The first solid state porphyrin-weak acid molecular complex: A novel metal free, nanosized and porous photocatalyst for large scale aerobic oxidations in water

Article abstract of DOI:10.1016/j.jcat.2018.06.011

The first solid state porphyrin dication of a weak carboxylic acid was synthesized through the reaction of meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (H₂TMPyP) supported on the sodium salt of Amberlyst 15 nanoparticles (nanoAmbSO₃

Full text of DOI:10.1016/j.jcat.2018.06.011

Journal of Catalysis 364 (2018) 394–405
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The first solid state porphyrin-weak acid molecular complex: A novel
metal free, nanosized and porous photocatalyst for large scale aerobic
oxidations in water
⇑
Akram Heydari-turkmani, Saeed Zakavi
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The first solid state porphyrin dication of a weak carboxylic acid was synthesized through the reaction of
meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (H2TMPyP) supported on the sodium salt of
Amberlyst 15 nanoparticles (nanoAmbSO3Na) with formic acid. The polymer-porphyrin hybrid com-
pound, nanoAmbSO3@H2TMPyP(HCOOH)2, was characterized by FT-IR and diffuse reflectance (DR) UV–
vis spectroscopy, EDX, BET, DLS, FESEM and TGA methods. Interestingly, the diacid was quite stable
towards decomposition to the corresponding porphyrin and carboxylic acid in neat water as well as in
acetonitrile. The protonation reaction was accompanied with the shift of the Soret band from 435 to
454 nm in DR UV–vis spectrum and a rapid color change from red brown to green. A particle size of
190 and <50 nm was estimated for the nanoparticles by DLS and FESEM, respectively. Also, the macrop-
orous structure of the catalyst was revealed by BET experiments. nanoAmbSO3@H2TMPyP(HCOOH)2 was
used as photosensitizer for the highly chemoselective and large scale aerobic oxidation of sulfides to the
corresponding sulfoxide in water within 3 h at room temperature. A conversion of ca. 95% was observed
for the oxidation of sulfides to their corresponding sulfoxide in water with a selectivity of ca. 100%.
The use of acetonitrile as solvent led to a significant decrease in the conversion values while retaining
selectivity. The higher efficiency of the catalyst in water was in accord with greater singlet oxygen
quantum yield (UD) value of the photosensitizer in this solvent (UD = 0.41) relative to that in acetonitrile
Received 24 March 2018
Revised 8 June 2018
Accepted 10 June 2018
Keywords:
Photocatalysis
Porphyrin diacid
Sulfides
Sulfoxides
1,5-Dihydroxynaphthalene
Singlet oxygen
Oxidation in water
(
UD = 0.08). Apparently, cooperative acid catalysis caused by the protic solvent and porphyrin diacid is
involved in the preference of water over acetonitrile as the solvent for efficient oxidation of sulfides
to sulfoxides. Furthermore, the photosensitizer was used for the efficient oxidation of 1,5-
dihydroxynaphthalene (DHN) in water within 15 min. The photocatalyst was remarkably stable towards
oxidative degradation so that it could be recovered and reused for at least 5 times without loss of activity.
It is noteworthy that large scale photooxidation of sulfides (TON ꢀ 5 Â 3000) and DHN (TON ꢀ 6 Â 1000)
were achieved in water, using nanoAmbSO3@H2TMPyP(HCOOH)2 as the photosensitizer.
Ó 2018 Elsevier Inc. All rights reserved.
1. Introduction
singlet oxygen production of metal free porphyrins as organic pho-
tosensitizers, a variety of photooxidation reactions were studied in
Porphyrins and derivatives have been extensively used as
photosensitizer in different applications such as photodynamic
therapy, water treatment, medicine and biological studies [1–5].
Recently, much attention has been given to the synthesis of novel
porphyrinic photosensitizers with the aim of increasing their
photocatalytic activity, product selectivity and oxidative stability
[6–9]. Due to the potential toxicity of most transition metals, con-
siderable attention was paid to the synthesis of metal free catalysts
[10–12]. On the other hand, owing to the high quantum yields of
the presence of porphyrin derivatives [13–15]. However, the
recovery and reuse of catalyst has been the key challenge for using
different compounds under homogeneous conditions [16]. Further-
more, the reduction of catalyst size to nanoscale is a common strat-
egy to overcome the slow kinetics of the reaction catalyzed by
heterogeneous catalysts [17–20]. On the other hand, the use of
water as the most green solvent is a key component of green chem-
istry [21]. Porphyrin macrocycle may be substituted at the central
nitrogen atoms as well as the peripheral positions [22,23]. In this
regard, porphyrin core may be metallated [9], methylated [24] or
protonated with different
tions may be readily prepared by the treatment of porphyrins with
r and p acceptors [25]. Porphyrin dica-
⇑
Corresponding author.
E-mail address: zakavi@iasbs.ac.ir (S. Zakavi).
https://doi.org/10.1016/j.jcat.2018.06.011
0021-9517/Ó 2018 Elsevier Inc. All rights reserved.

