Nano silica extracted from horsetail plant as a natural silica support for the synthesis of H3PW12O40 immobilized on aminated magnetic nanoparticles (Fe3O4@SiO2-EP-NH-HPA): a novel and effi

Article abstract of DOI:10.1007/s11164-020-04133-8

Abstract: In this work, nanomesoporous silica has been prepared from horsetail (Equisetum arvense), which is a medicinal plant, with high surface area. This natural silica has been applied as a support to prepare H₃PW₁₂O₄₀

Full text of DOI:10.1007/s11164-020-04133-8

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-020-04133-8
Nano silica extracted from horsetail plant as a natural
silica support for the synthesis of H3PW12O40 immobilized
on aminated magnetic nanoparticles (Fe3O4@
SiO2‑EP‑NH‑HPA): a novel and efcient heterogeneous
nanocatalyst for the green one‑pot synthesis
of pyrano[2,3‑c]pyrazole derivatives
Nina Hosseini Mohtasham1 · Mostafa Gholizadeh1
Received: 4 December 2019 / Accepted: 19 March 2020
© Springer Nature B.V. 2020
Abstract
In this work, nanomesoporous silica has been prepared from horsetail (Equisetum
arvense), which is a medicinal plant, with high surface area. This natural silica has
been applied as a support to prepare H3PW12O40 immobilized on aminated epibro-
mohydrin-functionalized Fe3O4@SiO2 nanoparticles (Fe3O4@SiO2-EP-NH-HPA) as
a highly powerful magnetically solid acid catalyst. Characterization of this nanocata-
lyst has been investigated by a variety of techniques such as FTIR, XRD, N2 adsorp-
tion–desorption, TEM, SEM–EDX, TGA, VSM, ICP-OES and elemental analysis.
This magnetically solid acid nanocatalyst has been successfully used for the one-pot
green synthesis of pyrano[2,3-c]pyrazole derivatives in an aqueous medium at room
temperature. The procedure resulted in structurally diferent pyrano[2,3-c]pyrazoles
in excellent yields in very low reaction times. Moreover, this magnetically solid acid
nanocatalyst can be simply recovered by an external magnetic feld and reused for
seven consecutive reaction cycles without signifcant loss of activity.
Graphic abstract
In this work, Fe3O4@SiO2-EP-NH-HPA as a highly powerful magnetically solid
acid nanocatalyst based on natural silica which was extracted from a medicinal plant
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4-020-04133-8) contains supplementary material, which is available to authorized users.
* Mostafa Gholizadeh
m_gholizadeh@um.ac.ir
1
Department of Chemistry, Faculty of science, Ferdowsi University of Mashhad, Mashhad, Iran
Vol.:(0123456789)
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N. Hosseini Mohtasham, M. Gholizadeh
was prepared and characterized using various techniques. The activity of the new
catalyst was tested for the synthesis of pyrano[2,3-c]pyrazole derivatives.
+
+
Keywords Horsetail plant · Natural silica · Heteropolyacid · Heterogeneous
catalyst · Pyrano[2,3-c]pyrazoles
Introduction
According to the “green and sustainable chemistry,” the application of the envi-
ronmentally benign reagents and reaction conditions is one of the most pleas-
ing developments in synthetic organic chemistry [1]. The green catalytic proto-
cols and environmentally friendly reaction conditions in which the use of volatile
organic solvents, toxic reagents, hazardous or harsh reaction conditions as well
as challenging and time-consuming wasteful separations has been avoided are of
signifcant importance to the chemists who make scientifc eforts to avoid the
annihilation of our blue planet for a sustainable future [2]. Hence, the improve-
ment in a novel, eco-friendly, nontoxic, inexpensive, stable and recyclable cata-
lysts with improved efciency has drawn a great deal of attention, regarding the
green chemistry demands [3–5]. Keggin-type heteropolyacids (HPAs), specif-
cally H3PW12O40, have attracted more attention due to their great acidity, high
catalytic activity and low toxicity leading to be known as eco-friendly compounds
[6]. Nevertheless, owing to their homogeneous structure, these compounds show
some limitations in their catalytic activity and separation from the reaction mix-
ture, which are identifed as poor stability and high solubility in polar solvents
[7]. Besides the formation of immense amounts of waste products, these disad-
vantages lead to wasting energy which is not in accordance with the concept of
“Green Chemistry.” Therefore, the heterogenization of these homogeneous cata-
lysts through their immobilization on various solid supports has been developed
as a main solution to overcome the problems caused by such catalysts [8, 9].
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Nano silica extracted from horsetail plant as a natural silica…
However, structural modifcations of the HPAs on the solid supports such as sil-
ica, carbon nanotube, titania and alumina which circumvent some of these prob-
lems have also been reported [10–18]. This process leads to the improvement in
the aspect of environmental compatibility, as well as economic aspect, in numer-
ous industrial manufacturing processes.
The unique properties of magnetite nanoparticles (Fe3O4) including potentiality
for large-scale production with low cost, high thermal and mechanical stability have
aroused a notably interest in the past decade. Understandably, their paramagnetic
structure allows simple and efcient separation from the reaction mixture which
skips over the standard fltration required in the separation of heterogeneous cata-
lysts. Magnetite nanoparticles (Fe3O4) can be separated from the reaction mixture
using an external magnet [19]. However, the implication of magnetic nanoparti-
cles has some problems, one of which is known as agglomeration [20]. Pleasantly,
this can be avoided by coating the catalyst surface with various organic or inor-
ganic shells including silica, polymers, biomolecules, metals, etc. [21, 22], among
which silica is the best choice due to its inherent properties such as thermal stability
against degradation, biocompatibility and high stability in aqueous media, besides
presenting a preferable surface area and therefore desirable loading of active sites
[23]. Nowadays, sol–gel processing or vapor-phase reaction using silicon alkoxides
such as TEOS (tetraethyl orthosilicate)/TMOS (tetramethyl orthosilicate) as preced-
ing materials is almost used to produce silica powders with extremely high level
of purity [24]. However, their preparations including the initial precursors are very
expensive, which results in their extreme usage limitation. Silica-rich plant materi-
als such as rice husk [25], wheat hull and wheat hull ash [26], bagasse ash [27],
lemon grass [28], etc., can be used as an alternative to prepare nanometer-size and
highly purifed silica based on high volume and low cost in an almost amorphous
form. Therefore, it is considered as a cheap, highly reactive silicon source for the
production of various silicon-based materials with new properties, which may be of
technical and industrial importance. Due to the presence of silica in association with
organic tissue, such as polysaccharides or proteins in these plants, an appropriate
strategy is required to extract silica from plant materials with specifc characteristics
(for instance, having large surface area and high porosity in comparison with the
native plant itself). Horsetail can be approximately defned as a feld of Equisetum
arvense and is a widely distributed plant in almost all parts of the world [29], and
it is known as one of the most densely silicifed plants in which silica is deposited
mostly in amorphous form [30] (Scheme 1). For decades, horsetail which is believed
to have some medicinal properties due to its high silica content has been extensively
applied in medicine [31–33].
Having attracted enormous attention in recent years, multicomponent reactions
(MCRs) are one-pot processes in which three or more components come together
to form a product containing substantial elements of all the reactants [34]. This
approach brings about especial advantages over common reactions, due to the con-
vergence, inherent atom economy, operational simplicity and shortness of their syn-
thetic pathway [35]. These reactions have recently acquired a new dimension to pre-
sent an invaluable protocol for the formation of massive libraries of drugs such as
compounds and drug discovery developments [36].
1 3

