Framework hydroxyls

In addition, some samples also gave a signal at 7.1 ppm from the residual NH4 cations the amount of the latter was determined by thermodesorption and subtracted from the intensity of line (3). Thus the sum of intensities of (2) and (3) gave the true total content of acidic hydroxyl groups. They had T2 of 60-75 ftscc, while sample 500 SB contained an additional free induction decay (FID) component due to extra-framework hydroxyls. 

Hopkins (161) found that a steady decrease in n-heptane cracking activity occurred over La- and Ca-exchanged Y zeolites as the catalyst calcination temperature was increased from 350° to 650°C. The lanthanum form was about twice as active as the calcium form. Reduction in activity with increasing activation temperature was attributed to removal of acidic framework hydroxyl sites as dehydration becomes more extensive. The greater activity of La—Y with respect to the calcium form was thought to result from the greater hydrolysis tendency of lanthanum ion, which would require more extensive dehydration to result in the same concentration of acidic OH groups as found on Ca—Y. 

In a previous paper (14) we have shown that by an adequate treatment of steam dealuminated Y zeolites with (NH4)2SiF6 it is possible to remove selectively the EFAL. The physicochemical characterization of these samples shows that the presence of EFAL is the responsible for the formation of superacid sites, as well as for the neutralization of a part of framework hydroxyls. 

Besides from the non-acidic SiOH and POH and the acidic HF and LF hydroxyls, the presence of two bands at 3612 and 3530 cm has been related to the presence of organic templates interacting with HF and LF framework hydroxyls respectively. 

Pt-mordenite catalyst for isomerizing nCs-Q alkanes with an optimal acidity (presence of very strong protonic sites due to interaction of framework hydroxyl groups with extraframework aluminum species)... 

It should also be noted that the decrease in the micropore volume induced by the grafting of the cationic zeolites is half that observed for the corresponding protonic zeolites. This result provides argument for the involvement of framework hydroxyl groups in the anchoring reaction. 

Dealumination and silicon-enrichment reaction of (NH SiF6 (AHFS) with zeolites Besides the hydrothermal method for preparation of ultra-stable Y zeolite (USY), Breck and Skeels[23] in 1983 invented a new secondary synthesis method for silicon-enriched zeolites. This method uses an ammonium hexafluorosilicate solution to remove the aluminum atoms from the framework structure of Y zeolite to the solution, and to insert silicon atoms back into the Al-removal vacancies in the framework so as to form a more or less perfect Y zeolite with a high Si/Al ratio. In comparison with the USY prepared by the hydrothermal method, the framework silicon-enriched Y zeolite obtained through the current technique possesses fewer framework hydroxyl vacancies, and the resulting zeolite has an ideal crystal lattice, and hence higher structural stability. Meanwhile, there... 

The remaining 0-H bands in Figure 2 lie at frequencies below the silanol stretch and have been attributed to the framework hydroxyl groups that exhibit Bronsted acidity. The prominent bands at 3610 cm-l for H-ZSM-5 and 3630 cm" for the AFS-H-ZSM-5 arise from the 0-H stretch of a particularly active M-OH-Si hydroxyl site (M = A1 and Fe, respectively). The shift to higher frequency when the Al(III) is replaced by Fe(III) reflects the reduced Bronsted acidity of AFS-H-ZSM-5 compared to that of H-ZSM-5 (1.0). ... 

Framework Hydroxyls. The structural nature of framework hydroxyls is particularly important because they probably act as Bronsted acid catalysts. The literature on catalysis is much too extensive to be reviewed here, and comments will be restricted to x-ray and infrared evidence on the location and nature of framework hydroxyls. Reviews of infrared data are given in this volume by Rabo and Poutsma and by Ward (see also 66, 67),... 

The structural framework of silica [5] is based on interconnected Si(-0->4 tetrahedra, with some OH terminations at the surface. Aluminas are built primarily of A1(-0-)4 tetrahedra and AK-O-) octahedra, with some surface-OH terminations [6]. The fundamental framework structure of aluminosilicates include primarily Si(-0-)4 and AK-O-) units plus charge compensating bridging or framework hydroxyl groups, namely structure I, which shows formal charges on oxygen and aluminum [7]. On the basis of these kinds of structures and various so-called defect structures that may include octahedral and three-coordinate aluminum sites, the investigation by NMR of a wide variety of nuclei can be anticipated. Numerous NMR studies based on Si, Al, H, H, and O have been reported on silica, alumina, and silica-alumina systems. 

Among the factors controlling catalytic activity a fundamental role is played not only by zeolite topology, but also by the type of cation, its location and coordination. Of particular importance is the location and population of proton sites of framework hydroxyls in hydrogen zeolites. 

Spectroscopic data (IR and NMR in particular), as well as the information given by quantum-chemical computations, can be extremely useful, when supported by structural data, for recognizing Bronsted sites. In our opinion, if all this information is taken into account together, it is possible in many cases to locate framework hydroxyls, even if neutron diffraction data are not at our disposal. 

Micro calorimetric studies of ammonia adsorption were used to investigate the acidic properties of the small-pore zeolite H-Rho [216]. Shallow-bed calcination of NH4-Rho at 673 K or 773 K produced a nearly pure H-Rho with highly acidic framework hydroxyls as BrOnsted acid sites having an initial heat of adsorption of ammonia of about 170 kj mol [217]. Shallow-bed calcination at 873 K and 973 K destroyed ca. 25% and ca. 50% of the framework hydroxyls, respectively, producing a small number of strongly acidic Lewis sites (ca. 185 kJ mol ) but also weakly acidic sites having a heat of adsorption of ammonia of ca. 60 kJ mol [216]. 

Because of the negatively charged framework of zeolites, basic framework hydroxyl groups do not exist [55]. Moreover, positively charged frameworks have not been reported for molecular sieve structures so far. Basic hydroxyl... 

In some instances the materials were also repeatedly subjected to this two-step treatment to enhance the degree of dealumination, e.g., in [89]. In the case of zeolites resistant to acids due to high framework Si/Al ratios, e.g.,mordenite and ZSM-5, acid leaching may also precede the hydrothermal treatment [ 10,23]. In this case, healing of framework hydroxyl nests left by aluminum release is the lattice-stabilizing process. The hydrothermal stability of steamed Y zeolites was found to increase upon removal of extra-framework aluminum by extraction with acids [94]. 

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