Highly Selective Synthesis of Acrylic Acid from Lactide in the Liquid Phase

Article abstract of DOI:10.1002/cssc.201800914

A new reaction system for the highly selective, hydrobromic acid catalyzed conversion of lactide into acrylic acid under mild conditions is reported. The applied liquid reaction system consists of a temperature-stable bromide-containing ionic liquid and 2-bromopropionic acid as a source of dry HBr, with no volatile organic solvent being used. This allows for the in situ removal of the formed acrylic acid, leading to an unmatched acrylic acid selectivity of over 72 % at full lactide conversion. Accounting for leftover reaction intermediates on the way to acrylic acid, which could be recycled in an elaborate continuous process, the proposed reaction system shows potential for acrylic acid yields well above 85 % in the liquid phase. This opens new avenues for the effective conversion of biogenic lactic acid (e.g., obtained by fermentation from starch) to acrylic acid. The resulting bio-acrylic acid is a highly attractive product for, for example, the diaper industry, where we expect consumers to be especially sensitive to aspects of sustainability.

Full text of DOI:10.1002/cssc.201800914

Accepted Article
Title: Highly selective synthesis of acrylic acid from lactide in liquid
phase
Authors: Jens Nagengast, Simon Hahn, Nicola Taccardi, Matthias
Kehrer, Julian Kadar, Dimitris Collias, Peter Dziezok, Peter
Wasserscheid, and Jakob Albert
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.201800914
Link to VoR: http://dx.doi.org/10.1002/cssc.201800914
A Journal of
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ChemSusChem
10.1002/cssc.201800914
Highly selective synthesis of acrylic acid from lactide in liquid phase
a
Jens Nagengast , Simon Hahn , Nicola Taccardi , Matthias Kehrer , Julian Kadar , Dimitris Collias ,
a
a
a
a
b
c
Peter Dziezok , Peter Wasserscheid and Jakob Albert
a,d
a*
a
Institute of Chemical Reaction Engineering, Friedrich-Alexander-University Erlangen-Nürnberg
FAU), Egerlandstrasse 3, 91058, Erlangen, Germany.
(
b
The Procter & Gamble Co, Materials Science Transformative Platform Technologies, Cincinnati,
USA.
c
Procter & Gamble Service GmbH, Sulzbacher Str. 40, 65824 Schwalbach am Taunus, Germany
d
Forschungszentrum Jülich, „Helmholtz-Institut Erlangen-Nürnberg für Erneuerbare Energien“ (IEK
1
1), Egerlandstrasse 3, 91058 Erlangen, Germany
Contact details of the corresponding author:
Phone: (+49)-(0)9131-85-67417 Fax: (+49)-(0)9131-85-27421; e-mail: jakob.albert@fau.de
Abstract
A novel reaction system for the highly selective, hydrobromic acid catalysed conversion of lactide into
acrylic acid under mild conditions is reported. The applied liquid reaction system consists of a
temperature-stable bromide-containing ionic liquid and 2-bromopropionic acid as source of dry HBr,
with no volatile organic solvent being used. This allows for the in situ removal of the formed acrylic
acid leading to an unmatched acrylic acid selectivity of over 72% at full lactide conversion. Accounting
for leftover reaction intermediates on the way to acrylic acid that could be recycled in an elaborate
continuous process, the proposed reaction system shows potential for acrylic acid yields well above 85%
in liquid phase. This opens new avenues for the effective conversion of biogenic lactic acid (e.g. obtained
by fermentation from starch) to acrylic acid. The resulting bio-acrylic acid is a highly attractive product
for, e.g. the diaper industry, where we expect consumers to be especially sensitive to aspects of
sustainability.
Keywords
Acrylic acid, lactide, lactic acid, phosphonium bromide, liquid phase
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ChemSusChem
10.1002/cssc.201800914
Introduction
Sustainable production of bulk chemicals from renewable feedstocks is gaining technological,
commercial, and societal interest as it opens ways to overcome our dependency on fossil feedstocks and
[1]
to reduce the emission of green-house gases (GHGs). Acrylic acid (AA) is a versatile bulk chemical
with main applications in the production of superabsorbent polymers (55%), and plastics and synthetic
rubber (30%). The production capacity of AA has exceeded 5 million tons per year placing AA among
the top 100 chemicals produced worldwide. The global AA market (> $ 8 billion) is assumed to increase
[2]
with a forecast growth of over 5% from 2016 to 2023. The current market price for AA is
[3]
$
1600 – $ 2200 per ton, depending on the product purity (crude- and glacial-grade AA).
Currently, commercial acrylic acid is predominately manufactured in a two-step oxidation process via
the intermediate acrolein with petroleum-based propene as feedstock. The first oxidation step from
propene to acrolein operates at 320 – 370°C, while the second step is carried out at slightly lower
temperatures (220 – 300°C). Both exothermic reactions are usually conducted in fixed-bed tubular
[4-7]
reactors (up to 22,000 tubes per reactor) using molybdenum-based heterogeneous catalysts. The two
step oxidation process from propene to AA has been optimised by several companies, e.g. Nippon
[8]
[9]
[10]
Shokubai , BASF or Ube Industries , reaching high propene and acrolein conversions (70 – 97%)
and high AA product selectivity (85 – 99%). Typically observed by-products are acetic acid (AcA),
propionic acid (PA), acetaldehyde (AcAld), acetone, carbon monoxide (CO), and carbon dioxide (CO
2
).
