Atomically Dispersed Co Catalyst for Efficient Hydrodeoxygenation of Lignin-Derived Species and Hydrogenation of Nitroaromatics

Article abstract of DOI:10.1021/acscatal.0c00239

Single-atom catalysts (SACs) have attracted much attention due to their outstanding catalytic performance in heterogeneous catalysis. Here, we report a template sacrificial method to fabricate an atomically dispersed Co catalyst; three kinds of silica templates with different microstructures (MCM-41, SBA-15, and FDU-12) were employed and the effect of pore structure of the templates on the dispersity of Co was investigated. The catalysts fabricated with different templates presented different Co dispersities, leading to distinguishing catalytic performance. The optimized Co1?NC-(SBA) catalyst with atomically dispersed Co displayed outstanding catalytic activity for the hydrodeoxygenation (HDO) of lignin-derived species as well as the hydrogenation of various nitroaromatics. The reaction mechanism of the HDO of vanillin was investigated by using density functional theory calculations as well.

Full text of DOI:10.1021/acscatal.0c00239

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Article
Atomically Dispersed Co Catalyst for Efficient Hydrodeoxygenation
of Lignin-derived Species and Hydrogenation of Nitro-aromatics
Longkang Zhang, Ningzhao Shang, Shutao Gao, Junmin Wang, Tao Meng, Congcong Du, Tongde
Shen, Jianyu Huang, Qiuhua Wu, Hai-Jun Wang, Yuqing Qiao, Chun Wang, Yongjun Gao, and Zhi Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.0c00239 • Publication Date (Web): 10 Jul 2020
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Atomically Dispersed Co Catalyst for Efficient Hydrodeoxygenation of
Lignin-derived Species and Hydrogenation of Nitro-aromatics
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Longkang Zhang, † Ningzhao Shang, † Shutao Gao, †* Junmin Wang, Tao Meng, Congcong
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Du, Tongde Shen, Jianyu Huang, Qiuhua Wu, Haijun Wang, Yuqing Qiao, * Chun Wang,
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Yongjun Gao, * and Zhi Wang,
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College of Science, Hebei Agricultural University, Baoding 071001, China
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College of Chemical and Environmental Science, Hebei University, Baoding 071000, China
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Clean Nano Energy Center, State Key Laboratory of Metastable Materials Science and
Technology, Yanshan University, Qinhuangdao 066004, China
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College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004,
China
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ABSTRACT
Single atom catalysts (SACs) have attracted much attention due to their outstanding catalytic
performance in heterogeneous catalysis. Here, we reported a template sacrificial method to fabricate
atomically dispersed Co catalyst, three kinds of silica template with different microstructure
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(MCM-41, SBA-15 and FDU-12) were employed and the impact of pore structure of the templates
on the dispersity of Co was investigated. The catalysts fabricated with different template presented
different Co dispersity, leading to distinguishing catalytic performance. The optimized Co @NC-
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(SBA) catalyst with atomically dispersed Co displayed outstanding catalytic activity for the
hydrodeoxygenation (HDO) of lignin-derived species as well as the hydrogenation of various nitro-
aromatics. The reaction mechanism of HDO of vanillin was investigated by using density functional
theory calculations as well.
KEYWORDS: single atom Co, lignin-derived species, vanillin, nitroarenes, hydrodeoxygenation,
hydrogenation, density functional theory
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INTRODUCTION
The conversion of biomass into value-added aromatic chemicals and liquid fuels has been
considered as an effective way to reduce the world dependence on fossil resources and environment
pollution simultaneously.1-4 Lignin, a representative example of biomass, can be transformed into
phenolic compounds such as bio-oil by fast pyrolysis approach. However, the high oxygen content
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and unsaturated degree of the obtained products always make them undesirable due to their high
viscosities, low energy density, low heating value, low chemical and thermal stability.5-6 Thus,
many strategies such as hydrotreating, hydrodeoxygenation (HDO), hydrocracking and steam
reforming of the model species of lignin-derived bio-oil have been proposed to reduce the oxygen
concentration.7-9 Among them, hydrodeoxygenation is an effective bio-oil upgrading strategy and
has received intensive attention.
Vanillin, a typical model substance of lignin-derived bio-oil, can be selectively converted to 2-
methoxy-4-methylphenol (MMP), which has been widely used as fragrances or the intermediates of
drugs.10 Owing to the distinguished catalytic performance of precious metal-based catalysts,
tremendous efforts have been devoted to develop high-performance noble metal catalysts such as
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and Pd14-18 for the HDO of vanillin. Nevertheless, the practical applications of noble
Au, Ru
metal-based catalysts are hindered by their prohibitive cost and low reserves. In this regard, the
development of highly efficient and economic catalysts for the upgrading of bio-oil is of great
significance. Several non-noble metal-based catalysts have been developed for the HDO of
vanillin.19-27 For example, Petitjean reported that Cu-PMO can convert vanillin into MMP with 90%
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yield at the condition of 180 °C, 4 MPa H , 18 h. Although Ni/NCB-900 can also catalyze the
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HDO of vanillin, but the selectivity of MMP was poor under mild reaction conditions. Jiang and
co-workers developed a non-noble metal catalyst named Co/N-C-600, in which the cobalt
nanoparticles were supported on nitrogen-doped carbon. Co/N-C-600 can converted vanillin into
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MMP with 99% yield in 8 h at 150 °C, 1 MPa H . Ga-doped Cu supported on H-nanozeolite-Y
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(Cu/HNZY) was fabricated by Verma et al and employed to catalyzing the HDO of vanillin. 99%
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selectivity was achieved with methanol as solvent after 2 h reaction at 160 °C. Vanillin was
converted to MMP with 87% yield in 12 h at the condition of 150 °C, 2.5 MPa H over Cu-Ni/CeO -
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SiO catalyst. ZnO/Co@N-CNT can also catalyze the HDO of vanillin, but the yield of MMP was
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low (89.1%) and the dosage of metal was large. Despite recent progress, the reported non-noble
metal-based catalysts still suffer one or more shortcomings, such as high catalyst loading, high H2
pressure, poor selectivity and long reaction time. Thus, exploring highly efficient non-noble metal-
based catalysts for the HDO of lignin-derived species still remains a challenge.
