Etherification of 5-hydroxymethylfurfural using a heteropolyacid supported on a silica matrix

Article abstract of DOI:10.1016/j.mcat.2020.111125

In this work, Preyssler-type heteropolyacids and their silica-included counterparts were employed in the etherification reaction of HMF and n-BuOH. Materials were synthesized with a Preyssler acid load of 12.5percent w/w using the sol-gel technique, which improved surface areas and modulated their acid strength. Prepared materials were used as heterogeneous solid acid catalysts in the selective etherification of 5-hydroxymethylfurfural (HMF) to 5-butoxymethylfurfural (5BMF). The high catalytic performance of the bulk Preyssler acids is related to their high acid strength, while selectivity related to the decrease in acidity by the inclusion effects. Different reaction parameters were optimized, with PWMo(12.5percent)&at;SiO₂ exhibited the highest catalytic activity with 89percent of HMF conversion and 73percent of 5BMF selectivity. The catalyst is reusable up to five cycles without noticeable decrease in selectivity.

Full text of DOI:10.1016/j.mcat.2020.111125

MolecularCatalysis494(2020)111125
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Etherification of 5-hydroxymethylfurfural using a heteropolyacid supported
on a silica matrix
Omar M. Portilla-Zuñigaa, José J. Martínezb, Mónica Casellaa, Daniela I. Licka, Ángel G. Sathicqa,
Rafael Luqued,e,*, Gustavo P. Romanellia,c,**
a Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. Jorge J. Ronco” (CINDECA-CCT La Plata-CONICET), Universidad Nacional de La Plata, Calle 47 No
257, B1900AJK La Plata, Argentina
b Escuela de Ciencias Químicas, Facultad de Ciencias. Universidad Pedagógica y Tecnológica de Colombia UPTC, Avenida Central del Norte, Tunja, Boyacá, Colombia
c CISAV. Cátedra de Química Orgánica, Facultad de Ciencias Agrarias y Forestales, Universidad Nacional de La Plata, Calles 60 y 119 s/n, B1904AAN La Plata, Argentina
d Departamento de Química Orgánica, Universidad de Córdoba, Campus de Rabanales, Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km 396, E14014, Córdoba, Spain
e Peoples Friendship University of Russia (RUDN University), 6 Miklukho Maklaya Str., 117198, Moscow, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this work, Preyssler-type heteropolyacids and their silica-included counterparts were employed in the
etherification reaction of HMF and n-BuOH. Materials were synthesized with a Preyssler acid load of 12.5% w/w
using the sol-gel technique, which improved surface areas and modulated their acid strength. Prepared materials
were used as heterogeneous solid acid catalysts in the selective etherification of 5-hydroxymethylfurfural (HMF)
to 5-butoxymethylfurfural (5BMF). The high catalytic performance of the bulk Preyssler acids is related to their
high acid strength, while selectivity related to the decrease in acidity by the inclusion effects. Different reaction
parameters were optimized, with PWMo(12.5%)@SiO2 exhibited the highest catalytic activity with 89% of HMF
conversion and 73% of 5BMF selectivity. The catalyst is reusable up to five cycles without noticeable decrease in
selectivity.
Biomass conversion
Preyssler heteropolyacids
5-Hydroxymethylfurfural
Silica matrix
Etherification
1. Introduction
Among them, HMF is a small molecule with high demand at in-
dustrial level [11,12], mainly because it possesses two functional
The use of biomass to obtain chemical products, biofuels, and en-
ergy has acquired an important role in the area of new sustainable
technologies. The processing of organic waste (mainly lignocellulosic)
is framed within the biorefinery concept. Biorefineries take advantage
of the multifaceted components present in biomass and the wide range
of technological pathways to convert primary feedstocks into products
including platform chemicals with higher added value [1–4]. The most
accepted industrial biorefineries are currently based on residues rich in
lignocellulosic biomass from wood, forestry, paper, and straw industries
[5,6]. The main products of the treatment of these raw materials in-
clude sugars, polyphenols, oils, and fats [7–10]. The most interesting
molecules from lignocellulosic biomass are commonly known as plat-
form molecules including glutamic acid, levulinic acid, 2 and 3-hy-
droxypropionic acids, succinic acid, fumaric acid, malic acid, glycerol,
sorbitol, xylitol, furfural (F), and 5- hydroxymethylfurfural (HMF).
groups that turn it into an invaluable platform for different transfor-
mations into high added value chemicals. The aldehyde group has been
modified using a wide range of catalysts to obtain amines [13], glu-
cosides [14], esters [15], chalcones [16], nitrones [17], Schiff bases
[18], dihydropyrimidinones [19], etc. Many of these products have
biological activities, some others are starting molecules for other re-
actions, while some are monomers for the synthesis of different poly-
mers with tunable properties [20–23].
In the case of the hydroxyl group, the reactions are mainly ester-
ifications [15,24–27] and etherifications [24,28–30], which have been
studied using several solid acid materials including nanocrystalline
zeolites [31–33], acid functionalized resins (Amberlyst-15 and Dowex
DR2030) [31,34], silica-SO3H [35], AlCl3 [36], silicotungstic acid
modified with Cs [37], phosphotungstic acid (HPW) [38,39], silico-
tungstic acid [40], HPW (40%) / MCM-41 [41], HPW/K-10 [42],
⁎ Corresponding author at: Departamento de Química Orgánica, Universidad de Córdoba, Campus de Rabanales, Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km
396, E14014, Córdoba, Spain.
⁎⁎ Corresponding author at: Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. Jorge J. Ronco” (CINDECA-CCT La Plata-CONICET), Universidad
Nacional de La Plata, Calle 47 No 257, B1900AJK La Plata, Argentina.
E-mail addresses: q62alsor@uco.es (R. Luque), gpr@quimica.unlp.edu.ar (G.P. Romanelli).
https://doi.org/10.1016/j.mcat.2020.111125
Received 18 May 2020; Received in revised form 13 July 2020; Accepted 15 July 2020
2468-8231/©2020ElsevierB.V.Allrightsreserved.

O.M. Portilla-Zuñiga, et al.
MolecularCatalysis494(2020)111125
heteropolyacids with exchanged silver (AgH2PW) [43].
water. 27 mL of H3PO4, 85% (0.02 mol), was then added dropwise to
the solution and the resulting mixture was refluxed for 24 h. Subse-
quently, 1 mL of concentrated nitric acid (65%) was added to the
mixture under constant stirring followed by the addition of 10 g of pure
KCl (0.13 mol). The obtained suspension was centrifuged for 15 min,
and the resulting solid was dissolved in 50 mL of hot distilled water and
kept under cooling (4 °C approx.) for 24 h. The precipitate obtained
(K14[NaP5W30O110]) was filtered and dried under vacuum at room
temperature. A 10% solution of the potassium salt obtained was eluted
through a previously activated Dowex® 50 W X8 hydrogen bed.
