Instant Cyclohexene Epoxidation Over Ni-TUD-1 Under Ambient Conditions

Article abstract of DOI:10.1007/s10562-020-03423-5

Abstract: To avoid the aggregation problem and activity loss of nickel oxide (NiO) nanoparticles (NPs) in organic reactions, NiO NPs were incorporated into TUD-1 mesoporous material. One-step sol–gel preparation was applied to prepare four samples of Ni incorporated in TUD-1 silica matrix with different Ni content. The four samples with Si/Ni ratio = 100, 50, 20, and 10 were characterized by means of elemental analysis, powder X-ray diffraction (XRD), Raman spectroscopy, N₂ sorption measurements, scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), and high-resolution transmission electron microscopy (HR-TEM). The characterization analysis showed that Ni²⁺ ions were incorporated into the silica matrix as individual isolated active sites at Ni content smaller than 2 wt%, and as nanoparticles of NiO when the loading is equal to or higher than 5 wt%. The size of NiO NPs inside the silica matrix is highly dependent on the Ni content, i.e. the size of NiO NPs when the loading was 5 wt% and 10 wt% was 5–10 and 40–60?nm, respectively. The catalytic activity of Ni-TUD-1 was investigated in the epoxidation reaction of cyclohexene at room temperature by using meta-chloroperoxybenzoic acid (m-CPBA) as an oxidant. The obtained results showed that Ni-TUD-1 exhibited superior activity in which 100% conversion of cyclohexene with > 90% selectivity towards cyclohexene oxide was obtained instantly. This result was found to benchmark not only the unsupported NiO nanoparticles, but also the reported catalysts at similar conditions. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-020-03423-5

Catalysis Letters
https://doi.org/10.1007/s10562-020-03423-5
Instant Cyclohexene Epoxidation Over Ni‑TUD‑1 Under Ambient
Conditions
1
2
1
1
3
Mohamed S. Hamdy · Nabil Al‑Zaqri · Taher Sahlabji · Murad Eissa · Mohammad Abu Haija ·
1
2
2
1
Abdullah M. Alhanash · Ali Alsalme · Fahad A. Alharthi · Mohamed Abboud
Received: 22 July 2020 / Accepted: 5 October 2020
©
Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
To avoid the aggregation problem and activity loss of nickel oxide (NiO) nanoparticles (NPs) in organic reactions, NiO
NPs were incorporated into TUD-1 mesoporous material. One-step sol–gel preparation was applied to prepare four samples
of Ni incorporated in TUD-1 silica matrix with diꢀerent Ni content. The four samples with Si/Ni ratio=100, 50, 20, and
1
0 were characterized by means of elemental analysis, powder X-ray diꢀraction (XRD), Raman spectroscopy, N sorption
2
measurements, scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), and high-resolution transmission
2+
electron microscopy (HR-TEM). The characterization analysis showed that Ni ions were incorporated into the silica matrix
as individual isolated active sites at Ni content smaller than 2 wt%, and as nanoparticles of NiO when the loading is equal
to or higher than 5 wt%. The size of NiO NPs inside the silica matrix is highly dependent on the Ni content, i.e. the size of
NiO NPs when the loading was 5 wt% and 10 wt% was 5–10 and 40–60 nm, respectively. The catalytic activity of Ni-TUD-1
was investigated in the epoxidation reaction of cyclohexene at room temperature by using meta-chloroperoxybenzoic acid
(
m-CPBA) as an oxidant. The obtained results showed that Ni-TUD-1 exhibited superior activity in which 100% conversion
of cyclohexene with>90% selectivity towards cyclohexene oxide was obtained instantly. This result was found to benchmark
not only the unsupported NiO nanoparticles, but also the reported catalysts at similar conditions.
