Magnetic horsetail plant ash (Fe3O4@HA): a novel, natural and highly efficient heterogeneous nanocatalyst for the green synthesis of 2,4,5-trisubstituted imidazoles

Article abstract of DOI:10.1007/s11164-021-04420-y

Horsetail plant ash (HA), as a natural source of mesoporous silica, has been prepared from the exposure of horsetail plant (Equisetum Arvense) to high temperature. In the present study, a new magnetically separable and also recoverable Fe₃O₄ nanoparticles were synthesized in the presence of natural horsetail plant ash (HA) as a support to result in Fe₃O₄@HA. FT-IR, XRD, TEM, SEM–EDX and VSM analysis were combined to characterize the morphology and structure of this novel synthesized nanocatalyst. This magnetically solid acid nanocatalyst showed an excellent catalytic activity for the synthesis of 2,4,5-trisubstituted imidazoles at room temperature in aqueous media. The procedure led to corresponding products in high to excellent yields and appropriate times. Additionally, this nanocatalyst can be easily recovered by a magnetic field and reused for six other consecutive reaction runs without noticeable loss of its catalytic efficiency. Based on this study, Fe₃O₄@HA is found to be an efficient, magnetically separable, recyclable, and green catalyst with natural source. Graphic abstract: In this work, horsetail plant ash was used as a natural source of mesoporous silica for the synthesis of Fe₃O₄@HA as a highly powerful magnetically solid acid nanocatalyst, which was fully characterized using various techniques. The activity of the newly synthesized nanocatalyst was tested for the synthesis of 2,4,5-trisubstituted imidazole derivatives.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s11164-021-04420-y

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-021-04420-y
Magnetic horsetail plant ash (Fe3O4@HA): a novel,
natural and highly efcient heterogeneous nanocatalyst
for the green synthesis of 2,4,5‑trisubstituted imidazoles
Nina Hosseini Mohtasham1 · Mostafa Gholizadeh1
Received: 2 December 2020 / Accepted: 27 February 2021
© The Author(s), under exclusive licence to Springer Nature B.V. 2021
Abstract
Horsetail plant ash (HA), as a natural source of mesoporous silica, has been pre-
pared from the exposure of horsetail plant (Equisetum Arvense) to high temperature.
In the present study, a new magnetically separable and also recoverable Fe3O4 nano-
particles were synthesized in the presence of natural horsetail plant ash (HA) as a
support to result in Fe3O4@HA. FT-IR, XRD, TEM, SEM–EDX and VSM analysis
were combined to characterize the morphology and structure of this novel synthe-
sized nanocatalyst. This magnetically solid acid nanocatalyst showed an excellent
catalytic activity for the synthesis of 2,4,5-trisubstituted imidazoles at room tem-
perature in aqueous media. The procedure led to corresponding products in high to
excellent yields and appropriate times. Additionally, this nanocatalyst can be eas-
ily recovered by a magnetic feld and reused for six other consecutive reaction runs
without noticeable loss of its catalytic efciency. Based on this study, Fe3O4@HA is
found to be an efcient, magnetically separable, recyclable, and green catalyst with
natural source.
Graphic abstract
In this work, horsetail plant ash was used as a natural source of mesoporous silica
for the synthesis of Fe3O4@HA as a highly powerful magnetically solid acid nano-
catalyst, which was fully characterized using various techniques. The activity of the
Extended author information available on the last page of the article
Vol.:(0123456789)
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N. Hosseini Mohtasham, M. Gholizadeh
newly synthesized nanocatalyst was tested for the synthesis of 2,4,5-trisubstituted
imidazole derivatives.
Keywords Horsetail ash · Nano magnetic catalyst · 2,4,5-Trisubstituted imidazoles ·
One-pot reaction
Introduction
Green chemistry principles are believed to be one of the efective and prevalent fac-
tors in today’s organic synthetic protocols which are primarily concerned with alter-
native reaction media [1]. Having undergone commercial development, these media
have become the basis of many of the cleaner chemical technologies. Green chemis-
try is mainly aimed to develop process selectivity, enlarge the use of starting mate-
rials, as well as replacing hazardous and stoichiometric reagents with eco‐friendly
catalysts in order to expedite the easy separation of fnal reaction mixtures, such as
catalyst recovery [2–4].
