The breakdown of reticent biomass to soluble components and their conversion to levulinic acid as a fuel precursor

Article abstract of DOI:10.1039/c3ra47923d

A biphasic system consisting of THF and water was studied to achieve the integrated conversion of cellulose and hemicellulose in lignocellulosic biomass to levulinic acid. As compared to previous studies using GVL as solvent, the utilization of a lower boiling point solvent, THF, also achieves the simultaneous hydrolysis of C₆ and C₅ carbohydrates in lignocellulosic biomass, and the results of simultaneous hydrolysis are comparable. Furthermore, it offers an alternative operation procedure after the hydrolysis. A distillation process is not only used to achieve the effective separation of the solid residue from the desired products, but it also helps in the complete isolation of furfural and formic acid from levulinic acid. Consequently, the utilization of by-product formic acid in the hydrogenation of furfural to furfuryl alcohol is explored, and the process is achieved with both model substrates and the feed from the lignocellulosic biomass feedstock. The hydrolysis of furfuryl alcohol gave C₅ carbohydrate-derived levulinic acid. We finally explored the integrated conversion with five biomass raw materials, and the total yield of levulinic acid was quite obviously promoted by the additional conversion of pentose. the Partner Organisations 2014.

Full text of DOI:10.1039/c3ra47923d

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The breakdown of reticent biomass to soluble
components and their conversion to levulinic acid
as a fuel precursor†
Cite this: RSC Adv., 2014, 4, 14985
*
Jiang Li, Dao-jun Ding, Lu-jiang Xu, Qing-xiang Guo and Yao Fu
A biphasic system consisting of THF and water was studied to achieve the integrated conversion of cellulose
and hemicellulose in lignocellulosic biomass to levulinic acid. As compared to previous studies using GVL as
solvent, the utilization of a lower boiling point solvent, THF, also achieves the simultaneous hydrolysis of C6
and C5 carbohydrates in lignocellulosic biomass, and the results of simultaneous hydrolysis are comparable.
Furthermore, it offers an alternative operation procedure after the hydrolysis. A distillation process is not
only used to achieve the effective separation of the solid residue from the desired products, but it also
helps in the complete isolation of furfural and formic acid from levulinic acid. Consequently, the
utilization of by-product formic acid in the hydrogenation of furfural to furfuryl alcohol is explored, and
the process is achieved with both model substrates and the feed from the lignocellulosic biomass
feedstock. The hydrolysis of furfuryl alcohol gave C5 carbohydrate-derived levulinic acid. We finally
explored the integrated conversion with five biomass raw materials, and the total yield of levulinic acid
was quite obviously promoted by the additional conversion of pentose.
Received 23rd December 2013
Accepted 11th February 2014
DOI: 10.1039/c3ra47923d
www.rsc.org/advances
solvent system consisting of biomass-derived solvent and
water.14 However, the content of C6 carbohydrates in natural
lignocellulosic biomass is no more than 55 wt%.15 If levulinic
1. Introduction
With the rapid decline of fossil fuel reserves, a growing number
of researchers have started to study the use of lignocellulosic
biomass to replace petroleum and to produce chemicals and
liquid fuels.1,2 Lignocellulosic biomass is mainly composed of
three components, cellulose, hemicellulose and lignin. Cellu-
lose and hemicellulose can be converted to ethanol, 5-hydrox-
ymethylfurfural (HMF),3 2,5-dimethylfuran (DMF),4 levulinic
acid and methyl furan,5 along with other energy platform
molecules. Levulinic acid, as one of twelve top sugar-derived
building blocks, can be converted to fuel additives such as
g-valerolactone (GVL)6–8 and valeric esters.9 Furthermore, levu-
linic acid-derived GVL can be converted to liquid alkenes in the
transportation fuel range.10 Thus, levulinic acid is considered as
a chemical bridge to connect biomass and petroleum.11
In the literature, the direct hydrolysis of biomass-derived C6
carbohydrates into levulinic acid has been studied as the main
pathway for generating levulinic acid from biomass, with
5-hydroxymethylfurfural as the intermediate.12 In this process,
the dehydration of C6 sugars such as glucose, starch or cellulose
is achieved with Brønsted acid or solid acid catalysts.13 The
reaction media could be water, organic solvent or a mixed
acid was only produced from C6 carbohydrates, the energy
efficiency would be only 22.0% in this levulinic acid-based
biomass to biofuel strategy.16 Recently, an integrated conversion
of both C6 and C5 carbohydrates in biomass to levulinic acid
was reported by Dumesic et al.17 The reaction was conducted in
a mixed solvent of biomass-derived GVL and water. The
hydrolysis of C5 carbohydrates to furfural and C6 carbohydrates
to levulinic acid occurred simultaneously. Then furfural was
hydrogenated to furfuryl alcohol, which was nally hydrolyzed
to levulinic acid. The additional conversion of C5 carbohydrates
to levulinic acid effectively improved the carbon utilization and
energy efficiency in the levulinic acid-based biomass to biofuel
strategy, and the solvent system consisting of GVL and water
was the key issue in this work. We proposed that more solvent
systems should be studied to further explore the advantages in
the integrated conversion.
