Efficient and catalyst free synthesis of acrylic plastic precursors: Methyl propionate and methyl methacrylate synthesis through reversible CO2 capture

Article abstract of DOI:10.1039/c9gc00413k

Methyl propionate (MP) and methyl methacrylate (MMA) are considered as industrially important precursors for large-scale acrylic plastic production. The existing industrial synthetic protocols for these precursors utilize expensive catalysts accompanied by toxic and explosive gases such as carbon monoxide, ethylene and hydrogen. Herein, we for the first time report highly selective, catalyst-free and room temperature synthesis of MP and MMA precursors through a reversible CO₂ capture approach involving an organic superbase. In short, initially an equimolar mixture of an organic superbase 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) and methanol was reversibly reacted with molecular CO₂ and the obtained switchable ionic liquid, [DBUH][MeCO₃], further reacted with an equivalent amount of propionic anhydride or methacrylic anhydride to form MP or MMA, respectively. These reactions were accomplished in different solvents such as DMSO and methanol, and in the case of methanol, separation of reaction products occurs from in situ formed DBU derivatives such as [DBU][propionate] or [DBU][methacrylate]. In the case of both MP and MMA syntheses, after the use of methanol as a solvent, good recovery of the alcoholic solution of esters was achieved where 85% and 92% yields of MP and MMA were obtained, respectively. Molecular DBU was recovered from its propionate and methacrylate salts using NaCl saturated alkaline solution. Furthermore, the recovered MMA with methanol was polymerised to poly-MMA using a benzoyl peroxide induced free radical polymerisation process. The synthesis and separation of MP or MMA and the recovery of DBU were monitored by NMR analysis. Hence, unlike DMSO, methanol not only functioned as a reagent in CO₂ capture and as a solvent medium in MP, MMA and poly-MMA syntheses but also assisted in the recovery of DBU from the reaction mixture. Most importantly, here we present a more efficient, safer and single solvent based alternative synthetic approach for the synthesis of acrylic plastic precursors MP or MMA compared to existing industrial methods. Also, no toxic or expensive catalysts were required.

Full text of DOI:10.1039/c9gc00413k

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DOI: 10.1039/C9GC00413K
Green Chemistry
COMMUNICATION
Efficient and catalyst free synthesis of acrylic plastic precursors:
methyl propionate and methyl methacrylate synthesis through
reversible CO2 capture
Received 00th January
20xx,
Accepted 00th January
20xx
Santosh Govind Khokarale,*a Jyri-Pekka Mikkolaa, b
DOI: 10.1039/x0xx00000x
Methyl propionate (MP) and methyl methacrylate (MMA) are considered as industrially important precursors
upon large-scale acrylic plastic production. The existing industrial synthetic protocols of these precursors
utilize expensive catalysts accompanied with toxic and explosive gases such as carbon monoxide, ethylene and
hydrogen. Herein, we for the first time report highly selective, catalyst-free and room temperature synthesis
of MP and MMA precursors through organic superbase involved reversible CO2 capture approach. In short,
initially equimolar mixture of organic superbase 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) and methanol
were reversibly reacted with molecular CO2 and the obtained switchable ionic liquid, [DBUH][MeCO3] further
reacted with an equivalent amount of propionic anhydride or methacrylic anhydride to form MP or MMA,
respectively. These reactions were accomplished in different solvents such as DMSO and methanol
whereupon, in case of methanol, separation of reaction products occurs from in-situ formed DBU derivatives
such as [DBU][propionate] or [DBU][methacrylate]. In case of both MP and MMA synthesis, after use of
methanol as a solvent, good recovery of alcoholic solution of esters were achieved where 85% and 92% yields
of MP and MMA were obtained, respectively. The molecular DBU was recovered from its propionate and
methacrylate salts using NaCl saturated alkaline solution. Further, the recovered MMA with methanol was
polymerised to poly-MMA using a benzoyl peroxide induced free radical polymerisation process. The synthesis
and separation of MP or MMA as well as recovery of DBU was monitored by NMR analysis. Hence, unlike
DMSO, methanol not only performed as a regent in CO2 capture and as a solvent media in MP, MMA and poly-
MMA synthesis but also assisted in the recovery of DBU from the reaction mixture. Most importantly, here
we represented a more efficient, safer and single solvent based alternative synthetic approach for the
synthesis of acrylic plastic precursors MP or MMA compared to existing industrial methods. Also, no toxic or
expensive catalysts were required.
www.rsc.org/
processes, acetone-cyanohydrin process (ACH sulfo process, C3
route), isobutene oxidation (C4 route) and alpha process (C2 route)
1. Introduction
Methyl methacrylate (MMA) is an industrially important
monomer/precursor for the synthesis of various acrylate based
plastics (e.g. poly-methylmethacrylate, PMMA, MMA-butadiene
copolymer) where currently 75 % of world’s MMA consumption is in
acrylic plastic production [1]. MMA is also widely used to produce
resins, plastic coatings, paints, adhesives, and other important fine
chemicals [1]. Acrylic plastic or PMMA is a optically clear
thermoplastic material and very versatile because of its durability,
impact, strength, colour and pattern possibilities and hence has
enormous applications in our day-to-day life including automotive,
construction, retail and electronics, aerospace industry, paints and
coatings etc. [2]. Considering the huge world demand of acrylic
plastic and hence MMA, enormous amounts of MMA is produced in
practice through various industrial processes. Among these
are sharing the major part of global MMA production [3]. Recently,
Evonik (leading industry for speciality chemicals) as well as Gao et al.