In the latter two cases, by-product formation results from further deep-oxidation of acrolein or acrylic
[11,12]
acid.
The propene feedstock for today’s AA production is mainly produced by steam-cracking of
naphta or as by-product of fluid-catalytic cracking of light crude-oil fractions. Hence, propylene and
[13]
thus AA production exclusively rely on fossil resources. Especially customers in the diaper industry,
however, are becoming more and more sensitive to questions of sustainability and ecology, a fact that
promotes the quest for alternative AA production routes based on renewable feedstocks.
In principle, there are two conceivable solutions for AA production from renewable resources. One is
[14,15]
to use renewable-based propene or acrolein in the already existing and established production route.
The second is the development of new processes from bio-based starting materials. Relevant substrates
[16,17]
[18]
3-hydroxypropionic acid (3-HPA), and acrylonitrile.
[19]
for the latter option are, e.g. glycerol,
Currently, the most promising substrate for the production of bio-AA is lactic acid (LA), which
represents a growing platform chemical in the bio-based economy due to a variety of attractive
applications, such as its use in the production of the renewable polymer polylactic acid (PLA). Over
[20,21]
9
0% of the current LA production is based on bacterial fermentation of carbohydrates.
The first attempt to produce AA from a LA derivative was based on pyrolysis and reported in 1935 by
[39]
Burns et al. However, the first attempts of LA dehydration to AA in gas-phase were reported in 1958
[22]
by Holmen et al. . Since then, several groups have investigated the optimised catalytic systems and
[23,24]
process operations for LA dehydration using modified Y-type zeolites or hydroxyapatite catalysts.
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ChemSusChem
10.1002/cssc.201800914
[5]
AA yields of up to 62% have been reported. Typical by-products at reaction temperatures of
50 – 400°C at ambient pressure in fixed-bed tubular reactors with diluted aqueous LA feed were CO,
AcAld, PA, AcA and 2,3-pentanedione (2,3-PD). In 2014, Ghantani et al. reported their investigations
3
[25a]
on non-stoichiometric calcium pyrophosphate catalysts for the dehydration of LA to AA.
These
authors reported experiments in fixed-bed tubular reactors in a temperature range from 325 – 400°C.
The feed concentration of LA ranged from 25 – 80 wt.% (aqueous) and the Weight Hourly Space
-1
Velocity (WHSV) was varied from 1 to 4.5 h . The best performing catalyst with a Ca/P ratio of 0.76
achieved an AA yield of 78% at full substrate conversion. The main side products were CO, CO , AcAld,
PA, and 2,3-PD. However, the authors did not provide detailed information about catalyst stability. In
2
2
014, also interesting results on the efficient production of AA by dehydration of LA over BaSO
4
with
[25b]
crystal defects were presented by Lyu et al.
Investigating the prepared catalysts in a fixed-bed quartz
-1
reactor at 350°C and under atmospheric pressure, feeding a 20 wt.-% aqueous LA at 1.2 mL h and a
-1
WHSV of 2.5 h gave AA yields of up to 78.8% at a conversion of 77.6%. In 2017, Zhang et al. reported
their investigations on the selective conversion of LA to AA over alkaline and alkaline-earth metal co-
[25c]
modified NaY zeolithes.
Performing their experiments in a vertical, downflow packed bed reactor at
-1
3
50°C and feeding an aqueous 20 wt.-% LA at 0.02 mL min , they achieved AA selectivities of up to
-1
8
4% at WHSVs from 0.48 to 4.8 h . After four reaction-regeneration cycles (>55 h time-on-stream) the
catalyst still delivered high AA selectivity of about 82%. So far the highest yields and selectivities of
AA obtained from the gas-phase dehydration of LA and its derivatives were reported and patented by
[26-28]
Collias and co-inventors at P&G.
The experiments were performed with amorphous and partially-
dehydrated polyphosphate catalysts MH2(1-x)PO(4-x) (where x varies from 0 to 1 and M is selected from a
sub-group of alkali metals) at reaction temperatures of approximately 375°C, total pressure of 25 bar,
and water partial pressure exceeding 3.4 bar with aqueous LA solution (20 wt.%). The reported yields
and the related selectivities of AA were in the range of 90% for long periods of reaction time. Observed
by-products in a decreasing order were AcAld, 2,3-PD, AcA, PA, diacrylic acid, and 1-hydroxyacetone.
[29]
[30]
Further approaches were based on dehydration of LA in near - and supercritical water.
