Synthesis of hierarchical mesoporous Mn-MFi zeolite nanoparticles: A unique architecture of heterogeneous catalyst for the aerobic oxidation of thiols to disulfides

Article abstract of DOI:10.1002/cctc.201300850

An efficient procedure for aerobic oxidation of thiols to disulfides catalyzed by new self-assembled hierarchical mesoporous Mn-MFI in the presence of air under solvent-free conditions as well as in aqueous medium is reported. The mesoporosity and Mn⁴⁺ loading, together with a highly crystalline microporous pore wall structure of the MFI framework were achieved through a newly designed hydrothermal process. This hydrothermal approach leads to hierarchical self-assembled mesoporous zeolite structures through isomorphous substitution of Si by Mn and Al. It is shown that Mn-containing mesoporous zeolites are capable to form disulfide bonds from thiols in the presence of air. The zeolitic materials were characterized by XRD, field-emission scanning electron microscopy, high-resolution TEM, X-ray photoelectron spectroscopy, ²⁹Si NMR, ²⁷Al NMR, and EPR spectroscopy, as well as AAS analysis and N₂ sorption studies. N₂ sorption analysis revealed high surface areas and narrow pore size distributions (1.2-6.0 nm) for different samples. The mesoporous Mn-ZSM-5 acted as an efficient heterogeneous catalyst with maximum catalytic activity in the benzenethiol conversion to diphenyldithiol. Copyright

Full text of DOI:10.1002/cctc.201300850

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DOI: 10.1002/cctc.201300850
Synthesis of Hierarchical Mesoporous Mn–MFI Zeolite
Nanoparticles: A Unique Architecture of Heterogeneous
Catalyst for the Aerobic Oxidation of Thiols to Disulfides
Astam K. Patra,[a] Arghya Dutta,[a] Malay Pramanik,[a] Mahasweta Nandi,[b] Hiroshi Uyama,[c]
and Asim Bhaumik*[a]
An efficient procedure for aerobic oxidation of thiols to disul-
fides catalyzed by new self-assembled hierarchical mesoporous
Mn–MFI in the presence of air under solvent-free conditions as
well as in aqueous medium is reported. The mesoporosity and
Mn4+ loading, together with a highly crystalline microporous
pore wall structure of the MFI framework were achieved
through a newly designed hydrothermal process. This hydro-
thermal approach leads to hierarchical self-assembled mesopo-
rous zeolite structures through isomorphous substitution of Si
by Mn and Al. It is shown that Mn-containing mesoporous zeo-
lites are capable to form disulfide bonds from thiols in the
presence of air. The zeolitic materials were characterized by
XRD, field-emission scanning electron microscopy, high-resolu-
tion TEM, X-ray photoelectron spectroscopy, 29Si NMR,
27Al NMR, and EPR spectroscopy, as well as AAS analysis and N2
sorption studies. N2 sorption analysis revealed high surface
areas and narrow pore size distributions (1.2–6.0 nm) for differ-
ent samples. The mesoporous Mn–ZSM-5 acted as an efficient
heterogeneous catalyst with maximum catalytic activity in the
benzenethiol conversion to diphenyldithiol.
Introduction
Disulfides are readily formed through covalent cross-linking
and these are abundant in numerous biological systems avail-
able in nature.[1] They are present in numerous native proteins
and they are involved in the stabilization of the folded form of
proteins. For many proteins, disulfide bridges are the most im-
portant requirement for having their proper biological func-
tion.[2] In the same biological context, the redox state of these
sulfide bonds has evolved into a signaling element for cells be-
cause they can be reversibly reduced and re-oxidized[3] and
thus can control many biological metabolisms. Cebollada
et al.[4] observed disulfide-bond isomerization in a single pro-
tein by single-molecule force spectroscopy. Reagents such as
cerium(IV) salts, chromium peroxide, permanganates, transition
metal oxides, ferric chloride, sodium chlorite, nitric oxide, dini-
trogen tetroxide copper nitrate complex, Cu nanoparticles, and
halogens among others have been utilized for oxidation of
thiols to disulfides.[5] However, with the increasing environmen-
tal concern, chemists have developed different heterogeneous
catalysts with molecular oxygen as the primary oxidant. Re-
cently, Corma and co-workers[6] reported Au–CeO2 as an effi-
cient catalyst for oxidizing thiols to disulfides with air in short
reaction time. However, the metal oxide supported gold nano-
particles exhibit leaching problems and thus these materials
usually face difficulties in recycling. Thus it was of our interest
to develop a mild and reusable catalyst for synthesizing disul-
fides from thiols.
ZSM-5-type zeolites are widely used in the petrochemicals
and fine-chemical industry for a series of catalytic reactions in-
volving cracking, alkylation, acylation, shape selective isomeri-
zation, radioactive ion sequestration, separation/purification
systems (for water softening), and many more.[7] Hydrothermal
stability, well-defined porosity, high BET surface area and the
ability to stabilize reactive metal ions in their microporous
framework[8] have made them unique in heterogeneous cataly-
sis. However, these zeolites with pore diameters of less than
1 nm have serious limitation for the diffusion of large molecule
(i.e., mass transport of reactants and products) in and out of
the pore structure and this affects the properties of zeolites.