Graphic Abstract
Keywords Cyclohexene · Instant epoxidation · Ni-TUD-1 · NiO nanoparticles · Superior activity · Sol–gel synthesis
*
Mohamed S. Hamdy
1
Introduction
m.s.hamdy@gmail.com
1
Catalysis Research Group (CRG), Department of Chemistry,
College of Science, King Khalid University, P.O. Box 9004,
Abha 61413, Saudi Arabia
The epoxidation reactions of cycloalkenes have attracted a
lot of interest because of the high industrial impact of the
epoxide products [1, 2]. Epoxides are the raw materials
for several products such as resins, paints, surfactants and
other intermediates for organic synthesis [3, 4]. Epoxides are
2
3
Department of Chemistry, College of Science, King Saud
University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
Department of Chemistry, Khalifa University of Science
and Technology, Abu Dhabi, United Arab Emirates
Vol.:(0
1
123456789)
3

M. S. Hamdy et al.
usually prepared through catalytic oxidation of cycloalkenes
at moderate temperature [5].
mesoporous material to avoid these drawbacks, and to inves-
tigate the eꢀect of Ni loading on the type of nickel active
sites and on the catalytic activity of Ni-TUD-1 nanocata-
lyst. Thus, four samples with diꢀerent Ni content were pre-
pared and characterized chemically and physically in order
to investigate the structure and morphology of Ni-TUD-1.
The prepared samples were used to catalyze the epoxidation
of cyclohexene at room temperature and the obtained results
are reported. The main signiꢁcance of the current work is
the obtained superior activity, in which almost 100% conver-
sion was obtained instantly. Moreover, high selectivity of
cyclohexene oxide was obtained as well.
The use of transition metal complexes as homogeneous
catalysts in the selective oxidation of hydrocarbon is a chal-
lenging and exciting scientiꢁc goal, because it is an eꢂcient
and inexpensive process [6]. Nickel metal has been incorpo-
rated in diꢀerent active sites of enzymes which are involved
in some oxidation processes [7]. These include acireductone
dioxygenases, CO dehydrogenase [8, 9], quercetin dioxyge-
nases [10], nickel superoxide dismutase [11], and acetyl-
coenzyme A(CoA) synthase [12]. However, the applied cata-
lysts are normally sacriꢁced in the reaction, because their
recovery is diꢂcult which increases the overall cost of the
catalytic process. Therefore, the designing of nickel-based
heterogeneous eꢂcient catalyst to catalyze the epoxida-
tion of cycloalkenes and replace the homogenous catalysts
became a target for several research groups.
2 Experimental
2.1 Synthesis
In the last few years, several techniques were applied to
prepare nanostructure materials which can be perfectly used
as catalysts [13–16]. Zeolites, e.g. ETS-10 and MOR, were
applied to accommodate active sites such as Ni [17] and
Co [18] to catalyze the epoxidation reactions; respectively.
Moreover, mesoporous silica exhibited interesting features
to support the diꢀerent active sites needed to catalyze the
epoxidation reactions. Nasseri et al. [19] reported the use
of silica to accommodate Fe O nanoparticles, and the
Four samples of Ni-TUD-1 with diꢀerent Si/Ni ratio were
synthesized via sol–gel surfactant-free technique, method
reported previously by Hamdy et al. [32]. Diꢀerent loadings
of Ni were incorporated into TUD-1 mesoporous silica using
diꢀerent Si/Ni ratios, 100, 20, 10, and 4. Basically, a dilute
solution of triethanolamine (TEA, 99%, Sigma-Aldrich) was
added as stepwise to a mixture of nickel nitrate hexahydrate
[Ni(NO ) ·6H O, 99%, Sigma-Aldrich] and silica precursor
3
4
3 2
2
prepared catalyst was applied to catalyze several alkenes
and cycloalkenes under mild conditions. In another study,
(TEOS, 98%, Aldrich) under vigorous stirring for few min-
utes followed by adding of tetraethylammonium hydroxide
(TEAOH, 25%, Sigma-Aldrich) to overall solution and still
under stirring for 2 h to 4 h until a gel formed at room tem-
perature. Then, the aging step expended to 24 h on vigorous
stirring followed by drying of aged mixture under 98 °C for
further 24 h. The mother gel composition was 1 TEOS: x
Ni(NO ) ·6H O:0.5 TEAOH:1 TEA:15H O (x=0.01–0.10).
Ghiami et al. [20] used the magnetically recoverable FeNi
3
nanoparticles incorporated silica in epoxidation of several
alkenes. Moreover, Bhaumik et al. reported the use of silica
to support Co(III) [21] and Cr [22], and the prepared cata-
lysts were used successfully in oxidation reactions. Finally,
Held et al. reported the vanadium modiꢁed mesocellular
silica foams to activate the epoxidation of propene [23]. As
seen, the synthesis of active, selective and stable catalyst for
epoxidation reaction still attracts a lot of interest.
