होम Catalysis Science & Technology The effect of potassium on Cu/Al 2 O 3...

The effect of potassium on Cu/Al 2 O 3 catalysts for the hydrogenation of 5-hydroxymethylfurfural to 2,5-bis(hydroxymethyl)furan in a fixed-bed reactor

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खंड:
8
साल:
2018
भाषा:
english
पत्रिका:
Catalysis Science & Technology
DOI:
10.1039/c8cy02017e
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Cite this: Catal. Sci. Technol., 2018,
8, 6091

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The effect of potassium on Cu/Al2O3 catalysts for
the hydrogenation of 5-hydroxymethylfurfural to
2,5-bisIJhydroxymethyl)furan in a fixed-bed reactor
Danxin Hu,†a Hualei Hu,†a Hao Zhou,b Guozheng Li,b Chunlin Chen,
Jian Zhang,*a Yong Yang,a Yaoping Hu,a Yajie Zhanga and Lei Wang*a

a

The highly efficient selective hydrogenation of 5-hydroxymethylfurfural (HMF) to 2,5bisIJhydroxymethyl)furan (BHMF) was achieved in a fixed-bed reactor by using inexpensive potassiumdoped Cu/Al2O3 catalysts, which were prepared via a successive incipient wetness impregnation method.
The characterization results revealed that the introduction of potassium could adjust the size of the copper
particles and modify the acid–base property of the catalyst, leading to a significant change in its perforReceived 28th September 2018,
Accepted 15th October 2018
DOI: 10.1039/c8cy02017e

mance for the selective hydrogenation of HMF to BHMF. The highest yield of BHMF (98.9%) was obtained
over the 1.5K–Cu/Al2O3 catalyst under the reaction conditions of 120 °C, 2.0 MPa of H2, and a weight
hourly space velocity (WHSV) of 1.0 h−1. The excellent performance could be explained by the fact that the
addition of an appropriate amount of potassium facilitated the dispersion of copper and reduced the acidity

rsc.li/catalysis

of the catalyst, which improved the catalytic activity and suppressed unwanted side reactions, respectively.

1. Introduction
With the diminishing availability of fossil resources and increasing environmental problems, the synthesis of chemicals
from renewable biomass resources is of considerable
interest.1–3 As a key platform molecule derived from sugars or
cellulose, 5-hydroxymethylfurfural (HMF) has been utilized to
produce a number of valuable derivatives, such as 2,5dimethylfuran (DMF), 2,5-dimethyltet; rahydrofuran (DMTHF),
2,5-furandicarboxylic acid (FDCA), 2,5-bisIJaminomethyl)furan
(BAF) and 2,5-bisIJhydroxymethyl)furan (BHMF).4–7 Among
these compounds, BHMF, which can be produced by the selective reduction of HMF, is widely used as an intermediate for
the synthesis of polymers, resins, fibers, drugs, and biofuels.8,9
To date, the selective hydrogenation of HMF to BHMF
with different hydrogen sources (especially H2) has been extensively studied over many kinds of heterogeneous
catalysts.8–10 Although the noble metal catalysts (Pt/MCM-41,
Au/Al2O3, RuIJOH)x/ZrO2, etc.) exhibited excellent catalytic
activity, high cost and limited resource restrict their application on an industrial scale.9,11–13 In this case, more attention
has been drawn to developing non-noble metal catalysts for

a

Ningbo Institute of Materials Technology & Engineering, Chinese Academy of
Sciences, 1219 Zhongguan West Road, Ningbo 315201, China.
E-mail: jzhang@nimte.ac.cn, wanglei@nimte.ac.cn
b
Technology Center, China Tobacco Henan Industrial Co., Ltd, Zhengzhou
450000, China
† The first two authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2018

