Immobilization of dihydrogen phosphate onto rice husk ash as a highly efficient and green catalyst for the synthesis of symmetrical N,N′-alkylidene bisamides

Article abstract of DOI:10.1007/s11164-019-03779-3

The preparation and characterization of an environmentally-benign and low-cost catalyst consisting of KH₂PO₄ supported on silica from rice husk ash is reported. The prepared catalyst was characterized by FT-IR spectroscopy, field emi

Full text of DOI:10.1007/s11164-019-03779-3

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-03779-3
Immobilization of dihydrogen phosphate onto rice husk
ash as a highly efcient and green catalyst for the synthesis
of symmetrical N,N′‑alkylidene bisamides
Hamid Reza Saadati‑Moshtaghin1
Received: 29 October 2018 / Accepted: 18 February 2019
© Springer Nature B.V. 2019
Abstract
The preparation and characterization of an environmentally-benign and low-cost
catalyst consisting of KH2PO4 supported on silica from rice husk ash is reported.
The prepared catalyst was characterized by FT-IR spectroscopy, feld emission scan-
ning electron microscopy, powder X-ray difraction and inductively coupled plasma.
The synthesized nanocomposite exhibited an excellent catalytic efciency in the
condensation reaction for the preparation of symmetrical N,N′-alkylidene bisamides
under solvent-free conditions.
Keywords Potassium dihydrogen phosphate · Rice husk ash · Supported catalyst ·
Symmetrical N,N′-alkylidene bisamides
Introduction
The choice of an efcient catalyst with appropriate support is one of the early con-
ceptions of green chemistry [1]. Recent interest in the heterogeneous catalyst stems
from environmental concerns. Potassium dihydrogen phosphate (KH2PO4) is a low-
cost, environmentally friendly and commercially available material which has been
widely employed in the agricultural, chemical, pharmaceutical and food industries
[2–5]; however, it has received much less attention as a catalyst. The high solubil-
ity of KH2PO4 is probably the most important obstacle for the widespread use of
it in catalytic applications. To overcome this constraint, a solid support should be
employed.
There are numerous reports in the literature for immobilizing catalysts on various
solid supports, such as MCM-41, montmorillonite, active carbon, Al2O3, zeolites, func-
tionalized polymer, resins and magnetic nanoparticles [6–13]. Indeed, the high cost,
* Hamid Reza Saadati-Moshtaghin
h_r_saadati@yahoo.com
1
Young Researchers and Elite Club, Islamshahr Branch, Islamic Azad University, Islamshahr,
Iran
Vol.:(0123456789)
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H. R. Saadati-Moshtaghin
complicated synthesis steps, and environmental issues of supports not only limit their
broad application but also encourage the exploration of new more promising supports.
In this regard, research on appropriate supports is highly desirable.
Rice husk (RH) is an abundantly available by-product of rice-milling in the agri-
culture industry. The combustion of RH produces rice husk ash (RHA), which is the
most economical source of silica. The silica content of obtained RHA is more than
94%, which can be increased to more than 99% by acid leaching, pyrolysis, and car-
bon-removing processes. The high specifc surface area and small particle size of pure
silica powders make RHA a valuable raw material for many applications, primarily as a
support for the heterogenization of a catalyst.
One of the signifcant classes of heterocyclic compounds are bisamide derivatives
which are present in many biologically signifcant substances and pharmaceutically
useful compounds. Symmetrical and asymmetrical N,N′-alkylidene bisamides and their
derivatives have been found to be key structural sub-units for the construction of pepti-
domimetic frameworks [14].
In this paper, I wish to report the use of RHA as a green, low-cost, and available
support for KH2PO4, and to evaluate the applicability of the obtained nanocomposite in
the synthesis of symmetrical N,N′-alkylidene bisamides under solvent-free conditions
(Scheme 1).
The commercial availability of the catalyst, using an abundantly available agricul-
tural waste as a catalyst support, the shortened reaction time, improved yields, and
avoidance of toxic organic solvents make this protocol practical and economically fea-
sible as well as exceptionally interesting.
Experimental
General
All chemicals were purchased from Merck or Aldrich and were used without further
purifcation. The RH sample was obtained from a local mill in Rasht (Guilan province,
Iran). Planetary ball mill equipment (Fritsch Pulverisette) with ten balls of 2 cm chrome
steel was applied for the preparation of nanoparticles of RHA. An 8400 s Shimadzu
spectrophotometer with KBr pellets was used to record Fourier transform infrared (FT-
IR) spectra. X-ray difraction (XRD) patterns were recorded with D8 Advance-Bruker
X-ray difractometer. The feld emission scanning electron microscopy (FESEM) was
carried out on a Tescan Mira3. Elemental analysis was performed by inductivity cou-
pled plasma optical emission spectroscopy on a Varian Australia model Vista-PRO
ICP spectrometer. The melting point of the catalytic reaction products was measured
Scheme 1 Synthesis of N,N′-alkylidene bisamides
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Immobilization of dihydrogen phosphate onto rice husk ash…
by using the capillary tube method with a Bamstead electrothermal type 9200 melting
points apparatus. Thin layer chromatography (TLC) was used for the catalytic reaction
progress monitoring. Reported yields refer to purifed products. The obtained prod-
ucts were characterized by comparison of the spectral and physical data with literature
values.
