A sequential Box-Behnken Design (BBD) and Response Surface Methodology (RSM) to optimize SAPO-34 synthesis from kaolin waste

Abstract

Kaolin waste from an industry in the Amazon region was selected as an economic source of Si and Al to optimize the synthesis of SAPO-34 molecular sieve. A sequential BBD, RSM and global desirability function (D) was used. The factors studied for optimization were the SiO2/Al2O3 molar ratio, the concentration of the structure-directing agent (SDA; morpholine) and the crystallization time. The responses evaluated in the synthesized SAPO-34 were the relative crystallinity (%), the enthalpy of formation of transition alumina (J.g−1) and the mass loss (%). The synthesis as a function of time and its products post-DSC and calcined at 1000 °C was performed to relate the enthalpy of formation of the transition alumina with the SAPO-34 formation. Prediction models and response surfaces were corroborated by the level values optimized by the desirability function. The ideal values based on the data used in this study included a crystallization time of 20 h, a xSiO2 value of 1.3 and a ySDA value of 1.6. The experimental validation based on the ideal values was within the established confidence level (±95%). The formation of the transition alumina at 985 °C influences the enthalpy of exothermic transformation identified at this temperature. Low values of this enthalpy are related to the effective formation of SAPO-34.