Highly crystalline poly(heptazine imides) by mechanochemical synthesis for photooxidation of various organic substrates using an intriguing electron acceptor – Elemental sulfur

Article abstract of DOI:10.1016/j.jcat.2017.02.029

Low-defect potassium poly(heptazine imide) (PHIK-BM) was engineered for application in photocatalytic oxidation of organic substrates. Mechanochemical pretreatment of a mixture of 5-aminotetrazole in LiCl/KCl eutectics using high-energy ball milling afforded a highly homogeneous mixture that, upon sequential thermolysis at 600?°C, gave nanosized particles of PHIK–BM. The photocatalytic activity of the free-standing PHIK–BM plates was assessed in the oxidation of benzyl alcohol to benzaldehyde under visible light irradiation using elemental sulfur as an electron acceptor. Both quantitative conversion (>99%) of benzyl alcohol and selectivity (>98%) with respect to benzaldehyde were achieved. The developed method was extended to aliphatic alcohol oxidation coupled with multicomponent Hantzsch 1,4-dihydropyridine synthesis. These 1,4-dihydropyridines were also photocatalytically oxidized by PHIK–BM to the corresponding substituted pyridines, with very good yields and under mild metal-free conditions.

Full text of DOI:10.1016/j.jcat.2017.02.029

Journal of Catalysis 350 (2017) 203–211
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Highly crystalline poly(heptazine imides) by mechanochemical
synthesis for photooxidation of various organic substrates using an
intriguing electron acceptor – Elemental sulfur
⇑
Aleksandr Savateev , Dariya Dontsova, Bogdan Kurpil, Markus Antonietti
Max-Planck Institute of Colloids and Interfaces, Department of Colloid Chemistry, Research Campus Golm, 14424 Potsdam, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Low-defect potassium poly(heptazine imide) (PHIK-BM) was engineered for application in photocatalytic
oxidation of organic substrates. Mechanochemical pretreatment of a mixture of 5-aminotetrazole in
LiCl/KCl eutectics using high-energy ball milling afforded a highly homogeneous mixture that, upon
sequential thermolysis at 600 °C, gave nanosized particles of PHIK–BM. The photocatalytic activity of
the free-standing PHIK–BM plates was assessed in the oxidation of benzyl alcohol to benzaldehyde under
visible light irradiation using elemental sulfur as an electron acceptor. Both quantitative conversion
(>99%) of benzyl alcohol and selectivity (>98%) with respect to benzaldehyde were achieved. The devel-
oped method was extended to aliphatic alcohol oxidation coupled with multicomponent Hantzsch
1,4-dihydropyridine synthesis. These 1,4-dihydropyridines were also photocatalytically oxidized by
PHIK–BM to the corresponding substituted pyridines, with very good yields and under mild metal-free
conditions.
Received 10 January 2017
Revised 21 February 2017
Accepted 23 February 2017
Keywords:
Carbon nitride
Poly(heptazine imide)
Mechanochemistry
Photocatalysis
Hantzsch 1,4-dihydropyridine synthesis
Photocatalytic oxidation
Elemental sulfur
Ó 2017 Elsevier Inc. All rights reserved.
1. Introduction
To overcome this problem, sulfur instead of oxygen could be
used as the electron scavenger, while oxidation of the alcohol
Photocatalysis is an attractive tool that enables chemical trans-
formation using light as a driving force [1]. Graphitic carbon nitride
(g-CN)-based polymeric semiconductors [2] are extensively stud-
ied as photocatalysts because they are able to harvest visible sun-
light as an abundant energy source, the precursors for carbon
nitride preparation are cheap and available, and carbon nitrides
have high chemical and thermal stability [3]. g-CN has been used
in the photocatalytic hydrogen evolution reaction [4], CO2 reduc-
tion [5], oxidation of sulfides [6], and many other reactions [7],
and also as a support for different catalysts [8]. Photocatalytic pro-
duction of carbonyl compounds from alcohols using O2 as an elec-
tron scavenger catalyzed by g-CN is reported in the literature as
well [9,10]. According to the accepted mechanism, dioxygen
receives two electrons and two protons, affording hydrogen perox-
ide [11]. The latter could be quite harmful for the reaction medium,
as OAO bond dissociation upon irradiation leads to highly reactive
hydroxyl radials [12]. These can initiate a series of undesired
chemical transformations that lower the selectivity of the photo-
catalytic reaction.
would then be performed very selectively by the photogenerated
holes. On the practical level this has not been explored, although
sulfur is an easy-to-handle solid soluble in many organic solvents
[13]. Similarly to dioxygen, sulfur can be reduced via a two-
electron process in the presence of proton donors to hydrogen sul-
fide [14]. Unlike H2O2, hydrogen sulfide is a gas and can easily be
removed from the reaction mixture at a slightly elevated tempera-
ture. Therefore, side reactions in which H2S could, in principle, par-
ticipate are minimized. Also, hydrogen sulfide is a valuable reagent
in organic synthesis—for example, in thiophenol production [15].