By far the least investigated possibility for bio-AA production from LA is its dehydration in liquid-
phase. Liquid phase processes promise milder reaction conditions in LA dehydration and thus thermally
induced side reactions, such as decarbonylation, decarboxylation, and oligomerisation may be strongly
reduced. Moreover, catalyst deactivation which is a frequently observed problem in the gas-phase LA
dehydration may be suppressed at milder conditions in a liquid-phase reaction. Furthermore, LA, as well
as AA and various by-products of the process, can be corrosive under the conditions present in some of
the gas-phase LA dehydration processes, leading to potentially additional investment cost of the
dehydration equipment. Last but not least, the energy demand of high temperature and/or high pressure
operation in gas-phase LA dehydration processes lead to additional operation cost. All these points make
alternative LA dehydration processes at mild thermal conditions in the liquid phase technically and
potentially economically highly attractive.
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ChemSusChem
10.1002/cssc.201800914
However, only two examples of liquid-phase LA dehydration to AA have been reported. In 2014,
Kuppinger et al. patented a process for the dehydration of hydroxycarboxylic acids in a liquid-phase
[31]
process. In the described transformation a LA solution (LA or 3-HPA; 60 – 90 wt.% in water) was
fed into a stirred round bottom flask containing mixtures of metal lactates, metal carbonates, and metal
phosphates of varying composition at temperatures between 160 – 300°C. Water and polymerization
inhibitor (e.g. phenothiazine or 4-methoxyphenol, 0.05 – 2.5 wt.%) were added to the molten salt
reaction mixture to prevent polymerisation. The reaction products were continuously removed from the
hot reaction zone at reduced pressure, condensed and collected. The maximum yield in AA was reported
to be 1.3% at a sump temperature of approx. 300°C, a reaction time of 5.3 h with K
2 4
HPO as salt additive
/
catalyst. Even though the used salt additives were similar to the active phosphate-based catalysts used
in gas-phase dehydration of LA, the reported yields of AA were thus very small. One can even speculate
whether the low reported yields may be the result of thermally-induced substrate decomposition.
Very recently, Terrade et al. have reported a novel method for the production of AA from fermentatively
[
32,33]
produced lactide and LA-oligomers.
These authors described a catalytic cracking of lactide in
]Br) in the presence of a strong acid (e.g.
bromide salts (e.g. tetraphenylphosphonium bromide, [PPh
4
methane sulfonic acid, MSA) and a high-boiling solvent (sulfolane). The described rearrangement of
lactide to AA was demonstrated at significantly lower temperatures (130 – 175°C) on relatively small
scale (batch-wise in 9 mL autoclaves) under 50 bar nitrogen pressure. After cooling to room
1
temperature, the reaction mixture was analysed by H NMR. At optimised reaction conditions, AA
yields of up to 58% were obtained after 10 h at 175°C at complete lactide conversion (SAA = 58%).
[33]
Water-free conditions were claimed to be crucial to achieve high yields of AA. Remarkably, the use
of monomeric lactic acid as feedstock for the proposed transformation is not described.
Mechanistically, Terrade et al. proposed a catalytic cracking of lactide into AA by in situ formed HBr
[32]
(
Scheme 1).
In a first step lactide reacts with HBr and forms the corresponding bromo species (1).
Further addition or elimination of HBr leads either to two 2-bromopropionic acid (2-BrPA) molecules
or the corresponding acrylate (2). Subsequently, (2) is cracked in the presence of HBr into AA and
2
-BrPA. As a last step all obtained 2-BrPA species form AA by eliminating HBr. They also observed
the secondary product 3-bromopropionic acid (3-BrPA), resulting from the addition of HBr to AA.
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ChemSusChem
10.1002/cssc.201800914
Scheme 1: Proposed mechanism by Terrade et al. for the catalytic cracking of lactide in the presence of
[32]
4
the IL [PPh ]Br and the strong acid MSA.
While the findings by Terrade et al. are highly remarkable and describe a very important step for liquid-
phase AA synthesis from lactide, it is obvious that the economics of such an alternative process for AA
production from biomass-derived lactide would greatly benefit from higher AA yields and less water
intolerant nature of the reaction system.
In this contribution, we present a novel liquid-phase reaction system for the efficient production of AA
from biogenic lactide. The reported technology benefits from mild reaction conditions (T < 200°C,
ambient pressure, liquid reaction phase) and straightforward product isolation by in situ vacuum
distillation. Most importantly, we report here that the technology opens in its optimised form a way to
produce AA from biogenic lactide and LA in over 80% yield. The work was developed in parallel and
[34,35]
independent from the above mentioned work by Terrade.
Results and Discussion
Optimised reaction system for converting LA into AA
In order to enable a high conversion of lactide to AA in a low temperature liquid phase process, it is
necessary to ensure activation of the reactant and stabilisation of the formed transition states and
intermediates to avoid detrimental decarbonylation, decarboxylation or oligomerisation pathways.
