Influência do tratamento pós-síntese sobre as propriedades texturais, acidez e atividade catalítica da zeólita ZSM-23 sintetizada com diferentes agentes direcionadores de estrutura

Abstract

The synthesis of ZSM-23 zeolite was studied using isopropylamine, ethylene glycol, pyrrolidine and N,N-dimethylformamide as structure-directing agents, with subsequent post-synthesis treatment (desilication and acid treatment), in order to obtain micro-mesoporous structures. The samples were characterized by X-ray diffraction, energy dispersive X-ray spectroscopy, thermal analysis, nitrogen adsorption-desorption, temperature-programmed ammonia desorption and scanning electron microscopy. The post-synthesis treatment promoted a drastic reduction of the SiO2/Al2O3 ratio (from 90 to 97 to 39 to 66), meaning the effective extraction of structural silicon atoms, with the increase of surface area values (from 37 to 80 m 2 ·g-1 for 48 to 154 m2·g-1) and total pore volume (from 0.13 to 0.31 cm3·g-1for 0.17 to 0.49 cm3·g-1), obtaining a bimodal pore distribution (micro and mesopores). There was a reduction in the density of weak acidity sites (from 237 to 370 μmol·g−1 to 165 to 245 μmol·g−1) for all catalysts, an increase in the density of strong acidity sites (from 94 to 134 μmol·g −1 to 207 to 226 μmol·g−1) for the samples from the IPA and PYR series and a decrease (from 148 to 223 μmol·g −1to 145 to 151 μmol·g−1) for those from the ETG and DMF series, due to partial dealumination of these structures. The modified samples showed superior catalytic activity (from 78 to 95% to 87 to 98%), less conversion decay (from 22 to 42% to 13 to 29%) and greater resistance to deactivation by coke (from 3 to 4% for 1 to 2%) in n-heptane cracking, and as such, greater effectiveness in reducing the decomposition temperature (from 428 to 467 °C to 416 to 436 °C) and lower activation energy level (from 133 to 176 kJ·mol–1 to 74 to 143 kJ·mol–1) in the process of degradation of ultra high molecular weight polyethylene.