Photocatalytic Coproduction of Deoxybenzoin and H2 through Tandem Redox Reactions

Article abstract of DOI:10.1021/acscatal.9b03651

Photocatalytic H₂ evolution from organic feedstocks with simultaneous utilization of photogenerated holes achieves solar energy storage and coproduces value-Added chemicals. Here we show visible-light H₂ production from benzyl alcohol (BAL) with controllable generation of deoxybenzoin (DOB) or benzoin (BZ) through tandem redox reactions. Particularly, DOB synthesis circumvents the use of expensive feedstocks and environmentally unfriendly catalysts that are required previously. Under the irradiation of blue LEDs, the key of steering the major product to DOB rather than BZ is to decrease the conduction band bottom potentials of the ZnIn sulfide catalysts by increasing the Zn/In ratio, which results in the dehydration of intermediate hydrobenzoin (HB) to DOB proceeding in a redox-neutral mechanism and consuming an electron-hole pair. As a proof of concept, this method is used to synthesize DOB derivatives in gram scale.

Full text of DOI:10.1021/acscatal.9b03651

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Article
Photocatalytic Coproduction of Deoxybenzoin
and H2 through Tandem Redox Reactions
Nengchao Luo, Tingting Hou, Shiyang Liu, Bin Zeng, Jianmin Lu, Jian Zhang, Hongji Li, and Feng Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b03651 • Publication Date (Web): 04 Dec 2019
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Photocatalytic Coproduction of Deoxybenzoin and H2 through Tandem
Redox Reactions
Nengchao Luo,1,2 Tingting Hou,3 Shiyang Liu,1,2 Bin Zeng,1,2 Jianmin Lu,1 Jian Zhang,1 Hongji Li,1,2
Feng Wang1*
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1 State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of
Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China
2 University of Chinese Academy of Sciences, Beijing 100049, P. R. China
3
School of Materials Science and Engineering, Central South University, Changsha 410083, P. R.
China
Corresponding author:
*Feng Wang, wangfeng@dicp.ac.cn
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ABSTRACT
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Photocatalytic H2 evolution from organic feedstocks with simultaneous utilization of photogenerated
holes achieves solar energy storage and coproduces value-added chemicals. Here we show visible light
H2 production from benzyl alcohol (BAL) with controllable generation of deoxybenzoin (DOB) or
benzoin (BZ) through tandem redox reactions. Particularly, DOB synthesis circumvents the use of
expensive feedstocks and environmentally unfriendly catalysts that are required previously. Under the
irradiation of blue LEDs, the key of steering the major product to DOB rather than BZ is to decrease
the conduction band bottom potentials of the ZnIn sulfide catalysts by increasing the Zn/In ratio, which
results in the dehydration of intermediate hydrobenzoin (HB) to DOB proceeding in a redox-neutral
mechanism and consuming an electron-hole pair. As a proof of concept, this method is used to
synthesize DOB derivatives in gram scale.
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Key words: Photocatalysis; dehydrogenation; dehydration; C−C coupling; deoxybenzoin; ZnIn2S4
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INTRODUCTION
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Photocatalytic transformation of organic feedstocks has emerged as a sustainable and environmentally
benign method to produce value-added products.1-3 By virtue of light irradiation, oxidative holes and
reductive electrons are generated in semiconductors, which results in oxidation and reduction of
organic moieties, respectively, and affords targeted products. Due to the formation of charges (i.e.
holes and electrons) that are in excited states, photocatalysis is able to achieve uphill reactions that are
usually possible at elevated temperatures for thermocatalysis.4-5 Therefore, if H2 is coproduced in
photocatalytic selective transformation of organic feedstocks, the whole reaction will maximize the
merit of photocatalysis. In this context, use of solar light to construct C−C bond with H2 production
receives much attention due to the straightforward production of value-added products from readily
available feedstocks without the use of hazardous catalysts and additives.6-7 Previous works have
achieved direct synthesis of ethylene glycol from methanol,8 ethane or benzene from methane,9-10
together with H2 production.11 These state-of-the-art processes demonstrate the tremendous potential
of photocatalysis in producing H2 with simultaneous utilization of small molecules as building blocks
to provide value-added products through C−H bond dehydrocoupling.
