Photocatalytic transfer hydrogenolysis of aromatic ketones using alcohols

Article abstract of DOI:10.1039/d0gc00732c

A mild method of photocatalytic deoxygenation of aromatic ketones to alkyl arenes was developed, which utilized alcohols as green hydrogen donors. No hydrogen evolution during this transformation suggested a mechanism of direct hydrogen transfer from alcohols. Control experiments with additives indicated the role of acid in transfer hydrogenolysis, and catalyst characterization confirmed a larger number of Lewis acidic sites on the optimal Pd/TiO2 photocatalyst. Hence, a combination of hydrogen transfer sites and acidic sites may be responsible for efficient deoxygenation without additives. The photocatalyst showed reusability and achieved selective reduction in a variety of aromatic ketones.

Full text of DOI:10.1039/d0gc00732c

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DOI: 10.1039/D0GC00732C
ARTICLE
Photocatalytic transfer hydrogenolysis of aromatic ketones using
alcohols
Hongji Li,a,b Zhuyan Gao,a,b Lijun Lei, a Huifang Liu, a Jianyu Han,a,b Feng Hong,a,b Nengchao Luo a and
Received 00th January 20xx,
Accepted 00th January 20xx
Feng Wang*a
DOI: 10.1039/x0xx00000x
A mild method of photocatalytic deoxygenation of aromatic ketones to alkyl arenes was developed, which utilized alcohols
as the green hydrogen donor. No hydrogen evolution during this transformation suggested a mechanism of direct hydrogen
transfer from alcohols. Control experiments with additives indicated the role of acid on the transfer hydrogenolysis, and
catalyst characterization confirmed a higher content of Lewis acidic sites on the optimal Pd/TiO2 photocatalyst. Hence a
combination of hydrogen transfer sites and acidic sites may be responsible for the efficient deoxygenation without additives.
The photocatalyst showed reusability and achieved selective reduction in a variety of aromatic ketones.
refluxing isopropanol using excess Raney nickel.20 Fernandes
and co-workers developed the deoxygenation of ketones
catalysed by oxo-rhenium complexes in 3-pentanol. Selective
Introduction
Developing green and atom-economic transformations is of
great importance for renewable production of fine chemicals
and fuels. Hydrodeoxygenation of aromatic ketones is one of
fundamental processes that has attracted considerable
attention due to its general applications in chemical synthesis
and biomass upgrading.1-4 To avoid using stoichiometric and
hazardous reductants such as Zn/Hg and hydrazine under harsh
partial hydrogenation to alkenes was realized under 170 oC and
no conversion was observed in other alcohols such as methanol,
ethanol and isopropanol.24 Despite this success, these methods
suffered from the limited types of alcohols and harsh refluxing
conditions due to difficulty in activation of alcohols under
thermal conditions. Herein we seek help from photocatalysis to
realize mild transfer hydrogenolysis using general alcohols. We
found that Pd/TiO2 catalysed hydrogenolysis of aromatic
ketones to alkyl arenes using ethanol (EtOH) as both hydrogen
donor and solvent under light irradiation (Scheme 1). Through
analysis of reaction process and characterizations of catalyst, a
photocatalytic hydrogen transfer mechanism was proposed.
This method provided a green route for deoxygenation of
aromatic ketones using alcohols.
conditions in the classical chemical reductions,5, catalytic
6
deoxygenation methods under mild conditions have been
developed.7, 8 In the presence of hydrogen gas (H2), Ru complex,
bimetallic Fe-Ru catalyst, supported Pd catalyst and Co-Ce/C
catalyst have been exploited to achieve general and selective
deoxygenation.9-12 Considering operational safety and facility,
liquid silanes which are by-products from silicon industry were
used to replace H2 as the hydride donor, allowing reduction to
occur under relative mild conditions.13-17 Even so, organic silicon
waste was generated during the silane-involved reduction.
Therefore more environmentally benign hydrogen donors are
urgently needed for sustainable deoxygenation reactions.
