The first non-acid catalytic synthesis of tert-butyl ether from tert-butyl alcohol using ionic liquid as dehydrator

Article abstract of DOI:10.1039/b300714f

Methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and isopropyl tert-butyl ether (IPTBE) have been synthesized for the first time over a non-acid ionic liquid as catalyst and dehydrator with high conversion (> 90%) and selectivity (> 90%) under mild conditions.

Full text of DOI:10.1039/b300714f

The first non-acid catalytic synthesis of tert-butyl ether from tert-butyl
alcohol using ionic liquid as dehydrator†
Feng Shi, Hai Xiong, Yanlong Gu, Shu Guo and Youquan Deng*
Center for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Lanzhou, 730000, China.
E-mail: ydeng@ns.lzb.ac.cn; Fax: +86-931-8277088
Received (in Cambridge, UK) 20th January 2003, Accepted 12th March 2003
First published as an Advance Article on the web 28th March 2003
Methyl tert-butyl ether (MTBE), ethyl tert-butyl ether
(ETBE), and isopropyl tert-butyl ether (IPTBE) have been
synthesized for the first time over a non-acid ionic liquid as
catalyst and dehydrator with high conversion ( > 90%) and
selectivity ( > 90%) under mild conditions.
Following ionic liquids were used in this reaction: emimBF4
(1-ethyl-3-methyl-imidazole), bmimBF4 (bmim = 1-butyl-
3-methyl imidazolium), dmimBF4 (1-decyl-3-methyl imidazo-
lium), cmimBF4 (1-cetyl-3-methyl imidazolium), bmimCl,
bmimPF6 and BPyBF4 (BPy = 1-butyl pyridinium). All ionic
liquids were synthesized according to previous papers17,18 with
small modifications and evaporated with a vacuum pump at 120
°C (5 mmHg) before use. All reactions were conducted in a 90
ml autoclave with a glass tube inside equipped with magnetic
stirring. In each reaction, 3 ml ionic liquid, 3 ml TBA and 6 ml
methanol (ethanol or isopropanol) were successively introduced
into the reactor without any additional organic solvent and
cocatalyst. Then the reaction proceeded at 130 °C for 3 h. After
reaction, a mixture of desired products, unreacted substrates and
byproducts (in some cases) could be separated from the ionic
liquids containing some water by simple distillation. The ionic
liquid was evaporated with vacuum pump at 120 °C (5 mmHg)
for 30 min to remove water and reused next time. Qualitative
analyses were conducted with a HP 6890/5973 GC-MS with
chemstation containing a NIST Mass Spectral Database.
Quantitative analyses were conducted with a HP 1790 GC
equipped with a FID detector.
All results of the etherization of TBA are listed in Table 1. As
shown, the effect of the substituted alkyl chain of the
imidazolium cations on the reaction was investigated first. Only
19% and 39% of conversions were achieved in emimBF4 and
bmimBF4 ionic liquids while even no byproduct was detected
(entries 1, 2). The best result was obtained when using
dmimBF4 as dehydrator and catalyst with a conversion of 93%
and a selectivity of 97% (entry 3). Nearly 100% of conversion
was obtained when the reaction was conducted at 175 °C while
the selectivity was only 80% because ca. 20% of TBA was
converted into isobutene (entry 4). Furthermore, some dimethyl
ether (about 10% based on the methanol) was also detected
under such reaction conditions, which indicated that the ionic
liquid exhibit higher catalytic activity at higher reaction
temperature and the dimethyl ether might be effectively
synthesized if appropriate reaction conditions were selected. At
As it is known, asymmetrical tertiary alkyl ethers, especially
methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE)
and isopropyl tert-butyl ether (IPTBE), are particularly useful
as octane improvers for gasoline due to the rapidly increasing
demand for lead-free octane boosters for gasoline1,2. MTBE,
ETBE or IPTBE are currently synthesized by reacting iso-
butylene with corresponding alcohols (synthesis route 1) and
are industriously manufactured over Amberlyst-153 with sat-
isfactory conversion and selectivity although several limitations
such as thermal instability and loss of acid sites were difficult to
overcome. At the same time, large amounts of work on zeolites
or zeolite supported catalysts4–6 were also conducted but all
studies mentioned above could not satisfy the enormous
increase of the demand of oxygenates in the coming years
because of the growing problem of the availability of isobutene
as a raw material.7 Therefore, to develop a process without
requiring isobutene as a building block would be advantageous
and an efficient process involving the reactions between
methanol or ethanol and tert-butyl alcohol (TBA) to afford
asymmetrical tertiary alkyl ethers is highly desirable because
TBA (synthesis route 2) is readily available commercially
through isobutane oxidation.7,8
(1)
(2)
In the previously reported papers, the catalysts selected for
the etherization of TBA were various solid acids including
heteropoly acid, titania supported phosphoric acid, acidic
montmorillonite clay and zeolite modified by fluorosulfonic
acid or fluorophosphoric acid, etc, but their catalytic perform-
ance could still not satisfy the requirement of industrial
application at this stage.9–12 The major problems associated
with these processes were the formation of large amount of
isobutene with an increase of TBA conversion because acids
were also effective catalysts for dehydration of TBA to yield
isobutene. Therefore, how to design and synthesize a catalytic
system that may avoid the dehydration of TBA has long been
pursued.
Table 1 Etherization results of tert-butyl alcohol in different ionic
liquidsa
Conversions
(%)
Selectivities
(%)
Entry
Ionic liquid
Alcohol
1
2
emimBF4
bmimBF4
dmimBF4
dmimBF4
cmimBF4
dmimBF4
bmimCl
bmimPF6
BPyBF4
dmimBF4
dmimBF4
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
19
39
93
~ 100
45
94
~ 0
~ 0
~ 0
95
~ 100
~ 100
97
80
~ 100
96
—
—
—
3
4b
5
6c
7
In this communication, it was found that an ionic liquid13,14
could be an effective dehydration agent for the etherization
reaction with a high yield if an appropriate ionic liquid was
used, especially if non-acid15,16 ionic liquids with BF42 anion,
was selected.
8
9
10
11
94
98
i-PrOH
91
a The byproduct was only isobutene. b The reaction was conducted at 175
°C. c The ionic liquid was used for the third time.
† This work was financially supported by the Natural Science Foundation of
China (No.20225309)
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CHEM. COMMUN., 2003, 1054–1055
This journal is © The Royal Society of Chemistry 2003