Owing to this major drawback associated with the zeolites, or-
dered mesoporous materials have attracted widespread re-
search for over a decade.[9] However, these materials do not
have the crystalline pore walls, which renders poor mechanical
and chemical stability.
[a] Dr. A. K. Patra, Dr. A. Dutta, M. Pramanik, Prof. A. Bhaumik
Department of Materials Science
Indian Association for the Cultivation of Science
2A & B, Raja S.C. Mullick Road, Jadavpur, Kolkata-700032 (India)
Fax: (+91)33-2473-2805
E-mail: msab@iacs.res.in
[b] Dr. M. Nandi
Integrated Science Education and Research Centre
Siksha Bhavana, Visva-Bharati, Santiniketan-731235 (India)
[c] Prof. H. Uyama
Department of Applied Chemistry
Graduate School of Engineering
Osaka University
These problems can be overcome by preparing porous inor-
ganic solids with high surface areas and pores in the meso-
scopic regime (ꢀ2–50 nm) bearing the zeolitic frameworks. In
the literature, several strategies are directed to synthesize
Yamadaoka 2–1, Suita 565–0871 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300850.
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mesoporous zeolites, such as the selective etching of presyn-
thesized zeolites to create mesopores in zeolite crystals with
acids such as HF or alkali. Different types of hard templates
such as mesoporous carbon, polystyrene spheres, carbon parti-
cles, are also used and nanosized zeolite crystallites are assem-
bled in the presence of soft templates, by which the meso-
pores are formed among the crystallites.[10] Tetrapropylammo-
nium hydroxide (TPAOH) is commonly employed as a template
for the synthesis of ZSM-5 zeolite. Pꢁrez-Ramꢂrez et al. have
shown that postsynthesis demetalation treatments often pro-
duce high-quality hierarchically mesoporous zeolitic materials
in terms of external surface, crystallinity, morphology, and
meso/microporosity.[11] In this context, Zhao and co-workers[12]
fabricated ordered porous zeolite materials by self-assembly of
zeolite nanocrystals. Tsapatsis and co-workers[13] established
a method for synthesizing zeolite nanocrystals through sponta-
neous hydrolysis of tetraethylorthosilicate in aqueous solutions
of TPAOH. These molecular-sieve crystals serve as a model for
the self-assembly of porous inorganic materials in the presence
of organic structure-directing agents. However, in this case the
reaction time is too long. Recently, Schwieger and co-work-
ers[14] synthesized mesoporous FAU-type zeolite by the self-as-
sembly of nanosheets. In this context, we attempted to synthe-
size self-assembled mesoporous ZSM-5 zeolite nanocrystals
through a new and simple synthetic pathway, which involves
two steps of hydrothermal treatment. In the first step, TPAOH
and citric acid/tartaric acid act as a scaffold, forming the self-
assembled mesopore zeolite nanoparticles, and in the second
step, steam-assisted TPAOH forms crystalline structures in the
presence of a NaClO4 promoter. The presence of mesopores
could be helpful to overcome the diffusion limitation of micro-
porous zeolite.
Figure 1. Small-angle XRD patterns of the samples a) MZ-1R, b) MZ-1RC,
c) MZ-2R and d) MZ-2RC.
broad peaks are consistent with the particle center to particle
center correlation lengths for all samples.[16] During calcination
of these nanoparticles the d-spacing decreases. This observa-
tion follows that of the conventional surfactant–templated
mesoporous materials, on which contraction of the pore wall
as well as d-spacings occurs during the removal of the tem-
plate molecules.[17]
The wide-angle XRD patterns of the Mn–ZSM-5 zeolite are
shown in Figure 2A. If the inorganic silica precursor hydrolyzed
to form nanoparticles at pH 6, the particles are amorphous
(Figure 2A, sample MZ-1). After the stream treatment in the
presence of NaClO4 at pH 12, the nanoparticles become crystal-
line (Figure 2A, sample MZ-1R). Here, NaClO4 acts as promoter
to enhance the crystallization of the silicate framework.[18] The
highly crystalline peaks for samples MZ-1R and MZ-1RC were
compared with the XRD pattern found in the literature, which
suggested that the materials have the characteristics of the
Herein, we report a new hydrothermal method for designing
the hierarchical self-assembled mesoporous–microporous Mn–
MFI zeolite and its use as a reusable heterogeneous catalyst
for aerobic oxidation of thiols to disulfides in the presence of
air as the oxidizing agent. However, each disulfide bond forma-
tion requires the removal of two electrons either by a small
molecular species or by a catalyst.[13] In the biological context,
disulfide bond formation in proteins is performed with molecu-
lar oxygen as the electron acceptor. Thus, oxidation of thiols
using air or oxygen is of great importance because of their sig-
nificance in converting thiols into industrially important chemi-
cals having biomedical applications.