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Production of deoxybenzoin (DOB) from benzyl alcohol (BAL) is such an example. DOB was
industrially prepared from phenylacetic acid and benzene by AlCl3 catalyzed C−C bond coupling. The
process requires functionalization of phenylacetic acid to phenylacetyl chloride by stoichiometric PCl3
or SOCl2 prior to the C−C bond coupling.12-13 Other reported methods required the pre-
functionalization of starting molecules or use expensive substrates alternatively (Scheme. S1).14-16
Thus, it is desirable to develop processes for DOB production using readily available and
environmentally benign feedstocks. Moreover, if H2 is also produced in the conversion of BAL to
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DOB, the whole process will achieve the coproduction of a clean energy vector and a very high value-
added product.17-19
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One way to produce DOB from BAL is by tandem dehydrocoupling and dehydration reactions.
Nevertheless, photocatalysis usually involves oxidation/reduction reactions induced by
photogenerated holes and electrons, respectively,4, 20 which limits its use in the synthesis of complex
products from readily available feedstocks. If redox catalytic sites work separately, BAL was finally
converted to benzoin (BZ) or benzil.4, 21 Therefore, rationally assembling oxidation and reduction
reactions in sequence is the key to produce DOB from BAL. A solution is to develop semiconductors
of which the properties can be continuously adjusted,22 thereby achieving the cooperation of oxidation
and reduction reactions.23-24 Ternary ZnIn2S4 is such a kind of semiconductor of which the band
structures can be adjusted either by replacing zinc or indium with other metals or by varying the ratio
of zinc and indium contents.25 The catalyst was used to catalyze coupling of phenylacetylene and
benzyl mercaptan,26 dehydrocoupling of benzylamine to imidazole,27 and methylfurans to diesel fuel
precursors by our group.25 Particularly, ZnIn2S4 can catalyze the self-transfer hydrogenolysis of lignin
β-O-4 models, which involves consecutive dehydrogenation of CH−OH moiety and reductive
hydrogenolysis of C−OAr bond.28 Therefore, rationally adjusting the band structures of ZnIn2S4
semiconductor has a great potential to produce DOB directly from BAL.
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Herein, by tuning the conduction band (CB) bottom potentials of ZnIn sulfides, we successfully
synthesized DOB from BAL with coproduction of H2 by visible light irradiation. This process was
directed to consecutive dehydrocoupling and dehydration reactions rather than dehydrogenation in the
second step to yield BZ. The dehydrocoupling of BAL to intermediate hydrobenzoin (HB) was realized
by one-step oxidation by holes followed by C−C coupling while the dehydration of HB was a redox-
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neutral process that consumed an electron-hole pair. The major product could be controlled to BZ if
ZnIn sulfides with larger CB bottom potential was used.
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EXPERIMENTAL SECTION
Materials. ZnSO4·7H2O (99.5%), InCl3·4H2O (99.9%), thioacetamide (TAA, 99%), cetyl trimethyl
ammonium bromide (CTAB), benzyl alcohol (BAL, 99%), 4-methoxyl acetophenone, Ru(II)Cl2(PPh3),
D2O, 4-methylbenzyl alcohol, 4-methoxylbenzyl alcohol, deoxybenzoin, benzoin, hydrobenzoin,
benzil, benzaldehyde, were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd.
Tetrabutylamine hexafluorophosphate (TBAHFP) was purchased from Alfa Aesar. All the reagents
were used as received without further purification.