Herein, we report the integrated conversion of both C6 and
C5 carbohydrates in lignocellulosic biomass to levulinic acid in
a biphasic system consisting of THF and water. THF could be
considered as a biomass-derived solvent, and it could be
produced from furfural by the combination of decarbonylation
and ring hydrogenation.18 Furthermore, the biphasic system of
THF and water has been used in many biomass conversion
processes, such as the dehydration of hexose and pentose to
HMF and furfural, respectively.19 Based on these previous
works, we explored the integrated conversion of both C6 and C5
Anhui Province Key Laboratory of Biomass Clean Energy, Department of Chemistry,
University of Science and Technology of China, Hefei 230026, China. E-mail:
fuyao@ustc.edu.cn; Fax: +86-551-63606689
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra47923d
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carbohydrates to levulinic acid in the biphasic system of THF crushed, screened to select the fraction of particles with a size
and water for the rst time. As shown in Scheme 1, C6 and C5 less than 0.3 mm. The dilute acid pretreatment was not con-
carbohydrates in biomass were rst hydrolyzed in one step to ducted in the conversion of biomass feedstocks. We carried out
levulinic acid + formic acid and furfural, respectively. This experiments to check the feedstock compositions and we
hydrolysis process was rstly investigated with model quoted the prices of biomass feedstocks which were reported by
substrates, then with raw biomass materials, and the results Dumesic et al.16a (see Table 1).
were comparable to values observed in the solvent system of
2.1.2 Analysis
GVL and water. However, as compared to GVL, the lower boiling
(a) Analysis of the liquid product. The quantitative analysis of
ꢀ
point of THF (66 C) allowed the product solution to be easily levulinic acid and formic acid was performed on a Waters 1515
separated into three fractions by one-step distillation. Lignin series HPLC (250*4.6 Agilent HC-C18 column) with a RI detector
and humins were precipitated as a solid residue. At the same using a 5 mM aqueous sulfuric acid solution as eluent. The total
time, furfural, formic acid, water and THF were isolated in the yield of levulinic acid in the biphasic system was calculated by
gaseous fraction, and levulinic acid, catalyst acid, and a little the yield of levulinic acid in the water phase plus the yield in the
water remained in the liquid fraction. There are two advantages organic phase. To determine the yields, the solution was diluted
of our separation step. Firstly, it was easy to operate and the 10 times in water, and then transferred to the HPLC.
solid residue could be isolated from the desired product effec-
The concentration of furfural was detected by HPLC using
tively. Secondly, aer the complete isolation of furfural and the external standard method. HPLC was performed on a
formic acid from levulinic acid, we could explore the utilization Hitachi L-2000 HPLC system equipped with two L-2130 pumps
of by-product formic acid in the hydrogenation of furfural to and an L-2455 photodiode array detector. Furfural was analyzed
furfuryl alcohol without any interference. We initially investi- by reversed-phase chromatography on an Alltech C18 packed
gated the model hydrogenation of furfural with formic acid, and column (250 ꢁ 4.6 mm, Alltech) at a column temperature of
ꢀ
then the hydrogenation of a feed from lignocellulosic biomass 30 C. The ow of the mobile phase was methanol and water
was also achieved. Furfuryl alcohol was nally hydrolyzed to (methanol–water ¼ 20 : 80) at a ow rate of 1.0 ml minꢂ1. The
give C5 carbohydrates-derived levulinic acid, and the total yield detection wavelength of furfural was 277 nm. The total yield of
of levulinic acid was quite obviously promoted by the additional furfural was also calculated by the yield of furfural in the water
conversion of pentose. Five biomass raw materials were inves- phase plus the yield in the organic phase.
tigated in our process, and the highest mass yield of levulinic
(b) Analysis of the water content. The analysis of the water
acid that could be attained was 27.7 wt% with the conversion of content was performed on a Karl Fischer Moisture Titrator, and
corn cob.
each sample was analyzed three times.
