Influence of properties of SAPO's on the one-pot conversion of mono-, di- and poly-saccharides into 5-hydroxymethylfurfural

Article abstract of DOI:10.1039/c3ra43197e

Synthesis of 5-hydroxymethylfurfural (5-HMF) from biomass derived mono- and poly-saccharides is gaining importance because of its usefulness in the preparation of important chemicals. In our work, we have synthesized several silicoaluminophosphate (SAPO)

Full text of DOI:10.1039/c3ra43197e

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Influence of properties of SAPO’s on the one-pot
conversion of mono-, di- and poly-saccharides into
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17156
5-hydroxymethylfurfural3
Prasenjit Bhaumik and Paresh Laxmikant Dhepe*
Synthesis of 5-hydroxymethylfurfural (5-HMF) from biomass derived mono- and poly-saccharides is gaining
importance because of its usefulness in the preparation of important chemicals. In our work, we have
synthesized several silicoaluminophosphate (SAPO) catalysts, and have shown that in the absence of any
other pH modifying reagents, those are active in converting mono- and poly-saccharides into 5-HMF under
biphasic reaction condition at 175 uC. Particularly, SAPO-44 catalyst showed the best activity in the
conversion of fructose to yield 78% 5-HMF with 88% selectivity. On the contrary, all other catalysts showed
lower yields (H-MOR: 63%, SAPO-5: 32%, 2DCT: 60%). Over SAPO-44, good yields for 5-HMF were
observed when glucose (67%), maltose (57%), cellobiose (56%) and starch (68%) were used as substrates.
Recycle study carried out with SAPO-44 catalyst in the fructose conversion reaction showed marginal
decrease in the activity up to 3rd run and then afterwards constant activity was observed up to 5th run
(1st: 78%, 2nd: 71%, 3rd: 66%, 4th: 65%, 5th: 65%). Catalyst characterizations revealed that SAPO catalysts
have higher hydrophilic nature than H-MOR (Si/Al = 10) and hence it is postulated that this property may
help in achieving better results. Further studies on the catalyst characterizations revealed that SAPO-44
undergoes modifications in its structure. However, ICP-OES data suggests that Al and/or P are not leached
out in the solution indicating that change in local environment around elements is possible. The influence
of acid amount, type of acid site etc. on the catalytic activity is discussed and found out that strong acid
sites are required to boost the 5-HMF yields.
Received 25th June 2013,
Accepted 16th July 2013
DOI: 10.1039/c3ra43197e
www.rsc.org/advances
acids,25,26 soluble metal halides,27,28 heteropoly acids29 and ionic
liquids.28,30,31 But due to the problems associated with homo-
1. Introduction
geneous catalysts solid acids such as, zeolites,13,32 ion-exchanged
resins,12,29,33,34 metal oxide,17,35 and Sn-Mont catalyst36 are used
to convert fructose into 5-HMF. Use of polar aprotic solvents such
as dimethyl sulfoxide (DMSO), dimethyl formamide (DMF) and
acetonitrile have proven to be better solvents compared to a
water-only system in HMF formation. Enhancement of 5-HMF
yield to 54% is possible in DMSO compared to a water-only
system (34% yield) using TiO2 nanoparticles.37 Ion-exchange
resin also showed good catalytic activity in fructose dehydrocy-
clization reaction (Amberlyst-15: 100% 5-HMF, Nafion-H: 75%
5-HMF) in DMSO.29 But the use of a high boiling solvent makes
the separation process for 5-HMF from the solvent unfavourable
and energy consumable. With this in mind miscible solvent
systems were studied. An acetone + DMSO system yielded better
5-HMF (90%) compared to a water + acetone system (73%) at 150
uC over an ion-exchange resin.33,34 Various metal oxides (Ta
oxide, Nb oxide) were also studied in a water + 2-butanol
system.35 A recent report shows that use of a Sn-Mont catalyst can
produce 79% 5-HMF yield from fructose at 160 uC using a
Solid acid catalysts are attracting
a lot of attention in
carbohydrate chemistry for the conversion of polysaccharides
and sugars into value-added chemicals. It is reported that solid
acid catalysts can hydrolyze polysaccharides like starch,1
cellulose2 and hemicellulose3 to yield monomer sugars. The
dehydration reaction of fructose yields 5-hydromethylfurfural (5-
HMF) over solid acid catalysts. 5-HMF, the ‘‘sleeping giant’’4 of
renewable intermediate chemicals is used to produce a versatile
range of important chemicals such as 2,5-furandicarboxyl acid,
one of the top 12 value added chemicals,5 2,5-dimethylfuran, 2,5-
dihydroxymethylfuran, 2,5-bis(hydroxymethyl)tetrahydrofuran
and liquid alkanes by undergoing oxidation,6–8 hydrogenation9,10
and Aldol condensation reactions.11,12 5-HMF is typically
synthesized from a variety of substrates such as fructose,13–15
glucose,14,16,17 starch,18,19 cellulose19,20 and inulin4,19,21 using
homogeneous catalysts such as mineral acids,22–25 organic
Catalysis & Inorganic Chemistry Division, CSIR-National Chemical Laboratory, Dr
Homi Bhabha Road, Pune, 411008, India. E-mail: pl.dhepe@ncl.res.in; Fax: +91-20
25902633; Tel: +91-20 25902024
tetrahydrofuran (THF)
+
DMSO solvent system.36 Further
Electronic supplementary information (ESI) available. See DOI: 10.1039/
3
improvement in 5-HMF yield is possible by incorporation of
c3ra43197e
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biphasic solvent where organic solvent extracts 5-HMF from the
aqueous solution thus limiting possible side reactions.