The exchanged solution was then brought to dryness by forced
evaporation under an air column to obtain H14[NaP5W30O110] (HPA).
The Preyssler heteropolyacid with molybdenum addenda atom
Heteropolyacids (HPAs) exhibit better catalytic properties as com-
pared to mineral acids and certain Lewis acids, being noncorrosive,
inexpensive, and eco-compatible as well as presenting fewer disposal
problems. In recent years, the development of various syntheses in-
volving Preyssler acid as catalyst for different transformations of bio-
mass derivatives, especially F and HMF, have been reported. Preyssler
acids were employed in multicomponent reactions [44], esterification
[45], and etherification [46] reactions.
Preyssler HPAs exhibit improved acidities as compared to Keggin
HPAs [47], which makes these materials a promising alternative for
acid catalysed reactions. The higher acid strength is associated to a
better catalytic efficiency, related to an increase in the number of
tungsten atoms (or the number of protons). Likewise, Preyssler HPAs
have a greater pH range of stability in solution [48,49] with respect to
Keggin HPAs [50]. Due to this hydrolytic stability, these HPAs can be
used in a wider range of materials. Furthermore, the solubility of
Preyssler HPAs is lower than that of Keggin HPAs [49], which re-
presents an advantage in heterogeneous catalysis due to reduced
leaching during catalytic tests and reuses. The replacement of tungsten
with a molybdenum atom also has notable effects. For example, the
yield in some esterification reactions increases, a relationship between
acidity and molecular symmetry distortion has been found [51]. Al-
though Preyssler acids can be directly employed as heterogeneous
catalysts in organic reactions, their stabilization on porous supports
improves their surface area with a favorable regulation of their acid
strength, promoting acid catalyzed reactions in short reaction times,
high conversion and selectivity (often with reduced formation of side
products).
The development of HMF ethers is an important line of research due
to their applications as fuels [31,52] and starting products for complex
molecules [52,53]. Commonly, the catalyzed etherification reaction of
5-hydroxymethylfurfural has been studied with Keggin HPAs using
ethanol as a reagent [54]. However, the use of n-butanol or other al-
cohols attracted our attention as an alternative to ethanol, because n-
butanol has comparably higher energy content, being also less hygro-
scopic [55]. Similarly, ethers derived from HMF and branched alcohols
including 5-(isopropoxymethyl) furfural (i-PMF) and 5-(t-butox-
ymethyl) furfural (t-BMF) exhibit superior blending properties when
compared to ethers obtained from linear analogues [56]. In this work,
the etherification reaction of HMF was attempted and optimized using
n-butanol and subsequently extended to other alcohols employing
Preyssler acids supported in a silica xerogel.
H
14[NaP5W29MoO110] (PWMo) was similarly synthesized using a 0.07/
8.3 mmol ratio following the same steps as for PW.
Similarly, the solids were supported in a silica matrix by means of a
sol-gel technique following a previously employed methodology [59].
Briefly, a mixture of Preyssler acid (PW) (0.665 g) and hydrochloric
acid (10 mL 0.005 M) was added to tetraethyl orthosilicate (13.00 g).
The mixture was stirred for 24 h at room temperature, then further
standing still for 5 days and dried under vacuum for one week at room
temperature after breaking the hydrogel formed with a Teflon stick. The
obtained powder was washed with ethanol 70 % (3 x 10 mL), distilled
H2O (3 × 10 mL), and then dried in vacuum for 24 h. PWMo(12.5%)@
SiO2 with molybdenum addenda atom was similarly synthesized.
2.3. Catalyst characterization
The textural properties of the solids were determined from N2 ad-
sorption–desorption isotherms at liquid nitrogen temperature using a
Micrometrics ASAP 2020 equipment. The acid strength was determined
by potentiometric titration using n-butylamine (0.025 N) solution at a
flow rate of 0.05 mL/min in 794 Basic Titrino Metrohm equipment
using a double junction electrode. The nature of acid sites was de-
termined by pyridine (Pyr) adsorption followed by FT-IR using an in
situ diffuse reflectance cell (Praying Mantis, Harrick). The protocols of
these techniques were described previously in reference [49].
Scanning electron microscopy characterization was performed in a
Philips 505 SEM using an accelerating voltage of 25 eV. Samples were
fixed as powder on a graphitized band and metalized with Au.
2.4. Etherification reaction
One mmol of substrate and 3 mmol of n-butanol with 40 mg of
catalyst (PW, PMoW, SiPW, SiPMoW) in 2 mL of dry toluene were
mixed in a 10 mL glass vessel sealed with a Teflon screw cap. The
mixture was stirred with a magnetic bar, and the heating source was a
thermostatic oil bath (100 °C). Once the reaction stopped, the solid was
separated by centrifugation and washed with toluene and absolute
ethanol for several times before reuse. The crude was filtered on a nylon
membrane (0.45 μm), and 100 μL aliquots were prepared with 5 mM of
decane in ethyl acetate for GC-FID analysis.
2. Materials and methods
2.1. Materials
Chemicals were purchased from Aldrich; toluene was distilled under
reduced pressure and dried with zeolites. The other chemicals were
used without further purification. The GC analysis on conversion and
selectivity was performed in a Shimadzu GC-2014 with FID detector
using the internal standard quantitation method. The internal standard
chosen was decane (5 mM) because it is stable, it is not found in the
solvents, and it has a different retention time than the components of
the crude mixture reaction. The compounds were identified by gas
chromatography–mass spectrometry (GC-MS) using a Shimadzu GCMS-
QP2010 Ultra, with Shimadzu AOC-20i injector and Agilent DB-1ht
0.25 mm x 30 m x0.10 μm column coupled to a mass detector.
3. Results and discussion
Typical isotherms of microporous materials were obtained from si-
lica xerogels (Fig. 1). The incorporation of Preyssler HPAs in SiO2
showed a slight increase in the mesoporosity of the material, most
evident with the inclusion of PWMo. This can be explained considering
the highest external surface area of this solid (Table 1). Although PWMo
(12.5%)@SiO2 exhibited the largest pore volume, the inclusion of HPAs
did not modify the pore size. This can be associated with a deposition of
PWMo on the external surface of SiO2. Comparably, PW and PWMo
were found to be non-porous materials.