3
2
2
2
After that, the result solid was gently grinded, hydrother-
mally treated in 100 ml Teꢃon-lined stainless-steel autoclave
under 178 °C for 4 h. Finally, calcination step was applied
of obtained solid under 600 °C for 10 h at a heating ramp
(1°/min) in air.
TUD-1 has been reported to be versatile mesoporous
materials with several applications [24]. One of the most
interesting application for TUD-1 is to be used as a support
for diꢀerent active sites in catalytic reactions [25]. Several
publications have reported the successful catalytic applica-
tions of TUD-1 supported transition metals such as Cr [26],
Mo [27], Cu [28], Fe [29], etc. The unique feature of TUD-1
is the possibility to tune the type of the metal active sites
supported in its framework based on metal loading percent-
age [30].
2.2 Characterization
The prepared Ni-TUD-1 samples were characterized by
using atomic absorption spectroscopy (AAS), XRD, Raman
spectroscopy, N sorption, SEM and HR-TEM. The applied
2
instruments and detailed measurements were reported in
reference [33].
In the ꢁrst publication for the current research group in
this line [31], Abboud has reported the use of NiO nano-
particles in the epoxidation of cyclohexene, and although
the high activity of NiO, however, problems of sintering,
agglomeration, and activity loss could not be avoided [31].
Here, diꢀerent loadings of Ni were incorporated into TUD-1
2.3 Catalytic Activity
The catalytic activity of Ni-TUD-1 was performed
as reported previously [31]. Briefly, 15 mg of Ni-
TUD-1 were added to a clean test tube with 3 ml of
1
3

Instant Cyclohexene Epoxidation Over Ni-TUD-1 Under Ambient Conditions
acetonitrile-dichloromethane mixture (1:1 v/v). Then, 60
μL (0.6 mmol) of cyclohexene were added to the above-
mentioned suspension under stirring. After 5 min, 156 mg
Table 1 Comparison between the Si/Ni ratio in the synthesis gel and
in the ꢁnal solid materials
a
Sample code
Si/Ni (synthe- Si/Ni (product)
sis)
Error %
(
0.9 mmol) of m-CPBA was added and this moment con-
sidered the 0 time of the reaction. Samples with a constant
volume (30 μL) were withdrawn from the reaction mixture,
TUD-1
Ni-1
Ni-2
Ni-5
Ni-10
∞
∞
0
100
50
20
10
101.1
49.93
20.12
10.07
1.09
− 0.14
0.60
0.70
ꢁ
ltered through hydrophobic membrane to remove the solid
catalyst and then analyzed by gas chromatography (GC).
Each reaction was run twice, and the average conversion
and selectivity are presented. Leaching measurement for Ni
were performed by using atomic absorption spectroscopy
aDetermined from AAS analysis
(
AAS), in which nickel was extracted with deionized water
and used for AAS.
Conversion of cyclohexene and selectivity towards
cyclohexene oxide (epoxide) were calculated according to
the following equations:
[
0
CXE] − [CXE]t
Ni-10
Ni-5
Conversion (%) =
× 100
[CXE]0
where [CXE] is the initial cyclohexene concentration,
o
[
CXE] is the concentration of cyclohexene after 5 s.
t
Ni-2
[
CXEO]
Selectivity (%) =
× 100
Ni-1
70
[
CXE] − [CXE]t
0
where [CXEO] is the cyclohexene oxide concentration.
On the other hand, the turnover frequency (TOF) was
calculated according to the equation
0
10
20
30
40
50
60
2 theta (degree)
Fig. 1 The XRD patterns of the four prepared Ni-TUD-1 samples, the
crystal planes of (111), (200) and (220) are pointed to the formation
of NiO cubic crystals as indexed in JCPDS card 78-0643
Moles of converted CXE
−
1
TOF (s ) =
Moles of NiO × 5 seconds
where TOF represents the number of converted moles of
CXE divided by the number of moles of Ni active phase per
reaction time which is 5 s.
