Mild Hydrogenation of α-Pinene Catalyzed by Ru Nanoparticles Loaded on Boron-doped Amphiphilic Core-Shell Mesoporous Molecular Sieves

Article abstract of DOI:10.1002/cctc.201802044

Highly dispersed and stable catalysts comprising Ru nanoparticles supported on boron-doped amphiphilic core-shell mesoporous molecular sieves (MMS?C@MMS?NH₂/B/Ru) with alkyl-modified hydrophobic silica core and NH₂-functionalized hydrophilic silica shell are successfully prepared for use in hydrogenation of α-pinene for the first time. Dodecyl-modified MMS?C₁₂@MMS?NH₂/B/Ru exhibits the best catalytic activity under mild hydrogenation conditions. The abundant ?NH₂ functional groups on the molecular sieve surface and their amphipathy allow the sieves to facilitate attachment of more Ru nanoparticles, and to simplify their dispersion in the water-organic reaction medium. Moreover, B-doped molecular sieves may adjust their acidity to meet the needs of α-pinene hydrogenation. Under mild reaction conditions (25 °C, 1 MPa H₂, and 1 h), α-pinene can be completely converted with 99 % selectivity to cis-pinane, because every nanocomposite is equivalent to a microreactor. The catalytic activity does not change much over 5 cycles, indicating that Ru nanoparticles are stably loaded on the molecular sieves.

Full text of DOI:10.1002/cctc.201802044

DOI: 10.1002/cctc.201802044
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Mild Hydrogenation of α-Pinene Catalyzed by Ru
Nanoparticles Loaded on Boron-doped Amphiphilic
Core-Shell Mesoporous Molecular Sieves
[a]
[a]
[a]
[a]
[a]
[a]
[a]
Fengli Yu, Lihua Xie, Fangzhu Wu, Bing Yuan, Congxia Xie,* Shitao Yu,* Xien Liu,
[a]
[b]
Lei Wang, and Dan Wang
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Highly dispersed and stable catalysts comprising Ru nano-
particles supported on boron-doped amphiphilic core-shell
Ru nanoparticles, and to simplify their dispersion in the water-
organic reaction medium. Moreover, B-doped molecular sieves
may adjust their acidity to meet the needs of α-pinene
mesoporous molecular sieves (MMSÀ C@MMSÀ NH /B/Ru) with
2
alkyl-modified hydrophobic silica core and NH -functionalized
hydrogenation. Under mild reaction conditions (25°C, 1 MPa H ,
2
2
hydrophilic silica shell are successfully prepared for use in
hydrogenation of α-pinene for the first time. Dodecyl-modified
MMSÀ C @MMSÀ NH /B/Ru exhibits the best catalytic activity
and 1 h), α-pinene can be completely converted with 99%
selectivity to cis-pinane, because every nanocomposite is
equivalent to a microreactor. The catalytic activity does not
change much over 5 cycles, indicating that Ru nanoparticles are
stably loaded on the molecular sieves.
12
2
under mild hydrogenation conditions. The abundant À NH
2
functional groups on the molecular sieve surface and their
amphipathy allow the sieves to facilitate attachment of more
[
6]
Introduction
Raney-Ni and Pd/C. However, the catalytic activity decayed
quickly owing to the aggregation of the nanoparticles. To prevent
the aggregation of metal nanoparticles, some stabilizers, such as
nonionic surfactants and polymers are usually added to the
Biomass utilization has been attracting increased attention
because it can ensure energy security, protect the environment,
and promote sustainable development of the eco-society.
Turpentine obtained from pine trees is one of the naturally rich
resources that contains more than 80% of α-pinene. Pinane,
obtained by the hydrogenation of α-pinene, is an important
chemical intermediate for the synthesis of valuable spices,
medicines, and other fine chemicals. Two species, including cis-
pinane and trans-pinane, can be obtained during the hydro-
genation of α-pinene, but cis-pinane exhibits higher reactivity
[1]
[7]
catalytic system. Our research group has also made progress in
the stabilization of metal nanoparticles with the use of some
polymers, such as PVA, F127, P123, and TPGS-1000. However, the
addition of stabilizers may lead to the formation of emulsions,
thus resulting in difficulties in product separation. Currently, the
developed alternative approach is to ensure the attachment of
nanoparticles on solid support materials, including amorphous
silica, activated carbon, and molecular sieves. These materials play
a significant role since they prevent the aggregation of metal
[2]
[8]
[
9]
[3]
[4]
relative to trans-pinane. Therefore, it is essential to obtain a high
yield of cis-pinane during the hydrogenation of α-pinene
processes.
