Hf-based metal organic frameworks as bifunctional catalysts for the one-pot conversion of furfural to Γ-valerolactone

Article abstract of DOI:10.1016/j.mcat.2019.04.010

One-pot conversion of furfural to the target product γ-valerolactone (GVL) is a challenging and meaningful part of biomass exploitation. Development of heterogeneous catalysts with both Br?nsted and Lewis acid properties has proved to be promising and useful because they offer an efficient way to tandem cascade reactions from furfural to GVL. Herein, we successfully synthesized sulfated DUT-67(Hf), a novel bifunctional catalyst, via a post-synthetic modification method. The prepared sulfated DUT-67(Hf) was characterized by XRD, SEM, TEM, N₂ adsorption-desorption, Elemental (N, C, H, S) analyses, situ infrared spectra of pyridine adsorptions, XPS, FT-IR, TG, and NH₃-TPD. The acidity of the catalysts could be adjusted by submersion in different concentrations of aqueous sulfuric acid, giving 0.01 mol/L sulfuric acid for 0.42 mmol/g acidity to 0.1 mol/L sulfuric acid for 2.16 mmol/g acidity. Sulfated DUT-67(Hf) possessed by 0.06 mol/L aqueous sulfuric acid exhibited optimal catalytic activity and showed an 87.1% yield of GVL under the conditions of 180 °C after 24 h of reaction. Moreover, the mechanism of introducing Br?nsted acid sites into DUT-67(Hf) and the better hydrogen donors was also investigated through contrasting experiments.

Full text of DOI:10.1016/j.mcat.2019.04.010

MolecularCatalysis472(2019)17–26
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Hf-based metal organic frameworks as bifunctional catalysts for the one-pot
conversion of furfural to γ-valerolactone
Wenke Lia, Zhe Caia, Hang Lia, Yu Shena, Yongqiang Zhub, Haichao Lib, Xubin Zhanga,⁎
,
Fumin Wanga,⁎
a School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, PR China
b College of chemistry and chemical engineering, Qinghai Nationalities University, Xining 810007, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
One-pot conversion of furfural to the target product γ-valerolactone (GVL) is a challenging and meaningful part
of biomass exploitation. Development of heterogeneous catalysts with both Brønsted and Lewis acid properties
has proved to be promising and useful because they offer an efficient way to tandem cascade reactions from
furfural to GVL. Herein, we successfully synthesized sulfated DUT-67(Hf), a novel bifunctional catalyst, via a
post-synthetic modification method. The prepared sulfated DUT-67(Hf) was characterized by XRD, SEM, TEM,
N2 adsorption-desorption, Elemental (N, C, H, S) analyses, situ infrared spectra of pyridine adsorptions, XPS, FT-
IR, TG, and NH3-TPD. The acidity of the catalysts could be adjusted by submersion in different concentrations of
aqueous sulfuric acid, giving 0.01 mol/L sulfuric acid for 0.42 mmol/g acidity to 0.1 mol/L sulfuric acid for
2.16 mmol/g acidity. Sulfated DUT-67(Hf) possessed by 0.06 mol/L aqueous sulfuric acid exhibited optimal
catalytic activity and showed an 87.1% yield of GVL under the conditions of 180 °C after 24 h of reaction.
Moreover, the mechanism of introducing Brønsted acid sites into DUT-67(Hf) and the better hydrogen donors
was also investigated through contrasting experiments.
Bifunctional catalyst
Biomass exploitation
Hf-based metal organic frameworks
γ-Valerolactone
Furfural
1. Introduction
catalyzed by SiO2-Al2O3, the resulting butene is further applied to
produce liquid C8-C16 alkenes [17]. GVL can also be used as a blending
Because of the gradual exhaustion of fossil resources and the in-
creasing concentration on the global climate change, enormous efforts
have been devoted to searching for an efficient method to reduce re-
liance on fossil resources. Biomass, which is inexpensive, renewable
and abundant in nature, is a sustainable raw feedstock to produce liquid
fuels and valuable chemicals that are conventionally derived from fossil
resource [1,2]. Among the various renewable resources, lignocellulosic
biomass, consisting of cellulose (40–50%), hemicellulose (20–35%) and
lignin (15–25%), is the most available and logical raw feedstock to yield
fuels, fuel additives and various kinds of high-value-added chemicals
[3–11]. As one of the most important industrial products derived from
hemicellulose, furfural (FUR), a colorless, sweet-smelling, mobile li-
quid, is currently deemed a potential platform chemical [12,13]. A
great diversity of value-added chemicals could be obtained from fur-
fural such as furfuryl alcohol (FA), furfuryl alkyl ethers (FEs), levulinic
acid (LA) and γ-valerolactone (GVL) [14–16]. Among those chemicals,
γ-valerolactone has been regarded as a valuable platform chemical and
serves numerous purposes. GVL can be converted to butene when
agent in conventional petrol and as a food additive [17,18]. Ad-
ditionally, people have made use of GVL as a solvent for biomass pro-
cessing, which led to significant progress in product yields and a more
convenient method for producing biomass-derived chemicals [19].
One-pot conversion of furfural to GVL involves complex cascade
reactions [10,16,20]. Roman-Leshkov et al. firstly demonstrated the
one-pot conversion from furfural to GVL through sequential catalytic
transfer hydrogenation (CTH) reaction and hydrolysis reactions. They
achieved it over the physical mixture of Zr-Beta zeolite and Al-MFI-ns
using 2-butanol as a hydrogen donor resulting in a high GVL yield of
78% after 48 h at 393 K [16]. The cooperation of Lewis and Brønst acid
sites catalysts plays an important role in the cascade reaction from
furfural to GVL, in which Zr-Beta zeolite supplies Lewis acids to cata-
lyze the CTH reaction and Al-MFI provides Brønsted acids to catalyze
the ring-opening reaction. Fan et al. recently showed that using the
combination of Au/ZrO2 and H-ZSM-5 as the catalyst, a high yield of
GVL from furfural up to 77.5% could be achieved [21]. Antunes et al.
reported that Sn Al-containing zeolite beta catalyst is also active for the
⁎ Corresponding authors.
