Conversion of carbohydrate biomass to γ-valerolactone by using water-soluble and reusable iridium complexes in acidic aqueous media

Article abstract of DOI:10.1002/cssc.201300245

Mild-mannered manipulation: A catalytic method for the conversion of carbohydrate biomass to γ-valerolactone in acidic aqueous media has been developed. The water-soluble iridium complexes were observed to be extremely catalytically active for providing γ-valerolactone in high yields with high TONs. The homogeneous catalysts can also be recycled and reused by applying a simple phase separation process. Copyright

Full text of DOI:10.1002/cssc.201300245

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DOI: 10.1002/cssc.201300245
Conversion of Carbohydrate Biomass to g-Valerolactone
by using Water-Soluble and Reusable Iridium Complexes
in Acidic Aqueous Media
Jin Deng, Yan Wang, Tao Pan, Qing Xu, Qing-Xiang Guo, and Yao Fu*[a]
Diminishing fossil fuel reserves and degradation of the envi-
ronment are strong driving forces for developing an efficient
process to convert renewable feedstocks into liquid fuels and
valuable chemicals. Biomass provides an abundant and
renewable alternative to fossil resources for the sustainable
production of energy and chemicals.[1] The largest part of
woody biomass is carbohydrates, which is, therefore, of partic-
ular interest for conversion into platform molecules that can
be utilized to produce liquid fuels, feedstock chemicals, and
fine chemicals.[2] Because of its versatile functions and benign
properties, g-valerolactone (GVL) is considered one of the most
promising reproducible platform molecules.[3] GVL can be con-
verted into transport fuels, fuel additives,[4] solvents,[5] food ad-
ditives, perfumes, and organic intermediates for the produc-
tion of fine chemicals.[6] In this respect, recent efforts have
been focused on the development of ecofriendly, efficient, and
inexpensive processes for the synthesis of GVL. One of the
most effective methods that can be used for the manufacture
of GVL is the hydrogenation of levulinic acid (LA), which can
be obtained through the hydrolysis of carbohydrate biomass.[7]
LA can be converted into GVL with molecular hydrogen or
formic acid (FA) with either heterogeneous[8] or homogeneous
catalysts.[9]
should be obtained from carbohydrate biomass feeds through
a simple and robust hydrolysis process. Secondly, for compati-
bility with the hydrolysis step, low concentrations of LA
formed in acidic aqueous medium should be reduced to GVL
without separation, evaporation, or pH adjustment by using
a highly efficient catalyst. Therefore, the development of
a highly efficient catalytic system, which can be reused in
strongly acidic aqueous solution, for the reduction of LA de-
rived from carbohydrate biomass is particularly important.
Himeda et al.[10] and Ogo et al.[11] reportedly used a series of
half-sandwich iridium complexes for hydrogenating carbon di-
oxide with high TONs in aqueous media. Hence, we deduced
that robust iridium complexes could be efficient catalysts for
the hydrogenation of biomass-derived LA in water. Herein, we
show that half-sandwich iridium complexes serve as highly effi-
cient and robust homogeneous catalysts for the generation of
GVL from various biomass-derived carbohydrates (i.e., cellu-
lose, starch, and sugars). More importantly, this water-soluble
homogeneous catalyst can be reused by applying a simple
phase separation for the selective reduction of low concentra-
tion LA with molecular hydrogen or FA in aqueous solution
without any additives.
We initially investigated the influence of the electronic and
steric hindrance effects of the substituents on the yields. The
initial experiments were performed at 1.01 MPa H2 and 1208C
in water in the presence of 0.01mol% of the half-sandwich
iridium catalyst (Scheme 1 and Table 1). After 4 h, for catalyst 1
without any substituent group, we observed that the GVL yield
was 31%. When a single carboxyl group was substituted at the
meta-position (catalyst 2), the yield decreased to 11%, and
with double carboxyl group substitution (catalysts 3 and 4),
the yield was even lower. This could be because the catalytic
efficiency of this reaction is closely related to the ligand
s-donor power of the substituent on the bipyridine ligand
backbone. Clearly, the electron-withdrawing groups weaken
the activity of the catalyst. Next, we replaced the carboxyl
group with hydroxyl functionality (catalyst 5) and obtained
a 96% yield of GVL. When we used stronger electron-donating
substituents, such as a methoxy group (catalyst 6), we ob-
tained the highest GVL yield of 98%. Steric hindrance was an-
other factor that influenced the catalytic efficiency, which ex-
plained the reduced activity of catalyst 7. The yield reduced
significantly for catalyst 8, which was likely caused by dehydro-
genation.[12] To validate this speculation, we used GVL as a raw
material with catalyst 8 under identical conditions, and we re-
ceived a 71% recovery of GVL and observed that LA was
formed during the reaction process. Interestingly, the activity
Typically, heterogeneous catalysis facilitates the separation
and recycling processes, but the catalytic efficiency is relatively
low and high temperatures, high pressure, or a combination of
both are required to ensure satisfactory conversions and
yields. In contrast, homogeneous catalyst has high catalytic ef-
ficiency with the turnover numbers (TONs), that is, moles of
substrate per mole of catalyst, of up to 71000.[9i] However, in
addition to the inherent limitations of the homogeneous cata-
lytic processes, it requires strict absence of water and an addi-
tional excess of alkali to improve the reduction kinetics and
minimize deactivation. Considering that the biomass hydrolysis
process has a rich water content and extremely high acidity,
there may be serious shortcomings in the energy consumption
and processing costs, as well as additional handling problems.
