Propane-1,2-diols from dilactides, oligolactides, or poly-L-lactic acid (PLLA): From plastic waste to chiral bulk chemicals

Article abstract of DOI:10.1002/chem.201302004

The preparation of racemic or enantioenriched propane-1,2-diol from dilactides, oligolactides, or poly-L-lactic acid (PLLA) is described. The transformation is carried out as tandem reactions in MeOH, covering hydrolysis and subsequent hydrogenation by using copper chromite as a catalyst. The starting material present undesired side products of the PLLA synthesis or PLLA waste. Copyright

Full text of DOI:10.1002/chem.201302004

DOI: 10.1002/chem.201302004
Communication
&
Hydrogenation
Propane-1,2-diols from Dilactides, Oligolactides, or Poly-l-lactic
Acid (PLLA): From Plastic Waste to Chiral Bulk Chemicals
Ivan A. Shuklov,*[a] Natalia V. Dubrovina,[a] Joachim Schulze,[b] Wolfgang Tietz,[b]
Klaus Kꢀhlein,[c] and Armin Bçrner*[a, d]
In memory of Karsten Krohn
removed.[1 g] Methods to use the meso-compound for the pro-
duction of other valuable chemicals are therefore of high inter-
est.
Abstract: The preparation of racemic or enantioenriched
propane-1,2-diol from dilactides, oligolactides, or poly-l-
lactic acid (PLLA) is described. The transformation is car-
ried out as tandem reactions in MeOH, covering hydrolysis
and subsequent hydrogenation by using copper chromite
as a catalyst. The starting material present undesired side
products of the PLLA synthesis or PLLA waste.
Since few years, we focus our research on the improvement
of the cost efficiency of the industrial PLLA manufacture.[4] Re-
ducing costs of the PLLA production, avoidance or beneficial
utilization of by-products, as well as an efficient recycling of
PLLA, are challenging goals within this framework.[1g,5]
The synthesis of propane-1,2-diol (propylene glycol) from
undesired lactides or PLLA waste could solve two problems at
once. First, it can greatly diminish the overall costs of PLLA
production. On the other hand, it offers an attractive and scal-
able approach to propane-1,2-diol based on renewable re-
sources. Commonly used propane-1,2-diol is produced industri-
ally from propylene through propylene oxide (HPPO process),[6]
and therefore it is crude oil derived. The annual production is
close to 1 Mio tons.[7] The main part is used for the manufac-
ture of polyesters or polyurethanes. Alternatively, it may serve
for the production of propylene carbonate, which is a valuable
and environmental friendly solvent for several applications.[8]
The production of propane-1,2-diol starting from lactic acid de-
rivatives would offer the possibility to change the feedstock
basis. In addition, since the generally applied enzymatic
venues to lactic acid produce enantiopure compounds, subse-
quent transformations may afford enantiopure propane-1,2-
diol with new sale prospects at the market than its racemic
mixture.
The application of biodegradable polymers in packaging or
one-way cutlery is one of the most important measurements
to reduce the vast amounts of waste worldwide. Poly-l-lactic
acid (PLLA) derived from natural l-(+)-(S)-lactic acid is one of
most promising material for such purpose.[1] It is assumed that
in near-future PLLA will have a similar potential like polyethyl-
ene terephthalate (PET), but with different field of applica-
tion.[1g] Unfortunately to date, the production costs are relative-
ly high in comparison to their petrochemically based ana-
logues.
PLLAs are usually produced by the ring-opening polymeri-
zation of (S,S)-dilactide.[1g] (S,S)-Dilactide is available by thermal
metal-catalyzed dimerization of (S)-lactic acid; the latter can be
derived by fermentation of corn, potato, or even garbage.[1g,2,3]
During the lactonization besides the desired (S,S)-dilactide, also
considerable amounts of meso-(S,R)-dilactide are formed,
which seriously affects the overall economy of the manufactur-
ing of PLLA on a large scale. meso-Dilactide impacts the prop-
erties of the resulting PLLA if incorporated and thus has to be
Herein, we report a simple one-pot protocol for the prepara-
tion of racemic or almost enantiomerically pure (S)-propane-
1,2-diol starting from different lactides, which give the diol in
excellent yield.
[a] Dr. I. A. Shuklov, Dr. N. V. Dubrovina, Prof. Dr. A. Bçrner
Leibniz-Institut fꢀr Katalyse an der Universitꢁt Rostock e.V.