Also, it can be photoelectrochemically converted in a second step
to sulfur and hydrogen to close a potential catalytic cycle [16].
Recently, we reported a new carbon-nitride-based material—
potassium poly(heptazine imide) (PHIK) (Fig. 1) [17]. It has the
highest VB potential (+2.54 eV) within the carbon nitride family
and is therefore able to oxidize water under visible light even with-
out a co-catalyst [18]. This makes PHIK a promising candidate to
explore in selective oxidation of organic substrates under visible
light irradiation.
Nanocrystalline PHIK is prepared by grinding a mixture of LiCl
and KCl with 5-aminotetrazole in a mortar. The product is obtained
after thermolysis at 550 °C. Notably, in the UV–vis absorption
spectrum a band extending up to 700 nm that corresponds to the
⇑
Corresponding author.
E-mail address: oleksandr.savatieiev@mpikg.mpg.de (A. Savateev).
http://dx.doi.org/10.1016/j.jcat.2017.02.029
0021-9517/Ó 2017 Elsevier Inc. All rights reserved.

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A. Savateev et al. / Journal of Catalysis 350 (2017) 203–211
Fig. 1. PHIK structure. A holey framework of PHIK is seen in the top view (left) and a stacked structure as in g-CN in the side view (right).
photon energy ꢀ1.74 eV is observed [19]. This band is potentially
defect-related n–
important role in water splitting.
MP porosimeter at 77.4 K. The specific surface areas were calcu-
lated by applying the Brunauer–Emmett–Teller (BET) model to
adsorption isotherms for 0.05 < p/p0 < 0.3 using the QuadraWin
5.11 software package. Elemental analysis (EA) was accomplished
as combustion analysis using a Vario Micro device. Scanning elec-
tron microscopy (SEM) images were obtained on a LEO 1550-
Gemini microscope. Transmission electron microscopy (TEM) was
performed on a CM200FEG (Philips) microscope operated at
200 kV. Samples were prepared by depositing a drop of a suspen-
sion of particles in ethanol on an amorphous carbon film. Optical
absorbance spectra of powders were measured on a Shimadzu
UV 2600 equipped with an integrating sphere. The emission spec-
tra were recorded on an LS-50B Perkin Elmer instrument. The exci-
tation wavelength was 350 nm. EDS investigations were conducted
on a Link ISIS-300 system (Oxford Microanalysis Group) equipped
with a Si(Li) detector at an energy resolution of 133 eV. Time-
resolved fluorescence measurements were performed using a
single-photon counting setup (TCSPC) with a Becker&Hickl PML
spectrometer (modified Oriel MS-125) with a laser repetition rate
of 2 MHz. The detector was a Becker&Hickl PML-16-C-1 (modified
Hamamatsu) multialkaline photomultiplier. The excitation wave-
length was 405 nm. Excitation was carried out using a pulsed laser
diode at ꢀ30 nJ/cm2 (LDH-P-C405, PicoQuant GmbH). The emission
was recorded in the range 460–600 nm, while the secondary detec-
tion of the excitation pulses was blocked with a 450 nm cutoff fil-
ter. Raw decay data presented as logarithms of photon counts
versus time were analyzed with PicoQuant GmbH data analysis
software (Germany). The decay times were extracted by means
of a reconvolution fit based on a double and triple exponential
model. Considering that
p⁄ transition, which, however, could play an
The specific surface area of a heterogeneous catalyst is another
important parameter that influences its performance. In this view,
PHIK with high surface area still remains a challenge—a material
with a surface area of 50 m2ꢁgꢂ1 has been reported so far [18].
We do not consider hard templating as a method to increase the
PHIK surface area, mainly because it requires a F- source to remove
the template after thermolysis [20]. In summary, an environmen-
tally friendly method of preparing structurally ordered PHIK with
a small number of defects and improved surface area is desired.
Mechanochemistry is a powerful tool that finds application in
different fields of material science [21]. In view of carbon nitride
chemistry, it has been already successfully used in g-CN synthesis
[22]. Sample contamination with metal particles is typically
unavoidable with high-energy ball milling [23]. However, it can
be turned to advantage, as it was shown previously to grow thick
multiwall and bamboolike carbon nanotubes [24,25]. To the extent
of our knowledge, metal-particles-induced nucleation processes
have not been studied for carbon nitrides. Thus, it would be
intriguing to combine mechanochemistry to create nucleation
seeds in situ with sequential ionothermal synthesis to obtain PHIK.
In this paper, we report on the preparation of PHIK with a small
number of defects and increased surface area due to nanosized
platelike morphology using a mechanochemical approach.
2. Materials and methods
Lithium chloride (ꢃ99%) was purchased from Carl Roth; 5-
aminotetrazole was purchased from Santa Cruz Biotechnology;
potassium chloride (ꢃ99.5) was purchased from Sigma Aldrich.