To minimize the cost of the GVL feed, two key problems re-
garding GVL production need to be resolved.[8d] First, LA
[a] J. Deng, Y. Wang, T. Pan, Q. Xu, Prof. Q.-X. Guo, Prof. Dr. Y. Fu
Anhui Province Key Laboratory of Biomass Clean Energy
Department of Chemistry, University of Science and Technology of China
Hefei 230026 (PR China)
Fax: (+86)-551-360-6689
E-mail: fuyao@ustc.edu.cn
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201300245.
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the turnover frequency (TOF), and it is also important to note
that the TOF of catalyst 6 is the highest reported to date (see
Table S1 in the Supporting Information).
It has been reported that half-sandwich iridium-complex cat-
alysts could be used to convert FA into hydrogen with highly
efficiency.[13] Also, FA is a byproduct that is formed in an equi-
molar amount with LA from the acidic hydrolysis of biomass.
From an economic and engineering perspective, we used FA
as an in situ source of hydrogen instead of using hydrogen gas
itself. Under otherwise identical conditions, the yield of GVL
from 1:2 (molar ratio) aqueous mixtures of LA and FA was
more than 99%. When the ratio was changed to 1:1.5, the
yield reduced to 85%, and with a ratio of 1:1.2, the yield re-
duced to 71%. Even when we prolonged the reaction time to
6 h, the yield only increased slightly to 76%. These results indi-
cated that excess FA was required in our reaction system, or
that we needed to add external hydrogen to enhance the ac-
tivity of the catalyst. Interestingly, when we simply increased
the catalyst loading from 0.01 to 0.1mol%, the catalyst
showed excellent activity at room temperature, and after 24 h
we obtained a GVL yield as high as 94%. To the best of our
knowledge, this represents the mildest reaction conditions to
achieve such a high yield in water with FA (Table S2, the
Supporting Information).
To test the reusability and the stability of catalyst 6, we
again conducted the LA conversion reaction under identical
conditions (Figure 1). After the reaction, GVL was extracted by
using ethyl ether or methyl tert-butyl ether, and separated
from the catalyst before it was reused in the next run. The
yield of GVL remained at 94% after 5 runs, which proved that
the catalytic activity did not decrease in this reaction system.
Therefore, the catalyst could be reused, which is crucial for
practical applications.
Scheme 1. The hydrogenation of LA and the structures of the different iridi-
um complexes used. (Reaction conditions: 5 mmol LA, 1.0 mL 0.5 mmolLÀ1
aqueous solution of catalyst (i.e., 0.01mol% cat.), 4.0 mL H2O, 1.01 MPa H2
(initial), 1208C, 4 h.
Table 1. Catalytic reduction of LA to GVL.[a]
Entry
Catalyst
Cat. loading
[mol%][b]
Reductant
Yield
[%][c]
According to previous reports, the IrÀH species could be
formed as an active catalytic intermediate during the reaction
process.[14] To investigate the stability of iridium complexes in
strong acidic solution, we used 4.0 mL of 0.5 molLÀ1 H2SO4
aqueous solution instead of water, with all other conditions
unchanged; the catalytic efficiency was not affected. Based on
this result, we predicted that half-sandwich iridium complexes
could be applied in the acidic hydrolysis of biomass. We tested
several types of sugars to study the catalytic performance of
catalyst 6 (Table 2). First, we obtained an acidic aqueous solu-
tion, which contained LA and FA, through the hydrolysis of
glucose (0.9 g in a 5 mL aqueous solution, catalyzed by
0.5 molLÀ1 H2SO4 at 1708C with 0.5 MPa N2). We then removed
any insoluble solid out of the mixture through filtration. The
filtered mixture was transferred into an autoclave containing
catalyst 6 (0.01mol%) and 1.01 MPa H2. GVL was generated in
97% yield based on the conversion of LA after 4 h at 1208C,
and the overall GVL yield was 45%. Therefore, we demonstrat-
ed the high efficiency of catalyst 6 for the conversion of glu-
cose to GVL. When we used fructose instead of glucose, we
obtained an overall yield of 61%, which was the highest
among all of the carbohydrate biomass sources that we tested.
