High-yield production of levulinic acid from cellulose and its upgrading to γ-valerolactone

Article abstract of DOI:10.1039/c4gc00737a

Direct catalytic conversion of cellulose to levulinic acid (LA) by niobium-based solid acids and further upgrading to γ-valerolactone (GVL) on a Ru/C catalyst were realized through sequential reactions in a reactor. Firstly, using aluminium-modified mesoporous niobium phosphate as a catalyst, cellulose can be directly converted to LA with as high as 52.9% yield in aqueous solution, even in the presence of the Ru/C catalyst. To the best of our knowledge, this is the best result over a heterogeneous catalyst so far. It was found that the type of acid (Lewis and Bronsted acids) and acid strength had an influence on the yield of LA; the doping of aluminium can enhance the strong Lewis and Bronsted acids, especially the strong Lewis acid, thus resulting in the increase of LA yield from cellulose as well as from glucose and HMF. Such an enhancement by a Lewis acid on LA yield from HMF was further confirmed by adding lanthanum trifluoroacetate [(TfO)₃La], a strong Lewis acid, in the catalytic system (HCl, (TfO)₃H, niobium phosphate), indicating that a suitable ratio of Lewis/Bronsted acid is important for higher selectivity to LA from HMF, as well as from cellulose. Then, after replacing N₂ with H₂, the generated LA in the reaction mixture can be directly converted to γ-valerolactone through hydrogenation over the Ru/C catalyst without further separation of LA. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00737a

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High-yield production of levulinic acid from
cellulose and its upgrading to γ-valerolactone
Cite this: DOI: 10.1039/c4gc00737a
Daqian Ding, Jianjian Wang, Jinxu Xi, Xiaohui Liu, Guanzhong Lu* and Yanqin Wang*
Direct catalytic conversion of cellulose to levulinic acid (LA) by niobium-based solid acids and further
upgrading to γ-valerolactone (GVL) on a Ru/C catalyst were realized through sequential reactions in a
reactor. Firstly, using aluminium-modified mesoporous niobium phosphate as a catalyst, cellulose can be
directly converted to LA with as high as 52.9% yield in aqueous solution, even in the presence of the Ru/C
catalyst. To the best of our knowledge, this is the best result over a heterogeneous catalyst so far. It was
found that the type of acid (Lewis and Brønsted acids) and acid strength had an influence on the yield of
LA; the doping of aluminium can enhance the strong Lewis and Brønsted acids, especially the strong
Lewis acid, thus resulting in the increase of LA yield from cellulose as well as from glucose and HMF. Such
an enhancement by a Lewis acid on LA yield from HMF was further confirmed by adding lanthanum
trifluoroacetate [(TfO)3La], a strong Lewis acid, in the catalytic system (HCl, (TfO)3H, niobium phosphate),
indicating that a suitable ratio of Lewis/Brønsted acid is important for higher selectivity to LA from HMF, as
well as from cellulose. Then, after replacing N2 with H2, the generated LA in the reaction mixture can
be directly converted to γ-valerolactone through hydrogenation over the Ru/C catalyst without further
separation of LA.
Received 24th April 2014,
Accepted 1st June 2014
DOI: 10.1039/c4gc00737a
www.rsc.org/greenchem
Generally, LA is produced from biomass under acidic con-
ditions or with acid catalysts via two approaches: furfuralcohol
Introduction
With the deterioration of the environment and the inevitable hydrolysis10 and direct biomass conversion.11 In the second
depletion of fossil resources, renewable and abundant approach, the feedstocks could be glucose, starch or cellulose.
biomass is considered to be a promising alternative to non- As the cheapest and most abundant feedstock, cellulose, com-
renewable natural resources for sustainable biofuels and bio- posed of glucose units linked by β-1,4-glucosidic bonds, was
chemicals in the future.1–3 Recently, extensive research studies firstly converted into water-soluble glucose with acidic
have focused on the conversion of biomass into fuels and catalysts, then the generated glucose was dehydrated into
chemicals. Among these explorations, an approach for convert- 5-hydroxymethylfurfural (HMF), and subsequently the formed
ing carbohydrate to levulinic acid (LA) is very important, HMF was further converted into LA and formic acid through
because as one of the 12 high-potential and versatile chemical the hydration and rearrangement process under the same
platform compounds,4 LA, as well as its derivatives, is widely acidic conditions (Scheme 1).
