Confinement of Ultrasmall Cobalt Oxide Clusters within Silicalite-1 Crystals for Efficient Conversion of Fructose into Methyl Lactate

Article abstract of DOI:10.1021/acscatal.8b03230

Chemocatalysis of sugars to methyl lactate (MLA) exhibits great advantages over the conventional fermentation approach because of its higher productivity and cost-effective separation process. However, widely used supported metal oxide catalysts suffer from deactivation resulting from sintering during the reaction and removal of coke at high temperatures. Herein, we report ultrasmall cobalt oxide clusters (～1.7 nm) stabilized within silicalite-1 crystals catalyst (CoO@silicalite-1), exhibiting superior catalytic activity and resistance to sintering for the conversion of fructose into methyl lactate. HAADF-STEM, EDS-mapping, and XRD experiments identify the existence of confined CoO clusters. XANES and Raman spectra demonstrated the covalent interaction between CoO and silicalite-1. Thanks to the ultrasmall CoO particle size (～1.7 nm), the CoO@silicalite-1 affords nearly 100-fold higher Co-mass-based activity (mg MLA/mg Co) compared with CoO or Co₃O₄ particles outside the silicalite-1 framework. More importantly, this catalyst exhibits good reuse performance via the removal of coke with facile calcination.

Full text of DOI:10.1021/acscatal.8b03230

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Article
Confinement of Ultrasmall Cobalt Oxide Clusters within Silicalite-1
Crystals for Efficient Conversion of Fructose into Methyl Lactate
Yue Yan, Zihao Zhang, Seong-Min Bak, Siyu Yao, Xiaobing Hu, Zulipiya Shadike, Chi-Linh Do-Thanh, Feng
Zhang, Hao Chen, Xilei Lyu, Kequan Chen, Yimei Zhu, Xiuyang Lu, Pingkai Ouyang, Jie Fu, and Sheng Dai
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03230 • Publication Date (Web): 13 Dec 2018
Downloaded from http://pubs.acs.org on December 14, 2018
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Confinement of Ultrasmall Cobalt Oxide Clusters within Silicalite-1
Crystals for Efficient Conversion of Fructose into Methyl Lactate
Yue Yan,a# Zihao Zhang,a# Seong-Min Bakd, Siyu Yaod, Xiaobing Hue, Zulipiya Shadiked,
Chi-Linh Do-Thanh,b Feng Zhangd, Hao Chena, Xilei Lyua, Kequan Chenf, Yimei Zhue,
Xiuyang Lua, Pingkai Ouyangf, Jie Fua*, Sheng Daib, c*
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a Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of
Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China
b Department of Chemistry, The University of Tennessee, Knoxville, TN 37996, USA
c Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
d Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, USA
e
Condensed Matter Physics and Material Science Department, Brookhaven National
Laboratory, Upton, New York 11973, USA
f
State Key Laboratory of Materials-Oriented Chemical Engineering, College of
Biotechnology and Pharmaceutical, Nanjing Tech University, Nanjing 211816, China
*Corresponding author: Jie Fu, E-mail: jiefu@zju.edu.cn
Sheng Dai, E-mail: dais@ornl.gov
# These authors contributed equally.
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Abstract: Chemocatalysis of sugars to methyl lactate (MLA) exhibits great advantages over
the conventional fermentation approach due to its higher productivity and cost-effective
separation process. However, widely used supported metal oxide catalysts suffer from
deactivation resulting from sintering during the reaction and removal of coke at high
temperatures. Herein, we report ultrasmall cobalt oxide clusters (~1.7 nm) stabilized within
silicalite-1 crystals catalyst (CoO@silicalite-1), exhibiting superior catalytic activity and
resistance to sintering for the conversion of fructose into methyl lactate. HAADF-STEM,
EDS-mapping, and XRD experiments identify the existence of confined CoO clusters.
XANES and Raman spectra demonstrated the covalent interaction between CoO and
silicalite-1. Thanks to the ultrasmall CoO particle size (~1.7 nm), the CoO@silicalite-1
affords nearly 100-fold higher Co-mass-based activity (mg MLA/mg Co) compared to CoO
or Co3O4 particles outside the silicalite-1 framework. More importantly, this catalyst exhibits
good reuse performance via the removal of coke with facile calcination.
