Catalytic Cracking of Lactide and Poly(Lactic Acid) to Acrylic Acid at Low Temperatures

Article abstract of DOI:10.1002/cssc.201700108

Despite being a simple dehydration reaction, the industrially relevant conversion of lactic acid to acrylic acid is particularly challenging. For the first time, the catalytic cracking of lactide and poly(lactic acid) to acrylic acid under mild conditions is reported with up to 58 % yield. This transformation is catalyzed by strong acids in the presence of bromide or chloride salts and proceeds through simple S_N2 and elimination reactions.

Full text of DOI:10.1002/cssc.201700108

DOI: 10.1002/cssc.201700108
Communications
Catalytic Cracking of Lactide and Poly(Lactic Acid) to
Acrylic Acid at Low Temperatures
Frꢀdꢀric G. Terrade,[a] Jan van Krieken,[b] Bastiaan J. V. Verkuijl,[b] and Elisabeth Bouwman*[a]
Despite being a simple dehydration reaction, the industrially
relevant conversion of lactic acid to acrylic acid is particularly
challenging. For the first time, the catalytic cracking of lactide
and poly(lactic acid) to acrylic acid under mild conditions is re-
ported with up to 58% yield. This transformation is catalyzed
by strong acids in the presence of bromide or chloride salts
and proceeds through simple SN2 and elimination reactions.
conversion of LA to AA is highly desired; however, this simple
dehydration reaction is surprisingly challenging.
The dehydration of alcohols is typically catalyzed by strong
acids.[5] Because LA bears a carboxylic acid function, the acid-
catalyzed dehydration of LA under mild and concentrated con-
ditions does not lead to acrylic acid but instead results in
linear oligomers and a small amount of lactide.[6] Under harsher
conditions, a range of products is formed, such as acetalde-
hyde, propionic acid, CO, and CO2.[7] The use of dehydration
catalysts based on rhenium or molybdenum complexes has
been reported to lead to traces of AA together with poly-
mers,[8] whereas in the presence of a reductant mainly propion-
ic acid is obtained.[9]
Environmental concerns and the impending scarcity of oil
resources result in increasing attention for the concept of a
biobased economy. To replace oil-based processes, energy-
efficient and economically competitive processes using bio-
based feedstocks have to be developed. Lactic acid (LA) is al-
ready an important platform chemical obtained by fermenta-
tion. Stimulated by technological improvement and the diversi-
fication of fermentation feedstocks, and driven by the increas-
ing demand for the biodegradable polymer poly(lactic acid)
(PLA), its production capacity is expected to rise in the coming
years, accompanied by a drop in price.[1] Besides PLA, value-
added commodities derived from LA such as alkyl lactates (sol-
vents) are already produced industrially. The cyclic diester lac-
tide is produced industrially from oligo(lactic acid): it is used as
a monomer for the production of PLA. LA can also be trans-
formed into propylene glycol, pyruvic acid, acetaldehyde, 2,3-
pentanedione, or acrylic acid (AA).[2] AA is an important feed-
stock for the production of various polymers applied in, for ex-
ample, plastics and elastomers as well as paints.[3] Petroleum-
based AA is relatively expensive owing to the relatively high
cost of the propene feedstock, the energy-demanding two-
step process, and purification.[4] A feasible process for the
The best systems reported so far for the direct dehydration
of LA to AA are based on heterogeneous phosphate or sulfate
catalysts in the gas phase (Scheme 1a).[10] The highest reported
[a] Dr. F. G. Terrade, Prof. Dr. E. Bouwman
Leiden Institute of Chemistry, Gorlaeus Laboratories
Leiden University
Scheme 1. (a) Direct dehydration of lactic acid. (b) Pyrolysis of 2-acetoxypro-
pionic acid. (c) Catalytic cracking of lactide (our work).
P.O. Box 9502
2300 RA Leiden (The Netherlands)
E-mail: bouwman@chem.leidenuniv.nl
[b] J. van Krieken, Dr. B. J. V. Verkuijl
Corbion, Central R&D, Product and Process Technology Department
P.O. Box 21
yields are in the range of 80%.[11] The main drawbacks of these
processes are their high temperature (300–4008C), the relative-
ly high dilution required to avoid the formation of non-volatile
oligomers of LA, and poisoning of the catalysts. Alternatively,
LA can be converted to AA in supercritical water without the
need of added catalysts. The polymerization of LA is sup-
pressed in this system. However, the temperature and pressure
required are very high (1000 bar) and AA yields are low (44%
selectivity at 23% conversion).[7b,12] Another reported two-step
process relies on the use of acetic acid: 2-acetoxypropionic
4200 AA Gorinchem (The Netherlands)
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under https://doi.org/10.1002/
cssc.201700108.
