Hierarchical porous organic polymer as an efficient metal-free catalyst for acetalization of carbonyl compounds with alcohols

Article abstract of DOI:10.1016/j.mcat.2017.10.022

An efficient melamine-porous organic polymer (M-POP) as N-functionalized organic polymer catalyst was synthesized with melamine and terephthalaldehyde by a facile microwave-assisted method. The synthesized M-POP exhibited a high specific surface area with hierarchical pore structure of both mesoporosity and microporosity. The rich N-moieties namely, C[dbnd]N and N–H from the melamine precursor were found to show hydrogen-bonding ability with various organic molecules such as carbonyl compounds. This was illustrated in the acetalization of aldehydes and ketones with methanol or ethylene glycol under mild conditions as a metal-free H-bonding catalyst with high product selectivity. The superior catalytic performance of M-POP was attributed to the availability of a large number of H-bonding sites both as H-donor and H-acceptor between the reactants and the catalyst.

Full text of DOI:10.1016/j.mcat.2017.10.022

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Editor’s choice paper
Hierarchical porous organic polymer as an efficient metal-free
catalyst for acetalization of carbonyl compounds with alcohols
Joong-Jo Kima, Cheang-Rae Lima, Benjaram M. Reddya,b, Sang-Eon Parka,∗
a Laboratory of Nano-Green Catalysis and Nano Center for Fine Chemical Fusion Technology, Department of Chemistry and Chemical Engineering, Inha
University, Incheon 402-751 Republic of Korea
b Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Hyderabad, 500 007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient melamine-porous organic polymer (M-POP) as N-functionalized organic polymer catalyst
was synthesized with melamine and terephthalaldehyde by a facile microwave-assisted method. The
synthesized M-POP exhibited a high specific surface area with hierarchical pore structure of both meso-
porosity and microporosity. The rich N-moieties namely, C N and N H from the melamine precursor
were found to show hydrogen-bonding ability with various organic molecules such as carbonyl com-
pounds. This was illustrated in the acetalization of aldehydes and ketones with methanol or ethylene
glycol under mild conditions as a metal-free H-bonding catalyst with high product selectivity. The supe-
rior catalytic performance of M-POP was attributed to the availability of a large number of H-bonding
sites both as H-donor and H-acceptor between the reactants and the catalyst.
Received 13 September 2017
Received in revised form 18 October 2017
Accepted 19 October 2017
Available online xxx
Keywords:
Porous organic polymer
Metal-free catalyst
H-bonding catalysis
© 2017 Elsevier B.V. All rights reserved.
Acetalization of carbonyl compounds
Introduction
Generally, in the nitrogen-based polymeric materials the cat-
alytic active site is primarily the unshared electron pair of nitrogen,
Porous organic polymers (POPs), a new class of highly cross-
linked amorphous polymers possessing high specific surface area
and nanopores, have emerged as versatile materials for adsorp-
tion and catalysis as a result of their high chemical, mechanical and
thermal stability, tunability, facile synthesis, and low cost [1–4].
These properties are very similar to those of metal–organic frame-
works (MOFs) and allow them for potential applications such as gas
storage, drug release, explosives detection, and catalysis [5–10]. In
particular, many studies were devoted recently towards the devel-
opment and utilization of nitrogen-based POPs as efficient catalysts
for various reactions of commercial significance [11,12]. The POPs
with high nitrogen contents in the framework possess a high poten-
tial for uptake of metal ions or for the immobilization of metal
nanoparticles [12,13]. Mesoporous carbon nitride (MCN) materials,
which require carburization process during the preparation, have
also been investigated and applied in a variety of catalytic reactions,
such as oxidation of cyclic olefins [14], Knoevenagel condensation
[15], and other applications [16,17].
