J. Li, et al.
AppliedCatalysisA,General591(2020)117385
2.2. Catalyst preparation
The Co/N-C catalysts were prepared by simultaneous pyrolysis of
cobalt and nitrogen precursors onto various supports under Ar at
350−1100 °C. The cobalt loading is 10 wt% unless otherwise specified.
Typical preparation is as follows: a certain amount of cobalt(II) acetate
tetrahydrate was added into ethanol (100 mL) at room temperature.
After a complete dissolution, melamine was added. Then activated
carbon (1 g) was added, and the solution was stirred at 60 °C for 16 h
followed by evaporation to remove the solvent. The solid was then
dried at 60 °C overnight. The final precursor was grounded into fine
powder, and then heated in a tubular furnace. An argon flow was used
at a flow rate of 100 mL min−1. The best Co/N-C catalyst is prepared
according to following temperature program: 20 °C hold for 60 min,
ramp 10 °C min−1 to 550 °C and then hold for 2 h. The catalysts are
denoted as Co/(N)-S-T, where S represents the type of support, and T
represents the pyrolysis temperature. The Co/N-C catalysts prepared
from different batches exhibited the same catalytic performance, de-
monstrating that our preparation method is highly reproducible.
Scheme 1. Conversion of fatty acids to alkanes via decarboxylation (Path 1) or
decarbonylation (Path 2), abbreviated collectively as DCOx, or hydro-
deoxygenation (HDO) (Path 3).
preserves the number of carbon atoms. Although HDO affords better
carbon atom economy and higher energy value of alkane products over
DCOx, the consumption of H2 in DCOx is relatively lower, making it less
costly and more energy-efficient [8]. However, previous Co-based cat-
alysts, e.g., Co/γ-Al2O3, Co/H-ZSM-5, and Co/clay, usually favor HDO
over DCOx (Table S1) [20–23]. Despite Co/SiO2 catalyst has re-
presented a DCOx favored selectivity, the result is buried in many other
HDO favored results over H-ZSM-22-supported Co catalysts, and the
reason for such selectivity regulation is not even investigated [24].
Thus, it is still desirable to unravel the mechanism controlling se-
lectivity to rationally develop DCOx favored Co catalysts.
2.3. Catalyst characterization
The morphology was characterized by Hitachi SU8010 scanning
electron microscopy (SEM, Japan) at 20 kV. (High-resolution) trans-
mission electron microscopy (TEM or HRTEM) and high-angle annular
dark field microscopy (HAADF-STEM) were performed on a JEOL JEM-
2100 F high-resolution transmission electron microscope operated at
200 kV. N2 adsorption measurements were performed on an
ASAP2020 M adsorption analyzer. The surface areas of catalysts were
calculated using the BET method in the range of relative pressures
between 0.05-0.20. BJH method is used to calculate the pore size and
distribution. XRD were operated on an X-ray diffractometer (TTR-III,
Rigaku Corp., Japan) with Cu Kα radiation (λ = 1.54056 Å). XPS was
performed on an X-ray photoelectron spectroscopy (ESCALAB 250Xi,
Thermo-VG Scientific, USA) with monochromatized Al Kα radiation
(1486.92 eV). The nitrogen content was detected by elemental analysis
(Eurovector EA 3000). The cobalt content was detected by ICP-AES
(Optima 7000DV, PerkinElmer Inc.). For NH3- or CO2-TPD tests, ap-
proximately 100 mg sample was loaded in a quartz reactor and then
heated at 500 °C under argon flow for 2 h. The adsorption of NH3 or CO2
was done at 40 °C for 1 h, and then the catalysts were flushed with
argon for 1 h. Subsequently, the sample was heated to 550 °C at a
heating ramp rate of 10 °C min−1. The desorbed NH3 or CO2 was
measured by a gas chromatograph (GC) with a thermal conductivity
detector (TCD). FTIR spectra were recorded on a Nicolet 8700 FTIR
spectrometer in the wavenumber range from 400 to 4000 cm−1. To
explore the chemical adsorption of stearic acid on surface of Co/N-C-
550 and Co/N-TiO2-550 catalysts, the catalysts were treated as our
previous studies. [13] 0.1 g catalyst was added into a 0.05 M stearic
acid n-hexane solution (20 mL), and then the mixture was stirred at
room temperature for 12 h. The solid was separated by centrifugation
with 15 times washing of n-hexane. The final solid was dried at 80 °C
overnight under N2.
Nitrogen-doping has recently been recommended as an effective
way to improve the catalytic activity of cobalt catalyst. Beller and co-
workers had reported pioneer work using Co/N-C catalysts towards
organic synthesis, such as nitroarenes reduction, mild epoxidation of
alkenes, selective oxidation of alcohol to esters, and hydrogenation of
terminal and internal olefins [38–41]. This nitrogen-doping promotion
effect has also been observed in some Co-catalyzed biomass conversions
[30,34,35,37]. The nitrogen dopant could introduce additional un-
electron transfer and substrate adsorption [42,43]. In this work, a ni-
trogen-doped carbon-supported Co catalyst with an average particle
size of 7.0 nm is fabricated via simultaneous pyrolysis of cobalt(II)
acetate and melamine onto activated carbon at 550 °C. Nitrogen-doping
induces the formation of highly dispersed Co NPs even at a high Co
loading of 10 wt% and after recycling tests. XRD showed that only the
(111) facet of α-Co0 is exposed at 550 °C, while additional (200) and/or
(220) facets are exposed at higher pyrolysis temperatures. The selective
exposure of the (111) facet of α-Co0 renders a DCOx favored reaction
route. The versatility of Co/N-C catalyst was successfully demonstrated
by conversion of various fatty acids and esters, and plant oil.
2. Experimental section
2.1. List of chemicals
2.4. Experimental procedure
Metal oxide supports such as α-Al2O3 (99.9 %), TiO2 (99 %), SiO2
(99.9 %), and ZrO2 (99 %), and solvents such as dodecane (98 %), and
decane (98 %) were purchased from Aladdin Reagent Co. Ltd. Cobalt(II)
acetate tetrahydrate (98.5 %) and ethanol (99.5 %) were purchased
from Sinopharm Chemical Reagent Co. Ltd. Melamine (98 %), activated
carbon (99 %), octadecane (98 %), 1-octadecene (90 %), octadecanol
(98 %), and octadecanal (95 %) were purchased from TCI Development
Co. Ltd.
The conversion of fatty acids was operated in stainless reactors
(50 mL) that purchased from Anhui Kemi Machinery Technology Co.,
Ltd. For a typical procedure, stearic acid (0.5 mmol), heterogeneous
catalyst (100 mg), and alkane solvent (20 mL) were loaded into a quartz
lining in the reactor. The reactor was then purged with hydrogen for
three times, and then purged with 4 MPa H2 at room temperature. The
reaction was set at reaction temperature for 8 h with a stirring speed of
800 rpm. After reaction, the gaseous phase was analyzed by gas
2