Modification of a sulphated zirconia catalyst using calcium carbide for conversion of plastic waste into non-aromatic liquid hydrocarbons
Almustapha, Muhammad Nurudeen
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Plastic consumption has increased 8% annually since the 1950 reaching an estimated 300 million tonnes in 2014, where more than 50% was discarded after single-use. Many recycling methods have been proposed to manage this growing waste, but most have practical, environmental and economic limitations. Catalytic conversion, a chemical recycling method using a suitable catalyst, has been suggested as a viable option since it can return plastic to a chemical feedstock, which is the aim of this work. Four potential catalysts, namely sulphated zirconia SZ, calcium carbide CC, molybdenum carbide MC and zirconium oxide ZO were tested for HDPE conversion, which was selected from five different polymer samples due to its high thermal resistance. It was found that only calcined SZ and calcined CC showed some impact HDPE conversion. However, SZ was marked with high coke yield while calcined CC had low conversion with virtually no coke. Therefore a hybrid catalyst was considered, where the SZ and the CC were mixed together on equal weight bases forming the hybrid catalyst SZ1CC1. Fixed bed pyrolysis showed an excellent HDPE conversion of virtually 100wt% using the hybrid catalyst at 410oC with 66.0wt% liquid yield against a 98.0wt% conversion with only 39.0% liquid yield for the pure SZ and no conversion in the case of HDPE only. The hydrocarbon composition of the liquid fraction obtained changed significantly from 58% aromatic and 16% paraffinic for the SZ to 74% olefinic and 23% naphthenic for the SZ1CC1. The improvement in liquid yield and selectivity to non-aromatic liquid was strongly linked to a modification in the acidic strength of the hybrid catalyst SZ1CC1. The moderation in acidity and textural properties, such as surface area and porosity were found to suppress excessive cracking and limiting secondary cracking reactions, that promotes high gas yield and aromatisation in the SZ. Overall, it is concluded that the SZ on its own, which had high acidic strength and large surface area, promoted secondary reactions during HDPE cracking that yielded aromatics. However, the hybrid catalyst SZ1CC1, which had moderate acid strength and low surface area halted any secondary reaction and terminated the cracking reaction at stages that produced only olefinic and naphthenic hydrocarbons.