A. Heydari-turkmani, S. Zakavi / Journal of Catalysis 364 (2018) 394–405
395
different acids in organic solvents and water. The red shifted
absorption bands of porphyrins diprotonated species make them
interesting alternatives to the corresponding free base porphyrins
[26]. However, limited attention has been devoted to developing
catalyst and photocatalyst based on porphyrins diprotonated
species for organic transformations [27,28]. Recently, we have
reported novel porphyrin photosensitizers formed by the molecu-
lar complexation of porphyrins with 2,3-dicholoro-5,6-dicyano-1,
4-benzoquinon (DDQ) and tetracyanoethylene (TCNE) [29]. The
presence of DDQ and TCNE was shown to have significant effect
on the photosensitizing ability of meso-tetra(aryl)porphyrins in
aerobic photooxidation of olefins [25]. Also, very recently we have
studied the photocatalytic activity of a series of meso-tetra(aryl)
porphyrins immobilized on Amberlyst 15 nanoparticles
(nanoAmb) in the form of diprotonated species [17]. In 2018, the
dications of H2TPP with different acids were used for chemoselec-
tive oxidation of sulfides to sulfoxides under homogeneous
conditions [30]. In the present study, meso-tetrakis(N-methylpyri
dinium-4-yl)porphyrin (H2TMPyP) was initially supported on the
nano-sized anionic Amberlyst 15 formed by the alkaline treatment
of this polymer. Then, the immobilized porphyrin was protonated
with formic acid to prepare the core protonated porphyrin
(Fig. 1). To our knowledge, it is the first report on the synthesis
and characterization of a porphyrin dication with a weak car-
boxylic acid. Interestingly, the dication was completely stable in
different solvents and water. Our previous studies [13] showed
that the dications of soluble porphyrins with formic acid are read-
ily decomposed to the corresponding free base porphyrins upon
dilution in organic solvents or exposure to water. Also, the dica-
tions cannot be dried under vacuum or air flow. This observation
gives evidence for remarkable impact of porphyrin immobilization
on the stability of porphyrin diacid with weak acids. Furthermore,
the solid compound is stable under light conditions and may be
stored for a long time. It is noteworthy that due to the low acid
strength of formic acid (pKa = 3.75) [31], the acid cannot reproto-
nate the neutralized Amberlyst 15 bed (pKa = À6.5) [32]. Moreover,
the major weakness of formic acid prevents its involvement in
unexpected side reactions. The nano-sized catalyst was used for
the aerobic photooxidation of sulfides in neat water as the solvent.
The selective oxidation of sulfides to the corresponding sulfoxide is
still a challenging synthetic problem that may be achieved through
singlet oxygen mediated photooxidation of sulfides under mild
conditions [33–35]. Herein, acid catalysis [36–38] induced by the
porphyrin diacid and the protic solvent is also involved in the
observed rate of the photooxidation reactions. It is noteworthy that
sulfoxides are important intermediates in organic synthesis phar-
maceutical industry [39,40]. Furthermore, the catalytic activity of
the immobilized porphyrin diacid in the oxidation of 1,5-
dihydroxynaphthalene (DHN) was investigated [41]. The product
of oxidation or photooxidation of DHN, i.e. 5-hydroxy-1,4-
naphthoquinone (Juglone) is a naturally occurring aromatic com-
pound which plays an important role in the pharmaceutical and
dye industries [42–44]. On the other hand, oxidative degradation
of DHN is of importance for the removal of phenolic and naphtholic
pollutants from wastewater [45]. The main objective of this study
is to present the effects of weak carboxylic acids on the photocat-
alytic activity and oxidative stability of porphyrins in photooxida-
tion of organic substrates in water under heterogeneous
conditions.
2. Experimental
2.1. Instrumental
A Pharmacia Biotech Ultrospec 4000 UV–vis spectrophotometer
and a Varian Cary 5000 UV–Vis–NIR absorption spectrometer were
used for preparation of UV–vis and diffuse reflectance UV–vis spec-
tra, respectively. 1H NMR spectra were obtained on a Bruker
Avance DPX-400 MHz spectrometer. The reaction mixtures were
analyzed using a Varian-3800 gas chromatograph with a HP-5 cap-
illary column and flame-ionization detector. FT-IR spectra were
prepared using a Bruker Vector 22 instrument. A Belsorp max
(Japan) instrument was employed for porosimetry analyses using
the nitrogen adsorption/desorption method. The mean diameter
and size distribution of the solid catalyst before and after loading
of porphyrin were measured by Zetasizer Nano ZS (Malvern Instru-
ments Ltd, United Kingdom) at 25 °C with a detection angle of 90°.
For morphology characterization by FE-SEM, the crushed Amber-
lyst beads obtained after 24 h were placed on a clean glass slide
and then vacuum-coated with gold. Digital images of the samples
were taken with HitachiS4160 field emission scanning electron
microscope operating at 20 kV.
2.2. Amberlyst 15 nanoparticles (nanoAmb)
Amberlyst 15 beads (dry, ion-exchange resin, ACROS) were stir-
red vigorously over a magnetic stirrer in ethyl acetate overnight at
room temperature to obtain the pale yellow powder of nanoAmb.
Details of nanoparticle characterization were described elsewhere
[17].
2.3. Preparation of porphyrins
Meso-tetra(4-pyridyl)porphyrin (H2TPyP) and 5,10,15,20-tetra
kis(N-methylpyridinium-4-yl)porphyrin tetra(p-toluenesulfonate),
H2TMPyP, were prepared and purified according to the literature
methods [46]. The UV–vis and 1H NMR spectral data of the used
porphyrin are as follows:
Fig. 1. Immobilization of H2TMPyP on nanoAmbSO3Na and its protonation with
formic acid.

396
A. Heydari-turkmani, S. Zakavi / Journal of Catalysis 364 (2018) 394–405
H2T(4-Py)P: 1H NMR (CDCl3, 400 MHz): d -2.93 (2H, br, s, NH),
used as the light source). In order to characterize the products of
the oxidation of sulfides, at different time intervals (Fig. 9) the
reaction mixture was extracted with ethyl acetate, rotary evapo-
rated, dissolved in CDCl3 and analyzed by 1H NMR and 13C NMR.
The NMR spectra confirmed the formation of sulfoxide as nearly
the exclusive product.
1-(methylsulfinyl)benzene: Methyl(phenyl)sulfane (248 mg,
2 mmol) was oxidized according to the above general
oxidation procedure using nanoAmbSO3@H2TMPyP(HCOOH)2
(0.66 Â 10À3 mmol). Crude material was separated by Thin-layer
chromatography (TLC) (1:4 ratio of ethyl acetate, n-hexane) to give
methyl phenyl sulfoxide; yield: (91%). 1H NMR (400 MHz,
chloroform-d): d = 7.56–7.38 (m, 5H), 2.61 (s, 3H). 13C NMR
(400 MHz, chloroform-d): d = 43.57 ppm (–CH3), 145.22 (–CS–),
123.26 (Co), 129.27 (Cm), 130.79 (Cp). (Supplementary material,
S3 (a,b)).