ꢁ 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution Non-Commercial NoDerivs License, which permits use and
distribution in any medium, provided the original work is properly cited,
the use is non-commercial and no modifications or adaptations are
made.
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acid is first produced by the esterification of LA and acetic acid
catalyzed by strong acids,[13] or by the reaction of lactide with
acetic acid catalyzed by a nickel catalyst at 2508C.[4] Pyrolysis
of 2-acetoxypropionic should lead to AA and acetic acid
(Scheme 1b). However, recycling of the acetic acid reagent/
coproduct is mandatory to achieve an economically viable
route,[4,11,13,14] impeding the development of an industrial pro-
cess.
Table 1. Rearrangement of lactide, oligo(lactic acid), and poly(lactic acid)
forming acrylic acid (AA) and the ring-opened intermediate 2.[a]
Entry Substrate Acid
Bromide source
T
Conv. AA
2
(1 mmol) [mmol] [mmol]
[8C] [%]
[mmol] [mmol]
1
2
3
4
5
6
7
8
9
lactide
lactide
lactide
lactide
lactide
lactide
lactide
HOTS TBAB
(0.5) (3.1)
HOMs TBAB
(0.5) (3.1)
HOMs TBAB
(2.4) (15)
130
93 0.04
63 0.06
93 0.10
0.13
0.27
0.42
0.44
0.44
0.35
0.35
0.26
0.26
130
130
Herein we report the direct formation of AA from lactide
(Scheme 1c): a yield of 58% AA was obtained from lactide
after 10 h at 1758C. Importantly, using this procedure AA can
be produced directly from PLA or oligo-LA, offering a new
route to the recycling of PLA yielding the high-valued product
AA (patent pending).
HOMs TBAB
(0.83) (5)
150 >99 0.32
150 >99 0.64
150 >99 0.62
150 >99 0.70
150 >99 0.46
150 >99 0.48
HOMs PPh4Br[b]
(0.83) (5)
2BrPA PPh4Br[b]
(0.5)
3BrPA PPh4Br[b]
(0.5) (5)
(5)
Our initial aim was to develop catalysts for the carbonylation
of LA to succinic acid, for which we investigated the Lapidus
system reported for the carbonylation of cyclohexanol.[15] This
catalytic system comprises a palladium source, a strong acid,
and the bromide-based ionic liquid tetrabutylammonium
bromide (TBAB). By analogy with the described mechanism for
related reactions,[16] we expected the formation of 2-bromopro-
pionic acid (2BrPA) as the first step of the mechanism. The con-
version of LA to 2BrPA in the presence of hydrobromic acid is
well known.[17] Unexpectedly, a control experiment at 1308C in
the absence of the palladium catalyst showed 24% conversion
of LA and the formation of AA in approximately 4% selectivity
with a large amount of LA oligomers and 2BrPA in 24% selec-
tivity, (Table S1, entry 1 in the Supporting Information). In the
absence of strong acids, no conversion was observed, and if
1 equivalent of water was added the conversion dropped
(Table S1, entries 2 and 3).
oligo-LA[c] HOMs PPh4Br[b]
(0.83) (5)
PLA[c]
HOMs PPh4Br[b]
(0.83) (5)
[a] Full analytical details and mass balance are given in the Supporting In-
formation. For ease of comparison, the quantities of material in this table
have been scaled to the use of 1 mmol lactide. Reaction conditions:
1308C, 16 h. [b] Sulfolane was used as a solvent (1 g sulfolane per g
PPh4Br). [c] Amount of PLA and oligo-LA corresponding to 1 mmol lac-
tide.
ing amount of HOTs was used because the HOTs used for this
study was the commercial monohydrate. Furthermore, 3-bro-
mopropionic acid (3BrPA), the addition product of HBr to AA,
was also detected in the reaction mixture.
Because the water formed by the dehydration of LA ap-
peared to be detrimental for the formation of AA, we decided
to use lactide as the substrate in this reaction. The main prod-
ucts found in the reaction mixtures of our investigations are
shown in Scheme 2.
To completely eliminate the adverse effect of water in subse-
quent reactions, anhydrous methanesulfonic acid was used
(HOMs, Table S3). Under these water-free conditions no LA was
formed. The selectivity towards the unsaturated compounds
AA and 2 was highest at 0.5 equivalents HOMs relative to lac-
tide (entry 2 in Table 1 and entry 3 in table S3: 5% selectivity
towards AA and 43% towards 2 at 63% conversion). The use
of larger amounts of HOMs resulted in an increase in conver-
sion but did not increase the selectivity towards AA and 2. An
increase in the bromide/lactide ratio resulted in increased con-
version up to 93% (when 15 equiv TBAB was used) but had
little or no effect on the selectivity: 40–45% 2 and 5% AA
were formed for Br/lactide ratios from 3 to 15 (Table S4 and
Table 1, entry 3).
