Photocatalytic hydrogenation coupling of acetone into pinacol using formic acid as hydrogen source

Article abstract of DOI:10.1246/cl.170781

We demonstrated photocatalytic hydrogenation coupling of acetone into 2,3-dimethyl-2,3-butanediol with formic acid as a hydrogen source. In this synergetic system, the conversion of acetone and formic acid was dramatically increased than that in the individual reaction system. Consequently, the selectivity of 2,3-dimethyl-2,3-butanediol was relatively high over that of NaTaO₃ and Na₂Ti₃O₇ photocatalysts. Additionally, in this acetoneformic acid reaction system, the presence of acetone can effectively restrain the generation of CO during the decomposition of formic acid.

Full text of DOI:10.1246/cl.170781

Received: August 18, 2017 | Accepted: September 18, 2017 | Web Released: September 23, 2017
CL-170781
Photocatalytic Hydrogenation Coupling of Acetone into Pinacol
Using Formic Acid as Hydrogen Source
Bao Y. Cao,1 Shan Xu,1 You L. Ren,1 Yan Yu,1 Jin Y. Guo,1 Li Zhang,2 Na Li,2 Guo C. Zhang,*1 and Chun S. Zhou*1
1Shaanxi Key Laboratory of Comprehensive Utilization of Tailings Resources, College of Chemical Engineering
and Modern Materials, Shangluo University, Shangluo 726000, P. R. China
2State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences,
Taiyuan, 030001, P. R. China
(E-mail: zgc316@126.com, slzhoucs@126.com)
We demonstrated photocatalytic hydrogenation coupling of
acetone into 2,3-dimethyl-2,3-butanediol with formic acid as a
hydrogen source. In this synergetic system, the conversion of
acetone and formic acid was dramatically increased than that
in the individual reaction system. Consequently, the selectivity
of 2,3-dimethyl-2,3-butanediol was relatively high over that
of NaTaO3 and Na2Ti3O7 photocatalysts. Additionally, in this
acetone-formic acid reaction system, the presence of acetone
can effectively restrain the generation of CO during the
decomposition of formic acid.
Photocatalytic organic reactions can experience naturally
unusual reaction channels under mild conditions; they are
generally mediated with neutral radicals, which can readily
couple together to derivate high-value chemicals through the
formation of C-C and C-N bonds.15 Many photocatalysts have
been tested as low cost and with high activity and stability under
irradiation. These studies effectively highlight the significant
incentives to develop new environmentally benign photocata-
lytic systems to use FA as a straightforward and highly
convenient hydrogen source. Kominami16 reported the photo-
catalytic reduction of benzonitrile to benzylamine in aqueous
suspensions of palladium-loaded titanium(IV) oxide, which
showed the possibility of using FA as a hydrogen source in
photocatalytic organic systems. As it is well known, both alkali
tantalate and titanate photocatalyst exhibited excellent photo-
catalytic ability. Our previous report showed that surface Na+
ions play an important role in the dehydrogenation coupling of
isopropanol and hydrogenation coupling of the acetone reaction
system, for its fast exchange and travel of H+ ions.17 Here,
we choose NaTaO3 and Na2Ti3O7 as the photocatalyst, which
might be much more active than Ta2O5 and TiO2 (P25) in the
combination reaction system.
In the present study, we conducted photocatalytic hydro-
genation coupling of acetone into 2,3-dimethyl-2,3-butanediol
(DMB, also called as pinacol), using FA as a hydrogen source
(Scheme 1). In this syngenetic reaction system, the active
hydrogen species produced from the dehydrogenation of FA
could directly perform as a feed for the hydrogenation of acetone
into 2-hydroxyisopropyl radicals, which coupled into DMB.
Moreover, both the conversion of acetone and FA and the
selectivity of target DMB have been improved by Na2Ti3O7
and NaTaO3 photocatalysts. Considering that the formed carbon
Keywords: Photocatalyst
| Hydrogenation coupling |
Formic acid
The increasing global demand for energy and decreasing
fossil fuel cause the urgent need to develop renewable energies.
Hydrogen is considerably important in the chemical industry
and might play a critical role in renewable energy technologies.1
However, hydrogen should be transported under high pressure
through pipelines or in tankers or cylinders, which limits its
widespread use.2-5 Formic acid (FA) as a hydrogen source has
many advantages with regard to handling, transport, and storage,
and it can be easily converted into hydrogen gas; these
properties are helpful for hydrogen storage and transportation.