8.15–8.17 (8Hm, dd), 9.06–9.07 (8Ho, dd), 7.90 (4H, m), 8.87
(8Hb, s); UV–Vis (k/nm) in CH2Cl2: 418 (Soret), 512, 547, 588,
646.
H2TMPyP: 1H NMR (DMSO d6, 400 MHz): À2.81 (2H, br, s, NH),
3.84 (12H, –CH3), 9.27–9.28 (8Ho, d), 9.48 (8Hb, s), 9.77–9.79
(8Hm, m) (see Supplementary material, S1 for the 1H NMR spec-
trum of the counteranion, p-toluenesulfonate), UV–Vis (k/nm)
in water: 420 (Soret), 515, 553, 582, 635.
2.4. Immobilization of H2TMPyP(HCOOH)2 on nanoAmbSO3Na
The sodium salt of nanoAmb was prepared by treatment of
Amberlyst 15 (1.5 g in 30 ml distilled water) with a 1 M sodium
hydroxide solution. The mixture was stirred magnetically for 24
h. After the required time, the solid was centrifugally separated.
The precipitate was thoroughly washed by distilled water in a cen-
trifuge tube until neutral and finally dried in a vacuum desiccator
(24 h). To prepare the immobilized porphyrin, an excess amount of
H2TMPyP was added to nanoAmbSO3Na in water. The mixture was
magnetically stirred for 5 h in dark and then the solid was sepa-
rated by centrifuge (5000 rpm). The solid residue was washed by
centrifuge with distilled water thoroughly until a colourless aque-
ous phase was observed. The remaining solutions were analyzed
by UV–vis spectroscopy to measure the non-immobilized por-
phyrin; a calibration curve (absorbance vs. concentration) obtained
for H2TMPyP in water was used to determine the non-supported
porphyrin. A maximum loading of 0.076 g H2TMPyP per one gram
of nanoAmbSO3 corresponding to a loading of 0.052 mmol gÀ1 was
measured. In a previous study [17], we have shown that a low
loading of porphyrin (0.003 mmol of porphyrin per 1 g of the poly-
mer, 0.003 mmol gÀ1) is more efficient in aerobic photooxidation of
olefins. Indeed, the maximum conversion of olefin to the oxidation
product was observed under this loading condition. Accordingly, in
the present study, we have examined different catalyst loadings
and a catalyst loading of 0.007 mmol gÀ1 (0.010 g of H2TMPyP
per 1 g of the polymer) was found to be the optimum one. It is
noteworthy that upon the addition of an amount of H2TMPyP less
than the maximum capacity of nanoAmbSO3Na, no non-
immobilized porphyrin remained in the solution after the cen-
trifuge separation of the solid residue.
1-(ethylsulfinyl)-4-methylbenzene:
Ethyl(p-tolyl)sulfane
(292.6 mg, 2 mmol) was performed according to the general proce-
dure using nanoAmbSO3@H2TMPyP(HCOOH)2 (0.66 Â 10À3 mmol).
Crude material was separated by TLC (1:4 ratio of ethyl acetate,
n-hexane) to give Ethyl phenyl sulfoxide; yield: (90%). 1H NMR
(400 MHz, chloroform-d): d = 7.60–7.45 (m, 4H), 2.91–2.82
(m, 1H), 2.77–2.68 (m, 1H), 1.15 (t, 3H). 13C NMR (400 MHz,
chloroform-d): d = 5.65 ppm (CH3CH2-), 21.35 (CH3Ph-), 49.73
(-CH2S-), 142.89 (-CS-), 128.87 (Cp), 123.78 (Co), 130.90 (Cm)
(Supplementary material, S4 (a,b)).
1-methoxy-4-(methylsulfinyl)benzene: (4-methoxyphenyl)
(methyl)sulfane (308.4 mg, 2 mmol) was performed according to
the general procedure using nanoAmbSO3@H2TMPyP(HCOOH)2
(0.66 Â 10À3 mmol). Crude material was separated by TLC (1:4
ratio of ethyl acetate, n-hexane) to give Methoxy phenyl sulfoxide;
yield: 95%. 1H NMR (400 MHz, chloroform-d): d = 7.42 (d, 2H), 6.85
(d, 2H), 3.65 (s, 3H), 2.51 (s, 3H). 13C NMR (400 MHz, chloroform-
d): d = 161.18 (-CS-), 136.41(Cp)., 125.59 (Cm), 114.87 (Co)., 55.37
(CH3S-), 43.60 (OCH3Ph-) (Supplementary material, S5 (a,b)).
1-chloro-4-(methylsulfinyl)benzene: (4-Chlorophenyl)(methyl)
sulfane (317.2 mg, 2 mmol) was performed according to the
general
procedure
using
nanoAmbSO3@H2TMPyP(HCOOH)2
(0.66 Â 10À3 mmol). Crude material was separated by TLC (1:4 ratio
of ethyl acetate, n-hexane) to give 4-chlorophenyl phenyl sulfoxide;
yield: (90%). 1H NMR (400 MHz, chloroform-d): d = 7.57–7.55
(m, 2H), 7.47–7.45 (m, 2H), 2.69 (s, 3H). 13C NMR (400 MHz,
chloroform-d): d = 144.29 (-CS-) ppm, 136.91 (Cp), 129.65 (Cm),
124.90 (Co), 43.84 (CH3Ph-), (Supplementary material, S6 (a,b)).
1-(butylsulfinyl)butane: Dibutylsulfane (292.6 mg, 2 mmol)
was performed according to the general procedure using
nanoAmbSO3@H2TMPyP(HCOOH)2 (0.66 Â 10À3 mmol). Crude
material was separated by TLC (1:4 ratio of ethyl acetate,
n-hexane) to give dibutyl sulfoxide; yield: (80%). 1H NMR (400
MHz, chloroform-d): d = 2.72–2.59 (m, 4H), 1.78–1.70 (m, 4H),
1.55–1.40 (m, 4H), 0.97 (t, 6H). 13C NMR (400 MHz, chloroform-
d): d = 52.09 (-CS-) ppm, 24.89 (-CH2-), 22.10 (-CH2CH3),
13.71 (-CH3) (Supplementary material, S7 (a,b)).
