Host structure and guest location

Until now, the most powerful contribution of neutron diffraction, even on powders, has been the guest location, especially for deuterated hydrocarbon molecules. As a matter of fact, one of the most important industrial applications of zeolites is hydrocarbon conversion catalysis and separation. The advantage of neutrons over X-rays comes from the fact that deuterium and carbon have similar scattering lengths (be = 6.65 fm, b = 6.67 fm) which involves participation of all the nuclei of the guest molecule in the structure factor. 

Right Equilibrium position of acetylene molecule in the NaA zeolite a) seen along the Oz axis, b) along the Ox axis. Ionic and covalent radii are used. Dashed circles sodium hidden by the molecule and coordinated to it 

In order to understand the diffusivity in the channel, Newsan [34] and Taylor [35] studied respectively benzene in L-zeolite and in ZSM-5 type zeolite. In L-zeolite, 

06 6 is found in the channel section, parallel to the channel wall, the n electron density facing a potassium cation which is slightly displaced from the anionic framework. In ZSM-5 the structural complexity of this framework makes the problem rather difficult. 

Taylor noticed that the diffraction pattern is very sensitive to the C6D6 contribution at the lower angles. He found three positions A, B, C A and B are in the intersection of the two kinds of channels (straight and wavy channels) the third one C is at a symmetry centre of a straight channel. At 78 K slight translational and orientational disorder remains. The packing 

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