Efficient, stable, and reusable silicoaluminophosphate for the one-pot production of furfural from hemicellulose

Article abstract of DOI:10.1021/cs400495j

Development of stable, reusable, and water-tolerant solid acid catalysts in the conversion of polysaccharides to give value-added chemicals is vital because catalysts are prone to undergo morphological changes during the reactions. With the anticipation that silicoaluminophosphate (SAPO) catalysts will have higher hydrothermal stability, those were synthesized, characterized, and employed in a one-pot conversion of hemicellulose. SAPO-44 catalyst at 170 C within 8 h could give 63% furfural yield with 88% mass balance and showed similar activity up to at least 8 catalytic cycles. The morphological studies revealed that SAPO catalysts having hydrophilic characteristics are stable under reaction conditions.

Full text of DOI:10.1021/cs400495j

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Letter
Efficient, Stable and Reusable Silicoaluminophosphate
for the One-pot Production of Furfural from Hemicellulose
Prasenjit Bhaumik, and Paresh Laxmikant Dhepe
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs400495j • Publication Date (Web): 09 Sep 2013
Downloaded from http://pubs.acs.org on September 13, 2013
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ACS Catalysis
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Efficient, Stable and Reusable Silicoaluminophosphate for the One-
pot Production of Furfural from Hemicellulose
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Prasenjit Bhaumik and Paresh L. Dhepe∗
Catalysis & Inorganic Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune
4
11008, India
KEYWORD: furfural, hemicellulose, hydrophilicity, SAPO, water-tolerant catalyst, zeolite
ABSTRACT: Development of stable, reusable and water tolerant solid acid catalysts in the conversion of polysaccharides
to give value added chemicals is vital since catalysts are prone to undergo morphological changes during the reactions.
With the anticipation that silicoaluminophosphate (SAPO) catalysts will have higher hydrothermal stability those were
synthesized, characterized and employed in the one-pot conversion of hemicellulose. SAPO-44 catalyst at 170 °C within 8
h could give 63% furfural yield with 88% mass balance and showed similar activity at least up to 8 catalytic cycles. The
morphological studies revealed that SAPO catalysts having hydrophilic characteristics are stable under reaction condi-
tions.
Utilization of renewable biomass, particularly non-
edible lignocellulosic materials consisting of cellulose,
hemicellulose and lignin serves considerable potential to
fulfil our future chemical needs. In this context, furfural,
one of the most important platform chemical derived
from biomass emerges as a potential building block for
its possible use in the synthesis of value-added chemicals,
fuels and intermediates such as, furfuryl alcohol, 2-
tivity (54% furfural yield at 170 °C), but are observed to be
14,15
unstable under the reaction conditions. Therefore care-
ful design of the stable catalyst in these reactions is re-
quired. In this respect, it is expected that silicoalumino-
phosphate (SAPO) catalysts can be potential catalyst
since they have higher hydrothermal stability (600 °C
1
16
under 20% steam).
In this communication, we reveal the synthesis of
SAPO-44, SAPO-11, SAPO-5 and SAPO-46 catalysts having
varying properties such as, pore diameter, Si/P ratio (Ta-
ble S1, Supporting Information), acid amount and type.
Further these catalysts were employed in the conversion
of hemicellulose to yield xylose, arabinose and furfural
methylfuran,
2-methyltetrahydrofuran,
phenol-
formaldehyde resin, furoic acid, maleic acid and linear
2-4
alkanes. Employment of homogeneous acid catalysts for
the dehydration of xylose, a C sugar (derived from hemi-
5
cellulose) to furfural are known however, those face seri-
ous drawbacks such as difficulty in recovering the cata-
(
Scheme 1). SAPO-44 catalyst exhibited remarkable activi-
5
-7
lyst, environmental issues, toxicity, corrosiveness etc.
ty for furfural synthesis and was observed to be stable
under reaction conditions.
To overcome these drawbacks, improvements in the con-
version technologies of xylose to yield furfural over solid
acid catalysts such as zeolites with various Si/Al ratio, Sn-
Scheme 1. Production of furfural from hemicellulose
using solid acid catalyst
8
-10
beta zeolite etc. are reported.
However, it would be
desirable to derive furfural directly from hemicellulose
(via xylose formation). Studies on two-pot method for
converting hemicellulose to xylose (by hydrolysis reac-
1
1-
tion) and furfural (by dehydration of xylose) are known.