Then, the immobilized porphyrin was protonated with an
excess amount of formic acid (beyond 1:2 M ratio of porphyrin
to acid) to prepare the core protonated porphyrin for 2 h. The
immobilization of porphyrin on the polymer was confirmed by dif-
fuse reflectance UV–vis and IR spectroscopy.
2.5. General oxidation procedure
Photooxidation of sulfides and DHN was performed in a double
walled cylindrical glass vessel (Supplementary material, S2). Water
was circulated through the outer jacket to maintain a constant
temperature. In a typical reaction photosensitizer (6.6 Â 10À4
mmol, 0.094 g) and sulfide (2 mmol) were added to 10 ml of the
desired solvent (acetonitrile, water or water/acetonitrile in 1:1
ratio) to obtain the 1:3000 M ratio of the catalyst to substrate. At
different time intervals, an aliquot of the solution was taken out
with a syringe and analyzed by GC. In the case of DHN, the catalyst
(0.0003 mmol) and substrate (0.003 mmol) were used. The
progress of the oxidation of DHN was monitored by UV–vis
spectroscopy; the oxidation of DHN was accompanied with the
decrease in the intensity of the absorption band at 301 nm
2.6. Singlet oxygen quantum yield determination
The U value of the photosensitizer was measured through
D
the reaction of the reactive oxygen species (ROS) with 1,3-
diphenylisobenzofuran (DPBF) as a quencher of singlet oxygen and
methylene blue (UD = 0.52 in water and acetonitrile) [47] as a refer-
ence photosensitizer. The equation UD
=
UsDtd  (ti  I
/
std tsitd  I)
proposed by Murata et al. was used to determine the U value. In this
D
(
(
e
e
= 7664 MÀ1 cmÀ1) and the increase of the band at 415 nm
equation, Ustd tsitd, Istd and I present the U of methylene blue, the
, ti,
D D
= 3811 MÀ1 cmÀ1). A 10 W white LED lamp was used as the light
rate of DPBF (8 Â 10À4 M) oxidation in the presence of the photosen-
sitizer and a 10 W red LED lamp, the rate of DPBF oxidation in the
presence of methylene blue, the number of photons absorbed by
source for the oxidation of DHN. In the case of sulfides, different
lamps (20 W blue, red and white LED lamps and sunlight were

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397
the standard solution and those absorbed by the photosensitizer,
3. Results and discussion
respectively [48].
3.1. Immobilization of porphyrin
The cationic porphyrin H2TMPyP was supported on the anionic
polymer by electrostatic interactions. The diffuse reflectance
UV–vis spectrum of nanoAmbSO3@H2TMPyP is shown in (Fig. 2).
The Soret and Q bands of the immobilized porphyrin appeared
at 435, 525, 558, 595 and 650 nm. The addition of formic acid to
the immobilized porphyrin led to change in the colour of the solid
from grey to green; diffuse reflectance UV–vis spectra of the green
solid (Fig. 2b, green curve) is in accord with the formation of por-
phyrin diacid. Fig. 2a (purple curve) demonstrates the UV–vis of
H2TMPyP in water with the Soret and Q bands at 420, 525, nm.
As seen in Fig. 2b (green curve), the Soret and Q bands of H2TMPyP
shift to longer wavelengths through protonation of porphyrin core
with HCOOH. Accordingly, the DR-UV–vis spectrum clearly shows
the immobilization of porphyrin on the polymer in the form of por-
phyrin diacid. Furthermore, the comparison of the IR spectra of
nanoAmbSO3@H2TMPyP and nanoAmbSO3@H2TMPyP(HCOOH)2
(Fig. 3) shows the appearance of a band at 1750 cmÀ1 that is
attributed to the presence of formic acid in the polymer structure.
Energy-dispersive X-ray spectroscopy (EDS) analyses and
elemental mapping were performed on nanoAmbSO3Na,
nanoAmbSO3@H2TMPyP and nanoAmbSO3@H2TMPyP(HCOOH)2
(Fig. 4). The results shown in Supplementary material, S8 demon-
strate uniform distribution of different elements on the solid
compound.
Fig. 2. (a) UV–vis spectra of H2TMPyP and the corresponding dication with formic
acid in water. (b) Diffuse reflectance UV–vis spectra of H2TMPyP immobilized on
nanoAmbSO3Na and its formic acid dication.
Also, the elemental analyses based on EDX measurements is in
accord with the formation of the respective compounds; the molar
ratio of oxygen to sulfur significantly increased from nanoAmbSO3-
Na to nanoAmbSO3@H2TMPyP(HCOOH)2. It is noteworthy that due
to the possible different hydrophilicity of nanoAmbSO3Na,
nanoAmbeSO3@H2TMPyP and nanoAmbSO3@H2TMPyP(HCOOH)2
there are some differences between the amounts of water
absorbed by these compounds.
3.2. DLS and SEM of nanostructures
Fig. 3. FT-IR spectra of nanoAmbSO3@H2TMPyP and nanoAmbSO3@H2TMPyP
(HCOOH)2 (KBr disk).
The formation of nanoAmbSO3Na and nanoAmbSO3@H2TMPyP
(HCOOH)2 was confirmed by dynamic light scattering (DLS) and
Fig. 4. EDS pattern and carbon, nitrogen, oxygen, sodium and sulfur elemental mapping captured for nanoAmbSO3@H2TMPyP(HCOOH)2.

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A. Heydari-turkmani, S. Zakavi / Journal of Catalysis 364 (2018) 394–405
Field-emission scanning electron microscopy (FESEM). As was
explained elsewhere [17], the polymer nanoparticles were pre-
pared by magnetically stirring overnight Amberlyst 15 beads in
water. In the present work, the acidic polymer was magnetically
stirred in a 1 M solution of NaOH. DLS measurements show a
particle size of 191 and 189.9 nm for nanoAmbSO3Na and
nanoAmbSO3@H2TMPyP(HCOOH)2, respectively with a homoge-
neous size distribution. Accordingly, the immobilization of por-
phyrin and the formation of porphyrin diacid had little or no
effects on the size of the polymer nanostructures. On the other
hand, FESEM images indicate a significantly smaller size (<50
nm) for nanoAmbSO3@H2TMPyP(HCOOH)2 (Fig. 5).