N. Hosseini Mohtasham, M. Gholizadeh
Scheme 1 Image of the horse-
tail plant
Pyrano-pyrazole derivatives which include wide range of biological activities such
as exhibiting analgesic, anti-infammatory potencies as well as being re-known as
vasodilators, hypotensive and hypoglycemic agents have been in focus of signifcant
research by scientists [37, 38].
In the current research after reviewing the functionalized magnetic nanoparticles
and also their efcient use as a magnetically separable catalyst, pure SiO2 having herbal
origin has been directly extracted from horsetail plant and was used for the frst time
as natural mesoporous silica support. As a result, acidic-functionalized magnetite
nanoparticles have been studied and H3PW12O40 has been immobilized over amine-
functionalized Fe3O4@SiO2 nanoparticles, which increased its activity in comparison
with pure H3PW12O40. Consequently, the catalytic scope of Fe3O4@SiO2-EP-NH-HPA
as a newly prepared magnetically nanocatalyst has been examined in the synthesis of
pyrano[2,3-c]pyrazole derivatives via one-pot multicomponent reactions using ethyl
acetoacetate, hydrazine hydrate, various aromatic aldehydes and malononitrile as start-
ing materials.
Notably, the present study aimed to improve the catalytic properties of heteropo-
lyacid with the use of natural silica as support and to show the high performance of
Fe3O4@SiO2-EP-NH-HPA as a reusable solid acid nanocatalyst, which furnished a
green and suitable strategy for synthesis of pyrano[2,3-c]pyrazole derivatives in short
reaction times.
1 3

Nano silica extracted from horsetail plant as a natural silica…
Experimental section
Materials and instruments
All chemical reagents and solvents were purchased from Merck and Sigma-Aldrich
chemical companies and used without further purifcation. All reactions and the
purity of obtained products were monitored by thin-layer chromatography (TLC)
utilizing aluminum plates coated with silica gel polygram STL G/UV 254 (Merck)
using n-hexane and ethyl acetate as eluents. The melting points of products were
determined with an electrothermal type 9100 melting point apparatus. Mass analysis
was performed by Agilent technology (HP) 5973, mass spectrometer operating at
an ionization potential of 70 eV. The 1H NMR and 13C NMR spectra were recorded
on a Bruker Avance, at 300 and 75 MHz, respectively, using TMS as an internal
standard and DMSO-d6 as the solvent. The Fourier-transform infrared (FTIR) spec-
tra were recorded from KBr disk within the region range of 400–4000 cm−1 using
an Avatar 370 FTIR Thermo Nicolet spectrometer. Wide-angle powder X-ray dif-
fraction (XRD) patterns were obtained on a PANalytical Company X’Pert Pro MPD
difractometer with Cu Ka (λ=0.154 nm) X-ray irradiation source in a 2θ range
between 10° and 80°. Surface areas of the samples were measured by adsorption
of nitrogen gas at 77 K and the application of Brunauer–Emmett–Teller (BET) cal-
culation, whereas pore size and pore-size distribution were determined using the
methods proposed by Barrett, Joyner and Halenda (BJH) and by Dollimore and Heal
(DH). A transmission electron microscope (TEM) was conducted on a Leo 912 AB
microscope (Zeiss, Germany) operated at 120 kV. Scanning electron microscopy
(SEM) images were taken using Leo1450 VP scanning electron microscope operat-
ing at an acceleration voltage 20 kV. Elemental compositions were determined with
an SC7620 energy-dispersive X-ray analysis (EDX) presenting a 133 eV resolution
at 20 kV. Thermal stability of the catalyst was investigated by thermogravimetric
analysis (TGA) using a Bahr STA-503 instrument at a heating rate of 10 °C min−1
under air atmosphere. The magnetic property of catalyst was recorded by the vibra-
tional sampling magnetometry (VSM) technique in 1.5 T external magnetic felds
at room temperature, by using magnetic Danesh Pajouh instrument. Inductively
coupled plasma optical emission spectroscopy (ICP-OES) was carried out on a
76004555 SPECTRO ARCOS ICP-OES analyzer. All yields refer to isolated prod-
ucts after purifcation by recrystallization.
Catalyst preparation
Preparation of silica from horsetail plant
Horsetail plant was purchased from the market, Iran, Mashhad, washed several
times with distilled water to remove any adhering and cleaving materials and
dried at room temperature for 24 h. Then, the dried plant was smashed and sieved
(80 mesh size), again washed with distilled water and dried at 110 °C for 5 h.
1 3

N. Hosseini Mohtasham, M. Gholizadeh
a
b
c
d
Scheme 2 Preparation of silica from horsetail plant, a) native horsetail plant, b) horsetail plant after
being smashed and sieved (80 mesh size) as green powder, c) sieved horsetail after treatment with HCl as
brown powder and d) extracted silica as white color ash
Fig. 1 FTIR spectrum of a
horsetail plant and b isolated
silica
Subsequently, purifed powdered horsetail (8 g) was taken in a round-bottom fask
and 400 mL 4 M hydrochloric acid was added to it. The resulting dispersion was
refuxed for 2 h at boiling temperature to remove all inorganic substances except
silica. After cooling, the supernatant liquid was discarded and the HCl-treated
horsetail was repeatedly centrifuged and washed with deionized water until the
pH of the water became nearly 7. The resulting mass was dried in an air oven at
50 5 °C followed by the calcination process to completely remove the organic
components. The calcination was carried out in the air atmosphere with a heating
rate of 1 K/min and 2 h holding time at 500 °C [29] (Scheme 2). The synthesis
of natural pure silica was successfully confrmed with FTIR and EDX analysis
(Figs. 1 and 9, respectively).
1 3