Results and Discussion
Mesophase and materials characterization
Small-angle powder XRD patterns of the as-synthesized and
calcined self-assembled mesoporous–microporous Mn–ZSM-5
and ZSM-5 samples are shown in Figure 1. In all cases, a single
broad diffraction peak at a small 2q value is observed, indicat-
ing the formation of a weak mesophase.[15] No distinctive
higher-order diffraction peaks were observed for the all sam-
ples. As-synthesized and calcined mesostructured samples of
MZ-1R exhibit peaks at 1.44 and 1.308, respectively. These
Figure 2. A) Wide-angle XRD patterns of MZ-1, MZ-1R, and MZ-1RC. B) Wide
angle XRD patterns of MZ-2R, MZ-2RC, MZ-3C, and MZ-4C.
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MFI zeolite ZSM-5 structure.[19] These typical ZSM-5 zeolites are
synthesized in the presence of tetrapropylammonium hydrox-
ide, which is a structure-directing agent for the MFI zeolites as
well as for maintaining the pH of the synthesis gel. The tetra-
propylammonium cation can have suitable interaction with the
zeolitic frameworks because of its geometric shape, size, and
charge distribution. The mesoporous ZSM-5 materials are also
crystalline in nature and have the same crystallographic fea-
tures, as shown in Figure 2B. Microporous MZ-3C and MZ-4C
samples are also crystalline with ZSM-5 crystallographic feature
are shown in Figure 2B.
seen in all the high-resolution TEM images. A close view of
these nanoparticles is presented in Figure 3d. From this image,
an average nanoparticle dimension of approximately 6 nm can
be suggested for this sample. An even closer view (Figure 3e)
of this sample reveals the periodic arrangement of micropores
in the Mn–ZSM-5 zeolite structure, templated by tetrapropy-
lammonium hydroxide.[1l] Lattice fringes corresponding to the
crystalline nanoparticles are relatively clearly seen in Figure 3 f.
Figure 3g–i represent the high-resolution images of ZSM-5
nanoparticles. Here the nanoparticles are clearly seen and the
average particle size is approximately 6 nm. From the low-reso-
lution images (Figure 3a–c and g–i), the self-assembly of nano-
particles with pores of dimension of approximately 4–5 nm
(white spots in Figure 3a–c and g–i) for the Mn–ZSM-5 and
ZSM-5 samples are suggested.
Nanostructure and morphology analysis
TEM images of representative mesoporous Mn–ZSM-5 and
ZSM-5 zeolites are shown in Figure 3. The images in Fig-
ure 3a–c are the respective high-resolution TEM images of the
Mn–ZSM-5 samples. The as-synthesized materials (Figure 3a,
MZ-1), steam-assisted materials (Figure 3b, MZ-1R), and cal-
cined materials (Figure 3c, MZ-1RC) are composed of self-as-
sembled zeolite nanoparticles. The nanoparticles are clearly
Further evidence in support of the self-assembled zeolite
nanoparticles in the samples prepared through the two-step
hydrothermal process is obtained from their respective field-
emission scanning electron microscopy (FESEM) images. In
Figure 4, FESEM images of calcined Mn-ZSM-5, ZSM-5, and mi-
croporous zeolite samples are shown. Uniform and homogene-
ous spherical nanoparticles of
approximately 5–6 nm in size are
observed for both samples pre-
pared by the two-step hydro-
thermal method. For the sam-
ples MZ-1RC and MZ-2RC, the
particles show hierarchical spher-
ical crystal morphology. The self-
assembly of these inorganic zeo-
lite nanoparticles are of practical
significance for the fabrication of
hierarchically porous structure.
On the other hand the micropo-
rous MZ-3C and MZ-4C zeolite
samples have a particular shape
(coffin-like). Edges of the coffin-
like particles are sharper than
MZ-1RC nanoparticle as a result
of the high synthesis tempera-
ture, suggesting that at high
temperature the crystallization is
favored.
Mesopore and micropore anal-
ysis by N2 adsorption
N2 adsorption–desorption iso-
therms of self-assembled meso-
porous and microporous zeolite
samples are shown in Figure 5
and Figure S1. The isotherms for
MZ-1R, MZ-1RC, MZ-2R, and MZ-
2RC could be classified as
Figure 3. TEM images of a) MZ-1, b) MZ-1R, and c) MZ-1RC, showing the self-assemble structure of nanoparticles
of Mn-ZSM-5 at different synthesis steps; d) close-up of part c; MZ-1RC, showing e) its micropore arrangement
and f) the crystalline lattice fringes; g) MZ-2, h) MZ-2R, and i) MZ-2RC, showing the self-assemble structure of
nanoparticles of ZSM-5 materials at different synthesis steps.
type IV characteristic of mesopo-
rous materials.[20] In these iso-
therms, between relative pres-
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Figure 4. FESEM images of hierarchical nanoparticles a) MZ-1RC, b) MZ-2RC,
c) MZ-3C, and d) MZ-4C.
