Sulfonated covalent triazine polymer loaded with Pd nanoparticles as a bifunctional catalyst for one pot hydrogenation esterification reaction

Article abstract of DOI:10.1016/j.jssc.2021.122417

Highly dispersed Pd nanoparticles over covalent triazine polymer functionalized with sulfonic acid groups (CTP-SO₃H/Pd) were prepared by facile Friedel-Crafts reaction, post synthetic sulfonation and Pd immobilization method. The prepared catalyst was characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂ adsorption-desorption, inductively coupled plasma - optical emission spectrometry (ICP-OES), elemental analysis and X-ray photoelectron spectroscopy (XPS). The sulfonic acid groups were grafted into the terphenyl backbone and the presence of triazine functionality within the framework enabled the uniform dispersion of palladium nanoparticles over the polymer network. When used as a bifunctional catalyst in one pot hydrogenation-esterification (OHE) reaction, the CTP-SO₃H/Pd exhibited good activity and stability. The performance of CTP-SO₃H/Pd is due to the surface-active acid/metal sites and was evident from the yield of the product in the reaction. The catalyst was easily recovered by filtration and recycle tests showed that it could be re-used for at least five repetitive runs with minor loss of catalytic activity suggesting its potential utility in OHE reaction. A plausible mechanistic pathway for OHE reaction over CTP-SO₃H/Pd was also proposed.

Full text of DOI:10.1016/j.jssc.2021.122417

Journal of Solid State Chemistry 302 (2021) 122417
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Sulfonated covalent triazine polymer loaded with Pd nanoparticles as a
bifunctional catalyst for one pot hydrogenation esterification reaction
a
b
a
a,*
A.Ahmed Raza , S. Ravi , S.Syed Tajudeen , A.K.Ibrahim Sheriff
a
Department of Chemistry, C. Abdul Hakeem College (Autonomous), Melvisharam, Ranipet- District, Tamil Nadu, 632509, India
Department of Chemical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, 03722, South Korea
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Highly dispersed Pd nanoparticles over covalent triazine polymer functionalized with sulfonic acid groups (CTP-
Covalent triazine polymer
Palladium nanoparticles
Heterogeneous catalyst
Bifunctional catalyst
Tandem reaction
SO
method. The prepared catalyst was characterized by powder X-ray diffraction (XRD), transmission electron mi-
croscopy (TEM), N adsorption-desorption, inductively coupled plasma - optical emission spectrometry (ICP-OES),
elemental analysis and X-ray photoelectron spectroscopy (XPS). The sulfonic acid groups were grafted into the
terphenyl backbone and the presence of triazine functionality within the framework enabled the uniform
dispersion of palladium nanoparticles over the polymer network. When used as a bifunctional catalyst in one pot
3
H/Pd) were prepared by facile Friedel-Crafts reaction, post synthetic sulfonation and Pd immobilization
2
OHE reaction
hydrogenation-esterification (OHE) reaction, the CTP-SO
3
H/Pd exhibited good activity and stability. The per-
formance of CTP-SO H/Pd is due to the surface-active acid/metal sites and was evident from the yield of the
3
product in the reaction. The catalyst was easily recovered by filtration and recycle tests showed that it could be re-
used for at least five repetitive runs with minor loss of catalytic activity suggesting its potential utility in OHE
reaction. A plausible mechanistic pathway for OHE reaction over CTP-SO H/Pd was also proposed.
3
1
. Introduction
performance in tandem processes [10–12]. In recent years, porous
organic polymers (POPs) have also been utilized as a platform to afford
multisite heterogeneous catalysts owing to their high surface area, robust
framework and easy functionalization [13–18]. Mostly, POPs are amor-
phous and have non uniform pores that are not well defined, making it
difficult to understand their behavior in catalysis [19]. Thus, it is highly
desirable to develop multisite catalysts with enhanced stability and
performance in tandem reactions for imperative industrial processes.
