Effect of kaolin chemical reactivity on the formation of ZSM-5 and its physicochemical properties

Abstract Two kaolins from different geological areas of South Africa were compared as precursors for the synthesis of ZSM-5. Their effects on the formation of ZSM-5 and its physicochemical properties were investigated. The mineralogical phases, morphology, surface area and porosity characteristics, acidity and chemical nature of the synthesised materials were investigated using XRF, XRD, HRSEM-EDS, N 2 physisorption (BET) analyses, NH 3 -TPD and 27 Al/ 29 Si MAS NMR. The two kaolins differed in structural order and XRD showed Fishoek (SK) kaolin to be more crystalline than the Grahamstown (BK) kaolin with Stoch indices of 0.97 and 1.17 respectively. The more disordered BK resulted in a faster crystallisation of ZSM-5. ZSM-5 derived from the two kaolins showed a remarkable difference in its physicochemical properties as well. N 2 physisorption confirmed a well-developed microporous structure manifested from the slower crystallisation kinetics of SK. The slower dissolution of reactants in the gel also resulted in a difference in the chemical properties as a highly acidic ZSM-5 was derived from SK (1313 μmol) as compared to BK (633 μmol). 27 Al MAS NMR studies clearly confirmed the presence of highly reactive Al species in the synthesis gel of BK derived ZSM-5 correlating the enhanced crystallisation kinetics observed with the kaolin structural order and chemical reactivity.