Insight into Aluminum Sulfate-Catalyzed Xylan Conversion into Furfural in a Γ-Valerolactone/Water Biphasic Solvent under Microwave Conditions

Article abstract of DOI:10.1002/cssc.201701290

A simple and efficient biphasic system with an earth-abundant metal salt catalyst was used to produce furfural from xylan with a high yield of up to 87.8 % under microwave conditions. Strikingly, the metal salt Al₂(SO₄)₃ e

Full text of DOI:10.1002/cssc.201701290

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Accepted Article
Title: Insight into aluminium sulfate catalyzed xylan conversion
to furfural in γ-valerolactone/water biphasic solvent under
microwave condition
Authors: Tao Yang, Yi-Han Zhou, Sheng-Zhen Zhu, Hui Pan, and Yao-
Bing Huang
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To be cited as: ChemSusChem 10.1002/cssc.201701290
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Insight into aluminium sulfate catalyzed xylan conversion to
furfural in γ-valerolactone/water biphasic solvent under
microwave condition
Tao Yang, Yi-Han Zhou, Sheng-Zhen Zhu, Hui Pan*, Yao-Bing Huang* [a]
Abstract: A simple and efficient biphasic system with earth-abundant
metal salt catalyst was reported to produce furfural from xylan with a
high yield up to 87.8% under microwave condition. Strikingly, the
metal salt Al2(SO4)3 exhibited excellent catalytic activity for xylan
conversion due to its appropriate combination of Lewis and Brønsted
acidity, and as well as a good phase separation promoter. The critical
Lignocellulose is mainly composed of cellulose, hemicellulose
and lignin, among which hemicellulose fraction takes up 25-35%
weight percentage of dry plants and rich in C5 sugars.[5] Xylan is
the main component of hemicellulose and made up by xylose
units through the glycosidic bond linkage. One of the most
important approaches to utilize xylan is converting it into furfural,
a platform molecule that can be directly used for the production of
a variety of organic chemicals such as furfuryl alcohol,[6] 2-
2-
role of SO4 anion was firstly emphasized, which determined the
aforementioned characteristic when combining with Al3+ cation. The
solvents with γ-valerolactone (GVL) as organic phase provided the
highest furfural yield, resulting from its good dielectric properties and
dissolving capacity that facilitated the absorption of microwave energy
and promoted mass transfer. Mechanistic studies suggested that the
methylfuran,[7]
2-methyltetrahydrofuran,[8]
maleic
acid,[9]
cyclopentanone[10] and levulinic acid/esters[11]
.
Thus, the
production of furfural from lignocellulose has gained considerable
interests during the past few decades.
xylan-to-furfural conversion proceeded mainly via
a
hydrolysis-
Generally, industrial production of furfural is mainly
accomplished through the hydrolysis/dehydration of biomass rich
in C5 sugars in aqueous solution over mineral acid catalysts such
as sulfuric acid and hydrochloric acid.[12,13] However, the reaction
efficiency was still not high (~50% furfural yield), not to mention
the association with environmental pollutions, equipment
corrosion, toxicity and catalyst recovery problems.[14] Later,
tremendous work was devoted to the development of more
efficient acidic catalysts for furfural production such as resins,[15,16]
sulfonated carbon/metal oxides,[17-19] mesoporous silica,[20,21]
zeolites[22,23] and heteropolyacids.24 But it should be noticed that
the reactions with these catalysts usually required higher reaction
temperature and longer reaction time to achieve satisfying
product yields. Recent researches highlighted the importance of
introducing Lewis (L) acid sites to the Brønsted (B) acid catalyst,
which can shift the reaction pathway from stepwise dehydration
to isomerization-dehydration, that can effectively promote furfural
yield and reduce the reaction temperature. For instance, Vlachos
and Lobo et al.[25] reported the use of Sn-Beta zeolite for the
conversion of xylose to xylulose at 383K, and further dehydration
to furfural in the presence of Brønsted acid HCl or Amberlyst-15.
Although the yield of furfural was low, this result just provided a
new method that allowed the mild production of furfural. Later,
Abu-Omar et al.[26] synthesized sulfonated carbonaceous material
that contain Brønsted acidic sulfonic groups and Lewis acidic
titania pores, which was successfully applied to the production of
furfural with ~50% yield. These results suggested the necessity of
co-existence of Lewis and Brønsted acid sites for the efficient
furfural production under mild condition.
isomerization-dehydration pathway and the hexa-coordinated Lewis
acidic [Al(OH)2(aq)]+ species were the active sites for xylose-xylulose
isomerization. Detailed kinetic studies of the subreaction for xylan
conversion revealed the regulatory role of GVL on the reaction rates
and pathways, by promoting the rates of the key steps involved for
furfural production and suppressed the side reactions for humin
production. Finally, Al2(SO4)3 catalyst was successfully used for the
production of furfural from several lignocellulosic feedstocks,
revealing its great potential of being used for other biomass
conversions.
