Mpg-C3N4-catalyzed selective oxidation of alcohols using O2 and visible light

Article abstract of DOI:10.1021/ja102866p

Mesoporous carbon nitride (mpg-C₃N₄) polymer can function as a metal-free photocatalyst to activate O₂ for the selective oxidation of benzyl alcohols with visible light, avoiding the cost, toxicity, and purification problems associated with corresponding transition-metal systems. By combining the surface basicity and semiconductor functions of mpg-C₃N₄, the photocatalytic system can realize a high catalytic selectivity to generate benzaldehyde. The metal-free photocatalytic system also selectively converts other alcohol substrates to their corresponding aldehydes/ketones, demonstrating a potential pathway of accessing traditional mild radical chemistry with nitroxyl radicals.

Full text of DOI:10.1021/ja102866p

Published on Web 11/02/2010
mpg-C3N4-Catalyzed Selective Oxidation of Alcohols Using O2 and Visible Light
Fangzheng Su,†,‡ Smitha C. Mathew,§ Grzegorz Lipner,§ Xianzhi Fu,† Markus Antonietti,‡
Siegfried Blechert,*,§ and Xinchen Wang*,†,‡
International Joint Laboratory, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou UniVersity,
Fuzhou, 350002, P. R. China, Max-Planck Institute of Colloids and Interfaces, Department of Colloid Chemistry,
Research Campus Golm, 14424 Potsdam, Germany, and Institut fu¨r Chemie, Technische UniVersita¨t Berlin, Strasse
des 17. Juni 135, D-10623 Berlin, Germany
Received April 6, 2010; E-mail: xcwang@fzu.edu.cn; blechert@chem.tu-berlin.de
for the whole family of substances), as a stable visible light
Abstract: Mesoporous carbon nitride (mpg-C3N4) polymer can
function as a metal-free photocatalyst to activate O2 for the selective
oxidation of benzyl alcohols with visible light, avoiding the cost,
toxicity, and purification problems associated with corresponding
transition-metal systems. By combining the surface basicity and
semiconductor functions of mpg-C3N4, the photocatalytic system can
realize a high catalytic selectivity to generate benzaldehyde. The
metal-free photocatalytic system also selectively converts other
photocatalyst for hydrogen and oxygen evolution.7 This easily
available organocatalyst features a semiconductor band gap of 2.7
eV corresponding to an optical wavelength of 460 nm.
Electrochemical analyses were therefore performed to study the
band positions of the g-C3N4 semiconductor. The studies revealed
a high oxidation potential of the conduction band (CB) at -1.3 V
at pH 7 vs the normal hydrogen electrode (NHE) (E°), while the
reduction level of the valence band (VB) is located at 1.4 V (pH
alcohol substrates to their corresponding aldehydes/ketones, dem-
onstrating a potential pathway of accessing traditional mild radical
chemistry with nitroxyl radicals.
7).8 These band positions are in agreement with a theoretical
calculation.7 A comparison of the band positions of g-C3N4 with
those of the current “golden” photocatalyst, TiO2, as well as some
redox potentials of reactive oxy-radicals, are illustrated in Scheme
S1 (Supporting Information). As revealed from these band char-
The use of solar radiant energy to drive organic reactions provides
a sustainable pathway for green synthesis and has gained significant
interest. However, most common organic molecules do not absorb
light in the visible light region (400-800 nm) that accounts for
43% of the incoming solar spectrum. This restricts photochemical
applications, and thus, it motivates the search for further approaches
to enable the direct utilization of visible light in organic synthesis.1
One approach is to activate target reactions via charge-transfer
catalysis mediated by photosensitizers, such as inorganic ions/
semiconductors and organometallic complexes.2 In the past, Ru-
(bipy)32+ and its derivatives have been widely exploited as visible-
light-harvesting complexes to initiate such photochemical reactions
as oxidation,3a reduction,3b cycloaddition,1b dehalogenation,1c and
asymmetric alkylation1a of organic matter. Dye molecules have also
been used to sensitize inorganic semiconductors via electron transfer
to achieve longer lifetimes of the excited states, such as in the well-
known case of the dye sensitized TiO2 solar cell.4 Recently, Zhao
and co-workers coupled dye/TiO2 photoredox catalysis with the
TEMPO (2,2,6,6-tetramethylpiperidinyloxyl) oxidation scheme to
activate oxygen for selective alcohol oxidation,5 a key transforma-
tion in laboratory and industrial syntheses.6 Their catalytic system
avoids toxic metal salts as oxidation catalysts, and the use of visible
light avoids the induction of the strong, nonselective hydroxyl
radicals, •OH. Although the excited states of some dye molecules
are stable against self-sensitized degradation in light-driven organic
synthesis, they have not found significant application in synthetic
transformations, partly due to the employment of precious metals
such as Ru.