Nano silica extracted from horsetail plant as a natural silica…
Preparation of Fe3O4 magnetic nanoparticles supported on extracted silica
In a typical synthesis, 2 g of natural synthesized silica was dispersed in 60 mL of
deionized water in a beaker and FeCl3·6H2O (5 g) and FeCl2·4H2O (2 g) were slowly
added to it. The mixture was stirred strongly at 80 °C to obtain a clear solution, and
then aqueous ammonia was added to it until the pH of 12 was obtained. The solution
was kept at 80 °C under vigorous stirring for further 30 min. After that, a bar mag-
net was held at the bottom of the beaker and all the nanoparticles got stuck to the
bottom, and the aqueous part was poured out. By using this technique, the nanopar-
ticles were washed repeatedly with deionized water and fnally dried in vacuum at
50 °C for 6 h. The material was designated as Fe3O4@silica (see scheme 3, Stage I).
Preparation of epibromohydrin‑functionalized Fe3O4@SiO2
Fe3O4@SiO2 NPs (1 g) was dispersed in 6 mL pure epibromohydrin by sonication
for 30 min. The mixture was heated at 60 °C with vigorous stirring. After 24 h, the
epibromohydrin-functionalized Fe3O4@SiO2 was separated by an external magnet
and washed with MeOH (5×10 mL) until the additional amount of epibromohydrin
was removed. Then, the obtained Fe3O4@SiO2-EP was dried at 40 °C under vacuum
for 18 h (Scheme 3, Stage II).
Preparation of ethylenediamine immobilized on epibromohydrin‑functionalized
Fe3O4@SiO2
To a suspension of Fe3O4@SiO2-EP (1 g) in 50 mL toluene, ethylenediamine (5 mL)
was added. The resulting mixture was stirred at 80 °C for 24 h. After completion,
the resulting Fe3O4@SiO2-EP-NH as a dark-brown precipitate was separated by an
external magnet, washed with MeOH and dried at 80 °C for 14 h (Scheme 3, Stage
III).
Scheme 3 Schematic representation of the synthesis of Fe3O4@SiO2-EP-NH-HPA (IV) nanocatalyst
1 3

N. Hosseini Mohtasham, M. Gholizadeh
General procedure for the preparation of H3PW12O40 loaded on the magnetic core
of Fe3O4@SiO2‑EP‑NH
Fe3O4@SiO2-EP-NH (1 g) was suspended in a methanolic solution of phospho-
tungstic acid (1 g in 50 mL), and the mixture was briefy sonicated and then stirred
at 60 °C for 24 h. The resulting solid was separated with an external magnet and
repeatedly washed with MeOH and dried at 60 °C for 8 h and designated as Fe3O4@
SiO2-EP-NH-HPA. The total synthesis is summarized in Scheme 3, Stage IV.
Catalytic activity
General procedure for the preparation of pyranopyrazoles
To a prestirred mixture of ethyl acetoacetate 1 (1 mmol) and hydrazine hydrate 2
(1 mmol) in 25 mL round-bottom fask in water (5 mL), various aromatic aldehydes
3(a–t) (1 mmol) and malononitrile 4 (1 mmol) along with Fe3O4@SiO2-EP-NH-
HPA (20 mg) as a catalyst were added. The resulting mixture was stirred at room
temperature for certain time periods as indicated in Table 4. After the reaction was
completed (monitored by TLC: n-hexane–ethyl acetate, 4:1), water was removed
from the reaction mixture under reduced pressure followed by dissolving the product
in warm ethanol (5 mL). The catalyst was then separated using an external magnet
and washed with ethanol and dried to be used in other reactions. Subsequently, the
solvent (ethanol) was evaporated to aford the pure product. And fnally, the isolated
compounds were characterized by melting point analysis, FTIR, mass, 1H NMR and
13CNMR spectroscopy.
Physical and spectral data of some products
6‑Amino‑3‑methyl‑4‑(4‑nitrophenyl)‑2,4 dihydropyrano[2,3‑c]pyrazole‑5‑carbonitr
ile(5d) White solid; Mp: 249–251 °C; FTIR (KBr, cm−1): 3477, 3284, 3229, 3118,
1
2196, 1650, 1594, 1401, 744; H NMR (300 MHz, DMSO-d6): δ (ppm) 1.82 (s,
3H, CH3), 4.85 (s, 1H, Aliphatic-H), 7.08 (s, 2H, NH2), 7.47–7.50 (d, 2H, Ar–H),
8.22–8.24 (d, 2H, Ar–H), 12.23 (s, 1H, NH). 13C NMR (75 MHz, DMSO-d6): δ
(ppm) 10.22, 36.35, 56.37, 97.03, 120.96, 124.37, 129.31, 136.34, 146.85, 152.57,
155.14, 161.61. MS (ESI) m/z for (297).
6‑Amino ‑3‑methyl‑4‑(3‑nitrophenyl)‑2, 4‑ dihydropyrano[2, 3‑ c]pyra‑
zole‑5‑carbonitrile(5e) White solid; Mp: 195–197 °C; IR (KBr, cm−1): 3473, 3224,
1
3121, 2195, 1654, 1596, 1400, 733; H NMR (300 MHz, DMSO-d6): δ (ppm) 1.83
(s, 3H, CH3), 4.90 (s, 1H, Aliphatic-H), 7.08 (s, 2H, NH2), 7.64–7.71 (m, 2H, Ar–H),
8.05 (s, 1H, Ar–H),8.13–8.17 (m, 1H, Ar–H), 12.23 (s, 1H, NH). 13C NMR (75 MHz,
DMSO-d6): δ (ppm) 10.25, 36.12, 56.59, 97.14, 121.0, 122.32, 122.47, 130.73,
134.87, 136.37, 147.30, 148.35, 155.16, 161.61. MS (ESI) m/z for (297).
1 3

