Efficient strategy for interchangeable roles in a green and sustainable redox catalytic system: IL/PdII-decorated SBA-15 as a mesoporous nanocatalyst

Article abstract of DOI:10.1039/d0nj05459c

Time and again, SBA-15-based composites as mesoporous materials and the incorporation of transition metals in them have been attracting dramatic attention in the field of catalysis due to their remarkable features. In this paper, the activity of SBA-15 supported ionic liquid-Pd(ii) has been investigated in the catalytic transfer hydrogenation of nitroarenes with formic acid as a hydrogen donor at room temperature in water medium, and the oxidation of benzyl alcohols to benzaldehyde derivatives under atmospheric oxygen at high temperature. This novel nanocatalyst was characterized by FT-IR, SA-XRD, BET, BJH, TGA, FE-SEM, TEM, and ICP as the most commonplace techniques for analyzing its characteristics to be revealed as truth. Furthermore, the EDX analysis illustrates the grafting of the ionic liquid-Pd(ii) into SBA-15. The catalyst showed high stability under reaction conditions, and can be recovered and reused for at least 15 and 6 reaction runs in oxidation and reduction reactions, respectively.

Full text of DOI:10.1039/d0nj05459c

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Efficient strategy for interchangeable roles in a
green and sustainable redox catalytic system:
IL/PdII-decorated SBA-15 as a
Cite this: New J. Chem., 2021,
45, 6682
mesoporous nanocatalyst
Samira Sadeghi,†a Meghdad Karimi, †a Iman Radfar,†a
Reza Ghahremani Gavinehroudi,a Dariush Saberi *b and Akbar Heydari*a
Time and again, SBA-15-based composites as mesoporous materials and the incorporation of transition
metals in them have been attracting dramatic attention in the field of catalysis due to their remarkable
features. In this paper, the activity of SBA-15 supported ionic liquid–Pd(II) has been investigated in the
catalytic transfer hydrogenation of nitroarenes with formic acid as
a hydrogen donor at room
temperature in water medium, and the oxidation of benzyl alcohols to benzaldehyde derivatives under
atmospheric oxygen at high temperature. This novel nanocatalyst was characterized by FT-IR, SA-XRD,
BET, BJH, TGA, FE-SEM, TEM, and ICP as the most commonplace techniques for analyzing its
characteristics to be revealed as truth. Furthermore, the EDX analysis illustrates the grafting of the ionic
liquid–Pd(II) into SBA-15. The catalyst showed high stability under reaction conditions, and can be
recovered and reused for at least 15 and 6 reaction runs in oxidation and reduction reactions,
respectively.
Received 7th November 2020,
Accepted 10th February 2021
DOI: 10.1039/d0nj05459c
rsc.li/njc
undesirable side reactions, is more noteworthy than other
green oxidants. These attributes encourage researchers to
1. Introduction
Within a survey among various porous materials, silicates could develop desirable and, of course, robust catalytic systems for
play particular roles in fine chemical industries.1–3 For the aerobic oxidation.22 However, due to several challenges,
running of miniaturized-scale syntheses, namely through pores industrial oxidation processes have extended far less than the
as nanoreactors,4–7 these pioneering catalysts have found a reduction processes.23
highlighted place across the chemistry world. Despite their
Concerning reduction reactions, molecular hydrogen is the
excellent aptitude to facilitate different reactions inside pores, most appropriate reducing agent. However, endangering its
for instance, condensation types,8–10 there is still a lack of application, storing problems, and restrictions on the use of
enough versatility. Hence, as a helpful tool to provoke chemists’ gaseous processes can be addressed as a unique source for its
minds, their structural modifications have always been merits and demerits. Therefore, the investigation of catalytic
embedded in their outlines. Requirements for the use of a systems based on alternative green hydrogen donors is one of
green reductant-oxidant11–16 for redox processes, based on the researchers’ vital interests in this field.11 In the middle of a
green chemistry protocols, have led researchers to have a vast number of options, formic acid with its intriguing properties,
replacement of the catalytic methods with traditional such as ease of transportation and use, availability, lack of
ones.17–20 One group of significant products via the partial hazardous wastes, economic efficiency, and high atom-economy,
oxidation of the carbon–hydrogen (C–H) bond, specifically is one of the most critical compounds used as a reducing agent.24
aldehydes, is of considerable importance as precursors.21 Air, Accordingly, various approaches and methods for developing
because of the accessibility, economic viability, more effortless suitable catalysts in reduction processes using this reductant have
exploitation than other reagents, and the low number of been studied.11
Since most homogeneous catalysts have not been reusable
in chemical processes, extensive endeavours have been allocated
a Chemistry Department, Tarbiat Modare University, P.O. Box 14155-4838, Tehran,
to studying different immobilization methods.25–27 Catalysts,
Iran. E-mail: heydar_a@modares.ac.ir; Tel: +98-21-82883444
including one or more grafted or inserted transition metals,
especially precious metals, such as palladium, are tremendously
regarded because of their ability to perform both oxidation and
b Department of Chemistry, Faculty of Nano and Bio Science and Technology,
Persian Gulf University, Bushehr 75169, Iran. E-mail: saberi_d@pgu.ac.ir
† These authors contributed equally.
