An Effective and Inexpensive Hf/ZSM-5 Catalyst for Efficient HMF Formation from Cellulose

Article abstract of DOI:10.1007/s10562-020-03441-3

Abstract: The production of 5-hydroxymethylfurfural (HMF) from cellulose is of great significance for the high-value utilization of biomass, although this route currently faces the challenge of low efficiency. In the present work, a series of effective (x)Hf/ZSM-5 catalysts were developed for HMF production from cellulose, and analyzed by BET, XRD, SEM, ICP, NH₃-TPD, Py-FTIR techniques. Driven by the (5)Hf/ZSM-5 catalyst, HMF with a yield of up to 67.5% and 17.2% of furfural are simultaneously obtained from cellulose in the H₂O_(NaCl)/THF biphasic system. Moreover, after four consecutive cycles, (5)Hf/ZSM-5 catalyst still maintained part of its catalytic activity. The 67.5% HMF yield achieved herein is one of the highest yields achieved in the conversion reaction using cellulose as a substrate, and the catalytic performance of the H₂O_(NaCl)/THF system containing the (5)Hf/ZSM-5 catalyst is even comparable to that of the expensive ionic liquid system. This result reflects the application prospect of (5)Hf/ZSM-5 catalyst in the future industrial production process of HMF. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-020-03441-3

Catalysis Letters
https://doi.org/10.1007/s10562-020-03441-3
An Efective and Inexpensive Hꢀ/ZSM‑5 Catalyst ꢀor Eꢁcient HMF
Formation ꢀrom Cellulose
1
1
1
1
1
1
Ningxin Wu · Manling Zhang · Xiaomei Pan · Jin Zhang · Lijing Gao · Guomin Xiao
Received: 30 July 2020 / Accepted: 24 October 2020
©
Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
The production of 5-hydroxymethylfurfural (HMF) from cellulose is of great signiꢀcance for the high-value utilization of
biomass, although this route currently faces the challenge of low eꢁciency. In the present work, a series of eꢂective (x)Hf/
ZSM-5 catalysts were developed for HMF production from cellulose, and analyzed by BET, XRD, SEM, ICP, NH -TPD,
3
Py-FTIR techniques. Driven by the (5)Hf/ZSM-5 catalyst, HMF with a yield of up to 67.5% and 17.2% of furfural are
simultaneously obtained from cellulose in the H O
/THF biphasic system. Moreover, after four consecutive cycles, (5)
(NaCl)
2
Hf/ZSM-5 catalyst still maintained part of its catalytic activity. The 67.5% HMF yield achieved herein is one of the highest
yields achieved in the conversion reaction using cellulose as a substrate, and the catalytic performance of the H O
/THF
(NaCl)
2
system containing the (5)Hf/ZSM-5 catalyst is even comparable to that of the expensive ionic liquid system. This result
reꢃects the application prospect of (5)Hf/ZSM-5 catalyst in the future industrial production process of HMF.
Graphic Abstract
Keywords Cellulose · HMF · Furfural · Hf/ZSM-5 · Solid acid
Electronic supplementary material The online version of this
1
Introduction
article (https://doi.org/10.1007/s10562-020-03441-3) contains
supplementary material, which is available to authorized users.
The issues of energy scarcity and environmental problems
caused by the rapidly rising resource consumption have
received considerable critical attention [1]. Therefore, in
*
Guomin Xiao
xiaogm426@gmail.com
Extended author information available on the last page of the article
Vol.:(0
1
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N. Wu et al.
order to meet the requirements of sustainable develop-
ment, studies on catalytic conversion of renewable biomass
resources to high-value chemicals have been widely con-
ducted [2, 3]. HMF, which is identiꢀed as one of the most
important biomass platform chemicals because of its active
functional groups, has the ability to be further transformed
into high value-added chemical products such as 2,5-furandi-
carboxylic acid [4], 2,5-dimethylfuran [5], levulinic acid [6]
and dihydroxymethylfuran [7]. Therefore, using biomass
resources as a substrate to prepare HMF products with high
yield and high selectivity is extremely valuable.
considerable yield of 46% HMF products. The comparison
of some C substrates to prepare HMF is summarized in
6
Table S1. Meanwhile, in the few studies, the excellent per-
formance of hafnium (Hf) element as the active center in the
formation of furan-based platform compounds has attracted
our attention. Hu et al. [19] reported that extremely 98%
yield of HMF was obtained from fructose over Hf-MOF
catalyst at 100 °C for 60 min. Xu et al. [20] used silica sup-
ported hafnium phosphide (HfP/SiO ) as catalyst, achiev-
2
ing 85% furfural yield from xylan at 180 °C for 60 min in
the water
/THF biphasic system. Therefore, considering
NaCl)
(
Notably, many eꢂorts have been devoted to the area of
catalytic conversion of biomass into HMF. High-yield HMF
products can usually be realized in the catalytic conversion
of simple biomass-derived monosaccharides such as fruc-
tose and glucose [7, 8]. However, as the main ingredient of
the food industry, the proportion of monosaccharides that
can be distributed for the HMF synthesis is quite limited.