Recently emerging hydrogen donors such as formic acid and
alcohols have been employed to realize green reduction
transformations via transfer hydrogenolysis.18-28 Alcohols are
readily available from diverse sources and they are considered
as a type of the optimal solvents with respect to green
chemistry.29-32 Utilization of alcohols as both solvent and
hydrogen donor can improve the atom economy and facility of
deoxygenation reactions. Initially, Zuidema and co-workers
developed the transfer hydrogenolysis of aromatic ketones in
Experimental
Catalyst preparation
Three samples of TiO2 loaded with Pd were prepared by a
photodeposition method using different Pd salts as
precursors.33 In a typical procedure, a 150 mL quartz reactor
equipped with a stir bar was loaded with the TiO2 (Degussa P25,
500 mg), Pd salts(Pd loading, 3 wt%) and solvent (50 mL
acid
redox
O
Pd/TiO2 as bifunctional photocatalyst
H
H
Ar
R
Ar
R
EtOH as hydrogen donor
room temperature
a. State Key Laboratory of Catalysis (SKLC), Dalian National Laboratory for Clean
Energy (DNL), Dalian Institute of Chemical Physics (DICP), Dalian 116023, China.
b. University of Chinese Academy of Sciences, Beijing 100049, China.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
Scheme 1 Photocatalytic transfer hydrogenolysis of aromatic
ketones using alcohol on bifunctional Pd/TiO2.
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dichloroethane and 3.2 mL methanol). Then the atmosphere Catalyst characterization
was changed to argon (Ar) before sealing the reactor. This
View Article Online
DOI: 10.1039/D0GC00732C
Transmission electron microscopy (TEM) was carried out using
a JEOL JEM-2100 electron microscope with an accelerating
voltage of 200 kV to observe the size and morphology of
nanoparticles. The carbon monoxide (CO) and pyridine
adsorption IR spectra were acquired on a Bruker 70 IR
spectrometer. The spectra of adsorbed CO or pyridine
molecules have subtracted the spectra of the samples prior the
adsorption as background. Quantification of acidity of
photocatalyst by the pyridine-adsorption IR was conducted as
follows: the Pd/TiO2 samples (21.1-21.8 mg) were placed in a
homemade IR cell and evacuated (P < 10-3 Pa) at 423 K for 0.5 h.
Then pyridine vapor was introduced to the cell at 303 K and left
for 1 h. The cell was then evacuated at 393 K for 0.5 h (P < 10-2
Pa) to remove the physically adsorbed pyridine and then cooled
mixture was irradiated at room temperature with homemade
LEDs lamps (λmax = 365 nm, 105 W) for 3 h. After completion of
photo-deposition, the samples were obtained from
centrifugation and washed with excess ethanol. These Pd/TiO2
samples with palladium nitrate (Pd(NO3)2), palladium acetate
(Pd(OAc)2) and palladium acetylacetonate (Pd(acac)2) as
precursors were denoted as Pd-N/TiO2, Pd-A/TiO2 and Pd-
AA/TiO2, respectively.
Besides the photodeposition method, an impregnation-
chemical reduction method was also used to prepare the
Pd/TiO2,34 which was denoted as Pd-IC/TiO2.
Catalytic activity measurement
In a typical run, a 5 mL quartz reactor equipped with a stir bar to 303 K. After collecting the spectra, the calculation of surface
was loaded with aromatic ketone (0.2 mmol), Pd/TiO2 (3 wt%, density of Lewis acidic sites was based on the following
푐 = 1.42 × 퐼퐴 × 푟2 ÷ 푤
35
10 mg) and alcohol (1 mL). Then the atmosphere was changed formula :
to argon before sealing the reactor. This mixture was irradiated Where c is the density of acidic sites [mmol/g(catalyst)], IA is the
at room temperature (fan cooling) with LEDs lamps (λmax = 365 integrated absorbance of assigned band (cm-1), r is the radius of
nm, 6 W) for 6 h. After completion of the reaction, the standard the sample disk (cm) and w is the mass of sample disk (mg).
solution (mesitylene) was added. After filtration using a Nylon
Besides, the IR spectra of pure Pd/TiO2 samples were
syringe filter, the solution was analysed by gas chromatography collected to compare with that of TiO2, no obvious difference
1
(GC, Agilent 7890B, DB-FFAP column). H and 13C NMR spectra suggested hardly any adsorbates on the prepared catalysts (SI,
of isolated compounds were measured on a Bruker AVIII 400 Figure S1).
spectrometer (1H: 400 MHz, 13C: 101 MHz).