Preparation of RHA
Rice husk was washed several times with tap water and rinsed with distilled water
to remove the impurities and then dried at room temperature for 24 h. RHA (white
silica powder) was produced by calcination of RH at 700 °C for 6 h in a furnace,
followed by ball milling at 500 rpm for 6 h. The collected RHA was then refuxed
with 6 M HCl for 6 h. Afterward, the mixture was centrifuged and washed with
copious amounts of deionized water until the neutralized precipitate was obtained.
The obtained RHA was subsequently dried at 70 °C and used as the solid support for
catalyst preparation.
Functionalization of RHA with thionyl chloride
The RHA was functionalized with thionyl chloride (RHACl) according to a previ-
ously reported method with some modifcations [15]. In a typical reaction, 3.0 g of
RHA was suspended in 50 mL of thionyl chloride and then the obtained slurry was
sonicated for 30 min and refuxed for 24 h. Afterward, the resulting solid was sepa-
rated by vacuum fltration, washed three times with methanol to remove the unre-
acted remnant of thionyl chloride and then dried at 60 °C.
Immobilizing of KH2PO4 on the surface of RHACl
KH2PO4 (3 mmol) was dissolved in 30 mL deionized water and then added drop-
wise to a suspension of RHACl (1.0 g) in 50 mL deionized water and stirred for 12 h
at room temperature. Finally, the H2PO4@RHA was separated, washed twice with
deionized water and then dried at 60 °C. Chemical analysis of the prepared nanoma-
terial showed 0.67 mmol/g dihydrogen phosphate loading for the RHA.
General procedure for the preparation of symmetrical N,N′‑alkylidene bisamides
H2PO4@RHA (0.05 g) was added to a mixture of benzaldehyde (1.0 mmol) and
benzamide (2.0 mmol). Then, the mixture was stirred in 80 °C in an oil bath under
solvent-free conditions for an adequate time. After completion of the reaction, which
was monitored by TLC, 5 mL of boiling ethanol was added to the mixture, and the
catalyst was easily separated by simple fltration. Then, the crude product was puri-
fed by recrystallization procedure in ethanol. The purity of the products was deter-
mined by comparison of their physical data with known compounds in the literature
[14, 16–18].
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H. R. Saadati-Moshtaghin
Scheme 2 Procedure for preparation of H2PO4@RHA
Fig. 1 FT-IR spectra of RHA, RHACl and H2PO4@RHA
Result and discussion
Catalyst preparation and characterization
Scheme 2 illustrates the procedure used for the preparation of the supported nano-
catalyst. In the frst step, RHA was prepared by pyrolyzation of RH in the furnace.
Then, the process was followed by acid leaching to remove impurities and to obtain
pure high-surface area and high-porosity silica. Finally, the external surface of RHA
was functionalized by thionyl chloride to prepare RHACl. The interactions between
the Cl groups of RHACl with potassium dihydrogen phosphate aforded H2PO4@
RHA.
FT‑IR spectroscopy
Figure 1 shows FT-IR spectra of the prepared RHA, RHACl, and H2PO4@RHA.
The strong bands appearing around 1080, 790 and 460 cm−1 are due to Si–O–Si
and Si–O–H bonds [11]. The broad band at about 3390 cm−1 is assigned to O–H
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Immobilization of dihydrogen phosphate onto rice husk ash…
stretching vibrations. The corresponding bending vibration of O–H was observed
at 1620 cm−1. The appearance of the Si–Cl stretching vibration band at 595 cm−1 in
the RHACl spectrum demonstrates the functionalization of RHA with the Cl groups.
As shown in the spectrum of the H2PO4@RHA, extra bands appeared in the surface-
modifed RHA, which correspond to H2PO4 and are associated with the PO−4 3 vibra-
tions around 694 cm−1 and 628 cm−1. Furthermore, the band at 2632 cm−1 is due to
P–O–H, and the P=O band cannot be seen because it appeared at 1100–1200 cm−1
and overlapped with the Si–O–Si stretching vibration. Accordingly, the FT-IR
spectroscopy confrms the surface modifcation of the RHA and the formation of
H2PO4@RHA.
