Unraveling the cation and anion effects and kinetics for ionic liquid catalyzed direct synthesis of methyl acrylate under mild conditions

Article abstract of DOI:10.1039/d0gc03133j

The direct synthesis of methyl acrylate (MA) from methyl acetate and trioxane at 350-380 °C is regarded as a supplementary route for the industrial propylene oxidation process; however, it suffers from rapid catalyst deactivation. Herein, a novel ionic liquid catalyzed mild liquid-phase system was developed for the direct synthesis of MA from methyl acetate and trioxane, where N,O-bis(trimethylsilyl)acetamide (BSA) was used as a probase for α-deprotonation and enol silyl etherification of methyl acetate. The trioxane decomposition to formaldehyde and methyl acetate enolization to 1-methoxy-1-trimethylsilyloxyethene proceeded with the catalysis of [Cation]Cl/MClx (M = Cu+, Fe3+, Zn2+ and Al3+) and [Cation]F, respectively. The cations and anions were observed to have significant effects on the yield and selectivity of MA, owing to the steric hindrance, acid site category and strength confirmed by pyridine probing FT-IR characterization. As a result, up to 60.2% yield with 94.6% selectivity of MA could be achieved when [N3,3,3,3]F and [N3,3,3,3]Cl/AlCl3 with 67 mol% AlCl3 were used in the presence of BSA at 25 °C. Kinetic studies indicated that the trioxane decomposition with the activation barrier of 41.2 ± 0.3 kJ mol-1 was the rate-determining step.

Full text of DOI:10.1039/d0gc03133j

View Article Online
View Journal
Green
Chemistry
Cutting-edge research for a greener sustainable future
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: G. Wang, Z. Li, C.
Li and S. Zhang, Green Chem., 2020, DOI: 10.1039/D0GC03133J.
Volume 20
Number
May 2018
Pages 1919-2160
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
9
7
Green
Chemistry
Cutting-edge research for a greener sustainable future
rsc.li/greenchem
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 1463-9262
PAPER
Paul T. Anastas et al.
The Green ChemisTREE: 20 years after taking root with the 12
principles
rsc.li/greenchem

Page 1 of 31
Green Chemistry
View Article Online
DOI: 10.1039/D0GC03133J
1
2
Unraveling the Cation and Anion Effects and Kinetics for Ionic Liquid
Catalyzed Direct Synthesis of Methyl Acrylate at Mild Condition
3
4
5
6
7
8
9
0
1
2
3
4
Gang Wang a, b, Zengxi Li a, b*, Chunshan Li c, d*, Suojiang Zhang c, d
a
School of Chemical Engineering, University of Chinese Academy of Sciences, Beijing, 101408,
China
b
CAS Key Laboratory of Green Process and Engineering, Institute of Process Engineering,
Chinese Academy of Sciences, Beijing, 100190, China
c
State Key Laboratory of Multiphase Complex Systems, Beijing Key Laboratory of Ionic Liquids
Clean Process, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190,
China
1
1
1
1
1
d
Innovation Academy for Green Manufacture, Chinese Academy of Sciences, Beijing, 100190,
China
Corresponding E-mail: zxli@ipe.ac.cn (Prof. Z.X. Li)
csli@home.ipe.ac.cn (Prof. C.S. Li)
1
1
1
1
1
2
2
2
2
2
2
5
6
7
8
9
0
1
2
3
4
5
The direct synthesis of methyl acrylate (MA) from methyl acetate and trioxane at 350-
o
380 C is regarded as a supplementary route for the industrial propylene oxidation
process, however, it suffers rapid catalyst deactivation. Herein, a novel ionic liquid
catalyzed mild liquid-phase system was developed for direct synthesis of MA from
methyl acetate and trioxane, where the N, O-bis(trimethylsilyl) acetamide (BSA) was
used as probase for α-deprotonation and enol silyl etherification of methyl acetate. The
trioxane decomposition to formaldehyde and methyl acetate enolization to 1-methoxy-
+
1-trimethylsilyloxyethene proceeded with the catalysis of [Cation]Cl/MCl (M = Cu ,
x
3
+
2+
3+
Fe , Zn and Al ) and [Cation]F, respectively. The cation and anion were observed
to have significant effects on yield and selectivity of MA, owing to the steric hindrance,
acid site category and strength confirmed by pyridine probing FT-IR characterization.
1

Green Chemistry
Page 2 of 31
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
As a result, up to 60.2% yield with 94.6% selectivity of MA could be achieved when
[N3,3,3,3]F and [N3,3,3,3]Cl/AlCl with 67 mol.% AlCl were used in the presence of
3
3
o
BSA at 25 C. The kinetic studies indicated the trioxane decomposition with activation
-1
barrier of 41.2 ± 0.3 kJ mol was the rate-determining step.
Keywords: Methyl acrylate, Ionic liquid, Aldol condensation, Liquid-phase system,
Kinetic
Introduction
Methyl acrylate (MA) as a very important chemical stock is widely used in the
field of paints, polyacrylate materials and acrylic plastics production. 1-3 The industrial
routes to MA are mainly based on two-step propylene oxidation, which includes
selective oxidation to acrolein first on Mo-Bi and then to acrylic acid on Mo-V catalyst,
and acrylonitrile hydrolysis. 4-7 However, the propylene oxidation process is recently
restricted by the gradual depletion resource of fossil oil and increase of petroleum price.
And the acrylonitrile hydrolysis technology needs large amounts of poisonous
hydrogen cyanide as raw material, leading to the production of equivalent mole of
1
1
1
1
1
1
1
1
1
1
2
2
2
8
ammonium sulfate wastes, which are relatively difficult to dispose. In recent years,
the coal-based chemicals and their downstream products, especially for formaldehyde,
trioxane, methyl acetate and methanol, are faced with the challenging problems of
overcapacity and commercial marketing. Hence, the development of new strategy for
production of MA based on the utilization of these low-cost and well-sourced feedstock
has stimulated researchers’ interests.
Aldol condensation as an effective and atom-economic reaction is widely used for
2

Page 3 of 31
Green Chemistry
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
C-C and C=C construction. 9-12 Since Ai reported the gaseous one-step synthesis of MA
from coal-based products of methyl acetate and formaldehyde (or trioxane) via aldol
o
13-16
reaction at 350-380 C, it has been paid more attention to.
Up to now, series of VPO
and Cs-P based acid-base bifunctional catalysts have been developed for this attractive
process. Feng developed a kind of VPO catalyst with high fraction of δ-VOPO entity
4
and density of medium strong acid sites, which was fabricated by employing poly
ethylene glycol additive, for conversion of methyl acetate and formaldehyde with the
highest formation rate of 19.8 μmol gcat-1 min for desired acrylic acid and MA. Guo
conducted this reaction on VPO composite and found the P/V ratio had great influence
on not only the physicochemical properties of catalyst, but also catalytic activity and
-1
17
1
1
1
1
1
1
1
1
1
1
2
2
2
o
selectivity, the highest yield of MA could reach 75.9 % at 370 C when the P/V ratio
was 1.2. 18 Yang modified the VPO compound with zirconium element, which would
be distributed in the surface adjacent layers and partially incorporated into VPO species,
to enhance the fraction of β-VOPO and distorted entity of (VO) P O , which could
4
2
2
7
promote the formation rate of MA and inhibit the carbon deposition. 19, 20 While Zhao
reported a VPO supported γ-Al O catalyst and found the yield of MA could be
2
3
enhanced from 35% to 42% when the γ-Al O support was pre-treated with phosphoric
2
3
acid. 21 Zuo performed the reaction on VPO catalyst in the oxygen atmosphere, which
could efficiently inhibit carbon deposition and increase the yield and selectivity of MA.
2
2
Zhang observed the loading percentage of Cs and P elements would affect the acid-
base properties of Cs-P/γ-Al O catalyst and the highest yield of MA could climb to
2
3
o
46.2% at 380 C when the Cs and P element was loaded as 5 wt.% and 10 wt.%,
3