E-mail addresses: tjzxb@tju.edu.cn (X. Zhang), wangfumin@tju.edu.cn (F. Wang).
https://doi.org/10.1016/j.mcat.2019.04.010
Received 27 December 2018; Received in revised form 26 March 2019; Accepted 8 April 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

W. Li, et al.
MolecularCatalysis472(2019)17–26
Scheme 1. Production Route from furfural to γ-valerolactone.
one-pot conversion of furfural to GVL using isopropyl alcohol as a hy-
drogen donor [22]. Li et al. used meso-Zr-Al-beta zeolite as a robust
catalyst for the one-pot conversion of furfural to GVL and a high GVL
yield of 90% could be obtained using isopropyl alcohol as a hydrogen
donor after 24 h at 393 K [23]. Winoto et al. reported on the one-pot
transformation of furfural to GVL using heteropolyacid supported on Zr-
Beta zeolite as catalyst, a GVL yield of 70% at 433 K after 24 h could be
obtained [24]. Furthermore, the one-pot conversion of furfural to GVL
could be carried out over ZrO2-SBA-15 [25].
The overall reaction pathway is depicted in Scheme 1. Firstly, the
conversion of furfural to furfuryl alcohol and its ether can be carried out
through a catalytic hydrogenation reaction using isopropyl alcohol as
the hydrogen donor catalyzed by the Lewis acid. Then the conversion of
furfuryl alcohol and its ether to isopropyl levulinate can be achieved
through ring-opening reactions with Brønsted acid sites. Isopropyl le-
vulinate is converted to 4-hydroxy pentanoate isopropyl ester (4-HPPE)
when catalyzed by Lewis acid sites. Then, the target product GVL is
obtained by lactonization of 4-HPPE. Although each individual reaction
has been intensively investigated in terms of various heterogeneous
catalysts and optimal conditions, a single heterogeneous catalyst that
can efficiently catalyze one-pot conversion of furfural to GVL is highly
desirable to reduce the cost of equipment and improve efficiency of
producing GVL.
Metal-organic frameworks (MOFs) have been developed for a broad
variety of applications such as gas sorption, condensation, membrane,
cycloaddition, alkylation, condensation reactions, carbene reactions,
isomerization, energy storage and catalysis [26–37]. As highly crys-
talline porous materials, MOFs are synthesized by using inorganic metal
ions or metal-containing clusters as nodes and organic moieties as lin-
kers through coordination bonds [38]. Large surface areas, tunable
structures and physicochemical properties are acknowledged as sig-
nificant advantages of metal-organic frameworks (MOFs) [39]. From
the viewpoint of catalytic science, large surface areas and accessible
porosities will provide a greater number of active sites for the reactant.
Sizable channels or cages contribute to reagents diffusion, which makes
the active sites more accessible to the reactant [28].
using bifunctional MOFs to catalyze one-pot conversion of furfural to
GVL has not yet been realized.
Inspired by the pioneer work [44,45] and our previous work [46],
we report the synthesis and characterization of bifunctional metal-or-
ganic frameworks (MOFs) sulfated DUT-67(Hf) and its application to
one-pot conversion from furfural to GVL. By introducing sulfate into
DUT-67(Hf), the prepared catalysts exhibit robust Brønsted acidity,
which allows Lewis acid and Brønsted acid sites within a single catalyst.
The synergy of Lewis acidity and Brønsted acidity enables the prepared
catalysts to catalyze the cascade reactions from furfural to GVL effi-
ciently [22]. To our knowledge, prior to this article, no report has been
focused on one-pot conversion of furfural to GVL using bifunctional
metal-organic frameworks (MOFs).
2. Materials and methods
2.1. Materials
The following chemicals were used in our experiment and pur-
chased from the suppliers: ZrCl4 (98%), 2,5-thiophenedicarboxylic acid
(H2TDC, 98%), HfO2 (99.5%), 1,4-benzenedicarboxylic acid (H2BDC,
98%), ZrO2 (98%), 2-Furaldehyde (99%), N-Methyl-pyrrolidone (99%),
formic acid (98%), sulfuric acid (98%), acetic acid (99.7%), propanal
(97%), 2-butanone (99%), benzaldehyde (99%), cyclopentanone
(97%), acetophenone (99.5%), 5-hydroxymethylfurfural (98%), HfO2
(98%), Amberlyst 15 and N,N-dimethylformamide (99.9%) were ob-
tained from Shanghai Aladdin Industrial Inc. Isopropyl alcohol (99.5%),
2-butyl alcohol (99%) and HfCl4 (98%) were purchased from Shanghai
Titan Scientific Co., Ltd. 5-hydroxymethylfurfural (98%) and ethyl le-
vulinate (99%) were bought from Meryer (Shanghai) Chemical
Technology Co., Ltd. Acetone (98%), methanol (99%), ethanol (99.5%)
and chloroform (98%) were purchased from Tianjin Yuanli Chemical
Co., Ltd. All the chemicals were used without further purification.
2.2. Catalyst preparation
Among the numerous series of metal-organic frameworks (MOFs),
Zr/Hf-based MOFs, which possess a wide range of structure types, ex-
cellent thermal abilities and chemical stability, have been used for the
CTH reaction of carbonyl compounds such as furfural, levulinic acid
and its ester [39,40]. From 2016, Valekar et al. first applied MOF-808 to
the catalytic transfer hydrogenation reaction of ethyl levulinate and
obtained a satisfactory yield [41]. Then, Rojas-Buzo et al. synthesized
Hf-MOF-808 and applied it to the Meerwein–Ponndorf–Verley reduc-
tion of several carbonyl compounds with excellent yields [42]. Kuwa-
hara et al. synthesized sulfonic acid-functionalized UiO-66 used for the
catalytic transfer hydrogenation of levulinic acid and its ester [43].
With these advantageous properties, Zr/Hf-based MOFs are expected to
be applicable catalysts for biomass transformation [40]. However,
2.2.1. Synthesis of UiO-66(Zr) and UiO-66(Hf)
UiO-66(Zr) and UiO-66(Hf) were prepared according to a typical
procedure [47,48]. UiO-66 samples were synthesized by sequentially
adding ZrCl4/ HfCl4 (3.104 g/2.268 g, 9.7 mmol), 2.203 g 35% HCl
(55.6 mmol) and 3.234 g H2BDC to a 250 mL conical flask containing
58.4 mL N, N-dimethylformamide. Then the mixture was sonicated for
20 min until completely dissolved. The resulting solution was trans-
ferred into a 100 mL Teflon kettle, which was subsequently heated at
160 °C for 16 h. After cooling to room temperature the resulting solu-
tion was centrifugation. The obtained powders were washed first with
DMF (3 x 30 mL) and then with ethanol (2 x 30 mL). Then the result
powders were dried in vacuum at 120 °C.
