Ionic-liquid-catalyzed efficient transformation of γ-valerolactone to methyl 3-pentenoate under mild conditions

Article abstract of DOI:10.1002/cssc.201200841

Green nylons! Acidic ionic-liquid catalysis for the transformation of g-valerolactone into methyl 3-pentenoate (M3P) is shown to be performed efficiently under mild conditions. M3P is obtained selectively from a reaction at 1708C for 3.5 h in the presence of an acidic ionic liquid that has a low vapor pressure, high thermal stability, and excellent catalytic performance. A possible reaction pathway for this conversion is also presented.

Full text of DOI:10.1002/cssc.201200841

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DOI: 10.1002/cssc.201200841
Ionic-Liquid-Catalyzed Efficient Transformation of
g-Valerolactone to Methyl 3-Pentenoate under Mild
Conditions
Fan-Xin Zeng,[a, b] Hai-Feng Liu,[a, b] Li Deng,*[a] Bing Liao,[a] Hao Pang,[a] and
Qing-Xiang Guo*[c]
It is argued that the base of the chemical and materials indus-
tries should be changed considerably from traditional fossil re-
sources to renewable feedstocks, with the aim of a sustainable
future.[1] Biomass is the only accessible non-fossil, natural
carbon source, which has drawn much attention from chem-
ists. Although there are many challenges regarding the optimal
routes for the production of chemicals and materials from bio-
mass with eco-friendly and economical goals, an effective way
to replace fossil resources is to produce important bulk chemi-
cals that already exist in the fossil value chain from biomass or
biomass-derived compounds.[2]
Studies regarding the dehydration and esterification of g-va-
lerolactone (GVL), a fascinating bio-based chemical,[6] into
methyl pentenoates and the subsequent hydrocyanation or hy-
droformylation led to another attractive pathway for the syn-
thesis of nylon monomers, which was proposed by Lange
et al.[7] Several patents disclosed the gas-phase catalytic syn-
thesis of methyl pentenoates including methyl 2-pentenoate
(M2P), methyl 3-pentenoate (M3P), and methyl 4-pentenoate
(M4P) from GVL and methanol using solid acids or solid bases
as catalysts.[8] Manzer[8d] used basic catalyst CsOAc/SiO2 for this
reaction, which resulted in 60% M4P that could be hydrofor-
mylated directly without double bond migration to generate
terminally functionalized products (Scheme 1).[9] In the work of
Lange et al., M3P was produced by using a catalytic distillation
method.[7] Compared to former gas-phase conversion, the cata-
lytic distillation allowed for a relatively low reaction tempera-
ture (2008C) and provided methyl pentenoates in high yield
(over 95%) because of the continuous removal of the methyl
Nylon is a family of condensation copolymers synthesized
from dicarboxylic acids and diamines or from lactams.[3] These
monomers are usually prepared from fossil chemicals such as
benzene, phenol, or butadiene, but can also be synthesized
from biomass-derived compounds through various newly de-
veloped catalytic methods.[2g] Heeres et al.[4] proposed a route
to bio-based caprolactam, the monomer of nylon-6, in which
versatile
biogenic
hydroxyl-
methylfurfural (HMF) was used
as the starting material and only
four steps were required, that is,
two steps less than the process
from benzene. By virtue of an
electrochemical method, adipo-
nitrile has been synthesized
from glutamic acid 5-methyl
ester through a two-step oxida-
tive decarboxylation by Fu
et al.[5]
Scheme 1. The synthetic pathways for the production of bio-based nylon monomers from GVL.
[a] F.-X. Zeng, H.-F. Liu, Dr. L. Deng, Prof. Dr. B. Liao, H. Pang
Key Laboratory of Cellulose and Lignocellulosics Chemistry
Guangzhou Institute of Chemistry,
Chinese Academy of Sciences
pentenoates formed, which shifted the thermodynamic equi-
librium towards the products. Despite the advantages of cata-
lytic distillation, the reaction time needed to achieve a high
product yield was too long. Therefore, the development of an
efficient catalytic system is required to ensure bio-based nylon
production is technically feasible.