Heteropoly acids are examined in a water + methyl iso-butyl
ketone (MIBK) solvent system at 115–120 uC and 74–78% HMF
yield was observed.38,39 Reactions carried out with zeolites in
water + MIBK solvent system shows that H-MOR is the best
catalyst (74% HMF yield) amongst all other zeolite catalysts
employed (HZSM-5, 45% 5-HMF yield; Hb, 32% 5-HMF
yield).13,32 For solid acid catalysts only a handful of reports are
available for the one-pot conversion of glucose into 5-HMF even
though glucose is cheaper than fructose. It is known that 5-HMF
formation from glucose has to proceed through fructose
formation and for this isomerisation reaction typically base
catalysts are employed. Hence it becomes important to find a
catalyst which is capable of carrying out isomerisation and
dehydration reactions to yield 5-HMF from glucose. Additionally,
the one-pot conversion of di- and poly-saccharides to 5-HMF is
scarcely reported. A catalyst recycling study is reported at 130 uC
for 1.5 h using a Glu-TsOH catalyst to convert fructose into ca.
90% yield of 5-HMF.40 Use of a Sn-Mont catalyst allowed the
reaction through six cycles with 53% 5-HMF yield and 98%
glucose conversion.36 Other reports on recycling with fructose are
available with catalysts under microwave heating.33,41 It is
important to note here that in addition to there being very few
reports most of the reports unfortunately fail to give details on
characterization and recyclability of the catalysts. We believe that
in sugar chemistry wherein water is an integral part of the
reactions (hydrolysis, dehydration) and high pressures and
temperatures are applied it would be challenging to use
structured catalysts (well defined porous structure) since those
have a tendency to collapse.3,42 It is also essential to achieve the
required properties of the amount of acid, hydrophilicity–
hydrophobicity, access to active sites etc. in the catalyst to obtain
high yields. Here we show that SAPO type catalysts can be
efficiently used in the conversion of mono-, di- and poly-
saccharides to yield 5-HMF with high selectivity (Scheme 1).
Further, we present a detailed study on catalyst characterization
and try to establish a catalyst property–activity correlation.
In the literature it is reported that SAPO materials are
hydrothermally stable up to ca. 600 uC under 20% steam.43 It is
also reported that H-MOR catalyst has a hydrothermal stability up
to ca. 240 uC.44 It is understood that hydrothermal stability
depends on various factors such as Si/Al ratio, synthesis
procedures, pore structures etc., hence direct comparison between
hydrothermal stability of SAPO and H-MOR can not be done. Yet it
was thought that SAPO catalysts may have more stability under the
reaction cconditions, hence we synthesized SAPO-44 and SAPO-5
catalysts and evaluated those in these reactions.
Scheme 1 Production of 5-HMF from poly-, di- and mono-saccharide using solid
acid catalyst.
Industries Pvt. Ltd, grade: MCI-1524, 65–78% Al2O3), fumed
silica (Aldrich) and orthophosphoric acid (Fisher Scientific)
were used.
SAPO-44 material was synthesized using
composition of 1.0CHA:1.0Al2O3:1.0SiO2:1.0P2O5:60.0H2O
where, CHA stands for cyclohexylamine.45 In
typical
a molar gel
a
synthesis, 4.60 g pseudoboehmite was added slowly (within 2
h) to diluted phosphoric acid (7.69 g H3PO4 + 12.50 g distilled
water) under vigorous stirring and gel A was formed. In
another container, 3.33 g CHA was added dropwise to 2.05 g of
fumed silica suspended uniformly in 23.50 g water and the
mixture was denoted as gel B. Now gel B was mixed under
stirring with gel A and the mixture was further homogenised (6
h stirring). The resulting solution was transferred to three
Teflon lined steel autoclaves and subjected to aging (crystal-
lization) for different time periods (0 h, 48 h and 176 h) at 200
uC under atmospheric pressure at static conditions. After
cooling the autoclave, the solid materials were filtered, washed
with distilled water and dried. The material was calcined at
500 uC at a heating rate of 1 uC min21 in a tubular furnace for 6
h in the presence of air (10 mL min21). Material with
crystallization times of 0 h, 48 h and 176 h were termed as
ZDCT, 2DCT and SAPO-44 respectively.
2. Experimental section
2.1 Synthesis and characterization of SAPO’s
SAPO-5 material was prepared using molar gel composition
of 1.0TEA:1.0Al2O3:0.4SiO2:1.0P2O5:50.0H2O where, triethyla-
mine is abbreviated as TEA.46 In a typical procedure, gel A was
Crystalline silicoaluminophosphate materials were synthe-
sized according to a published report.45,46 As a source of
Al2O3, SiO2 and P2O5, pseudoboehmite (Marathwada Chemical
prepared by mixing 1.00
g pseudoboehmite to diluted
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phosphoric acid (1.58 g orthophosphoric acid + 5.95 g distilled
water) in 2 h. To 0.69 g TEA, 0.16 g fumed silica was added to
get gel B. Both the gels were mixed together and made
homogeneous by stirring for 3 h at RT. The prepared gel was
aged in an autoclave with teflon liner for 4 h at 200 uC. After
cooling down the autoclave, the solid was filtered, washed and
dried. Final SAPO-5 material was obtained by calcining as-
synthesized solid at 500 uC for 6 h under air at 10 mL min21
flow rate.