18

W. Li, et al.
MolecularCatalysis472(2019)17–26
2.2.2. Synthesis of DUT-67(Zr) and DUT-67(Hf)
thermogravimetric (TG) curve was recorded.
In
a
typical synthesis [49,50], ZrCl4/ HfCl4 (0.466 g/0.641 g,
X-ray photoelectron spectroscopy (XPS) was performed on a Thermo
ESCALAB 250XI. Elemental (N, C, H, S) analyses of samples were per-
formed by EURO EA3000.
2 mmol) was added to the mixture of DMF (25 mL) and NMP (25 mL)
and dissolved by sonication 10 min. Then H2TDC (0,234 g, 1.34 mmol)
was added to the mixture and sonicated 5 min. After that formic acid
(9 mL, 240 mmol) was added. The resulting solution was placed into a
100 mL Teflon kettle for 48 h at 120 °C. After cooling to room tem-
perature, the resulting solution was centrifugation and obtained pow-
ders. The obtained powders were filtered and firstly washed with DMF
(3 x 30 mL) and then with ethanol (2 x 30 mL). The obtained powders
were dried in vacuum at 120 °C for 12 h before reaction.
The density of Lewis and Brønsted acid sites was collected on a
Thermo Nicolet-380 spectrometer at 4 cm−1 resolution by using pyr-
idine as the probe molecule at 4 cm−1 resolution. Before the test, ap-
proximately 25 mg sample was pressed into a 13 mm self-supported
wafer and this wafer was dried at 673 K for 3 h at 10-3 Pa. After that, the
wafer was purged with a stream of nitrogen to break the vacuum and
the background was scanned until the wafer was cooled to room tem-
perature. The wafer was purged with pyridine vapor and nitrogen for
0.5 h, respectively. Finally, the wafer was heated to 423 K at a rate of
10 K min−1 and kept at 423 K for 1.0 h to remove physisorbed pyridine.
The concentration of the Lewis and Brønsted acid sites was calculated
via Eqs. (1) and (2), respectively.
2.2.3. Post-synthetic modification of pristine DUT-67(Hf) [44]
As-synthesized DUT-67(Hf) powders were immersed in anhydrous
DMF for three days followed by water for three days, during which time
the solvent was exchanged three times per day. Then roughly 0.5 g of
water-exchanged DUT 67-(Hf) crystals were immersed in 50 mL of X
(X = 0.01, 0.03, 0.05, 0.06, 0.07, 0.09, 0.1, 0.2, 0.4 mol/L) sulfuric acid
for 24 h with continuous stirring. The crystals were then solvent ex-
changed with water for 12 h (water exchanged three times), quickly
exchanged with anhydrous acetone for 12 h (anhydrous acetone ex-
changed three times) and immersed in anhydrous chloroform for 12 h
(chloroform was exchanged three times). The chloroform in the solvent
exchanged crystals was removed in vacuum at 120 °C for 8 h. Sulfated
UiO-66(Hf) was sulfated by the same method.
1.42 × IAL × R2
cL(μmol·g-1) =
cB(μmol·g-1) =
(1)
W
1.88 × IAB × R2
(2)
W
cL and cB represent the concentration of Lewis and Brønsted acid sites,
respectively.IAL and IAB stand for the integrated absorbance peak at
1450 cm−1 for Lewis acid sites and peak at 1540 cm−1 for Brønsted
acid sites, respectively. R is the radius of the self-supported wafer. W is
the weight of the sample.
2.2.4. Catalysts naming rule
A series of sulfated DUT-67(Hf) were named as DUT-67(Hf)-X
(X = 0.01, 0.03, 0.05, 0.06, 0.07, 0.09, 0.1, 0.2, 0.4) where X re-
presented concentrations of sulfuric acid using for catalysts treatment.
Pristine DUT-67(Hf) represented DUT-67(Hf) that was not treated by
sulfuric acid. Sulfated UiO-66(Hf) was named by the same method.
2.4. Catalytic test and product analysis
In a typical experiment, 0.3 g catalyst, 0.05 mol/L furfural in 25 mL
isopropyl alcohol were added to a 50 mL stainless steel autoclave,
which was equipped with a temperature regulating device, a pressure
gauge and a magnetic stirrer. Then the reaction vessel was sealed,
purged, and pressurized with 0.6 MPa of N2 and then heated to the
desired temperature to initiate the reaction. The reaction was carried
out at a certain temperature for a desired time with magnetic stirring at
500 rpm. Then, the autoclave was cooled down to room temperature
with tap water. Then, the resulting solution was separated by cen-
trifugation. We collected the supernatant as the samples. The samples
that we obtained were quantitatively analyzed by GC (gas chromato-
graphy, Shimadzu 2010PLUS). The identifications of the compound
were tested by GC–MS (Shimadzu GCMS-QP 2010).The recyclable
catalysts were washed thoroughly with ethanol (5 × 30 mL) and dried
at 120 °C under vacuum. The conversion of furfural and the selectivity
of GVL were calculated by the following formula.
2.3. Catalyst characterization
X-ray diffraction (XRD) patterns were recorded on a D8-Focus,
Bruker X-ray diffractometer using a Cu Kα radiation at 40 KV and
40 mA (incident angle = 2°, λ = 1.541 Å, step size = 6° min−1). X-ray
diffraction (XRD) patterns were collected in the range of 2θ from 5° to
80° with a scanning rate of 6° min−1 and step size of 0.02°.
The specific surface area (SBET), pore volume and pore size dis-
tribution of the catalysts were obtained from N2 adsorption-desorption
isotherms measured on NOVA 2000 analyzer at −196 °C. Prior to the
measurement, samples were degassed at 150 °C under vacuum for 6 h.
The average pore diameter is given by 4 V total/SBET.