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Keywords: Biomass conversion; Silicalite-1; Methyl lactate; Cobalt oxide clusters; Sinter-
resistance.
1. Introduction
Biomass, as an inexpensive, abundant, and renewable source of organic carbon from the
earth, could produce liquid fuels and chemicals.1-3 Among these chemicals, methyl lactate
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(MLA) is one of the most important platform compounds, which has been widely used in the
food, chemical, green solvents, and pharmaceutical fields.4 To date, very few studies have
been reported for the direct production of MLA from carbohydrates using nonfermentive
approaches.5 However, the fermentation route results in the formation of large amounts of
gypsum, hampering large-scale operation due to the extensive separation and purification
process.6-7 MLA can be produced by the chemocatalytic esterification of lactic acid (LA,
obtained from carbohydrates) with the corresponding alcohol, but this two-step conversion
process is not economical.8-9
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Recently, much attention has been paid to the catalytic one-step conversion of fructose
(a basic monosaccharide derived from biomass) to MLA because of higher productivity and
cost-effective separation process compared to fermentation processes.5,6 Sn-Beta zeolite has
been successfully developed for the highly efficient production of MLA, however, the use of
hydrofluoric acid during the synthesis step significantly hinders its large-scale
applications.7,10-13 Homogeneous Co2+ catalysts have also been reported and exhibited
superior catalytic activity and less by-products, however, the sintering of heterogeneous
CoOx active site driven by the decrease of surface free energy at high temperature is still a
major problem.14,15 To minimize sintering, the strong metal-support interaction as well as the
deposition of a protective layer has been used to design catalysts with sinter-resistant
properties.16-18
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Metal oxides coating has been reported to prevent leaching and sintering of metal
particles for the aqueous-phase hydrogenation reaction.18,19 Additionally, high-dispersed
metals are usually stabilized by strong metal support interactions (SMSI) on carriers using
different strategies, such as metal cations coordination, organic ligand complexation, voids
or defects anchoring and metal–metal interactions.20-23 Quite recently, Xiao and co-workers
reported that a sinter-resistant noble metal (Pt, Pd, Rh, Ag) nanoparticle catalyst can be
achieved by immobilization within zeolite crystals.24 More importantly, these noble
metal@zeolite catalysts show long reaction lifetimes for the catalytic conversion of C1
molecules. Additionally, the Co2+ site has been reported as the homogeneous catalyst to
exhibit good catalytic activity for the conversion of fructose.25-27 Therefore, it is urgently
needed to develop highly thermally stable and cost-effective CoO active sites within the
zeolites crystals for the one-step production of MLA from fructose.
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Herein, we report, for the first time, CoO clusters encapsulated within silicalite-1
crystals catalyst (CoO@silicalite-1) synthesized using a steam-assisted method28 for the
conversion of fructose to MLA in methanol. The structure is characterized by transmission
electron microscopy (TEM), Fourier transform infrared (FTIR), N2 adsorption-desorption,
and X-ray diffraction (XRD). The reason for the immobilization of ultra-stable CoO cluster
within silicalite-1 crystals is suggested by Raman and X-ray adsorption near-edge
spectroscopy (XANES) results. The catalytic activity of CoO@silicalite-1 catalysts with
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different Co loadings and commercial CoOx catalysts were compared for the conversion of
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fructose to MLA.
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2. Experimental Section
2.1 Materials
Silica nanoparticles (Ludox, 30 wt.% in H2O), Co3O4 (99.5%) were purchased from
Mecklin Co., China. Co(NO3)2·6H2O (99%), polyvinyl pyrrolidone (PVP, K29-32), D-
xylose (98%), D-glucose (99.5%), Sucrose (AR) and D-fructose (99%) was purchased from
the Aladdin Industrial Corporation, China. Tetrapropyl ammonium hydroxide (TPAOH, 25
wt.% in H2O), sodium carbonate anhydrous (≥99.8%), NH4Cl (99.5%) were purchased from
Sinopharm Chemical Reagent Co., China. Glycolaldehyde dimethyl acetal (GADMA, 98%)
was purchased from Alfa Aesar. Methoxyacetaldehyde dimethyl acetal (MADDA, 98%),
1,3-dihydroxyacetone (98%) and pyruvaldehyde (40% solution in H2O) were purchased from
J&K Co., China. All reagents were used without further purification.