FA is widely used in the chemical, agricultural, and rubber
industries, as well as in power fuel cells for electricity
generation and automobiles.6-8 A number of highly active and
robust homogeneous catalysts selectively decompose FA into
H2 and CO2 near to room temperature.9 Notably, FA is a
commodity of chemical production from biomass resources,10
thereby making it a renewable and potentially valuable hydro-
gen source. Glucose can be converted into FA with an excellent
yield of 75% and purity of 95% at a mild temperature of 250 °C
in the presence of alkali.11 Moreover, FA, or its functional
derivatives, is widely used in organic chemistry to provide
active hydrogen. FA is also used as a reducing agent to obtain
substituted amines from nitroarene amine derivatives and
alcohols from carbonyl and its derivatives.12-14 Recently, FA
has been also applied in biomass conversion, such as production
of 2,5-dimethylfuran from methylfurfural, γ-valerolactone from
levulinic acid,12 1,2-propanediol from glycerol,13 and pyrroli-
dinone from levulinic acid.14 Nevertheless, these reaction
processes usually operate under high temperature and high
pressure. The development of efficient catalysts also remains
the main challenge for practical applications of FA because an
ideal sustainable catalyst should be recyclable, inexpensive,
and efficient for the dehydrogenation of FA under normal
temperature and pressure.
Scheme 1. Schematic of the hydrogenation coupling of
acetone into 2,3-dimethyl-2,3-butanediol using formic acid as
hydrogen source on light-excited photocatalyst surfaces.
Chem. Lett. 2017, 46, 1773–1776 | doi:10.1246/cl.170781
© 2017 The Chemical Society of Japan | 1773

Table 1. The photocatalytic data obtained from different photocatalyst and reactant recipesa
Reactants/mol
Acetone FA
Conversion/%
Acetone FA
4.6
Selectivity/%
Entry
Catalyst
Isopropanol
DMB
HMP
HXD
1
2
3
4
5
6
7
8
9b
10c
NaTaO3
NaTaO3
no
P25
Ta2O5
Na2Ti3O7
NaTaO3
NaTaO3
NaTaO3
NaTaO3
0.067
0
0
0
19.4
®
16.2
®
®
®
16.8
®
14.3
11.5
12.3
4.4
4.3
4.3
4.4
4.6
0.047
0.047
0.047
0.047
0.047
0.024
0.047
0.07
0
32.2
18.7
81.6
71.2
97.8
98.3
96.7
74.4
55.2
0.067
0.067
0.067
0.067
0.067
0.067
0.067
0.067
22.7
77.6
67.1
99.1
52.2
82.4
99.4
99.4
27.1
29.9
27.4
42.2
27.9
30.6
36.4
39.5
27.9
29.8
30.2
38.9
56.2
55.5
51.4
46.4
21.2
15.4
14.9
10.2
7.5
5.6
4.6
5.6
0.093
aReaction conditions: photocatalyst, 1.0 g; total volume of reactant aqueous solution, 200 mL; light, 300 W high-pressure Hg lamp
(- = 365 nm); temperature, ca. 25 °C; reaction duration, 12 h, under Ar atmosphere. bReaction 9 h. cReaction 6 h. The 2,3-dimethyl-2,3-
butanediol was abbreviated as DMB, 2-methyl-2-hydroxypropionic acid was abbreviated as HMP, 2,5-hexanedione was abbreviated as
HXD.
dioxide by the dehydrogenation of FA could be easily recycled
with the hydrogenation process by a suitable catalyst,18,19 the
overall process was atom-efficient, convenient, and a highly
optimized procedure to synthesize DMB.
0.067:0.024. Also, the conversion of acetone increased with
the increasing ratio of FA (Entries 7-10, Table 1). In particular,
both acetone and FA achieved a relatively high conversion
at 99.1% and 97.8% after a reaction of 12 h, when the molar
ratio of acetone to FA is 0.067:0.047 over the Na2Ti3O7
photocatalyst (Entry 6, Table 1). This promotive effect could
be attributed to the active hydrogen species produced from FA
could directly perform as a feed for hydrogenation coupling of
acetone with the photochemical process, as shown in eq 1 to
eq 5, Scheme 1. However, 2-methyl-2-hydroxypropionic acid
(HMP) and 2,5-hexanedione (HXD) by-products were generated
inevitably during the photochemical reaction process (in eq 6
and eq 7).
