Photogenerated singlet oxygen over zeolite-confined carbon dots for shape selective catalysis

Article abstract of DOI:10.1007/s11426-018-9417-4

Singlet oxygen as an activated oxygen species played an important role in organic synthesis. Suitable catalyst for converting ubiquitous oxygen molecule to singlet oxygen under mild conditions has attracted a wide range of attention. Herein, carbon dots h

Full text of DOI:10.1007/s11426-018-9417-4

SCIENCE CHINA
Chemistry
•ARTICLES•
https://doi.org/10.1007/s11426-018-9417-4
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Photogenerated singlet oxygen over zeolite-confined carbon dots
for shape selective catalysis
Qiu-Ying Yu1, Guang-Yao Zhai1, Tian-Lu Cui1, Hui Su1, Zhong-Hua Xue1, Jun-Jun Zhang1,
Peter J. Pauzauskie3, Shin-ichi Hirano2, Xin-Hao Li1 & Jie-Sheng Chen1*
1School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China;
2Hirano Institute for Materials Innovation, Shanghai Jiao Tong University, Shanghai 200240, China;
3Department of Materials Science & Engineering, Seattle WA 98195-2120, USA
Received December 4, 2018; accepted January 14, 2019; published online February 22, 2019
Singlet oxygen as an activated oxygen species played an important role in organic synthesis. Suitable catalyst for converting
ubiquitous oxygen molecule to singlet oxygen under mild conditions has attracted a wide range of attention. Herein, carbon dots
have been confined into mesopores of silicalite-1 nanocrystals framework and acted as active sites for generation of singlet
oxygen. The high oxygen-adsorption capacity of zeolite nanocrystals facilitated the photocatalytic generation rate of singlet
oxygen, outpacing the free-standing carbon dots for 14-fold. The integrated carbon dot-zeolite nanocrystal hybrid also exhibited
a special size-dependent selectivity for organic synthesis by using the in situ formed and confined singlet oxygen as active
oxygen species.
zeolite, photocatalysis, singlet oxygen, carbon dots, shape-selective
Citation:
Yu QY, Zhai GY, Cui TL, Su H, Xue ZH, Zhang JJ, Pauzauskie PJ, Hirano S, Li XH, Chen JS. Photogenerated singlet oxygen over zeolite-confined
carbon dots for shape selective catalysis. Sci China Chem, 2019, 62, https://doi.org/10.1007/s11426-018-9417-4
1 Introduction
dustrially important and environmentally friendly chemicals
as well as the synthesis of daily useful compounds [11].
Considering its high activity, the free singlet oxygen re-
leased from the surface of heterogeneous photocatalysts,
such as silicon nanostructures, graphenes, fewlayer black
phosphors and conjugated polymers, can react with various
exposed substrates, resulting in an uncontrollable selectivity
for specific oxidation reactions [13–20]. Developing effi-
cient strategies to control the reaction selectivity of photo-
generated singlet oxygen is thus highly alluring for the
artificial photosynthesis and in-situ photodynamic therapy
but has never been touch till now.
Herein, we report an easy but powerful method to anchor
visible-light active carbon dots inside the mesoporous sili-
calite-1 nanocrystals (C@S-1) for promoting the generation
of singlet oxygen in common solvents and, most importantly,
the well control in the size-dependent selectivity of the
Singlet oxygen stays at the forefront of chemical research
because of its unique reactivity as a synthetic reagent or a
reactive oxygen species for a wide range of applications from
selective oxygenation to cancer therapy [1–6]. With regards
to the potential applications of the singlet oxygen as a highly
active oxidant for sustainable organic synthesis and photo-
dynamic therapy, homogeneous initiators, including organic
dyes, the rational design of cheap, effective and reusable
initiators or photosensitizers is of key importance to generate
a controllable amount of singlet oxygen in common solvent
under mild conditions for real applications [7–12]. Visible-
light-responsible photocatalyst used in the heterogeneous
system is also a befitting choice for the producing of in-
*Corresponding author (email: chemcj@sjtu.edu.cn)
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019
chem.scichina.com link.springer.com

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Yu et al. Sci China Chem
confined singlet oxygen inside the pores of the zeolite for
organic synthesis. Moreover, the zeolite host has shown
advantage in adsorbing oxygen molecules to significantly
improve the catalytic performance of carbon dots for singlet
oxygen generation. The hierarchically porous structure of the
zeolite nanocrystals also ensured the mass-transfer efficiency
of the substrates for further selective oxidation reactions
[12,21]. The preparation of zeolite-confined carbon dots has
been reported previously by several groups and also our
group [22–26]. But as far as we know, this work is the first
example of controlling the selectivity of the as-formed
singlet oxygen inside open framework for organic synthesis.