this reaction. Zhu et al.14 found that the Cu-based catalysts
(Cu–ZnO and RANEY® Cu) displayed much higher selectivity
to BHMF than RANEY® Ni catalyst, which was attributed to
their high reactivity to CO bonds and relatively low reactivity toward CC hydrogenation. Cao et al.15 reported a Cu/
SiO2 catalyst for the hydrogenation of HMF and observed a
high yield of BHMF after 8 h of reaction at 100 °C. Recently,
Upare et al.16 have demonstrated that Cu–SiO2 nanocomposite materials were excellent catalysts for the selective
hydrogenation of HMF to produce BHMF. However, most of
these studies were conducted in batch reactors, the operation
of which is inefficient from the point of view of large-scale industrial production.17 In comparison, continuous operation
in a fixed-bed reactor is an attractive solution which can remarkably enhance the productivity by eliminating the time
required for charging, discharging, cleaning, and catalyst separation.18 Nevertheless, the relatively short contact time of reactants with the catalytic active species brings about a big
challenge to develop a highly active catalyst for practical use
in the fixed-bed reactor.19–22
Alumina-supported copper material (Cu/Al2O3) was one of
the most commonly used catalysts in many hydrogenation reactions, especially those in a continuous fixed-bed
process.23–25 For this catalyst, both the dispersion of the copper particles and the surface acid–base property were the key
factors that determine its catalytic performance.25,26 Usually,
to increase the activity and selectivity of the monometallic
loaded catalyst, alkali metals could be introduced to adjust
the size of the metal particles and modify the acid–base

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property of the supports.26–29 Herein, a series of potassiumdoped Cu/Al2O3 catalysts were synthesized and used for the
selective hydrogenation of HMF in a continuous fixed-bed
reactor. The effect of potassium on the physicochemical
properties of the catalysts was characterized by various
techniques, including X-ray diffraction (XRD), X-ray fluorescence (XRF) technique, X-ray photoelectron spectroscopy
(XPS), Brunauer–Emmett–Teller (BET) analysis, N2O chemisorption, dark-field transmission electron microscopy (TEM),
temperature-programmed desorption of NH3 (NH3-TPD),
temperature-programmed desorption of CO2 (CO2-TPD),
temperature-programmed reduction of H2 (H2-TPR) and
Fourier-transform infrared spectroscopy of chemisorbed CO
(CO-FTIR). The relationship between the hydrogenation performance of the catalysts and the amount of potassium was
investigated in detail.

2. Experimental
2.1. Materials
Al2O3 powder (SCFa-230) was provided by Sasol Germany
GmbH. 5-Hydroxymethylfurfural was purchased from Aladdin
Industrial Inc. (Shanghai, China). Ethanol (C2H6O, ≥99.7%),
copper nitrate trihydrate (CuIJNO3)2·3H2O, >99%), and potassium nitrate (KNO3, >99%) were obtained from Sinopharm
Chemical Reagent Co., Ltd. (Shanghai, China).
2.2. Catalyst synthesis
Potassium-doped Cu/Al2O3 catalysts with a nominal potassium loading of 0.5 to 5.0 wt% and Cu loading of 5.0 wt%
were prepared by a successive incipient wetness impregnation method. The Al2O3 powder was first tableted, crushed
and screened to obtain particles with diameters of 0.45–0.90
mm. The catalyst was then impregnated with quantitative
amounts of an aqueous solution of KNO3 at ambient temperature for 12 h before drying at 110 °C overnight. The dried
sample was calcined in air at 480 °C for 8 h. Then, the
potassium-doped Al2O3 sample was further impregnated with
quantitative amounts of an aqueous solution of CuIJNO3)2
·3H2O at ambient temperature for 12 h, followed by drying at
110 °C for 2 h and calcining again at 480 °C for 8 h.
2.3. Catalyst characterization
XRD patterns were recorded on a Bruker D8 ADVANCE X-ray
diffractometer using Cu Kα radiation. The compositions of
the catalysts were determined by using the XRF technique.
TEM imaging was performed using a high-resolution FEI F20
microscope in dark-field mode. The N2 physisorption isotherms were obtained at 77 K (ASAP-2020, Micromeritics).
The specific surface areas of the samples were evaluated
using the BET method. XPS spectra were recorded on an
Axis Ultra DLD (Shimadzu, Japan) and the C1s line at
284.8 eV was used as the reference to calculate the binding
energies (BEs).

