Heteropolyanion-based ionic liquids catalysed conversion of cellulose into formic acid without any additives

Article abstract of DOI:10.1039/c4gc01252f

Heteropolyanion-based ionic liquids with -SO₃H functionalized cations and PMo₁₁VO₄₀^4- anions were synthesized and applied in the conversion of cellulose into formic acid (FA). It was demonstrated that the as-prepared ILs showed enhanced activity for catalysing cellulose conversion to FA in the presence of oxygen in water compared to H₄PMo₁₁VO₄₀. A high FA yield over 50% accompanied by a FA concentration of ～10% in reaction solution was obtained at 180 °C and in an oxygen pressure of 1.0 MPa. The possible pathway of FA production from cellulose was investigated. It was indicated that the resulting ILs served as bifunctional catalysts with cations catalysing the cellulose hydrolysis to glucose and anions catalysing glucose oxidation to FA. These ILs may find more applications in cellulose conversion. This journal is

Full text of DOI:10.1039/c4gc01252f

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Heteropolyanion-based ionic liquids catalysed
conversion of cellulose into formic acid without
any additives†
Cite this: Green Chem., 2014, 16,
4931
Jilei Xu, Hongye Zhang, Yanfei Zhao, Zhenzhen Yang, Bo Yu, Huanjun Xu and
Zhimin Liu*
4−
Heteropolyanion-based ionic liquids with –SO3H functionalized cations and PMo11VO40 anions were
synthesized and applied in the conversion of cellulose into formic acid (FA). It was demonstrated that the
as-prepared ILs showed enhanced activity for catalysing cellulose conversion to FA in the presence of
oxygen in water compared to H4PMo11VO40. A high FA yield over 50% accompanied by a FA concen-
tration of ∼10% in reaction solution was obtained at 180 °C and in an oxygen pressure of 1.0 MPa. The
possible pathway of FA production from cellulose was investigated. It was indicated that the resulting
ILs served as bifunctional catalysts with cations catalysing the cellulose hydrolysis to glucose and anions
catalysing glucose oxidation to FA. These ILs may find more applications in cellulose conversion.
Received 5th July 2014,
Accepted 6th August 2014
DOI: 10.1039/c4gc01252f
www.rsc.org/greenchem
gen from biomass, and has been paid much attention. The FA
production from soluble carbohydrates such as glucose, fruc-
1. Introduction
With the requirements for sustainable development, the pro- tose, sucrose, and glycerol was reported by several groups, and
duction of fuel and chemicals from renewable biomass has relatively high FA yields were obtained.8–11,15 For example,
been paid more and more attention.1 Cellulose, the most the combination of Ru(OH)4/r-GO and FeCl3 catalysed the
abundant source of biomass, is regarded as a promising feed- conversion of glycerol to FA using O2 as an oxidant under
stock for chemicals because it is inedible by humans.2 The mild conditions.15 Keggin type catalysts H5PV2Mo10O40 and
conversion of cellulose into fuel and chemicals has been H4PVMo11O40 catalyzed the direct conversion of cellulose into
widely investigated in recent years. Cellulose can be readily cat- FA.8–12 Han and coworkers found that H4PVMo11O40 showed
12
alytically hydrolyzed into glucose using solid or liquid acids,3 better performance than H5PV2Mo10O40
.
More recently,
which are then converted into various value-added chemicals sodium metavanadate was reported to be highly effective for
via different reactions. For example, polyols including ethylene catalysing cellulose conversion to FA in acidic aqueous solu-
glycol,4 propylene glycol5 and sorbitol6 were produced from tion.13 Though some progress has been made, it is still desir-
the direct conversion of cellulose in the presence of H2 in able to develop efficient catalysts for cellulose conversion to
water. 5-Hydroxymethylfurfural (HMF) and its derivatives were FA, especially, with a high FA yield and high FA concentration.
