Conversion of fructose into 5-HMF: A study on the behaviour of heterogeneous cerium-based catalysts and their stability in aqueous media under mild conditions

Article abstract of DOI:10.1039/c5ra03358f

In this paper, we present the results of a study on 5-HMF production from fructose by means of heterogeneous catalysts in aqueous media. Mild conditions were used, setting the temperature between 393 K and 443 K. Cerium(iv) phosphates, different from other metal(iv)-phosphates, such as titanium and zirconium, have been characterized only recently. Ce-phosphates are quite complex structures as they show several arrangements. They undergo leaching of the phosphate group as phosphoric acid with consequent slow de-activation of the catalysts. The leaching rate depends on the nature and on the temperature of the calcination of the original phosphates. This opened the question whether the conversion was driven by the heterogeneous catalysts or by the soluble phosphoric acid. A specific test has demonstrated that the solid catalysts are responsible for the conversion of fructose into 5-HMF, more than the liquid phase. We have also demonstrated that the leached phosphate is substituted by fructose on the solid catalyst. A best yield of 52% with selectivity of 93% in batch and 24% in a flow reactor at 443 K (single pass) with a selectivity also >95% were obtained.

Full text of DOI:10.1039/c5ra03358f

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PAPER
Conversion of fructose into 5-HMF: a study on the
behaviour of heterogeneous cerium-based
catalysts and their stability in aqueous media under
mild conditions
Cite this: RSC Adv., 2015, 5, 26941
ab
Michele Aresta,b Carlo Pastore,†b Luigi di Bitonto,b
*
Angela Dibenedetto,
Antonella Angeliniab and Eugenio Quarantaa
In this paper, we present the results of a study on 5-HMF production from fructose by means of
heterogeneous catalysts in aqueous media. Mild conditions were used, setting the temperature between
393 K and 443 K. Cerium(IV) phosphates, different from other metal(IV)-phosphates, such as titanium and
zirconium, have been characterized only recently. Ce-phosphates are quite complex structures as they
show several arrangements. They undergo leaching of the phosphate group as phosphoric acid with
consequent slow de-activation of the catalysts. The leaching rate depends on the nature and on the
temperature of the calcination of the original phosphates. This opened the question whether the
conversion was driven by the heterogeneous catalysts or by the soluble phosphoric acid. A specific test
has demonstrated that the solid catalysts are responsible for the conversion of fructose into 5-HMF,
more than the liquid phase. We have also demonstrated that the leached phosphate is substituted by
fructose on the solid catalyst. A best yield of 52% with selectivity of 93% in batch and 24% in a flow
reactor at 443 K (single pass) with a selectivity also >95% were obtained.
Received 23rd February 2015
Accepted 6th March 2015
DOI: 10.1039/c5ra03358f
www.rsc.org/advances
polymers. Some important non-furanic compounds can also be
produced from 5-HMF: levulinic acid,25 adipic acid,26 1,6-hex-
anediol27 and caprolactone.28 The most convenient synthetic
method of 5-HMF is based on the isomerization of glucose to
fructose followed by dehydration of the latter in aqueous media
(Fig. 1).
The rst step is base catalyzed. The triple dehydration of
fructose is the way to 5-HMF and requires acid catalysts. Several
types of catalysts have been used in such process, such as:
mineral acids,29–32 ion exchange resins33,34 and H-form
zeolites.35,36
Mineral acids show a low selectivity in the production of
5-HMF (55–63%) due to the formation in water of humins
derived from secondary reactions of 5-HMF.29–32 The addition of
metal chlorides (Cr(III) or Al(III)) to the acid catalyzed dehydra-
tion system, improves the selectivity towards 5-HMF,37,38 but
further steps are required to purify the nal products at the end
of the reaction, with energy loss. The use of exchange resins
1. Introduction
The growing use of fossil-carbon (oil-, coal-, and gas) and its
derivatives that result in increasing emissions of CO2 and
progressive reduction of natural fossil-C resources, is pushing the
interest towards the use of renewables, implementing a bio-
renery approach.1–4 A new area of chemistry is growing that aims
at developing innovative catalytic reactions able to transform
renewable feedstock into chemicals and fuels that may substitute
those actually obtained from fossil carbon.5–8 An important
intermediate obtainable from cellulosic biomass is 5-hydrox-
ymethylfurfural (5-HMF) that can be prepared by dehydratation
of fructose. 5-HMF is dened as a “platform molecule” for the
production of chemicals and materials,9–13 and has also been
proposed as a key intermediate for fuels.14–16 Example of useful
derivatives are: 2,5-furandicarboxylic acid17–19 that may replace
terephthalic acid as a monomer in the preparation of plastics,
5-hydroxymethylfuroic
acid,20
bishydroxymethylfuran,21,22
2,5-dimethylfuran,23 alkoxymethylfurfurals24 and the diether of
5-HMF that nds application in the production of fuels and
aUniversity of Bari, Department of Chemistry, Via Orabona n. 4, 70126 Bari, Italy.
E-mail: angela.dibenedetto@uniba.it; Fax: +39 0805443606; Tel: +39 0805443606
bCIRCC, Via Celso Ulpiani n. 27, 70126 Bari, Italy
† Present Addresses: Water Research Institute (IRSA-CNR), National Research
Council, Via de Blasio 5, 70132 Bari, Italy.