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reduction.11,28,29 Fundamental studies on the employment of and sketched on a plot through a gradual heating rate of 20 1C
palladium as a catalyst on oxidation–reduction reactions by the minÀ1 over a temperature range of 25–1000 1C under a stream
aid of green oxidants-reductants, such as molecular oxygen– of purified N2. The SEM images were observed using Philips XL
hydrogen (O2/H2), indicate its extraordinary catalytic potential. 30 and S-4160 instruments with gold-aided coating, which was
However, the formation of palladium(0) and the debilitation of laid out on nanoparticles within the EDX test. The primary
the catalytic centres bring about the retardation of reaction results for the reaction progress were surveyed by gas
kinetics, which has been observed over time. Therefore, its chromatography (GC) (Yonglin 6100; BP-5; 30 m  0.25 mm Â
immobilization has two purposes: (i) heterogeneity to accelerate 0.25 mm) with toluene as a general solvent for injection, and
separation, (ii) prevention of the formation of Pd (0) and nitrogen as an inert carrier gas. All yields referred to isolated and
deactivating catalytic centres has been the basis of research. As purified products. 1H-NMR and 13C-NMR spectra were recorded
mentioned, ordered mesoporous silicates, especially functionalized on a Bruker Avance (DRX 400 MHz) in pure deuterated CDCl3
ones, have an outstanding potential due to their high surface solvent with a commercially introduced internal standard,
area with the hydroxyl groups to be used as much more efficient namely tetramethylsilane (TMS).
catalysts.30,31 In the present study, due to the advantages of
ordered mesoporous silica, SBA-15 was functionalized through
2.2. Synthesis of SBA-15
a solvent-free green pathway using the tetrachloropalladate Exploiting a general procedure out of the literature, the support
anion, [PdCl4]2À, based on ionic liquid. Various characterization SBA-15 prepared using Pluronic P-123 as a template was casted
techniques, such as FT-IR, SA-XRD, BET, BJH, TGA, SEM-EDX, by a diluting and initiating agent, TEOS (tetra ethyl oxy silane),
TEM, and ICP, have been carried out to assess the achievement for polymerization across such framework as a silica source,
of imposed corrections. As no solvent is used in this process, which was fulfilled in an acidic medium.32 In a typical procedure,
further steps, such as extraction to remove waste liquid is 4 g of the Pluronic P-123 surfactant was dissolved in 20 g of water,
eliminated. The functionalized ordered mesoporus silica was followed by adding 80 g of 2 M HCl. To the above solution, 8 g of
then used as a catalyst in the aerobic oxidation reaction of the TEOS was added gently, and stirring was continued for the next
C–H bonds of benzylic, aliphatic, and allylic alcohols. The highly 24 h at 40 1C. The gel mixture was then transferred to a
desirable efficiency of the catalyst in the aerobic oxidation polypropylene bottle, and hydrothermally treated at 100 1C for
process led us to investigate its ability in the green transfer 48 h. The solid residue was passed through a paper and then a
hydrogenation (TH) of nitroarenes. This reduction process was ceramic sieve, washed with water, and finally dried at 60 1C for
investigated in an aqueous medium using formic acid as a green 8 h. The volatile template was removed by calcination at 550 1C
hydrogen-donor. The results showed the high efficiency of the (heating rate = 2 1C minÀ1) for 6 h in the presence of air as an
catalyst for this process in green conditions, and at ambient in situ decomposer. The obtained porous material is denoted as
temperature. Besides, the reusability of the catalyst was estimated SBA-15 nanoparticles.
to be 15 and 6 times for the oxidation and reduction processes,
respectively, which confirms its high stability under both
associated conditions.
2.3. Synthesis of 3-methyl-1-(3-(trimethoxysilyl)propyl)-
1H-imidazol-3-ium
In a typical procedure for the preparation of the interior
decoration of the catalyst, trichloropropyltrimethoxysilane
(20 mmol) and N-methyl imidazole (20 mmol) were mixed
together and poured into a round-bottom-flask, as a container.
The mixture was stirred for 24 h at 100 1C under reflux and
2. Experimental section
2.1. Chemical, instrumentation and analysis
All starting materials and correlated apparatuses were purchased neutral gas atmosphere. The obtained yellow viscous liquid was
from credible commercial suppliers, and used without further dried at 80 1C and under vacuum for 24 h (Scheme 1A).
purification or manipulating in their proceeding styles, respectively.
2.4. Synthesis of (3-methyl-1-(3-(trimethoxysilyl)propyl)-
1H-imidazol-3-ium)2 monopalladium(II) tetrachloride
The catalyst was characterized using spectroscopic and different
affordable analytical techniques, including FT-IR, X-ray diffraction
(XRD), inductively coupled plasma (ICP), Brunauer–Emmett–Teller To the prepared ionic liquid (2 mmol), PdCl2 (0.17 g, 1 mmol)
(BET), Barrett–Joyner–Halenda (BJH), thermal gravimetric was added. A 6 h swirling was carried out at 100 1C under reflux
analysis (TGA), and scanning electron microscopy joined with and argon atmosphere until a transparent brown liquid
energy-dispersive X-ray analysis (SEM-EDX). The FT-IR spectra appeared. The obtained brown viscous liquid was dried at
reached an acquisition step of the basic spectra over the 80 1C under vacuum for 24 h (Scheme 1B).
region 400–4000 cmÀ1 with NICOLET IR100 FT-IR. Here,
2.5. Synthesis of Pd-sites linked onto SBA-15 (SBA-15-IL/Pd)
spectroscopic-grade KBr was harnessed. The powder X-ray
diffraction spectrum was marked at room temperature by a To the prepared PdII-containing ionic liquid (20 mmol), SBA-15
Philips X’pert 1710 diffractometer using Cu-Ka (a = 1.54056 Å) (0.25 g) was added and agitated overnight at 100 1C under
via charted Bragg–Brentano geometry (y À 2y). The ICP analysis reflux and argon atmosphere. The obtained brownish
was performed using a VISTA-PRO, CCD simultaneous ICP precipitate was centrifuged and washed three times with
system. The running of the thermal destruction was analysed ethanol. Finally, all remaining solvents and rinsed solutions
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melting point (m.p.; for solid products), FT-IR, 1H-NMR, and
13C-NMR. The spectral data of the known amines were checked
with their authentic references.
