Catalytic cracking of polyethylene liquefied oil over amorphous aluminosilicate catalysts

Polyethylene was thermally cracked at 425, 450, and 475 degrees C in nitrogen at atmospheric pressure for 10-120 min. The major components of the oil were straight-chain olefins which contained 4-25 carbons. Pyrolysis at lower temperatures for shorter reaction times produced straight chain olefins with a broad distribution of carbon atoms, and pyrolysis at 450 degrees C. for 60 min or under more severe conditions produced mainly straight-chain olefins containing 4-12 carbons. The reaction at 475 degrees C produced C-1-C-4 hydrocarbon gases and coke by the secondary cracking and polymerization. The oil was then cracked over catalysts at 500 degrees C for 2 s at a heating rate of 3000 degrees C/s using a Curie-point pyrolyzer in a flow of helium at atmospheric pressure. Metal-loaded and dealuminated Y-zeolites as well as aluminosilicate MCM-41 were prepared and examined as cracking catalysts, and their catalytic activities and selectivities were compared to those of conventional zeolite catalysts. Aluminosilicate MCM-41 of Si/2Al = 25 and Ni-HY zeolite of Si/2Al = 50 exhibited a mild cracking activity, giving products of the middle distillate range with no coke deposition. The metal-loaded and metal-free HY zeolites of Si/2Al = 16 exhibited an excessive cracking activity, producing C-5-C-7 olefins with coke deposition. n-Octadecane was used as the model compound and was cracked over the catalysts under the same conditions as those used for the polyethylene oil. The reactivity of the catalysts was well-correlated with the amount of NH3 adsorption per unit catalyst area. The metal-supported dealuminated Y-zeolites and aluminosilicate MCM-41 possessed less aluminum atoms and, as a result, a smaller number of acidic sites. The removal of aluminum atoms from the zeolite framework and the control of the acidity are thus key factors in achieving mild cracking of the paraffinic oil.