Scanning electron microscope (SEM) analysis was performed on a
FEI-Quanta 200F field-emission scanning microscope operated at 15 kV
with an EDX detector to determine the morphology of the prepared
samples. FT-IR analysis was carried out on a Nicolet 6700 FT-IR
Spectrometer.
initial moles of furfural − final moles of furfural
conversion(%) =
initial moles of furfural
(1)
× 100
moles of producti formed
initial moles of furfural
The acidic properties of the catalysts were measured using NH3-
TPD. The TPD profiles of catalysts were measured on a Micromeritics
PX 200 apparatus equipped with a thermal conductivity detector.
Samples were activated at 150 °C for 12 h in a helium flow, prior to the
adsorption step. Subsequently, activated samples were exposed to
10 vol. % NH3 in helium with a flow rate of 50 mL min−1 at 50 °C for
60 min to absorb NH3 onto the solid samples. Then physically adsorbed
NH3 gas were removed by purging with helium gas for 1 h at the same
temperature and flow rate. TPD data were recorded from 50 °C to
yieldi (%) =
× 100
(2)
(3)
yieldi
conversion
selectivity (%) =
× 100
i
3. Results and discussion
3.1. Catalyst characterization
300 °C with a heating rate of 5 °C min−1
.
The carbon depositions of the catalysts were determined by the
thermogravimetric analysis (TGA) using a Mettler Toledo TGA/DSC 2
instrument in the air atmosphere. Typically, 0.01 g of catalyst sample
was pre-treated at 373 K to remove the physical absorption water/or-
ganic and then heated to 773 K with a rate of 10 K min−1 in flowing air
To introduce Brønsted acid sites into DUT-67(Hf) and adjust the
acidity of sulfated DUT-67(Hf), different concentrations of aqueous
sulfuric acid (0.01 mol/L–0.1 mol/L) were employed for the acid
treatment step. The prepared sulfated DUT-67(Hf) samples were char-
acterized by XRD, SEM, TEM, TEM-EDS, N2 adsorption-desorption, XPS,
situ infrared spectra of pyridine adsorptions, Elemental (N, C, H, S)
(20 mL min−1).
Meanwhile,
the
temperature
dependent
19

W. Li, et al.
MolecularCatalysis472(2019)17–26
Fig. 1. (a) XRD patterns of sulfated DUT-67(Hf) treated with varied concentrations of sulfuric acid (top to bottom XRD patterns represent aqueous sulfuric acid
concentration of 0, 0.01, 0.03, 0.05, 0.06, 0.07, 0.09, and 0.1 mol/L). (b) FT-IR spectra of sulfated DUT-67(Hf) treated with varied concentrations of sulfuric acid (top
to bottom XRD patterns represent sulfuric acid concentration of 0, 0.01, 0.03, 0.05, 0.06, 0.07, 0.09, and 0.1 mol/L). (c) N2 adsorption-desorption isotherms of
sulfated DUT-67(Hf). (d) NH3-TPD profiles of sulfated DUT-67-(Hf).
analyses, TG analysis, FT-IR and NH3-TPD techniques. The influences of
the various concentrations of aqueous sulfuric acid on catalysts were
initially investigated by XRD analysis, as shown in Fig. 1a. XRD patterns
of sulfated DUT-67(Hf) coincided with that of pristine DUT-67(Hf) ex-
hibiting three distinct peaks at 2θ = 6.5°, 7.9°and 9.1° [49]. The mea-
surements showed that, XRD patterns for sulfated DUT-67(Hf) showed
no change after treatment by 0.01 mol/L-0.1 mol/L sulfuric acid. A
series of sulfated DUT-67(Hf) possessed complete crystalline structures.
The prepared sulfated DUT-67(Hf) was further characterized by FT-
IR. According to the reported literature, evidence for metal-bound
sulfate groups could be observed from infrared spectroscopy [44]. Four
new bands were observed at 1032 cm−1, 1138 cm−1, 1232 cm−1 and
1358 cm−1 from the infrared spectroscopy of sulfated DUT-67(Hf)
(Fig. 1b). The absorption bands appeared at 1138/1232 cm−1 and the
shoulder peaks at approximately 1358 cm−1 correspond to the sym-
metric and asymmetric stretching vibrations of O]S]O bonding, re-
spectively. The new band at 1032 cm−1 are associated with the
stretching mode of S]O bonding.
In addition, the NH3-TPD profiles of the samples demonstrated an
obvious difference between pristine DUT-67(Hf) and sulfated DUT-
67(Hf). As shown in Fig. 1d, pristine DUT-67(Hf) featured a broad
desorption peak centered at 160 °C and a small desorption peak cen-
tered at 205 °C. After acid treatment, these two desorption peaks were
enhanced remarkably, which indicated that sulfated DUT-67(Hf) ex-
hibited stronger acidity than pristine DUT-67(Hf). In the case of DUT-
67(Hf)-0.06, two desorption peaks were centered at 165 °C and 210 °C,
corresponding to presence of the weak and strong acid sites respectively
[24]. The desorption peak centered at 165 °C can be ascribed mainly to
the framework Hf and a few missing-linker carboxylate radical sites
adsorbing some NH3 molecules. In contrast, the existence of a new
broad desorption peak centered at 210 °C could be attributed to the
desorption of NH3 molecules of the sulfate group. The broad desorption
peak centered at approximately 210 °C was enhanced remarkably.
Taking all that into consideration, we can adjusted the acidity of sul-
fated DUT-67(Hf) by controlling the concentrations of immersed sul-
furic acid, which is supported by quantitative analysis of NH3-TPD. As
listed in Table 1, the acidity gradually increased from 0.28 to
Table 1
Surface area, volume and porosity of different catalysts.
Catalyst
SBET
(m2/g)
Vpore
Daverage (nm) Acidityd
a
b
c
(cm3/g)
(mmol/g)
Pristine DUT-67(Hf) 764
0.35
0.46
0.43
0.42
0.38
0.32
0.37
0.35
0.45
0.41
0.87
1.15
1.08
0.90
0.90
0.92
0.88
0.91
1.47
1.51
0.28
0.45
0.84
1.28
1.58
1.75
1.92
2.2
DUT-67(Hf)-0.01
DUT-67(Hf)- 0.03
DUT-67(Hf)-0.05
DUT-67(Hf)-0.06
DUT-67(Hf)-0.07
DUT-67(Hf)-0.09
DUT-67(Hf)-0.1
Pristine UiO-66(Hf)
UiO-66(Hf)-0.1
801
748
694
636
606
565.3
532
601
719
0.24
0.25
dAmount of acid sites were determined by NH3-TPD.
a
BET surface areas were determined by BET method.
