Z. Zhang et al. / Journal of Catalysis 373 (2019) 314–321
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Therefore, an alternative in situ hydrogenation method using
hydrogen donor from biomass derived primary alcohols (methanol,
ethanol), other than H2 or secondary alcohols has become a more
economical strategy [9,29–31]. However, the hydrogen evolution
from primary alcohols is usually more difficult kinetically and
requires crucial reaction condition. The well integration of H2 pro-
duction and hydrogenation centre is the crucial factor to enhance
the efficiency of in situ hydrogenation catalysts. In a recent
attempt, Riisager et al. employed a non-noble Cu-doped porous
metal oxide catalyst with methanol as solvent and hydrogen
donor. 48% DMF yield were achieved at 260 °C for 3 h, which was
far below other secondary alcohol systems [32]. Exploring suitable
catalytic systems with high efficiency in the selective hydrogen
evolution from ethanol, a greener biomass product, for the hydro-
genation of 5-HMF without excessively hydrogenating and open-
ing the furan ring is still a challenging problem in this area.
Herein, we report the development of a ternary CuZnCoOx cat-
alyst for highly efficient and selective hydrogenation of 5-HMF to
produce DMF with ethanol as the hydrogen source (Scheme 1).
The reaction behaviors of intermediates 5-HMF, 5-methyl furfural
3), CuCo and ZnCo were synthesized using the same method, in
which CuCo, CuZn and ZnCo refer to the reduced 20 wt% Cu/CoOx,
20 wt% Cu/ZnO and 20 wt% ZnO/CoOx respectively.
2.3. Experimental procedures
Catalytic activity tests: a certain amount of 5-HMF, ethanol and
catalyst were added into a 1.67 mL stainless steel autoclave pur-
chased from Swagelok, as shown in Fig. S1. The sealed reactor
was put in a fluidized sand bath (Techne SBL-2) which was pre-
heated to the desired temperature. After the reaction, the reactor
was cooled down to room temperature by immediately loading
water and then removing the solid catalyst by filtration. Recycling
experiments: the solid catalyst was separated by centrifugation,
after removal of the reaction liquid, washed with ethanol for sev-
eral times, dried at 40 °C in a vacuum oven to prevent oxidation,
and then reused for the next reaction run.
Ethanol dehydrogenation experiment: 3.5 mL ethanol and
15 mg catalyst were added into 8 mL stainless steel autoclave, then
the sealed reactor was pressurized with N2 three times to remove
air and charged with the N2 at atmospheric pressure. After reac-
tion, the reactor was cooled down to room temperature, and then
recording the final pressure in reactor and collecting the gaseous
products for GC analysis.
(5-MF),
5-methylfurfuryl
alcohol
(MFA)
and
2,5-bis
(hydroxymethyl)furan (BHMF) over ternary CuZnCoOx and their
corresponding binary catalysts were investigated. HRTEM, EDS-
mapping, XRD and H2-TPR were utilized to reveal the synergistic
effect of each component. Under optimized condition, the
CuZnCoOx catalyst achieved over 99% DMF yield in 5 h and exhib-
ited excellent recyclability.
2.4. Analysis method
The products were diluted to 10 mL with ethanol and then
quantified by using an Agilent 7890A gas chromatography
2. Experimental section
equipped with a 30 m  0.32 mm  0.25
carrier gas was nitrogen with a flow of 25 mL/min, the liquid vol-
ume was 1 L with a split ratio of 5:1 and the temperature of the
lm HP-5 column. The
2.1. Materials
l
injector and detector were 300 °C and 320 °C respectively. The
temperature program for analysis was: 30 °C kept for 4 min,
10 °C/min to 140 °C and 20 °C/min to 300 °C kept for 2 min. In
addition, an Agilent 1260 high performance liquid chromatographs
(HPLC) equipped with a Synergi Hydro-RP 80A column and a UV
detector was used to separate and quantify the intermediates 5-
MF and MFA. A 40 vol% methanol aqueous solution was employed
as mobile phase at 35 °C column temperature and 240 nm wave-
length with a flow rate of 0.6 mL/min. A Shimadzu GC-2018 gas
chromatography with Agilent PLOT 5A molecular sieve filled col-
umn was used to analyse the gaseous products. The oven temper-
ature was 60 °C, the volume was 1 mL with a split ratio of 1 and the
temperature of TCD detector and FCD detector were 100 °C and
180 °C respectively.
The conversion and selectivity of liquid product were based on
the external standard method. The molar conversion of reactant
was shown as the number of moles consumed divided by the initial
moles of the reactant, the selectivity of product was shown as the
number of moles of carbon in specific product divided by the num-
ber of moles of carbon in consumed reactant, the yield was shown
as the conversion multiply by selectivity. H2 content was also cal-
culated based on external standard method. Specifically, the actual
pressure (P1) after reaction was recorded at room temperature, and
volume of gases (V1) was also been estimated based on the volume
of the autoclave. Then, the actual volume of gases (V2) at 101.3 kPa
(P2) was determined by Clausius-Clapeyron equation (P1V1 = P2V2).
Finally, H2 molar content was obtained combined with V2 and per-
centage of H2 in gaseous products from GC. Each result was con-
firmed by repeated experiments for three times.
5-HMF (99%), 5-MF (98%), BHMF (99%), Co(NO3)2Á6H2O and Cu
(NO3)2Á3H2O were purchased from Aladdin Reagent Limited Com-
pany. Zn(NO3)2Á6H2O was purchased from Sigma Aldrich. MFA
(97%) was purchased from Thermo Fisher Acros Organics. DMF
(99%) was purchased from Shanghai Macklin Biochemical Technol-
ogy Co., Ltd. Ethanol, Na2CO3 and NaOH were purchased from Sino-
pharm Chemical Reagent Co., Ltd. All chemicals were used without
further purification.
2.2. Catalyst synthesis
The CuZnCoOx precursor was prepared by a co-precipitation
method. Typically, 16 mmol Co(NO3)2Á6H2O, Cu(NO3)2Á3H2O and
Zn(NO3)2Á6H2O with different Cu/Zn molar ratios were dissolved
in 200 mL deionized water with a mass composition of (Cu-ZnO)/
Co3O4 = 20/80. An alkali liquor composed of Na2CO3 and NaOH
according to the molar amount of alkali required for the metal pre-
cipitation was used as precipitant, and the molar ratio of OHÀ/CO32À
was determined as 2. The alkali was introduced into a 1 L round
bottom flask after being dissolved in 150 mL deionized water.
The resulting metal salt solution was added dropwise into alkali
liquor at 25 °C under vigorous stirring. After precipitation, the mix-
ture was heated to 90 °C for 3 h and aged for another 3 h in the
mother liquor, then washed and dried at 100 °C for 12 h. The as-
prepared precursor was calcined at 350 °C for 3 h to get the corre-
sponding CuO/ZnO/Co3O4. The obtained material was reduced in a
tube furnace under H2 atmosphere at 350 °C for 1 h with a ramp of
5 °C/min. After the reduction, the catalysts are pretreated in N2
overnight, and then immediately transferred into a 3 mL bottle
which was purged by N2 three times for next use of reaction
and characterization. The resulting samples were marked as
CuZnCo-x, in which x indicates the molar ratio of Cu/Zn in ternary
CuZnCoOx. Additionally, CoOx, Cu, ZnO, CuZn (Cu/Zn mole ratio is
2.5. Catalyst characterization
The ex-situ synchrotron X-ray diffraction (XRD) data of the
reduced CuZnCo-x samples in capillary were collected at 17BM