Preparation of ternary ZnxIn2S3+x (x = 0.1, 0.2, 0.4, 0.6 and 0.8). ZnIn sulfides were prepared with
referring to a literature procedure.29 Typically, ZnSO4·7H2O, InCl3·4H2O and CTAB (260.6 mg) were
dissolved in 20 mL of Milli-Q water in a 100-mL beaker and magnetically stirred for 30 min at room
temperature. TAA (604.8 mg) was then added into the above solution. After being stirred for another
30 min, the mixture was transferred to a 50-mL stainless Teflon-lined autoclave, tightly sealed and
placed in a 160 °C oven for 20 h. The autoclave was then naturally cooled to room temperature. After
being washed with absolute ethanol (4 × 25 mL) and Milli-Q water (2 × 25 mL), a yellow solid was
obtained after being dried in vacuum at 60 °C for 12 h.
Photocatalytic reactions. Photocatalytic reactions were conducted in a home-made LED photoreactor
as reported in our previous work.28 Briefly, 0.2 mmol of BAL or 0.1 mmol of HB, 10 mg of catalyst,
1.0 mL of MeCN and magnetic bar were added into the quartz reaction tube, followed by replacing
the atmosphere by Ar. The reaction tube was installed on the photoreactor. After irradiation for the
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desired time, the products were quantitatively analyzed.
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For Hammett plot, the reaction rate constants of BAL with substituents relative to that of BAL
were obtained by measuring the conversions of BAL and those with substituents at 0.25 h. For
hydrogen kinetic isotope effect (KIE), the conversions of BAL and BAL-d2 at 0.5 h were used to
calculate KIE value.
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Radical trapping experiments. The radical intermediates in the photocatalytic dehydrocoupling of
BAL were trapped in situ by 1,1-diphenylethylene in a scale-up reaction. Briefly, 5 mmol of BAL, 10
mmol of 1,1-diphenylethylene, 100 mg of catalyst, 50 mL of MeCN and magnetic bar were added into
the quartz reaction cell, followed by replacing the atmosphere by Ar. After irradiation for 50 h by an
86 W blue LEDs (452 nm, full width at half maximum = 10 nm), the mixture was analyzed by GC-
MS. Then the reaction mixture was filtered through a 0.22 μm Nylon syringe filter to remove the
catalyst, followed by purification by column chromatography to separate the adduct of radical
intermediate and 1,1-diphenylethylene.
1H NMR (400 MHz, CDCl3) δ 7.35 – 7.05 (m, 15 H), 4.41 (dd, J = 8.4, 5.1 Hz, 1 H), 4.09 (dd, J = 9.0,
6.8 Hz, 1 H), 2.52 – 2.28 (m, 2 H), 2.05 (s, 1 H). 13C NMR (101 MHz, CDCl3) δ 144.81, 144.78,
144.34, 128.71, 128.65, 128.64, 128.16, 127.97, 127.80, 126.44, 126.37, 126.11, 72.38, 47.65, 44.90.
Quantitative analysis of liquid phase products by gas chromatography (GC). BAL, DOB, BZ and
HB were quantified by GC equipped with a flame ionization detector (GC-FID, Agilent 7890A). 4-
Methoxy acetophenone as the internal standard was added into the reaction systems after photo-
irradiation for a desired time, followed by filtration through a 0.22 μm Nylon syringe filter to remove
the catalyst. The reaction system was then analyzed by GC. The following equation was used to
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calculate the yield of product.
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Yield of target product = moles of BAL converted to target product × 100%
(1)
moles of BAL added
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Quantitative analysis of H2. H2 was quantitatively analyzed by GC equipped with a thermal
conductivity detector (GC-TCD, Techcomp 7900) with Ar as the carrier gas. After reaction, He as the
internal standard was injected into the reaction system. After 10 minutes, about 15 μL of the gas phase
was sampled and analyzed. The produced H2 can be calculated from the following equation:
푉(He)
(
)
×
0.5965 × 퐼 H2
퐼(H2)
101.3
푉(He) mmol
mL
( )
H2
(2)
(3)
푛
=
×
8.314 × 298mmol = 0.0244 × 퐼(He) ×
퐼(He)
mL
Yield of H2 = 2 × moles of BAL converted to H × 100%
2
moles of BAL added
Where n(H2) is the produced H2 in the reaction system, mmol; I(H2) and I(He) are the TCD response
of H2 and He, respectively. V(He) is the injection volume of He. The yield of H2 was based on BAL
conversion to DOB, so 1 mole of H2 is produced when 2 moles of BAL is exclusively converted to
DOB.
