Non-plasmonic metal nanoparticles as visible light photocatalysts for the selective oxidation of aliphatic alcohols with molecular oxygen at near ambient conditions

Article abstract of DOI:10.1039/c6cc05186c

Nanoparticles (NPs) of Pd and Pt were used for the selective oxidation of aliphatic alcohols with molecular oxygen as an oxidant at near ambient temperatures under visible light irradiation. Distinct final products were obtained under identical reaction conditions, aliphatic esters formed over the Pd NPs while aldehydes formed over the Pt NPs. The reason for this different product selectivity is proven to be due to the much stronger interaction of Pd NPs with alcohol and aldehyde compared to Pt NPs. The photocatalytic activity is tuneable by light intensity or a moderate change in the reaction temperature.

Full text of DOI:10.1039/c6cc05186c

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View Article Online
DOI: 10.1039/C6CC05186C
Journal Name
COMMUNICATION
Non-plasmonic Metal Nanoparticles as Visible Light
Photocatalysts for Selective Oxidation of Aliphatic Alcohols with
Received 00th January 20xx,
Accepted 00th January 20xx
Molecular Oxygen at Near Ambient Conditions†
a
b
a
a
a
c
DOI: 10.1039/x0xx00000x
Tana Tana, Xiao-Wei Guo, Qi Xiao, Yiming Huang, Sarina Sarina, Phillip Christopher, Jianfeng
d d a
Jia, Haishun Wu and Huaiyong Zhu*
www.rsc.org/
Nanoparticles (NPs) of Pd and Pt were used for the selective the sole product. On the other hand, visible light irradiation
oxidation of aliphatic alcohols with molecular oxygen as oxidant at can significantly enhance the intrinsic catalytic performance of
near ambient temperatures under visible light irradiation. Distinct the NPs of non-plasmonic metals, such as Pd, Pt, and Ir at
final products achieved at identical reaction condition: aliphatic ambient temperatures for several reactions, although they
6
esters formed over Pd NP while aldehyde formed over Pt NP and cannot strongly absorb visible light via LSPR effect. It is of
the reason of this different product selectivity is proven to be the great interest to apply them to more challenging reactions
much stronger interaction of Pd NPs with alcohol and aldehyde such as activation of aliphatic alcohols and explore the
than Pt. The photocatalytic activity is tuneable by light intensity or mechanism of such photocatalysis.
moderate change of reaction temperature.
In the present study, we found the catalytic performance
of Pd and Pt NPs is significantly enhanced with low intensity
visible light irradiation and most importantly, distinctly
different final products are given: Pd NPs result in esters as
final product while Pt NPs result in corresponding aldehyde.
The interesting finding inspired us to explore the mechanism
for the photocatalysis and dependence of product selectivity
on the metals.
The selective oxidation of aliphatic alcohols to corresponding
aldehydes and esters is one of the most important and
fundamental transformations in organic synthesis. As the
1
alcohols being oxidized to product aldehyde (or ketones) or
acid, esterification between alcohols and acids may occur, and2
thus product selectivity is also one of the essential issues.
Various catalysts have been developed for this reaction
Pd and Pt NPs were loaded on phosphate anions
3
including transition-metal-complexes, and heterogeneous
exchanged
hydrotalcite
(with
a
formula
of
4
catalysts. However, commonly strong bases were used, and
[
Mg Al (OH) ]CO ·mH O and abbreviated as HT), which was
6
2
16
3
2
3−
the metal complex catalysts were not easily recycled after the
reaction. For the selective oxidation catalytic process, mild
reaction conditions – ambient temperature and atmospheric
pressure – is highly desired to guarantee satisfactory product
selectivity.
used as a basic supporting material (HT-PO4 ). Catalyst
preparation is given in the supporting information (SI).
3−
Compared to ZrO and HT, support of HT-PO significantly
2
4
increases the catalytic performance of Pd NPs (Table S1, SI).
