Methanol dehydration using MFI: The role of framework composition and binder

QD Chemistry

Munnoch, Alexander Leishman ; Hargreaves, Justin S.J.

University:University of Glasgow

Department:School of Chemistry

关键词: Methanol dehydration, dimethyl ether, zeolite, ZSM-5, MFI, binder, binder effects, sodium exchange, dealumination, steaming.;   

Others  :  http://theses.gla.ac.uk/4757/1/2013munnochphd.pdf

来源: University of Glasgow

【 摘 要 】

Methanol dehydration is currently of industrial interest as a means of dimethyl ether production. Pertinently, dimethyl ether may be used as an alternative LPG fuel. This reaction is acid catalysed and in this study a number of microporous catalysts, containing the unique MFI framework structure, have been investigated. Attention has also been directed towards the application of binders and their effects.Temperature ranges for selective dimethyl ether formation were found to be 150- 275 °C over H-ZSM-5, 300-350 °C over Na-ZSM-5 and 150-350 °C over γ-Al2O3 catalysts (where ZSM-5 = MFI zeolites). Comparison of commercial ZSM-5 materials with a wide range of SiO2/Al2O3 ratios (23-1500) showed methanol dehydration to increase with increasing framework aluminium content. B-MFI, Fe- MFI and Ga-MFI zeotypes were synthesised for comparison to zeolites. 11B and 71Ga MAS-NMR measurements showed boron and gallium to be tetrahedrally coordinated within their MFI frameworks. The following ranking of methanol dehydration activity: Al > Ga > Fe > B was observed. Large scale zeolite and zeotype applications require the use of binders such as silicas and aluminas to provide desirable physical properties. Despite their necessity in scale-up, zeolite binding and its effects have been largely ignored and there is a general misconception that they are inert components. Silica binders were found to decrease the activity of the catalyst by dilution, alkali metal poisoning and pore filling. Sodium content had a stronger negative effect than binder dilution and pore blockage on the methanol dehydration activity. Furthermore, increasing sodium content resulted in decreasing extra-framework aluminium formation. Arrenhius plots of sodium-exchanged zeolites showed increasing sodium content to decrease the number of zeolite active sites. The methanol dehydration activity of H-ZSM-5 (1500) was increased in the presence of an alumina binder and the method of binding had little effect on the catalytic activity. A small degree of aluminium insertion into the zeolite framework was indicated by 27Al and 29Si MAS-NMR in bound and aluminium impregnated systems. 27Al and 29Si MAS-NMR also showed the change of the nature of active sites in Arrenhius plots of extremely calcined and extremely steamed aluminium-impregnated ZSM-5 (1500) catalysts to be the result of zeolite dealumination.