2.2. Catalyst synthesis
Bulk
catalysts
of
H14[NaP5W30O110
]
(PW)
and
H
14[NaP5W29MoO110] (PMoW) were prepared according to previous
works [46,57,58]. PW was prepared via hydrothermal synthesis, where
30 g of Na2WO4·2H2O (0.13 mol) was dissolved in 20 mL of hot distilled
Previously, PW structures were reported to be preserved after in-
corporated into silica materials [59], because its presence without
2

O.M. Portilla-Zuñiga, et al.
MolecularCatalysis494(2020)111125
entries 2-3) exhibited a remarkably reduced selectivity to 5BMF. This
can be due to the occurrence of some polymerization reactions for
humin formation [56,60–61]. Besides, the catalyst could not be isolated
by a simple physical method, since a dark solid was also observed as
reaction side product (humin) and several products from furfural
cleavage and condensation reactions between the formed species [34].
It is well known that the catalytic performance in etherification
reactions using heteropolyacids is related to the acid strength of the
catalysts. While a higher acidity is associated with a higher conversion,
the selectivity to ethers substantially decreases. In this sense, to mod-
ulate the acidity, HPAs were incorporated into xerogels of SiO2 and the
reaction was conducted under solvent-free conditions (Table 2, entries
4-5).
The incorporation of the heteropolyacid into SiO2 decreased the
acidity of the final material due to the interaction of HPA protons with
the support. Consequently, these solids exhibited the lowest acid
strength (Table 1) but at the same time, the acid sites were better dis-
persed [62] improving results for etherification reactions. 5BMF for-
mation was favored in both systems included. Interestingly, the use of
silica as catalyst could provide a moderate conversion in the systems
(52%) with a high selectivity to 5BMF (81%) at prolonged times (10 h).
A strategy to displace the equilibrium and favor the formation of
5BMF is the use of a co-solvent. Toluene was chosen because it forms an
azeotrope, trapping water formed in the reaction and possibly being
able to avoid side reactions. Table 3 summarizes the effect of toluene on
catalytic activity, evidencing that its addition did not improve con-
version in the systems (Table 3, entries 2-6) with respect to results
obtained under solvent-free conditions (Table 2). However, differences
in selectivity to 5BMF in bulk HPAs and HPA-SiO2 materials were ob-
served. An increase in the formation of 5BMF was obtained when PW
and PW-Mo were employed as catalysts, probably due to the lowest
interaction of HMF with the acid sites in the reaction medium, which
avoided the formation of undesirable products. However, the presence
of micropores was not totally desirable, as demonstrated in the results
for PW@SiO2. Only in PWMo@SiO2, which had a higher external sur-
face area where the HPA was probably deposited, the selectivity to
5BMF increased.
Fig. 1. Typical isotherms obtained by physisorption of N2 at −196 °C of the
solids studied.
altering the five atoms of nonequivalent phosphorous was evidenced in
31 P MAS-NMR. The structure of Preyssler HPAs were preserved due to
a higher hydrolytic stability as compared to other heteropolyacid sys-
tems [48,49].
Figure S1 displays SEM images of silica-containing HPA materials.
The solids showed large and sharp fragments with an apparent size of
about 50-100 μm. However, PWMo-incorporated SiO2 materials ex-
hibited a larger number of small fragments that were probably asso-
ciated with HPA deposited onto the external surface of the silica.
The acid strength of the heteropolyacids included was also mea-
sured by potentiometric titration (Fig. 2). A higher initial potential is
associated with a higher acid strength, which can be seen in bulk het-
eropolyacids, but this value decayed almost threefold upon HPA in-
corporation into the SiO2 xerogel (Table 1). However, the percentage of
inclusion was 12.5 (w/w), which ensured a larger acidity as compared
to bulk HPAs.
Pyridine adsorption spectra are shown in Fig. 3. The band at
1445 cm-1, related to coordinately bonded pyridine adsorbed on Lewis
acid sites (L), was observed only in SiO2 and slightly in PW(12.5%)@
SiO2. For all the other solids, the stronger bands at 1540 and 1490 cm-1
were related to Brönsted acid sites (B) or co-adsorption on Lewis and
Brönsted sites. These bands were almost unaltered upon incorporation
of PW and PW-Mo.
Interestingly, the presence of toluene as co-solvent led to a simpli-
fied catalyst separation and favored the formation of OBMF (up to a
maximum of 32% selectivity at 80%-85% conversion). The results with
toluene could be related to the displacement from equilibrium (the
presence of toluene forms an azeotrope with formed H2O) and probably
a better stabilization of Brönsted acid sites also occurred in the presence
of n-butanol-toluene mixture.
The etherification of HMF with n-butanol (n-BuOH) was studied as a
test reaction. The distinct ethers that could be obtained were 5-butox-
Various alcohol:HMF ratios were employed using PWMo(12.5%)@
SiO2 as catalyst (Table 3, entries 7-11). Although with an excess of n-
BuOH the conversion increased to a maximum of 89%, the behavior of
the selectivity depended on the aldehyde/alcohol ratio (Table 3).
An equimolar mixture of HMF and n-BuOH led to obtaining equi-
molar amounts for both ethers. The formation of 5BMF required an
increase in alcohol concentration, reaching an optimum HMF/n-BuOH
ratio equal to 1:3. The formation of OBMF was indirect evidence of a
change in the interaction of HPA protons with n-butanol or HMF when
toluene was used as co-solvent.
ymethylfurfural
(5BMF),
5,5'(oxybis(methylene))-bis-2-furfural
(OBMF), and dibutyl ether (DBE). However, the two major products
detected and appropriately quantified were 5BMF and OBMF (Fig. 4).
Table 2 summarizes the results of conversion and selectivity using
bulk and supported Preyssler heteropolyacids at 1 h of reaction. In all
cases, the conversion was found to be quantitative, with the maximum
conversion reached within the first hour of reaction. When the reaction
was conducted in solvent-free conditions, bulk Preyssler-HPAs (Table 2,
Table 1
Textural properties of the Preyssler heteropolyacids bulk and included in SiO2 xerogels.
Catalyst
SBET
*
Micropore area (t-plot) (m2 g−1
)
External Surface (t-plot) (m2 g−1
)
Pv
Ps
(nm)*
U (mV)*
(m2 g−1
)
(cm3
g
−1)*
PW
PWMo
PW(12.5%)@SiO2
PWMo(12.5%)@SiO2
SiO2
10 <
10 <
362
448
358
-
.-
299
240
315
-
-
-
-
-
-
764.6
793.4
309.2
122.1
291.3
63
208
43
0.17
0.22
0.16
1.9
1.9
1.8
* Pv: SBET: surface area Pore volume, Ps: Pore size, U: Acid strength,
3

O.M. Portilla-Zuñiga, et al.
MolecularCatalysis494(2020)111125
Fig. 2. Acid strength by potentiometric titration with n-butyl amine of Preyssler heteropolyacids included in SiO2 xerogel.
