Si-O-Al bonds

Thermal dealumination. The method involves calcination of the ammonium (or hydrogen) form of the zeolite at relatively high temperatures (usually over 500°C) in the presence of steam. This results in the expulsion of tetrahedral aluminum from the framework into non-framework positions, but does not remove the aluminum from the zeolite. The process consists essentially in a high-temperature hydrolysis of Si-O-Al bonds and leads to the formation of neutral and cationic aluminum species (Figure 1A). 

Reaction with chelating agents. Such reactions have been used primarily for partial dealumination of Y zeolites. In 1968, Kerr (8,21) reported the preparation of aluminum-deficient Y zeolites by extraction of aluminum from the framework with EDTA. Using this method, up to about 50 percent of the aluminum atoms was removed from the zeolite in the form of a water soluble chelate, without any appreciable loss in zeolite crystallinity. Later work (22) has shown that about 80 percent of framework aluminum can be removed with EDTA, while the zeolite maintains about 60 to 70 percent of its initial crystallinity. Beaumont and Barthomeuf (23-25) used acetylacetone and several amino-acid-derived chelating agents for the extraction of aluminum from Y zeolites. Dealumination of Y zeolites with tartaric acid has also been reported (26). A mechanism for the removal of framework aluminum by EDTA has been proposed by Kerr (8). It involves the hydrolysis of Si-O-Al bonds, similar to the scheme in Figure 1A, followed by formation of a soluble chelate between cationic, non-framework aluminum and EDTA. 

Thus the probability of a Si-0 Si connecting bond is (Qs-Qa>/Qs = 1-1/R and of a Si-O-Al bond is QA/QS = 1/R where R = QS/QA = Si/Al atom ratio. This completely determines the environment of each atom and therefore the relative intensities of the five 29Si NMR lines in a structure consisting of some assumed distribution of building blocks. 

Here 1q4( ai) is the intensity of Q (nAl) species. Thus the Si NMR intensity of each Si-O-Al bond is equivalent to one-quarter Al atom, as each Al atom is surrounded by four other Si atoms. Figure 4.37 illustrates Si NMR of Y zeoHte sample and... 

The presence of water and amines at high temperatures may produce a process equivalent to a shallow bed calcination, and this could explain the formation of silanol groups by partial breaking of the Si-O-Al bonds in the zeolite framework. Probably, both types of occluded TEA exist in the zeolite. 

The most important factor in zeolite synthesis in the laboratory, or factory, is the rate of crystallization. Composition and concentration of the liquid solution acting on the solids is important to the process as is the absolute necessity of maximum disorder of the Si-O-Al bonds in the initial solids reacted (Zhdanov, 1970). It is thus evident that not only bulk chemical (equilibrium) factors are important in the initial crystallization of zeolites but also the. relative free energies of the reactants. It is apparent that zeolite equilibria are essentially aqueous i.e., that silicate equilibrium or approach to it is attained through reaction with solutions, and thus the solubilities of the solids present are of primary importance. If materials are slow to enter into solution they are essentially bypassed in the rapid crystallization sequence (Schwochow and Heinze, 1970 Aiello, et al , 1970). In most studies the zeolites precipitated from solution appear to respond to the laws concerning chemical activity of solutions (Zhdanov, 1970). 

Preparation of polv-alumazane. During impregnation the surface silanol groups of the silica are thought to react with A1C13 yielding Si-O-Al bond structures under evolution of hydro-... 

It is believed that, when steaming the gel at high temperatures, the V0+i attacks and breaks the Si-O-Al bonds promoting mullite formation and the collapse of the gel macroporous structure (3). The XRD pattern in Fig. 2B shows that mullite formation in the gel can be observed with only 1.5% V and when this occurs, there is a 81% decrease in surface area, Table 1. Mullite level increased with V-loadings, see Fig. 2. Data in the literature (20) indicates that when the steaming temperature is decreased to 730 C from 760 C (as in the present work) gel stability to V improved and only a 23% reduction in surface area was observed in a similar gel loaded with 1.5% V. Aluminosilicate gels are clearly less resistant than aluminas to V attack at hydrothermal conditions, Table 1. 

Oxycations of V (V02 or VO ) can easily penetrate the gel microspace and break Si-O-Al bonds causing the collapse of the gel microstructure and mullite formation even at low (1.5%) V levels. Excess V generates an x-ray amorphous, V205-like species which is highly hydrated and less susceptible to dnemical reduction than bulk... 

Zeolites and inorganic ion-exchange catalysts have been treated in reactive milling to alter their properties. For example, the amorphization of Zeolite A has been described by Kosanovic et al. [93] these authors observed a loss of crystallinity, a decrease of cation-exchange capacity, and an increase of solubility of A and Y zeolites which was caused by breaking of Si-O-Si and Si-O-Al bonds in the zeolite. 

Further, the polymerization constant ks (referring to the formation of a siloxane bond) and the ionization constant Ki are assumed not to be modified by the presence of aluminum atoms. The ka value, corresponding to the formation of a Si-O-Al bond, aas estimated by iteration according to values originating from a semi-quantitative experimental study (29) and aas found to be 35. The follo-aing results are based upon an example from this study. 

A closer look at the mechanism of hydrothermal dealumination (hydrolysis of Si-O-Al bonds) illustrates that a healing process also takes place during this transformation (3), but contrary to the chemical substitution highlighted above, the source of Si originates from the zeolite, and results in a partial destruction of the zeolites and Si migration. The mesoporosity created during the process is beneficial to the diffusion of the large molecules of oil. 

Obeying Loewenstein s rule (forbidden Al-O-Al linkages), zeolitic materials give rise to quite simple 27 1 NMR spectra, consisting of signals due to only one type of tetrahedral aluminium environment [A1 (OSi)4] besides evtl. octahedrally coradinated aluminium. No relationships between the chemical shifts and the Si, A1 ordering or Si/Al ratio have been established, whereas relations between the 27 1 chemical shifts and mean Si-O-Al bond angles exist, with shift values carefully corrected for quadrupolar shift contributions (11). 

Fig. 10.7. a) Si-O-Al bonding in zeolites b) representation of (a) with the intersections representing Si or A1 atoms and the lines the O bonded to them c) oxygen atoms in the ring. 

Figure 4.29. Effect of grinding on the Si spectra of the alumina-silica system. A. Mixture of gibbsite, Al(OH)3 and silica gel, and B. Diphasic gel of mullite composition. Note in both systems the evidence of Si-O-Al bond formation (peak at about — 84 ppm). Adapted from Temuujin et al.

MHz, much smaller than in sodium and calcium glasses (Table 6.11). This is probably because the increased strength of interaction with the bo of the trivalent ion decreases the Si-O-Al bond angle and hence decreases the Xq value. 

Zeolites with the pentasil structure are more V-tolerant than HY (or CREY) and collapse when steam-aged in the presence of V, forming cristobalite and vanadia. Thus, it is believed that vanadium (VO or V02 ) preferentially attack Si-O-Al bonds in zeolites. 

During steaming, hydrolysis products of compounds (such as V2O5) generates acids (such as H4V2O7) that further promote Si-O-Al bond breakage, dealumination reactions, and therefore lattice collapse. 

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