All were used without further purification.
i¼n
X
si
IPLðtÞ ¼
aieꢂt=
;
i¼1
where
s
i is the lifetime and ai is the amplitude of the ith component,
was calculated as
2.1. Characterization
the intensity-averaged fluorescence lifetime
s
i¼n
i¼n
1H NMR spectra were recorded on a Varian 400 MHz NMR spec-
trometer using the chloroform residual peak (d 7.26 ppm) as an
internal standard. An Agilent 6890 network GC system coupled
with an Agilent 5975 inert mass selective detector was used for
reaction mixture composition analysis. Powder X-ray diffraction
patterns were measured on a Bruker D8 Advance diffractometer
X
X
hs
i ¼
ais2i
=
aisi
:
i¼1
i¼1
A Retsch MM400 ball milling cup, Retsch 1.4112 (50 mL), and
steel ball (diameter 25 mm, weight 63 g) were used to grind
precursors.
equipped with a scintillation counter detector with CuKa radiation
(k = 0.15418 nm) applying a 2h step size of 0.05° and a counting
time of 3 s per step. FT-IR spectra were recorded on a Thermo Sci-
entific Nicolet iD5 FT-IR spectrometer equipped with an attenuated
total reflection unit at a resolution of 2 cmꢂ1. Nitrogen adsorption/
desorption measurements were performed after degassing the
samples at 150 °C for 20 h using a Quantachrome Quadrasorb SI-
2.2. LiCl/KCl eutectic synthesis
A steel ball mill cup was charged with LiCl (2.25 g) and KCl
(2.75 g). A steel ball was placed in the cup and the mixture was
homogenized using ball milling (20 sꢂ1, 2 min). The powder was
transferred into a porcelain crucible, covered with a lid, and placed

A. Savateev et al. / Journal of Catalysis 350 (2017) 203–211
205
in an oven. The temperature inside the oven was increased from 20
to 580 °C within 4 h under a flow of nitrogen, held at 580 °C for 4 h,
and allowed to cool to room temperature. The solid was removed
from the crucible and crushed into fine powder using ball milling.
The prepared eutectic is a hygroscopic solid that should be used
right after preparation. Yield: 4.97 g, 99%.
trifuge (13,500 minꢂ1, 1 min) and dried under vacuum, giving
27 mg of brown material.
2.6. Benzyl alcohol oxidation
A mixture of benzyl alcohol (0.5 mmol), PHIK-BM (20 mg), ele-
mental sulfur (48 mg), and ZnO (81 mg) in MeCN (6 mL) was stir-
red at +50 °C under N2 and irradiated with blue light (40 W,
465 nm) for 24 h. The catalyst was separated by centrifugation
(12,000 minꢂ1, 1 min) and washed with MeCN (2 ꢄ 1 mL). Wash-
ings were combined in a volumetric flask (10 mL), and ethyl ben-
zene (1 mL, 11.6 mgꢁmLꢂ1) was added as an internal standard. An
extra amount of MeCN was added into the volumetric flask until
the total volume of the solution reached 10 mL. The resultant solu-
tion was thoroughly mixed and analyzed by GC–MS.
2.3. PHIK-BM synthesis
A mixture of 5-aminotetrazole (0.99 g) and LiCl/KCl eutectics
(4.97 g) was placed in a steel ball mill cup. The steel ball was
inserted and the cup was closed. The mixture of precursors was
ground for 5 min at the operational frequency 25 Hz. The resultant
flourlike white powder was transferred into a porcelain crucible,
covered with a porcelain lid, and placed in an oven. The tempera-
ture inside the oven was increased from 20 to 600 °C within 4 h
(2.4 Kꢁminꢂ1) under a flow of nitrogen (15 Lꢁminꢂ1), after which
it was maintained at 600 °C for another 4 h. After that, the oven
was allowed to cool slowly to room temperature. The melt from
the crucible was transferred into a beaker, and deionized water
(50 mL) and a stir bar were added. The suspension was stirred at
room temperature for 4 h until it became homogeneous and no
agglomerated particles were seen. The solid was separated by cen-
trifugation (6500 minꢂ1, 12 min). The aqueous solution was care-
fully removed and the residue transferred into a 2 mL safe-lock
tube. The solid was washed with water (3 ꢄ 1.5 mL) using a cen-
trifuge (13,500 minꢂ1, 1 min) and dried under vacuum, giving
256 mg of the dark yellow material.