1
3
In these methods first hemicellulose in hot water is
converted into soluble oligomers, which later in a sepa-
rate reactor over solid acid catalysts can yield xylose and
furfural. Nonetheless, it is essential to produce furfural
directly from hemicellulose in a one-pot reaction instead
of using xylose or soluble oligomers as the substrates.
Recently, one-pot conversion of hemicellulose into xylose
and furfural over solid acid catalysts such as zeolites with
varying Si/Al ratio in presence of aqueous and biphasic
Hemicellulose conversions are carried out in a Parr au-
toclave under 2 bar nitrogen pressure and reaction mix-
ture was analyzed by GC and HPLC (for details, see Sup-
porting Information). With only water as a solvent over
1
4,15
media is shown.
Most of the zeolites showed good ac-
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SAPO-44 catalyst we observed 41±2% furfural yield along ble while ca. 92% of hemicellulose is converted. Under
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with 4±1% oligomers and 14±2% monomers (xylose, arab-
inose and glucose) formation at 170 °C within 8 h. In the
reaction, 86±2% conversion of softwood derived hemicel-
lulose could be successfully achieved however the total
mass balance was only ca. 59%. In comparison, over
HMOR (Si/Al=10) catalyst, 36±2% furfural yield was seen
with 87±2% conversion and ca. 52% mass balance.
similar reaction conditions, in the absence of catalyst very
poor (26±2%) furfural yield was attained and only ca. 53%
mass balance was observed. Possibility of the thermal
reactions (blank reaction) cannot be ruled out as pK
val-
w
ue of water is 13.99 at 25 °C and it decreases to 11.64 at 150
1
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°C and hence water can act as an acid. The difference in
the reaction results for blank and catalytic reactions clear-
ly emphasizes the fact that catalyst plays a major role in
diverting the reaction to produce desired products. Be-
sides SAPO-44, other SAPO series catalysts (SAPO-5,
SAPO-11, SAPO-46) were also evaluated for the reaction
and furfural yield in the range of 30-50% was obtained
over these catalysts (Figure 1). Although moderate yields
The presence of organic solvent along with water im-
15
proves the furfural formation hence it was decided to
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carry out the reactions in a biphasic solvent system. In
our study, significant improvement in furfural yield
(
63±2%) with water+toluene (1:1 v/v) solvent system over
SAPO-44 catalyst was observed when reactions were con-
(
44-56%) for furfural were possible using zeolite as a cata-
ducted at 170 °C for 8 h. Compared to this water+p-xylene
lyst, these catalysts cannot be reused effectively since
those undergo morphological changes as is shown earli-
(
55±2%) and water+methyl iso-butyl ketone (MIBK)
(
53±2%) system with 1:1 v/v ratio showed lower furfural
1
4,15
er.
yields. To understand the influence of organic solvent on
the yields of furfural we calculated partition co-efficient
of furfural in these solvents (Table S2, Supporting Infor-
mation). We found partition co-efficient of 0.73, 0.58 and
70
Oligomers
Xylose + Arabinose
Furfural
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0
.91 in water+toluene, water+p-xylene, and water+MIBK
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4
3
0
0
0
solvent system, respectively. In case of toluene and p-
xylene furfural yields are in good correlation with the par-
tition co-efficient observed. However, in water+MIBK
system partition coefficient is higher than even in wa-
ter+toluene but we observed less yields. This may be due
to higher miscibility of MIBK in water (19.1 g/L at 20 °C)
compared to toluene (0.47 g/L at 20 °C) and p-xylene
20
10
(
0.18 g/L at 20 °C) which leads to a situation wherein deg-
radation or side reactions of xylose and furfural are possi-
ble in presence of catalyst. Study on solvent stability un-
der reaction condition revealed that toluene is stable but
MIBK forms few of the degradation products which were
detected on FID-GC. Incidentally, in the greener aspect
also, toluene is considered as a preferred solvent as it has
a better life cycle assessment (LCA) value of 2 compared
0
No-catalyst HBeta
HMOR
HUSY SAPO-5 SAPO-11 SAPO-46 SAPO-44
Catalysts
Figure 1. Evaluation of catalysts for the conversion of soft-
wood hemicellulose to produce furfural. Reaction condition:
hemicellulose
(0.3
g),
catalyst
(0.075
g),
wa-
17
°
to p-xylene and MIBK (LCA=3 and 9, respectively). Tak-
ter+toluene=30+30 mL, 170 C, 8 h.
ing into account these points further reactions were car-
ried out with water+toluene system.
