Tetranuclear Zn complex covalently immobilized on sulfopropylsilylated mesoporous silica: An efficient catalyst for ring opening reaction of epoxide with amine

Article abstract of DOI:10.1016/j.mcat.2020.111220

Tetranuclear Zn complex, [Zn₄(L)₂(LH)₂((CH₃)₂SO)₂]·2CH₃OH·(CH₃)₂SO·H₂O (1) [where LH₃ = 3-(E)-(2-hydroxyphenylimino)methyl-4-hydroxy-5-hydroxymethylphenyl (LH₃)] was prepared and characterized by single-crystal X-ray analysis. Then, it was incorporated to the sulfopropylsilylated MCM-48, MCM-41 and SBA-15 mesoporous silica materials. The prepared materials were thoroughly characterized by XRD, FT-IR, N₂ adsorption/desorption analysis, NH₃-TPD, TGA, SEM-EDX and ICP-OES measurements. These catalysts were successfully used for the ring opening reaction of propylene oxide with morpholine producing the corresponding β-aminoalcohols. An abrupt increase in catalytic activities was observed after anchoring of the tetranuclear Zn complex onto the inner surface of sulfopropylsilylated mesoporous materials. Among all the catalysts, the tetranuclear Zn complex incorporated MCM-48 was found to be most active in this reaction system and gave maximum 95% conversion. Catalysis by the tetranuclear Zn complex immobilized on MCM-48 was totally heterogeneous and the catalyst was recyclable up to 4 times without leaching.

Full text of DOI:10.1016/j.mcat.2020.111220

Molecular Catalysis 497 (2020) 111220
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Tetranuclear Zn complex covalently immobilized on sulfopropylsilylated
mesoporous silica: An efficient catalyst for ring opening reaction of epoxide
with amine
,
Divya Jadava, Pooja Shuklaa, Rajib Bandyopadhyayb, Yoshihiro Kubotac, Sourav Dasa *,
,
Mahuya Bandyopadhyaya *
a Institute of Infrastructure, Technology, Research and Management, IITRAM, Maninagar, Ahmedabad, Gujarat, India
b School of Technology, Pandit Deendayal Petroleum University, Raisan, Gandhinagar, Gujarat, India
c Department of Material Science & Chemical Engineering, Yokohama National University, Yokohama, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
Tetranuclear Zn complex, [Zn4(L)2(LH)2((CH3)2SO)2]⋅2CH3OH⋅(CH3)2SO⋅H2O (1) [where LH3
= 3-(E)-(2-
Tetranuclear Zn complex
Mesoporous silica
Ring opening reaction
β-Aminoalcohols
hydroxyphenylimino)methyl-4-hydroxy-5-hydroxymethylphenyl (LH3)] was prepared and characterized by
single-crystal X-ray analysis. Then, it was incorporated to the sulfopropylsilylated MCM-48, MCM-41 and SBA-15
mesoporous silica materials. The prepared materials were thoroughly characterized by XRD, FT-IR, N2 adsorp-
tion/desorption analysis, NH3-TPD, TGA, SEM-EDX and ICP-OES measurements. These catalysts were success-
fully used for the ring opening reaction of propylene oxide with morpholine producing the corresponding
β-aminoalcohols. An abrupt increase in catalytic activities was observed after anchoring of the tetranuclear Zn
complex onto the inner surface of sulfopropylsilylated mesoporous materials. Among all the catalysts, the tet-
ranuclear Zn complex incorporated MCM-48 was found to be most active in this reaction system and gave
maximum 95% conversion. Catalysis by the tetranuclear Zn complex immobilized on MCM-48 was totally het-
erogeneous and the catalyst was recyclable up to 4 times without leaching.
Introduction
tetradentate in nature providing necessary free coordination sites. It
facilitates the formation of such complex which efficiently allows us to
Transition metal complexes are quite popular in the oxidation/
reduction of organic substrates and for the synthesis of fine chemicals,
but some disadvantages related to their use, such as difficult recovery
from the reaction mixture, low stability, high cost, limits their use,
especially from an industrial point of view. Their immobilisation on
solid supports leads to an important class of organic inorganic hybrid
materials with certain added advantages like selectivity, easy separation
from the reaction mixture and recycle capability [1–4]. These immobi-
lized materials can be utilized to develop new environment friendly
technologies.
study the role of multidentate ligands, geometry, labile groups as well as
its efficacy in the catalytic activity of the desired systems [5]. In case of
epoxide ring opening reaction, zinc complexes are effective in inducing
‘electrophilic activation’ of the epoxide ring. Besides, the strong elec-
trophilicity of the Zn2+ ion delocalized the negative charge of the oxy-
gen atom in the transition state of the epoxide ring opening reaction and
assisted the progress of the reversible reaction in the forward direction.
Basically, Zn2+ containing metal complexes due their flexibility in co-
ordination sites can play a delicate role in counterbalance between
Lewis acid and electrostatic activation of the substrate [6]. Sometimes,
substrate can be cooperatively activated by dual metal components
present in the polynuclear metal complexes [7,8].
Influence of zinc complexes arises from its excellent Lewis acidity,
rapid ligand exchange, being non-toxic and non-redox active making it
most obvious choice in catalytic studies. In order to deeply understand
these traits, one should understand and appreciate how the chemistry of
Zn2+ is influenced by its coordination sphere. Reported metal ion sys-
tems in most cases have Schiff- based ligands that are tridentate or
Till now, a wide varieties of hybrid solid supports have been used,
namely “bulk’’ supports including mesoporous silicas, carbon materials,
clay based materials and metal–organic frameworks or “nano-supports’’
like nano-silicas, carbon nano-tubes, magnetic iron oxide nano-particles,
* Corresponding authors.
