Crystal size and morphology

Finally, the nature of the crystalline microstmcture, ie, crystal size and morphology and the textural relationship among the crystals and glass, is the key to many mechanical and optical properties, including transparency/opacity, strength and fracture toughness, and machinabiUty. These microstmctures can be quite complex and often are distinct from conventional ceramic microstmctures (6). 

Optimizing the Cr layer also controls the crystal size and morphology. It was reported in 1986 (89,90) that the Cr underlayer thickness has a great influence on the coercivity of the Co—Ni—Cr layer. In most of the Hterature it can be found that with increasing Cr thickness the increases. Under ideal conditions and the right material combinations coercivities above 240 kA/m have been prepared. 

This paper presents results of an investigation into the effect which template type, sodium or potassium content, water content and aluminium source has on the crystal size and morphology of ZSM-5. The relationship between the crystal size and morphology, at similar Si/Al ratios, and performance for high pressure propene oligomerization is... 

Synthesis conditions used, crystal sizes and morphologies of ZSM-5 samples (see notes below)... 

Time-on-stream aetivity/selectivity and product distribution in the aromatization of C2+ hydrocarbons from natural gas over Ga H-ZSM-5 and H-GaAlMFI zeolites (having same bulk composition) at 600 C have been investigated. The zeolites were characterized for their FW-Si/Al and Si/Ga ratios and non-FW Ga by Si MAS NMR and bulk chemical analysis, for crystal size and morphology by SEM and also for their acidity by the chemisorption of pyridine at 400 C. The H-GaAlMFI zeolite shows higher activity and aromatics selectivity and slower deactivation due to coking in the aromatization of C2+ hydrocarbons from natural gas. 

The bulk and framework compositions of the H-GaAlMFI and Ga/H-ZSM-5 zeolites are given in Table 1. The zeolites were prepared by the procedures described elsewhere (6,9). The MFI structure of the zeolites was confirmed by XRD. The incorporation of Ga in the framework of GaAlMFI zeolite was confirmed by the Ga MAS NMR peak at +156 ppm. The framework (FW) Si/Ga and Si/Al ratios of the zeolites were obtained from their 29gi mAS NMR peaks, Si(0Ga or OAl) at about -112 ppm and Si(lGa or lAl) at about -104 ppm by the method described earlier (10). The crystal size and morphology of the zeolites was studied by SEM. The zeolite bulk chemical composition was determined by the chemical analysis of Ga, Al, Si and Na. Strong acid sites on the zeolites were measured in terms of the pyridine chemisorbed at 400 C, using the GC adsorption/desorption method (11). 

Molecular simulation approaches were used to investigate the effectiveness of template type on TS-1 crystal morphology. Calculations indicated that docking of hexane diamine on surfaces of TS-1 crystal strongly changes crystal size and morphology. It is also found that hexane diamine can pack together with TP A in TS-1 structure to play an important template effect. Simulation results consistent well with experimental results. 

This volume is a complete progress report on the various aspects of zeolite systhesis on a molecular level. It provides many examples that illustrate how zeolites can be crystallized and what the important parameters are that control crystallization. Forty-two chapters cover such topics as crystallization techniques gel chemistry crystal size and morphology the role of organic compounds and novel synthesis procedures. It offers a complete review of zeolite synthesis as well as the lastest finding in this important field. Contains benchmark contributions from many notable pioneers in the field including R. M. Barrer. H. Robson, and Robert Milton. 

The later works have shown that the porosity is not the sufficient factor of strength determination, because it is affected also by the pore structure and moreover, of the crystals size and morphology of the sohd phase. A significant compatibility of strength and porosity should be attributed to the correlation between these factors strength and total porosity. 

Recently, Yu and coworkers have successfully controlled the crystal size and morphology of the silicoaluminophosphate zeolite SAPO-34 (CHA) under microwave irradiation in the system of AljOj-P Oj-SiOj-TBAOH-H O by studying the synthetic factors such as the silica source, water content, crystallization time, and aging time [52]. Microwave-assisted synthesis proved to be an efficient approach to produce nanosized zeolite crystals [53]. As a comparison to traditional hydrothermal synthesis at 200°C for 72 h, the microwave-assisted synthesis of SAPO-34 needed only 1 h at 200°C, which greatly reduced the reaction time. 

---