Facile in situ Encapsulation of Highly Dispersed Ni@MCM-41 for the Trans-Decalin Production from Hydrogenation of Naphthalene at Low Temperature

Article abstract of DOI:10.1002/cctc.201801788

Ni@MCM-41 catalyst that has uniformly distributed, highly dispersed Ni nanoparticles (about 2.3 nm) was designed and successfully synthesized by in situ encapsulation of Ni in the channels of MCM-41. This catalyst exhibits excellent thermal stability and hydrogenation activity. Water insoluble nickel acetylacetonate (Ni(acac)₂) was first dissolved aqueous solution of cetyltrimethyl ammonium bromide (CTAB) and encapsulated in micelles of CTAB. Sodium silicate was used as a silicon source to rapidly hydrolyze and then wrap on the micelle surface to synthesize the MCM-41 zeolite. The MCM-41 zeolite encapsulating Ni(acac)₂ was synthesized within a short time (4 h) at 120 °C. Compared with conventional supported catalysts, thus 3 wt.% Ni@MCM-41 has ultra-small uniformly distributed Ni nanoparticles and exhibits improved activity for the hydrogenation of naphthalene to decalin at very low reaction temperatures. The TOF and the apparent activation energy of Ni@MCM-41 and the conventional catalysts (Ni/MCM-41 and Pt/MCM-41) were evaluated and compared. And the catalysis mechanism was analyzed. Furthermore, this Ni@MCM-41 catalyst offers an additional advantage of selectivity in decalin isomerization; 92 % trans-decalin selectivity was achieved at a wide temperature range.

Full text of DOI:10.1002/cctc.201801788

Accepted Article
Title: Facile in situ encapsulation of highly dispersed Ni@MCM-41 for
the trans-decalin production from hydrogenation of naphthalene
at low temperature
Authors: Xiaoyun Song, Qingxin Guan, Yu Shu, Xiaojing Zhang, and
Wei Li
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Link to VoR: http://dx.doi.org/10.1002/cctc.201801788
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ChemCatChem
10.1002/cctc.201801788
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Facile in situ encapsulation of highly dispersed Ni@MCM-41 for
the trans-decalin production from hydrogenation of naphthalene
at low temperature
[a]
[a]
[a]
[a]
[a] [b]
Xiaoyun Song, Qingxin Guan,* Yu Shu, Xiaojing Zhang, and Wei Li,**
Abstract: Ni@MCM-41 catalyst that has uniformly distributed, highly
dispersed Ni nanoparticles (about 2.3 nm) was designed and
successfully synthesized by in situ encapsulation of Ni in the channels
of MCM-41. This catalyst exhibits excellent thermal stability and
hydrogenation activity. Water insoluble nickel acetylacetonate
coal-based raw materials, much effort has gone into the
development of highly efficient supported catalysts for HDA.
According to many published papers, supported noble metal
catalysts (e.g., Ru,[1] Pd,[2] and Pt ) exhibit excellent
hydrogenation performance; however, unfortunately, large-scale
industrial applications are severely limited because of the high
cost and low reserves of noble metals. Furthermore, noble metal
catalysts are easily poisoned by sulfur compounds present in
fossil feedstock. In the research for less costly supported catalysts,
Ni catalysts have attracted much attention because of their low-
cost, rich-reserves, and sulfur-toleration.[4] Ni catalysts have been
widely reported; they have significant potential applications in
heterogeneous catalysis.[ Many unsupported Ni-based particles
with high dispersion and uniform size distribution have been
synthesized, and they often exhibit catalytic performances.[
However, these Ni materials tend to be unstable toward sintering
or aggregation in catalytic reactions; hence, they are unsuitable
for heterogeneous catalysis, especially for catalytic reactions in a
fixed-bed reactor. The idea of the preparation of supported Ni
catalysts is one of the themes of heterogeneous catalytic HDA
research. Of note here is that the hydrogenation of aromatic
compounds is an exothermic reaction and low reaction
temperature is more beneficial to the conversion of aromatics.
Unfortunately, a supported Ni catalyst has low activity at low
temperature and it is unstable during the hydrogenation
process.[7] Therefore, how to improve the low temperature activity
and stability of a supported Ni catalyst remains a significant
challenge.
[3]
2
(Ni(acac) ) was first dissolved aqueous solution of cetyltrimethyl
ammonium bromide (CTAB) and encapsulated in micelles of CTAB.
Sodium silicate was used as a silicon source to rapidly hydrolyze and
then wrap on the micelle surface to synthesize the MCM-41 zeolite.
The MCM-41 zeolite encapsulating Ni(acac)
a short time (4 h) at 120 °C. Compared with conventional supported
catalysts, thus wt.% Ni@MCM-41 has ultra-small uniformly
2
was synthesized within
3
5]
distributed Ni nanoparticles and exhibits improved activity for the
hydrogenation of naphthalene to decalin at very low reaction
temperatures. The TOF and the apparent activation energy of
Ni@MCM-41 and the conventional catalysts (Ni/MCM-41 and
Pt/MCM-41) were evaluated and compared. And the catalysis
mechanism was analyzed. Furthermore, this Ni@MCM-41 catalyst
offers an additional advantage of selectivity in decalin isomerization;
6]
92% trans-decalin selectivity was achieved at a wide temperature
range.