Table 2
Different catalyst performance of HMF etherification under solvent-free con-
ditions.
Entry
Catalyst
α (%)*
S5BMF
*
SOBMF*
1
2
3
4
5
6
Blank
PW
PWMo
PW(12.5%)@SiO2
PWMo(12.5%)@SiO2
SiO2**
-
-
23
8
82
65
81
-
100
100
100
100
52
2
2
6
29
9
Reaction conditions: 1 h; 40 mg of catalyst, 1:3 ratio of HMF:ButOH, 100 °C.
*α: conversion, S5BMF: 5-butoxyfurfural selectivity; SOBMF: OBMF selectivity
**The data were obtained at 10 h of reaction
Table 3
Effect of dry toluene on catalyst performance using several HMF/n-BuOH ratios.
Entry
Catalyst
HMF/n-BuOH
Conversion
S5BMF
SOBMF
Ratio
1
2
3
4
5
6
7
8
None
PW
PWMo
1:3
1:3
1:3
1:3
1:3
1:3
1:1
1:2
1:5
1:10
1:20
-
-
-
79
57
52
89
-
28
53
88
87
83
75
57
29
78
-
53
64
61
61
60
16
39
19
22
-
47
32
32
32
32
Fig. 3. Pyridine adsorption IR spectra of (a) SiO2, (b) PW, (c) PWMo, (d) PW
(12.5%)@SiO2, and (e) PWMo(12.5%)@SiO2.
PW(12.5%)@SiO2
PWMo(12.5%)@SiO2
SiO2
PWMo(12.5%)@SiO2
PWMo(12.5%)@SiO2
PWMo(12.5%)@SiO2
PWMo(12.5%)@SiO2
PWMo(12.5%)@SiO2
Fig. 5 shows the selectivity to 5BMF as a function of the conversion
level for the optimum catalyst PWMo(12.5%)@SiO2. The selectivity to
5BMF did not change with the conversion, reaching a maximum value
of ca. 80% for 5BMF. This behavior could confirm that the interaction
of the HPA proton with the hydroxyl groups of n-butanol or HMF was
important for the reaction. The interaction of the heteropolyacid was
previously reported to take place between the alcohol group and the
HPA proton [54]. In fact, a better degree of solvation of the Brönsted
acidic protons explained the reactivity of Keggin HPAs [54]. Previously,
we also found a stronger competition of n-butanol for HPA protons in
Keggin HPA supported on a magnetic core-shell which inhibited the
adsorption of other organic molecules modifying the selectivity ob-
tained [63]. Thus, the preferential interaction between the protons of n-
butanol seemed to be stronger than with HMF, and consequently, this
was reflected in the obtained selectivity.
9
10
11
Reaction conditions: 1 h; 40 mg of catalyst, 100 °C
*α: conversion, S5BMF: 5-butoxyfurfural selectivity; SOBMF: OBMF selectivity
became important at higher temperatures (> 110 °C) where the for-
mation of OBMF was favored in the presence of Brönsted acids
[49,56,64].
To ensure that these results were obtained in the absence of external
mass transfer, the etherification reaction was studied by modifying the
catalyst loading. Fig. 7 shows the initial rate as a function of the HPA
mass. A linear trend in the initial rate with an increase of the catalyst
amount used can be seen, which suggests the absence of external mass-
transfer resistance.
The recycling of PWMo(12.5%)@SiO2 was studied in five con-
secutive reaction runs. When the reaction finished, the catalyst was
separated from the crude mixture by centrifugation, washed several
times with ethyl alcohol, and utilized in another run. Fig. 8 shows the
The etherification of HMF with n-BuOH was studied in the tem-
perature range from 60 to 140 °C to corroborate a change in the ad-
sorption of reagents (Fig. 6). While the conversion increased with in-
creasing reaction temperature, the selectivity to 5BMF showed a
volcano-type dependence on temperature, and the selectivity to OBMF
Fig. 4. General scheme of the HMF etherifica-
tion reaction with n-butanol.
4

O.M. Portilla-Zuñiga, et al.
MolecularCatalysis494(2020)111125
Fig. 5. Selectivity to 5BMF as a function of the conversion level in the ether-
ification of n-BuOH with HMF. General conditions: 40 mg of PWMo(12.5%)@
SiO2, HMF/n-BuOH 1:3, 100 °C, 2 mL of toluene.
Fig. 8. Recycling of PWMo(12.5%)@SiO2. General conditions: 40 mg of PWMo
(12.5%)@SiO2, HMF/n-BuOH 1:3, 100 °C, 2 mL of toluene.
percentage conversion and the selectivity to 5BMF. The conversion
decreased gradually (ca. 40% of the initial activity after 5 cycles), with
only a minor decrease in selectivity. This result, due to ca. 10% HPA
leaching after 5 cycles, could explain the formation of 5BMF in-
dependently of the amount of catalyst employed.
Considering that the selectivity to 5BMF depends on proton stabi-
lization and availability, several alcohols were subsequently employed
in order to verify if the alcohol structure had any influence on the
catalytic activity. The resulting products were identified using GC-MS.
Table 4 shows the conversion and selectivity for highly substituted al-
cohols using PWMo(12.5%)@SiO2 as optimum catalytic system. In
general, the reaction gave good conversion and selectivity for primary
and benzyl alcohols (Table 4, entries 1 and 4). However, these results
did not clearly demonstrate that the selectivity was limited by the al-
cohol surface saturation on the heteropolyacid. Although an inductive
effect could explain the reactivity in linear alcohols, other products
were formed. When methanol or ethanol was employed, a consecutive
acetalization reaction occurred on the corresponding ether (Fig. 9).
With benzyl alcohol, the yield of the desired ether was 87%. However,
an electron-donating methyl group at the para-position substantially
reduced the formation of the desired ether, but with a strong electron
withdrawing the ether was not formed.
Fig. 6. Temperature effect on the conversion (□) and selectivity to 5BMF (○)
and OBMF (Δ). General conditions: 40 mg of PWMo(12.5%)@SiO2, HMF/n-
BuOH 1:3, 100 °C, 2 mL of toluene.
Besides, the obtained selectivity for more sterically hindered alco-
hols was very low (Table 4, entry 5).
3.1. Green metrics
In order to quantify the green metrics of the proposed synthetic
protocol, a number of green parameters were calculated and compared
with several examples found in the literature. The green parameters
Atom Economy, Reaction Yield, Inverse of Stoichiometric Factor,
Material Recovery Parameters, and Reaction Mass Efficiency are re-
presented in Fig. 10. In both figures, we can see that the proposed
solventless process is the closest to an ideal process. The full definition
of green parameters and settings can be found in the supplementary
material.