which can act as Lewis base site. Since mid-1960s, the POPs have
been recognized as organic species, which are capable of behaving
like organic reactants under appropriate conditions and facilitate
the chemical transformations of organic species [18,19]. Because
of their unique properties like hydrophobicity, good mechanical,
thermal and chemical stability, low cost, and low density [20], the
POPs have been recognized as promising catalysts for various reac-
tions [15,21–26]. Further, the POPs are an attractive ecofriendly
metal-free catalysts owing to their metal-free characteristics such
as non-corrosive property and non-toxicity [27]. In this context,
the melamine-based materials have received exceptional signifi-
cance because of their very rich nitrogen moiety. Also, melamine
is one of the cheap and easily available molecules with abun-
dant nitrogen moiety. Therefore, there is a lot of interest in recent
years on the synthesis of melamine-based materials for various
applications. For example, Tan et al. [28] reported mesoporous
polymelamine–formaldehyde as an efficient catalyst for chemo-
selective acetalization of aldehydes, Edwards et al. [29] described
melamine–formaldehyde polymer as an efficient ligand for the Pd-
catalyzed Suzuki–Miyaura coupling reaction, and Molla et al. [30]
reported a mesoporous polymelamine–formaldehyde stabilized Pd
nanoparticle as a proficient catalyst for the carbonylation of aryl
halides with amines. We have also successfully demonstrated the
∗
Corresponding author.
E-mail address: separk@inha.ac.kr (S.-E. Park).
http://dx.doi.org/10.1016/j.mcat.2017.10.022
2468-8231/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: J.-J. Kim, et al., Hierarchical porous organic polymer as an efficient metal-free catalyst for acetalization
of carbonyl compounds with alcohols, Mol. Catal. (2017), http://dx.doi.org/10.1016/j.mcat.2017.10.022

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catalytic application of melamine–terephthalaldehyde polymer in
the epoxidation of styrene and Knoevenagel condensation reac-
tions [21,31].
tion times by employing an efficient hierarchical pore-structured
melamine-terephthalaldehyde polymer having both secondary
amine (N − H) and tertiary amine (C N) moieties. These cata-
lysts were synthesized by the application of microwave irradiation
under mild conditions with an aim to achieve high catalytic activity
and selectivity towards the acetalization of carbonyl compounds
with alcohols. Characterization of the synthesized catalysts was
achieved by various techniques, namely, BET surface area, X-ray
diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR),
X-ray photoelectron spectroscopy (XPS), transmission electron
microscopy (TEM), solid-state NMR, and other. To support the char-
acterization and catalytic activity results, density functional theory
(DFT) measurements were also undertaken.
The microwave (MW) irradiation has received a tremendous
interest and it has been regarded as a novel synthesis tool for
making of various materials owing to numerous advantages such
as homogeneous nucleation to provide smaller and uniform crys-
tals having rather larger external surfaces, rapid decrease in the
synthesis time, phase controlled synthesis, facile morphology con-
trol, greener synthesis in an energy efficient manner, and so on
[32–35]. Besides these advantages, the application of MW tech-
nique for the synthesis of nanoporous materials allows relatively
facile incorporation of heteroatoms into the zeolitic frameworks,
more hydrophobicity, and higher thermal stability than those
of hydrothermally synthesized materials. The MW synthesized
porous materials were also found to show superior catalytic per-
formance in various industrially important reactions [36,37]. The
crystallization of microporous compounds under MW irradiation
was developed in the 1970s, and was characterized by mild reac-
tion conditions, low energy consumption, rapid reaction rate, small
particles, and uniform particle-size distribution. We have also
extensively exploited this method in the synthesis of various mate-
rials having defined porosity, mechanical strength, and very high
specific surface for different applications [38–40].
Acetalization of carbonyl compounds to facilitate acetal for-
mation from alcohol or diol has a wide range of applications in
many chemical industries [41]. The carbonyl function, as present
in aldehydes and ketones, is the most versatile functional group in
organic synthesis. During a synthetic sequence, a carbonyl group
may have to be protected against encounter by various reagents
such as acidic, basic, oxidizing, and reducing agents. Acetal forma-
tion can be achieved from alcohols or diols and used as protecting
group for carbonyl compounds in organic synthesis as they are sta-
ble with respect to hydrolysis by bases and many oxidizing and
reducing agents [41]. Therefore, protection of carbonyl compounds
by acetal formation via reaction with alcohols or diols is a com-
mon and useful technique for multistep synthesis in drug design,
organic and carbohydrate chemistry, pharmaceuticals, cosmetics,
and fragrances industries [42,43]. Many types of catalysts, including
conventional acids, solid acids, functionalized silica, metal cat-
alysts, small organic molecules, and natural materials (such as
kaolinite clay) have been reported to catalyze the acetal protec-
tion of carbonyls [44–50]. Although acetalization has been widely
reported and well investigated, it suffers from the drawbacks such
as the use of corrosive acid catalysts [48], the need for an excess
amount of drying agents, poor chemo-selectivity, and incompati-
bility with substrates containing acid-sensitive functional groups.
