Visible-light-driven photooxidation of alcohols using surface-doped graphitic carbon nitride

Article abstract of DOI:10.1039/c7gc00539c

Carbon-nanodot-doped g-C₃N₄ is used as a photocatalyst to promote the aerobic oxidation of alcohols and oxyfunctionalisation of activated hydrocarbons. A critical E-factor analysis of the current reaction system reveals its limitations en route to environmentally acceptable oxidation procedures.

Full text of DOI:10.1039/c7gc00539c

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Visible-Light-Driven Photooxidation of alcohols using surface-
doped graphitic carbon nitride
Wuyuan Zhang, *[a] Anna Bariotaki,[b] Ioulia Smonou,[b] and Frank Hollmann*[a]
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Carbon-nanodot-doped g-C3N4 is used as photocatalyst to pollutants and catalytic CO2 reduction.1 Applications for
promote the aerobic oxidation of alcohols and preparative organic synthesis are comparably few. For
oxyfunctionalisation of activated hydrocarbons. A critical E-factor example, Goettmann et al. reported g-C3N4 catalysed Friedel-
analysis of the current reaction system reveals its limitations en Crafts acylation.12 More recently, photocatalytic acetalisation
route to enviromentally acceptable oxidation procedures.
of aldehydes and ketones,13 hydrazine-driven reductions of
alkenes and alkynes was reported using g-C3N4.14, 15 Selective
oxidations especially of benzylic C-H-bonds have been
reported using mesoporous g-C3N4 together with N-OH-
cocatalysts,16-19 or using transition metal doped g-C3N4.14, 20-22
Also the oxidative coupling of amines has been reported.23
However, to the best of our knowledge, carbon-nanodot
doped g-C3N4 has so far not been evaluated as catalyst for
photocatalytic oxidation reactions. Therefore, we set out to
evaluate carbon-nanodot-doped g-C3N4 (CD-C3N4) as visible-
light-driven photocatalyst for the aerobic oxidation of alcohols
(Scheme 1).
In recent years, graphitic carbon nitride (g-C3N4) has received
substantial interest as photocatalyst for metal-free, visible-
light promoted reactions.1 It exhibits a graphite-like, layered
structure wherein tris-triazine units are connected through C-
N-bonds forming a two-dimensional layer. g-C3N4 can be
synthesized via various methods such as pyrolysis of urea or
other nitrogen-rich precursors or layer exfoliation of bulk
materials.2
Pure g-C3N4, however, is a rather poor photocatalyst, mainly
due to the fast recombination of photoexcited, charge-
separated states. Therefore, one focus of research lies on the
improvement of its photocatalytic properties by modulating
the potentials of g-C3N4’s conducting- and valence band.1
Particularly doping of g-C3N4 with other elements such as Y,3
Fe,4 Pt,5 Au/Pd6, 7 K, Ag,8, 9 C10 or carbon-nanodots11 and many
more has proven to be an efficient handle to modulate its
properties. Also, doping with carbon-nanodots appears
promising to increase the quantum efficiency of photocatalytic
processes.
Interestingly, g-C3N4 is mostly considered as photocatalyst for
(sun)-light driven water splitting, remediation of organic
Scheme 1. Photocatalytic aerobic oxidation using carbon nanodot-doped g-
C3N4 (CD-C3N4) as photocatalyst.
For the synthesis of g-C3N4 we followed the procedure by Tang
and coworkers24 due to the more porous structure of the
material and the resulting higher activity (due to increased
surface area). In short, calcination of urea at 600 oC for 4h gave
the desired mesoporous g-C3N4 as confirmed by TEM imaging,
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and X-ray diffraction (Figures S1 and S2). Next, carbon experiments. This observation is in line with previous findings
nanodots were synthesized via thermal decomposition of that CD-C3N4 is also an efficient H2O2 decomposition catalyst.11
sucrose.25 The latter were deposited on the g-C3N4 surface via The rate of the oxidation reaction exhibited a saturation-type
thermal treatment of both materials.11 The XRD pattern of the dependency on both the catalyst concentration (Figure 2) and
such-obtained composite material did not change significantly the intensity of the light source applied (Figure 3).
as compared to the starting material (g-C3N4) most probably
due to the amorphous character of the carbon nanodots
800
adsorbed. The UV/Vis spectrum showed the characteristic
increase in absorption at wavelengths below 600 nm (Figure
S3), and the BET measurement revealed a surface area of 105
m2/g (Figure S8).
600
Having both catalysts at hand, we next compared their
catalytic activity in the oxidation of benzyl alcohol to
benzaldehyde as model reaction (Figure 1). Due to the
400
volatility of benzaldehyde and poor water solubility of the
benzyl alcohol starting material we used a two-liquid phase
approach employing benzyl alcohol as the second organic
phase (phase ratio 3:7 organic : aqueous).