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The dispersion of the catalysts was determined by N2O
chemisorption. Firstly, the samples were reduced in an 8%
H2/Ar mixture at 300 °C for 2 h. After the samples were
cooled to 50 °C with He, pure N2O was introduced at a rate
of 30 cm3 min−1 for 30 min to oxidize all of the surface copper atoms to Cu2O. Then, the reduction of surface Cu2O to
Cu was conducted in an 8% H2/Ar mixture at 300 °C for 2 h.
CO-FTIR spectra of the catalysts were recorded using a
Thermo Scientific Nicolet 6700 FTIR spectrometer equipped
with a MCT detector. The sample was first heated and reduced at 300 °C under a H2 stream for 1 h. Then, the sample
cell was purged with Ar to remove hydrogen. CO sorption
experiments were conducted at 40 °C with a CO flow rate of
30 mL min−1. The spectra were collected every 5 min (from
5 min to 25 min). CO desorption was performed in an Ar
stream at the same temperature.
NH3-TPD of the samples was performed on an Auto Chem
II 2920 instrument equipped with a TCD detector. The samples were evacuated at 500 °C for 1 h in a flow of He before
cooling to 50 °C. The adsorption of NH3 was performed with
a flow of 10% NH3/Ar at 50 °C. NH3 desorption was
conducted between 100 and 600 °C with a heating rate of
10 °C min−1.
The basic properties of the samples were determined by
CO2-TPD using a TP-5080 chemisorption instrument
equipped with a MS detector. The samples were pre-treated
thermally at 500 °C for 1 h in a N2 flow of 50 mL min−1. After
cooling to 40 °C, the samples were exposed to a CO2 flow of
30 mL min−1 for 1 h. The desorption of CO2 occurred from
40 °C to 600 °C with a heating rate of 10 °C min−1.
H2-TPR of the samples was performed using an Auto
Chem II 2920 instrument equipped with a TCD detector. The
samples were heat-treated in a He flow at 200 °C for 1 h and
then cooled to room temperature. A gas mixture of 10% H2
in He was shifted and the hydrogen consumption was
recorded by TCD from 50 °C to 600 °C with a heating rate of
10 °C min−1.
2.4. Catalytic activity test
All evaluation experiments were carried out in a continuousflow fixed-bed reactor with a stainless steel tube (6.5 mm inner diameter). In each test, 5 g of catalyst was loaded into the
reactor and held in place in the middle of the reactor by glass
wool packed on both sides of the catalyst bed. Then, the catalyst was reduced in situ at 300 °C for 2 h in flowing H2 with a
rate of 100 mL min−1. After the reactor was cooled to the desired reaction temperature, the pressure of H2 was increased
from atmospheric pressure to 2.0 MPa. A mixture of HMF
and ethanol (HMF concentration of 30 g L−1) was then fed
into the reactor (WHSV = 1.0 h−1) with a co-fed H2 flow of
50 mL min−1. The liquid phase products were collected in a
gas–liquid separator at room temperature. The hydrogenation
products were periodically withdrawn from the gas–liquid
separator. Quantitative analysis of HMF and BHMF was
performed using an Agilent 1260 HPLC Infinity instrument

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equipped with a refraction index detector and an Agilent C18
column. The contents of HMF and BHMF were calculated
using an external standard method. The other products were
analyzed with an off-line gas chromatograph (GC7890,
Agilent, USA) using a DB-wax capillary column and a FID
detector.