Recently, nanostructured materials have been widely used for various
applications. The nanostructures offer advantages of high surface area,
synergistic interactions, and multiple functionalities towards biosensing,
environmental gas sensing, water remediation, photocatalysis and het-
erogeneous catalysis [20–24]. Covalent Triazine Polymers (CTPs), a class
of porous organic polymers, have been drawing much attention in het-
erogeneous catalysis due to their intriguing features including large
surface area, adjustable porous properties, easy tailorable chemistry, and
excellent stability. Unfortunately, the previous works using CTP-based
catalysts for organic transformations are focused only on studying
single-step reactions and multistep tandem reactions are not reported
[25–28]. To implement CTP as an efficient bifunctional catalyst for
tandem reactions, a possible way is to introduce two different catalytic
Catalysts with two or more active sites at juxtapositions trigger the
reactants to obtain the target product in organic synthesis through a
tandem process. These multisite catalysts carry out successive reactions
in one pot by evading the work up and isolation of intermediates, exhibit
high atom economy, reduce reaction time and energy consumption [1,2].
The design of catalyst for a tandem process involves careful choice of
catalytic active sites required for the entire sequence of the multistep
organic transformation. Furthermore, the active sites of the catalyst must
be close enough to carry out the transformation and at the same time they
should be spatially separated in order to avoid unnecessary interactions
[
3–5]. Normally, those functional groups of multisite catalysts are
embedded in the well-structured porous matrix for tandem synthetic
organic transformations. Porous silica materials grafted with multifunc-
tional sites have been widely studied in tandem reactions. However, the
instability in extremely acidic or basic solutions and limited functional
sites on the surface of these materials restrict their application in tandem
catalysis [6–9]. Besides, porous inorganic-organic hybrid materials such
as metal organic frameworks (MOFs) have been commonly applied as
multisite catalysts but the chemical and water instability limit their
*
Corresponding author.
E-mail address: drakis.che@gmail.com (A.K.Ibrahim Sheriff).
https://doi.org/10.1016/j.jssc.2021.122417
Received 27 April 2021; Received in revised form 7 July 2021; Accepted 8 July 2021
Available online 10 July 2021
0022-4596/© 2021 Elsevier Inc. All rights reserved.

A.Ahmed Raza et al.
Journal of Solid State Chemistry 302 (2021) 122417
functional sites like sulfonic acid groups and noble metal nanoparticles
attached in developing metal-acid bifunctional catalysts for
hydrogenation-esterification in one pot. For instance, Chen et al. devel-
oped a nanoreactor based on PtNi@ Metal organic framework composites
loading polyoxometalates for OHE to prepare anesthetics [41]. Zhang
et al. prepared palladium NPs supported on sulfonic acid functionalized
metal organic framework for the conversion of biomass to an important
and expensive pharmaceutical intermediate through OHE [42].
Metal-acid bifunctional systems such as Pt/HZSM-5 [43], Pt/SBA15-Pr-
(
3
NPs) into the framework. The sulfonic acid (–SO H) groups can be
grafted into the porous triazine framework as a pendant group by the
post-synthetic covalent modification to produce a solid acid catalyst
(
3
CTP-SO H) [29,30]. Such solid acid catalysts are superior to the con-
ventional mineral acid catalysts which are homogeneous and thus asso-
ciated with problems in separation and purification [31]. Furthermore,
the nitrogen functionality in CTPs could be effectively used for anchoring
catalytically active metal centers. CTPs anchored with palladium,
rhodium, silver, copper and ruthenium nanoparticles have been used for
catalytic reactions such as reduction of nitroarenes [32], hydrogenation
of benzene [33], oxidative esterification of alcohols [34], ipso-hydrox-
ylation [35] and oxidation of HMF [36], respectively. Therefore, these
features prompted us to study the utility of CTP as a bifunctional catalyst
by introducing both acid and metal active sites via post-synthetic
modification methods.
Hydrogenation and esterification are two important and essential
reactions in various fields of chemistry for the production of fine chem-
icals. Typical hydrogenation reaction is carried out using metal catalysts
such as nickel, palladium, or platinum [37,38], while esterification re-
action is performed using conventional mineral acids or solid acid cata-
lysts [39,40]. Recent literature shows that increasing interest have been
3
SO H [44], Cu-ZSM-5 [45] and Pd/Al-SBA-15 [46] have been reported
for catalytic upgrading of bio-oil through OHE reaction. Despite all the
studies, developing new tailor-made heterogeneous bifunctional cata-
lysts with high efficiency and selectivity for OHE reaction is still
desirable.