developed a new process for the production of MMA (LiMA, leading
to methacrylate) following a route involving direct oxidative
esterification of methacrolein, a condensation product between
propionaldehyde and formaldehyde. This newly developed process
is more simple as it is accompanied with high raw material
availability, less environmental impacts (reduction in CO2 emissions
by 40 %), efficient and selective catalysts and feasible process
parameters (e.g. reaction temperature ≤ 100 oC) compared to
existing industrial processes [4a,b]. Even though all these processes
are playing a vital role in MMA production, each of these processes
have their own drawbacks such as environmentally hazardous waste
production or use of toxic and flammable reagents etc. Hence, both
in industry and in academia researchers are focusing to improve the
current processes and/or inventing new and more environmentally
friendly processes for the MMA synthesis [3]. In these regards,
following the principles of “Green Chemistry”, such as use of less
a.Technical Chemistry, Department of Chemistry, Chemical-Biological Centre, Umeå
University, SE-90187 Umeå, Sweden
b.Industrial Chemistry & Reaction Engineering, Department of Chemical
Engineering, Johan Gadolin Process Chemistry Centre, Åbo Akademi University, FI-
20500 Åbo-Turku, Finland
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toxic reagents as well as less production of waste in downstream the overall economy as well as the safety of these processes. Hence,
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processes are the main targets currently under rigorous study.
catalytic processes with mild reaction conDdOitiIo: 1n0s.1a0n39d/Cn9eGuCtr0a0l4g13aKs
As already mentioned, the Alpha process (C2 process) developed atmosphere are comparatively safe upon industrial scale production.
by Lucite international is another well-established and industrially Furthermore, catalyst free synthesis of the desired product under
applied process for MMA production [5]. This two-step synthetic mild reaction conditions is the preferred alternative for several
protocol involves the synthesis of methyl propionate (MP) by industrial processes.
methoxycarbonylation of ethylene followed by condensation of MP
Reversible CO2 capture using strong organic bases or superbases
with formaldehyde to MMA (reaction scheme 1) [6a-d]. Following such as amidines and guanidines, in addition with various proton
the Alpha process, the industries in Singapore (Alpha 1, 2008, Lucite donors such as alcohols, water, alkanolamines etc. is not only an
international) as well as in Saudi Arabia (Alpha 2, 2017, Mitsubishi elegant technique for CO2 capture and storage (CCS) but it provided
Chemical Corporation and Saudi Basic Industries Corporation) a new solvent media in the form of special types of ionic liquids as
currently produce 100, 000 tons and 250, 000 tons of MMA per switchable ionic liquids [9a-c]. Mikkola et al. have used these types
annum, respectively.
of ionic liquids successfully as a solvent media upon biomass
pretreatment such as wood delignification [10a-c]. Apart from it,
recently similar reversible CO2 capture approach through organic
superbase involved process was applied in the catalyst free synthesis
of cellulose derivatives such as cellulose esters under mild reaction
conditions [11a-f]. Here, initially rapid and efficient dissolution of
crystalline cellulose was attained through its interaction with an
organic superbase 1, 8-diazabicyclo-[5.4.0]-undec-7-ene, DBU and
molecular CO2 [12a, b]. The obtained soluble cellulose further
reacted with various alkyl anhydrides to form its corresponding ester
derivatives. Apart from cellulose ester synthesis, Liu et al. discovered
that lower esters such as methyl acetate can also be synthesised
following this reversible CO2 capture approach. In this case, the
authors noted that during the cellulose ester synthesis, as a side
reaction, the previously synthesised [DBUH][MeCO3] formed due to
the interaction of DBU and MeOH with CO2 which also reacted with
acetic anhydride and formed methyl acetate [11b]. Hence, under
mild reaction conditions and under catalyst free reaction conditions,
various alcohols including biomolecules can be derivatized to their
esters using efficient organic superbase involved reversible CO2
capture reaction approach.
Scheme 1. The Alpha process, two step MP synthesis: a)
Methoxycarbonylation of ethylene to MP and, b) Condensation of
MP with formaldehyde to MMA
Even though the Alpha process is well established upon MMA
production, the synthesis of MP in the first step of the process is still
coupled with major drawbacks such as the use of expensive Pd-
phosphine homogeneous catalyst and handling of poisonous carbon
monoxide gas. To overcome these drawbacks, several processes
have been developed during the last decade aiming at either
recycling of the expensive catalyst or a new synthetic approach for
MP synthesis involving biomass derived precursors and cheaper
heterogeneous catalysts.
O
O
Fe-Ni/ZrO2
Mimicking the above described ester synthesis protocol, in this
report we are for the first time introducing a catalyst free synthesis
of MP and MMA, occurring at room temperature through reversible
CO2 capture by an organic superbase DBU (reaction scheme 3).
During the actual synthesis, initially [DBUH][MeCO3], (a switchable
ionic liquid) was synthesised through interaction of equimolar
mixture of DBU and methanol with molecular CO2. In the second
step, the formation of MP or MMA was achieved through interaction
of [DBUH][MeCO3] with equimolar amounts of propanoic (R = CH2-
CH3) or methacrylic anhydride (R = C(CH3)=CH2), respectively. The
molecular CO2 was released in the second step along with the
formation of [DBUH][propionate] or [DBUH][methacrylate] as a by-
product upon reaction during MP or MMA synthesis, respectively.