Volume of pores was estimated from BJH Adsorption cumulative volume of
b
pores.
c
Average pore size was estimated from the adsorption average pore dia-
meter.
20

W. Li, et al.
MolecularCatalysis472(2019)17–26
2.2 mmol/g with the increase in the concentration of aqueous sulfuric
acid from 0.01 to 0.1 mol/L. According to reported articles [51],
missing-linker carboxylate radical sites and hydroxyl linker within
hafnium clusters can act as Brønsted acid sites. However, their acidity is
much weaker than the sulfuric acid group. As a result, determined
Brønsted acid sites are mainly provided by metal-bound sulfate groups
linked with hafnium clusters.
In addition, N2 adsorption-desorption isotherms of these samples
featured typical isotherms of microporous solids which usually exhibit a
rapid increase in the low relative pressure area [43]. The amount of
adsorbed N2 decreases distinctly with the increase in the concentrations
of sulfuric acid (Fig. 1c). The physicochemical properties and textural
properties of samples are summarized in Table 1. The pristine DUT-
67(Hf) adopts a large surface area (764 m2/g) and total pore volume
(0.35 cm3/g). In addition, DUT-67(Hf)-0.01 featured the largest surface
area (801 m2/g). Other sulfated DUT-67(Hf) samples exhibited de-
creasing surface areas in accordance with the increase in the con-
centration of sulfuric acid. In the case of DUT-67(Hf)-0.1, it exhibited
total pore volume (0.35 cm3/g) and a smallest surface area (532 m2/g)
among the sulfated DUT-67(Hf).
SEM images show that the particles of both pristine DUT-67(Hf) and
sulfated DUT-67(Hf) adopt a diameter of approximately 0.6 μm with
uniform shapes and truncated octahedral morphology (Fig. 2a). Pristine
DUT-67(Hf) and sulfated DUT-67(Hf) exhibited smooth surfaces and
rough surfaces, respectively (Fig. 2b). It should be pointed out that
morphology of sulfated DUT-67(Hf) has been slightly broken due to the
acid treatment step. The TEM-EDS mapping images of the sulfated DUT-
67(Hf)-0.06 show the even connections of H2TDC with Hf species
(Fig. 2c) [51]. According to the images of TEM-EDS mapping, the dis-
tributions of element Hf and S are relatively concentrated (Fig. 2d).
Element Hf and S mainly derived from Hf clusters and H2TDC, re-
spectively (Fig. 2f). Moreover, sulfate anions chelating to a single haf-
nium atom also provide some element S. By contrast, the distributions
of element O and C are more dispersed (Fig. 2e).
One of the drawbacks of MOFs is relatively lower chemical and
thermal stability compared to zeolites, which limits its applications in
various fields [38]. Nevertheless, Zr/Hf-based MOFs exhibit good che-
mical, thermal, and mechanical stability because the bonds between Zr/
Hf and O are robust. Moreover, the higher coordination numbers of Zr/
Hf in the framework also contribute to its stability [47]. The thermal
stability of sulfated DUT-67(Hf)-0.06 was examined by TG analysis in
the range of 50–800 °C, as shown in Fig. 3. The decomposition of sul-
fated DUT-67(Hf)-0.06 from 50 °C to 350 °C is ascribed mainly to the
loss of H2O and SO42−. The weight of sulfated DUT-67(Hf)-0.06 de-
creased by approximately 23.5% in the temperature range of
350–650 °C, which illustrates that the framework of sulfated DUT-
67(Hf)-0.06 begins to break up at approximately 350 °C.
The node connectivity and proton topology in DUT-67(Hf) are
shown in Fig. S6 [40]. DUT-67(Hf) is a typical 8-connected metal-or-
ganic framework with a reo topology, in which each Hf cluster is 8-fold
connected to the H2TDC linker [48,49]. DUT-67(Hf) featured Hf6O8
clusters as nodes that need to absorb DMF and HCOO− to compensate
for the absent carboxylate. For 8-connected MOFs DUT-67(Hf), the
absorbed solvent can be removed after being dried in vacuum at 120 °C.
After activation, the Hf6O8 clusters used to absorb solvent to compen-
sate for absent carboxylate will become unsaturated. The activated
catalysts that expose more active sites exhibit better catalytic activity
than before.
DUT-67(Hf) consists of 8-connected Hf6O8 clusters and 2, 5-thio-
phenedicarboxylate as the ligand. Formic acid acts as modulator and
affects the secondary building units (SBUs). Hf6O6(OH)2(tdc)4(HCOO)2
is formed as the secondary-building unit (SBU) by connecting Hf clus-
ters to carboxylate terminated organic links. The approach to introdu-
cing Brønsted acid sites into DUT-67(Hf) is modifying secondary
building units (SBUs) with inorganic acidic sides, such as sulfuric acid
[44]. The number of Brønsted acid sites could be controlled by
adjusting the concentration of aqueous sulfuric acid. It can be seen in
Fig. 4, DUT-67(Hf)-0.06 exhibited stronger Brønsted acidity than pris-
tine DUT-67(Hf) [51]. In the process of submersion, HCOO− used to
balance charge is replaced by SO42-. The secondary building units
(SBUs) are modified into Hf6O6(OH)2(tdc)4(SO4)(H2O) (Fig. 5). In ad-
dition, according to the results of elemental analysis, the ratio of Hf/S
could be calculated (Table S4). Because the ligand 2,5-thiophenedi-
carboxylic acid contains the sulfur element, initial atomic Hf/S of
pristine DUT-67(Hf) is 0.95. After acid treatment, the ratio of Hf/S
decreased gradually. Based on the Hf/S of pristine DUT-67(Hf), we
could calculate the accurate name of DUT-67(Hf)-xxx SO4 (Table S4).
Moreover, TG analysis of those samples was also performed (Fig. S7).
Ratio of weight loss of those samples was summarized in Table S5.