In the present work, we report a new kind of bifunctional catalyst
with noble metal particles anchored on CTP framework functionalized
with acid sites (CTP-SO
drogenation esterification reaction (Scheme 2). In this catalyst system,
the Pd NPs acting as hydrogenation sites are surrounded by SO
3
H/Pd) (Scheme 1) specifically for one pot hy-
3
H
esterification sites of CTP framework, which assures that the hydroge-
nated product can be instantly esterified without any transfer restriction.
3
A model reaction was carried out using CTP-SO H/Pd as a catalyst and 4-
nitrobenzoic acid as a substrate. As expected, the OHE reaction produced
Scheme 1. Polymer synthesis, sulfonation & incorporation of Pd nanoparticles.
2

A.Ahmed Raza et al.
Journal of Solid State Chemistry 302 (2021) 122417
ꢁ
dried in a vacuum oven at 100 C for 3 h.
2.5. Characterization
CTP-SO
studies, elemental analysis, XPS and HR-TEM. Powder X-ray diffraction
patterns are obtained with a Shimadzu XD-D1 diffractometer using Cu K
3 2
H/Pd catalyst was characterized using PXRD, N sorption
α
ꢁ
ꢂ1
ꢁ
(
λ¼ 0.15406 nm) radiation at 0.2 min scan rate and 4–60 2θ scan
range. The polymer catalysts surface area and pore volumes were
calculated from N sorption isotherms at 77 K using a BELSORP-max,
2
Japan. Before sorption measurements, the samples were degassed
ꢁ
under vacuum overnight at 150 C. HR-TEM measurements were per-
formed with a JEOL JEM-3010 instrument (JEOL, Japan) high-resolution
transmission electron microscope. The chemical states of elements were
determined by X-ray photoelectron spectroscopy (XPS) for the CTP-
Scheme 2. One pot Hydrogenation-Esterification reaction.
SO
3
H/Pd sample operated on Kratos AXIS-SUPRA spectrometer (Kratos,
X-ray source and a hemispherical
benzocaine with almost 95% conversion and selectivity. The target
product benzocaine is a very important amino-ester-type anesthetic and
medical intermediate synthesized by a multistep process in traditional
industry [47,48].
U.K.) using a monochromatic Al K
α
analyzer. The catalytic reaction conversions were analyzed by using a gas
chromatograph (Agilent Technologies, 7890A, U.S.A.) fitted with an HP-
5 capillary column and a flame ionization detector.
2. Experimental section
2.6. General procedure for the one pot tandem reaction
One pot hydrogenation-esterification reaction was conducted using
CTP-SO H/Pd as a bifunctional catalyst. For this a 50 ml high pressure
3
2
.1. Materials
reactor was loaded with 1.2 mmol of 4-nitro benzoic acid, 40 mg of
Cyanuric chloride (Aldrich, 99%), para-terphenyl (Aldrich, ꢀ 99.5%),
ꢁ
dichloromethane (Aldrich, 99%), anhydrous aluminium chloride
(
(
catalyst, 0.1 MPa H
2
, 5 mL of ethanol. The reactor was heated at 80 C
under the stirring condition to complete the hydrogenation and esteri-
Aldrich, > 98%), chlorosulfonic acid (Aldrich, 99%), palladium acetate
Aldrich, 98%), sodium borohydride (Aldrich, ꢀ 98%), acetone (Aldrich,
ꢁ
fication process. After the reaction was halted, cooled to 5 C, and the
2
remaining H was evacuated slowly. The conversion of 4-nitrobenzoic
9
9%), 2-nitrobenzoic acid (Aldrich, > 95%), 3-nitrobenzoic acid
(
9
Aldrich, 98%), 4-nitrobenzoic acid (Aldrich, 99%), toluene (Aldrich, >
9%), methanol (Aldrich, > 99%) and ethanol (Aldrich, > 99%) were
purchased and used without further purification.
acid in the reactions was monitored by taking aliquots from the re-
actions mixture with filter syringe and analyzing them with GC fitted
with an HP-5 capillary column and a flame ionization detector.