obtained under acidic conditions in ionic liquids (ILs).7
Formic acid (FA) was obtained from cellulose conversion organic/inorganic anions, have unique properties, such as
under oxidation conditions.8–13
chemical and thermal stability, nonflammability, and immea-
ILs, a kind of molten salts composed of organic cations and
FA is a commodity chemical with various applications, surably low vapor pressure. In particular, they are designable
which is recently regarded as a hydrogen storage compound via changing cations and anions, thus providing the ILs with
because it can be readily decomposed into hydrogen under unique functionality. For example, heteropolyanion-based ILs
mild conditions.14 Therefore, the production of FA from were designed, which displayed high efficiency for esterifica-
biomass has been considered as an alternative to obtain hydro- tion between a carboxylic acid and an alcohol,16 and for deep
desulfurization of fuels.17–19 ILs have been shown to be prom-
ising solvents for the transformation of cellulose20 and for the
production of biodiesel.21
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid,
Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of
For the direct conversion of cellulose to FA, the catalysts or
Sciences, Beijing 100190, China. E-mail: liuzm@iccas.ac.cn; Fax: (+) 8610-62562821
catalytic systems should have the ability to catalyse cellulose
†Electronic supplementary information (ESI) available: The characterization of
ILs, including XRD, IR, 1H-NMR, 13C-NMR, 31P-NMR and ESI-MS analyses. See
DOI: 10.1039/c4gc01252f
hydrolysis into glucose and to catalyse glucose oxidation to FA.
In this work, we designed a series of heteropolyanion-based
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the resulting ILs were verified by means of NMR, ESI-MS (LC-MS
2010) and IR (TENSOR-27) analyses. 31P (400 MHz), 1H (400 MHz)
and 13C (100 MHz) NMR spectra were collected in D2O on a
Bruker Avance NMR (400 MHz) at ambient temperature.
2.3 Conversion of cellulose to FA
Typically, MCC (0.335 g, about 2 mmol glucose units), catalyst
(e.g., IL-3, 0.05 mmol, 0.13 g), and 5 mL water were sealed in a
high-pressure autoclave. The air in the reactor was removed by
charging O2 three times, and then O2 was charged into the
reactor up to the desired pressure (e.g., 1 MPa). The reactor
was moved to an oil bath of 180 °C, and was kept at this temp-
erature for 1 h. Subsequently, the vessel was cooled in ice-
water, and vented slowly. The gas products were collected by a
gas-balloon, analyzed by GC and detected by TCD. The liquid
products were collected by removing the solid catalyst via
centrifugation, and analyzed by HPLC equipped with UV and
RID detectors, using a HPX-87H column and sulfuric acid
aqueous solution (5 mmol L−1) as the mobile phase with
0.6 mL min−1 flow rate at 55 °C.
Scheme 1 ILs designed in this work. IL-N (N
= 1–4) stands for
[MIMPS]NH4−NPMo11VO40 (N = 1–4); IL-5: [PyPS]3HPMo11VO40; IL-6:
[TEAPS]3HPMo11VO40; IL-7: [BMIM]3HPMo11VO40.
.
4−
ILs with –SO3H functionalized cations and PMo11VO40
anions, as illustrated in Scheme 1. These ILs were used for cat-
alysing the cellulose conversion to FA in aqueous solution,
aiming at achieving high FA yield. The influence of reaction
time, reaction temperature, the amount of the used IL catalyst
on the cellulose conversion and FA yield was investigated. In
addition, the possible pathway of FA production from cellulose
conversion was explored.
3. Results and discussion
A series of heteropolyanion-based ILs with –SO3H functiona-
4−
lized cations and PMo11VO40 anions were successfully syn-
2. Experimental
thesized, and confirmed by NMR, ESI-MS and IR analyses (see
ESI†). These so-called ILs are solids at room temperature with
melting points higher than 100 °C, and are soluble in water.