Due to numerous efcient factors including magnetic and electrical proper-
ties, high specifc surface area, unique catalytic and wide applications in drug
delivery systems, targeted gene therapy, ion exchange separation, magnetic reso-
nance imaging, biosensors, magnetic data storage, and environmental remediation
and catalysis [5–7], magnetic nanoparticles (MNPs) such as magnetite (Fe3O4)
have attracted great interest among practitioners in green chemistry. It is worth
noting that the insoluble and paramagnetic nature of Fe3O4 enables trouble-free
separation of this nanocatalyst from the reaction mixture by applying an external
magnet eliminating the necessity of catalyst fltration which prevents loss of the
catalyst. Recent scholars, accordingly, have acquired immense and valuable inter-
ests within the synthesis and application of magnetically recoverable catalysts
as a fascinating choice to increase the efciency of heterogeneous nanocatalyst
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Magnetic horsetail plant ash (Fe3O4@HA): a novel, natural…
separation from the reaction mixture by simply utilizing a magnet [8]. Magnetite
nanoparticles are suitable to be chemically stable and uniform in size; however,
they trend to aggregate due to their nanoscale and strong interaction among each
other. Hopefully, this problem can be overcome by their coating with a layer as
the stabilizer, leading to the prevention of direct contacts among the nanoparti-
cles in addition to enhancing the stability of them [9–12].
In our earlier research, horsetail and horsetail ash as novel, inexpensive and
available heterogeneous natural mild solid acid catalysts as well as their interest-
ing properties and catalytic activities were completely discussed and investigated
[13]. It was further recommended that the horsetail ash could be used as support
for the preparation of a variety types of catalysts due to its high surface area and
high silica content that could serve as a natural silica resource having mesoporous
structure.
Throughout this century, Imidazole functional group and more particularly
2,4,5-trisubstituted imidazoles have received signifcant attention in organic syn-
thesis and have found many applications as a result of their biological and phar-
macological activities [14–20]. Imidazoles which are the core fragment of dif-
ferent natural products such as histamine, histidine, biotin, alkaloids, peptides,
and nucleic acids are classifed as an important class of heterocycles [21]. The
recently developed green chemistry and organometallic chemistry have increased
the utility and efcacy of imidazoles as green solvents in terms of ionic liquids
[22–24] and as N-heterocyclic carbenes [25, 26]. Also, it must be noted that in the
recent past, the synthesis of imidazole and its derivative has drawn much atten-
tion owing to their wide range of pharmacological activity in addition to their
industrial and synthetic applications. A considerable number of methods and pro-
cedures have been reported in the research literature for the one-pot synthesis of
2,4,5-trisubstituted imidazoles by cyclocondensation of benzil, aromatic aldehyde
and ammonium acetate, under distinct condition [27–43]
It is a wildly held view that all reported methods have their own merits, yet
most of them are closely associated with one or more shortcomings and weak-
nesses such as use of carcinogenic organic solvents, application of high temper-
atures, harsh and tough reaction conditions, poor yield and production, use of
expensive and toxic catalysts, lack of catalyst reusability, consumption of large
amounts of catalyst, and long-lasting reaction time. Consequently, due to the fact
that imidazoles are considered to have pharmaceutical signifcance, the establish-
ment of new catalytic systems in order to overcome these drawbacks and enable
simple, mild, green, and economical synthesis of imidazole derivatives would be
highly benefcial and advisable. In regard to the preceding discussion, one of the
remarkable highlights of the current paper is to introduce horsetail plant ash as a
source of silica and use it as a support on the surface of Fe3O4 which is present
in one nano-structure (Fe3O4@HA). In this research, the magnetic horsetail plant
ash has been used for the frst time to act as a recoverable catalytic system for
the synthesis of trisubstituted imidazoles by one-pot three-component reaction of
benzil, various aromatic aldehydes and ammonium acetate under aqueous condi-
tions (Scheme 1).
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N. Hosseini Mohtasham, M. Gholizadeh
Scheme 1 Synthesis of trisubstituted imidazoles using Fe3O4@HA as a green and recoverable nanomag-
netic catalyst
Experimental section
Materials and instruments
All chemical materials and solvents were purchased from Sigma-Aldrich and Merck
companies and were used without further purifcation. Subsequently, the products
were characterized through the comparison of their physical data, IR, 1H NMR, and
13C NMR spectra with known samples. The progress of the reaction was monitored
via thin layer chromatography (TLC) applying aluminum plates coated with silica
gel polygram STL G/UV 254 (Merck) using n-hexane and ethyl acetate as eluents.
