Shape selectivity grains

If the SMA is sufficiently close to Tm, an imposed stress is sufficient to cause pressure-induced austenite —> martensite phase transitions in selected grains of the alloy, relieving the stress through pseudo-elastic deformation of the softer martensite grains. Similarly, if the original austenite-shaped alloy is brought below Tm to convert it to malleable martensite form, many deformations of macroscopic shape leave the martensitic atoms close to their... 

In an investigation of PER synthesized without a template (66), it was shown that an acid treatment causes the removal of non-shape-selective acidic sites located in pores larger than the zeolitic pores or on the external surfaces of the ferrierite grains (66). Consequently, isobutylene selectivity... 

Effect of the Size of the Zeolite Grains Effect on Shape Select lyity PropeFt les ""... 

When refering to shape selectivity properties related to diffusivity, it seems obvious that the larger the zeolite grain, the higher will be the volume/sur f ace ratios and the shape selectivity, since the reaction will be more diffusion controlled. The external surface area represents different percents of the total zeolite area depending on the size of the grains which could be important if the active sites at the external surface also play a role in the selectivity. For instance in the case of toluene alkylation by methanol, the external surface acid sites will favor the thermodynamical equilibrium due to isomerization reactions (o m p-xylene - 25 50 25 at 400 C) while diffusivity resistance will favor the less bulky isomer namely the para-xylene. It may therefore be useful to neutralize the external surface acidity either by some bulky basic molecules or by terminating the synthesis with some Al free layers of siliceous zeolite. 

The effect of the crystal size on shape selectivity properties of zeolites has been nicely demonstrated by researchers at Mobil for ZSM-5 zeolite (53, 55-59). Many examples can be found in ref. 21j 22, 60, 61. Shape selectivity obviously increases with the size of the grains since the path length of the molecules increases. 

The grains are characterized by their shapes, initial characteristic sizes, and current sizes (at time f). The first fom columns of Table A. 1.1 of Appendix 1 provide the selected grain shapes and their characteristics. 

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