centered at 22.5° 2θ which is a characteristic of amorphous
silica material [34]. In the patterns of samples Ni-1 and
Ni-2, no more peaks were observed which indicates that
the Ni content was totally dispersed in the silica matrix of
TUD-1. However, in Ni-5 a very small peak at 43.3° 2θ, cor-
responds to the crystal plane (200), could be observed, while
in Ni-10 sample three small peaks at 37.3°, 43.3°, and 62.9°
3
Results
3.1 Characterization
2θ. These peaks can be attributed to the crystal planes (111),
The amount of Ni in the prepared materials was determined
from AAS analysis. The Si/Ni ratio in the synthesis gel is
compared with that after synthesis in Table 1. The obtained
results showed that the Ni content in the ꢁnal solid product
is almost identical to the amount of Ni which added dur-
ing the synthesis. The error percentage is very small and it
ranged from − 0.14 to 1.09%. This means that by applying
the sol–gel technique, no loss in Ni content was found in
the prepared samples and independently on the Ni loading.
In another word, the content of Ni incorporated in TUD-1
siliceous material is highly predictable.
(200) and (220) of NiO cubic crystal structure as indexed
in JCPDS card 78-0643 (cubic NiO) [35]. Therefore, it can
be concluded that NiO nanoparticles are formed when Si/
Ni ratio is less than 20.
The mesoporosity of the prepared materials was con-
ꢁrmed using N soprtion technique. Figure 2 (left panel)
2
presents the N isotherms of the four investigated samples.
2
All isotherms exhibited type IV isotherms which is a charac-
teristic of mesoporous materials [36]. The pore size distribu-
tion of the four samples is presented in Fig. 2-right panel,
which showed a narrow pore size distribution for the four
samples with a value between 3.8 and 4.5 nm, i.e. the pore
size of the four sample are lied in the “meso” pores size.
XRD patterns of the synthesized Ni-TUD-1 samples are
presented in Fig. 1. The four samples exhibited broad peak
1
3

M. S. Hamdy et al.
Fig. 2 The N sorption isotherms of the prepared Ni-TUD-1 (left panel), and the corresponding pore size distribution (right panel)
2
Table 2 The texture properties of the synthesized Ni-TUD-1 samples
Ni-10
Ni-5
Ni-2
Sample
Texture properties
Surface area
Pore volume
Pore size (nm)
2
3
(
m /g)
(cm /g)
TUD-1
Ni-1
Ni-2
Ni-5
Ni-10
655.2
512.2
461.2
480.9
470.9
1.71
0.67
0.68
0.71
0.58
4.51
4.12
4.45
4.47
3.85
1
400
1200
1000
800
600
400
200
The texture properties of the synthesized Ni-TUD-1 sam-
ples are listed in Table 2. Generally, the surface area of the
four Ni-TUD-1 samples are smaller than that of neat TUD-1
sample. This can be attributed to the eꢀect of Ni incorpora-
tion in the silica matrix. The surface area of Ni-TUD-1 is
-1
Raman shift (cm )
Fig. 3 Raman spectra of the four prepared Ni-TUD-1 samples, the
spectra were collected under ambient by using green laser for excita-
tion
2
ranged from 460 to 512 m /g, while the pore volume ranged
3
from 0.58 to 0.71 cm /g, and ꢁnally the pore size is ranged
from 3.8 to 4.5 nm.
produced by the condensation of surface hydroxyl groups
[38]. Furthermore, in Ni-10 sample, the collapsing of the
The phase composition of the prepared materials was
investigated by the means of Raman spectroscopy. Figure 3
presents the obtained Raman spectra of Ni-TUD-1 sam-
ples which are collected at room temperature in the range
−
1
peaks at 607 and 819 cm can be attributed to a decrease in
the concentration of reactive Si–OH in the surface which is
transformed to Si–O–Ni [39]. Similar behavior was observed
by Gbadamasi et al. when they incorporated Ni in SBA-15
mesoporous silica [37].
−
1
of 1500–200 cm . The spectra of Ni-TUD-1 exhibited
−
1
some characteristic peaks at 494, 607, 819 and 979 cm .