The quantification of hydrogen gas was performed with
helium (He) as the internal standard. After reactions, He (500
Results and discussion
μL) was injected into the reaction system and well mixed. Then
the mixed gas was analysed by GC equipped with thermal
conductivity detector (GC-TCD, Techcomp 7900) with Ar as the
carrier gas. The quantification of acetaldehyde was performed
via a 2,4-dinitrophenylhydrazine (DNPH) derivatization method
with p-chloroanisole as the internal standard. The derivatization
mixture was analysed by high performance liquid
chromatography (HPLC, waters XSelect HSS-PFP column).
This investigation on transfer hydrogenolysis started with
Table 1 Condition screening for transfer hydrogenolysis using
alcohols by photocatalysis. a
catalyst
alcohol
O
OH
h, Ar r.t.
AP
EB
PE
Convers
ion of
AP (%)
99
Yield
of EB
(%)
9
99
99
2
0
0
1
2
Yield
of PE
(%)
90
0
Feng Wang received his B.Sc. at
Zhengzhou University (1999) and
Ph.D. at Dalian Institute of
Chemical Physics, the Chinese
Academy of Sciences (2005). He
Entry
Catalyst
Solvent
1
2
3
4
5
6
7
8
9
Pd-N/TiO2
Pd-A/TiO2
Pd-AA/TiO2
Pd-IC/TiO2
TiO2
Pd-N/TiO2
Pd-N/TiO2
Pd-N/TiO2
Pd-A/TiO2
Pd-A/TiO2
Pd-A/TiO2
Pd-A/TiO2
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
1-PrOH
2-PrOH
MeOH
1-PrOH
2-PrOH
EtOH
99
99
99
92
52
78
92
99
0
spent
2005–2006
as
a
88
55
49
68
90
51
61
0
postdoctoral fellow at the
University of California-Berkeley in
USA and 2006–2009 at the
Hokkaido
University-Catalysis
42
27
99
0
Research Center in Japan. He
serves as a full professor and an
10
11
12b
92
99
0
independent PI at Dalian Institute of Chemical Physics (2009), a
joint professor in the State Key Laboratory of Catalysis at DICP
(2013), and is the director of the Division of Biomass Conversion
& Bio-Energy at DICP (2018 to present). He served as the Cheung
Kong Professor at Dalian University of Technology in 2016. His
current research focuses on heterogeneous catalysis and
biomass conversion.
0
aConditions: AP (0.2 mmol), catalyst (Pd 3 wt%, 10 mg), solvent
(1 mL), 365 nm LEDs, 6 h, room temperature; The conversion
and yields were quantified by GC using mesitylene as internal
standard. bDark conditions.