According to the reported literature, weight loss between 50 °C˜375 °C
mainly attribute to the loss of DMF, COOH− and SO42− [50]. Due to
the increase of SO42− coordinated to Hf clusters, weight loss between
50 °C–375 °C increase from 16.1% to 20.5%, corresponding to pristine
DUT-67(Hf) and DUT-67(Hf)-0.1, respectively. The post-synthetic
modification method could be achieved by submerging pristine DUT-
67(Hf) in aqueous sulfuric acids (0.01–0.4 mol/L).Then, the prepared
catalysts are activated under a dynamic vacuum and heating at 120 °C,
after which the sulfate anions used to coordinate via a bridging mode to
two hafnium atoms convert to exclusively the chelating mode to a
single hafnium atom. The water molecule absorbed on another hafnium
atom will participates in a hydrogen bond with the sulfate anion. The
reason for Brønsted acidity in DUT-67(Hf) is shown to consist of the
chelating mode of sulfate and the terminal water molecule [45].
3.2. One-pot conversion of furfural to γ-valerolactone
According to the reported literature and our experimental results
[16,21,42,52], the cascade reactions for one-pot conversion of furfural
to GVL needs both Lewis and Brønsted acids. On one hand, Lewis acid
sites, which are derived mainly from Hf4+, catalyze the CTH process
including the transformation of furfural to furfuryl alcohol and iso-
propyl levulinate to 4-HPPE. On the other hand, Brønsted acid sites,
which derive mainly from to a specific arrangement of adsorbed water
and sulfate moieties on the hafnium clusters, are chiefly effective for the
ring-opening reaction of furfuryl alcohol. With the synergy of Brønsted
and Lewis acid sites, one-pot conversion of furfural to GVL could be
achieved. However, as has been discussed in reported literature [16],
intensity of Brønsted acid has an influence on the CTH process.
Therefore, adjusting the intensity of the Brønsted acid of catalysts and
searching optimized conditions are outstanding issues. To solve those
problems we synthesized series of sulfated DUT 67-(Hf) with adjustable
acidity via post-synthetic modification method.
There are relatively few studies devoted to studying bifunctional
metal-organic framework used to catalyze the one-pot conversion of
furfural to GVL. To conduct in-depth research on the reaction, a series
of sulfated DUT-67(Hf) were employed in the experiments and the re-
sults are summarized in Table 2 Pristine DUT-67(Hf) with both Lewis
acid sites and few Brønsted acid sites cannot effectively catalyze the
conversion of furfural to GVL and showed 100% conversion of furfural
after 8 h of reaction to give furfuryl alcohol (FA) as a major product
with a 99% yield via a CTH reaction with isopropyl alcohol as the
hydrogen donor (Entry 1). In contrast, the sulfated DUT-67(Hf) can
effectively catalyze the conversion of furfural to GVL (Entry 2–10).
Sulfated DUT-67(Hf) processed by 0.06 mol/L aqueous sulfuric acid
exhibits optimal catalytic activity. Under identical reaction conditions
within 8 h at 180 °C, yields of GVL increased in the order of DUT-
67(Hf)-0.01 < DUT-67(Hf)-0.4 < DUT-67(Hf)-0.1 < DUT-67(Hf)-
0.09 < DUT-67(Hf)-0.2 < DUT-67(Hf)-0.03 < DUT-67(Hf)-
0.05 < DUT-67(Hf)-0.07 < DUT-67(Hf)-0.06. The highest GVL yield
was achieved over DUT-67(Hf)-0.06, completely converting furfural
and producing a 70.7% yield of GVL (Entry 5).
Initially, the main side products are furfural alcohol and its ester,
21

W. Li, et al.
MolecularCatalysis472(2019)17–26
Fig. 2. (a) SEM image of DUT-67(Hf). (b) SEM image of sulfated DUT-67(Hf). (c) TEM image of sulfated DUT-67(Hf)-0.06. (d) (e) (f) TEM-EDS mapping images of
sulfated DUT-67(Hf)-0.06.
isopropyl levulinate and 4-HPPE. With the increase in Brønsted acid
sites, the content of furfural alcohol and its ester were reduced greatly
in amount, which implied that the number of Brønsted acid sites has an
impact on the conversion of furfural alcohol and its ester to isopropyl
levulinate (Entry 2–4). When Brønsted acid sites reached a certain
level, furfural alcohol and its ester disappeared almost completely, and
the major side products are isopropyl levulinate and 4-HPPE (Entry 5).
However, the yields of GVL decreased when the intensity of Brønsted
acid passed beyond the appropriate level. At the same time, the content
of isopropyl levulinate increased, which suggested that superabundant
Brønsted acid sites can restrain the conversion of isopropyl levulinate to
GVL (Entry 6–10). Considering that the main side products are inter-
mediate products that can be converted to GVL with a longer reaction
time, we prolonged the reaction time to 20 h and the yield of GVL in-
creased markedly and reached 84.9% (Entry 12).
To achieve an incisive understanding of the mechanism of Brønsted
acid contained in the metal-organic framework, we did a series of
contrasting experiments (Table 3). As has been discussed above, DUT-
67(Hf) is a typical 8-connected metal-organic framework. By contrast, it
Fig. 3. TG curves of sulfated DUT-67(Hf)-0.06. TDC2− represents 2, 5-thio-
phenedicarboxylic acid radical.
22

W. Li, et al.
MolecularCatalysis472(2019)17–26
was obtained whether using pristine UiO-66(Hf) or sulfated UiO-66(Hf)
as catalysts. The major difference between the two catalysts is that
using sulfated UiO-66(Hf) as the catalyst will generate much more
furfuryl alcohol isopropyl ester. Based on reported literature [53], a
small quantity of Brønsted acidity can promote an ester exchange re-
action such as furfural alcohol to furfuryl alcohol isopropyl ester.
As has been discussed in the reported literature, length of the alkyl
chains of levulinate esters affects yields of GVL [43,54]. According to
results from GCeMS, the main by-product of one-pot transformation
from furfural to GVL was isopropyl levulinate. However, compared to
methyl levulinate and ethyl levulinate, isopropyl levulinate possesses
complex alkyl groups, which will hinder the reaction process. It has
been reported that ester exchange could be achieved among different
levulinate esters catalyzed by Brønsted acid. To improve the yield of
GVL, we added quantitative methanol or ethanol into the reaction
system and expected that the main by-product, isopropyl levulinate,
would be transformed into methyl levulinate or ethyl levulinate. As
shown in Table 3 the yield of GVL increased approximately 3.2% after
adding quantities methanol into the reaction system (Entry 3). How-
ever, when adding an identical quantity of ethanol into the solvent, the
yield of GVL increased by only 0.4% (Entry 4). The result is in accord
with the theory that methyl levulinate is easily converted into GVL
compared to ethyl levulinate. The yields of GVL declined when more
methanol or ethanol was added into the reaction system. These results
may arise from competitive adsorption between primary alcohols and
secondary alcohols (Entry 5–6). Too much primary alcohol may hinder
the secondary alcohol from contacting the catalyst.
