Novel Membrane Catalytic Reactor for Waste Polylactic Acid Recycling and Valorisation

Lead Research Organisation: University of Birmingham
Department Name: Chemical Engineering

Abstract

The disposal of plastic packaging represents a significant environmental problem; although recycling of plastics has increased in recent years, current recycling methods are mainly mechanical or chemical techniques that result in lower grade second life products and much material is also still disposed of to landfill. The introduction of plastics produced from biological sources such as plant derived sugars has potential to reduce reliance on fossil derived sources and decrease emissions of greenhouse gases associated with manufacture. Polylactide has emerged as one of the most promising biorenewable and biodegradable polymers which has uses in packaging, textile and biomedical applications. However the lack of a reliable method for recycling polylactide could limit its widespread application and market growth. A significant opportunity therefore exists to develop a process to depolymerise/degrade commodity PLA to produce value-added small molecules, such as lactate esters, via routes which have not previously been developed. Such molecules could be recycled to make new PLA or other value added chemicals, including solvents, fragrances and plasticizers.
We propose to address the above problems by developing a catalytic process for degradation/depolymerisation of PLA, integrated with a membrane separation to selectively isolate small molecule products within a specified molecular weight cut off range, as valuable products. Firstly the catalytic part of the process will be developed, building on previous work by Jones on salalen homogeneous catalysts, and including a work plan to select and design the best metal-support combinations to achieve a high conversion of PLA. Secondly, commercial membranes will be screened for the separation of product molecules, which will provide the necessary data to enable design and fabrication of bespoke membranes for a particular molecular weight cut off. We then aim to coat catalysts on to the membranes, so as to avoid the potential difficulties of working with, separating and reusing slurry catalysts. The tested catalyst and membrane designs will then be scaled up. A larger scale (1 litre) reactor will be constructed for carrying out crossflow membrane tests. The results of the studies will be used to develop kinetic models of the reaction and diffusion models for the membrane pore structure. A programme of activities for delivery of impact to the academic and industrial communities and the general public has been devised.
The work is expected to deliver new catalysts, supports and membrane designs and performance data. Laboratory scale up data will be used to determine how well these techniques work together and to deliver a process design that could be deployed to take the technology in to industrial production.
The proposed technologies are expected to deliver a number of potential benefits including reduced reliance on fossil derived plastics, potential to increase the market for bio-derived polymers, the production of value added chemicals such as ethyl lactate, novel catalyst and membrane designs, UK held intellectual property and patents.

Planned Impact

This three year programme is designed to have a significant impact to all, through producing a new world leading process for the degradation/depolymerisation of PLA - that can be used to produce value added feedstocks which can be recycled to make new polymers. This should enable a superior pathway for recycling of plastics and packaging rather than landfilling or processing to lower quality 'second life' products. The main beneficiaries will be:
(1) PEOPLE AND SOCIETY: Polymers derived from plant based sugars and subsequently recycled via depolymerisation could replace a number of petrochemical derived substances, leading to reduced environmental impact (giving a better environment for all) and lower reliance on fossil fuels. They have the advantage of being near carbon neutral (carbon dioxide released during use is offset by consumption during plant growth). Further research, such as in this project, is needed to develop catalysts and processes that can make bio derived polymers economically competitive with fossil fuel derived chemicals.
(2) COMPANIES AND MANUFACTURERS: A number of UK (and international) companies are expected to benefit from this research, leading to enhanced wealth creation, as well as company and job creation. Primary engagement with most companies will be through publications, press releases and conference/workshop presentations. The companies include:
a. Catalyst manufacturers. This project will provide these companies to more effective and efficient catalysts for degradation.
b. Reactor, membrane reactor and membrane manufacturers: The new membrane reactor technology and membrane materials will enable new manufactures to be created or will become new product lines for existing manufacturers.
c. Chemical manufacturers and suppliers: The process developed will provide a superior way of producing value added molecules such as lactate esters (initial value to Natureworks).
d. Biofeedstock suppliers (including waste dealers): This project will provide a new market, opening a new pathway for the utilisation of further renewable bioresources, which will be of use to companies producing bio-based polymers, e.g Natureworks.
(3) KNOWLEDGE AND SCIENTIFIC ADVANCES: This research programme will initiate and develop new research skills and a novel research at the interface of chemical and process engineering, chemistry, materials science and environmental engineering. The depolymerisation of plastics as a replacement to the currently non-renewable fossil fuel derivatives is an internationally important research area of wide interest and applicability. This grant will provide the platform to develop and applying new research skills and novel techniques, both experimental and computational in this area. These are interdisciplinary research skills that will enable the researchers we train to work from the level of molecular science up to the applied engineering level. These skills are transferable and will be used as the basis of building and extending further innovative research on other renewable chemical feedstocks.
IP and technology transfer will occur through the appropriate offices of the three partner universities, using a collaboration agreement between all partners prior to project start.
(4) SKILLS, RESEARCH CAPABILITY AND PEOPLE PIPELINE: The project will deliver two research fellows, who will be highly trained in a wide range of skills associated with biofeedstock processing, characterisation and assessment. They will provide the foundation for the development of further work in this area, creating a platform for this project team to be leaders in this area into the foreseeable future.