Nano silica extracted from horsetail plant as a natural silica…
6‑Amino‑4‑(4‑cyanophenyl)‑3‑methyl‑2,4‑dihydropyrano[2,3‑c]pyrazole‑5‑carboni‑
trile (5p) White solid; Mp: 228–230 °C; IR (KBr, cm−1): 3480, 3284, 3111, 2228,
2188, 1641, 1492, 558; 1H NMR (300 MHz, DMSO-d6): δ (ppm) 1.81 (s, 3H, CH3),
4.77 (s, 1H, Aliphatic-H), 7.05 (s, 2H, NH2), 7.39–7,42 (dd, 2H, Ar–H), 7.81–7,84
(dd, 2H, Ar–H), 12.21(s, 1H, NH); 13C NMR (75 MHz, DMSO-d6): δ (ppm) 10.20,
36.61, 56.48, 97.11, 110.09, 119.27, 121.0, 129.07, 133.08, 136.27, 150.53, 155.16,
161.59. MS (ESI) m/z for (276).
6‑Amino‑4‑(5‑bromo‑2‑hydroxyphenyl)‑3‑methyl‑2,4‑dihydropyrano[2,3‑c]pyra‑
zole‑5‑carbonitrile (5r) Pale yellow solid; Mp: 229–231 °C; IR (KBr, cm−1): 3456,
3429, 3354, 3214, 2189, 1658, 1478; 1H NMR (300 MHz, DMSO-d6): δ (ppm) 2.05
(s, 3H, CH3), 3.41 (s,1H, OH), 4.66 (s, 1H, Aliphatic-H), 6.81 (s, 2H, NH2), 6.95–
6.98 (d, 1H, Ar–H), 7.15 (s, 1H, Ar–H), 7.35–7.38 (d, 1H, Ar–H), 10.56 (s-broad,
1H, NH); 13C NMR (75 MHz, DMSO-d6): δ (ppm) 10.3, 29.1, 55.2, 105.0, 116.0,
118.4, 121.0, 126.6, 130.8, 131.6, 137.1, 148.2, 159.5, 160.2. MS (ESI) m/z for (346).
Elemental Analysis (CHN): C: 49.22, H: 3.46, N: 13.48%.
Results and discussion
Recently, the introduction and the designing of a better protocol for the synthesis
of highly efcient and environmentally friendly novel heterogeneous catalysts with
the use of green and safe reagents have become one of the most important parts of
our ongoing research program. Based on the information obtained from the studies
on Equisetum arvense, it was anticipated that this plant could be used as a plenti-
ful source of silica with high surface area and high porosity [30]. So in this work,
natural mesoporous silica with the high surface area was extracted and applied
for the synthesis of Fe3O4@SiO2-EP-NH-HPA with core–shell nanostructure as a
novel solid acid magnetic nanocatalyst (Scheme 2). The so-prepared magnetic nano-
catalyst was fully characterized using various techniques such as FTIR, XRD, N2
adsorption–desorption, TEM, SEM, EDX, TGA, VSM, ICP-OES and elemental
analysis. The obtained results are summarized in the following sections in which
all these techniques corroborate the successful preparation of the newly synthesized
mesoporous catalyst.
Characterization of novel Fe3O4@SiO2‑EP‑NH‑HPA
FTIR analysis
FTIR spectra are one of the methods which confrm the successful synthesis of the
nanocatalysts; therefore, they were used to analyze the synthesis of every step of the
nanocatalyst of this research. FTIR spectra of horsetail plant (Equisetum arvense)
and extracted silica are shown in Fig. 1. In the case of horsetail plant (Fig. 1a),
the peaks at 3460 and 1640 cm−1 are, respectively, attributed to the stretching and
1 3

N. Hosseini Mohtasham, M. Gholizadeh
Table 1 Specifc surface area
(SBET), pore volume and mean
pore diameter of horsetail plant,
isolated silica, Fe3O4@SiO2 (I)
and Fe3O4@SiO2-EP-NH-HPA
(IV)
Sample
SBET (m2 g−1
)
Total pore vol- Mean pore
ume (cm3 g−1
)
diameter
(nm)
Horsetail plant
Isolated silica
2.025
450
0.0094
0.66
7.63
8.12
6.77
5.68
Fe3O4@SiO2 (I) 245
0.56
Fe3O4@SiO2-EP- 180
0.35
NH-HPA (IV)
Fig. 2 FTIR spectra of a
isolated silica, b Fe3O4@SiO2
(I), c Fe3O4@SiO2-EP (II), d
Fe3O4@SiO2-EP-NH (III), e
pure H3PW12O40, f Fe3O4@
SiO2-EP-NH-HPA (IV) and g
seventh reused Fe3O4@SiO2-
EP-NH-HPA
bending modes of the Si–OH groups and the adsorbed water [39, 40]. The doublet
peaks appearing in the range of 2925 and 2855 cm−1 are characteristics of C–H
stretching vibrations, which prove the existence of aliphatic compounds in this
plant [41]. There is also a weak peak around 1734 cm−1, which was proposed to
be attributed to the C=O stretching of the related organic compounds [42]. The
depicted strong peaks at 1100, 813 and 468 cm−1 are assigned to Si–O–Si asymmet-
ric stretching, symmetric stretching and bending modes of silica, respectively [43],
which prove the presence of silica. This plant is regarded as a rich source of silica;
however, the presence of organic compounds and other metals causes the plant to
have a very small surface area (2.02 m2/g, Table 1), which limits its use as support
and further functionalization. As a result, we decided to extract pure silica having
a high surface area from the plant (450 m2/g, Table 1). The FTIR spectrum pre-
sented in Fig. 1b shows three signifcant peaks in 1098, 805 and 464 cm−1 that con-
frms all peaks that were related to the organic compounds have been lost and pure
silica was well synthesized. Furthermore, EDX analysis was carried out to prove
that metal constituents have been removed successfully from the plant (Fig. 9b).
The FTIR spectra of the fnal nanocatalyst which was synthesized in six steps along
with the seventh reused nanocatalyst are demonstrated in Fig. 2. As can be seen,
Fig. 2a presents the FTIR spectrum of synthesized pure silica mentioned above. As
presented in Fig. 2b, the main characteristic bond adsorption for the formation of
1 3

Nano silica extracted from horsetail plant as a natural silica…
iron nanoparticles is observed in the area of 581 cm−1 related to the stretching vibra-
tion of the Fe–O band [44]. Also, peaks observed at 1098 and 805 cm−1 which can
be correlated with the asymmetric and symmetric stretching of Si–O–Si confrm
the presence of silica coatings on the Fe3O4 nanoparticles. Successful coupling of
epoxy rings to the surface of silica-coated magnetic nanoparticles was demonstrated
by the stretching vibration of aliphatic CH2 groups of epibromohydrin at 2957 and
2875 cm−1 and C–O–C vibration stretching at 1240–1260 cm−1 [45]. In this case,
the vibration frequency of C–O–C was covered by the asymmetric vibration of
the Si–O–Si band. These results indicated that the Fe3O4@SiO2 surface had been
immobilized by covalent-bonded organic epoxy rings (Fig. 2c). Furthermore, the
ring opening of the epoxy ring with ethylenediamine was asserted by appearance of
the absorption broad bands around 3280 and 1550 cm−1 corresponding to the N–H
stretching and bending vibrations, respectively, which confrm the successful func-
tionalization of the Fe3O4@SiO2-EP with ethylenediamine (Fig. 2d) [46]. In Fig. 2e,
pure H3PW12O40 with a Keggin structure shows four strong bands at 1079 cm−1
(P–O), 980 cm−1 (W–O), the bands at 890 cm−1 and 802 cm−1 (W–O–W) and one
weak band at 524 cm−1 (W–O–P) [47]. As shown in the FTIR spectrum of fnal
nanocatalyst (Fig. 2f), after supporting the H3PW12O40 on the Fe3O4@SiO2-EP-
NH, the above-mentioned diagnostic vibrational peaks could be still found in the
FTIR spectrum. Also, the vibrations of tetrahedral P–O bonds of Keggin units may
overlap with the characteristic asymmetric stretching modes of Si–O bonds in the
SiO2 structure (at 1098 cm−1) [48]. As depicted in Fig. 2g, the FTIR spectrum of the
Fe3O4@SiO2-EP-NH-HPA nanocatalyst demonstrates that almost no change occurs
after being used seven times.
X‑ray difraction analysis
The crystalline phase and purity of the synthesized material were indicated
by the powder XRD patterns. First, the crystallinity of silica in horsetail plant
Fig. 3 XRD patterns of a horse-
tail plant and b extracted silica
1 3