Fig. 1 Synthesis of HMF starting from glucose.
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limits the formation of side-products through the selective
absorption of 5-HMF during the reaction (selectivity > 70%),
however long reaction times are necessary as low temperatures
are used with such catalysts (24–48 h, 363 K).39 Silica nano-
particles40 functionalized with acid (SO3H) and base (NH2)
groups are used as catalysts in the one-pot conversion of
cellulose to HMF. A yield of 19.2% is obtained aer 3 h of
reaction (393 K) but the reaction requires the use of ionic
liquids as reaction media (e.g. [EMIM]Cl). Heterogeneous acid
catalysts offer the advantage of their very easy separation from
the reaction products and potentiality of recycling, thus
appearing the most suitable catalysts for exploitation in
potential industrial processes. Several lanthanide(III) chlorides
(DyCl3, YbCl3, LaCl3, NdCl3 and EuCl3) have been tested in water
in presence of ionic liquids.41 These catalysts are preferred to
other transition metals for the lower toxicity and cost, although
they produce humins and other secondary products. TiO2 and
ZrO2 have been used as dehydration catalysts in the synthesis of
5-HMF from glucose and fructose.42,43 Good conversion yields
were obtained (60–80%), but the selectivity of such process was
very low (10–30%). A novel approach to deconstructing cellulose
into HMF integrating a sequential enzyme cascade technique in
an aqueous system with solid acid catalysis in an organic
solvent system was developed,44 but requires additional costs
for the management of the plants used. Zirconium and tita-
nium phosphates are very active catalysts in the dehydration of
fructose for 5-HMF synthesis:45–47 such systems were used in
aqueous media and no appreciable subsequent rehydration to
levulinic and formic acids occurred (selectivity ¼ 65–70%).
Although there is an abundant amount of literature available on
the production of 5-HMF, the conversion of fructose into 5-HMF
with heterogeneous catalysts is not fully understood and ques-
tions are open about the life of the catalysts. An issue is their
stability towards leaching that can deactivate the catalytic
materials and shorten their life. The direct conversion of
cellulose into 5-HMF is possible working at high temperature
(520 K) and using water as solvent, but the harsh conditions
required and the low stability of starting and nal organic
matter, lower the yield because of high loss of material. Our aim
is to understand the behavior of catalysts in the C6 to 5-HMF
conversion in order to design more stable ones, working under
real heterogeneous catalysis in water, in the mildest possible
conditions limiting the loss of C6 units as humins or any other
by-products. In this paper we present the results of a study on
newly synthesized Ce-phosphate used in the dehydration of
fructose for the synthesis of 5-HMF.
2.2. Catalysts preparation
2.2.1. Synthesis of Ce3(PO4)4 (CeP1). To a freshly prepared
cerium(IV) ammonium nitrate solution (10.695 g, 20 mmol, in
100 mL of distilled water), a potassium phosphate solution in
stoichiometric amount (P : Ce
¼
3 molar ratio; 5.66 g,
22.67 mmol, in 50 mL of distilled water) was added. The
precipitate was isolated and washed 6 times with water (10 mL
each). The wet residue was then dried in an oven at 403 K. 4 g of
cerium phosphates were nally isolated as an orange-yellow
powder. Elemental analysis: P calculated for Ce3(PO4)4: 15.5%;
found: 15.4%. When the solid was calcined, its color changed to
gray-green (823 K) and white (1173 K).
2.2.2. Synthesis of [(Ce(PO4) (HPO4)0.5(H2O)0.5)] (CeP2).
CeP2 was prepared following literature informations48 from
CeO2, H3PO4 and H2O using a ratio of PO4 : Ce of 2.4. 2.58 g of
CeO2 were mixed with 15 mL of a 1.96 M H3PO4 solution. The
pale yellow suspension was heated in an iron steel autoclave at
473 K for 5 h. The resulting light yellow product was ltered off,
washed ve times with distilled water (10 mL) and dried in a
N2 ow (1.98 g). Elemental analysis:
[(Ce(PO4)(HPO4)0.5(H2O)0.5)]: 16.0%; found: 15.8%. The calcined
solid was gray at 823 K and white at 1173 K.
P calculated for
2.2.3. Synthesis of [(Ce(PO4)1.5(H2O)(H3O)0.5(H2O)0.5)]
(CeP3). CeP3 was prepared following reported procedures.49
First, Ce(NO3)6(NH4)2 (2.74 g, 5 mmol) was dissolved in HNO3
(6 M, 48.3 mL), then phosphoric acid (6 M, 1.7 mL) was added.
The conditions corresponded to [Ce4+] ¼ 0.1 M, PO4/Ce ¼ 2, and
nal [HNO3] ¼ 5.8 M. This solution, aged at 368 K for 18 h,
yielded the desired product. The yellow solid obtained was
ltered off, washed ve times with distilled water (10 mL) and
dried in a N2 ow (1.4 g). Elemental analysis P calculated for
[(Ce(PO4)1.5(H2O)(H3O)0.5(H2O)0.5)]: 14.8%; found: 14.5%. The
calcined solid was gray at 823 K and white at 1173 K.