6.389 mW. Conversions of all substrates and yields of certain
products were demonstrated via gas chromatography or gas
chromatography-mass spectrometer analysis. The tempera-
ture-programmed desorption of oxygen (O2-TPD) was tested
using micromeritics AutoChem II with a thermal con-
ductivity detector (TCD).
2.3 Method
Synthesis of mesoporous C@S-1: Colloidal silica (Ludox,
40 wt% in H2O) and tetrapropyl ammonium hydroxide
(TPAOH, 25 wt% in H2O) with a molar ratio of 8:1 (SiO2/
TPAOH) were stirred at 70 °C to evaporate the water. 0.2 mL
of water (dry gel/H2O=10:1) was added to the as-obtained
product (2 g) and then reacted in a Teflon-lined autoclave at
130 °C for 48 h to get the mesoporous silicalite-1. After
crystallization, the zeolite was washed with distilled water
and then heated at 550 °C in N2 for 6 h to form the carbon
doped zeolite.
The etching process to release carbon dots from C@S-1:
one gram of C@S-1 sample was dissolved in a HF solution
(40 wt%) for 1 h. The obtained carbon dots were separated
via centrifugation and further washed by alcohol for three
times.
2 Experimental
2.1 Materials
Silica nanoparticles (Ludox, 40 wt% in H2O) were provided
by Alfa Aesar (USA). Tetrapropyl ammonium hydroxide
(TPAOH, 25 wt% in H2O) was purchased from Sinopharm
Chemical Reagent Co., Ltd. (China). Other chemicals were
purchased from Sigma-Aldrich (USA).
2.2 Instruments
Synthesis of mesoporous Coxi@S-1: The as-obtained
C@S-1 was further heated in air condition under 300 °C for
30 min to partially remove the carbon components.
Synthesis of mesoporous S-1: After crystallization of the
raw mesoporous silicalite-1, we washed the product using
ethanol to remove the TPAOH and carefully heated the
sample to 550 °C in an opened crucible in flow air for 6 h to
remove all organic residues.
Synthesis of carbon dots: 0.2 g of urea and 0.2 g of p-
phenylenediamine were added to 50 mL of water; then re-
acted in a Teflon-lined autoclave at 160 °C for 10 h. The
obtained solution was purified via silica column chromato-
graphy using a mixture of ethanol and ethyl acetate as the
eluent.
Photocatalytic oxidation of benzylamine: Photocatalysis
reactions of benzylamine were performed by dispersing
20 mg of catalyst and 0.1 mmol substrate into 5 mL of
CH3CN. The solution was then initiated by a 300 W Xe lamp
using a 420 UV-cut-off filter in O2 at room temperature. The
final products were identified by gas chromatography-mass
spectrometer (GC-MS) and quantified by GC with a FID
detector.
The oxidation of 2,2,6,6-tetramethylpiperidine (TEMP)
monitored by ESR analysis: The same reaction solution of
benzylamine oxidation were prepared, then 0.1 mmol TEMP
was added to the solution. ESR analysis was conducted un-
der the irradiation of a 420 nm light for 2 or 5 min.