SiO2, c-Al2O3, SiO2-Al2O3 were obtained from Aldrich and
H-MOR (Mordenite, Si/Al = 10) was obtained from Zeolyst
International.
The catalysts were characterized with XRD, NH3-TPD, ICP,
SEM, and solid state NMR techniques.
gation and the aqueous and organic layers were separated
from each other.
For the recycling study, the catalyst separated from the
reaction mixture was washed with water, dried in an oven and
calcined in air at 500 uC for 6 h. This catalyst was then used for
the next reaction.
The aqueous layer was analysed by HPLC, equipped with a
Pb2+ column (300 mm 6 7.8 mm, 80 uC), refractive index
detector (cell temperature of 40 uC) and Millipore water was
used as the mobile phase with 0.6 mL min21 flow rate. Organic
layer analysis was done on a Varian gas chromatograph,
equipped with BPX-5 column (50 m 6 0.22 mm ID) and flame
ionisation detector (FID).
Details on the calculations are mentioned below,
X-ray powder diffractions (XRD) were recorded in a Rigaku
Miniflex diffractometer using Ni-filtered monochromatic Cu-
Ka radiation (l = 1.5406 Å).
% Conversion = [{Initial mole of carbohydrate – unconverted
mole of carbohydrate (HPLC)}/Initial mole of carbohydrate] 6
100
Temperature programmed desorption (TPD) of NH3 was
carried out in a Micromeritics AutoChem-2920 instrument.
Typically, the catalyst was activated at 600 uC (4 uC min21) in a
helium flow (30 mL min21) for 1 h. The temperature was
decreased to 50 uC and NH3 was adsorbed by exposing the
samples to 10% NH3 in helium for 1 h. It was then flushed
with helium for another 1 h at 100 uC to remove all the
physisorbed NH3. Desorption of NH3 was carried out in a
helium flow (30 mL min21) by increasing the temperature
% Yield = {Mole of product formed (HPLC + GC)/Theoretical
mole of product} 6 100
All the conversions and yields mentioned in this manuscript
are calculated based on GC and HPLC results. In case of starch
conversions were calculated as follows,
from 100 to 600 uC at a rate of 10 uC min21
.
% Conversion = [{Initial weight of starch – recovered weight of
Inductively coupled plasma-optical emission spectroscopy
(ICP-OES) analysis was done on a Spectro Acros instrument
equipped with winlab software. The sample was prepared
using standard method.47
starch (after reaction)}/Initial weight of starch] 6 100
We considered 100% recovery of catalyst after reaction.
SEM micrographs of the samples were obtained on a Leo
Leica Cambridge UK Model Stereoscan 440 scanning electron
microscope. The samples were loaded on stubs and sputtered
with thin gold film to prevent surface charging and also to
protect from thermal damage due to an electron beam.
Solid state 29Si,31P and 27Al NMR spectra were recorded on a
Bruker Avence-300 MHz spectrometer, operated at a field of
7.06 tesla. A fine powdered sample was placed in a 4 mm
zirconia rotor and spun at 10 kHz for all the nuclei.
3. Results and discussion
3.1 Conversion of fructose to 5-HMF
Fig. 1 presents the results for fructose conversion into 5-HMF
using SAPO-44, SAPO-5, SiO2, c-Al2O3, and H-MOR (Si/Al = 10)
2.2 Catalytic reactions and analysis
D(-)-Fructose (s. d. fine, 99.5%), D(-)-glucose (s. d. fine,
.99.0%), maltose monohydrate (s. d. fine), cellobiose (s. d.
fine), potato starch (LOBA Chemie), 5-HMF (Aldrich, 99.0%),
MIBK (LOBA Chemie, 99.0%), and ethanol (Chanshu
Yangyuan Chemicals, 99.9%) were purchased and used as
received.
All the reactions were carried out in a 100 mL Parr autoclave.
In a typical reaction 0.5 g of carbohydrate (fructose, glucose,
maltose, cellobiose or starch, 10% wt/wt with respect to water
charge), 0.143 g catalyst (catalyst/substrate loading = 29%)
were mixed with water + organic solvent = 5 mL + 25 mL or 30
mL water/organic solvent in autoclave and heated at the
desired temperature under stirring. When the temperature of
the reactor reached just 5 uC below the desired temperature
that time was taken as zero time. After completion of the
reaction the reactor was cooled to room temperature. The
catalyst was separated from the reaction mixture by centrifu-
Fig. 1 Conversion of fructose using various solid acid catalysts. Reaction
conditions: fructose (10 wt%), catalyst (0.143 g), water + MIBK (1 : 5 v/v), 175
uC, 1 h.
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catalysts at 175 uC for 1 h. Zeolite H-MOR was taken as the
benchmark catalyst as it was shown to be the best catalyst
among the zeolites used (H-ZSM-5, H-b).13,32 SAPO-44 catalyst
showed 78% 5-HMF yield with 88% selectivity (89% conver-
sion). Besides 5-HMF, 6% glucose was also observed. Thus,
over SAPO-44 catalyst a 94% carbon balance was observed
hinting that loss of carbon due to degradation reactions is
much less. The elemental analysis of catalyst revealed that ca.
