Development of Al-MCM-41 catalyst for cracking of high density polyethylene and polypropylene wastes

Abstract

Al-MCM-41 was prepared using different silica sources: sodium metasilicate pentahydrate, colloidal silica and a mixture of colloidal silica and sodium hydroxide solution. The catalysts were prepared by two-step hydrothermal crystallization at 100°C and 125°C. The organic template was removed from the as-synthesized samples by calcination in a muffle furnace at 540°C. The calcined solid products were treated with a solution of ammonium chloride at various concentrations prior to calcination again. The untreated and treated samples were characterized by X-ray power diffraction, scanning electron microscope, ICP-AES, 27Al-MAS-NMR, nitrogen adsorption and ammonia-temperature programmed desorption techniques. The optimal concentration for ammonium chloride solution is 0.03 M. Catalytic cracking of HDPE and PP over various Al-MCM-41 catalysts were studied under different conditions. The cracking of HDPE is more difficult than that of PP. When Al-MCM-41 was used as catalyst, the conversions of both plastics greatly increase compared to that in the absence of catalyst. The conversions and yields of gas fraction and liquid fraction depend on the reaction temperature and the Si/Al ratios in catalyst. The product selectivity is affected only slightly. The gas fraction obtained by HDPE cracking composes mainly propene and n-butane and by PP cracking is mainly propene. The liquid fraction obtained by cracking of both types of plastic composes mainly in the range from C7 to C8. The catalysts with various Si/Al ratios after the treatment with ammonium chloride provides higher yield of liquid fraction and lower yield of coke than that before the treatment. The used Al-MCM-41 catalyst can be regenerated by simple calcination and has activity almost the same as the fresh catalyst.