E-mail addresses: souravdas@iitram.ac.in (S. Das), mahuyabandyopadhyay@iitram.ac.in (M. Bandyopadhyay).
https://doi.org/10.1016/j.mcat.2020.111220
Received 30 July 2020; Received in revised form 6 September 2020; Accepted 7 September 2020
Available online 1 October 2020
2468-8231/© 2020 Elsevier B.V. All rights reserved.

D. Jadav et al.
Molecular Catalysis 497 (2020) 111220
Scheme 1. Ring opening reaction of propylene oxide with morpholine.
etc. [9]. The promising properties of high surface area, narrow pore size
distribution and flexible pore diameters (>2 nm) of ordered mesoporous
materials have attracted enough consideration for their potential ap-
plications in adsorption, separation, tailored delivery devices for
bio-applications, chemical sensing and catalysis. In acid catalyzed re-
actions like alkylation, acylation, esterification, biodiesel production to
redox reaction such as oxidation of alcohols, alkenes, sulfides and hy-
drogenation of acids, aldehydes and ketones and alkenes/alkynes also
including asymmetric applications, such materials have been exten-
sively used [10–19].
chemical reactions, we have prepared new tetranuclear open-cubane
shaped Zn complex and it was anchored to the sulfopropylsilylated
mesoporous silica supports (MCM-48, MCM-41 and SBA-15). Owing to
high surface area which offers effective number of active sites than non-
crystalline amorphous silica [58,59], mesoporous crystalline materials
are preferred as a support for functionalization. The prepared materials
were thoroughly characterized using different physicochemical char-
acterization techniques to have a clear image of the successful anchoring
of Zn complex in mesoporous silica support. In our present work, we
have synthesized and used tetranuclear Zn-complex to incorporate in
sulfonic acid functionalized mesoporous silica and explored their cata-
lytic activities in epoxide ring opening reaction.
The combination of Schiff based complexes with functionalized
porous materials can be a promising approach towards the industrially
important catalytic reactions. S. Sharma et al. [20] synthesized high
order of dioxygen binding to the immobilized CoII complexes in porous
materials (90% for P-1. py[CoII]) and used them as heterogeneous
oxidation catalysts for substituted phenol. There are reports on
aminopropyl-functionalized SBA-15, as a support for anchoring tri
(8-quinolinolato) iron complexes [21] and mono- and polynuclear
carbonyl compounds of Rh, Ir, encapsulated and different oxovanadium
and other metal complexes in the super cages of Y zeolites [22–28]. As
reported, oxovanadium(IV) and copper(II) complexes are more stable as
dimmer [29–32]; the zeolitic pore allows them to exist as stable
monomeric active center by preventing dimerization. There are also
other reports on synthesis and catalytic properties of Cu (II) complex
modified MCM-41 material in variety of chemical conversions [33–35].
Epoxides are significant class of synthons which has profound use in
synthetic organic chemistry [36–38]. Epoxide ring opening reactions
lead to the formation of industrially important multipurpose products.
The presence of nucleophiles, such as amines, ammonia, alcohols, phe-
nols, water, thiols, etc. is required for this reaction to attack the sub-
strate. Ring opening of epoxide by amine yields β-amino alcohols
(Scheme 1) which are industrially important for the production of bio-
logically active natural/unnatural materials and also can be used as a
chiral ligand in asymmetric synthesis [39–42]. β-amino alcohols can also
be used as a potential inhibitor of the anti-tubercular target N-acetyl-
transferase [43]. Traditionally the epoxide ring opening reactions are
carried out at elevated temperature, however, at higher temperature the
formation of unwanted side reactions [44] cannot be avoided. In most of
the cases, weak nucleophile reacts slowly and yields low regioselective
products. Some reported studies demonstrate the use of excess amount
of sulfamic acid, modified montmorillonite clay, mesoporous carbon,
ionic liquid, cobalt(II) chloride, polymeric rare earth complexes, nano-
porous aluminosilicate and Lewis acid as a catalyst in epoxide ring
opening reaction performed at elevated temperature, high pressure and
long reaction time [45–52]. Henceforth, developing a method which
will work at mild condition is always an effort for the scientific frater-
nity. Researchers are already working in this area to achieve higher yield
at mild and solvent free reaction system [53–57].
Experimental
Synthesis and preparation of sulfopropylsilylated MCM-48, MCM-41 and
SBA-15
Mesoporous MCM-48, MCM-41 and SBA-15 materials were synthe-
sized as per the conditions mentioned in the literature [60]. MCM-48,
MCM-41 and SBA-15 materials were synthesized using molar gel
composition of 1.0 TEOS:0.7 CTACl:0.5 NaOH:64 H2O, 1.0 TEOS:0.12
CTABr:0.23 NaOH:130 H2O and 1.0 TEOS:0.017 Pluronic123:6.1
HCl:165 H2O respectively. The as-made materials were calcined at 550
◦C for 5 h.
Post-synthetic modification was carried out by using 3-mercaptopro-
pyltrimethoxysilane (MPTES) using literature procedure by our group
[61]. Appropriate amount of catalyst was vacuum dried and refluxed at
110 ◦C for 12 h with excess of MPTES. The solid material was recovered
and oxidation of thiol group was done by using excess of 30% H2O2
solution for 24 h followed by acidification with excess of 0.2 M H2SO4.
Finally, the solid powder was filtered off, washed with water, dried at 60
◦C. The schematic diagram for the functionalization process is explained
in Scheme 3.
Synthetic procedure for the preparation of complex
The ligand (E)-2-(hydroxymethyl)-6-(((2-hydroxyphenyl) imino)
methyl)-4-methylphenol (LH3) was prepared from a known procedure
[62]. Zinc acetate dihydrate as well as 2-aminophenol were received
from Sigma Aldrich, India and were used as such. The base tetra butyl
ammonium hydroxide (TBA-OH) was obtained from SRL Chemicals,
India. The other reagents were purchased from commercial sources and
utilized without any further purification. Complex was prepared by
using above mentioned ligand and the procedure is described as below.