2 | J. Name., 2012, 00, 1-3
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acetophenone (AP) as the substrate and EtOH as the solvent
and hydrogen donor, and different Pd/TiO2 photocatalysts were
tested (Table 1). When Pd-N/TiO2 was employed, a high
selectivity (90%) of phenethanol (PE) product was observed
(entry 1). When Pd-A/TiO2 and Pd-AA/TiO2 were used, a
stoichiometric amount of ethylbenzene (EB) was obtained
(entries 2, 3). The catalytic performance of Pd-IC/TiO2 was
similar with Pd-N/TiO2, and partial hydrogenation to alcohol
product was observed (entry 4). The support TiO2 could catalyse
partial reduction of AP to PE but in a moderate yield (55%),36, 37
and no EB was obtained indicating loaded Pd was essential for
full hydrogenolysis (entry 5). Then the feasibility of other alcohols
was tested. In the presence of Pd-N/TiO2, the high selectivity of
PE product was kept in methanol (MeOH), 1-propanol (1-PrOH)
and 2-propanol (2-PrOH) though lower conversion was observed
in MeOH and 1-PrOH (entries 6-8). However, the type of alcohols
affected the selectivity of EB product in the presence of Pd-A/TiO2
(entries 9-11). Only 2-PrOH exhibited similar hydrogen donating
ability with ethanol. In view of that ethanol is more available than
2-PrOH, thus ethanol was used for further investigation. The
water-contained ethanol was also used for this transformation in
view of that abundant bio-ethanol from biomass refinery contains
water. A small amount of water (5 equiv) did not affect the EB
generation, however, the selectivity of EB decreased as further
increasing the amount of water (SI, Figure S4). Meanwhile, Pd-
A/TiO2 and Pd-AA/TiO2 showed the equally catalytic performance,
thus one of them (Pd-A/TiO2) was selected for further studies. The
heterogeneous Pd-A/TiO2 was successfully recycled for two
times,
View Article Online
Scheme 2 Substrate scope of aromatic ketones for Pd-A/TiO2
catalysed transfer hydrogenolysis usingDeOtI:h1a0n.1o0l39u/nDd0GerC0p0h73o2tCo
irradiation.
however, it deactivated in the third recycle (SI, Figure S5). No
conversion under dark conditions indicated that this transfer
hydrogenolysis was photo-induced (Table 1, entry 12).
With the optimized conditions in hand, the substrate scope
of aromatic ketones was explored (Scheme 2). Firstly, aryl
methyl ketones with different substituent groups on aryl ring
were tested. The ketone with electron-donating group (-MeO)
was more readily converted to corresponding alkyl arene (2a,
83% yield) than those with electron-withdrawing groups (2b
and 2c, 44-63% yields). The existence of phenolic hydroxyl did
not affect the transfer hydrogenolysis (2d-f, 54-93% yields)
which showed a promising application on hydrogenation of
lignin-derived monomers. Then aryl ketones with substituent
groups on aliphatic carbon were tested. Aryl ketones with
secondary carbon were still tolerable under current
hydrogenolysis conditions (2g-i, 57-94% yields). However, aryl
ketones with the tertiary and quaternary carbon could not
undergo efficient hydrogenolysis (2j and 2k, 0-16% yields),
suggesting that the steric hindrance has a remarkable effect on
transfer hydrogenolysis in this heterogeneous system. The
lignin β-O-4’ ketone model was selectively deoxygenated at
carbonyl group with trace amount of ether bond cleavage (2h,
94% yield). This phenomenon was different from reported
methods on transfer hydrogenolysis of lignin substrates to
monomers.38, 39 The possible reason for retaining ether bond
under current conditions maybe the fast hydrogenation of
carbonyl group to hydroxyl group, which could increase the
bond energy of ether bond.40 Hence, this transfer
hydrogenolysis method can be applied in the selective
O
Pd-A/TiO2
EtOH
H
H
R'
R'
R
R
h
Ar, 12 h, r.t.
deoxygenation
of
side-chains
in
lignin
without
1
2
depolymerization.
There are two possible pathways for hydrogen transfer from
alcohol to substrates. One is that alcohol dehydrogenates to
release hydrogen gas, then hydrogen gas is activated on the
Pd/TiO2 and hydrogenates the ketone substrates.41 The other
pathway is that hydride species is directly generated by
activation of alcohol on the Pd/TiO2 and then hydrogenates the
substrates.26 To elucidate the hydrogen transfer pathway, the
time curve of this reaction was performed on Pd-N/TiO2 and Pd-
A/TiO2 (Figure 1). It is observed that PE was gradually generated
on Pd-N/TiO2 and then hardly decomposed as reaction time was
prolonged (Figure 1a). While PE was generated initially then
rapidly converted to EB on Pd-A/TiO2 suggesting that PE was the
intermediate of EB product (Figure 1b). Meanwhile, as for both
catalysts, the hydrogen release did not occur until 80%
conversion of AP was observed. The delay of hydrogen
evolution suggested that these hydrogen transfer reactions may
undergo via a direct hydride generation from the alcohol.