3,3′,5,5′-Tetramethylbenzidine (TMB) oxidation: TMB
(5 mg) was added to 3 mL of HAc/NaAc buffer solution
The powder X-ray diffraction (XRD) patterns were recorded
on a Rigaku D/Max 2550 X-ray diffractometer (Japan) with
Cu Kα radiation (λ=1.5406 Å) with a tube current of 30 mA
and a tube voltage of 40 kV. The scanning electron micro-
scope (SEM) images were taken on a JEOL JSM-6700F field
emission scanning electron microscope (Japan). Transmis-
sion electron microscopy (TEM) and high resolution trans-
mission electron microscope (HRTEM) images were
obtained on a TALOS F200X transmission electron micro-
scope (FEI, USA). The nitrogen adsorption/desorption
measurements were performed on an ASAP2020 Ac-
celerated Surface Area and Porosimetry (Micromeritics Inc.,
USA). The surface areas were calculated by the Brunauer-
Emmett-Teller (BET) method, the pore size distributions of
micropores and mesopores were calculated by the Horvath-
Kawazoe (HK) and Barret-Joyner-Halenda (BJH) methods,
respectively. UV-Vis absorption spectra were measured on a
Hitachi U-4100 instrument (Japan). The GC analysis was
performed on Shimadzu GC-2014 gas chromatograph (Ja-
pan). The PL spectrum was tested in the FLSP920 with the
excitation wavelength of 368 nm. The content of carbon of
C@S-1 was determined by elemental analysis (Vario EL
Cube, Germany). The X-ray photoelectron spectroscopy
(XPS) measurements were conducted on a Kratos Axis Ultra
DLD spectrometer (UK). The electron spin resonance
spectroscopy (ESR) was conducted with Bruker EMX-8/
2.7C (Germany), with the following setting: center field,
3520 G; microwave frequency, 9.875 GHz; power,

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Yu et al. Sci China Chem
(0.2 M:0.2 M) with 1 mL of H2O. Then 20 mg of C@S-1
(carbon content: 0.058 mg) or 0.058 mg CD was added to the
solution. The reaction was conducted in air at room tem-
perature. The reaction process at different time intervals was
monitored via UV-Vis absorption analysis.
O2-TPD analysis: 0.03 g samples were purged with pure
He at 200 °C for 1 h, and then cooled to room temperature.
After being saturated with O2-He, the samples were flushed
with pure He and then heated to 210 °C at a ramp rate of
10 °C.
TEM observation of the released carbon dots via HF etching
(Figure S4) with a lattice spacing of 0.21 nm (Figure 1(c)
inset), which is identical with the (100) facet of graphite. The
elemental analysis (Table S1) results confirmed that the
carbon dots were partially removed for the Coxi@S-1 sam-
ple via a mild calcination process, resulting in slightly in-
creased mesopore volume (Figure 1(f)) according to the N2
adsorption-desorption analysis and also lowered carbon
contents (Table S1). XPS suggested a similar chemical
structure of carbon dots integrated inside the C@S-1 and
Coxi@S-1 samples (Figure S6). Pristine S-1 was prepared in
opened air at 550 °C.
3 Results and discussion
It is surprising that the introduction of carbon dots did not
block the micropores of the mesoporous S-1 nanocrystals in
this work. According to the N2 adsorption/desorption ana-
lysis results (Figure S7), the Barret-Joyner-Halenda (BJH)
pore size distributions (Figure 1(e, f) and Table S2) directly
present a gradual decrease in the pore volume of typical
intracrystal mesopores centered at 2.4 nm for samples from
pristine S-1 via partially etched Coxi@S-1 to C@S-1, whilst
the micropore volume slightly increases as the nanocarbons
also bear more or less micropores. Moreover, the strong in-
teraction between carbon species and zeolite framework as
reveal by ESR spectroscopy (Figure 2(d)) also lead to the
formation of unexpected defects, presumably increasing the
micropore volume of C@S-1. All these results demonstrate
the distribution of the as-formed carbon dots mainly in the
intracrystal mesopores of C@S-1 nanocrystal.
