Síntese da zeólita MCM-22 a partir de sistema reacional contendo sódio e potássio e desenvolvimento de catalisadores ácidos com topologia MWW modificada.

Abstract

MCM-22 is a zeolite with a porous structure identified by the MWW code, which crystallization occurs through the formation of a lamellar precursor. Although the MCM-22 synthesis has been extensively investigated, some important parameters for its preparation have not yet been systematically studied. In this context, the present work aimed to evaluate the influence of the aluminum source and the presence of sodium and potassium cations in the reaction mixture on the MCM-22 crystallization and develop acid catalysts with MWW topology modified by MCM-22 desilication and delamination of its lamellar precursor. The hydrothermal method was used to synthesize the said zeolite, varying the aluminum source (aluminum hydroxide, sodium aluminate, and pseudoboehmite) and the relative amount of sodium and potassium present in the reaction mixture, under static conditions. The desilication process was conducted using sodium hydroxide solutions with different concentrations, whereas delamination was performed by swelling of the lamellar precursor and subsequent exfoliation by sonication. The catalysts acidity were evaluated by ammonia thermal programmed desorption. The XRD results showed that among the aluminum sources used the aluminum hydroxide was more effective to synthesize MCM-22 under the experimental conditions employed, and that the simultaneous presence of sodium and potassium in the reaction mixture increased zeolite crystallinity and crystallization rate, when potassium corresponded to 45% of the total alkali metal on a molar basis. The micrographs of MCM-22 samples showed that the morphology and particle size were affected by the different ratios between sodium and potassium evaluated. The XRD, N2 physical adsorption and SEM analyses of the desilicated and delaminated materials evidenced the MWW topology sensitivity to alkaline treatments and that the lamellar precursor monolayers exfoliation process did not occur integrally. The acid catalysts with structures modified by desilication and delamination processes presented lower acidity and weaker acid sites compared to the MCM-22 without structural changes, with the greatest reduction being observed in the partially exfoliated catalyst.