Green Chemistry
Page 4 of 31
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
respectively. 23 Jiang carried out the reaction on Cs-P/γ-Al O catalyst with 5 wt.% Cs
2 3
and 10 wt.% P loading in fluidized bed reactor, which solved the problems of rapid
deactivation and regeneration of catalyst, and the yield of MA could still remain 39.5%
2
4
after 1000 h of lifetime evaluation. Zuo utilized ZSM-5 instead of γ-Al O as support
2
3
to modulate the acid-base properties of Cs-P catalyst and the maximum yield of MA
o
25, 26
could attain 43% at 370 C.
He prepared Cs supported silica catalyst for MA
synthesis and conducted DFT calculation, revealing the appropriate amount of Cs was
required since much higher loading would lead to excessive decomposition of methyl
acetate and MA. 27, 28 Yan reported a Cs/SBA-15 catalyst with weak Lewis acid-base
pairs loaded on the surface, the highest yield and selectivity of MA could reach 46.0%
and 95.0%, respectively. 29
1
1
1
1
1
1
1
1
1
1
2
2
2
Except for the VOP and Cs-P catalysts, Bao reported barium based catalysts
including Ba/Al O , Ba/γ-Ti-Al O and Ba-La/Al O for MA production, and the yield
2
3
2
3
2
3
could reach up to 45.1% with a selectivity of 90.2%. 30-32 Although it has achieved
progress in vaporous one-step synthesis of MA from methyl acetate and trioxane (or
formaldehyde) via heterogeneous catalysis, the problem of rapid catalyst deactivation
and regeneration at high temperature still confuses the researchers. Very recently, our
group developed the mild liquid-phase catalytic system for direct synthesis of MA from
methyl acetate and trioxane (or methanol) with the highest 90.7% yield and 91.8%
selectivity, and found the α-deprotonation of methyl acetate and trioxane
decomposition (or methanol dehydrogenation to formaldehyde) were the crucial steps.
3
3-38
However, both of the trimethylsilyl trifluoromethanesulfonate and dibutylboryl
4

Page 5 of 31
Green Chemistry
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
trifluoromethanesulfonate for methyl acetate activation are not stable at lab condition
and require the experimental operation under inert atmosphere. In addition, the
generated silicon and boron wastes are difficult to recycle, and the effects of cation and
anion of ionic liquid catalyst are still unknown.
It is reported that the decomposition of trioxane can proceed with catalysis of
strong Lewis acid 33-36, and the α-deprotonation and enolization of ester will undergo
with catalysis of fluoride ion in the presence of probase N, O-bis(trimethylsilyl)
acetamide (BSA) 39, 40. Stimulated by these discoveries, a novel and efficient ionic
liquid catalyzed reaction system was developed for direct synthesis of MA from methyl
o
1
1
1
1
1
1
1
1
1
1
2
2
2
acetate and trioxane in the presence of BSA at 25 C, which can be operated under lab
atmosphere. The effects of cation and anion of ionic liquid on yield and selectivity of
MA were investigated and demonstrated with pyridine probing FT-IR. The reaction
pathway for MA production was proposed based on the mechanistic experiments and
intermediates determined by GC-MS. Also, the kinetic studies were conducted to obtain
the parameters and activation barriers, which will promote to have a better
understanding on this new process. In addition, the ionic liquid catalyst can be recycled
and the silicon compounds generated during reaction can be recovered for BSA
preparation.
Experimental
Main materials
Methyl acetate (purity ≥ 99.0%), AgF (purity of 99.0%) and N, O-
bis(trimethylsilyl) acetamide (BSA, purity ≥ 97.0%) were purchased from J&K
5

Green Chemistry
Page 6 of 31
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
Scientific Ltd., China. Dry dichloromethane (purity ≥ 99.5%) and trioxane (purity of
99.5%) were provided by Xilong Chemical Co., Ltd., China. Octane (purity ≥ 99.5%)
was supported by Aladdin Industrial Co., China. All of the ionic liquids (purity ≥ 99.0%)
were supplied by Linzhou Keneng Materials Technology Co. Ltd., China. It should be
noted that the reactants used for synthesis reactions should be dehydrated by 5 Å zeolite.
Preparation of ionic liquid catalysts
The fluoride-type ionic liquids ([Cation]F) were synthesized from the
corresponding chloride-type ionic liquids ([Cation]Cl) and AgF, and the general
procedure was described as follows. The aqueous AgF (1.02 eq.) solution was dropwise
added into the ethanol solution of [Cation]Cl (1 eq.) at room temperature under
vigorous stirring. Then the generated precipitates were filtered off and the filtrate was
1
1
1
1
1
1
1
1
1
1
2
2
2
o
placed under high vacuum at 40 C to remove the solvent. After that, the obtained crude
[Cation]F ionic liquid was dissolved in ethanol and then filtered off again to remove
o
the remaining AgF. Afterwards, the filtrate was placed under high vacuum at 40 C
1
9
again, giving the needed [Cation]F ionic liquid with required purity confirmed by F-
NMR (Table S1, ESI).
+
3+
2+
3+
The Lewis acid-type ionic liquids ([Cation]Cl/MCl , M = Cu , Fe , Zn and Al )
x
were prepared from metal chloride and corresponding [Cation]Cl ionic liquids
according to the following procedures. In a glove box, the suitable amount of metal
chloride was charged into a flask, then the flask was sealed and kept in the alcohol bath
o
under -20 C. Afterwards, the [Cation]Cl ionic liquid, which had been dried under high
o
vacuum at 90 C for 24 h, in a needed molar ratio was added dropwise into the flask
6

Page 7 of 31
Green Chemistry
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
under vigorous stirring and kept it for 4 h.
Synthesis reaction
The synthesis reactions for catalytic evaluation were carried out in a 50 mL round-
bottomed flask equipped with condenser and magnetic stirrer under atmospheric
pressure. The air and moisture inside it should be purged out by charging N before
2
adding the reagents and the reaction mixture was kept in a temperature-controlled water
bath. A typical reaction solution was consisted of methyl acetate, BSA, ionic liquid
catalyst ([Cation]F and [Cation]Cl/MCl ) and octane which was used as the internal
x
standard for quantitative analysis. To keep the concentrations consistent, solutions for
synthesis reactions with other reagents were prepared to have the same concentration
1
1
1
1
1
1
1
1
-1
as that of methyl acetate (0.1 mol L ). The methyl acetate in a needed molar ratio to
trioxane, octane and 5 wt.% ionic liquid catalyst were mixed in the solvent of CH Cl
2
2
prior to being charged into the flask. Then BSA with the needed ratio to methyl acetate
was added dropwise into the solution under the suitable stirring speed. The product
samples collected periodically were qualitatively and quantitatively analyzed on MS
and GC part of GC-MS. The yield and selectivity of MA were defined as equations (1)
and (2).
Cmethyl acetate, o - Cmethyl acetate, t
1
1
8
9
Yield of MA =
× 100%
(1)
(2)
Cmethyl acetate, o
CMA, t
Selectivity of MA =
× 100%
Cmethyl acetate, o - Cmethyl acetate, t
2
2
0
1
Where the Cmethyl acetate, o is the initial concentration of methyl acetate before
reaction, Cmethyl acetate, t and CMA, t is respect to the concentration of methyl acetate and
7

