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キーワード:
sulfated zirconia; catalysis; alkane isomerization; defects
要旨:
Motivated by the hypothesis that catalytic activity of sulfated zirconia (SZ) is essentially governed by defects in the materials tetragonal structure, this study uses the n-butane isomerization as a model system to analyze the effect of structural imperfections of the material on its properties, catalytic performance, and long term stability. Defects, as high energy sites on the catalyst surface, are considered as a prerequisite for the formation of active centers. These centers are thought to be composed of a combination of surface defects and nearby closely spaced disulfate groups.
The preparation and, in particular, the calcination were investigated systematically. The chemical properties were varied gradually through the promoter content (thulium) inside the structure with automated co-precipitation and the sulfate content on the catalyst´s surface with a subsequent incipient wetness impregnation. The samples were characterized by nitrogen adsorption isotherms, XRD refinement, SEM, EDX, TEM, HRTEM, IR, UV-vis, and n-butane isomerization. Additionally, characterization methods newly for the system adapted were used: for the relative apparent defect density (combination of TEM and XRD), for the Lewis to Brønsted acid site (L/B) ratio (CO adsorption and IR spectroscopy), and for the long term deactivation rate constant.
The investigated catalysts are highly active but do not show a steady state even after 250 h. It could be characterized with a linearized form of a second order deactivation model starting after the initialization of the reaction, from which precise deactivation rate constants could be determined.
It was found that intrinsic defects in the tetragonal structure and not the structure itself determine the surface properties of an active SZ catalyst, and-, accordingly, its catalytic performance and stability in the isomerization of light alkanes is controlled by lattice imperfections. The defect density inside the tetragonal lattice, as well as side isolation of this phase, can be governed by controlled preparation and calcination.
A stabilized defect structure together with a low L/B ratio of surface acid sites are the key features of high-performance SZ catalysts. This knowledge can be used to tune-, and tailor highly active catalysts with specifically desired bulk and surface characteristics for industrial applications.