Therefore, there is an increasing interest in developing an acetaliza-
tion protocol that is mild, greener, chemo-selective, cost-effective,
and atom efficient with ease of catalyst recycling and no exces-
sive use of water scavenging reagents [47]. On those lines, the
H-bonding motif catalysts have been identified as a promising
option for this protocol [51]. The hydrogen bonding catalysts are
often simple to make and could be synthesized in high enantiomeric
purity [52]. New reactions catalyzed by H-bonding motifs have also
been investigated with an increasing pace, including asymmet-
ric variants of commonly used organic synthesis reactions, such
as aldol addition, Diels-Alder cycloaddition, and Mannich reaction
[53]. It is known that for H-bonding motif, the nitrogen atoms could
be used as catalytic active sites, because nitrogen can form H-bond
as a proton acceptor [54,55]. Kotke and Schreiner studied the acetal-
ization reaction of carbonyl compounds with alcohols in acid-free
condition over thiourea as a homogeneous catalyst [56].
Experimental
Preparation of catalysts
Melamine monomer was procured from Tokyo Chemical Indus-
try Co. Ltd., terephthalaldehyde from Merck, dimethyl sulfoxide
(DMSO) from Sigma Aldrich, dichloromethane (DCM) and acetone
from Duksan Chemicals respectively. All chemicals were guaran-
teed reagent grade (GR) and used without further purification.
The basic melamine porous organic polymer (M-POP) synthetic
procedure adopted in this study was similar to the earlier reported
literature [21,57,58]. However, a significant modification was made
in the synthetic procedure by adopting microwave-assisted heat-
ing which reduced the preparation time enormously and resulted
in superior catalysts. In brief, 577 mg (4.575 mmol) melamine and
460 mg (3.430 mmol) terephthalaldehyde were mixed together and
dissolved in 20 mL of DMSO under nitrogen atmosphere. After the
reagents were dissolved in DMSO clearly, the mixture solution was
loaded in a Teflon coated pressure vessel for microwave irradiation
(MARS-5 from CEM Co. Ltd.). The optimized microwave irradiation
treatment was carried out at 160 ◦C for 2 h with 400 W microwave
power. The formed product was cooled to room temperature, fil-
tered and washed with excess dichloromethane, cooled water, and
acetone. The resulting product was dried under vacuum for 12 h at
200 ◦C.
Characterization of M-POP
X-ray diffraction pattern was obtained on a Rigaku MiniFlex
X-ray diffractometer with CuK␣ radiation source in the 2␪ range
of 5–50◦ with a 2␪ step size of 0.02◦ and a step time of 2.4 s at
ambient temperature. The as-synthesized M-POP was pretreated at
200 ◦C for 6 h before N2 adsorption-desorption isotherm measure-
ments at liquid nitrogen temperature (−196 ◦C) on a Micromeritics
ASAP 2020 instrument. The specific surface area was calculated
by Brunauer-Emmett-Teller (BET) method from desorption data
in the relative pressure (P/P0) range = 0.040–0.25. The total pore
volume was estimated from the amount of adsorbed nitrogen at
the relative pressure of 0.98. The pore size distribution was cal-
culated by applying Barret–Joyner–Halenda (BJH) method from
desorption branch. Infrared spectroscopy (IR) measurements were
made on a Bruker VERTEX 80 V FTIR unit at ambient conditions.