Figure 5. A) N2 adsorption (full symbols)–desorption (empty symbols) iso-
therms of a) MZ-1RC, b) MZ-2RC, c) MZ-3C, and d) MZ-4C. B) Respective pore
size distributions using NLDFT model. w=non-local density functional
theory.
sures P/P0 of 0.01–0.40, the adsorption amount grad-
ually increases for both zeolite samples, whereas the
Table 1. Physicochemical properties of self-assembled mesoporous–microporous zeo-
lite and microporous zeolite samples.
microporous zeolites (MZ-3C and MZ-4C) become sa-
turated beyond P/P0 >0.04. The BET surface area for
as-synthesized MZ-1R and MZ-2R are 110.7,
104.9 m2 gꢁ1 and the respective pore volumes are
0.176, 0.166 cm3 gꢁ1 respectively. The BET surface
areas of the calcined MZ-1RC, MZ-2RC, MZ-3C, and
MZ-4C materials are 517.6, 456.6, 338.1, and
332.0 m2 gꢁ1 and their pore volumes 0.321, 0.249,
0.172, and 0.155 cm3 gꢁ1 respectively. All the data are
summarized in Table 1. From Table 1, it is seen that
the porosity of the as-synthesized MZ-1R is mainly
owing to interparticle porosity. The mesopore
volume (0.165 cm3 gꢁ1) stems from the interparticle
porosity that is dominant over the micropore struc-
ture. However, if we calcined the zeolite to remove
the TPAOH template, the surface area and the micro-
pore volume increased as a result of generation of
[c]
[d]
[e]
[f]
[g]
[h]
Sample
Si/Mn
[a] [b]
SBET
[m2 gꢁ1
Sext
Vtot
Vmi
Vm
Dpw
]
[m2 gꢁ1
]
[cm3 gꢁ1
]
[cm3 gꢁ1
]
[cm3 gꢁ1
]
[nm]
MZ-1R
MZ-1RC
MZ-2R[i]
MZ-2RC[i] 20 22.9 456.6
MZ-3RC
MZ-4RC
20 23.2 110.7
20 23.4 517.6
20 22.8 104.9
108.0
143.0
102.0
55.7
43.0
15.6
0.176
0.321
0.166
0.249
0.172
0.155
0.011
0.157
0.010
0.143
0.124
0.133
0.165
0.164
0.156
0.106
0.048
0.022
6.00
4.60
5.97
2.60
1.20
1.18
20 22.1 338.1
332.0
–
–
[a] Si/Mn ratio used in the reaction mixture. [b] Si/Mn ratio measured by AAS analysis
of the respective products. [c] BET specific surface area. [d] External surface area, mea-
sured by t-plot analysis. [e] Total pore volume, derived at P/P0 =0.98. [f] Microporous
volume, determined by the t-plot analysis. [g] Mesopore volume, calculated as the dif-
ference between Vtot and Vmi. [h] Pore width, calculated by nonlocal density functional
theory (NLDFT) method. [i] Si/Al ratio used in the reaction mixture.
micropores in the framework. This result was also supported
by adsorption–desorption studies on ZSM-5 (MZ-2R and MZ-
2RC). The characteristics of MZ-3C and MZ-4C materials fol-
lowed those of conventional microporous zeolite materials.
Pore size distributions of these samples obtained by employ-
ing NLDFT model calculations (with N2 adsorption on silica as
a reference)[21] suggested that as-synthesized MZ-1R materials
have an average pore width of 6 nm, whereas the calcined
MZ-1RC have an average pore width of 4.6 nm. The N2 sorp-
tion studies on the ZSM-5 materials (MZ-2R and MZ-2RC) re-
vealed similar features. In contrast, the microporous MZ-3C
and MZ-4C materials exhibited average pore widths of 1.2 and
1.18 nm, respectively. The pore widths obtained from this N2
sorption analyses agree well with the values obtained from
TEM image analysis performed independently.
Framework characterizations: XPS, MAS NMR, and EPR
analyses
To understand the surface composition and valence states of
the elements present in the nanomaterials, X-ray photoelec-
tron spectroscopy (XPS) is one of the most important charac-
teristics tools. XPS provides important information about the
surface electronic structure as well as valence state of the ele-
ments present in the materials. The XPS result of the hierarchi-
cal Mn–ZSM-5 (MZ-1RC) nanoparticles is shown in Figure 6.
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in this XPS profile. In the high-resolution Mn spectrum (Fig-
ure 6b), two peaks at binding energies of 641.6 and 653.2 eV
are observed for Mn 2p3/2, Mn 2p1/2, respectively. This is charac-
teristic of Mn4+ in Mn–ZSM-5.[23] The high-resolution XP spectra
of O 1s and Si 2p1/2 are shown in Figure 6c and d.[24] Further-
more, on the basis of the quantification of Mn, Si, and O peaks
by CasaXPS software, the atomic ratio of Si and Mn is approxi-
mately 22.65, which is supported by the AAS analysis results.
We analyzed the solid-state by 29Si and 27Al magic-angle
spinning (MAS) NMR spectroscopy to understand the chemical
environment of these nuclei. The 29Si NMR and 27Al NMR spec-
tra of different mesoporous and microporous zeolites are
given in Figure 7 and 8, respectively. In Figure 7, the 29Si NMR
spectrum of the MZ-4C sample reveals only one peak at
Figure 7. 29Si NMR spectra of a) MZ-4C, b) MZ-1RC, and c) MZ-2RC.