Therefore, the abundant and cheap cellulose is logically the
most suitable substrate for the production of HMF [9]. The
robust crystal structure of cellulose is the main diꢁculty
hindering its eꢁcient conversion. Referring to the previ-
ous studies, there is now a general consensus that acidic
catalysts promote the degradation of cellulose [10, 11]. It is
widely recognized that the conversion of cellulose to HMF
is a synergistic reaction catalyzed by Brønsted and Lewis
acid. This process successively undergoes the depolym-
erization of cellulose catalyzed by Brønsted acid to form
glucose, the isomerization of glucose catalyzed by Lewis
acid to produce fructose, and the subsequent dehydration
of fructose catalyzed by Brønsted acid to generate HMF
the excellent catalytic performance of H-ZSM-5 and Hf ele-
ment, we have developed an idea to combine the beneꢀcial
eꢂects of the two through a simple impregnation method to
prepare a catalyst suitable for the production of furan-based
platform compounds.
To the best of our knowledge, Hf modiꢀed H-ZSM-5
catalysts have not been synthesized and used in the pro-
duction of HMF from cellulose. In the present work, after
screening by reaction conditions, the optimal 67.5% HMF
yield was obtained through using (5)Hf/ZSM-5 in H O
/
2
(NaCl)
THF system at 190 °C for 120 min, one of the highest HMF
yields achieved so far in using cellulose as a substrate. In
addition to HMF, furfural, another useful and important
chemical, was also generated in the (5)Hf/ZSM-5-catalyzed
cellulose conversion process, and the highest yield is 18.3%.
Moreover, the inherent structure of the (5)Hf/ZSM-5 catalyst
remained unchanged after four cycles. These satisfactory
results indicate the industrial application prospects of the
Hf/ZSM-5 catalyst.
[
12]. In response to this catalytic mechanism, a variety of
eꢂective acidic bifunctional catalysts were employed in the
process of catalyzing the synthesis of HMF from cellulose.
Nakajima et al. [13] received 12% HMF yield by using sole
Nb O ·nH O as Lewis acid solid catalyst, and the yield rose
2 Experimental
2.1 Materials
2
5
2
to 48% through adding H PO as Brønsted acid sites. Swift
Cellulose (microcrystalline, 435, 236-250G) was obtained
from Sigma-Aldrich (Shanghai China). 5-hydroxymethyl-
furfural (HMF) and furfural standard products were pur-
chased from Aladdin (Shanghai, China). Hafnium chloride
3
4
et al. [14] reported that vary two types of acid sites (e.g.
in Sn-BEA and HCl) promoting the HMF formation and
a plateau of HMF yield reached through adding Brønsted
acid sites.
(HfCl ) was supplied by Sigma Reagent (Shanghai, China).
4
H-ZSM-5 zeolite is a well-known microporous material,
has the advantages of available pore structure, high surface
area and favorable thermal and hydrothermal stability. Fur-
thermore, H-ZSM-5 zeolite itself processing both Lewis and
Brønsted acid sites, and the acidity can be easily modiꢀed
by metal modiꢀcation (inside or outside the framework) [15,
Tetrahydrofuran (THF) and NaCl were purchased from Sin-
opharm Chemical Reagent Co., Ltd (Shanghai, China). The
H-ZSM-5 zeolite was obtained from The Catalyst Plant of
Nankai University Catalyst Co., Ltd. in Tianjin, China. The
bought H-ZSM-5 was calcined in muꢄe at 550 °C for 2 h
+
to assure its protonic form (H cation), and all purchased
1
6], wherefore ZSM-5 based catalyst is frequently used in
chemicals were used without further puriꢀcation.
the formation of HMF. Moreno-Recio et al. [17] utilized
H-ZSM-5 zeolite as a catalyst to catalyze glucose conver-
sion, and obtained 42% HMF yield at 195 °C for 30 min.