Green Chemistry
Page 8 of 31
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
MA after reaction.
For the understanding of reaction pathway, the synthesis reaction in the absence
of [Cation]Cl/MCl and the treatment of methyl acetate with BSA and [Cation]F in the
x
medium of CH Cl were also performed under similar condition. The decomposition of
2
2
trioxane (0.1 mol/L) without methyl acetate catalyzed by [N3,3,3,3]Cl/MCl (5 wt.%)
x
o
in the solvent of CH Cl was conducted at 25 C. While the synthesis of MA from 1-
2
2
methoxy-1-trimethylsilyloxyethene (0.1 mol/L) and trioxane (0.1 mol/L) with catalysis
o
of [N3,3,3,3]Cl/MCl (5 wt.%) in the solvent of CH Cl was conducted at 25 C.
x
2
2
Product analysis and quantification
1
1
1
1
1
1
1
1
1
1
2
2
2
The product samples were quantitatively analyzed on GC part of GC-MS (QP
2020, Shimadzu) equipped with an Rtx-5MS column (30 m × 0.32 mm × 0.25 μm) by
using octane as internal standard. Each component in reaction mixture was identified
by comparison with the standards and MS information. The initial temperature of oven
o
o
o
-1
was 40 C and hold for 2 min, then increased to 300 C at the rate of 15 C min and kept
o
o
for 5 min. The injector and detector temperature was 300 C and 250 C. The relative
mass correction factor of each component to octane was obtained from the standard
mixture samples. For other compounds without the standard sample, it was estimated
with effective carbon number method. 41, 42
Characterization methods
1
9
The F-NMR analysis was conducted on a Bruker AVANCE instrument (600
MHz) and the spectra were recorded at ambient temperature. All of the pyridine probing
FT-IR spectra were obtained from a Nicolet Fourier transform infrared
8

Page 9 of 31
Green Chemistry
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
spectrophotometer at room temperature and the samples were prepared by mixing
pyridine, which was firstly dried with KOH and distilled over 4 Å zeolite, with ionic
liquid catalyst in the volume ratio of 1:5 under inert atmosphere.
Results and discussion
Catalytic system evaluation
First of all, the ionic liquid catalyzed system for direct synthesis of MA from
o
methyl acetate and trioxane in liquid-phase at 25 C was constructed and evaluated, and
the results were shown in Table 1. Without probase BSA (Entry 1, 2 and 3), no product
of MA can be detected even if [N3,3,3,3]F is used in combination with
1
1
1
1
1
1
1
1
1
1
2
2
2
[N3,3,3,3]Cl/AlCl ionic liquid. In the presence of BSA, there’s still no MA is recorded
3
on GC-MS when [N3,3,3,3]F (Entry 4) or [N3,3,3,3]Cl/AlCl (Entry 5) ionic liquid is
3
used alone. Only if the [N3,3,3,3]F is utilized with [N3,3,3,3]Cl/AlCl ionic liquid in
3
the existence of BSA (Entry 6), 60.2% yield of MA is achieved with a selectivity of
94.6%, which is comparable to the bulk VPO catalytic system 17-20 and much higher
than that obtained under the catalysis of Cs-P supported materials at 350-380 C 23-29.
As reported in literatures, the trioxane should be decomposed into formaldehyde
monomer, which then undergoes aldol condensation with the enolate of methyl acetate
for MA production, and the fluoride ion can catalyze the cleavage of Si-O in BSA to
generate the strong onium amide base, which is very efficient for α-deprotonation and
enol silyl etherification of ester. 33, 40 When the reactions were conducted in the absence
of BSA (Entry 1, 2 and 3), the onium amide base would not generate for α-
deprotonation and enol silyl etherification of methyl acetate, so the aldol condensation
9
o

Green Chemistry
Page 10 of 31
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
8
between formaldehyde and the enolate of methyl acetate for production of MA could
not proceed. Very similarly, the onium amide base will not produce without catalysis
of [N3,3,3,3]F (Entry 5), despite the BSA exists in the reaction system and
formaldehyde monomer will generate with catalysis of [N3,3,3,3]Cl/AlCl . Although
3
the strong onium base will form with the catalysis of [N3,3,3,3]F for α-deprotonation
and enol silyl etherification of methyl acetate (Entry 4), the trioxane unit can’t undergo
aldol condensation with the enolate of methyl acetate. Therefore, the [N3,3,3,3]F must
be used in combination with [N3,3,3,3]Cl/AlCl ionic liquid in the presence of BSA for
3
9
0
production of MA.
1
1
Table 1. Catalytic system evaluation for MA synthesis from methyl acetate and trioxane a
O
O
O
+
1/3
O
O
O
O
1
Entry
Ionic liquid catalyst b
[N3,3,3,3]F
probase Yield of MA /%
Selectivity of MA /%
1
2
3
4
5
-
0
0
-
[N3,3,3,3]Cl/AlCl3
-
-
[N3,3,3,3]F and [N3,3,3,3]Cl/AlCl3
[N3,3,3,3]F
-
0
-
BSA
BSA
BSA
0
-
-
[N3,3,3,3]Cl/AlCl3
0
6
[N3,3,3,3]F and [N3,3,3,3]Cl/AlCl3
60.2
94.6
a
1
1
1
2
3
4
Reaction condition: 0.1 mol/L trioxane, methyl acetate and BSA (ntrioxane : nmethyl acetate: nBSA = 1:1:1)
o
in 20 mL CH Cl with 5 mol.% of catalyst at 25 C for 3 h.
b
2
2
The fraction of AlCl in [Cation]Cl/AlCl ionic liquid is 67 mol.%.
3
3
1
1
5
6
Effects of cation of [Cation]F and [Cation]Cl/AlCl ionic liquids
3
The studies on fluoride ion catalyzed probase method for α-deprotonation of ester
1
0