The melting points were evaluated by an electrothermal type 9100 melting point
apparatus. Mass analysis was performed by Agilent technology (HP) 5973, mass
1
spectrometer functioning at an ionization potential of 70 eV. The H NMR and 13C
NMR spectra were recorded on a Bruker Avance, at 300 and 75 MHz, respectively,
utilizing TMS as an internal standard and DMSO-d6 as the solvent. The Fourier
transform infrared (FT-IR) spectra were recorded from KBr disk within the region
range of 400 to 4000 cm−1 utilizing an AVATAR 370 FT-IR Therma Nicolet spec-
trometer. Crystallinity and textural patterns of the catalyst were investigated by XRD
analysis applying a Panalytical company X’Pert Pro MPD difractometer with Cu
Ka (λ=0.154 nm) X-ray irradiation source in a 2Ѳ range between 10˚ and 80˚. A
transmission electron microscope (TEM) analysis was performed on a Leo 912 AB
microscope (Zeiss, Germany) at an accelerating voltage of 120 kV. Scanning elec-
tron microscopy (SEM) images and energy-dispersive X-ray spectro scopy (EDX)
results were obtained from Leo1450 VP operated at an accelerating voltage of
20 kV. The magnetic property of catalyst was examined by a vibrating sample mag-
netometer (VSM) technique in 1.5 T external magnetic felds at room temperature
using magnetic Danesh pajoh instrument. All yields refer to isolated products after
being purifed by recrystallization.
Preparation of magnetic horsetail plant ash (Fe3O4@HA)
As stated in the preparation stage of our earlier research [13], in the initial phase
horsetail plant was purchased from the market, washed several times to remove any
adhering materials and then dried at room temperature for 24 h. Afterward, the dried
plant was smashed and sieved (80-mesh size) and again washed and dried at 110 °C
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Magnetic horsetail plant ash (Fe3O4@HA): a novel, natural…
for 5 h. Accordingly, mesoporous horsetail ash was prepared by thermal process-
ing of the horsetail at 400 ˚C for 12 h. Following this, Fe3O4@HA was synthesized
by adding FeCl3.6H2O (5 g) and FeCl2.4H2O (2 g) to a suspension of horsetail ash
(1 g) in 60 mL of deionized water in a beaker. Afterward, we let the mixture stir
strongly at 80 °C in order to obtain a clear solution, and then aqueous ammonia was
dropped very slowly into the mixture until the pH of 12 was obtained. The resulting
black solution was kept at 80 °C under vigorous stirring for further 30 min. At a
later stage, the obtained magnetic nanoparticles were separated from solution by a
powerful magnet and were washed repeatedly for three times with deionized water
and eventually dried in vacuum at 60 °C for 8 h to result in a novel nanocatalyst.
Throughout the synthesis, horsetail ash acted as a stabilizer and after the synthesis,
Fe3O4 nanoparticles were functionalized by horsetail ash (Scheme 2).
General procedure for the synthesis of 2,4,5‑trisubstituted imidazoles
Fe3O4@HA (10 mg) was added to a mixture of benzil 1 (1 mmol), various aromatic
aldehydes 2 (1 mmol) and ammonium acetate 3 (2 mmol) in 3 mL H2O and was
stirred at room temperature in an appropriate time as reported in Table 2. Later,
when the reaction was completed (monitored and checked by TLC: n‐hexane/ ethyl
acetate, 7/3), water was removed from the reaction mixture under reduced pressure,
and in the following step, the reaction mixture was dissolved in hot ethanol (5 mL)
and the catalyst was removed magnetically by means of an external magnet and then
it was washed with ethanol and dried to use it in other reactions. Thereafter, the
reaction mixture was allowed to get cool at room temperature to recrystallize from
ethanol to yield the pure fnal product. All the compounds were recognized and con-
frmed on the basis of their melting points as well as being compared with those
reported in the literature and also via the spectroscopic method using FT-IR, Mass,
1H and, 13C NMR.