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3. Results and discussion
3.1. Catalyst characterization
Fig. 1 shows the XRD patterns of the calcined samples. Three
diffraction peaks at 2θ = 37.6°, 45.9°, and 66.9° corresponding to γ-Al2O3 were observed on the unsupported sample.30
The Cu/Al2O3, 0.5K–Cu/Al2O3, and 1.5K–Cu/Al2O3 catalysts
exhibited similar diffraction patterns to that of Al2O3, indicating that the structure of γ-Al2O3 was preserved after the impregnation. Moreover, no diffraction peaks corresponding to
CuO and K2O could be detected, suggesting that both CuO
and K2O were well dispersed in Al2O3 crystals or the formed
particles were too tiny to be detected by XRD.28 When the potassium contents increased to 3.0 and 5.0 wt%, the diffraction peaks assignable to CuO (35.5°, 38.8°, and 48.6°)
appeared in their XRD patterns.28 This demonstrates that an
excessive amount of potassium is apt to cause the aggregation of copper species into bigger particles.
The dark-field TEM images and relevant particle size distributions of the reduced samples are shown in Fig. 2. It can
be seen that the copper particles are uniformly dispersed in
the support matrix. For the Cu/Al2O3 sample, the size of the
copper particles was distributed from 1 nm to 9 nm with an
average size of 4.1 nm. After the addition of 0.5 and 1.5 wt%
potassium, the average size of the copper particles decreased
to 4.05 nm and 3.49 nm, respectively. This indicates that
the introduction of potassium (≤1.5 wt%) could facilitate
the formation of copper particles with smaller size. Nevertheless, further increasing the potassium content triggered a
gradual increase in the average size from 3.49 nm to 4.25 nm
(Fig. 2c–e), indicating a negative effect of excess potassium

Fig. 2 HAADF/STEM images (left) and the relevant particle size
distributions (right) of the reduced samples. (a, Cu/Al2O3; b, 0.5K–Cu/
Al2O3; c, 1.5K–Cu/Al2O3; d, 3.0K–Cu/Al2O3; e, 5.0K–Cu/Al2O3).

Fig. 1 XRD patterns of the calcined samples.

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(≥3.0 wt%) on the dispersion of copper. This evidence clearly
suggests a volcano plot of copper dispersion toward the
amount of potassium content (Table 1).
Table 1 lists the physicochemical properties of Cu/Al2O3
and K–Cu/Al2O3 samples. For all catalysts, the copper contents were quite close to the corresponding nominal values
(5.0 wt%). After the loading of copper on the support, both
the BET surface area and the pore volume of the Al2O3

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Table 1 Chemical compositions and physicochemical properties of the Al2O3 and K–Cu/Al2O3 samples

Sample

Cua (wt%)

Ka (wt%)

SBETb (m2 g−1)

Vpc (cm3 g−1)

Cu dispersiond (%)

Number of acid sitese
(mmol of NH3 g−1)

Al2O3
Cu/Al2O3
0.5K–Cu/Al2O3
1.5K–Cu/Al2O3
3.0K–Cu/Al2O3
5.0K–Cu/Al2O3

—
5.13
5.19
5.25
5.00
5.36

—
0
0.56
1.47
3.20
5.34

235.01
195.03
193.03
191.19
186.11
178.28

0.51
0.43
0.44
0.41
0.40
0.37

—
12.6
26.6
60.5
32
4.6

—
0.65
0.56
0.47
0.43
0.36

a

Determined by XRF. b Calculated by the BET method. c Estimated from a single point adsorption at a relative pressure of 0.973.
by N2O chemisorption. e Determined by NH3 desorption.

