I: Solid Materials for Low Temperature Thermochemical Water Splitting. II: Structure-Property Relationships on the Zeolite Catalyzed Conversion of Methanol to Light Olefins

Citation

Bhawe, Yashodhan (2013) I: Solid Materials for Low Temperature Thermochemical Water Splitting. II: Structure-Property Relationships on the Zeolite Catalyzed Conversion of Methanol to Light Olefins. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/05FG-JG39. https://resolver.caltech.edu/CaltechTHESIS:11282012-141927745

Abstract

This thesis describes two separate projects. The first part of the thesis involves the synthesis of materials for enabling thermochemical water splitting at temperatures below 1000ºC, while the second part focuses on the structure-property relationships for the catalytic conversion of methanol-to-olefins.

In the first project, metal oxide clusters are impregnated in a silica support and tested as catalysts for the thermochemical splitting of water at temperatures below 1000ºC. These supported catalysts are able to do either the oxidation or reduction half cycle, but not both. Thermodynamic analyses reveal that for most common metal / metal oxide pairs, a thermochemical water splitting cycle can not occur in two steps below 1000ºC. Thus, a multi-step thermochemical cycle is developed using Mn₃O₄ and Na₂CO₃. This new cycle can be closed at temperatures that do not exceed 850ºC. Additionally, three metal spinels (Mn, Fe and Co based) are investigated with three alkali metal carbonates (Li, Na and K) for both thermochemical water splitting and thermochemical CO₂ reduction. The manganese, sodium system is found to be the optimal combination for water splitting.

The second project explores the effects that zeolite structure has on the product selectivity in the methanol-to-olefins (MTO) reaction. After first ensuring that literature results on the more common MTO catalysts could be reproduced, the effects of several zeolite structure features on product selectivity are elucidated. Structural features such as channel width and channel eccentricity, cage size (tested on the LEV, CHA and AFX frameworks) and framework composition (tested on the LEV, AEI, CHA and AFX frameworks) are explored. It is found that the product selectivity does not have a strong correlation with channel width and eccentricity. Ethylene selectivity did increase with a reduction in cage size, while propylene selectivity is a maximum with the CHA cage. The most consistent theme noted is that between the aluminosilicates (zeolites) and the silicoaluminophosphates (SAPOs), the effect of temperature on the C₃₌/C₂₌ is the same (if the aluminosilicate shows an increase in C₃₌/C₂₌ with an increase in temperature, so does the silicoaluminophosphate, etc.).

Thesis Files