Mott-Schottky measurements. 20 mg of Zn0.2In2S3.2 or Zn0.6In2S3.6 was dispersed in 2 mL of MeCN
and vigorously stirred for 16 h followed by ultrasonication for 0.5 h. Thereafter, the suspension was
added to a clean fluorine-doped tin oxide (FTO) glass (ultrasonically treated in acetone, ethanol and
H2O for 0.5 h, respectively) followed by spin coating (WS-650MZ-23NPP) at 1500 r min-1 for 30 s.
Then the FTO was dried naturally at r.t. for re-spin coating. The above steps were repeated for 15 times
for each of the two materials to form a proper film of Zn0.2In2S3.2 or Zn0.6In2S3.6 on the FTO glass.
Finally the FTO glass coated with sample was dried in vacuum at 90 °C for 12 h. The electrochemical
measurements of the prepared Zn0.2In2S3.2 or Zn0.6In2S3.6 electrode were conducted in 0.1 M
TBAHFP/MeCN solution with an electrochemical workstation (PARSTAT MC). A three electrode
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system with platinum-plate electrode as the counter electrode and Ag/AgCl electrode as the reference
electrode was used for measurements. The frequency, scanning voltage range and test points were
1000 Hz, −0.72 V−0 V and 80, respectively.
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RESULTS AND DISCUSSSION
Photocatalyst contrast
Photocatalytic coproduction of H2 and DOB from BAL was tested with some commonly used
semiconductor photocatalysts for comparison. Their BET surface areas and XRD patterns were
obtained and shown in Table S1 and Figure S1. In these photocatalysts, BiVO4, Bi2WO6 and mpg-
C3N4 (Table 1, entries 1~3) showed low activity in converting BAL to DOB and H2. Under the
irradiation of UV LEDs (365 ± 5 nm, 6 W ), anatase TiO2 loaded with Pt by impregnation method
converted BAL to benzaldehyde (BA) and H2 in 73% and 238% yields, respectively (Table 1, entry
4), affording very minor C−C bond coupling products. The missing carbon together with the very high
H2 balance (1.56) may be caused by the over-oxidation of BAL to CO2 or related acid, which has been
reported in other work.30 Irradiation of In2S3 with blue LEDs (450 ± 5 nm, 6 W) for 12 h converted
BAL to BA (6% yield) and H2 (13% yield, Table 1, entry 5). Hydrobenzoin (HB), formed via
dehydrocoupling of two BAL molecules, was also generated despite of 2% yield. Reaction over CdS
produced HB (31% yield) and H2 (44% yield) as the major products. Other products were
deoxybenzoin (DOB) and trimers (Table 1, entry 6), similar to the reaction results in previous
literatures.4, 21 By contrast, when the reaction was conducted over Zn0.2In2S3.2 ternary sulfide, BZ and
H2 were the major products (61% and 172% yields, respectively, Table 1, entry 7), the corresponding
H2 balance was determined to be 1.00 (Scheme S2). The yield of DOB was also much higher (30%
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yield) than that over CdS catalyst. The best result was achieved over Zn0.6In2S3.6 with 64% yield of
DOB produced (Table 1, entry 8). H2 was produced in 134% yield, lower than that over Zn0.2In2S3.2
catalyst. This result is consistent with the higher yield of DOB over Zn0.6In2S3.6 catalyst since BAL
conversion to DOB produces 1 equivalent of H2 while 2 equivalents when BAL is converted to BZ
(Scheme S2). The byproducts BZ and trimers were produced in 16% and 13% yields, respectively.
Carbon balance and hydrogen balance of this reaction were 95% and 0.99, respectively, the missing
carbon may go to oligomers due to C−C bond coupling. The reaction did not proceed in the dark (Table
1, entry 9), demonstrating a photocatalytic process.