The nitrogen sorption isotherms of the solid catalysts (Fig. 6)
are in accord with the macroporous structure of the catalyst.
Also, significant decrease in BET pore volume (Vtotal) and surface
area (SBET) values was observed after the immobilization and
protonation of the porphyrin (Table 1). Thermal gravimetry analy-
sis (TGA) of nanoAmbSO3Na, nanoAmbSO3@H2TMPyP and
nanoAmbSO3@H2TMPyP(HCOOH)2 are shown in (Fig. 7). The
changes below ca. 100 °C show dehydration or other small
molecule loss. As seen from Fig. 7 and Supplementary material,
S9, the weight loss in this region is more pronounced in the case
of nanoAmbSO3Na and nanoAmbSO3@H2TMPyP. This observation
seems to be due to the larger amount of the adsorbed
water molecules in these structures compared to that of
nanoAmbSO3@H2TMPyP(HCOOH)2. TGA curves display the major
Fig. 6. N2 adsorption/desorption isotherms of nanoAmbSO3Na and the immobilized
porphyrin.
weight-loss stage above 450 °C. In the case of the three compounds
the weight loss occurs at ca. 450 °C. Also, comparable weight losses
are observed for nanoAmbSO3@H2TMPyP(HCOOH)2, nanoAmbSO3-
Na and nanoAmbSO3@H2TMPyP. Accordingly, the protonation of
porphyrin core had little effect on the thermal stability of the
immobilized porphyrin.
Fig. 5. FE-SEM (A and B) and DLS characterization of nanoAmbSO3Na and nanoAmbSO3@H2TMPyP(HCOOH)2.

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399
Table 1
reaction in the presence of nanoAmbSO3@H2TMPyP(HCOOH)2 gave
sulfoxide as the nearly exclusive product. This observation seems
to be due to the stabilization of the persulfoxide intermediate by
hydrogen bond formation with porphyrin diacid (vide infra) [48].
It is noteworthy that the lack of such hydrogen bonds leads to
the formation of a cyclic persulfoxide through the nucleophilic
attack of the peroxide residue on the sulfur atom of the persulfox-
ide intermediate. The migration of oxygen atom following this step
leads to the formation of sulfone as the product. Furthermore, sta-
bilization of the sulfoxide product caused by hydrogen bond for-
mation with hydrogen bond donors such as porphyrin diacid
prevents further oxidation of sulfoxide to sulfone [49]. The steric
hindrance due to the hydrogen bond donors attached to the sulfox-
ide product was also proposed to be involved in the increased sul-
foxide selectivity [50]. In accord with these observations,
conducting the oxidation of methyl phenyl sulfide in water led to
a significantly increased conversion values and reaction rates. In
other words, cooperative acid catalysis caused by the protic solvent
and porphyrin diacid is involved in the preference of water over
acetonitrile as the solvent for efficient oxidation of sulfides to sul-
foxides. It should be noted that the small size of water molecule
makes it a more efficient hydrogen bond donor than porphyrin dia-
cid. The oxidation of methyl phenyl sulfide was also studied in the
presence of nanoAmbSO3@H2TMPyP in water (entry 9). Herein,
methyl phenyl sulfoxide obtained as the sole product, but the cat-
alyst was subject to extensive oxidative degradation under the
reaction conditions. In a control experiment, ethanol was added
to the reaction mixture (entry 10) and a ca. 10% increase in the con-
version value was observed. This observation seems to be due to
the increased solubility of methyl phenyl sulfide in water caused
by dissolved ethanol. On the other hand, ethanol is expected to
be a better hydrogen bond donor [51] and therefore a stronger acid
catalyst than the former. This in turn would increase the rate of
photooxidation reaction (vide infra).
BET analysis of nanoAmbSO3Na and the immobilized catalyst.
a
b
Photosensitizer
SBET (m2 gÀ1
)
Vt (cm3 gÀ1
)
nanoAmbSO3Na
nanoAmbSO3@H2TMPyP
nanoAmbSO3@H2TMPyP(HCOOH)2
107
47
20
0.914
0.401
0.179
a
SBET is the specific surface area.
Vt is the total pore volume.
b
Fig. 7. TGA of nanoAmbSO3Na, nanoAmbSO3@H2TMPyP and nanoAmbSO3
@H2TMPyP(HCOOH)2 under nitrogen atmosphere at a heating rate of 10 K/min.
3.3. Photooxidation of alkyl aryl sulfides
NanoAmbSO3@H2TMPyP(HCOOH)2 has the potential to be used
as a red shifted heterogeneous photocatalyst for the oxidation of
organic compounds. In order to elucidate the influence of protona-
tion on the photocatalytic performance of nanoAmbSO3@H2TMPyP,
the oxidation of methyl phenyl sulfide was conducted in the pres-
ence of nanoAmbSO3Na, nanoAmbSO3@H2TMPyP and the formic
acid protonated counterpart (Table 2) under different reaction con-
ditions (Scheme 1).