Prior to being used as catalysts, these ILs were examined in
aqueous solution at an oxygen pressure of 5.0 MPa and at
200 °C. There were no products detectable in both liquid and
gas phases, indicating that the as-prepared ILs were tolerable
to oxidation conditions. They were applied in catalysing the
MCC conversion in the presence of oxygen in aqueous solu-
tions, and the results are listed in Table 1.
2.1 Materials
Methylimidazole, pyridine, triethylamine, butyl methyl imid-
azole, concentrated sulfuric acid, glyceraldehyde, and acetic
acid were provided by Beijing Chemical Company. 1,3-Propane-
sulfone, Na2HPO4, NaVO3, Na2MoO4·2H2O, 1,2-propanediol,
1,3-propanediol, glyoxal, glycolic acid, oxalic acid, and formic
acid were provided by J&K Scientific Ltd (Beijing, China).
Microcrystalline cellulose (MCC) was provided by Alfa Aesar
company. All the chemicals were used as received.
The MCC conversion was carried out at an oxygen pressure
of 1.0 MPa and at 180 °C. It was demonstrated that the tested
2.2 Synthesis of ILs
Molybdovanadophosphoric acid (H4PMo11VO40·32.5H2O) was catalysts except for IL-7 were effective for MCC conversion, and
first synthesized based on the reported procedures.22 Methyl- FA was the main liquid product accompanied by trace
imidazole functionalized with –SO3H (MIMPS) was synthesized amounts of acetic acid (AA) and glycolic acid (GOA). CO2 was
as follows. Methylimidazole (9.02 g, 0.11 mol) and 1,3- the sole product detected in the gas phase. Compared to
propanesulfone (12.21 g, 0.1 mol) were dissolved in toluene H4PMo11VO40, the ILs, IL-N (N = 1–4) afforded improved FA
(100 mL), and the mixture was stirred at 50 °C for 24 h under a yields (Table 1, entries 1, and 2–5). Particularly, IL-3 gave the
N2 atmosphere. The resulting white precipitate was separated highest FA yield, 51.3%, under the experimental conditions.
from the mixture via filtration, washed with ether three times, However, these IL catalysts gave declined MCC conversion in
and dried in a vacuum overnight. Similarly, –SO3H functiona- the order: IL-1 > IL-2 > IL-3 > IL-4. These results suggest that
lized pyridinium (PyPS) and TEAPS were prepared via similar the cation species present in the reaction system influenced
procedures using the corresponding pyridinium and triethyl- the activity of the catalysts for cellulose conversion. To confirm
this, IL-5 and IL-6, with PMo11VO40 as anions and [PyPS]+
4−
amine as starting materials, respectively.
The heteropolyanion-based ILs were prepared via a similar and [TEAPS]+ as the main cations, respectively, were examined
procedure as follows. In a typical experiment to prepare IL-3, to catalyse the MCC conversion under similar conditions. It
0.1 mol H4PMo11VO40·32.5H2O and 0.3 mol MIMPS were dis- was demonstrated that both IL-5 and IL-6 were also effective
solved in 20 mL water, and the mixture was stirred for 24 h at for catalysing the MCC conversion to FA, showing a little
room temperature. The water was removed via reduced distilla- better performance than H4PMo11VO40 (Table 1, entries 1, 6
tion, giving the product as a solid. The chemical structures of and 7). However, IL-7 that had no –SO3H in its cation did not
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Table 1 Conversion of MCC catalysed by the as-prepared ILs in the presence of oxygena
Yield/%
FA
Entry
Catalyst
T/°C
t/h
Conv./%
AA
GOA
CO2
1
2
3
4
5
6
7
H4PMo11VO40
IL-1
IL-2
IL-3
IL-4
IL-5
IL-6
IL-7
IL-3
180
180
180
180
180
180
180
180
180
160
180
1
1
1
1
1
1
1
1
1
4
1
100
100
98
93.3
92
95.7
100
15.2
90.1
89.5
91.2
35.5
37.6
43.8
51.3
38.1
36.5
38.8
5.2
1.7
0.3
0.5
0.6
0.4
2.6
4.7
0.8
8.3
0.3
0.5
0.2
0.0
0.0
0.1
0.1
0.0
0.0
0.2
1.4
0.1
0.1
62.6
62.1
53.7
41.3
53.4
56.6
56.5
7.7
8b
9c
10
11d
49.7
35.7
49.5
30.7
53.4
41.5
IL-3
IL-3
a Reaction conditions: MCC, 0.335 g; the molar ratio of glucose units in MCC to IL was about 40 : 1; water, 5 mL; O2, 1 MPa. b Glucose, 0.36 g.
