Metal-ion-exchanged Zeolites for Applications in Endothermic Reforming PDF Download

Author: Yu-Hao Yeh
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Languages : en
Pages : 0

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Book Description
A significant challenge in the design of future hypersonic aircraft is engine cooling and one solution that has been proposed is to use the fuel as a coolant. The amount of energy that can be taken up by the thermal heat capacity of the fuel is limited but additional heat can be removed if the fuel is reformed via endothermic reactions prior to entering the combustion chamber. Two possible reactions that have been investigated in this thesis are the acid-catalyzed cracking reactions that occur in acidic zeolites and the aromatization reactions that occur over Ga- and Zn-exchanged zeolites. The first part of this dissertation was aimed at understanding the adsorption properties of alkanes on acidic zeolites in order to understand the reactions on these materials. Calorimetric measurements with CH4 on three zeolites with widely differently pore dimensions demonstrated heats decreased strongly with increasing pore size. Calorimetric measurements for a series of alkanes on H-ZSM-5 demonstrated that hydrogen bonding increased in a regular manner with the size of the alkane. In the second part of this dissertation, supercritical, high-pressure reactions of n-hexane over H-ZSM-5, with and without the addition of Pt, Ga, or Zn, were studied and reaction endothermicities were determined from the product distributions and from actual heat-flow measurements. The nature of the catalytic sites in these materials was also investigated using simultaneous Temperature-Programmed-Desorption (TPD) and Thermogravimetric analysis (TGA) of propyl amines. Adsorption studies showed that, at low ion-exchange levels, less than 0.5 Zn/Al, each added Zn cation in H(Zn)ZSM-5 displaced one Brønsted-acid site. FTIR of adsorbed acetonitrile-d3 and calorimetric measurements of adsorbed CO at 195 K indicated that the exchanged Zn cations form Lewis-acid centers. The heat flows measured directly for reaction at 60 bar and both 673 and 773 K indicated the acid-catalyzed cracking of n-hexane over H-ZSM-5 zeolite catalysts was ineffective for endothermic reforming applications. Aromatization reactions over H(Zn)-ZSM-5 exhibit much better endothermicities under these conditions. Measurements of the product distributions showed that the reaction endothermicity for H(Zn)-ZSM-5 at lower conversions was likely due to formation of significant amounts of benzene, toluene, and xylene but that these were converted to higher molecular weight products at high conversions. In the development of new catalysts, careful attention must be paid to the product selectivities. The present thesis, with systematic studies of metal-exchanged zeolites, should greatly increase our understanding of how metal cation at ion-exchange sites affect the reactions. The advances will help in predicting reactions and broadening the application, not only to use zeolites for endothermic reforming but in other important reforming reactions in the future.