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Single atom catalysts (SACs), as a rising star in catalysis, have attracted broad attention since it
shows the merits of maximize atomic utilization, well-defined mononuclear structures and
remarkably promoted catalytic properties.28-30 The SACs have already been employed in the field of
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electrocatalysis (CO reduction, O reduction, H reduction reaction ), photocatalysis (CO
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reduction34-35 and H evolution reduction ) and catalytic hydrogenation reaction (alkynes and
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styrene38 hydrogenation, nitroarene hydrogenation ), oxidation reaction (benzene oxidation,
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glucose oxidation,41 selective oxidation of alcohols42 and C-H bonds ) and other coupling
reactions.44-45 Thanks to the properties of higher metal loading and exposing more active sites on
the support surface of the obtained SACs, sacrificial hard template method received more attention
for the preparation of SACs compared with other thermal condensation strategies.46-49 For example,
Xu et al 50 employed Fe-phenanthroline as the precursor and SBA-15 as the hard template, after
cacilined at 950 °C followed by alkali and acid leaching, the obtained Fe SAs/MC exhibited
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excellent ORR activity and durability. Zhang and co-workers fabricated single atom catalyst Ni-
N-C by employing Ni-phenanthroline as the precuesor and MgO as the template, which was active
for hydrogenation of various unsaturated substrates and extraordinary durability for cellulose
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conversion under harsh conditions. ISAS-Co/OPNC and Co-N-C were also fabricated by hard
template assistant method and exhibited outstanding catalytic performance for catalyzing reaction.
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Until now, only single atom Ru catalyst, Ru /mpg-C N , was reported for the HDO of vanillin.
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Considering the economic benefits and catalytic efficiency, non-noble metal SACs with improved
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catalytic activity and selectivity are highly desirable.
In this paper, Co supported on nitrogen-doped carbon catalysts were fabricated with
Co(NO ) ·6H O and glycine as the precursor and three kinds of SiO (MCM-41, SBA-15 and FDU-
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12) with different microstructure as the hard template, respectively. The impact of pore structure of
the templates on the dispersity of Co was investigated for the first time. The coordination of metal
with ligands as well as the introduction of hard template might increase the dispersion of metal and
endow the catalysts with more exposed active sites and high surface area, which might render the
catalysts with enhanced activity. After calcinated at 800 °C followed by alkali leaching, atomically
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dispersed Co supported on porous nitrogen-doped carbon (Co @NC-(SBA)) was obtained with
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SBA-15 as the template (Scheme 1). The Co @NC-(SBA) was characterized by high-angle annular
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dark field aberration-corrected scanning transmission electron microscopy (HAADF-STEM) and X-
ray absorption spectroscopy (XAS). Co @NC-(SBA) manifested outstanding catalytic performance
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for the hydrodeoxygenation of lignin-derived species as well as the hydrogenation of series
nitroarenes.
Scheme 1. Schematic illustration for the formation of Co @NC-(SBA).
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EXPERIMENTAL SECTION
Preparation of FDU-12. The ordered pouous silica FDU-12 was prepared according to the
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reported method reported by Bao et al. Briefly, 0.50 g of Pluronic F127, 0.60 g of (TMB) and 2.5
g of KCl were dissolved in 30 ml of 2 M HCl at 15 ± 0.1°C. After 2 h stirring, 2.08 g of tetraethyl
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orthosilicate (TEOS) was added to this solution. After stirring for 24 h at 15 C, the mixture was
transferred into an autoclave and heated at 100°C for 24 h. The as-made products were obtained by
filtration and dried at room temperature in air. For the high-temperature hydrothermal treatment,
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mixture was heated at desired temperature for another 48 h. The obtained solid was calcinated at
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50 °C for 4h (5 °C/min, N ) to remove the organic templates.
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Preparation of Series Catalysts. Co @NC-(SBA) was fabricated by using a support-sacrificing
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method. Briefly, 2 g SBA-15 and 2 g glycine were first dissolved in a solution of 100 mL water and
00 mL ethanol, then 0.85 mmol Co(NO ) ·6H O in 10 mL water was mixed with the above
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solution. The resulting pink suspension was refluxed at 80°C for 12 h followed by removing the
solvent via rotary evaporation at 70°C. The obtained powder was calcinated at 800°C for 2 h
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5 °C/min, N ) followed by etching SBA-15 template and Co nanoparticles with 6 M NaOH at 60°C
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for 24 h.