Under the catalysis of bimodal H-ZSM-5 zeolite, Nandi-
wale et al. [18] used cellulose as a substrate to achieve a
2.2 Catalyst Preparation
(x)Hf/ZSM-5 catalysts were synthesized by a simple wet-
ness impregnation method. The typical synthesis procedure
1
3

An Eꢀective and Inexpensive Hf/ZSM-5 Catalyst for Eꢁcient HMF Formation from Cellulose
−
1
was as follows: 2 g H-ZSM-5 and 0.036 g (1 wt%) HfCl4
were mixed in 20 mL deionized water and stirred at room
temperature for 12 h. After stirring, the mixture was dried by
evaporation at 80 °C, and then be put into an oven overnight.
Finally, the target catalyst was obtained through calcining
at 550 °C for 3 h in the air atmosphere and masticating into
powder. The (x)Hf/ZSM-5 catalysts with diꢂerent Hf ele-
ment loading (1 wt%, 3 wt%, 5 wt%, 7 wt%, 10 wt%) was
denoted as (1)Hf/ZSM-5, (3)Hf/ZSM-5, (5)Hf/ZSM-5, (7)
Hf/ZSM-5 and (10)Hf/ZSM-5 respectively. The actual Hf
content in the catalysts was determined by ICP-AES and the
results are collected in Table 1.
200 °C in He ꢃow (40 mL min ) for 1 h. Next, after the cat-
alyst cooling to 50 °C, NH was adsorbed for 0.5 h through
3
exposing the catalyst to NH atmosphere. Then, the catalyst
3
was purged with He ꢃow for 1 h to remove the physisorbed
NH . Finally, desorption of NH was conducted in He ꢃow
3
3
−
1
−1
(40 mL min ) from 50 to 700 °C (10 °C min ).
Pyridine adsorption infrared spectroscopy (Py-FTIR)
was applied to detect the Lewis and Brønsted acidic sites of
the catalysts through a Frontier Fourier Transform Infrared
Instrument. After evacuating at 400 °C and 0.06 Pa for one
hour, the sample was next exposed to pyridine vapor for one
hour at room temperature, and ꢀnally outgassed at 150, 250
and 350 °C. The IR spectra were recorded at a resolution of
−
1
−1
−1
2.3 Catalyst Characterization
4 cm in the range of 1700 cm to 1400 cm .
The GC–MS was used for by-products analyzing. A GC
(Agilent Technologies 7890A) was used at a rate of 10 °C
The elemental analysis of the samples was tested by ICP-
AES (Perkin OPTIMA 5300DV).
−
1
min from 40 to 280 °C, and MS analysis (Agilent Tech-
nologies 5975C) employed a Triple-Axis Detector.
N adsorption–desorption isotherms were recorded
2
at − 196 °C on an automatic 3H-2000PS1 gas sorption
system (BeiShiDe Instrument Technology Co., Ltd., Bei-
jing, China). The speciꢀc surface area was calculated by
Brunauer–Emmett–Teller (BET) equation, and the pore
volume and pore size distribution were measured by Bar-
rett-Joyner-Halenda (BJH) method. Prior to analysis, the
catalysts were degassed under vacuum at 200 °C for 12 h to
remove moisture and volatile impurities.
2.4 Typical Procedure of Catalytic Test
In a typical process, 100 mg cellulose, 50 mg (1)Hf/ZSM-5,
200 mg NaCl and solvent (H O/THF, 1/4 mL) were mixed
2
together into a 15 mL pressure-resistant glass tube, heating
and stirring in oil bath mixer. Start timing once the speciꢀed
temperature was reached and stop heating upon predeꢀned
time. After reaction, the mixture should be cooled to room
temperature and then separated by centrifugal ꢀltration for
further analysis.
The XRD patterns were determined by X-ray diꢂractiom-
eter (XRD, Rigaku Ultima IV), employing Cu–Kα radiation
(
k=0.154 nm, 40 kV, 40 mA) with a rate of 10°/min, scan-
ning range from 5° to 80°.
Scanning electron microscope (SEM, Inspect F50, FEI,
USA) images were obtained to assess the morphology,
microstructure of samples and the element distribution.