Figure 8. 27Al NMR spectrum of MZ-2RC, showing octahedral (Oh) and tetra-
hedral (Td) centers.
ꢁ116.7 ppm characteristic of Q4 [Si(OSi)4] silica with equivalent
silicon atoms indicating that the materials is composed of only
SiO2. In contrast, sample MZ-1RC exhibits three maxima at
ꢁ115.1, ꢁ109.2, and 103.4 ppm characteristic of Q4, Q3, and Q2
silica [Si(OSi)n(OH)4ꢁn], respectively, with nonequivalent silicon
atoms indicating isomorphous substitution of silicon by man-
ganese(IV) in the framework. The MZ-2RC sample exhibits
three maxima at ꢁ115.2, ꢁ109.2, and ꢁ103.2 characteristic of
Q4, Q3, and Q2 silica [Si(OSi)n(OH)4ꢁn] with nonequivalent silicon
atoms caused by aluminum(III) substitution in the frame-
work.[25] In the 27Al MAS NMR spectrum (Figure 8), the peaks at
Figure 6. a) X-ray photoelectron spectra of MZ-1RC; b–d) high-resolution
X-ray photoelectron spectra of Mn, O, and Si atoms, respectively.
The binding energies of the elements obtained in the XPS
analysis are corrected for specimen charging by referencing
the O 1s line to 532 eV.[22] The survey XPS profile (Figure 6a)
revealed that the Mn–ZSM-5 (MZ-1RC) samples are composed
of Mn, Si, and O. The Mn–ZSM-5 nanoparticles exhibit major
peaks at 84.2, 102.0, 154.3, 532.0, 641.6, 653.2, 669.2, 746.6,
and 769.1 eV, which correspond to Mn 3s, Si 2p1/2, Si 2s, O 1s,
Mn 2p3/2, Mn 2p1/2, Mn LMM, O KLL, and Mn 2s, respectively,
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approximately 57.2 and 3.8 ppm are indicative of tetrahedrally
coordinated aluminum in the framework and octahedral non-
framework aluminum, respectively, in good agreement with lit-
erature data.[25b,26]
Table 2. Mn–ZSM-5-catalyzed oxidation reaction of thiol to disulfide
under air without solvent at 353 K.[a]
The EPR spectra of the mesoporous Mn–ZSM-5 catalysts are
shown in Figure 9. The spectra were recorded for the catalyst
MZ-1RC before (Figure 9a) and after (Figure 9b) the catalytic
Entry
Catalyst
t
[h]
Conversion[b]
[%]
Yield[c]
[%]
1
MZ-1RC
MZ-1RC
MZ-2RC
MZ-3C
MZ-4C
meso-MnO2
6
6
6
6
6
6
87
0
0
78
0
27
84
0
0
77
0
26
2[d]
3
4
5
6
[a] Reaction conditions: thiophenol (10 mmol), water (2 g), and Mn–ZSM-
5 (30 mg, 0.0327 mmol Mn). [b] Calculated on the basis of recovered
product. [c] Yield of isolated pure product. 1H and 13C NMR data of the
products are given in the Supporting Information. [d] Reaction performed
in N2 atmosphere.
Figure 9. EPR spectra of a) MZ-1RC and b) MZ-1RC after one cycle of cataly-
sis. g=Landꢁ g-factor, can give information about a paramagnetic center’s
electronic structure.
reaction. In both cases, a typical nearly symmetric single wide
line with a g value of 1.994 and 2.004, respectively, is
a common feature of Mn4+ and the result is in good agree-
ment with literature data.[27] This result further suggested that
Mn sites are strongly bound in the zeolitic framework and re-
tains the oxidation state of manganese after the catalytic
reaction.
Figure 10. FTIR spectra of a) thiophenol and b) diphenyl disulfide.
Catalysis
product diphenyl disulfide is a colorless crystalline solid. Sever-
al materials such as hierarchical mesoporous MZ-1RC, MZ-2RC,
microporous MZ-3C, and MZ-4C were used as catalysts for this
reaction. The conventional H-ZSM-5 and Si-ZSM-5 materials
were also screened for this catalytic desulfurization reaction
(Supporting Information, Section S2). The reaction conditions
and the corresponding results are shown in Table 2. Among
these hierarchical mesoporous zeolites, MZ-1RC has the high-
est catalytic activity (entry 1). ZSM-5 is not able to oxidize
thiols to disulfides with air. Similarly, a control reaction showed
that MZ-4C, without any added metal, was not able to catalyze
the disulfide formation reaction (entry 5). Interestingly, also in
the absence of air (entry 2) the oxidation reaction could not
proceed. Further, when mesoporous MnO2 was used as the
catalyst under identical reaction conditions, only a very poor
yield of diphenyldithiol was observed (entry 6). These result
suggest that hierarchical porosity and a zeolite-based nano-
structure are crucial for this catalytic reaction.
