One-pot conversion of furfural to gamma-valerolactone in the presence of multifunctional zirconium alizarin red S hybrid

Article abstract of DOI:10.1016/j.apcata.2021.118203

A multifunctional Zr-containing catalyst (FM-Zr-ARS) was successfully synthesized by a modulated hydrothermal synthesis route. Systematic characterization results supported the presence of robust porous inorganic-organic frameworks stabilized by the strong coordination interaction of Zr⁴⁺ with oxygen-rich functional groups in Alizarin red S (ARS). Moreover, the -O-Zr-O- network in the FM-Zr-ARS structure formed a rich content of acid-base sites. In addition, the inherent sulfonic groups in ARS made the FM-Zr-ARS hybrids possess Br?nsted acid sites. Therefore, under the synergistic catalysis of the multiple functional sites, FM-Zr-ARS showed remarkably high catalytic activity for γ-valerolactone (GVL) production from levulinate esters and furfural. Finally, 72.4 % and 97.7 % yields of GVL were obtained in the conversion of furfural and ethyl levulinate, respectively, after 8 h of reaction at 433 K. On the basis of the role of different functional sites, a plausible catalytic mechanism was presented for the conversion of biomass-derived furfural to GVL.

Full text of DOI:10.1016/j.apcata.2021.118203

Applied Catalysis A, General 621 (2021) 118203
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Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
One-pot conversion of furfural to gamma-valerolactone in the presence of
multifunctional zirconium alizarin red S hybrid
a
a,
a
,
b
a
Qingrui Peng , Haijun Wang *, Yongmei Xia , Xiang Liu
a
School of Chemical and Materials Engineering, Jiangnan University, 1800 Lihu Avenue, Wuxi, Jiangsu, 214122, China
State Key Laboratory of Food Science and Technology, Jiangnan University, 1800 Lihu Avenue, Wuxi, Jiangsu, 214122, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
A multifunctional Zr-containing catalyst (FM-Zr-ARS) was successfully synthesized by a modulated hydrothermal
synthesis route. Systematic characterization results supported the presence of robust porous inorganic-organic
Biomass valorization
One-pot reaction
4+
frameworks stabilized by the strong coordination interaction of Zr with oxygen-rich functional groups in
γ-Valerolactone
multifunctional hybrid catalyst
Modulated hydrothermal (MHT) synthesis
Alizarin red S (ARS). Moreover, the -O-Zr-O- network in the FM-Zr-ARS structure formed a rich content of acid-
base sites. In addition, the inherent sulfonic groups in ARS made the FM-Zr-ARS hybrids possess Brønsted acid
sites. Therefore, under the synergistic catalysis of the multiple functional sites, FM-Zr-ARS showed remarkably
high catalytic activity for γ-valerolactone (GVL) production from levulinate esters and furfural. Finally, 72.4 %
and 97.7 % yields of GVL were obtained in the conversion of furfural and ethyl levulinate, respectively, after 8 h
of reaction at 433 K. On the basis of the role of different functional sites, a plausible catalytic mechanism was
presented for the conversion of biomass-derived furfural to GVL.
1
. Introduction
The energy crisis and environmental problems caused by the over-
developed and realized a satisfactory yield of GVL. In terms of a com-
bined catalyst system, solid Lewis acid sites provided by Tin-, Sn-, and
Zr-containing [16,17] catalysts are typically used to catalyze
Meerwein-Ponndorf-Verley (MPV)-type hydrogen transfer reactions,
while solid Brønsted acids provided by aluminum-containing acid zeo-
lites, such as H-beta, H-ZSM-5 [14], or H–Y zeolites [18] are usually
applied in hydrolysis reactions.
exploitation and consumption of fossil resources have become crucial
issues facing human society [1–3]. Therefore, exploring the efficient use
of carbon-containing renewable resources and upgrading biomass plat-
form molecules (e.g., furfural, levulinate esters, 5-hydroxymethylfurfu-
ral) to value-added chemicals is of great importance [4–6]. Among
various biomass-derived compounds, gamma-valerolactone (GVL) is
considered the most promising renewable chemical, possesses unique
physicochemical properties and has wide applications in the food and
chemical industries [7–12]. The direct production of GVL from ligno-
cellulose is complicated and technically difficult. Compared with the
lignocellulose protocol [11,13,14], the cascade conversion of furfural to
GVL is more approachable. However, the realization of the cascade
conversion of furfural requires a suitable catalyst, on which different
active acid-base sites are required to work cooperatively. A strategy for
achieving this one-pot cascade process is the use of a combined catalyst
system. Bui et al. first reported a Lewis acid Zr-Beta and mesoporous
Brønsted acid Al-MFI zeolite combined catalyst system that could ach-
ieved a high GVL yield (78 %) after 48 h of reaction at 393 K [15].
Following this work, several combined catalyst systems have been
Apart from the combined catalyst system, there was an alternative
strategy to use multifunctional (bi-functional) catalysts, which were
prepared by the way of simultaneous incorporation of different (both)
functionalities within the same zeolite framework [17,19–25]. For
example, Winoto et al. successfully prepared a Sn-Al-Beta 7 (Si/Sn = 63
and Si/Al = 473) catalyst with an optimized functional catalytic site
ratio through a multiple-step post-synthesis strategy. These materials
achieved more than 60 % GVL yield after 24 h of reaction at 453 K in
2-butanol media [19]. Winoto and coworkers recently successfully
synthesized similar Zr-Beta materials by the introducing a hetero-
polyacid on the surface of Zr-Beta to improve the physicochemical
properties of the multifunctional material. Eventually, HPW- loaded
Zr-Beta achieved a nearly 70 % GVL yield after 24 h of reaction at 433 K
[24]. These catalysts exhibited remarkable catalytic activity on the
transformation of furfural to GVL. The development of a more efficient,
*
Corresponding author.