The progress of both steps, i.e. process of reversible CO2 capture and
MP or MMA synthesis, were monitored by NMR analysis. The
separation of reaction products such as MP or MMA from the
reaction mixture, recovery of molecular DBU as well as
polymerisation of recovered MMA to poly-MMA is also described in
this report.
MeOH, 220 o
C
OMe
OMe
OH
Methyl propionate
Methyl lactate
Scheme 2. Hydrodeoxygenation of biomass derived methyl lactate to
MP.
Riisager et al. used a Brønsted acid ionic liquid as an efficient reaction
media for methoxycarbonylation of ethylene to MP where the ionic
liquid was involved in the stabilisation as well as separation of the
expensive Pd-complex from the reaction mixture via a biphasic
reaction approach [7 a,b]. The same authors further explored this
ionic liquid involved metal complex system in the vapour phase
synthesis of MP by developing a supported ionic liquid phase (SILP)
technology. In both liquid as well as vapour phase approaches, a
higher than ≥ 99 % yield of MP was obtained as the used Pd-
phosphine complex was highly chemo-selective for MP instead of the
alternative copolymerised products. On the other hand, in order to
avoid the use of carbon monoxide as well as expensive Pd-phosphine
catalyst upon MP synthesis, Shunmugavel et al. developed a
heterogeneous catalyst based approach for the MP synthesis where
cheaper base metal catalysts were used for selective
hydrodeoxygenation of biomass derived methyl lactate with 77% MP
o
yield, at 220 C in 12 hour (scheme 2) [8]. Even though these new
processes are promising to simplify the MP synthesis, still the use of
expensive catalysts as well as ionic liquids along with poisonous
carbon monoxide gas in case of the Alpha process and high
temperature reaction approach, slow reaction kinetics and use of
explosive H2 gas in hydrodeoxygenation of methyl lactate influence
2 | J. Name., 2012, 00, 1-3
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MMA were further confirmed by means of NMR analyses. In case of
reaction mixtures containing DMSO as a soDlvOeIn: t1,0t.1h0e3V9Nie/CwM9ARGrtCiacl0ne0aO4lny1lsi3niKes
was performed using a capillary containing D2O as an internal
standard. Upon use of reaction mixture with MeOH as a solvent, the
NMR analysis was carried out with DMSO-d6 as the internal standard.
The types of chemical species formed upon synthesis of MP or MMA
are depicted in Fig. 1. Further MP was also synthesized in DMSO
solution containing the 13C-SIL.
2.2.3 Separation of MP or MMA from reaction mixture
The separation of MP or MMA and recovery of DBU was performed
only for the reaction proceeding with alcoholic solutions of SILs. After
-
complete conversion of MeCO3 (confirmed by NMR analysis), MP or
Scheme 3. a) Switchable ionic liquid synthesis and, b) MP or MMA
synthesis
MMA along with the excess methanol was separated from the
[DBUH][propionate] or [methacrylate] by high vacuum distillation at
40 C. In order to avoid the loss of MP or MMA and methanol, the
reservoir used for the collection of MP or MMA with methanol was
kept in a cold trap containing dry ice (solid CO2) and acetone. The
purity of the obtained products MP or MMA with methanol as well
as remaining [DBUH][propionate] or [methacrylate] were confirmed
by NMR analysis (with DMSO-d6 as an internal standard).
o
2. Experimental
2.1 Chemicals and methods
2.1.1 Chemicals
1,8-Diazabicyclo-[5.4.0]-undec-7-ene (DBU, ≥99.0%, GC analysis),
propanoic anhydride (≥99.0%), methyl propionate (≥99.0%),
methacrylic anhydride (94.0%), methyl methacrylate (99.0%), d6-
DMSO (anhydrous, 99.9 atom% D and CDCl3 (99.96 atom% D) were
purchased from Sigma Aldrich while methanol (≥99.0%), acetone
(≥99.0%) and dimethyl sulfoxide (≥99.0%) were purchased from VWR
chemicals and used further without purification. CO2 and 13C
enriched CO2 gas bottles (>99.99%) were obtained from AGA AB
(Linde Group) and used without further purification.
2.2.4 Yield of MP and MMA
The amount of alcoholic solution of MP or MMA recovered after
separation from the reaction mixture was interpreted by Equation 1,
where VR and VE represent the recovered and theoretically expected
volumes of alcoholic solution of MP or MMA, respectively. Further,
the yields of MP or MMA were obtained by using Equation 2, where
nR and nE denote the number of moles of recovered and theoretically
expected number of moles of MP or MMA, respectively. The
recovered and expected moles of MP or MMA were calculated using
the results from gas chromatography (GC) in an Agilent 7820 N
instrument equipped with a HP-5 column (30 m length, 0.3 mm
internal diameter, and 0.25 µm of film thickness) and the FID
detector. Helium gas was used as a carrier gas. The GC injection port
2.1.2 NMR and FTIR analysis
The composition of the reaction mixtures was confirmed by means
of NMR analysis using the Brucker’s Avance 600 MHz instrument. In
these analyses, as per requirement, either glass capillary filled with
D2O or DMSO-d6 or CDCl3 were used as internal standards. The
obtained data was further processed with TopSpin 3.2 software. The
Attenuated Total Reflectance-Fourier Transform Infrared
Spectroscopy (ATR-FTIR) technique was used for the analysis of
alcoholic solutions of MP and MMA by Bruker Vertex 80v FT-IR
spectrometer equipped with a DTGS detector.