2.5 Product Analysis
Quantitative analysis of HMF and furfural in the liquid phase
was determined by HPLC (Shimadzu LC-2010AHT) with
a XDB-C18 column at ultraviolet wavelength of 280 nm at
35 °C, and the mobile phase was a mixed solution with 70
vol% methanol and 30 vol% water ꢃowing at a speed of 0.5 mL
The NH -temperature-programmed desorption
3
(
NH -TPD) was carried out to estimate the acidity of (x)Hf/
3
ZSM-5 catalysts, using a TP-5076 catalyst analyzer (Xian-
quan Industrial and Trading Co., Ltd., Tianjin, China). In
a typical process, 100 mg catalyst was ꢀrstly degassed at
−1
min (the legend of HPLC is shown in Fig. S1). The yields
Table 1 Physicochemical
properties of H-ZSM-5 and (x)
Hf/ZSM-5 catalysts
SBETa (m /g) Vporesb (cm /g) Dmeanc (nm)
2
3
Hf (wt%)d
Si (wt%)d
Catalysts
H-ZSM-5
Hf/Si
343.3
326.6
305.6
304.7
296.2
280.9
0.270
0.233
0.224
0.208
0.199
0.187
0.134
2.62
2.61
2.96
2.97
3.15
3.20
3.30
–
–
–
(
(
(
(
(
1)Hf/ZSM-5
3)Hf/ZSM-5
5)Hf/ZSM-5
7)Hf/ZSM-5
10)Hf/ZSM-5
0.85
2.5
4.3
5.7
7.9
2.1
2.5
2.6
2.8
2.4
2.6
1.4
0.3
1.0
1.5
2.4
3.0
1.5
Recycled (5)Hf/ZSM-5 167.8
a
BET surface area was determined from N adsorption isotherm
Volume of pore was determined from single point desorption method
Average pore size was determined from the desorption average pore diameter(4 V/A by BET)
Determined by ICP-AES
2
b
c
d
1
3

N. Wu et al.
of HMF and furfural were deꢀned according to the following
equations:
3 Results and Discussion
3.1 Characterization of the Catalysts
moles of HMF produced
HMF yield from cellulose(mol%) =
× 100
moles of glucose units
The ICP results were shown in Table 1, testifying the actual
Hf loading of catalysts were close to expected value.
The XRD patterns of the parent H-ZSM-5 zeolite and
synthesized (x)Hf/ZSM-5 catalysts are shown in Fig. 1. It
can be seen from Fig. 1 that the diꢂraction peaks displayed
by the (x)Hf/ZSM-5 catalysts are consistent with the par-
ent H-ZSM-5 zeolite, indicating the introducing of Hf ele-
ment had no signiꢀcant eꢂect on the framework of cata-
lysts. Meanwhile, diꢂraction signals of bulk hafnium oxide
species were not detected, suggesting no accumulation and
sintering of Hf element occur during preparation, and Hf
element in (x)Hf/ZSM-5 catalysts was uniformly dispersed
into the zeolitic framework.
Furfural yield from cellulose(mol%)
moles of furfural produced
=
× 100
moles of glucose units
(
10)Hf/ZSM-5
(
7)Hf/ZSM-5
5)Hf/ZSM-5
Figure 2 displays the SEM images of H-ZSM-5 and (x)
Hf/ZSM-5 catalysts. In all samples, (x)Hf/ZSM-5 catalysts
showed a well-deꢀned hexahedral morphology, consistent
with the structure of crystalline H-ZSM-5 zeolite particles.
Additionally, the obvious attachment appeared with intro-
duction of Hf element, indicating Hf element was success-
fully loading on the surface of H-ZSM-5 zeolite. And from
the result of SEM–EDS (Fig. S2), the Hf element shown was
uniformly dispersed on (x)Hf/ZSM-5 catalysts. These results
are consistent with the above-mentioned XRD analysis.
(
(
3)Hf/ZSM-5
1)Hf/ZSM-5
(
HZSM-5
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
2θ (°)
Fig. 1 XRD patterns of H-ZSM-5 and (x)Hf/ZSM-5 catalysts
Fig. 2 SEM images of H-ZSM-5 and (x)Hf/ZSM-5 catalysts
1
3

An Eꢀective and Inexpensive Hf/ZSM-5 Catalyst for Eꢁcient HMF Formation from Cellulose
Figure S3 shows the N adsorption–desorption isotherms
Figure 4 presents results of Py-FTIR for the (x)Hf/ZSM-5
catalysts, the absorption bands attributed to Lewis and Brøn-
2
of H-ZSM-5 and (x)Hf/ZSM-5, all samples exhibited type
IV isotherms with a clear hysteresis loop, showing a typi-
cal adsorption of microporous H-ZSM-5 and the presence
of mesoporous structure [21]. It can be found from Table 1
that as the Hf element content continues to increase, the
−
1
−1
sted acid sites are found at 1450 cm and 1545 cm , and
−1
another band observed at 1490 cm is related to both Lewis
and Brønsted acid sites. The quantitative analysis of cata-
lyst acidity is summarized in Table 2. It can be clearly seen
that, compared with the parent H-ZSM-5 zeolite, when Hf
element is introduced, the total acidity of the formed (x)Hf/
ZSM-5 catalysts drops suddenly. This result can be attrib-
uted to the active Hf species on the surface covering the acid
sites of the H-ZSM-5 carrier itself [24, 25]. Then with the
increase of Hf content, the total acidity of the series of (x)
Hf/ZSM-5 catalysts gradually increased, indicating that the
active Hf species itself is an acidic active center. Meanwhile,
it can be seen from Table 2 that the ratio of the Brønsted to
Lewis acid sites (B/L ratio) in the (x)Hf/ZSM-5 catalysts
has a trend of ꢀrst decreasing and then increasing, which
demonstrates the acidity variation was tuned by adjusting
Hf amount during preparation. Consequently, it is proven
that (x)Hf/ZSM-5 catalysts possess both Lewis and Brønsted
acid sites, and have the ability to be bifunctional catalysts for
the transformation of cellulose into HMF.