(c) Analysis of the solid product. Aer ltration, the solid
residue was collected and dried at 60 ꢀC in air. The solid residue
was weighed and its SEM image is shown in Fig. S2.†
(d) Calculation of the products. The yields of furfural and
levulinic acid in the model hydrolysis were dened as follows:
2. Experimental
2.1 Materials and analysis
2.1.1 List of chemicals. Xylan from birch wood (xylose
residue >90%, Sigma-Aldrich), cellulose (Alfa Aesar), levulinic
acid (99%, Aladdin Reagent Co. Ltd.), hydrochloric acid, formic
acid, sodium chloride, furfural, glucose and xylose (Sinopharm
Chemical Reagent Co. Ltd.), furfuryl alcohol (98%, Aladdin
Reagent Co. Ltd.). THF was purchased from Sinopharm
Chemical Reagent Co. Ltd., and anhydrous THF was obtained
by reuxing THF with sodium. The anhydrous THF was used
when studying the effect of water content in the solvent.
The biomass feedstocks used in this work were collected
from Hefei in China. Each biomass feedstock was air-dried,
Yield ¼ (moles of furfural/levulinic acid produced)/(moles of
sugar units in initial xylan/cellulose) ꢁ 100%
All the yields of furfural and levulinic acid aer the hydro-
lysis of lignocellulosic biomass were detected aer the distilla-
tion, and they were dened as follows. The weight of
hemicellulose was considered as the weight of total pentose,
and the weight of cellulose was considered as the weight of total
hexose.
Scheme 1 Illustration of the integrated conversion of C6 carbohydrates and C5 carbohydrates in lignocellulosic biomass to levulinic acid in a
biphasic solvent system consisting of THF and water.
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Table 1 Compositions (in wt%) and prices of biomass feedstocks used in our research
Corn stover
Bagasse
Poplar
Pine
52.82
10.6
29.50
7.7
Corn cob
Cellulose
Hemicellulose
Lignin
36.00
21.36
18.67
23.97
100
41.27
21.91
17.32
19.5
100
40.00
48.15
17.25
27.17
7.43
100
50.70
36.21
37.16
7.05
19.58
100
Others
Total
100
57.32
Price16a $ per dry ton
83.00
—
hydrate, sodium lauryl sulphate, 2-ethoxyethanol, disodium
hydrogen phosphate, decahydronapthalene and sodium sul-
phite. Following this, the remaining solid was ltered,
washed and then incubated with an amylase solution over-
night at 37 ꢀC. Aer incubation, the residue was washed with
acetone and dried at 105 ꢀC for 4 hours. The nal residue was
weighed. The NDF is the percent of the original material that
is residue. The acid detergent ber (ADF) method determines
the cellulose plus lignin content and consequently, the
difference between NDF and ADF provides a determination of
hemicellulose. The ADF method involves the autoclaving of
Molar yield of furfural ¼ (weight of furfural after distillation)/
(weight of hemicellulose in biomass raw material) ꢁ 132/96
ꢁ 100%
Molar yield of levulinic acid ¼ (weight of levulinic acid after
distillation)/(weight of cellulose in biomass raw material) ꢁ 162/
116 ꢁ 100%
Mass yield ¼ (weight of furfural/levulinic acid after distillation)/
(weight of biomass raw material) ꢁ 100%
2.2 General procedure for detecting the composition of
lignocellulosic biomass
the sample in
a solution of cetyltrimethylammonium
bromide, sulphuric acid, and decahydronapthalene at 121 ꢀC
for 15 minutes. The residue is ltered, washed, rinsed with
acetone and then dried at 105 C for 4 hours. The ADF is the
From every crushed biomass feedstock at least 200 g was
collected to ensure the composition of biomass feedstock was
similar in each experiment. The composition of the ligno-
cellulosic biomass used in our experiments was determined
by the method reported in the literature,20 and the results are
listed in Table 1. The methods are based on the use of neutral
and acid detergents for the removal of soluble carbohydrates,
proteins and tannins. The neutral detergent ber (NDF)
method provides a measure of the total cell-wall material
(cellulose, hemicellulose and lignin). In this test, a lignocel-
lulosic biomass sample of known weight was autoclaved for
15 minutes at 121 ꢀC in a neutral solution containing
ethylene-diamine tetra acetate dihydrate, sodium borate deca-
ꢀ
percentage of the original material that is residue. Lignin may
be found separately in the ADF fraction by removal of lignin
by permanganate oxidation. This is called the acid detergent
lignin (ADL) method. The residue from the ADF method is
digested with 72% sulphuric acid for 3 hours and then
washed, dried and weighed. The sample is then ashed at 550
ꢀC for 3 hours and then weighed again. The ADL is the change
in weight during ashing as a percentage of the original straw
sample. Hemicellulose content is found by the difference
between NDF and ADF and the cellulose content is the
difference between ADF and ADL.
Table 2 Model investigation of one-step acid hydrolysis of cellulose and xylana
Yield [mol%]
Entry
T1 [ꢀC]
t1 [min]
T2 [ꢀC]
t2 [min]
NaCl [wt%]
Vorg./Vaq.