decreased somehow due to the partial pore filling by the copper component.23 A similar phenomenon has been frequently
observed on many supported catalysts.31–33 With the increase
in the amount of potassium, both the BET surface area and
the pore volume of the catalysts gradually decreased. Pore
blockage of the support by K2O has been reported in the literature and the trends observed with the increased amount of
potassium indicate that similar effects are dominating here
as well.28,34
The copper dispersion on the catalysts was measured by
N2O adsorption–decomposition and the data are shown in
Table 1. For the undoped Cu/Al2O3 catalyst, 12.6% dispersion
of copper was detected. As the potassium content reached 1.5
wt%, the copper dispersion dramatically increased to 60.5%.
Similar to the tendency of TEM tests, further increase of the

d

Calculated

potassium content to 3.0 and 5.0 wt% caused a rapid decrease in copper dispersion to 32% and 4.6%, respectively. In
general, the dispersion of metals was closely related to the
metal–support interaction.25,35,36 Hence, XPS and CO-FTIR
were used to identify the chemical environment of the Cu/
Al2O3 and K–Cu/Al2O3 catalysts.
The states of Cu over the reduced Cu/Al2O3 and K–Cu/
Al2O3 catalysts were observed via XPS. Fig. 3a displays the
Cu 2p spectra of the reduced catalysts. The predominant
peak at lower binding energy (<933 eV) was ascribed to the
reduced copper species, and the peak at a binding energy of
933–935 eV was assigned to Cu2+ of CuO species.37,38 Moreover, the presence of CuO species on the surface of the catalyst was also confirmed by the intense satellite peak in the
binding energy range of 940–945 eV, which could be

Fig. 3 Cu(2p) XPS (a), CO-FTIR (b), NH3-TPD (c), and CO2-TPD (d) profiles of the samples (a and b: reduced samples; c and d: calcined samples).

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attributed to the oxidation by O2 when the reduced catalyst
was exposed to air.36–38 With the addition of potassium, the
Cu 2p peaks for the catalysts slightly shifted to a lower
binding energy, which was probably caused by the increased
Cu electron cloud densities.39,40 The strong interaction between the copper species and the doped potassium species
might be responsible for this phenomenon.39–41 Fig. 3b
shows the FT-IR spectra of CO adsorbed on the reduced
samples. The Cu/Al2O3 catalyst displayed a sharp peak at
2100 cm−1 corresponding to CO adsorbed on Cu.42 With the
increase in potassium content, the spectra of K–Cu/Al2O3
catalysts gradually red-shifted from 2100 to 2092 cm−1. A
similar red shift has been observed on potassium-doped Pd/
SiO2–Al2O3, sodium-doped Pd/SiO2, and alkaline earth
metal-modified Cu/Al2O3 catalysts, which was caused by the
direct interaction between the metal and the doped
ions.43–45 Accordingly, the red shift of absorbed CO bonds
on the K–Cu/Al2O3 catalysts could be attributed to the
electronic enrichment of the copper species by interacting
with doped potassium species.45
Based on the XPS and CO-FTIR results, it is clear that the
introduced potassium might enhance the interactions between the copper species and the potassium-doped supports
by changing the electronic structures of the metal components.35,36 This enhancement would facilitate the copper dispersion on the 0.5K–Cu/Al2O3 and 1.5K–Cu/Al2O3 catalysts.
However, the excess potassium would also occupy the surface
adsorption sites of the support, which is unfavourable for the
dispersion of copper in the subsequent synthesis procedure.23,46 As a consequence, such a negative effect became
pronounced, resulting in decreased copper dispersion on the
3.0K–Cu/Al2O3 and 5.0K–Cu/Al2O3 catalysts.
NH3-TPD and CO2-TPD were performed to investigate the
effect of potassium on the surface acidic and basic properties
of the catalysts. It can be seen in Fig. 3c (NH3-TPD profiles)
that the Cu/Al2O3 catalyst exhibited two desorption peaks (labeled α and β), which could be ascribed to the ammonia
adsorbing onto the weak and relatively stronger acid sites, respectively.47 Both the temperature of the β peak and the total
amount of the acid sites gradually decreased with the increase in the potassium content, indicating that the introduction of potassium could effectively reduce the acidity of the
catalysts. In the CO2-TPD profile of the Cu/Al2O3 catalyst
(Fig. 3d), only one broad peak (I) at the temperature of 98 °C
could be observed, corresponding to the CO2 adsorbing on
the weak basic sites. With the increase in the potassium content, the intensity of the CO2 desorption peak (I) significantly
increased, demonstrating the increase in the number of basic
sites.23 Besides peak I, a high-temperature desorption peak
(II), which represented CO2 desorption on the stronger basic
centers, was present in the CO2-TPD profile of the 5.0K–Cu/
Al2O3 catalyst. The addition of the basic metal (potassium)
was responsible for the increased amount and strength of the
basic sites.
H2-TPR was used to investigate the effect of potassium
on the reduction behavior of the catalysts. As shown in

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Fig. 4 H2-TPR profiles of the calcined samples.