X‑ray difraction (XRD) study
XRD patterns provide further information on the preparation of H2PO4@RHA.
Figure 2 depicts the X-ray difraction patterns of RHA and H2PO4@RHA. The
intense difraction appearing at 2θ of ca. 23° indicates that the silica of RHA has an
amorphous nature [12]. The XRD pattern of H2PO4@RHA displays both small and
broad traces, and these additional weak difractions can be attributed to the supported
KH2PO4 indicating that the dihydrogen phosphate anion species are stabilized on the
surface of the RHA.
FESEM study
The particle size and surface morphology of H2PO4@RHA have been revealed
by FESEM micrographs, as illustrated in Fig. 3. As can be seen, the support has
an irregular surface. Furthermore, FESEM micrographs illustrate an average par-
ticle size of about 25 nm for H2PO4@RHA. In addition, energy-dispersive X-ray
Fig. 2 XRD spectra of a RHA and b H2PO4@RHA
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H. R. Saadati-Moshtaghin
Fig. 3 FESEM images of H2PO4@RHA
Fig. 4 EDX analysis of H2PO4@RHA
spectroscopy (EDX) was conducted to investigate the elemental analysis (Fig. 4).
The EDX results show the coexistence of Si, O, and P. The presence of P reveals
−
that H2PO4 is supported on the surface of the RHA. This result confrmed the
chemical analysis of H2PO4@RHA.
Investigation of catalytic activity of H2PO4@RHA in the synthesis of symmetrical
N,N′‑alkylidene bisamides
The synthesized H2PO4@RHA were used for the preparation of symmetrical N,N′-
alkylidene bisamides under solvent-free conditions. The best experimental route was
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Immobilization of dihydrogen phosphate onto rice husk ash…
Table 1 Efect of catalyst
Entry
Catalyst (g)
Time (min)
Yield (%)
amount on the preparation of
N,N′-alkylidene bisamides
1
2
3
4
5
0
180
15
15
15
15
24
42
68
87
84
0.01
0.03
0.05
0.1
Reaction conditions: benzaldehyde and benzamide (1.0, 2.0 mmol)
were mixed at 80 °C under solvent-free conditions
Table 2 Efect of temperature
on the preparation of N,N′-
alkylidene bisamides
Entry
Temperature (°C)
Yield (%)
1
2
3
4
25
60
80
24
63
87
82
100
Reaction conditions: benzaldehyde and benzamide (1.0, 2.0 mmol)
were mixed in the presence of 0.05 g catalyst under solvent-free con-
ditions
Table 3 Efect of reaction time
on the preparation of N,N′-
alkylidene bisamides
Entry
Time (min)
Yield (%)
1
2
3
4
5
5
10
15
20
30
27
46
87
88
90
Reaction conditions: benzaldehyde and benzamide (1.0, 2.0 mmol)
were mixed in the presence of 0.05 g catalyst at 80 °C under solvent-
free conditions
procured via optimization of the catalyst amount, reaction time for various catalysts.
A simple and efcient procedure was applied for the synthesis of symmetrical N,N′-
alkylidene bisamides in the presence of H2PO4@RHA. To optimize the conditions,
the condensation reaction of benzaldehyde (1.0 mmol) and benzamide (2.0 mmol) at
80 °C under solvent-free conditions was used as the model reaction to examine the
efect of the H2PO4@RHA catalyst (0–0.1 g). Table 1 shows the efect of catalyst
amount on the synthesis of symmetrical N,N′-alkylidene bisamides. According to
the obtained results, in the absence of the catalyst, only a trace amount of the desired
product was obtained after 180 min. It was observed that the yield% was increased
with enhancing the catalyst amount and that 0.05 g of catalyst was enough to reach
the maximum yield.
The efect of temperature was studied by the reaction of benzaldehyde (1.0 mmol)
and benzamide (2.0 mmol) in the presence of 0.05 g catalyst for 15 min (Table 2).