Levulinic acidd
Furfurale
1
2
3
4
5
6
7
8
110
110
110
110
110
—
110
110
110
110
110
110
110
40
40
40
40
40
—
40
40
40
40
40
40
40
200
200
200
200
200
200
200
200
220
180
200
200
200
20
20
20
20
20
20
10
40
20
20
20
20
20
—
5
0
47.3
0
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
1 : 1
2 : 1
53.8(1.625)
58.8(2.885)
48.5(4.865)
47.2(7.13)
57.0
47.1
53.8
49.1
46.0
42.0(2.735)
58.6(5.493)
59.2(11.19)
65.1(27.36)
47.5
52.5
44.1
37.8
46.6
10
20
30
10
10
10
10
10
10
10
10
9
10
11b
12
13c
44.2
51.5
45.4
52.7
26.5
59.0
a
Reaction conditions: 2 g cellulose, 1 g xylan, a 0.8 M hydrochloric acid solution and THF (water + THF ¼ 60 ml), were stirred in a 150 ml autoclave
made of zirconium alloy. b Use of 0.4 M HCl. c Use of 0.8 M H2SO4. d The value in parentheses is the partition coefficient of HMF. e The value in
parentheses is the partition coefficient of furfural.
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Table 3 Investigation of one-step acid hydrolysis of lignocellulosic biomassa
Yield [mol%]
Entry
Substrate
HCl [M]
T1 [ꢀC]
t1 [min]
T2 [ꢀC]
t2 [min]
NaCl [wt%]
Vorg./Vaq.
Levulinic acid
Furfural
1
2
3
4
Xyloseb
Bagasse
Bagasse
Bagasse
0.8
0.4
0.4
0.4
—
—
40
40
40
200
200
200
210
20
20
20
20
10
10
10
10
2 : 1
2 : 1
2 : 1
2 : 1
—
76.9
40.1
66.3
60.9
110
110
110
58.5
44.3
55.6
a
Reaction conditions: 4 g bagasse, a hydrochloric acid solution and THF (water + THF ¼ 60 mL), was stirred in a 150 mL autoclave made of
zirconium alloy. b The substrate is 1 g xylose.
temperature of the autoclave was elevated to the target
temperature in 35 min. Aer the reaction was nished, the
autoclave was cooled to room temperature with ice. The product
solution was transferred to the distillation step.
2.3 General procedure for model reactions
2.3.1 Model hydrolysis. For typical runs, cellulose (2 g),
xylan (1 g) and NaCl (mass depends on the reaction condition)
was loaded in a 150 ml Zr alloy autoclave, then a hydrochloric
acid aqueous solution (20 ml) and THF (40 ml) were added to
the autoclave. The autoclave was ushed with nitrogen, and the
pressure of the remaining nitrogen was 0.5 MPa. The autoclave
was stirred at 850 rpm, and the temperature of the autoclave
was elevated to the target temperature in 35 min. Aer the
completion of reaction, the autoclave was cooled to room
temperature with ice. The yields of levulinic acid and furfural
were determined by HPLC.
2.3.2 Model distillation. For model distillation, levulinic
acid (1 g), furfural (0.5 g), formic acid (0.5 g) were added into a
solvent system consisting of water (20 ml) and THF (40 ml). The
weights of these chemicals were close to the values in the
product solution aer the hydrolysis of lignocellulosic biomass.
During the distillation, furfural and formic acid were easily
removed with THF and water, and then they were collected in a
cold trap. Levulinic acid was kept in the solution. Furfural and
formic acid could be removed completely with distillation at
50 ꢀC, leaving less than 1% furfural and formic acid in the
solution.
2.4.2 One-step distillation of the product solution. The
operation of the distillation of the solution aer the hydrolysis
of the lignocellulosic biomass was the same as the model
distillation. Furfural and formic acid could be removed with
THF and water during the distillation, while lignin and humins
were precipitated simultaneously to be easily isolated from the
levulinic acid solution by ltration. The solution of furfural and
formic acid was further converted to levulinic acid solution.
2.4.3 Conversion of furfural and formic acid to furfuryl
alcohol. The optimum solvent in the model hydrogenation was
the mixture of water and THF (1 : 19). However, aer distilla-
tion, the water content in the gaseous fraction was much higher
than this value. An extraction process was needed to decrease
the water content. For the extraction operation, NaCl (the ratio
of it to biomass feedstock was 2 : 3) was rstly added to the
distilled solution to saturate the water phase. Then the water
phase was further extracted twice, so that the water content in
the extracted organic phase was just 5% (detected by Karl
Fischer Moisture Titrator). Furfural was extracted completely
into the organic phase, and the extraction efficiency of formic
acid was about 60%. Aer the adjustment of the water content,
additional formic acid was added to achieve a 1 : 2 molar ratio
of furfural to formic acid. Then the hydrogenation of furfural by
formic acid was carried out and the reaction conditions were
similar to the model hydrogenation.