[10]
nanoparticles.
Among the developed supporting materials,
mesoporous molecular sieves (MMSs) have become a research
hotspot in catalysis, because their high-specific surface area could
allow the attachment of additional metal nanoparticles, while the
large, uniform pore structure, can achieve molecular shape-
Metal nanoparticles possess an excellent catalytic activity for
the hydrogenation reaction owing to their quantum sizes and
[5]
surface effects. For example, Ni nanoparticles exhibited an
[11]
excellent selectivity for cis-pinane which was better than those of
selective catalysis.
However, pure silica MMS cannot be adequately dispersed in
the organic phase owing to its inherent hydrophilicity. This
drawback can be resolved via the surface modification or
functionalization of MMS through the binding of the organic
[a] Dr. F. Yu, L. Xie, F. Wu, B. Yuan, Prof. C. Xie, Prof. S. Yu, Prof. X. Liu,
Prof. L. Wang
State Key Laboratory Base of Eco-Chemical Engineering
Taishan Scholar Advantage and Characteristic Discipline Team of Eco-Che-
mical Process and Technology
Qingdao University of Science and Technology
Qingdao, Shandong 266042 (P. R. China)
E-mail: xiecongxia@126.com
[12]
functional groups with the massive SiÀ OH on its surface. A more
important strategy is the doping of heteroatoms in molecular
[13]
sieves to improve their acidities or oxidizabilities.
MMSs, owing to their specific hydrophilic/hydrophobic properties,
acidity, or oxidizability, have been extensively used in the
Modified
yushitaoqust@126.com
b] Prof. D. Wang
[
[14]
State Key Laboratory of Biochemical Engineering
Institute of Process Engineering
Chinese Academy of Sciences
Beijing, 100190 (P. R. China)
adsorption-separation, catalysis, drug delivery, and sensors.
To-this-date, no prior report has investigated metal nano-
particles which have been loaded on an amphiphilic and doped
periodic mesoporous molecular sieve (PMMS) with a core-shell
structure. The present work first reports the synthesis of ruthenium
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.201802044
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Scheme 1. Preparation of the catalyst MMSÀ C@MMSÀ NH /B/Ru.
2
2
Figure 1. STEM of MMSÀ C12@MMSÀ NH /B/Ru and STEM-EDS mappings showing the elemental distribution of Si, Bi, and Ru.
(Ru) nanoparticles loaded on boron atoms-doped PMMS with
chloride (CTAC) as the template in a methanol aqueous
solution, and tetramethoxysilane (TMOS) and alkyl trimethox-
ysilane (R-TriMOS) as the co-precursors, alkyl-functionalized
hydrophobic silica core (MMSÀ C) was synthesized in alkaline
amphipathy and a core-shell structure. PMMS is composed of an
alkyl-modified hydrophobic mesoporous silica core and an amino-
functionalized
hydrophilic
mesoporous
silica
shell
(
MMSÀ C@MMSÀ NH ). The catalyst of MMSÀ C@MMSÀ NH /B/Ru
conditions. Secondly, NH -functionalized hydrophilic silica shell
2
2
2
used for hydrogenation of α-pinene has several superior advan-
(MMSÀ NH ) was formed subsequently in the presence of the
2
tages. The abundant À NH in the prepared PMMS allows the
same template, i.e., TMOS and 3-aminopropyl trimethoxysilane
2
attachment of additional Ru nanoparticles, while the amphipathy
of the molecular sieve ensures its proper and adequate dispersion
in the water-organic reaction medium. The doping of boron atoms
into the molecular sieve may adjust both the acidity and selectivity
of the catalyst to meet the need of the reaction. Accordingly, Ru
nanoparticles are well dispersed on the obtained PMMS with
excellent stabilization. Therefore, the prepared catalyst of
(NH -TriMOS). Thirdly, boric acid was added in the amphiphilic
2
MMSÀ C@MMSÀ NH prepared above to prepare the doped
2
PMMS (MMSÀ C@MMSÀ NH /B). Finally, the in situ reduction of
2
the Ru precursor by NaBH resulted in the target catalyst of
4
MMSÀ C@MMSÀ NH /B/Ru. In consideration of the modification
2
of catalyst with organic functional groups, the reduction by H2
with strict conditions is not used.
