Catalytic transformation of the gases evolved during the thermal decomposition of HDPE using acid-activated and pillared clays

HDPE was thermally decomposed in a thermobalance and the evolved gases were passed through a bed of catalyst. Two acid activated clays of different structure were used, ie. sepiolite (SEP), which is a fibrous clay mineral, and smectite (K10) which is a layered mineral. In addition two pillared smectites were studied one which was pillared with an Al species (AZA) and the other with an Al/Fe-species (FAZA). The thermal and catalytic decomposition of HDPE was studied under dynamic (35 to 650 degrees C at 10 degrees C min(-1)) and isothermal (60 min at 420 degrees C) conditions. The evolved gases were analysed using TC-OTM-GC-MS. The thermal decomposition of HDPE yielded characteristic quartets of peaks assigned to n-alkanes, alk-l-enes, alk-x-enes and alpha,omega-dienes in the range C-4-C-20 Species of higher molecular weight than C-20 were not detected. All four catalysts converted the alkenes present in the thermally generated off gases into light gases and aromatic species including respectable quantities of toluene, xylenes, tri- and tetra-methylbenzenes. Ethylbenzenes and naphthalenes were produced to a lesser extent. AZA and FAZA produced the largest yield of aromatics and sepiolite is considered to produce significant quantities of low molecular weight gases in the isothermal process. The product distribution over sepiolite suggested that there were fewer dehydrocyclisation sites on this catalyst. None of the catalysts used was capable of cracking the saturated alkanes.