2.2 Catalyst synthesis
The Co@silicalite-1 catalysts were synthesized according to a steam-assisted method.
Briefly, different amounts of Co(NO3)2·6H2O were mixed with 0.1g PVP in deionized water
and stirred to a clarified solution. Then, colloidal silica and tetrapropyl ammonium hydroxide
(TPAOH) were added into the solution with a molar ratio of 8:1. After stirred 15 mins, the
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solution was evaporated at 50 °C to remove the water. The obtained dry gel was transferred
to a 50 mL Teflon cup, which was placed in a 100 mL Teflon-lined autoclave with water
added outside the cup to create the steam for hydrothermal synthesis, as shown in Figure S1.
Afterwards, the autoclave was heated to 130 °C for 72 h for a moisture-assisted hydrothermal
crystallization. The obtained solid was washed by deionized water and filtrated for three
times, after drying under 60 °C for 12 h, the template was removed by calcination in air at
550 °C for 6 h. Before catalytic reaction, the alkali-treatment was applied to tune the pore
size of catalysts with aqueous solutions of 0.1M Na2CO3 at 60 °C for 2 h, then filtered,
washed with deionized water and dried at 100 °C for 12 h; Finally, the solid was ion-
exchanged with 1 M NH4Cl at 70 °C for three times, after calcined in air at 550 °C for 6 h,
the Co@Silicalite-1 was obtained without further reduction. A series of Co@Silicalite-1
catalysts were identified as Cox, where x is the content of cobalt calculated from ICP-AES
results in Table S1. It might be assumed that the Co loading was limited by the silicon sites,
and the excess Co(NO3)2 was reinstated by steam vapor during the hydrothermal
crystallization and filtrated after washing.
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2.3 Experimental procedures
The batch reactor was made of stainless steel with a volume of 14 mL. In a typical
experimental run, 0.06 g fructose, 0.03 g catalyst and 6 mL of methanol were added into the
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reactor. Then, the sealed reactor was put into a furnace, which had been previously heated to
the setting temperature. The reactor was removed at fixed time intervals of setting times and
quickly placed into cool water to quench the reaction. The products were collected and rinsed
in a 25 mL volumetric flask by methanol and then filtered through a 0.22 μm filter membrane.
The calculated conversion of the reactants and the yield of the products were based on the
equivalent amount of carbon, and the experimental result was the average value of two
replicate experiments.
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3. Results and discussion
3.1 Characterizations
Scheme 1 shows a general route for the synthesis of CoO@Silicalite-1 with different Co
loadings. Co(NO3)2, polyvinylpyrrolidone, tetrapropylammonium hydroxide (TPAOH), and
Ludox first formed a starting dry gel via rotary evaporation. The obtained dry gel was
transferred to a 50 mL Teflon cup, placed in a 100 mL Teflon-lined autoclave with water
added outside the cup to create the steam for hydrothermal synthesis, as shown in Figure S1.
After crystallization and calcination, ultrasmall CoO clusters (~1.7 nm) encapsulated within
silicalite-1 crystals were formed, however, excessive Co loading lead to the aggregation of
CoOx particles and collapse of the silicalite-1 structure. A series of CoO@silicalite-1
catalysts were identified as Cox, where x is Co content determined by inductively coupled
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plasma-atomic emission spectroscopy (ICP-AES) in Table S1.