For comparison, Co@NC-(FDU) and Co@NC-(MCM) were synthesized by the above method
except that SBA-15 was replaced by FDU-12 and MCM-41, respectively. Cu@NC-(SBA) and
Ni@NC-(SBA) and Fe@NC-(SBA) were prepared in the same way of Co @NC-(SBA) except that
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.85 mmol Cu(NO ) ·4H O, 0.85 mmol Ni(NO ) ·6H O and 0.85 mmol Fe(NO ) ·9H O were
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added to replace Co(NO ) ·6H O, respectively.
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Co@NC was prepared as follows: 2 g glycine were dissolved in a solution of 50 mL water and 50
mL ethanol, then 251 mg Co(NO ) ·6H O in 10 mL water was mixed with the above solution. The
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obtained pink solution was kept stirring for 6 h at room temperature followed by rotary evaporation
at 70 °C to remove the solvent. The pink powder was calcined at 800 °C for 2 h at a heating rate of
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°C/min under the protection of N and then cooled to room temperature naturally. The obtained
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material was grinded into powder, named as Co@NC.
Procedure for the Hydrogenation Reaction. The catalytic hydrogenation of lignin-derived
species was carried out in a 50 mL Teflon-lined autoclave. Typically, 0.5 mmol of substrate, 30 mg
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of Co @NC-(SBA) and 4 mL of water were added into the autoclave, which was purged with
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nitrogen gas and then purged with hydrogen for four times, respectively. The final H pressure was
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set at 1 MPa. The autoclave was heated to 140 °C for a certain time with stirring at 1000 rpm. Cold
water was employed to cool down the autoclave quickly after the reaction finished. The products
were extracted by dichloromethane and analyzed by GC-MS. The catalyst was separated by
filtration and washed with ethanol, and then dried at 60°C for further use.
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The hydrogenation of nitroarenes was carried out in a 50 mL Teflon-lined autoclave. Typically,
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autoclave which was purged with nitrogen gas and then purged with hydrogen for four times,
respectively. The final H pressure was set at 1 MPa. The autoclave was heated to 90 °C for a
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was separated by filtration and washed with ethanol, and then dried at 60 °C for further use.
RESULTS AND DISSCUSION
Characterization of the Catalysts. High resolution transmission electron microscope (HRTEM)
images showed that MCM-41, SBA-15 and FDU-12 with different pore size all exhibited ordered
pore structure (Figure 1, a-c) and the Barrett-Joyner-Halenda (BJH) median pore width of MCM-41,
SBA-15 and FDU-12 are 2.66, 11.10 and 17.20 nm, respectively (Table S1 and Figure 2a-b). The
Brunner-Emmet-Teller (BET) surface area of MCM-41, SBA-15 and FDU-12 were 1065, 619 and
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475 m /g, respectively (Table S1).
When the templates were etched away, Co@NC-(MCM) fabricated with MCM-41 as the
template exhibited layered structure and no obvious pores can be observed (Figure 1d). Co @NC-
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(SBA) fabricated with SBA-15 as the template possessed an ordered porous structure (Figure 1e).
HRTEM image of Co@NC-(FDU) (Figure 1f) fabricated with FDU-12 as the template showed that
its pore structure was complementary to that of FDU-12, but the pores of Co@NC-(FDU) partly
collapsed. The BET surface area and pore size of the prepared catalysts were listed in Table S1 as
well. Co @NC-(SBA) possessed largest BET surface area, pore size as well as pore volume. No
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obvious cobalt particles were detected in the HRTEM images of the prepared catalysts, signifying
the cobalt was anchored on the support with high dispersity. Besides, the X-ray diffraction (XRD)
patterns of Co@NC-(MCM), Co @NC-(SBA) and Co@NC-(FDU) only displayed a broad peak at
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θ = 24°, corresponding to the (002) plane of graphitic carbon, no diffraction peak of Co was found,
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which further demonstrated the well dispersion of the cobalt sites (Figure S1). As for Co@NC
prepared in the absence of a hard template, obvious cobalt particles were observed in the HRTEM
image (Figure S2). XRD patterns of Co@NC showed distinct peaks at 44.2°, 51.5°, and 75.8°
(Figure S1), which correspond to the (111), (200) and (220) planes of metallic Co (JCPDS No. 15-
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0806), respectively. The BET surface area, pore volume and pore size of Co@NC were 433 m /g,
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.51 cm /g and 2.25 nm, respectively (Table S1, entry 7).
Figure 1. HRTEM images of MCM-41 (a), SBA-15 (b), FDU-12 (c), Co@NC-(MCM) (d), Co @NC-(SBA) (e)
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and Co@NC-(FDU) (f). HAADF-STEM images of Co@NC-(MCM) (g), Co @NC-(SBA) (h) and Co@NC-(FDU)
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(i).
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Furthermore, high-angle annular dark-field scanning transmission electron microscopy (HAADF-
STEM) was performed to characterize the dispersion of Co in the prepared catalysts. Both Co
nanoparticles or clusters were observed in Co@NC-(MCM) (Figure 1g) and Co@NC-(FDU)
(Figure 1i). As shown in Figure 1h, the observed brighter spots (highlighted by red circles) were
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identified as isolated Co single sites in Co @NC-(SBA), and no cobalt cluster or nanoparticles was
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Figure 2. N sorption isotherms (a) and BJH pore size distributions (b) of the employed SiO templates. N
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sorption isotherms (c) and BJH pore size distributions (d)of the series catalysts.