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
Fax: (+49)381-1281-5202;
Alcoholysis hydrogenation of di- and oligolactides
The ring-opening hydrogenation of dilactides is an interesting
challenge for the synthesis of propane-1,2-diol. To the best of
our knowledge, this tandem reaction has not been described
in the literature to date.[9] In contrast, the hydrogenation of
none-cyclic lactic acid esters, such as alkyl lactates, has been al-
ready established.[10,3b] The reaction requires normally high
temperatures and hydrogen pressures.[10a,b] For the reduction,
alcohols are commonly applied as solvent.
E-mail: ivan.shuklov@catalysis.de
armin.boerner@catalysis.de
[b] Dr. J. Schulze, W. Tietz
ThyssenKrupp Uhde GmbH
Am Haupttor, Bau 3668, 06237 Leuna (Germany)
[c] Prof. Dr. K. Kꢀhlein
Fasanenstrasse 14, 65799 Kelkheim (Germany)
[d] Prof. Dr. A. Bçrner
Institut fꢀr Chemie der Universitꢁt Rostock
Albert-Einstein-Strasse 31, 18059 Rostock (Germany)
Our initial screening experiments revealed that dilactides are
resistant to hydrogenation conditions at low temperatures up
to 1008C by using heterogeneous ruthenium catalysts, such as
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201302004.
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Communication
5% Ru on charcoal, 5% Ru on Al2O3, and 5% Ru with 0.25%
Pd on charcoal. Noteworthy, only transesterification took place
in alcoholic medium. The rates of the ring cleavage of the lac-
tide and the following alcoholysis were mainly dependent on
the nature of the catalyst support. Thus, ruthenium on alumini-
um oxide promoted the reaction much more efficient than
ruthenium on charcoal. With the former, a ratio of methyl lac-
toyl lactate to methyl lactate of 1:2 was obtained, whereas
with the latter—a ratio of 10:1 was obtained after 4 h at 908C.
These observations inspired us to find reaction conditions, in
which the alcoholysis of dilactides proceeds fast giving lac-
tates, which are immediately subjected to the hydrogenation.
Indeed, preliminary experiments with copper chromite (Cu/
Cr) and barium-promoted copper chromite (Cu/Cr/Ba)[10a,11,12]
as catalysts showed that rac-dilactide can be converted into
propane-1,2-diol at 1508C and 150 bar in methanol with excel-
lent chemoselectivity and in high yield (Table 1, runs 1–4). Re-
markably, products of deoxygenation were not observed at all.
The barium-promoted copper chromite catalyst was superior
in terms of separation by centrifugation.
Scheme 1. Alcoholysis hydrogenation of rac-dilactide in methanol.
Scheme 2. Diastereomeric lactides used as substrates.
The reaction in ethanol proceeded much slower. Thus, full
conversion of 0.5 g rac-dilactide (3.5 mmol) in 5 mL ethanol
was observed only after 15 h. At higher concentrations of di-
lactide, complete conversion was not reached within this time.
Unfortunately, propane-1,2-diol was not suited for the direct
determination of the enantiomeric-excess (ee) values by GC at
chiral columns. Therefore, it was converted into the corre-
sponding bis(phenylurethane) derivative and analyzed by
HPLC analysis on Chiralcel OD-H. Similarly useful revealed the
formation of the bis(trifluoroacetate) derivative, which was
subjected to GC analysis on a Lipodex E column.[13]
Table 1. Alcoholysis hydrogenation of stereoisomeric dilactides.[a]
Run Dilactide Load [g] t [h] T [8C] Catalyst
Yield[b] [%]
1
2
3
4
rac
rac
rac
rac
rac
rac
0.15
0.15
0.15
0.15
1.0
1.0
1.0
0.5
1.0
5
5
5
5
5
10
10
15
15
15
15
125
150
150
150
90
99
99
150
150
150
150
Cu/Cr (33 wt%)
Cu/Cr (33 wt%)
Cu/Cr (133 wt%)
40
70
99
Cu/Cr/Ba (130 wt%) 99
Cu/Cr/Ba (130 wt%) 19
Cu/Cr/Ba (133 wt%) 99
Cu/Cr/Ba (133 wt%) 99
Cu/Cr/Ba (130 wt%) 99
Cu/Cr/Ba (130 wt%) 99
Cu/Cr/Ba (130 wt%) 99
Cu/Cr/Ba (130 wt%) 99
5[c]
6
7[d] rac
8
9
10
11
rac
rac
(S,S)
meso
Next, the temperature dependency of the stereoselectivity
in the hydrogenation of (S,S)-dilactide was studied. It was
found that the ee of the formed propane-1,2-diol is strongly af-
fected by the reaction temperature (Table 2). At 1508C, the hy-
drogenation procedure gave exclusively racemic product. A de-
crease of the reaction temperature to 1258C gave (S)-propane-
1,2-diol with 74% ee. At 1008C, almost no epimerization took
place, and (S)-propane-1,2-diol was obtained with 98% ee.