The measured wavelength range was from 4000 to 400 cm−1. X-
ray photoelectron spectroscopy measurements were made on a
Thermo Scientific Instrument with monochromated Al K␣ radi-
ation source. The transmission electron microscope images were
obtained on a JEM 2100F, JEOL instrument equipped with a slow-
scan CCD camera and at an accelerating voltage of 400 kV. The solid
13C CP TOSS and 15N CP-MAS nuclear magnetic resonance spectra
were recorded on a Bruker Avance II solid state 500 MHz spec-
The present investigation was undertaken against the afore-
said background. Herein, we report the acetalization of carbonyl
compounds with alcohols under mild conditions in short reac-
Please cite this article in press as: J.-J. Kim, et al., Hierarchical porous organic polymer as an efficient metal-free catalyst for acetalization
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that the formation of condensation polymer is complete and it is
free from monomeric impurities [60,61].
The N2 adsorption-desorption isotherm along with the pore size
distribution curve are shown in Fig. 2. The Fig. 2 clearly reveals the
presence of a hysteresis loop at the relative pressure (P/P0) of above
0.4. The adsorption-desorption isotherm and the pore size distribu-
tion analysis data suggest the existence of significant mesoporosity
along with micropores [62]. The typical mesoporosity in the synthe-
sized material is introduced due to the rapid polymerization when
microwave irradiation is employed. The organic monomers play a
key role in the absorption of microwave irradiation and dissipate
the energy absorbed to the surroundings [34]. Water is formed as a
byproduct during the polymerization of melamine and terephtha-
laldehyde. The H2O being highly dielectric, it continuously attempts
to reorient in the electromagnetic radiation and is expected to help
in the creation of mesoporosity. This was the major advantage in
the application of microwave irradiation during the synthesis. From
N2 adsorption-desorption isotherms, the specific surface area was
calculated in the relative pressure (P/P0) range of 0.04–0.25. Thus
obtained BET surface area was found to be 811 m2g−1 (t-plot micro
surface area = 695 m2g−1 and meso surface area = 116 m2g−1) indi-
cating a high degree of cross linking in the M-POP which is also
evident from the pore size distribution curve (Fig. 2) and TEM
micrograph (Fig. S1, Supporting Information). The total pore vol-
ume was calculated from the amount of adsorbed N2 at the relative
pressure of 0.99 and was found to be 0.59 cm3g−1. These observa-
tions clearly signifies the importance of microwave irritation in the
synthesis of M-POP in the present investigation.
Fig. 1. X-ray diffraction pattern of M-POP at ambient conditions.
trometer. The solid 1H MAS NMR spectra was recorded on a Digital
Bruker Avance III 400 MHz spectrometer.
Acetalization reaction
Before conducting the acetalization reaction, the M-POP catalyst
was pretreated in vacuum at 200 ◦C overnight for condition-
ing. All catalytic reactions were performed in the EYELA WORLD
ChemiStation PPS-1511 personal organic synthesizer. All carbonyl
compounds (aldehydes and ketones) with methanol or ethylene
glycol were taken in 50 mL Pyrex vials having magnetic stirring bar
for conducting the reactions. The reactants, 1 mmol with 100 mmol
of methanol or ethylene glycol were mixed to which a 10 mg of cat-
alyst was added and mixed thoroughly. The reaction mixtures were
placed in the personal organic synthesizer system with stirring pro-
vision at 40 ◦C for 2 h. After completion of the reaction, reaction
mixtures were separated from the catalysts. The separated reac-
tion mixtures were analyzed by Agilent 7890N GC equipped with
HP-5 capillary column and FID. Both injector and detector temper-
ature were maintained at 250 ◦C. The GC analysis program adopted
was as follows: the starting temperature of 50 ◦C was kept for 1 min
and then raised to 250 ◦C at a rate of 20 ◦C min−1 and maintained
there for 3 min. Products were identified with the help of GC–MS.