acteristics, light-excited electrons in the CB of melon possess a
large thermodynamic driving force to reduce O2 (E°(O2/•O2-) )
-0.16 V), but the potential of the photogenerated hole in the VB
is inadequate to oxidize -OH to hydroxyl radicals (E°(-OH/•OH)
) 2.4 V). These features provide an argument that g-C3N4 might
act as a suitable candidate for photooxidation and related transfor-
mations in aqueous media, in which TiO2 photocatalysis can achieve
selective oxidation of aromatic alcohols to aldehydes and miner-
alization to CO2 with UV light.9
When we extended our original work to organic functional group
transformations, we discovered a remarkable selectivity (>99%) of
bulk g-C3N4 (surface area: ca. 8 m2/g) for the oxidation reaction of
benzyl alcohol to benzaldehyde with visible light, yet with a
moderate catalytic conversion under the applied standardized
conditions (5%, see entry 1, Table 1). The system presented here
is however metal-free, using cheap and environmentally benign
molecular oxygen as a primary oxidant.
To make the carbon nitride polymer semiconductor more valuable
for heterogeneous (photo)catalysis, an increase of its specific surface
area is important. Porosity was introduced into the bulk g-C3N4 by
using silica nanoparticles as templates10 (the resultant mesoporous
polymeric g-C3N4 is denoted as mpg-C3N4 in the following),
rendering the materials a high mesopore surface area (ca. 200 m2/
g) and accessible surface sites for interfacial (photo)reactions. It is
underlined that mpg-C3N4 is functionally not only a semiconductor
but also a solid state organic reagent with its surface-terminating
amino groups and lone-pair N heteroatoms of the poly(tri-s-triazine)
framework.
Recently, we have introduced a solid-state semiconductor based
on polymeric melon (a slightly disordered precursor of graphitic
carbon nitride, g-C3N4; for simplicity, we use the latter notation
Such N-containing graphitic-like carbon materials can in principle
show the ability to reductively adsorb O2,11 being a potential carrier
for catalytic oxidations. We were thus inspired to employ mpg-
C3N4 to activate O2 with visible light for liquid-phase oxidation of
benzyl alcohol, a model reactant often employed to evaluate the
activity of the oxidation of alcohols.12
† Fuzhou University.
‡ Max-Planck Institute of Colloids and Interfaces.
§ Technische Universita¨t Berlin.
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10.1021/ja102866p  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 16299–16301 16299

C O M M U N I C A T I O N S
A series of experiments was conducted at 373 K using O2 (8
bar) in a closed autoclave with a quartz window for coupling in
the incident photon flux (see Experimental Section in the Supporting
Information) to test the synergistic reactivity of mpg-C3N4, dioxy-
gen, and photonic energy (Table 1). In the absence of the mpg-
C3N4 photocatalyst, benzyl alcohol (BA) was not converted to the
corresponding aldehyde, even with light irradiation under a high
pressure of oxygen for 3 h (entry 5, Table 1). Under the same
conditions, mpg-C3N4 catalyzes the oxidation to benzaldehyde with
>99% selectivity and 57% (visible light irradiation for 3 h) and
38% (UV irradiation for 1 h) (entries 2 and 3, Table 1) conversion
yield, respectively. In the absence of light, a negligible thermocata-
lytic conversion (<1%, entry 4, Table 1) of BA was observed for
the same system. These results demonstrate that the selective
oxidation is clearly due to heterogeneous organo-photocatalysis.
The photocatalytic system also works when the oxygen is replaced
by air, thus enabling aerobic oxidation of BA with 44% conversion
and >99% selectivity to benzaldehyde (entry 6, Table 1).
the conversion rate (57%) of BA was five times higher than that
(11%) in the absence of oxygen (entry 7, Table 1), while keeping
the selectivity at >99%.
Fluorinated solvents which are known to dissolve a larger amount
of oxygen14 turned out to be favorable for the reaction. When
perfluorohexane was used as solvent in an initial experiment under
nonoptimized conditions, 46% conversion of BA was achieved
already at 25 °C with 1 atm of O2. Under the same conditions, BA
conversion in a partly fluorinated solvent, trifluorotoluene, was 21%
(entries 11 and 12, Table 1). The activity in toluene (entry 9, Table
1) was still lower, while in acetonitrile a significantly decreased
selectivity was observed (68%, entry 10, Table 1), with benzoic
acid as the only side product. This is presumably due to the
solvolysis of O-O bonds in polar media,15 making the reaction
less surface controlled.