4. Conclusions
Fig. 7. Effect of the amount of catalyst on the initial rate in the etherification of
n-BuOH with HMF. General conditions: HMF/n-BuOH 1:3, 100 °C, 2 mL of to-
luene.
Heteropolyacids (PWMo and PW) and silica-included counterparts
were active as catalysts in the etherification reaction of HMF and n-
BuOH. The catalyst acid strength could be directly related to the se-
lectivity to 5BMF. Less acidic species, that is, the included species PW
(12.5%)@SiO2 and PWMo(12.5%)@SiO2, were found to be optimum
5

O.M. Portilla-Zuñiga, et al.
MolecularCatalysis494(2020)111125
Table 4
Conversion and selectivity for a range of alcohols in the etherification of HMF using PWMo (PWMo(12.5%)@SiO2) as catalyst.
Entry Alcohol
1
Ether
Conversion Selectivity (%) MS (m/z, %)
88
73
182 (M+, 20), 153 (30), 125 (10), 109 (83), 97 (80), 81 (45), 69 (7), 53 (23), 41 (17).
2
3
4
100
100
100
27
33
87
140 (M+, 23), 139 (4), 112 (7), 111 (100), 95 (3), 81 (30), 68 (7), 53 (62), 45 (11).
154 (M+, 20), 126 (5),125 (80), 109 (90), 97 (100), 81 (79), 69 (40), 53 (82), 41 (68), 40 (3).
216 (M+, 20), 187 (3), 170 (1), 138 (1), 110 (35), 91 (100), 81 (50), 78 (25), 65 (20), 53 (33)
5
100
5
262 (M+, 2), 246 (1), 185 (1), 175 (1), 110 (45), 91 (100), 77 (15), 65 (10), 58 (15), 51 (15), 43
(20).
6
7
77
25
25
182 (M+, 1), 167 (5), 153 (1), 126 (18), 109 (100), 97 (25), 81 (15), 57 (40), 53 (20).
100
230 (M+, 5), 201 (1), 134 (7), 124 (15), 110 (40), 105 (100), 91 (74), 81 (76), 77 (40), 65 (20), 53
(50).
8
100
-
-
Fig. 9. Tandem etherification – acet-
alization of HMF.
Fig. 10. Radial pentagon for green metrics calculated for the etherification of HMF with n-BuOH.
catalysts to carry out the reaction with greater selectivity and less by-
product production. PW and PWMo species were highly active under
the investigated reaction conditions but also led to by-product forma-
tion (humins). The reaction also showed a marked effect of the solvent
on the process, improving the selectivities of PW and PWMo catalysts
when dry toluene was employed as co-solvent (which also facilitated
catalyst recovery and minimized humin formation). Optimum systems
could also be employed in the condensation of other alcohols with
HMF, producing a variety of ethers with important structural diversity.
The catalysts could be reused up to five times with a reduction in ac-
tivity (90% to 50% conversion) but with a minor decrease in 5BMF
selectivity.
CRediT authorship contribution statement
Omar M. Portilla-Zuñiga: Investigation, Writing - original draft.
José J. Martínez: Writing - review & editing, Supervision. Mónica
Casella: Supervision. Daniela I. Lick: Investigation. Ángel G. Sathicq:
Conceptualization, Methodology, Writing
-
review
&
editing,
Supervision, Funding acquisition. Rafael Luque: Writing - review &
editing, Supervision. Gustavo P. Romanelli: Conceptualization,
Methodology, Writing - review & editing, Supervision, Project admin-
istration, Funding acquisition.
Declaration of Competing Interest
Funding
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
This work was supported by CONICET (PIP 003), ANPCyT (PICT
2013-0409), UNLP, and ERANET-LAC-1. The publication has been
prepared with support from RUDN University Program 5-100.
6

O.M. Portilla-Zuñiga, et al.
MolecularCatalysis494(2020)111125
Appendix A. Supplementary data
Macromolecules. 47 (2014) 3306–3316, https://doi.org/10.1021/ma500433e.
[24] G.G. Xu, P.P. Pagare, M.S. Ghatge, R.P. Safo, A. Gazi, Q. Chen, T. David,
A.B. Alabbas, F.N. Musayev, J. Venitz, Y. Zhang, M.K. Safo, O. Abdulmalik, Design,
Synthesis, and Biological Evaluation of Ester and Ether Derivatives of Antisickling
Agent 5-HMF for the Treatment of Sickle Cell Disease, Mol. Pharm. 14 (2017)
3499–3511, https://doi.org/10.1021/acs.molpharmaceut.7b00553.
[25] F. Li, X.L. Li, C. Li, J. Shi, Y. Fu, Aerobic oxidative esterification of 5-hydro-
xymethylfurfural to dimethyl furan-2,5-dicarboxylate by using homogeneous and
heterogeneous PdCoBi/C catalysts under atmospheric oxygen, Green Chem. 20
(2018) 3050–3058, https://doi.org/10.1039/c8gc01393d.
[26] C. Xiong, Y. Sun, J. Du, W. Chen, Z. Si, H. Gao, X. Tang, X. Zeng, Efficient con-
version of fructose to 5-[(formyloxy)methyl]furfural by reactive extraction and in-
situ esterification, Korean J. Chem. Eng. 35 (2018) 1312–1318, https://doi.org/10.
1007/s11814-018-0025-9.
[27] M. Krystof, M. Pérez-Sánchez, P. Domínguez De María, Lipase-catalyzed (Trans)
esterification of 5-hydroxymethylfurfural and separation from HMF esters using
deep-eutectic solvents, ChemSusChem. 6 (2013) 630–634, https://doi.org/10.
1002/cssc.201200931.
[28] Q. Cao, J. Guan, G. Peng, T. Hou, J. Zhou, X. Mu, Solid acid-catalyzed conversion of
furfuryl alcohol to alkyl tetrahydrofurfuryl ether, Catal. Commun. 58 (2015) 76–79,
https://doi.org/10.1016/j.catcom.2014.08.030.
[29] D. Padovan, A. Al-Nayili, C. Hammond, Bifunctional Lewis and Brønsted acidic
zeolites permit the continuous production of bio-renewable furanic ethers, Green
Chem. 19 (2017) 2846–2854, https://doi.org/10.1039/c7gc00160f.
[30] D.R. Chaffey, T.E. Davies, S.H. Taylor, A.E. Graham, Etherification Reactions of
Furfuryl Alcohol in the Presence of Orthoesters and Ketals: Application to the
Synthesis of Furfuryl Ether Biofuels, ACS Sustain. Chem. Eng. 6 (2018) 4996–5002,
https://doi.org/10.1021/acssuschemeng.7b04636.