Moreover, acetaldehyde was continuously generated along
with the conversion of AP that indicated ethanol was the
hydrogen donor responsible for the hydrogen transfer to
substrate.
H
H
H
H
H
H
MeO
MeO
O
F
2a, 83%a
2b, 44%
2c, 63%
OMe
H
H
H
H
H
H
MeO
MeO
HO
HO
HO
2e, 93%
2f, 54%
2d, 72%
H
H
H
H
H
H
2g, 79%
2h, 57%b
2i, 94%
H
H
H
H
H
H
O
2l, 94b
2j, 16%
2k, trace
Conditions: substrate, 0.2 mmol; Pd-A/TiO2, 10 mg; EtOH, 1 mL; Ar, room
temperature, 365 nm LEDs, 12 h; GC yields with mesitylene as the internal
standard. aIsolated yield. bSubstrate, 0.1 mmol; DCE/EtOH (1:1).
This journal is © The Royal Society of Chemistry 20xx
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To investigate the key factor of catalysts on the selectivity of
hydrogen transfer products, different additives were tested
(Figure 2). The individual Pd-N/TiO2 delivered PE as a major
product (Figure 2a). The addition of acetic acid induced the
further hydrogenolysis to EB. Meanwhile, the sodium acetate
Journal Name
O
catalyst
ethanol
OH
View Article Online
DOI: 10.1039/D0GC00732C
h, Ar r.t.
AP
(a)
EB
PE
100
80
60
40
20
0
100
80
60
40
20
0
AP
O
OH
catalyst
O
OH
+
H2
+
+
+
PE
h Ar, r.t.
AP
EB
PE
AD
(a)
0.7
AP
EB
PE
H2
EB
0.6
0.5
0.4
0.3
0.2
0.1
0.0
100
None
HNO3 BF3
HOAc NaOAc
H3PO4
80
60
40
20
0
AD
(b)
100
80
60
40
20
0
100
80
60
40
20
0
AP
EB
0
1
2
3
PE
Time (h)
(b)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
AP
EB
PE
H2
100
80
60
40
20
0
None
Py
t-BuONa
HNO3 NaNO3
AD
Figure 2 The effect of additives on the transfer hydrogenolysis on Pd-
N/TiO2 (a) and Pd-A/TiO2 (b). Conditions: substrate (0.2 mmol),
catalyst (Pd 3 wt%, 10 mg), additives (0.2 mmol), ethanol (1 mL), 365
nm LEDs, 6 h, room temperature.
did not affect the products distribution. This comparison
indicated that protonic acid may promote the hydrogenolysis of
PE. Indeed, phosphoric acid also increased the yield of EB.
However, nitric acid showed no effect on the Pd-N/TiO2
catalysed hydrogenolysis. In previous reports, nitrate ion could
be hydrogenated to nitrogen gas or ammonia on Pd/TiO2 under
photo-irradiation.42 Thus nitrate ion in nitric acid may block the
hydrogenolysis of PE.43, 44 Besides, boron trifluoride (BF3) also
efficiently promoted the full conversion of AP to EB. These
experiments indicated the presence of a protonic or Lewis acid
could assist the Pd-N/TiO2 to realize the total hydrogenolysis. In
the case of Pd-A/TiO2, EB was obtained without additives
(Figure 2b). The variation of products was observed when a base
or the nitrate ion was added. This variation in the presence of
bases (pyridine and sodium t-butoxide) suggested the acidic
sites on Pd-A/TiO2 may be responsible for the hydrogenolysis of
PE thus blocking of acidic sites hinders the dehydroxylation. The
variation of products in the presence of nitrate ion further
proved that nitrate ion hindered the hydrogen transfer to PE
even to AP.