MMSÀ C@MMSÀ NH /B/Ru exhibits an excellent catalytic activity,
In combination with the catalytic activity experiments, the
structure of the prepared dodecyl (C ) modified catalyst of
2
high selectivity, and good recyclability in the hydrogenation of α-
pinene. In particular, the catalyst MMSÀ C @MMSÀ NH /B/Ru can
1
2
MMSÀ C @MMSÀ NH /B/Ru was fully characterized. Accordingly,
12
2
12
2
effectively catalyze α-pinene to cis-pinane in very mild hydro-
genation conditions because of the formation of the three-phase
interface microreaction environments.
high-angle annular dark field scanning TEM (STEM), and
corresponding energy dispersive X-ray (EDX) elemental map-
ping images were acquired (Figure 1). The EDX images clearly
displayed the distribution of the Si, B, and Ru elements. The
results confirmed that the B atoms were well doped in the
skeleton of the molecular sieves, and the Ru atoms were
uniformly immobilized on the surface of the molecular sieves,
thus indicating that Ru existed in the nanoparticles rather than
in aggregated (nanocluster) forms.
Results and Discussion
Preparation and Characterization of the Catalyst
Scheme 1 illustrates the overall preparation procedure of the
target catalyst. Firstly, by using hexadecyl trimethyl ammonium
The prepared silica core MMSÀ C has an average diameter
1
2
of 235.5 nm (Figure S1, Supporting Information). After the outer
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modified molecular sieves. It was demonstrated that the doping
of boron atoms still maintained the primary pore structure and
its integrity. However, after the doping of B atoms and the
loading of Ru nanoparticles, the peak intensity gradually
declined. This indicated the incorporation of B and Ru made the
pores less regular.
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[15]
Zeta potential is an important index of colloidal dispersion
stability. When the absolute value of zeta potential is higher
than 30, the colloid is generally considered stable. The zeta
potential of the formed MMSÀ C @MMSÀ NH /B/Ru in aqueous
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solution was examined and yielded a value of À 36.7 mV
(Figure S3) that confirmed a good stability of the prepared
catalyst. The unique hydrophilicity/hydrophobicity of the cata-
lyst was also examined by the quantification of the water
droplet contact angle (Figure S4). Elicited results indicated that
the water droplet contact angle was approximately 91° for
MMSÀ C @MMSÀ NH /B/Ru, thus revealing the fact that the
1
2
2
catalyst was amphiphilic.
FT-IR spectra of various molecular sieves are shown in
Figure S5. The characteristic absorption peaks at 2960, 2920,
Figure 2. SEM and TEM images of MMSÀ C12@MMSÀ NH
2
(a and c – left) and
MMSÀ C12@MMSÀ NH /B (b and d - right).
2
À 1
and 1138 cm in the IR spectrum of MMSÀ C @MMSÀ NH
1
2
2
showed that the organic group C was successfully introduced
1
2
in the molecular sieves. The new absorption peak near
À 1
silica shell was formed, the average diameter of the prepared
1310 cm for MMSÀ C @MMSÀ NH /B corresponded to the BÀ O
1
2
2
MMSÀ C @MMSÀ NH was found to 386 nm (Figure S2). Accord-
bond, which showed that B atoms were successfully doped in
the skeletons of the molecular sieves. After the loading of Ru
nanoparticles, the structure of MMSÀ C @MMSÀ NH /B/Ru was
12
2
ing to the SEM images of MMSÀ C @MMSÀ NH and
1
2
2
MMSÀ C @MMSÀ NH /B shown in Figure 2 (a and b), the doping
12
2
12
2
of the boron atom in the molecular sieves gave rise to increased
surface roughness but maintained a uniform spherical structure.
Comparison of the TEM images of MMSÀ C @MMSÀ NH and
in line with MMSÀ C @MMSÀ NH /B.