[31] S. Alipour, H. Omidvarborna, D.S. Kim, A review on synthesis of alkoxymethyl
furfural, a biofuel candidate, Renew. Sustain. Energy Rev. 71 (2017) 908–926,
https://doi.org/10.1016/j.rser.2016.12.118.
[32] S. Shinde, C. Rode, Cascade Reductive Etherification of Bioderived Aldehydes over
Zr-Based Catalysts, ChemSusChem. 10 (2017) 4090–4101, https://doi.org/10.
1002/cssc.201701275.
[33] M.M. Antunes, S. Lima, A. Fernandes, A.L. Magalhães, P. Neves, C.M. Silva,
M.F. Ribeiro, D. Chadwick, K. Hellgardt, M. Pillinger, A.A. Valente, MFI Acid
Catalysts with Different Crystal Sizes and Porosity for the Conversion of Furanic
Compounds in Alcohol Media, ChemCatChem. 9 (2017) 2747–2759, https://doi.
org/10.1002/cctc.201601236.
[34] M. Balakrishnan, E.R. Sacia, A.T. Bell, Etherification and reductive etherification of
5-(hydroxymethyl)furfural: 5-(alkoxymethyl)furfurals and 2,5-bis(alkoxymethyl)
furans as potential bio-diesel candidates, Green Chem. 14 (2012) 1626–1634,
https://doi.org/10.1039/c2gc35102a.
[35] B. Liu, Z. Zhang, One-pot conversion of carbohydrates into 5-ethoxymethylfurfural
and ethyl d-glucopyranoside in ethanol catalyzed by a silica supported sulfonic acid
catalyst, RSC Adv. 3 (2013) 12313–12319, https://doi.org/10.1039/c3ra41043a.
[36] Y. Yang, C. Hu, M.M. Abu-Omar, Conversion of glucose into furans in the presence
of AlCl3 in an ethanol-water solvent system, Bioresour. Technol. 116 (2012)
190–194, https://doi.org/10.1016/j.biortech.2012.03.126.
[37] G. Raveendra, A. Rajasekhar, M. Srinivas, P.S. Sai Prasad, N. Lingaiah, Selective
etherification of hydroxymethylfurfural to biofuel additives over Cs containing si-
licotungstic acid catalysts, Appl. Catal. A Gen. 520 (2016) 105–113, https://doi.
org/10.1016/j.apcata.2016.04.017.
[38] Y. Yang, M.M. Abu-Omar, C. Hu, Heteropolyacid catalyzed conversion of fructose,
sucrose, and inulin to 5-ethoxymethylfurfural, a liquid biofuel candidate, Appl.
Energy. 99 (2012) 80–84, https://doi.org/10.1016/j.apenergy.2012.04.049.
[39] H. Wang, T. Deng, Y. Wang, Y. Qi, X. Hou, Y. Zhu, Efficient catalytic system for the
conversion of fructose into 5-ethoxymethylfurfural, Bioresour. Technol. 136 (2013)
394–400, https://doi.org/10.1016/j.biortech.2013.02.110.
[40] P. Che, F. Lu, J. Zhang, Y. Huang, X. Nie, J. Gao, J. Xu, Catalytic selective ether-
ification of hydroxyl groups in 5-hydroxymethylfurfural over H4SiW12O40/MCM-41
nanospheres for liquid fuel production, Bioresour. Technol. 119 (2012) 433–436,
https://doi.org/10.1016/j.biortech.2012.06.001.
[41] A. Liu, Z. Zhang, Z. Fang, B. Liu, K. Huang, Synthesis of 5-ethoxymethylfurfural
from 5-hydroxymethylfurfural and fructose in ethanol catalyzed by MCM-41 sup-
ported phosphotungstic acid, J. Ind. Eng. Chem. 20 (2014) 1977–1984, https://doi.
org/10.1016/j.jiec.2013.09.020.
[42] A. Liu, B. Liu, Y. Wang, R. Ren, Z. Zhang, Efficient one-pot synthesis of 5-ethox-
ymethylfurfural from fructose catalyzed by heteropolyacid supported on K-10 clay,
Fuel. 117 (2014) 68–73, https://doi.org/10.1016/j.fuel.2013.09.072.
[43] Y. Ren, B. Liu, Z. Zhang, J. Lin, Silver-exchanged heteropolyacid catalyst
(Ag1H2PW): An efficient heterogeneous catalyst for the synthesis of 5-ethox-
ymethylfurfural from 5-hydroxymethylfurfural and fructose, J. Ind. Eng. Chem. 21
(2015) 1127–1131, https://doi.org/10.1016/j.jiec.2014.05.024.
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2020.111125.
References
[1] F. Cherubini, The biorefinery concept: Using biomass instead of oil for producing
energy and chemicals, Energy Convers. Manag. 51 (2010) 1412–1421, https://doi.
org/10.1016/j.enconman.2010.01.015.
[2] E. de Jong, G. Jungmeier, Biorefinery Concepts in Comparison to Petrochemical
Refineries, in: A. Pandey, R. Höfer, M. Taherzadeh, M. Nampoothiri, C. Larroche
(Eds.), Ind. Biorefineries White Biotechnol. 1st ed., Elsevier B.V., 2015, pp. 3–33, ,
https://doi.org/10.1016/B978-0-444-63453-5.00001-X.
[3] R. Ravindran, S. Hassan, G. Williams, A. Jaiswal, A Review on Bioconversion of
Agro-Industrial Wastes to Industrially Important Enzymes, Bioengineering. 5 (2018)
93, https://doi.org/10.3390/bioengineering5040093.
[4] F. Federici, F. Fava, N. Kalogerakis, D. Mantzavinos, Valorisation of agro-industrial
by-products, effluents and waste: Concept, opportunities and the case of olive mill
waste waters, J. Chem. Technol. Biotechnol. 84 (2009) 895–900, https://doi.org/
10.1002/jctb.2165.
[5] R. Kumar, S. Singh, O.V. Singh, Bioconversion of lignocellulosic biomass:
Biochemical and molecular perspectives, J. Ind. Microbiol. Biotechnol. 35 (2008)
377–391, https://doi.org/10.1007/s10295-008-0327-8.
[6] J.B. Binder, R.T. Raines, Simple chemical transformation of lignocellulosic biomass
into furans for fuels and chemicals, J. Am. Chem. Soc. 131 (2009) 1979–1985,
https://doi.org/10.1021/ja808537j.
[7] L.A. Pfaltzgraff, M. De Bruyn, E.C. Cooper, V. Budarin, J.H. Clark, Food waste
biomass: A resource for high-value chemicals, Green Chem. 15 (2013) 307–314,
https://doi.org/10.1039/c2gc36978h.