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X-ray diffraction (XRD) patterns in Figure 1 and S2 were collected with the λ of 0.45226
Å, which was much lower than traditionally used 1.54 Å. As a result, 2θ value in our XRD
results shifted to the lower angle. XRD results revealed the characteristic peaks (ranging from
2 to 8 degrees) ascribed to MFI zeolite structure in Cox (x=0~5.87), which indicated the
formation of silicalite-1. For Co6.50 catalyst, MFI structure was collapsed completely and
Co3O4 crystalline structure was discovered in Figure 1, suggesting excessive Co loading
destroy the structure of silicalite-1 and promote the formation of Co3O4. A representative
transmission electron microscope (TEM) image of Co1.21 in Figure 2d revealed the ordered
porous structure of silicalite-1. The electronic energy-loss spectroscopy (EELS)
measurements in Figure 2a confirmed the existence of ultrasmall CoO with the silicalite-1
framework. The CoO clusters in Co1.21 were further identified as the brighter spots in Figure
2b-c using a high-angle annular dark-field scanning transmission electron microscope
(HAADF-STEM). Several CoO clusters were highlighted by red circles and arrows.
Moreover, the average CoO particle size in Co1.21 was found to be 1.7 nm in Figure 2c.
The silicalite-1 framework with MFI zeolite structure was also verified by Raman spectra
(band at 372 cm-1) and Fourier transform infrared (FTIR) spectroscopy (bands at 550 cm-1
and 1242 cm-1) results in Figure 3a, and Figure 4 respectively. The energy-dispersive X-ray
spectroscopy mapping (EDS-mapping) analysis of Co1.21 in Figure 2e suggested an even
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distribution of Co and O, as well as the existence of isolated CoO clusters. The CoO clusters
should be stabilized in the intersectional void spaces due to narrow MFI channels.29 XRD
results in Figure 1 further demonstrated that no CoO and Co3O4 particles can be found in
Co1.21 catalyst. In comparison, the MFI structure was collapsed completely, and a Co3O4
particles were formed in the Co6.50 catalyst, revealed by TEM (Figure 2f) and XRD (Figure
1) results. This suggested that excessive Co loadings destroy the structure of silicalite-1, and
Co2+ was further oxidized to Co3O4 during the calcination process. The Raman peaks at 458
cm-1 and 497 cm-1 corresponded to Co3O4 was also discovered in Co6.50, but no bands ascribed
to silicalite-1 was seen in Figure 3a. EDS-mapping result of Co6.50 in Figure 2g also proved
the existence of large Co3O4 particles.
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The N2 adsorption-desorption isotherms and pore size distributions of silicalite-1, Co1.21
,
Co1.68 and Co6.50 were shown in Figure 5a. A type I and IV isotherm with a plateau starting at
a very low relative pressure and remarkable hysteresis loop was discovered in silicalite-1,
indicating the hierarchical porous structure. With the increase of Co loading, the adsorption
amount at low relative pressure gradually decreased, indicating the reduction of micropores.
The pore size distribution by Barrett-Joyner-Halenda (BJH) model was shown in Figure 5b.
The co-existence of micropores and mesopores in silicalite-1, Co1.21, Co1.68 samples was
discovered, however, Co6.50 exhibited only mesopores, furthering confirming the collapse of
silicalite-1 framework. The textural properties were summarized in Table S2. The specific
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surface area decreased significantly after modification with Co (silicalite-1, 935 m2/g; Co1.21
,
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462 m2/g; Co1.68, 349 m2/g; Co6.50, 284 m2/g;). Additionally, the micropore surface area also
decreased from 678 m2/g (silicalite-1) to 45 m2/g (Co6.50) when the Co content increased.
Amorphous mesoporous SiO2 became the main structure in Co6.50, in accordance with the
XRD and TEM results.
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X-ray absorption spectroscopy (XAS) was carried out to investigate the correlation
between structural characteristics. Co K-edge XANES spectra for Co1.21 and Co6.50 are shown
in Figure 3b, alongside the spectra of the reference materials CoO and Co3O4.30 Different
spectral features were observed with varying Co loadings. First, the average Co valence was
slightly increased (i.e., oxidized) when the Co loading is increased (Co1.21Co6.50) and
becomes more similar to the valence of Co3O4 (average valence of ~2.7). Interestingly, a
spectral difference between Co1.21 and Co6.50 was observed from the shoulder peak (B),
related to the ligand-to-metal charge transfer process.31 The dominant B peak intensity
indicated the increased degree of covalent character of Co(3d)-O(2p) in the CoO@Silicalite-
1 materials. Therefore, the strong intensity of peak B for the Co1.21 might be the strong
interaction between CoO and silicalite-1.