1
2
2
29
Moreover, the spectrum of Si MAS-NMR in Figure S6a
showed there were three types of Si in the B-doped molecular
sieves. The chemical shifts near À 106.134 corresponded to Si
(OSi) À OH and Si(OSi) . The other peak at À 71.677 was ascribed
1
2
2
MMSÀ C @MMSÀ NH /B in Figure 2 (c and d) indicates that the
12
2
pore channels of the doped molecular sieves are still intact and
not damaged. The high-resolution SEM image of
MMSÀ C @MMSÀ NH /B/Ru shows an average diameter of
3
4
to SiÀ C. This also demonstrated that the organic group C was
12
1
1
successfully introduced to the molecular sieves. The B MAS-
NMR spectrum in Figure S6b shows that there are two types of
B in the B-doped molecular sieves. The chemical shift of À 1.430
12
2
2
.5 nm for the loaded Ru nanoparticles.
XRD patterns of various molecular sieves are shown in
Figure 3. The small-angle diffraction peak near 2θ=2.3° was
corresponding to BO indicated that most of the B atoms were
4
characteristic for the mesoporous structure, and existed in all
doped in molecular sieves. Otherwise, a few B atoms existed in
the form of BO with three coordinations at the chemical shift
3
of 12.400.
TG-DTG curves of MMSÀ C @MMSÀ NH /B/Ru are shown in
1
2
2
Figure S7a. There were four parts of weightlessness for the B-
doped molecular sieves. The removal of the adsorbed moisture
mainly occurred at approximately 187°C, and the decomposi-
tion of residual organic template agents was in the range of
1
87–400°C. With increasing temperature, organic functional
groups grafted in molecular sieves became cracking and
shedding in the range of 400–610°C. When the temperature
became higher than 610°C, the removal of B atoms began.
However, the loss of B atoms was not found for nondoped
molecular sieves based on the comparison of the TG curves of
MMSÀ C @MMSÀ NH and MMSÀ C @MMSÀ NH /B/Ru (Fig-
1
2
2
12
2
ure S7b). Therefore, the B-doped molecular sieves exhibit good
stability at temperatures under 400°C.
N
adsorption-desorption
isotherms
of
2
Figure 3. XRD patterns of (a) MMSÀ C12@MMSÀ NH
2
, (b) MMSÀ C12@MMSÀ NH
2
/
MMSÀ C @MMSÀ NH /B in Figure S8a show a typical type IV
1
2
2
B, and (c) MMSÀ C12@MMSÀ NH
2
/B/Ru.
adsorption-desorption isotherm characteristics with a specific
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surface area of 13 m /g and a specific pore volume of 0.06 cm /
[a]
Table 1. Catalytic activity of different modified catalysts.
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g. The BJH pore size distribution curve of MMSÀ C @MMSÀ NH /
1
2
2
Catalyst
Ru content
wt%]
Conversion
[%]
Selectivity
[%]
B in Figure S8b shows that the molecular sieves doped with B
atoms had two types of different pore structures with different
sizes. However, the vast majority of the pore structures were
uniform with sizes in the range of 2–2.5 nm.
[
MMS/Ru
1.62
1.99
2.25
3.76
3.47
3.65
3.74
35.72
34.26
44.35
63.56
73.59
99.71
97.15
98.50
98.23
99.21
99.80
99.01
99.00
98.82
MMSÀ C12/Ru
MMSÀ NH /Ru
2
MMSÀ C12@MMSÀ NH
2
/Ru
According to the NH -TPD spectra of the molecular sieves
3
MMSÀ C @MMSÀ NH /B/Ru
8
2
depicted in Figure S9, the peak indicating the desorption of the
molecular sieves doped with B atoms was enhanced dramati-
cally, and the area of the desorption peak increased consid-
erably relative to SiO and MMSÀ C @MMSÀ NH , thus indicating
MMSÀ C12@MMSÀ NH
2
/B/Ru
MMSÀ C @MMSÀ NH /B/Ru
1
8
2
[
a] Reaction conditions: 40°C, 1 MPa, 2 h.
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that the B-doping led to a significant increase of the acid
amount. In addition, a strong acidic center was produced owing
to the B-doping according to the temperature desorption curve
achieved at temperatures above 400°C.