The FTIR spectrum of M-POP (Fig. S2, SI) confirms that the poly-
merization is near to perfection by the absence of (C O) stretching
vibrations corresponding to the aldehyde functionality from the
precursor terephthalaldehyde. The absence of bands at 3470 and
3420 cm−1 (NH2 stretching) that correspond to the primary amine
group of melamine also supports the polymer formation. The
peaks centred at 1575, 1485, and 1375 cm−1 correspond to the
−1
hetero-aromatic triazine ring. The bands at 1172 and 1025 cm
correspond to the secondary amine polymer framework as per the
literature [21].
In order to further investigate the elemental composition of
nitrogen and carbon in the M-POP, the XPS analysis was carried
out. Fig. 3 shows the C 1 s and N 1 s XP spectra of M-POP. The
individual components were deconvoluted and the peak assign-
ments were made based on their chemical shifts [58]. The C 1 s
XP spectrum showed the typical peaks at 284.8 and 286.9 eV, and
another peak at 288.2 eV which can be attributed to the existence
DFT calculations
of sp2
C C owing to the aromatic terephthalaldehyde, C N of the
The density functional theory (DFT) calculations were per-
formed by Material Studio 6.0 software from Accelrys. Firstly,
geometry optimization was performed by Becke, three-parameter,
Lee-Yang-Parr (B3LYP) functional method [59]. The B3LYP func-
tional method includes Hartree-Fock exchange to reduce the
interaction error. After each material’s structure was optimized by
B3LYP functional method, the binding energy of the reagent and
catalyst was calculated. For calculating H-bond binding energy, the
binding energy states with and without H-bond were considered.
heteroaromatic triazine ring, and C N from the secondary amine
polymer framework of M-POP, respectively. The deconvoluted N
1 s XP spectrum exhibited two distinct peaks at around 398.1 eV
which correspond to the C − N C functionality arising from the tri-
azine ring, and another peak at 399.1 eV related to sp3
[58].
C N bond
In order to further evaluate the structure of M-POP, we have
utilized the 13C and 15N solid state NMR spectroscopy. The solid
state 13C CP TOSS NMR spectrum (Fig. S3, SI) exhibited three res-
onances at 167, 124, and 54 ppm respectively. The 13C NMR peak
at 167 ppm is due to the triazine ring of M-POP, the other peak
at 124 ppm is related to the aromatic carbon of the terephtha-
laldehyde’s benzene ring, and the other peak at 54 ppm is due
to the carbon of secondary amine group of M-POP’s triazine ring
[17,55]. The solid state 15N CP-MAS spectrum of M-POP showed
two major resonances at 99 and 176 ppm respectively (Fig. S4, SI).
The peak at 99 ppm is associated to the nitrogen in the triazine
ring of M-POP, and the peak at 176 ppm is due to the secondary
Results and discussion
Characterization of porous melamine-terephthalaldehyde
polymer (M-POP)
The synthetic procedure of M-POP implemented in the present
investigation is schematically presented in Scheme 1. The wide
angle XRD pattern of the synthesized M-POP in the 2␪ range of
5–50 ◦ is shown in Fig. 1. As can be noted from Fig. 1, a broad peak
centred at 2␪ = 21◦ confirms the formation of an amorphous poly-
meric network. Absence of characteristic crystalline peaks endorses
amine. The peak at 275 ppm is due to the imine peak (
C N C-)
that was from the melamine and terephthalaldehyde’s condensa-
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Scheme 1. Synthetic procedure of M-POP.
Fig. 2. N2 adsorption-desorption isotherms and pore size distribution curve of the M-POP obtained as per the procedure described in the experimental section.
Fig. 3. The C 1 s and N 1 s XP spectra of M-POP obtained as per the procedure described in the experimental section.
tion product [21,61]. Based on the information obtained from FTIR,
XPS, 13C and 15N solid-state NMR and other techniques, it can be
concluded beyond doubt that there is a formation of polymeric
M-POP.
use of solvents at 40 ◦C for different times as shown in Scheme 2.
The catalytic results pertaining to the acetalization of carbonyl
compounds with MeOH are presented in Table 1. The results per-
taining to acetalization of aldehydes are listed from entry 1–6,
and the acetalization of ketones are at entry 7 and 8 (Table 1).