3.4. Catalyst to substrate optimum molar ratio
The immobilized catalyst was employed for the oxidation of
sulfides. In a very recent paper [30] it was shown that under homo-
geneous conditions, the free base porphyrin has a decreased photo-
catalytic activity and photooxidative stability compared to those of
the corresponding dication. Also, the reaction time for a conversion
of 48% was 24 h that was much longer than that in the case of the
As seen from Table 2, the use of nanoAmbSO3@H2TMPyP led to
the formation of sulfoxides and sulfone in ca. 2:1 M ratio (sulfoxide
selectivity = 67%). Under the same conditions, conducting the
Table 2
Effect of catalyst to substrate molar ratio and solvent on the oxidation of methyl phenyl sulfide.a
d
Entry
Catalyst
Catalyst:sulfide
molar ratio
Solvent
Time
Conversionb (%)
Sulfoxide (Sulfone)
Selectivity(%)
TONc(TOF, hÀ1
)
1
2
3
4
5
6
7
8
nanoAmbSO3Na
nanoAmbSO3@H2TMPyP
0:1000
1:1000
1:1000
1:1000
1:1000
1:1000
1:2000
1:3000
1:3000
1:3000
1:3000
1:3000
CH3CN
CH3CN
CH3CN
H2O
H2O
H2O
H2O
H2O
H2O
H2O
3 h
3 h
3 h
1 h
2 h
3 h
2 h
2 h
2 h
2 h
3 h
3 h
N.R
N.R
67
N.R
25 (12)
32 (<1)
49 (<1)
91 (<1)
97 (<1)
60 (<1)
65 (<1)
44 (<1)
55 (<1)
45 (<1)
91 (<1)
370 (123)
320 (106)
490 (490)
910 (455)
970 (323)
1200 (600)
1950 (975)
1320 (660)
1650 (825)
1350 (450)
2730 (910)
nanoAmbSO3@H2TMPyP(HCOOH)2
nanoAmbSO3@H2TMPyP(HCOOH)2
nanoAmbSO3@H2TMPyP(HCOOH)2
nanoAmbSO3@H2TMPyP(HCOOH)2
nanoAmbSO3@H2TMPyP(HCOOH)2
nanoAmbSO3@H2TMPyP(HCOOH)2
nanoAmbSO3@H2TMPyPe
ꢁ100
ꢁ100
ꢁ100
ꢁ100
ꢁ100
ꢁ100
ꢁ100
ꢁ100
ꢁ100
ꢁ100
9
10
11
12
nanoAmbSO3@H2TMPyPf,g
nanoAmbSO3@H2TMPyP(HCOOH)2
nanoAmbSO3@H2TMPyP(HCOOH)2
H2O:CH3N
H2O
a
A 20 W white LED lamp was used as the light source.
All reactions were repeated three times, analyzed by GC and the average values were reported. The formation of sulfoxide as the main product was also confirmed by 1
b
H
NMR (Fig. 9).
c
TON demonstrates the number of moles of the product obtained per one mole of the catalyst.
TOF shows the turnover number of the reaction per time unit.
An oxidative degradation of ca. 50% was observed for the catalyst at the end of the first run.
For the photooxidation reaction performed in the presence of ethanol; catalyst and alcohol was used in 1:1 M ratio.
An oxidative degradation of ca. 70% was observed for the catalyst.
d
e
f
g

400
A. Heydari-turkmani, S. Zakavi / Journal of Catalysis 364 (2018) 394–405
Scheme1. The optimized conditions for aerobic sulfoxidation of methyl phenyl catalyzed by nanoAmbSO3@H2TMPyP(HCOOH)2.
dication. It was shown that the protonation of porphyrin core leads
to a remarkable increase in the photocatalytic activity and oxida-
tive stability of the porphyrin photosensitizer.
However, the absence of aryl group at the sulfur atom led to a
decrease in the reactivity of the sulfur atom. In the case of dibutyl
sulfide, sulfone was also obtained that was attributed to the acid
character of the aliphatic CH protons the formation of persulfoxide
led to an increase in the acidity of the CH bonds adjacent to the
SOOÀ group of the persulfoxide [38,49] (Table 3, entry 5). This in
turn leads to an increase in the electron density at the carbon atom
adjacent to the sulfur atom and consequently leads to increased
reactivity of the sulfur group towards overoxidation to the corre-
sponding sulfone compound. The reaction was also performed
using air as the oxygen source (Table 4) and a decrease in the rate
of the oxidation reaction was observed; the reaction time for
nearly complete conversion of methyl phenyl sulfide to the
corresponding sulfoxide increased from 3 to 5 h.
It is noteworthy that the use of air as the molecular oxygen
source lowers the cost of the overall process and is in line with
the green chemistry principles. The risk of ignition will be
considerably increased by even a few percent oxygen enrichment
of the atmosphere [52]. As was expected, no reaction was observed
in under nitrogen atmosphere.
Previous studies showed that the catalyst to substrate molar
ratio and solvent play crucial roles in the efficiency of the photoox-
idation reactions conducted in the presence of porphyrinic photo-
sensitizers. In order to study the effects of these parameters, the
oxidation of methyl phenyl sulfide was performed under different
conditions (Table 2 and Fig. 8). As seen from Table 2, the use of
water as solvent led a significant increase in the conversion value
in comparison with that achieved in acetonitrile with no effect
on product selectivity. Indeed, very little sulfone product was
obtained for reaction times less than 3 h. Also, the use of 1:1 mix-
ture of water and acetonitrile caused a two times increase in the
conversion (Table 2, entry 11). The oxidation reaction was also
studied using catalyst and methyl phenyl sulfide in 1:1000 to
1:5000 M ratios. Although the maximum conversion was observed
in the case of the 1:1000 ratio, the use of 1:3000 M ratio led to
much higher turnover number (TON) and frequencies (TOF) of
the reaction. Accordingly, the 1:3000 M ratio (entry 12) and a reac-
tion time of 3 h were used as the optimum reaction conditions. It is
noteworthy that no catalyst degradation or leaching was observed
within a reaction time of 3 h. In order to find the effects of the reac-
tion time on the product selectivity, the oxidation of methyl phenyl
sulfide was also conducted for a reaction time of 6 h.
The immobilized catalyst may be recycled and reused at least 5
times without any detectable loss in the catalytic activity. Further-
more, no catalyst degradation or leaching was observed in a time
interval of 3–5 h. The degree of degradation of the porphyrinic cat-
alyst was only ca. 40% after the fifth cycle (Fig. 10 and Supplemen-
tary material, S10).
The oxidation of different sulfides was studied to find the influ-
ence of the groups attached to sulfur atom (Table 3). It is observed
that the presence of Cl and OCH3 group at the phenyl group has
little or no effect on the reactivity of sulfide. It should be noted
that –OCH3 is an efficient electron-releasing group to phenyl ring
and in electrophilic substitution of benzene ring is an activating
group directing to the ortho/para positions.