In addition to these monomeric C6 sugars, we tested the activi-
ty of catalyst 6 for the conversion of sucrose, starch, and cellu-
1
2
3
4
5
6
7
8
9[d]
10
11
12
13[e]
14[f]
1
2
3
4
5
6
7
8
6
6
6
6
6
6
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.001
0.01
0.01
0.01
0.01
0.1
1.01 MPa H2
1.01 MPa H2
1.01 MPa H2
1.01 MPa H2
1.01 MPa H2
1.01 MPa H2
1.01 MPa H2
1.01 MPa H2
1.01 MPa H2
2.0 equiv. FA[b]
1.5 equiv. FA[b]
1.2 equiv. FA[b]
1.2 equiv. FA[b]
2.0 equiv. FA[b]
31
11
2.1
1.9
96
98
77
21
78
99
85
71
76
94
[a] Reaction conditions: LA (5 mmol), aqueous solution of catalyst
(1.0 mL), H2O (4.0 mL), 1208C, 4 h. [b] Catalyst loading and amount of FA
based on the amount of LA. [c] Calculated by using GC (all selectivities
were over 99%). [d] 36 h. [e] 6 h. [f] 258C, 24 h.
of catalyst 6 was high even when the catalyst loading was re-
duced from 0.01 to 0.001mol%; after 36 h we still obtained
a 78% GVL yield under 1.01 MPa H2. To our best knowledge,
this is the highest TON (78000) reported under mild reaction
conditions for the hydrogenation of LA. Another crucial prop-
erty regarding the economic potential of the catalyst system is
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mild temperatures and pres-
sures. We observe TONs up to
78,000 at 1208C and 1.01 MPa
H2, which is the highest TON re-
ported to date. The catalyst also
shows high activity for the gen-
eration of GVL from aqueous
mixtures of LA and FA, when
excess FA or extra hydrogen is
supplied, under the mildest con-
ditions reported so far. Further-
more, through a simple phase
separation, this water-soluble
homogeneous catalyst can be
reused for the selective reduc-
tion of low-concentration LA in
aqueous solutions. Therefore,
this catalyst can be used for the
generation of GVL from various
biomass-derived carbohydrates,
indicating its superiority in prac-
tical applications for the industri-
al production of GVL.
Figure 1. Reuse of catalyst 6 for the hydrogenation of LA in aqueous solution after a phase separation to recycle
the catalyst. The organic phase (light grey) contained the product and the aqueous phase (dark grey) contained
the catalyst.
Experimental Section
LA (99%), FA (98%), and GVL
(98%) were purchased from Alad-
din Reagent Co. Ltd. (Pentamethyl-
cyclopentadienyl) iridium(III) chlo-
ride dimer [(Cp*IrCl2)2, 98%] was
purchased from Suzhou Sinocom-
pound Technology Co., Ltd. Silver
sulfate was purchased from Sino-
pharm Chemical Reagent Co. Ltd.
Fructose (99%), glucose (99%), su-
crose (99%), starch (soluble), and
cellulose (microcrystalline) were
purchased from Alfa Aesar. 2,2’-bi-
pyridine, 4,4’-dicarboxy-2,2’-bipyri-
dine, 5,5’-dicarboxy-2,2’-bipyridine,
and 4,4’-dimethoxy-2,2’-bipyridine
were purchased from TCI. 5-car-
Table 2. Dehydration of carbohydrates[a] and the subsequent reduction of carbohydrate-derived LA in the pres-
ence of catalyst 6.[b]
Entry
Carbohydrate
FA yield [mol%][c]
LA yield [mol%][c]
GVL yield [mol%][d]
1
2
3
4
5
glucose
fructose
sucrose
starch
51
68
58
52
43
46
62
53
46
37
45 (97)
61 (98)
52 (99)
42 (92)
34 (92)
cellulose
[a] Reaction conditions: carbohydrate (0.9 g), H2SO4 aqueous solution (0.5 molLÀ1, 5 mL), N2 (0.5 MPa), 1708C,
1 h. [b] Reaction conditions: filtrate from hydrolysis (4 mL), aqueous solution of catalyst 6 (0.5 mmolLÀ1, 1 mL),
H2 (1.0 MPa), 1208C, 4 h. [c] Yield for the initial carbohydrate hydrolysis, consistent with reference [6b]. [d] Over-
all yield for the two-stage conversion of the biomass-derived carbohydrate. The yield based on the conversion
of LA is shown in parenthesis.
