Nonframework aluminum

It has already been mentioned that the formation of ultrastable Y zeolites has been related to the expulsion of A1 from the framework into the zeolite cages in the presence of steam (8,9), and the filling of framework vacancies by silicon atoms (11,12). This results in a smaller unit cell size and lower ion- exchange capacity (6). It also results in a shift of X-ray diffraction peaks to higher 20 values. Ultrastable Y zeolites prepared with two calcination steps (USY-B) have a more silicious framework than those prepared with a single calcination step (USY-A). Furthermore, since fewer aluminum atoms are left in the USY-B framework, its unit cell size and ion-exchange capacity are also lower and most of the nonframework aluminum is in neutral form (18). 

The most widely used zeolite in petroleum refining so far is Y zeolite. Currently, REUSY zeolite is the main active component of RFCC catalysts. However, in the course of hydrothermal preparation of ultrastable Y zeolite, nonframework aluminum debris formed by dealumination could block the channels thus influencing the ion-exchange ratio of rare earth as well as the accessibility of active sites [2],... 

Recently, RIPP has developed a proprietary method to modify the properties of ultrastable Y zeolite via a treatment for cleaning its pores (CP) [3], which include the selective removal of nonframework aluminum from zeolite pores by a novel acid treatment at optimized pH and temperature conditions. 

Tables 5.1 and 5.2 hst the main physicochemical properties of the modified zeolite characterized by a series of analyzing methods. XRF, XRD, and Al NMR results listed in Table 5.1 showed that with the increasing intensity of CP treatment, nonframework aluminum was ranoved gradually with httle influence on zeohte framework (unit cell size (UCS) changed little), thus the relative crystallinity increased. The removal of nonframework aluminum can also be verified by the FT-IR results shown in Figure 5.1, in which it can be seen that after CP treatment the intensity of the small peak at wave number 3660-3690 cm characterizing nonframework hydroxyl groups decreased step by step.

The modified zeolite was then ion exchanged with rare earth to prepare structure optimized Y zeolite (SOY). Due to the removal of nonframework aluminum debris from zeolite pores, SOY can obtain a rare earth content of about 8-10 wt% (by weight RF2O3), which is much higher than that of traditional RFUSY zeolite (2-4 wt%). ACF evaluation results showed that SOY zeolite catalysts can perform higher... 

NHj adsorption microcalorimetry was used by Shannon et al. [225] to follow the changes in acid sites of a H Y zeolite during dehydroxylation, framework dealumina-tion, and the formation of nonframework aluminum species. 

Perhaps an overly simple mechanism has been proposed by Kerr to explain the formation of the nonframework aluminum found in ultrastable faujasites prepared by the two methods just described 22,26). 

Equation (13) works well for materials with framework Si/Al ratios below ca. 10. For more siliceous zeolites, the 29Si MAS NMR spectrum is dominated by the Si (4 Al) signal and the estimation of composition becomes inaccurate. In these cases, the relative amounts of framework and nonframework aluminum can be estimated from 27A1 NMR spectra (Section HI,I). 

J. Scherzer Since we can expect that OH groups attached to nonframework aluminum will absorb in the same general region of the IR spectrum where absorption bands of structural hydroxyl groups of fau-jasite-type zeolites occur, their identification would be difficult because of possible band overlapping. This is true especially for materials with Structure III and IV since we have found that their IR spectra show a rather broad band in the 3600 cm"1 region. 

The preparation and application of practical catalysts usually require exposure to thermal or hydrothermal conditions that induce some degree of framework cation hydrolysis. In the case of zeolites, the hydrothermal manipulation of the aluminum between crystal framework and extra framework sites is the preferred method to optimize zeolite acidity and catalytic performance. The chemistry of these materials is complex. Namely, the change from framework aluminum to nonframework-aluminum species affects the intrinsic acidity of the remaining framework aluminum sites. In addition, the nonframework aluminum usually displays a catalytic activity of its own. Therefore, the interpretation of catalytic data obtained with such catalysts requires a detailed knowledge of the crystal chemistry, including the amorphous debris formed from framework aluminum hydrolysis. 

Nonframework aluminum in zeolites, which has typically an octahedral AlO coordination, gives rise to signals at about Oppm this means it is well separated from the tetrahedral... 

An important application of Al MAS NMR is the detection and characterization of nonframework aluminum species formed by various thermal or hydrothermal treatments of zeolites, as is usually applied in the dealumination processes or the preparation of the acidic H-forms of zeolites. Provided that all aluminum is visible in the spectrum, such that no signal intensity is lost as a result of very strong quadrupole line broadening, the relative proportions of framework and nonframework A1 in zeolites can be directly determined from the intensities of the signals at about 60 and Oppm. 

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