Introduction
Aromatic compounds present in diesel lead to diesel with a lower
cetane number and increased particulate pollutants in exhaust
gases; hence, the removal of aromatic hydrocarbons from fuel is
considered an urgent task. Naphthalene is generally used as a
model compound for the hydrodearomatization (HDA) reaction.
Naphthalene is also a major component of light cycle oil and coal
tar, and it can be hydrogenated to produce tetralin and decalin.
Hence, for the reasons of producing environmental friendly fuels
and meeting demands for the conversion of fine chemicals from
For a supported catalyst, the particle size of the active
component/s is a crucial parameter, as it influences the activity,
selectivity, and lifetime of the catalyst.[8] Smaller metal
nanoparticles are often more effective because they are able to
expose more active sites. Although many methods have been
described for the synthesis of supported catalysts with small metal
particles, results indicate that their use still leads to uncontrollable
size distribution or relatively poor stability in catalytic processes.[
To overcome existing limitations, encapsulation is considered an
9]
[
[
a]
b]
Dr. X. Song, Prof. Q. Guan, Y. Shu, X. Zhang, Prof. W. Li
College of Chemistry, State Key Laboratory of Elemento-Organic
Chemistry, Key Laboratory of Advanced Energy Materials Chemistry
attractive strategy to design highly dispersed metal nanoparticle
catalysts with excellent stability.[10] Jian Zhang et al.[11]
constructed
a defined structure (Pd@Beta) by fixing Pd
(
Ministry of Education), Nankai University, Tianjin 300071, China
nanoparticles inside Beta zeolite, which exhibited enhanced
hydrogenation selectivity and stability. This work offered a new
concept for the design of durable catalysts with high selectivity.
Mesoporous silica type MCM-41, possesses a large surface area,
narrow pore size distribution, and good thermal stability. MCM-41
is therefore considered as an ideal catalyst support candidate.
The synthetic approach to MCM-41 involves multidentate binding
E-mail: qingxinguan@nankai.edu.cn; weili@nankai.edu.cn
Prof. W. Li
Collaborative Innovation Center of Chemical Science and
Engineering (Tianjin), Nankai University, Tianjin 300071, China
Supporting information for this article is given via a link at the end of
the document.
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ChemCatChem
10.1002/cctc.201801788
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of silicate oligomers to the cationic surfactant and charge density
matching of surfactant–silicate.[12] Therefore, MCM-41 can be
considered a good candidate for supporting metal nanoparticles
inside the pores. Indeed, it has often been used as a support for
generating noble metal nanoparticles. Xijie Lin and coworkers[13]
introduced Pd nanoparticles into the channels of MCM-41, which
led to the formation of highly dispersed and stable Pd species.
This synthetic method may be useful for non-noble metals with
nanosizes <5 nm, which are otherwise difficult to reach and
control.
of the hexagonal structure for Ni@MCM-x are well discerned in
small-angle XRD patterns. Thus, the incorporation of Ni by the in
situ encapsulation method dose not destroy the symmetry of the
porous framework. The intensities of the major peaks decrease
slightly, however, with increasing Ni content, which is probably
because a small fraction of the mesopores are filled, and block Ni
incorporation. Besides this, there is a slight change in the
diffraction peak positions for Ni@MCM-x compared with blank
MCM-41, which is probably related to a slightly larger pore size
for the Ni@MCM-x.
Here, we demonstrate a simple and efficient method for the in
situ encapsulation of Ni nanoparticles in MCM-41 channels. The
detailed synthesis route is shown in Scheme 1. In this strategy,
the cetyltrimethyl ammonium bromide (CTAB) surfactant is not
only used as a micelle template for the formation of MCM-41, but
also utilized for grafting Ni species effectively. Sodium silicate is
Furthermore, wide-angle XRD patterns of Ni@MCM-x were
also detected and compared with Ni/MCM-41 (see Figure 1, inset).
Typical peaks of Ni appear in the XRD patterns of Ni/MCM-41.
However, the patterns of Ni@MCM-x samples only reveal a broad
2
SiO diffraction peak; no clear peak belonging to Ni is observed.
This result indicates that the Ni particles of Ni@MCM-x may be
too small and highly dispersed in the channels to be detected.
And this supposition is confirmed by the dispersion results
calculated from CO chemisorption. As Table 1 shows, the
dispersion of Ni@MCM-x is much higher than that of Ni/MCM-41,
which suggests that our encapsulation method can effectively
improve the dispersion of Ni particles.
used as
a silicon source to rapidly hydrolyze and then
encapsulate the micelle to synthesize the MCM-41. Ultimately,
the newly prepared Ni@MCM-41 catalyst has ultra-small
uniformly distributed Ni nanoparticles, and completely different
activity and selectivity compared with the corresponding
conventional Ni/MCM-41.
Scheme 1. Illustration of the in situ Ni@MCM-41 synthesis and HDA reaction.