4% carbon is present on the spent catalyst. The formation of
substrate) solutions keeping the S/C ratio constant. The
increase in concentration decreased the 5-HMF yield to 66%
compared to 78% observed with 10% (wt/wt) solution. This
decrease may be attributed to the formation of condensation
products or degradation reactions as the selectivity to 5-HMF
was decreased to 73% even if conversion (91%) was almost
same as that with 10% (wt/wt) solution (89% conversion, 88%
selectivity).
In the presence of acidic catalysts, it is observed that the
reaction yields formic and levulinic acids and is thus
hampering the 5-HMF yields. To overcome formation of these
acids from 5-HMF in the literature use of organic solvents such
as MIBK, 2-butanol, DMSO, ethyl acetate, THF etc. is reported.
Our study showed that with only water as solvent, 39% 5-HMF
yield (59% selectivity) is obtained with SAPO-44 catalyst within
1 h at 175 uC. Under similar reaction conditions, fructose
conversion was carried out using only ethanol (fructose and
5-HMF can dissolve) and we observed 56% yield of 5-HMF.
Later, a biphasic (water + MIBK) system was found to be
suitable to achieve high yields (78%). To check the effect of the
water + MIBK ratio on the 5-HMF yield and selectivity,
reactions were conducted with 1 : 1 (v/v) and 1 : 5 (v/v) solvent
ratios. It was observed that with 1 : 5 (v/v) ratio a higher yield
of 5-HMF (78%) was seen compared to 1 : 1 (v/v) ratio (47%).
Though in both the reactions 5-HMF selectivity was almost
same (ca. 85%). Similar results of change in solvent ratio were
observed in earlier work.12,19
5-HMF was confirmed by 1H NMR (Fig. S1, ESI ). Under similar
3
reaction conditions, H-MOR and SAPO-5 catalysts showed 63%
and 32% 5-HMF yields, respectively. Furthermore, with
H-MOR 72% selectivity for 5-HMF (88% conversion) and 6%
glucose formation (79% carbon balance) was observed. The
reaction without catalyst showed 29% 5-HMF yield with 74%
fructose conversion. Since SAPO and H-MOR catalysts contain
Si, Al and P we carried out reactions with SiO2 and Al2O3.
However, we observed poor yields of 5-HMF with these
catalysts. This implies that the combination of Si, Al and P
gives rise to Brønsted acid sites in the catalysts and is
important for the reaction. Formation of humins in these
reactions is known from the literature,48 however, it is difficult
to analyse those with GC and HPLC so we could not quantify
this. But it is interesting to note that with SAPO-44 as catalyst a
very high yield for 5-HMF is observed. Though SAPO-5 has a
larger pore size (0.80 nm) than SAPO-44 (0.43 nm) its activity is
lower.38 We acknowledge here that the presence of small pore
opening in SAPO-44 may hinder the access of active sites by
substrate molecules. Additionally, the difference in activity
between SAPO-5 and SAPO-44 might be due to the variation in
the properties of the catalysts such as amount and type of acid
sites, hydrophilicity-hydrophobicity (see discussions on cata-
lyst characterization). To make sure that all the results are
reproducible we have done all the reactions in triplicate and
we observed ¡2% variation in the results (conversion,
selectivity, yield and carbon balance).
3.3 Hydrophilicity-hydrophobicity
It is important to note here that the total acid amount in SAPO-
44 and H-MOR is the same (1.2 mmol g21, Table 1), yet H-MOR
gave a lower yield of 5-HMF (63%) than SAPO-44 (78%) and
that selectivity was lower in the case of H-MOR (72%)
compared to SAPO-44 (88%). This indicates that not only the
acid amount but other factors are also playing an important
role in achieving higher activity in SAPO-44. Over SAPO-5
catalyst 34% 5-HMF yield was observed, however with poor
selectivity. To understand the catalyst behaviour, we under-
took a comparative study on the hydrophilicity-hydrophobicity
property of all three catalysts (H-MOR, SAPO-5, and SAPO-44)
under similar conditions. In three different test tubes equal
amounts of H-MOR, SAPO-5 and SAPO-44 were taken and to it
water was added. Then to these test tubes carbon tetrachloride
was added (d = 1.58 g mL21 at 25 uC) and maintaining 1 : 1 v/v
ratio of solvents. We could easily see that immediately after
addition of carbon tetrachloride the SAPO-44 went to the
aqueous layer (upper layer) giving a clear organic phase
(Fig. 2). However, H-MOR was almost equally distributed in
both the layers. The SAPO-5 catalyst was distributed more in
the water layer compared to the organic layer. After 10 min of
settling time some of the H-MOR was still present in the
organic layer (lower layer). It is noteworthy that though the
density of SAPO-44 (0.61 g mL21) is higher than that of H-MOR
(0.35 g mL21) still it was present in the upper layer (water).
Similarly, most of the SAPO-5 was observed in the water layer
after 10 min settling time. This reveals that the hydrophilicity
of the catalysts is in the following order, SAPO-44 . SAPO-5 .
H-MOR.
