Microwaving plastic into hydrogen and carbons

The development of feasible routes for the valorization of waste plastics is an urgent challenge to be solved. Now, a strategy is introduced for the selective production of hydrogen-rich gas and multi-walled carbon nanotubes in a single-step process using an FeAlOx catalyst and microwave irradiation.

The low degradability and ever-increasing generation of waste plastics is having a serious impact on the environment, especially on the marine ecosystem1. Accordingly, urgent solutions are required for the implementation of sustainable and practical waste plastic management strategies. Within this scenario, thermochemical valorization routes are an interesting option for the production of fuels and chemicals2,3. Recently, two-step processes combining pyrolysis with in-line catalytic steam reforming of pyrolysis volatiles have been developed, showing potential for the complete conversion of plastics to hydrogen and high-value structured carbon materials, such as carbon nanotubes (CNTs)4,5,6. However, the development of these processes is conditioned by the requirement of expensive catalysts and their fast deactivation.

Now, writing in Nature Catalysis, Peter Edwards, Tiancun Xiao, John Thomas and collaborators have developed a simple single-step strategy as an alternative to the previously reported sequential processes7. Polymer degradation takes place in a microwave reactor under batch conditions. The key point in this process is the in situ utilization of an inexpensive iron and aluminium oxide (FeAlOx) prepared by the citric acid combustion method. Interestingly, this oxide showed a better performance than other tested catalysts based on more-expensive and toxic metallic phases, such as Ni and Co.

It is noteworthy that FeAlOx has two different roles in the process. On the one hand, it acts as a microwave absorbent to avoid the limitations associated with the low microwave absorption capacity of plastics; on the other, it catalyses polymer conversion towards valuable products. The use of microwaves for heat transfer promotes the selective heating of active catalyst particles instead of heating the whole catalyst–polymer mixture, as in the case of conventional catalytic pyrolysis reactors. Notably, a remarkable temperature gradient of more than 300 °C, between the polymer and catalyst particles, was observed experimentally. This fact improves process selectivity because secondary reactions are hindered and polymer degradation only takes place by contact with the catalyst. The random scission mechanism can therefore be avoided since depolymerization is controlled by the catalyst. Fig. 1 summarizes the proposed mechanism for the conversion of plastics to hydrogen and CNTs.

Fig. 1: Proposed mechanism for the conversion of plastics to hydrogen and CNTs.

FeAlOx particles selectively absorb microwave radiation and activate the catalyst for the first step of polymer degradation into intermediates hydrocarbons. Next, these intermediates are further transformed to hydrogen and multi-walled carbon nanotubes (MWCNTs). HDPE, high density polyethylene; PP, polypropylene; PS, polystyrene.

The attained pyrolysis conditions ensured a hydrogen recovery of 97% when reforming high density polyethylene (HDPE). This accounts for a hydrogen production of \({{11}\,{{\mathrm{g}}_{{\mathrm{H}}_{2}}}}\)/100 gplastic with a gas yield higher than 60 wt%. Furthermore, hydrogen concentration was higher than 90% in the produced gas, with minor impurities of C1–C5 hydrocarbons and CO. The remarkable efficiency of the FeAlOx catalyst is evident in view of the low yield of oil obtained, below 3 wt%, compared to that obtained without catalyst where oil was the main product. Thus, a significant fraction of the carbon contained in the waste plastics was transformed into carbonaceous materials including multi-walled carbon nanotubes (MWCNTs), with this product being the prevailing one with a yield above 30 wt%.

Thorough experimental work has been conducted to understand the role played by main process conditions and develop the process for the selective production of hydrogen and MWCNTs. Thus, the optimum plastic/FeAlOx ratio was determined. In fact, it should have values in the 1 to 2 range, given that higher catalyst loads led to lower hydrogen production. The performance of FeAlOx has also been evaluated by treating other polymers such as polypropylene (PP) and polystyrene (PS). A hydrogen production of \({{10}\,{{\mathrm{g}}_{{\mathrm{H}}_{2}}}}\)/100 gplastic was obtained in PP conversion, whereas that obtained with PS was below \({{6}\,{{\mathrm{g}}_{{\mathrm{H}}_{2}}}}\)/100 gplastic. These results are explained by the lower hydrogen content of PS in relation to polyolefins. The operation in five successive reaction cycles showed a reduction in the hydrogen production capacity of FeAlOx catalysts.

A highly interesting feature of the process proposed by Peter Edwards et al. lies in its simplicity and economic viability — a single-step process using low-cost catalysts. Moreover, a high selectivity towards valuable products was also achieved while the method performed well with plastics of different composition. The production of hydrogen and CNTs from waste plastics is a topic of increasing interest in the literature. However, it is usually performed in two-step processes in batch regime, consisting of pyrolysis and in-line catalytic conversion of pyrolysis volatiles in separate reactors. The catalysts proposed for this process are mainly based on Fe and Ni metallic phases on a variety of supports5,7. In addition, the same strategy of sequential pyrolysis and in-line catalytic treatment was used for exclusively producing hydrogen from waste plastics. For instance, Ni-based catalysts were employed in a steam atmosphere to promote the conversion of plastic-derived hydrocarbons to H2 and CO2, taking advantage of the high activity of these catalysts for steam reforming and water–gas shift reactions. Productions higher than \({{30}\,{{\mathrm{g}}_{{\mathrm{H}}_{2}}}}\)/100 gplastic were reported operating under optimum conditions6,8,9, although in this case hydrogen is the sole value-added product. Therefore, besides having a higher operational complexity and using more expensive catalysts, these two-step processes are also unable to valorize the carbon contained in the spent plastic when compared to the strategy proposed by Peter Edwards et al.

The proposed process is an original contribution to the current waste plastics valorization field. However, it should be noted that, at this stage, the study is a proof of principle, as it has been performed in a laboratory-scale unit operating in batch regime. Accordingly, the challenges associated with the scale-up of this strategy remain unsolved. This is also the case for the aforementioned alternative technology based on a two-step process10,11. Detailed studies are therefore required prior to the full-scale development of the process, especially those involving the evaluation of catalyst stability, MWCNT separation from the catalyst and the development of strategies for continuous operation.


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Correspondence to Gartzen Lopez.

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Lopez, G., Santamaria, L. Microwaving plastic into hydrogen and carbons. Nat Catal 3, 861–862 (2020).

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