3. Results and discussions
3.1. Validation of the synthesized SBA-15-IL/Pd
For the assessment of the precision of our first theoretical
modification and synchronizing with former reports, a wide
range of analyses were applied. For studying the prepared
SBA-15-IL/Pd by different fundamental instrumental techniques,
such as SA-XRD, FT-IR, BET, TGA/DTA, SEM, EDX, TEM, and
ICP-AES, the quality and quantity information of the composite
structure were measured.
Scheme 1 Synthesis of the catalytic system.
3.1.1. FT-IR (Fourier-transform infrared) spectroscopy. The
non-decorated SBA-15 includes a peak related to the Si–O bond
in its infrared spectrum similar to that of silica. The (3-methyl-
1-(3-(trimethoxysilyl)propyl)-1H-imidazol-3-ium)2 monopalladium(II)
tetrachloride has functional groups, such as aliphatic C–H,
aromatic C–H, aliphatic C–C, aromatic C–C, aromatic CQC,
aromatic CQN, aliphatic C–N, and aromatic C–N. The
mentioned functional groups are quite apparent in FT-IR
spectroscopy, and they are expected to remain unchanged after
functionalizing SBA-15 (in fact, modifying the internal cavity
surfaces). The FT-IR spectra of SBA-15 before (SBA-15) and after
functionalization (SBA-15–Pd) were compared point-by-point,
and as a result, the formation of the desired structure was
verified (Fig. 1a). The FT-IR spectra of SBA-15 (black curve),
IL–Pd (blue curve), and SBA-15-IL–Pd (red curve) have been
illustrated in Fig. 1a. The peak at 1091 cmÀ1 is related to the
stretching vibration of the Si–O bond and is apparent in all IR
graphs for compounds that contained a silicate basis. This peak
is present in all three IR diagrams, indicating the integrity and
stability of the SBA-15 structure. The broad peak observed at the
3300–3600 cmÀ1 region is ascribed to the O–H stretching
vibration in Si–OH and water molecules absorbed on the
were wiped out at 80 1C under vacuum condition for 24 h
(Scheme 1C).
2.6. General imposed approach for the aerobic oxidation of
alcohols using SBA-15-IL/Pd
Benzyl alcohol (1 mmol) was added gradually to a mixture of
the catalyst, SBA-15-IL/Pd, (40 mg, 3 mol% of Pd), and NaOH
(1 mmol) in water (2 mL). The reaction mixture was whirled for
six hours at 80 1C inside of a condensed-air chamber, and thin-
layered chromatography (TLC) was used to check the finalized
reaction. After the perfecting of the reaction, the catalyst was
separated from the reaction mixture by centrifugation. As a
work-up step, it was washed four times with ethanol. The final
mixture was added to a new flask and diluted with ethyl acetate,
and the organic phase containing the product was decanted.
Finally, the organic layer was isolated by the aid of column
chromatography on a silica gel. All refined compounds were
1
checked out using the melting point test, FT-IR, H-NMR, and
13C-NMR. The spectral data of the known compounds were
compared with the reported patterns and a number of previous
reports.
2.7. General procedure for the transfer hydrogenation of
nitroarenes to corresponding amines using SBA-15-IL/Pd
As a reverse role of the catalyst in promoting the oxidation
reaction, in a typical transfer hedrogenation (TH) reaction,
nitrobenzene (0.5 mmol) and formic acid (1 mmol) were
combined together by making an aqueous solution (2 ml) in
a glass vial. Then, using 20 mg of the added catalyst, the
mixture was stirred at room temperature in contact with air
for 3 h. Thin-layered chromatography (TLC) and GC were
performed to ensure reaction completion. By the end of the
completion of the reduction, the catalyst was separated by
centrifugation, and the product underwent an eluting procedure
by applying an aqueous–organic extraction stage (H2O–ethylacetate).
In the organic phase, the aniline product was separated and
identified after the separation of the ethyl acetate solvent using
a rotary evaporator and passing through the silica gel column.
The identification of all compounds was rendered using the
Fig. 1 (a) FT-IR spectra, (b) SA-PXRD and PXRD. (c) Nitrogen adsorption–
desorption isotherms, and (d) TGA pattern of SBA-15 and functionalized
SBA-15.