While the previously published work by Terrade et al. uses a combination of high-melting [PPh
mp = 295°C) and sulfolane (vapour pressure = 112 mbar at 200°C), we tried to avoid the use of any
volatile solvent to allow the in situ distillation of products from the reaction mixture without solvent
loss. We chose tetrabutylphosphonium bromide ([PBu ]Br) as bromide source and stabiliser for the
4
]Br
(
4
4
reaction that has a melting point of 100°C. In independent experiments, we confirmed that [PBu ]Br is
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ChemSusChem
10.1002/cssc.201800914
[34,35]
stable in the targeted temperature range of 140 – 220°C.
Due to the liquid nature of all components
under reaction conditions, we performed our experiments without any additional solvent. Table 1 shows
[33]
a comparison of two independent experiments using conditions mentioned by Terrade et al. (entry 1)
[34]
and our independently developed solvent-free benchmark system (entry 2).
Both entries were
prepared at ambient conditions and heated up to the desired reaction temperature of 150°C. The
composition of the reaction mixture used in entry 1 led to an increased pressure in the reaction setup of
3
bar. With our solvent-free conditions used in entry 2, the system pressure did not increase at 150°C.
In our optimised reaction protocol (entry 2), we used 2-BrPA, a reaction intermediate formed during the
reaction, as source of acidity. For this purpose we prepared 2-BrPA independently according to a
[36]
recently published protocol and added it in a molar concentration of 5% to the reaction mixture. Note
that 2-BrPA is converted during the reaction to AA and liberates water-free HBr that acts as the acidic
catalyst. In our conversion and selectivity calculations we account for the added 2-BrPA as feedstock
molecule. By using 2-BrPA as source of acidity we keep the system simple as no extra chemical is
introduced to the reaction mixture to create the required system acidity. As seen from the results in
Table 1, our simplified reaction system shows higher AA yields in the batch-mode reactions than
[33]
Terrade et al. under comparable reaction conditions. Moreover, our benchmark system requires long
reaction times of 48 h but less dilution of the reactant leading to improved space-time-yields (STY).
Table 1: Bromide assisted transformation of lactide to AA - Comparison of the system mentioned by
[33]
Terrade at al. to our solvent-free system at ambient pressure in batch mode reactions.
Entry
Bromide
source
Acid PressureSolvent T Time X(Lactide) Y(AA)
a
Molar ratio
source [bar]
[°C] [h]
150 16
150 48
[%]
> 99
> 99
[%]
[33]
1
sulfo-
lane
-
[
PPh
4
]Br
]Br
14
6
MSA
3
1
35
2
[PBu
4
2-BrPA
55
Reaction conditions: lactide as reactant (0.69 mmol for entry 1, 8.37 mmol for entry 2), acid source (0.58 mmol
MSA for entry 1, 5 mmol 2-BrPA for entry 2), ionic liquid (3.47 mmol [PPh ]Br for entry 1, 100 mmol [PBu ]Br
4
4
a
for entry 2), additional solvent (1.5 mL sulfolane for entry 1). Molar ratio of bromide salt and solvent (if used) to
LA feedstock.
Molar ratio variation
We already noticed in the early stages of the optimisation of the reaction system that the amount of
bromide is a crucial parameter for the obtained AA yield. Therefore, we investigated the dependency of
the bromide salt to lactide ratio on the AA yield systematically. The results are shown in Figure 1. The
amount of substrate is given in so-called lactic acid equivalents (LAe’s). All experiments were
performed at 150°C in a batch reaction setup. The data shown were recorded for 48 hours reaction time.
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ChemSusChem
10.1002/cssc.201800914
60
50
40
30
20
10
0
1:2
1:1
2:1
3:1
6:1
Molar ratio IL:LAe
4
Figure 1: Influence of the molar ratio IL:LAe on the AA yield; reaction conditions: 100 mmol [PBu ]Br,
X mmol lactide, 5 mmol 2-BrPA, 150°C, 48 h, ambient pressure.
From the results it is very clear that an excess of bromide salt is required in the reactor to realise high
AA yields. Obviously, this excess is needed to reduce the vapour pressure of the HBr catalyst and to
stabilise the reaction intermediates in a way that avoids extensive side reactions. A key requirement is
to stabilise the carbocation intermediate that is most relevant for the undesired decarbonylation forming
AcAld. For our further experiments we decided to work with a bromide to LAe ratio of 2. We considered
this ratio to be the best compromise between high AA yield and volumetric productivity of the reaction
system.
Temperature variation
Temperature is a crucial parameter in the reaction system under investigation both for reaction rate and
product selectivity. Figure 2 shows the results of our time-resolved temperature variation experiments
in the temperature range between 140°C and 220°C.
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ChemSusChem
10.1002/cssc.201800914
5
5
4
4
3
3
2
2
1
1
5
0
5
0
5
0
5
0
5
0
5
0
5
140 °C
50 °C
60 °C
170 °C
80 °C
90 °C
200 °C
10 °C
20 °C
1
1
1
1
2
2
-
0
1
2
3
4
5
6
7
time
/ h
Figure 2: Time-resolved temperature variation experiments and their corresponding AA yields;
reaction conditions: 100 mmol [PBu
4
]Br, 25 mmol lactide, 5 mmol 2-BrPA, 140 – 220°C, 7 h, ambient
pressure.