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Table 1. Catalytic performance of some typical semiconductors in photocatalytic coproduction
of H2 and DOB from BAL.a
Zn0.6In2S3.6
OH
Ar, blue LEDs, MeCN,
O
O
OH
OH
OH
BAL
H2
+ Trimers +
+
+
DOB
BZ
HB
Yield (%)
DOB BZ
c
Conversion
H2
Entry
Catalyst
Trimer
s
b
(%)
balance
H2
BA
HB
1
2
BiVO4
Bi2WO6
mpg-C3N4
Pt/P25
5
7
4
3
0
0
3
13
3
0
4d
5
99
238
13
73
6
1
3
1
2
1.56
0.96
0.94
1.00
0.99
In2S3
11
6
CdS
39
44
1
2
31
5
7
Zn0.2In2S3.2
Zn0.6In2S3.6
> 99
> 99
172
134
9
30
64
61
16
8
2
13
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Zn0.6In2S3.6
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a Reaction conditions: 0.2 mmol of BAL, 10 mg of catalyst, 1.0 mL of MeCN, Ar atmosphere, 6 W
blue LEDs (455 ± 5 nm), 12 h. b The calculation of H2 yield was based on the conversion of BAL to
DOB (stoichiometric ratio of H2 and DOB is 1:2), so the theoretical yield of H2 is 100% when DOB is
produced in 100% yield. c H2 balance = yield (H2) / [2×yield (BA) + yield (DOB) + 2×yield (BZ) +
yield (HB) + 8/3×yield (trimers)]. d 6 W LEDs (365 ± 5 nm). e Reaction in the dark. The procedures
for the preparation of control catalysts can be found in Supporting Information.
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Figure 1. Influence of Zn/In ratio on product yields in photocatalytic dehydrocoupling of BAL to DOB
and BZ. Reaction conditions: 0.2 mmol of BAL, 10 mg of catalyst, 1.0 mL of MeCN, Ar atmosphere,
6 W blue LEDs (455 ± 5 nm), 9 h.
Variation of Zn/In ratio of ternary ZnIn sulfides greatly altered selectivity towards DOB and BZ
according to Table 1, entries 7 and 8. As a result, a higher Zn/In ratio favors the formation of DOB.
The influence of Zn/In ratio was further studied. ZnxIn2S3+x with various x were prepared by the same
hydrothermal method except varying the relative contents of zinc and indium precursors (Table S1).
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According to X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and
transmission electron microscope (TEM) results (Figure S2 and S3), the structures of the series of
ternary ZnIn sulfides are similar to hexagonal ZnIn2S4,29 but contains minor β-In2S3 phase.31 Their
band gaps widen from 2.27 to 2.40 eV when x varies from 0.1 to 0.8 (Figure S4). The ternary
ZnxIn2S3+x with x = 0.1, 0.2, 0.4, 0.6 and 0.8 were employed for photocatalytic dehydrocoupling of
BAL to DOB (Figure 1). When Zn0.1In2S3.1 (Zn/In = 0.05) was used, DOB was produced in 24% yield
and the major product was BZ (65% yield). Other products were mainly BA (7% yield), implying
predominant dehydrogenation than dehydration over Zn0.1In2S3.1 catalyst. Reaction over Zn0.2In2S3.2
offered similar distributions of products as previously discussed. By contrast, ZnIn sulfide with a
higher zinc content (x ≥ 0.4) produced 59% DOB yield accompanied by the BZ yield decreased to 8%.
Therefore, we show that the main product can be switched between DOB and BZ by altering the zinc
content of ZnIn sulfide catalysts.
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Exploration of reaction path
Figure 2. Time profile of photocatalytic dehydrocoupling of BAL over Zn0.6In2S3.6 catalyst. Reaction
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conditions: 0.2 mmol of BAL, 10 mg of Zn0.6In2S3.6, 1.0 mL of MeCN, Ar atmosphere, 6 W blue LEDs
(455 ± 5 nm). All the data points are the average of two separated experiments. Curves drawn on top
of data are guides to the eye.
