Synthesis of ZSM-5 from impure and beneficiated Grahamstown kaolin : effect of kaolinite content, crystallisation temperatures and time

Abstract

Raw kaolin obtained from Grahamstown, South Africa was beneficiated using wet sieving and filtration before use in the synthesis of ZSM-5 zeolite via hydrothermal treatment at various temperatures (120–190 °C) and times (24–96 h) under autogenous pressure. The impacts of kaolinite content and crystallisation parameters (i.e. temperature and time) on ZSM-5 formation were investigated. The mineralogical phases, morphology, sur-face area and porosity characteristics of the synthesised materials were investigated using XRF, XRD, FT-IR, HRSEM-EDS and N2 physisorption (BET) analyses. Catalytic tests were carried out in a fixed bed reactor, WHSV of 8 h−1 and a temperature of 350 °C at atmospheric pressure using 1-hexene as a feed. Beneficiation decreased the impurity content in the clay and increased kaolinite content which resulted in the formation of pure ZSM-5 zeolite. XRD and HR-SEM revealed that ZSM-5 synthesised from raw kaolin contained quartz and formed a ZSM- 5/quartz coated composite. Temperature and time were critical in controlling phase purity and crystallinity of ZSM-5. At a synthesis temperature of 190 °C ZSM-5 transformed into a more thermodynamically stable quartz phase. The optimum condition for synthesising pure ZSM-5 from this clay was found to be 150 °C for 48 h. Morphology, surface area and porosity of ZSM-5 changed with an increase in crystallisation time. ZSM-5 with a high external surface area and presence of mesopores were observed in products synthesised at 150 °C for 24 h. Microporosity further developed as time increased and the ZSM-5 became more crystalline. The catalyst performance studies indicated that the ZSM-5 synthesised from raw and beneficiated kaolin at crystallisation conditions 150 °C and 48 h had good 1-hexene transformation activity and high selectivity to gasoline range hydrocarbons. The presence of quartz in the catalyst enhanced catalyst stability and selectivity to C10+ hydrocarbons.