2.3.3 Model hydrogenation of furfural with formic acid.
The model hydrogenation was carried out to determine the
optimum hydrogenation conditions (Table 3). All the catalytic
experiments were carried out in a 50 ml Parr autoclave made of
zirconium alloy. Furfural, formic acid, catalyst and solvent were
rstly added to the autoclave. Then N2 was used to exclude the
air. The mixture of substrates and catalyst was heated to the
desired temperature in 20 min with vigorous stirring.
2.4.4 Hydrolysis of furfuryl alcohol to levulinic acid. The
hydrolysis of furfuryl alcohol to levulinic acid could be easily
achieved by reuxing furfuryl alcohol with hydrochloric acid in
a mixed solvent of THF and water, and the yield of levulinic acid
was about 80%.
2.4 General procedure for integrated conversion of
lignocellulosic biomass in the biphasic system
While mixing the furfuryl alcohol solution from step c and
The integrated conversion of lignocellulosic biomass to levu- the levulinic acid aqueous solution from step b, the hydrolysis
linic acid in our research contained four main steps. of furfuryl alcohol occurred with heating. The supplement of
2.4.1 One-step hydrolysis of lignocellulosic biomass to catalyst hydrochloric acid was needed because some hydro-
levulinic acid and furfural. For typical runs, lignocellulosic chloric acid was removed out in the distillation step, and the
biomass feedstock (4 g), NaCl (2 g), 0.4 M hydrochloric acid levulinic acid yield attained could reach 75% in this step.
aqueous solution (20 ml), and THF (40 ml) were added to a
The experiment to obtain the total yield of levulinic acid was
150 ml Zr alloy autoclave. Then the autoclave was ushed with carried out on a smaller-scale (1 g biomass feedstock), otherwise
nitrogen, and the pressure of the remaining nitrogen was the solution volume in the hydrogenation of furfural will exceed
0.5 MPa. The autoclave was stirred at 850 rpm, and the 100 ml which cannot be operated in our autoclave at the time.
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Table 4 The results of the hydrogenation of furfural to furfuryl alcohol
with formic acid as the hydrogen sourcea
t
Conversion Yield Selectivity
Entry Additive Catalyst T [ꢀC] [h] [%]
[%]
[%]
1
2
3
4
—
—
Pd/C
Ru/C
Pd/C
Ru/C
Ru/C
Ru/C
Ru/C
Ru/C
Ru/C
Ru/C
Ru/C
90
90
90
90
90
90
90
90
6
6
6
6
6
6
8
6
6
6
6
60.2
11.9
65.8
70.8
99.3
99.8
99.5
74.1
99.2
56.4
99.5
19.1
0.4
8.3
47.9
99.3 100
79.3
94.3
69.4
96.6
42.1
99.0
31.7
3
12.7
67.6
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
5b
6c
7c
8d
9d
10e
11f
79.5
94.7
93.6
97.4
75.7
99.5
110
110
110
a
Typical reaction conditions: furfural (2 mmol), FA (8 mmol), THF
(20 ml), Pd/C or Ru/C (0.4 g), additive was 10 mol% to the formic
b
c
acid. Change the solvent to 19 ml THF and 1 ml H2O. Furfural
d
(10 mmol), FA (40 mmol). Furfural (2 mmol), FA (4 mmol).
e
f
Furfural (2 mmol), FA (2 mmol). Feed from 1 g biomass raw
material, additional 0.5 mmol formic acid was supplemented.
3. Results and discussions
3.1 Model acidic hydrolysis in the biphasic system
We rstly studied the acidic hydrolysis of biomass-derived
carbohydrates. The simultaneous hydrolysis of C6 and C5
carbohydrates in a mixed solvent system of GVL and water has
been reported recently. In our research, we rstly carried out the
model hydrolysis of cellulose and xylan (mass ratio was 2 : 1,
similar to the mass ratio in the lignocellulosic biomass) to
check the occurrence of one-step hydrolysis in our biphasic
system.