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nanoparticles in the preparation process. In the FT-IR spectrum solids that revealed the identity of the SBA-15 structures. A sharp
of treated SBA-15 (after complete preparation), there is no peak capillary condensation step confirmed by a narrow pore size
in the range of 2900–3000 cmÀ1, which confirms the absence of distribution in the mesoporous range appeared at P/P0 = 0.5–0.8
the surfactant Pluronic P123 used in its preparation process, or as the fractional pressure, and proved the presence of a regular
its complete removal from the structure during the separation mesopore in the network of the catalyst (Fig. 1c).36 The results
and calcination process. After the functionalization of SBA-15 obtained for the analysis of the textural characteristics (BET
using the (3-methyl-1-(3-(trimethoxysilyl)propyl)-1H-imidazol-3- surface area, average pore size, and pore volume) of the samples
ium)2 monopalladium(II) tetrachloride, the discriminating are summarized in Table 1. As shown in Table 1, the specific
peaks of this ligand or ionic liquid were appeared in the surface area and the pore volume of bar SBA-15 decreased after
infrared spectrum of the noted catalyst. In particular, the IR the functionalization, proving the immobilization of (3-methyl-1-
absorption peaks at 1500–1700 cmÀ1 and 1300–1500 cmÀ1 (3-(trimethoxysilyl)propyl)-1H-imidazol-3-ium)2-monopalladium(II)
ranges could be allocated to the nasym(CQC) and nsym(CQC) tetra chloride inside of the SBA-15 pores.
stretching modes of the imidazole aromatic ring of 3-methyl-1-
(3-(trimethoxysilyl)propyl)-1H-imidazol-3-ium, in similar differential thermal) analysis. The thermal stability of functio-
sequence. The observation of a peak around 1593 cmÀ1 could nalized SBA-15 with (3-methyl-1-(3-(trimethoxysilyl)propyl)-1H-
be eloquently assigned to the stretching vibration along the imidazol-3-ium)2 monopalladium(II) tetrachloride was
3.1.4. TG/DTA (thermo-gravimetric degradation via
CQN bond presented in the imidazole, which after IL evaluated by TG/DTA analysis at the temperature range of
formation, is shifted to 1627 cmÀ1. Such a phenomenon can 25 1C to 800 1C (Fig. 1d). Moreover, the ratio of organic
conclude that the IL formation has occurred. The presence of a matter to mineral content in the catalyst was calculated by
peak in the 2250–2400 cmÀ1 region could also be another this technique. According to the diagram, the first step,
evidence of the conjugated CQN functional group being including the slight weight loss (6 w%) below 200 1C, can be
shifted to higher frequencies because of the IL formation. considered to correspond to the loss of water and organic
Moreover, the vibrational frequency found at 3130 cmÀ1 is groups absorbed physically. The second step, which indicates
related to the aromatic C–H group (imidazole). The bands at 24% weight loss at the range of 200–400 1C, is most likely due to
2977, 2923, and 2877 cmÀ1 belong to the aliphatic (propyl the decomposition of organic molecules (imidazolium
chain) C–H bonds in the FT-IR spectrum.
ionic liquids) grafted to the SBA-15 surface. In the third step,
3.1.2. SA-PXRD (small-angle powder X–ray diffraction). there is a 20% weight loss at the range of 400–800 1C because of
Pattern Fig. 1b indicates the SA-PXRD patterns of SBA-15 organic residue contents. Through the TGA analysis, the
before and after modification by this palladium-activated organic motif, particularly the 3-methyl-1-propyl-1H-imidazol-3-
linker. The SA-PXRD pattern of the bar SBA-15 shows ium chloride content of the functionalized SBA-15 nanoparticle
three distinguishing Bragg peaks below 2.5, indicating its was evaluated to be 200 mg gÀ1. The amount of 200 mg per
well-ordered mesostructure that expanded toward two- 1000 mg equals 1.25 mmol of 3-methyl-1-propyl-1H-imidazol-3-
dimensional layers. Three strong peaks at 2y = 0.986, 1.630, ium chloride and 0.75 mmol Pd(0) per 1000 mg of modified
and 1.859 with d-spacings of 89.47, 54.15, and 47.49 Å, catalyst.
respectively, were assigned to reflections of the 100, 110, and
3.1.5. SEM (scanning electron microscopy). We investigated
200 associated with the hexagonal structure. In other words, the the morphology, size, and texture of SBA-15 (Fig. 2a), and
100 and 110 peaks are related to the long silica sequence functionalized SBA-15 (Fig. 2b) by SEM (Fig. 2). These images
(the length of the honeycomb hexagons), and the 200 peak demonstrate that the bare SBA-15 has a worm-like morphology
was assigned to the crystal plates associated with the hexagonal and a striated appearance, indicating the presence of surfaces
widths (the width of the honeycomb hexagons). After caused by surfactants during structure synthesis. The SEM
functionalization, the correlation between the reduction of analysis indicated that the synthetic SBA-15-IL–Pd had a unified
the peak intensity and the overall decrease in the electron surface of worm-shaped nanoparticles. These micrographs
density contrast confirmed the Pd–IL’s placement on the silica confirmed the formation of the free form and functionalized
wall.33–35 It also shows a prominent effect by loading the ligand SBA-15 arranged in a style of curved worm.
and the metal. The diffraction peak (100) width and intensity
3.1.6. Energy-dispersive X-ray spectroscopy (EDX). An EDX
have become broader and weaker, respectively, and the change spectrum can prove the presence of stabilized Pd(II) ions and
to 2y values related to the SA-PXRD pattern of pure SBA-15. The other expected elements inside the functionalized SBA-15
blockage of a part of the open anchor catalyst pores decreased pores. It gives the atomic percent of elements that are present
the long-range order of SBA-15.