E-mail addresses: wanghj@jiangnan.edu.cn, wanghj329@outlook.com (H. Wang).
https://doi.org/10.1016/j.apcata.2021.118203
Received 30 September 2020; Received in revised form 30 April 2021; Accepted 5 May 2021
Available online 7 May 2021
0
926-860X/© 2021 Elsevier B.V. All rights reserved.

Q. Peng et al.
Applied Catalysis A, General 621 (2021) 118203
sustainable, and low-cost catalysts without a complex preparation pro-
cess for the conversion of biomass-derived furfural to GVL is also
needed. Recently, various Zr-containing functional hybrid materials
applied in Meerwein-Ponndorf-Verley (MPV) reduction of bioderived
aldehydes were reported [26–31]. These functional catalytic hybrid
materials prepared using organic ligands from natural sources can pro-
vide rich content functional sites and generate porous frameworks with
good stability after chelation with metal ions. On the basis of previously
reported research, we envisioned a multifunctional catalytic material
constructed using organic molecules derived from renewable resources
and featuring acid/basic properties for the conversion of
biomass-derived furfural to GVL. Alizarin red S (1, 2-dihydroxy-9,
%) and furfuryl alcohol (FAL, >98.0 %) were obtained from Shanghai
Aladdin Bio-Chem Technology Co., Ltd. All chemicals were used without
further treatment. Deionized water was used throughout the
experiment.
2
.2. Syntheses of formic acid modulated zirconium alizarin red S (FM-Zr-
ARS)
4
Fifteen milliliters of ZrCl (6 mmol) and 60 ml of ARS (4 mmol)
solution were prepared. The solutions were mixed, and 10 ml of formic
acid was added. The resulting solution was transferred into a stainless-
steel autoclave lined with Teflon and heated at 433 K for 12 h. Then,
the obtained precipitate was filtered and washed 15 times with deion-
1
0-anthraquinone-3-sulfonate, ARS, Fig. 1a) is one of the most widely
used and most popular analytical reagents [32–35], which can be
directly derived from alizarin (AZN, Fig. 1b). Because of the presence of
a p-quinone group and adjacent hydroxyl groups in the molecular
structure of ARS [36], it can coordinate with various metal ions without
a complicated reaction process to form a catalytic material with po-
tential catalytic activity. Moreover, these oxygen-containing functional
groups in ARS may exhibit potential acid-base sites after coordination
interactions with metal ions.
4
ized water and 5 times with ethanol to remove the remaining ZrCl or
ARS. For convenience, we abbreviate FM-Zr-ARS (1:1.5) as FM-Zr-ARS
in this paper. Other Zr-Alizarin red S hybrids were prepared by chang-
4
ing the initial ZrCl /Alizarin red S mole ratios of 1:1 and 2:1. These
catalysts were denoted as FM-Zr-ARS (1:1) and FM-Zr-ARS (2:1),
respectively.
2
.3. Syntheses of zirconium alizarin (Zr-AZN)
The present research is realizing a one-pot catalytic process for the
conversion of biomass-derived furfural to GVL using a multifunctional
FM-Zr-ARS, which was synthesized by a facile modulated hydrothermal
The synthetic procedures for Zr-AZN hybrids was based on that for
FM-Zr-ARS through a slight change. Briefly, 2 mmol of AZN was added
to methanol (100 mL) containing 4 mmol of sodium hydroxide and
(
MHT) synthesis approach. The intentional and controlled introduction
of acid regulators in the synthesis of Zr-FM-ARS can improve the dis-
advantages of low specific surface area and porosity caused by con-
ventional hydrothermal synthesis methods [37]. Previous studies have
reported that the introduction of modulator molecules in the
metal-organic framework (MOF) synthesis process may decrease crystal
aggregation, which may help overcome diffusion limitations [38,39].
Therefore, we hope to optimize the synthesis process of the catalyst and
maximize the catalytic activity of the catalyst using formic acid as an
acid regulator. Compared with Zr-ARS, FM-Zr-ARS modified with formic
acid has a larger pore diameter, higher surface area and more Lewis
acid-base content. Eventually, a GVL yield of up to 72.4 % was obtained
under mild conditions (433 K, 8 h). We anticipate that the approach of
using ARS as the building blocks to construct multifunctional catalysts
will open new possibilities for exploring efficient use of renewable cat-
alytic materials and the promoting the conversion of biomass resources.
4
stirred until completely dissolved. Then, 2 mmol of ZrCl methanol so-
lution was mixed with the AZN solution. The resulting suspension was
filtered and thoroughly washed with hot water and methanol. Finally,
the precipitate was dried under vacuum at 353 K for 12 h. Unless
otherwise stated, the following discussion is preferably selected from Zr-
AZN for comparison.
2
.4. Syntheses of zirconium alizarin red S (Zr-ARS) and UiO-66(Zr)
Except for the omission of formic acid, the synthesis procedure of Zr-
ARS was identical to that of FM-Zr-ARS. For comparison, conventional
UiO-66 (Zr) was synthesized using a solvothermal method in which re-
action mixtures of equimolar quantities of ZrCl
4
and 1,4-benzenedicar-
◦
boxylic acid in DMF were heated at 120 C for 24 h [40].