3
-
PO4 modified HT itself did not show any catalytic activity for
the oxidative esterification of 1-octanol, indicating that alcohol
oxidation took place on the metal NPs. Increasing metal NP
content results in better performance (Table S2, SI). 5 wt% Pd
and Pt NPs exhibit optimal photocatalytic activity for the
model reaction, achieving excellent conversion rate under light
irradiation, respectively.
Recently, we found that Au-Pd alloy NPs supported on
hydrotalcite are able to efficiently catalyze direct esterification
of aliphatic alcohols under visible light at 50 °C in 1 atm of O2
without adding base. In the alloy (NP) photocatalyst main role
of gold is to achieve intensive visible light absorption via
localized surface plasmon resonance (LSPR) effect. Esters are
5
Transmission electron microscopy (TEM) images and the
light absorption spectra of Pd, Pt photocatalysts and support
are shown in Fig. 1. The metal NPs are well dispersed on the
support with the average diameter of 2-3 nm (Fig. 1C). Pd and
Pt NPs exhibit strong absorption in the ultraviolet and visible
range of the spectrum. While, the HT-PO4 support displays
very weak absorption at wavelength above 400 nm, indicating
that most of the visible irradiation energy is absorbed by the
a.
School of Chemistry, Physics and Mechanical Engineering, Science and Engineering
Faculty, Queensland University of Technology, Brisbane, QLD 4001 (Australia). E-
mail: hy.zhu@qut.edu.au
Key Laboratory of Bio-Based Material Science and Technology, Ministry of
Education, Northeast Forestry University, Harbin 150040 (China)
Department of Chemical & Environmental Engineering, University of California,
Riverside, Riverside, California 92521 (United States).
School of Chemical and Material Science, Shanxi Normal University, Linfen 041004
b.
c.
3-
d.
(
China).
†
Electronic Supplementary Information (ESI) available: Experimental section,
photocatalysts characterization and additional supplementary figures and tables.
See DOI: 10.1039/x0xx00000x
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J. Name., 2013, 00, 1-3 | 1
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COMMUNICATION
Journal Name
metal NPs. More characterization data are given in SI, Fig. S1- diols to yield different lactones. The oxidative lactonization of
View Article Online
DOI: 10.1039/C6CC05186C
S3.
diols is highly desirable to both academic and industrial
chemists because lactones are ubiquitous among natural and
8
synthetic organic compounds. 1, 5-pentanediol and cis-2-
butene-1, 4-diol were successfully oxidized to corresponding
lactones with high yields under air atmosphere. Notably, the
yield for oxidation of 1, 5-pentandiols to lactones on Pt NPs is
relatively low. Similarly to the unique behavior of Pd in primary
alcohol conversion to esters with light, in the case of reaction
1
5, Pd exhibited unique selectivity to the lactone, only under
illumination.
Table 1 Photocatalytic Base-Free Direct Oxidation of Aliphatic
a
Alcohols by Using Pd and Pt NPs
OH
R'
R= alkyl; R'= H, alkyl
O
O
Photocatalysts, hv
R
R
O
R
+
R
R'
ester
aldehyde, ketone
3
-
Fig. 1 (A) TEM image of the 5 wt% Pd@HT-PO
4
catalyst. (B) TEM
3-
b
image of the 5 wt% Pt@HT-PO
4
catalyst. (C) Particle size distribution
of the Pd and Pt NPs. (D) Normalized diffuse reflectance
ultraviolet−visible (DR-UV/vis) extinction spectra of the photocatalysts.