Catalytic Hydrogenation of α-Pinene
XPS spectra were afforded in Figure 4. The binding energy
changes of the C, O, and N atoms in the molecular sieves before
and after the loading of Ru nanoparticles were also detected by
XPS. The binding energy of the C atoms elicited no any changes
The prepared Ru nanoparticles loaded on different MMSs were
used to catalyze the hydrogenation of α-pinene. As shown in
Table 1, nonmodified MMS/Ru showed low-catalytic activity
owing to the inherent hydrophilicity of the Si material and low
Ru loading. However, alkyl-modified MMSÀ C /Ru did not
(Figure S10). However, ΔE=0.45 and 0.60 eV are observed for
1
2
the O and N atoms, respectively (Figures S11 and S12), which
improve the conversion but yielded a lower conversion rate.
The reason was that the bulky alkyl impeded the contact of Ru
nanoparticles with the substrate. NH -modified MMSÀ NH /Ru
demonstrate that Ru nanoparticles are mainly bonded to À OH
and À NH on the surface of the molecular sieves. Moreover, the
2
2
2
possibility that À NH could adsorb more Ru nanoparticles was
improved the conversion process in spite of its very low
2
higher.
lipophilicity. This was because À NH adsorbed more Ru nano-
2
particles compared to À OH. Amphiphilic MMSÀ C @MMSÀ NH /
1
2
2
Ru with a high Ru loading yielded a higher conversion.
However, B-doped amphiphilic MMSÀ C@MMSÀ NH /B/Ru com-
2
paratively displayed the best catalytic activity, and the
modification of different carbon chains on MMSÀ C@MMSÀ NH /
2
B/Ru had a considerable influence on the catalytic hydro-
genation. The length of the carbon chain determined the
hydrophobic ability of the amphiphilic molecular sieves. Longer
carbon chains with a larger steric hindrance were unfavorable
to the establishment of contacts with the substrate. Shorter
carbon chains with relatively poor hydrophobicities were not
conducive to the dispersion of the catalyst in the water-organic
system. According to Table 1, MMSÀ C @MMSÀ NH /B/Ru modi-
1
2
2
fied with dodecyl trimethoxysilane (C ) elicited the best
1
2
catalytic activity. For the hydrophilic group of amphiphilic
molecular sieves, the commonly used aminopropyl (3-amino-
propyl trimethoxysilane) possessed excellent hydrophilicity and
good coordination properties that enhanced the stability of the
metal nanoparticles.
It has been reported that Pd and Ni nanoparticles exhibited
good catalytic activity in the hydrogenation of α-pinene with
harsh reaction conditions. However, when Pd and Ni nano-
particles were loaded on amphiphilic molecular sieves doped
with B atoms, the corresponding catalysts exhibited very low
catalytic activity in the hydrogenation of α-pinene (Table S1).
Interestingly, Pd and Ni nanoparticles were replaced by Ru
nanoparticles that resulted in very high α-pinene conversion
and in increased cis-pinane selectivity. The most important
reason for this finding is attributed to the severe hydrogenation
conditions of Pd and Ni.
Figure 4. XPS spectra of MMSÀ C12@MMSÀ NH
2
/B/Ru including a) full spec-
trum scanning (top) and b) local amplification (bottom).
In addition, the different precursors of Ru nanoparticles,
including ruthenium nitrosyl nitrate, tri-ruthenium dodecacar-
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Figure 5. Optimization of hydrogenation conditions.
bonyl, and ruthenium trichloride, were also investigated. The
results showed that the prepared catalysts using the first two
precursors had almost no catalytic activity, and the catalyst
prepared by the ruthenium trichloride precursor exhibited a
very high catalytic activity. Owing to the large sizes of the
molecules of ruthenium nitrosyl nitrate and tri-ruthenium
dodecacarbonyl, they could not easily enter the pore channels
of the molecular sieves. Therefore, the formed Ru nanoparticles
only were on the surface of the molecular sieves, which led to
their easy removal.
For the hydrogenation of α-pinene catalyzed by
MMSÀ C @MMSÀ NH /B/Ru, the reaction conditions included the
1
2
2
Si/B molar ratio, catalyst dosage, water volume, H pressure,
2
reaction temperature, and the reaction time, which were all
optimized. The results are shown in Figure 5 (a–f).