Publications

10 25 50
 
Description In order to understand the depolymerisation of polylactic acid, a key objective of this grant, it is first necessary to understand how it polymerises. The group of Jones at Bath are developing complexes for application as catalysts for the ring opening polymerisation of rac-lactide. A paper has already been published on the development of Mg(II), Li(I) and Zn(II) complexes based on two families of amine bis(phenolate)s for this purpose. High molecular weight polylactide was produced and isolated from the products prepared using these catalysts and they were structurally characterised. The development and characterisation of the polymerisation catalysts formed a background on which to develop catalysts that can perform the reverse operation of depolymerisation, for application in polymer recycling. It was subsequently found that an imino phenolate Zn(1)2 catalyst is active for the depolymerisation of PLA into methyl lactate using methanol and THF as co-reactant/solvent mixture. The effects of PLA concentration (0.05-0.2 g/mL), reaction temperature (40-130 degrees C) and catalyst concentration (4-16 wt%) on conversion, yield and selectivity were studied and the results statistically analysed. Temperature and catalyst concentration were found to have the largest effects. A reaction mechanism for the production of methyl lactate was proposed and a kinetic model fitted to the experimental data. A second series of Zn(II) complexes were prepared based on propylenediamine Schiff bases and fully characterised. These complexes were also applied to the ring-opening polymerisation of L-lactide with emphasis on industrial conditions, achieving high conversion. The more active Zn(II) catalysts were applied to PLA degradation to alkyl lactacte under mild conditions Zn(A-B)2 demonstrated high activity and selectivity in this process with PLA being consumed within 1h at 50 degrees C. Zn(C-D)2 were shown to be less active. Zn(A-B)2 catalysts showed some success for scale up. Initial results also showed these catalysts could be applicable to different polymers such as the degradation of poly(ethylene terephthalate) and mixed feeds, highlighting the broader applicability of the systems presented. Further studies highlighted that different alcohols could be used in the depolymerisation including propanol and butanol in addition to ethanol, although the longer chain alcohols gave rise to slower rates of reaction.
Exploitation Route The aim of the project is to develop catalyst and separation systems for the depolymerisation of polymers such as polylactic acid cups and packaging. The findings will be used by chemists and chemical engineers to develop further catalysts and membrane separation systems for product recycling and could potentially be taken up by our industrial partners Natureworks who manufacture such polymers industrially. Follow on proposals have been prepared based on mixed polymers and circular economy concepts.
Sectors Chemicals,Education,Energy,Manufacturing, including Industrial Biotechology

URL https://www.sciencedirect.com/science/article/pii/S0022328X1730503X?via%3Dihub#gs2
 
Description Sustained media reports on the dangers of single use plastic and the need to find new ways of using, disposing and recycling plastics have kept this issue in the public eye over the last 18 months (to February 2020). Individuals are therefore seeking to change their behaviour, request companies to change or reduce their packaging, and companies are seeking to reduce their use of single use plastics or find new ways of recycling it. There is also increasing interest in circular economy, in which value-added products are manufactured from waste. Polylactic acid is a renewable polymer that is increasingly being used in replacement applications, for example drinking cups at the National Trust. However after use these can take a long duration to break down via composting and thus new methods of chemical recycling are sought, as delivered by the techniques developed under this project. The catalysts and process developed provide impact in terms of the potential for industry to take forward such techniques to implement chemical recycling on a centralised or distributed scale. In turn, the process will produce value added products such as ethyl lactate, which can be applied as a solvent. Via conferences, publications and press reports we have disseminated the findings of the project to a wider audience, leading to some enquiries about the process from industry. We have also liaised regularly with our Project Partner Natureworks, and provided them with reports on the depolymerisation techniques as tested on PLA samples they supplied. The project has thus provided the catalysts, characterisation, kinetic models and information required for industry to further scale up the process for industrial applications. In turn, increasing use of chemical recycling will benefit society by reducing the amount of fossil fuel extraction required to create polymers from fossil reserves, decreasing material sent to landfill or incineration and thus reducing emissions of greenhouse gases. Challenges to overcome achieving impact include persuading companies to invest in alternative technologies and materials.
First Year Of Impact 2019
Sector Chemicals,Education,Energy,Manufacturing, including Industrial Biotechology
Impact Types Societal,Economic,Policy & public services

 
Description Natureworks 
Organisation NatureWorks
Sector Private 
PI Contribution Discussions via email concerning project direction and outcomes.
Collaborator Contribution Donation of plastic samples for laboratory depolymerisation studies, providing advice, technical comments on manuscripts prior to publication.
Impact DOI : 10.1002/cssc.201902755 DOI: 10.1021/acscatal.8b04863
Start Year 2017
 
Description Seminar on Reactors Scale Up and Separations organised by Prof. Wood under the IChemE Catalysis Special Interest Group 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact 70 delegates attended the day, which included a series of invited talks, including one about the outcomes of this project on plastics recycling via chemical methods. It sparked a lot of discussion and interest, for example for future collaborations and seminars.
Year(s) Of Engagement Activity 2019
URL https://www.icheme.org/membership/communities/special-interest-groups/catalysis/events/reactors-scal...