Guangzhou 510650 (PR China)
Fax: (+86)2085232917
E-mail: dengli@gic.ac.cn
[b] F.-X. Zeng, H.-F. Liu
In this study, catalytic distillation was adopted and a series
of acidic ionic liquids were used as the catalysts. Ionic liquids
are commonly considered as a class of switchable, robust, and
eco-friendly solvents and catalysts for various organic reac-
tions.[10] For the catalytic distillation of GVL, the advantages of
acidic ionic liquids include the ultra-low volatility and miscibili-
ty with GVL and methanol, which enables the catalysts to
University of Chinese Academy of Sciences
Beijing100039 (PR China)
[c] Prof. Q.-X. Guo
Department of Chemistry
University of Science and Technology of China
Hefei 230026 (PR China)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201200841.
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remain in the reactor during distillation and form homoge-
neous mixtures, which enhances the catalytic performance.
This reaction was successfully performed in the presence of
acidic ionic liquids within 3.5 h at 1708C, which resulted in
M3P as the main product with a maximum selectivity of 88%
and total methyl pentenoates with a yield over 90%. In the
presence of acidic catalysts, the dehydration followed Zaitsev’s
rule (Scheme 2). Although M3P is not the most convenient
substrate for hydroformylation to form the precursor of nylon
and transferred into the collector. However, the turnover fre-
quency (TOF, based on M3P) was enhanced almost twofold by
increasing the methanol feed rate, even at lower reaction tem-
peratures compared with the previous study.[7] Based on re-
ports regarding the excellent performance of the fluorine-func-
tionalized acidic resin Nafion, we tested CF3SO3H (Table 1,
entry 2) as it contains similar acidic sites. The catalytic perfor-
mance of CF3SO3H was greater than that of H2SO4, and
CF3SO3H could be distilled out from reactor with methanol and
products when most of the GVL
was exhausted. Another strong
acid, Keggin-type polyoxometa-
late (H3PW12O40; Table 1, entry 3),
resulted in comparable perfor-
mance to CF3SO3H. For these
two catalysts, M3P was preferen-
tially formed and the conjugated
isomer M2P could be generated
from M3P through acid-cata-
lyzed CÀC double bond migra-
tion. Two bases, NaOH and
CsCO3 (Table 1, entries 4 and 5)
were also tested, but they exhib-
ited no catalytic activity.[8d]
As shown in Table 1, acidic
ionic liquids resulted in GVL con-
versions ranging from 29 to 98%
(Table 1, entries 6–15). In detail,
for ionic liquids 1b and 2b
(Table 1, entry 7, 10) containing
À
HSO4 as the counter ion, 29
and 38% of GVL were converted,
Scheme 2. The catalytic distillation process and the acidic ionic liquids examined in this work.
respectively, and the yield of
monomers (5-formyl methyl pentanoate), it has previously
Table 1. Product selectivity for the catalytic distillation of GVL in the pres-
been converted to the precursor by using a platinum-based
catalyst containing a large-bite-angle diphosphine ligand.[9]
Moreover, in principle the olefin metathesis reaction
(Scheme 1) could transform M3P into dimethyl 3-hexenedioic
ester, which could be used as a precursor for adipic acid.[11]
A clear advantage of replacing hydroformylation with an olefin
metathesis reaction is the avoidance of using toxic CO.[9]
ence of different catalysts.[a]
Entry
Catalyst
Conversion of
GVL [mol%]
Selectivity [mol%]
TOF[b]
[hÀ1
]
M2P
M3P
M4P
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
H2SO4
78
92
99
2
0
9
7
0
0
0
0
0
0
0
0
0
0
0
0
0
62
74
74
0
0
14
6
2
33
6
19
12
11
0
0
5
0
0
11
3
0
5
9
8
0
0
2
0.2
0.1
2
0.3
0.4
8
7
8
5
0
CF3SO3H
H3PW12O40
NaOH
[c]
Acidic ionic liquids have strong acidity and low vapor pres-
sure, thus ten were investigated in this study (Scheme 2). The
acidic ionic liquids were prepared from their zwitterionic pre-
cursors by adding the corresponding acids; they were charac-
Cs2CO3
3
1a
1b
1c
2a
2b
2c
3
4
5
6
53
29
34
54
38
36
94
98
97
95
0
1
terized by thermogravimetric analysis (TGA), FTIR, and H NMR
9
spectroscopy (Figures S1–S3, see the Supporting Information).
The TGA results indicated that the acidic ionic liquids were
stable at the temperatures used in this study.