−
1
The obtained peak at 979 cm is related to the stretch-
ing mode of the surface Si–OH group, moreover, the peak
The morphological structure was investigated by SEM
analysis. The micrographs of Ni-TUD-1 samples are present
in Fig. 4. The SEM micrographs of samples Ni-1, -2, and
-5 showed irregular-shape particles which is characteristic
of amorphous TUD-1 siliceous material [39]. Moreover, no
other phase(s) could be detected as an indication for the
−
1
around 819 cm can be attributed to the stretching mode of
Si–O–Si linkage [37]. On the other hand, the sharp peaks
−
1
at 494 and 607 cm can be assigned to the four-membered
and three-membered siloxane rings, which is normally
1
3

Instant Cyclohexene Epoxidation Over Ni-TUD-1 Under Ambient Conditions
Fig. 4 SEM micrographs of Ni-5 and Ni-10 samples, the micrographs were obtained by using JSM 7600 F, JEOL instrument
homogeneity of these three samples. On the other hand, the
Ni-10 sample maintained the same morphological structure
of other Ni-TUD-1 samples, however, few NiO NPs could
be observed outside TUD-1 framework. The formation of
NiO NPs in TUD-1 pores is in agreement with XRD and
Raman results.
EDX analysis was performed to investigate the purity of
the Ni-TUD-1 samples. The obtained results are presented in
Fig. 5. Only three elements were observed in the EDX graph
which are Si, O and Ni. No more elements were present as
an indication for the high purity of the prepared samples.
HR-TEM analysis was performed to investigate the pres-
ence and the distribution of NiO nanoparticles through the
silica matrix of TUD-1. Ni-1 and Ni-2 samples did not show
any ordered phase, only the sponge-like structure of TUD-1
was visible in the micrographs (not presented here). This is
an indication that Ni ions were totally incorporated in the sil-
ica matrix as isolated active sites, e.g. (≡Si–O–Ni–O–Si≡),
Fig. 5 EDX analysis for Ni-5 sample, the peaks of only Si, O and Ni
are clearly visible
or as poly moieties e.g. (≡Si–O–(Ni–O–Ni) –O–Si≡).
i.e.>50 nm. Figure 6-bottom panels show the well-distrib-
uted NiO NPs incorporated in TUD-1 matrix.
n
However, in Ni-5 sample, HR-TEM micrographs showed
the presence of highly ordered NiO NPs (black spots) with
average size of 2–5 nm (Fig. 6-top panels). More interest-
ingly, in Ni-10 sample, the NiO NPs exhibited bigger size,
TGA and DTA analysis are performed for Ni-TUD-1
samples to investigate the thermal stability of the prepared
material at elevated temperatures. The thermograms of the
1
3

M. S. Hamdy et al.
Fig. 6 Top panels: The HR-
TEM micrographs of Ni-5
sample. Bottom panels: The
HR-TEM micrographs of Ni-10
sample
prepared Ni-TUD-1 samples are depicted in Fig. 7a and b.
The TGA and DTA curves clearly indicated that Ni-TUD-1
samples, with diꢀerent Ni content, exhibited a high thermal
stability. Two main weight loss steps are observed in TGA
thermograms (Fig. 7a). The ꢁrst step occurs at 80–250 °C
is attributed to loss of adsorbed and crystallized water from
TUD-1 pores and NiO surface, with corresponding peaks in
DTA (Fig. 7b). The second step occurs at>250 °C, and this
is due to silica condensation.
DR-UV–Vis spectroscopy was applied to study the coor-
2
+
dination environment of Ni ions in the silica matrix of
TUD-1. The spectra of the prepared Ni-TUD-1 samples were
collected at room temperature using Kubelka–Munk func-
tion and plotted in Fig. 8. From the spectra, an intensive
band at 263 nm could be observed in addition to a strong
shoulder at 314 nm, and ꢁnally two broad absorption bands
at 415 and 720 nm were also recognized. The absorption
band at 263 nm, which exhibited maximum intensity in Ni-1
Fig. 7 TGA (a) and DTA (b) pattern of the four prepared Ni-TUD-1 samples
1
3

Instant Cyclohexene Epoxidation Over Ni-TUD-1 Under Ambient Conditions
6
5
4
3
2
1
0
1
00
Cyclohexene conversion
Epoxide selectivity
TOF (1/s)
80
60
40
20
0
Ni-10
Ni-5
No catalyst TUD-1
NiO(C)
NiO(Fl)
Ni-TUD-1 No oxidant
Ni-2
Fig. 9 The cyclohexene conversion %, the epoxide selectivity, and the
TOF of the epoxidation reaction under diꢀerent reaction conditions
Ni-1
2
00
300
400
500
600
700
800
TUD-1 silica, or (3) without oxidant with Ni-1 catalyst.