2.2.4. Synthesis of Ce(IV) phosphotungstate. Ce3(PW12O40 4
)
was prepared by adapting the technique used for the synthesis
of the relevant iron compounds.50,51 An aqueous solution of
H3PW12O40 (19.2 g, 6.67 mmol, in 100 mL of distilled water) was
added dropwise to 2.74 g (5 mmol) of (NH4)2Ce(NO3)6 dissolved
in HNO3 solution (6 M, 48.3 mL). The yellow suspension
obtained was kept under reux for 5 h. The resulting solid was
ltered off, washed with distilled water (5 ꢀ 100 mL) and dried
under a nitrogen ux for 12 h. A ne yellow powder (19.5 g) was
isolated. Elemental analysis P: calculated for Ce3(PW12O40)4:
1.03%; found: 0.95%.
2.3. Catalytic tests
In order to avoid the presence of oxygen during the catalytic
tests, all operation were carried out under nitrogen. The
kinetics of conversion of fructose into 5-HMF at a xed
temperature (in the range 393–443 K), was studied in a 100 mL
2. Experimental section
2.1. Materials
Cerium ammonium nitrate $98% (by titration); potassium steel reactor equipped with a withdrawal valve and an elec-
phosphate tribasic monohydrate $95%; cerium(IV) oxide fused, trical heating jacket. 3.2 g of fructose were dissolved in 64 mL
pieces, 3–6 mm, 99.9% trace metals basis; phosphoric acid 85% of distilled water in a glass reactor, in which 0.5 g of the
wt in H2O, 99.99% trace metals basis; iron(III) nitrate non- catalyst under study and a magnetic stirrer were placed. The
ahydrate $98%; fructose $99%; glucose were ACS grade glass-reactor was then transferred into the steel reactor, that
reagents purchased from Sigma Aldrich.
was closed and heated to the reaction temperature. At xed
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intervals of time, 0.5 mL of the solution were withdrawn
through the sampling valve and analyzed.
For comparing the activity of the different catalysts under
study, a 4x-parallel stainless steel autoclave (Fig. 2)52,53 was
used. One position of the reactor was always used for a
control made of 0.1 g of fructose and 2 mL of distilled water.
In the three other positions, samples containing a variable
amount of fructose, 2 mL of distilled water and the studied
catalyst were loaded.
Fig. 3 Catalytic tests of the three different cerium phosphates in the
dehydration of fructose. Reaction conditions: 3.2 g of fructose, 0.5 g of
catalyst, 64 mL of water, temperature ¼ 423 K.
2.4. Analytical methods
5-HMF was analyzed by using a Jasco HPLC with an UV
detector at 284 nm and a ion-exclusion column Phenomenex
Rezex RHM-Monosaccharide H+ (8% cross-linked sulfonated
styrene–divinylbenzene) 300 ꢀ 7.8 mm at 343 K. A 0.005 N
solution of sulphuric acid was used as the mobile phase. The
ow rate was 0.6 mL minꢁ1. The concentration of fructose,
3. Results and discussion
3.1. Catalytic activity of cerium(IV) phosphates in the
dehydration of fructose for the synthesis of HMF
glucose and other by-products were determined using a RI Different cerium phosphates (namely, Ce3(PO4)4, [(Ce(PO4)
detector.
(HPO4)0.5(H2O)0.5)] and [(Ce(PO4)1.5(H2O)(H3O)0.5(H2O)0.5)],
FTIR-DRIFT spectra were recorded with a Shimadzu Prestige labeled as CeP1, CeP2 and CeP3 through the test, respectively)
21 instrument equipped with the Shimadzu DRS-8000 basic were tested in the dehydration of fructose to 5-HMF. The
apparatus modied for our purposes.54 Elemental analysis were kinetics of reaction (Fig. 3) carried out at 423 K shows that
carried out on mineralized samples. 31P NMR analysis were cerium phosphates accelerate the reaction: in 3 h 30 min the
made using a 400 MHz Varian NMR spectrometer at 162.0 MHz. same amount of product was obtained as in 9 h by pure thermal
The surface characterization was carried out by using the Pulse reaction and higher selectivity.
ChemiSorb 2750 Micromeritics instrument. The analyses of the
The three different phosphates show a different activity
acid sites were carried out using NH3 as probe-gas using 100 mg under such conditions, and differ for their stability. The reac-
of catalyst. The samples were pre-treated under N2 (30 mL tion proles show that aer 5 h 30 min of reaction a plateau is
minꢁ1) ow and 373 K. The Pulse Chemisorb was performed reached corresponding to a yield of ca. 43–52% 5-HMF. It is
with NH3 gas using He as carrier gas (30 mL minꢁ1). The TPD worth to emphasize that although in absence of catalysts the
were performed under He ow at 30 mL minꢁ1
.