Initially, when water was used as the solvent, no furfural was
detected while the yield of levulinic acid achieved 47.3%
(Table 2, entry 1). The absence of furfural was most likely due to
its reduced stability. In the literature, the hydrolysis of pentose
usually occurs under milder reaction conditions (lower reaction
temperature and lower acid concentration), and furfural was
less stable in high reaction temperature.21 When the reaction
conditions were suitable for the generation of levulinic acid, all
the furfural was condensed to humins. Some previous investi-
gations have shown that biphasic systems consisting of organic
solvent and water can effectively stabilize HMF and furfural,
thereby improving the selectivity of HMF and furfural from
carbohydrates.22 However, the simultaneous production of lev-
ulinic acid and furfural from C6 and C5 carbohydrates in
biphasic systems have not yet been reported. As compared to
other common organic solvents, THF showed the best combi-
nation of partition coefficient of HMF in biphasic systems and
HMF selectivity.19a We proposed that THF could also stabilize
furfural to improve the selectivity of furfural, so a biphasic
system consisting of THF and water was chosen in our opti-
mization process. The NaCl salt was added into the water phase
to increase the partition coefficient of unstable furfural, and the
effect of the amount of NaCl was studied (Table 1, entries 2–5).
The yield of furfural increased with the increasing
Fig. 1 The yields of furfural and levulinic acid after the one-step
hydrolysis of baggase (A), poplar (B), corn cob (C), corn stover (D) and
pine (E). Reaction conditions: 4 g biomass raw material, 20 ml aqueous
solution (10 wt% NaCl and 0.4 M HCl), 40 ml THF. T1 ¼ 110 ꢀC, T2
¼
210 ꢀC.
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Table 5 The results of the conversion of various lignocellulosic biomass feedstocks to levulinic acida
Best reaction
time during
hydrolysis
Yield of levulinic
acid from C6
Yield of levulinic
acid from C5
Total yield of
levulinic acid [wt%]
Mass yield of solid
residued [wt%]
Entry
Feedstock
carbohydratesb [wt%]
carbohydratesc [wt%]
1
2
3
4
5
Bagasse
Corn stover
Poplar
Pine
Corn cob
25
15
15
25
25
17.2 (58.1)
16.1 (62.6)
16.1 (46.4)
17.5 (46.2)
16.4 (63.2)
9.1 (58.1)
7.8 (40.3)
5.8 (54.9)
4.5 (42.3)
11.3 (54.9)
26.1
23.9
21.9
22.0
27.7
23.8
24.1
27.3
35.2
16.1
a
All the operation processes for lignocellulosic biomass conversion were described in the Experimental section. b The value in parenthesis is the
molar yield of levulinic acid from C6 carbohydrate in the lignocellulosic biomass. c The value in parenthesis is the molar yield of furfural from C5
carbohydrate in the lignocellulosic biomass. d The solid residue consisted of unreacted lignin and humins generated in the hydrolysis process.
concentration of NaCl. However, the tendency of the yield of of the detection of the hydrolysis product was greatly interfered.
levulinic acid was not the same as the furfural yield, and the Hence, before the investigation of the hydrolysis of lignocellu-
highest yield of levulinic acid was observed with a NaCl content losic biomass, a simple and appropriate method was sought to
of 10 wt%. We found that the increase of NaCl content raised isolate lignin and humins from the production solution which
both the partition coefficient of furfural and HMF (the data was was also a necessary step in the process of lignocellulosic
shown in Table 2). Therefore, the yield of furfural increased biomass conversion.
while more furfural was extracted to the organic phase.
However, while more HMF was present in the organic phase, azeotrope with water, and the boiling points were 97.5 C and
difficulties surrounding the contact of HMF with the acid 107.3 C, respectively. These boiling points were far below the
We noticed that both furfural and formic acid could form an
ꢀ
ꢀ
catalyst in the water phase led to a decrease in the yield of boiling point of levulinic acid (b.p. ¼ 246 ꢀC), so we tried to
levulinic acid. By considering both the yields of furfural and separate the product solution by vacuum distillation at a low
levulinic acid, we selected a NaCl content of 10 wt% as the temperature. The model distillation of the mixture of levulinic
optimized concentration of the salt. Both the yields of furfural acid, formic acid and furfural was rstly carried out, and more
and levulinic acid were nearly 60%. The reaction temperature detailed conditions are shown in the Experimental section.
also had a great impact on the hydrolysis yield. The hydrolysis During the distillation, furfural and formic acid were easily
should be performed at 110 ꢀC for 40 minutes to achieve a separated with THF and water, and levulinic acid remained in
higher yield of furfural. This was most likely due to a pre- the solution. While the distillation temperature achieved 50 ꢀC,
depolymerization of xylan to xylose at this temperature (Table 2, furfural and formic acid could be removed completely from
entries 3 and 6). The best nal hydrolysis temperature was 200 levulinic acid (less than 1% furfural and formic acid remained
ꢀC, and the optimized reaction time was 20 min (Table 2, entries in the liquid fraction).