3.2 Effect of reaction parameters in fructose reactions
It is suggested in the literature that the temperature may
influence the degradation or side reactions of fructose and
5-HMF. Hence reactions were conducted at 165 and 185 uC to
check the effect of temperature on the 5-HMF yield. The
reaction carried out over SAPO-44 for 1 h at 165 uC (43% yield,
56% conversion, 77% selectivity) and 185 uC (45% yield, 92%
conversion, 49% selectivity) gave poor yields of 5-HMF
compared to the reaction carried out at 175 uC (78% yield).
The poor yield of 5-HMF at 165 uC might be because of lower
conversion (56%) and at 185 uC is due to predominant
degradation reactions (49% selectivity). H-MOR and SAPO-5
catalysts gave 43% yield (69% conversion, 62% selectivity) and
28% yield at 165 uC respectively.
To know if the reaction atmosphere has any effect on the
conversions and yields of 5-HMF, reactions were performed
under nitrogen and air atmosphere. In both reactions similar
activity with SAPO-44 catalyst was observed (78 ¡ 2% yield at
175 uC after 1 h).
It is desirable to use concentrated solutions of substrates
and hence reactions were carried out using 10% (wt/wt, 5 g
water/0.5 g substrate) and 30% (wt/wt, 5 g water/1.5 g
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Table 1 NH3-TPD study of catalysts
Acid amount distribution (mmol g21
)
Catalyst
Total acid amount (mmol g21
)
100–250 uC (weak)
250–350 uC (moderate)
350–500 uC (strong)
SAPO-44 (Fresh)
SAPO-44 (Spent)
ZDCT
2DCT
SAPO-5
SiO2
c-Al2O3
SiO2–Al2O3
H-MOR
1.2
0.5
0.3
0.4
0.8
0.1
0.4
0.6
1.2
0.7
0.5
0.3
0.4
0.8
0.1
0.1
0.3
0.5
0
0
0
0
0
0
0.3
0.3
0
0.5
0
0
0
0
0
0
0
0.7
Later, we used a water-MIBK (1 : 1 v/v ratio) system to check
how the catalyst distribution would be under the reaction
conditions. MIBK (d = 0.8 g mL21 at 25 uC) was added to
H-MOR, SAPO-5 and SAPO-44 already dispersed in water. It can
be seen that a maximum amount of SAPO-44 came in the water
layer and very little amount of SAPO-44 remains at the
interface of MIBK-water. This is because MIBK has some
solubility in water (16.6 g L21 at 30 uC, 12.2 g L21 at 90 uC)49.
On the other hand most of the H-MOR remains in the MIBK
layer (Fig. 2). Similarly, SAPO-5 was seen distributed equally in
both the layers after immediate addition of MIBK. However,
after 10 min settling time SAPO-5 was observed at the interface
of both the layers but more in the organic layer. This study
indicates that SAPO-44 is more hydrophilic in character than
SAPO-5 and H-MOR and this is due to local electronegativity
differences in the framework Si, Al and P in SAPO-44 and also
due to presence of ‘P’ in the framework. Moreover, it is
reported in the literature that in different SAPO materials the
hydrophilicity varies.43
It is known that fructose is soluble in water and thus higher
a concentration of catalyst in the water layer will be useful for
the conversion of fructose into 5-HMF and suppressing the
5-HMF degradation or condensation reactions once it is
extracted into the organic layer. On the other hand, if a higher
concentration of catalyst is present in the organic layer then it
will adversely affect the 5-HMF yields: first, a lower concentra-
tion of catalyst present in the water layer means that fructose
may undergo non-catalytic reactions (refer Fig. 1) to give lower
yields of 5-HMF. Secondly, whatever quantity of 5-HMF is
formed in the water layer, once it is extracted into the organic
layer it will undergo degradation or condensation reactions
(see section 3.9).
As shown in Fig. 2, H-MOR has a tendency to remain in the
organic phase and because of that 5-HMF can undergo further
degradation reactions on the catalytically active sites of
H-MOR catalyst.
A similar study is also done with 1 : 5 (v/v) ratio of water +
MIBK as this ratio is used in the reactions. With a 1 : 5 (v/v)
ratio similar results were observed as were seen with 1 : 1 (v/v)
ratio.
The above study has been done to the best of our knowledge
for the first time to correlate the catalyst activity and property
in this reaction, which may be helpful in tailoring the catalyst.
The poor yield of 5-HMF in SAPO-5 cannot be attributed to the
hydrophilic character of the catalyst alone. In the following
sections, further studies are discussed to understand the
effects of catalyst properties on the reactions.
3.4 Effect of catalyst morphology
To understand the correlation between catalytic activities with
morphology-property of catalyst (SAPO-44) a study on synthesis
time vs. activity was undertaken. It was also contemplated that
a similar hydrophilic nature can be observed for all the
samples of the catalysts synthesized with different crystal-
lization times. In view of this, during the synthesis of SAPO-44
Fig. 2 Study on hydrophilicity-hydrophobicity property of catalysts. The line
mark on the test tube is to show the boundary between two layers.
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than SAPO-44 (78% yield, 88% selectivity). Hence we may
conclude that higher number of strong acid sites may also
help degradation reactions. In case of SAPO-5 catalyst the
absence of strong acid sites may be responsible for lower
activity. Considering all the above discussions it is possible to
say that in the formation of 5-HMF, catalyst properties like
acid amount, ratio of weak and strong acid sites, hydro-
phobicity-hydrophilicity play important role. Overall, SAPO-44
has optimum properties which are required to achieve higher
5-HMF selectivity.