2.7. Diethyl 2,4,6-trimethylpyridine-3,5-dicarboxylate 2a
A mixture of ethyl 3-oxobutanoate (0.23 g, 1.8 mmol), NH4HCO3
(70 mg, 0.9 mmol), PHIK-BM (100 mg), S8 (240 mg, 7.5 mmol), and
ZnO (405 mg, 5 mmol) in EtOH (40 mL) was stirred under N2 at
+50 °C under blue LED (40 W, 465 nm) irradiation for 24 h. Reac-
tion progress was monitored by GC–MS. The catalyst was sepa-
rated by centrifugation (12,000 minꢂ1, 2 min) and washed with
ethanol (3 ꢄ 2 mL). Ethanol washings were combined, the solvent
was evaporated under reduced pressure, and the residue was dried
under vacuum. MeCN (40 mL) was added to the residue, followed
by the addition of PHIK-BM (100 mg), S8 (240 mg), and ZnO
(405 mg). The suspension was stirred under blue LED (40 W,
465 nm) irradiation under N2 for another 18 h. The catalyst was
separated by centrifugation (12,000 minꢂ1, 2 min) and washed
with MeCN (3 ꢄ 2 mL). Washings were combined and concen-
trated under vacuum, affording an orange oil. The oil was washed
with hexane (3 ꢄ 5 mL) and extracts were combined and concen-
trated under vacuum furnishing 200 mg of a yellow oil. Yield:
85%. 1H NMR (400 MHz, CDCl3): d = 4.41 (q, J(H,H) = 5.3 Hz, 4H;
CH2), 2.52 ppm (s, 6H; CH3), 2.27 ppm (s, 3H; CH3), 1.39 ppm (t, J
(H,H) = 5.3 Hz, 6H; CH3).
2.4. NS synthesis
A mixture of 5-aminotetrazole (0.5 g), lithium chloride (1.13 g),
and potassium chloride (1.38 g) was brought together in a steel
ball milling cup. The steel ball was inserted and the cup was closed.
The mixture of precursors was ground for 5 h at the operational
frequency 25 Hz. The temperature of the ball milling cup was
increased to 80 °C. The cup was allowed to cool to room tempera-
ture. The cup content (2.917 g, gray solid) was transferred into a
beaker and water (40 mL) was added. The resultant mixture was
stirred at room temperature for 1 h until salts and unreacted pre-
cursor were dissolved. The colloidal solution was subjected to dial-
ysis (MWCO: 1 kD). Water was evaporated and the residue was
dried under vacuum overnight, furnishing 2.7 mg of dark gray
powder.
2.8. Diethyl 2,6-dimethylpyridine-3,5-dicarboxylate 2b
A mixture of diethyl 2,6-dimethyl-1,4-dihydropyridine-3,5-dic
arboxylate (62.4 mg, 0.25 mmol), PHIK-BM (20 mg), elemental sul-
fur (48 mg, 1.5 mmol), and ZnO (81 mg, 1 mmol) in MeCN (6 mL)
was stirred at +50 °C under N2 and blue LED (40 W, 465 nm) irra-
diation for 24 h. The catalyst was separated by centrifugation
(12,000 minꢂ1, 1 min) and washed with MeCN (3 ꢄ 2 mL). Acetoni-
trile washings were combined and concentrated under vacuum,
affording a white solid. Yield: 59 mg, 95%. 1H NMR (400 MHz,
CDCl3): d = 8.68 (s, 1H; CH), 4.40 ppm (q, J(H,H) = 5.4 Hz, 4H;
CH2), 2.85 ppm (s, 6H; CH3), 1.42 ppm (t, J(H,H) = 5.4 Hz, 6H; CH3).
2.5. PHIK-BM–NS synthesis
A mixture of 5-aminotetrazole (0.5 g), lithium chloride (1.13 g),
and potassium chloride (1.38 g) was brought together in a steel
ball milling cup. The steel ball was inserted and the cup was closed.
The mixture of precursors was ground for 5 h at the operational
frequency 25 sꢂ1. The temperature of the ball milling cup was
increased to 80 °C. The cup was allowed to cool to room tempera-
ture and the contents (2.898 g, gray solid) were transferred into a
porcelain crucible, covered with a porcelain lid, and placed in the
oven. The temperature inside the oven was increased from 20 to
600 °C within 4 h under a flow of nitrogen (15 Lꢁminꢂ1), after
which it was maintained at 600 °C for another 4 h. After that, the
oven was allowed to cool to room temperature. The melt from
the crucible was transferred into a beaker; deionized water
(50 mL) and a stir bar were added. The suspension was stirred at
room temperature for 1 h until it became highly homogeneous
and no agglomerated particles were seen. The solid was separated
by centrifugation (6500 minꢂ1, 12 min). The aqueous solution was
carefully removed and the residue transferred into a 2 mL safe-lock
tube. The solid was washed with water (3 ꢄ 1.5 mL) using a cen-
3. Results and discussion
At first, we decided to gain insight into the role of high-energy
ball milling in precursor pretreatment. To do this, a mixture of 5-
aminotriazole, LiCl, and KCl was ball milled for 5 h. After the unre-
acted precursor was separated, the residue (abbreviated as NS,
nucleation seeds) was analyzed. NS chemical composition is shown
in Table S1. The material contains 15.1 wt.% of iron and 6.9 wt.% of
chromium, but also, small amounts of Ni (0.5 wt.%) and Mo (0.5 wt.