Based on the results of BET data, HRTEM and HAADF-STEM analysis, we can conclude that
the SiO templates with different pore structure directly affect the morphologies of the prepared
2
catalysts and the dispersity of cobalt. For Co@NC-(MCM), Co(NO ) ·6H O-glycine precursor was
3 2
2
hard to transfer into the narrow pore of MCM-41, part of Co(NO ) ·6H O-glycine precursor might
3
2
2
adsorbe on the surface of MCM-41, which led to the agglomeration of cobalt during the
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carbonization process. For FDU-12, although the Co(NO ) ·6H O-glycine precursor can migrate
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into the pores of FDU-12 easily, but the relatively large pores of FDU-12 did not have a strong
confinement and anchoring effect for Co complex during the pyrolysis process, resulting in the
formation of cobalt cluster. However, when SBA-15 was employed as the template, the
Co(NO ) ·6H O-glycine can be anchored in the pores of SBA-15 with moderate pore size via
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hydrogen bonding interaction between the silicon hydroxyl in SBA-15 and the amino and carboxyl
groups in glycine. The confinement effect of SBA-15 with proper pore structure might prevent the
cobalt from aggregation during the carbonization process, guaranteeing the cobalt were
homogenously dispersed on the ordered nitrogen-doped carbon support after etching the template
away. The results of inductively coupled plasma atomic emission spectrometry (ICP-AES) can also
provide some evidence. The Co content in Co@NC-(MCM), Co @NC-(SBA) and Co@NC-(FDU)
1
measured by ICP-AES were 5.3 wt%, 3.2 wt% and 3.8 wt%, respectively, where Co@NC-MCM
showed highest content of Co as compared with Co@NC-SBA and Co@NC-FDU. The results
indicated the formation of more Co clusters in the catalysts when MCM-41 and FDU-12 were
employed as the templates. In addition, compared with MCM-41 and FDU-12, the unique three-
dimensional interconnected channel structure of SBA-15 would provide Co @NC-(SBA) with
1
much more ordered pores, leading to expose more catalytic active sites.
X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure
(EXAFS) measurements were performed to analyze the electronic structure and coordination
environment of Co @NC-(SBA). As shown in Figure 3a, the absorption edge position of Co @NC-
1
1
(SBA) was located between that of CoO and Co O , suggesting that Co in the Co @NC-(SBA)
3
4
1
2+
3+ 57
carried a positive charge and the valence state of the Co was between Co and Co . The Fourier-
transformed (FT) k -weighted EXAFS spectrum of Co @NC-(SBA) displayed a strong peak at 1.35
3
1
Å, corresponding to the Co-N or Co-C (Figure 3b), the absence of Co-Co coordination peak (2.18 Å)
further excluded the presence of metallic cobalt nanoparticles and confirmed that the Co in
5
8
Co @NC-(SBA) sample was atomically dispersed. Furthermore, the EXAFS wavelet transform
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(WT) plot (Figure 3c) of Co @NC-(SBA) showed only one intensity maximum at around k = 4 Å ,
1
corresponding to the Co-N/C coordination. Compared with the WT plots of Co foil, CoO, Co O
3
4
and CoPC (cobalt porphyrin) in Figure 3c, no WT maximum corresponding to Co-Co bonding can
be observed in the R range from 1~3 Å.59-60 Quantitative EXAFS fitting was then conducted to
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extract the structural parameters and obtain the precise coordination configuration of Co atom for
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Co @NC-(SBA). The Co K-edges k χ (k) oscillation curve in Figure 4d suggested the distinct local
1
atomic arrangement of Co @NC-(SBA), which was consistent with reported literature.61-62 The
1
best-fitting analyses (Figure 3e) clearly presented a main peak originating from Co-N/C first shell
coordination. As shown in Table S2, the Co-N shell presented a coordination number of 5.3 at a
distance of 1.91 Å. The error of the best-fitting results was within an acceptable error.
Figure 3 XANES spectra (a) and Fourier transform (FT) EXAFS at the Co K-edge (b) of Co foil, CoO, Co O ,
3
4
3
CoPC and Co @NC-(SBA). Wavelet transform (WT) for the k -weighted EXAFS signals of Co foil, CoO, Co O ,
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3
4
CoPC and Co @NC-(SBA) (c). EXAFS spectra of Co @NC-(SBA) and Co foil at K space (d). The corresponding
1
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EXAFS fitting curves of Co @NC-(SBA) at R space (e), the inset of (e) is the schematic model of Co @NC-
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(SBA), Co (pink), N (blue), O (red), C (grey).
The chemical states of Co and N in Co @NC-(SBA) were also analyzed by X-ray photoelectron
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spectroscopy (XPS). As shown in Figure S3a, the binding energy (BE) of Co 2p3/2 was located at
6
3
781.1 eV, corresponding to the Co-N sites. The N 1s XPS spectrum can be deconvoluted into
x
three peaks at 398.3, 400.7 and 404.8 eV (Figure S3b), corresponding to pyridinic N, graphitic N
52
and chemisorbed N-oxide, respectively. The predominance of pyridinic N may coordinate with
cobalt and served as an anchor for stabilizing the atomically dispersed cobalt sites more efficiently.