2
speciꢀc surface area (from 320.6 to 280.9 m /g) and pore
3
volume (from 0.233 to 0.187 cm /g) of the corresponding
(
x)Hf/ZSM-5 catalysts gradually decrease. The reason for
this result is that the introduced Hf species blocked the pores
and channels of the parent H-ZSM-5 zeolite [22]. Accom-
panied by the gradual increase in the average pore diameter
of the (x)Hf/ZSM-5 catalysts (from 2.61 to 3.30 nm), this
phenomenon is caused by the existence of excessive avail-
able mesopores due to the blocking of micropores [19, 23].
In order to evaluate the acidic characteristics of the (x)
Hf/ZSM-5 catalysts, NH -TPD analysis technology was
3
employed and the analysis results are shown in Fig. 3. There
were two desorption peaks observed in parent H-ZSM-5
zeolite around 190 °C and 450 °C, corresponding with weak
and moderate acid sites respectively [24]. With the initial
introduction of Hf element (1 wt% and 3 wt%), desorption
peaks at 670 ℃, which are attributed to strong acid sites,
appeared in the correspondingly formed (1)Hf/ZSM-5 and
3.2 Determination of Suitable Hf Content on (x)Hf/
ZSM‑5 for Cellulose Conversion
(
3)Hf/ZSM-5 catalysts. Unexpectedly, after the loading of
Hf element was further increased (5 wt%, 7 wt% and 10
wt%), the strong acid sites in the corresponding (5)Hf/ZSM-
It can be seen from the above analysis that the acidic prop-
erties of (x)Hf/ZSM-5 catalysts could be easily tuned by
altering the amount of Hf element. In order to seek a suitable
acid catalyst for the conversion of cellulose to HMF, various
(x)Hf/ZSM-5 catalysts with diꢂerent Hf content have been
investigated. A biphasic system consisting deionized water
and THF was selected here as it has been proven to be ben-
eꢀcial for HMF formation [26]. As can be seen from Fig. 5,
the HMF yield was less than 15% when there was no catalyst
5
, (7)Hf/ZSM-5 and (10)Hf/ZSM-5 catalysts disappeared.
The result indicates that the (x)Hf/ZSM-5 catalysts with low
loading of Hf species can derive some strong acid sites due
to their high dispersibility. For (x)Hf/ZSM-5 catalysts with
high Hf loading, due to the partial agglomeration of active
Hf species, the strong acid sites formed are covered or dis-
appeared. Similar phenomena have also appeared in other
reported literatures [22–24].
1
545
1490
(B+L)
1450
(L)
(
B)
Weak acid
Moderate acid
(10)Hf/ZSM-5
HZSM-5
(
7)Hf/ZSM-5
(5)Hf/ZSM-5
3)Hf/ZSM-5
(
1)Hf/ZSM-5
3)Hf/ZSM-5
Strong acid
(
(
(
5)Hf/ZSM-5
7)Hf/ZSM-5
(1)Hf/ZSM-5
(
(
10)Hf/ZSM-5
HZSM-5
1
580 1560 1540 1520 1500 1480 1460 1440 1420 1400
100
200
300
400
500
600
700
-
1
Temperature (°C)
Wavenumber (cm )
Fig. 3 NH -TPD proꢀles of H-ZSM-5 and (x)Hf/ZSM-5 catalysts
Fig. 4 Py-FTIR spectra of H-ZSM-5 and (x)Hf/ZSM-5 catalysts
3
1
3