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H. R. Saadati-Moshtaghin
Table 4 Synthesis of diferent N,N′-alkylidene bisamides
Entry
Aldehyde
Yield%
Melting point
Measured
Reported [references]
1
2
3
4
5
6
7
8
C6H5CHO
2-ClC6H4 CHO
3-ClC6H4 CHO
4-ClC6H4 CHO
87
89
92
90
89
81
89
86
80
81
71
93
96
94
218–219
242–244
217–219
240–242
192–194
198–200
223–225
256–258
193–195
232–234
240–241
258–260
228–230
255–257
217–219 [14]
241–242 [18]
221–222 [14]
243–245 [17]
195–197 [16]
203–206 [16]
226–228 [14]
252–254 [16]
188–190 [17]
228–229 [18]
242–243 [16]
259–261 [14]
232–235 [16]
259–261 [16]
2,4-(Cl)2 C6H3 CHO
2,6-(Cl)2 C6H3 CHO
3-BrC6H4 CHO
4-BrC6H4 CHO
3-(OMe) C6H4 CHO
4-(OMe) C6H4 CHO
4-CH3C6H4 CHO
2-NO2C6H4 CHO
3-NO2C6H4 CHO
4-NO2C6H4 CHO
9
10
11
12
13
14
Reaction conditions: benzaldehyde derivatives, and benzamide (1.0, 2.0 mmol) were mixed in the pres-
ence of 0.05 g of catalyst at 80 °C for 15 min under solvent-free conditions
Table 5 Comparison of catalytic activity of H2PO4@RHA with some reported catalysts for the synthesis
of N,N′-alkylidene bisamides
Entry catalyst
Amount of catalyst Yield (%) Time (min) Condition
References
1
2
3
4
5
6
H2PO4@RHA
H14[NaP5W29MoO110
B(HSO4)3
p-Toluene sulfonic acid 0.1 mmol
0.05 g
0.15 g
0.05 mmol
87
86
85
95
93
85
15
90
45
60
35
20
Solvent-free This work
CH3OH [14]
Solvent-free [16]
Solvent-free [17]
Solvent-free [19]
Solvent-free [20]
]
Silzic
0.3 mg
2.5% mol
H7[(P2W17O61)
Fe(H2O)]
7
8
ZnCl2
Montmorillonite
2% mol
0.035 g
91
85
7 h
80
Toluene
Solvent-free [22]
[21]
This revealed that the yield% was increased by enhancing the reaction temperature
from 25 to 80 °C. However, the yield% was decreased by increasing the temperature
from 80 to 100 °C. Therefore, 80 °C was selected as the best temperature for the
reaction.
Table 3 shows the efect of time on the preparation of symmetrical N,N′-
alkylidene bisamides in the presence of 0.05 g catalyst at 80 °C under solvent-
free conditions. As can be seen, with lengthening the reaction time, the yield%
was increased. Nevertheless, by increasing the reaction time from 15 to 30 min,
the yield% increased with a low slope. Therefore, the reaction time of 15 min was
selected as the best time for the reaction.
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Immobilization of dihydrogen phosphate onto rice husk ash…
Fig. 5 The reusability of H2PO4@RHA
Scheme 3 The proposed mechanism for the preparation of N,N′-alkylidene bisamides
The generality of the present methodology was examined by using a range of
benzaldehyde derivatives in the desired reaction under optimum conditions. The
results are summarized in Table 4. In all cases, the intended product was obtained
with good yields.
To the best of our knowledge, several methods and catalysts have been reported
for the preparation of symmetrical N,N′-alkylidene bisamides [14, 16–18, 21, 22].
However, it seems that the present procedure is a unique casedue to the shorter
1 3

H. R. Saadati-Moshtaghin
reaction time and higher yield (Table 5). Furthermore, operation simplicity, cost-
efectiveness, reusability of the catalyst and avoidance of toxic organic solvents
are other notable advantages of the present procedure.
The reusability of the catalyst is a key factor to consider for a solid material as
a potent catalyst in industry. To study the reusability, the catalyst was separated
from the reaction mixture by simple fltration and washed with deionized water.
Then, the catalyst was dried in a vacuum oven at 60 °C for 3 h and reused for fve
runs (Fig. 5).
Scheme 3 illustrates the possible reaction pathway for the preparation of N,N′-
alkylidene bisamides in the presence of H2PO4@RHA. It is conceivable that the
carbonyl group is activated by H+ from the supported H2PO4 against nucleophilic
attack. Afterward, the nucleophilic addition of amides to activated aldehyde fol-
lowed by the loss of H2O generates an intermediate form, which is further acti-
vated by H2PO4@RHA. Then, the nucleophilic addition of a second molecule of
amide to the activated intermediate form afords the symmetrical N,N′-alkylidine
bisamide products.
Conclusion
In the present study, an afordable and efcient catalyst was synthesized by immobi-
lization of KH2PO4 onto RHA. The prepared nanocatalyst displayed supreme cata-
lytic efciency for the preparation of N,N′-alkylidene bisamides. In comparison with
previously reported methods, the present nanocatalyst provides an easy and conveni-
ent methodology for the preparation of N,N′-alkylidene bisamides. The results indi-
cate that the presented method is the most convenient route with regard to operation
simplicity, high yields, low reaction time, cost-efectiveness, simple work-up, reus-
ability and recyclability of catalyst, and the avoidance of toxic organic solvents.
Acknowledgements I am grateful for the invaluable support of my family. I would like to extend sincere
thanks to Young Researchers and Elite Club.
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