2.7. Titration method for quantification of total acid concentration
2
.2. Synthesis of CTP
To quantify the acid sites in catalyst, all the sulfonic acid-
The covalent triazine polymer (CTP) was prepared by Friedel-Crafts
functionalized covalent triazine polymers were back titrated in a non
aqueous acetonitrile (ACN) solvent, following a previously reported
protocol [49]. In a typical titration experiment, a mixture of the polymer
catalyst (~25 mg) and a [base þ indicator] solution (20 mL) in ACN (25
mM pyridine þ0.02 mM p-naphtholbenzein) was stirred vigorously at
room temperature for 4 h. The resulting mixture was then filtered
reaction of cyanuric chloride (1.47 g, 8 mmol) with para-terphenyl
2.3 g, 11.2 mmol) in dichloromethane (100 ml) under reflux and in the
presence of anhydrous AlCl (3.00 g, 24.2 mmol) as a catalyst for 12 h.
(
3
After thoroughly washing the obtained precipitate with DCM, methanol,
and water (3–4 times) to completely remove the unreacted starting
precursors and impurities, the solid was collected by filtration and vac-
uum dried for 4 h.
through a 0.2
μm PTFE syringe filter. Three aliquots (3 ꢃ 5 mL) of the
filtrate were back-titrated using a 0.1 M solution of perchloric acid in
acetic acid.
2
.3. Sulfonation of CTP
3
. Results and discussion
The functionalization with acid groups, of the CTP was carried out by
3
simple treatment with chlorosulfonic acid in DCM, yielding CTP-SO H.
X-ray diffraction was carried out to identify the patterns of the sample
For this, solution of chlorosulfonic acid (0.2 ml) in DCM (2.0 ml) was
phases as shown in (Fig. 1). The synthesized CTP exhibited a broad
ꢁ
added dropwise to a dispersion of CTP (0.15 g) in DCM (5.0 ml) at 0 C.
ꢁ
ꢁ
diffraction peak from 6 to 38 , indicating a structure with partial crys-
tallinity and shows a certain degree of ordering of CTP. This diffraction
Then the mixture was allowed to stand at room temperature for two days,
and then the reaction mixture was quenched with 250 mL of ice cold
water. The resulting product was washed repeatedly with DCM, meth-
anol and water to get rid of unreacted chlorosulfonic acid, filtered and
3 3
peak can be also observed in CTP-SO H and CTP-SO H/Pd, evidencing
the retention of CTP structure against post-synthetic modification.
Meanwhile, the diffraction pattern of CTP-SO
0.1 for the Pd nanoparticles corresponding to (111) plane, indicating
3
H/Pd shows a peak at
ꢁ
dried at 120 C.
ꢁ
4
the high dispersion of Pd NPs on the triazine framework. The decrease of
the reflection intensity of CTP-SO H and CTP-SO H/Pd with respect to
2
.4. Pd immobilization
3
3
the parent CTP is a consequence of the inclusion of guest molecules in the
framework.
Transmission electron microscopy (TEM) analysis was used to predict
3
the dispersion of palladium nanoparticles in the CTP-SO H/Pd catalyst. It
is clear from the TEM images that the Pd particles are of nanoscale and
the particle size distribution calculations predict the size of 3.0 nm
(Fig. 2). Moreover, CTP has served as an effective platform for the
CTP-SO
the addition of 0.1 M Pd(OAc)
temperature for 30 min, then 0.1 M NaBH
dropwise under constant stirring for another 2 h to reduce Pd to Pd .
Then, the obtained solid is filtered, washed repeatedly with methanol,
3
H (0.05 g) was well dispersed in DCM (40 mL) followed by
solution (1.0 mL) and stirred at room
solution (2.0 mL) added
2
4
2þ
0
3
acetone and water. The black solid (CTP-SO H/Pd) thus obtained was
3

A.Ahmed Raza et al.
Journal of Solid State Chemistry 302 (2021) 122417
Pd is due to the stepwise functionalization on CTP framework. Pore size
distribution (PSD) estimated by non-local density functional theory
(
NLDFT) of the CTP and derived materials is shown in Fig. 3 (b). These
PSD patterns exhibited pore diameters ranging from 0.6 nm to 5.5 nm for
CTP and derived materials. A peak pore diameter of 0.6 nm suggested
that the materials are microporous along with small mesoporosity [50].