3.1.3. Nitrogen adsorption–desorption test. Nitrogen
adsorption–desorption, namely BET (Brunauer–Emmett–Teller), Table 1 Textural properties of SBA-15 and SBA-15-IL/Pd samples
measurements were carried out for SBA-15 (black curve) and
SBA-15-IL–Pd (red curve) to evaluate their casted properties. The
isotherms and pore size distributions are given and depicted in
Fig. 1c. All of the materials exhibited type IV isotherms, and H1
hysteresis loops were detected as a characteristic of mesoporous SBA-15-IL/Pd 318.47
Langmuir specific
Name of
sample
BET surface surface area
Pore volume Pore size
area (m2
g
À1) (m2 gÀ1
)
(cm3 gÀ1
)
(Å)
SBA-15
736.85
3994.5489
1291.07
1.27
0.241
69.39
21.71
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3.2. Application of SBA-15-IL/Pd for the selective oxidative
transformation of alcohols and nitroaromatic compound
reduction
3.2.1. Optimization of the aerobic oxidation of alcohols.
After the preparation and characterization of the catalyst, its ability
was investigated in the aerobic oxidation of alcohols. Indeed, it is
necessary to have a catalyst containing Pd(II) to run the reactions,
and gain the best results. Recently, controlling the conversion of
Pd(II) to Pd (0) has been a considerable challenge because this
prevents the unique effect of a catalyst on the progression of the
reaction. In several studies, the use of disparate ligands and co-
catalysts has been considered.38–40 Moreover, many efforts have
been assigned to investigate methods based on condition
adjustment, not using auxiliary agents.41,42 According to the articles
reported so far, many efforts have been made to find optimal
oxidation conditions in an aqueous medium. Different parameters,
such as the base, catalyst loadings, solvent, and temperature, have
been considered for the optimization step (Scheme 2).
As a model reaction, we investigated the oxidation of benzyl
alcohol to benzaldehyde in the company of SBA-15-L/Pd and air
bubbling at room temperature. First, different solvents were
examined to determine their effect on the yield of the model
reaction. Water was selected as the best solvent for the aerobic
oxidation reaction. Notably, toluene as a solvent had a similar
effect, but due to the adherence to the requirements dictated by
the green chemistry protocols, water was eventually used as the
optimal solvent for the remaining processes. Interestingly, the
catalyst had limited freedom to process in polar solvents, which
could be repressed due to disturbance at the catalyst centers
and catalyst poisoning (Fig. 3a). These studies also confirm that
Fig. 2 SEM images of (a) SBA-15, (b) SBA-15-IL/Pd. (c) Weight percent of
various elements on the surface of SBA-15-IL–Pd from EDX analysis.
(d) EDX studied area. TEM illustration of (e) SBA-15SBA-15 and (f) SBA-
15-IL/Pd.
on the surface of the catalyst. As shown in Fig. 2c, the EDX
analysis approved the successful synthesis of SBA-15-IL–Pd with
0.16% palladium. The distribution of other elements was also
obtained, including Cl, 0.57; Si, 2.45; N, 15.64; O, 23.02; C,
58.16%.
3.1.7. Transmission electron microscopy (TEM). TEM
images of SBA-15 before and after binding of the silica-ended
linker to the silicate surface were investigated to identify the
size, morphology, and shape of the synthesized catalysts
(Fig. 2e and f). Based on the investigated images, it can be
seen that the dark halation inside the pores confirms the
immobilization of (3-methyl-1-(3-(trimethoxysilyl)propyl)-1H-
imidazol-3-ium)2 monopalladium(II) tetrachloride in the pores.
However, as observed in Fig. 2f, the assemblages of (3-methyl-1-
(3-(trimethoxysilyl)propyl)-1H-imidazol-3-ium)2 monopalladium(II)
tetrachloride are visible on the surface of SBA-15.32,37
3.1.8. Inductively coupled plasma (ICP). The capacity of
the catalyst about palladium in SBA-15-IL/Pd was measured by
ICP-AES analysis. In a typical procedure, 1 g of the catalyst
was dispersed in a mixture of hydrochloric acid and nitric
acid (HCl/HNO3, 3/1 v/v ratio), and then the remaining
precipitate was separated by centrifugation. The remaining
solution containing Pd2+ was analyzed by ICP. Eventually,
the captivated palladium that is inserted inside of the
catalyst was calculated to be equal to 0.73 mmolÁgÀ1, which is
in good agreement with the results obtained from the TGA
analysis.
Scheme 2 Model reaction for the aerobic oxidation of alcohols.
Fig. 3 Optimization of the base, catalyst loading, solvent temperature and
catalyst heterogenity effect for the aerobic oxidation of alcohols (a–e
respectively) and catalyst reusability (f).