used in medicine, agriculture, food, cosmetics, and spice
The chemical degradation of cellulose has been investi-
industries, and biofuels probably in future. For example, levu- gated extensively,12 and many of the studies were conducted
linate esters can be used as solvents in industrial extraction, in strongly acidic aqueous solutions13–17 or in the presence of
spices in cosmetics and medicine for injection.5,6 Moreover, ionic liquids,18–20 due to the robust structure of cellulose.
γ-valerolactone (GVL), produced through selective hydrogen- Therefore, preparation of the solid acid catalyst with higher
ation of LA or levulinate esters, is also considered as an impor- activities under mild conditions is meaningful. Recent reports
tant platform chemical and can be used as food additive, drug, have reported a series of acid catalysts that may be effective in
solvent and the precursor of methyl-tetrahydrofuran, which cellulose hydrolysis, such as metal salts, sulfonated carbon,
is used as a very popular fuel additive currently being (layered transition) metal oxides, heteropoly acid, sulfonated
investigated.7–9
silica nano/meso composites, niobium phosphate, and so
on.21–46 However, in most of these studies, sugars or water-
soluble oligosaccharides, but not cellulose, were used as sub-
strates for the production of LA.
Previous studies showed that the type of acid (Brønsted and
Lewis acids, as well their ratio), acid amount and acid strength
Key Lab for Advanced Materials, Research Institute of Industrial Catalysis East China
University of Science and Technology, # 130, Meilong Road, Shanghai, 200237,
P. R. China. E-mail: gzhlu@ecust.edu.cn, wangyanqin@ecust.edu.cn
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concentration, similar to the results obtained using glucose
and HMF as the substrates.15,16 V. Ordomsky et al.44 compared
the performances of a series of solid acid catalysts and found
that higher acid amount leads to higher LA selectivity when
using solid acids as catalysts.
Subsequently, the hydrogenation of LA to GVL is convenient
and 100% yield of GVL, even on the industrial scale, can be
easily achieved in the presence of the Ru/C catalyst. S. G.
Wettstein et al.9 have reported that a high GVL yield of 75%
was achieved from cellulose in a water–GVL biphasic system
using HCl and Ru–Sn/C as hydrolysis and hydrogenation cata-
lysts, respectively. This approach for cellulose conversion elimi-
nates the need to separate the final product from the solvent,
because the GVL product is the solvent.
In our previous studies, mesoporous niobium phosphate
exhibited excellent performances in the conversion of fructose,
xylose, and even cellulose in pure water and a biphasic system
under mild conditions because of its suitable Lewis
and Brønsted acidity, as well as its hot-water-tolerant prop-
erty.33,38,47,48 Herein, we continue our study to investigate the
catalytic performance of mesoporous niobium phosphate in
the direct conversion of cellulose to LA and the influence of
modification of the acidic properties by doping with alumi-
num on the selectivity of LA. Besides, the direct upgrading of
the generated LA in the aqueous solution into GVL is investi-
gated as well.
Scheme 1 The reaction pathway of converting cellulose to levulinic
acid and GVL in water.
in solid catalysts had an influence on the selectivity to LA from
cellulose or glucose conversion. F. Chambon et al.25 investi-
gated the performance of a series of heterogeneous solid acid
catalysts in the hydrolysis of cellulose in hot water and found
that the catalyst containing much more Lewis acid, such as
ZrW and AlW, had higher cellulose hydrolysis level, but the
main product was lactic acid. The highest LA yield was
achieved over sulfated zirconia (ZrS), which has both Brønsted
and Lewis acid sites. These results indicate that large amounts
of solid Lewis acids can promote cellulose depolymerisation,
and Brønsted acid would enhance LA production.