Under these conditions, the rate of the reaction was lowered.
After that, the hydrogenation of hexalactide was studied to
broaden the scope of the reaction. The substrate was synthe-
sized by using a modified procedure of Chisholm et al. from
rac-dilactide.[14] Thus, a mixture of stereoisomeric hexalactides
was obtained from the reaction of sodium tetraphenylborate-
hexalactide with N,N,N’,N’-tetramethylethylenediamine. The
mixture was azeotropically dried with benzene.
1.0
1.0
[a] Reaction conditions: 150 bar H2, 5 mL MeOH. [b] Yield based on GC.
[c] 90 bar H2. [d] Recycled catalyst from run 6 was used.
In turn, the reaction was optimized. It was found that up to
0.2 g rac-dilactide (1.4 mmol) per 1 mL of methanol can be
converted into propane-1,2-diol (runs 4,8,9). The transforma-
tion below 1008C was slow (run 5). It should be noted that
methanol acts as a solvent as well as a reagent, but it was not
consumed during the transformation. In all experiments, full
conversion of dilactide was achieved. Methyl lactoyl lactate
and methyl lactates were the only by-products observed in
those cases, in which the reaction was not complete
(Scheme 1). The observation of these esters gave proof that
both are intermediates in the transformation to propane-1,2-
diol. Under the employed reaction conditions, all stereoisomers
of dilactide could be converted into the final product
(Scheme 2).
Table 2. Alcoholysis hydrogenation of (S,S)-dilactide.[a]
Run
t [h]
T [8C]
Yield[b] [%]
ee[c] [%]
Recycling of catalyst was possible without loss of activity
(runs 6–7). Equal yields of propan-1,2-diol were obtained in the
reaction of racemic dilactide, (S,S)-, and meso-dilactides under
the same reaction conditions (runs 9–11). In the presence of
water or free lactic acid, the catalyst was deactivated and met-
allic copper precipitated on the walls of the autoclave.
1
2
3
12
12
15
150
125
100
100 (82)
90
80
0
74
98
[a] Reaction conditions: 5 mL MeOH, 0.5 g (S,S)-dilactide, 133 wt% Cu/Cr/
Ba, 150 bar H2. [b] Yield based on GC (isolated yield in parentheses).
[c] Determined by GC at Lipodex E.
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We were pleased to see that in the subsequent alcoholysis
hydrogenation under the conditions described above also hex-
alactide reacted to give exclusively propane-1,2-diol in an ex-
cellent yield (Scheme 3).
Scheme 3. Alcoholysis hydrogenation of hexalactide.
Alcoholysis hydrogenation of poly-l-lactic acid polymers
(PLLA)
Scheme 4. Hydrogenation of PLLA and epimerization as side reaction.
The results discussed above gave us hope that not only dimers
and oligomers, but also PLLA can be successfully subjected to
our tandem-reaction protocol. This transformation attracts par-
ticular attention by employment of waste of PLLA.
mized reaction conditions and by using enantiomerically pure
lactides, the reaction gives (S)-propane-1,2-diol with high enan-
tioselectivity. The new transformation allows the use of lactic
acid derivatives, which are commonly produced from renewa-
ble resources for the production of the bulk chemical propane-
1,2-diol and, therefore, it is a contribution to sustainable
chemistry.
The recycling of PLLA was intensively investigated over the
past decade. Different procedures, such as hydrolysis with
water or aqueous solutions,[15,16] alcoholysis with alcohols and
their mixtures,[17–19] reaction with amines,[20] as well as thermal
degradation,[21–24] are known. The transformation into propane-
1,2-diol has not been studied to date.
Experimental Section
Our experiments showed that the hydrogenation procedure
coupled with prior alcoholysis proceeds smoothly also with
polymeric lactides with a molecular weight >15000 Da. As has
been already noted with dilactides, the ee of the resulting (S)-
propane-1,2-diol is dependent on the hydrogenation tempera-
ture (Table 3). At 1008C, the enantioenriched product was ob-
tained with 90% ee.
All reagents, unless otherwise mentioned, were purchased from
commercial sources and used without additional purification. Sol-
vents were dried and freshly distilled under argon before use.
Copper chromite was purchased from Aldrich. Barium-promoted
copper chromite was purchased from STREM Chemicals and was
finely powdered before use. For the determination of the enantio-
meric ratio, GC was used at columns with chiral stationary phases
(Lipodex E column) from Macherey–Nagel GmbH.