%) are present. Metals’ EDX mapping patterns (Fig. S1) are similar,
suggesting that these elements belong to the same phase. A higher
C/N ratio of NS, 0.421 versus 0.172 for pure 5-aminotetrazole,
could be explained by partial polymerization of the precursor. This

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A. Savateev et al. / Journal of Catalysis 350 (2017) 203–211
is in agreement with the FTIR spectrum of NS (Fig. S2a). Peak
broadening, when compared with 5-aminotetrazole FTIR spectrum,
implies that NS is a low-molecular-weight polymer. NS is an amor-
phous material, as evidenced by its PXRD pattern (Fig. S2b). A
broad peak at ꢀ29° that could be attributed to the layer-like struc-
ture of PHIK is present [18]. Therefore, the ball milling cup acts as a
near-room-temperature mechanoreactor in which partial poly-
merization of 5-aminotetrazole occurs, along with metallic nucle-
ation seeds deposition.
In order to synthesize highly crystalline PHIK and not contami-
nate it with transition metals, i.e., keep the photocatalyst metal-
free, a mixture of 5-aminotetrazole and LiCl/KCl eutectics was
ground using ball milling only for a short time [26]. A highly homo-
geneous flourlike white powder was obtained. Potassium poly
(heptazine imide), abbreviated as PHIK-BM, was prepared by heat-
ing this ball-milled precursor mixture at 600 °C for 4 h in the oven.
The reference material, PHIK-MG, was synthesized using manual
grinding with a pestle in a mortar according to the procedure
developed before [18]. Visual examination of the solidified melts’
cross sections (Fig. S3) obtained right after oven treatment indi-
cates that PHIK-BM is a porous homogeneous brownish solid,
while PHIK-MG has white inclusions of pure LiCl/KCl eutectics
incorporated into the PHIK bulk. This implies that ball milling is
more efficient in terms of precursor mixture homogenization then
manual grinding. After the salts were removed by extensive wash-
ing with water, powders were dried and analyzed.
Elemental composition of the PHIK-BM (Table S2) is close to the
previously reported values [18]. Potassium content in the sample is
7.8 wt.%. A C/N weight ratio (0.663) slightly higher than the theo-
retical value of 0.572 could be explained by higher oxygen content
as a result of partial surface hydrolysis upon aqueous work-up. It is
important to note that due to short contact time of the precursor
with the ball milling cup, PHIK-BM was free of iron and other tran-
sition metals.
FTIR spectra of PHIK-BM and PHIK-MG perfectly repeat each
other (Fig. S4) and are identical to the FTIR spectrum of the PHIK
reported before [18]. The broad band in the range 2400–
3700 cmꢂ1 is partially due to the OAH stretching of highly acidic
OH groups and to NAH stretching in primary and secondary ami-
nes [27]. The peaks at 1577 and 1656 cmꢂ1 are due to NAH bend-
ing vibration in secondary HNC2 and primary H2NC amino groups,
respectively. The peaks in the range 1100–1500 cmꢂ1 correspond
to the stretching vibrations of CAN and C@N bonds in the polymer.
The bands at 989 and 1062 cmꢂ1 are related to the symmetric and
asymmetric vibrations of NC2 bonds, respectively, of the KANC2
groups [28]. The breathing mode of the heptazine ring is responsi-
ble for the peak at 802 cmꢂ1
PXRD patterns of the PHIK-MG and PHIK-BM are shown in
Fig. 2. They are similar to each other and to those reported previ-
ously [18,19].
The resolution of the PHIK-BM diffraction pattern was improved
over that for PHIK-MG, implying that ball-mill-induced seeding
indeed ameliorates the final material structural order. The peak
at 8.2°, corresponding to a distance of 1.1 nm, due to diffraction
from the (100) plane [18] (Fig. 1) is more pronounced, indicating
that crystal growth along this direction is more favored, producing
particles with platelike morphology (see TEM and AFM images
below). Additionally, instead of one broad peak at 27° due to inter-
layer stacking, two separate peaks at 2h 27.1° and 28.2° are
observed in the PHIK-BM X-ray pattern.
.