2
. Experimental
2.5. Catalytic reduction of ethyl levulinate to GVL
2
.1. Chemicals
In this step, EL was chosen as the reaction substrate to evaluate the
reduction performance of the prepared catalysts and observe the influ-
ence of each active site on the catalytic performance of FM-Zr-ARS. All
experiments were performed in a 20 ml Teflon-lined stainless-steel
autoclave equipped with a magnetic stir bar under oil-heating condi-
tions. Typically, 1 mmol of selected intermediates (i.e., ethyl levulinate)
was added to 5 ml of alcohol solution (i.e., isopropanol, IPA) with
varying amounts of catalysts (100ꢀ 200 mg), and then the reactor was
Furfural (FF, ≥99.0 %), isopropanol (2-propanol, ≥99.7 %), formic
acid (≥88.0 %), sodium hydroxide (≥96.0 %) and zirconium dioxide
(
≥99.0 %) were received from Sinopharm Chemical Reagent Co., Ltd,
China. Alizarin red S (ARS, ≥96.0 %), Alizarin (AZN, >97.0 %), decane
≥99.5 %), ethyl levulinate (EL, >99.0 %), γ-valerolactone (GVL, >98.0
(
Fig. 1. Chemical structures of the two compounds: (a) Alizarin red S and (b) Alizarin (For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article).
2

Q. Peng et al.
Applied Catalysis A, General 621 (2021) 118203
heated to and kept at the desired temperature for a certain time. After
the oil bath was heated to the target temperature, the reactor was
immediately placed under magnetic stirring to initiate the reaction.
After the reaction was completed, the reactor was quickly removed from
the oil bath and cooled with cold water to terminate the reaction. The
quantification of reduction products was conducted on a GC-9790 (Fuli
Analytical Instrument Co. Ltd., China) equipped with an FID detector
reaction system after the reaction proceeded for 5 h. Then, without the
FM-Zr-ARS catalyst, the FF conversion reaction ran for another 11 h.
Subsequently, ICP-OES was used to measure the Zr species immersed in
the reaction mixture system. The reusability test of FM-Zr-ARS catalyst
was performed as follows: After each cycle of catalytic reactions, the FM-
Zr-ARS was separated by centrifugation, and successively washed with
isopropanol and acetone for 3 times. After dried under vacuum at 353 K
for 12 h, the resulting solid sample was employed for the next cycle
under the identical reaction conditions.
and SE-54 capillary column (30.0 m×0.32 mm × 0.25
μm). GC–MS
(
Bruker SCTONSQ-456-GC) was used for identification of products.
Calibration curves were obtained from stock solutions, using decane as
an internal standard. Catalyst performance was evaluated according to
the following equations:
3. Results and discussion
3
3
.1. Catalyst characterization
0
n ꢀ n
i
i
X
i
=
× 100
0
n
i
.1.1. Textural and structural properties
To gain detailed characterizations of the catalysts, the textural and
n
j
0
Y
j
= n × 100
morphological properties of the prepared materials were characterized
by SEM, SEM-EDS, TEM, N adsorption-desorption, XRD and FT-IR
i
2
where Xi and Yj are the mean conversion of i and yield toward j,
respectively. Subscripts i and j denote reactants and products respec-
tively, and superscript 0 refers to the initial conditions. Data are pre-
sented as mean value of at least three measurements; all the SDs were
less than 5%.
techniques. As depicted in Fig. 2a and b, FM-Zr-ARS samples consist of
irregular nanoparticles with no uniform shape. Scanning electron mi-
croscopy with corresponding element mapping analysis confirmed that
the S, Zr and O elements were uniformly dispersed on the surface of the
2
FM-Zr-ARS sample (Fig.2d). The N adsorption/desorption isotherms of
FM-Zr-ARS displayed an IV-type isotherm with an obvious H3-hysteresis
loop (Fig. 3a), indicating that FM-Zr-ARS is a mesoporous material. [29].
The surface area of FM-Zr-ARS is much smaller than that of UIO-66,
possibly because of the use of water as a solvent in the synthesis pro-
cess, because the hydrothermal treatment would inevitably lead to the
accumulation of crystal water in the pore walls of the agglomerates
formed by the particles [37]. Compared with the conventional
2
.6. Full catalytic reaction assessment
Zr-AZN (without sulfonic acid groups) and FM-Zr-ARS were selected
to convert furfural into GVL within
a prolonged reaction time
(
0.5ꢀ 24 h) to evaluate the beneficial aspect of the inherent sulfonic
groups of FM-Zr-ARS toward a GVL production.
2
metal-organic framework UiO-66(Zr) and ZrO in a bulky structure,
Similar to the previous procedure, 1 mmol of furfural, 5 ml of iso-
propanol, and 200 mg of catalysts were loaded into the reactor. After the
oil bath was heated to the desired temperature (433 K), put the reactor
in the oil bath and initiate the reaction with magnetic stirring, the
products at different time points were sampled and analyzed during the
monitoring time (0.5ꢀ 24 h). The method of product quantitative anal-
ysis is consistent with short-time reaction analysis.
FM-Zr-ARS had a centered BET surface as well as the largest average
pore diameter (5.2 nm) (Fig. 3a, Table S1). The larger pore channels in
FM-Zr-ARS compared to Zr-ARS may be more conducive to the diffusion
of the reaction substrates in the pore walls, allowing the reaction sub-
strate to more efficiently approach the active site and quickly participate
in the reaction. From the powder XRD pattern of FM-Zr-ARS, no obvious
2
crystal diffraction peaks similar to those of ZrO were found. The broad
diffraction peaks indicated that the prepared FM-Zr-ARS has an amor-
phous structure, which was consistent with the SEM results.