o
and detector temperature were set at 300 C. The temperature
programme during analysis was maintained as follows: Initially the
column temperature was maintained at 50 C for 1 min and then a
O
o
o
temperature ramp from 50 C to 100 C with a heating rate of 4.5
oC/min was commenced. After reaching 100 C, temperature was
o
2.2. Synthesis and separation of MP and MMA and recovery of DBU
further increased with rate of 20 oC/min to 300 oC and the
temperature was held at this temperature for 7 min. During the
actual MP or MMA synthesis, 3 ml of 50 vol. % of alcoholic solution
of DBU was used. Based on reaction scheme 3, we know the amount
of MP or MMA can be form is equivalent to DBU in the alcoholic
solution. Therefore corresponding amount of the MP or MMA was
used during calibration by GC analysis. During the calibration, various
amounts of MP or MMA were mixed with mesitylene (the internal
standard) and further diluted with DMSO. The obtained calibration
curves are shown in the supporting information S4. Finally, the yield
of MP or MMA were calculated in the recovered alcoholic solutions
with the help of the calibration curves and Equation 2.
2.2.1 Synthesis of [DBUH][MeCO3] or SIL with and without dimethyl
sulfoxide (DMSO) solvent
The [DBUH][MeCO3] (SIL) was synthesized via an interaction of CO2
molecule with DBU and methanol with or without DMSO. During the
actual synthesis, CO2 gas was bubbled under stirring in the DMSO
solution containing equimolar amounts of DBU and methanol or in
alcoholic solution of DBU (50 vol. % of DBU in methanol), at room
temperature. In both cases, pale yellow clear solutions were
obtained and their compositions were further analysed by NMR
analysis. Furthermore, the SIL was also synthesized with 13C isotope
enriched CO2 molecule (tracer technique). The NMR analysis was
carried out with D2O filled in a glass capillary swimming in the
reaction mixture with DMSO solvent or with DMSO-d6 for reaction
with methanol solvent.
% ꢀꢁꢂꢃꢄꢅ ꢂꢆ ꢇꢈꢉꢂꢊꢂꢈꢋꢉ ꢌꢂꢈꢃꢅꢋꢂꢄ ꢂꢆ ꢍꢎ ꢂꢏ ꢍꢍꢀ ꢏꢐꢉꢂꢑꢐꢏꢐꢒ
ꢓꢔ
=
ꢖ ꢗꢘꢘ
… . (ꢗ)
ꢓꢕ
2.2.2. Synthesis of MP or MMA
ꢚꢔ
ꢚꢕ
% ꢙꢋꢐꢈꢒ ꢂꢆ ꢍꢎ ꢂꢏ ꢍꢍꢀ =
ꢖ ꢗꢘꢘ
… . (ꢛ)
Equimolar amounts of anhydride precursors such as propionic or
methacrylic anhydride based on DBU amounts were slowly added
under stirring to previously synthesized DMSO or alcoholic solution
of SIL, at room temperature. The reaction mixture was kept under
stirring for 1h at room temperature and the formation of MP and
2.2.5 Synthesis of poly-MMA from recovered MMA
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The obtained MMA with methanol was further converted to poly-
MMA by free radical polymerisation using benzoyl peroxide as the
radical initiator. The reaction mixture containing an alcoholic
solution of MMA (in methanol) and the free radical (5 wt. % based
on MMA amount) were refluxed at 70 oC. The reaction was quenched
after 4h as highly viscous liquid formation was observed. After
cooling of the reaction mixture, a white precipitate was slowly
settling at the bottom of the reaction flask. To purify the product, the
white precipitate was further dissolved by addition of minimum
amount of acetone and the product was further re-precipitated by
addition of excess of methanol under vigorous stirring. The obtained
white precipitate was separated from the reaction mixture by
filtration and washed 3-4 times with methanol. After drying
overnight, at room temperature, the white product was further dried
in vacuum for 3 h at 70 oC and its purity as poly-MMA was confirmed
by NMR analysis (with CDCl3 as an internal standard).
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DOI: 10.1039/C9GC00413K
Figure 1. Chemical species formed in MP and MMA synthesis
2.2.6 Recovery of DBU
The DBU was recovered from [DBUH][propanoate] or [methacrylate]
by using alkaline NaCl saturated aqueous solution following a
previously reported procedure [11b, 11f]. Initially, saturated NaCl
aqueous solution was prepared by mixing NaCl in water under
stirring and the required amount of solution was taken to make 4 wt.
% solution of NaOH in a NaCl saturated solution. This alkaline NaCl
saturated solution was further added to [DBUH][propanoate] or
[methacrylate] and the mixture was stirred for 30 min. at room
temperature. The DBU was extracted with ethyl acetate and
separated from the aqueous phase by using a separating funnel. The
DBU was obtained from organic phase after removing ethyl acetate
by rotation evaporator under high vacuum and its purity was
confirmed by NMR analysis. The recovery percentage (weight %) of
DBU was calculated from the recovered amount and initial amount
used for CO2 capture in the reaction.
