Upcycling a plastic cup: One-pot synthesis of lactate containing metal organic frameworks from polylactic acid

Article abstract of DOI:10.1039/c9cc02861g

Waste PLA can be upcycled to metal organic frameworks of potential high value in a one-pot synthesis scheme, where PLA depolymerisation occurs in situ. Three homochiral lactate based frameworks were successfully synthesised and characterised from PLA as a feed source, including ZnBLD. The chiral separation ability of ZnBLD was maintained.

Full text of DOI:10.1039/c9cc02861g

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Upcycling a plastic cup: one-pot synthesis of
lactate containing metal organic frameworks from
Cite this:DOI: 10.1039/c9cc02861g
polylactic acid†
Received 12th April 2019,
Accepted 29th May 2019
ab
c
Benjamin Slater,
Matthew R. Hill
So-On Wong,a Andrew Duckworth,a Andrew J. P. White,
b
ad
and Bradley P. Ladewig
*
DOI: 10.1039/c9cc02861g
rsc.li/chemcomm
Waste PLA can be upcycled to metal organic frameworks of potential metric tons of virgin plastic was produced, 76% of which is now
high value in a one-pot synthesis scheme, where PLA depolymerisation waste.15 Recently, PET has been used as a terephthalic acid source
occurs in situ. Three homochiral lactate based frameworks were for the synthesis of several MOFs. PET was successfully depoly-
successfully synthesised and characterised from PLA as a feed source, merised under microwave irradiation before being used as a linker
including ZnBLD. The chiral separation ability of ZnBLD was maintained. source in solvothermal synthesis of two terephthalate based MOFs.16
Subsequently, reports highlighted the synthesis of several MOFs
Chirality is a phenomenon present throughout nature, for example directly from PET as terephthalate source (without a prior depoly-
the chiral, helical structure of DNA.1 Enantiopure chemicals are merisation step), utilising either microwave,17 or conventional heat-
required on industrial scales for a wide variety of industries such as ing techniques.18,19 This process of upcycling waste polymer
pharmaceuticals, where one enantiomer of a drug may be meta- receptacles such as PET bottles and cups, has the potential to be
bolised differently to its mirror image.2 Metal Organic Frameworks applied to other waste polymer materials for the synthesis of MOFs.
(MOFs) are a series of porous coordination polymers consisting of ZnBLD [Zn2(bdc)(L-lactate)(dmf)]Á(DMF) (bdc = 1,4-benzenedi-
metal nodes coordinated to organic ‘linker’ ligands.3 MOFs have carboxylate), (dmf = N,N-dimethylformamide) is a homochiral
several promising applications from sensing,4 to separations,5,6 MOF synthesised from zinc nitrate hexahydrate, terephthalic
including chiral separations.7,8 A significant number of reported and L-lactic acids in a 2 : 1 : 1 molar ratio respectively.7 Hypothe-
MOFs are synthesised from terephthalic acid as an organic tically, ZnBLD could be synthesised from PET as terephthalic
linker.9,10 A search of the Cambridge Structural Database (Conquest) acid source and polylactic acid (PLA) as the lactic acid source.
for all structures containing 1,4-benzenedicarboxylate revealed 2028 MOFs 1201 [Ca14(L-lactate)20(acetate)8(C2H5OH)(H2O)] and 1203
reported structures, of which a very high proportion are MOFs.11,12 [Ca6(L-lactate)3(acetate)9(H2O)] are synthesised from calcium acetate
Polyethylene terephthalate is a polyester consisting of terephthalic and L-lactic acid in a 1 : 2 and 1 : 1 molar ratio respectively.20 In
acid and ethylene glycol in a 1 : 1 molar ratio. Research efforts have addition to ZnBLD, there is the possibility that these frameworks
focused on uses for excess PET in the environment, and alternative can be synthesised from PLA as the lactic acid source. Schemes S1
recycling methods, for example, utilisation of PET in asphalt and S2 (ESI†) depict the depolymerisation schemes for PET and
pavement,13 and biological based recycling.14 This is likely to have PLA to the corresponding monomers. PLA is also a polyester
been influenced by growing concerns about the environmental formed from the polymerisation of lactic acid. In the poly-
impact of plastic where it is estimated that as of 2015, 8300 million merisation process of both PLA and PET, one water molecule is
removed in each ester linkage formed (1 mole of water per mole of
lactic acid/1 mole of water per mole of terephthalic acid). Therefore,
the minimum molar quantity of water must be present in the 1-pot
polymer based MOF synthesis procedures in order to successfully
depolymerise the polymers.