This indicates that the TUD-1 solid support accommodates
the diꢀerent Ni active sites does not promote the epoxida-
tion of cyclohexene. In the presence of bulk nickel oxide
Wavelength (nm)
Fig. 8 DR-UV–Vis spectra of the prepared Ni-TUD-1 samples
expressed in terms of Kubelka–Munk function against the measured
wavelength
[
NiO(C)], 65% of cyclohexene was converted with a total
selectivity of 58% towards cyclohexene oxide (the desired
epoxide product). Moreover, in the case of using ꢃower-like
mesoporous nickel oxide [NiO(Fl)] [31] 91% conversion was
obtained with 53% selectivity towards the epoxide product.
The calculated TOF of NiO(C) and the NiO(Fl) was 0.53 and
sample and its intensity is decreased with Ni loading, can be
2
+
related to the presence of isolated Ni species in the silica
framework [40]. Moreover, the 314 nm shoulder, which
exhibited a maximum intensity at high Ni loading (samples
−1
0.88 s , respectively. More importantly, when Ni-10 sample
was used almost 100% cyclohexene conversion was obtained
with 90.5% selectivity towards the desired epoxide product.
2
−
2+
Ni-5 and Ni-10), can be attributed to the O (2p)→Ni (3d)
2
+
charge transfer transition of octahedral Ni species in NiO
lattices [41]. Finally, the two broad bands at 415 and 720 nm
−1
The TOF of Ni-10 sample was 5.97 s , which is almost 11.2
times higher than that of NiO(C) sample.
2+
can be attributed to the Ni ions in octahedral local environ-
ment in NiO [42].
3.2 Catalytic Activity
3.2.2 The Efect oꢀ Ni Content
The catalytic activity of the entire Ni-TUD-1 samples was
examined in the cyclohexene epoxidation reaction at room
temperature by using m-CPBA as an oxidant. The major
products of the liquid phase epoxidation of cyclohex-
ene are cyclohexene oxide (desired), 2-cyclohexen-
The eꢀect of Ni content in TUD-1 mesoporous material on
the catalytic performance of Ni-TUD-1 nanocatalyst was
investigated. The four prepared samples with Ni loading
from 1 to 10 wt% were used to catalyze the epoxidation
reaction under the same reaction conditions and the obtained
results are plotted in Fig. 10. The conversion % of cyclohex-
ene was found to be increased with Ni content. Ni-1 and
Ni-2 catalyzed the conversion of 90% and 95% of cyclohex-
ene, respectively, while Ni-5 and Ni-10 samples catalyzed
the total conversion; i.e. 100%, of cyclohexene after only 5 s.
The selectivity was found to be slightly decreased with
increasing Ni content, this can be related to the over oxida-
tion reaction of epoxide which open the oxirene ring of the
cyclohexene oxide. On the other hand, the turn over fre-
quency (TOF) was found also to be decreased with increas-
ing the Ni content, in other words, the catalytic activity per
Ni atom is decreased. This can be related to the increasing
the size of the Ni active site (from isolated Ni ion to NiO
nanoparticles) and decrease the overall surface area available
1
1
-ol, 2-cyclohexen-1-one, 1,2-cyclohexanediol, and
-(t-butylperoxy)-2-cyclohexene (undesired).
3.2.1 Blank Experiments
A series of blank reactions were performed to conꢁrm the
importance of Ni-TUD-1 to catalyze the epoxidation reac-
tion. It is important to mention that the data reported in this
section was obtained after only 5 s from starting the reac-
tion. Figure 9 presents the obtained results of cyclohexene
conversion %, epoxide selectivity % and the turnover fre-
−
1
quency TOF (s ) over diꢀerent reaction conditions. The
obtained data clearly show that no conversion was obtained
in the following cases: (1) without catalyst, (2) with blank
1
3