2
.7. Catalyst recycling and heterogenous study
In addition, the structural discrepancy between FM-Zr-ARS and ARS
For the leaching experiment, the catalyst was filtered from the
Fig. 2. Characterization of FM-Zr-ARS. SEM images (a, b), TEM images (c), and TEM -EDS image (d) with elemental mapping.
3

Q. Peng et al.
Applied Catalysis A, General 621 (2021) 118203
Fig. 3. N
2
adsorption–desorption isotherm (a), XRD patterns (b), FT-IR spectra (c), UV–vis DRS spectra (d), TG (e) and DTG (f) curves.
was studied using FT-IR techniques. In contrast to the spectrum of the
weakened, especially the 9-C = O absorption band, possibly because
ꢀ 1
4+
ARS ligand (Fig. 3c), a band appeared at 1520 cm in the FM-Zr-ARS
1ꢀ OH and 9-C = O on the ARS ligand coordinate with Zr to form a
–
–
–
spectrum, corresponding to
ν(C
–
C) coupled with
ν(C
–
O), or
ν(C
–
C)
six-membered chelating ring. On the basis of these considerations, a
possible structure for FM-Zr-ARS was proposed (Fig. 9).
–
–
+
ν(C
O). This new band is similar to the previously reported results,
where chelation in acetylacetonate ion (acac)ꢀ bound to metals [41]
was proposed, further confirming that the 1ꢀ OH and 9-C = O groups in
3.1.2. Stability and acid-base properties
4
+
ARS coordinate with Zr [33,42]. Similarly, a splitting of the band at
The comparison of the thermal stability of FM-Zr-ARS and ARS in
Fig. 3e–f, shows that the pyrolysis peak of the ARS appeared at 642 K,
while the corresponding pyrolysis temperature of FM-Zr-ARS increased
to 777 K. The TG characterization results confirmed that the strong
ꢀ
1
4+
1
442 cm indicated the coordination interaction of Zr with carbonyl
oxygen and the adjacent hydroxyl groups in ARS, in which the 1-C-C and
-C-O bands are probably due to double bonds with a certain amount of
1
coordination between ARS and Zr4 ions improved the thermal stability
+
single-bond character [42]. Furthermore, the intensity of the bands in
FM-Zr-ARS related to 10-C = O and 9-C = O (1668 and 1636 cmꢀ ) was
1
of FM-Zr-ARS. Compared with the UV–vis spectrum of ARS (Fig. 3d), the
Fig. 4. CO
2
-TPD (a), NH
3
-TPD (b), pyridine-adsorbed FT-IR spectra (c), XPS survey scan (d), Zr 3d XPS spectra (e) and O 1 s XPS spectra (f).
4

Q. Peng et al.
Applied Catalysis A, General 621 (2021) 118203
absorbance peak of FM-Zr-ARS exhibits broad absorption maximums at
For our initial tests, EL was chosen as the substrate to finish this reaction,
because the conversion process of the intermediate product levulinate
esters to GVL tends to be more time consuming than the furan ring-
opening reaction and catalytic hydrogenation of furfural. Among the
different zirconium-containing compounds (Table 1), the prepared FM-
Zr-ARS showed the highest ethyl levulinate conversion (99.4 %) and
lower and higher wavelengths in the ranges of 200–400 nm and 400 to
7
00 nm, respectively. These changes are usually caused when
complexation occurs at the peri- or ortho-hydroxycarbonyl group of ARS
[
36,43], which indicates the coordination interaction between Zr4 and
+
ARS (Fig. 3d).
ꢀ 1
ꢀ 1
To obtain more insight into the electronic states of the hybrid cata-
lysts, XPS was used to observe the local environment of Zr, S and O in
GVL formation rate (1221
μmol g
h ) at 433 K within 8 h. For
catalyst FM-Zr-ARS, the best molar ratio of metal to ligand was 1.5:1.
Compared with Zr-ARS, FM-Zr-ARS can obtain a higher EL conversion
and GVL selectivity in the conversion of EL. We speculated that the acid-
base content over the catalyst surface may be the major factor ac-
counting for this difference. To better understand the universality of the
FM-Zr-ARS and ZrO
2
. Fig. 4e shows that the binding energy values of Zr
3/2 and Zr 3d5/2 in FM-Zr-ARS slightly increased from 182.0 and
82.8 eV to 182.8 and 185.2 eV, respectively, compared to that of ZrO
3
d
1
2
.