Very interestingly, the new Raman peaks at 290 cm-1, 469cm-1 and 688 cm-1 were
discovered in Co1.21 and Co1.66, which should be attributed to the twisting vibrations and
stretching vibrations of the Co–O or Co–Si bonds. Combined with XANES, Raman spectra,
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and temperature-programmed reduction (TPR, Figure S3) results, this covalent interaction
between CoO and silicalite-1 was the main factor for the immobilization of CoO clusters
within silicalite-1 crystals. Linear combination fitting of the XANES spectrum for Co6.50
(Figure S4) reveals that the structure of Co6.50 might be a hybrid of Co1.21 and Co3O4.
Figures 3c and 3d show the k2-weighted EXAFS spectra ((k)) and the respective
Fourier transforms of the Co1.21 and Co6.50, as well as the reference Co3O4. In the case of
Co1.21, the amplitude of the oscillations at a higher k region (about 7–12k) is significantly
reduced compared with the amplitudes of the others. This result indicated the CoO might be
well dispersed into the silicalite-1 crystals in the form of ultrasmall clusters, as observed from
the TEM images. With the increase in Co loading, the oscillation of the Co6.50 sample at
higher k regions becomes more pronounced and resembles that of the Co3O4 reference. From
the Fourier transforms (Figure 3d), the contribution of the longer radial distance (high R) was
prominent with the increase in Co loading, and a high degree of similarity of local structure
to Co3O4 was observed for Co6.50, consistently with the Raman and XANES results. The
covalent interaction between CoO and silicalite-1 should be the reason for the stability and
not further oxidation of CoO clusters. Increasing the Co loading on silicalite-1 resulted in the
collapse of the silicalite-1 framework, then leads to the formation of Co3O4 by oxidation of
CoO.
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As for why cobalt stabilized at 2+ state in Co1.21, we suspected that cobalt nitrate (+2)
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was used as the precursor of Co. Thereafter, CoO was obtained during calcination process,
and CoO was not further oxidized to Co3O4 due to the strong interaction between Co and Si
confirmed by XANES and Raman spectra results. In comparison, the structure of silicalite-1
was destroyed in a high Co loading (6.50), and CoO was further oxidized to Co3O4 without
the covalent interaction between Co and Si. Additionally, the CoO particle size (~1.7 nm),
larger than the dimensions of silicalite-1 cage, was due to that CoO particles was in-situ
prepared during the synthesis of silicalite-1. As a result, CoO particles was encapsulated
within silicalite-1 crystals, not dispersed in the silicalite-1 cage, which was in good agreement
with the previous study.24
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3.2 Catalytic activity of different catalysts
To investigate the catalytic activity of CoO@silicalite-1 catalysts for the conversion of
fructose to MLA, the experiment without catalysts was first carried out, and the yield of MLA
was very low. The yield of MLA over pure silicalite-1 was also low due to the absence of
Brønsted acidity (Table 1, entry 1, 18.4%).32 The reason was that Lewis and Brønsted acidity
was found to the common factor for the production of MLA from sugars.33 In comparison,
the commercial CoO and Co3O4 was found to give a higher MLA yield of 37.0 and 34.6%,
respectively (entries 2 and 3). Although oxidized Co catalysts exhibited a comparable activity
to the reported MgO, CaO and ZrO2 catalysts,34 their Co-mass-based activity (MBA, mg
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MLA/mg Co) were less than 1. Additionally, Co6.5, mainly Co3O4 dispersed on mesoporous
SiO2, exhibited a MBA of 7.9 (entry 10). Significantly, a MBA of 620.9 over Co0.13 (entry 5)
was obtained, substantially higher than un-supported CoO, Co3O4 and Co6.5 catalysts.
This “volcano” type variation in catalytic activity was due to the ultrasmall CoO active
site and introduction of Brønsted acidity in CoO@silicalite-1 (seen from FTIR-pyridine
adsorption results in Table S3). The highest MLA yield of 51.2% was discovered over Co1.21
with a MBA value of 100.0 (Table 1, entry 6). To confirm the optimum reaction condition,
the MLA yields and conversion of fructose over Co1.21 at different temperatures and time
were investigated, as shown in Figure 6. The MLA yields kept increasing with reaction time
increased from 0 to 24 h at 160 °C, however, the conversion of fructose was around 80%. A
complete conversion could be achieved at 180 °C after 18 h with the highest yield of 52%.