c MCC, 0.653 g; IL-3, 0.26 g. d IL-3 was reused for the third time. Conversion was calculated based on the mass difference of MCC before and after
conversion.
catalyse MCC conversion, and only showed a little activity for within 80 min. The FA yield increased with reaction time and
glucose oxidation under the same conditions (Table 1, entry 8). reached 51.3% at 60 min, while the selectivity of FA changed a
This implies that the –SO3H group in the cations of the as- little. Prolonging the reaction time to 80 min, the FA yield
synthesized ILs (e.g. IL-1 to IL-5) was required and played an almost remained unchanged with a slight decrease in selecti-
important role in the conversion of MCC. Among the tested vity. Under the tested conditions, GOA and AA were hardly
ILs, IL-3 displayed the highest performance for the production detected.
of FA from MCC conversion under the same experimental con-
The oxygen pressure was another important factor to influ-
ditions. Moreover, a high FA concentration of ∼10 wt% was ence the MCC conversion, the yield and the selectivity of FA.
obtained (Table 1, entry 9), which showed great potential for In the tested oxygen pressure range of 0–2 MPa, both the yield
the scale production of FA. Thus IL-3 was used as the catalyst of FA and the conversion of MCC increased with the oxygen
to study the influence of reaction parameters on the MCC con- pressure in the range of 0–1 MPa, and the highest FA yield of
version and FA production, and further to explore the possible 51.3% was achieved at 1 MPa, as shown in Fig. 2. Further
formation mechanism of FA from MCC conversion.
increasing oxygen pressure to 1.5 and 2.0 MPa, MCC converted
The cellulose conversion was performed at 160 °C. It was completely, while the FA yield declined. These results indicate
demonstrated that longer reaction time (e.g. 4 h) was required that high pressure is favourable to MCC conversion, while it is
for obtaining a high cellulose conversion, however, with a rela- unfavourable to the production of FA. It should be noted that
tively lower FA yield (Table 1, entries 10 vs. 4). This suggests AA and GOA were detected in high yields around 10% at
that 180 °C was a suitable temperature for cellulose conversion 0.5 MPa, while the yield of AA and GOA decreased to 0.5% at
to FA in this work.
higher pressure, suggesting that the intermediates were
Fig. 1 shows the dependence of MCC conversion and FA further converted to FA or CO2 quickly under higher pressure.
yield on the reaction time. It is clear that MCC converted The above results indicate that an oxygen pressure of 1 MPa
rapidly in the initial stage and the conversion reached 100% was suitable for the FA production from MCC conversion.
Fig. 1 Dependence of MCC conversion, FA selectivity and yields of FA
and intermediates on the reaction time. Other reaction conditions:
180 °C, 1.0 MPa, the molar ratio of the glucose unit in MCC to IL-3 was 40.
Fig. 2 Dependence of MCC conversion, FA selectivity, and yields of FA
and intermediates on the O2 pressure. Other reaction conditions:
180 °C, 1 h, the molar ratio of the glucose unit in MCC to IL-3 was 40.