The higher positive charge of the zirconium species in FM-Zr-ARS means
that the Lewis acidity of the zirconium center in FM-Zr-ARS would be
enhanced [44]. In addition, a peak at 168.7 eV belonging to the S 2p
species in FM-Zr-ARS indicates that the linkages of sulfonic groups with
zirconium were formed after the hydrothermal process [45,46]. In
addition, this structure not only maintains the Brønsted acidity of the
sulfonate, but also increases the Lewis acid-base content at the same
time. (Fig. 4d). However, the O 1 s binding energy is higher than that of
cooperative effect of acid-base sites on the MPV reduction of EL, ZrO
2
and UiO-66(Zr) were tested (Table 1, Entries 2–6). UiO-66(Zr), a typical
Zr-based metal-organic framework, only afforded a GVL yield of 72 %
2
(entry 7). Compared with ZrO , the higher contents of both acidic (0.51
vs 0.16 mmol/g) and basic (0.97 vs 0.24 mmol/g) species but slightly
lower acid/base molar ratio (0.53 vs 0.67) in UiO-66(Zr) cannot effec-
tively restrain the generation of IPL (entry 8). As a counterpart of UiO-66
(Zr), FM-Zr-ARS showed relatively high catalytic activity with high EL
selectivity (97.9 %), and moderate GVL formation rates
ZrO
2
(531.8 eV vs 529.9 eV), illustrating a lower negative charge on
oxygen species in the Zr-O-C framework of FM-Zr-ARS (Fig. 4f) [47]. The
4
+
ꢀ 1 ꢀ 1 ꢀ 1
higher strength of Lewis acid sites (Zr ) in FM-Zr-ARS benefitted
(3450
μ
mol g
h ) and TOF (0.89 h ). To better understand the
transfer hydrogenation, while the lower basicity (O2 ) was favorable for
suppressing side reactions, such as condensation, especially those
caused by strong basic species [47,48]. The C1s XPS peak for FM-Zr-ARS
can be fitted to four subpeaks at around 284.7, 285.9 285.4, 286.8 and
ꢀ
cooperative effect of acid-base sites on the MPV reduction of EL, pyri-
dine (Py) and benzoic acid (BA) were used to passivate acid and base
sites, respectively. Table 1 shows that an 81 % yield of GVL could be
obtained from Py-treated FM-Zr-ARS, which was 14 % lower than that of
FM-Zr-ARS without Py treatment, and 25 % higher than that of BA-
poisoned FM-Zr-ARS. This result confirmed that the effect of Lewis ba-
sicity on the MPV reduction reaction is more pronounced than that of
Lewis/ Brønsted acidity. Similar results can also be observed by
comparing Zr-ARS and FM-Zr-ARS. The FM-Zr-ARS catalyst with a
higher content of basic sites exhibited a higher EL conversion and GVL
formation rate than the Zr-ARS catalyst. In addition, the catalytic ac-
tivity and selectivity of the prepared FM-Zr-ARS are close to or even
better than those of Zr-based catalytic materials reported in recent
literature (Zr-tannin, Zr-HA, Zr-RSL and Zr-LS) (Table S1). The excellent
catalytic performance indicated that each functional site has a syner-
gistic catalytic effect in the FM-Zr-ARS hybrid, and after the coordina-
2
88.9 eV, which corresponds to C–C/C = C, C–S, C-O-C/C = O, and
O–C = O, respectively [45,46]. These assignments are consistent with
the structure of FM-Zr-ARS shown in Fig. 9.
The acidic and basic properties of the prepared materials were
investigated via NH
3
-TPD and CO
2
-TPD methods. As shown in Fig. 4a
/NH desorption area than that of
and b, FM-Zr-ARS showed larger CO
2
3
UiO-66, indicating that FM-Zr-ARS has more acid-base sites. Compared
with UiO-66, the total acid content of FM-Zr-ARS was slightly higher
ꢀ 1
than that of UiO-66 (0.55 vs 0.51 mmol g ), but the content and
strength of the basic sites of FM-Zr-ARS were substantially better than
those of UiO-66 (Fig. 4a, Table S1). These results further indicate that
the building block (ARS) in the hybrids was beneficial, not only helping
stabilize the strength of the Lewis acid sites derived from Zr but also
increasing the total content and strength of the Lewis base sites in FM-Zr-
ARS. In addition, the presence of the Lewis acid and Brønsted acid sites
in FM-Zr-ARS was further confirmed via pyridine-FTIR (Fig. 4c). Ac-
cording to previous reports [21], the FTIR-active bands generated by the
Bronsted acid sites of the catalysts usually appeared at 1490, 1550 and
tion interaction of Zr4 with ARS, a rich content of acid-base sites is
+
created in the resulting -Zr-O-Zr- network (Fig. 9).
3.2.2. Effect of modulated hydrothermal synthesis method on FM-Zr-ARS
hybrids
As mentioned before, FM-Zr-ARS showed a better catalytic perfor-
mance for the CTH reaction and cascade conversion of furfural than Zr-
ARS. To obtain more insight into the activity discrepancy between these
two materials, the prepared FM-Zr-ARS and Zr-ARS were characterized
by BET, TG and FTIR measurements (Fig. 5a-d). The FT-IR spectra
proved that the chemical structures of FM-Zr-ARS remained no evident
change compared to those of Zr-ARS, but some subtle differences could
still be observed, such as the intensity of the bonding water band at
ꢀ
1
1
1
640 cm , while adsorption on the Lewis acid site usually occurs at
ꢀ 1
450, 1490 and 1610 cm .These characteristic absorption bands
appear in the FM-Zr-ARS spectrum and agreed well with previous
studies [20,21,24]. Fig. 4c shows that the pyridine molecules were
strongly bound with Brønsted acid and Lewis acid sites even after the
desorption temperature increased from 373 K to 473 K, indicating strong
Brønsted acid and Lewis acid sites in FM-Zr-ARS. Quantitative calcula-
tions showed that the ratio of the Brønsted to Lewis acid sites of
FM-Zr-ARS was 0.23 (Table S2). The appropriate strength of the
Brønsted acid would help facilitate the ring-opening of the furans, which
was crucial for the upgrading of biomass-derived furfural to GVL [14,18,
ꢀ 1
around 3400 cm in the FM-Zr-ARS spectrum being slightly weaker
than that of the Zr-ARS spectrum (Fig. 5f). This result is consistent with
the TG curve, which shows that FM-Zr-ARS lost intramolecular water
molecules in the range of 373ꢀ 473 K (Fig. 5c). Correspondingly, the
thermal decomposition temperature of FM-Zr-ARS is slightly lower than
that of Zr-ARS. This may be because the decrease in intramolecular
water molecules reduces the crystallinity of the material to some extent
and affects the structural characteristics of FM-Zr-ARS, which changes
the thermal stability [49]. By adding the regulator molecule formic acid
during the synthesis process, the physicochemical properties of
FM-Zr-ARS also show obvious changes. Compared with Zr-ARS,
2
4]. Furthermore, the pyridine-FTIR results further indicated that the
organic moiety in this catalyst played an essential role informing strong
Lewis acid-base sites and Brønsted acid sites after the coordination
interaction with Zr4 [40].