To make the process more effective, it is desirable to
use concentrated solutions of the substrates. Therefore,
we carried out the reactions with 1, 3, 5 and 10 wt% hemi-
cellulose concentration (with respect to water) keeping
S/C ratio same. Results showed that furfural formation
remains constant (62±1%) when 1, 3 and 5 wt% hemicellu-
loses concentrations solutions were used. However, with
the use of 10 wt% hemicellulose solutions lower furfural
yield (42%) was observed. This might be due to the for-
mation of by-product (humin) which can be recognized
from the dark brown color of reaction solution obtained
after reaction.
The catalytic activities of various solid acid catalysts in
the conversion of hemicellulose are presented in Figure 1.
The SAPO-44 catalyst showed the best catalytic perfor-
mance by yielding 63±2% furfural (ca. 68% selectivity)
directly (one-pot) from softwood derived hemicellulose.
Along with furfural over SAPO-44 catalyst, formations of
ca. 7±1% oligomers and ca. 6±2% xylose+arabinose were
also observed. Softwood hemicellulose consists of ca. 15%
glucose and hence under reaction conditions it is con-
verted to yield 5% fructose and 2% 5-hydroxymethyl fur-
fural (HMF). Incidentally, we also observed 5% of uncon-
verted glucose. Isomerization of glucose to fructose over
Lewis acid sites and further dehydration of fructose to
HMF over Brönsted acid sites is possible since both the
acid sites are available on SAPO-44 catalysts as is evi-
denced by IR studies. The possibility of participation of
Lewis acid sites in the isomerization of glucose is already
Figure S1 (Supporting Information) plots the studies on
effect of temperature on the reactions. Reaction carried
out at 160 °C yields 31±2% furfural, 42±2% monomers and
25±1% oligomers. If reactions are conducted at 170 and 180
°
C, similar furfural yields (63±2%) were observed. This
suggests that to reach valuable performance, reactions
should be done at least at 170 °C. When reactions with
furfural as a substrate were carried out with or without
1
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discussed in the literature. In view of the above results,
over SAPO-44 catalyst, ca. 88% mass balance is achieva-
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ACS Catalysis
To understand the morphology of catalysts (fresh and
catalyst no measurable conversions were observed indi-
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cating that furfural does not undergo any conversion un-
der reaction condition.
spent) X-ray diffraction (XRD) study was carried out (Fig-
ure 3). The XRD pattern for SAPO-44 showed characteris-
tic peaks for CHA structure. The XRD patterns for SAPO-
7
0
5
, SAPO-11 and SAPO-46 also showed characteristic pat-
60
50
terns for AFI, AEL and AFS structures, respectively. All
the XRD patterns coincided well with those in the litera-
20-23
ture.
For spent SAPO-44 catalyst XRD peak pattern
matches well with the fresh SAPO-44 catalyst. This
demonstrates that the crystal structure of catalyst re-
mains same and that it is stable under reaction condi-
tions. XRD patterns for fresh and spent HMOR are pre-
sented in Figure S2 (Supporting Information). It can be
seen from the peak pattern that spent HMOR has lower
peak intensity compared to fresh catalyst and this in turn
indicate morphological changes. Moreover in our earlier
report it is shown by various physico-chemical characteri-
zation (XRD, SEM, N2-sorption, ICP-OES, NH3-TPD and
solid state NMR) techniques that zeolites undergo mor-
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1st
2nd
3rd
4th
5th
6th
7th
8th
Recycle Runs
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phological changes.
Figure 2. Recycle runs for SAPO-44 catalyst. Reaction condi-
Table 1. Physico-chemical properties of catalysts
tion: hemicellulose (0.3 g), SAPO-44 (0.075 g), wa-
°
ter+toluene=30 +30 mL, 170 C, 8 h.
