Síntese da zeólita SSZ-13 com o uso combinado de agentes direcionadores de estrutura e cristais sementes

Abstract

Zeolites have been widely used in industry due to their excellent properties, such as high surface area, acidity, among others. The development of different synthesis strategies makes it possible to improve the synthesis methodology and the physicochemical properties of zeolitic structures. Recent studies have been developed to optimize the synthesis of SSZ-13 zeolite (reduction of crystallization time, use of less expensive reagents, among others) in order to expand the range of applications of this structure. Therefore, the present research addressed the optimization of the synthesis of SSZ-13 zeolite using choline chloride, hexamethylenetetramine, betaine, 1-(cis-3- chloroallyl)-3,5,7-triazo-1-azoniaadamantane chloride and of 5-ethyl-1-azo-3,7- dioxabicyclo[3.3.0]octane as structure-directing agents combined with seed crystals. The seed crystals, with CHA structure, were synthesized from an inorganic gel obtained by the interzeolitic transformation of faujasite zeolite. Some important synthesis parameters for obtaining SSZ-13 zeolite were evaluated, such as: directing molar ratio/SiO2 = 0.10, 0.40 and 0.8, OH/SiO2 molar ratio = 0.4 and 0.8, percentage of seed crystals 2.5% and 10% and crystallization times between 24 to 72 hours. The samples were characterized by X-ray diffractometry (XRD), energy dispersive X-ray spectroscopy (EDX), thermal analysis (TG/DTG), scanning electron microscopy (SEM), nitrogen adsorption-desorption and ammonia desorption at programmed temperature (TPD-NH3). The catalysts that presented better textural properties were submitted to the catalytic test using the cracking of n-hexane as a model reaction. The XRD results revealed that among the structure- directing agents used, only choline chloride and hexamethylenetetramine were effective for the formation of pure SSZ-13 with 24 hours and 48 hours of crystallization, respectively. It was found that for OH/SiO2 = 0.4 there was no formation of SSZ-13 at all times studied. For choline chloride syntheses, the increase in alkalinity (OH/SiO2 = 0.8) promoted nucleation and crystal growth leading to the formation of SSZ-13 with high purity and crystallinity, regardless of the percentage of seed crystals. However, the syntheses with hexamethylenetetramine were only efficient with the percentage of seed crystals of 10%. The EDX results showed that the samples containing the SSZ-13 phase presented very close values of SiO2/Al2O3 ratio, between 9 and 11, indicating a restricted range of SSZ-13 zeolite formation. The samples showed morphology of intergrown cubic crystal clusters and textural and acidic properties characteristic of the CHA structure. The catalytic activity results showed that the sample synthesized with choline chloride suffered a greater deactivation of parts of its acid sites by the deposition of coke, resulting in a lower conversion (about 21%) when compared to the sample synthesized with hexamethylenetetramine, which presents a total acidity lower, but achieved a conversion of approximately 25% and a coke content of only 8.3%.