[Zn4(L)2(LH)2((CH3)2SO)2]⋅2CH3OH⋅(CH3)2SO⋅H2O
(1)
Ligand LH3 (0.041 gm, 0.1593 mmoles) was dissolved in methanol
and after stirring for fifteen minutes, Zn(OAc)2⋅2H2O (0.0073 gm,
0.0398 mmoles) was added. To it three moles of TBA-OH was added
Inspired by the versatile application of metal complex incorporated
porous silica materials as a catalyst in different industrially important
2

D. Jadav et al.
Molecular Catalysis 497 (2020) 111220
Scheme 2. Synthesis of the homometallic Zn4 complex (1).
Scheme 3. Preparation of Zn complex incorporated sulfopropylsilylated mesoporous silica.
giving yellow colored precipitate immediately. After stirring for twelve
hours, the solution was evaporated in vacuum and dissolved in DMSO
and dichloroform. One week later, yellow colored crystals suitable for
single crystal X-ray diffraction were obtained (Scheme 2). Yield:
0.034gm, 53.01% (based on Zn). Anal. Calcd. C68H79N4O18S3Zn4
(1597.91): C, 43.12; H, 5.59; N, 4.44. Found: C, 42.98; H, 4.05; N, 5.60.
EVO II TG 8120 (Rigaku) instrument. The sample was heated from room
temperature to 800 ◦C at 10 ◦C min–1 in a dry air flow of 30 mL min–1
.
The temperature programme desorption (NH3-TPD) was carried out
with a chemisorption analyzer (BELCAT-B, MicrotracBEL) and a thermal
conductivity detector (TCD). The fresh sample (50 mg) was preheated at
600 ◦C for 1 h prior to measurement under He flow at 50 cm3 (NTP)
min–1 and then it was cooled before being exposed to ammonia. NH3
adsorption was performed at 100 ◦C using 5% NH3/He flow at 50 cm3
(NTP) min–1, and the TPD profiles were collected by increasing the
temperature range from 100-500 ◦C at a ramp rate of 10 ◦C min–1 under
a He flow at 30 cm3 (NTP) min–1. Total number of acid sites was
determined from the area of the h-peak [63] in the TPD profiles. The
Si/Zn molar ratios were measured by inductively coupled
plasma-atomic emission spectroscopy (ICP-AES; ICPE-9000, Shimadzu).
The FT-IR spectroscopic analysis (PerkinElmer, Spectrum Two) was
performed to evident the successful grafting of Zn complex to the mes-
oporous supports. The IR spectra was recorded at room temperature at
resolution of 2 cm–1 in the range of 500–4000 cm–1 with the diamond
crystal. The morphological study and elemental analysis of the zeolite
catalysts were performed by Field Emission Scanning Electron Micro-
scopy (FE-SEM with EDX; JSM-7600 F, JEOL). Elemental analysis was
measured using Thermoquest CE instrument CHNS-O, EA/110 model.
NMR spectra with CDCl3 solutions were performed by a JEOL JNM
LAMBDA 400 model spectrometer operating at 400 MHz.
Zn complex incorporation to the mesoporous silica
Sulfopropylsilylated mesoporous silica was used as a host to immo-
bilize tetranuclear cubane shaped Zn complex to prepare the organic-
inorganic hybrid materials (Scheme 3). Prior to the anchoring, 400 mg
of sulfopropylsilylated mesoporous silica was dried at 80 ◦C for 3 h and
then 40 mg of Zn complex was mixed with it in dry toluene followed by
refluxing at 110 ◦C for 18 h. The solid product was then recovered by
filtration, washed thoroughly with toluene and dried at 80 ◦C for 12 h.
Zn complex incorporated MCM-48, MCM-41 and SBA-15 materials were
denoted as M-48-S-Zn, M-41-S-Zn and S-15-S-Zn respectively.
Characterization
The crystallinity and phase purity of the prepared materials were
investigated by powder X-ray diffraction (XRD; Ultima-IV, Rigaku) via
Cu Kα
radiation at 40 kV and 20 mA. Nitrogen (–196 ◦C) adsorption-
desorption isotherm analysis was performed on BELMAX, Micro-
tracBEL instrument measured for samples pre-treated at 150 ◦C for 4 h.
Thermogravimetric analysis (TGA) was performed on a Thermo plus
3

D. Jadav et al.
Molecular Catalysis 497 (2020) 111220
Fig. 1. (a). Molecular structure of complex 1
(Hydrogen atoms except those of protonated
ligand are omitted for clarity). (b) Coordination
framework of 1 displaying open half cubane
like structure. Selected bond distance (Å) and
bond angle (o) parameters: Zn(1) - O(2) = 2.015
(5), Zn(1)- O(1) = 2.009(5), Zn(1)- O(4) =
2.037(5), Zn(1)- N(1) = 2.056(7), Zn(1) - O(14)
= 2.053(6), Zn(2) – N(3) = 2.023(6), Zn(2) – O
(4) = 2.244(5), Zn(2) – O(7) = 1.994(6), Zn(2)
– O(8) = 2.259(5),), Zn(2) – O(2) = 2.089(5),
Zn(2) – O(3) = 2.101(6), Zn(3) – O(4) = 2.259
(5), Zn(3) – O(8) = 2.222(5), Zn(3) – O(11) =
2.082(5), Zn(3) – O(5) = 2.010(6), Zn(3) – O
(12) = 2.112(5), Zn(3) – N(2) = 2.031(6), Zn(4)
– O(10) = 2.013(5), Zn(4) – N(4) = 2.058(7),
Zn(4) – O(13) = 2.052(6), Zn(4)-O(11) = 2.017
(5), Zn(4) – O(8) = 2.047(5). Zn(1) – O(4) – Zn
(2) = 95.1(2), Zn(1) –O(2) – Zn(2) = 100.8(2),
Zn(1) – O(4) – Zn(3) = 111.2(2), Zn(2) – O(4) –
Zn(3) = 100.5(2), Zn(3) – O(8) – Zn(2) =
101.18(19), Zn(4) – O(8) – Zn(2) = 110.9(2),
Zn(4) – O(8) – Zn(3) = 95.8(2), Zn(4) – O(11) –
Zn(3) = 101.2(2).