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Targeting DOB, the time profile of photocatalytic dehydrocoupling of BAL over Zn0.6In2S3.6
catalyst was studied (Figure 2). After lighting on, BAL was rapidly converted, accompanied by the
formation of HB and H2. HB showed a maximum yield (62% yield) after the LEDs irradiation for 4 h.
Afterwards, HB diminished and the main product DOB, byproducts BZ and trimers appeared, together
with the formation of additional H2. The time profiles of DOB and BZ show induction periods at the
same time range, implying that DOB and BZ are produced from HB in consecutive reactions.21 BZ
generation can be rationalized by the second-step dehydrogenation of the hydroxyl group in HB
(Scheme S3) by photogenerated holes as described in previous work.21 DOB is the dehydration product
of HB, but the mechanism is less known as photocatalytic dehydration is rarely reported.32 After
reaction for 12 h, the yields of the three liquid phase products all reached constant values with total
conversion of BAL.
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Control Experiments
O
O
H
Zn0.6In2S3.6
blue LEDs
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6
7
8
OH
OH
OH
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(b)
(c)
+
+
+
64% yield
O
22% yield
O
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OH
OH
H
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O
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O
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O
Deuterium Experiment
H
D
D
D
D
OH
Zn0.6In2S3.6
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(d)
+
OH
BAL-d2
Scheme 1. Investigation of reaction path. Reaction conditions: 0.2 mmol of BAL, 10 mg of Zn0.6In2S3.6,
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1.0 mL of MeCN, Ar atmosphere, 6 W blue LEDs (455 ± 5 nm), 9 h.
HB as the intermediate for the formation of DOB and BZ was confirmed by control experiments.
With HB subjected to reaction over Zn0.6In2S3.6 catalyst under the irradiation of blue LEDs (455 nm),
DOB and BZ were formed in 64% and 22% yields, respectively, similar to the case of BAL substrate
(Scheme 1a). Other byproducts were not detected in GC and might be oligomers due to homo-coupling
of two HB after the cleavage of the benzylic C−H bond. Reaction over Zn0.2In2S3.2 catalyst afforded
BZ in 61% yield (Scheme 1b). The above results well explain the variation of product yields over the
ZnIn sulfides with different Zn/In ratios in Figure 1. When the reaction was conducted over Zn0.6In2S3.6
catalyst in the dark, HB was only slightly converted with very low yields of DOB and BZ (Scheme
1c), implying dehydration of HB to DOB proceeds in a photocatalytic manner.
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Figure 3. Radical capturing results of photocatalytic dehydrocoupling of BAL to DOB. (a) Reaction
scheme of radical capturing by 1,1-diphenylethylene. (b) Gas chromatogram of the reaction mixture.
Reaction conditions: 100 mg of Zn0.6In2S3.6, 5 mmol of BAL, 10 mmol of 1,1-diphenylethylene, 50
mL of MeCN, Ar, 86 W LEDs (452 ± 10 nm), 84 h.
The reaction mechanism was studied by isotope label experiment, replacing the benzylic hydrogen
atoms by deuterium (Scheme 1d). The involvement of only one deuterium atom in DOB is consistent
with the reaction process that dehydrocoupling of BAL forms HB followed by the dehydration and
enol-keto tautomerism to DOB. BZ also contains one deuterium atom linked to carbon, consistent with
the dehydrocoupling-dehydrogenation mechanism for its formation. Substituents at the para-position
of BAL show irregular and marginal influence on the reaction rates (Figure S5), confirming a neutral
intermediate (radical) involved in the benzylic C‒H bond cleavage.33-34 The slower reaction rate of
BAL with −OMe substituent may be ascribed to the larger polarity of –OMe which competes with –
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CH2OH group for adsorption on catalyst surface. The radical in photocatalytic BAL dehydrocoupling
was captured by 1,1-diphenylethylene (DPE, Figure 3). The addition of 2 equivalents of DPE nearly
inhibit the reaction and only a small amount of DOB was produced (Figure 3a), consistent with a
radical mechanism for the photocatalytic BAL dehydrocoupling. Besides, the direct detection of DPE-
BAL illustrates that photocatalytic dehydrocoupling of BAL to HB involves direct C−H bond
activation by photogenerated holes, forming radical R-BAL (Figure 3b). Of note, the hydrogen kinetic
isotope effect (KIE) of photocatalytic dehydrocoupling of BAL was determined to be 1.25 ± 0.04
(Scheme S4a), demonstrating that C−H bond activation is not involved in the rate-limiting step.35
Accompanied by the reactions of photogenerated holes, the generated protons produced from C−H
bond cleavage was reduced to H2 by photogenerated electrons (Scheme S3).