Although, the higher reaction rate was observed at 200 °C, the occurrence of side reactions
such as dehydration and aggregation leaded to the decrease in MLA yield.35,36 The conversion
of fructose with different fructose/catalyst ratios over Co1.21 at 180 °C was also performed,
and a lower fructose/catalyst ratio contributed a higher MLA yield at the same reaction
conditions (Table S4). The possible reaction pathway and intermediates (1,3-
dihydrooxyactone: DHA; pyruvaldehyde; methyl pyruvate) were shown in Figure S5. With
the increase of Co loading from 0.13 to 6.50, the yield of MLA increased firstly then
decreased. The increase in MLA yield from Co0.13 to Co1.21 was attributed to the introduction
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of more active sites (CoO clusters). The decrease of MLA yield with further increase of Co
loading was ascribed to the formation of larger particles and collapse of silicalite-1
framework, revealed by TEM and XRD results. In addition to MLA, the by-products
glycolaldehyde dimethyl acetal (GADMA) and methoxyacetaldehyde dimethyl acetal
(MADDA) in Figure S5 were also detected in our reaction system, but the total yield was
less than 5%. The other by-products were almost not found in the final products, which leaded
to a relatively low carbon balance. The low carbon balance was ascribed to the
polymerization and dehydration reaction to humins owing to the presence of the strong
acidity.5,37
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In order to figure out the correlation of catalyst activity with structures, we have
compared the catalytic performance of silicalite-1, CoO, Co3O4 and Co1.21 for the conversion
of different intermediates (DHA and pyruvaldehyde in Figure S5). As shown in Table S5,
the yield of MLA over blank silicalite-1 from the conversion of both DHA and
pyruvaldehyde was very low, which was similar to the result without catalysts used (~12.5%).
In comparison, CoO, Co3O4 and Co1.21 showed almost the same MLA yield from the
conversion of DHA after 6 h compared with the results from the conversion of fructose after
18 h. Moreover, these three catalysts gave the higher MLA yield from the conversion of
pyruvaldehyde, since side reactions (isomerisation) were effectively suppressed. The above
results exhibited that blank silicalite-1 had almost no catalytic activity towards to the
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conversion of fructose and intermediates due to the low acidity. Oxidation state of cobalt was
useful for the retro-aldol reaction, dehydration and esterification, and the catalytic activity of
CoO was slightly better than Co3O4. The rate-determining step (retro-aldol reaction) could
be catalyzed by CoO, and the silicalite-1 provided a place for the ultrasmall CoO clusters due
to the formation of Si-Co bonds. Additionally, the conversion of other monosaccharides and
disaccharide sucrose over Co1.21 were also discussed, shown in Table S7. The MLA yield
from the conversion of fructose and sucrose exhibited a similar result, much higher than that
obtained from the conversion of glucose and xylose.
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3.3 Catalyst stability
The stability of the catalyst is of great importance for commercial processes. The spent
catalysts were recovered by filtration and washed thoroughly with methanol and dried with
or without calcination in air. The yield of MLA over spent Co1.21 catalyst without calcination
decreased significantly after four reaction cycles (52% to 23%, Figure S6, Table 1, entry 11).
The thermogravimetric (TGA) analysis in Figure S7 revealed that the carbon deposition
should be responsible for the reduced reactivitiy. On the contrary, the spent Co1.21 calcination
at 550 °C for 6 h maintained original activity (about 50% MLA yield) even after being used
four times (Figure S6, Table 1, entry 12). Therefore, the catalytic activity of spent Co1.21
catalyst can be easily recovered by the facile calcination, and no CoO clusters aggregation
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can be found in Figure S2 due to the strong covalent interaction between CoO clusters and
silicalite-1.