N. Hosseini Mohtasham, M. Gholizadeh
(Equisetum arvense) and silica derived from HCl treatment followed by calci-
nation is studied by X-ray difraction. In horsetail plant (Fig. 3a), a broad peak
appeared around 2θ equal to 22° indicating that the silica of the plant was mainly
in the form of amorphous [49]. As can be seen in Fig. 3b, silica extracted from
Equisetum arvense after HCl treatment followed by calcination shows several
crystalline peaks attributed to α-quartz besides a prominent broad peak of amor-
phous silica based on the powder difraction pattern [50]. Quartz is the only crys-
talline component detected in these samples. The crystalline nature of Fe3O4@
SiO2 along with X-ray difraction patterns of Fe3O4@SiO2-EP-NH-HPA and the
seventh recovered Fe3O4@SiO2-EP-NH-HPA is demonstrated in Fig. 4. Accord-
ing to curve 4b, six characteristic peaks at the values of 2θ equal to 30.4 (220),
35.8 (311), 43.8 (400), 54.2 (422), 57.2 (511) and 62.8 (440) correspond to the
characteristic peaks of cubic structure Fe3O4 MNPs (JCPDS: 19-0629) [51],
revealing a high phase purity of magnetite. Also, a new broad peak at 2θ = 22
related to the amorphous silica shell together with some crystalline α-quartz
peaks with low intensity was observed. The mean crystalline sizes of Fe3O4
nanoparticles are calculated to be 30 nm using Debye–Scherrer’s equation. As
illustrated in Fig. 4c, despite the immobilization of organic segment and form-
ing of Fe3O4@SiO2-EP-NH-HPA, the crystallinity of Fe3O4 nanoparticles is well
retained throughout the process, and in comparison with curve 4b, no remark-
able change occurred in the number of peaks in curve 4c. In addition, due to its
dispersion in the catalyst matrix, H3PW12O40 showed no peaks in the XRD pat-
tern. Hence, it can be concluded that immobilization of H3PW12O40 nanoparticles
cannot lead to disruption of the formation of a regular catalyst crystal network.
The presence of H3PW12O40 nanoparticles was verifed through the analysis of
ICP and EDX techniques. The obtained results achieved from the XRD pattern of
the seventh recovered Fe3O4@SiO2-EP-NH-HPA nanocatalyst indicate that there
was no signifcant diference in the chemical structure of catalyst before and after
seven times reuse (Fig. 4d).
Fig. 4 XRD patterns of a
extracted silica, b Fe3O4@SiO2
NPs (I), c Fe3O4@SiO2-EP-NH-
HPA (IV) and d seventh reused
Fe3O4@SiO2-EP-NH-HPA (IV)
1 3

Nano silica extracted from horsetail plant as a natural silica…
N2 adsorption–desorption studies
The structural and textural properties of isolated silica and Fe3O4@SiO2-EP-NH-
HPA (IV) were investigated through the N2 adsorption and desorption isotherms
results. The nitrogen sorption isotherm of isolated silica (Fig. 5a) shows a typi-
cal type IV isotherm with an H1 hysteresis loop, indicating the characteristic of
mesoporous silica network [52]. Furthermore, the corresponding pore-size dis-
tribution from a BJH analysis of the sorption isotherms shows a narrow size dis-
tribution peaked at around 6 nm (Fig. 5b). In addition, the adsorption–desorption
curve for Fe3O4@SiO2-EP-NH-HPA (Fig. 5c) displayed the characteristics of type
II isotherm with H3-type hysteresis loop demonstrating the existence of mesoporous
or macroporous structure according to the IUPAC classifcation [53]. Moreover,
the BJH calculations revealed a uniform pore-size distribution accompanied by a
high-intensity peak, which ascertains the high-regularity pore-size distribution for
Fig. 5 N2 adsorption–desorption isotherms and pore-size distributions for a, b isolated silica and c, d
Fe3O4@SiO2-EP-NH-HPA (IV), respectively
1 3

N. Hosseini Mohtasham, M. Gholizadeh
nanocatalyst (Fig. 5d). The specifc surface areas and pore volume and mean pore
diameter of horsetail plant, isolated silica, Fe3O4@SiO2 and Fe3O4@SiO2-EP-NH-
HPA were determined by applying the BET equation, and the numerical data are
listed in Table 1. As shown in Table 1, the isolated silica sample had a maximum
specifc surface area of 450 m2 g−1 in comparison with native horsetail, and all of
the functionalized materials had a smaller surface area, pore size and pore volume,
which can be easily realized due to the fact that the occupation of some pores by
anchored organic segments carried out successfully.
TEM analysis
The size and morphology of Fe3O4@SiO2-EP-NH-HPA nanocatalyst were investi-
gated by TEM technique corroborating the homogeneous spherical core–shell mor-
phology for Fe3O4@SiO2-EP-NH-HPA (Fig. 6a, b). Moreover, the TEM image of
seventh reused Fe3O4@SiO2-EP-NH-HPA catalyst, shown in Fig. 6c, d revealed that
Fig. 6 TEM image of a, b Fe3O4@SiO2-EP-NH-HPA (IV) and c, d seventh reused Fe3O4@SiO2-EP-NH-
HPA (IV)
1 3