Product yield (%)
Ester
Substrate
Catalyst
Aldehyde
Pd
Pt
100 (21) 0 (42)
0 ( 0 )
OH
1
2
3
4
5
6
7
8
9
1
1
56 (47)
Various aliphatic alcohols were oxidized by using 5 wt%
3-
3-
Pd
Pt
92 (13)
0 ( 0 )
0 (44)
44 (38)
Pd@HT-PO4 and 5 wt% Pt@HT-PO4 photocatalysts, without
any base additive. For comparison, control experiments of
dark reactions were conducted without irradiation, while the
other reaction conditions were kept identical. As shown in
Table 1, the results show clearly that light irradiation
significantly enhanced the catalytic performances of the Pd NP
catalyst. From a study of 7 different primary aliphatic alcohols
OH
Pd
Pt
84 (18)
0 ( 0 )
0 (37)
62 (42)
OH
Pd
Pt
52 (34)
0 ( 0 )
3 ( 8 )
36 (24)
OH
Pd
Pt
47 (28)
0 ( 0 )
6 (10)
24 (12)
OH
(
entries 1-7), good to excellent yields of corresponding
OH
OH
Pd
100 (14) 0 (19)
aliphatic esters, up to 100%, were achieved under irradiation
at 60 °C. Much lower alcohol conversion and relatively higher
selectivity to another product, aldehydes, were observed for
the reactions in the dark. In comparison to the results reported
in the literature, the photocatalytic process show significant
advantages of high ester yield attained at low temperature
Pd
91 (22)
9 (29)
c
OH
Pd
Pt
―
―
96 ( 0 )
27 ( 0 )
d
c
OH
Pd
Pt
―
―
92 ( 0 )
17 ( 0 )
d
2a,4b
without base.
The dependence of photocatalytic activity on
c
OH
Pd
Pt
―
―
100 ( 0 )
20 ( 0 )
light intensity showed that the product yield increases linearly
with the increasing light intensity (Fig. S4), because more
photons contribute to driving the reactions under irradiation
0
1
d
c
OH
Pd
Pt
―
―
94 ( 0 )
18 ( 0 )
d
7
of higher intensity.
c
Pd
Pt
―
―
100 (42)
88 (42)
Visible-light irradiation also enhanced the performance of
Pt NPs photocatalyst for those reactions (Table 1), but Pt NPs
exhibited distinctly different product selectivity compared to
Pd NPs. Aldehydes were observed as the sole final product
under same reaction conditions.
The Pd and Pt NPs are also effective for the oxidation of
secondary aliphatic alcohols to ketones (entries 8-13). The
yields of ketones using the Pd photocatalyst, up to 100%, are
even significantly higher than for the primary alcohols under
even mild conditions with air as oxidant. Notably, no catalytic
activities were observed for those reactions undertaken in the
dark (entries 8-11) by both Pd and Pt photocatalysts. Cyclic-
12
13
OH
e
c
Pd
Pt
―
―
O
100 (18)
36 ( 9 )
OH
d
O
HO
HO
OH
Photocatalysts, hv
14
c
Pd
Pt
93 (18)
30 (14)
―
―
d
O
O
Photocatalysts, hv
O
O
OH
+
1
5
d
Pd
Pt
100 (0)
0 (0)
0 (100)
0 (0)
d
a
3-
Reaction conditions: 5 wt% Pd/Pt@HT-PO
4
photocatalyst, 50 mg;
alcohols turn out to be more easily oxidized than aliphatic reactant, 0.1 mmol; solvent, 2 mL of α,α,α-trifluorotoluene; 1 atm of
O ; environment temperature, 60°C; reaction time, 24 h; and light
2
alcohols under the same reaction conditions.
The successful self-esterification of aliphatic alcohols over
Pd NPs also inspired us to explore the selective oxidation of
2 b
intensity, 0.6 W/cm . The values in parentheses are the results in the
c
d
e
dark. air, 50°C, 6 h; air, 50°C, 24 h; air, 50°C, 12 h.