In PHIK-BM UV–vis absorption spectrum (Fig. 3a), the band due
to
p
–
p
⁄ transitions is observed at ꢀ470 nm [29] and is blue-shifted
compared with PHIK-MG (ꢀ490 nm). It was shown previously that
distortion of the carbon nitride structure from perfectly planar as a
result of layers buckling [30] and deviation of the ring units from
trigonal symmetry gives rise to the broad band at ꢀ600 nm involv-
ing transitions between nonbonding orbitals of N- and O-
containing groups and the LUMO level of carbon nitride [31]. It is
also in the agreement with our results that more structurally
ordered PHIK-BM exhibits lower absorption due to n–
tions. Optical band gaps (BG) were determined from Tauc plots
(Fig. S5), taking
⁄ transitions as the principal band for calcula-
p⁄ transi-
p–p
tion. The BG value of PHIK-BM is 2.73 eV, while that of PHIK-MG is
2.62 eV. The BG expansion could be explained by particle size
decrease (see below) [32]. Time-resolved PL spectra are shown in
Fig. 3b. Average fluorescence lifetimes for the PHIK-BM and
PHIK-MG materials extracted from the fitting function are 0.62
and 0.48 ns, respectively. These values are significantly smaller
than those reported for g-CN, implying that exciton recombination
in PHIK is much more favorable than in g-CN [33]. In room-
temperature PL spectra (Fig. 3c), obtained after materials excita-
tion with a 350 nm wavelength, the emission peak of PHIK-MG is
observed at 496 nm, while that of PHIK-BM is blue-shifted to
489 nm. PHIK-BM demonstrates higher PL intensity than PHIK-
MG, which is a consequence of higher crystallinity of this material.
The specific surface area of PHIK-BM, 89 0.8 m2ꢁgꢂ1, was cal-
culated applying the BET model to the N2 sorption isotherm
(Fig. S6). The surface area of PHIK-MG calculated to be 40 m2ꢁgꢂ1
.
The gain of surface area is due to particle size decrease, as also seen
from the electron microscopy images (Fig. 4a, b).
Introduction of nucleation seeds into a mixture of precursors
provides many more centers of crystallization than the seedless
method. It results in a totally different morphology. PHIK-BM is
represented by a few hundred nm long sheets weakly agglomer-
ated together, while PHIK-MG exhibits a ‘‘coral reef” morphology
(Figs. S7c, S7d) with the crystals growing from a presumably only
weakly ordered primary structure. The PHIK-BM particles can
easily be separated by brief sonication in ethanol, as is seen from
a transmission electron microscopy (TEM) image (Figs. 4a, S8a,
S8b). Visual analysis of TEM images reveals that the particle shape
distribution is not uniform—there are larger plates (ꢀ200 nm long)
as well as nanoparticles (ꢀ20–30 nm). The holey structure of PHIK-
BM due to alternating lines of potassium can be discerned in both
large and small particles (Fig. S8c). Atomic force microscopy (AFM)
proves that generally two types of particles exist: ‘‘short” with a
height below ꢀ20 nm, and ‘‘tall” above 45 nm (Fig. 4c, d).
To assess the photocatalytic performance of PHIK-BM, we have
selected a test reaction of benzyl alcohol oxidation to benzalde-
hyde under visible light irradiation (Scheme 1). The results of cat-
alytic tests are summarized in Table 1.
Fig. 2. Powder X-ray patterns of PHIK-BM and PHIK-MG.

A. Savateev et al. / Journal of Catalysis 350 (2017) 203–211
207
Fig. 3. UV–vis (a), time-resolved PL (b), and steady state PL (c) spectra of PHIK-BM and PHIK-MG.
Fig. 4. TEM image with SAED as inset (a); SEM image (b); AFM image (c) with corresponding cross sections (d) of PHIK-BM particles: blue line for tall and red for short
particles.

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A. Savateev et al. / Journal of Catalysis 350 (2017) 203–211
properties, e.g., exposed facets and surface area. Finally, mpg-CN,
with a surface area of 180 m2ꢁgꢂ1, gave benzyl alcohol conversion
of only 37.9% (entry 12). We attribute this to the less oxidative
VB position of mpg-CN compared with PHIK, which creates a smal-
ler thermodynamic driving force for substrate oxidation. No con-
version of benzyl alcohol was observed in the dark. Similarly, we
did not detect any products of benzyl alcohol degradation in the
absence of electron acceptor.
Scheme 1. A model reaction of benzyl alcohol oxidation under visible light
irradiation.
The proposed mechanism of the benzyl alcohol oxidation is
depicted in Scheme 2; it is in agreement with the mechanism in
which O2 instead of S8 is used as an electron acceptor and was
studied elsewhere [11]. In the first stage, PHIK-BM absorbs a pho-
ton, producing excited short-living PHIK-BM⁄ (structure ‘‘A”) with
bound hole–electron pairs. Then sulfur receives an electron from
the conduction band (CB) of PHIK-BM, giving a polysulfide radical
anion [Sn]Å,ꢂ while benzyl alcohol oxidation occurs via electron trans-
fer to the PHIK-BM valence band. The polysulfide radical anions were
extensively studied and characterized elsewhere [36]. We assume
that the polysulfide radical anion [Sn]Åꢂ abstracts a hydrogen atom
from the radical cation ‘‘B” giving rise to hydropolysulfide anions
[HSn]ꢂ and benzyl-like cations ‘‘C”. Hydrogen abstraction from dif-
ferent substrates by sulfur radicals was theoretically studied before
and is a common theme [37]. Finally, in the last step, anion [HSn]ꢂ
acts as a base. It abstracts a proton from the cation ‘‘C”, producing
benzaldehyde. The final rearrangement gives H2S, which evolves as
a gas.