Further investigation of the surface properties of developed polymers is
also implied by elemental analysis (EA) and inductively coupled plasma -
optical emission spectrometry (ICP-OES). CTP-SO
mmol/g of nitrogen and 3.14 mmol/g of sulfur, CTP-SO
3
H contains 4.97
H/Pd revealed
3
an amount of 4.65 mmol/g of nitrogen and 2.99 mmol/g of sulfur as
determined by elemental analysis. The Pd loading amount on
CTP-SO
The chemical bondings and electronic state of metal in CTP-SO
was confirmed by the X-ray photoelectron spectroscopy (XPS) as shown
in Fig. 4. N 1s of CTP-SO H/Pd (Fig. 4(a)) exhibited a binding energy
3
H/Pd is determined to be 0.518 mmol/g by ICP-OES.
3
H/Pd
3
peak at 397.7 eV that can be attributed to triazinic nitrogen similar to
pyridinic nitrogen (C-N¼C). S 2p fitting XPS spectrum (Fig. 4(b)) shows
two binding energy peaks at 166.2 eV and 165.1 corresponding to sulfur
bonding environment with a spin-split doublet, S 2P1/2 and S 2P3/2. This
confirms the successful grafting of sulfonic acid groups onto the polymer
3 3
Fig. 1. Powder X-ray diffraction pattern of CTP, CTP-SO H and CTP-SO H/Pd.
dispersion of the nanoparticles and thereby preventing the agglomera-
tion or precipitation of Pd from solvent.
The N adsorption-desorption isotherm of the catalyst samples are
2
shown in Fig. 3 (a) and physiochemical, surface properties of synthesized
covalent triazine polymers are summarized in Table 1. All the three
[
51]. Pd 3d spectra of the Pd NPs on CTP (Fig. 4 (c)) is resolved into two
spin-orbit pairs with 3d3/2 binding energies of 340.1 eV and 338.2 eV,
and with 3d5/2 binding energies of 335.2 eV and 333.1 eV, respectively.
This suggests that employed loading conditions favor complete reduction
3 3
samples (CTP, CTP-SO H and CTP-SO H/Pd) exhibited same type I iso-
therms with strong adsorption in the low pressure region, suggesting the
microporous nature of the materials. Moreover, desorption shows hys-
teresis due to reversible and/or irreversible gas uptake in the pores. The
BET surface area and pore volume of the samples were calculated to be
2þ
of Pd and metallic Pd (0) is bound to the nitrogens of CTP [52,53].
4
. Catalytic results of CTP-SO
3
H/Pd
2
2
2
3
3
9
01 m /g, 487 m /g, 223 m /g and 0.489 cm /g, 0.245 cm /g, 0.167
The characterization results suggest that the bifunctional CTP-SO
3
H/
3
cm /g for CTP, CTP-SO
decrease in surface area and pore volume of CTP-SO
3
H, CTP-SO
3
H/Pd respectively. The considerable
H and CTP-SO H/
Pd consisting of metal/acid active sites is successfully constructed
through Friedel-Crafts reaction followed by stepwise functionalization. It
3
3
3
Fig. 2. TEM images of CTP-SO H/Pd (a) and (b) at different magnifications; (c) size distribution of Pd nanoparticles.
Fig. 3. (a) N
2
adsorption desorption isotherm at 77K of CTP, CTP-SO
4
3 3
H and CTP-SO H/Pd, (b) Pore size distribution.

A.Ahmed Raza et al.
Journal of Solid State Chemistry 302 (2021) 122417
Table 1
Physiochemical and surface properties of covalent triazine polymers.
S
BET (m [2]/g)
Pore volume (cm [3]/g)
N (mmol/g)a
S (mmol/g)a
Pd (mmol/g)b
Acid density (mmol/g)c
CTP
CTP-SO
CTP-SO
901
487
223
0.489
0.245
0.167
6.81
4.97
4.65
–
3.14
2.99
–
–
–
3.17
3.05
3
3
H
H/Pd
0.518
a
Determined by elemental analysis.
Determined by ICP-OES.