X-ray crystallography
Crystallographic data for the 1 were collected with a Bruker Apex II
CCD diffractometer with graphite monochromated Mo-Kα radiation (λ
=0.71073 Å). Data processing was completed with the help of SMART
[64] and SAINT [64] processing program. SADABS [65] was used for
absorption correction, and SHELXTL [65,66] was used for space group,
structure determination and least-squares refinements on F2. The crystal
structures were solved and refined by full-matrix least-squares methods
against F2 by using the program SHELXL-2014 [67] and Olex-2 [68]
software. The hydrogen atoms were set at calculated positions and
refined with the help of riding model. The non-hydrogen atoms present
in the complex were refined by anisotropic displacement parameters.
Diamond 3.1e [69] was used to obtain the crystallographic structures.
The solvent accessible voids present in 1 were assumed to be filled with
certain disordered molecules of both methanol and DMSO which were
unable to be modeled. Utilizing “PLATON/ “SQUEEZE” [70,71] pro-
gram to remove those disordered solvent molecules (complex 1,
2CH3OH, 1(CH3)2SO), 1H2O). Total solvent-accessible void is of volume
is 743 Å3 (per unit cell) which coincides to total electron count removed
to 171 per unit cell. The total electron removed are 85 electrons per
molecule (Z = 2). The crystal data can be acquired from the Cambridge
Crystallographic Data Centre free of cost as CCDC 2002850 (1) from
www.ccdc.cam.ac.uk/data_request/cif. Details of crystallographic data
and structure refinement parameters are given in Table S1.
Scheme 4. Two different binding mode of ligands [LH]2ꢀ for Zn(II) ions.
thickness and 0.25 mm-i.d.). Mass fragment of the products (around 145
m/z) were observed in GC–MS analysis which clearly confirm the for-
mation of regioisomer products (1-Morpholinopropan-2-ol and 2-Mor-
pholinopropan-1-ol). After removal of catalyst, reaction filtrate was
further characterized by 1H NMR and mixture of both the regioisomers
were observed in the data.
Results and discussion
X-Ray crystal structure [Zn4(l)2(LH)2((CH3)2SO)2]⋅2CH3OH⋅
(CH3)2SO⋅H2O (1)
Catalytic reaction
Single crystal X-ray diffraction analysis reveals that homometallic
tetranuclear complex 1 crystallizes in the triclinic system with P-1 space
group (Z = 2). A perceptive view of the molecular structure of 1 is
displayed in Fig. 1a. Bond parameters associated with 1 are given in the
caption of Fig. 1. The structural analysis of 1 imparts that the crystal-
lographic independent unit contains one tetranuclear unit, [Zn
(LH)4((CH3)2SO)2]. Molecular complex of 1 comprises a double open-
cubane type [Zn4(μ3-O)2(μ2-O)2]4+ core which is constructed by four
di-deprotonated Schiff base ligands [HL2ꢀ ]. Here it is noteworthy to
mention that in terms of binding capability two pairs of nonequivalent
ligands [HL2ꢀ ] are present in the system. Each ligand from a pair, che-
Epoxide ring opening reaction of propylene oxide with morpholine
was performed under solvent free condition using a series of prepared Zn
complex grafted sulfopropylsilylated mesoporous materials. For a
typical reaction procedure, required amount of catalyst was activated at
80 ◦C for 1 h. Propylene oxide, morpholine and catalyst were taken in a
round bottom flask and the reaction mixture was allowed to stir at 70 ◦C
using reflux condenser. The catalyst was filtered off from the resulting
liquid and the liquid was subjected to Gas chromatographic analysis
(Thermo Fisher Trace-1310, TG 5 column). A series of reactions were
performed to optimize the reaction parameters to maximize the reactant
conversion. The products were identified by means of GCMS (Perkin
lates two Zn(II) ions via utilizing its all potential binding mode in μ2
-
Elmer Clarus 500) using DB-5 MS Ui Column (30 m-long, 0.25
μm
η1
:
η1
:
η2
:η
1-O, N, O, O fashion, while other ligand form a different pair
4

D. Jadav et al.
Molecular Catalysis 497 (2020) 111220
Fig. 2. Powder XRD patterns of (a) calcined, (b) sulfopropylsilylated, (c) Zn complex incorporated materials, (d) high angle XRD pattern of Zn complex incorporated
materials and (e) pure Zn complex: (A) MCM-48, (B) MCM-41 and (C) SBA-15.
accommodates three Zn(II) ions utilizing its partial coordination binding
equatorial bond distances vary from 1.993(6) – 2.260(4) Å, while the
axial bond lengths fall in the range from 2.023(6) – 2.087(5) Å.
Collected coordination action of all four HL2ꢀ ligands form a open-
cubane type [Zn4(μ3-O)2(μ2-O)2]4+ core where, the corners of the
cubane are occupied by Zn(II) atoms and bridging phenoxy O atoms.