Introduction
Recently, the utilization of renewable biomass resources to
produce chemicals and biofuels has great potential to reduce the
society’s dependence on traditional fossil sources, as well as the
emission of greenhouse gas.[1-3] Lignocellulosic feedstocks are
primarily considered as the suitable source for this goal due to
their worldwide availability and large annual production, which
can also avoid the issue of chemical and food competition.[4]
[a]
[b]
Dr.T Yang, Dr.Y.-H. Zhou, Sheng-Zhen Zhu, Prof.H. Pan, Prof.Y.-B.
Huang
College of Chemical Engineering
Nanjing Forestry University
Longpan Road 159# Nanjing China
E-mail: hyb123@mail.ustc.edu.cn
Title(s), Initial(s), Surname(s) of Author(s)
Department
Recently, commercially available metal salt catalysts (e.g. AlCl3,
CrCl3) have greatly enriched the family of Lewis/Brønsted site-
containing catalysts, which showed excellent reactivities in the
Institution
Address 2
isomerization
and
hydrolysis/dehydration
reactions.[27,28]
Successful examples with such conversions can be easily found
in the glucose-to-fructose or xylose-to-xylulose conversions, and
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followed by their further dehydration to 5-hydromethylfurfural
(HMF) or furfural, respectively.[27-29] Appropriate combination of
cation and anion could provide various metal salt catalysts that
differ in B/L ratio and reactivity, which would meet different
demands of acidity of multitudinous biomass reactions. Strikingly,
some earth abundant metal salt catalyst such as Al salts also
possess several other appealing advantages such as commercial
available, inexpensive, easy to handle and environmental benign.
For example, Hu and Abu-Omar et al.[30] reported the use of AlCl3
for the conversion of xylan, xylose and biomass, reaching high
furfural yields up to 75% in a water-tetrahydrofuran biphasic
medium in the presence of NaCl under microwave condition. On
the basis of this, Luo et al developed a AlCl3 catalyzed two step
progress for the selective dissolution and conversion of
pubescens biomass, and high yields of furfural (39.1%) and
levulinic acid (48.3%) were obtained.[31] Since furfural was
unstable in aqueous solution at thermal condition, the organic
solvent was introduced to form a biphasic system with water,
which could continuously extract furfural from aqueous phase,
thus preventing the undesired side reactions. This effect was
further promoted by the addition of inorganic salt (e.g. NaCl) to
aqueous phase, which could significantly increase the partition
coefficient of furfural in organic phase.
Although significant advances have been made on the
development of metal salt catalytic systems for the production of
furfural, it’s not difficult to find that the reported metal salt catalysts
focused almost exclusively on chlorine salts. In our previously
reported works,[32,33] sulfate salts (i.e. Al2(SO4)3) were
demonstrated to possess high reactivities, even better than
chlorine salts, in biomass derived alcoholysis/esterification
reactions due to their good Lewis and Brønsted acidity in
methanol solution. These results encouraged us to further
investigate their reactivities for the conversion of xylan to furfural
in biphasic solvent. And, to the best of our knowledge, no sulfate
salt catalyst has been used for furfural production until now. Our
preliminary experimental result showed that aluminum sulfate
provided a high furfural yield from xylan in biphasic solvents under
microwave conditions. However, the entire reaction details
including active catalytic species, role of biphasic solvents,
kinetics of tandem reactions and reaction pathways are still
unavailable, which motivated us to systematically study this
promising catalytic system.
the study about catalyst’s recyclability and application in the
conversion of real biomass materials were also involved.