HMF yield increased as the reaction time increased the selec-
13C cross-polarization magic angle scanning (CP-MAS) tivity is very low. Aer 24 h the conversion of fructose may be as
measurements on the freshly prepared and used catalysts high as 30% but the selectivity is lower than 30%.
were performed at 151 MHz on a Bruker AVANCE 600 apparatus
Such trend of reaction, is similar to that of zirconium and
equipped with a MAS II Pneumatic Unit. The solid sample was titanium phosphates reported in the literature,45–47 indicating
packed in a 4 mm zirconia rotor with a Kel-F cap and spinned at Ce-phosphates as new candidates for the production of 5-HMF.
7 kHz during the analysis (delay time ¼ 4 s, determined through
A fully surface characterization has been carried out: CeP1,
an inversion-recovery experiment on the proton that allows to CeP2, CeP3 show about the same surface area (Table 1).
measure longitudinal and spin–lattice T1 relaxation times).
Fig. 4a shows the TPD proles for NH3 release from CeP1,
CeP2, CeP3. It is evident that CeP1 and CeP3 are richer in strong
sites, while CeP2 is richer in medium strength sites.
Fig. 4b shows that there is a parallel trend between the yield
in 5-HMF and the total number of acid sites. Fig. 4c shows the
correlation of strong acid sites to the production of 5-HMF.
Apparently, both medium and strong sites play a role in 5-HMF
production. Noteworthy, analyzing the fructose conversion vs.
Table 1 BET specific surface area for Cerium(IV) phosphates
BET surface
Entry
Catalysts
area (m2 gꢁ1
)
1
2
3
CeP1
CeP2
CeP3
16.7
16.2
18.9
Fig. 2 4x-parallel stainless steel autoclave.
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Fig. 4 TPD profiles for NH3 release from CeP1, CeP2, CeP3 and the correlation of the total number of acid sites and strong acid sites to the
formation of 5-HMF.
HMF formation, we found that the selectivity towards 5-HMF
decreases from 79% to 68% aer 6 h of reaction. Simulta-
neously, we have investigated the effect of calcination of the
three catalysts listed above at two different temperatures: 823 K
and 1173 K. The catalytic activity was measured at 408 K for 3 h
using the 4x reactor (Fig. 2). At the end of each reaction cycle,
the solutions were separated and analyzed by HPLC, while the
residual solid was recovered, washed with water and used for a
new catalytic cycle using a fresh solution of fructose (Fig. 5–7).
Fig. 6 Recycling test of CeP2: with no calcination (black), calcined at
The experimental data reported in Fig. 5–7 clearly show that
the calcination at 823 K improves the resistance of the catalyst
(compare the percentage of HMF formed in the different calci-
nation conditions). In particular, in the rst run CeP2 and CeP3
calcined at 823 K are also more active than the non-calcined
materials. The catalytic activity of CeP3 calcined at 823 K is
always higher than that of the non-calcined sample in the four
consecutive runs. CeP2 shows a variation of the activity
depending on the number of runs: at the fourth run, it is more
active than the non-calcined sample. Conversely, aer the
calcination at 1173 K, the three cerium phosphates become less
active than the non calcined samples and the samples calcined
at 823 K, also if their activity remains quite constant through the
four consecutive runs.
823 K (grey) and 1173 K (white). Reaction conditions: 0.1 g of fructose,
0.016 g of catalyst, 2 mL of water, temperature ¼ 408 K, reaction time
¼ 3 h.
ꢂ the activity of the catalysts is in the order CeP3 > CeP1 >
CeP2;
ꢂ CeP2 decreases its activity at any cycle;
ꢂ CeP1 reaches a plateau aer the rst run, and then main-
tains its activity;
ꢂ catalysts calcined at 1173 K maintain their activity quite
constant during the three cycles, but are less active.
In order to gather information on possible changes occur-
ring to the catalysts during the runs, the IR-DRIFT spectra were
registered. At the end of each recycling test, the residual cata-
lysts were recovered, washed with distilled water for three times
(3 mL) and dried under a nitrogen ow.
The results are reported in Fig. 8 and 9 which show that the
calcination at 1173 K affords less modiable catalysts than the
calcination at 823 K. The most signicant changes increasing
Fig. 3–7 clearly show that:
ꢂ for all catalysts, calcined and non-calcined, there is a
substantial reduction of activity aer the rst cycle;
ꢂ catalysts calcined at 823 K maintain their activity for longer
time than others;
Fig. 5 Recycling test of CeP1: with no calcination (black), calcined at Fig. 7 Recycling test of CeP3: with no calcination (black), calcined at
823 K (grey) and 1173 K (white). Reaction conditions: 0.1 g of fructose, 823 K (grey) and 1173 K (white). Reaction conditions: 0.1 g of fructose,
0.016 g of catalyst, 2 mL of water, temperature ¼ 408 K, reaction time 0.016 g of catalyst, 2 mL of water, temperature ¼ 408 K, reaction time
¼ 3 h.
¼ 3 h.
