References. 4. 1. 1. 75. The anionic exchange groups are on the surface of the zeolite as follows: (1) Hydrogen-bonded surface oxygens; (2) Sulfate group; (3) Al2+ or H3+; (4) Type B cations are exchanged with the cationic form of the [AlO4]4- tetrahedron (see figure 19). 2. 2. 2. 75. Zeolites are porous crystalline aluminosilicates; they have a uniform pore-size distribution and channels of molecular dimensions in their solid state.Zeolite molecular sieves are synthesized by the hydrothermal crystallization of aluminosilicate systems containing alkali, alkali earth, transition, and rare-earth elements. In aluminosilicate systems, the silicon and the aluminum are not equally available, and/or there is an imbalance in the rates of their formation and deposition onto the growing crystal. The amount of silicon and aluminum deposition is controlled by the rate of addition of the reagents and the acidity of the reaction. The system also contains alkalies, the composition of which affects the hydrothermal stability of the system. For example, bromide ion is always present in the alkalified systems. The total alkali content is controlled so that the gel is stable and forms crystals by hydrothermal treatment.
In a thermal analysis, a sample is heated at a controlled rate and the temperature at which the sample undergoes a sudden mass change is the thermal stability temperature (figure 6). Thermo-gravimetric analysis (TGA or TGA-DSC) and differential scanning calorimetry (DSC) are based on the same principle, but TGA-DSC can provide a good measure of the temperature at which a polymer undergoes phase separation, while TGA is more likely to detect a material’s decomposition in the absence of a phase transition to a separate phase. d2c66b5586