We then investigated whether TBAB might be replaced by
other sources of bromide ions; the use of tetraphenylphospho-
nium bromide (PPh4Br), 1-ethyl-3-methylimidazolium bromide,
1-butylpyridinium bromide, or tetrabutylphosphonium bro-
mide as bromide sources afforded similar or lower selectivities
and conversions (Table S5). Contrary to the other bromide
sources, PPh4Br is not an ionic liquid under the reaction condi-
tions (melting point: 2958C), and in the reactions with this salt,
sulfolane was added as a solvent. In the sulfolane/PPh4Br mix-
ture the use of other (anhydrous) strong acids such as trifluor-
omethanesulfonic acid or meta-phosphoric acid afforded simi-
lar results as with HOMs. Weak acids such as acetic acid or
Scheme 2. Species observed in the reaction mixtures.
When lactide was heated in TBAB in the presence of p-tolu-
ene sulfonic acid (HOTs), AA was obtained in 2% selectivity,
and compound 2, the ester of AA and LA, in 14% selectivity at
93% conversion (Table 1, entry 1). Subsequently, we tried to
optimize the reaction for the selective formation of AA. The
effect of the quantity of HOTS on the outcome of the transfor-
mation was investigated (Table S2). Not surprisingly, an increas-
ing amount of lactide was hydrolyzed to LA when an increas-
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oxalic acid were inefficient because neither bromo-substituted
nor unsaturated species were formed (Table S6).
2BrPA was formed with only 7% selectivity under otherwise
identical conditions.
Higher selectivity towards AA and 2 was obtained when the
reaction temperature was raised to 150 or 1758C with TBAB or
PPh4Br as bromide sources (Table S7 and Table 1, entries 4 and
5). Unfortunately, when the reaction was run at 1758C for 16 h,
the mass balance decreased to 60% (for both bromide sour-
ces), and when the reaction was run at 2008C, the mass bal-
ance dropped to 30% in TBAB or a mere 2% in PPh4Br. A ten-
tative explanation for the loss of mass balance may be the for-
mation of polyacrylates under these harsh conditions. Further-
more, TBAB partially decomposes at these higher tempera-
tures, as indicated by the observation of butyl bromide in the
reaction mixture. The PPh4Br/sulfolane system is more suitable
for the use under harsher reaction conditions because of the
higher thermal stability of these species.
We hypothesized that bromopropionic acid itself can also be
used as a source of acid because it can eliminate HBr to afford
AA.[18] The rearrangement of lactide also occurred in the ab-
sence of a strong acid but with 2BrPA added at the beginning
of the reaction (Table S10, entries 1–3): when lactide
(0.69 mmol) and 2BrPA (0.35 mmol) were stirred in BrPPh4/
sulfolane for 16 h at 1508C, the resulting reaction mixture was
found to contain AA (0.54 mmol), 2 (0.31 mmol), and 2BrPA
(0.10 mmol). The compound 3BrPA could also be used
(Table S10, entries 4 and 5): a reaction of lactide (0.69 mmol)
and 3BrPA (0.35 mmol) in BrPPh4/sulfolane for 16 h at 1508C
resulted in
a
mixture containing AA (0.62 mmol),
2
(0.31 mmol), and 3BrPA (0.18 mmol).
Finally, oligo-LA and PLA were submitted to the PPh4Br/
sulfolane/HOMs reaction medium at 1508C for 16 h. Starting
from oligo-LA (100 mg), AA (23 mg) and 2 (26 mg) were ob-
tained. Additional signals in the acrylate region of the NMR
spectrum were attributed to the tri-ester 3 (comprising two LA
units and one acrylate unit, see Scheme 2 and Figure S2),
which was obtained in 11 mg yield (Table S11, entry 1). When
PLA (100 mg) was used as a starting material, AA (24 mg), 2
The rearrangement of lactide was monitored over 10 h at
1758C in the HOMs/PPh4Br/sulfolane reaction medium
(Figure 1 and Table S8). Under these conditions, lactide quickly
(26 mg) and
3 (16 mg) were obtained (Table 1, entry 9;
Table S11, entry 2). Besides pure commercial PLA, to show that
this transformation can be applied for the recycling of PLA, we
also used a piece of plastic cutlery (consisting of 67% PLA and
33% inert filler) from the university canteen. The results were
virtually identical to those obtained when bulk PLA was used
(Table S11, entry 3).