88
71
73
63
0
7
19
22
19
0
For comparison, we began our investigation with the cata-
lytic distillation of 10 g GVL in the presence of 0.18 g H2SO4
(Table 1, entry 1) at 1708C with a methanol feed rate of
10 mLhÀ1. As a result, 78% of GVL was converted into methyl
pentenoates. There was almost no GVL left in the reactor after
a 3.5 h run, the unreacted GVL was vaporized with methanol
MIMPS
[a] Reaction conditions: GVL (10 g, 0.1 mol) with catalyst (3.6 mmol,
based on protons) at 1708C with methanol feed rate 10 mLhÀ1. [b] Turn-
over frequency, calculated based on M3P generated per proton in the ini-
tial 1 h. [c] 1.8 mmol.
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M3P was only 2%. Similar results
were also obtained in the pres-
ence of p-toluene sulfonic acid
functionalized ionic liquids 1c
and 2c (Table 1, entries 8 and
11). By using 1a (Table 1,
entry 6), the conversion of GVL
increased slightly to 53%; 14%
M3P and 5% M4P were ob-
tained. 2a (Table 1, entry 9) also
showed comparable catalytic
performance. These results sug-
Table 2. Catalytic distillation of GVL in the presence of 3 under different conditions.[a]
Entry
T
[8C]
Methanol feed
GVL conversion
[mol%]
Selectivity [mol%]
rate [mLhÀ1
]
M2P
M3P
M4P
MMP
MPA
1
2
3
4
5
6
7[b]
150
190
210
170
170
170
170
10
10
10
2
5
15
10
52
96
85
30
88
97
96
0
2
4
3
0
0
0
20
75
62
40
74
62
78
8
16
13
13
10
18
13
13
0
0
0
3
1
0
0
0
0
1
0
17
5
[a] GVL (10 g, 0.1 mol) with 3 (1.8 mmol) for 3.5 h. [b] 10 g GVL added into reactor every 4 h during a 24 h run.
À
gested that CF3SO3 was the
preferable anion for the catalytic distillation of GVL. By combin-
ing CF3SO3H and three germinal zwitterionic organic salts, 3, 4,
and 5 were synthesized. The conversion of GVL was clearly im-
proved to 94, 98, and 97% by 3, 4 and 5 (Table 1, entries 12–
14), respectively. Moreover, they also increased the selectivity
for M3P to 88, 71, and 73%, respectively. Note that in the pres-
ence of 3, only 7% M4P was generated and the isomer ratio of
M3P/M4P was over 12. The good discrimination between M3P
and M4P may facilitate further highly selective syntheses of
nylon monomers through the metathesis method we pro-
posed above. With regard to the high activity of H3PW12O40,
ionic liquid 6 was also used, which resulted in a medium yield
of M3P (Table 1, entry 15). A control experiment showed that
the zwitterionic salt (MIMPS; Table 1, entry 16) was incapable
of catalyzing this reaction. Additionally, for the tests involving
Figure 1. The structures of the by-products MMP and MPA formed from the
catalytic distillation of GVL.
low conversion of GVL at 2108C could be attributed to more
GVL vaporizing and entering into the collector with methanol.
By lowering the methanol feed rate to 2 mLhÀ1, (Table 2,
entry 4) only 30% GVL was converted and 3% M2P was
formed. At feed rates of 5 and 15 mLhÀ1, (Table 2, entries 5
and 6) the conversion of GVL was improved and the selectivity
for M3P was slightly lower than that at the rate of 10 mLhÀ1
(Table 1, entry 12). Moreover, higher methanol feed rates in-
creased MMP formation. These results suggested that the
methanol feed rate played a key role in the catalytic distillation
of GVL.
À
acidic ionic liquids containing CF3SO3 and PW12O403À, no M2P
was isomerized from M3P. These results revealed that acidic
ionic liquids synthesized through the combination of strong
acids and zwitterionic salts did not only maintain high catalytic
activity, but also exhibited good selectivity.
Compared with H2SO4, CF3SO3H, and H3PW12O40, the acid-
functionalized ionic liquids provided much less charcoal prod-
ucts and remained homogeneous and semitransparent; how-
ever, the color of these ionic liquids turned dark after 3.5 h run
time. For all tests conducted under 1708C, the weight loss of
methanol was 1–2 wt%. To verify the formation of dimethyl
ether, a liquid nitrogen trap was used to collect samples for
gas chromatography (GC) and NMR analysis; however, no di-
methyl ether was detected.
In an extended 24 h catalytic run (Table 2, entry 7), we re-
loaded fresh GVL into the reactor every 4 h and performed the
reaction at 1708C with a methanol feed rate of 10 mLhÀ1
.