Two general trends are noticeable from Figure 2. As expected, the AA formation rate accelerates with
increasing temperature shortening the reaction time to, e.g. 40% AA yield, from 7 h at 170°C to less
than 30 min at 210°C. Equally important is the finding that at temperatures above 180°C, AA
decomposition in the system becomes important. This decomposition process is triggered by the actual
AA concentration in the reaction system so that independent of the temperature level the maximum AA
yields obtained are relatively similar at 36 to 50%. Gas analysis after 7 h reaction time indicated that
[37,38]
AA decarboxylation to ethylene and CO
2
is the major AA decomposition process.
This was further
confirmed by independent AA decomposition experiments in our reaction system with AA purposely
added to the reaction phase (see ESI, Figure S1). Thus, the obtained maximum AA yield is the result of
a trade-off between simultaneous AA production and AA decomposition.
Table 2 shows an example of a more detailed product analysis of the reaction at 190°C. Along with AA,
longer-chain acrylate oligomers (acrylates), 2-BrPA and 3-BrPA have been monitored quantitatively.
The corresponding data for the other reaction temperatures are given in the ESI, Tables S1-S9.
Table 2: Time-resolved lactide conversion and composition of the reaction mixture in a batch LA de-
hydration experiment at 190°C.
Reaction time [h]
Y(AA)
%]
Y(acrylates)
[%]
Y(2-BrPA)
[%]
Y(3-BrPA)
X(lactide)
[%]
[
[%]
0
0
1
4.9
7.9
2.9
57.2
33.3
15.5
3.5
0.9
99.8
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ChemSusChem
10.1002/cssc.201800914
2
3
4
5
6
7
44.9
44.8
41.2
36.6
30.9
26.8
6.8
2.2
0.9
0.5
0.2
0
1.4
0.7
0.5
0.3
0
2.1
2.1
1.7
1.3
0.9
0.7
> 99.9
> 99.9
> 99.9
> 99.9
> 99.9
> 99.9
0
4
Reaction conditions: 100 mmol [PBu ]Br, 25 mmol lactide, 5 mmol 2-BrPA, 190°C, 0 – 7 h, ambient pressure.
More than 50% of the lactide was converted during heating the reaction mixture to 190°C (taking about
0 min in our setup) and almost full lactide conversion was reached after one hour reaction time.
1
Acrylates are formed in the early stages of the reaction and are later converted to AA. This confirms
that oligomer formation from lactide is a simultaneous pathway that still leads with high probability to
AA formation. It is worth mentioning that the same acrylate peaks were also observed in the reaction
[32]
system reported by Terrade et al. These authors assumed the observed acrylates to be acrylate dimers
or trimers. Regarding the carbon mass balance, up to 55% of the carbon in the lactide feed is found in
the products analysed in the reaction mixture at full conversion. The missing part is due to the formation
2
of volatile products, in particular CO and AcAld from decarbonylation, and ethylene and CO from
decarboxylation indicating an intrinsic selectivity problem of the batch reaction protocol employed. 2-
BrPA is the added acid source to the reaction mixture and is consumed during the reaction. 3-BrPA is
the secondary product of the addition reaction of HBr to the product AA, and this reaction is reversible
under the applied conditions. Consequently, the 3-BrPA formation is dependent on the concentration of
AA and HBr in the reaction solution. With ongoing AA decomposition to ethylene and CO
the 3-BrPA yield also decreases as less and less AA is available for its formation.
2
over time,
From all these results we concluded that a suitable in situ separation of the AA product from the reaction
mixture should boost the overall AA yield significantly. In our optimised reaction matrix, such in situ
product removal by distillation is greatly facilitated by the absence of volatile organic solvent. Note that
AA is the lightest boiling substance (bp = 139°C at ambient pressure).
Consequently, our next set of experiments aimed at identifying the best conditions for in situ removal
of AA from our reaction system by vacuum distillation. The corresponding results are shown in Table 3
for a 3 h reaction at 190°C and different pressure levels in the reactor. For comparison the mean values
of five batch experiments at ambient pressure (batch experiments without in situ product distillation)
are given as entry 6.
Table 3: Results of the lactide to AA conversion at 190°C with in situ product removal by vacuum
distillation varying the applied pressure – isolated products have been distilled out of the reaction sys-
tem; in addition, the composition of the remaining reaction mixture is given.
Composition of reaction mixture
Isolated products
Total
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ChemSusChem
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Entry Pressure Y(AA) Y(acry- Y(2-
Y(3- Y(AA) Y(acryl- Y(2-
Y(3- Y(AA)
[
mbar]
[%]
lates) BrPA) BrPA)
[%]
ates)
[%]
0.3
0.3
0.8
0
BrPA) BrPA)
[%]
[
%]
[%]
0.6
0.6
0.6
0.8
1.1
1.4
[%]
0
[%]
3.7
1.6
0.8
0
[%]
3.4
2.4
1.5
1.8
0
1
2
3
4
5
6
2
5
2.4
5.3
3.4
3.8
2.5
3.8
3.6
4.5
54.0
62.6
72.6
25.4
2.3
56.4
67.8
81.1
47.2
33.4
46.8
0.1
0.3
0.7
2.6
2.8
10
8.5
20
21.8
31.1
46.8
50
0
0
1023
-
-
-
-
4
Reaction conditions: 100 mmol [PBu ]Br, 25 mmol lactide, 5 mmol 2-BrPA, 190°C, 3 h; X(lactide, entry 1) =
8
2%, X(lactide, entry 2 – 6) = 100%).