Page 11 of 31
Green Chemistry
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
conducted by Teng and Kondo revealed the cation in generated onium amide base
would affect the yield and selectivity of product. 39, 43 Thus, the effects of cation in
fluoride and Lewis acid-type ionic liquids were investigated and the cation in these two
kinds of ionic liquids were kept the same to ensure the unambiguous result analysis. In
this part of work, three types of cation including N, N-disubstituted imidazolium, N-
substituted pyridine and quaternary ammonium were selected for this catalytic system
and the results were shown in Fig. 1.
Yield
Conversion of methyl acetate
Selectivity Conversion of trioxane
1
00
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
[
]
]
]
]
]
]
]
]
]
]
]
]
y
y
y
y
2
,
3
4
8
,
M
M
M
,
,
I
I
M
I
P
P
P
P
2
,
3
8
,
I
4
E
B
H
O
,
,
M
M
M
[
[
[
2
,
3
4
8
,
M
[
,
,
E
P
2
3
8
4
[
[
B
H
[
N
N
N
N
[
[
[
[
Cation
8
9
Fig. 1. Effects of cation of [Cation]F and [Cation]Cl/AlCl ionic liquids on catalytic performance
3
1
0
1
2
3
4
5
6
7
at the reaction condition of 0.1 mol/L trioxane, methyl acetate and BSA (ntrioxane : nmethyl acetate: nBSA
= 1:1:1) in 20 mL CH Cl , 5 mol.% of [Cation]F and [Cation]Cl/AlCl (with 67 mol.% AlCl )
1
1
1
1
1
1
1
2
2
3
3
o
ionic liquids, 25 C, 3 h. The full names of cation abbreviated in the figure are as follows: [EMIM]
(1-Ethyl-3-methylimidazolium), [PMIM] (1-Propyl-3-methylimidazolium), [BMIM] (1-Butyl-3-
methylimidazolium), [HMIM] (1-Hexyl-3-methylimidazolium), [EPY] (N-ethylpyridinium),
[BPY] (N-butylpyridinium), [HPY] (N-hexylpyridinium), [OPY] (N-octylpyridinium), [N2,2,2,2]
(Tetraethyl ammonium), [N3,3,3,3] (Tetrapropyl ammonium), [N4,4,4,4] (Tetrabutyl ammonium),
[N8,8,8,8] (Tetraoctyl ammonium)
1
1
2
8
9
0
As it can be seen that the cation of [Cation]F and [Cation]Cl/AlCl ionic liquids
3
has significant influence on the yield and selectivity of MA. And it also reveals a very
interesting phenomenon that the yield and selectivity of MA and the conversion of
1
1

Green Chemistry
Page 12 of 31
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
methyl acetate increase first to the maximum value and then decrease with the extension
of substituted alkyl chain length. The catalytic performance of these cation series on
MA production follow the order of N-substituted pyridine < N, N-disubstituted
imidazolium < quaternary ammonium. Among the cation tested, the [N3,3,3,3]
exhibited the highest performance with the 60.2% yield and 94.6% selectivity towards
MA. However, the conversion of trioxane is not affected by the cation, indicating the
trioxane decomposition is determined by the anion of [Cation]Cl/AlCl that exhibits
3
strong Lewis acidity.
It has been confirmed the activation and breakage of Si-O bond in BSA undergoes
with the catalysis of [Cation]F, generating the acetamide anion which interacts with the
cation to form onium amide base. Although the catalytic activity of [Cation]F derives
from the fluoride ion, the steric hindrance and molecular voidage of cation will increase
with the extension of substituted alkyl chain length, 44-46 which will enhance the
capacity of BSA molecules in ionic liquid clusters and promote the interaction with
catalytic centers. As a result, the transformation of BSA into onium amide base can be
accelerated, contributing to the α-deprotonation and enol silyl etherification of methyl
acetate. However, the further extension of alkyl chain length will increase the steric
hindrance of onium amide base, leading to the inhibition of α-deprotonation of methyl
acetate to enolate for the followed aldol reaction between the enolate of methyl acetate
and formaldehyde. This explanation is consistent with the experimental fact that the
yield of 1-methoxy-1-trimethylsilyloxyethene produced from the enol silyl
etherification of methyl acetate is affected by the steric hindrance of cation in [Cation]F
1
1
1
1
1
1
1
1
1
1
2
2
2
1
2

Page 13 of 31
Green Chemistry
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
ionic liquid (Table S2, ESI). So it can be concluded that the cation with suitable steric
hindrance can not only conduce to the generation of onium amide base from BSA, but
also promote the enol silyl etherification of methyl acetate.
Effects of metal ion in [N3,3,3,3]Cl/MCl ionic liquid
x
It has been believed that when [Cation]Cl ionic liquid was mixed with metal
chloride, the [Cation]Cl/MCl shows Lewis acidity, however, the strength will be
x
affected by the metal ion. 40 Herein, series of [N3,3,3,3]Cl/MCl (M = Cu , Fe , Zn
+
3+
2+
x
3
+
and Al ) ionic liquid with 67 mol.% metal chloride was used in combination with
[N3,3,3,3]F for catalytic synthesis of MA from methyl acetate and trioxane, and the
results were shown in Fig. 2. Also, the decomposition of trioxane to formaldehyde
catalyzed by these four ionic liquids without methyl acetate were conducted and the
results were summarized in Table S3 (ESI). The catalytic performance of these ionic
liquid series on both trioxane decomposition and MA production can be ranked as
[N3,3,3,3]Cl/CuCl < [N3,3,3,3]Cl/FeCl < [N3,3,3,3]Cl/ZnCl < [N3,3,3,3]Cl/AlCl .
1
1
1
1
1
1
1
1
3
2
3
In addition, the catalytic activity of these ionic liquids on synthesis of MA from 1-
methoxy-1-trimethylsilyloxyethene, which is the enolate of methyl acetate, and
trioxane follows the same trend, which is shown in Table S4 (ESI).
1
3

Green Chemistry
Page 14 of 31
View Article Online
DOI: 10.1039/D0GC03133J
1
00
100
80
60
40
20
0
Selectivity
Yield Conversion
8
6
4
2
0
0
0
0
0
3
l
C
l 3
l 2
l
lC 3
u
C
C
n
e
l/
C
/F
Z
l/
A
l
l/
C
C
]
]
C
C
,
3
,3
]
,3
]
,3
,
,3
,3
,3
,
3
,3
,3
,3
3
3
3
3
N
N
[
[
N
N
[
[
Lewis acid-type [N3,3,3,3]Cl/MCl ionic liquid
1
2
3
4
x
Fig. 2. Effects of metal ion in [N3,3,3,3]Cl/MClx on catalytic performance at the reaction
condition of 0.1 mol/L trioxane, methyl acetate and BSA in 20 mL CH Cl , 5 mol.% of
2
2
o
[N3,3,3,3]F and [N3,3,3,3]Cl/MCl with 67 mol.% MCl , 25 C, 3 h.
x
x
5
6
7
8
9
0
1
2
3
4
5
6
7
To interpret this observed experimental phenomenon, the pyridine probing FT-IR
characterization was conducted to measure the acid properties of these ionic liquid
series and the results were presented in Fig. 3. It can be found that [N3,3,3,3]Cl/AlCl3
and [N3,3,3,3]Cl/CuCl have both Brønsted and Lewis acid sites, however,
[N3,3,3,3]Cl/FeCl and [N3,3,3,3]Cl/ZnCl have only Lewis acid sites. By comparing
3
2
1
1
1
1
1
1
1
1
the Lewis acid site densities of these four ionic liquids, it follows the order of
[N3,3,3,3]Cl/CuCl < [N3,3,3,3]Cl/FeCl < [N3,3,3,3]Cl/ZnCl < [N3,3,3,3]Cl/AlCl ,
3
2
3
which agrees with the trend of catalytic performance. Although the Brønsted acid sites
in [N3,3,3,3]Cl/AlCl and [N3,3,3,3]Cl/CuCl ionic liquids are also effective for the
3
decomposition of trioxane into formaldehyde, it has no catalytic activity on
condensation of 1-methoxy-1-trimethylsilyloxyethene with formaldehyde to produce
MA. Similar results were also obtained in the aldol condensation between methyl
acetate and benzaldehyde. 40 Thus, it can be concluded that the Lewis acid-type
1
4