Nano silica extracted from horsetail plant as a natural silica…
Fig. 7 Particle size distribution
histogram of Fe3O4@SiO2-EP-
NH-HPA (IV)
no signifcant change was detected in the morphology of the catalyst even after seven
recoveries. Also, particle size distribution histogram of the nanocatalyst which was
calculated according to the TEM analysis is presented in Fig. 7. This diagram was
obtained by measuring 150 separated particles and displayed that the particle sizes
of Fe3O4@SiO2-EP-NH-HPA in irregular geometric shape were about 9–32 nm
and the average diameter of the nanoparticles was about 15 nm. The attained TEM
results were in accordance with the small-angle XRD, as well as with data obtained
from nitrogen adsorption–desorption analyses.
SEM analysis
The size and surface morphology of native horsetail plant, isolated silica and
Fe3O4@SiO2-EP-NH-HPA catalyst were further determined using emission scan-
ning electron microscopy (SEM). As illustrated in Fig. 8, silicon was found at its
highest amount in the outer edge of both native horsetail plant and isolated silica
(Fig. 8a, b, respectively). The results presented by the SEM image of isolated silica
(Fig. 8b), which were achieved through HCl treatment followed by calcination and
resulted in elimination of mineral and organic compounds, surprisingly illustrated
that the original structure of horsetail plant was still perfectly retained even though
the merging of its rows has occurred owing to shrinkage, indicating that the accu-
mulation of silica over the whole outer surface forms a continuous layer of siliceous.
SEM images of the prepared nanocatalyst reveal that the core–shell nanoparticles
have spherical morphology with uniform size (Fig. 8c). Also, resulting images indi-
cate that the sizes of the catalyst particles are on the nanometer scale having a mean
diameter of about 9–32 nm with good disparity. These results are in good accord-
ance with the particle size extracted from the XRD, TEM and nitrogen adsorp-
tion–desorption analyses.
EDX analysis
Energy-dispersive X-ray spectroscopy (EDX) was conducted as another powerful
tool to completely confrm the successful synthesis of all steps of catalyst. The EDX
spectrum of horsetail (Equisetum arvense) (Fig. 9a) shows the presence of high con-
centrations of carbon related to organic compounds (e.g., cellulose, hemicellulose,
pectin) and alkali and alkaline-earth metals, for instance Si, Ca, K, Mg, S, Al, as
1 3

N. Hosseini Mohtasham, M. Gholizadeh
Fig. 8 SEM images of a horse-
tail plant, b isolated silica and c
Fe3O4@SiO2-EP-NH-HPA (IV)
MNPs
the main components [29]; however, the concentration of silica was revealed to be
higher than that of other metals. To extract silica with high surface area, HCl treat-
ment followed by calcination was performed on Equisetum arvense to completely
remove all inorganic and organic compounds except silica. Figure 9b reveals the
successful synthesis of silica, and it is worth noting that no impurity elements were
detected in the structure. The EDX spectrum of Fe3O4@SiO2-EP-EN-HPA (IV)
(Fig. 9c) shows the presence of Si, C, Fe, P, N and W proving the successive surface
functionalization of the silica-encapsulated magnetite nanoparticles, which confrms
the structure of the fnal nanocatalyst.
1 3

Nano silica extracted from horsetail plant as a natural silica…
Fig. 9 EDX analysis of a horsetail plant, b isolated silica and c Fe3O4@SiO2-EP-NH-HPA (IV) MNPs
Thermal analysis
To determine the thermal stability of Fe3O4@SiO2-EP-NH-HPA as well as the per-
cent of organic functional groups attached to the surface of the magnetic nanopar-
ticles, thermogravimetric analysis (TGA) was conducted and the obtained curve is
illustrated in Fig. 10. The TGA plot of Fe3O4@SiO2-EP-NH-HPA depicts two main
1 3

N. Hosseini Mohtasham, M. Gholizadeh
Fig. 10 TGA thermogram of
Fe3O4@SiO2-EP-NH-HPA (IV)
weight loss steps from the samples in the temperature range of 30–600 °C. As can
be seen, the frst weight loss of about 3% at a temperature below 200 °C was related
to the removal of physically trapped water molecules and the second weight loss of
about 15% in the temperature range of 200–600 °C was attributed to the thermal
decomposition of the organic parts that were grafted on the surface of the nano-
catalyst [54]. Based on TGA results, the amount of organic segments supported on
Fe3O4@SiO2 MNPs is estimated to be 1.2 mmol g−1, which is in good agreement
with the results obtained via elemental analysis data (CHNs). These results pre-
sented in Table 2 confrmed the loading amount of organic layer immobilized on
Fe3O4@SiO2 was 1.2 mmol g−1 (C=2.36%, N=2.25%), demonstrating that organic
ligand attached successfully to the surface of Fe3O4@SiO2 core–shell NPs.
Magnetic measurement
Paramagnetic particles are highly valuable for their magnetic separation, so the mag-
netic behavior of Fe3O4@SiO2 and Fe3O4@SiO2-EP-NH-HPA nanocatalyst was car-
ried out by VSM (vibrating sample magnetometer) technique at room temperature.
As shown in Fig. 11, the saturated magnetization values of Fe3O4@SiO2 (Fig. 11a)
and Fe3O4@SiO2-EP-NH-HPA (Fig. 11b) are 44, and 16.5 emu/g, respectively.
Compared to Fe3O4@SiO2 NPs, the saturation magnetization intensity of Fe3O4@
SiO2-EP-NH-HPA NPs was decreased, which was ascribed to organic functional
Table 2 Thermogravimetric analysis (TGA) and elemental analysis (EA) results
Sample
Percentage of weight loss (%) Organic grafted segments
Elemental
analysis
(%)
(mmol g−1
)
C
N
Fe3O4@SiO2-EP- NH-HPA 15
1.2
2.36 2.25
1 3

Nano silica extracted from horsetail plant as a natural silica…
Fig. 11 Magnetization curves
of a Fe3O4@SiO2 (I) and b
Fe3O4@SiO2-EP-NH-HPA (IV)
groups grafted on the surface of Fe3O4@SiO2. It is worth pointing out that Fe3O4@
SiO2-EP-NH-HPA still maintained a good magnetism and could be separated easily
by an external magnet from the reaction mixture, which is important for the mag-
netic separation and reusability of the magnetic catalyst. Also, it is worth mention-
ing that no remarkable hysteresis, remanence or coercivity was detected in the mag-
netization curves revealing superparamagnetic properties for nanocatalyst resulting
in easy separation from the reaction system [55, 56].
ICP‑OES analysis
Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis was
applied to realize the exact amount of tungsten in the fresh and reused nanocata-
lyst. This technique showed that 0.42 and 0.40 mmol of tungsten were anchored on
1.000 g of the fresh and seventh reused nanocatalyst, respectively. According to the
obtained results, no signifcant leaching of tungsten from the surface of Fe3O4@
SiO2-EP-NH-HPA NPs was observed even after seven recycle runs.
Application of Fe3O4@SiO2‑EP‑NH‑HPA
After characterizing the newly prepared nanocatalyst, it was aimed to investigate its
catalytic activity, as a magnetically recyclable heterogeneous solid acid nanocatalyst
for the one-pot four-component synthesis of dihydropyrano[2,3-c]pyrazole deriva-
tives. First of all, in order to achieve the optimum conditions for this reaction, model
substrates ethyl acetoacetate (1 mmol), hydrazine hydrate (1 mmol), malononitrile
(1 mmol), and 4-nitrobenzaldehyde (1 mmol) were selected and various parameters
such as the amount of catalyst, reaction temperature, reaction time, type of solvent
system as well as solvent-free conditions were evaluated. The results of all optimi-
zation experiments for the model reaction are listed in Table 3. The completions of
the reactions were monitored by TLC. As can be seen, the best result was obtained
using 20 mg of Fe3O4@SiO2-EP-NH-HPA (IV) at room temperature in an aqueous
1 3