Mechanistic considerations
The various intermediates found in the reaction mixtures
helped us to devise a mechanism for the transformation of lac-
tide to AA, as shown in Scheme 3. The cyclic lactide ester is
first opened by a proton and a bromide ion to give 2-(2’-bro-
mopropanoyloxy)propanoic acid (1) (first nucleophilic substitu-
tion). Compound 1 then either eliminates HBr to afford 2-
(acryloyloxy)propanoic acid (2) (first elimination) or may react
with a second equivalent of HBr to yield two equivalents of
2BrPA. Compound 2 can also be hydrobrominated to afford AA
and 2BrPA (second nucleophilic substitution). Finally, 2BrPA
eliminates HBr to yield AA (second elimination reaction). The
presence of small amounts of 3BrPA in the reaction mixture
could be the result of the reversible addition of HBr to AA.
At high temperatures, secondary alkyl halides can be trans-
formed to alkenes through the E1 mechanism involving
a carbo-cationic intermediate. However, in the case of 2BrPA,
the involvement of such a carbo-cationic intermediate is not
likely owing to its a-position with respect to the carboxylic
acid group. A strict E2-type mechanism for this elimination
step is also not very probable owing to the extremely acidic re-
action medium. An E2C mechanism with bromide ions acting
as a weak base is more likely to be operative.[19] Alternatively,
an intramolecular carboxylic acid-assisted E1-type elimination
Figure 1. Rearrangement of lactide to acrylic acid at 1758C as a function of
time. Reaction conditions: lactide (0.69 mmol), HOMs (0.83 equiv), PPh4Br
(5 equiv), sulfolane. Lines are only given as a guide to the eye.
disappeared from the reaction mixture: 97% was consumed
within 2 h. The selectivity towards 2 reached a maximum after
2 h (50% selectivity at 94% conversion) and then decreased,
whereas the selectivity towards 2BrPA increased to 14% in the
first hour and then slowly decreased to reach 0% after 10 h.
The amount of 3BrPA reached a maximum at 4 h with 7% se-
lectivity and then remained stable until the end. The selectivity
towards AA increased continuously over the 10 h reaction time
to reach 58%. In the first few hours of the reaction, the overall
mass balance was initially quite good, but after 6 h it de-
creased significantly.
The chloride salt PPh4Cl can also be used for the reaction, al-
though the selectivity towards rearrangement products is
lower than when PPh4Br is used. Whereas AA and 2 were ob-
tained in 32 and 44% selectivity at full conversion in PPh4Br,
under otherwise identical conditions AA and 2 were obtained
in 11 and 29% selectivity at 86% conversion in PPh4Cl (see
Table S9). A significant amount of 2-chloropropionic acid
(2ClPA) was formed in the reaction (26% selectivity), whereas
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Scheme 3. Proposed mechanism for the catalytic cracking of lactide by strong acid and bromide salts.
pathway involving a hypothetical oxiranone intermediate has
also been proposed.[20]
ic acid (1), 2-(acryloyloxy)propanoic acid (2), and 2-bromopro-
pionic acid are intermediates in the conversion of lactide to AA
and that simple SN2 and elimination mechanisms are involved.
Selectivities to acrylates up to 64% were obtained (up to 58%
to AA). Attempts to obtain higher selectivities resulted in lower
mass balances, probably owing to polymerization of the acryl-
ates. To circumvent this limitation, continuous removal of AA
as it forms seems to be a promising area to be explored be-
cause AA has the lowest boiling point of all species involved in
this reaction. In that respect, it is foreseen that the catalytic
system (bromide salts, acid, and solvent) could be reused after
separation of AA by continuous distillation. This new process is
strictly different from previously described processes because
it is formally a rearrangement and not a dehydration. Contrary
to other processes, the reaction temperature is much milder
(130–1758C vs. 300–5008C), and even oligomers or polymers
of lactic acid can be used as substrates. The relatively high
cost of the lactide starting material could be a major drawback
for the industrial application of our reaction. Thus, a process
based on the use of oligomers of lactic acid or mixtures of
oligomers and racemized lactide as starting materials will have
a higher economic viability. Moreover, poly(lactic acid) is an in-
teresting substrate for recycling to the value-added product
AA.
The transformation of one equivalent of lactide to two
equivalents of AA formally is not a dehydration reaction but
rather cracking catalyzed by protons and bromide ions. Molec-
ular HBr is not expected to be present as such in the reaction
mixtures: it is likely to be fully dissociated because of its low
pKa (À9) compared to the pKa of HOTs or even of protonated
acids or esters (pKa in the range of À3 to À2).
