Thermal Stability of the Framework

It is not easy to predict which frameworks will remain intact upon template removal, and which will collapse, but it should be borne in mind that all open frameworks are less stable (when empty) than dense crystalline forms, and barriers to recrystallisation are kinetic rather than thermodynamic. High-temperature treatment of microporous solids eventually results in recrystallisation to dense ceramics rather than other microporous solids. For example, the magnesium form of zeolite P transforms to magnesian cordierite, and aluminophosphates transform to dense AIPO4 polymorphs. 

An interesting example of formation of a zeolite structure by the thermal treatment of a precursor outside of the hydrothermal autoclave is the observed 

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The higher thermal stability of the exchanged phase is a direct consequence of the presence of NH4 molecules in the channels the decomposition of the molecules, at about 450 °C, and the loss of NFI3" ions, leave in the channels H+ protons to balance the framework charges. While Na2+ ions tend to weaken the Si-O bonds, the presence of FI+ protons contribute to stabilize the structure balancing framework negative charges. 

The thermal stability of mesoporous frameworks substantially increases with an increase in the wall thickness and pore size, which can be varied even for the same template by changing the processing conditions. Ozin et al.55 developed a way to prepare crystalline titania films with a 2D-hexagonal architecture by replacement of ethanol in the Pluronic-containing precursor solution with more hydrophobic butanol-1. The latter promotes phase separation at low surfactant-to-titania ratios, resulting in thicker pore walls, which are more compatible with the crystal growth during subsequent calcination. 

The revolutionary zeolite cracking catalyst (synthetic Linde X and Y) was introduced commercially over 28 years ago, but considerable effort is still being expended on the improvement of its stability and catalytic properties. Decreasing the aluminum content of the zeolite framework and the replacing the rare-earth with the hydrogen form have greatly increased activity at the expense of stability. The thermal stability of the faujasites is fairly well understood, while the reasons for the increased catalytic activitity are still not fully known. 

Aluminium can be isomorphously substituted for silicon in the framework of zeolite Y by hydrothermal treatment of the dealuminated (ultrastabilised) zeolite with aqueous solutions of strong bases at elevated temperatures. The extent and efficiency of the reaction depend on the temperature, duration of treatment and especially on the kind and concentration of the basic solution. The degree of crystallinity and the thermal stability of the products are primarily controlled by the composition of the parent material. 29Si and 27A1 magic-angle-spinning NMR (MAS NMR) indicates that the extent of realumination is determined by the number of available tetrahedral Si(OAl) sites. 

Properties of zeolites are intimately related to the type of occupancy of the tetrahedral sites. Modification of the composition of the framework by increasing the silicon content increases the thermal stability of the samples. The catalytically active centres in zeolites are the acidic (Bronsted) hydroxyl groups associated with tetrahedrally coordinated framework aluminium atoms. Catalytic activity is thus strongly dependent on the concentration and location of aluminium in the framework. 

The calcosilicate zeolite-like crystal material CAS-1 was hydrothermally synthesized and the thermal stability of the samples were investigated. The effects of composition of raw materials, reaction temperature and alkali metals on the synthesis of CAS-I were addressed. Cation exchange reactions and their influences on the thermal stability of CAS-I framework structure were also studied. The samples were characterized by XRD, TEM, SEM, DT-TGA, AAS and chemical analysis. The results showed that CAS-1 could be obtained from a wide range of composition of raw materials and reaction conditions. The cations have great influence on the thermal stability of the CAS-I framework structure. 

The difference in thermal stability between framework structures of different samples indicated that CAS-1 had the capability of reversible ion exchange and the cations greatly influenced the thermal stability of CAS-1. The thermal stability of K-CAS-1, which were obtained from Na-CAS-1 equilibrated with KCl, is much stronger than that of Na-CAS-1. At the same time, the fact that the thermal stability of the as-synthesized CAS-1 between that of Na-CAS-1 and K-CAS-1 reveals the presence of Na in the as-synthesized CAS-1. And the results well correspond to those from the chemical analysis, suggesting that the as-synthesized CAS-1 contains Na cations introduced by the addition of colloidal silica. 

A mass-spectral study performed on Sc(acac)3, Sc(dpm)3 and La(dpm)3 vapors shows that, due to volatility and thermal stability, this group of S-drketonates is suitable for low-temperature gas-phase transport of metals. The similarity of the mass spectra of Sc, Y and La -diketonates may point to a similarity of the processes involved in the thermal destruction of the molecular species present in vapor. Within the framework of crystal field theory, an unpaired electron of a central metal ion (Sc +, Y + or La + with the nrf configuration) occupies a orbital, which has the lowest energy in D2h symmetry. This results in stabilization of the ML2 radical and enhances the thermal stability of the complex. 

The size and shape of the channels and cavities, the location of cations and the distribution of A1 or other T-atoms affect the adsorption, rate of diffusion and the size of the catalyzed product. The Si/Al ratio, the concentration of the various ring sizes and the nature and location of cations determine the thermal and hydrothermal stabilities of the framework. The ability to determine these structural features is necessary in order to understand the physical and catalytic properties which can also be affected by faulting crystallite size and loading. 

Table 1 summarizes the data on the thermal stability of the hydroxyl groups in faujasites and mordenites. The table contains the results derived from the measurements of the relative intensity of the hydroxyl bands at 3640 cm l and 3610 cm l as a function of the calcination temperature for faujasites and mordenites, respectively, vrith different amounts of the framework Al. Included also are the data calculated from the concentration of OH groups foimd by H-D exchange and from the high temperature weight loss based on thermogravimet-ric analysis. 

Figure 5.25. Changes in the NMR spectrum of metakaolinite on the room temperature formation of sodium polysialate geopolymer and its subsequent service at high temperatures. Note the immediate conversion to tetrahedral A1 upon room-temperature polymerisation, and the thermal stability of the tetrahedral A1 in the geopolymer framework structure. The 13 ppm resonance at 1300°C arises from a small amount of corundum (a-Al203). Based in part on Barbosa et al. (2000).

Silica materials have been studied extensively because of the structural flexibility of silica (through Si04 tetrahedral connections), easy control of hydrolysis and polymerization of silica species, high thermal stability of silica framework, easy modification of the silica surface, and well known silica and zeolite chemistry. Amorphous silica is also the main inorganic component for certain natural materials obtained from bioassembly, such as diatoms. Various mesoporous silica materials have been reported, which are very important for both fundamental research and applications. 

The intracrystalline channel cavity-pore-cage system in zeolites is surrounded by the lattice and therefore is submitted to the zeolite crystal field. This results in solvent-like and even electrolyte-type properties. One has seen above how cations could be easily exchangeable. It may also exist an interaction between any occluded ionic compound and the zeolitic framework. Salts, especially salts of univalent anions, have been shown to penetrate the zeolite structure and fill the available space even if the openings of the cavities (as the 0 -ring of 0.24 nm in size in sodal te cage of Y zeolite) is smaller than the size of the anion (CIO, NO for instance). The interesting feature is then the enhanced thermal stability of the occluded salt. 

Additional elements or functional groups may be incorporated into the anionic framework of the VOPO class in order to increase framework stability or to provide additional stmctural flexibility. The presence of aluminum in the framework, for example, results in enhanced thermal stability of the phases. Borate serves not only as a framework constituent but as an effective solubilizer of the metal oxide components. Addition of fluoride to silicates and aluminophosphates and gaUophosphates (188-191) results in enhanced mineralization and induces crystallization in neutral and acidic pH. While VOPO phases incorporating such additional constituents are relatively unexplored, the few... 

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