The Co 2p3/2 peaks of Co@NC-(MCM) (Figure S3c) with a BE of 779.6, 780.9 and 782.2 eV were
attributed to Co O464 and the Co 2p3/2 peak with a BE of 781.1 eV was attributed to Co-N . The Co
3
x
2
p3/2 peaks of Co@NC-(FDU) (Figure S3d) with a BE of 780.0 and 782.1 eV can be assigned to
6
4
CoO and the Co 2p peak with a BE of 781.1 eV was attributed to Co-N . The content of Co-N
x
3
/2
x
in Co@NC-(MCM) and Co@NC-(FDU) were about 6.6% and 14.4%, respectively, which is much
lower than that in Co @NC-(SBA). No metallic Co peak was found in the spectrum Co@NC-
1
(MCM) and Co@NC-(FDU), the presence of cobalt oxide was due to the surface oxidation of Co
nanoparticles or clusters in Co@NC-(MCM) and Co@NC-(FDU) during storage in the air.65
Vanillin Hydrodeoxygenation. The HDO of vanillin was selected as a model substrate to
examine the catalytic activity of the series catalysts. The catalytic conditions including different
catalysts, reaction temperature, initial H pressure and solvent were optimized. The catalysts
2
fabricated with different microstructure and degrees of Co dispersity exhibited distinguishing
catalytic performance for the HDO of vanillin (Table 1). No conversion was observed with Co@NC
contained large Co nanoparticles as catalyst (Table 1, entry 1), which was fabricated by directly
carbonization of Co(NO ) ·6H O-glycine complex complex without a template. 20.3% yield of
3
2
2
MMP was obtained in 3 h when the reaction was performed with Co@NC-(MCM) as catalyst, the
yield was only increased to 25.3% when the reaction time was extended to 6 h (Table 1, entries 2-3).
For Co@NC-(FDU) catalyst, 32.6% and 57.6% yield of MMP were achieved after 3 h and 6 h
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reaction, respectively (Table 1, entries 5-6). The results revealed that the cobalt in the form of
nanoparticles or clusters displayed a relatively poor catalytic activity toward the HDO of vanillin.
To our delight, vanillin can be transformed to MMP with 99.2% yield in H O-MeOH (1:1, v/v) over
2
Co @NC-(SBA) at 140 °C, 1 MPa H within 3 h (Table 1, entry 4). According to the results of XPS,
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the content of Co-N in Co @NC-(SBA) is much higher than that in Co@NC-(MCM) and
x
1
Co@NC-(FDU), which further demonstrated the Co-N sites were the active sites for the HDO of
x
vanillin. We also prepared Fe@NC-(SBA), Ni@NC-(SBA) and Cu@NC-(SBA) to investigate the
effect of the metal type on the catalytic performance (Table 1, entries 7-9), only Fe@NC-(SBA)
was active for the HDO of vanillin, 7.5% yield of MMP was achieved in 3 h.
Table 1. HDO catalytic performance of vanillin a
Time Conv. MMP Yield
Entry
Catalysts
(
h)
3
3
6
3
3
6
3
3
3
(%)
(%)
1
2
3
4
5
6
7
8
9
Co@NC
0
0
Co@NC-(MCM)
Co@NC-(MCM)
25.7
31.6
100
43.2
88.9
8.2
0
20.3
25.3
99.2
32.6
57.6
7.5
0
Co @NC-(SBA)
1
Co@NC-(FDU)
Co@NC-(FDU)
Fe@NC-(SBA)
Ni@NC-(SBA)
Cu@NC-(SBA)
0
0
a
Reaction conditions: 0.5 mmol vanillin, 3.25 mol% Co,1 MPa H ,140 °C, 2 mL H O, 2 mL MeOH.
2
2
The reaction pathway of the HDO of vanillin over Co @NC-(SBA) in H O-MeOH (1:1, v/v) was
1
2
further investigated (Figure 4a). In the initial reaction stage, the reaction was carried out with a mild
increase in the conversion of vanillin and the yield of MMP. The yield of HMP increased until 30
min and then decreased slowly. 68.1% conversion of vanillin and 53% yield of MMP were achieved
within 1 h. When the reaction time prolonged to 3 h, vanillin was completely converted and the
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selectivity for MMP was 99.2%. It should be mentioned that the formation of 4-hydroxymethyl-2-
methoxyphenol (HMP) and 2-methoxy-4-(methoxymethyl) phenol (DMP) were relatively low
(<10%) in the whole reaction process (Figure 4a, S4).
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Figure 4. Optimization of the reaction conditions for HDO reaction of vanillin over Co @NC-(SBA). Reaction
1
conditions: 0.5 mmol vanillin, 30 mg Co @NC-(SBA) (3.25 mol% Co): Effect of reaction time. 1 MPa H , 2 mL
1
2
H O, 2 mL MeOH, 140 °C (a). Effect of reaction temperature. 1 MPa H , 2 mL H O, 2 mL MeOH, 3 h (b). Effect
2
2
2
of H pressure. 2 mL H O, 2 mL MeOH, 140 °C, 3 h (c). Effect of the type of solvent. 1 MPa H , 4 mL solvent,
2
2
2
1
40 °C, 3 h (d).