Figure 2. 13C NMR spectra for the synthesis of [DBUH][MeCO3], SIL
from DBU, methanol and CO2 in DMSO. a) DBU and MeOH, and b)
[DBUH][MeCO3].
3. Result and discussion
The MP synthesis process further continued whereupon
propionic anhydride (based on amount of DBU or methanol used)
was added in DMSO solution of SIL. After addition of propionic
anhydride, the immediate release of molecular CO2 was observed
and as represented in the reaction scheme 3, the interaction of SIL
with the anhydride precursor was confirmed through release of
molecular CO2. After stirring the reaction mixture at room
temperature for 1h, the 13C NMR analysis was performed to confirm
the formation of MP. As shown in Fig. 3, after addition of an
equivalent amount of propionic anhydride, the signal related to the
3.1.Synthesis of MP with and without DMSO solvent and recovery
of DBU
3.1.1. Synthesis of MP with DMSO solvent
The reversible CO2 capture by DBU along with equivalent amounts of
methanol in a DMSO solvent was performed and the transformation
of molecular DBU to its ionic form i.e. [DBUH][MeCO3] or SIL was
confirmed by 13C NMR analysis. As shown in Fig. 2, after the SIL
formation the peaks belonging to the carbon atoms of DBU at
positions 6 and 9 got shielded by 5-6 ppm whereas the peak
belonging to carbon atom at positon 7 got de-shielded by 5 ppm.
Also, the peaks related to methyl group in the free methanol
disappeared at 47.44 ppm and new peaks for methyl group and
-
MeCO3 anion at 155.97 ppm disappeared and the peak related to
carbonyl carbon in MP appeared at 174 ppm. Further, the formation
of MP was confirmed by comparing the 13C NMR spectra of the
reaction mixture with commercially available MP. In case of in-situ
formed [DBUH][propionate], the signal belongs to sp2-hybridised
carbon i.e. C-7’’ in DBUH+ cation and carbonyl carbon in [propionate]
anion are observed at 165.31 and 177.34 ppm, respectively. 1H NMR
analysis of the reaction mixture was also carried out and the spectra
confirmed the formation of MP after interaction between the SIL and
propionic anhydride (supporting information S1-c and d).
-
carbonyl carbon in MeCO3 anion were observed at 50.06 and 155.97
ppm, respectively. Hence, it is evident that the SIL got formed after
equivalent interaction of DBU, methanol and CO2 with each other.
Further, the formation of SIL was also confirmed by 1H NMR analysis
(supporting information S1-a and b). Similar observations based on
13C NMR analysis for the formation of SIL have also been described
in previously reported articles [11b].
Now, in order to confirm the origin of the molecular CO2 released
during the MP synthesis, 13C isotope enriched CO2 was used. Initially,
the SIL was prepared after interaction of equimolar mixture of DBU
and methanol with 13C-CO2. The resultant SIL was further analysed
by means of 1H and 13C as well as two-dimensional NMR analysis. As
shown in Fig. 3-b, the NMR spectrum of SIL formed with 13C-CO2 gave
-
rise to a high intensity of carbonyl carbon in the MeCO3 anion
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compared to C-7’ of the protonated DBU molecule. As shown in the
reaction scheme 3-a, molecular CO2 reacts during the SIL formation
through chemical interaction with the in-situ formed methoxide
anion.
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DOI: 10.1039/C9GC00413K
Figure 4. H-13C HMBC NMR spectra of [DBUH][MeCO3] in DMSO
1
obtained with a) CO2 and b) 13C-CO2.
As [DBUH][propionate] formed in the reaction mixture, we can
assume that the [DBUH]+ cation might not interfere in the MP
formation process. On the other hand, it seems that the formation of
-
MP preceded after interaction between MeCO3 anion and propionic
anhydride. In the previous section we have shown that the ester
synthesis proceeds through the release of CO2 after the
-
decomposition of the MeCO3 anion. However, a release of molecular
CO2 after addition of propionic anhydride and further interaction of
methoxide anion with propionic anhydride was still not confirmed.
As already mentioned, the formation of MP after interaction of
propionic anhydride with SIL is mimicked in the cellulose ester
synthesis process. In the previous reports, concerning the synthesis
of these cellulose derivatives is not adequately explained since the
interaction between cellulose alkyl carbonate anion (formed after
interaction between cellulose, DBU and CO2) and alkyl anhydride
occurs before ester formation via the release of CO2. Hence, in MP
synthesis one needs to understand the release of molecular CO2 from
Figure 3. 13C NMR spectra of a) DBU and MeOH, b) [DBUH][MeCO3]
with 13C-CO2 reaction mixture in DMSO: MP and [DBUH][propionate]
obtained after reaction of propionic anhydride with c) 13C-
[DBUH][MeCO3] and d) [DBUH][MeCO3]
-
MeCO3 anion after addition of propionic anhydride. The reaction
-
scheme 4 represents a plausible interaction between MeCO3 anion
and propionic anhydride during MP synthesis. As shown in reaction
-
scheme 4a, it is possible that the MeCO3 anion is in equilibrium with
Further, to emphasize the chemical interaction of CO2 and
methoxide anion during the SIL formation, two-dimensional long
range 1H-13C HMBC (Heteronuclear Multiple Bond Correlation) NMR
analysis of both reaction mixtures containing SIL formed with the 13C-
CO2 and “normal” CO2 was carried out. As shown in Fig. 4-a, the
correlation peak observed at the position 3.21/156.12 ppm is
associated with a long range correlation between protons and
the methoxide anion and molecular CO2 and the equilibrium was not
disturbed until the addition of propionic anhydride. However, after
the addition of propionic anhydride this equilibrium seems to be
disturbed whereupon the methoxide anion got consumed via its
interaction with the anhydride molecule (scheme 4b). Therefore, this
might give rise to the release of CO2 in the reaction mixture.