Usually, coked zeolites were considered as inactive cata-
lysts and thus regenerated to remove the unwanted coke. In
this work, we found that the carbon dots confined inside the
S-1 nanocrystals were visible light responsible with potential
application as photocatalysts, remaining as the long-term
interests of our group and others to extend the applications of
carbon-zeolite nanocomposites [22–26]. The C@S-1 sample
exhibited broad light absorption along the whole visible light
range (Figure S8) and an emission peaks around 450 nm by
the excitation at 368 nm (Figure S9). The O2 temperature-
programmed desorption (O2-TPD) results (Figure 2(a)) fur-
ther confirmed that the microporous zeolite framework could
efficiently adsorb oxygen molecules, the adsorption capacity
of which was not obviously changed by the introduction of
carbon dots [28].
As depicted in Figure 1(a), the carbon dots were generated in
N2 gas by direct pyrolysis of the left tetra-N-propylammo-
nium hydroxide (TPAOH) inside the micropores, which was
originally used to direct the growth of mesoporous silicalite-
1 nanocrystals (raw S-1) via a modified Kirkendall growth
process (Figures 1(b) and S1–S3, Supporting Information
online) [21,27]. The formation of the nanocarbons did not
deconstruct the crystalline structure of the zeolite, as de-
monstrated by the HRTEM image (Figures 1(b) and S2) and
the XRD analysis results (Figure S5). The highly integrated
carbon dots (Figure 1(c)) has a mean size of 4 nm and
homogeneously distributes along the whole zeolite nano-
crystal (Figure 1(d)) with a content estimated to be 0.29% by
the elemental analysis results (Table S1, Supporting In-
formation online). The formation of carbon dots inside the
C@S-1 and Coxi@S-1 samples were well confirmed by the
As a result, we initially tested the possibility of C@S-1
sample as an oxygen gas collector and also a heterogeneous
photo-initiator (Figure 2(b)) for the synthesis of singlet
oxygen from oxygen gas in water by using TMB as the probe
(Figure S10) [29]. Both pristine carbon dots (CD) [30] and
C@S-1 could initiate the formation of singlet oxygen under
fixed conditions and visible light irradiation. Surprisingly,
the amount of as-formed singlet oxygen over the C@S-1
photocatalyst, as reflected by the absorption peak intensity of
the oxidized TMB (Figure 2(c)), was about 15 times higher
Figure 1 (a) The synthesis processes for typical C@S-1, partially etched
Coxi@S-1 and bare S-1 samples. HRTEM (b, c) and HAADF C-element
mapping (d) images of typical C@S-1 sample (inset: HRTEM of a single
CD released via etching the zeolite with HF). Pore size distribution (e) and
pore volumes (f) of the C@S-1, Coxi@S-1 and S-1 samples (color online).

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Yu et al. Sci China Chem
the optimized solvent acetonitrile with better selectivity
(>99%) (Table S3), C@S-1 catalyst exhibited the highest
conversion rate (75%) among all metal-free catalysts tested
within 6 h in this work (Figure 3(a), Tables S4, S5 and Figure
S11). The bare S-1 was totally inactive under fixed condi-
tions (Table S4, entry 4), and partially etched sample
Coxi@S-1 could only selectively convert 24% of the ben-
zylamine (Table S4, entry 3). Usually, the activity of zeolite-
embedded nanocatalyst was more or less depressed by the
blocking effect of the zeolite micropores with relatively poor
mass transfer efficiency. The fact that free CD could only
give much lower conversion (50% within 6 h) again suggests
the key role of the zeolite framework in significantly pro-
moting the photocatalytic generation of singlet oxygen and
thus overcoming the blocking effect of micropores.