Zn⋅⋅⋅⋅Zn distances within Zn4O4 cubane cores are different and are in the
range of 2.018(5) to 2.260(4) while the average bond angle of Zn-O-Zn is
106.01◦. The average bond angle of Zn-O-Zn and Zn⋅⋅⋅⋅Zn distances
within Zn4O4 cubane cores is almost comparable with the reported ones
[73,74].
in μ3
-
η3
:
η1
:
η
1-O, N, O mode (Scheme 4). Fig. 1b unveils a simplified
layout of coordination environment around the four Zn(II) ions. The
overall structure of 1 contains two structurally distinct Zn(II) ions. The
metal ions, Zn1 and Zn4 are five-coordinate and shows similar square
pyramidal geometry, while the other two metal centres, Zn2 and Zn3 are
present in a distorted octahedral geometry. The Addison distortion [72]
indexes of Zn1 and Zn4 are found to be τZn1 = 0.1394 and τZn4
=
0.10435, respectively which further indicates that Zn1 and Zn4 centres
possess distorted square pyramidal geometries. The basal planes of the
square pyramid ligated by one imine nitrogen and two phenoxide oxy-
gen atoms of the partially deprotonated ligand [HL]2ꢀ and the μ3-phe-
Structural Integrity studies by characterization
noxido oxygen atom of another deprotonated ligand [HL]2ꢀ
.
The apical position of the square pyramid is occupied by dimethyl
sulphoxide molecule. The basal bond lengths of the square pyramid are
in the range of 2.006(5) Å to 2.057(7) Å and the average apical bond
distance is 2.055 Å. On the other hand, Zn2 and Zn3 centres adopt
distorted octahedron geometry where basal plane is achieved by the
imine nitrogen, phenoxido, μ3-phenoxido oxygen atoms of one HL2ꢀ
ligand and μ2-phenoxido oxygen atoms from another HL2ꢀ ligand, and
one of the axial position is occupied by oxygen atoms from the benzyl
alcohol groups from later ligand and other axial position is occupied by
μ3-phenoxido oxygen atoms of another HL2ꢀ ligand (Fig. S1). The
The powder XRD patterns of unmodified and modified materials are
depicted in Fig. 2, high angle XRD patterns are shown in the inset of each
of the figures. Low angle diffraction patterns of MCM-48, MCM-41 and
SBA-15 show well resolved characteristic reflections revealing pure
mesoporous phase. In systematic studies of mesoporous/sorbate system
it is already proved that with the tethering of the functional groups into
the pores and surface of the host molecules decreases the peak intensity
in the diffraction patters [75]. The higher the electron density of the
sorbate molecules the lower the peak intensity, however, the successive
anchoring of the molecules restores the scattering contrast leading to the
Fig. 3. N2 adsorption/desorption isotherm of (a) sulfonic acid functionalized, (b) Zn complex incorporated (A1) MCM-48, (B1) MCM-41 and (C1) SBA-15 materials.
Pore size distribution curve of (a) sulfonic acid functionalized (b) Zn complex incorporated (A2) MCM-48, (B2) MCM-41 and (C2) SBA-15 materials.
5

D. Jadav et al.
Molecular Catalysis 497 (2020) 111220
N2 adsorption/desorption isotherms of MCM-48, MCM-41 and SBA-
15 are shown in Fig. 3. The adsorption/desorption isotherms of Zn
complex incorporated mesoporous silica are compared with sulfopro-
pylsilylated mesoporous silicas. All the isotherms are of type IV with
steep increase around p/p0 0.3, which is more prominent in case of all
the calcined materials (not shown here) and for functionalized MCM-41
and SBA-15 (Fig. 3 B & C), not very sharp for MCM-48 catalyst. The
presence of mesopore assembly is confirmed by a sharp increase of
adsorbed gas volume due to capillary condensation in a partial pressure
of nitrogen ranging between p/p0 0.1 and 0.8, indicative of narrow and
undeviating pore size distribution [60,76–80], which is likely for mes-
opores. The pore size distribution curve (Fig. 3) of acid modified and
Table 1
Characteristics of various unmodified and modified mesoporous materials
Catalyst
Surface
area[a]
(m2/g)
Pore
Pore
Si/Zn
Acid
volume[b]
(cc/g)
diameter[b]
(nm)
ratio[c]
concentration
(mmol/g)[d]
MCM-48
MCM-
48-
1351
590
1.1
2.3
2.1
–
–
–
0.61
0.44
SO3H
MCM-
48-
515
0.43
1.7
18.3
0.824
SO3H-
Zn
MCM-41
MCM-
41-
1088
850
0.99
0.62
2.5
2.3
–
–
–
0.3
acid
and
Zn
modified
material
was
obtained
from
Barrett-Joyner-Halenda (BJH) method. The reduction of specific surface
area, average pore diameter and pore volume were observed for all the
samples may be due to the pore filling by tethering of the functionalized
materials [81] and are summarized in Table 1. The deterioration of the
sharpness in the isotherm of functionalized MCM-48 materials may be
due to the decrease of uniformity of the structure after the acid and
Zn-complex modifications. Results clearly indicate that Zn complex
covalently tethered to the external and internal pore surfaces of all three
materials, which was further confirmed by other physico-chemical
characterizations.
SO3H
MCM-
41-
715
0.51
2.0
10.6
0.373
SO3H-
Zn
SBA-15
SBA-15-
SO3H
SBA-15-
SO3H-
Zn
852
661
0.96
0.85
6.4
6.2
–
–
–
0.33
512
0.77
6
17.6
0.33
SEM images of all three modified materials reveal that there was no
apparent change in morphology of the materials observed after Zn
complex immobilization into the mesoporous materials. Fig. 4 Shows the
SEM image of M-48-S-Zn was found in spherical morphology with par-
[a]determined by the BET method, [b]determined by the BJH method, [c]from
ICP-OES analysis, and [d]calculated from NH3-TPD measurement.
diffraction properties of the starting materials again showing the intact
mesoporous silica framework after sulfone as well as Zn4 complex
modifications consecutively. The scattering contrast and the resolution
of the X-ray powder diagram was reduced in each functionalization step
directly proves the successful grafting of the sulfonic acid onto the
surface of mesoporous silica by surface silanolic ꢀ OH groups followed
by covalently linked of Zn-complex with –SO3H groups.
ticle size of approx. 200 nm (0.2 μm).
