Cristobalite synthesis

Silica is of particular importance because of its use as a stable catalyst support with low acidity and its relationship to zeolite catalysts, which will be discussed in chapter 4. Silicon is an abundant material in the earth s crust and occurs in various forms including silica. Silica is also polymorphous with the main forms being quartz, cristobalite and trydimite. The stable room temperature form is quartz (Si02). Recently, a new family of stable silica-based ceramics from chemically stabilized cristobalites has been described using electron microscopy (Gai et al 1993). We describe the synthesis and microstructures of these ceramic supports in chapters 3 and 5. 

Note that the presence of cristobalite as side phase does not influence this behavior (samples 4 and 9). The only consequence due to its formation (probably induced by other variables) is the decrease of the overall Si content in the gel. This does not seem to affect the Nu-10 synthesis efficiency, provided the gel composition is recalculated adequately (Table II). 

Impurities. Of course, the presence of impurities in a sample will have a dramatic effect on the XRD characteristics. Zeolite preparations, as synthesized, can contain both organic and inorganic impurities. After washing and calcination, many impurities become amorphous, and the resulting XRD powder pattern will clearly show changes from the as-synthesized material. Some impurities, however, are stable to calcination and can make identification and characterization of the material (especially a new material) rather difficult. This is particularly true for cases where only a small number of samples, prepared in a narrow synthesis regime, are available for XRD examination. Common impurities found in zeolite preparations are the stable silicates, quartz and cristobalite. 

The results depicted in Figure 8 further indicate that, the more siliceous the synthesis mixtures are, the earlier cristobalite starts to form. Hence, if a ZSM-23 with relatively high SiO2/AI2O3 ratio is to be synthesized, a careful optimization of crystallization time and, perhaps crystallization temperature should be made to obtain pure products. Increasing the aluminium content of the gel to SiOp/A Oo ratios below ca. 50 also yields ZSM-23 but contaminated with ZSM-5 of a peculiar morphology (Figure 10). 

Compared with the amount of organic solvent, the amount of water in the solvothermal synthesis system is little. However, it plays an extremely important role in the successful formation of the final structures. Previous studies revealed that the synthetic reaction would not happen without the presence of this small amount of water. Another direct proof showing the importance of this small amount of water is the variation of structure type crystallized from the aluminum isopropoxide-phosphoric acid-triethylamine-tetra-ethylene glycol-water (extra added) system with the variation of the amount of extra added water, keeping the other parameters constant. By a gradual increase in the amount of the extra added water, one-dimensional aluminophosphate chain, two-dimensional aluminophosphate sheet, three-dimensional aluminophosphate open-framework JDF-20 and AIPO4-5, and dense-phase cristobalite sequentially crystallized from this system under conditions of identical crystallization temperature and time. 

Unlike quartz, cristobalite has an open structure, allowing some fraction of silicon (2-3%) to be replaced by other elements, such as, Al, Na, or Ca. Still, 95% of the mineral is formed by Si02. The natural cristobalite does not exist in concentrations that make mining feasible therefore it is produced by synthesis (see separate section on cristobalite). Both minerals are found in volcanic rocks, but quartz, which constitutes 12.5% of the Earth s crust, is found eveiywhere, since it does not change or erode. Sandstone is one of the sources of quartz. 

Phospham or phosphonitrilic amide, PNNH, is a fascinating material also isoelectronic with Si02 [213], Its formula suggests a relationship to LiPN2 with the filled high cristobalite structure, but it is amorphous to X-ray analysis. Phospham has been used in the synthesis of phosphorus-containing nitrides [111], There is an extensive literature on phosphonitrile polymers (for a review, see [214]). 

Table 8 shows a compilation of the most important parametes influencing M-SH synthesis and their effects on (a) the rate of formation and (b) the formation of impurities. All crystalline substances that do not belong to the desired M-SH are designated foreign phases. These phases include otha- M-SHs as well as three-dimensionally cross-linked Si02 products such as cristobalite and quartz. Some of the parameters will be discussed in the following subsections. 

The sequence of consecutive products and also the morphology of the synthesis products can be altered by addition of inorganic salts to the synthesis mixture [36,73]. Addition of LiCl, KCl, RbCl, and MgCl2 retards nuclei formation. However, it accelerates recrystallization to cristobalite, kenyaite, and quartz [76]. According to (DE-OS 31 23 000), the formation of magadiite from silica gel and sodium hydroxide solution is accelerated by copper sulfate and ammonia. Bergk et al. [73] observed that the sulfate anion shortens the incubation period of the magadiite crystallization process much more than nitrate or chloride anions. 

Mineralizer. A substance that, even though present in only a small amount, assists the formation and/or crystallization of other compounds during firing. A small amount of alkali, for example, mineralizes the conversion of quartz to cristobalite boric oxide acts as a mineralizer in the synthesis of spinel (MgAl204) the presence of iron compounds facilitates the growth of mullite crystals. 

Gittleman et al. studied in more detail the synthesis of all-silica SSZ-24 [108]. In agreement with earlier studies they found that the use of the trimethyl-1-aminoadamantyl cation and fiuned silica as Si02 source is essential for the formation of pure SSZ-24. Moreover, they also confirmed that the molar ratios template/Si02 and OH-/Si02 should exceed values of 0.15 and 0.25, respectively [108]. Especially for lower pH or alkali content, the formation of non-zeolitic silicate phases such as quartz, cristobalite and layered silicates are reported to be favored. 

Habersberger et al. reported for the first time the synthesis of V-ZSM-48 with hexamethylene diamine as a structure directing agent, VCI3 and NaOH. The product was found to coUapse to cristobalite upon calcination in air at 550 °C [ 104]. 

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