N. Hosseini Mohtasham, M. Gholizadeh
Table 3 One-pot,
four-component
synthesis
of
6-amino-3-methyl-4-(4-nitrophenyl)-2,4-
dihydropyrano[2,3-c]pyrazole-5-carbonitrile(5d) under diferent reaction conditions
Entry
Catalyst(g)
Solvent
T (°C)
Time (min)
Conversion %
1
2
3
4
5
6
7
8
–
–
100
25
25
70
25
25
25
25
25
25
25
120
35
3
3
3
25
20
120
55
35
50
<5
52
99
99
99
78
85
<30
0
85
Fe3O4@SiO2-EP-NH-HPA (0.005)
Fe3O4@SiO2-EP-NH-HPA (0.02)
Fe3O4@SiO2-EP-NH-HPA (0.02)
Fe3O4@SiO2-EP-NH-HPA (0.04)
Fe3O4@SiO2-EP-NH-HPA (0.02)
Fe3O4@SiO2-EP-NH-HPA (0.02)
Fe3O4@SiO2-EP-NH-HPA (0.02)
Isolated SiO2(0.02)
Water
Water
Water
Water
EtOH
MeCN
–
Water
Water
Water
9
10
11
Fe3O4 (0.02)
H3PW12O40 (0.02)
45
medium (Table 3, entry 3). When the amount of catalyst was lower, the yield of
the product decreased (Table 3, entry 2). Further increase in the amount of cata-
lyst had no efects on the yield and the reaction time (Table 3, entry 5), and also,
in the absence of catalyst, lower product yield was observed, even after prolonged
reaction time (Table 3, entry 1). For choosing the reaction media, various solvents
such as H2O, EtOH and MeCN and also solvent-free conditions were used. In these
experiments, the most promising result was obtained when water was employed as
the solvent; therefore, water was selected as the encouraging solvent for the reaction.
Then, to optimize the reaction temperature, three diferent temperatures (25, 70 and
100 °C) were employed for the reaction and the best result was obtained at 25 °C.
By increasing the temperature from 25 to 100 °C, no noticeable increase in the yield
was observed. Furthermore, to estimate the efect of the nanocatalyst species on
the reaction progress, isolated SiO2, Fe3O4 and H3PW12O40 were separately used as
catalysts in the model reaction under similar optimized conditions (Table 3, entries
9, 10, 11). The catalytic performance of Fe3O4 as catalyst was checked in model
reaction indicating that Fe3O4 due to having Lewis acidic properties could produce
the product in trace amounts. (Table 3, entry 10). Further analyses of the results
pointed out that when isolated SiO2 was used separately as a catalyst in model reac-
tion, no product was obtained revealing the fact that just having high surface area
is not sufcient for promoting model reaction and this clearly showed that the reac-
tion was catalyzed by the acidic moiety (Table 3, entry 9). Moreover, by keeping
the other conditions the same, the efectiveness of applying pure H3PW12O40 as a
catalyst was also investigated. However, the result was far from satisfactory in this
context (Table 3 entry 11). Even though HPA is a homogenous strong acid in nature
and is expected to have more activity, the catalytic activity of Fe3O4@SiO2-EP-NH-
HPA (IV) was a great deal higher and provided better results due to its conversion to
nanometric counterpart. Therefore, the immobilization of HPA is highly efective in
reaction progress due to having more surface atoms in comparison with the conven-
tional one which results in more activity of the designed catalyst.
1 3

Nano silica extracted from horsetail plant as a natural silica…
After optimization of the reaction conditions, in order to determine and
investigate the scope and efciency of the novel synthesized Fe3O4@SiO2-
EP-NH-HPA (IV) as the next step, the focus of the study turned to apply this
nanocatalyst in the synthesis of a series of 6-amino-2,4-dihydropyrano[2,3 c]
pyrazol-5- carbonitrile derivatives. As a result, this catalyst was added (under
optimized condition mentioned before) to a mixture of ethyl acetoacetate 1,
hydrazine hydrate 2 in the reaction with various aromatic aldehydes 3 and malo-
nonitrile 4 in water (5 mL) at room temperature, to aford the corresponding
products in high yields and in short reaction times (Table 4). Interestingly, a
variety of aryl aldehydes bearing electron-withdrawing or electron-donating
substituents (ortho-, meta- and para-substituted) were very encouraging in this
reaction and gave the product in good to excellent yield (89–99%). In addition,
using benzaldehydes with electron-withdrawing groups (at C4), the product was
obtained in higher yields. This was achieved because, in the frst step of the
reaction mechanism, electron-withdrawing groups facilitated the nucleophilic
addition to the carbonyl position of the aromatic aldehyde. Evidently, electron-
releasing groups at C4 decreased the yield of the fnal product. It is worth men-
tioning that the formation and purity of products were confrmed by their melt-
ing point determination being in good accordance with the literature values and
the structures of some of the products were well characterized by using FTIR,
mass, 1H NMR and 13C NMR spectral data.
Finally, in order to compare the efciency of this synthesized magnetic nano-
catalyst with some of the other previously explored methods for this MCR, a
comparison was made and the model reaction of ethyl acetoacetate, hydra-
zine hydrate, malononitrile and 4-Nitro benzaldehyde was considered as a
representative example, and the results are classifed in Table 5. As shown in
Table 5, Fe3O4@SiO2-EP-NH-HPA(IV) has remarkably improved the synthesis
of pyrano[2,3-c]pyrazole derivatives and acted convincingly superior over the
reported methods concerning the reaction time and exhibited broad applicability
in terms of yields.
Based on the research fndings, one can propose a plausible synergic cata-
lytic mechanism for the synthesis of dihydropyrano[2,3-c] pyrazole derivatives
in the presence of Fe3O4@SiO2-EP-NH-HPA (IV) by the literature illustrated in
Scheme 4 [63, 73]. It is believed that the reaction proceeds in a catalytic cycle
which involves the synthesis of pyrazolone intermediate, Knoevenagel conden-
sation, Michael addition and then intramolecular cyclization. As presented in
scheme 4, both condensation of ethyl acetoacetate (A) with hydrazine hydrate
(B) and the Knoevenagel condensation of the aryl aldehyde (C) with the nitrile
anion (E) formed by the removal of acidic hydrogen from malononitrile were
simultaneously promoted to produce the pyrazolone (G) and the intermediate
olefn (F). Intermediate (G) was enolized in the presence of the catalyst to form
compound (H). Subsequent Michael-type addition of (H) and (F) in the presence
of the catalyst resulted in the in situ formation of intermediate (I). Finally, it
underwent an intramolecular cyclization followed by tautomerization so that it
could aford the desired dihydropyrano[2,3-c] pyrazole derivatives (J).
1 3