a Barrer Centre, Department of Chemical Engineering, Imperial College London,
London SW7 2AZ, UK. E-mail: Bradley.ladewig@imperial.ac.uk
b CSIRO, Private Bag 10, Clayton South MDC, Victoria 3169, Australia
c Department of Chemistry, Imperial College London, Molecular Sciences Research
PLA is commonly found in biodegradable single use plastic
items such as plastic cups. Although it is often considered as a
recyclable polymer, PLA waste is typically treated by methods
such as composting, hydrolysis and natural decomposition.21
Pure lactic acid is stable as a solid below its melting point of
16.8 1C,22 but under standard conditions will melt and absorb
water. Consequently, the price of pure L-lactic acid (required for
Hub, White City Campus, London, W12 0BZ, UK
d Institute for Micro Process Engineering, Karlsruhe Institute of Technology,
76344 Eggenstein-Leopoldshafen, Germany
† Electronic supplementary information (ESI) available: Experimental details and
additional results and discussion. CCDC 1908241. Raw XRD spectra and other
supporting data are available from the open data repository: https://doi.org/10.
5281/zenodo.2638093. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c9cc02861g
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Scheme 1 Synthesis of three homochiral lactate based MOFs from PLA
cups and single crystal X-ray diffraction structures of: (A) chiral channels of
MOF-1201 viewed along the b axis, (B) view along the a axis of MOF-1203
and (C) view along the a axis of ZnBLD–dPLA, (perspective used for artistic
effect), Ca and Ca2+ polyhedra in green, O in red, C in grey, N in blue and
Zn in pink. H atoms have been omitted for clarity.20
the synthesis of ZnBLD and MOFs 1201 and 1203) is consider-
ably more expensive than the more stable but lower purity
aqueous solution.‡ Other studies have previously reported the
synthesis of PLA–MOF composites for disposable electronic
applications,23 but, to the best of our knowledge, PLA has not
been used as lactic acid ligand source for MOF preparation.
From herein, we report the synthesis and chiral separation
properties of ZnBLD, and the syntheses of MOF-1201 and MOF-
1203 by replacing L-lactic acid with the equivalent molar ratio of
PLA illustrated in Scheme 1. All of these frameworks have a
molar excess of water from the metal salts (calcium acetate
monohydrate and zinc nitrate hexahydrate). However, ZnBLD is
synthesised from DMF with no requirement for this to be
Fig. 1 Experimental and predicted powder X-ray diffraction traces of: (A)
ZnBLD and ZnBLD–dPLA and (B) MOFs 1201 and 1203 and MOFs 1201-
dPLA and 1203-dPLA.
anhydrous. As DMF is hygroscopic, having DMF seasoned by patterns using pure L-lactic acid as feed source, to confirm phase
atmospheric water vapour can significantly increase the water purity, these results are displayed in Fig. 1. The phases of ZnBLD
content. Contrastingly, MOFs 1201 and 1203 require anhydrous and MOFs 1201 and 1203 were obtained. There were slight
ethanol and methanol respectively. Therefore, it is expected variations in the PXRD traces of ZnBLD and ZnBLD–dPLA. The
that PLA will show considerably different depolymerisation relative intensity of the Zn(bdc)ÁxH2O (8.61 2y) phase (caused by
behaviours in each solvent. Table S1 (ESI†) shows the composition partial hydrolysis of the framework), had a relatively higher
of the frameworks synthesised based on the adjusted molar ratios intensity in the ZnBLD–dPLA crystals. We suspected that this
for the PLA synthesised MOFs. The suffix dPLA is used to describe was caused by a higher concentration of this phase in the
MOFs synthesised from waste PLA (dPLA = depolymerised poly- ZnBLD–dPLA crystals than that of ZnBLD, which may be a
lactic acid). Additional, unsuccessful attempts were made to consequence of using PLA as feed source instead of L-lactic acid.
synthesise ZnBLD from waste PET, this is most likely because In order to ensure consistency with our results, the same batch
hydrolysis of PET requires higher temperatures (above 200 1C) than of as-synthesised crystals were used for all characterisation and
the synthesis temperature of ZnBLD (110 1C) and is typically separation experiments. The PXRD traces of MOFs 1201-dPLA
performed in aqueous media,24 this is outlined in the ESI.† The and 1203-dPLA were less crystalline than the pure lactic acid
synthesised frameworks are characterised and compared with their synthesised counterparts. Additionally, the product was obtained
counterparts synthesised from pure lactic acid and finally, the as a mixture with a gel like by-product, which is commonly
enantioseparation ability and guest molecule loading of ZnBLD– prepared from calcium acetate and alcohol. The crystal structure
dPLA is compared with ZnBLD.
of ZnBLD–dPLA is reported in the ESI,† this structure is iso-
The dPLA frameworks powder X-ray diffraction (PXRD) traces structural with ZnBLD (previously reported) with the same unit
were recorded and compared with the predicted and experimental cell and space group (P222121).7
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Fig. 3 Enantiomeric excesses obtained from enantioselective adsorption
in ZnBLD and ZnBLD–dPLA.