0
1
2
3
Time (h)
Figure 1 Time curves of photocatalytic transfer hydrogenolysis of
acetophenone using ethanol on Pd-N/TiO2 (a) and Pd-A/TiO2 (b).
Conditions: substrate AP (0.2 mmol), catalyst (Pd 3 wt%, 10 mg),
ethanol (1 mL), 365 nm LEDs, room temperature.
To verify the ability of protonic acid as a hydrogen donor,
the reactions using protonic acid as additives in acetonitrile
were performed (SI, eq S2-4). With addition of formic acid, trace
amount of EB and 10% yield of PE were obtained. However, no
hydrogenated products were obtained with addition of
phosphoric acid. These experiments suggested the usage of
proton as hydrogen donor needs a proper hole sacrificial agent,
such as alcohols and formate anion.
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experiments, the methyl group was also deuteVraietweAdrticwleiOthnlinae
lower labelling extent than AP (EB-D3, EB-D4 and EB-D5). The
O
H
H
D
D
D
D
Pd-A/TiO2
Yield
72%
DOI: 10.1039/D0GC00732C
H
H
H
H
D
D
C2D5OD
CH3
...
...
H
H
D
formation of benzylic C−H and primary C−D bonds were further
365 nm, 6 h
r.t., Ar
1
AP
proved by the H NMR analysis (SI, Figure S6). A possibility is
EB-D2
EB-D0
EB-D5
EB-D0 : EB-D1 : EB-D2 : EB-D3 : EB-D4 : EB-D5
that the deoxygenation of AP may undergo via the styrene
intermediate, where the aliphatic C−H bond would be activated
and deuterated. However, no styrene was detected in the dark
reaction of PE catalysed by Pd-A/TiO2 (SI, eq S1). As proposed in
Ru complex catalysed system,9 a reversible keto−enol
tautomerization and following hydrogenolysis processes may
occur, which could increase the labelling on methyl of AP with
respect to PE. Even so, if following hydrogenolysis of PE
underwent via direct dehydroxylation, only EB-D3 would be
obtained and this is not the case. Hence, an acid-participated
H/D exchange without free styrene generation during reduction
of PE may occur to induce further labelling in the product.21
Different precursors affected the properties of catalysts.
Through comparison of TEM data from two photocatalysts
(Figure 3, a-b), it was observed that the average size of Pd
particles on Pd-A/TiO2 (4.2 nm) was a bit larger than that of Pd-
N/TiO2 (3.5 nm). Then carbon monoxide (CO) adsorption IR
spectra were used to characterize the valence state of Pd
particles (Figure 3c). Only Pd0 species (2000~1900 cm-1 for
bridging adsorption and around 2087 cm-1 for linear adsorption)
was observed on Pd-A/TiO2.45, 46 However, both Pd2+ (2172 and
36 : 87 : 100 : 44 : 28 : 14
OH
Pd-A/TiO2
C2D5OD
Yield
99%
CH3
EB-D0 : EB-D1 : EB-D2 : EB-D3 : EB-D4 : EB-D5
H
365 nm, 6 h
r.t., Ar
19 : 61 : 100 : 26 : 11 : 4
PE
OD
D
dehydroxylation
Pd-D
CH3
CH3
D
D
PE
EB-D2
O
D
dehydroxylation
D
CH3
C
D
H2
OH
OH
EB-D3
D
Pd-D
C
CH2
D
H2
acid
H+
O
acid
D
acid
- DHO
D
H
DHO
acid
...
D
H
Pd-D(H)
EB-D0, EB-D1, EB-D4, EB-D5
Scheme 3 Isotopic labelling experiments and a proposed pathway for 2119 cm-1 for linear adsorption) and Pd0 species were observed
on Pd-N/TiO2.47, 48 These observations are in consistent with
previous reports where small size particle of Pd contained Pd0
and Pd2+ species while the large size particle only contained Pd0
species.49-51 The possible reason for residual Pd2+ species and
photocatalytic transfer hydrogenolysis on Pd-A/TiO2. Conditions:
substrate (0.2 mmol), Pd-A/TiO2 (Pd 3 wt%, 10 mg), ethanol-D6 (1
mL), 365 nm LEDs, 6 h, room temperature.