3.6 Catalyst recycles study
The fresh SAPO-44 catalyst showed 78% 5-HMF yield but in the
recycling study, a decrease in the activity was observed (50%
5-HMF yield). This might be due to the adsorption of
carbonaceous deposits on the catalytically active sites as we
observed a total of 94% carbon balance (see section 3.1).
Elemental analysis of the catalyst also showed the presence of
carbon on the spent catalyst. To remove the deposits from the
catalyst, it was calcined (550 uC for 12 h) and used in further
reactions. As observed in Fig. 4, with fresh catalyst 78% 5-HMF
yield was observed which decreased to 71% in the 2nd run.
These results prove that removal of carbonaceous deposits is
necessary to achieve higher activity in subsequent runs. In the
3rd recycle run, 66% yield was obtained. However, beyond the
3rd run the activity remained almost constant up to the 5th
run (4th run: 65%, 5th run: 65%). The decrease in activity in
recycle runs can be attributed to the fact that SAPO-44
undergoes minor morphological changes (see section 3.10
for discussion on catalyst characterization). The NH3-TPD
study (Table 1) reveals that in 2DCT (weak, 0.4 mmol g21) and
spent SAPO-44 catalyst (0.5 mmol g21) almost similar total
acid amount and same type of acid site (weak) is present and
hence both the catalysts offered comparable activities (65 and
60%). The H-MOR catalyst showed a decrease in activity in
each consecutive run up to the 5th recycle run (1st run: 63%,
2nd run: 40%, 3rd run: 34%, 4th run: 30%, 5th run: 28%).
Fig. 3 Effect of catalyst morphology on fructose conversion. Reaction conditions:
fructose (10 wt%), catalyst (0.143 g), water + MIBK (1 : 5 v/v), 175 uC, 1 h.
catalyst, samples were withdrawn after 0 day crystallization
time and 2 days crystallization time. The samples were named
ZDCT and 2DCT, respectively (see section 2.1 for details on
synthetic procedure). Reactions were carried out with ZDCT
and 2DCT catalysts along with SiO2–Al2O3 catalyst at 175 uC for
1 h (Fig. 3). The ZDCT catalyst (amorphous analogue of SAPO–
44) showed 43% yield of 5-HMF whereas SiO2–Al2O3 (amor-
phous analogue of H-MOR) gave only 13% 5-HMF yield
(mainly form black solids). Though SiO2–Al2O3 (0.6 mmol
g
21) has a higher total acid amount than ZDCT (0.3 mmol g21
)
(Table 1, Fig. 3) the catalytic activity was lower in SiO2–Al2O3. It
is noteworthy that in 2DCT catalyst the total acid amount is
less (0.4 mmol g21) compared to H-MOR (1.2 mmol g21),
however similar activity was observed for both the catalysts (ca.
60%) (Fig. 3). Moreover, it is interesting to note that although
the complete structure is not formed in 2DCT catalyst, it
showed good activity compared to a structured catalyst like
H-MOR. This may be attributed to more hydrophilic nature of
SAPO materials than H-MOR.
3.5 Effect of acid amount and type of acid site
Fig. 3 shows the results for different catalysts with same
catalyst loading (0.143 g, S/C weight ratio constant). The 2DCT
and ZDCT catalysts had a total acid amount of 0.4 and 0.3
mmol g21 and the yields were 60% and 43%, respectively. The
SAPO-44 catalyst had a total acid amount of 1.2 mmol g21 and
showed 78% yield. To compare the catalytic results reactions
were performed using similar acid amounts (S/C molar ratio
constant). However, even by increasing the charge of the 2DCT
and ZDCT catalysts in the reactions, the yields for the catalysts
remained same. This may emphasize that to boost the 5-HMF
yields further, strong acid sites are required which are not
present in ZDCT and 2DCT catalysts but present in SAPO-44
(Table 1). If this is true then the reaction over H-MOR catalyst
higher activity should have been obtained compared to SAPO-
44 since H-MOR has more strong acid sites (0.7 mmol g21
)
compared to SAPO-44 (0.5 mmol g21). However, results show
that H-MOR gives lower activity (63% yield, 72% selectivity)
Fig. 4 Recycle study of SAPO-44 catalyst. Reaction conditions: fructose (10 wt%),
SAPO-44, water + MIBK (1 : 5 v/v), 175 uC, 1 h.
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After the 4th recycle run the catalyst showed activity similar to
the non-catalytic reaction (29% yield).