Results and Discussion
Screening of the catalysts
A variety of metal salt catalysts were tested for the conversion
xylan under microwave condition with biphasic GVL/H2O solvent
as reaction medium (Table 1). Since xylan was not easily detected
in the reaction mixture, we only provided the yield of different
reaction products. A blank experiment without any catalyst was
firstly conducted and only afforded 1.1% yield of furfural,
excluding the self-catalysis or thermal degradation of xylan in the
reaction solvents (Table 1, entry 1). When the typical Lewis acidic
metal salt SnCl4 was used as catalyst, the furfural yield was
greatly promoted to 70.9%, accompanied with 2.7 and 1.8%
xylose and xylulose, respectively (Table 1, entry 2). These two
side products were also frequently detected in the reactions with
other metal salt catalysts, which were indicative of possible
reaction pathways involved for xylan conversion. To further
investigate the states of metal salt in aqueous solution (a.q.), pH
values of the metal salt aqueous solutions were detected. SnCl4
provide an acidic solution with an initial pH of 1.54 and final pH
1.52, respectively. This result indicated that SnCl4 underwent
hydrolysis in water and generated Brønsted acid proton H+, which
also greatly contributed to xylan conversion with high furfural yield.
In case of CrCl3, a moderate furfural yield of 59.1% was obtained
with higher pH values, as compared to SnCl4 (a.q.) (Table 1, entry
3). It’s also worth noting that sulfate type salt Cr2(SO4)3, which
was rarely used in biomass conversions, provided higher furfural
yield of 70.5% with even lower solution’s pH values (Table 1, entry
4). This result clearly showed that the anion had an important role
in determining the catalyst’s reactivity, possibly by affecting the
acid strength of the metal salt solution. For Cu salts, both sulfate
and chlorine salts showed inferior activities toward the formation
of furfural, but with considerable amount of xylose (Table 1,
entries 5-6). According to the previous work,[34] isomerization of
xylose to xylulose was the rate-determining step for the
production of furfural, which could proceed over proper Lewis
acidic catalysts. And the cationic species, originated from the
hydrolysis of cations, were mainly responsible for the Lewis
acidity of the reaction system. Based on above results, it can be
inferred that Cu cationic species lack sufficient Lewis acidity that
cannot efficiently catalyze xylose to xylulose conversion under the
identical condition.
In this study, we explored the aluminum sulfate catalyzed xylan
conversion in a biphasic solvent system to produce furfural under
microwave condition. A serious of metal salt catalysts were
compared to examine their reactivities in the conversion.
Optimized experiments were involved to provide comprehensive
characteristics of the xylan conversion. The chlorine salt and
sulfate salt were directly compared to figure out their difference in
the catalyzed process. Biphasic solvents, especially the different
organic phases, were elaborated to understand the key role in this
conversion. Kinetics of tandem reactions were simulated to
differentiate the activation energies under microwave condition.
The actual forms of catalyst species were demonstrated by
electrospray ionization mass spectrometry/mass spectrometry
(ESI-MS/MS), 27Al-NMR and experimental simulations. Finally,
2-
For Fe salts with Cl- and SO4 anions, they exhibited
distinguished activities in the xylan conversion, offering 50.2%
and 84.2% yield of furfural, respectively (Table 1, entries 7-8).
This may be originated from their different acid strength in
aqueous solution, as can be seen from their pH values. This result
further confirmed the key role of anion in determining the
hydrolysis degree of metal salts, thereby providing distinct acidity
of the solution. As for Al salts, the commonly used AlCl3 only
provided 51.5% yield under the identical reaction condition, and a
considerable amount of xylose and xylulose were also detected
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(Table 1, entry 9). Instead, Al2(SO4)3 showed remarkable activity,
yielding 87.8% furfural with negligible side products (Table 1,
entry 10). The pH values, on the other side, were much lower than
that of the AlCl3 system. The use of Al(NO3)3 showed even lower
reactivity, yielding only 20.4% furfural (Table 1, entry 11). The
comparison of reactivates of the three Al salts clearly highlighted
the importance of anions in determining the hydrolysis of metal
salt, generating different Brønsted acidty in the reactions.
or xylulose was detected (Table 1, entry 13). These experiments
provided direct evidence about the critical role of Lewis acid in the
xylan conversion. By the way, when single Sn-beta was used in
the model reaction, only 10.8% furfural was generated, excluding
the significant contribution of single Sn-beta to the high yield
(Table 1, entry 14). This result, on the other side, also highlighted
the importance of Brønsted acid in the reactions, by accelerating
the xylan hydrolysis and the subsequent dehydration reactions
involved in xylan conversion.
Based on the above catalyst screening, it’s not difficult to find
that both the pH value (Brønsted acidty, determined by the
hydrolysis of cations and anions) and the catalytic species (Lewis
acidic species, originated from cation hydrolysis) were key factors
for the high yield xylan conversion in the biphasic systems.