Fig. 4. Situ infrared spectra of pyridine adsorptions of pristine DUT-67(Hf) and
DUT-67(Hf)-0.06.
Fig. 6 depicts the time profiles of the yields of products in the one-
pot conversion of furfural to GVL catalyzed by DUT-67(Hf)-0.06. Under
the standard reaction conditions, the conversion of furfural reaches
100% within 4 h of reaction. Furfural alcohol, sec-propyl furfuryl and
isopropyl levulinate are observed as the major intermediate products at
the incipient stage of reaction (within 4 h). With the further evolution
of the reaction, the yields of FA/FE and isopropyl levulinate gradually
decrease, while the yield of GVL increases respectively. Concurrently,
the amount of 4-HPPE is also observed and gradually becomes one of
the major by-products. When the reaction time is prolonged to 12 h and
longer times, the yields of GVL increases much more slowly than before,
maybe because it is difficult to transform isopropyl levulinate to GVL
due to long alkyl chains retarding ring formation reaction. Finally, the
GVL yield can reach 87.1% after 24 h under the optimized reaction
condition.
Fig. 5. The structural transformation from pristine DUT-67(Hf) to DUT-67(Hf)-
0.1SO4. Atom labeling scheme: Hf, blue polyhedral; C, gray; O, red, S, yellow.
All H aatoms are omitted for clarity. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this
article.)
is commonly known that UiO-66(Hf) is typical 12-connected metal-or-
ganic framework [47]. The formation of its secondary building unit
(SBU) is Hf6O4(OH)4(BDC)6, which means all Hf4+ contained in UiO-
66(Hf) Hf6O8 clusters are saturated. Therefore, it is difficult to in-
troduce sulfate anions and terminal water ligand into UiO-66(Hf) via a
post-synthetic modification method. This is supported by the results of
NH3-TPD and contrasting experiments. According to the NH3-TPD test,
the acidity of pristine UiO-66(Hf) and sulfated UiO-66(Hf) were
0.24 mmol/g and 0.25 mmol/g respectively. After acid treatment, the
acidity of UiO-66(Hf) only shows a slight increase, consistent with the
observation from contrasting experiment (Entry 1–2). Low GVL yield
3.3. Catalyst recycle and characterization
3.3.1. Heterogeneous property of the sulfated DUT-67(Hf)
To examine the heterogeneous properties of sulfated DUT-67(Hf),
we removed the catalyst from the reaction system after reaction for 4 h,
and the resulting mixture was subjected to reaction for an additional 4 h
(Fig. 7). Based on the results of gas chromatography, no further reaction
Table 2
One-pot conversion of furfural to GVL catalyzed by sulfated DUT-67(Hf).
Entry
Catalyst
Time (h)
Conversion (%)
Selectivity (%)
GVL yield (%)
1
2
3
4
5
6
7
8
Pristine DUT-67(Hf)
DUT-67(Hf)-0.01
DUT-67(Hf)-0.03
DUT-67(Hf)-0.05
DUT-67(Hf)-0.06
DUT-67(Hf)-0.07
DUT-67(Hf)-0.09
DUT-67(Hf)-0.1
DUT-67(Hf)-0.2
DUT-67(Hf)-0.4
DUT-67(Hf)-0.06
8
8
8
8
8
8
8
8
8
8
24
100
99.1
100
100
100
100
100
100
100
100
100
2.1
2.1
32.3
48.9
61.7
70.7
64.1
48.3
41.4
39.7
32.6
87.1
32.0
48.9
61.7
70.7
64.1
48.3
41.4
39.7
32.6
87.1
9
10
12
Reaction condition: 0.05 mol/L furfural in 25 mL isopropyl alcohol, catalysts 0.3 g, 180 °C.
23

W. Li, et al.
MolecularCatalysis472(2019)17–26
Table 3
One-pot conversion of furfural to GVL by various hydrogen donors.
Entry
Catalyst
solvent
Yield %
FA
FE
IPL
4-HPPE
GVL
1a
2a
3b
4b
5b
6b
Pristine UiO-66(Hf)
UiO-66(Hf)-0.1
DUT-67(Hf)-0.06
DUT-67(Hf)-0.06
DUT-67(Hf)-0.06
DUT-67(Hf)-0.06
isopropyl alcohol
isopropyl alcohol
isopropyl alcohol + methanol(22.5:2.5)
isopropyl alcohol + methanol(20:5)
isopropyl alcohol + ethanol(22.5:2.5)
isopropyl alcohol + ethanol(20:5)
79.3
46.9
0
0
0
11.9
40.2
0
0
0
2.1
4.9
8.1
10.2
11.2
10.1
0
0
3.3
4.7
3.4
4.3
2.7
8.0
88.1
82.1
85.3
79.6
0
0
a
Reaction condition: 0.05 mol/L furfural in 25 mL isopropyl alcohol, catalysts 0.3 g, 180 °C and 8 h.
Reaction condition: 0.05 mol/L furfural in 25 mL solvent, catalysts 0.3 g, 180 °C and 20 h.
b
powders obtained were dried under vacuum at 120 °C prior to the next
reaction. However, the yield of GVL decreased remarkably and by-
product such as 2-propyl furfuryl ether began to accumulate. Based on
the results of NH3-TPD, acidity of sulfated DUT-67(Hf) decreased from
1.578 mmol/L to 0.732 mmol/L after the reaction. To solve this pro-
blem, recovered powders were subjected to the post-synthetic mod-
ification method again and reused under the same conditions. As shown
in Fig. 8, recovered catalysts after the acid regeneration showed higher
GVL yields than before. The catalysts, which were treated by the acid
regeneration method after each cycle, were able to be reused at least
four times with few decrease in performance.