Table 3. Stereoselectivity of the alcoholysis hydrogenation of PLLA.[a]
Alcoholysis hydrogenation of rac-dilactide
Run
t [h]
T [8C]
Yield[b] [%]
ee[c] [%]
dl-Dilactide (1.00 g, 6.9 mmol) barium-promoted copper chromite
(1.33 g, 133 wt%) and absolute MeOH (5 mL) were placed in auto-
clave (10 mL). The autoclave was rinsed three times with H2, and
then a pressure of 150 bar H2 was pressed. The reaction mixture
was stirred 12 h at 1508C. The H2 pressure was kept constant be-
tween 148 and 153 bars. The autoclave was cooled to RT and venti-
lated. The reaction mixture was diluted with MeOH (5 mL), and the
catalyst was removed by centrifugation (15 min, 4500 rpm). The re-
sulted reaction mixture was concentrated under reduced pressure.
The blue-colored crude product (1.03 g) consisted of propane-1,2-
diol with about 5% MeOH (13C NMR spectrum). The pure product
was obtained as colorless liquid (0.88 g, 82%) by distillation at
101–1028C and 8 mbar.
1
2
3
15
15
12
100
125
150
50
99
99 (88)
90
40
0
[a] Reaction conditions: 5 mL MeOH, 0.15 g PLLA, Cu/Cr/Ba (133 wt%),
150 bar H2. [b] Yield based on GC (isolated yield in parentheses). [c] Deter-
mined by GC at Lipodex E.
It seems that at elevated temperature, the epimerization oc-
curred mostly with lactoyl lactates or longer-chain oligomers
of lactic acid. Enantiomerically pure propane-1,2-diol turned
out to be stable under these conditions. Because the cleavage
of PLLA with methanol proceeded slower than the alcoholysis
of dilactides with the former substrate, a lowered enantioselec-
tivity in the final product was noted (Scheme 4).
Preparation of hexalactide
To a suspension of [(CH3CHC(O)O)6]NaBPh4 (2.30 g, 3.10 mmol) pre-
pared from rac-lactide[14] in CH3CN (20 mL) was added TMEDA
(3.0 mL, 20 mmol), and the reaction mixture was heated at reflux
at 908C for 20 h. After filtration, the filtrate was evaporated to give
a brown sticky solid. The solid was extracted with CHCl3 (2ꢂ
In conclusion, we have developed a new and highly efficient
protocol for the transformation of di-, oligo-, or polylactides
(PLLA) into propane-1,2-diol based on a tandem reaction cov-
ering alcoholysis and subsequent hydrogenation. Under opti-
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Communication
[3] a) S. Inkinen, M. Hakkarainen, A.-C. Albertsson, A. Sodergard, Biomacro-
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15 mL). The organic phases were combined and concentrated
under reduced pressure. The brown oil was passed through a SiO2
plug by using CHCl3 as eluent to give 0.40 g of a waxy solid. The
material was passed through a 1.0ꢂ17 cm SiO2 column over CHCl3,
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Alcoholysis hydrogenation of hexalactide
Hexalactide (0.20 g, 0.5 mmol), barium-promoted copper chromite
(0.26 g, 133 wt%), and absolute MeOH (5 mL) were placed in a au-
toclave (10 mL). The alcoholysis hydrogenation was carried out as
was described for rac-dilactide. The reaction mixture was diluted
with MeOH (5 mL), and the catalyst was removed by centrifugation
(40 min, 4500 rpm). The resulted reaction mixture was concentrat-
ed under reduced pressure. The product was obtained as colorless
liquid (0.15 g, 72%).
[6] See, for example: http://www.chemistryinnovation.co.uk/stroadmap/
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Alcoholysis hydrogenation of PLLA
A grinded one-way glass made from PLLA (0.50 g), barium-promot-
ed copper chromite (0.67 g, 133 wt%) and absolute MeOH (5 mL)
were placed in autoclave (10 mL). The alcoholysis hydrogenation
was carried out as described for rac-dilactide. The reaction mixture
was diluted with MeOH (5 mL), and the catalyst was removed by
centrifugation (40 min, 4500 rpm). The resulted reaction mixture
was concentrated under reduced pressure. The product was ob-
tained as colorless liquid (0.46 g, 88%).
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Derivatization of propane-1,2-diol
Trifluoroacetic acid anhydride (0.75 g, 3.6 mmol) was added slowly
at 08C to propane-1,2-diol (0.1 g, 1.3 mmol) from the hydrogena-
tion experiments. The reaction mixture was stirred 60 min at RT.
Then it was diluted with dichloromethane (5 mL) and washed
twice with aqueous NaHCO3 (2ꢂ10 m). The organic phase was sep-
arated und dried over MgSO4. The solvent was removed under re-
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Keywords: hydrogenation
· lactides · propane-1,2-diol ·
synthetic methods · tandem reactions
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