V. Choudhary et al.39 also found that Lewis acid, which is
CrCl3 in their research, would speed up the isomerisation of
glucose, but its selectivity to HMF and LA was low when used
by itself. Thus, the interplay of Lewis and Brønsted acids is
important for the conversion of cellulose to LA, and excellent
yield may be obtained by using a solid acid with suitable Lewis
and Brønsted acidity.
Experimental section
Chemicals
All chemicals were purchased from Sinopharm Chemical
Reagent Co. Ltd, except cellulose and the Nb precursor,
niobium tartrate; the former was bought from Fluka Analytical
Co. Ltd and the latter was prepared in our laboratory according
to the literature.32,33 All purchased chemicals were of analytical
grade and used without further purification.
R. Weingarten et al.42 deeply investigated the selectivity of
HMF and LA with solid metal(IV) phosphate catalysts and
liquid acids by controlling the acid concentration and glucose
conversion in the same level, and adjusting the molar ratio of
B/L acid from 0 to 1. It was also found that Lewis acid would
speed up the conversion of glucose, while the selectivity to
HMF and LA is low. The LA selectivity had an obvious
enhancement when the molar ratio of B/L acid was adjusted
from 1 : 3 to 1 : 1 using homogeneous or solid acid catalysts.
These results are similar to those of F. Chambon et al.,25
giving us the enlightenment of designing the solid acid cata-
lyst with strong acid sites and a suitable B/L acid molar ratio.
In another method, the kinetic model of cellulose hydro-
lysis to LA was studied using hydrochloric acid by J. C. Shen
et al.14 and using sulfuric acid by B. Girisuta et al.13 Both of
the research studies made the conclusion that the reaction of
cellulose hydrolysis needs severe conditions such as higher
temperature, higher acid concentration or lower substrate
Catalysts preparation
Mesoporous niobium phosphate was synthesized according to
our previous report.33 Typically, 0.01 mol of diammonium
hydrogen phosphate was dissolved in 20 mL water and
adjusted to pH 2 using phosphoric acid; then 20 mL of 0.5 M
niobium tartrate (pH = 2) was added to the above solution
with stirring. The mixed solution was slowly poured into
the previously prepared aqueous solution of cetyltrimethyl
ammonium bromide (CTAB), which contains 1.0 g of CTAB in
13 mL water, and the final pH was around 2. A large amount
of niobium-phosphorus precipitation appeared immediately
from the solution, and the mixture was stirred for an
additional 60 min at 35 °C, and then transferred into a Teflon-
lined autoclave and aged at 160 °C for 24 h. The solid was fil-
tered after cooling down, washed with distilled water and
dried at 100 °C overnight. Finally, the catalyst was obtained by
calcination at 500 °C for 5 h in air to remove organics. The syn-
thesis of aluminium-modified porous niobium phosphate was
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similar to the above procedure except for the addition of a cal- stirring. N2 was used as a protective gas and kept at a pressure
culated amount of aluminium precursor, which was prepared of about 0.8 MPa. The zero time was taken when the Teflon-
by dissolving aluminium hydroxide in 10 mL of 1.5 M oxalic lined stainless steel autoclave was placed in the heating
acid solution.
mantle under magnetic stirring, which had already been
heated to the set temperature. After reaction, the catalyst was
collected by centrifugation and washed with water and ethanol
several times, and then dried at 100 °C for 24 hours for the
next run. The supernatant liquid in the centrifugation tube
was analyzed by an Agilent 1200 Series HPLC based on the
external standard method. All analytical data were the averages
from three repeated reactions, and the error was less than 1%
compared with the average data. The soluble polymer in the
liquid was separated by evaporation of water at 100 °C, and the
insoluble humins were obtained by filtering. Together with the
unused and used catalysts, the soluble polymer and insoluble
humins were weighed and analyzed by EDS to determine the
element content.