As can be noted from entry 1 and 2, the acetalization of ben-
zaldehyde was highly effective proving the selective products
within the short duration of 1 h. On the other hand, as shown
at entry 3 and 4 (4-methoxybenzaldehyde) and entry 5 and 6
(trans-cinnamaldehyde), the conversion of aldehydes was less at
Acetalization of carbonyl compounds with MeOH or ethylene
glycol
The acetalization of carbonyl compounds with MeOH or ethy-
lene glycol employing M-POP catalyst was carried out without the
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Table 1
Acetalization of carbonyl compounds with MeOH using M-POP catalyst.
Entry
Substrate
Time (h)
Conversion (%)
Selectivity (%)
1
2
1
2
94
94
>99
>99
3
1
75
96
4
5
2
1
80
68
98
91
6
7
2
2
76
65
90
98
Acetone
8
2
12
>99
Reaction conditions: 10 mg catalyst (M-POP), 100 mmol MeOH, 1 mmol carbonyl compound, temperature 40 ◦C; products were analyzed by GC and GC–MS.
of aldehydes was higher than the conversion of ketones. However,
the acetal formation selectivity is higher than 90% with both alde-
hydes and ketones. There results demonstrate that the M-POP is
a highly promising catalyst for selective acetalization of carbonyl
compounds.
In Table 2, the results of acetalization of carbonyl compounds
with ethylene glycol (EG) are listed. The substrates used in this
investigation were benzaldehyde, 4-methoxybenzaldehyde, and
trans-cinnamaldehyde respectively. The acetalization of aldehydes
with EG showed similar results with that of methanol as presented
in Table 1. In other words, the conversion of aldehyde and the
selectivity of acetal formation with EG is almost similar to that
of acetalization of aldehyde with MeOH. However, the acetaliza-
tion reaction time was much longer than 2 h (12 and 24 h). It was
observed that the conversion of aldehydes and the selectivity of
acetal formation were not increased rapidly. However, the final
results were almost similar with varying reaction times. The cat-
alytic results clearly indicated that the acetalization of carbonyl
compounds could be achieved in short reaction times with M-POP
as the promising catalyst. It was also noted from this study that M-
POP could be utilized for acetalization reaction with many kinds of
carbonyl compounds and alcohols with varying reaction times. It
should be pointed out here that the acetylazation was carried out
at a much lower temperature of 40 ◦C and reasonably good results
were obtained within 2 h.
Scheme 2. Acetalization of carbonyl compounds with methanol or ethylene glycol
using the M-POP.
1 h reaction time and increased with increase of reaction time
to 2 h. As expected, the conversion of benzaldehyde was higher
than the other aldehydes, because benzaldehyde is more reactive
than 4-methoxybenzaldehyde and trans-cinnamaldehyde. Due to
the inductive effect owing to the presence of OCH3 group in
4-methoxybenzaldehyde its reactivity is lower than the benzalde-
hyde. In the case of trans-cinnamaldehyde, the C C bond is existed
between aromatic ring and the carbonyl group. In general, the order
of inductive effect is as follows: −OR > −C C− > −H. It is a known
fact that the trans-cinnamaldehyde is more stable than benzalde-
hyde and 4-methoxybenzaldehyde because of inductive effect.
It is interesting to note in the case of aldehydes that the acetal
formation selectivity is higher than 90% irrespective of the nature
of the aldehyde [63]. However, in the case of ketones (Table 1,
entry 7 and 8), the conversion was lower than aldehydes (65 and
12% respectively). Although the catalytic reaction conditions were
same, the conversion of ketones was decreased. As known, ketones
are more stable compounds than aldehydes hence the conversion
Catalyst recyclability test
For ascertaining the recyclability of the catalyst, the M-POP
catalyst was recovered and reused after conducting the acetal-
ization reaction. After the reaction, the catalyst was recovered
by filtration and washed with DCM, excess distilled water, and
acetone multiple times. The washed catalyst was dried under vac-
uum at 200 ◦C for 12 h. Thus dried M-POP catalyst was directly
utilized in the acetalization reaction once again. Repeated acetal-
ization reaction experiments were conducted with benzaldehyde
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Table 2
Acetalization of aldehydes with ethylene glycol using M-POP catalyst.
