X-ray diffraction, of solids

The thermal imidization of a polyamic acid film (PMDA-ODA or BPDA-ODA) obtained by casting an NMP solution leads to an amorphous polyimide. Two different teams have shown that a polyamic acid solutions in NMP heated at 200°C for a short time (20 min) gives polyimide particles fully cyclized and highly crystalline, as shown by X-ray diffraction and solid 13C NMR spectroscopy.151152 The chemical imidization of the same solution gives only amorphous particles. The difference between the cyclization of a solution and a casted film in the same solvent is intriguing. In the case of the solution, the temperature and the heating time are lower than in the case of the casted film as a consequence, a less organized structure would be expected for the particle. 

The purpose of this section is to describe recent achievements in time-resolved X-ray diffraction from liquids. Keeping the scope of the present chapter in mind, neither X-ray diffraction from solids nor X-ray absorption will be discussed. The majority of experiments realized up to now were performed using optical excitation, although some recent attempts using infrared excitation were also reported. The main topics that have been studied are (1) visualization of atomic motions during a chemical reaction, (2) structure of reaction intermediates in a complex reaction sequence, (3) heat propagation in impulsively heated liquids, and (4) chemical hydrodynamics of nanoparticle suspensions. We hope that the actual state-of-the-art will be illustrated in this way. 

Structural study by solid state NMR and powder X-ray diffraction of the pure silica chabazite through water intrusion-extrusion processes... 

Friedel-Crafts reactions involving electrophilic substitution of aromatic compounds have been reported on solid base catalysts such as thallium oxide and MgO. The rates of benzylation of toluene by benzyl chloride over MgO nanocrystals were found to be of the order CP-MgO > CM-MgO > AP-MgO.56 An important observation in the study was that x-ray diffraction of the spent catalyst... 

As described above, most solid-state reactions are heterogeneous, in the sense that reactant and product are in different solid phases. In many of these, product crystals first appear as nuclei that grow at the expense of the parent crystal. On the other hand, there are some solid-state reactions that are not accompanied by a phase change and for which, therefore, analogy with a solid-state transformation is not plausible. Such reactions are of particular interest in several respects They make possible conversion of a single crystal of reactant to a single crystal of product they enable study, for example by X-ray diffraction, of the structures of the parent and product molecules as functions of the degree of conversion in more or less constant environments and one can elucidate from them the constraints that the parent crystal imposes both on the reaction pathway and on the conformation of the product. It is in connection with the latter that this subject is of particular interest in the present context. This class of processes has been discussed by Thomas (183). 

Bostrom D. (1988). Experimental studies of (Ni,Mg)—and (Co,Mg)—olivine solid solutions by single crystal X-ray diffraction and solid state emf method. Ph.D. thesis. Department of Inorganic Chemistry, University of Umea, Umea, Sweden. 

Nye then turns to Polanyi s brilliant work on the X-ray diffraction of polymers and solids in general. In this field he was more successful, although some of his ideas were also developed independently by other scientists. Polanyi s main problem in this area was lack of support from his colleagues in Berlin. This was partly because his ideas were too advanced for the 1920s, but also reflects Polanyi s inability (or at least unwillingness) to follow the current trends. 

X-RAY DIFFRACTION OF ORDERED CRYSTALS, LIQUIDS AND DISORDERED SOLIDS... 

Most structural determinations are X-ray diffractions of single crystals which refer to the solid state. These geometries may be influenced by crystal packing forces and various disorders, and the atomic positions derived by X-ray diffraction analysis do not correspond to equilibrium positions but rather to centroids of distributions obtained by... 

Dealuminated Y zeolites which have been prepared by hydrothermal and chemical treatments show differences in catalytic performance when tested fresh however, these differences disappear after the zeolites have been steamed. The catalytic behavior of fresh and steamed zeolites is directly related to zeolite structural and chemical characteristics. Such characteristics determine the strength and density of acid sites for catalytic cracking. Dealuminated zeolites were characterized using X-ray diffraction, porosimetry, solid-state NMR and elemental analysis. Hexadecane cracking was used as a probe reaction to determine catalytic properties. Cracking activity was found to be proportional to total aluminum content in the zeolite. Product selectivity was dependent on unit cell size, presence of extraframework alumina and spatial distribution of active sites. The results from this study elucidate the role that zeolite structure plays in determining catalytic performance. 

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