+
3
3
.2. Catalytic performance of FM-Zr-ARS
2
.2.1. Production of γ-valerolactone from ethyl levulinate catalyzed by FM-
FM-Zr-ARS showed a much larger BET surface area (134 vs 102 m /g)
3
Zr-ARS
Catalytic transfer hydrogenation is one of the means to evaluate the
catalytic performance of catalysts in the cascade conversion of furfural.
and total pore volume (0.44 vs 0.34 cm /g), as well as a larger average
pore diameter (5.2 vs 3.8 nm) (Table S1). We speculate that formic acid
may act as a frame network linker or space-filling agent that directs pore
5

Q. Peng et al.
Applied Catalysis A, General 621 (2021) 118203
Table 1
a
MPV reduction of EL to GVL over various catalysts .
ꢀ
1
ꢀ 1
Entry
Catalyst
T
Time
(h)
Conv.
(%)
GVL selec.
(%)
IPL selec.
(%)
GVL formation rate (
μmol g
TOF (h
c
)
Carbon balance
(%)
Base/acid
hꢀ 1
b
d
(
℃)
)
ratio
1
2
3
4
5
6
FM-Zr-ARS
Zr-ARS (1.5:1)
UiO-66
150
150
150
150
150
150
150
160
8
99.4
96.3
89.2
16.4
90.7
64.2
90
98.3
95.3
80.1
75.0
89.5
87.7
91
1.7
3.9
17.8
35
8.7
9.1
–
1221
1147
893
0.77
0.87
0.72
0.51
–
100
99
98
100
98
98
–
1:0.53
1:0.59
1:0.61
1:0.66
–
8
8
ZrO
2
8
153
e
f
FM-Zr-ARS
FM-Zr-ARS
Zr-LS
8
1015
704
8
–
–
9
1
[30]
8
1149
2300
1.96
0.46
1:0.32
1:0.7
0
ZrPN
10
93
98.9
–
–
[
31]
27]
53]
26]
54]
55]
1
1
1
1
1
1
Zr-HA
150
150
160
150
180
11
4
99.5
95
84.6
90.5
95.7
97
–
–
–
–
383
–
–
–
–
–
–
–
–
–
–
–
[
2
Zr-tannin
Zr-RSL
1075
337
1:0.39
[
3
12
12
5
92.4
100
96.07
–
–
–
[
4
10Cu-5Ni/
808
[
Al
Cu
AlOOH
2
O
3
5
2
(OH)
2
CO
3
/
90.51
1287
[
a
b
c
d
e
f
Reaction conditions: 1 mmol of EL in 5 ml isopropanol, 100 mg of catalyst, 423 K.
GVL formation rate = (mol of formed GVL)/(catalyst amount × time).
TOF= (mole of GVL)/(mole of acidꢀ base site content×time).
Base/acid ratios of the catalysts were calculated using NH
Acid sites of FM-Zr-ARS were poisoned by pyridine.
Base sites of FM-Zr-ARS were poisoned by benzoic acid.
3 2
- and CO -TPD.
Fig. 5. N
2
2 3
adsorption–desorption isotherm (a), FT-IR spectra (b), TG (c), DTG (d), CO -TPD (e) and NH -TPD (f) curves.
formation. In addition, the CO
2
-TPD calculation of FM-Zr-ARS shows
kinetic studies were performed at three different temperatures (403 K,
423 K, 443 K). In Scheme S1, the possible main reaction pathways for the
cascade conversion of furfural into GVL in isopropanol medium over FM-
Zr-ARS and Zr-ARS are proposed. The rate constant of each step can be
derived by pseudo-homogeneous kinetic model [50]. Each step is fitted
to a first-order equation, assuming stirred batch reactor operation, ac-
cording to the following model equations for the reaction network:
that the acid-base content substantially differs between FM-Zr-ARS and
Zr-ARS. The total amount of basic species of FM-Zr-ARS increased from
ꢀ
1
0
.83 to 1.04 mmol g , and the total amount of acidic species increased
ꢀ 1
from 0.83 to 1.04 mmol g (Table S1). This phenomenon is possibly due
to the absorption of some HCOO- into the inner cavities of FM-Zr-ARS,
which may also alter the acidity and basicity of the catalyst.
dCFF
dt
3
.2.3. Kinetics analysis
= ꢀ k
1
∗CFF ꢀ k
7
∗CFF
For more in-depth analysis of catalytic action of FM-Zr-ARS and Zr-
ARS in the cascade transformation of furfural into GVL. The reaction
6

Q. Peng et al.
Applied Catalysis A, General 621 (2021) 118203
dCFAL
dt
3.2.4. Time course of the integrated conversion of biomass-derived furfural
to GVL over FM-Zr-ARS
=
=
=
=
k
1
∗CFF ꢀ k
2
∗CFAL ꢀ k
5
6
∗CFAL
∗CFIE
A preliminary study showed that FM-Zr-ARS had excellent perfor-
mance for the MPV reduction of EL, and the integrated conversion of
biomass-derived furfural to GVL was conducted in isopropanol solution.