3.8 Di- and poly-saccharides as substrates for 5-HMF synthesis
For the conversion of b-glucose dimer (cellobiose) to 5-HMF
researchers have used homogeneous catalysts such as mineral
acids,19,20 mineral acid with Sn-beta zeolite and metal chloride
in ionic liquids.28,36,58,59 Cellobiose conversion over HT/
Amberlyst-15 catalyst at 120 uC yields 35% 5-HMF.14 The Sn-
W oxide catalyst is also known to convert dimers yielding 39%
5-HMF after 18 h.57 Recently, it was shown that Sn-Mont
catalyst converts cellobiose to 42% 5-HMF.36 Yet another
report showed that from maltose (dimer of a-glucose) 14%
5-HMF yield can be obtained under microwave heating using
TiO2 NPs.37
We used SAPO-44 for the one-pot conversion of dimers of
glucose such as maltose and cellobiose to yield 5-HMF and the
results are summarized in Table 2. In all the reactions we
observed complete conversion of substrates. As seen, at 175 uC
SAPO-44 converts maltose to 5-HMF with 57% yield within 4 h
reaction time. Along with 5-HMF, 14% yield for glucose and
fructose was obtained. On the contrary, H-MOR gave 41% and
2DCT gave 44% 5-HMF yield under similar reaction condi-
tions. While cellobiose is used as a substrate 56% yield of
5-HMF, along with 12% yield of glucose and fructose at 175 uC
within 6 h of reaction time was observed over SAPO-44. The
2DCT catalyst showed 44% 5-HMF yield and H-MOR gave 43%
5-HMF yield under similar reaction conditions.
Few reports are available in the literature on the one-pot
conversion of glucose polymer such as starch to 5-HMF with
homogeneous mineral acids19 or metal chloride in ionic
liquid.18,58 Production of 5-HMF from starch using hetero-
geneous Sn-beta zeolite mixed with HCl is reported.60 Another
report showed use of Sn-W oxide to produce 41% 5-HMF from
starch within 36 h.57
We used SAPO-44 catalyst for starch conversion which at 175
uC and within 6 h showed 68% yield of 5-HMF (Table 2). Along
with 5-HMF, formation of glucose (15% yield) and fructose
(7% yield) was also observed. However, when the reaction was
conducted for 8 h 5-HMF yield decreased to 61%, which may
be due to the occurrence of side reactions which leads to
brown solid (humins).40 Under similar reaction conditions
H-MOR (41%) and 2DCT (40%) were also active to give 5-HMF.
To the best of our knowledge this is the first time that using
3.7 Glucose as a substrate for 5-HMF synthesis
It would be desirable to use cheaply and abundantly available
glucose as a substrate to synthesize 5-HMF. However, glucose
forms a stable pyranose ring structure in the aqueous phase
and thus is difficult to convert. The first step in 5-HMF
production from glucose would be an isomerisation reaction
to give fructose. This is typically achieved over base cata-
lysts.50–55 There are reports on the use of mineral bases,
carbonate and hydroxide forms of hydrotalcite,50 alkali metal
exchanged zeolites,51 metallosilicate,52 zirconia53,54 and alu-
minate resin55 for the isomerization of glucose. It is demon-
strated that the use of Lewis acid Sn-beta zeolite can isomerize
glucose to fructose and further addition of mineral acid
produces a very low yield of 5-HMF (11%).56 It is mentioned
that an intramolecular hydride shift mechanism may be
responsible for the isomerisation of glucose to fructose in
Sn-beta catalyst.50 Additionally, there are few reports on using
sulphated zirconia-alumina,15 Sn-W oxide,57 HT/Amberlyst-
15,14 phosphoric acid treated metal hydroxide,35 Sn-Mont36 to
convert glucose into 5-HMF by either using Lewis acid
properties of catalysts or combining base-acid properties in a
catalyst. But these reaction systems suffer from disadvantages
like lower yields, longer reaction times, no recyclability of
catalyst, use of mineral acid with solid acid etc. To overcome
these drawbacks we used SAPO-44 catalyst in the conversion of
glucose.
With glucose as a substrate, SAPO-44 catalyst gave 67%
5-HMF yield (83% conversion of glucose, 81% selectivity) after
4 h at 175 uC. Besides 5-HMF we observed fructose (13%) in the
reaction mixture which was not a surprise as it is an
intermediate product. Further, various other catalysts were
used for the glucose conversion to yield 5-HMF [SiO2 (30%),
SAPO-5 (28%), 2DCT (44%) and H-MOR (49%)]. Nonetheless
these catalysts showed lower activity than SAPO-44. It is
noteworthy to point out here that SAPO-44 could convert
glucose efficiently into 5-HMF without addition of any other
catalyst. We assume that similar type of mechanism which is
responsible for Sn-beta catalyst50 may be predominant in
SAPO-44 catalyst.
Table 2 Dehydration of glucose dimer and polymer to 5-HMFa
Substrate
Reaction Time (h)
Catalyst
Glucose yield (%)
Fructose yield (%)
5-HMF yield (%)
Maltose
Maltose
Maltose
Maltose
Cellobiose
Cellobiose
Cellobiose
Cellobiose
Starch
4
4
4
4
6
6
6
6
6
6
6
6
No catalyst
H-MOR
2DCT
SAPO-44
No catalyst
H-MOR
2DCT
SAPO-44
No catalyst
H-MOR
2DCT
17
1
2
5
21
2
2
7
17
10
8
8
2
3
9
9
3
3
5
7
8
7
7
22
41
44
57
15
43
44
56
29
41
40
68
Starch
Starch
Starch
SAPO-44
15
a
Reaction conditions: substrate (10 wt%), 175 uC, water + MIBK (1 : 5 v/v).
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only solid acid catalyst a 68% yield of 5-HMF is achieved
directly from starch.