7–10). A decrease in the concentration of the acid and the mass
The distillation of the feed from the hydrolysis of lignocel-
ratio of the organic phase to water phase led to a reduction of lulosic biomass was also carried out, and bagasse was rstly
the yields of the hydrolysis products (Table 2, entries 11 and 12). studied as the biomass feedstock in this section. Before the
The use of sulphuric acid gave an obviously lower yield of lev- distillation, the weights of the hydrolysis products were detec-
ulinic acid. Overall, we demonstrated that the one-step hydro- ted, furfural (0.414 g), formic acid (0.412 g) and levulinic acid
lysis of xylan and cellulose could also be achieve in the biphasic (0.635 g). It should be noted that in the detection step, due to
system, and the molar yield of levulinic acid was 58.8% while the interference of solid residues, the detection results of the
the furfural molar yield was 58.6%. These results were compa- hydrolysis products varied over a large range (more than 10%)
rable to values observed in the solvent system of GVL and water. even with the same sample. The result we gave was just one of
them. Aer distillation, the product solution was separated into
three fractions at a distillation temperature below 50 ꢀC. The
gaseous fraction contained furfural (0.369 g), formic acid
3.2 Distillation of product solution
The results of the model hydrolysis of cellulose and xylan (0.444 g), THF and water. The liquid fraction contained levulinic
provided the possibility of the one-step hydrolysis of lignocel- acid (0.686 g) and the catalyst acid with less than 1% (about
lulosic biomass in the biphasic system. When we turned to 0.5%) furfural and formic acid remaining, and the last solid
study the hydrolysis of the lignocellulosic biomass, we found it fraction was the solid residue consisting of lignin and humans.
was more complex than the model reaction because of its The distillation process could be reproduced successfully.
complex structure which comprised of cellulose, hemicellulose Though the detection of hydrolysis products before distillation
and lignin. In addition, though the unreacted lignin and the by- is not precise due to interference, the comparison of these
product humins dissolved very well in the organic phase aer results partially demonstrated that the one-step distillation was
the hydrolysis process, they were precipitated immediately with also achieved with the feed from biomass raw materials. Over-
the addition of water at the detection step, and the precision all, aer the simultaneous hydrolysis, a one-step distillation not
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only effectively separated lignin and humins from the desired acid to 1 equivalent led to a decrease in both the conversion of
products, but it also achieved the isolation of furfural and for- furfural and the selectivity of furfuryl alcohol.
mic acid from levulinic acid.
Aer the model hydrogenation, we studied the hydrogena-
tion of furfural in the feed from lignocellulosic biomass
conversion. In the model hydrogenation, the best solvent was a
mixed solvent of THF and water (v/v ¼ 19 : 1). However, aer
distillation, the water content in the gaseous fraction was
higher than this value (nearly 20%). Therefore, the hydroge-
nation process was depressed, and both the conversion of
furfural and the selectivity of furfuryl alcohol were very low. To
solve this problem, an extra extraction operation was added
before the hydrogenation process of the feed from biomass.
Aer the extraction operation, furfural was completely
extracted into the organic phase, and the water content was
successfully decreased to 5%. However, the extraction effi-
ciency of formic acid was not very good (about 60%), so a
supplement of formic acid was needed. Our previous work had
shown the possibility of the production of formic acid from
biomass-based carbohydrates,23 so the supplementary formic
acid could also be considered as biomass-derived. Finally, the
hydrogenation of the feed from biomass raw material was
successfully achieved (Table 4, entry 11), the selectivity of
furfuryl alcohol was 99.5% with a 99.5% conversion of furfural
under the same reaction conditions as the model reaction. The
reaction solution initially contained about 1 mmol furfural
and 1.5 mmol formic acid, and an additional 0.5 mmol formic
acid was supplemented. This hydrogenation process was not
so effective yet further improvements in the catalytic efficiency
are on the way.
3.3 One-step hydrolysis of biomass raw materials
The effective isolation of lignin and humins led to a feasible
detection of the hydrolysis products from lignocellulosic
biomass, and we started to optimize the reaction conditions
during biomass hydrolysis. In this section, the detection of
hydrolysis products was achieved aer the distillation, and only
acetic acid was observed as a by-product (less than 5 wt%).
Bagasse was rstly used as a substrate. In contrast to the model
hydrolysis, we found that a lower HCl concentration (0.4 M) and
ꢀ
a higher reaction temperature (210 C) were favourable for the
hydrolysis of lignocellulosic biomass, and the results are listed
in Table 3. Under the above reaction conditions, the yields of
furfural and levulinic acid versus reaction time are shown in
Fig. 1. The yield of furfural decreased with the increase of
reaction time, but the yield of levulinic acid uctuated with the
reaction time. We choose 25 min as the best reaction time and
both the molar yields of levulinic acid and furfural were about
58%. The molar yield of formic acid and levulinic acid were
about 1.1 : 1 in the hydrolysis of C6 carbohydrates, but in our
research we found that the ratio of formic acid to levulinic acid
was about 1.6 : 1. The extra portion of formic acid was most
likely due to the hydrolysis of the formyl group in hemicellulose.