Results and discussion
Characterization of catalyst
Based on the method shown in Scheme 2, the modifcation of Fe3O4 NPs with horse-
tail ash as a source of silica was acquired resulting in the preparation of Fe3O4@HA
Scheme 2 General route towards the synthesis of Fe3O4@HA
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N. Hosseini Mohtasham, M. Gholizadeh
as a novel solid acid magnetic nanocatalyst. Both the successful construction and the
structure of this synthesized nanocatalyt were fully identifed and verifed by using
various spectroscopic and microscopic techniques including Fourier transform infra-
red (FT-IR) spectroscopy, X-ray difraction analysis (XRD), transmission electron
microscopy (TEM), scanning electron microscopy (SEM) images, energy-dispersive
X-ray spectroscopy (EDX), and vibrating sample magnetometer (VSM). The FT-IR
spectra of a) horsetail plant, b) horsetail plant ash, c) magnetic horsetail plant ash
and d) 6th reused Fe3O4@HA are illustrated in Fig. 1. As indicated in the FT-IR
spectrum of horsetail plant (Fig. 1a), the most typical and standard broad band in
the region from 3500 to 3300 cm−1 comes from O–H stretching vibrations of silanol
OH groups and the H–O–H vibration of water molecule adsorbed on the silica sur-
face [44]. In addition, the distinctive bands at 2923 and 2851 cm−1 are related to the
asymmetric and symmetric methylene C–H stretching vibrational frequencies from
aliphatic saturated compounds, respectively [45]. A weak peak around 1734 cm−1
was determined to be associated to the C=O stretching of the related organic com-
pounds [46]. Moreover, the absorption band at 1636 cm−1 was attributed to the
bending vibrational modes of the surface-attached hydroxyl groups (Si–OH) [47].
The sharp peak at 1100 cm−1 could be referred to the Si–O–Si asymmetric stretching
vibration, while the characteristic bands at 813 and 468 cm−1 were allocated to the
symmetric and bending vibration of Si–O–Si bond, respectively [48]. A closer look
at Fig. 1a, b reveals signifcant changes in FT-IR spectra. As illustrated in Fig. 1b,
the horsetail ash is acquired through the exposure of horsetail plant to high tem-
perature resulting in the removal of all organic group peaks. In contrast to the curve
shown in Fig. 1b, a new strong peak appeared at around 578 cm−1 in Fig. 1c that
Fig. 1 FT-IR spectra of (a) Horsetail plant, (b) Horsetail plant ash, (c) Magnetic horsetail plant ash
(Fe3O4@HA), and (d) 6th reused Fe3O4@HA
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Magnetic horsetail plant ash (Fe3O4@HA): a novel, natural…
can be ascribed to the vibration of Fe–O bonds confrming the Fe3O4 phase, which
is consistent to XRD analysis [49]. As demonstrated in Fig. 1c, the encapsulation of
the Fe3O4 core with the silica shell has been carried out efectively. In addition, the
FT-IR spectra of the recovered catalyst after being used six times are illustrated in
Fig. 1d. It is worth noting that the acquired data represented no signifcant diference
between the FT-IR spectra of as-prepared and that of the recovered nanocatalyst
confrming the stability of the nanocatalyst under the reaction conditions.
In order to acquire information about the structural properties and crystallinity of
the horsetail ash as well as that of encapsulated magnetite nanoparticles (Fe3O4@
HA), X-ray difraction (XRD) analysis was performed and the comparative X-ray
patterns were observed as shown in Fig. 2a, b. The characteristic small and sharp
peak (at 2θ equal to 22° angle) was found in the XRD pattern of horsetail ash which
could be attributed to the silica that is in α-quartz phase (Fig. 2a). Powder XRD
analysis of magnetic horsetail ash (Fig. 2b) shows six peaks at 2θ values=30.1°,
35.4°, 43.1°, 53.4°, 57° and 62.6° which correspond to the (220), (311), (400),
(422), (511) and (440) refections were in good agreement with the standard pattern
for crystalline magnetite with cubic structure as evidenced by the database of mag-
netite in the Joint Committee on Powder Difraction Standards [JCPDS] (JCPDS
card: 19–629) [50], revealing a high phase purity of magnetite. It is worth noting
that the new peak with low intensity at 2θ=22 which was observed in Fig. 2b could
be attributed to the characteristic peak of horsetail ash. This evidence indicates that
the coating process did not result in the phase change of Fe3O4 nanoparticles and
corroborates the fact that the silica coating strategy is indeed successful in generat-
ing a protective silane shell around the magnetic core. Besides, the average crystal-
line size of Fe3O4@HA NPs which was calculated using Debye–Scherrer equation
[51] is estimated to be 20 nm.