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no aerobic oxidation product has been produced without a alcohols led to the more favorable products (Table 2, entries 1e
base, and NaOH could lead to the best results (Fig. 3b). and 1f), and the electron-donating moieties had a slightly contrary
Subsequently, the model reaction was investigated using impact (Table 2, entries 1b–1d). To investigate the steric effects on
different catalyst loadings. 40 mg of the catalyst led to the best the performance of the secondary alcohols, 1-phenylethan-1-ol
results because we did not observe more significant progress by (reduced form of acetophenone) was selected as a sample
increasing the amount of catalyst. This slight increase is not a substrate. It underwent the imposed oxidative condition
plausible justification for a 25% rise in the catalyst mass. successfully with high yield, containing a little steric hindrance
Therefore, 40 mg was chosen as the optimal amount. It is (Table 2, entry 1n). Nevertheless, diphenylmethanol (reduced
probable that with increasing the mass, the catalyst starts to form of benzophenone), known as
a bulky 21 alcohol,
perform as an absorbent instead of a catalyst because of its demonstrated a poor conversion into the product (Table 2,
porous structure (Fig. 3c). Further tests at higher temperatures entry 1o). The 11 and 21 aliphatic alcohols that are the problematic
indicated that increasing the temperature to 80 1C had a class of alcohol substrates in the usual aerobic oxidation reactions
positive effect on the reaction process (Fig. 3d). Increasing worked well in these optimal reaction conditions (Table 2, entries
the temperature can affect the reaction process in several ways. 1j–1m). However, the secondary aliphatic alcohols unveiled
When the temperature climbs up, the solubility of the significantly low achievement because of the steric hindrance
precursors in the aqueous medium increases.43 In other words, (Table 2, entry 1m). There should be an emphatic viewpoint that
the increased temperature leads to the diffusion of the species excellent chemoselectivity was observed regarding the primary
into the pores and more exposure to catalytic centres. In alcohols, and just the aldehyde as a unique product was captured
contrast, a further increase of the temperature reduces the under these reaction conditions. After this time, the feasibility of
solubility of oxygen in the reaction medium. As a result, a greater the formation of acid or other corresponding byproducts was
reaction progress was not observed. Eventually, the highest eliminated from the first estimations or predictions.
output in the field of product for the model reaction was gained
using NaOH (1 equiv.) as a base, 0.04 g of catalyst (3 mol% of the previous literature, the selective oxidation of alcohols to
Pd(II)), and H2O as solvent at 80 1C after 6 h (Scheme 2). aldehydes relies on an essential medium, such as using NaOH
3.2.2. Aerobic oxidation mechanism study. According to
In another experiment, catalysts such as (1-butyl-3-methyl-1H- to increase the oxidative activity of the catalysts, and can cause
imidazol-3-ium)2 monopalladium(II) tetrachloride, Na2PdCl4 and an enhancement around the selectivity towards BzH.44–46 From
PdCl2 did not have a remarkable influence on model reaction additional investigations, it was confirmed that the optimized
efficiency. Indeed, using (1-butyl-3-methyl-1H-imidazol-3-ium)2 basicity for the alteration of benzyl alcohol to benzyl aldehydes
monopalladium(II) tetrachloride, Na2PdCl4, and PdCl2 as the is B9.2. Increasing it to a basic strength of B9.5 can lead to the
catalyst yielded about 49, 31, and 21% of the desired products formation of benzoic acid.47 Therefore, the adjustment of the
(Fig. 1e). The model reaction was surveyed via other comparable optimized basicity is remarkably essential to obtain the highest
Pd-based catalysts. However, these catalysts were not as effective selectivity. Mori et al. reported that the reaction would occur if
as SBA-15-IL/Pd.
the hydroxyl (OH) group of benzyl alcohol was coordinated with
The performance and effectiveness of green SBA-15-IL/Pd the unsaturated Pd species. Subsequently, a Pd–alcoholate
were evaluated using various substrates with electron-donor species, as a roughly stable complex, was formed. Then, during
and electron-withdrawer groups. The results of the efficiency the hydride abstraction reaction, aldehydes and Pd–hydride
and reaction time for the selective oxidation of alcohols to (temporarily stable in contact with air molecules) were produced
aldehydes are shown in Table 2. All of the substrates were as products.48 The Pd–hydride species remains on its reaction
successfully converted to the corresponding aldehydes at high pathway, and makes a Lewis acid–base coordination with O2,
conversion ratios (87–97%) without any byproducts, such as thereby releasing the Pd species, and forming O2 and H2O.
surplus oxidation products or aromatic acids, within a reaction Additional investigations agreed on the same outcome. Prati
time of 6 h.
et al. also suggested that surface oxygen can dissociate benzyl
Therefore, the most general, familiar, and eminently sensitive alcohol homolytically, and produce an alkoxy radical as the
to the oxidation of alcohol derivatives were studied in the optimal intermediate Ph–CH2–Oꢀ on the active site of noble metal
redox conditions (Table 2). The accompanying of the electron- surfaces. Then, H of Ph–CH2–Oꢀ was coordinated to vacant Pd
withdrawing groups (EWG) on the phenyl ring of the benzylic sites to cause it to pass through oxidative carbonylation at the
benzylic site. These sites are necessary for benzaldehyde
embodying.49–52 In this study, the categorizations of the catalysts
were settled in acid/base bi-functional heterogeneous division.
Interaction between Pd2+ and the –OH of NaOH was considered
as the Brønsted-Lowry base, while –N+= played a role as the Lewis
acid site. The dominant mechanism is supposed to be an E1cB
elimination process. In more detail, the single O–H bond of the
alcohols was broken up by free coordinative Pd sites, and led to
the conversion of benzyl alcohol to alkoxide, which is its
Scheme 3 Model reaction for the reduction of the nitroaromatic
compounds.
conjugate base. Next, Pd would undergo a b–H elimination,
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Table 2 Concocting of various carbonyl derivatives through SBA-15-IL/Pd catalyzed aerobic C–H oxidationab
a
Reaction conditions: alcohol (1 mmol), NaOH (1 equiv.), 0.04 g of catalyst (3 mol% of Pd(II)) and H2O (2 ml) at 80 1C for 6 h under air bubbling
b
condition. The yields refer to the isolated pure products.
