Synthesis and characterisation of high silica zeolites with MOR and MFI framework type from South African coal fly ash.

Abstract

High-silica zeolites are porous, aluminosilicate materials known for high thermal stability; making these materials favourable for application in various industrial processes involving elevated temperatures. The synthesis of zeolites from alternative feedstock such as coal fly ash has been investigated previously. Coal fly ash is a waste by-product of the coal combustion process, which is main source utilised for energy generation in South Africa. Coal fly ash is an ideal alternative feedstock for zeolites due to the large annual production as well as the rich mineral content (high in silicon and aluminium). Coal fly ash also contains a range of other elements which is known to result in environmental problems such as excessive land-use as well as air, soil, surface water and ground water pollution; including inorganic cations that may influence the zeolite crystallisation process. However, coal fly ash has successfully been utilised for the synthesis of a range of low-silica zeolites. High-silica zeolite synthesis from alternative feedstock has usually focused on the synthesis of zeolite ZSM-5. Furthermore, these synthesis routes commonly utilised additional silicon sources and/or a purification step by reflux treatment with a chelating agent to enhance the feedstock Si/Al ratio prior to crystallisation. This adds complexity to the conversion of coal fly ash to zeolite process, in terms of cost, energy and time.