Benzyl alcohol conversion after only 4.22 h was already 64.1%.
Using these data, the apparent photon-to-benzyl alcohol conver-
sion efficiency of the PHIK-BM-based catalytic system was calcu-
lated to be 1.38% (see ESI for calculation details).
The formation of hydrogen sulfide as a sulfur reduction product
was confirmed by the test reaction with AgNO3. The latter was
transformed into insoluble Ag2S (Fig. S9).
Both high conversion and selectivity of benzyl alcohol oxidation
prompted us to explore the developed catalyst in reactions with
other molecules. Ethanol was selected as an even more challenging
substrate than benzaldehyde. In order to trap acetaldehyde gener-
ated in situ, ethanol oxidation was coupled with Hantzsch ester
We have started screening reaction conditions using 10 mol.% of
PHIK-BM at 20 °C. In this case, benzyl alcohol conversion was 81.3%
and selectivity with respect to benzaldehyde 95.5% (entry 1). The
main impurity was presumably thiobenzoic acid, which formed
upon H2S nucleophilic addition to the carbonyl group. Conversion
has increased to 84.4% and selectivity to 99.5% (entry 2) when
the reaction was performed at +50 °C. When the loading of PHIK-
BM was doubled (20 mol.%), benzyl alcohol conversion increased
only to 90.7% (entry 3), while selectivity remained nearly the same
(99.8%). However, the quantitative conversion, 99.2%, of benzyl
alcohol along with high selectivity, 98.4%, was achieved when
ZnO (1 mmol) was added to the reactor (entry 4). One of the pos-
sible explanations of the role of ZnO could be that it acts as a sor-
bent of hydrogen sulfide [34]. The developed catalytic system also
demonstrated excellent stability (entries 5–7). Even after the
fourth cycle, benzyl alcohol conversion remained at the quantita-
tive level, 99.5%, with excellent selectivity, 97.5–99.3%.
Almost no photocatalytic activity was observed when ZnO was
used alone without PHIK-BM (entry 8), implying that under given
conditions benzyl alcohol oxidation is exclusively catalyzed by
PHIK-BM, while ZnO acts only as an auxiliary agent. Negligible con-
version (0.2%) of benzyl alcohol was also observed when S8 was
used without any photocatalyst (entry 9), though
a-sulfur itself
could serve as a photocatalyst under different conditions [35].
When the reaction was performed under oxygen (1 bar) as electron
acceptor (entry 10), conversion of benzyl alcohol was significantly
lower (29.0%). Moreover, selectivity decreased to 70.5% due to a
number of side reactions initiated by highly reactive HOÅ species,
which in turn are formed upon H2O2 decomposition. The main
by-product is benzoic acid, which is formed from benzaldehyde
upon its further oxidation.
Benzyl alcohol conversion using PHIK-MG under identical con-
ditions was 50.8% (entry 11). The higher activity of PHIK-BM com-
pared with PHIK-MG is presumably due to different shape of the
crystallites, namely plates, which in turn is reflected in catalyst
synthesis, which opens
a route to 1,4-dihydropyridine, 1a
(Scheme 3). Dihydropyridines are used in pharmacy, for example,
as calcium channel agonists [38], but are equally important in
organic synthesis as hydrogen source in
a,b-unsaturated ketone
hydrogenation [39], aldehyde reductive amination [40], acetylene
Table 1
Benzyl alcohol oxidation: photocatalyst screening and reaction conditions optimization.a
Entry
Catalyst
Electron acceptor
T, °C
Conversion, %b
Selectivity, %c
1
2
3
4
5
6
7
8
PHI-K (10 mol.%)d
PHI-K (10 mol.%)
PHI-K (20 mol.%)
PHI-K (10 mol.%) + ZnO (1 mmol)
PHI-K (10 mol.%) + ZnO (1 mmol)e
PHI-K (10 mol.%) + ZnO (1 mmol)f
PHI-K (10 mol.%) + ZnO (1 mmol)g
ZnO (1 mmol)
S8
S8
S8
S8
S8
S8
S8
S8
S8
20
50
50
50
50
50
50
50
50
50
50
50
81.3
84.4
90.7
99.2
99.6
99.5
99.5
1.6
95.5
99.5
99.8
98.4
97.5
98.6
99.3
100
9
—
0.2
100
h
10
11
12
PHI-K (10 mol.%)
PHIK-MG (10 mol.%) + ZnO (1 mmol)
mpg-CN (10 mol.%) + ZnO(1 mmol)
O2
S8
S8
29.0
50.8
37.9
70.5
99.5
99.6
a
b
c
Benzyl alcohol: 0.5 mmol; MeCN: 6 mL; S8: 1.5 mmol.