followed by a cleavage of the Pd–C–H bond from the alkoxide. This phenomenon indicates the importance of the presence of
After these two steps, the –OH– of NaOH caught the protons (H) Pd catalytic centers in the reaction. Also, quinoline as a Lewis
from Pd-hydride and formed H2O. Finally, the iminium site base, which can also act as a catalyst toxin, drastically reduces
(–N+Q) reacted with molecular oxygen to regain its Pd–O form. the catalytic activity. This indicates the importance of the
Eventually, the coordinative Pd sites were free from any ligand, coordination of the corresponding alcohol to the active catalytic
completing the catalysis circle. An overall proposed mechanism centers of Pd for performing the reaction. In the presence of
is depicted in Fig. 5a. Inhibitors, such as thiophenol, quinoline, TEMPO, as a radical scavenger, the alkoxy radical is trapped and
benzoic acid, HCl, cyclohexanone, and 2,2,6,6-tetramethyl- the reaction does not progress. The presence of benzoic acid in
piperidine 1-oxyl (TEMPO) with different concentrations, have competition with alcohol did not significantly affect the oxida-
been used to further investigate the mechanism of the aerobic tion reaction, indicating that this inhibitor does not interfere
oxidation reaction of benzyl alcohol, and performed using the with the oxidation reaction. Since the oxidation reaction in the
prepared catalyst under optimized conditions. As can be seen presence of this catalyst is associated with the release of HCl, it is
from the results of the reaction under optimal conditions and in predicted that the reaction progress will be slow in the acidic
the presence of inhibitor agents, in the presence of thiophenol medium. The reaction dependency on the base also confirms
as a sulfur agent with a high affinity for the active catalytic this result. These results indicate the prominent role of
centers of Pd, the catalyst is poisoned. With increasing amount palladium in catalyzing the oxidation reaction, and factors such
of this inhibitor, the catalyst activity decreases significantly. as the coordination of alcohol to the active catalytic centers of
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palladium and the need for the presence of a base in this
reaction.
Since SBA-15-IL–Pd led to the high efficiency in the model
reaction, we employed various nitroaromatic derivatives to be
3.2.3. Optimization of the reduction of nitroaromatic com- hydrogenated by this catalyst (Table 3). As shown in Table 3,
pounds. Considering the desirable results and stability of halogens (such as chloro, bromo, and iodo) as substituents on
Pd–IL–SBA-15 within the aerobic oxidation reaction of alcohols, the phenyl ring decreased the yield of the reaction, and
we decided to investigate the performance of the catalyst for the prolonged the reaction time (Table 3, entry 2h, 2i, and 2m).
transfer hydrogenation of nitroaromatic compounds in green In contrast, electron-donating (–OCH3, –CH2OH, –OH) substituents
orientation. Room temperature conditions and aqueous increased the yield of the reaction (Table 3, entries 2b, e, f, g, j,
medium were chosen as a model reaction for the reduction of k, l, and 2n). We also compared the reduction of nitrobenzene
nitroarenes to evaluate the efficiency of SBA-15–IL/Pd as a in the presence of –OCH3 and –NH2 substituents in the
catalyst in the presence of HCOOH as a hydrogen-donor. meta and para positions using SBA-15-IL/Pd. Accordingly, the
Initially, the reduction reaction was investigated in alpha- reduction of 3-methoxynitrobenzene produced 3-methoxy-
hydrogenated alcohols as a solvent. The first assumption was aniline after 5 h (Table 3, entry 2f), whereas the same substrate
based on the role of the abovementioned alcohols as a solvent with a methoxy group on the para position gave 4-methoxy-
and reducing agent, simultaneously. However, this showed no aniline after 2 hours (Table 3, entry 2g). The same trend was
desirable results (Fig. 4a). Then, other various solvents were repeated for the –NH2 group in the meta and para positions
investigated using formic acid as a green reducing agent. As (Table 3, entries 2c–2e). The catalyst also quickly catalyzed the
shown in the optimization figure, the highest efficiency and reduction of a bulky substrate, pyrene, with a good yield
catalytic performance were achieved in the aqueous medium (Table 3, entry 2o).
(Fig. 4a). According to the results, the more reduction in
solvents’ polarity happens, the less efficiency will be obtained.
This phenomenon can be due to the lower solubility of base
Table 3 List of various aniline derivatives that passed an attainment of the
reduction of nitroaromatics in the presence of SBA-15-IL/Pdab
used in these media reactions. Organic bases performed better
than inorganic ones, and their efficiency was almost consistent
with their strength. Among the organic bases, 1,8-
diazabicycloundec-7-ene (DBU) had the best performance.
Interestingly, we also ran the model reaction in the absence
of base, and aniline was successfully obtained with
a
comparable yield (Fig. 4b). Regarding the amount of catalyst,
we investigated different masses and concluded that when
0.02 mg of catalyst was used in the model reaction, the highest
yield was observed (Fig. 4c). According to the results, by the
addition of 0.02 g of catalyst, the rate of the reaction and
conversion values met a higher range for the desired products
(Fig. 4c). Eventually, the highest product yield in the model
reaction was obtained using 20 mg of the catalyst (1.5 mol% of
Pd(II)) when the reaction components were immersed in H2O as a
solvent at room temperature after 3 h (Scheme 3).
a
Reaction conditions: nitro compound (1 mmol), 20 mg of catalyst (1.5
mol% of Pd(II)), and H2O as the solvent at the temperature of the
peripheral conditions for 3 h. The yields refer to the isolated pure
b
Fig. 4 Optimization of the solvent, base and amount of catalyst for the
reduction of nitroaromatic compounds (a–c) and catalyst reusability (d).
products.