Page 15 of 31
Green Chemistry
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
[N3,3,3,3]Cl/MCl ionic liquid can catalyze not only the decomposition of trioxane into
x
formaldehyde, but also the condensation between the enolate of methyl acetate (1-
methoxy-1-trimethylsilyloxyethene) and formaldehyde to produce MA. And the metal
ion in [N3,3,3,3]Cl/MCl will affect the Lewis acid site density, which is required for
x
this catalytic system.
[
N3,3,3,3]Cl/AlCl3
N3,3,3,3]Cl/FeCl3
[N3,3,3,3]Cl/ZnCl2
[N3,3,3,3]Cl/CuCl
[
Lewis acid site Bronsted acid site
1
300
1400
1500
Wavenumber cm
Fig. 3. Pyridine probing FT-IR spectra of [N3,3,3,3]Cl/MCl ionic liquid with 67 mol.% of MClx
1600
-1
6
7
8
x
+
3+
2+
3+
(M = Cu , Fe , Zn and Al )
9
0
1
2
3
4
5
6
7
Effects of anion species in [N3,3,3,3]Cl/AlCl ionic liquid
3
1
As reported by Kou that the anion species existing in [Bmim]Cl/AlCl ionic liquid
3
1
1
1
1
1
1
1
and the Lewis acid strength would be affected by the molar fraction of AlCl , changing
3
-
-
47
from AlCl to Al Cl with the molar fraction increasing from 50% to 67%. To
4
2
7
investigate the effects of anion species on catalytic activity, the [N3,3,3,3]Cl/AlCl3
ionic liquid with 50 mol.%, 55 mol.%, 60 mol.% and 67 mol.% of AlCl were prepared
3
for catalytic synthesis of MA from methyl acetate and trioxane. It can be seen from Fig.
4 that both of the trioxane decomposition and MA production can be promoted by
increasing the molar fraction of AlCl from 50% to 67%, revealing the anion species of
3
1
5

Green Chemistry
Page 16 of 31
View Article Online
DOI: 10.1039/D0GC03133J
-
-
1
Al Cl shows much higher activity than that of AlCl .
2
7
4
1
00
100
80
60
40
20
0
Selectivity
Yield
Conversion
8
6
4
2
0
0
0
0
0
Anion species in [N3,3,3,3]Cl/AlCl3
-
-
-
-
AlCl4
AlCl + Al Cl
2
Al Cl7
4
2
7
0
.50
0.55
0.60
0.65
0.70
Molar fraction of AlCl in [N3,3,3,3]Cl/AlCl3
3
2
3
4
5
Fig. 4. Effects of molar fraction of AlCl in [N3,3,3,3]Cl/AlCl on catalytic performance at the
3 3
reaction condition of 0.1 mol/L trioxane, methyl acetate and BSA in 20 mL CH Cl , 5 mol.% of
2
2
o
[N3,3,3,3]F and [N3,3,3,3]Cl/AlCl , 25 C, 3 h.
3
6
7
8
9
0
1
2
3
4
5
6
7
Then the pyridine probing FT-IR characterization for these [N3,3,3,3]Cl/AlCl3
ionic liquids with different molar fraction of AlCl were performed and the results were
3
presented in Fig. 5. It unravels that with the increasing molar fraction of AlCl , the peak
3
for the coordination of pyridine at the position of Lewis acid sites shifts to higher
wavenumber, suggesting their interaction becomes more intense at higher molar
1
1
1
1
1
1
1
1
-
-
fraction, namely the Lewis acidity of Al Cl is much stronger than that of AlCl .
2
7
4
Therefore, it can be considered that the predominant anion species in
[N3,3,3,3]Cl/AlCl affected by the molar fraction of AlCl has significant effects on
3
3
both trioxane decomposition and MA production, owing to the change in strength of
-
Lewis acidity. And the anion species of Al Cl , which exhibits stronger Lewis acidity,
2
7
shows higher catalytic performance. Similar phenomenon can be observed by
comparing the reported results of silyl enol ether with boron enol ether reaction system.
1
6

Page 17 of 31
Green Chemistry
View Article Online
DOI: 10.1039/D0GC03133J
3
3-36
1
6
7 mol.% AlCl3
5 mol.% AlCl3
60 mol.% AlCl3
50 mol.% AlCl3
5
Lewis acid site
1400
1300
1500
1600
-1
Wavenumber cm
2
3
Fig. 5. Pyridine probing FT-IR spectra of [N3,3,3,3]Cl/AlCl with different molar fraction of AlCl
3
3
4
5
6
7
8
9
0
1
2
3
4
5
6
Reaction pathways
As mentioned above, the trioxane should be decomposed into formaldehyde
monomer before proceeding condensation with the enolate of methyl acetate to form
MA. When the reaction was conducted with the catalysis of [Cation]F, no formaldehyde
and MA were detected, but the 1-methoxy-1-trimethylsilyloxyethene as enolate of
methyl acetate and N-trimethylsilyl acetamide were recorded on GC-MS (MS
information are provided in ESI). Only [Cation]F was used in combination with
[Cation]Cl/MCl , the formaldehyde and MA generated. Actually, the [Cation]Cl/MCl
1
1
1
1
1
1
1
x
x
with Lewis acidity is also effective for trioxane decomposition except for protonic acid
(Table S3). In addition, the reaction between 1-methoxy-1-trimethylsilyloxyethene and
trioxane with catalysis of [N3,3,3,3]Cl/MCl in the medium of CH Cl can also gave
x
2
2
the product of MA with almost 100% selectivity after 2.5 h (Table S4). The GC-MS
analysis for the components in the terminal reaction mixture showed the reaction
1
7

Green Chemistry
Page 18 of 31
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
between methyl acetate and trioxane with catalysis of [Cation]F and [N3,3,3,3]Cl/MClx
gave the main product of MA. Otherwise, a small amount of byproducts of
trimethylsilyl acetate and trimethylsilyl acrylate were detected (MS information were
provided in ESI).
O
O
TMS
O
N
O
H
O
O
[
Cation]
O[Cation]
+ TMS F
O
TMS
+
TMS F
N
O
(
onium amide base)
HCHO
Si OH
Si OH
O
O
O
TMS
TMS
O
TMS
TMS
[
Cation]Cl/MClx
O
[
Cation]F
O
N
CH OH
3
O
(
BSA, probase)
(
1-methoxy-1-trimethylsilyloxyethene)
(MA)
5
6
Scheme 1. Proposed reaction pathways for ionic liquid catalyzed direct synthesis of MA from
7
methyl acetate and trioxane in mild liquid-phase
8
On the basis of these above experimental findings and confirmed substrates, the
9
0
1
2
3
4
5
6
7
8
9
reaction pathway for MA production in this mild liquid-phase catalytic system was
proposed as Scheme 1. The probase of BSA is firstly transformed into the strong onium
amide base with catalysis of [Cation]F, followed by the α-deprotonation of methyl
acetate to onium enolate and generation of N-trimethylsilyl acetamide. 39, 43 The highly
active onium enolate will transform to 1-methoxy-1-trimethylsilyloxyethene through
the interaction with trimethylsilyl group, during which the [Cation]F can be recycled.
Meanwhile, the trioxane is decomposed into three units of formaldehyde with catalysis
of [Cation]Cl/MCl . Then the [Cation]Cl/MCl catalyzed condensation between the
1
1
1
1
1
1
1
1
1
1
x
x
generated formaldehyde and 1-methoxy-1-trimethylsilyloxyethene, which is also called
as Mukaiyama reaction, proceeds to produce MA and trimethylsilanol. While the
generated trimethylsilanol will undergo transesterification with methyl acetate and
1
8