The doping of the molecular sieves with B atoms may adjust
its acidity to meet the needs of the hydrogenation process of α-
pinene. The catalysis results of B-doped catalysts with different
Si/B molar ratios are shown in Figure 5a. It is observed that the
lower and the higher Si/B molar ratios were unfavorable to the
hydrogenation of α-pinene. When the Si/B molar ratio was
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Figure 8. Depiction of the hydrogenation reaction process of α-pinene.
recyclability of the catalyst for the hydrogenation of α-pinene
was investigated, and the results are shown in Figure 6. The
conversion and selectivity did not change during the five
recycling times. However, when the catalyst was reused six
times, the conversion of α-pinene significantly decreased. The
Ru content in the catalyst was determined by ICP-AES
detection. The freshly prepared catalyst contained 3.649 wt%
Ru, but 3.585 wt% Ru was still detected after the catalyst was
reused six times, thus indicating that Ru nanoparticles could be
loaded in a stable manner on the modified molecular sieves.
The decrease of the catalytic activity is attributed to the carbon
deposition in the holes of the molecular sieves.
Figure 6. Recyclability of the catalyst (25°C, 1 MPa, 1 h).
between 30 and 100, the corresponding catalysts yielded a
better catalytic activity both in terms of a higher conversion
rate and a higher selectivity. In other words, the hydrogenation
of α-pinene required a catalytic acidity within a certain range.
To minimize the number of B atoms used, the selected Si/B
molar ratio was set to 100.
Figure 5e shows the effects of the reaction temperature on
hydrogenation. The conversion of α-pinene could reach 96%
even at 15°C, which indicates that the catalyst exhibited high
catalytic activity. For an α-pinene mass of 0.3 g, the best
hydrogenation conditions were as follows: a catalyst mass equal
Mechanism of Catalysis
to 7 mg, water volume equal to 1 mL, a H pressure of 1 MPa, a
2
reaction temperature equal to 25°C, and a reaction time of 1 h.
The α-pinene was completely converted with a cis-pinane
selectivity of 99%.
With the use of the same experimental conditions, the
industrial catalysts used for the hydrogenation of α-pinene
included Ru/C, Pd/C, and Raney Ni were compared with the
novel Ru catalyst loaded on the molecular sieves (Table S2). The
novel Ru catalyst obviously exhibited the best catalytic activity
compared to the currently used industrial catalysts.
The greatest advantage of the new catalytic system was
attributed to the increased reactivity at very mild hydro-
genation conditions. Figure 7 shows the microscopic image
changes of the oil-water biphasic interface during the reaction
process. It can be clearly observed that the bubbles gradually
migrate into the oil phase from the water phase, and gradually
break. The released hydrogen reacts with α-pinene in the oil
phase on the surface of the solid catalyst. In simpler terms, the
catalytic reaction progressed at the gas-liquid-solid interface.
Therefore, every nanocomposite was equivalent to a micro-
reactor as shown in Figure 8. There were numerous micro-
After the reaction, the catalyst was centrifuged, washed,
dried, and was then applied to the subsequent reaction. The
Figure 7. Microscopic images of the oil-water biphasic interface during the process of the reaction.
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reactors in the catalytic system, which resulted in increased
hydrogenation efficiency. Moreover, because of the sp hybrid-
spatially closer to the double bond of the α-pinene from the
endo-surface. The amphipathy of the molecular sieves ensured
that the catalyst was well dispersed in the water-organic
reaction medium. The catalytic hydrogenation reaction pro-
gressed in the gas-liquid-solid interface. Every nanocomposite
was equivalent to a microreactor, and resulted in an increased
hydrogenation efficiency in mild reaction conditions. This study
provided a potential new method for mild and highly efficient
hydrogenation.
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ization of the B atom, there was an unoccupied orbital that led
to an easier access to the π-orbital of α-pinene. That was to say,
the double bond of α-pinene was activated by the catalyst.
Thus, H atoms were more easily added to the double bond of
α-pinene. Therefore, the hydrogenation reaction had lower
activation energy, and the hydrogenation condition was very
[16]
mild.
Additionally, the steric hindrance of the molecular
sieves allowed only the endo-surface of the α-pinene to contact
with the Ru nanoparticle catalyst, as shown in Figure 9. There-
fore, the new catalytic system also exhibited a high selectivity Experimental Section
for cis-pinane.