There were no significant changes in the morphology of M-41-S-Zn
and S-15-S-Zn as well (not shown here) show the intactness of the
structure even after modification. SEM-EDX analysis of a selected area of
Zn complex modified sulfopropylsilylated MCM-48 material is shown in
Fig. 4 B. The peaks of S, Si and Zn elements in the sample is clearly
visible here. There is a clear Zn signal at 8.7 Kev for ZnK
α. Apart from
In order to check the Zn peaks in the mesoporous silica materials due
to anchoring of the Zn-complex, high angle X-ray diffraction analysis
was also recorded upto 50˚2θ, and compared with pure Zn complex to
confirm the anchoring of Zn complex into mesoporous silica, which are
shown in the inset of respective figures. All the modified materials
showed peak at around 7.7˚2θ which is well matched with the pattern of
pure Zn complex, with slight shift of˚2θ value of Zn modified samples
towards right angle which may be attributed to the change of chemical
composition of the Zn complex after covalently linked with –SO3H
groups in mesoporous materials. This observation confirms the presence
of Zn complex in modified porous materials without any degradation.
This fact was further confirmed by IR, EDX and ICP analyses also and
explained in the later sections.
this the signals for ZnL and ZnKβ are also present. These results indicate
α
that Zn complex is covalently linked with the –SO3H group anchored in
the mesoporous materials. From the EDX analyses the wt % of Zn present
in the materials were calculated. M-48-S-Zn material showed 3.70% of
Zn metal in the material whereas M-41-S-Zn and S-15-S-Zn showed 2.31
and 3.35% respectively. These estimations are done to have a rough
calculations of bulk elements present in the materials or to confirm the
presence of Zn or S in the hybrid materials. We have also used ICP an-
alyses to calculate the exact amount of Zn and S present in the modified
materials.
ICP-OES results are summarized in Table 1. Result showed that
highest Si/Zn ratio of 18.3 was in M-48-S-Zn with compare to M-41-S-Zn
(10.6) and S-15-S-Zn (17.6) materials. 3D structure of MCM-48 material
Fig. 4. (A) FE-SEM image and (B) EDX analysis of M-48-S-Zn material.
6

D. Jadav et al.
Molecular Catalysis 497 (2020) 111220
Fig. 5. (A) FT-IR spectra of (a) M-48-S-Zn, (b) M-41-S-Zn, (c) S-15-S-Zn and (d) pure Zn complex. (B) NH3-TPD profile of (a) M-48-S-Zn, (b) M-41-S-Zn and (c) S-15-
S-Zn.
Table 2
TOF, acidity and morpholine conversion with different catalyst amount.
Catalyst
Catalyst
Morpholine
Acid concentrationa
(mmol/g)
TOF
amount (mg)
conversion (%)
(hꢀ 1
)
5
65
84
95
58
77
86
52
64
78
263
170
96
M-48-S-
Zn
10
20
5
0.824
0.373
0.330
519
344
192
525
323
197
M-41-S-
Zn
10
20
5
S-15-S-
Zn
10
20
aCalculated from NH3-TPD measurements.
Fig. 6. Comparison of various catalysts in epoxide ring opening reaction. Re-
action conditions: temperature, 70 ◦C; catalyst, 20 mg; time, 6 h; propylene
oxide, 10 mmol; morpholine, 10 mmol.
may have more accessibility to the active sites hence maximum amount
of Zn was anchored to the internal and external pore surfaces of MCM-
48. We can correlate these data with the N2 adsorption isotherm of M-
48-S-Zn material. As the loading of Zn complex is maximum in MCM-48
material, the uniformity of the pore structure is somewhat disturbed,
which is clearly reflected in the decrease of steepness of the curve in the
range of p/p0 0.2 to 0.8 in the material than the other two.
than the other two. We have compared the activities with sulfopro-
pylsilylated catalysts also with less acidity as revealed by TPD analyses
and discussed in the later section. Additionally, to investigate the ther-
mal stability of prepared material and to study the amount of catalyst
loading, thermogravimetric analysis was performed. The results are
depicted in Fig. S2. The small weight loss around 5% was observed
below 150 ◦C which was due to the presence of physiosorbed water in all
three materials. The other weight loss in the temperature range of
200–600 ◦C was assigned to the decomposition of covalently bonded Zn
complex with sulfonic acid group with propyl linkage of mesoporous
materials. In this temperature range, around 15–17% weight loss was
observed for all three materials.
In order to confirm the linkage of Zn complex to the sulfopropylsi-
lylated mesoporous silica materials, FT-IR spectra of pure Zn complex
and Zn complex incorporated mesoporous materials were compared and
shown in Fig. 5 A. All the materials exhibit broad bands at 787 and 1050
cmꢀ 1attributed to Si–O–Si bending and stretching vibrations as well as
silanolic OH groups [61,82] respectively. At 1390 cmꢀ 1 deformation
vibration band of ꢀ CH2 are also found for all the materials, which is
ascribed from the propyl group linked to silanolic OH group due to
presence of sulfonic acid group in the materials. Symmetric vibration of
Comparison of various catalysts on epoxide ring opening reaction
–
–
–SO H groups as well as asymmetric vibration of S O is found around
3
1000–1200 cmꢀ 1 wave length range respectively, but due to overlay
with the Si–O–Si band at 1050–1150 cmꢀ 1 this peak cannot be sepa-
rated. The IR spectrum of pure Zn complex displayed band at 1530 cm-1
The efficiency of proposed modification method can be clearly un-
derstood after evaluation of catalytic activities in epoxide ring opening
reaction of propylene oxide with morpholine using sulfopropylsilylated
catalysts, Zn complex incorporated catalysts and also in the absence of
catalyst. Fig. 6 shows the comparison of various catalysts in epoxide ring
opening reaction.
due to C N stretching vibration and the peak at 1450 cm-1 correspond
–
–
–
O
to carbon-carbon stretching vibration of the aromatic rings. The Zn
stretching frequency is observed at 720 cmꢀ 1
.