Determined by titration method.
b
c
3
Fig. 4. XPS binding energy peaks of (a) N 1s, (b) S 2p and (c) Pd 3d; (d) Molecular functional sites of CTP-SO H/Pd.
is apparent that this polymer made of metal/acid bifunctional sites can
perform as a bifunctional catalyst in one pot hydrogenation esterification
reaction in which the metal nanoparticles serve as hydrogenation sites
and sulfonic acid groups act as esterification sites. To investigate the
catalytic activity of the developed catalyst, the conversion of p-nitro-
benzoic acid to benzocaine was chosen as a model reaction. For this, the
reaction was carried out under optimized conditions with 1.2 mmol of 4-
is obtained with 98% selectivity. This demonstrates that for the OHE
reaction the Pd NPs and sulfonic acid groups should be adjacent to each
other. Thus, sulfonated CTPs form the basis for multisite catalysts, con-
taining both basic coordination sites for metal nanoparticles and surface
acidity within one sole system. The triazine rings in CTP framework not
only stabilize the loading of metal nanoparticles but also restrict metal
agglomeration and subsequent deactivation [54].
nitro benzoic acid, 40 mg of catalyst, 0.1 MPa H
2
, 5 mL of ethanol, and at
The effect of reaction parameters such as amount of catalyst and
temperature were studied for the OHE reaction to optimize the reaction
conditions. For this 1.2 mmol of 4-nitro benzoic acid in 5 mL of ethanol
ꢁ
80 C for 8h. Under these conditions, catalyst screening tests were also
carried out to explore the role of two active sites. The reaction was car-
ried out in the absence of catalyst or in the presence of CTP, CTP/Pd,
was taken and the OHE reaction was carried out for 8 h under 0.1 MPa H
2
CTP-SO
3
H and CTP-SO
3
H/Pd. The catalytic results of all the catalysts are
at different temperatures and by varying the catalyst amount. The results
of effect of amount of catalyst on % yield and conversion of OHE using
CTP-SO H/Pd is shown in Fig. 6. The yield and conversion were low
3
when minimum amount of the catalyst was used. When the amount of
catalyst was gradually increased, the yield and conversion were also
increased due to increase in the number of catalytically active sites. A
maximum yield and conversion of 94% and 95% were achieved when the
amount of catalyst added was 40 mg. No change in the yield and con-
version were observed when the amount of catalyst was increased to 50
shown in Fig. 5. In the absence of catalyst, no reaction occurred. The
reaction was carried out using bare CTP without any functionally active
sites and as expected, there was no reaction. Only the hydrogenated
product 4-aminobenzoic acid (P
as catalyst. The CTP grafted with sulfonic acid groups (CTP-SO
ded the esterified product 4-nitrobenzoate (P ) with 98% selectivity. To
our delight, when both Pd NPs and sulfonic acid groups are embedded
into CTP (CTP-SO H/Pd), the target product ethyl 4-aminobenzoate (P
b
) was obtained when CTP/Pd was used
3
H) yiel-
c
3
d
)
5

A.Ahmed Raza et al.
Journal of Solid State Chemistry 302 (2021) 122417
Fig. 5. Catalytic selectivity results of the one-pot tandem reaction with different catalysts; where P
a
- 4-nitrobenzoic acid, P
b
-4-aminobenzoic acid, P
c
-ethyl 4-nitro-
ꢁ
d
benzoate and P -ethyl 4-aminobenzoate. Conditions: 1.2 mmol of 4-nitro benzoic acid, 40 mg of catalyst, 0.1 Mpa H
2
, 5 mL of ethanol, and at 80 C for 8h.
Fig. 6. Effect of amount of catalyst on tandem catalytic conversion of 4-nitro-
Fig. 7. Effect of temperature on tandem catalytic conversion of 4-nitrobenzoic
acid to 4-amino benzoic acid. Conditions: 1.2 mmol of 4-nitro benzoic acid,
40 mg of catalyst, 0.1 MPa H , 5 mL of ethanol for 8 h; where P -4-amino-
benzoic acid to 4-amino benzoic acid; conditions: 1.2 mmol of 4-nitro benzoic
ꢁ
acid, 0.1 MPa H
2
, 5 mL of ethanol, and 80 C for 8 h; P
b
-4-aminobenzoic acid,
2
b
P
c
-ethyl 4-nitrobenzoate, P
d
-ethyl 4-aminobenzoate and Ptc-total conversion.
c d tc
benzoic acid, P -ethyl 4-nitrobenzoate, P -ethyl 4-aminobenzoate and P -
total conversion.
mg. The effect of temperature on OHE reaction was investigated under
the same reaction conditions with 40 mg of catalyst (Fig. 7). The reaction
temperature was gradually increased from room temperature to 80 C.