To explore the role of active sites in the entire reaction pathways, the
time course of the product distribution was investigated over FM-Zr-ARS
and Zr-AZN at 433 K (Fig. 7a-b). As shown in Fig. 7a, furfural was
initially reduced to furfuryl alcohol (FAL) through the catalytic transfer
hydrogenation reaction and underwent nearly 100 % conversion 0.5 h.
Subsequently, FAL was etherified to isopropyl furfuryl ether (FIE). When
the concentration of FIE reached a certain level (within 1 h), the hy-
drolytic ring-opening reaction occurred rapidly. During this time, iso-
propyl levulinate and γ-valerolactone were detected in the reaction
solution. With further reaction progress, only approximately 1% of
furfural could be detected. At this time, a certain amount of byproducts
dCFIE
dt
k
2
∗CFAL ꢀ k
3
∗CFIE ꢀ k
dCIPL
dt
k
3
∗CFIE ꢀ k
4
∗CIPL
dCGVL
dt
k
4
∗CIPL
dChumins
dt
=
k
5
∗CFAL + k
6
∗CIPL + k
7
∗CFAL
where C is the molar concentration of each component, and ki is the
apparent rate constant of step i. The Nelder–Mead method was used to
optimize the rate constants by minimizing the following objective
function:
′
were observed, such as 2,2 -difurfuryl ether (4% yield) and di-isopropyl
ether, which were generally formed by Zr-containing catalysts [24]. In
addition, only a small amount of β-angelica lactone was detected during
the reaction, indicating that only a small amount of FAL undergoes
{
}
np
∑
∑[
]
2
F
obj
=
C
m,n,calc ꢀ Cm,n,exp
m
n=1
hydrolysis to β-angelica lactone or -angelica lactone and that hydro-
α
genation to GVL was extremely slow. These results agreed well with
previous reports [23]. Notably, under the identical reaction conditions,
the yield of GVL obtained from Zr-AZN could reach 9% after reaction for
m m
where C ,n,calc and C ,n,exp are the calculated and experimental con-
centrations of component m at a given reaction time n, respectively. The
resulting apparent first-order rate constants for the FM-Zr-ARS and Zr-
ARS catalysts are reported in Table S3. The proposed pseudo-first
order kinetic model provides a good fit to the experimental datasets
for all catalysts (Fig.S3).
8
h at 433 K (Fig. 7b). In addition, with prolonged duration, a large
amount of FIE (61 % yield) and a certain amount of isopropyl levulinate
13 % yield) were observed, and these intermediate products were
generally formed by acid ꢀ base couple sites with high strength [24,25].
This result suggested that in the absence of a Brønsted acid (SO H),
(
The MPV reduction of furfural to furfuryl alcohol (k1) is the fastest of
all the catalytic steps, while the MPV reduction of isopropyl levulinate
3
Zr-AZN could not efficiently catalyze the ring-opening of FIE, and FIE
was generally obtained as the exclusive product. This result also indi-
cated that even though Lewis acid-base couple sites were more impor-
tant than Brønsted acid sites in two major intermediate conversion steps
viz., furfural to FAL and conversion of IPL to GVL, the Brønsted acid
generated by sulfonic acid in the hybrids (FM-Zr-ARS) was more crucial
than the Lewis acid-base couple sites in the acid-catalyzed ring-opening
of FIE [46]. Therefore, the appropriate strength of Bronsted acid is very
important. The catalysts with a suitable ratio of Brønsted acid to Lewis
acid sites contributed largely to the high FAL conversion and high yield
of GVL (Table S2). It is precise because of the efficient synergy of
Brønsted acid and Lewis acid sites that FM-Zr-ARS exhibited a high FAL
conversion rate and obtained a satisfactory GVL yield.
(
k4) is a relatively slow process (Fig. 6a–c). The isomerization of iso-
propyl furfural to isopropyl levulinate (k3) in the cascade reaction is the
rate-limiting step because the isomerization step requires the partici-
pation of both Brønsted and strong Lewis acid sites. Therefore, the
inherent sulfonic acid group in the ARS ligand is crucial because it acts
as the Brønsted acid required by the cascade reaction. In addition, for
each step of the cascade reaction, FM-Zr-ARS has a higher reaction rate
than that of Zr-ARS. The formic acid molecules can remove part of
intramolecular water molecules in the process of synthesizing materials,
thereby generating extra Lewis acid-base sites and more open materials.
Therefore, FM-Zr-ARS has a wider pore system than Zr-ARS. These
changes greatly facilitate the substrates to reach the active sites located
at the internal surface and thus increasing the reactivity scope of the
materials. In addition, after adjustment by formic acid, higher acid-base
loading in FM-Zr-ARS also benefits cascade conversion, because the
MPV reduction step in the cascade reaction is mainly promoted by Lewis
acid and base synergistic catalysis. (Scheme 1)
Considering that the hydrogen donors play an important role in the
cascade conversion of furfural to GVL, a series of hydrogen donors are
also screened for this reaction. As shown in Fig. S4, the GVL yield of
different alcohols over FM-Zr-ARS was in the order of 2-propanol >2-
butanol > 1-butanol > ethanol ≈ methanol. Among these solvents, 2-
propanol appears to be better hydrogen donors than other alcohols.
Previous studies also substantiated that secondary alcohols were supe-
rior to primary alcohols for the MPV reaction [15]. GC–MS analysis
Fig. 6. Apparent first-order pseudohomogeneous kinetic constants (ki) at 403,423 and 443 K for steps 1, 2, 3, and 4 in the reaction of furfural over FM-Zr-ARS and Zr-
ARS catalysts.