It is very remarkable that the expected beneficial effect on AA yield is indeed observed by applying this
in situ product removal strategy, but only in a relatively narrow pressure window. The optimum pressure
was found under the applied conditions at 10 mbar where more than 81% yield of AA was obtained with
7
2.6% AA yield isolated in the cooling trap of the vacuum line. Applying higher vacuum to the reactor,
to our first surprise, led to a significant reduction in the substrate conversion (82% lactide conversion at
mbar compared to full conversion in entries 2-6) and lower AA yield. As reason for this observation,
2
we identified the HBr removal from the reaction system under the too harsh vacuum conditions as the
reason for this observation. HBr forms in the early stages of the reaction from the 2-BrPA additive and
is immobilised in the liquid bromide salt matrix. This immobilisation works effectively unless the
reaction pressure gets significantly below 10 mbar. If significant amounts of HBr are removed from the
reaction mixture by the applied vacuum, the reaction rate slows down and a smaller quantity of AA is
formed. However, if the applied pressure is too high, AA yields are reduced. This is due to the fact that
the vapour pressure of AA in the bromide salt matrix is significantly reduced and pressures significantly
above 10 mbar do not remove the AA product effectively from the reaction mixture. As a consequence,
the above-described thermal decomposition of AA proceeds despite the reduced pressure in the reactor.
Note that also small amounts of acrylates (0.3 – 0.8%), 2-BrPA (0.8 – 3.7%) and 3-BrPA (1.5 – 3.4%)
are removed from the reactor by the vacuum. In the commercial process it would be suitable to separate
these relatively small amounts from AA and recycle these compounds back to the reactor to avoid loss
of HBr equivalents and feedstock from the reaction system.
In a further set of experiments, we aimed at demonstrating that the same beneficial effect of in situ AA
removal can also be realised by applying nitrogen strip gas to the reactor instead of vacuum. The results
are presented in Table 4 for the comparable experiments at 190°C and 3 h reaction time.
2
Table 4: Results of the strip gas experiments varying the used amount of N -strip gas; divided into non-
isolated and isolated yields.
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ChemSusChem
10.1002/cssc.201800914
Composition of reaction mixture
Entr Strip gas Y(A Y(acrylate Y(2- Y(3-
Isolated products
Y(A Y(acrylate Y(2- Y(3-
Total
Y(A
A)
y
[mLn/mi
n]
A)
[%]
18.8
9.9
s) [%]
BrPA BrPA
) [%] ) [%]
A)
s) [%]
BrPA BrPA
) [%] ) [%]
[%]
34.4
64.6
52.0
[%]
53.1
74.5
56.6
1
2
3
250
2.5
2.5
2.9
0.5
0.4
0.4
1.1
1.1
0
0.3
1.1
0.6
0.1
0
0.6
0
500
1000
4.6
0.2
0.3
4
Reaction conditions: 100 mmol [PBu ]Br, 25 mmol lactide, 5 mmol 2-BrPA, 190°C, 3 h.
Overall, similar trends were observed by applying strip gas as in the case of the vacuum distillation
experiments. 500 mL /min proved to be the best compromise between system integrity (no removal
of HBr) and AA removal to avoid thermal AA decomposition. The obtained overall AA yield of 74.5%
64.6 % isolated yield) proves that strip gas has a very positive effect on the product yield, although the
n
N
2
(
absolute values of AA yields are somewhat lower compared to those from the optimised vacuum
systems. From a practical view point, the use of strip gas would facilitate the up-scaling and the
designing of the plant.
Bromide salt recycling
To confirm that the thermal stability of the low melting [PBu
4
]Br is sufficient for the required long term
]Br at 250°C
stability or recyclability of our reaction system, we performed stability tests with pure [PBu
4
for 168 h (see ESI, Table S10) and studied the stability of the melt in repetitive use. In total, the same
bromide salt was used in four consecutive batch recycle experiments (see ESI, Figure S2 for
1
representative H NMR spectra for one cycle). The results are presented in Table 5. All reactions were
performed at 190°C for 3 hours. After each run the products were removed from the reaction matrix by
-2
subjecting the system to 1 x 10 mbar pressure at 220°C for 4 hours. The reaction matrix remained the
same and fresh 2-BrPA and lactide were added for the next run. The somewhat harsher product
distillation procedure compared to the in situ removal studies were applied to remove all residual
materials from the previous reaction from the reaction system to establish a comparable starting point
for each individual recycle experiment.