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Table 2 Oxidations of other substrates using IL-3 as the catalysta
Yield/%
FA
Entry
Substrates
Conv./%
AA
OA
GOA
CO2
1
2
3
4
5
6
7
8
Glucose
1,2-PO
1,3-PO
Glyoxal
GLY
GOA
OA
FA
AA
Glucose
100
88.8
1.2
100
100
100
100
2.8
54.7
22.5
0.1
74.2
39.2
30.0
15.6
97.2
0.0
21.4
50.0
0.0
0.0
25.6
0.0
0.0
0.0
99.9
8.8
1.4
0.0
0.0
0.0
0.1
0.1
0.0
0.0
0.0
0.0
6.7
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.2
15.8
16.3
1.1
25.8
35.0
69.9
84.4
2.8
9
0.1
100
0.1
46.2
10b
44.8
Fig. 3 Dependence of MCC conversion, FA selectivity, and yields of FA and
intermediates on the molar ratio of the glucose unit in MCC to IL-3. Other
reaction conditions: 180 °C, 1 h, 1.0 MPa, the mass of MCC was 0.335 g.
a Reaction conditions: substrate, 2 mmol; the molar ratio of substrate
to catalyst, 40 : 1; water, 5 mL; 1 MPa; 180 °C;1 h. b H4PMo11VO40 as
the catalyst.
Fig. 3 shows the effects of the molar ratio of the glucose unit
in MCC to IL-3 (labeled MCC : IL-3) on the MCC conversion and
FA selectivity and the yield in the MCC : IL-3 range of 40–160. It
was demonstrated that the conversion of MCC decreased with
the ratio of MCC : IL-3 from 40 to 160, and the FA yield decreased
as well. Notably, the yields of AA and GOA increased slightly with
the reduction of the amount of IL-3 in the reaction system.
These results indicate that increasing the IL-3 amount is favour-
able to the conversion of MCC, and to the production of FA.
In addition, the reusability of IL-3 was investigated. It was
demonstrated that the cellulose conversion and FA yield
decreased little after it was reused 3 times (Table 1, entry 11),
suggesting that this IL was relatively stable for catalysing cellu-
lose conversion under the experimental conditions.
The conversion of cellulose into FA might involve two steps:
the hydrolysis of cellulose to glucose and the further oxidation
of glucose to FA. To confirm this, the hydrolysis of MCC was
investigated in the presence of IL-3 without O2 at 180 °C for
1 h. It was found that 14% MCC was converted, and glucose
was the main product accompanied by a small amount of fruc-
tose and others including HMF, levulinic acid and FA. This
indicates that IL-3 was effective for catalysing the hydrolysis of
MCC to glucose in the absence of O2, and it catalysed the
further conversion of glucose to fructose and others as well.
Since FA was present during the process of MCC conversion,
the hydrolysis of MCC catalysed by IL-3 in the presence of FA
(2 wt%) was tested under the O2-free conditions. It was indi-
cated that the MCC conversion increased to 30%, suggesting
that the presence of FA promoted the hydrolysis of cellulose.
Considering that no glucose was detected in the reaction solu-
tion of MCC conversion to FA catalysed by IL-3 in the presence
of oxygen, it can be deduced that the oxidation of glucose cata-
lysed by IL-3 occurred rapidly. Therefore, the MCC hydrolysis
to glucose was the rate-controlling step for the production of
FA from the MCC conversion.
version catalyzed by IL-3. This may be ascribed to the high
concentration of glucose, which resulted in more side reac-
tions catalysed by IL-3.
To explore the production mechanism of FA, the oxidation
of the detected intermediates and possible intermediates
including GOA, glyceraldehyde (GLY), glyoxal, and OA was
examined in the presence of IL-3 under the same conditions,
and the results are listed in Table 2. The oxidations of 1,2-pro-
pylene glycol (1,2-PO) and 1,3-propylene glycol (1,3-PO) indi-
cated that the pinacol structure was very important for the
oxidation reaction, which determined the broken site of C–C
(Table 2, entries 2 and 3). GLY and glyoxal were the two pro-
ducts of the reverse condensation of glucose and the inter-
mediates of the glucose oxidation.8 Glyoxal could be oxidized
effectively to FA in a yield up to 74.2%, with CO2 as the sole
byproduct (Table 2, entry 4). In contrast, the oxidation of GLY
produced less FA; however, more AA was obtained in a high
yield of 25.6% (Table 2, entry 5).