In the conversion of cellulose to GVL, the mixture of 0.4 g
solid acid catalyst, 0.04 g Ru/C (5%), 0.5 g cellulose and 10 mL
water was loaded into the Teflon-lined stainless steel autoclave
and reacted for 24 h as above; then it was quenched in cool
water and N2 gas in the autoclave was replaced with H2 gas
and the reaction was restarted. The analysis of the product
from the hydrogenation reaction was carried out with the
same HPLC.
The analysis of the reaction products was carried out in a
HPLC apparatus (Agilent 1200 Series) equipped with an ion-
exclusion column Aminex HPX-87H (Bio-Rad), eluting with an
aqueous solution of sulphuric acid as the mobile phase. The
LA and other products were analyzed using a refractive index
detector (Agilent G1362A) in the HPLC.
For the calculation of the carbon balance, humins and
other solid carbon-based compounds were separated by cen-
trifugation (together with solid catalyst) and then analyzed by
TG and EDX; for the part of soluble polymers, they were
obtained by evaporating water, formic acid and LA and then
analyzed for carbon by EDX. The analysis of humins and
soluble polymers may not be accurate, but still gives infor-
mation for carbon distribution.
Characterizations
Nitrogen adsorption–desorption isotherms of the catalysts
were measured at −96 °C on a NOVA 4200e analyzer (Quanta-
chrome Co. Ltd). Before the measurements, all samples were
outgassed at 180 °C for 12 h under vacuum to remove moisture
and volatile impurities. The BET method was used to calculate
the specific surface area, the pore size distribution curve was
calculated by the Barrett–Joyner–Halenda (BJH) method
through the desorption branch of the isotherm and the total
pore volume (Vt) was estimated at a relative pressure of 0.95.
The pyridine adsorption infrared (IR) spectra were recorded
on a Nicolet NEXUS 670 FT-IR spectrometer, with 32 scans at
an effective resolution of 4 cm−1. 30 mg of the catalyst was
pressed into a self-supporting disk and placed in an IR cell
attached to a closed glass-circulation system. The catalyst disk
was dehydrated by heating at 400 °C under vacuum in order to
remove physisorbed moisture. The IR spectrum background
was recorded at room temperature when the cell cooled down.
Pyridine vapor was then introduced into the cell at room temp-
erature until equilibrium was reached. Subsequent evacuations
were performed at 100, 200, 300 and 400 °C for 60 min fol-
lowed by spectral acquisitions at room temperature using the
background recorded before. The quantification of acid sites
was performed using the same expressions as those in the
research article of Angela S. Rocha:49
ꢀ
ꢁ
π
r2
w
CL ¼ KLA1450
¼
A1450
IMECL
and
ꢀ
ꢁ
π
r2
w
CB ¼ KBA1540
¼
A1540
IMECB
CL and CB are the concentrations of Lewis acid sites and
Brønsted acid sites in μmol g−1; A1450 and A1540 are the inte-
grated areas of bands at 1450 and 1540 cm−1 in the original
data of FTIR spectra, as shown in the Results and discussion
section. KL and KB are molar extinction constants for Lewis
and Brønsted acid sites; IMECL and IMECB are integration
molar extinction coefficients, 2.22 and 1.67 cm μmol−1 for
Lewis and Brønsted acids, respectively; r is the wafer radius in
cm and w is the wafer weight in g of the self-supporting cata-
lyst disk.