As shown in Fig. 3, the morphology and size distribution of Fe3O4@HA
were calculated utilizing transmission electron microscopy (TEM). It can be
Fig. 2 XRD patterns of (a) Horsetail ash and (b) Fe3O4@HA
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N. Hosseini Mohtasham, M. Gholizadeh
Fig. 3 TEM images of (a) Fe3O4@HA and (b) 6th reused Fe3O4@HA
simply inferred from the TEM image of fresh Fe3O4@HA that the most prepared
nanoparticles are spherical in shape (Fig. 3a). It also indicated the presence of
prominent layers encapsulating the Fe3O4-NPs which proves the fact that the
synthesized MNPs have core–shell structure. Based on distribution histogram
of Fe3O4@HA (Fig. 4), the particle sizes of Fe3O4@HA in irregular geometric
shape were found to be about 8–33 nm and the average diameter of nanoparticles
was estimated to be 20 nm validating the results deduced from the XRD. Fur-
thermore, the TEM image of Fe3O4@HA catalyst after being used six times, as
shown in Fig. 3b indicated no obvious changes in the morphology structure of the
catalyst even after experiencing six recoveries.
Scanning electron microscopy (SEM) images were recorded so as to analyze
the morphology of the as-synthesized nanocatalyst. The SEM images of Fe3O4@
HA which are presented in Fig. 5 indicate that nanoparticles are well dispersed
Fig. 4 Particle size distribution histogram of Fe3O4@HA
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Magnetic horsetail plant ash (Fe3O4@HA): a novel, natural…
Fig. 5 SEM images of Fe3O4@HA
and they also demonstrate a regularly spherical morphology structure of nanome-
ter-sized particles with an average diameter of 21 nm.
Additionally, the energy-dispersive X-ray (EDX) analysis was applied to calculate
the elemental composition of the Fe3O4@HA MNPs. The EDX spectral pattern pre-
sented in Fig. 6, strongly demonstrates the chemical characterization of the typical
catalyst sample with peaks for the O, Ca, Si, and Fe elements, verifying the con-
struction and good distribution of Fe3O4@HA nanoparticles. Apart from that, the
obtained results afrm the efective and successful grafting of horsetail ash onto the
surface of Fe3O4. Also, it should be noted that no impurity elements were identifed
in the nanocatalyst structure.
A vibrating sample magnetometer (VSM) was implemented with the purpose of
analyzing the magnetic properties of Fe3O4@HA nanoparticles at room tempera-
ture. In the VSM magnetization curve of Fe3O4@HA nanoparticles, no hysteresis
Fig. 6 EDX analysis of Fe3O4@HA
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N. Hosseini Mohtasham, M. Gholizadeh
Fig. 7 The magnetization curve of Fe3O4@HA nanoparticles
was detected, and the remanence and coercivity are inconsequential and negligible,
signifying the superparamagnetism of this nanomaterial (Fig. 7) [44, 52]. The satu-
ration magnetization value of Fe3O4@HA was about 47 emu g−1 displaying that the
magnetic sensitivity of the prepared catalyst is strong enough to be simply recycled
from the reaction mixture by an external magnetic feld which is substantial for both
magnetic separation and reusability of the magnetic catalyst. Therefore, it could
have promising applications as a renewable catalyst.
Catalytic study
After Fe3O4@HA was characterized as a novel nano magnetic catalyst, we tried
to investigate its catalytic efectiveness and productivity towards the synthesis of
2,4,5-trisubstituted imidazoles. For the purpose of achieving this objective, in the
initial stage, a model reaction was selected upon the reaction of benzil (1 mmol),
ammonium acetate as nitrogen source (2 mmol) and benzaldehyde (1 mmol) and
following that, the role of various efcient parameters including the amount of
catalyst, reaction temperature, reaction time, type of solvent system as well as
solvent‐free conditions was assessed in order to obtain the best outcomes regard-
ing yield and reaction time. The fndings of this optimization process are reported
in Table 1. During the initial phase, the model reaction was investigated in several
solvents such as H2O, EtOH, MeCN, toluene and also solvent‐free conditions in
the presence of 0.01 g of the catalyst with the aim of choosing the reaction media.