Catalyst
Weak acid Strong
Total acid Surface
The SAPO-44 catalyst could be reused at least eight
times almost without loss of any activity (Figure 2). The
catalyst recovered from the reaction mixture was simply
washed with water and without any further treatment was
subjected to next reaction (for detail, see Supporting In-
formation). After 8 h of reaction time, the same conver-
sion (ca. 91%) and furfural yield (64%) were obtained
amount
100-
50°C)
mmol/g)
acid
amount
(350-
amount
(mmol/g)
area
a
2/
b
(
(m g)
2
a
500°C)
(
a
(mmol/g)
Fresh
SAPO-44
0.7
0.6
0.8
0.3
0.4
0.5
0.5
1.2
1.1
369
353
309
42
th
even after 8 reuse of catalyst. Concurrently to affirm the
Spent
SAPO-44
0.5
0
reusability of SAPO-44 catalyst and to check the activity
at initial reaction time reactions were carried out for 2 h.
The result shows that almost similar furfural yield (16±2%,
2 h) can be obtained in all the 8 recycle runs. To explain
this behavior detailed catalyst characterization studies
were conducted. To compare, recycle studies with HMOR
catalyst were also carried out and we observed ca. 10%
Fresh
SAPO-5
0.8
0.3
0.8
1.2
Fresh
SAPO-11
0
st
Fresh
SAPO-46
0.4
0.7
132
decrease in the activity after each catalytic run (1 run:
nd
rd
51%, 2 run: 43%, 3 run: 33%).
Fresh
528
HMOR
Fresh SAPO-46
(
Si/Al=10)
Fresh
HUSY
(Si/Al=15)
0.1
0.5
0.7
0.6
0.9
909
761
Fresh SAPO-11
Fresh SAPO-5
Fresh Hβ 0.2
(
Si/Al=19)
a
b
NH -TPD study, N -sorption study.
3
2
Spent SAPO-44
Fresh SAPO-44
35
TPD-NH was carried out to compare the amount of ac-
3
id sites of the SAPO and zeolite samples (Table 1). The
total acid amount is decreased in the following order,
SAPO-44 = HMOR > Hβ > SAPO-46 = SAPO-5 > HUSY >
SAPO-11. While the number of sites with strong acid
strength decreased in the sequence of, HMOR = Hβ >
SAPO-44 =HUSY > SAPO-46. In case of SAPO-5 and
SAPO-11 only weak acidity is observed while in other cata-
lysts both weak and strong acidity is seen. Thus, this dif-
ference in acid amount may influence the catalytic activi-
5
10
15
20
25
30
40
o
2θ
Figure 3. XRD patterns of SAPO catalysts.
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ty as over SAPO-46 higher activity (51%) is observed than smaller pore diameter (0.43 nm) in SAPO-44 that hinders
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SAPO-5 (35%) though both have same total acid amount.
The possibility of SAPO-44 undergoing any morphologi-
cal changes during reaction is ruled out as we observed
only marginal difference between the acid amount in
fresh and spent catalyst (Table 1). It would be unfair to
compare the catalyst activity and acidity correlation be-
tween SAPO and zeolite and also only among different
zeolites since during reaction those undergo morphologi-
the access of pyridine molecules (Figure S6, Supporting
Information). This data was in same line with the earlier
literate report which described the same observation.
24
The cubic morphology for SAPO-44 (Figure S7, Sup-
porting Information) observed in SEM photographs
25
matches well with the literature. Apparently we did not
observe any change in morphology in fresh and spent
catalysts, which indicates that catalyst is stable under
reaction conditions. The possibility of leaching of Al
and/or P was also verified by subjecting the SAPO-44 cat-
alysts to ICP-OES analysis. As summarized in Table S3
(Supporting Information), no leaching of Al and/or P dur-
ing reaction was observed and the result could be equated
to the theoretical value (for detail, see Supporting Infor-
mation). ICP analysis of reaction mixture also supports
our above observations (Table S3, Supporting Infor-
mation). Specific surface area were measured for all cata-
lysts and described in Table 1. Porosity analysis for both
fresh and spent SAPO-44 is carried out and the result
shows that fresh SAPO-44 has a pore diameter of 0.45 nm
and spent SAPO-44 has a pore diameter of 0.49 nm. This
data implies that the SAPO-44 catalyst is stable under the
reaction condition. However, the fact that SAPO-44 has
small pore opening suggest that hydrolysis of hemicellu-
lose may occur on the external acid sites of the catalysts
and once saccharides are produced those may undergo
further reactions by diffusing inside the pores. All the
characterization studies strengthen the fact that SAPO-44
catalyst is highly stable catalyst.