-
carbonyl carbon in the MeCO3 anion of the SIL. Also, similar
observation was made for the SIL obtained with 13C-CO2 (Fig. 4-b).
Hence, the NMR analysis proves that during the SIL synthesis the CO2
-
molecule is chemisorbed upon the MeCO3 anion formation. After
the addition of propionic anhydride in a DMSO solution of the SIL,
the MP and [DBUH][propionate] formed upon release of molecular
CO2. As shown in Fig. 3-c, after addition of propionic anhydride, the
signal at the position 156.12 ppm disappeared while a new intense
signal, at the position 123.4 ppm, appeared. The new signal belongs
to molecular CO2 [13]. As shown in Fig. 3-c and d, respectively, the
signal for molecular CO2 obtained after MP synthesis from 13C-
[DBUH][MeCO3] has a very high intensity compared to the one when
“normal” (non-13C) CO2 was used. Also, no high intensity signal
appeared for 13C enriched CO2 containing species between 160 and
180 ppm after the MP synthesis (Fig. 3-c). Hence, we can conclude
Scheme 4: Plausible mechanism for MP synthesis.
Now, as shown in Fig. 5, the MP formed after disappearance of
-
MeCO3 anion in the reaction mixture. In order to confirm this,
different equivalents such as 0.5, 0.8 and 1 (on the basis of DBU or
methanol) of propionic anhydride were added to the DMSO solution
of the SIL. The formation of MP was monitored with 13C NMR analysis
and the obtained spectra are summarized in Fig. 6. After regular
-
-
addition of propionic anhydride, the signals belonging to the MeCO3
that the molecular CO2 originated from the MeCO3 anion upon the
MP synthesis process.
anion steadily decrease while the signals related to the MP and
propionate anion increased accordingly. Hence, the formation of MP
in the reaction mixture can be monitored by NMR analysis on the
-
basis of signals appearing for the MeCO3 anion. Considering this
-
aspect, as shown in Fig. 5 a and b, the MeCO3 anion got completely
converted to MP.
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1h, the formation of MP was confirmed by 13C NMVRiewanAartilcylseiOs.nliAnes
shown in Fig. 7, similar to previous experiments where MP was
DOI: 10.1039/C9GC00413K
-
synthesized in DMSO, a complete transformation of MeCO3 anion to
MP was also observed in the alcoholic solution. Hence, the synthesis
of MP following reaction scheme 3 is not only limited to a polar non-
protic solvent such as DMSO, but it can proceed in polar protic
solvents like alcohol i.e. in this case methanol. Further, methanol not
only performed as a solvent and reagent for CO2 capture but also
served as a role of solvent media to accomplish the MP synthesis
process.
Figure 5. 13C NMR spectra for the synthesis of MP from DMSO
solution of [DBUH][MeCO3] and propionic anhydride, a)
[DBUH][MeCO3], b) reaction mixture in DMSO: MP and
[DBUH][propionate] and, c) commercially available MP (peak
represented by the asterisk belongs to carbonyl carbon in
[DBUH][propionate])
The separation of MP from the reaction mixture along with
recovery of the DMSO as well as DBU superbase are equally
important aspects if process economy is concerned. Considering the
lower boiling point of MP (79.8 OC), its separation from the reaction
mixture is possible through a low temperature distillation process.
On the other hand, DMSO has a boiling point of 189 OC and hence its
recovery from the reaction mixture is more energy-intensive
compared to MP. The presence of DMSO in the reaction mixture can
also influence the recovery of DBU. Therefore, instead of searching
new techniques for DMSO separation from reaction mixture, here we
decided to replace the DMSO with excess of methanol in the starting
reaction mixture during SIL synthesis and similar alcoholic solution of
SIL was used further for MP synthesis.
Figure 7. 13C NMR spectra for the synthesis of MP from alcoholic
solution of [DBUH][MeCO3] and propionic anhydride,
a)
[DBUH][MeCO3], b) reaction mixture in MeOH: MP and
[DBUH][propionate] and, c) commercially available MP, (peak
represented by the asterisk belongs to carbonyl carbon in
[DBUH][propionate]).
3.1.3. Separation of MP from reaction mixture and recovery of
DBU
The DMSO free synthesis of MP was carried out to replace the
expensive DMSO and to ease the separation of MP from the reaction
mixture as well as the recovery of DBU. Considering the lower boiling
point of MP and methanol, their separations from the reaction
mixture were performed under high vacuum where both MP and
methanol were collected in a cold trap kept in acetone-dry ice
solution. The separation of reaction components i.e. MP, methanol
and [DBUH][propionate] were confirmed by 13C NMR analysis. As
shown in Fig. 8, the pure form of MP and methanol got separated
1
from the [DBUH][propionate] following high vacuum distillation. H
NMR spectra as well as FTIR spectra of the recovered alcoholic
solution of MP are shown in supporting information S5 and S7-a,
respectively. Further, 93 % recovery of alcoholic solution of MP was
achieved after distillation whereas 85 % yield of MP was obtained
after distillation.