Entry
1
Substrate
Time (h)
1
Conversion (%)
94
Selectivity (%)
>99
2
3
2
95
95
>99
>99
24
4
5
6
7
1
79
79
80
81
96
98
91
90
2
12
24
8
1
80
83
83
87
97
9
2
95
10
11
12
24
98
>99
Reaction conditions: 10 mg catalyst (M-POP), 100 mmol ethylene glycol, 1 mmol aldehyde, temperature 40 ◦C; product was analyzed by GC and GC–MS.
Table 3
Recyclability of the catalyst.
H-bonding motif in acetalization reaction
As mentioned in the earlier paragraphs, the H-bonding motif for
catalytic reactions can solve some of the disadvantages of acid cat-
alysts [21]. The M-POP is a high nitrogen content material which
can be explored for H-bonding catalysis. The triazine’s nitrogen
and the secondary amine can form H-bond with carbonyl group
(−CHO) and hydroxyl group (−OH). To estimate the H-bonding
energy of carbonyl compounds and alcohol with M-POP, the DFT
calculations were utilized. From DFT calculations, it was found that
there are three kinds of H-bonds possible between MeOH and the
catalyst. One H-bond is between secondary amine’s nitrogen and
the hydroxyl group’s hydrogen. This H-bond’s binding energy was
calculated to be −0.008 kcal mol−1 (Fig. S5a). Another H-bond is
between secondary amine’s hydrogen and the hydroxyl group’s
oxygen. The corresponding H-bond binding energy was found to be
−0.3 kcal mol−1 (Fig. S5b). The other H-bond is between triazine’s
nitrogen and the hydroxyl group. This binding energy was found
to be −4.7 kcal mol−1 (Fig. S5c). This H-bond is strongest and it is
expected that H-bond of M-POP and MeOH is preferred to triazine’s
nitrogen and hydroxyl group’s hydrogen.
Number of cycle
Conversion of
aldehyde (%)
Selectivity of
acetal (%)
1
2
3
94
92
89
>99
>99
98
Reaction conditions: 10 mg catalyst (M-POP), 100 mmol MeOH, 1 mmol benzalde-
hyde, temperature 40 ◦C, time 2 h.
and MeOH at 40 ◦C for 2 h. The recyclability tests were conducted
more than 3 times with the same catalyst. It was observed from
these experiments that there is no big change in the M-POP abil-
ity for acetalization reaction as shown in Table 3. The conversion
of benzaldehyde was almost 89% and the acetal product selectivity
was 98% in the 3rd cycle. As can be noted from Table 1 (entry 2), the
conversion of benzaldehyde was not decreased substantially and
the selectivity of acetal formation was almost same after 3rd recy-
cle of the catalyst. These results confirm the usefulness of M-POP
catalyst for large scale application of acetalization in the industrial
organic synthesis. Further work is under active progress on those
lines.
In the case of benzaldehyde and M-POP, it is possible to have one
H-bond between benzaldehyde and the M-POP. The typical H-bond
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Fig. 4. Proposed mechanism of acetalization reaction over M-POP.
with binding energy between benzaldehyde and M-POP is shown in
Fig. S6. From the DFT calculations, one could propose the catalytic
mechanism of acetalization reaction over M-POP as shown in Fig. 4.
The benzaldehyde can form H-bond with secondary amine, and
MeOH can form H-bond with secondary amine and triazine’s nitro-
gen. To demonstrate H-bonding effect with M-POP and the reagent,
the 1H solid state MAS NMR analysis was utilized (SI). Such anal-
yses were already exploited by various researcher groups in the
literature [64], especially in the bio-macromolecules research area
[65]. The solid state NMR measurements supported in favour of the
mechanism as shown in Fig. 4 in line with literature [63,64]. How-
ever, further studies are necessary in order to endorse the proposed
mechanism. Nevertheless, M-POP exhibited good catalytic activity
for acetalization of various carbonyl compounds with methanol or
ethylene glycol under mild conditions which is a good evidence for
its catalytic action.
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