The reaction temperature played a significant role in the conversion of vanillin and selectivity of
MMP. As shown in Figure 4b, the HDO of vanillin was not occurred when the temperature was
lower than 100 °C. HMP and DMP were formed when the reaction temperature was between
1
00 °C and 140 °C. The increment of temperature led to the improvement of vanillin conversion
and MMP selectivity. Vanillin was completely converted to MMP when the reaction was executed
at 140 °C. The impact of initial H pressure on the hydrodeoxygenation of vanillin was also
2
investigated at 140 °C for 3 h. As shown in Figure 4c, increasing the initial H pressure can
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accelerate the conversion of vanillin and the yield of MMP with the H pressure increase from 0.25
2
MPa to 1 MPa, which can be ascribed to the capacity for adsorption of H . Complete conversion of
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vanillin and excellent MMP yield of 99.2% was achieved with 1 MPa H pressure.
2
The influence of reaction solvent for the HDO of vanillin over Co @NC-(SBA) catalyst was
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further conducted (Figure 4d). Among the various solvent examined, high conversion of vanillin
was achieved in the common solvents including water, isopropanol (IPA), ethanol, methanol,
tetrahydrofuran (THF) and ethyl acetate (EA). When water was employed as the solvent, 99%
conversion was achieved in 2 h, no HMP and other by-products were detected, whereas the mass
balance of MMP was only 64%. Besides, when HMP was selected the substance, HMP was
completely converted in 1 h with water as the solvent and the MMP yield was 52%. According to
the previous reports, the loss in mass balance was mainly caused by the polymerization of the
substrates and the oligomer was formed during the polycondensation process.20, 66 When methanol
was employed as the solvent, the mass balance was 100% when the reaction time prolonged to 3 h,
but the conversion of vanillin and the yield of MMP were 90% and 86.8%, respectively. Vanillin
was completely converted to MMP with 99.2% yield in 3 h when methanol-H O (1:1, v/v) was used
2
as the solvent. It can be concluded from above results that the selectivity of MMP was mainly
affected by the reaction temperature, reaction time and solvent rather than H pressure.
2
Scheme 2. Reaction pathways of hydrodeoxygenation of vanillin.
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For comparison, control experiment for hydrogenolysis of HMP was conducted and the results
were shown in Figure S5. Nearly 100% conversion of HMP was achieved in the first 40 min. As the
reaction conducted further, the main initial product, DMP, was gradually converted to MMP via
hydrogenolysis. These results indicated that the transformation of HMP to MMP was much easier
than that of vanillin to MMP. So, when vanillin was used as substrate, the content of HMP and
DMP during the whole reaction was low (Figure 4a).
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0
To further reveal the reaction mechanism, the HDO of the mixture of vanillin and HMP was also
conducted under the optimal reaction conditions. When the mole ratio of vanillin and HMP was 1:1
(0.25 mmol vanillin and 0.25 mmol HMP) and 9:1, the conversion of vanillin after 2 h reaction was
1
2% and 18%, respectively, which were much lower than that with only vanillin as substrate; while
HMP was coverted completely. The results demonstrated that the presence of HMP may inhibit the
conversion of vanillin to some extent.
The kinetic studies were further conducted to obtain more insights into the conversion of vanillin
to MMP. The calculated activation energy of the hydrogenation of vanillin to HMP and the
hydrogenolysis of HMP to MMP based on the slope value of Arrhenius plots (Figure S6) were
130.2 kJ/mol and 86.5 kJ/mol, respectively. The activation energy of the HMP hydrogenolysis
reaction was lower than that of vanillin hydrogenation, which further suggested that the
hydrogenolysis of HMP was more easily than the hydrogenation of vanillin.
The possible reaction pathways of the HDO of vanillin were proposed in Scheme 2. Previously, it
has been reported that two reaction pathways existed for the transformation of a carbonyl group into
a methyl group in aqueous solution, MMP can be obtained by the direct hydrogenolysis of vanillin
(Scheme 2, path 1) or the hydrogenation of vanillin into HMP, and subsequent hydrogenolysis of
1
7
HMP into MMP (Scheme 2, path 2a). As for the HDO of vanillin in methanol media, it was
reported that the reaction can also be proceeded via the path 2a; moreover, the presence of DMP
25
revealed that another pathway existed, in which DMP was formed by the o-methylation of HMP,
followed by the hydrogenolysis of DMP to produce MMP (Scheme 2, path 2b). On the basis of the
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time course reaction data and the kinetic studies, we proposed that MMP was mainly obtained via
the hydrogenation-hydrogenolysis route (Scheme 2, path 2).
For other lignin-derived species, o-vanillin, syringaldehyde and acetovanillone can be completely
converted to corresponding HDO products on Co @NC-(SBA) under the optimized conditions
1
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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2
3
4
5
6
7
8
9
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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(Table 2, entries 1-3). After 3 h reaction, isoeugenol was transformed into 2-methoxy-4-
propylphenol with 99% yield (Table 2, entry 4). 5-hydroxymethylfurfural (HMF) can also be
transformed to 2, 5-dimethylfuran (DMF) with 99% conversion and 82.6% selectivity at mild
conditions (Table 2, entry 5). Besides, cinnamyl alcohol was converted to phenylpropanol at mild
condition with 91.4% yield (Table 2, entry 6).
Table 2. The hydrogenation reactions of serious ligin-derived species over Co @NC-(SBA)a
1
Conv.
%)
Sel.
(%)
Entry
1
Substrates
Products
(
CHO
OH
CH3
OH
99
99
O
O
CHO
CH3
2
99
99
O
O
O
O
O
OH
OH
O
O
3
4
99
99
99
99
99
99
HO
HO
HO
HO
O
O
O
O
5b
6c
OH
82.6
91.4
O
O
OH
H
a
Reaction conditions: 0.5 mmol substrate, 30 mg Co @NC-(SBA) (3.25 mol% Co), 2 mL H O, 2 mL MeOH,
1
2
b
c
1
40 °C, 1 MPa H 3 h. 2 MPa H , 4 mL EtOH, 180 °C, 4 h, 2 MPa H , 4 mL EtOH, 120 °C, 2 h.