Compound 1 remains a hypothetical intermediate because it
has not been observed. It is thought to quickly eliminate HBr
to afford 2. The reaction of 1 with HBr to yield two equivalents
of 2BrPA cannot be ruled out and could contribute to the low
concentration of 1. It has been observed that the concentra-
tion of 2BrPA and 2 in the reaction mixture increases in the be-
ginning of the reaction and then decreases (Figure 1), which
supports their role as reaction intermediates for the conversion
of LA to AA. The role of 2BrPA as an intermediate and a source
of HBr was further confirmed by an experiment in which no
strong acid was added but in which 2BrPA was added at the
start of the reaction. Similarly, 3BrPA can also be used as
a source of HBr, indicating that the addition of HBr to AA is re-
versible under our reaction conditions.
Increasing the concentration of sulfonic acid (HOTs·H2O/lac-
tide >0.5 or MsOH/lactide >1) in the reaction mixture led to
an increase in the formation of 2BrPA but a decrease in the for-
mation of 2 and AA. These observations suggest that the elimi-
nation reactions are hindered by high concentrations of pro-
tons, which hamper the ability of bromide ions to act as weak
bases.
Experimental Section
Representative experimental procedure (corresponding to entry 5
in Table 1): (S,S)-lactide (100 mg, 0.69 mmol), methanesulfonic acid
(37.6 mL, 0.58 mmol, 0.83 equiv), tetraphenylphosphonium bromide
(1.45 g, 3.47 mmol, 5 equiv), and sulfolane (1.5 mL) were intro-
duced in the glass inset of an autoclave equipped with a stirring
bar. The autoclave was closed, pressurized with 50 bar N2, and
then heated to 1508C (temperature of the heating mantle) for 16 h
under magnetic stirring (400 rpm). Then, the autoclave was placed
in an ice bath for 30 min before being vented and opened.
1H NMR spectroscopy of the crude reaction mixture in deuterated
DMSO, using the signal of the sulfonic acid as an internal standard,
showed the complete consumption of lactide and the formation of
acrylic acid (0.44 mmol, 32% selectivity), 2 (0.31 mmol 44% selec-
tivity), 2-bromopropionic acid (0.08 mmol, 6% selectivity), and 3-
bromopropionic acid (0.09 mmol, 7% selectivity).
Compared to the bromide ion, chloride is a better nucleo-
phile in aprotic environments (but not so good in protic envi-
ronments); it is less acidic and an inferior leaving group. As
a result, a significant amount of 2-chloropropionic acid (2ClPA)
was formed in the reaction (23%) with a chloride salt, whereas
6% 2BrPA was formed under otherwise identical conditions;
clearly, the elimination of HCl from 2ClPA is more difficult than
the elimination of HBr from 2BrPA. This observation corrobo-
rates the proposed mechanism.
To conclude, we disclosed a promising new strategy for the
conversion of lactide, oligo(lactic acid) or poly(lactic acid) to
acrylic acid (AA). This transformation is catalyzed by strong
acids in the presence of bromide salts. Preliminary mechanistic
investigations suggest that 2-(2’-bromopropanoyloxy)propano-
To avoid gaseous HBr leaving the reaction mixture the initial reac-
tions were performed at 50 bar N2 pressure. However, reactions
performed at autogenic pressure in closed autoclaves, or at normal
pressure in Schlenk glassware led to identical results.
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Communications
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Keywords: acrylic acid · biobased economy · lactic acid ·
renewable resources · sustainable chemistry
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Manuscript received: January 20, 2017
Revised: March 13, 2017
Accepted Article published: April 4, 2017
Final Article published: && &&, 0000
ChemSusChem 2017, 10, 1 – 6
www.chemsuschem.org
5
ꢁ 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

COMMUNICATIONS
F. G. Terrade, J. van Krieken,
B. J. V. Verkuijl, E. Bouwman*
&& – &&
Catalytic Cracking of Lactide and
Poly(Lactic Acid) to Acrylic Acid at
Low Temperatures
Sweet diapers! A radical new and effi-
cient synthetic method is presented,
which converts lactide to acrylic acid in
a convenient one-pot/one-step process.
Acrylic acid is a highly valued chemical
and a starting material for superabsorb-
ent polymers (SAPs), which finds their
prime application in diapers. The tech-
nique presented here is an important
step towards full biobased SAPs and
turns lactide into a key platform chemi-
cal in the biobased arena.
&
ChemSusChem 2017, 10, 1 – 6
www.chemsuschem.org
6
ꢁ 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!