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Table 2 Elemental analysis of cerium phosphates before and after the
fourth catalytic run
Experimental P (%)
Aer 1st and
% Loss
Catalysts
Starting catalyst
5th run
aer 5 runs
CeP1
CeP2
CeP3
15.4
15.8
14.5
11.6–9.8
7.2–5.9
11.6–11.0
36.4
62.6
24.1
powder while they are yellow when not calcined and grey
when calcined at 872 K. The thermal treatment at 1173 K may
produce a mixture of Ce(III)–Ce(IV) compounds, as demonstrated
by XPS analysis that shows a composition 75% of CePO4 and
25% of Ce(P3O9). This is in line with what reported by Nazaraly
et al.,56 who used TGA for following the reduction of Ce(IV) to
Ce(III). These gures suggest that upon the use in catalysis there
is a modication of the structure and the composition of the Ce-
phosphate. In order to shade light on such modication the
catalytic materials were analyzed for their P-content. The cata-
lyst recovered aer the fourth run were isolated by ltration,
washed with water and dried under a nitrogen ow as discussed
above. The samples were mineralized with concentrated nitric
acid and the solution worked up as described in the literature
for the colorimetric determination of phosphorus.57 The results
of the elemental analysis shown in Table 2 conrm the loss of
phosphate during the catalytic runs: so the nature of the cata-
lysts aer their use in the dehydration of fructose signicantly
changes.
Noteworthy, CeP3, undergoes the less signicant change,
while CeP2 is the species that is most affected. It was, thus,
extremely important to dene if the real active sites were on the
solid catalysts or in solution, and to characterize the nature of
the leached P-species.
To answer this question, the activity of two phosphates that
are putative species formed in solution upon release of P from
the catalysts were investigated, namely: H3PO4 and NaH2PO4.
The dependence of the activity of such species on their
concentration in solution is shown in Fig. 10, working at 408 K.
This conrms that the dehydration of fructose is promoted by
Bronsted acidity and strong acids are more effective than weak
Fig. 8 Comparison of the IR-DRIFT spectra of cerium phosphates
calcined at 823 K before (black) and after (grey) their use in catalysis.
the calcination temperature from 823 K to 1173 K consist in the
strong modication of the IR spectrum in the region where the
n(OH) is observed. The position and the intensity of bands at
3500 cmꢁ1 is strongly modied upon calcination and upon
reaction.
The higher calcination temperature leads to almost
complete loss of water as demonstrated by the strong reduction
of the intensity of the above-mentioned band.
Both CeP1 and CeP2 calcined at 823 K show a deep change in
the region 3500–3000 and 1250–1000 cmꢁ1, where the n(OH)
and d(OH) occur. CeP3 shows less important change aer use in
catalysis, most probably due to its peculiar structure. Its higher
stability is joint to a higher activity aer the rst run as dis-
cussed above.
When the sample calcined at 1173 K are considered, one can
say that CeP1 and CeP2 are less hydrated than the same samples
calcined at 823 K, are more stable during catalytic runs, but, as
demonstrated by the XPS spectra,55 the reduction of the cerium
atoms from Ce(IV) to Ce(III) occurs: the Ce(III) component at
885.7 and 903.3 eV is quite evident in the XPS spectra. Indeed,
aer calcination at 1173 K, the samples appear a white-grey
Fig. 10 Dependence of the activity in catalysis of H3PO4 and NaH2PO4
on their concentration, in the synthesis of HMF. Reaction conditions:
Fig. 9 Comparison of the IR-DRIFT spectra of cerium phosphates 0.1 g of fructose, 2 mL of water, temperature ¼ 408 K, reaction time
calcined at 1173 K before.
¼ 3 h.
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Table 3 Amount of phosphates leached (determined by HPLC) and pH of the solutions using CeP3 in presence and absence of fructose before
and after heating at 373 K for 3 h, and 31P NMR signals of the solutions isolated after a catalytic run
Time zero
Aer heating
E
Catalyst
System
Amount of PO4 (mg)
pH
Amount of PO4 (mg)
pH
31P signal ppm
1
2
3
4
No catalyst
No catalyst
CeP3
Only water
Fructose solution
Only water
—
—
1.47
1.48
5.06
5.10
2.50
2.30
—
—
2.51
14.54
3.88
3.97
2.10
1.71
—
—
0.12
0.09
CeP3
Fructose solution
acids. In fact, the dissociation constant for the rst and second during runs 2–4, that is in agreement with the yield of HMF in
dissociation of phosphoric acid is Ka1,H PO ¼ 7.1 ꢀ 10ꢁ3 and the same runs. As Table 3 shows, the presence of fructose
3
4
Ka2,H PO ¼ 7.1 ꢀ 10ꢁ8 at 298 K. The low value of the pH of the enhances the release of phosphoric acid (compare entries 3 and
ꢁ
4
2
solution aer catalysis (ca. 2.5) suggests that H3PO4 is formed in 4). In order to conrm such nding in non-catalytic conditions,
solution more than H2PO4ꢁ, that is in line with the fact that the CeP2 was heated in a fructose aqueous solution at 373 K (at such
dehydration is acid-catalysed.
temperature no conversion of fructose is observed) for 3 h.