Recently, thiol coupling or disulfide bond formation reactions
have attracted considerable research interest, because of the
abundance of these disulfide compounds in protein chemistry
and their biological functionalities. In contrast to other cova-
lent bonds, disulfides are highly dynamic under physiological
conditions. Although in proteins a much more complex scenar-
io is predicted with multiple disulfides, in simple disulfide
bond formation reactions the interaction between redox
agents and sulfide, which leads to only one disulfide bond, is
relatively straightforward. In this context, manganese is well
studied in the literature as a radical initiator.[12d,28] Herein, the
disulfide bond formation reaction was studied with several cat-
alysts under various reaction conditions over Mn-containing
materials. Herein, the oxidation of benzenethiol to diphenyldi-
sulfide was chosen as model reaction to evaluate the catalytic
activity of different catalysts (Table 2). This disulfide bond for-
mation reaction was confirmed by FTIR analysis of the prod-
ucts. From the spectra in Figure 10 it is clearly seen that one ꢁ
SꢁH bond has disappeared, corresponding to the peak at
2566.8 cmꢁ1, which is absent in spectrum (b), and one new
ꢁSꢁSꢁ bond has formed with the appearance of peak at
466 cmꢁ1 in spectrum (b). The progress of this reaction can be
monitored easily as the reactant thiophenol is liquid and the
The reusability of the self-assembled mesoporous Mn–ZSM-5
nanoparticles catalyst in the disulfide bond formation reaction
was also examined by using thiophenol as a reference in aque-
ous medium. The kinetics of the recycled catalyst was evaluat-
ed for three repetitive runs. The results are shown in Figure 11,
suggesting good catalytic efficiency. We also varied the sub-
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1RC is shown in Figure S4, indicating steady progress of the re-
action up to 6 h reaction time together with the retention of
high disulfide selectivity. In order to check the heterogeneity
of the process and possibility of leaching we performed high-
resolution TEM (HRTEM), FTIR, and powder XRD analyses of the
reused catalyst MZ-1RC. Characterization results Figure S5–S7)
suggested retention of hierarchical porosity and zeolitic frame-
work after the catalytic aerobic oxidation of thiols to disulfides.
From these results, we can conclude that our hierarchical
mesoporous Mn–ZSM-5 nanomaterial is highly efficient in cata-
lyzing thiols to disulfides under mild and eco-friendly reaction
conditions. Thus, hierarchically porous zeolitic framework and
Figure 11. Kinetics plot demonstrating the recycling efficiency of the meso-
porous MZ-1RC catalyst.
Table 3. Mn–ZSM-5 (MZ-1RC) catalyzed oxidation reaction of thiols to disulfides in the
presence of air in aqueous reaction medium at 343 K.
strate (Table 3) and observed 83.2, 82, 81.2, and
65.3% yields (entries 2–5) for methyl, chloro, bromo,
and carboxylic acid derivatives of thiophenol, respec-
tively, over MZ-1RC under identical reaction condi-
tions. However, under similar conditions, aliphatic
thiols gave a very low yield (entry 6). Differently sub-
stituted thiophenol compounds were studied here,
and the MZ-1RC catalyst exhibited good catalytic ac-
tivity in all cases (entries 1–6) with high TON values
in aqueous medium. We also performed the same re-
action with thiophenol, 4-chlorothiophenol, and thio-
glycolic acid under solvent-free conditions (entries 7–
9), and the catalyst exhibited very good conversions,
together with high TON values. The reaction per-
formed in organic solvent such as ethanol also gave
good results for disulfide bond formation (Table 3
entry 10, 11).Thus, our catalyst is active in aqueous as
well as organic medium. To understand the potential
of our new mesoporous self-assembled Mn–ZSM-5
nanoparticle, we performed the same reaction with
bulk MnO2 as the catalyst (entry 12), which gave
moderate conversion but a very low TON. However,
the reaction in homogeneous system such as with
MnCl2–K2CO3 as the catalyst (entry 13) did not give
any product. We also performed the reaction in ab-
sence of the catalyst (entry 14), and absolutely no re-
action took place.
Entry Reactant
Product
t
Yield[a] TON[b]
[h] [%]
1
2
3
4
6
6
6
6
84.0
83.2
82.0
81.2
256.8
254.4
250.7
248.3
5
6
6
6
65.3
45.1
199.6
137.9
7[c]
8[c]
6
6
80.0
77.9
244.6
238.2
9[c]
6
34.6
105.8
10[d]
11[d]
12[e]
13[f]
14[g]
8
8
6
6
6
72
78
22.0
–
220.0
238.5
15.0
–
We also performed the recycling experiments with
the hierarchical mesoporous Mn MFI catalyst for di-
sulfide bond formation reaction in the presence of
water as the dispersion medium. Results are shown
in Figure S2, suggesting very high recycling efficiency
of the catalyst after five consecutive cycles. Further,
we measured the selectivity of diphenyldisulfide at
different reaction times over MZ-1RC and found that
despite a steady decrease, it remained very high
(over 96%, Figure S3) during the course of the reac-
tion. Importantly, apart from diphenyldisulfide the
major side product in this reaction is benzenesulfonic
acid. Similarly, the conversions and product selectivi-
ties for the oxidation of 4-chlorothiophenol over MZ-
–
–
[a] Yield of isolated pure product. [b] TON (turnover number)=number of moles of sub-
strate converted/number of moles of active site of the catalyst. [c] Reaction was per-
formed in solvent-free conditions. [d] Reaction was performed in ethanol as solvent.