N. Wu et al.
Table 2 Acidity of H-ZSM-5
Catalysts
H-ZSM-5
Leiws acid sites Brønsted acid
Total acidity
(µmol/g)
Acid density
Ratio of Brøn-
sted to Lewis
and (x)Hf/ZSM-5 catalysts
2 a
(
µmol/g)
sites (µmol/g)
(µmol/m )
153.9
37.36
918.13
81.49
1072.03
118.85
207.62
232.8
278.51
382.57
3.12
0.36
0.68
0.76
0.94
1.36
5.97
2.18
1.09
1.04
1.11
1.66
(1)Hf/ZSM-5
(3)Hf/ZSM-5
(5)Hf/ZSM-5
(7)Hf/ZSM-5
(10)Hf/ZSM-5
99.18
108.45
119.15
147.03
238.82
113.65
131.48
143.75
The amount of acid sites was determined by Py-FTIR at 150 °C
a
Acid density=(total acidity/SBET
)
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
ratio. This property makes the three processes of cellulose
depolymerization release glucose, glucose isomerization to
form fructose, and fructose dehydration to produce HMF
under the catalysis of the (5)Hf/ZSM-5 catalyst, always in
the optimal dynamic balance [14, 27]. Therefore, (5)Hf/
ZSM-5 was screened as the optimal catalyst for the process
of preparing HMF from cellulose.
3.3 Optimization of Reaction Parameters
3
.3.1 The Efect oꢀ Reaction Temperature and Reaction
Time on the HMF Yield
After determining the optimal Hf content, the eꢂect of reac-
tion temperature and reaction time on the catalytic conver-
sion of cellulose was further discussed. Primarily, the reac-
tions were monitored as a function of time from 1 to 5 h at
diꢂerent temperature range from 170 to 200 °C, every one
hour and 10 °C as a step, settled in Fig. 6. It was obvious
that the reaction temperature exhibited a signiꢀcant eꢂect
on HMF yield. The three-step tandem for transportation of
cellulose to HMF are endothermic [28, 29], thus higher tem-
perature are beneꢀcial to the formation of HMF. As shown
in Fig. 6, when the reaction proceeded for 1 h, HMF yield
increased with temperature rising from 25.2% (170 °C) to
57.4% (200 °C). However, when the reaction temperature
was extended to 2 h, the HMF yield achieved at 190 °C was
the highest, as high as 60.0%. Moreover, under this standard
(190 °C, 2 h), further increasing the reaction temperature
and prolonging the reaction time all resulted in a decrease
in the HMF yield. This phenomenon implied that the vio-
lent reaction conditions provided excessive reaction energy,
accelerating side reactions such as HMF rehydration and
condensation occur more readily, led to a decrease in HMF
yield and an augment of soluble polymer [30] and humins
[31]. Therefore, the reaction conditions of 190 °C and 2 h
were determined to be the most suitable after a comprehen-
sive screening, which was also used as a test standard for the
remaining parameter evaluation process.
Fig. 5 HMF yield over no catalyst, H-ZSM-5 and (x)Hf/ZSM-5 cata-
lysts. Reaction conditions: cellulose (100 mg), catalyst (50 mg), H O
2
(
1 mL), THF (4 mL), NaCl (200 mg), 180 °C, 180 min
in this reaction, which was intensely lower than that of pos-
sessing catalysts, suggesting the necessary of catalysts for
high cellulose conversion. When (1)Hf/ZSM-5 catalyst (1
wt% Hf content) was used, only 35.5% HMF yield was pro-
vided, lower than that of unmodiꢀed H-ZSM-5 zeolite. The
possible reason is that the adjustment of Hf element altered
the acidic properties of catalyst, which could be proven by
the sharp drop of total acidity from Py-FTIR results. Moreo-
ver, although Hf itself is active site (conꢀrmed by NH -TPD
3
results), the addition of Hf element still covered lots of origi-
nal acid sites. While the Hf element in the (x)Hf/ZSM-5 cat-
alyst increasing slowly, the catalytic eꢁciency improved and
approached a plateau with increasing total acidity. The (5)
Hf/ZSM-5 catalyst (5 wt% Hf content) exhibited the optimal
catalytic performance in all (x)Hf/ZSM-5 samples, obtain-
ing 52.0% yield of HMF, four times over than that with no
catalyst. This result indicates that among the series (x)Hf/
ZSM-5 catalysts, the (5)Hf/ZSM-5 catalyst has the most
suitable acidic properties, including total acidity and B/L
1
3

An Eꢀective and Inexpensive Hf/ZSM-5 Catalyst for Eꢁcient HMF Formation from Cellulose
7
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5
5
4
4
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3
2
2
0
5
0
5
0
5
0
5
0
5
0
7
6
5
0
0
0
1
1
1
2
70 °C
80 °C
90 °C
00 °C
40
30
20
10
0
5
10
15
20
25
Substrate Concentration (wt%)
1
2
3
4
5
Reaction Time (h)
Fig. 8 Eꢂect of substrate dosage on the conversion of cellulose to
HMF. Reaction conditions: (5)Hf/ZSM-5 (25 mg), H O (1 mL), THF
2
(
4 mL), NaCl (200 mg), 190 °C, 120 min
Fig. 6 Eꢂect of reaction time at diꢂerent temperatures on the HMF
yield. Reaction conditions: cellulose (100 mg), (5)Hf/ZSM-5
(
50 mg), H O (1 mL), THF (4 mL), NaCl (200 mg)
2
the occurrence of side reactions such as fragmentation of
furan compounds, the condensation between products and
reactants or intermediates, and promote the formation of
humins and soluble polymers [32, 33].