-
The induction of a formal, presumably still surface-mediated •O2
by mpg-C3N4 was confirmed by electron spin resonance (ESR) and
the DMPO (5,5-dimethyl-1-pyrroline N-oxide) spin trap technique
in methanol.16 There is no ESR signal in the dark (Figure S2), but
a gradual evolution of clear ESR signals for DMPO-•O2- adducts
was observed with visible light irradiation. Note that we do not
detect DMPO-•OH adducts in water (Figure S3). On the basis of
the aforementioned results, the electron transfer reactions involved
in the selective photooxidation of alcohols using mpg-C3N4 with
oxygen are proposed in Scheme 1.
Table 1. Study of Reaction Conditionsa
Con.
[%]b
Sel.
[%]
Solvent
hv
O2 source
1c
2
trifluorotoluene
trifluorotoluene
trifluorotoluene
trifluorotoluene
trifluorotoluene
trifluorotoluene
trifluorotoluene
trifluorotoluene
toluene
+
+
+
-
+
+
+
-
+
+
+
+
O2 (8 bar)
O2 (8 bar)
O2 (8 bar)
O2 (8 bar)
O2 (8 bar)
Air (8 bar)
-
5
57
38
<1
0
44
11
<1
40
70
46
21
>99
>99
>99
-
3d
4
5e
6
-
Scheme 1. Electron Transfer Reactions with mpg-C3N4
>99
>99
-
>99
68
>99
>99
7f
8f
-
9
O2 (8 bar)
O2 (8 bar)
O2 (1 bar)
O2 (1 bar)
10
11g
12g
acetonitrile
perfluorohexane
trifluorotoluene
a In a typical reaction mpg-C3N4 (50 mg) was added to the solvent
(10 mL) in a 100 mL stainless steel autoclave with a quartz window.
The O2 source was introduced and the closed autoclave was heated to
100 °C, followed by visible light irradiation for 3 h. b Conversion rates
were determined by GC with an FID detector. c Using bulk g-C3N4.
d Initiated with UV for 1 h. e Reference experiment without catalyst.
f Reference experiments in the absence of O2. g Performed in a closed
glass reactor with 1 atm of O2 at 25 °C.
To test the reusability of the catalyst system, the used mpg-C3N4
was washed with ethanol, dried at 80 °C, and then re-employed with
a new reactant mixture. Gradual deactivation of the catalyst was
observed during four consecutive experiments while about one-third
of the activity was lost, overall. However, the mpg-C3N4 can be
completely recovered simply by washing with 0.2 M NaOH. This result
not only indicates surface protonation as the deactivation mechanism
but also proves the promoting role of surface basic sites in photoca-
talysis. The total production of benzaldehyde in this series summed
up to 2.7 mmol, and the turnover number (TON) was calculated to be
100, confirming that the reaction proceeds catalytically.
The dependence of activity on the wavelength of the incident
light was also investigated. The activity of mpg-C3N4 corresponds
well with its UV/vis diffuse reflectance spectrum (Supporting
Information, Figure S1), providing an extra confirmation that the
reaction is really enabled by photocatalysis.
Some organic substrates can undergo photocatalytic dehydro-
genation to produce H2, namely the formally abstracted H+ accepts
the photogenerated electrons from the CB of the photocatalyst while
the formal negative organic fragment left captures the photogener-
ated hole from the VB. This is indeed found for BA in our system,
when the reaction was carried out in the absence of oxygen. The
conversion rate for the photocatalytic dehydrogenation of BA is
lower with 11% under standard conditions, however, with >99%
selectivity toward benzaldehyde (entry 7, Table 1). Because the
interfacial transfer of the light-induced electron is considered to
induce photochemical reaction in heterogeneous photocatalysis,13
it is expected that the electron transfer can be promoted by the
introduction of more electrophilic species like O2 (∆G ) -110 kJ
mol-1 per electron transferred) as compared to a proton (∆G )
-85 kJ mol-1) to capture the electron. As some oxygen is
presumably chemically preadsorbed on the mpg-C3N4 via Lewis
acid-base interaction, a potential acceleration by the acid-base
interaction is assumed by us. Indeed, in the presence of oxygen,
Scheme 2. Kinetic Isotope Effect
The kinetic isotope effect of this reaction was studied using
monodeuterated benzyl alcohol (Scheme 2) to gain more insight
into the mechanistic details. The crude aldehyde obtained after the
reaction was converted to the corresponding hydrazone derivative
and analyzed using ESI mass spectroscopy. Results showed that
the abstraction of hydrogen during the reaction is 3.3 times faster
compared to that of deuterium. This clearly proves that the
secondary abstraction of H/D as indicated in Scheme 1 is a rate-
determining step in the oxidation of alcohol.