Figure 1. Small-angle XRD patterns of MCM-41, Ni/MCM-41, and Ni@MCM-x
(
x=1.5, 3.0). Inset: wide-angle XRD patterns.
Results and Discussion
2
As Figure 2 shows, the N absorption–desorption isotherms of
samples are typical Ⅳtype with monolayer adsorption, which is a
typical characteristic of mesoporous materials. There is a sharp
ICP results (Table S1) reveal that the actual Ni loadings in the
Ni@MCM-3.0 and Ni@MCM-1.5 samples are 3.0% and 1.5%,
respectively. In terms of appearance, the Ni@MCM-x is a different
increase in N
2
adsorption within a narrow range of relative
colour to Ni/MCM-41, synthesized using
a
traditional
pressure (0.3 ‒ 0.4), arising from capillary condensation of N
2
impregnation method. The color of the matrix material (MCM-41)
is white, whereas the color of Ni/MCM-41 is dark black. The latter
may be attributed to the Ni species located on the surface of the
inside the mesopores. It is well known that a steep hysteresis
loops between adsorption and desorption always indicates a
narrow mesopore size distribution. For all samples, the adsorption
path coincides with the corresponding desorption path and no
hysteresis loops are observed. This is in accordance with what
reported for MCM-41 in the literature.[ This result suggests that
all samples here possess ordered mesopores. This is consistent
with the pore diameter distribution shown in Figure 2 (inset).
Textural properties of samples are tabulated in Table 1. There is
sample.[12] However, Ni@MCM-x shows a light gray color. This is
attributed to not much Ni present on the outer surface. This result
is taken as an initial indication of the encapsulation of Ni in the
channels of MCM-41. A similar appearance of Pd nanoparticles
supported on B-MCM-41 has been reported by Chatterjee.[12]
Successful synthesis of the mesoporous MCM-41 structure was
confirmed by XRD patterns (Figure 1). Three characteristic peaks
14]
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ChemCatChem
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only a slight decrease in the surface area (SBET) of Ni@MCM-1.5
the external surface of MCM-41. By contrast, in Ni@MCM-x
(Figure 3c, e), the Ni nanoparticles are small (average size 2.3
nm); almost no large particles were found. Most Ni nanoparticles
are confined within the channels of MCM-41, which indicates that
the channels of MCM-41 play a role in the encapsulation of Ni
nanoparticles. More importantly, Ni nanoparticles remain highly
dispersed in the channels of MCM-41, although the Ni content
increases from 1.5 wt.% to 3.0 wt.%. Results also indicate that the
Ni nanoparticles of Ni@MCM-3.0 and Ni@MCM-1.5 are the same
size. This size is controllable and does not unaffected by the Ni
loadings. The Ni dispersion was further confirmed by STEM
pictures, including BF and HAADF images (Figure 4).
Furthermore, the existence of Ni in Ni@MCM-x was identified by
EDS (Figure 3d). Hence, we can confidently state that, as
expected, our synthesis route is effective in encapsulating Ni into
the channels of MCM-41.
2
–1
2
–1
(
902 m g ) compared with blank MCM-41 (969 m g ), owing to
Ni incorporation. Although the SBET of Ni@MCM-3.0 shows an
decrease of 14% with respect to MCM-41, its SBET is still much
higher than that of Ni/MCM-41 (24%). Interestingly, the average
pore size decreases slightly from 2.9 to 3.1 nm, which may be
attributed to the encapsulation of Ni. This result is in good
agreement with that the XRD peaks shift to a smaller angle and
the pore diameter distribution (Figure 2 inset) shift to a larger size.
Therefore, the introduction of Ni particles affects the MCM-41
structure to some extent. Notwithstanding such a small difference,
2
the overall results of N sorption suggest that Ni@MCM-x possess
a mesoporous structure with ordered channels.
Figure 2. Nitrogen adsorption-desorption isotherms and pore diameter
distribution (inset) of as-prepared samples.
Table 1. Textural properties of samples.
[
a]
d[b] (nm)
[c]
total
Sample
S
(
BET
2
V
Metal
dispersion
3
[d]
m /g)
(cm /g)
MCM-41
969
738
902
823
2.4
2.4
2.4
2.6
0.93
--
Ni/MCM-41
Ni@MCM-1.5
Ni@MCM-3.0
0.68
36%
51%
50%
0.84
0.75
[
a] surface area was calculated with BET method. [b] average pore size
determined by means of BJH method. [c] total pore volume obtained at P/P
0.999. [d] was estimated according to the number of CO molecules
adsorbed and the CO uptake was listed in Table 2.
0
=
Figure 3. TEM images of (a) MCM-41, (b) Ni/MCM-41, STEM images of (c)
Ni@MCM-3.0 under a bright field operation mode and its EDS analysis (d), and
STEM images of (e, f) Ni@MCM-1.5 under dark field mode. Inset of (f): HRTEM
of Ni nanoparticle gives the crystal plane of the (111) for Ni.