From the above results, it may be supposed that cellulose was
hydrolysed to glucose first in the presence of IL-3, and then
glucose was rapidly oxidized to C2 intermediates, such as
glyoxal, which were oxidized further to FA. In the case of glucose
oxidation, since the initial concentration of glucose in aqueous
solution was much higher than that produced in the cellulose
hydrolysis process, the glucose isomerization may occur to
produce fructose, which could produce C3 intermediates
(e.g. GLY) via reverse condensation. GLY would be dehydrated to
pyruvaldehyde under acidic conditions which could be oxidized
to AA and formic acid/CO2. This might be the reason why the
yield of AA was so high when glucose was oxidized. Low yields of
FA were obtained in the cases of GOA and OA oxidation (Table 2,
entries 6 and 7), suggesting that the carboxyl group could be
converted into CO2 rather than FA. Therefore, it can be supposed
that carboxylic acids may not be the main intermediates of FA.
The stability of FA under the reaction conditions was exam-
ined, and it was found that only 2.8% FA was degraded
accompanied by CO2 as the sole product (Table 2, entries 8).
This suggests that the experimental conditions were suitable
for FA production. Similarly, AA was also tolerant to oxidation
The oxidation of glucose catalysed by IL-3 was investigated.
It was found that glucose was mainly converted to FA
accompanied by AA and a small amount of GOA, and oxalic
acid (OA) (Table 2, entry 1). It is worth noting that the AA yield
was much higher than that obtained in the case of MCC con-
4934 | Green Chem., 2014, 16, 4931–4935
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glucose oxidation to FA. These ILs may find more applications
in cellulose conversion, and the related work is under way in
our laboratory.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (no. 21125314, 21321063, 21373242).
Scheme 2 Possible pathway for conversion of cellulose to FA.
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under the experimental conditions, suggesting that AA was a
byproduct rather than an intermediate in the cellulose conver-
sion (Table 2, entries 9). From these results, it can be deduced
that the CO2 produced in the cellulose conversion process was
from the side reactions rather than the degradation of FA.
For the comparison purpose, H4PMo11VO40 was examined
to catalyse glucose oxidation, and more CO2 in a yield of
46.2% was obtained (Table 2, entries 10), suggesting that
H4PMo11VO40 was powerful for catalysing the oxidation of the
intermediates directly to CO2. Thus, it can be deduced that the
cations of ILs played an important role in the glucose oxi-
dation, which suppressed the overoxidation of the intermedi-
ates to CO2 under the reaction conditions. This also can
explain the improvements of the FA yield as the heteropoly-
anion-based ILs were used as catalysts.
Based on the above results and analysis, a possible pathway
for the cellulose conversion into FA was proposed, as illus-
trated in Scheme 2. Cellulose was first hydrolysed into glucose
catalysed by the acidic ILs. Then glucose was converted into
glyoxal under acidic conditions or isomerized to fructose
which was further converted into GLY through reverse conden-
sation reaction. Subsequently, the resulting intermediates were
oxidized to FA, CO2 and AA catalysed by the anion of the ILs. It
was reported that V was the active center as H5PMo10V2O40 was
used to catalyse the glucose oxidation.8 In this work, V in the
anions of the as-prepared ILs may be the active center for
glucose oxidation, while the cations of the ILs may play an
important role in catalysing the hydrolysis of cellulose. This
means that the as-prepared ILs were dual catalysts with cations
catalysing the cellulose hydrolysis to glucose and anions cata-
lysing glucose oxidation to FA.
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