The mass yield of the solid residue was 19.12 wt%. Finally, the
hydrolysis conditions were further applied to the conversion of
other lignocellulosic biomass feedstocks and the results were
are shown in Fig. 1. These results demonstrated that lignocel-
lulosic biomass could undergo one-step hydrolysis in the
biphasic system.
3.5 Hydrolysis of furfuryl alcohol and integrated conversion
of lignocellulosic biomass
Aer the optimization of the model hydrogenation, the
hydrolysis of furfuryl alcohol was studied. This reaction could
3.4 Hydrogenation of furfural with formic acid
Aer the optimization of the hydrolysis reaction conditions, we be easily achieved by reuxing furfuryl alcohol and hydro-
started to study the conversion of furfural in the gaseous frac- chloric acid in a mixed solvent of THF and water, and the yield
tion (aer distillation) to furfuryl alcohol through a Ru-cata- of levulinic acid could reach 80%. The hydrolysis of furfuryl
lyzed process. The gaseous fraction aer distillation was a alcohol from lignocellulosic biomass was achieved by mixing
mixed solution of furfural, formic acid, THF and water, and the the furfuryl alcohol solution (from section 3.4) with the liquid
hydrogenation of furfural to furfuryl alcohol with formic acid as fraction aer distillation (from section 3.2). We found that a
hydrogen source was investigated. A model hydrogenation was supplement of catalyst acid (0.1 g, half of initial value) was
rstly studied, and a mixed solvent system of THF and water needed to achieve the hydrolysis and the yield of levulinic acid
was used as the solvent (see Table 4). The hydrogenation of achieved was 75%.
furfural with 4 equivalents formic acid was used to explore the
Finally, we studied the integrated conversion of ve biomass
optimized ratio of water to THF. While the ratio of water to THF feedstocks to levulinic acid. The process started from 1 g
was 1 : 19 (the effect of water content in the solution is shown in biomass raw material. The total mass yield of levulinic acid
Fig. S1†), the 100% selectivity of furfuryl alcohol was achieved from bagasse was 26.1%, and this yield was much higher
with 99.3% conversion of furfural, and 10 mol% NaOH was than the yield of levulinic acid from only C6 carbohydrates
added in the solution (Table 4, entry 5). However, in our (17.2 wt%). The extension of this process to other lignocellu-
hydrolysis process, the molar ratio of formic acid and furfural in losic biomass feedstocks was studied, and the results were lis-
the hydrolysis solution was generally about 2 : 1, so the amount ted in Table 5. The biggest improvement in the yield of levulinic
of formic acid was further reduced to 2 equivalents of furfural in acid was achieved for the conversion of corn cob (16.4 wt% to
the model hydrogenation. Aer optimizing the reaction condi- 28.3 wt%). However, the molar yields of levulinic acid from C6
tions, we found that the selectivity of furfuryl alcohol could carbohydrates in poplar and pine were only 46.4% and 46.2%,
reach 97.4%, while the conversion of furfural was 99.2% and these results may be due to the difficulty associated with the
(Table 4, entry 9). Further decreases in the amount of formic depolymerization of the wood structure.
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Overall, we have reported the integrated conversion of ligno-
cellulosic biomass to levulinic acid in a biphasic system con-
sisting of THF and NaCl aqueous solution. The one-step
hydrolysis of C6 and C5 carbohydrates in the lignocellulosic
biomass was rstly achieved to give a product solution that
contained furfural, formic acid, levulinic acid and lignin, and
the hydrolysis process was operated with both model substrates
and biomass raw materials. The yields of the hydrolysis prod-
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any interference. The hydrogenation process was eventually
achieved and the product furfuryl alcohol was nally hydrolyzed
to levulinic acid to achieve the integrated conversion of ligno-
cellulosic biomass to levulinic acid. The highest mass yield of
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the additional conversion of the hemicellulose fraction. Thus,
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achieves the simultaneous hydrolysis of C6 and C5 carbohy-
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also offers an alternative operation procedure for the integrated
conversion of biomass to levulinic acid.
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Acknowledgements
The authors are grateful to the National Basic Research Program
of China (2012CB215305, 2013CB228103), NSFC (21325208,
21361140372, 21172209), FRFCU (WK2060190025), SRFDP
(20123402130008), CAS (KJCX2-EW-J02) and Fok Ying Tung
Education Foundation for nancial support.
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14992 | RSC Adv., 2014, 4, 14985–14992
This journal is © The Royal Society of Chemistry 2014