[e] Reaction was performed with bulk MnO2 as the catalyst. [f] Reaction was performed
with K2CO3–MnCl2 as the catalyst. [g] Reaction was performed without catalyst.
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highly reactive Mn4+ at the pore surface facilitate the interac-
tion between the reactant molecule and the active site of the
catalyst.
mesoporous structure. The hierarchical zeolitic materials have
high BET surface areas, good pore-wall stability and possess re-
active Mn4+ sites, which could activate the sulfurꢁhydrogen
bond cleavage and help to form the disulfide bond from thiols
in the presence of air. Hierarchical mesoporous Mn–ZSM-5 ex-
hibits very high catalytic efficiency in aqueous medium. High
catalytic efficiency of mesoporous Mn–ZSM-5 reported herein
for the synthesis of disulfides may open up new opportunities
in heterogeneous catalysis for the synthesis of value-added di-
sulfides.
A probable mechanistic pathway for this disulfide bond for-
mation reaction is shown in Scheme 1. Here the reaction
mechanism can be expressed as Equation (1)–(3).[13]
4 RSH þ O2 ! 2 RSꢁSR þ 2 H2O
2 RSH þ O2 ! RSꢁSR þ H2O2
H2O2 ! H2O þ 1=2 O2
ð1Þ
ð2Þ
ð3Þ
Experimental Section
Following the reaction as in Equation (1), the thiol RSH
moiety adsorbed on the Mn–ZSM-5 nanoparticle surface indu-
ces oxidative addition of the metal center to give MnꢁSR and
Synthesis of self-assembled hierarchical mesoporous–micro-
porous zeolite nanoparticles
Mesoporous zeolites were synthesized in the following two-step
procedure. Step one: In a typical synthesis, citric acid (2.1 g,
10 mmol, Merck) was dissolved in tetrapropylammonium hydroxide
(16.6 g, TPAH, Sigma Aldrich, 25% in water). The solution was
stirred for 30 min. Then, tetraethyl orthosilicate (TEOS, 10.4 g,
50 mmol, Sigma Aldrich, 99.5%) was added and the mixture was
stirred again for 30 min. Then, MnCl2·6H2O (0.5 g, 2.5 mmol, Merck)
was dissolved in distilled water (5.0 g) with H2O2 (1 g) and this solu-
tion was slowly added to the above solution. The pH of the solu-
tion was approximately pH 6 and the resulting mixture was stirred
for 2 h. The molar ratio in the precursor solution was TEOS/MnCl2/
TPAOH/H2O=50:2.5:20:1000. Then the mixture was transferred
into a Teflon-lined stainless-steel autoclave and hydrothermally
treated at 443 K for 48 h. The solid was collected by filtration,
washed with water, dried at RT under vacuum and the material has
been designated as MZ-1. Step two: A portion of 3 g of MZ-1 com-
posite materials (Si/Mn=20) were soaked with water (20 g) and
NaClO4·H2O (0.875 g, 6.25 mmol, Merck) was added. The pH of the
solution was adjusted by TPAOH at approximately pH 12. Then the
resulting mixture was transferred into a Teflon-lined stainless-steel
autoclave and hydrothermally treated at 443 K for 48 h. The solid
was collected by filtration, washed with water, dried at RT under
vacuum and the material has been designated as MZ-1R. These as-
prepared MZ-1 and mesoporous MFI-type zeolite nanomaterials
were calcined at 773 K for 6 h to form the corresponding tem-
plate-free materials and these have been designated as MZ-1C and
MZ-1RC, respectively.
Scheme 1. Probable reaction mechanism for the mesoporous Mn–ZSM-5 cat-
alyzed aerobic oxidation of thiols using molecular oxygen as oxidant.
C
MnꢁH species. Two neighboring RS radicals then combine
with each other at the metal surface, making possible the for-
mation of the disulfide compound RSꢁSR. The MnꢁH inter-
mediate will reduce molecular oxygen O2 and produce hydro-
gen peroxide, which is decomposed into water and O2 accord-
ing to Equation (3) and closing up the catalytic cycle. An EPR
study (Figure 9) revealed that the manganese has the same ox-
idation state before and after the reaction, which supports the
possible catalytic cycle. The reaction performed in N2 atmo-
sphere did not shown any catalytic activity. This result demon-
strates that the catalyst was only active in the presence of
oxygen and again supports our possible catalytic cycle.
In a similar procedure, ZSM-5 was prepared by maintaining the
molar ratio of TEOS/AlCl3/TPAOH/H2O=50:2.5:20:1000 and the as-
synthesized, twice treated, calcined, and twice treated calcined ma-
terials are designated as MZ-2, MZ-2R, and MZ-2C, MZ-2RC, respec-
tively.