2, 2 2
Density Functional Theory (DFT) Calculations. DFT calculations were further applied to
uncover the intrinsic influence of Co electronic structure on the catalytic activity of CoN . Three
x
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typical Co-N models with different coordination numbers, including Co-N (a CoN moiety
x
4
4
embedded in graphene layer), Co-N O and Co-N (possible structures when the coordination
4
5
number is five) (Figure S7),67-68 were built to conduct the calculation by using first principle method.
The computational details were provided in the Supporting Information (Table S3-S4, Figure S8-
S12). For comparison, Co particles (Co12 cluster) were used to simulate the Co@NC catalyst.
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0
According to the optimized structures, compared with the Co-N bond length (1.86 Å) in CoN sites,
4
the Co-N bond length (1.89 Å) in the Co-N or Co-N O site was closest to that of EXAFS fitting
5
4
results (1.91Å). The adsorption energies of the main species (H , H*, vanillin, HMP, DMP and
2
MMP) involved in the reaction process on the four possible active sites, Co-N , Co-N O, CoN and
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Co particles, were calculated and the results were shown in Table S3. Calculation results revealed
that the adsorption energies of the main species on Co were too large and led to the catalyst
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deactivation. The conclusion was consistent with the experimental results. In addition, DFT
calculations also confirmed that the adsorption energy of HMP on CoN sites was lower than that of
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vanillin (Table S3), suggesting that HMP is more easily adsorbed and then activated (Figure S8).
The result was consistent with the experimental result: in detail the transformation of HMP to MMP
was easier than that of vanillin to HMP. So, the reaction pathway most probably undergo a
hydrogenation-hydrogenolysis route.
The adsorption energies of H on the three active sites were low (-0.14 eV~ -0.38 eV) (Table S3,
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entry 2). The adsorption distances between Co and H on the active sites were about 3.0 Å (Table
2
*
S4, entry 1). The bond lengths of H molecule and the adsorbed H on Co N O or CoN had no
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evident difference (Table S4, entry 2), which indicated that the interaction between hydrogen
molecule and CoN O or CoN was weak, and there was no obvious chemical interaction between
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them, so the adsorption of H on CoN O or CoN belonged to physically adsorption. The bond
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length of hydrogen molecule adsorbed on CoN sites was 0.799 Å (Figure S9), which was larger
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than that of free hydrogen molecule (0.751 Å). This result indicated that the Co @NC-(SBA)
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catalyst could indeed activate hydrogen molecule. However, the activation of H on the model
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catalyst was not easier than that on metal cobalt nanoparticle catalyst because the system energy
increases slightly (about 0.11 eV) in contrast to the energy of free catalyst and hydrogen. However,
in the presence of vanillin, homolytic cleavage of H-H bond of the activated hydrogen occurred
(Figure S10). And one active H* is captured by carbonyl oxygen of vanillin and the other H* is left
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on the cobalt site of CoN sites (Figure S10, step 1). The energy in this step has a slight increase
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(0.1 eV), which can be achieved easily at high reaction temperature.
According to the results reported in the literatures, although the adsorption energy of H on the
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efficiently.69-70 The dissociated H can be adsorbed on Co atoms and the nearby O or N atoms to
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form Co-H, O-H and N-H species, respectively. The d band center model is a good descriptor of
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the adsorbate-metal interaction. The greater the energy difference between the d-band center of
the active sites and the corresponding Fermi level (E = 0), the weaker the adsorption of the
f
adsorbate on the active sites. In order to discuss the role of d-orbital electron involved in the
reactions, the occupied d-band center of the three active sites were calculated (Table S4, entry 4).
The results suggested that the adsorption energies of the active H were relatively low (Table S4,
entry 5) and the adsorption of the active H on the active sites was weak, which facilitated the
desorption of H from the catalyst sites and enhanced the catalytic reaction.
The Bader charge analysis of the Co-N , Co-N O and CoN sites were shown in Table S5.
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According to the calculation data, the charge of Co atom was greatly affected by the outer
coordination atom. Because of the large electronegativity of O and N atoms, the charge of Co atom
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increased from 0.83 e on Co-N sites to above 1.0 e on Co-N O sites and Co-N sites. Vanillin was
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easier to be adsorbed and H* radicals were easier to be desorbed on the Co-N O sites and Co-N
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sites because the increased positive charge of Co. The Bader charge analysis also approved the
results of adsorption energy.
Hydrogenation Reaction of Nitroarenes. To verify the application potential of Co @NC-(SBA),
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the hydrogenation reaction of nitro-compounds to corresponding aromatic amines was further
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investigated since they play an important part in the manufacture of dyes, pharmaceuticals,
pigments as well as their intermediates.72-74 The hydrogenation of p-nitrochlorobenzene was
employed as the model reaction to optimize the reaction conditions. As shown in the Table S6, both
the activity and selectivity were significantly affected by the H pressure, reaction temperature, time
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and solvent.