Page 19 of 31
Green Chemistry
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
methyl acrylate, leading to the consumption of reactant and decrease in selectivity of
MA. Among all these steps, the trioxane decomposition, onium amide base generation
and methyl acetate deprotonation are significantly crucial for the formation of MA in
this mild liquid-phase reaction system as we considered.
Kinetic studies
With the understanding of cation and anion effects on catalytic performance of
[Cation]F and [Cation]Cl/MCl -type ionic liquids, the kinetic studies on this whole
x
transformative process was conducted. As described in Scheme 1, the main reaction
networks include generation of onium base, enolization of methyl acetate to 1-methoxy-
1-trimethylsilyloxyethene, decomposition of trioxane into formaldehyde monomer, and
condensation between 1-methoxy-1-trimethylsilyloxyethene and formaldehyde to MA.
To simplify the kinetic investigation, the trioxane decomposition into formaldehyde
without methyl acetate was carried out separately. It has reached a general agreement
that the trioxane observes a two-step decomposition mechanism on either Brønsted or
Lewis acid sites, namely the C-O activation caused ring opening and formation of a
linear trioxymethylene intermediate, followed by further decomposition into three
formaldehyde units. 33, 48 Fig. 6a shows the concentration evolution of trioxane during
1
1
1
1
1
1
1
1
1
1
2
2
2
o
decomposition reaction without methyl acetate at 20-35 C, which fits the first-order
model well in the region of acceptable deviation less than 5%, and the corresponding
rate constants are summarized in Table 2. The pre-exponential factor and activation
5
-1
barrier of trioxane decomposition can be calculated as 1.80×10 min and 41.2 ± 0.3
-1
kJ mol from the Arrhenius plot, as shown in Fig. 6b. Compared with the activation
1
9

Green Chemistry
Page 20 of 31
View Article Online
DOI: 10.1039/D0GC03133J
-1
49
1
2
3
4
barrier of 120.1 kJ mol obtained in concentrated hydrochloric solution, it can be
considered [N3,3,3,3]Cl/AlCl with 67 mol.% AlCl is an efficient ionic liquid catalyst
3
3
for the trioxane decomposition into formaldehyde, which will provide supplementary
reference in the research field of trioxane decomposition.
-
3.8
2.5
2.0
1.5
1.0
0.5
0.0
o
o
(a)
20 C
25 C
o
o
3
0 C
35 C
-4.0
-4.2
-4.4
-4.6
-4.8
-5.0
3
.20
3.25
3.30
3.35
000 K/T
3.40
3.45
0
20
40
60
80
100
120
5
6
Reaction time (t) /min
1
Fig. 6. Concentration evolution of trioxane (a) and Arrhenius plot (b) for kinetic investigation of
7
8
trioxane decomposition at the reaction condition of 0.1 mol/L trioxane in 20 mL CH Cl and 5
2
2
mol.% of [N3,3,3,3]Cl/AlCl with 67 mol.% AlCl
3
3
o
9
Table 2 Rate constants of trioxane decomposition at 20-35 C
o
3
-1
T / C
Kinetic equation
ln (CTrioxane, initial/CTrioxane, t) = (8.13 ± 0.05) ×10 t
Rate constant (k ) ×10 /(min )
1
-3
2
2
3
3
0
5
0
5
8.13 ± 0.05
10.80 ± 0.06
14.21 ± 0.10
18.53 ± 0.19
-3
ln (CTrioxane, initial/CTrioxane, t) = (10.80 ± 0.05) ×10 t
-3
ln (CTrioxane, initial/CTrioxane, t) = (14.21 ± 0.10) ×10 t
-3
ln (CTrioxane, initial/CTrioxane, t) = (18.53 ± 0.19) ×10 t
2
0

Page 21 of 31
Green Chemistry
View Article Online
O
DOI: 10.1039/D0GC03133J
k1
3
HCHO
(B)
(1-1)
O
O
O
(
A)
TMS
O
K2
TMS
+
TMS
+
TMS
(1-2)
N
N
(D)
(
C)
(E)
O
O
O
O
TMS
K3
TMS
+
(
1-3)
O
N
N
H
(H)
O
(
F)
(D)
(G)
O
O TMS
K4
TMS
E)
+
(
1-4)
O
(G)
O
(I)
(
O TMS
O
k5
+
HCHO
+ TMS OH
(1-5)
(1-6)
(1-7)
O
O
(
I)
(B)
(J)
(K)
O
O
k6
TMS
+
TMS OH
+ CH OH
3
O
k-6
O
(
M)
(
F)
(K)
(L)
O
O
k7
TMS
+
TMS OH
+ CH OH
3
O
k-7
O
(
M)
(
J)
(K)
(N)
1
2
3
Scheme 2. Proposed elemental steps for direct synthesis of MA from methyl acetate and trioxane
with catalysis of [N3,3,3,3]F and [N3,3,3,3]Cl/AlCl in the presence of BSA
3
4
5
6
7
8
9
0
1
Based on the kinetic studies on trioxane decomposition and the reaction pathways
illustrated in Scheme 1, the elemental steps including trioxane decomposition (1-1),
onium amide base generation (1-2), methyl acetate enolization (1-3 and 1-4), aldol
condensation (1-5) and transesterification (1-6 and 1-7) were proposed in Scheme 2. In
addition, the steps of onium amide base generation, methyl acetate enolization and
transesterification were considered as reversible. Compared with the much slower
condensation between 1-methoxy-1-trimethylsilyloxyethene and formaldehyde, both of
onium amide base generation and methyl acetate enolization steps, which are also
1
1
2
1

Green Chemistry
Page 22 of 31
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
involved in highly active intermediates of D, E and G, are regarded as fast equilibrium.
So the equilibrium equations can be expressed as (3)-(5) according to the steady state
theory.
D
E
2 C
C C = K C
(3)
(4)
(5)
G 3 D F
C C = K C C
H
I 4 E G
C = K C C
So the concentration of intermediate I can be expressed by equation (6).
K2K3K4CCCF
8
I
C =
(6)
CH
9
0
Then the governing series of design equations can be obtained from (7) to (16).
dCA
1
1
= -k C
(7)
(8)
1
A
dt
dCB
dt
k5K2K3K4CBCCCF
CH
1
2
3
4
5
6
= 3k C -
1 A
dCC
dt
k5K2K3K4CBCCCF
CH
1
1
1
1
1
= -
(9)
(10)
(11)
(12)
dCF
k5K2K3K4CBCCCF
CH
= -
6 F K -6 L M
- k C C + k C C
dt
dCH k5K2K3K4CBCCCF
=
dt
CH
dCJ k5K2K3K4CBCCCF
=
7 J K -7 M N
- k C C + k C C
dt
CH
dCK k5K2K3K4CBCCCF
=
6 F K -6 L M 7 J K -7 M N
- k C C + k C C - k C C + k C C (13)
dt
CH
dCL
1
1
7
8
-6 L M
= k C C - k C C
(14)
(15)
6
F
K
dt
dCM
6
F
K
-6
L
M
7
J
K
-7 M N
= k C C - k C C +k C C - k C C
dt
2
2