Table 5: Results of recycling the reaction system in four consecutive batch experiments.
Run
1
Y(AA) [%]
45.8
Y(acrylates) [%]
Y(2-BrPA) [%]
Y(3-BrPA) [%]
4.9
3.8
6.1
5.7
1.5
1.4
1.8
1.4
2.8
2.7
3.5
2.8
2
49.4
3
44.6
4
49.3
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ChemSusChem
10.1002/cssc.201800914
Reaction conditions: 100 mmol [PBu
purification conditions after each individual experiment: 1 x 10 mbar, 220°C, 4 h.
4
]Br, 25 mmol lactide, 5 mmol 2-BrPA, 190°C, 3 h, ambient pressure;
-
2
All results consistently demonstrate that the reaction matrix is stable and does not lose performance over
the total operation time (see ESI, Tables S11 & S12 for more details). There are some variations from
run to run but these are in the same range as the variations observed in four consecutive runs with fresh
bromide salt in every run. We attribute these variations to small changes in heating-up times and reaction
temperatures.
Experimental
4
Batch experiments converting lactide and 2-BrPA to AA in [PBu ]Br
All batch experiments were performed in a 100 mL three-necked glass reactor connected to a reflux
condenser. The desired amount of solid lactide (PURALACT®, L, L- lactide, polymer grade, Corbion
Purac Co., Lenaxa, KS) was added to a mixture of 34.6 g solid tetrabutylphosphonium bromide (PBu
4
Br;
1
00 mmol, 98%; Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany; catalog # 189138) and 0.8 g of
liquid 2-bromopropionic acid (2-BrPA; 5 mmol, 99%; Sigma-Aldrich Chemie GmbH, Taufkirchen,
Germany; catalog # B78300) at room temperature and atmospheric conditions. The reaction mixture
was then heated to the desired temperature under continuous stirring with a magnetic stir bar at a speed
of 800 rpm. During reaction, evaporating components of the mixture were refluxed using a cold trap at
2
2
5°C. Gaseous products passing the cold trap (mainly CO, acetaldehyde, ethylene, and CO ) were routed
to the off-gas. After a reaction time of 3 hours at the selected temperature, the hot reaction mixture was
1
allowed to cool down to room temperature and analysed via off-line H-NMR (JEOL ECX 400 MHz).
4
Lactide to AA reaction with in situ AA removal in [PBu ]Br
All experiments with in situ removal of AA were performed in a 100 mL three-necked glass reactor
connected to a bridge and a product collection unit. The applied product collection unit contained two
consecutive cooling traps, the first at 0 to – 50 °C and the second at -197 °C, respectively. A desired
amount of solid lactide (PURALACT®, L, L- lactide, polymer grade, Corbion Purac Co., Lenaxa, KS)
4
was added to a mixture of 34.6 g of solid tetrabutylphosphonium bromide (PBu Br; 100 mmol, 98%;
Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany; catalog # 189138) and 0.8 g of liquid 2-bromo-
propionic acid (2-BrPA; 5 mmol, 99%; Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany; cata-
log # B78300) at room temperature and atmospheric conditions. The reaction mixture was then heated
to the desired temperature under continuous stirring with a magnetic stirrer at a speed of 800 rpm. By
applying vacuum or strip gas to the reactor after reaching the desired temperature, the in situ removal of
AA from the reaction system was initiated. All volatile products were condensed in a cooling trap. Ad-
ditionally a downstream cooling trap or washing bottle filled with water was employed in the vacuum /
strip gas experiments to guarantee the collection of all products. After the reaction temperature of 3
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ChemSusChem
10.1002/cssc.201800914
hours, the hot reaction mixture was allowed to cool down to room temperature and the collector was
slowly warmed up to room temperature by removing the coolant. The inhibitor 4-methoxyphenol
(
MEHQ) was added to the collector in order to prevent polymerisation during the thawing. Gaseous
products evaporating from the condenser during warm-up were routed to the off-gas. The reaction mix-
1
ture and distillate were then analysed via off-line H-NMR (JEOL ECX 400 MHz).
Analysis, quantification & calculation
1
Product analysis was carried out with H-NMR spectroscopy (JEOL EX 400 MHz, 8 scans). All chem-
1
using H (residual) chemical shifts of the deu-
ical shifts were recorded relative to the peak of Si(CH
3
)
4
terated solvent DMSO-d6 as a secondary standard. Data analysis was performed with the software JEOL
Delta 5.0.4.