Conversion of benzyl alcohol according to GC–MS data using ethylbenzene as an internal standard.
Selectivity with respect to benzaldehyde formation.
d
e
f
The molar weight of PHIK-BM taken for calculation is 438 gꢁmolꢂ1 (Fig. 1).
Second run.
Third run.
Fourth run.
g
h
O2 pressure 1 bar.

A. Savateev et al. / Journal of Catalysis 350 (2017) 203–211
209
Scheme 2. A tentative mechanism of benzyl alcohol oxidation by PHIK using elemental sulfur as an electron scavenger.
Scheme 3. Hantzsch 1,4-dihydropyridine synthesis using acetaldehyde generated in situ.
aniline tandem hydroamination/asymmetric transfer hydrogena-
tion [41], etc.
ethanol to a solvent more stable against oxidation, acetonitrile
(Scheme 4).
Dihydropyridine 1a is formed, as proven by GC–MS, presumably
by the mechanism reported in the literature [42]. In agreement
with the predicted high oxidation power of PHIK-BM, we found
that after 24 h the reaction mixture contained 53% pyridine 2a
(Scheme 4) and 36% dihydropyridine 1a. In addition, 11% ethyl 2-
ethyl-3-oxobutanoate, the product, which formed presumably by
Knoevenagel adduct reduction by 1,4-dihydropyridine 1a, was
detected in the reaction mixture (Scheme S2) [43]. It is known that
Knoevenagel condensation between carbonyl groups and CH acids
is catalyzed by bases and precedes 1,4-dihydropyridine ring forma-
tion [44]. On the other hand, carbon nitride is known to act as a
solid base to catalyze Knoevenagel condensation, here via the pres-
ence of negatively charged nitrogen imides and amino groups [45].
Therefore, in the targeted complex cascade reaction described in
Scheme 3, PHIK-BM acts simultaneously as a photocatalyst and
as a solid base.
Quite often, not 1,4-dihydropyridine, but the aromatic pyridine
ring is a target molecule of Hantzsch ester synthesis, as it is a main
component of a number of pharmaceuticals, dyes, and agrochemi-
cals [46]. Pyridines are synthesized from the corresponding 1,4-
dihydropyridines via oxidative dehydrogenation [47] using such
systems as Co(OAc)2/H2O2 [48] or transition-metal chloride/
NaNO2/TEMPO [49].
Pyridine 2a was isolated with 85% yield, while unsubstituted
pyridine 2b was obtained with 95% yield. A lower yield of pyridine
2a can be explained by worse interaction of the starting substrate
1a with the PHIK surface due to the presence of a methyl group in
the fourth position of the 1,4-dihydropyridine ring.
The high yields are further quantitative evidence that the sys-
tem indeed goes very effectively through the 1,4-dihydropyridine
step, which we infer to isolate for practical reasons. The example
of a complex photocatalytic reaction cascade presented in this
work, however, demonstrates the high potential of PHIK-BM for
application in organic synthesis.
4. Conclusions and outlook
High-energy ball milling is a useful tool for preparing a highly
homogeneous mixture of 5-aminotetrazole and LiCl/KCl eutectics
and also for introducing nucleation seeds. These seeds, upon subse-
quent heating of precursors, provide centers of crystallization,
altering the morphology of PHIK from ‘‘coral reef”-like to separate
plates. Photocatalyst screening tests were performed in the model
reaction of benzyl alcohol oxidation to benzaldehyde. Excellent
benzyl alcohol conversion and selectivity of benzaldehyde forma-
tion were achieved when elemental sulfur was used as an electron
acceptor. According to the proposed mechanism of sulfur-assisted
benzyl alcohol oxidation, chemical shortcuts between intermedi-
ates do exist. Therefore, our next step is aimed at the modification
of the catalytic system to improve apparent photon-to-compound
conversion efficiency of the whole process.
In current work, we did not attempt to isolate 1,4-
dihydropyridine 1a, but aimed to obtain the corresponding pyri-
dine 2a. This was accomplished by switching from redox-active
Elemental sulfur has advantages as an electron scavenger: (1)
Unlike silver nitrate, which upon reduction is transformed into
Ag0, blocking the catalyst active site, the product of sulfur reduc-
tion, hydrogen sulfide, can easily be removed from the reaction
mixture, leaving the catalyst intact. (2) Elemental sulfur, com-
pared, for example, with ammonium persulfate, is a much milder
oxidant that receives electrons from the photocatalyst CB and does
not react with the substrate. (3) the solubility of S8 in organic
solvents is much higher than that of inorganic electron sacrificial
Scheme 4. 1,4-Dihydropyridine 1a, 1b oxidation using PHIK-BM as a photocatalyst
and S8 as an electron acceptor. The isolated yields of pyridines 2a, 2b are given in
parentheses.

210
A. Savateev et al. / Journal of Catalysis 350 (2017) 203–211
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This research did not receive any specific grant from funding
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2017.02.029.
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