Page 23 of 31
Green Chemistry
View Article Online
DOI: 10.1039/D0GC03133J
dCN
dt
1
2
3
4
5
6
7
8
-7 M N
= k C C - k C C
(16)
7
J
K
After that, the computational regression for the mechanism-based kinetic model
was performed by using Runge-Kutta method on MATLAB software, during which the
temperature and concentration distribution under all the reaction conditions were
considered as uniform. Fig. 7 shows the concentration evolution of methyl acetate and
o
MA over reaction time at 20-35 C, which are also compared with the results calculated
from kinetic model. The obtained parameters from model regression with experimental
concentrations are summarized in Table 3.
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0.08
(
a)
(b)
0.06
o
0.04
o
2
2
3
3
0 C
20 C
o
o
5 C
25 C
o
o
0 C
30 C
0.02
o
o
5 C
35 C
dot: experimental
line: model
dot: experimetal
line: model
0.00
0
20 40 60 80 100 120 140 160 180
Reaction time /min
0
20 40 60 80 100 120 140 160 180
Reaction time /min
9
0
1
2
1
Fig. 7. Concentration evolution of methyl acetate (a) and MA (b) over reaction time at the reaction
1
1
condition of 0.1 mol/L trioxane, methyl acetate and BSA in 20 mL CH Cl , 5 mol.% of
2
2
o
[N3,3,3,3]F and [N3,3,3,3]Cl/AlCl , 20-35 C.
3
o
1
1
1
1
1
1
1
3
4
5
6
7
8
9
It can be seen from Fig. 7 that the concentrations of methyl acetate at 20-35 C
decrease rapidly in 140 min and then the tendency becomes slower. For the
o
o
concentration of MA, it increase fast before 140 min at 20 C and 25 C, and then the
trend becomes stable. However, it appears a peak value and then decrease over reaction
o
o
time at 30 C and 35 C, due to the transesterification with trimethylsilanol. According
to the equilibrium and rate constants presented in Table 3, it can be concluded that the
generation of onium amide base and deprotonation of methyl acetate are both
2
3

Green Chemistry
Page 24 of 31
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
endothermic, while the formation of 1-methoxy-1-trimethylsilyloxyethene is
exothermic. In addition, the transesterification of methyl acetate with trimethylsilanol
is easier than that of MA due to the electron donor effect of C=C bonds in MA. By
comparing the experimentally detected concentrations with the calculated data from
kinetic model, the deviation is less than acceptable 5%, suggesting the established
o
model and obtained parameters should be believable at the temperatures from 20 C to
o
7
8
35 C and reaction time before 180 min.
o
Table 3 The obtained kinetic parameters at 20-35 C
o
Temperature / C
2
0
25
30
35
Parameter
3
-1
k ×10 /(min )
8.13 ± 0.05
8.39 ± 0.08
0.54 ± 0.01
1.80 ± 0.02
5.16 ± 0.06
4.40 ± 0.05
5.87 ± 0.08
3.29 ± 0.04
5.06 ± 0.09
10.80 ± 0.06 14.21 ± 0.10 18.53 ± 0.19
1
2
K ×10
8.82 ± 0.11
0.55 ± 0.01
1.65 ± 0.01
6.60 ± 0.09
6.80 ± 0.10
8.65 ± 0.10
5.21 ± 0.07
7.66 ± 0.11
9.26 ± 0.10
0.57 ± 0.02
1.52 ± 0.01
8.36 ± 0.13
9.71 ± 0.09
0.59 ± 0.01
1.40 ± 0.01
10.52 ± 0.11
2
K3
K4
2
-1
-1
k ×10 /(L mol min )
5
3
-1
-1
k ×10 /(L mol min )
10.35 ± 0.18 15.57 ± 0.31
12.59 ± 0.19 18.10 ± 0.31
6
3
-1
-1
k ×10 /(L mol min )
-6
3
-1
-1
k ×10 /(L mol min )
8.13 ± 0.15
12.49 ± 0.19
7
3
-1
-1
k ×10 /(L mol min )
11.43 ± 0.21 16.84 ± 0.32
-7
9
0
1
2
3
4
5
6
With these rate and equilibrium constants at different temperatures in hand, the
1
1
1
1
1
1
1
activation barriers and enthalpies of related reaction steps can be calculated from
Arrhenius and Van’t Hoff plots, which are shown in Fig. 8 and Table 4. It reveals that
the thermal effects of these equilibrium reactions are relatively small due to the low
enthalpy, which is also consistent with the fact of room-temperature reaction.
Compared with the trioxane decomposition and condensation steps, the
transesterification of methyl acetate and MA with trimethylsilanol show higher
activation barrier. With these kinetic and thermodynamic informatics, this novel ionic
2
4

Page 25 of 31
Green Chemistry
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
liquid catalyzed mild liquid-phase system for direct synthesis of MA from methyl
acetate and trioxane can be understood in more details. Besides, it will be regarded as
a reference for the new catalyst design and catalytic system development in this
research area.
-
-
-
-
-
2
3
4
5
6
1
(
a)
k5
k6
k-6
k7
k-7
(b)
K1
K2
K3
0
-1
-2
-3
3
.20
3.25
3.30
3.35
3.40
3.45
3.20
3.25
3.30
3.35
3.40
3.45
5
6
1000 K/T
1000 K/T
Fig. 8. Arrhenius (a) and Van’t Hoff (b) plot for calculation of activation barrier and enthalpy
7
Table 4 Activation barrier, pre-exponential factor and enthalpy of related reaction steps
Reaction step
Pre-exponential factor Activation barrier /(kJ mol-
Enthalpy /(kJ mol-
1)
1)
5
-1
A→B
1.80×10 min
41.2 ± 0.3
-
-
C↔D + E
1.68
3.01
7.3 ± 0.2
D + F↔G + H
E + G↔I
-
4.2 ± 0.4
1.02×10-2
-
-12.6 ± 0.3
5
-1
B + I→J + K
1.15×10 L mol min
35.6 ± 0.6
63.2 ± 1.1
56.3 ± 0.8
66.7 ± 0.7
60.1 ± 0.9
-
-
-
-
-
8
-1
F + K→L + M 8.22×10 L mol min
7
-1
L + M→F + K 6.42×10 L mol min
9
-1
-1
J + K→M + N 2.56×10 L mol min
8
M + N→J + K 2.62×10 L mol min
8
9
0
1
Recycling test of ionic liquids
After the reaction, the mixture of [N3,3,3,3]F and [N3,3,3,3]Cl/AlCl catalysts can
3
1
be separated and recovered through high reduced pressure distillation method, owing
to the special property of ultralow vapour pressure of ionic liquids. Then the catalyst
1
2
5