3
.3.2 The Efect oꢀ Catalyst Dosage and Substrate
Concentration on the HMF Yield
It can be seen from Fig. 8 that when the initial concentra-
tion of cellulose is 5 wt% and 10 wt%, the obtained HMF
yields are at a high level, corresponding to 64.0% and 67.5%,
respectively. As the substrate concentration further increased
to 15 wt% and 20 wt%, the realized HMF yields dropped to
53.5% and 44.7%. To make matters worse, the cellulose with
a concentration of 25 wt% gives only 11.5% of the low-level
HMF yield. The swift decline of HMF yield in agreement
with literature reported [34], that high initial cellulose con-
centrations probably contribute to more sugar monomers
and intermediates in reaction system, which increase side
reactions between these molecules and HMF. Therefore, for
the (5)Hf/ZSM-5-catalyzed conversion of cellulose to HMF,
the appropriate catalyst dosage and substrate concentration
were determined to be 2.5 wt% and 10 wt%, and the optimal
HMF yield achieved under these conditions was an impres-
sive 67.5%.
It is anticipated to obtain a satisfactory HMF yield in a low
catalyst dosage and high cellulose concentration, hence the
eꢂect of catalyst dosage and initial substrate on the cel-
lulose conversion process was studied and the results are
correspondingly summarized in Figs. 7 and 8. When the
dosage of (5)Hf/ZSM-5 catalyst was 2.5 wt%, the yield of
HMF reached maximum value of 67.5%, further increases
in the catalyst dosage (5.0 wt%, 7.5 wt%, 10.0 wt%) led
to a decrease in HMF yield. It is because there are exces-
sive available active sites in the system, which accelerate
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
3.4 Analysis of By‑Products
In the (5)Hf/ZSM-5-catalyzed cellulose conversion process,
in addition to achieving a satisfactory HMF yield, another
valuable result appeared. That is, another high value-added
chemical furfural is also generated simultaneously and the
yield range is as high as 16.7–18.3% (Table 3). The forma-
tion of furfural from cellulose may be due to the retro-aldol
reaction of fructose intermediates in the catalytic process,
which results in the production of xylose [35]. It can be seen
from Table 3 that a furfural yield of 17.2% was achieved
under the reaction conditions of 190 °C for 2 h. It means
1
.0
2.5
5.0
7.5
10.0
Catalyst Dosage (wt%)
Fig. 7 Eꢂect of catalyst dosage on the conversion of cellulose to
HMF. Reaction conditions: cellulose (100 mg), H O (1 mL), THF
2
(
4 mL), NaCl (200 mg), 190 °C, 120 min
1
3

N. Wu et al.
Table 3 Eꢂect of reaction temperature and time on the yields of fur-
6
5
0
0
fural
Entry
Temperature/°C
Time/min
Furfural yield/%
1
2
3
4
5
6
7
8
190
190
190
190
190
170
180
200
60
120
180
240
300
120
120
120
16.7
17.2
17.7
18.3
18.3
17.5
17.5
17.1
40
30
20
10
0
Reaction conditions: cellulose (100 mg), (5)Hf/ZSM-5 (50 mg), H O
2
(
1 mL), THF (4 mL), NaCl (200 mg)
1
st
2nd
3rd
4th
H-ZSM-5
Cycle
that the total yield of furyl products simultaneously achieved
under this condition is 84.7% (HMF: 67.5%, Furfural:
Fig. 9 The recycled (5)Hf/ZSM-5 catalyst and the parent H-ZSM-5
catalyst for HMF production from cellulose. Reaction conditions: cel-
17.2%). For microcrystalline cellulose with a complex struc-
lulose (100 mg), catalyst (50 mg), H O (1 mL), THF (4 mL), NaCl
2
(
200 mg), 190 °C, 120 min
ture and high degree of polymerization, it is particularly rare
to achieve such a high furan-based product yield through
catalytic conversion, which further conꢀrms the excellent
catalytic performance of the prepared (5)Hf/ZSM-5 catalyst.