Results and discussion
In our previous work, mesoporous niobium phosphate was
synthesized with high surface area and narrow pore size distri-
bution, and exhibited excellent activities in the conversion of
fructose and xylose in pure water under mild conditions
because of the higher acid amount, co-existence of Lewis and
Brønsted acidity, as well as its hot-water-tolerant pro-
perties.33,38,47 The total amount of acid was around 1000 μmol
g−1 and the molar ratio of B/L acid was ca. 1.8 at 373 K from
Catalytic reactions
In the conversion of cellulose to LA, a batch reactor was used. Py-FTIR, the latter of which was much higher than those of AlW,
Typically, a mixture of 0.4 g catalyst, 0.5 g cellulose and 10 mL ZrW, ZrP, and SnP (B/L ratio normally lower than 1.0).25,42
water was loaded into a Teflon-lined stainless steel autoclave Meanwhile, LA was always co-produced in these reactions
and placed into a temperature-controlled electric heating although with low selectivity. Generally, high acid concen-
mantle with a thermocouple probed detector and magnetic trations and reaction temperatures can accelerate the reaction
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rate and enhance LA yield.13,14 Hence, in this work, the reac-
tion temperature was increased to 180 °C, and ca. 40% yield of
LA was obtained after 24 h from 5 wt% cellulose solution cata-
lyzed by mesoporous niobium phosphate. This yield of LA
was comparable with that of 38.6% using 0.9 mol L−1 HCl as a
catalyst under 160 °C from ca. 0.8 wt% cellulose solution14 and
higher than those reported by R. Weingarten et al.,42 in which
zirconium (ZrP) and tin (SnP) phosphates, or other solid acid
catalysts such as metal oxides and heteropoly acid containing
Al, W, Zr, Cs and Sn were used as catalysts for glucose conver-
sion to LA under 160 °C.25 Such an excellent performance may
be attributed to the unique Brønsted and Lewis acid property
of niobium phosphate.
Table 1 Chemical composition and BET properties of porous niobium
catalyst synthesized with different amounts of aluminium
SBET
/
Pore size/
nm
Catalyst
m2 g−1
Nb/at%
Al/at%
P/at%
H-ZSM-5
318
233
145
102
50
0.55
3.8
7.6
7.3
9.6
—
—
—
1.12
2.12
2.49
—
NbOPO4 (A)
Al-NbOPO4 (B)
Al-NbOPO4 (C)
Al-NbOPO4 (D)
20.94
21.01
18.98
17.45
20.95
21.39
20.82
18.61
Considering pure niobium phosphate with a high B/L
molar ratio (ca. 1.8), we would like doping of aluminium in
niobium phosphate to enhance Lewis acid because aluminium is
always considered as a strong Lewis acid site, and samples with
different amounts of aluminium were prepared using the same
method. The samples doped with 5 and 10% aluminium were
referred to as samples B and C; as a comparison, pure
niobium phosphate was named A. Another catalyst D was also
prepared by lowering the amount of CTAB (ca. a quarter of the
initial on catalyst D) during the hydrothermal synthesis
process. ICP analysis and N2 sorption were conducted to inves-
tigate the change of the composition and structural para-
meters after aluminium introduction and the results are
presented in Fig. 1 and Table 1, respectively. It is found that
the molar ratios of Nb/P in different samples were nearly the
Fig. 2 Py-FTIR spectra of catalyst A (a) and catalyst D (b).
Table 2 The weak/medium/strong acid sites amounts of catalysts A, B,
C and D calculated from pyridine adsorption infrared (Py-FTIR) spectra
Niobium phosphate
A
B
C
D
same, close to 1, while the surface area decreased gradually Brønsted acid/
with the doping of aluminium. The acidic properties (type,
strength and amount) were also studied through pyridine
Weak
373–473 K 113.0 127.9 109.3 122.4
μmol g−1
Medium 473–673 K 397.6 268.0 253.4 136.1
Strong
Weak
>673 K
Total
373–473 K
130.9 215.2 323.7 234.1
641.5 611.1 686.4 492.6
adsorbed infrared spectra (Py-FTIR) and the data are collected Lewis acid/
68.2
57.9
51.7
75.2 119.4
31.2
μmol g−1
Medium 473–673 K 231.3 201.2
Strong
in Fig. 2 and Table 2. It is found that the strong Brønsted and
>673 K
Total
45.6 112.7 296.9 318.7
345.1 371.8 423.8 469.3
Lewis acid sites increased with increasing the amount of Al,
especially the strong Lewis acid sites, which increased from
45.6 to 296.9 μmol g−1, indicating that aluminium is indeed a
fer from a medium acid site to a strong acid site, which is
unclear till now.
