Aluminum sites

Unit Cell Size (UCS). The UCS is a measure of aluminum sites or the total potential acidity per unit cell. The negatively-charged aluminum atoms are sources of active sites in the zeolite. Silicon atoms do not... 

Aluminum siting in the framework of silicon rich zeolites. A ZSM-5 study... 

The location of boron or aluminum sites in zeolites is of utmost importance to an understanding of the catalytic properties. Due to the inherent long-range disorder of the distribution of these sites in most zeolites, it is difficult to locate them by diffraction methods. The aforementioned methods to measure heteronuclear dipolar interactions can be utilized to determine the orientation between the organic SDA and A1 or B in the framework. The SDA location may be obtained by structure refinement or computational modeling. For catalytic reactions, the SDA must be removed from the pores system by calcination. 

In cases where the antisite defects are balanced, such as a Ga atom on an As site balanced by an As atom on a Ga site, the composition of the compound is unaltered. In cases where this is not so, the composition of the material will drift away from the stoichiometric formula unless a population of compensating defects is also present. For example, the alloy FeAl contains antisite defects consisting of iron atoms on aluminum sites without a balancing population of aluminum atoms on iron sites. The composition will be iron rich unless compensating defects such as A1 interstitials or Fe vacancies are also present in numbers sufficient to restore the stoichiometry. Experiments show that iron vacancies (VFe) are the compensating defects when the composition is maintained at FeAl. 

At high temperatures the spinel MgAl204 can take in excess alumina to a composition of approximately 70 mol% A1203 (Fig. 4.5). (a) What are the possible formulas that fit the composition of this spinel Write the defect formation equation for the reaction if the excess A1 is (b) distributed over both magnesium and aluminum sites and (c) only over aluminum sites. Assume that there is no electronic compensation in the insulating oxide. 

Solomon (3, h, 5.) reported that various clays inhibited or retarded free radical reactions such as thermal and peroxide-initiated polymerization of methyl methacrylate and styrene, peroxide-initiated styrene-unsaturated polyester copolymerization, as well as sulfur vulcanization of styrene-butadiene copolymer rubber. The proposed mechanism for inhibition involved deactivation of free radicals by a one-electron transfer to octahedral aluminum sites on the clay, resulting in a conversion of the free radical, i.e. catalyst radical or chain radical, to a cation which is inactive in these radical initiated and/or propagated reactions. 

Equation (3)]. Polymerization of PO also proceeds rapidly to give the low molecular weight PPO [J/n = 4050 g mol— 1.2]. Since the molecular weight of polymer indicates that less than 10% of aluminum sites are active for producing the polymer, an oligomeric aluminum complex is supposed to be an active species although the characterization is not fully achieved. In this system, exclusion of a coordinated compound such as THE is essential for the high catalytic activities. Thus, aluminum complex prepared by the reaction of APBus and methylphosphonic acid in the presence of THE exhibits much less catalytic activity. 

There are two aluminum sites in the unit cell one has sixfold coordination and the other threefold coordination. The chromium preferentially goes into the sixfold coordinated position and has an energy-level scheme like that of ruby. 

The activity of parent H-mordenite is essentially Bronsted activity, and aluminum sites are effective proton counters (2). In parent H-mor-denite (11.2 wt % alumina), acid centers are present at a density of 100 times those in aluminum-deficient H-mordenite (0.1 wt % alumina). 

Jacobs and Uytterhoeven (199) and Maher et al. (195) indicated that 38 aluminum atoms remained in framework positions in ultrastable samples. It was suggested that each aluminum site in ultrastable zeolites gives rise to one strong acid site (179). 

Mild steaming enhanced the activity in the MPV reaction and was accompanied by an IR band at 3782 cm 1 assigned to an OH group attached to an Al.40 A recent infrared study gives convincing evidence that this 3782 cm 1 band is related to a 885 cm1 band and belongs to a defect Lewis-acidic aluminum site grafted to the Beta framework.41... 

To conclude this section, let us consider the results of ab initio calculations of the chemisorption of a H2 molecule on a trigonal aluminum site (16). Two types of shift of the stretching vibration frequencies of H2 were observed in... 

Figure 2 A portion of the faujasite framework. Each vertex represents either a sdicon or an aluminum site, each line represents a bridging oxygen. The opening into the supercage is approximately 8 A in diameter it consists of 12 metal sites and 12 bridging oxygens...

One of the disadvantages of EXAFS in general is that it is an averaging technique. Thus it is very important to make sure the zeolites are very well defined. Other techniques such as Al MAS NMR spectroscopy and in situ infrared spectroscopy are needed to ensure single aluminum sites. Depending on the data quality it is also possible to define more than one type of site however, in this case... 

Table 3. Structural data for AI13, GaAhs and GeAhs selenate or sulfate crystals. The subscript abbreviations are T = tetrahedral O = octahedral. The standard deviation for each bond parameter, when available, is given in parentheses and corresponds to the last place in the value. Data for AIb and GaAln are from Parker et al. (1997) and data for the GeAfs are from Lee et al. (2001). A range of values is given for the P2-Off and P2-OH sites in the GeAl molecule because two crystallographically distinct octahedral aluminum sites appear in the structure of this crystal, but not in the Alls or GaAli2 salts.

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