Preparation procedure of microporous zeolite materials
In a typical synthesis, TEOS (10.4 g, 50 mmol, Sigma Aldrich,
99.5%) was dissolved in TPAOH (16.6 g). The solution was stirred
for 30 min. Then, MnCl2·6H2O (1.01 g, 5 mmol, Merck) was dis-
solved in distilled water (5.0 g) with H2O2 (1 g) and this solution
was slowly added to the above solution. NaClO4·H2O (0.875 g,
6.25 mmol, Merck) was added. The pH of the solution was adjusted
to approximately pH 12 by addition of TPAOH solution and the re-
sulting mixture were stirred for 2 h. The molar ratio in the precur-
sor was TEOS/MnCl2/TPAOH/H2O=50:5:20:1000. Then the mixture
was transferred to a Teflon-lined stainless-steel autoclave and hy-
Conclusions
From our experimental results we can conclude that self-as-
sembled hierarchical mesoporous Mn–ZSM-5 and ZSM-5 spher-
ical nanoparticles can be synthesized through a two-step hy-
drothermal method. Citrate and tartrate anions are legated
with the zeolite nanoparticles through covalent interaction
and thus control the growth of the zeolitic nanoparticles. The
nanoparticles are self-assembled and form the hierarchical
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drothermally treated at 443 K for 48 h. The solid was collected by
filtration, washed with water, dried at RT under vacuum and the
material has been designated as MZ-3. Material MZ-4 was synthe-
sized by the same procedure without MnCl2·6H2O adding. All these
as-prepared materials were calcined at 773 K for 6 h to form the
corresponding microporous MFI-type zeolites and they have been
designated as MZ-3C and MZ-4C.
alyst was dry under vacuum. To know the stability of the reused
catalyst, we have examined the TEM, FTIR, and XRD analysis of
reused catalyst. The results are shown in the Supporting Informa-
tion, Figure S5, S6, and S7, respectively.
Characterizations technique
The conventional H-ZSM-5 sample was synthesized following the
literature procedure.[29] The details procedure is given in the ESI
Section 3.
Powder XRD patterns of the samples were recorded on a Bruker
D-8 Advance diffractometer operated at 40 kV voltage and 40 mA
current using CuKa (l=0.15406 nm) radiation. TEM images were re-
corded in a JEOL2010 TEM operated at 200 kV. A JEOL JEM6700F
FESEM was used for the determination of particle morphology. Ni-
trogen sorption isotherms were obtained by using a Beckman
coulter SA3100 surface area analyzer at 77 K. Prior to the measure-
ment, the samples were degassed at 423 K for 3 h. Manganese
content in different zeolites were measured by using a Shimadzu
AA-6300 atomic absorption spectrometer fitted with a double-
beam monochromator. 1H and 13C NMR experiments were per-
formed on a Bruker DPX-300 NMR spectrometer. EPR measure-
ments were performed on a Bruker EMX EPR spectrometer at
X-band frequency (9.46 GHz) at RT (298 K). Solid state MAS–NMR
studies were performed by using Chemagnetics 300 MHz CMX 300
spectrometer. FTIR spectra were recorded on a Perkin–Elmer Spec-
trum100 spectrophotometer. X-ray photoelectron spectroscopic
(XPS) measurements were performed on an Omicron nanotech op-
erated at 15 kV and 20 mA current by using monochromatic Al Ka
X-ray source.
Procedure for thiols to disulfides formation reaction
The reaction was performed in a 50 mL round-bottom flask fitted
with a water condenser and placed in an oil bath at 343 K under
vigorous stirring. For
a typical reaction, liquid thiophenol
(10 mmol), water (2 g), and Mn-ZSM-5 catalyst (0.03 g) were mixed
together in the presence of air. The progress of the reaction was
monitored by TLC. At the end of the reaction, the heterogeneous
mixture was cooled down to room temperature. The mixture was
diluted with Et2O (10 mL) and extracted with Et2O. The organic
layer was washed with 2% sodium hydroxide solution and brine
solution. Then the organic layer was dried over Na2SO4 and evapo-
rated to dryness to give the colorless crude product. The isolated
1
crude product was characterized by H and 13C NMR (shown in the
Supporting Information, Section 1), respectively.
The experiments were also performed under solvent-free condi-
tions, under which thiophenol (10 mmol) and Mn-ZSM-5 catalyst
(0.03 g) were mixed together. The experiments were performed at
343 K in the presence of air.
Acknowledgements
AKP, AD, and MP thank CSIR, New Delhi for their respective
senior research fellowships. AB wishes to thank DST New Delhi
for providing instrumental facilities through DST Unit on Nano-
science and SERB project grants.
Hot-filtration test
To confirm the heterogeneous nature of catalyst and catalytic ac-
tivity bound to the solid phase, a hot-filtration test was performed.
The thiophenol (10 mmol) and mesoporous MZ-1RC (0.03 g) were
mixed together with water (2 g) in an oil bath at 343 K under vigo-
rous stirring. The catalyst was filtered off (from the hot reaction
mixture) after 8 h. Then after the reaction 46% conversion was
Keywords: hierarchical porosity · manganese · mesoporous
materials · self-assembly · zeolites
1
achieved (isolated product and H NMR) and the reaction was con-
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Received: October 8, 2013
Published online on December 2, 2013
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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