M. S. Hamdy et al.
1
00
5
4
3
2
0
0
0
0
8
6
4
2
0
0
0
0
0
10
0
Cyclohexene conversion
Epoxide selectivity
Fig. 11 The type of diꢀerent active sites present in Ni-TUD-1, the
2
+
Ni-1
Ni-2
Ni-5
Ni-10
isolated Ni ions and the nanoparticles of NiO
Ni-TUD-1 samples
NiO NPs are much larger than that as seen from the HRTEM
images, which means that the formed nanoparticles are
not located inside the pores of TUD-1. To understand this
contradiction, the synthesis mechanism of the metal oxide
incorporated TUD-1 must be carefully investigated. First,
the synthesis is carried out in one-step sol–gel process, in
which TEA (the template) s forming complexes with silica
and with Ni in the same time. Second, it is believed that the
silica matrix is formed and surrounded the Ni-TEA com-
plexes which will be oxidized during calcination step to
nickel oxide nanoparticles. To prove this hypothesis, TUD-1
was used to encapsulate two types of zeolite nanoparticles in
[43, 44], and the nanoparticles of zeolite was much bigger
than the pores of TUD-1. The obtained materials consisted
of well distributed nanoparticles surrounded by the silica
matrix of TUD-1. Moreover, although the pores were par-
tially blocked due to the presence of nanocrystals, but it
was also enough accessibility to reach the active sites and
catalyze the reaction.
Fig. 10 The cyclohexene conversion %, the epoxide selectivity, and
the TOF of the epoxidation catalyzed by Ni-TUD-1 samples
for catalysis. Similar behavior was reported by Hamdy et al.
related to Mo-TUD-1 catalysts [27].
3.2.3 The Stability oꢀ Ni in TUD‑1 Matrix
The organic phase after the reactions was separated by cen-
trifugation and analyzed by inductive coupled plasma (ICP)
elemental analysis to measure the amount of Ni ions leached
from the catalyst. The obtained results show the presence
of Ni ions in the organic phase after the reaction. The per-
centage of Ni present in the organic phase was found to be
highly dependent on the Ni content. The amount of Ni ions
identiꢁed in the organic phase after ꢁrst run, catalyzed by
Ni-1 and Ni-10, was found to be 8.9% and 12.4% of the
original Ni loading, respectively. These results proved that
2+
Ni ions are not strongly bonded to silica matrix and can be
partially removed from the framework during the epoxida-
tion reaction.
4.2 Proposed Mechanism
According to previous mechanistic studies and theoretical
predictions for the epoxidation of oleꢁns catalyzed by sup-
ported transition metals [45–47], and based on the above
4
Discussion
II
results, it could be deduced that the Ni species are the active
4.1 The Prepared Ni‑TUD‑1 Samples
sites in the epoxidation of cyclohexene using Ni-TUD-1 as
a catalyst and m-CPBA as an oxidant. The mechanisms pro-
posed for the epoxidation of oleꢁns (including cyclohexene)
using supported or unsupported transition metals as cata-
lysts, and peroxides as oxidant, suggested the formation of
Metal-Oxo (M-Oxo) species from metal-bonded activated
peroxide Metal-OOR [48–50]. Then, the oxygen atom in
M-Oxo is transferred to the C=C double bond. Therefore, a
possible mechanism for the epoxidation of cyclohexene over
Ni-TUD-1 catalyst using m-CPBA as oxidant is proposed,
and this reaction could involve two diꢀerent mechanistic
pathways.
Combining all the obtained characterization data, it can be
concluded that Ni-TUD-1 materials are mesoporous mate-
2
+
rial with amorphous nature mostly. At low Ni loading, Ni
ions are highly dispersed in the silica framework as isolated
active sites. While at high Ni loading, NiO nanoparticles are
formed inside the pores of silica matrix. It was found that
the size of NiO nanoparticles is directly proportional with
Ni content. Figure 11 shows the type of active sites present
in Ni-TUD-1.
It is interesting to mention that the pore size as observed
from these N sorption isotherms of these TUD-1 samples
As shown in Fig. 12 (Path 1), m-CPBA ꢁrst adsorbed on
the surface of Ni-TUD-1 and be bonded to Ni(II) species to
2
varies from 4 to 5 nm, however the size of Ni(II) clusters or
1
3

Instant Cyclohexene Epoxidation Over Ni-TUD-1 Under Ambient Conditions
O
Cl
OH
m-CBA
O
Ni nanoparticles
NiII
O
Cl
O
H+
Oxo
+
m-CPBA
m-CPBA
NiII
NiIV
Ni-TUD-1
A
B
Path 2
O
O
O
O
Path 1
Cl
NiIV
O
O
O
O
C
O
NiII
NiIV
E
D
Fig. 12 Proposed mechanism for the epoxidation of cyclohexene over Ni-TUD-1 using m-CPBA as oxygen donor
II
form Nickel-bonded activated m-CPBA [Ni –O–O–R, (A)].
