DDR framework

As one would have expected based on its structure, aluminum-free deca-dodecasil 3R readily adsorbs small molecules like CO2, N2O and NH3 [20]. On the other hand, alkanes and alkenes with three or four carbon atoms show only retarded adsorption at 296 K, while 1,3-butadiene is readily adsorbed under these conditions. This peculiar behavior has been attributed to the more elliptical cross section of the diolefin due to the presence of sp -hybridized carbon atoms. The former obviously has a better fit to the elliptical windows in the DDR structure than the circular cross section of saturated hydrocarbons [20]. In the following, the synthesis of materials with the DDR framework topology will be described in more detail. 

So far, essentially two templates have been reported to promote the crystallization of materials with the DDR framework topology, viz. 1-aminoadamantane (ADA) and methyltropiniumiodide. Their structures are depicted in Fig. 1. Both are relatively bulky molecules which, after hydrothermal synthesis, are encapsulated inside the large cavities of the DDR structure and have to be removed by calcination in an oxygen-containing atmosphere at temperatures of at least 813 K. Materials which have been prepared using 1-aminoadamantane are denoted as deca-dodecasil 3R [20,22] or Sigma-1 [18], while zeolite ZSM-58 is prepared using methyltropiniumiodide [16,17,19]. 

Fig. 1. Templates reported in the literature for the synthesis of zeolites with the DDR framework topology...

Mechanistic framework to develop dose-damage relationships (DDRs)... 

There has been successful development of DDRs for both C-Mn and Mn-Mo-Ni steels.The approach has been to base the mathematical forms of the expressions within the DDR, and the inclusions of particular input variables (e.g. Cu content, irradiation temperature) on mechanistic understanding (such as the framework depicted in Fig. 9.41). The values of the associated coefficients were then determined by fitting to the surveillance database. Different DDRs have been developed in different... 

It is not yet clear whether the simple mechanistic framework presented at the beginning of this section can be modified to incorporate the hardening from Mn-Ni-Si clusters or whether the observations of such clusters may be interpreted as evidence of a mechanism that is not yet fully incorporated into existing DDRs. 

The rest of the catalysts shown in Table 5.1 all consist of small-pore zeolites with 8-ring openings but a variety of framework structures such as CHA, LEV, ERI, and DDR. When fresh, all the catalysts exhibit excellent SCR activity achieving nearly 100 % NOx conversion at 250 °C similar to the Cu/beta and the Cu/ZSM-5 catalysts. These smaU-pore zeolite supported Cu SCR catalysts however, exhibit much higher hydrothermal stability. After 750 °C/24 h or 900 °C/1 h aging, they still achieve very high NOx conversion efficiency. In fact, no obvious performance degradation occurs for most of the catalysts. 

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