The acid concentration of prepared materials due to incorporation of
Zn complex was calculated from NH3-TPD measurements and NH3-TPD
profile is depicted in Fig. 5 B. As shown in Table 2, the total acidity of
prepared materials was calculated and estimated by quantity of
ammonia adsorbed. Total acid concentration was calculated from the
NH3 peak above 350 ◦C which was derived from the physically adsorbed
ammonia on the surface of mesoporous silica. Calculated acidity of M-
48-S-Zn catalyst was 0.82 mmol/g which was higher with compare to
other two catalysts as well as from sulfopropylsilylated mesoporous
materials (0.61 mmol/g), and it was clearly reflected in the catalysis
result (Section 4.2.), as M-48-S-Zn showed maximum catalytic activity
Without any catalyst the reaction showed only 10% conversion of the
reactant, whereas there were not much differences in conversion with
sulfopropylsilylated mesoporous materials (15–25 %) also. There was a
drastic change of catalytic activities when Zn-complex modified sulfo-
propylsilylated hybrid materials were used as catalysts. 95% reactant
conversion was achieved with M-48-S-Zn material whereas M-41-S-Zn
and S-15-S-Zn showed 86 % and 78 % conversion respectively. For
comparison, another hybrid material by using Zn-acetate salt, impreg-
nated with MCM-48-SO3H was also prepared. 40% morpholine con-
version was found, which clearly indicates the significance of Zn
complex impregnation instead of Zn acetate salt in sulfopropylsilylated
7

D. Jadav et al.
Molecular Catalysis 497 (2020) 111220
Scheme 5. A. A plausible mechanism of ring opening reaction of propylene oxide with moprholine via SN2 route using Zn complex incorporated mesoporous silica as
a catalyst [84]. B.
8

D. Jadav et al.
Molecular Catalysis 497 (2020) 111220
Fig. 8. Conversion of morpholine for different reaction time. Reaction condi-
tions: temperature, 70 ◦C; catalyst, 20 mg; propylene oxide, 10 mmol; mor-
pholine, 10 mmol.
Fig. 7. Conversion of morpholine using different catalyst amount. Reaction
conditions: temperature, 70 ◦C; time, 6 h; propylene oxide, 10 mmol; mor-
pholine, 10 mmol.
mesoporous siica materials. The reactivity of a hybrid material depends
on the type of ligands attached with the available site of centre metal ion
to fulfil the coordination no after forming the bond with siliceous ma-
terials [83]. Here Zn-complex anchored with functionalized mesoporous
material plays the key role in determining the high activity.
Above results clearly demonstrate the importance of Zn complex
¨
immobilized (Lewis acid) catalysts over sulfopropylsilylated (Bronsted
acid) catalysts in the proposed reaction system. In this reaction, we have
only calculated reactant conversion and selectivity of the products was
not calculated as we were unable to separate the products from the re-
action mixture. The contribution of the two mechanism (SN2 and SN1)
depends on substrate, steric restriction, and reaction conditions. In the
case of SBA-15, the space inside the pore is too large to give steric re-
striction. The substrate in this work is propylene oxide, which means the
substituent is methyl group. The methyl group has some ability to sta-
bilize the adjacent carbenium ion, but phenyl group has stronger ability
to do so and has more tendency to choose SN1 mechanism. On the other
hand, the tendency to choose SN2 mechanism depends on the strength of
nucleophile. Investigation of substituents and nucleophile is future work
and at this stage we focused on the catalyst preparation and its ability to
catalyse the ring-opening of propylene oxide as a primary investigation
in this work. The Lewis acidic nature of Zn ion in the tetranuclear Zn
complex plays a significant role in this reaction and are the active sites
for this organic-inorganic hybrid mesoporous materials. In the proposed
epoxide ring opening reaction, ring opening of propylene oxide in
presence of prepared lewis acid catalyst occurs according to possible SN1
and SN2 mechanism which leads to the formation of regioisomer prod-
ucts. 1-Morpholinopropan-2-ol product was formed via SN2 route while
2-morpholinopropan-1-ol product was obtained via SN1 route. The
plausible mechanism for this reaction is depicted in Scheme 5A and B.
Fig. 9. Conversion of morpholine at different reaction temperatures. Reaction
conditions: catalyst, 20 mg; time, 6 h; propylene oxide, 10 mmol; morpholine,
10 mmol.
calculated based on the acidity estimated from the TPD analyses for all
the catalysts to have an idea about the availability of active sites from
the catalytic point of view. TOF is considered as the number of catalytic
series per unit time, mainly depends on the catalyst, temperature,
pressure, and concentration [85]. Turnover frequency (TOF) values are
summarized in Table 2. To check the efficiency of prepared catalysts in
the proposed reaction, it has been found that TOF values of all the hybrid
materials decreased with increase of the catalyst amount from 5 mg to
20 mg as well as with increase of reactant conversion. All the materials
are found to be proficient in the proposed reaction system and also
showed higher TOF values. Maximum TOF values were obtained for
S-15-S-Zn catalyst and M-41-S-Zn catalysts have comparable values,
though, M-48-S-Zn has comparatively lower TOF values under same
reaction condition. However, catalytic result implies the prominent ac-
tivity of M-48-S-Zn catalyst in the proposed reaction system. This may be
due to the reason that MCM-48 has 3D framework and so has more
available active sites in its structure. Therefore, when Zn complex was
anchored to the MCM-48 material, may be higher extent of Zn complex
was covalently tethered to the surface with compare to MCM-41 and
SBA-15 material. Also the higher acid concentration was observed in
MCM-48 material which is in good agreement with the above mentioned
reason. Acidity of the M-48-S-Zn catalyst was found to be higher
compared to other two catalysts; however, the higher TOF and best
conversion are not fully contributed by the acidity of the material, rather
Reaction optimization
In order to attain the maximum conversion of reactant, reaction was
optimized by changing reaction parameters like catalyst amount, time
temperature etc. and the effect of reaction conversion on catalyst
amount is shown in Fig. 7. 50–65% conversion was achieved when only
5 mg of catalyst was used, the conversion was increased simultaneously
as amount of catalyst was increased. M-48-S-Zn catalyst showed 95%
conversion using 20 mg of catalyst whereas M-41-S-Zn and S-15-S-Zn
catalysts showed 86 % and 78 % conversion respectively with same
reaction conditions. Reaction was also made using 30 mg of catalyst but
there was not much change in the activities compare to 20 mg of
catalysts.