The product was obtained in 99, 75, 65, 55 and 25% yields, respectively (Table 1,
entries 2–6). As a result, H2O was found to be the most efective solvent for this
reaction due to the fact that other solvents did not generate satisfactory results in
comparison to it. Then the model reaction was tested in the presence of various
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Magnetic horsetail plant ash (Fe3O4@HA): a novel, natural…
Table 1 Optimization of various reaction parameters for the synthesis of 2,4,5 Triphenyl-1H-imidazole
(4aa)
Entry
Catalyst (g)
Solvent
T (˚C)
Time (min)
Conversionb%
1
2
3
4
5
6
7
8
–
–
100
25
25
25
25
25
25
25
70
25
25
110
3
<10
99
75
65
55
25
65
99
99
87
45
Fe3O4@HA (0.01)
Fe3O4@HA (0.01)
Fe3O4@HA (0.01)
Fe3O4@HA (0.01)
Fe3O4@HA (0.01)
Fe3O4@HA (0.005)
Fe3O4@HA (0.04)
Fe3O4@HA (0.01)
Horsetail ash (0.01)
Fe3O4 (0.01)
H2O
EtOH
MeCN
Toluene
–
H2O
H2O
H2O
H2O
H2O
20
20
45
80
35
3
3
12
100
9
10
11
aReaction condition: benzil (1 mmol), benzaldehyde (1 mmol), ammonium acetate (2 mmol) and solvent
(3 mL). bIsolated yield
amounts of the catalyst in H2O as solvent in order to determine the efect of the
catalyst amount. Based on data displayed in Table 1, the reaction yield increased
when the amount of the catalyst enhanced from 0.005 to 0.01 g and the best result
was attained in the presence of 0.01 g of the catalyst. Moreover, reducing the
amount of catalyst led to a decrease in the yield of the product (Table 1, entry 7).
Notably, further increase in the amount of catalyst had no impacts on the yield
and the reaction time (Table 1, entry 8) and also, no product was detected in the
absence of the catalyst even after prolonged reaction time (Table 1, entry 1). At a
later stage, the efect of reaction temperature was analyzed and the best result was
achieved at 25 °C. By increasing the temperature from 25 to 70 °C, no signifcant
rise in the yield was observed (Table 1, entry 9). The obtained results revealed
that the best outcomes were acquired when 0.01 g of catalyst was utilized at
room temperature in an aqueous medium (Table 1, entry 2). Moreover, horsetail
ash and Fe3O4 were separately used as catalysts in the model reaction under the
same optimized conditions, in order to evaluate the impact of the nanocatalyst
species on the reaction progress (Table 1, entries 10, 11). The catalytic perfor-
mances of Fe3O4 as catalyst were examined in model reaction and the obtained
results revealed that Fe3O4 based on having Lewis acidic properties could pro-
duce the product in trace amounts (Table 1, entry 11). According to the analy-
ses performed on the horsetail ash which were fully investigated in our previous
study, the horsetail ash was found to act as an acid catalyst itself whose catalytic
features have been proved as Lewis acid. Consequently, further analyses of the
received data indicated that when horsetail ah was applied separately as a catalyst
in model reaction in the current research, the reaction was relatively good which
yield to a higher product. All things considered; however, the catalytic activity
of Fe3O4@HA was signifcantly higher leading to better outcomes. We attribute
the obtained results to several factors including the conversion of horsetail ash to
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N. Hosseini Mohtasham, M. Gholizadeh
nanomagnetic counterpart as well as the presence of Fe together with other met-
als existing in horsetail ash which have increased the catalytic activity of Fe3O4@
HA.
Subsequently, with the optimal reaction conditions in hand, to investigate the ef-
ciency and applicability of Fe3O4@HA as a magnetic nanocatalyst in the prepara-
tion of 2,4,5-trisubstituted imidazole derivatives, benzil 1 were treated with various
aryl aldehydes 2 and ammonium acetate 3 in water (3 mL) at room temperature, to
gain the corresponding products with good yields and short reaction times (Table 2,
entries 1–16). Interestingly, the required imidazole products were achieved in excel-
lent yields for both electron-rich and electron-poor aromatic aldehydes. Heteroar-
omatic aldehydes such as thiophene-2-carboxaldehyde and 1-naphthaldehyde also
provided the corresponding products in high yields. The characterization of all the
synthesized 2,4,5-trisubstituted imidazoles has been carried out through melting
points and spectroscopic techniques such as FT‐IR, 1H NMR and 13C NMR.