The molar ratio x
1
of a binary mixture of two substances 1 and 2 was determined as shown in equation
(
1). Conversion (X
r p
) and yields (Y ) of all performed experiments were determined via the integrals of
1
the corresponding peaks in the H NMR spectrum, using the following equations (with I = value of the
area under a specific peak, H = number of corresponding protons of the specific molecule, r = reactant,
p = product, ꢀ = stoichiometric coefficient):
ꢃꢂ
1
ꢁꢂ
=
=
ꢃ + ꢃ
ꢅ ∗ ꢆ
ꢄ ꢂ
ꢄ
ꢂ
+
1
ꢅꢂ ∗ ꢆ
ꢄ
ꢃꢈ,ꢉ − ꢃꢈ
ꢃꢈ
ꢇ =
ꢈ
= 1 −
ꢃꢈ,ꢉ
ꢃꢈ,ꢉ
2
-BrPA is added to reaction mixture for in situ generation of HBr, it is, however, also a reaction inter-
mediate which can be converted to AA. Therefore, it has been considered in all calculations of yield and
selectivity as a reactant. The reactant lactide was expressed in all calculations in lactic acid equivalents
(
LAe’s). As lactide is the cyclic dimer of lactic acid, 1 mmol of lactide corresponds to 2 mmol lactic
acid equivalents. Consequently, the yield in the experiments is calculated as follows:
(
ꢃ − ꢃꢋ,ꢉ) |ꢌꢈ|
(ꢃ − ꢃꢋ,ꢉ)
ꢋ
ꢋ
ꢊꢋ =
=
ꢃꢈ,ꢉ
ꢌꢋ
2ꢃꢍꢎꢏꢐꢑꢒꢓ + ꢃꢄꢔꢕꢈꢖꢗ
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ChemSusChem
10.1002/cssc.201800914
We have verified some of our NMR analysis of the composition of the reaction mixture using GC and
HPLC methods. The latter analyses were carried out in the laboratories of our industrial partner P&G.
These comparisons revealed very small deviations (+/-1 %) between the applied methods. For reasons
1
of practicality we decided to use the H-NMR spectroscopic analysis throughout the study as described
above.
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ChemSusChem
10.1002/cssc.201800914
Conclusion
Our contribution reports a novel and solvent-free reaction system for the highly selective conversion of
lactide to acrylic acid. The work builds on the remarkable finding that HBr is a very suitable reagent to
form 2-bromopropionic acid from lactide. This intermediate undergoes effective dehydrobromination to
form acrylic acid and reform the HBr catalyst (see Figure 3). From this mechanistic understanding,
[
4
PBu ]Br has been identified as a very suitable reaction matrix for this kind of transformation as it
reduces the HBr pressure significantly and provides a high concentration of bromide in the system for
stabilising cationic reaction intermediates that would otherwise lead to undesired decarbonylation
pathways.
Figure 3: Mechanistic view on the HBr-catalysed conversion of lactide to AA as further developed in
this work - next to the desired rearrangement to AA, decarbonylation to AcAld, thermal decarboxylation
to ethylene and consecutive hydrobromination to 3-BrPA is shown.
Our results show that thermal AA decomposition causes a major problem in the quest for high AA yields
from lactide (see Figure 3). By removing AA in situ as light-boiling component from the reaction
mixture by either vacuum or strip gas, the AA yield could be increased to an unprecedented level of
more than 81% (10 mbar, 190°C, and 3 h reaction time). Taking into account that the reaction mixture
in this specific result still contains 6.5% of 2-BrPA, 3-BrPA, and acrylates that can be recycled to the
reactor to be transformed consecutively to AA, AA yields of well over 85% appear realistic with the
presented reaction and product separation system. These high selectivities point strongly to the
interpretation that the remaining challenge in the HBr catalysed transformation of lactide to AA in
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ChemSusChem
10.1002/cssc.201800914
bromide salts is more the full suppression of the consecutive AA decarboxylation and less the 2-BrPA
decarbonylation to acetaldehyde.
Our work indicates that the pressure level during in situ AA removal from the reaction system is a critical
parameter, optimised AA yields are only possible in a relatively narrow pressure range. This is due to
the fact that too low pressures remove the HBr catalyst from the reaction system thus slowing down the
transformation significantly. In contrast, too high pressures do not provide the necessary driving force
for effective removal of the AA product from the reaction mixture with significant AA decarboxylation
in the reaction system as negative consequence. Also, we demonstrated that the bromide salt [PBu
used is stable over at least four reaction cycles and long term stability tests at 250°C for 168 hours.
4
]Br
The presented process operates at mild conditions (< 220°C, and ambient or reduced pressure) in the
liquid phase. Compared to the known gas-phase processes for AA production from LA, temperatures
are much lower, and consequently potential corrosion issues are much easier to solve. For example, at
temperatures below 200°C operation of the process in polymer or polymer-lined reactors is well suitable.
All in all, our work has opened a new route for efficient conversion of biogenic lactide into acrylic acid
with hitherto unmatched selectivities. It therefore constitutes a realistic and scalable way for a future
production of bio-acrylic acid from renewable feedstock, such as starch.
Acknowledgements
We are very grateful for excellent scientific and financial support from our industrial collaboration part-
ner Procter & Gamble. In addition, financial support by the Bundesministerium für Ernährung und Land-
wirtschaft through the Fachagentur Nachwachsende Rohstoffe e.V. (FNR, Project No. 22009614) is
gratefully acknowledged. Infrastructural support has been provided by the Erlangen Cluster of Excel-
lence “Engineering of Advanced Materials”.
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