The intrinsic catalytic activities or turnover frequencies were also
9

D. Jadav et al.
Molecular Catalysis 497 (2020) 111220
Fig. 10. (A) Heterogeneity test and (B) Recycle test of M-48-S-Zn catalyst. Reaction conditions: temperature, 70 ◦C; catalyst, 20 mg; time, 6 h; propylene oxide, 10
mmol; morpholine, 10 mmol.
other factors such as acid strength as well as framework structure have
significant contribution towards the catalytic activity [86].
Conclusion
In this work, we have synthesized open cubane shaped Zn4 complex
Reaction time is an important parameter to study the mechanism of
reaction and so several reactions were performed for different durations
to optimize reaction time. The catalytic performance of prepared cata-
lysts displayed in Fig. 8 demonstrate that less amount of morpholine was
converted into the product when reaction was performed for 1 h and it
was increased rapidly as reaction time was increased up to 6 h. Mor-
pholine conversion of all three catalyst proved that this reaction is time
dependent and within 6 h of reaction time 95% morpholine conversion
was achieved by M-48-S-Zn.
utilizing multidentate Schiff base ligand which was further character-
ized by single crystal X-ray diffraction analysis. Later, this complex
incorporated mesoporous silica materials were successfully prepared
and anchoring of the complex was confirmed through thorough char-
acterization techniques using powder XRD, N2 adsorption/desorption
analysis, SEM-EDX, FT-IR, NH3-TPD, TGA and ICP-OES analysis. Meso-
porous phase remains intact and homogeneous even after modification
confirmed through powder XRD patterns. Successful immobilization of
Zn complex to the sulfopropylsilylated mesoporous silica was confirmed
by FT-IR spectroscopy, SEM-EDX, and ICP-OES analyses that showed the
presence of Zn metal in the hybrid materials. The decrease in surface
area, pore volume and pore diameter after modification was consistent
with the presence of the organometallic complex inside the pore. The
prepared catalysts, which were thoroughly characterized, were found to
be highly active for ring opening reaction of propylene oxide with
morpholine, and also noticeably M-48-S-Zn catalyst was found to be
truly heterogeneous and reusable up to 4 times.
The effect of reaction temperature on the morpholine conversion was
studied by varying reaction temperature and the results are summarized
in Fig. 9. Reactions were performed for three different temperatures in
order to achieve the maximum conversion of morpholine. Results indi-
cate that, when the reactions were performed at 50 ◦C, morpholine
conversion was less (40 % for M-48-S-Zn, around 20 % for M-41-S-Zn
and S-15-S-Zn) for all three catalysts. As the temperature was increased
to 60 ◦C, conversion of morpholine was also increased and on further
increase in the temperature to 70 ◦C, 95 % morpholine conversion was
found.
CRediT authorship contribution statement
Heterogeneity of the catalyst
Divya Jadav: Methodology, Writing - original draft. Pooja Shukla:
Methodology. Rajib Bandyopadhyay: Visualization, Investigation.
Yoshihiro Kubota: Characterization, Reviewing. Sourav Das:
Conceptualization, Reviewing. Mahuya Bandyopadhyay: Conceptual-
ization, Supervision, Reviewing, Editing.
In order to check the heterogeneity of the prepared M-48-S-Zn
catalyst two tests were performed, filtration test and recycle (Fig. 10). In
the filtration test, Reaction was started at optimized conditions and after
1 h catalyst was removed from the reaction mixture by filtration and
reaction was continued for further 5 h. As shown in the Fig. 10 A, there
was no change in the conversion after removal of catalyst and after
completion of the test (after 6 h) the morpholine conversion obtained
was 55% which was well matched with the conversion on 1 h of reaction
time (54%). Results clearly indicate the true heterogeneity of prepared
M-48-S-Zn catalyst and confirm no leaching of active site into the re-
action medium during the reaction path. Recycle test was also carried
out to check the efficiency of prepared catalyst and it was performed for
the M-48-S-Zn catalyst in the optimized reaction conditions. After each
catalytic run, the catalyst was recovered by filtration, washed with
acetone and dried at 80 ◦C. The catalyst was used for four catalytic cy-
cles under the same reaction and regeneration conditions.
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
Divya Jadav and Pooja Shukla are thankful to IITRAM for providing
financial support and infrastructural facilities. We would like to
acknowledge IIT Gandhinagar for SEM-EDX analysis, and Mr. Kengo
Takama of Yokohama National University for his technical assistance.
As shown in Fig. 10 B, no drastic change in morpholine conversion
was observed and the catalytic activity was sustained even after 4th run
with 80 % conversion over M-48-S-Zn catalyst. Slight gradual decrease
in the conversion may be attributed to the deactivation of catalysts in
successive runs which may have occurred due to the deposition of the
product or substrate within the mesopore [60].
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2020.111220.
10

D. Jadav et al.
Molecular Catalysis 497 (2020) 111220
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