Valorisation of plastic waste via metal-catalysed depolymerisation

Abstract

Metal-catalysed depolymerisation of plastics to reusable building blocks, including monomers, oligomers or added-value chemicals, is an attractive tool for the recycling and valorisation of these materials. The present manuscript shortly reviews the most significant contributions that appeared in the field within the period January 2010–January 2020 describing selective depolymerisation methods of plastics. Achievements are broken down according to the plastic material, namely polyolefins, polyesters, polycarbonates and polyamides. The focus is on recent advancements targeting sustainable and environmentally friendly processes. Biocatalytic or unselective processes, acid–base treatments as well as the production of fuels are not discussed, nor are the methods for the further upgrade of the depolymerisation products.

Recent Advancements in Plastic Packaging Recycling: A Mini-Review

“…Plastic depolymerization may be carried out in the presence of different catalysts such as strong mineral acids, bases, organocatalysts, enzymes, and metal catalysts in homogeneous or heterogeneous phase [147].…”

“…Hydrogenolysis is widely employed for the depolymerization of PET in the presence of hydrogen and homogeneous Milstein-type Ru-PNN complexes which are highly reactive toward the C=O double bonds of PET to give 4-benzenedimethanol (BDM) in 99% yield at 160 • C in 48 h (Table 1, entry 1), while they are ineffective in the presence of PP and PE [147,[153][154][155]. More complex phosphine ligands have also been tested, but the economic viability on an industrial scale seems to be rather limited [147] (Table 1, entries 2-3). Two very important studies have been published on the efficient conversion of postconsumer PET to benzene, toluene, and xylenes by reportedly "unlocking hidden hydrogen in the ethylene glycol part" with Ru/Nb 2 O 5 catalyst [156,157].…”

Zinc‐Catalyzed Depolymerization of the End‐of‐Life Poly(ethylene 2,5‐furandicarboxylate)

“…An option to circumvent the competitive situation can be the application of efficient recycling methods. On the other hand, for b) recycling techniques have been installed as resource‐efficient tool for reuse of plastics, e. g. mechanical recycling, downcycling, chemical recycling [10–18] . Especially, chemical recycling of end‐of‐life polymers has been discussed as a resource‐efficient and sustainable methodology [19,20] .…”

“…On the other hand, for b) recycling techniques have been installed as resource-efficient tool for reuse of plastics, e. g. mechanical recycling, downcycling, chemical recycling. [10][11][12][13][14][15][16][17][18] Especially, chemical recycling of endof-life polymers has been discussed as a resource-efficient and sustainable methodology. [19,20] Noteworthy, chemical recycling can be beneficial for solving issues a) and b).…”

Closing the Carbon Loop in the Circular Plastics Economy

“…Several reviews have addressed back-to-monomer molecular recycling by thermal, chemical, and (bio)catalytic unzipping of polymer chains. [457,459,464,491,[511][512][513][514][515][516][517][518][519][520][521][522][523][524][525][526][527]72,79,80,82] Similarly to polymerization catalysis in industrial polymer manufacturing processes, depolymerization catalysis plays a key role in back-to-monomer molecular recycling. [80,468,491,515,526,528] In contrast to polymers such as polyolefins, acrylics, and vinyl polymers, all of which have hydrolytically stable C-C linkages in their backbones, polycondensation-and polyaddition-based polymers contain ester, amide, urethane, and carbonate groups that enable monomer recovery by hydrolysis and solvolysis.…”