Alkali tantalate crystals have been well proven to exhibit
good catalytic ability for water splitting under ultraviolet
irradiation.20,21 In this study, the NaTaO3 photocatalyst exhibited
nanosized plate or cubic morphologies (Supporting Information
(SI), Figure S1) and a typical orthorhombic single crystalline
structure (SI, Figure S2). Na2Ti3O7 nanotube was prepared
from a hydrothermal reaction of P25 with NaOH. Transmission
electron microscopic (TEM) images showed that the prepared
Na2Ti3O7 photocatalyst exhibits nanotube morphologies, lengths
of several micrometers, and a narrow diameter of about 10 nm
(SI, Figure S3). X-ray diffraction (XRD) showed less intense
and broad peaks at 2ª = 24, 28, and 48°, caused by Na2Ti3O7
(SI, Figure S4), which is in good agreement with the PDF files:
72-0148. Ultraviolet-visible absorption spectrum of the NaTaO3
and Na2Ti3O7 photocatalysts exhibited an electron absorption
edge at about 320 and 380 nm, respectively (SI, Figures S5 and
S6), which corresponded to band gaps of 3.86 and 3.26 eV.
Photocatalytic reactions were evaluated in an argon-
degassed aqueous medium at ambient temperature using a
200 mL photoreactor (SI, Figure S7). Table 1 and Table S1
present the experimental data obtained from different reactant
recipes and conditions. As shown in Entry 2, Table 1, for the
individual FA reaction, the photocatalytic oxidation decompo-
sition of FA over the NaTaO3 photocatalyst has low efficiency
with a conversion of 32.2%, and the main products are CO2,
CO, and H2 gases,22 as shown in Table S1, which are consistent
with our previous report.17 Also, acetone individual reaction
displayed photolysis behavior in the same conditions, which
released large amounts of CO and CH4 gases, with substantially
low selectivity of DMB (16.2%, Entry 1, Table 1), as obtained
in our previous studies.17 Remarkably, the conversion of both
acetone and FA increased significantly in the acetone-FA
synergetic reaction system (Entries 3-10, Table 1). As shown
in Entry 7, Table 1, the conversion of acetone and FA achieved
52.2% and 98.3% after reaction of only 12 h over the NaTaO3
photocatalyst, when the molar ratio of acetone to FA is
hv
HCOOH ꢀ! H• þ •COOH
ð1Þ
ð2Þ
hv
•COOH ꢀ! H• þ CO2
•
hv
CH3COCH3 ꢀ! CH3COCH2 þ H•
ð3Þ
ð4Þ
H• þ CH3COCH3 ! CH3COHCH3
•
2CH3COHCH3 ! ðCH3Þ2COHCOHðCH3Þ2
ð5Þ
ð6Þ
•
CH3COHCH3 þ •COOH ! ðCH3Þ2COHCOOH
•
•
2CH3COCH2 ! CH3COCH2CH2COCH3
ð7Þ
Furthermore, the hydrogenation coupling of acetone into
DMB using FA as a hydrogen source over the semiconductor
photocatalyst could significantly improve the selectivity of
DMB and isopropanol (Entries 3-7, Table 1), respectively. The
results were far better than that obtained using only acetone
(Entry 2, Table 1, which produced CO, CO2, CH4, and HXD by-
products) and only FA reaction systems (Entry 1, Table 1, which
produced CO and HMP by-products). Interestingly, the forma-
tion of HMP and HXD by-products could be restrained with the
presence of the NaTaO3 or Na2Ti3O7 photocatalyst, which was
more active than blank reaction (Entry 3). These effects linked
with multiple functions, which tuned well the photocatalytic
reaction kinetics and channels:
hv
Semiconductor ꢀ! hþ þ eꢀ
ð8Þ
1774 | Chem. Lett. 2017, 46, 1773–1776 | doi:10.1246/cl.170781
© 2017 The Chemical Society of Japan

HCOOH þ hþ ! hþ þ •COOH
•COOH þ hþ ! Hþ þ CO2
ð9Þ
ð10Þ
ð11Þ
H
þ þ CH3COCH3 þ eꢀ ! CH3COHCH3
•
CH3COHCH3 þ H ! CH3CHOHCH3
ð12Þ
ð13Þ
ð14Þ
•
CH3CHOHCH3 þ hþ ! CH3COHCH3 þ Hþ
•
2CH3COHCH3 ! ðCH3Þ2COHCOHðCH3Þ2
•
For one reason, during the photochemical reaction process,
various kinds of radicals were produced; these radicals could
readily couple together to form the C-C bond. In the first step,
UV excitation of the semiconductor photocatalyst produced
electron-hole pairs (h+/e ), as shown in eq 8. Subsequently, the
¹
Figure 2. The acetone reactant and isopropanol, DMB
products as a function of reaction time. Reaction conditions:
0.067 mol acetone and 0.047 mol formic acid, after reaction of
12 h, adding 0.04 mol acetone.
