| Co-Investigator(Kenkyū-buntansha) |
WAKAHIRA Yoshiroh Ohkura Riken Co.Ltd., Technical Department, Member, 技術部, 部員
KATADA Naonobu Tottori University, Faculty of Engineering, Research Associate, 工学部, 助手 (00243379)
KIM Jong-Ho Tottori University, Faculty of Engineering, Associate Professor, 工学部, 助教授 (60240751)
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| Research Abstract |
Temperature-Programd Desorption (TPD) of ammonia is a convenient method to quickly measurement of acidic property of zeolite. However, it is usually confused by readsorption of ammonia, and this complicates the interpretation. Various conditions were examined for the TPD on various zeolites in the present study, and these experiments clarified that the TPD was controlled by an equilibrium between gaseous and adsorbed ammonia. A theoretical equation was derived based on this equilibrium. The entropy change, which was assumed to consist of the phase-transformation and gaseous mixing terms, was determined from the experiments under various conditions. The former term was almost constant on the zeolites with various crystal structures and compositions, and consistent with Trouton's rule of thermodynamics. These support the validity of the theory. This theoretical equation gave the method to determine the acid amount and strength from one-point experiment.
Moreover, a method to determine the
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acid amount, strength and its distribution from one experiment with curve-fitting of the observed curve and the curve simulated based on the theoretical equation is proposed. This method also could separate the peaks due to ammonia desorbed from acid site from that due to the weakly adsorbed ammonia. Analysis of acidic property on weakly acidic zeolites such as Y and beta types became possible by this method.
The determined acid amount confirms a simple principle that isomorphous substitution of one aluminum atom generates one Brphinsted acid site, and one sodium atom can block one acid site. The determined acid strength had a narrow distribution on each zeolite, and was mainly controlled by the crystal structure as mordenite>ZSM-5>beta>Y.The strength was unchanged by varying the composition by dealumination or varying the ion-exchange ratio. This proposes a very simple principle that the acid strength of zeolite is controlled by the crystal structure, and hence homogeneous on one zeolite.
The present study thus establishes the experimental and analytical methods to exactly and quickly determine the acidic property, i.e., acid amount, strength and its distribution, of zeolite. Less
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