Fig. 4, reduction peaks in the temperature range of 170–200
°C were observed in all of the samples, which could be attributed to the reduction of CuO under our experimental
conditions. The Cu/Al2O3 catalyst gave a reduction peak at
the temperature of 184 °C. After the addition of 0.5 and 1.5
wt% potassium, the reduction peaks shifted to the lowtemperature direction, indicating that the CuO species were
easily reduced, while for the 3.0K–Cu/Al2O3 and 5.0K–Cu/
Al2O3 catalysts, the reduction temperature increased to 189
°C and 200 °C, respectively. This means that the addition of
excess potassium increased the difficulty in the reduction of
the CuO species. In general, the highly dispersed copper
species can be more easily reduced than the bulk CuO particles due to their relatively higher surface area exposed to
H2.24 Although the 3.0K–Cu/Al2O3 sample possessed a
smaller average size of copper particles, its reduction temperature (189 °C) was higher than that of the Cu/Al2O3 sample. This indicates that the size of the copper particles was
not the only factor influencing the reduction process here. It
has been reported that the interactions between K2O and
CuO could result in an inhibiting effect on CuO reduction.28
Clearly, the higher the amount of potassium introduced into
Cu/Al2O3, the more difficult the reduction of the CuO species. As for the 5.0K–Cu/Al2O3 catalyst, both the largest average size of the copper particles and the highest potassium
content were responsible for its highest reduction temperature (200 °C).
3.2. Catalytic performance
During the metal-catalyzed hydrogenation of HMF, multiple
reactions could occur due to the marvelous structure of
HMF composed of a furan ring and hydroxyl and carboxyl
groups.9 As shown in Fig. 5, apart from the hydrogenation
of the carboxyl group, the occurrence of side reactions (such
as the hydrogenation of the furan ring, the hydrogenolysis
of the C–O bond, the ring-opening of the furan ring, and
the etherification of HMF derivatives) might lead to the
formation of 5-methylfurfural (MF), 5-methyl-2-furanmethanol

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Fig. 5 Reaction pathways for the conversion of HMF.

(MFM), 2,5-dimethylfuran (DMF), ring-opening rearrangement products (RORPs), and etherification products
(EPs).10,48,49 Hence, suppressing the unwanted side reactions
is essential to obtain the high-yield production of BHMF
from HMF.
Considering that a high temperature above 140 °C might
promote the hydrogenolysis reaction,14,50 the influence of the
temperature (ranging from 80 °C to 120 °C) on the catalytic
performance of the Cu/Al2O3 catalyst was investigated first.
As shown in Fig. 6, both the conversion of HMF and the
selectivity of BHMF gradually increased with the increase in
the reaction temperature. Although 93.4% selectivity of
BHMF was obtained at 120 °C, the conversion of HMF was
only 62.6%. It is clear that the catalytic activity of the Cu/
Al2O3 catalyst was insufficient under this reaction condition.
When potassium was introduced into the catalysts, a dramatic change in the reaction results at 120 °C could be observed (Fig. 7). With the increase in potassium content to
1.5 wt%, the conversion of HMF increased to 99.2%. However, further increasing the potassium content (≥3.0 wt%)
led to a rapid decrease in the HMF conversion. Overall, the

Fig. 6 Catalytic performances of the Cu/Al2O3 catalyst in the selective
hydrogenation of HMF to BHMF under different reaction temperatures.
Reaction conditions: 5.0 g catalyst, 30 g L−1 HMF/ethanol, WHSV = 1.0
h−1, 2.0 MPa H2.

