Synergic chemical analysis — the coupling of TG with FTIR, MS and GC-MS: 2. Catalytic transformation of the gases evolved during the thermal decomposition of HDPE using acid-activated clays

High-density polyethylene, HDPE, has been thermally decomposed in a thermobalance and the evolved gases passed through a bed of catalyst. The resulting transformation products were trapped and subsequently identified using GC-MS. Two smectites of different octahedral compositions were acid-activated under conditions known to produce catalysts of differing activity. Three different treatments were used for each smectite, thus providing six samples for evaluation. The thermal and catalytic decomposition of HDPE was studied under isothermal (60 min at 420°C) and dynamic (35–650°C at 10°C min−1) conditions. The thermal decomposition of HDPE yielded characteristic quartets of peaks in the chromatogram which were assigned to n-alkanes, 1-alkenes, x-alkenes and α,ω-dienes in the range C4–C22. Species of higher molecular weight than C22 were not detected. All six catalysts converted the alkenes present in the thermally generated off gases into light gases and aromatic species. Mono-, di- and trimethylbenzenes were the most abundant aromatic species although small quantities of ethylbenzenes and naphthalenes were produced. More aromatics were produced at 420°C than at temperatures up to 650°C, whereas the reverse was true for branched alkanes. The total conversion of HDPE increased with both the extent of acid treatment and the process temperature, whereas the proportion of aromatics produced was greatest for catalysts prepared using short acid-treatment times.