Another possibility that the as-formed singlet oxygen
molecules diffuse out of the zeolite crystal as free active
species to trigger the oxidation reactions was directly ex-
cluded in our system by the excellent size-dependent se-
lectivity of the C@S-1 catalyst (Figure 3(b)). Under standard
conditions, C@S-1 catalyst could not oxidize 3,4,5-tri-
methoxybenzylamine and mesitylmethanamine, whose mo-
lecule sizes (7.3×7.8 Å2 and 6.2×6.9 Å2) are larger than that
of the micropores (pore size: 5.3×5.6 Å2) of silicalite-1. On
the contrary, free CD catalyst could make the oxidation of
these two substrates process smoothly. This observation
again confirms that: (1) the carbon dots were mostly an-
chored inside the intracrystal mesopores of the C@S-1
samples; (2) most importantly, the as-formed singlet oxygen
molecules were also confined inside the zeolite nanocrystals,
indicating an effective method to directly modify their re-
Figure 2 (a) O2-TPD curves of C@S-1 and S-1. (b) The reaction scheme
for TMB oxidation over the C@S-1 catalyst or free CD. (c) The time course
of singlet oxygen generation monitored by the intensities of UV-Vis ab-
sorption peaks of the oxidized TMB over C@S-1, Coxi@S-1 and CD.
Reaction condition: 5 mg TMB, 3 mL of HAc/NaAc buffer solution
(0.2 M: 0.2 M), 1 mL of H2O, catalysts: 20 mg C@S-1 or 0.058 mg of CD
with the same carbon contents or 20 mg Coxi@S-1, visible light: λ>
420 nm,25 °C, air. (d) ESR spectra of the C@S-1, bare S-1 and the me-
chanical mixture of CD and S-1 (CD+S-1) (color online).
than that over CD. It should be noted that bare S-1 is inactive
for the oxidation of TMB (Figure S10), again speaking for
the main contribution of carbon dots to the formation of
singlet oxygen. On the contrary, there is a consensus in the
literature that zeolite-coated nanocatalysts usually exhibited
depressed catalytic conversion as compared with bare na-
nocatalysts due to the relatively lower mass diffusion effi-
ciency of certain substrates inside the narrow micropores/
channels [31,32]. All these results rather speak for a nano-
confinement effect by the intracrystal mesopores of the
zeolite host on accelerating the transformation of the oxygen
molecules. Indeed, the highly integrated structure of the
carbon dots and zeolite framework in the C@S-1 generate
more defect-induced paramagnetic centers (e.g. localized
radicals) as compared with the sample of a mechanical
mixture of free carbon dots and bare S-1 nanocrystals ac-
cording to the ESR signals at g=2.003 (Figure 2(d)) [23,33].
Enlightened by the accelerated formation of singlet oxygen
inside the microscale environment of C@S-1, we further
extend the application of the as-formed singlet oxygen for
organic synthesis, exemplified with the oxidative coupling
reaction of amines to imines here. The imines are important
ligands or intermediates for organic synthesis and are usually
prepared at elevated temperatures and with moderate to good
selectivity [34]. Using molecule oxygen and visible light in
Figure 3 (a) Benzylamine oxidative coupling reaction over C@S-1 and
CD. (b) Shape selective catalysis over C@S-1 and CD. Reaction condi-
tions: 0.1 mmol substrate, 20 mg of C@S-1 or 0.058 mg of CD (with the
same carbon contents), 5 mL of acetonitrile, 1 atm O2, 6 h, 25 °C, visible
light: λ≥420 nm (color online).

5
Yu et al. Sci China Chem
action selectivity.
2,2,6,6-tetramethylpiperidine-N-oxyl, from the in-situ oxi-
dation of TEMP by singlet oxygen, with a g-value of 2.006
and a hyperfine splitting constant (aN=1.6 mT) [18,37].
Possible pathway for the formation of imines via the for-
mation of aldimine [38,39] from the oxidation of benzyla-
mine by singlet oxygen was presented in Figure 4(e).
Our study also referred to the coupling of a variety of
amines to confirm the functional group tolerance of C@S-1
(Table S4). Benzylamines with electron-donating (CH3 or
OCH3) and electron-withdrawing (Cl or CF3) substituent
group were all converted to corresponding imines over
C@S-1 catalyst under standard conditions (Table S4, entries
9–13). The C@S-1 catalyst also showed excellent conver-
sion of heterocyclic ring amines with sulfur heteroatoms
(Table S4, entry 14), which are usually toxic to noble-metal-
containing nanocatalysts.
