Thermal hydrodehalogenation of 2,4-dibromophenol by polymeric materials

摘要

Pyrolysis appears to be an emerging option allowing recovery of useful products from wastes of electric and electronic equipment (WEEE) because of the high value of potentially accessible products such as precious metals or coke (in the residue), fuel and chemicals (pyrolysis oil and gases). However, contamination of oil by harmful compounds remains a severe issue and has a strong impact on material recycling and thermal treatment: Bromine-containing phenols are classic examples of harmful compounds emitted during pyrolytic recycling of printed circuit boards, and their dehalogenation is an area of continuing scientific interest. Pyrolysis of 2,4-dibromophenol (DBP) with high density polyethylene (HDPE), low density polyethylene (LDPE), polybutadiene (PBD), polystyrene (PS), polyamide 6 (PA-6), polyamide 6,6 (PA-6,6) and polyacrylonitrile (PAN) at 290-370℃ for 20min results in the successive transformation of DBP to monobromophenols and further to phenol and HBr, together with small amounts of alkylphe-nols, depending on the reaction temperature and reactivity of polymers. The formation of PBDDs and PBDFs is strongly retarded despite of the structure of DBP, which favours their development. The rate of hydrodebromination decreased in the series LDPE > HDPE > PBD > PS ≈ PA-6 > PA-6,6 > PAN. The hydrode-bromination process transforms the polymers studied in the highly polyaromatized char. The formation of the pyrolysis products is in favour of a radical hydrodebromination mechanism.