200
0
500
400
300
200
100
0
0
24
48
time [h]
Figure 2. Influence of the catalyst loading on the rate of the photocatalytic oxidation of
benzyl alcohol. [CD-C3N4] = 5(ꢁ), 10 (ꢀ), 25 (ꢀ) g L-1. Conditions: two phase reaction
with 700 µL of water + 300 µL of benzyl alcohol, 30 oC and oxygen atmosphere under
visible light illumination using Setup 1 (λ > 400 nm).
800
600
400
200
0
0
12
24
36
48
time [h]
Figure 1. Photocatalytic oxidation of benzyl alcohol to benzaldehyde using g-C3N4 (ꢀ)
and CD-C3N4 (ꢀ) as photocatalyst. Conditions: 5 g L-1 of photocatalyst, two phase
reaction: 700 µL of water + 300 µL of benzyl alcohol, 30 oC and oxygen atmosphere
under visible light illumination using Setup 1 (λ > 400 nm).
As shown in Figure 1, CD-C3N4 excelled over g-C3N4 both in
terms of activity and robustness. Not only was the initial
product formation rate roughly two times higher but also the
long term-stability of the reaction: the reaction rate with g-
C3N4 levelled off significantly after several hours whereas with
CD-C3N4 linear product accumulation was observed for at least
48h. Overall, with CD-C3N4 more than 500 mM of product
accumulated corresponding to a product to catalyst ratio of
more than 4:1 (g g-1), under non-optimized conditions.
0
6
12
18
24
time [h]
Figure 3. Influence of the light intensity on the rate of the photocatalytic oxidation of
benzyl alcohol. Light intensity of 79 (ꢀ), 197 (ꢀ), 341 (ꢀ) W cm−2. Conditions: 5 g L-1 of
photocatalyst, two phase reaction with 700 µL of water + 300 µL of benzyl alcohol, 30
oC and oxygen atmosphere under visible light illumination using Setup 2 (λ > 400 nm).
It is worth mentioning here, that in the absence of either the
photocatalyst or a light source, no noticeable conversion of the
starting material was observed. Also, hydrogen peroxide as by-
product was observable in trace amounts only throughout the
In case of increasing catalyst concentrations, we suspect the
decreasing transparency of the reaction mixture to account for
this observation. The converging reaction rate at increasing
light intensities may well be attributed to oxygen diffusion
becoming overall rate-limiting. It should be mentioned here
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that for the latter experiments we utilised a specialized light- Especially allylic alcohols were converted at excellent rates and
setup to control the light intensity (Setup 2, Figure S5). Despite selectivities while benzylic alcohols were converted somewhat
the much higher product formation rate attainable with this slower and non-activated alcohols such as cyclohexanol were
system (Figure 3) we decided to perform the following rather sluggish substrates. This is roughly in-line with the
experiment using the cheap white-light bulb in order to enable general bond-dissociation energies of the C-H bonds oxidised.
simple reproduction by others (Setup 1, Figure S4). However, it also should be taken into account that the
Nevertheless, the productivities shown in Figure 3 (using the reactions reported in Table 1 have been obtained from two-
simple light source) of more than 0.2 gproduct g-1
h-1 liquid phase systems and that, depending on the partitioning
catalyst
demonstrate the preparative potential of the photochemical coefficient of the starting material, the aqueous
alcohol oxidation system. concentrations may vary very significantly thereby influencing
We investigated the recyclability of CD-C3N4 by performing the reaction kinetics.
benzyl alcohol oxidation reactions followed by filtration, The preparative applicability of the proposed photocatalytic
washing and re-loading with reaction medium (Figure S6). As a oxidation was exemplarily demonstrated at the oxidation of
result CD-C3N4 could be recycled at least 5 times. From linear carveol to carvone. Performing this reaction at 6.8 mmol-scale
regression of the initial rates, a catalyst deactivation of less (1.03 g) gave more than 95% conversion into the desired
than 4% per cycle was estimated. product (GC yield) and 0.773 g of isolated carvone (74.8 %
Encouraged by these results we further explored the product isolated yield) under non-optimised reaction- and DSP
scope of the reaction system (Table 1).
conditions.
An E-factor analysis26 of this reaction revealed the current
limitations of this reaction setup from an environmental point-
of-view (Table 2). The ‘classical’ E-factor (including the
weighable compounds only) of the overall reaction is rather
moderate (144) with solvents (used both for the reaction and
for the extraction of the product) contributing over 95% to the
total E-factor. Obviously, dichloromethane used in this
reaction, is not acceptable and will be substituted by more
acceptable solvents in future studies.27 Also decreasing the
contribution of water (e.g. by further increasing the
concentration of the starting material) will be highly desirable.