The catalysts recovered after four cycles were characterized by XRD,
N2 adsorption-desorption, XPS characterization and NH3-TPD. In order
to explore surface chemical states of elements and investigate the role
of Hf element in the reaction, we have added some XPS characterization
of series of sulfated DUT-67(Hf). As shown in Fig. S8, the binding en-
ergy values of Hf 4d in fresh DUT-67(Hf)-0.06 were higher than those of
recycled DUT-67(Hf)-0.06, indicating higher positive charge on Hf
atoms in the fresh DUT-67(Hf)-0.06. The results also demonstrated that
Lewis acidity of recycle catalyst was weaker than fresh catalyst, which
is correlated to the reusability test of catalysts. As shown in Fig. 9, the
structure of the recovered catalyst after four cycles retained no evident
change by the XRD technique. The results of N2 adsorption-desorption
showed that the recovered catalyst after four cycles exhibited a smaller
surface area (519 m2/g) than before (636 m2/g). By contrast, the total
pore volume of recovered catalyst remained no significant change
(0.40 cm3/g). According to the results of NH3-TPD, the recovered cat-
alysts after the acid regeneration method exhibited enough acidity
(1.6 mmol/g) to catalyze the ring opening reaction of furfuryl alcohol
or its esters.
Fig. 6. Yields of products within 24 h in the reaction of furfural with isopropyl
alcohol in the presence of DUT-67(Hf)-0.06. Reaction condition: 0.05 mol/L
furfural in 25 mL isopropyl alcohol, 0.3 g catalysts, 180 °C. FA = furfuryl al-
cohol, FE = sec-propyl furfuryl ether, IPL = isopropyl levulinate, and GVL = γ-
valerolactone, 4-HPPE = 4-hydroxy pentanoate isopropyl ester.
Fig. 7. (a) Time–yield plots for the CTH reaction of furfural catalyzed by DUT-
67(Hf)-0.06. Reaction conditions: 0.05 mol/L furfural in 25 mL isopropyl al-
cohol, 0.3 g catalysts, 180 °C (b) The line shows the GVL yields after removing
the solid catalyst from 4 h to 20 h.
was observed after the catalyst was removed, which substantiated that
sulfated DUT-67(Hf) acted as the heterogeneous catalyst.
3.3.2. Reusability of the sulfated DUT-67(Hf)
Fig. 8. Reusability of sulfated DUT-67(Hf)-0.06 in the conversion of furfural to
GVL. Reaction condition: 0.05 mol/L furfural in 25 mL isopropyl alcohol, 0.3 g
catalysts, 180 °C and 20 h.
The catalyst recycling was conducted with sulfated DUT-67(Hf)-
0.06 under the conditions of 180 °C for 20 h. Initially, the catalyst was
recovered by filtration and washed with ethanol (3 × 30 mL). Then, the
24

W. Li, et al.
MolecularCatalysis472(2019)17–26
As shown in Fig. 10, we found that secondary alcohols such as isopropyl
alcohol and 2-butanol have a very good effect compared to primary
alcohols such as ethanol and methanol. Moreover, isopropyl alcohol
appears to be the best hydrogen donor for the reaction. By contrast,
using 2-butanol as the hydrogen donor showed a low yield of GVL.
Based on the reported knowledge and our experimental experience, the
elongation of the alkyl chains of levulinate esters affects the yield of
GVL, which might be due to steric hindrance of the alkyl groups.
3.5. Reaction mechanism study
A possible mechanism for sulfated DUT-67(Hf) catalyzing the one-
pot conversion of furfural to GVL was proposed (Fig. 11) [25,55–57].
First, isopropyl alcohol is absorbed on sulfated DUT-67(Hf) and inter-
acts with the Hf4+-O2− contained in the Hf clusters, which leads to the
separation of isopropyl alcohol into the corresponding alkoxide and
hydrogen. Concurrently, the carbonyl group in furfural can be activated
by Hf4+-O2−. Then the hydrogen transfer takes place between the ac-
tivated carbonyl group and the separated alcohol leading to the for-
mation of furfural alcohol or its ester through a six-member inter-
mediate. Next, furfural alcohol or its ester are catalyzed by Brønsted
acid sites and transformed into LA or its esters through a ring-opening
reaction. 4-hydroxypentanoate is obtained from LA and its esters
through a similar six-member intermediate. In the end, 4-hydro-
xypentanoate is transformed into the desired product GVL through in-
tramolecular esterification or transesterification.
Fig. 9. XRD patterns of fresh sulfated DUT-67(Hf)-0.06 and recovered sulfated
DUT-67(Hf)-0.06.
4. Conclusions
In summary, porous bifunctional metal-organic framework Hf-based
DUT-67 was synthesized via a post-synthetic modification method and
tested for one-pot conversion of furfural to produce GVL using isopropyl
alcohol as the hydrogen donor and solvent. Sulfated DUT-67(Hf)-0.06
exhibits optimal catalytic activity for one-pot conversion of furfural to
GVL. With the synergy of Lewis acid and Brønsted acid, a 100% con-
version of furfural and a 84.9% yield of GVL were obtained, catalyzed
by sulfated DUT-67(Hf)-0.06 at 180 °C after 20 h. The mechanism of
introducing Brønsted acid sites into DUT-67(Hf) was modifying the
molecular decoration of the zirconium clusters with aqueous sulfuric
acid. The sulfated DUT-67(Hf) featured high thermal and chemical
stability and can be reused at least four cycles with high GVL yields. To
improve the yield of GVL, we used a mixture of primary alcohol and
secondary alcohol as hydrogen donors. The results showed that the
yield of GVL increased slightly using a certain proportion of primary
alcohols and secondary alcohols as hydrogen donor.
Fig. 10. Yields of products over 20 h in the reaction of furfural with various
alcohol in the presence of DUT-67(Hf)-0.06. Reaction condition: 0.05 mol/L
furfural in 25 mL isopropyl alcohol, 0.3 g catalysts, 180 °C.
3.4. Catalytic effect of sulfated DUT-67(Hf) on different Alcohols
Considering that the hydrogen donor has an important influence on
the CTH reaction, a set of hydrogen donors was tested for the reaction.
Fig. 11. Possible mechanism for producing GVL from furfural catalyzed by DUT-67(Hf)-0.1SO4.
25

W. Li, et al.
MolecularCatalysis472(2019)17–26
Acknowledgements
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This work was supported by the National Basic Research Program of
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