good strong Lewis acid centre, while the medium acid sites
decreased with increasing the Al doping, maybe due to a trans-
The catalytic performances of various catalysts including
H-ZSM-5, a strong solid acid with a molar ratio of Si/Al = 25, in
the conversion of cellulose to LA are presented in Fig. 3. It can
be seen that the LA yield increased with the increase of alu-
minium amount in the catalyst, but the conversions of cellu-
lose are similar, all above 90%. This kind of tendency may
imply that the increase of strong Brønsted and Lewis acids by
aluminium doping will promote the generation of LA from
cellulose and the highest LA yield of 52.9% can be obtained by
using catalyst D, which contains 2.49% aluminium. This value
is 12% higher than that from pure mesoporous niobium phos-
phate as a catalyst, and 24% higher than that from H-ZSM-5.
Even though such a value of LA yield is still lower than that of
60–70% from liquid acids (e.g., HCl) in the biphasic phase,9 it
is already much higher than those obtained from any other
kind of solid acid catalyst investigated so far. The analytical
data were the average from three repeated reactions, and the
error was less than 1% compared with the average data.
Fig. 1 N2 adsorption/desorption isotherms of catalysts A, B, C and D.
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Table 3 Chemical composition and BET properties of used catalyst D
and humins
SBET
/
Pore
size/nm
m2 g−1
Nb/at%
Al/at%
1.29
P/at%
21.54
Recycled catalyst D
39
8.4
18.98
C/at%
H/at% O/at%
Humins
Soluble polymers
—
—
—
—
59.18
43.76
8.26
16.42
32.10
38.94
carbon in humins and soluble polymers are ca. 60% and 44%,
respectively, the former of which was separated and analyzed
by EDS and are shown in Table 3, and the latter was calculated
from the structure of humins41 and monomers of oligo-
saccharides. In addition, no lactic acid, HMF, glucose or other
small organic molecules were detected in the reaction solu-
tion, and the main by-products are humins and soluble poly-
saccharides. According to previous studies,25,39 it is believed
that small molecules, such as lactic acid would appear only
when there is not enough Brønsted acid in the reaction
system. Once again, the importance of the bonding between
Brønsted and Lewis acids is emphasized. It is also notable that
in the products distribution catalyzed by these four niobium-
based catalysts, the percentage of soluble oligomers and poly-
mers remains stable, and the increase of FA and LA would
result from the decrease of humins.
Fig. 3 Cellulose conversion and LA yield from catalysts A, B, C, D, and
H-ZSM-5 (Al/Si = 1/25). Reaction conditions: 0.5 g of cellulose, 0.4 g of
catalyst, 10 g of H2O, 180 °C, 24 h. Calculated from the conversion of
anhydroglucose unit in cellulose.
To investigate the influence of strong acidity, especially
strong Lewis acids on the selectivity/yield of LA, the LA selecti-
vity/yield from glucose and HMF catalyzed by these four cata-
lysts were investigated and compared with cellulose conversion
and presented in Fig. 5. In all cases, the conversion of all sub-
strates is above 95%. It is found that the LA yield from HMF
had the same tendency as that from cellulose, indicating that
the solid acid catalyst with more aluminium content, that is,
stronger Lewis acid sites could increase the LA yield. The
strong Lewis acid would enhance the selectivity to LA in HMF
conversion was never mentioned before and will be further
confirmed by using a strong liquid Lewis acid later. The result
from glucose has the same tendency, too, but conflicted with
the previous reports,25,39,43 especially the studies reported by
V. V. Ordomsky et al.,43 where the high molar ratio of B/L acid
would promote the formation of LA from glucose. This surpris-
ing result may be due to the high B/L molar ratio (ca. 1.2–1.8)
in niobium phosphate, much higher than those other reported
catalysts (lower than 1.0), so the addition of aluminium
increased the strong Lewis acid and thus enhanced the selecti-
Fig. 4 The distributions of carbon elements in the products by using
various acid catalysts under the same conditions as in Fig. 3. Calculated
from the total number of carbon atoms.
