2 was 39 % and 75 %,
respectively (Table S2, entry 13). The conversion of
cinnamaldehyde increased to 72 % and the selectivity of
cinnamyl alcohol increased to 90% over Pt-1a/SiO2 (Table S2,
entry 14). Aspartic acid just improved the selectivity of Pt/C,
Figure 1 TEM (a, e) and HRTEM (b) images of Pt-1a/n-Al2O3, TEM image of Pt/n-Al2O3
while the activity of Pt-1a/C was lower than Pt/C (Table S2,
(c), TEM image Pt-1a/n-Al2O3 after recycled five times (d), EDS images of Pt-1a/n-Al2O3
(f-i).
entries 15 and 16).
Table
cinnamaldehydea
1 The performances of different catalysts for the hydrogenation of
Although the sizes of Pt particles in Pt-1a/n-Al2O3 and Pt-
1a/mesoporous-Al2O3 were similar, the selectivity to the
desired product over Pt-1a/n-Al2O3 was much higher than that
over Pt-1a/mesoporous-Al2O3 (Table 1, entries 5 and 7). The
contents of Pt in Pt-1a/n-Al2O3 and Pt-1a/mesoporous-Al2O3
were 0.68% and 2.53%, respectively, as determined by ICP.
The surface active metal atoms in Pt-1a/n-Al2O3 and Pt-
1a/mesoporous-Al2O3 were 9.69% and 8.02 %, respectively.
The TOF value of Pt-1a/n-Al2O3 was 3746 h-1 and was also
much higher than 1571 h-1 of Pt-1a/mesoporous-Al2O3. The
TOF value was calculated based on the surface active metal
atoms of the Pt and the data of entries 4 and 7 in table 1.
XPS characterization was carried out to identify the surface
electronic states of the catalysts. Figure S3a shows the Al2p
peak together with the Pt4f. The Al2p peak at 74.8 eV was a
typical binding energy of the Al3+ state in Al2O3.43 The Pt4f
spectra (Figure S3a) for the Pt/n-Al2O3 could be resolved into
two spin-orbit pairs with 4f7/2 binding energies of 74.5 eV and
75.9 eV, and with 4f5/2 binding energies of 71.4 eV and 72.7
O
OH
OH
O
+
+
2a
3a
3a'
3a''
Selectivity (%)c
t
Conversi
on(%)b
50
Entry
1
Catalyst
(h)
2
3a
3a’
40
49
4
3a’’
Pt/n-Al2O3
46
38
93
94
91
38
19
12
14
13
3
2
3
4
5
6
7
8
Pt/mesoporous-Al2O3
Pt-1a/n-Al2O3
1
53
1
45
Pt-1a/n-Al2O3
2
76
3
3
Pt-1a/n-Al2O3
3
99
3
6
Pt-1a/mesoporous-Al2O3
Pt-1a/mesoporous-Al2O3
Pt-1a/mesoporous-Al2O3
1
56
48
49
48
14
32
40
2
98
3
99
a
Reaction conditions: cinnamaldehyde 1 mmol, PH2 = 10 atm, catalyst 30 mg,
methanol 0.5 mL, water 0.1 mL, 30 oC. b Conversion of 3a was determined by GC.
c Selectivity was determined by GC using n-decane as the internal standard.
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eV, respectively. The peaks at 75.9 eV (Pt 4f7/2) and 72.7 eV contact with the Pt particles, resulting in hydrogenation C=C
(Pt 4f5/2) are attributed to the oxidation state of Pt(II). The groups. The lower activity of Pt-1a/mesoporous-Al2O3 than Pt-
peaks of 74.5 eV and 71.4 eV are attributed to reduced 1a/n-Al2O3 originated mainly from the poorer mass transfer in
metallic Pt(0). The Pt4f spectrum shifted to lower binding the mesopores than the non-porous support.
energy direction for Pt-1a/n-Al2O3 in comparison with Pt/n-
Al2O3. It was obvious that the modification of aspartic acid
resulted in enrichment of electrons on the metal surfaces,
which is favourable to activation of the electrophilic C=O group
by enhancing its adsorption, leading to higher activity and
selectivity to the unsaturated alcohols. To be more accurate,
the Pt4d spectra were also analysed because the Pt4f signal
was overlapped with Al2p signal. The Pt4d5/2 spectra show
the indicative peaks of elemental Pt and oxidized Pt at
314.6ꢀeV and 317.6 eV for Pt/n-Al2O3, respectively, and they
also shifted to lower binding energy direction for Pt-1a/n-Al2O3
which were 314.3 eV and 317.2 eV, respectively (Figure S3b).