3.9 Possible degradation reaction pathways
It was rather disappointing that in the literature none of the
reports claimed 100% selectivity and 100% yield for 5-HMF
formation. This indicates that besides fructose to 5-HMF
reactions, the catalysts are active for the other side reactions
such as: 1) Conversion of 5-HMF into degradation products or
2) Conversion of fructose to unwanted products or 3)
Condensation reactions between fructose and 5-HMF. To
understand which of these reactions are playing major role
in decreasing the selectivity for 5-HMF, we carried out a 5-HMF
stability study at 175 uC for 1 h with H-MOR, SAPO-44, 2DCT
and SAPO-5 catalysts. The results showed that 5-HMF does not
undergo any reaction on any of these catalysts and hence the
possibility of further degradation of 5-HMF is ruled out. The
study on the second possibility of conversions of fructose
cannot be performed since in any case fructose will give rise to
5-HMF and other products. The third possibility is the
formation of condensation products arising from fructose
and 5-HMF. To study this we carried out reactions with
simultaneous addition of fructose and 5-HMF as substrate. It
was observed that 5-HMF and fructose do undergo condensa-
tion reactions and that H-MOR catalyst is more active for these
reactions compared to SAPO-44 (for details on experiments
and discussions please refer to the ESI ). This is not surprising
3
since the majority of H-MOR is present in the organic phase
(hydrophilicity-hydrophobicity study) and the presence of
strong acid sites in the catalyst (Table 1).
Fig. 5 Powder XRD pattern of catalysts.
3.10 Catalyst characterization study
H-MOR sample compared to fresh H-MOR sample lower peak
intensity for all the peaks was observed. Moreover, it can be
seen that the highest intense peak due to (202) at 2h = 25.8u
present in the fresh H-MOR sample is absent in the spent
H-MOR sample. This clearly indicates that during the reaction
H-MOR catalyst undergoes serious morphological changes
compared to SAPO-44.
The ICP-OES study shows that the amount of Al (fresh, 22.5
ppm; spent, 21.7 ppm) and P (fresh, 24.6 ppm; spent, 25.3
ppm) remains similar in the fresh and spent catalysts.
Moreover, the absence of Al and/or P in the reaction solution
confirms that elements are not leached out during the
reaction. These analyses indicate that since Al and/or P are
not leached out the local environment around Si, Al and P has
altered.
The XRD patterns for fresh and spent SAPO catalysts are
presented in Fig. 5. The peak characteristics of AFI morphol-
ogy are observed for SAPO-5 catalyst.61 In case of SAPO-44,
peak pattern matches well with the CHA morphology.62 After a
gel was obtained (formed by mixing Si, Al and P precursors)
immediately a sample was taken (filtered, washed, dried) and
the XRD was performed to reveal formation of a tridymite
amorphous phase (ZDCT, 0 h).63 The diffraction peaks due to
AFI morphology start appearing as crystallization time is
increased (SAPO-5, 4 h) and further increase in time yields
SAPO-44 with CHA morphology (176 h).63 Therefore, it is
expected that 2DCT sample withdrawn after 48 h crystal-
lization time may have an intermediate phase between AFI and
CHA or contribution from both the phases. The XRD pattern of
2DCT sample shows few of the peaks similar to AFI
morphology, however, their intensities are less. At the same
time in the XRD pattern of 2DCT sample it was observed that a
few extra peaks start appearing which may give rise to CHA
morphology. In case of spent SAPO-44 material, the XRD
pattern matches well with the fresh SAPO-44 material,
however, peak intensities are decreased implying that crystal-
linity of the SAPO-44 is lower. A similar phenomenon was also
observed in case of 2DCT catalyst samples. It is important to
note here that though peak intensities in spent catalysts
(SAPO-44 and 2DCT) are reducing none of the peak(s)
Fig. S3 (ESI ) presents the SEM images for fresh and spent
3
SAPO-44 catalyst. SEM images of both fresh and spent SAPO-44
indicate the presence of cubic morphology with similar edge
length.
Solid state 27Al, 31P and 29Si MAS NMR spectra were
recorded to understand the local environment in SAPO-44. In
31P MAS NMR spectra we observed a peak at 231.5 ppm in
both fresh and spent SAPO-44 catalysts (Fig. S4, ESI ). This
3
peak is characteristic of tetrahedrally co-ordinated phosphor-
ous [P(4Al)] in the SAPO-44 framework. Careful observation
suggests that in spent catalyst the peak has broadened which
may imply that few other P species are forming. The 31P NMR
completely vanished. Fig. S2 (ESI ) shows the XRD patterns
3
for the fresh and spent H-MOR samples. In the XRD of spent
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spectra of 2DCT catalyst also showed a broad peak at 231.5
Acknowledgements
ppm (Fig. S4, ESI ). The 27Al MAS NMR spectrum shows a
3
We acknowledge Drs. C. V. V. Satyanarayana and R. Nandini
Devi for helpful discussions. This work is financially
supported by the Department of Science and Technology
(DST), India. PB thanks the University Grant Commission
(UGC), India, for a Research Fellowship.
major peak at 38.1 ppm which can be assigned to the presence
of tetrahedrally co-ordinated aluminium in the framework of
fresh and spent SAPO-44 (Fig. S5, ESI ).45 Additionally, in both
3
fresh and spent catalyst a minor peak was observed at ca.
215.0 ppm for octahedral Al species. In fresh SAPO-44 catalyst,
peak broadening was observed which may indicate that few
other Al (5-cordinated, distorted 4-cordinated) species may
present.
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