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Studies of photocatalytic dehydration mechanism
Figure 4. HB (0.1 mmol) conversion in the presence of hole trapping reagent (0.2 mmol of HCOONa)
or electron trapping reagent (0.2 mmol of K2S2O8), photo-irradiation for 1 h. Reaction conditions: 0.2
mmol of substrate, 10 mg of Zn0.6In2S3.6, 1.0 mL of MeCN, Ar atmosphere, 6 W blue LEDs (455 ± 5
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The dehydration mechanism of intermediate HB to targeted product DOB was studied by radical
trapping experiments (Figure 4). Since the valence band (VB) top potentials of the series of ZnIn
sulfides lie above the oxidation potential of hydroxyl groups,36-37 we considered that HB conversion
to either DOB or BZ is initiated by C−H bond activation, in the same way as that of BAL. Addition of
hole trapping reagent (HCOONa) into the reaction system accelerated HB conversion to DOB,
illustrating that oxidation of HB by photogenerated holes was not involved in the rate-limiting steps,
which was proved by the secondary KIE value (1.17 ± 0.12, Scheme S4b) for the cleavage of the C−H
bond of HB. The slight increase of DOB yield from 64% to 71% (prolong the reaction time to 4 h)
implies that reduction reactions, more precisely the reductive cleavage of C−OH bond, induced by
photogenerated electrons are likely to steer the selectivity of dehydration products. In agreement with
this, addition of electron trapping reagent K2S2O8, that competes with HB or related intermediates to
be reduced, dramatically suppresses the formation of DOB but favors BZ formation.
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Figure 5. Mott-Schottky plots, the violet and golden yellow lines are tangent lines. The inset shows
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the equivalent Randles circuit used to fit the electrochemical impedance data, where Rs is the total
series resistance, Rp is the parallel resistance due to the charge transfer, and Cscl is the space-charge
layer capacitance.
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The reaction results of ZnIn sulfides with different Zn/In ratios can be rationalized by their
reactivity in the reductive cleavage of C−OH bond. Mott-Schottky method was used to measure the
CB bottom potentials of the ZnIn sulfides in MeCN solution, which are located at potentials 0.4 V
more negative than the corresponding flat band potentials (Ef, Figure 5).38-39 The obtained result of
Zn0.6In2S3.6 was −2.85 V, smaller than that of Zn0.2In2S3.2 (−2.68 V), in agreement with previously
reported results that a higher Zn/In ratio corresponds to a lower CB bottom potential of the ZnIn
sulfide,40 which results in the high reduction ability of the catalyst. Therefore, a higher Zn/In ratio
favored the reductive cleavage of C−OH bond for the formation of DOB. However, when 1-
phenylethanol was subjected to reaction, 2,3-diphenylbutane-2,3-diol was the only product due to C−C
bond coupling (Scheme S4c), implying that the catalysts cannot directly reduce the C−OH bond of
HB. Therefore, the ZnIn sulfides were likely to reductively cleave the C−OH bond of radical R1b
(Figure 6) that was generated from HB through C−H bond cleavage. Although CB bottom potential is
crucial for the reductive cleavage of C−OH bond, a relatively large light intensity is also important for
the photocatalytic dehydration of HB to DOB since solar light cannot afford DOB as the main product
(Scheme S5).