Table 3. Substrates scope of the hydrogenation reaction of nitro-compounds. a
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OH
NH2
NH2
H N
2
NH2
HO
NH2
1
.5 h, 99%
1.5 h, 99%
1.5 h, 99%
2 h, 99%
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NH2
NO2
O
NH2
OHC
NH2
2 h, 99%
2 h, 99%
2 h, 99%
2 h, 99%
9b
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15
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NH2
Cl
NH2
Br
NH2
I
NO2
2 h, 99%
1 h, 99%
2 h, 99%
2 h, 99%
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3
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4
NH2
O
NC
NH2
NH2
O
NH2
2 h, 99%
2 h, 99%
2
h, 99%
2
h, 99%
a
b
Reaction conditions: 0.5 mmol substrate, 28 mg Co @NC-(SBA), 4 mL ethanol, 90 °C, 1 MPa initial H .
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40 °C.
The hydrogenation of series nitroarenes were carried out under the optimized conditions (90 °C,
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1 MPa H , ethanol as the solvent, 0.5 mmol substrate, 28 mg Co @NC-(SBA)) and the results were
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summarized in Table 3. Most of the functionalized nitroarenes can be converted to corresponding
aromatic amines within 2 h. The series nitroarenes with electron donating groups such as p-amino,
p-hydroxyl, o-hydroxyl, p-methyl, p-ethyl and p-nitroanisole can be transformed to the
corresponding amines with 100% yield within 2 h (Table 3, entries 2-7). Nitro-compounds with
electron absorption groups such as p-formyl, p-halogen, p-acetyl, m-acetyl, p-cyano and 1-
nitronaphthalene achieved 99% conversion and 99% selectivity too (Table 3, entries 8, 10-16).
Furthermore, the aldehyde group on p-nitrobenzaldehyde was transformed to methyl as well when
the reaction was conducted at 140 °C (Table 3, entry 9), the results were in good consistence with
the HDO reaction of vanillin.
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One of the greatest concerns regarding the use of SACs for liquid-phase reactions is its stability.
Generally, the hydrogenation of p-nitrochlorobenzene was conducted to verify the reusability of
Co @NC-(SBA). As shown in Figure S13a, after ten continuous runs, the conversion and
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selectivity of p-nitrochlorobenzene kept at 94% and 94%, respectively. HAADF-STEM image of
the recycled Co @NC-(SBA) displayed that the cobalt was still atomically dispersed and no cluster
1
was observed (Figure S13b), indicating the outstanding reusability and high stability of the Co-Nx
bonds.
In addition, we summarized the materials recently reported for catalyzing the HDO of vanillin
and hydrogenation of nitro-compounds, the results were listed in Table S7 and Table S8,
respectively. The catalytic performance for the HDO of vanillin of Co @NC-(SBA) was
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comparable or even superior to some of the previously reported noble metal catalysts (Table S7,
entries 1-4) or higher than the reported non-noble catalysts (Table S7, entries 6-16). Besides, the
catalytic performance for the hydrogenation of nitro-compounds on Co @NC-(SBA) was
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comparable or superior to the reported non-noble catalysts as well (Table S8, 7-12). Those results
further demonstrated the highly catalytic activity of Co @NC-(SBA) in the heterogeneous
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CONCLUSIONS
A hard-template sacrificial strategy was developed to prepare SACs Co catalyst Co @NC-(SBA)
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by employing Co(NO ) ·6H O-glycine complex as the precursor and SBA-15 as the template. For
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comparison, the catalysts fabricated by MCM-41 and FDU-12 presented obviously difference of Co
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dispersity. Among all the prepared catalysts, atomically dispersed Co catalyst Co @NC-(SBA)
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exhibited the best catalytic activity, which was not only active for the hydrodeoxygenation of
lignin-derived species with excellent yield, but also exhibited highly catalytic activity for the
hydrogenation of various nitroarenes under mild conditions. Co @NC-(SBA) also manifested good
1
reusability for the heterogeneous hydrogenation reaction. The highly catalytic activity and good
stability of Co @NC-(SBA) were attributed to the atomical dispersity of Co and its CoN O
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structure.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI:
Chemicals and reagents, apparatus, DFT Calculation details, XRD and XPS patterns of the series
catalysts, HRTEM image of Co@NC, catalyst recycle and HAADF-STEM of reused Co @NC-
1
(SBA), surface area and pore size of the series SiO templates and catalysts, EXAFS data fitting
2
results of Co @NC-(SBA), optimization of the hydrogenation of nitro-compounds, summary of
1
reported literatures for the HDO of vanillin and hydrogenation of nitro-compounds
AUTHOR INFORMATION
†
These authors contributed equally to this work.
Corresponding Author
Chun Wang, Phone: +86-312-7528291. Fax: +86-312-7528292. E-mail: wangchun@hebau.edu.cn,
*
chunwang69@126.com.
Shutao Gao, E-mail: gaoshutao@hebau.edu.cn
Yuqing Qiao, E-mail: qiaoyq@ysu.edu.cn.
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Yongjun Gao, E-mail: yjgao@hbu.edu.cn.
ORCID
Chun Wang:
Yongjun Gao:
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial supports from National Natural Science Foundation of China (31671930, 21773053),
Natural Science Foundation of Hebei Province (B2020204003, B2019204023, B2018204145,
B2016204131, B2015204003) and the One Hundred Talent Project of Hebei Province
(E2016100015) are acknowledged. The DFT calculation was carried out on TianHe-2 at LvLiang
Cloud Computing Center of China.
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