photogenerated holes oxidized the FA and produced hydrogen
protons (H+) and hydrocarboxyl radicals (•COOH) (eq 9).23,24
The hydrocarboxyl radicals can be further oxidized into H+ and
CO2 (eq 10). Therefore, the selectivity of HMP can be
significantly improved in the presence of Na2TiO3 and NaTaO3
photocatalysts (Entries 3-7, Table 1). As shown in Figure S8,
the photocatalyst can accelerate the photocatalytic oxidation
dehydrogenation of hydrocarboxyl radicals into H+; conse-
quently, the coupling of hydrocarboxyl radicals with 2-hydroxy-
isopropyl radicals to form HMP by-product was efficiently
avoided.
with reaction time. Therefore, acetone addition can promote the
conversion of isopropanol into DMB.
Finally, the decomposition of FA follows two main path-
ways: (a) decarbonylation, which results in carbon monoxide
and water (1:1); and (b) decarboxylation, which provides a 1:1
mixture of carbon dioxide and hydrogen.24
For the other reason, a single-charge process was required
for photocatalytic hydrogenation coupling of acetone into DMB.
Although two-charge reactions possibly occurred in the combi-
nation process to form isopropanol, it would not affect DMB
selectivity. As shown in Figure 1, when the molar ratio of FA
to acetone was 0.047:0.067, acetone was converted into DMB
and isopropanol after reaction of 12 h, and the selectivity values
of isopropanol and DMB are 30.60% and 55.5% (Figure 1 and
Table 1), respectively. With the increase in reaction time, the
selectivity of isopropanol gradually decreased; on the contrary,
the selectivity of DMB gradually increased. Such a change was
explained by the fact that the produced isopropanol can be
further converted into DMB through the dehydrogenation
coupling of isopropanol and hydrogenation coupling of acetone,
which are reported in a previous article.24 This result can be also
confirmed by adding 0.047 mol of acetone into an acetone-FA
reaction system (the molar ratio of FA to acetone is 0.067:0.047,
FA is excessive). As shown in Figure 2, both the added acetone
and isopropanol decreased, whereas DMB gradually increased
hv
HCOOH ꢀ! CO þ H2O
ð15Þ
ð16Þ
hv
HCOOH ꢀ! CO2 þ H2
This result implied that a kinetic competition exists between
the decomposition of FA via CO and CO2 channels. The
experimental results showed that the addition of acetone could
tune the reaction channel to produce CO2 with a relatively high
n(CO2)/n(CO) value (Figure 3). For one reason, the presence of
photocatalyst can accelerate the decomposition of FA, as shown
in eq 9 and eq 10, as shown in Figure S8. For the other reason,
as shown in eq 15 and eq 16, there are two equilibria of the
formic acid decomposition reaction. As the active hydrogen
species produced from FA could directly perform as a feed for
the hydrogenation coupling of acetone, leading to a dramatically
promotive effect on the decomposition of FA into hydrogen and
CO2.
In summary, we demonstrated the photocatalytic hydro-
genation coupling of acetone into DMB using FA as a hydrogen
source. In this synergetic system, the conversion of acetone and
FA was dramatically increased than that in individual reaction
systems because of active hydrogen species produced from FA
could directly perform as a feed for hydrogenation coupling of
acetone, which caused the promotive effect on the decom-
position of FA into active hydrogen species. Meanwhile, it
manifested a relatively high selectivity of the DMB product
because of the accelerated process of photocatalytic oxidation
dehydrogenation of FA into active hydrogen species and the
ability of converting the produced isopropanol into DMB on the
excited NaTaO3 and Na2Ti3O7 photocatalysts. Finally, in this
combination system, acetone could help tune the reaction
channel of FA to the way which we expected, which
demonstrated that FA was a superior candidate of hydrogen
source in photocatalytic organic synthesis. This work demon-
strates another paradigm of using FA as a straightforward and
Figure 1. The conversion of acetone and selectivity of
isopropanol and DMB as function of reaction time.