The recovery of molecular DBU from its ionic form i.e. from
[DBUH](propionate] was performed using alkaline NaCl saturated
aqueous solution. Similar alkaline solvent system previously used for
the deprotonation of DBU from its ionic forms during cellulose esters
[11b, 11f]. Even though this method was not well optimized, it is
frequently used to recover DBU from its ionic forms. As described in
the experimental section, after the treatment of [DBUH][propionate]
with alkaline NaCl saturated solution, molecular DBU was extracted
from alkaline solution with ethyl acetate. The DBU was further
recovered after separation of ethyl acetate by distillation and its
purity was confirmed by 13C NMR analysis. As shown in supporting
information S8, the purified form of molecular DBU was obtained
Figure 6. 13C NMR spectra for the synthesis of MP from DMSO
solution of [DBUH][MeCO3] and propionic anhydride, a)
[DBUH][MeCO3], reaction mixture containing DMSO, SIL and, b) 0.5
c) 0.8 and d) 1 eq. propionic anhydride.
3.1.2. Synthesis of MP in methanol (without DMSO)
The SIL in methanol was prepared after bubbling CO2 in DBU and a
methanol mixture containing 50 vol. % of DBU. The formation of
[DBUH][MeCO3] was confirmed by 13C NMR analysis and the
obtained spectra are summarized in supporting information S2.
Further an equivalent amount of propionic anhydride (based on
DBU) was added dropwise at room temperature to the reaction
mixture an immediate CO2 release was observed. After stirring for
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with 89 wt.% recovery. Hence, MP synthesis can be proceed with the
recovery of DBU superbase.
View Article Online
Figure 9. 13C NMR spectra for the synthesDisOoI:f10M.1M03A9/fCr9oGmC0D0M41S3OK
solution of [DBUH][MeCO3] and methacrylic anhydride, a)
[DBUH][MeCO3], b) reaction mixture in DMSO: MMA and
[DBUH][methacrylate] and, c) commercially available MMA (peak
represented by the asterisk belongs to carbonyl carbon in
[DBUH][methacrylate]).
3.2.2 Synthesis of MMA in methanol (without DMSO),
separation of MMA and recovery of DBU
Similar to MP synthesis, the process of synthesis of MMA was also
performed in methanol. Initially, the SIL was synthesised in methanol
followed by the addition of an equivalent amount of methacrylic
anhydride to synthesise MMA. As shown in Fig. 10, after comparing
the NMR spectra of the reaction mixture with commercially available
MMA, it was confirmed that MMA formed after interaction of SIL
with the methacrylic anhydride in methanol. After complete
-
conversion of the MeCO3 anion, the MMA and methanol were
separated from the reaction mixture under high vacuum and
[DBUH][methacrylate] was collected to recover the DBU superbase.
Figure 8. 13C NMR spectra for the separation of MP and methanol
from [DBUH][propionate], a) reaction mixture in MeOH: MP and
[DBUH][propionate], b) recovered MP and MeOH from reaction
mixture and, c) recovered [DBUH][propionate].
3.2.Synthesis of methylmethacrylate (MMA) in with and without
DMSO solvent and recovery of DBU
3.2.1. Synthesis of MMA with DMSO
The MMA synthesis was proceeded with similar experimental
procedure that used for MP synthesis using methacrylic anhydride as
the anhydride precursor. In this case, methacrylic anhydride was
added to the previously synthesised DMSO solution of SIL and the
reaction mixture was stirred for 1h. 13C NMR analysis of the reaction
mixture was further performed to confirm the formation of MMA. As
shown in Fig. 9,13C NMR spectra demonstrates that after the addition
-
of methacrylic anhydride, the MeCO3 anion got completely
consumed and, accordingly, MMA formed in the reaction mixture
and signal belonging to the carbonyl carbon in MMA appeared at
167.39 ppm. Further, the formation of MMA was confirmed after Figure 10. 13C NMR spectra for the synthesis of MMA from alcoholic
comparing the NMR spectra of the reaction mixture and solution of [DBUH][MeCO3] and methacrylic anhydride, a)
commercially
available
MMA.
The
formation
of [DBUH][MeCO3], b) reaction mixture in MeOH: MMA and
[DBUH][methacrylate] was also confirmed by NMR analysis where [DBUH][methacrylate] and, c) commercially available MMA (peak
sp2-hybridised carbon i.e. C-7’’ in [DBUH] cation and carbonyl carbon represented by the asterisk belongs to carbonyl carbon in
in methacrylate anion appeared at 165.40 and 171.64 ppm, [DBUH][methacrylate]). (peak represented by asterisk is belongs to
1
respectively. H NMR spectra shown in supporting information S3 carbonyl carbon in [DBUH][methacrylate]).
also confirmed the formation of MMA as a result of the interaction
between [DBUH][MeCO3] and methacrylic anhydride.
The separation of MMA and methanol from the reaction mixture
was further confirmed by NMR analysis. As shown in Fig. 11, it is
possible to separate MMA with methanol from the
[DBUH][methacrylate]. Further, the 1H NMR analysis of the
separated alcoholic solution of MMA was also carried out and the
results are summarised in supporting information S6. The purity of
the recovered alcoholic solution of MMA was also confirmed by FTIR
analysis and the spectra can be found in supporting information S7-
b. Finally, 90% of the alcoholic solution of MMA was recovered and
10% lost due to the evaporation of both MeOH and MMA during the
distillation. In the recovered distillate, 92 % yield of MMA was
obtained.