Eventually, the efectiveness of our presented protocol was investigated carefully
in a comparative manner with some methods having been formerly reported for the
synthesis of desired molecules. Considering the data reported in Table 3, a high
competitive potential of the catalytic system described in the current paper com-
pared with other previously reported systems was observed.
Following the patterns provided in the previous reports [40, 67] and in regard to
our fndings, a simple and satisfying mechanism for the synthesis of 2,4,5-trisubsti-
tuted imidazoles was proposed in Scheme 3. Initially, the reaction was considered
to begin by an in situ generated ammonia from ammonium acetate. Meanwhile, the
carbonyl groups of aldehyde [A] and benzil [B] were activated by Fe3O4@HA to
provide intermediates [C] and [D], respectively. At a later stage, imine intermedi-
ates [E] and [F] were generated by nucleophilic addition of ammonia to activated
aldehyde [C] and activated benzil [D], respectively. Then after, imine intermediate
[E], condensed with the carbonyl group of [F] to produce carbocation [G]. Carboca-
tion [G], right after the attack of the nitrogen atom of imine to the carbon atom of
the iminium-positive center, supplies the cyclization with dehydration to form iso-
imidazole [H], which successively rearranges the trisubstituted imidazoles by a [1,
5]-H shift.
Due to the fact that the reusability and easy recovery are among the most desir-
able and appealing characteristics of nanomagnetic catalysts, throughout our study,
we focused on the reusability feature of Fe3O4@HA that is actually required and
necessary for industrial applications. As a result, the reutilization trait of Fe3O4@
HA as a nanocatalyst was examined in the model reaction (Fig. 8). Following the
completion of the reaction, in order to separate nanocatalyst, hot ethanol was added
to the reaction mixture. Then, the magnetic catalyst was readily isolated applying an
external magnet, washed with ethanol and fnally weighed for the further use under
the same reaction conditions. As refected in Fig. 8, it should be noted that after the
catalyst was reused for six times in the same reaction and under the identical condi-
tions, only an insignifcant diminish in the yield of product was observed, indicating
that the recovered catalyst after being used for six runs had no obvious change in
structure as well as morphology, referring to the FT-IR spectra and TEM images in
comparison with the fresh one.
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Magnetic horsetail plant ash (Fe3O4@HA): a novel, natural…
Table 2 Synthesis of 2,4,5-trisubstituted imidazole derivatives catalyzed by Fe3O4@HAa
Entry Aldehyde
1
Product
(4a)
Time (min) Yieldb (%) M.P. °C (Lit.Ref.
)
3
99
274–275 (275–276 [53])
2
5
96
261–263 (262–264 [54])
(4b)
3
4
7
6
97
99
191–193 (190–192 [53])
233–234 (232–233 [55])
(4c)
(4d)
5
6
10
9
95
93
267–269 (267–270 [56])
231–233 (230–233 [57])
(4e)
(4f)
7
8
10
20
91
92
296–298 (298–299 [58])
229–231 (232–233 [59])
(4 g)
(4 h)
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N. Hosseini Mohtasham, M. Gholizadeh
Table 2 (continued)
Entry Aldehyde
9
Product
(4i)
Time (min) Yieldb (%) M.P. °C (Lit.Ref.