N. Hosseini Mohtasham, M. Gholizadeh
Table 4 Synthesis of pyrano[2,3-c]pyrazole derivatives catalyzed by Fe3O4@SiO2-EP-NH-HPA(IV)
M.P.b °C (Lit. References)
Time
(min)
Entry
Aldehyde (Ar)
Product
Yielda (%)
1
5
96
243–245 (245–256 [57])
(5a)
2
3
4
99
233–235 (234–236 [58])
(5b)
5
97
246–248 (247–248 [59])
(5c)
4
3
99
249–251 (249–251 [60])
(5d)
(5e)
5
6
6
7
95
91
195–197 (194–196 [58])
206–207 (207–209 [61])
1 3

Nano silica extracted from horsetail plant as a natural silica…
Table 4 (continued)
M.P.b °C (Lit. References)
Time
(min)
Entry
Aldehyde (Ar)
Product
Yielda (%)
(5f)
7
8
90
172–174 (171–173 [62])
(5g)
8
7
92
221-223 (220-221[60]
)
(5h)
9
6
91
254–256 (253–256 [63])
206–208 (204–205 [64])
(5i)
10
9
90
(5j)
11
3
6
96
95
179–181 (179–180 [57])
220–222 (218–220 [63])
(5k)
(5l)
12
1 3

N. Hosseini Mohtasham, M. Gholizadeh
Table 4 (continued)
Entry Aldehyde (Ar)
M.P.b °C (Lit. References)
Time
(min)
Product
Yielda (%)
13
14
15
9
10
7
94
209–211 (211–212 [57])
(5m)
93
251–253 (250–252 [65])
214–216 (215–217 [66])
(5n)
92
(5o)
16
6
97
228–230 (230–240 [63])
167–169 (162–164 [67])
(5p)
17
11
89
(5q)
18
10
96
229–231 (227–228 [57])
201–203 (200–201 [68])
(5r)
19
7
98
(5s)
(5t)
20
6
90
190–192 (192–194 [59])
1 3

Nano silica extracted from horsetail plant as a natural silica…
1 3

N. Hosseini Mohtasham, M. Gholizadeh
Scheme 4 Proposed mechanism for the preparation of the corresponding pyrano[2,3‐c] pyrazole deriva-
tives in the presence of Fe3O4@SiO2-EP-NH-HPA (IV)
Fig. 12 Synthesis of 6-amino-
3-methyl-4-(4-nitrophenyl)-2,4
dihydropyrano[2,3-c]pyrazole-
5-carbonitrile (5d) in the
presence of reused Fe3O4@
SiO2-EP-NH-HPA (IV)
1 3

Nano silica extracted from horsetail plant as a natural silica…
Reusability of the catalyst
For practical applications of heterogeneous systems, the lifetime of the catalyst and
its level of reusability are very important factors. In this regard, the reusability of
Fe3O4@SiO2-EP-NH-HPA (IV) was investigated for the model reaction under opti-
mized reaction conditions (Fig. 12). It is worth noting that in each process, after the
completion of the reaction, the catalyst was easily recovered by the use of an exter-
nal magnet, washed with ethanol and subsequently reused in the next run after being
dried. It was realized that the nanocatalyst could be reused efectively for at least
seven runs with insignifcant deterioration in activity, which is ascribed to the stable
functionalized groups on the catalyst. The obtained results from the FTIR (Fig. 2g),
XRD pattern (Fig. 4d), TEM images (Fig. 6c, d) and ICP-OES analysis of the sev-
enth recovered catalyst clearly indicated that there was not a great diference in the
chemical structure of catalyst before and after seven runs.
Catalyst leaching study
In order to make sure that the catalytic activity of the nanocatalyst originated from
immobilized functions within Fe3O4@SiO2-EP-NH-HPA (IV) and not from the
drained segments, especially H3PW12O40, in the reaction mixture, a leaching test
was applied, and the results are presented in Fig. 13. To this aim, Fe3O4@SiO2-EP-
NH-HPA (IV) (0.02 g) was added on model reaction under the optimized conditions.
After having continued the reaction for 1.5 min, consequently, the catalyst was sepa-
rated from the reaction mixture by the use of an external magnetic feld, in which
the product yield was reported to be 45%. Then, the resulting fltrate was allowed to
react for a further 1.5 min, while the progress of the reaction was being monitored
by TLC. Surprisingly, the results of the monitoring revealed that after the separation
of the catalyst, almost no signifcant increase in the fnal conversion was observed.
This afrmed the satisfactory steadiness of the Fe3O4@ SiO2-EP-EN-HPA (IV) in
the condensation reaction along with no destruction of the catalyst during the cata-
lytic reaction. Also, these data suggested that H3PW12O40 was strongly supported
on the Fe3O4@SiO2-EP-NH-HPA (IV) network, and no leaching of H3PW12O40 has
been occurred confrming its accordance with the ICP-OES results.
Fig. 13 Hot fltration test for the model reaction using Fe3O4@SiO2-EP-NH-HPA (IV)
1 3

N. Hosseini Mohtasham, M. Gholizadeh
Conclusion
To conclude, the fndings of the current research revealed that the natural
mesoporous SiO2 having a high surface area can be easily extracted from horsetail
plant as raw medicinal material and used for the frst time as natural silica support.
This natural silica, having an herbal origin, was applied to prepare Fe3O4@SiO2-
EP-NH-HPA (IV) as a magnetically heterogeneous solid acid nanocatalyst, which
was then fully characterized by diferent analytical methods. The results obtained
from the characterization confrmed the satisfactory synthesis of Fe3O4@SiO2-
EP-NH-HPA (IV) with spherical core–shell and mean diameters of 9–32 nm as a
magnetic nanocatalyst. Subsequently, this green and newly synthesized nanocatalyst
was applied for the one-pot synthesis of dihydropyrano[2,3-c] pyrazole derivatives,
exhibiting excellent activity and stability, simple and efective separation using an
appropriate external magnet leading to minimizing the loss of catalyst during sep-
aration and its reusability for several times with no signifcant loss of activity. It
is recommended that the application of such synthetic methodology for using this
medicinal plant with rich silica content, as a more practical alternative to the exist-
ing methods, is highly promising and provides a better scope for the preparation of
pure silica by considering economical benefts, which would be in good accordance
with the innovation of green chemistry.
Acknowledgements We are thankful to Ferdowsi University of Mashhad Research Council for fnancial
support to this work (Grant No. p/3/45801).
Compliance with ethical standards
Confict of interest The authors declare that they have no confict of interest.
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