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Figure 6. Proposed reaction mechanism of photocatalytic dehydration of intermediate HB to DOB.
The proposed reaction mechanism of HB dehydration to DOB was shown in Figure 6.
Photogenerated holes oxidize the C−H bond of HB, delivering radical R1b (Figure 6) that transfers to
CB and is reduced to 1,2-diphenylethenol (DPE) by photogenerated electrons. Tautomerism of DPE
finally forms DOB. Besides, DOB production from HB involves hole-oxidation and electron-reduction
reactions where kinetic factors also counts, thus, the reported method of varying CB bottom potential
of ZnIn sulfide only improved DOB yield from 24% to 64%.
Scale-up photocatalytic preparation of DOB derivatives
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Figure 7. Scale-up reactions of photocatalytic coproduction of substituted DOB and H2. p-Methoxy
BAL (a) and p-methyl BAL (b) as substrates. Reaction conditions: 10 mmol of substrate, 250 mg of
Zn0.6In2S3.6, 50 mL of MeCN, Ar atmosphere, 86 W blue LEDs (452 ± 10 nm). Inserts show the
substrates. The yields of products were quantified by GC. Curves drawn on top of data are guides to
the eyes.
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The tandem dehydrocoupling-dehydration method was used to synthesize DOB derivatives in
scale-up reactions (10 mmol of substrate). Methoxyl and methyl groups were chosen due to the
abundance of them in pharmaceutics. Besides, benzylic methyl group is very active in many
photocatalytic systems,2, 25 thus, the co-presence of benzylic methyl group can be used to evaluate the
selectivity of the catalyst. Figure 7 shows the time profiles in converting p-methoxy BAL and p-methyl
BAL to their corresponding DOB. The product distributions were similar to that of BAL with dominant
production of DOB with substituents. Consistent with photocatalytic BAL to DOB, vicinal diols were
the intermediates, further confirming the previously proposed reaction mechanism. Particularly, the
formed H2 was 147 mL in the case of p-methoxy BAL, higher than that of p-methyl BAL (134 mL of
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H2), in agreement with the lower yield of substituted DOB for p-methoxy BAL. The isolated yields of
1,2-bis(4-methoxyphenyl)ethanone and 1,2-di-p-tolylethanone were 58% and 65%, respectively. The
simultaneous production of H2 and substituted DOB in gram scale demonstrates the tandem
dehydrocoupling-dehydration method in the transformation of readily available feedstocks to value-
added products including clean energy vectors and chemicals.
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CONCLUSIONS
In summary, we achieved the visible-light photocatalytic production of H2 from BAL with
simultaneously production of DOB, which improved the value of feedstocks. The dehydrocoupling of
BAL to HB was the first step involved in the transformation and achieved by selective cleavage of the
benzylic C−H bond by photogenerated holes. Subsequent dehydration of HB was the key to the desired
product DOB, which was a redox neutral process that consumed an electron-hole pair. Increasing the
Zn/In ratio decreased the CB bottom potentials of the ZnIn sulfides, thus facilitating the reduction of
hydroxyl groups and directing the major product to DOB under the irradiation of the blue LEDs.
Therefore, the main product can be varied between DOB and BZ by altering Zn/In ratio of the ZnIn
sulfides. This work puts forward a way to produce renewable H2 from readily available feedstocks that
are simultaneously converted to very valuable products by tandem redox photoreactions.
AUTHOR INFORMATION
Corresponding Author
* F.W. E-mail: wangfeng@dicp.ac.cn
Notes
There are no conflicts to declare.
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Supporting Information. Catalyst characterization, including XRD, TEM images and some catalytic
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results. This material is available free of charge via the Internet at http://pubs.acs.org.
ACKNOWLEDGMENT
We acknowledge financial support from the National Natural Science Foundation of China (21721004,
21690084, 21690080), the Strategic Priority Research Program of Chinese Academy of Sciences
(XDB17020300, XDB17000000). Computing resource from the National Supercomputing Center in
Tianjin, China is gratefully acknowledged.
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Graphical abstract
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