Table 1. Effect of different metal salts on the conversion of xylan to furfural
under microwave irradiationa
Entry
Catalyst
Xylose
[%]
Xylulose
[%]
Furfural
[%]
Initial
pH
Final
pH
1
2
None
SnCl4
2.2
2.7
4.3
1.6
11.9
28.7
11.7
3.3
4.8
0.7
8.1
43.4
0
0
1.8
5.7
8.3
3.5
2.9
4.5
0.4
10.9
0.3
15.1
0
1.1
6.77
1.54
3.10
2.71
3.39
3.55
3.23
1.91
3.36
2.71
3.79
1.24
1.24
5.82
1.52
2.18
1.99
2.66
3.10
2.63
1.15
2.54
1.24
3.63
1.28
1.24
Influence of reaction conditions
70.9
59.1
70.5
49.2
32.5
50.2
84.2
51.5
87.8
20.4
33.7
90.1
Figure 1a showed the results of xylan conversion at different
reaction temperatures varied from 110 oC to 150 oC. The reaction
conducted at lower reaction temperature 110 oC provided 35.9%
yield of furfural, associated with 41.7% and 14.1% side products
xylose and xylulose, respectively. As the reaction temperature
increased, the yield of furfural gradually increased and reached
the plateau at 130 oC (87.8%), during which xylose and xylulose
gradually decreased to a negligible amount. These changes
suggested that the reaction rates increased with increasing
reaction temperature. However, higher temperature caused slight
decreases in furfural yields and the brown color of the reaction
mixture, possibly ascribing to the instability of furfural which may
undergo decomposition, cross-/self-polymerization between
products and reactant/intermediates at elevated reaction
temperatures.[35,36] Additionally, a lower reaction temperature of
110 oC could also provide a high furfural yield of 85.3% by
extending the reaction time to 60 min. The catalyst loading
actually determined the concentration of acidic species in the
reaction medium. As shown in figure 2b, the reaction with 5 mol%
catalyst loading provided 65.1% yield of furfural together with
14.5% and 6.9% xylose and xylulose, respectively. When the
catalyst loading was doubled to 10 mol%, the highest furfural was
obtained and the side products were not detected. Excessive
catalyst loading slightly decreased the furfural yield, possibly due
to high density of active sites that accelerated the side reactions
(rehydration or condensation, etc.).[37] The solid to liquid ratio was
also found to affect the reaction. Fig. 1c provided the results of
different xylan loading in the reactions with a fixed metal salt
catalyst loading, which could also probe the catalyst’s efficiency.
In lower xylan loading (solid/liquid=0.1), the reaction system
provided even higher furfural yield of 88.1% without any
detectable side products, as compared to the aforementioned
optimal solid/liquid ratio of 0.2. When the xylan loading was scaled
up 5 times of the model reaction (solid/liquid=1.0), the reaction
could also finish with a high furfural yield of 82.3%. Further
increasing the xylan loading to 50 mmol (25 times,
solid/liquid=5.0) could also provide a moderate furfural yield of
72.5%. The decrease in furfural yield may lie in the two aspects:
1) rising xylan concentration greatly increased the viscosity of
aqueous phase, which would easily make the aqueous phase
3
CrCl3
4
Cr2(SO4)3
CuCl2
5
6
CuSO4
FeCl3
7
8
Fe2(SO4)3
AlCl3
9
10
11
12
13
Al2(SO4)3
Al(NO3)3
H2SO4
H2SO4 +
Sn-beta
0
14
Sn-beta
1.5
5.1
10.8
4.98
4.26
aReaction Conditions: Xylan 2 mmol, catalyst (20 mol%, based on Mn+ or
H+), 8 mL GVL, 2 mL H2O, 400 W, 130 oC, 30 min. All the yields were
calculated based on the products both in organic and aqueous layers. b: 50
mg Sn-beta was used. pH values were measured with complete dissolution
of metal salts in water at room temperature (initial pH), and after the deep
hydrolysis at 130 oC for 30 min under microwave condition (final pH).
To further investigate the possible roles of Brønsted and Lewis
acid species in the reaction system, a certain concentration of
H2SO4 aqueous solution with the same pH value as that in the
final aqueous solution of Al2(SO4)3 system in Table 1, entry 10
and then subjected to xylan conversion. However, furfural was
only produced in 33.7% yield and with much of xylose production
(43.4%) (Table 1, entry 12). This result revealed that the catalyst
without Lewis acid sites just could not efficiently catalyzed the
xylose isomerization step, which would then inhibit the
subsequent xylulose dehydration to furfural. By adding the
typically used Lewis solid catalyst Sn-beta to the above system,
furfural was produced with a high yield of 90.1%, and no xylose
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