Fig. 2 Organic compositions of ZnBLD and ZnBLD–dPLA determined by
NMR spectroscopy.
this study,26 and to the best of our knowledge, has not been
investigated in other studies. Our hypothesis is based on the
As-synthesised MOF crystals of ZnBLD, MOFs 1201 and 1203 assumption that smaller crystals will reach maximum separation
and their dPLA counterparts were digested in DMSO-d6/DCl capacity faster than larger ones.
before nuclear magnetic resonance (NMR) spectra were recorded
Additionally the loading of the four adsorbed racemates was
to determine the organic composition of the frameworks which determined by 1H NMR spectroscopy in the ZnBLD and ZnBLD-
could be compared with the expected formulas. Comparing the dPLA frameworks. In this process, after adsorption of each
spectra of ZnBLD and ZnBLD–dPLA, (Fig. 2) there was a slight racemate, the MOF crystals were filtered and subsequently
difference in the ratio of terephthalic acid: lactic acid (1: 0.89 dissolved in DMSO-d6/DCl before NMR spectra were recorded.
and 0.73 respectively). We can therefore conclude that there is a Characteristic proton resonances of the framework terephthalic
higher concentration of Zn(bdc)ÁxH2O in ZnBLD–dPLA as acid and selected peaks from each racemate were integrated in
observed in the PXRD traces.
order to determine the loading (where the loading of racemates
As-synthesised MOFs 1201 and 1203 produced consistent was compared with 1 molecule of the framework terephthalic
organic compositions with the expected formulas for these acid), further details are reported in the ESI.† Fig. S5 (ESI†)
MOFs but the dPLA synthesised counterparts contained higher displays the loading results from the adsorbed chiral guest
concentrations of acetate and solvent (Fig. S1 and S2, ESI†). This molecules. There were slight variations in guest molecule
is caused by the formation of the amorphous gel like phase.
The chiral separation ability of ZnBLD–(dPLA) was compared neither framework consistently outperformed the other.
with ZnBLD for four racemates (1-phenylethanol, 2-butanol, We have shown, for the first time that polylactic acid can be
loading when comparing the two frameworks, but overall,
limonene and 2-methyl-2,4-pentanediol) using an adsorption, utilised as a feed source for the synthesis of lactate containing
desorption method. Whereby, each framework was soaked in MOFs such as ZnBLD and MOFs 1201 and 1203. By using this
neat solutions of the racemates, filtered and soaked in dichloro- method, we were able to upcycle post-consumer use PLA
methane to desorb the adsorbed guest species. Subsequent analysis products into potentially high value materials. We demonstrate
of the supernatant by chiral gas chromatography revealed that it is possible to bypass the requirement to purchase
the enantiomeric excess of the separation process, and using expensive high purity lactic acid and instead use waste plastic
Table S3 (ESI†), the preferentially adsorbed enantiomers can be drinking cups. The ZnBLD–dPLA framework upholds the
determined. Fig. 3 shows that the separation ability of the dPLA equivalent or enhanced chiral separation properties as its
synthesised frameworks were not negatively impacted by the counterpart synthesised from pure chemicals without an over-
modified synthesis method for the racemates tested. Interest- all reduction in loading. This can considerably reduce the cost
ingly, there was a significant increase in the enantiomeric excess of manufacturing of these products. We expect that by altering
for the limonene separation when using the ZnBLD–dPLA frame- the synthesis of MOFs 1201 and 1203, to increase the PLA
work. The ZnBLD 1-phenylethanol and 2-butanol enantiomeric depolymerisation efficiency, these MOFs can be synthesised
excess values are previously reported.25 Further investigation is effectively without obtaining the amorphous gel, and with the
required to better understand the selectivity increase effecting equivalent crystallinity of the originally reported MOFs as
limonene. We hypothesise that this is caused by the reduction in shown for ZnBLD.
crystal size of ZnBLD–dPLA (see Images S3 and S4, ESI†). The
BDS acknowledges CSIRO and Imperial College London for
enantiomeric excess for a mandelate based homochiral MOF has financial support, and thanks Dr Jason Hallett for discussions
been shown to increase with time from 1–7 days. However, the on PLA waste. The authors acknowledge Dr Charles Romain for
influence of the crystal size was not taken into consideration in providing PLA cups.
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2019: 1 L 80% aqueous solution L-lactic acid = d38.50(GBP), 10 g 98%
L-lactic acid = d121(GBP).
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