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Fig. 7 Effect of K content on the catalytic performances of K–Cu/
Al2O3 catalysts in the selective hydrogenation of HMF to BHMF.
Reaction conditions: 5.0 g catalyst, 30 g L−1 HMF/ethanol, WHSV = 1.0
h−1, 2.0 MPa H2, and 120 °C.

catalytic activity of the K–Cu/Al2O3 catalysts changed with the
size of the copper particles (Fig. 2), which could be interpreted that small metal particles are usually more active than
large metal particles.51–53 As a result, the 1.5K–Cu/Al2O3 catalyst which possessed the smallest size of copper particles
gave the highest catalytic activity.
In order to further evaluate the activity of the catalysts in
this work, the specific reaction rate (mmol h−1 gCu−1) normalized to the amount of Cu in the 1.5K–Cu/Al2O3 catalyst and
other Cu-based catalysts tested in the literature were
compared.14–16,48–50 The reaction rate was calculated
according to the following formula: A = cF/w,54 where c is the
conversion of HMF, F is the HMF flow rate in mmol h−1 (for
batch reactors, F is the amount of HMF divided by the reaction time), and w is the weight of copper in grams. These
catalysts could be classified into two categories: the
unsupported metal catalysts (Cu–ZnO,14 Cu/PMO,50 CuZn
nanoalloy,48 CuIJ50)–SiO2,16 and RANEY®@Cu (ref. 17)) and
the supported metal catalysts (Cu/SiO2 (ref. 15) and 1.5K–Cu/
Al2O3). As shown in Fig. 8, the reaction rate of HMF on the

Fig. 8 Reaction rate of the Cu-based catalysts in the selective hydrogenation of HMF to BHMF.

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Table 2 The activity and selectivity of Cu/Al2O3 and K–Cu/Al2O3 catalysts in the hydrogenation of HMF

Conversion
of HMF (%)

Selectivity (%)

Catalyst

BHMF

5-MF

MFM

RORP

EP

Unknown

Cu/Al2O3
0.5K–Cu/Al2O3
1.5K–Cu/Al2O3
3.0K–Cu/Al2O3
5.0K–Cu/Al2O3

62.6
69.3
99.2
88.0
63.7

93.4
95.6
99.7
98.0
52.5

3.4
2.6
0.2
1.4
3.1

0.3
0.1
0
0
0.2

1.8
1.0
0
0.3
1.0

0.8
0.3
0
0.1
0.3

0.3
0.4
0.1
0.2
42.9

Reaction conditions: 5.0 g catalyst, 30 g L−1 HMF/ethanol, WHSV = 1.0 h−1, 2.0 MPa H2, 120 °C.