The zeolite nanocrystals also act as separable and reusable
hosts for recycling the used C@S-1 catalyst. The used C@S-
1 sample has similar XPS C1s spectra (Figure 4(d)) to that of
fresh sample, suggesting the high stability of the confined
carbon dots during the photocatalytic reactions. As expected,
the reused C@S-1 catalyst exhibited repeatable activity un-
der fixed conditions (Table S4, entries 15 and 16).
In order to glean the real role of the photo-initiated and
confined singlet oxygen to trigger the oxidation reaction, we
try to capture the singlet oxygen-related reaction inter-
mediate during the reaction progress. Moreover, the wave-
length dependence (Figure 4(b) and Table S7) of the catalytic
conversion provided by the C@S-1 catalyst further demon-
strated the reactive singlet oxygen species were indeed in-
itiated by the photo-responsive carbon dots integrated inside
the crystal (Scheme S1, Supporting Information online). The
phenomenon that the introduction of butylated hydro-
xytoluene (BHT), mannitol and catalase did not obviously
disturb the yields of imine under fixed conditions (Figure 4
(a) and Table S6) directly excluded the contribution of pos-
sible oxygen-containing radicals to the catalytic conversion
of amine [35,36]. It is interesting that only carotene quench
the oxidation of amine, suggesting the formation of singlet
oxygen for further catalytic reactions. Indeed, the formation
of the photo-initiated singlet oxygen over the C@S-1 catalyst
during the catalytic reaction was directly confirmed by the
typical 1:1:1 triplet ESR signal (Figure 4(c)) of as-formed
4 Conclusions
In summary, we prepared a carbon dots/zeolite-based nano-
composite by selectively confining the carbon nanocatalysts
inside the intracrystal mesopores of zeolite nanocrystals.
Such a highly integrated carbon/zeolite can function as a
highly effective photo-initiator of singlet oxygen using
oxygen gas and visible light. The confinement effect of the
zeolite framework on the as-formed singlet oxygen species
affords them unexpected shape selectivity for organic
synthesis. Meanwhile, zeolite host improved the singlet
oxygen generation by enhancing the pre-adsorption of the
oxygen molecules inside the open framework of the zeolite
nanocrystals. Our research opens the new vista of zeolite-
based materials as effective modifiers to control the se-
lectivity of reactive oxygen species for organic synthesis in
green chemistry and also for possible applications in artifi-
cial photosynthesis, light energy harvesting and even photo-
therapy.
Figure 4 (a) The photocatalysis conversions of benzylamine over the
C@S-1 catalyst in the presence of different sacrificial agents. (b) The
wave-length-dependent conversions of benzylamine over the C@S-1 cat-
alyst (Red line: the UV-Vis spectra of C@S-1 after Kubelka-Munk tran-
sition). (c) ESR spectra of the TEMP-based intermediates after the addition
of 0.1 mmol TEMP to the C@S-1-based standard reaction within different
time periods. (d) The XPS C1s spectra of the fresh and used C@S-1 cat-
alyst. (e) The proposed reaction mechanism for the oxidative coupling of
amines by photogenerated singlet oxygen over C@S-1 (S and T represent
the singlet and triplet states of the excited electrons photogenerated under
visible light, ISC: intersystem crossing) [15]. Reaction conditions:
0.1 mmol benzylamine, 20 mg of C@S-1, 5 mL of acetonitrile, 1 atm O2,
6 h, 25 °C, visible light: λ≥420 nm for (a, c) or varied light irradiation for
(b) (color online).
Acknowledgements This work was supported by the National Natural
Science Foundation of China (21720102002, 21722103, 21673140),
Shanghai Basic Research Program (16JC1401600), SJTU-MPI partner
group, Shanghai Eastern Scholar Program and Shanghai Rising-Star Pro-
gram (16QA1402100). We thank Prof. Huan-Ming Xiong and his co-
workers (Department of Chemistry, Fudan University, Shanghai 200433,
China) for providing CD for this study.
Conflict of interest The authors declare that they have no conflict of
interest.

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Yu et al. Sci China Chem
Supporting information The supporting information is available online at
http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors.
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