Noteworthy, the pH of a solution of 0.01 g of H3PO4 in 4 mL Again we found that the amount of leached phosphate in
of water is 1.69 (very close to the value found for the solution presence of fructose is ve times that leached when only water
formed upon leaching of phosphate, entry 4, Table 3) while the was used. In order to ascertain whether fructose may link to the
pH of a solution of 0.01 g of NaH2PO4 in 4 mL of water is 4.18, catalyst, we have carried out CP-MAS spectra of the solid cata-
that does not match any of the values found for the solution of lysts before and aer use in catalysis. The presence of the
the leached phosphate.
organic matter linked to the catalysts surface has been
Fig. 10 shows a clear difference of activity for the two putative demonstrated by solid state 13C CPMAS NMR spectrum. The
phosphate-species formed in solution upon release of P from spectrum of the catalyst recovered aer the rst run and washed
the catalysts. H3PO4 is much more active than its conjugated with water shows two broad signals centred at about 68 and
ꢁ
base H2PO4
.
118 ppm that have to be compared with the sharp signals of
solid fructose at 68, 71.7, 74.1 ppm and 102.35 ppm. The broad
The presence of H3PO4 was conrmed also by HPLC and 31
P
NMR (see Table 3). The 31P signals also show that H3PO4 is shape of the NMR signals of the catalyst aer the catalytic run
formed. In fact, NaH2PO4 shows the 31P resonance at ꢁ3.5 ppm maybe due to the high dispersion of the organic matter and to
in water.58 Another interesting information given by Table 3 is its low amount. It is worth to note that the catalyst itself does
that the release of H3PO4 is enhanced in presence of fructose not show any signal in the range reported above aer an
(14.54 mg) with respect to pure water (2.51 mg).
acquisition time equal to that of the catalyst aer catalysis.
Clearly, the polyol interacts with the catalyst and favours the
The deactivation of the catalyst is, thus, due to the loss of
release of phosphate through a linkage to the Ce-atoms. In order phosphate and to the insurgence of bonds of the organic
to gain further insight into the role in catalysis of the released species with Ce in a new mixed inorganic–organic compound.
acids and the solid, the solution was separated from the solid The nature of such modied catalysts is still under investigation
catalyst aer the rst run, and catalytic tests were made using in our laboratory.
separately the solution and the solid (408 K, 0.1 g of fructose, 3 h
In conclusion, it is clear that the catalysts used leach phos-
reaction time, the total volume was adjusted with water at 2 mL). phoric acid during their use in the dehydration of fructose and
The results are listed in Table 4 that shows that the solid is more the latter enhances the leaching of phosphorus as H3PO4.
active than the solution in producing 5-HMF (>85%), the released Concurrently, fructose modies the original phosphate into an
phosphates afford only marginal production of 5-HMF.
inorganic–organic species containing less phosphate: most
Anyway, the catalysts undergo a structural change that is probably, the C6 species is bonded to the surface causing a
quite important during run 1, and remains quite constant lower activity. Such changes take place mostly during the rst
run and is not as much important in consecutive runs. In fact,
increasing the number of the catalytic runs, the leaching of
Table 4 5-HMF production using the recovered solid and the solution
phosphate continues, but at much lower extent, until reaching a
obtained after a catalytic run. Reaction conditions: 0.1 g of fructose, 2
maximum aer ve runs. The reduction of the activity of the
mL of water, temperature ¼ 408 K, time ¼ 3 h
catalysts depends on the nature of the original species and on
5-HMF production yield (%)
Solution
the temperature of calcination. The temperature of 823 K seems
to be the most correct, reaching a good compromise of structure
stabilization and activity. Higher temperature might affect the
oxidation state of Ce, reducing the original Ce(IV) to Ce(III) much
less active in catalysis.
Catalysts
Residual solid
CeP1
CeP2
CeP3
1.1
1.2
0.8
6.4
8.2
11.5
26946 | RSC Adv., 2015, 5, 26941–26948
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Table 5 Continuous production of HMF from fructose using CeP3 as
catalyst. 1 mL of solution collected and analysed
Starting
T
fructose
Flow
HMF
Fractions yield%
HMF
selectivity%
E (K) (mg mLꢁ1
)
(mL minꢁ1
)
1 403 10
2 423 10
3 443 10
4 443 10
0.20
0.20
0.10
0.15
1–3
1–4
1
1
2
0.00
0.00
4.14
2.28
8.9
0.00
0.00
95
93
96
5 443 10
6 443 10
0.20
0.30
1
2–4
1
13.90
23.9
3.67
2.62
93
95
94
93
Fig. 11 Kinetic study using Ce(IV) phosphotungstate in the dehydration
of fructose. Reaction conditions: 3.2 g of fructose, 0.5 g of catalyst,
64 mL of water, temperature ¼ 423 K.
2–4
3.2. Catalytic activity of cerium(IV) phosphotungstate
The catalytic activity of Ce3(PW12O40)4 was also tested working
at 423 K: aer 3 h only 10% of HMF was formed (Fig. 11) with a
low selectivity of 33% ca.
4. Conclusions
In this work, the possibility of developing a process to manu-
facture 5-HMF in water under convenient conditions was
investigated, using cerium phosphates as catalysts. The kinetics
of reaction show that aer 5 h 30 min of reaction a plateau in
the production of HMF is reached with a yield of 43–52% and a
selectivity of 93%.