A turnover number greater than 250 (based on M3P) was
achieved by using 3 as the catalyst. The overall yield of M3P
was 75% (yield=conversionꢁselectivity) and the selectivity for
M3P reached 78%. Because of the low volatility and high ther-
mal stability, most of 3 remained in the reactor and could be
easily recovered.
Next we examined the effect of temperature and methanol
feed rate on the reaction using 3 as the catalyst. As shown in
Table 2, the reaction did not proceed adequately at 1508C
(Table 2, entry 1), leaving a large amount of GVL in the reactor
after a 3.5 h run. The selectivities for M3P and M4P were only
20 and 8 %, respectively. Additionally, two unexpected by-
products, methyl 4-methoxypentanoate (MMP) and 4-methoxy-
pentanoic acid (MPA), were detected by GC–MS (Figure 1). The
mass spectra of them and plausible fragmentation pathways
are shown in Figure S4. The formation of a considerable
amount of MMP indicated that the elimination of water or
methanol to form a double bond could not proceed easily at
1508C. When the reaction was performed at 1908C and 2108C
(Table 2, entries 2 and 3) MMP and MPA were not detected;
however, isomerization of M3P to M2P occurred. The relatively
Because of the formation of MMP and MPA, we examined
the conversion of GVL with methanol in a Teflon-lined auto-
clave. As a result, only 2% methyl pentenoates were obtained
from GVL (Table 3, entry 1). However, GVL was converted
Table 3. Different substrates with methanol were tested in an autocla-
ve.[a]
Entry
Substrate
Product distribution [mol%]
GVL
pentenoates
MMP+MPA
1
2
3
GVL
MMP
M3P
53
51
0
2
8
58
43
26
1
[a] After reaction of substrate (1 g), methanol (3 g), and 3 (1.8 mmol)
loaded into a Teflon-lined autoclave and heated to 1708C for 3 h.
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Acknowledgements
The authors are grateful to NFSC,
Youth Innovation Promotion Asso-
siation, CAS; Ph.D Programs
Foundation of the Ministry of Edu-
cation of China; and Foshan
Centre for Functional Polymer
Materials, CAS for financial sup-
port.
Scheme 3. The proposed reaction pathway for the acid-catalyzed distillation of GVL.
mainly to MMP and MPA. Replacing GVL with MMP as the sub-
strate, 51% GVL was generated and methyl pentenoates were
still only minor products (Table 3, entry 2). Moreover, when
M3P reacted with methanol, no GVL was observed, only 1%
MMP was generated, and 40% of M3P was converted into
products that were not detectable by GC (Table 3, entry 3).
These results indicated that M3P was prone to forming poly-
meric by-products rather than returning to GVL in the pres-
ence of a catalyst. Thus, under catalytic distillation conditions,
the nonvolatile by-products were mainly attributed to the pen-
tenoates that were not effectively removed by the methanol
vapor. Based on the product distribution, it is believed that
there is a dynamic equilibrium between GVL and MMP in
methanol and the latter is the key intermediate for this reac-
tion, in which M3P is generated through elimination of metha-
nol from MMP. Terminal olefin isomer M4P was generated as
a minor product because of thermodynamic effects, and it is
reasonable to consider that M2P was generated from M3P
through a secondary isomerization process. Therefore, a possi-
ble reaction pathway for the acid-catalyzed transformation of
GVL into methyl pentenoates is presented in Scheme 3.
Keywords: acidity · distillation · ionic liquids · polymers · g-
valerolactone
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Experimental Section
The procedures for the preparation of the ionic liquids are provid-
ed in the Supporting Information. The tests were performed in
a 25 mL two-necked flask equipped with a 10 cm rectification
column and a water-cooled condenser. The reaction flask was
loaded with GVL (10 g, 0.1 mol) and acidic catalyst (3.6 mmol, on
the basis of protons), heated up to the desired temperature and
fed with methanol at 2–15 mLhÀ1 under an argon atmosphere. The
resulting distillate was sampled and analyzed by a Shimadzu 2010
GC equipped with an ATX-35 (30 mꢁ0.32 mmꢁ0.25 um) capillary
column. A Shimadzu GC–MS was also used to identify the prod-
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ucts. MMP was synthesized according to the procedure reported
by Horvath et al.[12]
Received: November 5, 2012
Published online on March 6, 2013
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