In fact, preliminary experiments using neat reagents (i.e. CD-
C3N4 suspended in pure benzyl alcohol or cyclohexanol)
showed an even faster product accumulation than in biphasic
system (Figure S7). Probably this is also to be attributed to a
higher O2 solubility in these media than in aqueous systems.
Another advantage of using neat reagents is that extraction
can be omitted as physical methods to separate the product
(e.g. distillation) are sufficient.
Table 1. Examples for CD-C3N4-catalyzed, photocatalytic alcohol oxidations.
Rate [g g-1 h-1]
Product
Product [mM][a]
223
0.059
60.1
73.0
40.2
0.020
0.023
0.012
12.7/1.9a
228.1/41a
0.003/0.001a
0.055/0.023a
However, the ‘hidden’ E-factor contributors demand more
attention en route to an environmentally acceptable reaction
system. Using setup 2 enabled us to quantify the power input
(197 W for 90 h) and energy used for the illumination reaction
(17.7 kWh). According to the European Energy Agency this
701.2
108.4
0.193
28
corresponds to CO2 emission of approximately 9.9 kg CO2
and an E-factor contribution of 12.800 obviously ‘outshining’
the values discussed above. Of course the current setup has
not been optimised for efficient utilisation of light and further
geometric optimisation together with the increase of the
reagent payload will certainly reduce this number to
acceptable values. Also, provided the aspirational trend
towards renewable energies continues, less CO2 emissions and
0.040
0.093
247.3
252.4
thereby
a
reduced ‘CO2’-E-factor may be assumed.
0.087
Furthermore, using sunlight will almost entirely eliminate this
contribution.
Reaction conditions: 5 g L-1 of photocatalyst, two phase reaction with 700 µL of
water + 300 µL of alcohol, 30 oC and oxygen atmosphere under visible light
illumination using Setup 1 (λ > 400 nm) for 24h. a) product concentration in
aqueous phase.
Also, it should not be forgotten that the preparation of the
photocatalyst (though exhibiting very low classical E-factors) is
based on high-temperature calcination processes.
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18.1
30.3+37.7
28.14
87.4
/
0.0067
Table 2. Estimation of the wastes generated in the photobiocatalytic oxidation of
carveol to carvone.
E-factor contribution
Contributor
0.008+0.010
0.011
[kg kg-1]
Reaction
Water
CD-C3N4
38.8
0.26
12.800
CO2 from light source
DSP
21.25
0.008
CH2Cl2
MgSO4
102.8
1.9
10.3/9.1b
3.0
0.002/0.005 b
0.0007
Overall, despite the potential of photochemical, aerobic
oxidation we prefer to refrain from calling the current
procedure green or environmental benign.
Reaction conditions: 5 g L-1 of photocatalyst, two phase reaction with 700 µL of water +
300 µL of alkane, 30 oC and oxygen atmosphere under visible light illumination using
Finally, we evaluated oxidation/oxyfunctionalisation of non-
functionalized C-H bonds (Table 3). In general, the same trend
in reaction rate was observed here as well whereas the
reaction rates were significantly lower than observed for
corresponding alcohols. This is in line with the higher C-H-bond
dissociation energy of these non-functionalized C-H bonds.
Furthermore, accumulation of the intermediate alcohol
product did not occur (generally the alcohol product
accounted for less than 25% of the final product) indicating
that the initial C-H-bond oxidation is overall rate-limiting.
Setup
2 (λ > 400 nm) for 24h. a: Selectivity = [aldehyde/ketone] / ([alcohol] +
[aldehyde/ketone]) %; b: product concentration in aqueous phase.
Conclusions
With the current contribution we demonstrate that simple
metal-free CD-C3N4 is very suitable and recyclable
a
photocatalyst for the oxidation of primary and secondary
alcohols to the corresponding aldehydes and ketones.
Furthermore, also extension of this concept to the
corresponding alkanes appears feasible, albeit at reduced
efficiencies.
Table 3. Examples for CD-C3N4-catalyzed, photocatalytic oxyfunctionalisations.
Ongoing mechanistic studies will reveal a more detailed
understanding of the reaction and put the basis for optimised
catalysts and reaction setups en route to truly practical
catalysts.
Product
[mM]
Selectivitya
%
Rate
Product
[g g-1 h-1]
The critical E-factor analysis of the current reaction setup will
guide our further studies en route to truly environmentally
acceptable oxidation processes.
16.3
77
0.004
19.0
83.3
0.0065
Notes and references
1. W.-J. Ong, L.-L. Tan, Y. H. Ng, S.-T. Yong and S.-P. Chai, Chem.
Rev., 2016, 116, 7159-7329.