Chem. Lett. 2017, 46, 1773–1776 | doi:10.1246/cl.170781
© 2017 The Chemical Society of Japan | 1775

L. Mleczko, Green Chem. 2015, 17, 2904.
F. Joó, ChemSusChem 2008, 1, 805.
M. Grasemann, G. Laurenczy, Energy Environ. Sci. 2012, 5,
8171.
4
5
6
K. Tedsree, T. Li, S. Jones, C. W. A. Chan, K. M. K. Yu,
P. A. J. Bagot, E. A. Marquis, G. D. W. Smith, S. C. E.
Tsang, Nat. Nanotechnol. 2011, 6, 302.
Y. Himeda, Green Chem. 2009, 11, 2018.
A. Boddien, H. Junge, Nat. Nanotechnol. 2011, 6, 265.
M. J. Climent, A. Corma, S. Iborra, Green Chem. 2014, 16,
516.
7
8
9
10 S. Liang, P. Monsen, G. B. Hammond, B. Xu, Org. Chem.
Front. 2016, 3, 505.
11 J. Hou, J.-H. Xie, Q.-L. Zhou, Angew. Chem., Int. Ed. 2015,
54, 6302.
12 L. Deng, Y. Zhao, J. Li, Y. Fu, B. Liao, Q.-X. Guo,
ChemSusChem 2010, 3, 1172.
13 I. Gandarias, P. L. Arias, S. G. Fernández, J. Requies, M. E.
Doukkali, M. B. Güemez, Catal. Today 2012, 195, 22.
14 Y. Wei, C. Wang, X. Jiang, D. Xue, Z.-T. Liu, J. Xiao, Green
Chem. 2014, 16, 1093.
15 M. Fagnoni, D. Dondi, D. Ravelli, A. Albini, Chem. Rev.
2007, 107, 2725.
Figure 3. The n(CO2)/n(CO) production-time functions in
different reaction systems. Conditions: 0.047 mol formic acid for
individual reactions and 0.067 mol of acetone and 0.047 mol of
formic acid for combination reaction.
highly convenient hydrogen source in photocatalytic organic
synthesis and provides an efficient and environmentally friendly
approach for DMB synthesis.
16 K. Imamura, T. Yoshikawa, K. Nakanishi, K. Hashimoto, H.
Kominami, Chem. Commun. 2013, 49, 10911.
17 B. Cao, J. Zhang, J. Zhao, Z. Wang, P. Yang, H. Zhang, L.
Li, Z. Zhu, ChemCatChem 2014, 6, 1673.
18 S.-M. Lu, Z. Wang, J. Li, J. Xiao, C. Li, Green Chem. 2016,
18, 4553.
This work was supported by National Natural Science
Foundation of China (No. 21703134), Science and Technology
Development Projects of Shangluo City (No. SK2015-20) and
Project of the Key Laboratory of Scientific Research in Shaanxi
Province Department of Education (No. 16JS029).
19 Z. Zhang, S. Hu, J. Song, W. Li, G. Yang, B. Han,
ChemSusChem 2009, 2, 234.
20 H. Kato, K. Asakura, A. Kudo, J. Am. Chem. Soc. 2003, 125,
3082.
Supporting Information is available on http://dx.doi.org/
10.1246/cl.170781.
References
21 X. Li, J. Zang, J. Phys. Chem. C 2009, 113, 19411.
22 S. Kakuta, T. Abe, ACS Appl. Mater. Interfaces 2009, 1,
2707.
23 D. S. Muggli, M. J. Backes, J. Catal. 2002, 209, 105.
24 A. Gazsi, G. Schubert, P. Pusztai, F. Solymosi, Int. J.
Hydrogen Energy 2013, 38, 7756.
1
2
3
D. Haeseldonckx, W. D’Haeseleer, Int. J. Hydrogen Energy
2007, 32, 1381.
S. Uhm, H. J. Lee, Y. Kwon, J. Lee, Angew. Chem., Int. Ed.
2008, 47, 10163.
W. Supronowicz, I. A. Ignatyev, G. Lolli, A. Wolf, L. Zhao,
1776 | Chem. Lett. 2017, 46, 1773–1776 | doi:10.1246/cl.170781
© 2017 The Chemical Society of Japan