The results further indicate the electron enrichment on the Pt
Figure
2 Possible adsorption mode of Pt-1a/n-Al2O3 (a), Pt/n-Al2O3 (b) and Pt-1a/
surface because of the modification of aspartic acid.
mesoporous-Al2O3 (c).
The extended X-ray absorption fine structure (EXAFS) was
also used to study the catalysts. The EXAFS spectra (Figure S4)
show that Pt-1a/n-Al2O3 exhibited a distinct Pt-N peak.27 The
length Pt-Al bond changed from 2.50 Å in Pt/n-Al2O3 to 2.42 Å
in Pt-1a/n-Al2O3.44-45 This indicates that Pt particles contacted
more tightly on the support in Pt-1a/n-Al2O3. This is
understandable because aspartic acid interacts strongly with
both the support and Pt particles by hydrogen bonding and
coordination, respectively, which enhances the interaction
between the support and the Pt particles at the Pt/support
interface.
The reusability of the Pt-1a/n-Al2O3 was studied. After the
reaction, the catalyst was easily separated through
centrifuging. It was used for the next run after washing and
drying. The catalyst could be reused at least five times without
notable loss of activity (Figure S8). The TEM characterization of
the catalyst after reused five times (Figure 1d) indicated that
the morphology of the catalyst did not change considerably.
Considering the excellent performance of Pt-1a/n-Al2O3, we
studied the selective hydrogenation of other α,β-unsaturated
aldehydes to the corresponding unsaturated alcohols, and the
results are presented in Table 2. It can be known that Pt-1a/n-
The
temperature-programmed
reduction
(TPR)
characterization showed that only one single major reduction
peak could be observed for the Pt/n-Al2O3 and Pt-1a/n-Al2O3
catalysts (Figure S5). The peak was observed at relatively
higher temperature for the Pt-1a/n-Al2O3 than that for the
Pt/n-Al2O3, indicating that the interaction between the Pt
particles and the support was stronger in Pt-1a/n-Al2O3
because of the modification of aspartic acid. The results from
TPR and EXAFS techniques are consistent.
Al2O3 could promote the hydrogenation of
a series of
substrates with moderate condition to high conversion and
excellent selectivity towards the corresponding unsaturated
alcohols. Citral is a mixture of two isomers: 50% neral (cis-
isomer) and 50% geranial (trans-isomer). The selectivity of the
corresponding product all can reach above 95% for these two
isomers. Interestingly, the double bond geometry can survive
after the hydrogenation of carbonyl group.
As discussed above, Pt-1a/n-Al2O3 was much more active
and selective than Pt/n-Al2O3 and Pt-1a/mesoporous-Al2O3 for
the reaction. On the basis of the results above, we can discuss
the reasons for the interesting phenomenon. The better
activity of Pt-1a/n-Al2O3 than Pt/n-Al2O3 resulted mainly from
the uniform dispersion of Pt particles in the former (Figures 1a
and 1c). The much higher selectivity of Pt-1a/n-Al2O3 than
Pt/n-Al2O3 is attributed to the difference in steric hindrance, as
shown schematically in Figures 2a and 2b. There existed a layer
of aspartic acid on the Pt particles. Thus, steric hindrance is
much more significant for hydrogenation of C=C than C=O
because C=O group can approaches the Pt particles when the
reactant molecule is perpendicular to the surface of the Pt
particles, as shown in Figure 2a. However, both C=C and C=O
can contact the Pt particles on Pt/n-Al2O3 easily, and thus the
selectivity was low. The better selectivity of Pt-1a/n-Al2O3 than
Pt-1a/mesoporous-Al2O3 can be discussed by comparing
Figures 2a and 2c. When the reactant molecules pass the Pt
particles in the mesopores, the C=C groups are forced to
In conclusion, Pt-1a/n-Al2O3 was a very active, selective, and
stable catalyst for the selective hydrogenation of unsaturated
aldehydes to unsaturated alcohols. The Pt nanoparticles could
be immobilized stability on the support because aspartic acid
interacts strongly with both the support and the Pt
nanoparticles. The Pt nanocatalysts on the non-porous support
are much more active and selective for this kind of reaction.
The aspartic acid and the non-porous property of the support
played key roles for the unusual catalytic performance of the
catalysts. Aspartic acid molecules adsorbed Pt particles had
significant steric effect for the C=C bond hydrogenation and
could modify electronic state of the metal particles, which
improved the selectivity of the reaction. We believe that the
route to combine non-porous supports and modifiers can also
be used to design highly selective nanocatalysts for some
other reactions.
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