2. J. Liu, H. Wang and M. Antonietti, Chem. Soc. Rev., 2016, 45,
2308-2326.
3. Y. Wang, Y. Li, X. Bai, Q. Cai, C. Liu, Y. Zuo, S. Kang and L. Cui,
Catal. Commun., 2016, 84, 179-182.
4. Q. Wang, A. Chen, X. Wang, J. Zhang, J. Yang and X. a. Li, J.
Mol. Catal. A: Chem., 2016, 420, 159-166.
5. X. Li, W. Bi, L. Zhang, S. Tao, W. Chu, Q. Zhang, Y. Luo, C. Wu
and Y. Xie, Adv. Mat., 2016, 28, 2427-2431.
6. C. Han, L. Wu, L. Ge, Y. Li and Z. Zhao, Carbon, 2015, 92, 31-
40.
3.0
12.6
3.9
86.2
0.001
0.0044
0.002
/
/
7. T. Xiong, W. Cen, Y. Zhang and F. Dong, ACS Catal., 2016, 6,
2462-2472.
8. Y. Fu, T. Huang, L. Zhang, J. Zhu and X. Wang, Nanoscale,
2015, 7, 13723-13733.
9. R. Kuriki, H. Matsunaga, T. Nakashima, K. Wada, A.
Yamakata, O. Ishitani and K. Maeda, J. Am. Chem. Soc., 2016,
138, 5159-5170.
4.1
7.6
/
0.0018
76.9
10.0023
4 | J. Name., 2012, 00, 1-3
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10. Y. Wang, X. Bai, H. Qin, F. Wang, Y. Li, X. Li, S. Kang, Y. Zuo
and L. Cui, ACS Appl. Mater. Interfaces, 2016, 8, 17212-
17219.
11. J. Liu, Y. Liu, N. Liu, Y. Han, X. Zhang, H. Huang, Y. Lifshitz, S.-
T. Lee, J. Zhong and Z. Kang, Science, 2015, 347, 970-974.
12. F. Goettmann, A. Fischer, M. Antonietti and A. Thomas,
Angew. Chem. Int. Ed., 2006, 45, 4467-4471.
13. M. Abdullah Khan, I. F. Teixeira, M. M. J. Li, Y. Koito and S. C.
E. Tsang, Chem. Commun., 2016, 52, 2772-2775.
14. S. Verma, R. B. N. Baig, M. N. Nadagouda and R. S. Varma,
ACS Sus. Chem. Eng., 2016, 4, 1094-1098.
15. R. B. N. Baig, S. Verma, R. S. Varma and M. N. Nadagouda,
ACS Sus. Chem. Eng., 2016, 4, 1661-1664.
16. P. Zhang, J. Deng, J. Mao, H. Li and Y. Wang, Chinese J. Catal.,
2015, 36, 1580-1586.
17. F. Z. Su, S. C. Mathew, G. Lipner, X. Z. Fu, M. Antonietti, S.
Blechert and X. C. Wang, J. Am. Chem. Soc., 2010, 132,
16299-16301.
18. B. Long, Z. Ding and X. Wang, ChemSusChem, 2013, 6, 2074-
2078.
19. P. Zhang, Y. Wang, J. Yao, C. Wang, C. Yan, M. Antonietti and
H. Li, Adv. Synth. Catal., 2011, 353, 1447–1451.
20. H. Han, G. Ding, T. Wu, D. Yang, T. Jiang and B. Han,
Molecules, 2015, 20, 12686.
21. Z. Ding, X. Chen, M. Antonietti and X. Wang, ChemSusChem,
2011, 4, 274-281.
22. S. Verma, R. B. Nasir Baig, M. N. Nadagouda and R. S. Varma,
ACS Sus. Chem. Eng., 2016, 4, 2333-2336.
23. F. Su, S. C. Mathew, L. Möhlmann, M. Antonietti, X. Wang
and S. Blechert, Angew. Chem. Int. Ed., 2011, 50, 657-660.
24. D. J. Martin, K. Qiu, S. A. Shevlin, A. D. Handoko, X. Chen, Z.
Guo and J. Tang, Angew. Chem. Int. Ed., 2014, 53, 9240-9245.
25. J. Pan, Y. Sheng, J. Zhang, J. Wei, P. Huang, X. Zhang and B.
Feng, J. Mater. Chem. A, 2014, 2, 18082-18086.
26. R. A. Sheldon, Green Chem., 2017, 19, 18-43.
27. P. G. Jessop, Green Chem., 2011, 13, 1391-1398.
28 http://www.eea.europa.eu/ (accessed on 19.02.2017).
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