TEM images are shown in Figure 3. They not only confirm the
above results, but also give direct insight into the presence and
size distribution of Ni nanoparticles, including the structural
properties of samples. The ordered mesoporous structure of
The mechanism can be rationalized by the role of CTAB
surfactant in the grafting of Ni species inside the mesopores. The
synthesis process and mechanism are shown in Scheme 1. In our
‘
blank’ MCM-41 (Figure 3a) can be clearly distinguished. In an
image of Ni/MCM-41 (Figure 3b), many bulky Ni particles exist on
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method, CTAB is used as micelle template for the formation of
MCM-41, and Ni(acac)
Interestingly, a clear solution is obtained when water insoluble
Ni(acac) is dissolved in aqueous solution of CTAB as a result of
the interaction between Ni(acac) and CTAB. As is well known, a
2
is employed as the source of Ni species.
2
2
micelle of CTAB can form in aqueous solution as a result of the
ordered uniform aggregation of its organic group. Acetylacetone
(
acac) ligand of Ni(acac)
2
can interact with the organic group of
CTAB,[ and hence, the Ni(acac)
can enter the hydrophobic
2
15]
core of surfactant micelles. In other words, Ni species are
encompassed by CTAB and grafted into micelles.
2
Figure 5. The UV-vis spectra of different solutions: solution of Ni(acac) and
CTAB before (a) and after (b) adding NaOH, and CTAB aqueous (c).
The XRD (Figure S1) and XPS (Figure S2) results of Ni/MCM-
4
1 and Ni@MCM-3.0 before reduction show that Ni species are
in the form of Ni oxide in both samples before reduction. The
XPS spectra of Ni/MCM-41 and Ni@MCM-3.0 samples after
reduction were recorded under identical conditions; the results
are shown in Figure 6. The predominant peaks, centered at
8
52.7 eV, 870.8 eV, and 860.2 eV, can be assigned to metallic
0
0
Ni (Ni )2p3/2, Ni 2p1/2, and a satellite peak. A further peak at
2
+
8
54.6 eV is assigned to Ni , which may be attributed to the
surface NiO.[
16]
XPS is commonly used to analyze the
distribution of elements within a surface of (to the depth of)
several nanometers. When considering a reason why the Ni
intensity of the XPS spectra of Ni@MCM-3.0 is too weak to
distinguish, it is possible that most of the Ni nanoparticles are
confined and highly dispersed in the channels of MCM-41, which
means that the Ni nanoparticles in this sample is lower than the
detection limit of XPS. The metal Ni in Ni@MCM-41 after
reduction has been confirmed by HRTEM results (Figure 3f). In
a word, the XPS results combined with characterization of wide-
angle XRD, and HRTEM, confirm the metal Ni in Ni/MCM-41
and Ni@MCM-3.0 samples after reduction. Therefore, being
Figure 4. TEM (a, b) and STEM images under bright (c) and dark (d) mode for
Ni@MCM-3.0.
To gain a better understanding of the existent state of Ni
species in the formation process of MCM-41,
a parallel
experiment was carried out. In this experiment, to avoid
interference from a silica precipitate and simulate the alkaline
conditions, NaOH was used as a replacement for sodium silicate.
2
reduced in H at 600 °C for 2 hours can ensure a complete
reduction of nickel oxides.
The pH value of the Ni(acac)
to 13 by adding NaOH. The UV-vis spectra of the solution before
pH = 8) and after (pH = 13) adding NaOH, as well as an aqueous
2
/CTAB solution was adjusted from
8
(
solution of CTAB, are compared in Figure 5. There is no apparent
change, irrespective of whether the pH value was 8 or 13, which
illustrates that alkaline conditions have little effect on Ni(acac) . In
2
other words, the Ni species remains stable within CTAB micelles
of a MCM-41 precursor. In the next step, when the precursor was
2
calcined at 600 °C in air and reduced in H , the organic
components were decomposed. Meanwhile, Ni atoms were
released and aggregated together. Therefore, Ni particles were
generated and confined within the channels of MCM-41, as
observed in the TEM images.
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the interaction strength between Ni and a MCM-41 support can
be determined according to the reduction temperature. All H -TPR
2
profiles of samples (Figure 8) show two reduction stages.
Obviously, Ni@MCM-3.0 and Ni@MCM-1.5 samples show similar
peak profiles, suggesting the same interaction between Ni and
MCM-41 in both samples. Compared with Ni/MCM-41, both
stages of Ni@MCM-x are shifted to a higher temperature, which
indicates a stronger interaction between Ni and MCM-41 in both
samples. This strong interaction leads to a fixation of Ni into MCM-
41 channels in the Ni@MCM-41 composite. The extraordinary
thermal stability of Ni@MCM-3 (Figure 7) can be reasonably
attributed to the fixed structure of Ni and the silica support.[
According to a previous study, a low reduction temperature peak
is associated with bulky and aggregative NiO particles on the
surface of MCM-41.[18] And a peak at higher temperature is
attributed to smaller and more dispersed NiO particles, which
interact strongly with the mesoporous MCM-41 support. Hence,
from this result, combined with the above analysis of thermal
stability (Figure 7), we can summarize by stating that the function
of confining Ni within the channels of MCM-41 results in not only
high dispersion of Ni particles, but also the prevention of these
particles from aggregation and sintering.