After the reaction was completed, the organic phase
32% loss of HMF yield after four cycles of use (from 60.0%
to 28.0%). Compared with the 14.1% HMF yield obtained
in the blank control experiment without catalyst addition,
the forth recycled (5)Hf/ZSM-5 could still achieve approxi-
mately twice of that number, implying the partial remaining
of catalytic activity.
(THF) and the water phase in the biphasic system were sepa-
rated, and the by-products dissolved therein were detected.
The GC–MS analysis result of the organic phase showed
that the soluble by-products formed in the cellulose conver-
sion catalyzed by (5)Hf/ZSM-5 were C -C molecular frag-
In order to recognize the reasons of catalyst deactivation,
the texture and acidity properties of recycled (5)Hf/ZSM-5
catalyst have been analyzed. As can be seen from Table 1,
sharply reduced speciꢀc surface area and pore volume indi-
cate the collapse of pore structure. Meanwhile, the loss of Hf
element proves the leaching of active sites [20]. Therefore,
it is reasonable to believe that the collapse of the framework
and the loss of active components are the causes of the deg-
radation of the performance of the (5)Hf/ZSM-5 catalyst,
and these phenomena usually occur with high probability
under severe hydrothermal conditions (190 ℃).
4
15
ments and polymers, such as C H O , C H O, C H O
4
8
2
7
14
12 10
5
and C H O. In the aqueous phase, glucose, as a cellulose
1
5
24
depolymerization monomer, was detected with negligible
yield (<4%). Apart from these soluble by-products, humins
as insoluble by-products has also been found, which can be
directly observed from the dark brown organic phase and
brown substances on the catalysts after the reaction (Fig.
S4). All these detected by-products directly proved the
existence of side reactions such as resiniꢀcation, fragmen-
tation and condensation during the formation of HMF from
cellulose.
3.5 Catalytic Stability of (5)Hf/ZSM‑5
4 Conclusion
The reusability of heterogeneous catalysts is one of its most
important advantage, can not only evaluate the structural
stability of (5)Hf/ZSM-5, but also one of the criteria for
large scale HMF production. After each cycle, the (5)Hf/
ZSM-5 catalyst was recovered by centrifuging, washing with
deionized water and ethanol three times respectively, drying
at 80 °C overnight and calcining in air at 450 °C for 3 h to
eliminate the eꢂects of residual carbon deposition. As shown
in Fig. 9, with increasing number of cycles, the catalytic per-
formance of (5)Hf/ZSM-5 lowered gradually, resulting in a
In summary, a series of (x)Hf/ZSM-5 catalysts were success-
fully synthesized by simple impregnation method, among
which the (5)Hf/ZSM-5 catalyst exhibited an excellent
catalytic performance for HMF production from cellulose.
Under the optimal conditions, an outstanding 67.5% HMF
yield could be achieved using (5)Hf/ZSM-5 as catalyst in
H O/THF (1/4 mL) system at 190 °C for 120 min. At the
2
same time, furfural in the high-yield range, soluble C –C
4
15
molecules and insoluble humus were detected and identiꢀed
as by-products. Furthermore, after four cycles of use, the
1
3

An Eꢀective and Inexpensive Hf/ZSM-5 Catalyst for Eꢁcient HMF Formation from Cellulose
framework structure and catalytic performance of the (5)
Hf/ZSM-5 catalyst are partially maintained, conꢀrming the
potential of (5)Hf/ZSM-5 in conversion cellulose to HMF
and further application.
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(
2016) Chem Eng J 303:22–30
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Acknowledgement This work was financially supported by
the National Key R&D Program of China (No. 2019YFB1504003),
and National Natural Science Foundation of China (No. 21676054).
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Aꢀliations
1
1
1
1
1
1
Ningxin Wu · Manling Zhang · Xiaomei Pan · Jin Zhang · Lijing Gao · Guomin Xiao
1
School of Chemistry and Chemical Engineering, Southeast
University, Nanjing 211189, China
1
3