The possibility of reusing a catalyst like cerium(IV) phos-
phates is linked to the leaching of phosphates. Tests on the
recoverability of the catalyst and their use in consecutive runs,
show that CeP3 is the most stable catalyst showing a decent
activity in the dehydratation of fructose, undergoing the less
signicant changes. Finally, catalytic tests using CeP3 in a ow
reactor afforded a best yield of 23–24% at 443 K (single pass)
with a selectivity over 93–96%. Further studies are in progress in
order to increase the reaction yield and characterize the mate-
rials aer the consecutive runs.
3.3. Test in a ow reactor
Having identied CeP3 as the most active and resistant catalyst
using the batch process, a ow-reactor was used for the
production of HMF from fructose. Such study contributed to
nd the best operative conditions for converting fructose into
HMF. The apparatus is represented in Fig. 12.
Both the reservoir containing the fructose solution and the
cylinder where the reaction solution was collected were gradu-
ated so that it was easy to make a (volume)mass-balance.
The pump used allowed to work with a very precise ow
(<0.1 mL minꢁ1). The reaction column was lled with cerium
phosphates (ca. one gram mixed with glass micro-beads), and
heated with an electric jacket. The reaction solution was
collected in several fractions of 10 mL and analyzed using
HPLC. Table 5 shows that below 443 K there is no conversion of
fructose. The best operative conditions are 443 K with a ow
rate is 0.20 mL minꢁ1 (entry 5).
Acknowledgements
This work was carried out within the Projects MIUR-PON 01966
Enerbiochem (synthesis and characterization of catalysts) and
CTN01_00063_49393 Cluster Project REBIOCHEM (develop-
ment of the ow reactor).
Increasing the reaction ow, the yield of HMF decreased, but
selectivity of the process grew up, with a lower formation of
humins and other co-products. Increasing the starting concen-
tration by 5 times, the conversion yield did not improve: the overall
conversion yield remained in the range 14–16% in the best cases,
while a reduction of selectivity was again veried.
Notes and references
1 Biorenery: from biomass to chemicals and fuels, Renery of the
future: feedstock, processes, products, ed. M. Aresta, A.
Dibenedetto and F. Dumeignil, De Gruyter, 2012, pp. 19–48.
2 J. V. Mitchell and B. Mitchell, Energy Policy, 2014, 64, 36.
3 R. F. Aguilera, Energy Policy, 2014, 64, 134.
4 C. McGlade, J. Speirs and S. Sorrel, Energy, 2013, 55, 571.
5 R. A. Sheldon, Green Chem., 2014, 16, 950.
6 M. J. Climent, A. Corma and S. Iborra, Green Chem., 2014, 16,
516.
7 S. Perathoner and G. Centi, Energy, 2014, 61, 719.
¨
8 R. Schlogl, ChemSusChem, 2010, 3, 209.
Fig. 12 Apparatus used for the continuous production of HMF from
fructose.
9 S. P. Teong, G. Yi and Y. Zhang, Green Chem., 2014, 16, 2015.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 26941–26948 | 26947

View Article Online
RSC Advances
Paper
10 L. Nishita, R. K. Narasimha, S. N. Atul, K. Ganesh and 35 H. Suqin, C. Feipeng, J. Fuqiang, L. Yan, W. Jianmei and
C. Satyanarayana, J. Chem. Sci., 2014, 126, 403.
Y. Gai, Faming Zhuanli Shenging, CN 102212047 A, 2011.
36 Y. Du, Q. Liu, H. Cao and C. Bai, Faming Zhuanli Shenging,
CN 101434589 A, 2009.
37 H. Xianglin, D. Tiansheng and Z. Yulei, Faming Zhuanli
Shenqing, CN 101941957 A, 2011.
11 S. Dutta and S. Pal, Biomass Bioenergy, 2014, 62, 182.
12 A. Corma, S. Iborra and A. Velty, Chem. Rev., 2007, 107, 2411.
13 X. Tong, Y. Ma and Y. Li, Appl. Catal., A, 2010, 385, 1.
14 D. Liu and E. Y. X. Chen, ACS Catal., 2014, 4, 1302.
15 H. Cai, C. Li, A. Wang and T. Zhang, Catal. Today, 2014, 234, 38 S. De, S. Dutta and B. Saha, Green Chem., 2011, 13, 2859.
59.
39 H. E. Vandam, A. P. G. Kieboom and H. Vanbekkum, Starch,
1986, 38, 95.
16 C. V. McNeff and D. T. Nowlan, US Pat. 8697893 B2, 2014.
17 S. Siankevich, G. Savoglidis, Z. Fei, G. Laurenczy, 40 W.-H. Peng, Y.-Y. Lee, C. Wu and K. C.-W. Wu, J. Mater.
D. T. L. Alexander, N. Yan and P. J. Dyson, J. Catal., 2014,
315, 67.
Chem., 2012, 22, 23181.
˚
41 T. Stahlberg, M. G. Sørensen and A. Riisager, Green Chem.,
18 A. Villa, M. Schiavoni, S. Campisi, G. Veith and L. Prati,
ChemSusChem, 2013, 6, 609.
19 L. Lu, C. Tianming and Z. Junping, Faming Zhuanli
Shenging, CN 101891719 A, 2010.
20 J. J. Pachemo and M. E. Davis, Proc. Natl. Acad. Sci. U. S. A.,
2014, 111, 8363.