leads to the formation of the peroxo metallocycle (E). After
the breakage of this intermediate (E), the corresponding
IV
O–O bond was cleaved and m-CBA and Ni -Oxo (B) were
IV
II
formed. Ni -Oxo reacted then with a second molecule of
epoxide is formed, and the starting catalyst Ni -TUD-1 is
IV
m-CPBA to form Ni -peroxo compound (C). The formed
recovered. For higher Ni content, the cyclohexene epoxide
selectivity was steadily decreased (Fig. 10). This result can
be explained by the opening of the epoxide ring with water,
catalyzed by the acid character of the metallo-silicates.
intermediate (C) would react with C=C of cyclohexene to
aꢀord the peroxo metallocycle (D). In the last step, cyclohex-
ene oxide is formed by the cleavage of the intermediate (D)
IV
and Ni -Oxo (B) is recovered. Meanwhile, a second path-
way can take place also in parallel with path 1, which is the
4.3 Comparison with Other Catalysts
II
direct epoxidation of cyclohexene by N –O–O–R (Path 2).
II
It is reported that Ni –O–O–R can undergo itself the epoxi-
To have a fair comparison with other heterogeneous cata-
lysts, diꢀerent operating parameters, such as reaction tem-
perature, applied oxidant, and solvent have to be the same.
The diꢂculty of obtaining a consistent comparison is that
only a very limited number of articles reported the metal-
catalyzed heterogeneous epoxidation of cyclohexene using
m-CPBA as oxygen donor at room temperature. The com-
parison between Ni-TUD-1 and the other reported heteroge-
neous catalysts are presented in Table 3.
dation reaction of cyclohexene [45]. According to previous
mechanistic studies performed for the epoxidation of oleꢁns
II
catalyzed by supported metal oxide, [51, 52]. Ni active sites
in Ni-TUD-1 catalyst are electron-poor metal centers, would
II
increase the oxygen electrophilicity in N –O–O–R peroxide,
making this complex an eꢂcient oxygen-transfer agent. The
nucleophilic attack of C=C double bond of the cyclohex-
II
ene on an oxygen atom of the peroxy complex N –O–O–R
Table 3 Comparison between
Entry
Catalyst
Time (min)
Cyclohexene con-
version (%)
Epoxide selectiv- Ref
ity (%)
the catalytic activity of diꢀerent
heterogeneous catalysts in the
oxidation of cyclohexene by
using m-CPBA as an oxidant
at 25 °C
1
2
3
4
5
6
Ni-TUD-1 (Ni-10)
NiO bulky
<0.1
<0.1
<0.1
240
180
180
100
91
65
95
90
94
90.5
93
Current
[31]
[31]
[19]
[20]
[53]
NiO ꢃower like
Fe O /SiO
58
99
3
4
2
FeNi @SiO
99
99
3
2
III
Co @Fe O /SiO
3
4
2
1
3

M. S. Hamdy et al.
The comparison clearly shows that Ni-TUD-1 is a very
promising catalyst related to the applied reaction time, the
reaction was performed totally within 5 s.
14. Salavati-Niasari M, Ghanbari D, Loghman-Estarki MR
(
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Four samples of mesoporous silica incorporated Ni were
prepared successfully by using hydrothermal sol–gel
method. The characterization data showed that at low Ni
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Q (2019) Mol Catal 463:8–15. https://doi.org/10.1016/j.
mcat.2018.11.006
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+
loading samples, only isolated Ni were present in the
framework of TUD-1, while at high Ni loading NiO NPs
were formed inside the mesopores of TUD-1 samples. The
produced materials exhibited an extraordinary catalytic
activity in the epoxidation of cyclohexene at room tempera-
ture. Instant cyclohexene conversion was obtained when
m-CPBA was applied as an oxidant with a selectivity more
than 90% towards cyclohexene oxide. The superior activity
benchmarked other reported catalysts under similar reaction
conditions.
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Acknowledgements The authors are sincerely acknowledged the
Deanship of Scientiꢁc Research at King Khalid University for fund-
ing this work through a research group Project Number R.G.P1/89/40.
Moreover, the authors extend also their appreciation to the Deanship
of Scientiꢁc Research at King Saud University for funding this work
through research group no (RSP-2020/160).
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