Obtenção de materiais com estrutura hierárquica de poros através da dessilicação da Zeólita Zsm-22

Abstract

The present work deals with the generation of mesoporosity in ZSM-22 zeolite (TON structure). In a first part the synthesis of ZSM-22 was studied through a methodology similar to that presented by Valyocsik (1990), followed by an addition of an amphiphilic trimethoxyphenylsilane organosilane (TMPHS) in the synthesis gel, and by another substitution of 20% of the silica required for the synthesis by silanized silica beads by TMPHS. In the second part, the formation of mesoporosity by post-synthesis treatment with NaOH solution was studied, varying the concentration of 0.2, 0.4 and 0.6 mol.L-1 at a temperature of 70 °C per 1hour. The zeolites obtained were characterized by X-ray diffractometry (XRD), nitrogen adsorption at -196 °C, scanning electron microscopy (SEM), thermogravimetric analysis (ATG), absorption spectroscopy in the infrared region with Fourier transform (FT-IR) and desorption of ammonia at the programmed temperature. The catalysts that presented the best textural properties were submitted to the catalytic test with the n-hexane cracking model reaction. The diffractograms of the synthesized samples showed that they presented the main peaks referring to the TON structure, and that the desilicated samples maintained in their entirety their crystalline structure. The thermogravimetric curves presented four mass loss events, reaching values close to 10% of total loss. Infrared identified absorption bands characteristic of zeolitic materials. The micrographs of the TON-P sample showed crystal agglomerates in cylindrical form 2 μm in length. The pore diameter distribution of the desilicated samples presented a distribution in the range of 2 to 100 nm, proving one of additional mesoporosity due to the desilication process, with a higher formation of mesopores for the samples submitted to the treatments with higher OH- concentration, which caused an increase in the total acidity by the reduction of the Si/Al ratio. The desilicated samples presented a lower conversion compared to the precursor, but with higher stability in the conversion rate, with selectivity to formation of propene and isobutane.