Further,
the
DBU
was
recovered
from
[DBUH][methacrylate] alkaline and NaCl saturated aqueous solution
with 82 wt.% of recovery and its purity was confirmed by NMR
analysis (supporting information S9). Hence, similar to MP synthesis,
methanol can be used as a reagent as well as solvent system upon
synthesis of MMA.
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Hence, catalyst free synthesis of both MP and MMA are
Journal Name
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described in this report. The reaction proceeds under mild reaction
conditions and in a single solvent system. In industrial processes,
both MP and MMA syntheses are usually dependent on the
availability of different toxic and expensive reagents as well as
catalysts. Also, these processes are often accompanied with harsh
reaction conditions such as high temperature, pressure and
explosive and toxic gas environments. On the other hand, the
reagents used in the process for both MP and MMA synthesis such
as propionic and methacrylic anhydrides, methanol and CO2 gas are
less expensive and toxic as well as easily available compared to
reagents currently used in industrially applied processes. The organic
superbase used, i.e. DBU, is comparatively expensive compared to
other reagents used in this process. However, as shown, the DBU can
be recovered at the end of the process in its pure form using a cheap
alkaline solvent treatment. Hence, in overall, the method developed
for the synthesis of MP and MMA can be considered as a more
sustainable and greener compared to mainstream industrial
processes and can be explored at pilot as well as industrial scale.
DOI: 10.1039/C9GC00413K
Figure 12. 13C NMR spectra (in CDCl3) for the synthesis of poly-MMA
from MMA in methanol, a) MMA and, poly-MMA
Conclusions
In conclusion, in the present work, we successfully achieved the
synthesis of acrylic plastic precursors namely methyl propionate and
methyl methacrylate as well as poly-MMA in single solvent system
such as methanol. The switchable ionic liquid obtained after
chemisorption of molecular CO2 by organic superbase DBU and
methanol further reacted with either propionic or methacrylic
anhydride and resulted to highly selective synthesis of methyl
propionate or methyl methacrylate, respectively. This synthetic
approach was fruitfully accomplished in both DMSO as well as
methanol where the progress of reaction was not influenced by the
choice of the solvent system. However, in contrast to DMSO as a
solvent, methanol facilitated an efficient separation from the
reaction mixture along with MP or MMA. High recovery level of
alcoholic solutions of both esters were achieved and 85 and 92%
yields of MP and MMA, respectively, were obtained. Further, the
recovered MMA with methanol was polymerized to poly-MMA with
87% yield using free radical polymerization process. Hence, here
methanol not only served as reagent and reaction media for CO2
capture process but also assisted in the form of solvent in MP, MMA
and poly-MMA synthesis. The 87 wt. % or 82 wt. % recovery of the
comparatively expensive DBU was further also fruitfully achieved
from [DBUH][propionate] or [DBUH][methacrylate], respectively, by
using an alkaline NaCl saturated aqueous solution. Hence, in this
report, highly selective, catalyst free, single solvent based, and hence
economically favourable synthesis of both MP and MMA is
represented. Being catalyst free, room temperature, cheaper and
renewable solvent based process, this approach of acrylic plastic
precursor synthesis can be explored at industrial level to tackle the
drawbacks of currently ongoing industrial processes.
Figure 11. 13C NMR spectra for the separation of MMA and methanol
from [DBUH][methacrylate]. a) reaction mixture in MeOH: MMA and
[DBUH][propionate], b) recovered MMA and MeOH from reaction
mixture and, c) recovered [DBUH][methacrylate].
3.3 Synthesis of poly-MMA from recovered MMA
Considering the applicability of the MMA for the synthesis of poly-
MMA, the free radical polymerization was proceed with recovered
MMA (with MeOH) in the presence of benzoyl peroxide. As shown in
1
Fig. 12, the H NMR analysis represent the pure form of poly-MMA
formed with 87% yield from the recovered MMA which was
previously synthesized from SIL and methacrylic anhydride via
release of molecular CO2. The synthesis of poly-MMA from the
recovered MMA was not optimized further as synthesis of polymeric
compound was carried out to demonstrate the applicability of
methanol as a signal solvent media for both MMA and poly-MMA
synthesis.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We wish to thanks Dr. Mattias Hedenström and Dr. Tobias
Sparrman for their assistance in the NMR analysis. This work is
part of activities of the Technical Chemistry, Department of
Chemistry, Chemical-Biological Centre, Umeå University,
Sweden as well as the Johan Gadolin Process Chemistry Centre
at Åbo Akademi University in Finland. The Swedish Research
8 | J. Name., 2012, 00, 1-3
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Council (Drn: 2016-04090), Bio4Energy programme, Kempe
Cramail, and M. A. R. Meier, ACS Sustainable Chem. Eng. 2018,
View Article Online
DOI: 10.1039/C9GC00413K
Foundations and Wallenberg Wood Science Center under
6, 8826-8835
auspices of Alice and Knut Wallenberg Foundation are gratefully 13 J. K. Moore, M. D. Guiver, N. Du, S. E. Hayes, and M. S. Conradi,
acknowledged for funding this project.
J. Phys. Chem. C 2013, 117, 22995−22999
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