Title : Microwave transformation of high-density polyethylene to hydrogen: the role of CoFeAlOx catalyst
Abstract:
High-density polyethylene (HDPE) represents a persistent environmental burden due to its exceptional durability. Catalytic decomposition offers a transformative pathway to valorize this waste into valuable energy products. However, the challenge of selective bond cleavage in polymers necessitates tailored catalytic approaches that specifically target C-C and C-H bonds to produce hydrogen and methane as primary outputs. A detailed study was conducted on the microwave catalytic decomposition of high-density polyethylene in an inert argon atmosphere. Three variants of CoFeAlOx catalysts were prepared for the study by sol-gel method with different stoichiometric molar ratios of metals (0:1:2; 0.1:1:2; 0.5:1:2 and 1:1:2 Co:Fe:Al), where FeAlOx served as the support matrix. The FeAlOx matrix provided efficient microwave energy transfer and basic catalytic activity. Cobalt was added to enhance the selectivity of the catalytic decomposition of the polymer chain. This enhanced selectivity led to the efficient conversion of HDPE to a mixture of hydrogen-enriched gases and light hydrocarbons, liquid hydrocarbon fractions, and a solid carbonaceous residue.
The experiments were carried out in a specially - designed microwave reactor equipped with a dual magnetron with a maximum power of 1800 W, which provides a perfectly homogeneous microwave field without the formation of localized hot spots. The temperature regime was meticulously maintained at precisely defined temperatures of 300, 400 and 500 °C with precise control by an infrared thermocouple that continuously monitored the temperature and adjusted the magnetron power according to a pre-set temperature program, thus ensuring high process reproducibility. The CoFeAlOx catalyst played a pivotal role in the process due to its distinctive properties, functioning as both an effective microwave energy absorber and a selective decomposition catalyst.
The primary benefit of the method employed is the integration of rapid and effective heating directed at the catalytic centres, accompanied by a reduction in the overall energy consumption of the process. The study yielded novel insights into the mechanisms of microwave-assisted catalytic degradation of polymers, with particular emphasis on the influence of the catalyst composition (especially the Co/Fe ratio) on the selectivity of the overall process. The results obtained represent a significant contribution to the development of advanced methods for the valorisation of waste polymers into valuable chemical feedstocks and alternative fuels, where the synergistic effect of microwave heating and the catalytic properties of the CoFeAlOx material play a key role. Consequently, the research under discussion opens up new perspectives for the efficient recycling of plastics, with the possibility of producing energetically valuable materials at relatively low temperature requirements.
Funding: The financial support of the OP JAK project "INOVO!!!" (No. CZ.02.01.01/00/23_021/0008588) supported by the Ministry of Education, Youth and Sports of the Czech Republic and co-financed by the European Union and the REFRESH - Research Excellence For REgion Sustainability and High-tech Industries project (No. CZ.10.03.01/00/22_003/0000048) via the Operational Programme Just Transition under the European Union are gratefully acknowledged. Experimental results were accomplished by using Large Research Infrastructure ENREGAT supported by the Ministry of Education, Youth and Sports of the Czech Republic under project No. LM2023056.
