An investigation into the catalytic cracking of LDPE using Py-GC/MS

摘要

Pyrolysis coupled with gas chromatographic separation and mass spectrometry detection (Py-GC/MS) has been used to study the catalytic degradation of low-density polyethylene (LDPE). This novel approach allowed rapid screening of the catalytic activity of different acid solids and permitted a straightforward analysis of the resulting products. The results obtained are comparable to those reported by other authors while very small sample and catalysts masses (less than 0.2 mg) were required. For the purpose of this work, three acid solids differing in their textural and acid properties (micrometer HZSM-5, nanocrystaline n-HZSM-5 and Al-MCM-41) were synthesized, characterised for their chemical and structural characteristics and tested for their thermal stability between 550 and 800℃. Thermogravimetric (TG) analysis was then employed to evaluate their catalytic activity in the degradation of pure LDPE. This activity was related to their capacity to shift the degradation reaction to lower temperatures. Py-GC/MS analysis of pure LDPE at 700℃ generated a pyrogram with characteristic triplets corresponding to straight chain diene, alkene and alkane hydrocarbons of varying lengths. Catalytic degradation of LDPE over micrometer and nanocystalline HZSM-5 zeolites generated a similar range of degradation products with a marked increase in the light olefins and aromatic fractions (e.g. benzene, toluene, xylene) and complete elimination of heavier olefin and paraffin hydrocarbons. Despite its high catalytic activity, as determined by TG analysis, mesoporous Al-MCM-41 exhibited no shape selectivity in the products generated with a low proportion of aromatics and a higher content of olefin and paraffin species.