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16300 J. AM. CHEM. SOC. VOL. 132, NO. 46, 2010

C O M M U N I C A T I O N S
Table 2. Selective Oxidation of Alcohols
As we never observe molecular H2O2, a more reactive oxidation
agent than O2, H2O2, is presumably quickly rebound and undergoing
a second, similar two-electron cycle on the catalyst, which leads
to the formation of (more stable) water. Indeed, by adding, in a
reference experiment, H2O2 instead of O2 to the system, an even
higher turnover of benzylalcohol to benzaldehyde resulted (as
described in the Supporting Information), however, with lower
selectivity. We attribute this lower selectivity to the higher overall
concentration of perhydrol, while only the catalyst-surface bound
species are sufficiently mild to be highly selective.
In conclusion, we have shown that mpg-C3N4 can function as a
photocatalyst to activate O2 for the selective oxidation of benzyl
alcohols under visible light irradiation. By combining the surface
basicity and semiconductor functions of mpg-C3N4, the photocata-
lytic system can realize a high catalytic selectivity to generate
benzaldehyde. The system also selectively converts other alcohol
substrates to their corresponding aldehydes/ketones.
Time
[h]
Con.
[%]
Sel.
[%]
R1
R2
1
2
3
4
5b
6
7
8
9
Phenyl
Phenyl
H
CH3
H
H
H
3
3
3
3
1.8
3
3
5
3
57
77
86
79
100
80
92
35
32
>99
>99
90a
>99
95c
>99
64d
>99
90d
4-Methylphenyl
4-Chlorophenyl
4-Methoxyphenyl
4-Methylbenzoate
PhCHdCH2
Pentyl
H
H
CH3
Cyclopropyl
Phenyl
a Acid (10%) was formed. b 4-Methoxybenzylalcohol 0.65 mmol.
c Acid (5%) was formed. d Benzaldehyde (36%) was detected.
e 1-(1,2-Dicyclopropyl-2-phenylethyl)benzene (10%) was formed.
The whole set of experiments demonstrated a potential pathway
of accessing traditional mild radical chemistry, as it is otherwise
only done via nitroxyl radicals, such as TEMPO. The “mild” nature
of the mpg-C3N4 photocatalyst, together with its unique electronic
and surface properties, had already found some interesting applica-
tions in metal-free coordination chemistry and metal-free hetero-
geneous catalysis18 but now obviously can be extended to a broad
variety of related problems including O2 activation, organic
photosynthesis, or reductive CO2 fixation.
The effect of substitution on the aromatic ring has also been
studied. Both electron-withdrawing and electron-donating substit-
uents have been found to enhance the rate of the reaction (Table
2). These results also strongly support a radical mechanism. We
also examined the chemoselectiviy of the current system using some
additional substrates incorporating both aliphatic and benzylic
alcohols. The system showed high chemoselectivity toward the
benzylic alcohol moiety (Table S1).
A radical clock experiment using the cyclopropyl-phenylcarbinol
however failed. Oxidation of the substrate resulted in the formation
of the corresponding ketone with an intact cyclopropyl ring. This
is a strong indication that either no solution-based radicals are
relevant for the reaction or the reaction runs through an alkoxide
anion R to the benzyl radical which considerably decelerates the
rate of ring-opening.17 Another possibility is that the initial radical
formation is on the oxygen atom which immediately forms the CdO
bond through abstraction of a hydrogen radical.
Acknowledgment. Supported by NSFC (21033003), 973 Pro-
gram (2007CB613306), NCET (07-0192), PCSIR(0818), and the
Cluster of Excellence “Unifying Concepts in Catalysis” coordinated
by TU Berlin and funded by DFG.
Supporting Information Available: Experimental details and more
characterization and reaction results. This material is available free of
charge via the Internet at http://pubs.acs.org.
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Scheme 3. Proposed Potential Oxidation Mechanism
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)
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JA102866P
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J. AM. CHEM. SOC. VOL. 132, NO. 46, 2010 16301