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4. Conclusions
In summary, ultrasmall CoO clusters (~1.7 nm) stabilized within silicalite-1 crystals
catalyst have been successfully synthesized and used for the conversion of sugar to MLA in
near-critical methanol. The ultrasmall CoO clusters was revealed using HAADF-STEM,
EDS-mapping, and XRD experiments. Additionally, a strong covalent interaction between
CoO and silicalite-1 was validated and considered as the main reason for the confinement of
CoO clusters. The CoO@silicalite-1 catalyst exhibits a nearly 100-fold higher Co-mass-
based activity for the production of MLA compared to CoO or Co3O4 particles outside the
silicalite-1 framework. The metal particle aggregation and collapse of silicalite-1 framework
with the increase of Co loading leads to the relatively poor catalytic activity. Under the
optimum reaction conditions, the 52.0% MLA yield was achieved at 180 °C over Co1.21
.
More importantly, this catalyst exhibits superior reuse performance with the removal of coke
by facile calcination.
Supporting Information
Additional results of catalyst characterization, analysis method, possible reaction pathway,
characterization results (XRD, ICP, TGA, N2 adsorption-desorption, TPR, pyridine-FTIR
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and Co K-edge XANES spectrum) and a series of catalytic performance of Co1.21
.
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Acknowledgments
This work was supported by the National Natural Science Foundation of China (No.
21436007, 21676243, 21706228), the Zhejiang Provincial Natural Science Foundation of
China (No. LR17B060002), and the Fundamental Research Funds for the Central
Universities (No. 2018QNA4038). SD was supported by the Division of Chemical Sciences,
Geosciences, and Biosciences, Office of Basic Energy Sciences, US Department of Energy.
The work at Brookhaven National Laboratory was supported by the U.S. Department of
Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle
Technologies, under Contract Number DE-SC0012704.
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Scheme 1. Synthesis route of CoO@silicalite-1 with different Co loadings
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Co
O
:
PDF#42-1467
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Silicalite-1: PDF#44-0696
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
Co O :
3 4
Silicalite-1: PDF#44-0696
PDF#42-1467


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Co6.50
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Co1.66
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Figure 1. XRD pattern of blank silicalite-1, Co1.21, Co1.66 and Co6.5 catalysts.
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Figure 2. HADDF-STEM images (a-c) of Co1.21 and corresponding EELS profiles (a) acquired from the
frames I, II and statistical analyses of the mean particle size (c); A representative TEM image of Co1.21
(d) and Co6.50 (f); STEM-EDS elemental mapping of Co1.21 (e) and Co6.50 (g) images, CoO clusters are
marked by red circled in (c) and red arrows in (b), Co (red), Si (yellow), O (green).
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Co1.66
Co1.21
Silicalite-1
Co3O4
300
400
500
600
700
800
900
1000
Raman shifts/cm-1
c
b
d
Figure 3. (a) Raman spectra of silicalite-1, Co1.21, Co1.66, Co6.50, Co3O4, and CoO. (b) Co K-edge
XANES spectra of Co1.21 and Co6.50 in comparison with the reference spectra for CoO and Co3O4. (c)
k2-weighted (k) spectra and (d) Fourier transform (FT) magnitude of the k2-weighted EXAFS spectra
of Co1.21 and Co6.50 with the reference Co3O4 spectra. The FT was not phase corrected, so the actual
bond length may be ~0.2–0.4 Å longer.
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Silicalite-1
Co0.63
Co1.21
Co1.66
8
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Co3.51
Co5.87
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Co6.50
500
750
1000
Wavenumber/cm-1
1250
1500
Figure 4. FTIR spectroscopy of CoO@Silicalite-1 materials with different Co loading.
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8
800
600
400
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0
Silicalite-1
Co1.21
Co1.66
Co6.50
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0.0
0.2
0.4
0.6
0.8
1.0
P/P0
(a)
Silicalite-1
Co1.21
0.08
0.06
0.04
0.02
0.00
4
Co1.68
Co6.50
3
2
1
0
0
10
20
30
40
50
Pore Size/nm
0.5
0.6
0.7
0.8
Pore Size/nm
(b)
Figure 5. N2 adsorption-desorption isotherms (a) of silicalite-1, Co1.21, Co1.68 and Co6.50 samples and
corresponding pore size distribution (b).
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