The distribution of carbon element in the products was vity to LA from HMF. It means that there is a balance of Lewis
analyzed by HPLC and thermal analysis, and the result is pre- and Brønsted acids for the conversion of HMF/glucose/cellu-
sented in Fig. 4. Cellulose conversion is above 95% when lose to LA, not the higher B/L ratio, a better selectivity to LA.
using a niobium-based catalyst and 93% for H-ZSM-5, indicat- On the other hand, the final LA yield from glucose is lower
ing that cellulose can be fully converted using these solid acid than that from cellulose with all four catalysts. This may be
catalysts. The ratio of carbon atoms in formic acid and levuli- due to the side reactions during the conversion of glucose to
nic acid analyzed by HPLC is always 1 : 5 in solution (of equal HMF, which happens much easier at a high concentration of
molecule amounts), which is consistent with the report that glucose. While in the conversion of cellulose, glucose just
LA and formic acid are equimolarly produced. The contents of existed as an intermediate with very low concentration, which
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Fig. 6 LA generation from HMF catalyzed by different catalysts under
the conditions of 180 °C, 24 h. The total acid amount was controlled at
0.0004 mol in 10 mL of water.
Fig. 5 Comparison of LA production from different substrates catalyzed
by four catalysts, under the same reaction conditions as in Fig. 3.
prevents the formation of humins or soluble polymers and
therefore leads to high LA yield.
It is also worth noting that the pH value in the system
before the reaction started was 6.1 (measured after 2 h of stir-
ring) while the final LA solution was around 1.8, which means
that the continuing increase of the Brønsted acid ratio in the
system was caused by the dissociation of LA (–COOH) in water,
and the acid catalyst was stable in water.
The enhancement of a strong Lewis acid to the selectivity of
LA was further confirmed by the addition of a liquid Lewis
acid, lanthanum trifluoroacetate [(TfO)3La] in the catalytic
system. The pH value of (TfO)3La in a 0.01 mol L−1 aqueous
solution was 5.42, close to that of distilled water, meaning that
it remained as a solid Lewis acid in water and almost no H+
ions generated through the hydrolysis of this salt. Three cata-
lytic systems were compared in HMF conversion, with and
Fig. 7 Products distribution in various reaction temperatures. Con-
without (TfO)3La, including homogeneous acids, HCl, trifluoro- ditions: cellulose 0.5 g, catalyst D 0.4 g, H2O 10 g, reaction time 24 h.
acetic acid [(TfO)3H] and a solid catalyst with a high B/L ratio,
mesoporous niobium phosphate (catalyst A), the results are
presented in Fig. 6. All the three catalysts showed an enhance-
ment in LA yield when (TfO)3La was added to the aqueous
solution, indicating an addition of Lewis acid can enhance the
selectivity of LA from HMF although the mechanism is
unclear. All these results explained the enhancement of LA
yield from cellulose with aluminium-doped niobium phos-
phate as a catalyst.
The influence of reaction temperature on products distri-
bution was also investigated to optimize the reaction condition
and is presented in Fig. 7. Clearly, a temperature higher than
140 °C is needed for the production of LA, because at this
temperature, a high amount of unreacted cellulose (ca. 40%)
and unhydrolyzed water-soluble compounds were present as
shown in the products distribution. Other intermediates,
including glucose, HMF and oligosaccharide also existed. At
160 °C, more LA was produced with less cellulose left. When
the temperature was further increased to 180 °C, higher LA
yield was obtained, up to 52.9%. Further increasing tempera-
ture to 200 °C, the LA yield decreased, which may be attributed
to the speeding up of the side reactions and lead to the
formation of more solid humins.