the same time, the conversion decreased to 45% while no
isobutene formed if cmimBF4 was used as catalyst (entry 5).
All the results mentioned above show that an imidazolium
cation with a substituted alkyl chain of about ten carbons was
the best catalyst for this etherization reaction. The experiment
results also showed that no longer equilibrium time for the
reaction achieving high conversion was needed and no catalyst
deactivation was observed when the ionic liquid was reused
several times because similar conversion and selectivity were
obtained in comparison with that of the ionic liquid used for the
first time (entry 6), this result was much better than that reported
previously.11
Surprisingly, no reaction occurred when ionic liquids
bmimCl, bmimPF6 and BPyBF4 were used as catalysts (entries
7–9). This showed that both the anions and cations of the ionic
liquids had a strong impact on the catalytic activity. Good
results were also obtained when dmimBF4 catalyst was further
used in the synthesis of other two important tert-butyl ethers,
i.e. ETBE and IPTBE (entries 10, 11), which conversions were
95% and 91% with more than 90% of selectivities.
As to the reaction mechanism of this process, it is not clear at
this stage. The understanding of the mechanism of catalysis in
ionic liquids is still in its infancy. The experimental results
showed that poor results were obtained over the ionic liquids
either with stronger acidic BMImPF6 or with stronger basic
bmimCl in comparison with the excellent catalytic performance
given by non-acid or neutral dmimBF4. The ionic liquids are
composed only of ions, such ionic nature can affect the course
of the reactions. It can be conjectured that the strong
electrostatic field possessed by the dialkyl imidazolium ionic
liquids mediated with length of side alkyl chain substituents,
may play an important role in initiating the desired reaction and
a traditional acid or base catalytic mechanism was not involved
in this process.
In summary, this, to the best of knowledge, is the first report
of a non-acid and highly effective ionic liquid catalyst system
for synthesis of tert-butyl ether, and it was also the first time to
show that an ionic liquid could also be an effective dehydration
agent. Further investigation about the reaction mechanism is
now on-going.
Notes and references
1 G. J. Hutchings, C. P. Nicolaides and M. S. Scurrel, Catal. Today., 1994,
23.
2 K. P. Jong, W. Bosch and T. D. B. Morgan, Surf. Sci. Catal., 1995, 96,
15.
3 S. Gao and J. B. Moffat, Catal. Lett., 1996, 42, 105.
4 R. Le Van Mao, R. Carli, H. Ahlafi and V. Ragaini, Catal. Lett., 1990,
6, 321.
5 P. Chu and G. H. Kunl, Ind. Eng. Chem. Res., 1987, 26, 365.
6 A. Bylina, J. M. Adams, S. H. Graham and J. M. Thomas, J. Chem. Soc.
Chem. Commum., 1980, 1003.
7 J. F. Knifton and J. R. Sanderson, USPatent., 5220078.
8 J. F. Knifton, Pei-Shing E. Dai, USPatent., 5716896.
9 M. A. Salomón, J. Coronas, M. Menéndez and J. SantamarAia, Appl.
Catal. A: gen., 2000, 200, 201.
10 J. F. Knifton and J. C. Edwards, Appl. Catal. A: gen., 1999, 183, 1.
11 G. D. Yadav and N. Kirthivasan, Chem. Commun., 1995, 203.
12 Q. H. Xia, K. Hidajat and S. Kawiq, J. Catal., 2002, 211, 566.
13 T. Welton, Chem. Rev., 1999, 99, 2071.
14 J. Dupont, G. S. Fonseca, A. P. Umpierre, P. F. P. Fichtner and S. R.
Teixeira, J. Am. Chem. Soc., 2002, 124, 4228.
15 Y. Chauvin and H. Oliver-Bourbigou, Chem. Tech., 1995, 25, 26.
16 P. Wasserscheid and W. Keim, Angew. Chem. Int. Ed., 2000, 39,
3772.
17 A. Bontempi, E. Alessio, G. Chanos and G. Mestroni, J. Mol. Catal.,
1987, 42, 67.
18 P. Bonhote, A. P. Dias, N. papageorgiou, K. Kalyanasundaram and M.
Grätzel, Inorg. Chem., 1996, 35, 1168.
CHEM. COMMUN., 2003, 1054–1055
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