boxy-2,2’-bipyridine,[15]
4,4’-dihy-
droxy-2,2’-bipyridine,[16] 6,6’-dihy-
droxyl-2,2’-bipyridine,[17] and 6,6’-
dimethoxyl-2,2’-bipyridine[18] were
synthesized according to the previ-
ously reported procedures.
lose. For sucrose, we observed near-complete conversion of LA
to GVL. The yield of GVL from LA was also very high in the
starch and cellulose experiments. For the conversion of carbo-
hydrate biomass to GVL we obtained an overall yield of 52%,
42%, and 34% for sucrose, starch, and cellulose, respectively.
These results demonstrated that catalyst 6 could be used for
the generation of GVL from carbohydrate biomass resources
with the addition of hydrogen, without the need for any sepa-
ration or pH adjustment.
Catalytic conversion of LA into GVL with hydrogen: LA (5 mmol),
aqueous solution of catalyst (0.5 mmolLÀ1, 1 mL), and water (4 mL)
were added to a 10 mL zirconium alloy high-pressure reaction
tube, and stirred at a rate of 800 rpm under 1.01 MPa H2 for the
specified reaction time. The mixture of substrates and catalyst
were heated to the desired temperature in less than 15 min. The
liquid products were diluted with methanol and analyzed by using
GC on a Shimadzu GC-2014 gas chromatograph equipped with
a capillary column (DM Waxm 30 mꢁ0.25 mm) and a flame ioniza-
tion detector. The identification of the products was performed by
In summary, we report that half-sandwich iridium complexes
can be used to convert LA into GVL in excellent yields under
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using GC–MS. An internal standard (dimethyl formamide) was used
to determine the amount of GVL produced.
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Catalytic conversion of LA into GVL with FA: LA (5 mmol), FA
(6–10 mmol), aqueous solution of catalyst (0.5 or 5 mmolLÀ1
,
1 mL), and water (4 mL) were added to a disposable tube with
a plastic screw cap top. The catalytic conversion proceeded as de-
scribed above, but in the absence of hydrogen.
Dehydration of carbohydrate biomass with H2SO4 and subse-
quent hydrogenation: Carbohydrate, that is, cellulose, starch,
sucrose, glucose, or fructose (0.9 g) was loaded into a 25 mL
Hastelloy-C high-pressure Parr reactor, and H2SO4 (5 mL,
0.5 molLÀ1) was then added. Under 0.507 MPa N2, the autoclave
was quickly heated to 1708C, and vigorously stirred for 1 h. After
the reaction, the insoluble solid byproducts were removed through
filtration. The hydrolysis filtrate containing LA and FA (4 mL) was
transferred to a 10 mL zirconium alloy high-pressure reaction tube
containing an aqueous solution of catalyst 6 (1 mL, 0.5 mmolLÀ1),
which was then heated to 1208C with 1.01 MPa H2 for 4 h. Finally,
the products were analyzed by using GC as described above. The
concentrations of LA and FA were determined by using HPLC; this
system consisted of a Waters 1525 pump, a Waters 5C18-PAQ
column (4.6ꢁ250 mm), and a Waters 2414 refractive index detec-
tor. H2SO4 (5 mm) was used as the mobile phase at flow rate of
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1 mLminÀ1
.
Acknowledgements
This work was supported by the National Basic Research Pro-
gram of China (2012CB215305), the National Natural Science
Foundation of China (21172209) and Chinese Academy of Science
(KJCX2-EW-J02).
Keywords: biomass
· homogeneous catalysis · iridium ·
sustainable chemistry · g-valerolactone
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Received: March 19, 2013
Published online on && &&, 0000
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J. Deng, Y. Wang, T. Pan, Q. Xu, Q.-X. Guo,
Y. Fu*
&& – &&
Conversion of Carbohydrate Biomass
to g-Valerolactone by using Water-
Soluble and Reusable Iridium
Mild-mannered manipulation: A cata-
lytic method for the conversion of car-
bohydrate biomass to g-valerolactone in
acidic aqueous media has been devel-
oped. The water-soluble iridium com-
plexes were observed to be extremely
catalytically active for providing g-valer-
olactone in high yields with high TONs.
The homogeneous catalysts can also be
recycled and reused by applying
Complexes in Acidic Aqueous Media
a simple phase separation process.
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 5
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6
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These are not the final page numbers!