1.5K–Cu/Al2O3 catalyst was 150 mmol h−1 gCu−1, which was
notably higher than that of other Cu-based catalysts. It is
known that in contrast to the unsupported metal catalysts,
the supported metal catalysts often possess the higher dispersion of the active metals due to the presence of the supports
maintaining the small metal particles.55,56
Since the catalytic reactions generally occur on the surface
of the metals, for the catalyst with a given mass of a metal,
the high dispersion of the active phase always guarantees a
high reaction rate and effective utilization of metal.57,58 In
this work, copper was supported on alumina, and more importantly, the introduction of an appropriate amount of potassium effectively improved copper dispersion. This resulted
in the superior reaction rate of HMF on the 1.5K–Cu/Al2O3
catalyst to that on the unsupported ones. However, as a similar supported catalyst, the Cu/SiO2 sample showed poor activity because too much copper (56 wt%, about 10 times more
than that in this work) impregnated on the SiO2 support
inhibited the metal dispersion.15
Besides the catalytic activity, the selectivity is also a key
determinant for the high-yield production of BHMF. It can be
seen in Fig. 7 that the selectivity of BHMF showed a volcanic
curve with increasing amount of potassium. It is noteworthy
that the maximum selectivity of BHMF (99.7%) was achieved
on the 1.5K–Cu/Al2O3 catalyst. In order to verify the reason
for the change in the BHMF selectivity, the product distributions of the Cu/Al2O3 and K–Cu/Al2O3 catalysts are given in
Table 2. 5-MF, MFM, RORP, and EP were detected in the
products of the Cu/Al2O3 catalyst, suggesting the occurrence
of hydrogenolysis, ring-opening rearrangement, and etherification reactions (Fig. 5). Increasing the potassium content
to 1.5 wt% (1.5K–Cu/Al2O3) significantly decreased the selectivity of these by-products. It has been demonstrated that the
ring-opening and etherification reactions could be catalyzed
by the acid sites.59–61 Moreover, the acid sites were also confirmed to play a crucial role in the hydrogenolysis of the saturated C–O bond.16 Compared with the Cu/Al2O3 catalyst, the
acidity of the 1.5K–Cu/Al2O3 catalyst was weakened by the introduction of potassium, which suppressed the occurrence of
the side reactions.
It should be noted that further increasing the potassium
content (≥3.0 wt%) led to an observable decrease in the selectivity of BHMF. Although the acidity of the 5.0K–Cu/Al2O3
catalyst was the weakest among all of the samples, the selectivity of BHMF on this catalyst was the lowest. It has been

This journal is © The Royal Society of Chemistry 2018

demonstrated that the basic sites could also catalyze the
hydrogenolysis,
ring
opening,
and
etherification
reactions.2,61–63 Considering the difference in the basic properties in the CO2-TPD profiles (Fig. 3d), it is reasonable that
the formation of 5-MF, MFM, RORP, and EP was promoted
by the stronger basic sites created in the 5.0K–Cu/Al2O3 sample. Moreover, there are many unknown products detected on
the same sample, suggesting the occurrence of some unknown reactions except for the hydrogenation. There is no
doubt that these unknown reactions would enhance the conversion of HMF. In this case, although the 5.0K–Cu/Al2O3 catalyst possessed the larger size of the copper particles, the
HMF conversion was slightly higher than that of the Cu/Al2O3
catalyst.
Based on the above discussion, it can be concluded that
the product distribution of the catalytic hydrogenation of
HMF was strongly dependent on the acid–base property of
the catalysts. The side reactions were highly suppressed on
the 1.5K–Cu/Al2O3 catalyst due to its appropriate acid–base
property. Combined with the fastest reaction rate originating
from the smallest copper particles, the excellent yield of
BHMF (98.9%) was achieved on the 1.5K–Cu/Al2O3 catalyst.

4. Conclusions
In this work, a series of K–Cu/Al2O3 catalysts with different
potassium contents were prepared via a successive incipient
wetness impregnation method and used for the selective hydrogenation of HMF in a fixed-bed reactor. The results indicated that the introduction of potassium could effectively adjust the size of the copper particles and the acid–base
property of the catalysts. The 1.5K–Cu/Al2O3 catalyst with the
highly dispersed copper particles and the appropriate acid–
base property exhibited an excellent performance for the conversion of HMF to BHMF (98.9% in yield). The results of this
work would help in developing catalysts for the industrial
production of BHMF from HMF.

Conflicts of interest
There are no conflicts to declare.

Acknowledgements
This work was financially supported by the Key Research Program of Frontier Sciences of CAS (No. QYZDB-SSW-JSC037),

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Paper

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the Key Research Program of the Chinese Academy of
Sciences (ZDRW-CN-2016-1), the Zhejiang Provincial Natural
Science Foundation of China (LR16B030001), the K. C. Wong
Education Foundation (rczx0800), and the Ningbo Science
and Technology Bureau (2017A610230). Thanks are due to
Guoxin Chen, Yan Liu and Haitao Yu for assistance with the
TEM imaging.

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