Mesoporous materials with enhanced porosity and acidity to obtain clean fuels from low-density polyethylene (LDPE) cracking

Abstract

Mesoporous materials with Si/Al ratio 7, 19 and 23 have been synthesized from ZSM-5 zeolite precursors prepared from tetrapropylammonium cations-containing clear aluminosilicate solutions, in the presence of the surfactant cetyltrimethylammonium bromide, and by employing different precursors of secondary mesoporosity or mesoporogen agents, such as nanometric carbon particles (BP700 and BP2000) and the biopolymer chitosan. The synthesized materials were characterized by X-ray diffraction, FT infrared spectroscopy, thermogravimetric analysis, electron microscopy, chemical analysis and nitrogen adsorption. It has been found that the effect of the pore-forming agents depends on the Si/Al ratio of the material, it is very small for Si/Al = 7, while for Si/Al above 19, there is a substantial increase of the secondary mesoporosity due to the mesoporogen agent, being this effect more evident for the sample with Si/Al = 23. For this material, in addition to the porosity associated to pores with a narrow size distribution centered at 3.7 nm due to the surfactant, secondary mesopores with a size distribution centered at 20–30 nm for carbon nanoparticles and in the range 4–10 nm for chitosan, are also detected. The acidity of the samples increases with their Si/Al ratio, and no significant effect of the porogen agent on the acidity has been found. The catalytic properties of these materials were tested in the conversion of m-xylene and in the cracking of low-density polyethylene (LDPE). Noticeable differences in the activity and selectivity of the gas/liquid products obtained in the catalytic cracking of LDPE were observed.