3.5. Effect of the light source
The oxidation of methyl phenyl sulfide was performed using
different light sources (see Supplementary material, S11 for the
Fig. 9. 1H NMR spectrum of the reaction mixture in CDCl3 at different time
intervals; after the required time, the organic phase was extracted with ethyl
acetate, dried with rotary evaporator and dissolved in CDCl3.
Fig. 8. The formation of methyl phenyl sulfoxide as the main product upon the
oxidation of methyl phenyl sulfide.

A. Heydari-turkmani, S. Zakavi / Journal of Catalysis 364 (2018) 394–405
401
Table 3
Photooxidation of different Sulfides.a
Entry
Substrate
Product
Conversionb
[Selectivity] (%)
Photostability (%)
>95
TON
TOF (hÀ1
)
1
91
2730
910
[100]
2
3
90
[100]
>95
>95
2700
2700
900
900
90
[100]
4
5
95
[100]
>95
>95
2850
2400
950
800
80
[90]
15
[10]
a
Catalyst and sulfides were used in a 1:3000 M ratio in water.
GC yield; all products were characterized by 1H NMR and 13C NMR (Supplementary material, S3–S7).
b
Table 5
Table 4
Effect of light source on the photooxidation of methyl phenyl sulfide under the
Photocatalytic oxidation of methyl phenyl sulfide under nitrogen, air and oxygen
optimum conditions.a
atomosphere.a
TON (TOF, hÀ1
)
Light source
Conversion (%)
Selectivity (%)
TON [TOF (hÀ1)]
Atmosphere
Time
Conversion
[Sulfoxide selectivity] (%)
White LED (20 W)
Blue LED (20 W)
Red LED (20 W)
Sunlight
91
46
20
88
100
100
100
100
2730 [910]
1380 [460]
600 (200)
2640 (880)
O2
Air
Air
N2
3 h
3 h
5 h
3 h
91 [100]
47 [100]
93 [100]
Trace
2730 (910)
1410 (470)
2790 (465)
–
a
See the footnotes of Tables 2 and 3 for more details.
a
See the footnotes of Tables 2 and 3 for more details.
with the higher intensity of the absorption band of porphyrin dia-
cid in the Soret band region. However, there is no large difference
between the conversion values achieved in the presence if the blue
and red LED lamps. The latter shows that the excitation at the Soret
and Q bands region are both involved in the photosensitizing
process.
3.6. Proposed mechanism
The oxidation of organic substrates in the presence of por-
phyrinic photosensitizers may proceed through a singlet oxygen
mediated or charge transfer mechanism [53]. Control experiments
(Supplementary material, S12a) confirm that no reaction occurs in
the absence of the photocatalyst. The oxidation of sulfides usually
occurs by the intermediacy of a persulfoxide species (Scheme 2)
[49]. The increased sulfoxides selectivity in the presence of por-
phyrin diacid in comparison with that observed in the case of the
immobilized free base porphyrin is probably due to the acid catal-
ysis by porphyrin diacid. Furthermore, the significantly higher con-
version value in water compared to that in acetonitrile seems to be
due to the hydrogen bond stabilized persulfoxide intermediate as
well as the increased singlet oxygen quantum yield of the photo-
sensitizer in water relative to that in acetonitrile (vide infra);
herein, porphyrin diacid and water cooperatively stabilize the
intermediate and direct the reaction towards the formation of per-
sulfoxide as the sole product. The addition of furfuryl alcohol as a
Fig. 10. Reusability of nanoAmbSO3@H2TMPyP(HCOOH)2 under the optimized
reaction conditions.
emission spectra of the light sources) and under Sunlight irradia-
tion (Table 5). It is observed that in spite of large changes in the
conversion value, the selectivity of the reaction towards sulfoxide
was completely independent of the light source. The higher effi-
ciency of the blue LED lamps versus the red LED one is in accord

402
A. Heydari-turkmani, S. Zakavi / Journal of Catalysis 364 (2018) 394–405
Scheme 2. Proposed mechanism for photooxidation of sulfides; (A) acid catalysis by porphyrin diacid and water; (B) sulfone formation.
singlet oxygen quencher led to a significant decrease in the conver-
sion of methyl phenyl sulfide to the oxidation product [54]. Also,
the irradiation of catalyst with a red LED lamp in presence of
1,3-diphenylisobenzofuran (DPBF) in water led to fast decay of
the band due to DPBF at ca. 420 nm (Fig. 11). It is noteworthy that
the later is
a well-known singlet oxygen quencher [55,56].
However, a conversion of 6% was achieved under this condition.
On the other hand, running the oxidation reaction in the presence
of 1,4-benzoquinone as a scavenger of superoxide anion radical led
to a ca. 7% decrease in the conversion (Supplementary material,
S12b). Accordingly, singlet oxygen is proposed to be the key reac-
tive intermediate in this catalytic system. However, the formation
of superoxide anion radical is also involved as the minor reaction
pathway. Furthermore, hot filtration test of the reaction mixture
(Fig. 12) showed that the oxidation reaction was terminated com-
pletely by catalyst removal. Also, the results confirmed the absence
of catalyst leaching during the oxidation reaction.
Fig. 12. The effect of removal of the catalyst (hot filtration) after 1 h on the progress
of the oxidation reaction.
Fig. 11. The change in the UV–vis spectrum of DPBF (a) in the presence of nanoAmbSO3@H2TMPyP(HCOOH)2 in water using a 10 W red LED lamp as the light source. Fig. b
shows the changes at the kmax of DPBF vs. time.

A. Heydari-turkmani, S. Zakavi / Journal of Catalysis 364 (2018) 394–405
403
3.7. Singlet oxygen quantum yield
oxygen necessitates the entrance of triplet oxygen into the por-
phyrin sites bounded to the polymeric matrix. On the other hand,
the exit of singlet oxygen from the polymeric matrix prevents
the quenching of singlet oxygen before reaction with DPBF used
for the measurement of UD value.