2
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A kinetic study of the oxidative esterification of 1-octanol
COMMUNICATION
View Article Online
DOI: 10.1039/C6CC05186C
3−
using 5 wt% Pd@HT-PO4 photocatalyst was conducted under
irradiation with a short wavelength LED (400 nm), a long
wavelength LED (620 nm) and in the dark (Fig. S5, SI). Time
course for the reaction conversion and product selectivity
shows that the conversion of the 1-octanol increased as the
reaction proceeded. The conversion of reaction and ester
selectivity under LED irradiation of 400 nm wavelength, are
higher than that under the irradiation of 620 nm wavelength
and that in the dark. Aldehyde was the main intermediate
during the reaction and no product selectivity to ester
exhibited in early reaction stage (0-8 h) for all reaction
conditions. This indicated that the alcohol is firstly oxidized to
aldehyde and then the aldehyde further condensed with
alcohol molecules to yield ester.
4 8
Fig. 3 The local density of states (LDOS) of butanal (C H O) adsorbed
on Pd (111) surface and Pt (111) surface. The geometries and
adsorbate states as shown in the Figure were calculated by DFT
method with generalized gradient approximation functional PBE.
To understand why illuminated Pd is uniquely selective for
ester formation, infrared emission spectroscopy (IES)
measurements of 1-pentanol absorbed on Pd and Pt catalysts
were performed, clearly showing the difference in binding
It is also important to consider how light is mediating the
affinity of Pd and Pt NPs with alcohol (see Fig. S6 in SI). Peaks catalytic reactions. One possible pathway by which the light
-1
-1
-1
at 2870 cm , 2935 cm and 2960 cm , which are attributed to irradiation can enhance the photocatalytic activity is that the
the saturated C-H stretching vibrations of 1-pentanol (Fig. S7, light absorption of metal NPs generates energetic electrons
SI) adsorbed on the surface of Pd NPs, are still obvious when (hot electrons), which may migrate to the unoccupied LUMO
temperature increased to 400°C. Whereas these peaks are of the reactant molecules, inducing reaction of the molecules
6
a,10
negligible in spectra of Pt NP catalyst plus 1-pentanol and HT- (Fig. 2A).
The prerequisite of this mechanism is the hot
3
-
9
PO4 plus 1-pentanol. The results indicate that the adsorption electrons have sufficient energy to migrate to the LUMO.
of 1-pentanol on the Pd NP surface is much stronger than that Nevertheless, when the strong chemisorption of the reactant
on Pt NP surface. The adsorption energies of 1-butanol and (or intermediate) molecules on the metal NP surface occurs,
butanal on Pd and Pt NPs, respectively were also estimated by hybridization of metal electron states and adsorbate molecule
density functional theory (DFT) simulation. The results (Table orbitals forms bonding and antibonding states (Fig. 2B). In
S3, SI) indicate that both the reactant alcohol and intermediate such system the light absorption can cause direct
aldehyde adsorb more strongly to the Pd NP surface than to photoexcitation of electrons from the hybridized bonding state
1
1
1
2
the Pt surface.
to antibonding state, inducing the reactions.
If the reactions are induced by the hot electron migration
According to the DFT simulation, the aldehyde and alcohol
have similar adsorption energies on Pd NPs, indicating that (Fig. 2A), Pd and Pt NPs should have exhibited similar catalytic
both would be on the surface after sufficient aldehyde has activity, based on the several similar factors, such as work
been produced, which would enable enhanced ester formation functions (5.2-5.6 eV for Pd and 5.1-5.9 eV for Pt), particle size
under illumination. Whereas on Pt NPs, the adsorption of distribution (Fig. 1C) and light absorption at visible light region
aldehyde is much weaker than that of corresponding alcohol. It (Fig. 1D). But this is not in agreement with the experimental
is more likely that alcohol would dominate the surface of Pt results. Hence, another factor – strength of interaction of the
NPs under same reaction conditions. Light irradiation is unable reactant and intermediate molecules with Pd or Pt NP is
to enhance the aldehyde adsorption. Under harsh conditions, playing crucial role to the photocatalytic activity
13
aldehydes on Pt NPs are readily over-oxidized to CO2.
enhancement.