Green Chemistry
Page 26 of 31
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
mixture was recycled for synthesis reaction and the catalytic activity can still remain
80% of the fresh one after ten times’ reuse, which is shown in Fig. 9. And the generated
silicon compounds of N-trimethylsilyl acetamide and trimethylsilanol can be reused for
synthesis of BSA, 50 despite it has not been conducted in this work. Compared with the
in-situ generated ionic liquid catalyzed liquid-phase system, 33-36 the catalyst and
activation reagent of methyl acetate, namely the probase BSA, can be recycled,
although the yield of MA achieved in this work is lower.
Yield
Selectivity
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
Reuse time
8
9
Fig. 9. Recycling test of [N3,3,3,3]F and [N3,3,3,3]Cl/AlCl ionic liquids at the reaction condition
3
1
1
0
1
of 0.1 mol/L trioxane, methyl acetate and BSA in 20 mL CH Cl , 5 mol.% of [N3,3,3,3]F and
2
2
o
[N3,3,3,3]Cl/AlCl , 25 C.
3
1
1
1
1
1
1
1
2
3
4
5
6
7
8
Conclusion
In this work, an ionic liquid catalyzed mild liquid-phase system was developed for
direct synthesis of MA from methyl acetate and trioxane, during which the
decomposition of trioxane into formaldehyde and deprotonation of methyl acetate
+
3+
2+
proceeded with catalysis of [Cation]F and [Cation]Cl/MCl (M = Cu , Fe , Zn and
3
+
Al ) ionic liquid, respectively. Both of cation and anion of ionic liquid have significant
effects on yield and selectivity of MA, resulting from the steric hindrance, Lewis acid
2
6

Page 27 of 31
Green Chemistry
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
site density and strength. The side reactions of transesterification between methyl
acetate (or MA) with generated trimethylsilanol will decrease the yield and selectivity
of MA. As a result, up to 60.2% yield with 94.6% selectivity toward MA was achieved
by using [N3,3,3,3]F and [N3,3,3,3]Cl/AlCl with 67 mol.% AlCl in the presence of
3
3
o
BSA at 25 C. The kinetic studies revealed the activation barrier of trioxane
-1
decomposition (41.2 ± 0.3 kJ mol ) was higher than that of condensation between 1-
-1
methoxy-1-trimethylsilyloxyethene and formaldehyde (35.6 kJ mol ), indicating the
production of MA was determined by formaldehyde generation.
Acknowledgements
1
For this study, we need to thank the financial support of Key Research Program of
Frontier Sciences, CAS (No. QYZDB-SSW-SLH022), National Natural Science
Foundation of China (No. 21878293, 21676270), Innovation Academy for Green
Manufacture, Chinese Academy of Sciences (No. IAGM-2019-A14) and the K. C.
Wong Education Foundation (No. GJTD-2018-04).
1
1
1
1
1
1
1
1
1
2
2
2
Notes and References
1 X. Miao, C. Fischmeister, P. H. Dixneuf, C. Bruneau, J. -L. Dubois and J. -L.
Couturier, Green Chem., 2012,14, 2179.
2 A. Rybak and M. A. R. Meier, Green Chem., 2007, 9, 1356.
3 K. M. Meek, T. R. Eaton, N. A. Rorrer, D. G. Brandner, L. P. Manker, E. M. Karp,
M. J. Biddy, A. D. Bratis, G. T. Beckham and A. K. Naskar, Green Chem., 2018,
20, 5299.
4 Z. Zhai, B. G. Andrew and A. T. Bell, J. Catal., 2013, 308, 25.
2
7

Green Chemistry
Page 28 of 31
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
5 L. L. Zhou, L. Wang, S. J. Zhang, R. Y. Yan and Y. Y. Diao, J. Catal., 2015, 329,
431.
6 B. Y. Jo, S. S. Kum and S. H. Moon, Appl. Catal. A: Gen., 2010, 378, 76.
7 M. Haecker, S. Wrabetz, J. Kronert, L. -I. Csepei, R. Naumann d’Alnoncourt, Y. V.
Kolen’ko, F. Girgsdies, R. Schlol and A. Trunschke, J. Catal., 2012, 285, 48.
8 R. Wilczynski and J. J. Juliette, Kirk-Othmer Encyclopedia of Chemical Technology,
Hoboken Inc., 2006, 227.
9 A. L. Zhu, T. Jiang, D. Wang, B. X. Han, L. Liu, J. Huang, J. C. Zhang and D. H.
Sun, Green Chem., 2005, 7, 514.
1
1
1
1
1
1
1
1
1
1
2
2
2
10 H. L. Fan, Y. Y. Yang, J. L. Song, G. D. Ding, C. Y. Wu, G. Y. Yang and B. X.
Han, Green Chem., 2014, 16, 600.
11 R. Mestres, Green Chem., 2004, 6, 583.
12 P. N. Davey, S. A. Forsyth, H. Q. N. Gunaratne, C. Hardacre, A. McKeown, S. E. J.
McMath, D. W. Rooney and K. R. Seddon, Green Chem., 2005, 7, 224.
13 Y. Traa, Chem. Commun., 2010, 46, 2175.
14 M. Ai, Appl. Catal. A Gen., 1990, 63, 365.
15 M. Ai, J. Catal., 1988, 112, 194.
16 M. Ai, J. Catal., 1987, 107, 201.
17 X. Z. Feng, B. Sun, Y. Yao, Q. Su, W. J. Ji and C. -T. Au, J. Catal., 2014, 314, 132.
18 X. P. Guo, D. Yang, C. C. Zuo, Z. J. Peng, C. S. Li and S. J. Zhang, Ind. Eng. Chem.
Res., 2017, 56, 5860.
19 D. Yang, C. Sararuk., H. Wang, S. J Zhang, Z. X. Li and C. S. Li, Ind. Eng. Chem.
2
8

Page 29 of 31
Green Chemistry
View Article Online
DOI: 10.1039/D0GC03133J
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
Res., 2018, 57, 93.
20 D. Yang, G. Wang, H. Wu, X. P. Guo, S. J. Zhang, Z. X. Li and C. S. Li, Catal.
Today, 2018, 316, 122.
21 H. Zhao, C. C. Zuo, D. Yang, C. S. Li and S. J. Zhang, Ind. Eng. Chem. Res., 2016,
55, 12693.
22 C. C. Zuo, T. T. Ge, G. Wang, X. P. Guo, C. S. Li and S. J. Zhang, Chem. Eng.
Technol., 2018, 41, 1331.
23 G. L. Zhang, H. H. Zhang, D. Yang, C. S. Li, Z. J. Peng and S. J. Zhang, Catal. Sci.
Technol., 2016, 6, 6417.
1
24 S. F. Jiang, C. S. Li, H. N. Chen, D. Yang and S. J. Zhang, Ind. Eng. Chem. Res.,
2017, 56, 9322.
1
1
1
1
1
1
1
1
1
2
2
2
25 C. C. Zuo, T. T. Ge, X. P. Guo, C. S. Li and S. J. Zhang, Micropor. Mesopor. Mat.,
2018, 256, 58.
26 C. C. Zuo, C. S. Li, T. T. Ge, X. P. Guo and S. J. Zhang, Can. J. Chem. Eng., 2017,
95, 2104.
27 T. He, Y. X. Qu and J. D. Wang, Catal. Lett., 2019, 149, 373.
28 T. He, Y. X. Qu and J. D. Wang, Ind. Eng. Chem. Res., 2018, 57, 2773.
29 J. B. Yan, C. L. Zhang, C. L. Ning, Y. Tang, Y. Zhang, L. L. Chen, S. Gao, Z. L.
Wang and W. X. Zhang, J. Ind. Eng. Chem., 2015, 25, 344.
30 Q. Bao, T. T. Bu, J. B. Yan, C. L. Zhang, C. L. Ning, Y. Zhang, M. M. Hao, W. X.
Zhang and Z. L. Wang, Catal. Lett., 2017, 147, 540.
31 Q. Bao, H. Qi, C. L. Zhang, C. L. Ning, Y. Zhang, Y. Jiang, Y. F. Wu, W. Y. Gui
2
9