The singlet oxygen quantum yield (UD) of photosensitizers, a
quantitative measurement of the efficiency of photosensitizers to
use the absorbed light to convert triplet molecular oxygen into sin-
glet oxygen (1Dg) species, is as an important parameter to evaluate
the efficiency of novel photocatalysts [57]. The UD value of
nanoAmbSO3@H2TMPyP(HCOOH)2 was measured by using DPBF
as the quencher of singlet oxygen and methylene blue (UD = 0.52
in water and acetonitrile) [47] as a reference photosensitizer. The
experimental details and equations used for the measurement of
UD value were described elsewhere [58]. The Kinetics curves used
for the UD measurements and the changes in the absorption spec-
trum of DPBF in the presence of the photosensitizers in water and
acetonitrile are shown in Fig. 13d; a ca. 5 fold increase in the
observed rate constant of BPBF photooxidation is observed from
acetonitrile to water. In spite of the fact that singlet oxygen has a
3.8. Oxidation of 1,5-dihydroxynaphthalene
Oxidation of DHN to Juglone was studied in acetonitrile, water
and a 1:1 mixture of water and acetonitrile (Fig. 14, Table 6 and
Supplementary material, S13). As seen from Table 6, the oxidation
reaction completed in 15 min in water as solvent. The use of the
1:1 ratio of acetonitrile and water increased the reaction time to
45 min. Also, conducting the oxidation reaction in neat acetonitrile
led to a remarkable decreased in the rate of the reaction. Again, the
higher efficiency of the photooxidation reaction is in accord with
higher UD value in water compared to that in acetonitrile. As
was observed in the case of sulfides, the use of water instead of
acetonitrile led to a remarkable decrease in the reaction rate
from 180 to 15 min. The oxidation of DHN to Juglone was sug-
gested to occur through a hydroperoxide intermediate (Scheme 3)
[43,62,63]. Accordingly, the hydrogen bond formation ability of
water which stabilizes the hydroperoxide intermediate seems to
be also involved in higher conversion values observed in water.
Furthermore, previous studies revealed the increased reaction
rates under weak acidic conditions [62]. It is noteworthy that the
presence of porphyrin diacid as the photosensitizer leads to weak
acid conditions in this catalytic system.
much shorter lifetime in water (2
most organic solvents (30, 69 and 75
l
s) in comparison with that in
ls in acetonitrile, toluene
and dichloromethane, respectively) [59,60], a UD value of 0.41
was measured in water that is significantly greater than that in
neat acetonitrile (UD = 0.08) which correlates with higher effi-
ciency of the photosensitizer in water (Table 2). The increased
UD value of the polymer supported porphyrin in water seems to
be due to the polymer swelling in water caused by the hydrophilic-
ity of Amb [61] which makes the immobilized porphyrin more
accessible to triplet oxygen. Furthermore, polymer swelling can
facilitate the entrance and exit of oxygen molecules into and out
of the polymer phase. It is noteworthy that the formation of singlet
Fig. 13. The change in the UV–vis spectrum of DPBF upon irradiation with a 10 W red LED lamp in the presence of nanoAmbSO3@H2TMPyP(HCOOH)2 in water (a) and
acetonitrile (b). C and d show the rate of DPBF photooxidation in water and acetonitrile, respectively.

404
A. Heydari-turkmani, S. Zakavi / Journal of Catalysis 364 (2018) 394–405
acid in solid state. Furthermore, in contrast to the dication of por-
phyrins with weak acids, the immobilized porphyrin diacid was
highly stable toward decomposition to the fragments upon expo-
sure to different solvents including acetonitrile, ethyl acetate,
water and acetonitrile/water mixed solvent. NanoAmbSO3@H2-
TMPyP(HCOOH)2 was used as a novel photosensitizer for large
scale aerobic photooxidation of sulfides and DHN under heteroge-
neous conditions in water, acetonitrile and water/acetonitrile at
room temperature. The remarkable increase in the efficiency of
nanoAmbSO3@H2TMPyP(HCOOH)2 for the oxidation of sulfides in
water is probably due to the ability of the protic solvent and pro-
tonated porphyrin to stabilize the persulfoxide intermediate
through hydrogen bond formation with the persulfoxide interme-
diate. Furthermore, water induced swelling of the polymer support
may be also involved in the increased catalytic performance of the
catalyst. Interestingly, the immobilized porphyrin diacid showed a
remarkable oxidative stability under reaction conditions. Also, it
was recovered and reused at least 5 times in the oxidation of sul-
fides to achieve complete and highly selective oxidation of the
organic substrates to their corresponding sulfoxide within 3 h. It
is noteworthy that a maximum porphyrin degradation of ca. 40%
was observed after the fifth cycle. On the other hand, the oxidation
of DHN completed within a reaction time of 15 min in water. Run-
ning the oxidation of sulfides in the presence of furfurylalcohol and
1,4-benzoquinone showed the involvement of singlet oxygen and
superoxide anion radical as the major and minor reactive oxidant
species, respectively.
Fig. 14. Photooxidation of DHN in water catalyzed by nanoAmbSO3@H2TMPyP
(HCOOH)2 as the photosensitizer (PS). The formation of Juglone as the product was
also confirmed by 1H NMR (Supplementary material, S14).
Table 6
Photooxidation of DHN.a,b
c
Entry
Solvent
Time (min)
U
kobs (sÀ1
)
Yieldd (%)
D
1
2
3
4
CH3CN
CH3CN: H2O
H2O
180
45
0.08
–
0.41
0.41
0.0076
–
0.039
0.039
84
98
98
98
15
H2O
90e
a
b
c
Catalyst (0.0003 mmol) and DHN (0.003 mmol) were used in 1:10 M ratio.
A white LED lamp (10 W) was used as the light source.
Acknowledgements
The observed rate constants for the oxidation of DPBF (Fig. 13d).
d
On the basis of the absorption change at the kmax (427 nm,
e )
= 3811 cmÀ1 MÀ1
Financial Support of this work by the Institute for Advanced
Studies in Basic Sciences (IASBS) is gratefully acknowledged.
of the product.
e
Large scale conditions using 0.0003 mmol catalyst and 0.3 mmol DHN.
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2018.06.011.
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