17]
Figure 6. XPS spectra of Ni/MCM-41 and Ni@MCM-3.0 samples.
The advantage of our encapsulation method is that not only are
highly dispersed Ni nanoparticles generated inside the channels
of MCM-41, but also the sintering of Ni nanoparticles is prevented.
Therefore, the thermal stability of this catalyst is obviously
improved. Generally, high temperature treatment leads to an
aggregation or growth of Ni nanoparticles. After treatment at
800 °C for 1 h, the conventional Ni/MCM-41 is found to have Ni
nanoparticles with larger particle sizes (Figure 7a) than in the as-
prepared sample (Figure 3b). This may be caused by the weak
interaction between Ni2+ and MCM-41, which causes the Ni
nanoparticles to be sintered and aggregated on the external
surface of MCM-41 during the calcination and reduction process.
On the other hand, Ni nanoparticles in Ni@MCM-3.0 (Figure 7b)
remain inside channels, with no sintering after thermal treatment,
and the particle size is similar to that of the fresh sample (Figure
3c). Therefore, as expect, Ni@MCM-3.0 shows extraordinary
thermal stability. This result is attributed not only to the fact that
most of the Ni particles are confined within the channels, but also
to the strong interaction between Ni and MCM-41 support.
2
Figure 8. H -TPR of Ni/MCM-41, and Ni@MCM-x samples.
It is well known that the dissociative hydrogen adsorption
capacity of a catalyst is related to its electronic structure.[19] This,
combined with the H -TPR results, we can deduce that the
2
electronic properties of Ni can be modified through this in situ
method.
Test (Table S2) shows that nickel oxide possesses almost no
activity on hydrodearomatization. Based on the results of H
2
-TPR,
all catalysts described here in this manuscript were reduced by H
2
Figure 7. TEM images of (a) Ni/MCM-41 and (b) Ni@MCM-41-3.0 after thermal
treatment.
after 2 h at 600 °C; the complete reduction of NiO before the HDA
reaction is therefore ensured. It is acknowledged that the size of
the metal component is vital to the catalytic efficiency, and that
the thermal stability is important for heterogeneous catalysis;
therefore, an enhanced hydrogenation performance can be
expected here.
2
H -TPR is an effective technique for obtaining information on
the interaction strength between metal species and supports, in
addition to the reducibility of different metal species. Therefore,
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Naphthalene was used as a model compound for the HDA
reaction. The catalytic performance of Ni@MCM-x was evaluated
by the hydrogenation of naphthalene. The hydrogenation process
is illustrated in Figure 9,[9a] and the structure models optimized by
DFT method are shown in Figure S3. The main product is fully
saturated decalin and the by-product is partially hydrogenated
tetralin. For comparison, conventional catalysts were also tested,
specifically, 3 wt.% Ni/MCM-41 and 0.3 wt.% Pt/MCM-41 (TEM,
Figure S4). HDA results indicate that the conversion and
selectivity of all catalysts tend to increase at temperatures below
a
Furthermore, the apparent activation energy (E ) of catalyst
was evaluated from the Arrhenius plots (Figure 11) at
naphthalene conversion <20% to avoid the mass diffusion
limitation. E
Pt/MCM-41 exhibits the highest activity with the lowest E
Ni@MCM-3 displays a lower E compared with Ni/MCM-41,
a
values are tabulated in Table 3. As expected,
a
.
a
suggesting a higher catalytic performance. This indicates that the
encapsulation of Ni in MCM-41 channels is greatly beneficial for
the development of catalytic activity. These results further
confirms the advantage of encapsulation of Ni. Of note is that
260 °C, but decrease above that. This is because the
a
these E results are in good agreement with the results of catalytic
hydrogenation of aromatics is an exothermic reaction, and a low
temperature is thermodynamically favored. At lower temperatures
activity.
(
140 – 260 °C), Ni@MCM-3.0 displays 100% conversion as well
Table 2. TOF of HDA at 120 °C, LHSV =3 h–1
.
as 100% selectivity for decalin (Figure 10). However, the highest
selectivity for decalin, by Ni/MCM-41, is only 92.3%. Ni/MCM-41
shows incomplete conversion of naphthalene below 220 °C.
Therefore, in terms of HDA activity, it is considerably improved by
encapsulating Ni into the channels of MCM-41. In terms of decalin
selectivity, that exhibited by Ni@MCM-3.0 is much higher than
that of the Pt/MCM-41 catalyst over the temperature range
considered in the present study(Figure 10b). Overall, the
conversion and decalin selectivity are 100% over quite a wide
range of test temperatures (140 – 260 °C) for Ni@MCM-3.0.
However, for Ni/MCM-41 and Pt/MCM-41, there is a sharp decline
when the temperature decreases as a result of a decrease in
catalytic activity at low temperatures. Ni@MCM-3.0 displays
outstanding activity not only at higher, but also lower
temperatures.