The conversion of 4-nitrobenzoic acid to benzocaine can occur
through two routes as follows (i) hydrogenation followed by esterifica-
tion in which p-aminobenzoic acid is the intermediate; (ii) esterification
followed by hydrogenation in which ethyl 4-nitrobenzoate is the inter-
mediate . With the objective of knowing the actual catalytic mechanism
ꢁ
From the results, it is evident that low temperature does not have any
significant effect on the OHE reaction. When the temperature was
ꢁ
increased to 80 C, maximum yield and conversion of 96% and 97% were
achieved. The above studies show that the hydrogenation is faster
compared to esterification and the latter is the rate determining step.
Increase in the amount of catalyst and temperature favors the esterifi-
cation step.
of this conversion using CTP-SO
were obtained (Fig. 8). The yield of esterified product P
lower than the hydrogenated product P throughout the catalytic reac-
tion (i.e.) hydrogenation is kinetically faster and esterification is slower
3
H/Pd, the yield–time curves of products
c
was found to be
b
6

A.Ahmed Raza et al.
Journal of Solid State Chemistry 302 (2021) 122417
Fig. 8. Product yields and total conversion of OHE reaction on CTP-SO
3
H/Pd with respect to time. Reaction conditions: 4-nitro benzoic (1.2 mmol), ethanol (5 ml),
ꢁ
and CTP-SO
3
H/Pd catalyst (40 mg) at 80 C for 8 h under 0.1 MPa H
2
; where P
b
-4-aminobenzoic acid, P
c
-ethyl 4-nitrobenzoate, P
d
-ethyl 4-aminobenzoate and Ptc
-
total conversion.
and rate determining step. The reaction produced 19%, 9% and 4% of P
and P respectively with a total conversion of 35% at 1 h. The yields of
intermediates P & P first gradually increased and then decreased with
time. When the reaction was continued, the yield of target product P and
total conversion continuously increased with time. Maximum yield of
target product P and total conversion of 95% and 94% were obtained at
h of reaction time. Thus the optimum reaction conditions for the OHE
b
,
These molecules produced the corresponding 2-aminoethylbenzoate and
3-aminoethylbenzoate with 94% and 97% GC yield owing to the
remarkable catalytic activity of CTP-SO H/Pd. It is worth mentioning
3
that irrespective of the position of the reaction sites, the bifunctional
catalyst smoothly transformed the substrates to the corresponding target
product in high yields.
P
c
d
b
c
d
d
8
A plausible reaction mechanism for one pot hydrogenation-
reaction are 4-nitro benzoic (1.2 mmol), ethanol (5 ml), and CTP-SO
3
H/
esterification of para nitrobenzoic acid to benzocaine is proposed keep-
ing in mind the previously reported literature [55–58]. As illustrated in
Scheme 3, a two step mechanism is proposed in which the first step is
hydrogenation followed by esterification. The first step hydrogenation
ꢁ
Pd catalyst (40 mg) at 80 C for 8 h under 0.1 MPa H
2
. The product yield
& P are low
–
time curves also show that the yields of intermediates P
b
c
throughout the reaction which prove that hydrogenated or esterified
intermediate immediately undergoes subsequent reaction by adjacent
þ ꢂ
begins with the adsorption of H on the surface of Pd as H and H . These
2
catalytically active sites to give the target product P
d
.
are subsequently added to electron rich oxygen and electron deficient
nitrogen respectively resulting in the formation of para nitrosobenzoic
acid with the elimination of water molecule. Successive addition of the
To test the generality of the present reaction, 2-nitrobenzoic acid and
-nitrobenzoic acid were subjected to OHE under optimized conditions.
3
3
Scheme 3. Plausible mechanism of the OHE reaction over CTP-SO H/Pd catalyst.
7

A.Ahmed Raza et al.
Journal of Solid State Chemistry 302 (2021) 122417
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
The authors are thankful to C. Abdul Hakeem College (Autonomous)
for providing chemicals, laboratory facility and financial support. A.
Ahmed Raza would like to acknowledge Dr. Seenu Ravi (Yonsei Uni-
versity, Republic of South Korea) for his support in characterization and
application studies.
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CRediT authorship contribution statement
A.Ahmed Raza: Conceptualization, Methodology, Validation, Formal
analysis, Investigation, Writing – original draft. S. Ravi: Conceptualiza-
tion, Methodology, Validation, Formal analysis, Writing – review &
editing. S.Syed Tajudeen: Writing – review & editing. A.K.Ibrahim
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