21 J. Ohyama, A. Esaki, Y. Yamamoto, S. Arai and A. Satsuma,
RSC Adv., 2013, 3, 1033.
22 M. Balakrishnan, E. R. Sacia and A. T. Bell, Green Chem.,
2012, 14, 1626.
23 S. Nishimura, N. Ikeda and K. Ebitani, Catal. Today, 2014,
232, 89.
24 Y. Roman-Leshkov, C. J. Barrett, Z. Y. Liu and J. A. Dumesic,
Nature, 2007, 447, 982.
2010, 12, 321.
42 X. Qi, M. Watanabe, T. M. Aida and R. L. Smith Jr, Catal.
Commun., 2008, 9, 2244.
43 I.-J. Kuo, N. Suzuki, Y. Yamauchi and K. C.-W. Wu, RSC Adv.,
2013, 3, 2028.
44 Y.-C. Lee, S. Dutta and K. C.-W. Wu, ChemSusChem, 2014, 7,
3241.
45 V. Ordomsky, J. Van der Schaaf, J. C. Schouten and
T. A. Nijhuis, ChemSusChem, 2013, 6, 1697.
46 Y. Hiroyuki and A. F. Salak, Jpn. Kokai Tokkyo Koho, JP
2007196174 A 20070809, 2007.
47 F. Benvenuti, C. Carlini, P. Patrono, A. M. R. Galletti,
G. Sbrana, M. A. Massucci and P. Gallic, Appl. Catal., A,
2000, 194, 147.
´
25 B. D. Mullen, C. M. Leibig, L. A. Kapicak, D. L. Bunning, 48 M. Nazaraly, G. Wallez, C. Chaneac, E. Tronc, F. Ribot,
S. M. Strand, D. J. Brunelle, D. M. Rodwogin,
R. P. Shirtum, A. J. Louwagie and D. J. Yontz, PCT Int.
Appl. WO 2013078391 A1, 2013.
M. Quarton and J.-P. Jolivet, J. Phys. Chem. Solids, 2006, 67,
1075.
´
49 M. Nazaraly, C. Chaneac, F. Ribot, G. Wallez, M. Quarton
26 T. R. Boussie, E. L. Dias, Z. M. Fresco, V. J. Murphy,
and J.-P. Jolivet, J. Phys. Chem. Solids, 2007, 68, 795.
J. Shoemaker, R. Archer and H. Jiang, US Pat. 8669397 B2, 50 K. Shimizu, R. Uozumi and A. Satsuma, Catal. Commun.,
2014. 2009, 10, 1849.
27 J. Tuteja, H. Choudhary, S. Nishimura and K. Ebitani, 51 D.-L. Long, R. Tsunashima and L. Cronin, Angew. Chem., Int.
ChemSusChem, 2014, 7, 96. Ed., 2010, 49, 1736.
28 T. Buntara, S. Noel, P. H. Phua, I. Melian-Cabrera, J. G. de 52 A. Dibenedetto, A. Angelini, L. di Bitonto, E. De Giglio,
Vries and H. J. Heeres, Angew. Chem., Int. Ed., 2011, 50, 7083. S. Cometa and M. Aresta, ChemSusChem, 2014, 7, 1115.
29 M. Watanabe, K. Yashiro and T. Nishi, PCT Int. Appl., WO 53 M. Aresta, A. Dibenedetto, L. di Bitonto and A. Angelini,
2013146085 A1, 2013. Patent MI2013A001136, 2013.
30 Q. Wai, H. Renliang, S. Rongxin and H. Zhimin, Faming 54 M. Aresta, A. Dibenedetto, C. Pastore, A. Angelini, B. Aresta
´
´
and I. Papai, J. Catal., 2010, 44–52, 269.
Zhuanli Shenging, CN 101724664 A, 2010.
31 S. T. Hansen, J. M. Woodley and A. Riisager, Carbohydr. Res., 55 M. Aresta, A. Dibenedetto, C. Pastore, C. Cuocci, B. Aresta,
2009, 344, 2568. S. Cometa and E. De Giglio, Catal. Today, 2008, 137, 125.
32 Z. Zhao, C. Li, Z. Zhang and Y. Du, Faming Zhuanli 56 M. Nazaraly, G. Wallez, C. Chaneac, E. Tronc, F. Ribot,
Shenqing, CN 101386611 A, 2009. M. Quarton and J.-P. Jolivet, Angew. Chem., 2005, 117, 5837.
33 J. Perez-Maqueda, I. Arenas-Ligioiz, O. Lopez and 57 C. H. Fiske and Y. Subbarow, J. Biol. Chem., 1925, 66, 375.
´
´
´
´
´
˜
´
58 A. Aldrey, A. Macıas, R. Bastida, G. Zaragoza, G. Rama and
J. Fernandez-Bolanos, Chem. Eng. Sci., 2014, 109, 244.
34 X. Qi, M. Watanabe, T. M. Aida and R. L. Smith, Green Chem.,
2008, 10, 799.
´
M. V. Lopez, Org. Biomol. Chem., 2012, 10, 5379.
26948 | RSC Adv., 2015, 5, 26941–26948
This journal is © The Royal Society of Chemistry 2015