Catalysts
Metal
content
COuptake
Conversion
TOF (h–1
)
(μmol g–1)
Ni@MCM-3.0
Ni/MCM-41
Pt/MCM-41
3.0%
3.0%
0.3%
6.1
4.4
1.0
20.0%
12.6%
10.3%
31.4
18.3
65.8
Figure 9. Reaction pathway of naphthalene hydrogenation.
To study the efficacy of the catalysts further, the TOFs were
calculated and compared (Table 2). Pt/MCM-41 shows optimal
–1
catalytic efficiency with the highest TOF value (65.8 h ) because
of the excellent catalysis nature of the noble metal. Ni@MCM-3.0
(
28.0% conversion) exhibits a much higher conversion than 0.3
wt.% Pt/MCM-41 (10.3% conversion). This is a rather exciting
result, given that the TOF of Pt/MCM-41 catalyst is twice that of
Ni@MCM-41 catalyst. It is acknowledged that noble components
are usually used with a loading of <1% in industrial applications
because of their high cost and low reserves. This is in contrast to
the loading of non-noble metals, which tends to be >10%. So the
above results about 3 wt.% Ni@MCM-41 and 0.3 wt.% Pt/MCM-
Figure 10. (a) Conversion and (b) selectivity of catalysts at LHSV =1 h–1
.
41 suggest that Ni@MCM-41 has great potential as an alternative
to noble metals in heterogeneous catalysis.
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Table 3. The trans-decalin selectivity of catalysts at the same conversion of 100%.
Ea (kJ mol–1
)
a
Catalysts
Trans-selectivity (%)
1
60 °C
180 °C
92.4
--
200 °C
220 °C
92.3
240 °C
260 °C
92.4
86.0
--
Ni@MCM-3.0
Ni/MCM-41
Pt/MCM-41
103.5
152.5
59.4
92.4
--
92.4
--
92.3
86.3
--
80.1
--
57.1
56.9
58.7
[a] The trans-decalin selectivity calculated by calculating the percentage of trans-decalin in decalin (trans- + cis-) product.
encapsulation of Ni particles inside the channels of MCM-41 has
a beneficial effect on trans-decalin selectivity.
To the best of our knowledge, there is, as yet, no confirmed
mechanism for isomer selectivity.[21] Some direct evidence can
now be used to address this (the reaction pathway). We propose
the following reason for isomer selectivity. It can be speculated
that the in situ synthesis method causes a stronger electronic
2
interaction between Ni and MCM-41, as determined from the H -
TPR results (Figure 8), which subsequently causes an increased
electron density of the Ni species.[12] Hence, the Ni species can
interact more strongly with HDA intermediates absorbed on the
MCM-41 support. These intermediates are formed after the
hydrogenation of tetralin to decalin. This hydrogenation step is
structure sensitive. More importantly, it is a key step in decalin
isomer selectivity.[
22]
Ni@MCM-3.0 shows remarkable trans-
decalin selectivity, which might be the result of the stronger
adsorption of the intermediate (A), as illustrated in Figure 9.
Furthermore, the encapsulation structure of Ni@MCM-3.0
facilitates the adsorption and enrichment of naphthalene in the
channels of MCM-41 and promotes the desorption of trans-
decalin rather than the cis-decalin or tetralin by-product.[17] Overall,
confining Ni within the channels of MCM-41 not only results in
excellent catalysis activity, but also offers an advantage in terms
of isomerization selectivity.
As is well known, the HDA reaction is one of the most important
reactions used in modern-day oil refineries; decalin is produced
from naphthalene. Generally, decalin is manufactured
commercially, using a conventional HDA catalyst, and the product
is almost a 50/50 mixture of trans-/cis-decalin. Many published
-
1
Figure 11. Arrhenius plots of catalysts at LHSV =3 h .
According to the analytical results described above, we can
confidently state that the encapsulation structure of Ni@MCM-3.0
can significantly improve the catalysis efficiency of HDA. Ni
particles in Ni@MCM-3.0 are ultra-small in size and highly
dispersed but much larger and agglomerated in Ni/MCM-41,
which has been confirmed in TEM analysis. In addition,
Ni@MCM-3.0 shows a much higher COuptake (Table 2) than that
of Ni/MCM-41, suggesting Ni@MCM-3.0 possesses more active
sites than Ni/MCM-41. Considering that the Ni@MCM-3.0 and
Ni/MCM-41 catalysts have similar Ni loadings and the same
mesoporous silica support, the higher activity of Ni@MCM-3.0
can be reasonably attributed to its smaller encapsulated Ni
particles, which expose more active sites.[20] Besides, XPS results
of samples before (Figure 6) and after (Figure S5) catalysis
suggest that there is no Ni species change occured in the
hydrogenation. As illustrated in Figure 8, decalin has two isomers,
trans-decalin and cis-decalin. Interestingly, when at a same
conversion of 100%, Ni@MCM-3.0 shows a significantly higher
catalytic selectivity for trans-decalin than other tested catalysts
papers have focused on the isomerization activity of supported
metal catalysts in naphthalene hydrogenation,[
23]
and those
results indicate that high isomerization selectivity is always
associated with the use of noble metals and/or harsh reaction
conditions. Unfortunately, a trans-decalin selectivity which >92%
is rarely achieved in such processes. Therefore, indications are
that the Ni@MCM-3.0 catalyst may be a promising and effective
alternative to noble metal catalysts in selective naphthalene
hydrogenation.
