THERMOCATALYTIC DEHYDROGENATION OF PLASTIC WASTES ASSISTED BY ZnCl2-BASED MOLTEN SALTS

Molten salt activated catalytic carbonization is an innovative method that can be used to achieve zero CO2 emissions and promote a circular carbon economy by utilizing organic waste. An interesting outcome would involve transforming the carbon content of the low-value waste into valuable solid carbon products while simultaneously extracting the hydrogen content as a low-carbon, hydrogen-rich fuel gas. We are investigating the feasibility of utilizing ZnCl2-based molten salts for the carbonization process of a plastic waste model, specifically high density polyethylene (HDPE). In the work herein, the use of pure molten ZnCl2 in batch reactors was studied for this purpose. A proper reaction apparatus was designed and realized. The ZnCl2 was provided by VWR Chemicals (>97% of purity) and C16-C18 saturated hydrocarbons and synthetic polyolefins were used as model compounds to carry out a preliminary investigation of the effect of reaction temperature and time, on the performances of the process evaluated in terms of yield of gaseous hydrogen and solid carbon in the residue. A dehydrogenation index (DHI), defined as the ratio between the mols of H atoms globally found in the gas phase and that of H atoms in the initially charged substrate was adopted to measure the dehydrogenation activity of the molten salt mixture. TGA analyses of pure ZnCl2 and high density polyethylene (HDPE) and of their binary mixtures demonstrated an improvement of the mass loss rate that is a clear indication of the catalytic activity of ZnCl2 . Exploratory carbonization experiments of high density polyethylene showed that, when the reaction temperature increased from 300 to 400 °C for 30 min, the gaseous H2 yield increased from 0,01% to 1,1 %w/w (the limiting theoretical value being 14% w/w) and the DHI increased from 0 to 1.84. The preliminary results indicate that ZnCl2 can catalyze the dehydrogenation of polyolefins. However the first outcomes suggested a possible role of the phase behavior of the polymer/molten salt system on the performances of the process. In fact, the DHI strongly increased to 10.7 with a gaseous H2 yield of 2.4% in experimental trials performing stirring the reactor at 200 rpm at 400°C for 30 min. Additional investigation is necessary to characterize the carbonaceous waste and to explore operational limits of the process.