W. Liao, et al.
MolecularCatalysis482(2020)110756
Nomenclature
DMTHF 2,5-(dimethyl)tetrahydrofuran
BHMF
MFA
THF
2,5-bis(hydroxymethyl)furan
5-methylfurfurylalcohol
tetrahydrofuran
S-CNT
CNT
sulfur-modified carbon nanotubes
carbon nanotubes
HMF
DMF
5-hydroxymethylfurfural
2,5-dimethylfuran
5-MF
5-methylfurfural
reaction mechanism in the process of HMF decarbonylation [37,38].
Chen et al. reported selective hydrogenation of HMF to DHMTHF with
96 % yield over palladium catalyst supported on mesoporous graphitic
carbon nitride (Pd/mpg-C3N4) [39]. Although Pd-based catalysts de-
monstrated high activity in hydrogenation of HMF, the selectivity to
DMF was limited in most cases. Doping noble metals with abundant
metal elements seems to be an ideal solution to improve the catalytic
performance (activity, selectivity, and durability), meanwhile which is
likely capable to minimize the usage of noble metals [40,41]. It was
hypothesized a Pd-Co bimetallic composite could be a potential catalyst
for conversion HMF to DMF, which may share the high activity of Pd-
based catalysts as well as the high selectivity of Co-based catalysts.
In this work, highly active and selective Pd-Co bimetallic catalysts
supported on sulfur-modified carbon nanotubes (PdCox/S-CNT) for the
hydrogenolysis of HMF to DMF under mild condition (120 °C, 0.3 MPa
H2) were investigated. A series of PdCox/S-CNT catalysts with different
atomic ratios of Pd/Co were prepared. The interactions of Pd and Co9S8
particles as well as the effect of the support were implemented in order
to shed light on the structure-performance relationship. The influence
of reaction parameters, including reaction temperature, H2 pressure,
dosage and reaction time, on the catalytic performance were thor-
oughly investigated.
TPR measurements were carried out on a TP-5076 adsorption appa-
ratus, a hydrogen-argon (10 % H2, volume percentage) mixture was
used (30 mL min–1) as reducing atmosphere, the heating rate was 5 °C
min–1. XPS was performed on a Thermo K-Alpha + spectrometer with
an Al K Alpha (1486.8 eV) source. The microstructure of PdCo8/S-CNT
was investigated by SEM (JSM-7900 F) and TEM (JEM100-CXII). TGA
was conducted on a thermal analyzer (STA409PC) in air and the tem-
perature was linearly raised at a rate of 10 °C min–1
.
2.3. Catalytic hydrogenolysis of HMF
Hydrogenolysis of HMF was carried out in a 50 mL autoclave fitted
with a magnetic stirrer. In a typical experiment, 20 mL HMF/THF so-
lution (2 mmol HMF), 0.120 g tetradecane (as GC internal standard)
and specified amount of catalyst were added to the reactor. The reactor
was sealed and then purged with hydrogen five times, and finally
pressurized to the specified pressure. The reactor was heated to the
desired temperature and stirred at a rate of 1000 rpm. After reaction,
the reactor was cooled down with ice-water bath to room temperature,
catalyst was separated by centrifugation. All products were identified
by a GC–MS (Agilent 7890 equipped with a HP-5 column and an FID
detector). The organics mixture were analyzed with a GC (Agilent
7890B; HP-5 column, 30.0 m × 320 μm × 0.25 μm; FID detector)
according to internal standard method to obtain conversion and se-
lectivity.
2. Experimental
2.1. Preparation of catalyst
3. Results and discussion
The sulfur-modified nanotubes (S-CNT) were prepared by a solvent
impregnation and melt-coat method [42]. Typically, 48.0 mg of sub-
limed sulfur (Sinopharm Chemical reagent Co., Ltd.) was ultrasonically
dissolved in 20 mL toluene and then 160.0 mg of CNT (multi-walled, ID:
5−10 nm, OD: 10−30 nm, Shanghai Macklin biochemical Co., Ltd.)
was added. The mixture was sonicated for 1 h to break aggregates and
disperse the CNT. Then, the solvent was evaporated at room tempera-
ture and the sample was ground thoroughly in an agate mortar. The
mixture in a sealed vial was heated to 155 °C in an oven for 6 h.
The PdCox/S-CNT catalysts were prepared by a typical wetness
impregnation method. As a representative of PdCo8/S-CNT, 2.53 mL
PdCl2 (Sinopharm Chemical reagent Co., Ltd.) aqueous solution (11.84
mg mL–1) and 0.6563 g Co(NO3)2·6H2O (Sinopharm Chemical reagent
Co., Ltd.) were dissolved in 20 mL mixture solution of methanol and
water (volume ration of 1:1). The modified S-CNT was dispersed in the
solution under stirring. After stirring for 5 h at room temperature, the
solution was dried overnight at 80 °C, subsequently reduced at 400 °C
for 2 h with flowing H2/N2 (H2 volume percentage of 10 %, 30 mL
min–1) in tube furnace before used as catalysts. As comparison material,
Pd/S-CNT, Co/S-CNT, and PdCo8/CNT without S-modification have
been synthesized with the similar method in our laboratory. The Co9S8/
CNT catalyst was prepared according to literature reported previously
3.1. Composition and structural characterization
The structural identification of materials was performed by XRD. As
shown in Fig. 1, Pd/S-CNT and CNT all exhibited characteristic dif-
fractions at 26.1 and 43.3°, attributed to graphitized carbon from CNT
[42,44,45]. No diffractions of metallic Pd or palladium oxides were
2.2. Catalyst characterization
The surface area and the pore size distribution were measured by
nitrogen adsorption-desorption at 77 K on a NOVA 3000e device. The
XRD patterns of samples were obtained on a XRD Rigaku, Smart Lab3
powder diffractometer with Cu Kα radiation (40 mA, 50 kV). The H2-
Fig. 1. XRD patterns of different catalysts and supports.
2