7

Q. Peng et al.
Applied Catalysis A, General 621 (2021) 118203
Scheme 1. A plausible way to convert furfural to GVL in isopropanol medium.
Fig. 7. Time course of the production of GVL from furfural over FM-Zr-ARS(a) and Zr-AZN(b). Reaction conditions: 0.2 g catalyst, 1 mmol furfural in 5 ml of
isopropanol, 433 K.
revealed that
a
large amount of bulky by-products, from the
catalyst on the reaction is very similar to that of temperature. As the
amount of catalyst increases, the yield of GVL only maintains slow
growth, instead of a higher increase, and the selectivity of GVL gradually
decreases with time (Fig.S2a). At high catalyst loading, the decrease in
yield might be due to the conversion of GVL into further upgraded
products on available acidic sites of the catalyst [23]. Therefore, 26 g
cat. /L was the optimum catalyst loading for this reaction. Previous
studies have shown that strong acidic of the catalysts and the higher
temperatures promoted this type of side reaction [21,51,52]. Under
optimized reaction conditions, the highest 83 % yield of GVL was ob-
tained after 12 h of reaction at 433 K over the FM-Zr-ARS catalyst
(Fig. S2b).
self-condensation of the aldehyde or reaction of aldehyde with levuli-
nate esters, are formed when using primary alcohols as hydrogen donors
[
21]. Hence, these results support the suitability of 2-propanol as a
solvent for the overall cascade conversion of furfural to GVL.
The effect of reaction parameters was optimized maximize the yield
of GVL. To study the reaction parameters, first, the effect of reaction
temperature was studied, and the results are presented in Table S4,
including the GVL yield from 393 to 453 k. This yield modestly increased
greatly with reaction temperature. For example, increasing the reaction
from 433 K to 453 K, only increased the yield of GVL from 66 % to 69 %.
In addition, multiple undesired byproducts were generated during this
time, such as diisopropyl ether, polymeric furfurals, or furfural de-
rivatives, which were probably formed during the self-condensation
between furfural and alcohols or the reaction between furfural and
levulinate esters. Taking all factors into consider, 433 K was selected as
the optimal reaction temperature. Likewise, the effect of the amount of
3.2.5. Catalyst recycling and heterogeneity study
In addition, the heterogeneity of FM-Zr-ARS was investigated by
filtering out the catalyst from the reaction after reacting at 433 K for 5 h
(Fig. 8a). The reaction did not occur in the absence of catalyst even after
8

Q. Peng et al.
Applied Catalysis A, General 621 (2021) 118203
Fig. 8. Recycling(a) and leaching(b) test in cascade conversion of furfural to GVL over FM-Zr-ARS. Reaction conditions: 0.2 g catalyst, 1 mM furfural in 5 ml of
isopropanol, 8 h, 433 K.
Fig. 9. Plausible structure of multifunctional Zr-ARS hybrid.
another 11 h. The reaction filtrate was subjected to ICP analysis, and the
low leaching content of zirconium (<3 ppm) and S species (<7 ppm)
indicated the integrity of FM-Zr-ARS during the reaction. The recycla-
bility test of FM-Zr-ARS was conducted at 433 K, the weight loss of the
catalyst after fifth recovery was less than 2%. The FM-Zr-ARS could still
achieve a GVL yield greater than 65 % after five consecutive operations
and had no noticeable decrease in catalytic performance (Fig. 8b). FT-IR
the proper ratio of Lewis acid to base sites in this present paper was also
of great significance for the production of GVL from furfural. Compared
with Zr-ARS, FM-Zr-ARS, which possessed a higher porosity and BET
surface area offered the optimum balance of basicity and acidity in the
MPV reduction reaction. Finally, the prepared FM-Zr-ARS could effec-
tively catalyze furfural being converted to GVL in a moderate yield of
72.4 % under mild conditions (433 K, 8 h). In summary, this research
may provide a new method for synthesizing other multifunctional hy-
brids catalyst in biomass valorization.
2
and N adsorption-desorption analyses showed that the Zr-O bond
remained intact between the fresh and reused FM-Zr-ARS catalysts, but
the structure changed slightly. This change can be attributed to the
adsorption of organic species (e.g., isopropoxide) by the catalyst during
the reaction, which may delimit the access of substrates to the active
sites in the catalytic process and decrease catalytic activity (for example,
CRediT authorship contribution statement
Qingrui Peng: Conceptualization, Methodology, Data curation,
Writing - original draft. Haijun Wang: Writing - review & editing, Su-
pervision. Yongmei Xia: Writing - review & editing. Xiang Liu: Writing
2
the specific surface area decreased from 134 to 97 m /g, and the average
pore diameter decreased from 5.2 nm to 3.4 nm) (Table S1).
-
review & editing.
4
. Conclusions
Declaration of Competing Interest
There are no conflicts to declare.
Acknowledgement
In summary, we present a multifunctional hybrid catalyst FM-Zr-ARS
that was successfully synthesized by a facile modulated hydrothermal
synthesis route. The efficient synergy between acid and base sites in FM-
Zr-ARS exhibited remarkable catalytic activity toward two major in-
termediate CTH reaction steps. Furthermore, the Brønsted acid sites
3
originating from the -SO H in ARS also played a crucial role in the furan
This research was financially supported by MOE & SAFEA for the 111
ring-opening reaction. Except for the acid-base strength and content of,
Project (B13025).
9

Q. Peng et al.
Applied Catalysis A, General 621 (2021) 118203
Appendix A. Supplementary data
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