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Materials: Hexadecyl trimethyl ammonium chloride (CTAC), tetrame-
thoxysilane (TMOS), octyl trimethoxysilane, dodecyl trimethoxysi-
lane, octadecyl trimethoxysilane, 3-aminopropyl trimethoxysilane
Conclusions
(APTS), and RuCl ·3H O were purchased from the Aladdin Industrial
3 2
Corporation or the Shanghai Siyu Chemical Technology Co., Ltd.
Sodium hydroxide (NaOH), boric acid (H BO ) and sodium borohy-
3
3
B-doped amphiphilic core-shell mesoporous molecular sieves
dride (NaBH ) were obtained from the Sinopharm Chemical Reagent
4
MMSÀ C@MMSÀ NH /B were successfully prepared with the use
2
Co., Ltd. Additionally, α-pinene (98%) was supplied by Jiangxi
Hessence Chemicals Co., Ltd., and hydrogen (99.99 wt%) was
obtained from Qingdao Airichem Specialty Gases & Chemicals Co.,
Ltd. All solvents were purified before use. Doubly deionized water
was used in all experiments.
of an alkyl-functionalized hydrophobic silica core and a NH -
2
functionalized hydrophilic silica shell. The doping of B atoms to
the molecular sieves still maintained the primary pore channel
and the integrity of the structure. Ru nanoparticles were loaded
in a stable manner on MMSÀ C@MMSÀ NH /B to form the
Compound characterizations: The structure of the catalyst was
characterized using a Nicolet 510P Fourier transform infrared
2
hydrogenation catalyst of MMSÀ C@MMSÀ NH /B/Ru. The exis-
2
spectrometer (FT-IR) using the KBr method (frequency range from
tence of abundant À NH on the surface of the molecular sieves
2
À 1
4
000 to 400 cm ), and with an Agilent V NMRS 600 magic angle
could achieve the attachment of an increased number of Ru
2
9
spinning nuclear magnetic resonance spectrometer ( Si MAS-NMR
and B MAS-NMR). The morphology of the catalyst was inves-
nanoparticles. The catalyst MMSÀ C@MMSÀ NH /B/Ru was firstly
11
2
used to catalyze the hydrogenation of α-pinene to cis-pinane. It
exhibited an excellent catalytic activity, high selectivity, and
good recyclability under mild conditions. When B atoms were
doped in the skeletons of the molecular sieves, the acidity
values of the sieves were adjusted, and the catalyst was
tigated with a JEOL JEM-2100 transmission electron microscope
(TEM) and a JSM-6010LV scanning electron microscope (SEM). The
high-angle, annular dark field scanning TEM (STEM), and the
corresponding energy dispersive X-ray (EDX) elemental mapping
images were recorded on TALOS F200X. The zeta potential was
examined by a Malvern particle size analyzer (Nano S90). The water
droplet contact angle was quantified using a JC2000 contact angle
meter. Rigaku D/max-2500PC X-ray diffractometer (XRD) was used
to study the crystal structure of Ru. Thermal gravity (TG) and
differential thermal gravity (DTG) curves were recorded on a
Netzsch-TG209 thermogravimetric analyzer. A NH3 temperature
programmed desorption curve (NH -TPD) was used to determine
3
the acidity of the catalyst with the use of a BET-3000 adsorption
instrument. The N2 adsorption-desorption isotherm was used to
determine the porous structure of the catalyst by a micromeritics
ASAP 2020 adsorption instrument. The content of Ru in the catalyst
was measured by inductively coupled plasma atomic emission
spectrometry (ICP-AES). The valence state of Ru was determined by
a Kratos Axis Ultra DLD X-ray photoelectron spectroscopy (XPS).
Laser scanning confocal microscopy (Leica SP8) was used to study
the bubble images during the hydrogenation reaction. The hydro-
genation product was analyzed by a GC-9790 gas chromatography
instrument with an FID detector.
Preparation of amphiphilic MMSÀ C@MMSÀ NH : CTAC (0.88 g) was
2
dissolved in 225 mL of an aqueous solution of methanol with a
weight fraction of 55.56%. NaOH solution (625 mL, 1 mol/L) was
then added. The mixture was put in a 45°C water bath and stirred
at 600 r/min for 1 h. Subsequently, 160 mL of TMOS and 81 mL of
the corresponding alkyl trimethoxysilane (octyl trimethoxysilane,
dodecyl trimethoxysilane, or octadecyl trimethoxysilane) were
dropped under stirring. After 2 h of stirring, an alkyl-functionalized
hydrophobic core (MMSÀ C) was formed. A volume of 179 mL of
TMOS and a volume of 11 mL of APTS were added to the MMSÀ C
Figure 9. Proposed hydrogenation mechanism of α-pinene.