[8] A. Corma, G.W. Huber, L. Sauvanaud, P. O’Connor, Processing biomass-derived
oxygenates in the oil refinery: Catalytic cracking (FCC) reaction pathways and role
of catalyst, J. Catal. 247 (2007) 307–327, https://doi.org/10.1016/j.jcat.2007.01.
023.
[9] M. Benish-Weisman, T. Steinberg, A. Knafo, Genetic and environmental links be-
tween children’s temperament and their problems with peers, Isr. J. Psychiatry
Relat. Sci. 47 (2010) 54–61, https://doi.org/10.1038/nature05923.
[10] S. Czernik, A.V. Bridgwater, Overview of Applications of Biomass Fast Pyrolysis Oil,
Energy & Fuels. 18 (2004) 590–598, https://doi.org/10.1021/ef034067u.
[11] A.A. Rosatella, S.P. Simeonov, R.F.M. Frade, C.A.M. Afonso, 5-
Hydroxymethylfurfural (HMF) as a building block platform: Biological properties,
synthesis and synthetic applications, Green Chem. 13 (2011) 754–793, https://doi.
org/10.1039/c0gc00401d.
[12] B.F.M. Kuster, 5-Hydroxymethylfurfural (HMF). A Review Focussing on its
Manufacture, Starch-Stärke. 42 (1990) 314–321, https://doi.org/10.1002/star.
19900420808.
[13] Z. Xu, P. Yan, K. Liu, L. Wan, W. Xu, H. Li, X. Liu, Z.C. Zhang, Synthesis of Bis
(hydroxylmethylfurfuryl)amine Monomers from 5-Hydroxymethylfurfural,
ChemSusChem. 9 (2016) 1255–1258, https://doi.org/10.1002/cssc.201600122.
[14] J.N. Tan, M. Ahmar, Y. Queneau, HMF derivatives as platform molecules: Aqueous
Baylis-Hillman reaction of glucosyloxymethyl-furfural towards new biobased ac-
rylates, RSC Adv. 3 (2013) 17649–17653, https://doi.org/10.1039/c3ra43369b.
[15] H. Sugimura, M. Kikuchi, S. Kato, W. Sekita, I. Sasaki, Practical synthesis of mu-
mefural, a component of Japanese apricot juice concentrate, Tetrahedron. 72
(2016) 7638–7641, https://doi.org/10.1016/j.tet.2016.10.026.
[16] Y.H. Wang, H.H. Dong, F. Zhao, J. Wang, F. Yan, Y.Y. Jiang, Y.S. Jin, The synthesis
and synergistic antifungal effects of chalcones against drug resistant Candida albi-
cans, Bioorganic Med. Chem. Lett. 26 (2016) 3098–3102, https://doi.org/10.1016/
j.bmcl.2016.05.013.
[17] L. Cisneros, P. Serna, A. Corma, Selective reductive coupling of nitro compounds
with aldehydes to nitrones in H2 using carbon-supported and -decorated platinum
nanoparticles, Angew. Chemie - Int. Ed. 53 (2014) 9306–9310, https://doi.org/10.
1002/anie.201402878.
[18] J. van Schijndel, L. Canalle, D. Molendijk, J. Meuldijk, Exploration of the Role of
Double Schiff Bases as Catalytic Intermediates in the Knoevenagel Reaction of
Furanic Aldehydes: Mechanistic Considerations, Synlett. 29 (2018) 1983–1988,
https://doi.org/10.1055/s-0037-1610235.
[19] W. Fan, Y. Queneau, F. Popowycz, HMF in multicomponent reactions: Utilization of
5-hydroxymethylfurfural (HMF) in the Biginelli reaction, Green Chem. 20 (2018)
485–492, https://doi.org/10.1039/c7gc03425c.
[20] E. Gubbels, L. Jasinska-Walc, C.E. Koning, Synthesis and characterization of novel
renewable polyesters based on 2,5-furandicarboxylic acid and 2,3-butanediol, J.
Polym. Sci. Part A Polym. Chem 51 (2013) 890–898, https://doi.org/10.1002/pola.
26446.
[21] L. Wu, R. Mincheva, Y. Xu, J.M. Raquez, P. Dubois, High molecular weight poly
(butylene succinate-co-butylene furandicarboxylate) copolyesters: From catalyzed
polycondensation reaction to thermomechanical properties, Biomacromolecules. 13
(2012) 2973–2981, https://doi.org/10.1021/bm301044f.
[22] A. Rodríguez, M. Brijaldo, L. Rache, L. Silva, L. Esteves, Common Reactions of
Furfural to scalable processes of Residual Biomass, Ciencia en Desarrollo 11 (1)
(2020), https://doi.org/10.19053/01217488.v11.n1.2020.10973.
[23] C.H.R.M. Wilsens, J.M.G.A. Verhoeven, B.A.J. Noordover, M.R. Hansen, D. Auhl,
S. Rastogi, Thermotropic polyesters from 2,5-furandicarboxylic acid and vanillic
acid: Synthesis, thermal properties, melt behavior, and mechanical performance,
[44] O.M. Portilla-Zuñiga, Á.G. Sathicq, J.J. Martínez, S.A. Fernandes, T.R.M. Rezende,
G.P. Romanelli, Synthesis of Biginelli adducts using a Preyssler heteropolyacid in
silica matrix from biomass building block, Sustain. Chem. Pharm. 10 (2018) 50–55,
https://doi.org/10.1016/j.scp.2018.09.002.
[45] D.M. Ruiz, G.P. Romanelli, P.G. Vázquez, J.C. Autino, Preyssler catalyst: An effi-
cient catalyst for esterification of cinnamic acids with phenols and imidoalcohols,
Appl. Catal. A Gen. 374 (2010) 110–119, https://doi.org/10.1016/j.apcata.2009.
11.037.
[46] A. Páez, H.A. Rojas, O. Portilla, G. Sathicq, C.A.M. Afonso, G.P. Romanelli,
J.J. Martínez, Preyssler Heteropolyacids in the Self-Etherification of 5-
Hydroxymethylfurfural to 5,5′-[Oxybis(methylene)]bis-2-furfural Under Mild
Reaction Conditions, ChemCatChem. 9 (2017) 3322–3329, https://doi.org/10.
7

O.M. Portilla-Zuñiga, et al.
MolecularCatalysis494(2020)111125
1002/cctc.201700457.
20120053355. 2012. Mar, 18.