H
e
Pd
e
CB
H+
h
TiO2
H+ CH3CHO
+
h+
VB
CH3CH2OH
O
OH
acid
H
H
H
H
H
H
Pd
Pd
O
O
O
hydrogen transfer centers
Ti4+ Ti
O
hydrogen transfer centers
Ti
Figure 3 Characterization of Pd/TiO2 samples. TEM images and Pd
size distribution histograms of Pd-N/TiO2 (a) and Pd-A/TiO2 (b). (c) IR
spectra of CO adsorption on Pd/TiO2 samples. (d) IR spectra of
pyridine adsorption on Pd/TiO2 samples.
Lewis acidic centers
Scheme 4 Proposed mechanism of Pd-A/TiO2 photocatalysed
transfer hydrogenolysis of aromatic ketone using ethanol as
hydrogen donor.
Then isotopic labelling experiments were performed to
investigate the trend of hydrogen transfer (Scheme 3). It was
observed that not only carbonyl carbon but also the adjacent
methyl carbon of AP were deuterated in the ethanol-D6.
Unlabelled EB (EB-D0) was also detected with a considerable
amount. Besides, when PE was subjected into the labelling
smaller size of Pd-N/TiO2 is that the nitrate ion could be reduced
by photo-induced electrons on TiO2 during photo-deposition
(Figure S3) thus it competes with reduction of Pd2+ cation.42-44
The external acid or base additives dramatically affected the
hydrogen transfer ability of Pd/TiO2. Then pyridine adsorption
IR spectra were employed to characterize the acidity of Pd-
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4.
Z. Zhang, J. Song and B. Han, Chem. Rev., 2016, 117, 6834-
A/TiO2 and Pd-N/TiO2 (Figure 3d). Only Lewis acidic sites (1445
View Article Online
52, 53
cm-1) were observed on both catalysts.35,
And the
6880.
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54-57
proposed (Scheme 4).50,
Under irradiation of light, the
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photogenerated holes on TiO2 oxidized the ethanol to
acetaldehyde and proton. The photogenerated electrons on
TiO2 migrated to Pd0 particles and reduced the proton to
activated hydrogen species (Pd-H). Then the ketone substrate
was reduced to alcohol intermediate by Pd-H. In the presence
of abundant Lewis acidic sites on TiO2, the alcohol intermediate
was dehydrated and further hydrogenated to ethylbenzene by
Pd-H. Thus Pd-A/TiO2 acted as the bifunctional photocatalyst
containing hydrogen transfer sites and Lewis acidic sites, which
realized the transfer hydrogenolysis using alcohols without
addition of other additives.
11.
12.
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Conclusions
Herein we developed a photocatalytic transfer hydrogenolysis
of aromatic ketones using alcohols as hydrogen donor at room
temperature. The Pd-hydride species could be generated from
activation of alcohols under light irradiation and reduced
substrates directly. The detection of benzylic alcohol
intermediate suggested a stepwise reduction pathway. The
combination of Lewis acidic sites and hydrogen transfer sites on
Pd/TiO2 may be the key to achieve efficient deoxygenation
without external additives.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21721004, 21961130378, 21690080), the
Ministry of Science and Technology of the People's Republic of
China (2018YFE0118100), the "Strategic Priority Research
Program of the Chinese Academy of Sciences" (XDB17000000)
and Dalian Science and Technology Innovation Fund
(2019J11CY009).
27.
28.
29.
30.
31.
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Green Chemistry
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Photocatalytic deoxygenation of aromatic ketones to alkyl arenes was developed on Pd/TiO2 using
DOI: 10.1039/D0GC00732C
alcohols as the green hydrogen donor.