The catalytic cracking of polyethylene has been studied over two natural clays and their pillared analogues with a view toward assessing their suitability in a process for recycling plastic waste to fuel. Although these clays were found to be less active than US-Y zeolite around 600 K, at slightly higher process temperatures, they were able to completely decompose polyethylene. Their yields to liquid products were around 70%, compared to less than 50% over US-Y zeolite. Moreover, the liquid products obtained over the clay catalysts were heavier. Both of these facts are attributed to the milder acidity of clays, as the very strong acidity characterizing zeolites leads to overcracking. Furthermore, this milder acidity leads to significantly lower occurrence of hydrogen-transfer secondary reactions compared to US-Y zeolite, and as a consequence, predominantly alkenes were the products over the clay catalysts. An additional advantage of these catalysts is the considerably lower amount of coke formed.

No abstract

The catalytic cracking of polyethylene over an Al pillared saponite, an Al pillared montmorillonite, and their regenerated samples was studied in a semi-batch reactor. Pillared clays were able to convert completely polyethylene in gaseous and liquid hydrocarbons, showing low coking levels. The selectivity and yield to liquid hydrocarbons were high, as the mild acidity of pillared clays avoided excessive cracking to small molecules. Regenerated catalyst samples showed practically identical levels of conversion and selectivity with fresh pillared clay samples. Furthermore, they produced hydrocarbons with practically the same distribution as the fresh samples, confirming that pillared clays can be completely regenerated. Both facts of high yield to liquid products and regenerability make pillared clays potential catalysts for an industrial process of catalytic cracking of plastic waste. The effect of the heating program on the liquid product quality and distribution is also investigated, using two different temperature programs with the same levels of temperature steps, but different duration. The boiling point distribution of the liquid products formed during the second interval of the shorter program was intermediate between this of the lighter liquid produced in the first 10 min of the longer program and the heavier liquid produced between 10 and 20 min. This result clearly shows the importance of the polymer state during each temperature stage.