DFT calculation of the LDOS for butanal adsorbed on Pd
and Pt surface (Fig. 3) shows that molecular orbitals of the
adsorbed aldehyde can hybridize with metal d states of both
14
Pd and Pt NPs, forming bonding and antibonding states.
However, the hybridization of butanal with Pd surface is much
better than that with Pt surface. The energy level of π
antibonding orbital is only 1.37 eV above the Fermi Level when
butanal adsorbed on Pd(111) surface, while the energy level is
1
.83 eV above the Fermi Level when butanal is adsorbed on
Pt(111). It is much easier to excite further reaction of aldehyde
NPs under light irradiation. (B) Direct photoexcitation of bonds formed on Pd NPs than that on Pt NPs. Therefore, the large
between non-plasmonic transition metal surfaces and strongly difference in the interaction with aldehyde between Pd NPs
Fig. 2 (A) Indirect photoexcitation of non-plasmonic transition metal
13
chemisorbed adsorbates.
and Pt NPs is the main reason why Pd photocatalyst yields
ester while Pt photocatalyst yields aldehyde.
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COMMUNICATION
To further support the proposed mechanism, we
conducted the reactions using a mixture of alcohol and and Pt NPs supported on PO4 exchanged hydrotalcite exhibit
aldehyde on Pd and Pt catalysts (Table 2). Oxidative superior visible light photocatalytic activity to the selective
esterification of 1-pentanol and 1-hexanol with octanal were oxidation of aliphatic alcohols at a near ambient temperature
investigated. Up to 20% of pentyl-octanoate and 9% hexyl- without any additives. Moreover, the product selectivity of the
octanoate were achieved by Pd NPs at first 8 h, while no ester reaction varies with the catalyst metals: esters are the final
products were observed in the same reactions using the Pt NP products with the Pd NP catalyst while aldehyde are produced
Journal Name
In summary, this study shows that non-plasmonic Pd NPs
View Article Online
DOI: 10.1039/C6CC05186C
3
−
catalyst.
on Pt NPs. The reaction is facilitated by the photoexcitation of
hybridized electronic states formed by chemisorption of the
Table 2 Photocatalytic base-free adirect esterification of reactant on metal particle surfaces. The adsorption of the
different alcohols with octyl aldehyde
alcohol and aldehyde on the metal particles determines the
product selectivity. The mechanism is different from that of
plasmonic metal NP photocatalysts. We may apply the visible
light photocatalysis of non-plasmonic metal NPs to selective
catalytic reactions that cannot be achieved exclusively with
thermal energy input.
Ester
product
Reaction
time
Yield (%)
Alcohol
Aldehyde
Pd
20
20
31
Pt
0
0
8
h
Pentyl-
octanoate
1
1
-Pentanol Octanal
12h
1
6h
0
8
h
9
15
18
0
0
0
Hexyl-
octanoate
-Hexanol
Octanal
12h
Notes and references
1
6h
a
3-
4
Reaction conditions: 5 wt% Pd/Pt@HT-PO
photocatalyst, 50 mg;
1
S. E. Davis, M. S. Ide and R. J. Davis, Green Chem., 2013, 15,
alcohol, 0.1 mmol; aldehyde, 0.1 mmol; solvent, 2 mL of α,α,α-
trifluorotoluene; 1 atm of air; environment temperature, 60 °C;
reaction time, 8 h, 12 h, 16 h; and light intensity, 0.6 W/cm .
17.
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selective oxidation of primary aliphatic alcohols on Pt and Pd
NPs as illustrated in Scheme 1. First, alcohol molecule
adsorbed on the metal nanoparticles surface because of strong
binding affinity. Light irradiation can promote the cleavage of
O−H bond of adsorbate to form metal alkoxide and metal
hydride species(I and i). Light excited electrons can further
facilitate H elimination from the α-H of metal alkoxide species
3
4
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the strong chemisorption, and further react with adsorbed
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2
4
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9
Scheme
1
Proposed reaction pathway of oxidative
esterification of aliphatic alcohols
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