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3.2.4. Mechanism study of the reduction of nitroaromatic the active catalytic center of Pd is a necessity for the reaction.
compounds. Previous studies have assumed that, as the first Unlike the oxidation reaction, the presence of TEMPO has little
step, the formate ion (HCOOÀ) is absorbed on the surface of the effect on the reaction. In terms of benzoic acid, there is a
Pd-catalyst and forms Pd(HCOOÀ).53,54 Thoroughly, upon competition between benzoic acid and formic acid, reducing
bubbling of the unbound CO2 molecule and in situ formation the efficiency of the reduction reaction. Thus, the coordination
of an activated metal hydride species (Pd–hydride; labeled as of formic acid to palladium is quite essential to perform the
Pd(HÀ)ad), ammonium cations acting as Brønsted acid reaction. Acidic media have little effect on the reaction. Also,
protonate these hydride species and create hydrogen and this reaction does not rely on the presence of a base. Moreover,
ammonium (amine) species. Alternatively, substrate molecules a significant decrease in the reaction efficiency in the presence
compete with RNH3+ ions for the Pd(HÀ)ad sites. Following the of cyclohexanone as a hydride scavenger55 was observed,
hydride transfer to the NO2-group, the resulting intermediate indicating the primary role of hydride generation during the
+
captures the proton and RNH3 formed, which is associated reaction mechanism. These results indicate the key role of Pd
with the release of the ammonium species (maybe in the form in catalyzing the reduction reaction, and factors such as the
of ammonia gas). As a vital stage, these species, in the case of coordination of nitroarene and formic acid to the active
the isolated systems, re-enter the catalytic cycle. Eventually, the catalytic centers of Pd, and the formation of hydrides in this
aniline product is excreted as an oily layer (Fig. 5b). Regarding reaction.
the reduction reaction, the same inhibitors with different
3.2.5. Catalyst reusability. Considering the importance of
concentrations were used to determine its mechanism for environmentally and eco-friendly catalytic systems, we concentrated
nitrobenzene reduction and under optimized conditions. As on investigating the stability and reusability of the catalyst
shown in Fig. 5b, by using thiophenol as a sulfur-containing under the optimal reaction circumstances. The efficiency of
agent with a high affinity for the active catalytic centers of Pd, the catalyst was surveyed in the model reaction. After completing
the catalyst is polluted. As the amount of this inhibitor the model reaction, in order to separate the heterogeneous
increases, the catalytic activity of the catalyst decreases. This catalyst, centrifugation was used, then washed with ethanol,
result confirms the great importance of the Pd catalytic centers and dried at 60 1C under vacuum suction. Finally, it was reused
as catalytic sites in the reaction. Similarly, the usage of quino- in the next run. As shown, after 15 (Fig. 3f) and 6 (Fig. 4d) cycles
line as a Lewis base and inhibitor reduces the catalytic activity for the oxidation and reduction reactions, respectively, just
dramatically, indicating that the coordination of nitroarene to
a trivial dwindle in the catalytic activity was observed.
Fig. 5 Proposed mechanism for the aerobic oxidation (a) and reduction (b).
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the mixture using centrifugation, and the residue solution was
shaken under the reaction conditions. The monitoring of the
reaction mixture by TLC witnessed no considerable progress.
These results indicated that only a slight amount of Pd species
might exist in the solution phase. Altogether, these data
confirmed the high stability and capability of the catalyst under
several tandem reaction conditions.
4. Conclusion
All in all, we have demonstrated a general, convenient, green,
and highly efficient principle for the aerobic oxidation of
alcohols and transfer hydrogenation of nitroaromatic compounds
in the presence of SBA-15-IL/Pd as a robust and highly stable
catalyst. There are numerous advantages of the green oxidants/
reductants, which are quite valuable because these compounds
not only work as environmentally-benign alternatives to
mineral oxidants, but can also be quickly produced and used.
Other positive aspects of this catalytic system study include a
significant improvement in the substrate and functional group
sustainability and stability, highly preferable attack, environ-
mental and economic efficiency, recovery, and reuse compared to
former published methods. In spite of these unique properties,
this catalytic methodology can have specific pharmaceutical
applications, and lead to the synthesis of important biological
compounds that bring safety, environmental, and financial
benefits of paramount importance into the field of catalytic
chemistry.
Conflicts of interest
There are no conflicts to declare.
Fig. 6 FE–SEM and TEM images, and FT-IR, SA-PXRD and PXRD of
SBA-15-IL/Pd after oxidation (a, c, e and f, respectively) and reduction
(b, d, g and h, respectively) reactions.
Acknowledgements
We acknowledge Tarbiat Modares University for partial support
of this work.
The morphology and stability of the catalysts against the self-
agglomeration were checked after the 15th and sixth runs
(Fig. 6). As shown in Fig. 6a–h, the FE-SEM and TEM images
and FT-IR, SA-PXRD and PXRD spectra of the recovered hetero-
geneous catalyst (after the 15th and sixth cycle) revealed that
almost all SBA-15–L/Pd particles were uniquely wormlike in
shape and stable in structure, as well as the intact catalyst. As
a result, the accumulation of the particles was negligible.
Moreover, the applied catalyst was investigated by Inductively
Coupled Plasma (ICP) analysis after the last run to determine the
Pd leaching content. Accordingly, the amount of loaded Pd on
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