The reusability of catalyst D is also investigated and pre-
sented in Fig. 8. The conversion of cellulose decreased gradu-
ally, while the selectivity to LA remained stable except for the
second time, which may be due to the rearrangement of the
acid sites on the solid acid catalysts in aqueous solution,
which is the common characteristic for most of the solid acid
catalysts.33 The gradual decrease of the catalytic activity may
be caused by the deposition of humins on the surface of the
catalyst or the leaching of Al, Nb or P elements. To confirm the
influence of humins deposition, the used catalyst was treated
with hot methanol and found that a certain degree of catalytic
activity was really recovered, indicating the influence of
humins deposition. Furthermore, the used catalyst was
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pressure higher than 3 MPa to make the reaction going fast
enough. It is also interesting to find that the yield of GVL is
higher than that of LA (56.9% vs. 52.9%), which may be due to
the strong adsorption of LA on the surface of the catalyst,
which reduced concentration of LA in solution; while during
hydrogenation, the adsorbed LA turned into GVL, and then
was released back into water. To verify this assumption, 0.4 g
catalyst D, 10 g water and 0.215 g LA (0.185 mol L−1, as high as
60% LA yield from 0.5 g cellulose) were loaded in a reactor and
kept at 180 °C for 24 h, then cooled down and analyzed by
HPLC and GC-MS. It is found that the amount of LA dropped
to 0.174 mol L−1 in solution, while no by-products were
detected, indicating the adsorption of LA. When replacing LA
with the same amount of GVL, the yield of GVL dropped from
0.185 mol L−1 to 0.183 mol L−1, much lower than that of LA.
These results confirmed the assumption of the adsorption of
LA on the catalyst, and once converted to GVL by hydrogen-
ation, most of them would be released back into the solution.
Thus, the yield of GVL looks higher than that of LA.
Fig. 8 Catalyst reusability. Reaction conditions: 0.5 g of cellulose, 0.4 g
of catalyst D, 10 g of H2O, 180 °C, 24 h.
collected and analyzed by thermal gravimetric analysis,
Py-FTIR, EDS and nitrogen sorption. The BET surface area of
the recycled and re-calcined catalyst D was around 39 m2 g−1
,
Conclusions
smaller than the fresh one (50 m2 g−1), while the pore size did
not change obviously, compared with the fresh catalyst D. EDS
analysis showed that the contents of Nb and P were almost the
same as the fresh one, but Al showed an obvious loss. Py-FTIR
of used catalyst D also showed the decrease of the total
acid amount, especially the decrease of strong acids. Thus, the
leaching of Al could be another reason for the decrease of
cellulose conversion. In the future, we would enhance the cata-
lyst stability by using the new synthesis method or incorporate
other Lewis acid sites into NbOPO4.
A high yield of 52.9% of LA was achieved over Al-doped meso-
porous niobium phosphate in aqueous solution, which is
better than any other solid acid catalysts reported so far. The
high selectivity and yield of LA were attributed to the strong
acid strength and a suitable B/L acid molar ratio (1.2 : 1) of
this catalyst. The doping of aluminium in niobium phosphate
enhanced the strong Lewis and Brønsted acids, especially the
strong Lewis acid and led to the increase in LA yield from
cellulose as well as from glucose and HMF. The addition of
(TfO)3La into the catalytic system containing another catalyst
(HCl, (TfO)3H, niobium phosphate) further confirmed a suit-
able ratio of Brønsted/Lewis acid is important for higher
selectivity of LA from HMF, as well as from cellulose. After pro-
duction of LA in aqueous solution of cellulose, it can be com-
pletely converted to γ-valerolactone through hydrogenation
over the Ru/C catalyst without further separation.
The production of GVL comes from the hydrogenation of
LA (Fig. 9) after cellulose hydrolysis for 24 h without separ-
ation. It can be found that hydrogenation of LA needs a H2
Acknowledgements
This project was supported financially by the NSFC of China
(no. 21101063 and 21273071), the Science and Technology
Commission of Shanghai Municipality (13520711400,
13JC1401902, 10dz2220500) and the Fundamental Research
Funds for the Central Universities, China.
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