[47] Masahiro Sadakane, Yutaro Ichi, Yusuke Ide, Tsuneji Sano; Thermal Stability and
Acidic Strength of Preyssler-Type Phosphotungstic Acid, H14[P5W30O110Na] and It’s
Catalytic Activity for Hydrolysis of Alkyl Acetates, Z. Anorg. Allg. Chem. 637 (14-
15) (2011) 2120–2124, https://doi.org/10.1002/zaac.201100427.
[48] Rahim Hekmatshoar, Majid M. Heravi, Sodeh Sadjadi, Hossein A. Oskooie,
[56] O.H. Pardo Cuervo, S.P. Simeonov, A.F. Peixoto, M.D. Popova, H.I. Lazarova,
G.P. Romanelli, J.J. Martínez, C. Freire, C.A.M. Afonso, Efficient Continuous
Production of the Biofuel Additive 5-(t-Butoxymethyl) Furfural from
5‐Hydroxymethylfurfural, Energy Technol. 7 (2019) 1900780, , https://doi.org/10.
1002/ente.201900780.
F. Fatemeh, Bamoharram; Catalytic performance of Preyssler heteropolyacid,
[57] Y. Jeannin, J. Martin-Frere, D.J. Choi, M.T. Pope, The Sodium Pentaphosphato(V)-
Triacontatungstate Anion Isolated as the Ammonium Salt, in: A.P. Ginsberg (Ed.),
Inorganic Syntheses, 2007, https://doi.org/10.1002/9780470132586.ch20.
[58] O. Portilla-Zúñiga, Á. Sathicq, J. Martínez, H. Rojas, E. De Geronimo, R. Luque,
G.P. Romanelli, Novel Bifunctional Mesoporous Catalysts Based on Preyssler
Heteropolyacids for Green Pyrrole Derivative Synthesis, Catalysts 8 (2018) 419,
https://doi.org/10.3390/catal8100419.
[59] Á.G. Sathicq, D.M. Ruiz, T. Constantieux, J. Rodriguez, G.P. Romanelli, Preyssler
heteropoly acids encapsulated in a silica framework for an efficient preparation of
fluorinated hexahydropyrimidine derivatives under solvent-free conditions, Synlett.
25 (2014) 881–883, https://doi.org/10.1055/s-0033-1340845.
[60] H. Shen, H. Shan, L. Liu, Evolution Process and Controlled Synthesis of Humins with
5‐Hydroxymethylfurfural (HMF) as Model Molecule, ChemSusChem 13 (2020) 513,
https://doi.org/10.1002/cssc.201902799.
[61] O.H.P. Cuervo, H.A. Rojas, L.A. Santos, T.C. Ramalho, G.P. Romanelli,
J.J. Martínez, Etherification of Hydroxymethylfurfural with Preyssler
Heteropolyacids Immobilized on Magnetic Composites, Chemistry Select. 3 (2018)
5526–5533, https://doi.org/10.1002/slct.201801051.
[62] W. Yang, J. Billy, Y.B. Taârit, J.C. Védrine, N. Essayem, H3PW12O40 supported on Cs
modified mesoporous silica: catalytic activity in n-butane isomerisation and in situ
FTIR study. Comparison with microporous CsxH3-xPW12O40, Catal. Today. 73
(2002) 153–165, https://doi.org/10.1016/S0920-5861(01)00508-9.
[63] A. Escobar, Á Sathicq, L. Pizzio, et al., Biomass valorization derivatives: Clean es-
terification of 2-furoic acid using tungstophosphoric acid/zirconia composites as
recyclable catalyst, Process Saf Environ Prot 98 (2015) 176–186, https://doi.org/
10.1016/j.psep.2015.07.008.
[64] P. Zhang, J. Yang, H. Hu, D. Hu, J. Gan, Y. Zhang, C. Chen, X. Li, L. Wang, J. Zhang,
Catalytic self-etherification of 5-hydroxymethylfurfural to 5,5′(oxy-bis(methylene))
bis-2-furfural over zeolite catalysts: effect of pore structure and acidity, Catal. Sci.
Technol. (2020), https://doi.org/10.1039/D0CY00733A.
14-
[NaP5W30O110
]
in liquid phase alkylation of phenol with 1-octene, Catalysis
Communications 9 (2008) 837–841, https://doi.org/10.1016/j.catcom.2007.09.
007.
[49] Wataru Ninomiya, Masahiro Sadakane, Yutaro Ichi, Toshiya Yasukawa,
Ken Ooyachi, Tsuneji Sano, Wataru Ueda, Influence of structural differences and
acidic properties of phosphotungstic acids on their catalytic performance for acy-
lation of pyruvate ester to α-acyloxyacrylate ester, Catalysis Today 164 (2011)
107–111, https://doi.org/10.1016/j.cattod.2010.10.047.
[50] Ivanka Holclajtner-Antunović, Danica Bajuk-Bogdanović, Marija Todorović, Ubavka
B. Mioč, Joanna Zakrzewska, Snežana Uskoković-Marković, Spectroscopic study of
stability and molecular species of 12-tungstophosphoric acid in aqueous solution,
Can. J. Chem. 86 (2008) 996–1004, https://doi.org/10.1139/v08-138.
[51] M. Arabi, M.M. Amini, M. Abedini, A. Nemati, M. Alizadeh, Esterification of
phthalic anhydride with 1-butanol and 2-ethylhexanol catalyzed by hetero-
polyacids, J. Mol. Catal. A Chem. 200 (2003) 105–110, https://doi.org/10.1016/
S1381-1169(03)00043-8.
[52] J.C. Serrano-Ruiz, R. Luque, A. Sepúlveda-Escribano, Transformations of biomass-
derived platform molecules: From high added-value chemicals to fuels via aqueous-
phase processing, Chem. Soc. Rev. 40 (2011) 5266–5281, https://doi.org/10.1039/
c1cs15131b.
[53] A.L. Marshall, P.J. Alaimo, Useful products from complex starting materials:
Common chemicals from biomass feedstocks, Chem. - A Eur. J. 16 (2010)
4970–4980, https://doi.org/10.1002/chem.200903028.
[54] E. Ahmad, T.S. Khan, M.I. Alam, K.K. Pant, M. Ali Haider, Understanding reaction
kinetics, deprotonation and solvation of Brönsted acidic protons in heteropolyacid
catalyzed synthesis of biorenewable alkyl levulinates, Chem. Eng. J. 400 (2020)
125916, https://doi.org/10.1016/j.cej.2020.125916.
[55] Jin-Ku C, Kim, S. Y., Lee, D. H., Kim, D., Jung, S., inventors, Gyeonggi-do, KR
Chungcheongnam-do, KR Seoul, KR - Daejeon, KR Seoul, KR, assigned. Method for
producing biofuel using marine algae-derved galactan. U.S Patent Application
8