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solution during stirring and placed in a 35°C water bath. After Conflict of Interest
stirring for 12 h, the mixture was aged for another 12 h. The formed
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precipitate was separated, desiccated, and then extracted by 30 mL
The authors declare no conflict of interest.
of anhydrous ethanol to remove the organic template agent in an
oil bath at 80
°
C during stirring at 500 r/min. The anhydrous ethanol
was replaced by the fresh anhydrous ethanol every 12 h. After three
extractions, the precipitate was centrifuged and dried under
vacuum at 50°C to obtain the target mesoporous molecular sieve
with an amphiphilic and core-shell structure, which was abbrevi-
Keywords: hydrogenation · α-pinene · Ru nanoparticles · B-
doped molecular sieves · amphipathy
ated as MMSÀ C@MMSÀ NH .
2
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catalysts loaded on the boron-doped amphiphilic mesoporous
molecular sieves were prepared by using the impregnation method.
A mass of 0.0500 g MMSÀ C @MMSÀ NH , 0.0020 g H BO , and
1
[
12
2
3
3
4
0.0150 g RuCl ·3H O were dissolved in 4 mL of ethyl acetate. After
3 2
supersonic dispersion for 30 min, the mixture was put in a water
bath at 40°C and stirred at 500 r/min for 12 h. Subsequently, the
mixture was centrifuged, and the supernatant was removed.
Another 4 mL of ethyl acetate was added. Sodium borohydride
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0.0300 g) was added slowly in a dropwise manner in 1 mL of an
anhydrous ethanol solution, and the reaction was allowed to occur
for 2 h. After three centrifugation separations with the use of
anhydrous ethanol as a detergent, the collected precipitates were
dried under vacuum at 50°C for 4 h to obtain the catalyst
MMSÀ C @MMSÀ NH /B/Ru.
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Hydrogenation of α-pinene: A typical hydrogenation procedure was
executed based on which 0.30 g α-pinene, 7 mg
MMSÀ C @MMSÀ NH /B/Ru catalyst, and 1 mL H O, were added to a
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5 mL autoclave with a polytetrafluoroethylene liner. After sealing
the autoclave, the atmosphere in the autoclave was replaced by H2
for four times at 2.5 MPa, and H was then subsequently filled at
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9] Y. Liu, L. Li, S. W. Liu, C. X. Xie, S. T. Yu, J. Mol. Catal. A 2016, 424, 269–
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1
.0 MPa. The mixture in the autoclave was stirred for 1 h at 25°C.
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product phase was separated for GC analysis. The catalyst
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Acknowledgements
The authors are very grateful for the financial support from the
National Natural Science Foundation of China (No. 31270615), the
Taishan Scholar Foundation of Shandong Province of China (Nos.
ts201511033 and ts201712045), and the Open Project of the
Chemistry Department in Qingdao University of Science and
Technology of China (No. QUSTHX201811).
5
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Manuscript received: December 15, 2018
Revised manuscript received: January 10, 2019
Accepted manuscript online: January 11, 2019
Version of record online: ■■■, ■■■■
ChemCatChem 2019, 11, 1–9
www.chemcatchem.org
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© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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FULL PAPERS
Take core! Highly dispersed and
stable Ru nanoparticle catalysts
loaded on boron-doped amphiphilic,
core-shell, mesoporous molecular
sieves are successfully prepared and
used to catalyze the hydrogenation
of α-pinene to cis-pinane with an
excellent catalytic activity, high selec-
tivity, and good recyclability, in mild
experimental conditions given that
every nanocomposite is equivalent
to a microreactor.
Dr. F. Yu, L. Xie, F. Wu, B. Yuan, Prof. C.
Xie*, Prof. S. Yu*, Prof. X. Liu, Prof. L.
Wang, Prof. D. Wang
1
– 9
Mild Hydrogenation of α-Pinene
Catalyzed by Ru Nanoparticles
Loaded on Boron-doped Amphi-
philic Core-Shell Mesoporous
Molecular Sieves
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