)
25
93
257–259 (258–260 [31])
10
11
10
94
205–207 (204–207 [60])
261–263 (260–262 [54])
(4j)
12
98
(4 k)
12
13
15
8
95
92
230–232 (232–233 [61])
229–231(228–231 [57])
(4 l)
(4 m)
(4n)
14
15
16
12
20
15
87
90
91
255–257 (256–258 [62])
295–297 (297 [27])
(4o)
(4p)
261–263 (260–262 [60])
aReaction condition: benzil (1 mmol), aromatic aldehydes (1 mmol) and ammonium acetate (2 mmol) in
the presence of 10 mg of Fe3O4@HA in 3 mL H2O at room temperature. bIsolated yield
1 3

Magnetic horsetail plant ash (Fe3O4@HA): a novel, natural…
Table 3 Comparison of catalytic activity of Fe3O4@HA with other catalysts previously reported in the
literature for synthesis of compound 4aa
Entry Catalyst
Reaction Condition
Time (min) Yieldb (%) Ref
1
2
[Hbim]BF4 c(4 mmol)
100 ˚C
60
95
97
[34]
[41]
Yb(OPf)3 d(0.004 mmol)
HOAc, 2 mL; perfuorodeca- 6 h
lin (C10F18, cis and trans-
mixture), 1.5 mL, 80 ˚C
3
4
I2 (5 mol%)
EtOH (2 mL), 75 ˚C
15
45
99
97
[29]
[63]
Sulphanilic acid (10 mol%) EtOH / H2O (1:1 v/v;
20 mL), 80 ˚C
5
6
7
8
DABCO (0.7 mol %)
Scolecite (2 wt%)
-
-
t-BuOH (10 mL), 60–65 ˚C
EtOH (15 mL), refux
Lactic acid (1 mL), 160 ˚C
12 h
35
180
5
92
96
90
93
[38]
[64]
[53]
[65]
Microwave irradiated at
150 W and 120 ˚C
9
10
11
-
EtOH, refux
EtOH (1 mL), 78 °C
40
2 h
77
95
98
[66]
[67]
[62]
Fe3O4@g-C3N4e (20 mg)
BO3H3f (5 mol%)
EtOH / H2O (5:5 mL), Ultra- 30
sonication, r.t
12
13
[P4-VP]-Fe3O4g (100 mg)
Fe3O4 (5 mol%)
100 ˚C
35
98
96
[54]
[40]
EtOH (10 mL), Ultrasonica- 25
tion, r.t
14
15
16
17
18
(NBS)h (15 mol%)
Silica sulfuric acid (0.5 g) water (30 ml), refux
Al-MCM-41 (10 mg)
KH2PO4i (5 mol%)
Fe3O4@HA
120 ˚C
50
4 h
45
40
3
89
73
91
93
99
[61]
[68]
[69]
[70]
120 ˚C
EtOH, refux
H2O (3 mL), r.t
This Work
b
aReaction condition: benzil (1 mmol), benzaldehyde (1 mmol) and ammonium acetate (2 mmol). Iso-
lated yield. c1-Butyl imidazolium tetrafuoroborate. dYtterbium perfuorooctanesulfonate. eMagnetic gra-
f
phitic carbon nitride. Boric acid. gCross-linked poly(4-vinylpyridine) supported Fe3O4 nanoparticles.
hN-bromosuccinimide. iPotassium dihydrogen phosphate
Conclusion
To conclude, this study aimed to prepare horsetail ash from the exposure of horse-
tail to a high temperature at 400 °C. Based on the data acquired from the analysis
of horsetail ash, it can be concluded that it possess acidic properties which are due
to the presence of metallic ions in it, indicating that it could act as a potential lewis
acid catalyst whose catalytic activity was completely examined and investigated in
our earlier research. In the present work, it is highly recommended that the horsetail
ash could be used as a natural silica resource having mesoporous structure. Accord-
ingly, studies have been carried out to analyze the practical applications of horse-
tail ash as good support for the synthesis of magnetic horsetail ash and Fe3O4@
HA as a magnetically heterogeneous solid acid nanocatalyst was prepared which
was subsequently fully characterized by various analytical methods. This green and
1 3

N. Hosseini Mohtasham, M. Gholizadeh
Scheme 3 Proposed mechanistic pathway for the preparation of 2,4,5-trisubstituted imidazole deriva-
tives in the presence of Fe3O4@HA
Fig. 8 Synthesis of 2,4,5 Triphenyl-1H-imidazole (4a) in the presence of reused Fe3O4@HA
new synthesized magnetic nanocatalyst was applied for the one-pot synthesis of
2,4,5-trisubstituted imidazole derivatives. The signifcant advantages of this meth-
odology are characterized as excellent activity and stability, simple and efective
1 3

Magnetic horsetail plant ash (Fe3O4@HA): a novel, natural…
separation utilizing an appropriate external magnet resulting in minimizing the loss
of catalyst during separation and its reusability for several times with no great loss
of activity which make the present synthetic methodology more convenient and
environmentally friendly in nature.
Supplementary Information The online version contains supplementary material available at https://doi.
org/10.1007/s11164-021-04420-y.
Acknowledgements We are thankful to Ferdowsi University of Mashhad Research Council for fnancial
support to this work (Grant no. p/3/45801).
Declarations
Confict of interest The authors declare that they have no confict of interest.
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Magnetic horsetail plant ash (Fe3O4@HA): a novel, natural…
Authors and Afliations
Nina Hosseini Mohtasham1 · Mostafa Gholizadeh1
* Mostafa Gholizadeh
m_gholizadeh@um.ac.ir
1
Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran
1 3