(
Table 3). Ni@MCM-3.0 maintains a selectivity for trans-decalin
of 92% over the reaction temperature range of 160 – 260 °C. On
the contrary, both Ni/MCM-41 and Pt/MCM-41 catalysts have
much lower trans-decalin selectivity. It can be concluded that the
Conclusions
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ChemCatChem
10.1002/cctc.201801788
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In summary, an efficient and simple in situ method for the
controlled encapsulation of Ni particles inside the channels of
sample of Pt/MCM-41 with 0.3 wt.% Pt content was prepared by
impregnation with H PtCl solution.
2 6
2
MCM-41 is reported. In this method, Ni(acac) is employed as Ni
Characterization of catalysts
source and grafted into the micelle template of CTAB. Sodium
silicate is used as a silicon source to rapidly hydrolyze and then
wrap on the micelle surface to synthesize MCM-41 zeolite. The
The Ni content of catalysts was determined by inductively coupled plasma
atomic emission spectroscopy (ICP-AES) on an Agilent 710-ES. X-ray
diffraction (XRD) data were recorded using Rigaku SmartLab equipment,
2
MCM-41 zeolite encapsulating Ni(acac) is synthesized at 120 °C
within 4 h, which also significantly reducing the synthesis time.
Homogeneously distributed Ni nanoparticles with an average size
of 2.3 nm are formed and highly dispersed inside the pores of
MCM-41 after removal of the template. As a result of the
encapsulation, Ni@MCM-41 displays extraordinary thermal
stability. Results of evaluation of catalytic activity indicate that 3
wt.% Ni@MCM-41 is an excellent catalyst for the HDA reaction of
naphthalene to decalin. Its activity is even much higher than that
of conventional 0.3 wt.% Pt/MCM-41catalysts. Furthermore, it
shows high isomer selectivity for decalin; 92% trans-decalin
selectivity was recorded over a wide reaction temperature range
of 160 – 260 °C. Due to its superior catalytic performance together
with its simple synthesis route, this Ni@MCM-41 catalyst can be
potentially applied to a large-scale production. This in situ method
should not only largely simplify synthesis routes to supported
metal catalysts, but also lead to improved catalytic activity. Hence,
it may open opportunities for the development of highly active
non-noble metal catalysts for efficient heterogeneous catalysis
and the selective production of isomers.
with Cu Kα radiation.
determined with Belsorp-Max equipment at 77 K. All samples were
pretreated at 150 °C in N for 10 h. The surface area and pore volume
were calculated using the BET equation. Pore diameter distribution were
determined by means of the nonlocal density functional theory (NLDFT)
method; the parameters were set as PSD fitting: no assumption, surface
element: oxygen, and pore type: cylinder. Average pore size were
calculated by BJH. Transmission electron microscopy (TEM) and energy
dispersive spectroscopy (EDS) measurements were carried out on a JEOL
JEM-2800. UV-vis spectra was recorded with a Nicolet iS-50 spectrometer
N
2
adsorption–desorption isotherms were
2
(Thermo Scientific, Waltham, MA, USA). X-ray photoelectron
spectroscopy (XPS) analysis was carried out with an Axis Ultra DLD
spectrometer (Kratos Analytical Ltd., UK) equipped with an Al Kα X-ray
source (250 W). Tests were carried out with an analyzer pass energy of
1
88 eV for survey scans and 30 eV for detailed elemental scans. The
temperature programmed reduction of (H -TPR) and CO pulse
chemisorption were determined with a Micromeritics Chemisorb 2750 gas
adsorption instrument. For the analysis of H -TPR, the NiO@MCM-x
sample was pretreated in N at 200 °C for 2 h. In the test process, the
samples were heated to 800 °C at a rate of 10 °C min–1, in 10% H
/Ar. For
the measurement of CO pules chemisorption, the sample was first
pretreated in 10% H /Ar at 600 °C for 2 h. After the sample had cooled to
room temperature, pulses of 10% CO/He in a He carrier (25 mL min–1
H
2
2
2
2
2
2
)
were periodically injected at 30 °C, through a loop tube, until the adsorption
of the catalyst reached saturation. Dispersion was calculated from CO
Experimental Section
chemisorption assuming crystallites were hemispherical,
stoichiometry (i.e., 1 molecule of CO interacts with one exposed atom of
Ni) and the crystallites were reduced.
a
1:1
Sodium silicate (Na 0.7 mol kg–1, Si 2.5 mol kg–1, H
purchased from Tianjin Chemicals (Tianjin, China) Scientific Company.
Nickel acetylacetonate (Ni(acac) ), naphthalene, CTAB, nickel nitrate
hexahydrate, and H PtCl were purchased from Shanghai Sigma-Aldrich
Shanghai, China). All materials were used as received. Deionized water
O 80 wt.%; >99%) was
2
2
2
6
Determination of Catalytic performance
(
was used as throughout.