1
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cal changes as shown earlier.
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Since in all the reactions water is used as a reaction
media we postulated that the presence of water may play
an imp0rtant role in deciding why catalysts behave differ-
ently. Hence we examined the effect of water on the cata-
lyst morphology by studying the hydrophilic character of
SAPO-44 and HMOR catalyst. The studies were carried
out with zeolite, HMOR as amongst all zeolite samples it
has a lowest Si/Al ratio and thus is expected to be more
hydrophilic in nature. We took same amount of HMOR
and SAPO-44 catalysts in two similar test tubes and to it
water and organic solvents (either toluene, d=0.86 g/mL
at 25 °C or carbon tetrachloride, CCl , d=1.58 g/mL at 25
4
°
C) were added by maintaining 1:1 v/v ratio (Figure S3,
Supporting Information). Both the test tubes were stirred
vigorously to mix well with the water and organic solvent
layers. As observed in Figure S3 (Supporting Information),
after vigorous stirring most of the SAPO-44 is present in
water layer however; HMOR is largely present in organic
layer. Although the density of SAPO-44 (0.61 g/L) is
higher than that of HMOR (0.35 g/L), SAPO-44 was ob-
served to be separated from lower CCl layer due to its
4
In conclusion, it is shown that SAPO-44 catalyst is very
efficient in converting cheaply and abundantly available
hemicellulose directly in a one-pot method into furfural
with very high yields. The work allows us to eliminate
processing hemicellulose in a separate reactor to obtain
water soluble fractions first and then further process the-
se fractions in another reactor to obtain furfural. Various
physico-chemical characterizations revealed that SAPO-
higher hydrophilicity. Because of higher hydrophilicity in
SAPO-44 catalyst, it prefers to remain in water phase and
thus is completely available for hydrolysis and dehydra-
tion reactions and at the same time may suppress the
degradation reactions. On the contrary HMOR catalyst
present in both the layers hampers the hydrolysis-
dehydration reaction rates and also increases the chances
of degradation reactions. This property of SAPO-44 may
also help in achieving higher activity and reusability of
catalyst.
4
4 is highly stable catalyst and can be reused at least 8
times without losing any activity. The strong hydrophilic
nature of SAPO-44 compared to zeolite (HMOR) can be
helpful in achieving higher activity. This work tries to
establish the fact that in several processes wherein water
is an essential part of reaction system, SAPO type of cata-
lysts having high hydrophilic character and hydrothermal
stability can be effectively used.
TGA-DTG profile (Figure S4, Supporting Information)
for SAPO-44 shows 7.9% mass loss at 92.7 °C, which cor-
responds to loss of water. Further loss of 0.8% at 400 °C is
due to decomposition of remaining structure directing
agent (SDA). Further increase in temperature up to 1000
°
C did not show any loss indicating that catalyst is stable.
ASSOCIATED CONTENT
AUTHOR INFORMATION
-
1
Presence of both Brönsted acid (νmax/cm 1634 and
-
1
1542), Lewis acid (νmax/cm 1612 and 1452) and
-
1
Brönsted+Lewis acid (νmax/cm 1490) sites in SAPO cata-
lysts was proved with pyridine IR analysis (Figure S5,
Supporting Information). In SAPO-44 catalysts, the peaks
occur at almost similar positions. Though the concentra-
tion of acid sites in SAPO-44 measured through TPD-NH3
is higher compared to SAPO-5, SAPO-11 and SAPO-46,
the peak intensity of IR bands for the Lewis and Brönsted
acid sites in SAPO-44 are much smaller than those ob-
served in other SAPO catalysts. This might be due to the
Corresponding Author
*E-mail: pl.dhepe@ncl.res.in; Fax: +91-20 25902633; Tel: +91-
20 25902024
ACKNOWLEDGMENT
This work is financially supported by Department of Science
and Technology (DST), India. PB thanks University Grant
Commission (UGC), India for research fellowship.
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ACS Catalysis
Supporting Information. Detailed description of catalyst
synthesis, characterization techniques, materials, experi-
mental process, calculations, partition co-efficient calcula-
tion, temperature effect on reaction, catalyst physico-
chemical properties are provided. This material is available
free of charge via the Internet at http://pubs.acs.org.
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Table of Contents
Hemicellulose Furfural Hemicellulose Furfural
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Fresh SAPO-44
Spent SAPO-44
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