The HDA activities of Ni@MCM-3 and Ni@MCM-1.5 were determined by
hydrogenating naphthalene in a continuous-flow fixed-bed microreactor.
For comparison, Ni/MCM-41 (3 wt.% Ni) and Pt/MCM-41 (0.3 wt.% Pt)
were also evaluated under with the same conditions. Prior to the reaction,
the catalyst was pelleted, crushed, and sieved to 20–40 mesh. It was then
Catalyst preparation
Ni@MCM-41 was prepared by mixing sodium silicate, CTAB, deionized
water and Ni(acac) in a molar ratio of 1 SiO : 0.12 CTAB : 40 H O : 0.003-
.004 Ni(acac) . The synthesis was carried out as follows. First, CTAB and
an appropriate quantity of Ni(acac) were dissolved in water under strong
2
2
2
loaded into a fixed-bed microreactor and reduced in situ with H
00 °C for 3 h. The reaction material was 5% naphthalene in cyclohexane.
Test conditions were the following: pressure 3 MPa, liquid hourly space
velocity (LHSV) 1 h–1, and V(H
)/V(oil) = 600. Liquid products were
collected and analyzed with a GC-MS instrument (7890A/5975C; Agilent)
equipped with an HP-5 column. The apparent activation energy (E ) was
2
flow at
0
2
6
2
agitation until a clear solution was obtained. The resulting mixture was held
at 75 °C for 0.5 h. Second, sodium silicate was added. After further
agitation for 0.5 h, the resulting solution was cooled to 60 °C. Third, the pH
value of the solution was adjusted to 10 by the dropwise addition of HCl.
After further agitation for 3 h, the mixture was transferred to an autoclave
and heated to 130 °C for 5 h. Fourth, the solid product was filtered, washed
five times with deionized water, and then dried at 80 °C. Finally, the
prepared material was calcined in air at 600 °C for 5 h to fully remove the
CTAB template to yield NiO/MCM-41. The Ni@MCM-41 catalyst was
obtained after reduction at 600 °C for 2 h in a H
passivated in a gas mixture (99.5% N and 0.5% O
before exposure to air. Ni@MCM-41 catalysts with 3.0 wt.% and 1.5 wt.%
Ni content were prepared; these were abbreviated as Ni@MCM-x, where
2
a
calculated when the naphthalene conversion was < 20% (LHSV 3 h–1),
using the Arrhenius empirical equation [equation 1]:
퐸푎
ln훾 = − + ln퐴
(1)
푅푇
where γ (mol s–1 g–1) is the reaction rate, R is the universal gas constant,
T is the temperature (in Kelvin) and A is the pre-exponential factor. The
turnover frequency (TOF) was calculated at 120 °C for LHSV 3 h–1, using
the following equation [equation 2]:
2
flow. It was then
2
) at 100 °C for 4 h
2
‘x’ represents the Ni content.
ꢁ
ꢂꢃ
푋ꢂ
ꢀ
푂퐹 =
(2)
푊
퐶ꢄ푢푝푡푎푘푒
A ‘blank’ MCM-41sample was prepared in the same way as reported
above, with the exception of no Ni(acac)
Ni/MCM-41 with 3.0 wt.% Ni content was prepared by a wetness
impregnation method with an aqueous solution of Ni(NO . Similarly, a
2
addition. For comparison,
where FA0 is the rate of reactant fed into the reactor (μmol h–1), XA is the
reactant conversion (%), W is the catalyst weight (g), and COuptake is the
uptake of chemisorbed CO (μmol g–1).
3
)
2
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ChemCatChem
10.1002/cctc.201801788
FULL PAPER
[
[
22] R. A. Ortega-Domínguez, H. Vargas-Villagrán, C. Peñaloza-Orta,
Acknowledgements
K. Saavedra-Rubio, X. Bokhimi, T. E. Klimova, Fuel. 2017, 198,
110-122.
This work was financially supported by the NSFC (21878162,
23] a) M. Luo, Q. Wang, G. Li, X. Zhang, L. Wang, T. Jiang, Catal. Sci.
Technol. 2014, 4, 2081-2090; b) H. Yang, H. Chen, J. Chen, O.
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21603107, U1403293), MOE (IRT-13R30 and 113016A), NSFT
(
16YFZCGX00020), the Research Fund for 111 Project (B12015).
Keywords: decalin • hydrogenation • MCM-41 • nickel •
naphthalene
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Entry for the Table of Contents
FULL PAPER
Encapsulation of ultra small Ni
particles in the channels of MCM-41.
In catalysis hydrogenation of
X. Song, Q. Guan,* Y. Shu, X. Zhang,
W. Li*
naphthalene, this catalyst shows a
high low-temperature activity and
excellent selectivity for decalin isomer.
Page No. – Page No.
Facial in situ encapsulation of highly
dispersed Ni@MCM-41 for the trans-
decalin production from
hydrogenation of naphthalene at low
temperature
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