Conventional dealumination

Macedo et al. [227] studied HY zeolites dealuminated by steaming, and found that the strength of intermediate sites decreased with increasing dealumination for Si/Al ratios varying from 8 to greater than 100. For comparison, isomorphously substituted HY, which is free of extra-framework cationic species, possesses more acid sites than conventionally dealuminated solids with a similar framework Si/Al ratio [227], This is because some of the extra-framework aluminum species act as charge-compensating cations and therefore decrease the number of potential acid sites. 

These results provide evidence that conventional dealumination or dehydroxylation at elevated temperatures forms strong Lewis acid sites at the... 

In agreement with previous studies, microcalorimetry confirms that in steamed products, most of the strong acid sites are poisoned by cationic extra-framework A1 species. These sites can be recovered by an optimized post-steaming acid leaching. Isomorphously substituted HY which is free of extra-framework cationic species possesses more acid sites than conventional dealuminated solids with similar framework Si/Al ratio. 

Conventional dealumination. NH is known to be a non-selective probe it can interact with Bronsted as well as with Lewis sites (30). Under such conditions the amounts of ammonia which interact strongly (P = 1 torr) with the (HT), (HTA1) and (HTA2) solids cannot be directly attributed to the neutralization of the framework Bronsted sites. To address that problem a. possible preliminary appro t is to evaluate NHg/Al and NH /Al ratios (where Al and A1 stand respectively for the total amount of aluminium present in the solids and the amount of framework A1 atoms). 

This general scheme has the advantage of predicting the behaviour of other solids not studied here. It thus tqtpears ftn- instance that it will be very difficult to improve the porosity of hi silica zeolites by conventional dealumination treatments if the as-synthesized zeolites have a low density of structural defects. 

An uitrastabie or a dealuminated zeolite (USY) is produced by replacing some of the aluminum ions in the framework with silicon. The conventional technique (Figure 3-9) includes the use of a high temperature (1,300-1,500°F [704-816°C]) steam calcination of... 

As a conclusion of this section, it can be said that the method used has to be carefully chosen according to the sample studied and/or the expected results. Conventional XRD may be sufficient to localise a single cation species in a dehydrated zeolite whereas for bicationic zeolites more elaborate techniques like anomalous XRD or MAS and MQMAS NMR may be necessary. If the focus of the study is more on the influence of adsorbed molecules on the distribution of the cations, neutron scattering may be needed to complete the work. Finally, highly dealuminated zeolites may be difficult to study with diffraction techniques, in this case NMR techniques may be the best available option. 

The microwave technique has also been found to be a potential method for the preparation of the catalysts containing highly dispersed metal compounds on high-porosity materials. The process is based on thermal dispersion of active species, facilitated by microwave energy, into the internal pore surface of a microporous support. Dealuminated Y zeolite-supported CuO and CuCl sorbents were prepared by this method and used for S02 removal and industrial gas separation, respectively [5], The results demonstrated the effective preparation of supported sorbents by micro-wave heating. The method was simple, fast, and energy-efficient, because the synthesis of both sorbents required a much lower temperature and much less time compared with conventional thermal dispersion. 

Zeolite-based processes have gradually displaced conventional ones, involving supported H3P04 or A1C13 as catalysts, in the manufacture of cumene, the raw material for phenol production [1, 6, 39]. A three-dimensional dealuminated mordenite (3-DDM) catalyst was developed by Dow Chemical for this purpose [39]. Dealumination, using a combination of acid and thermal treatments, increases the Si/Al ratio from 10-30 up to 100-1000 and, at the same time, changes the total pore volume and pore-size distribution of the mordenite. The... 

The chemical shifts are taken relative to the aluminium of a fully ionised salt. i.e. the chemical state [A1(H20)5]. With this convention, hexa-co-ordinated aluminium resonates between -15 and +15 ppm and tctra-co-ordinated aluminium between 40 and 75 ppm. Some authors identify penta-co-ordinated aluminium atoms in the region of 40 ppm but this attribution remains subject to caution. It thus appears possible, using aluminium NMR, to monitor, qualitatively, the distribution of these various species during dealumination (self-steaming. acid attack, washing), for example, in a mordenite H (cf. spectrum in Fig. 13.7). 

Dealuminated M-Y zeolites (Si/Al = 4.22 M NH4, Li, Na, K, Cs) were prepared using the dealumination method developed by Skeels and Breck and the conventional ion exchange technique. These materials were characterised by infrared spectroscopy (IR) with and without pyridine adsorption, temperature-programmed desorption (t.p.d.) of ammonia. X-ray difiracto-metry (XRD) and differential thermoanalysis (DTA). They were used for encapsulation of Mo(CO)5. Subsequent decarbonylation and ammonia decomposition was monitored by mass spectrometry (MS) as a function of temperature. The oxidation numbers of entrapped molybdenum as well as the ability for ammonia decomposition were correlated to the overall acidity of the materials. It was found that the oxidation number decreased with the overall acidity (density and/or strength of Bronsted and Lewis acidity). Reduced acidity facilitated ammonia decomposition. 

Figure 2 demonstrates the variations in the Nai values per unit cell (u.c.) for hydrogen faujasites and mordenites preheated at a temperature between 400 and 1000°C. The results clearly show that considerable extraction of the framework A1 occurs at temperatures as low as 500-600°C. For example, 65-75% of the framework A1 was lost by NH4Y zeolites and mordenites with conventional Si/Al ratios, heated to 800°C. Reducing the initial A1 content of the frameworks makes the release of the aluminum on heat treatment increasingly difficult, and for the dealuminized samples DY4.8 and DM62 removal of the framework A1 does not exceed a 5% level. 

The dealumination of zeolite MCM-22 by calcination or hydrothermal treatment has been investigated by conventional multinuclear solid-state NMR and checked by ultrahigh-field NMR experiments (19.6 T) with a fast spin rate (19.1 kHz). The presence or variation of different species, sueh as silanol nest, 4-coordinated framework, 6-coordinated and 5-coordinated extraframework aluminium, during or after dealumination has been detected, and their... 

Strong Acid Sites of Dealuminated Y Zeolites Prepared by Conventional Treatments and Isomorphous Substitution... 

The acidity of dealuminated Y zeolites prepared either by conventional treatments or isomorphous substitution has been characterized in detail through a microcalorimetric study of the adsorption of ammonia. 

It is important at this stage to emphasize that the isomorphic substitution by (NH J SiF gives solids with limited dealumination levels up to about 60 %, which corresponds to unit cell parameters not smaller than about 2.442 nm (26,27), whereas a total framework dealumination could be easily achieved by conventional treatments (unit cell parameter down to 2.423 nm (4)). It is not entirely clear at the moment, whether this limited dealumination level for the (IS) solids results from the experimental conditions we used or is specific to the fluorosilicate technique. Nevertheless this situation renders difficult the detailed comparison between the acidic properties of solids with identical framework dealumination levels prepared by the two techniques. However, as will be seen later, important trends can be derived. 

The Bronsted acid sites of HY zeolites dealuminated either by conventional treatment (steaming + acid leaching) or isomorphous substitution (fluorosilicate) have been characterized at each step of the preparation procedures through IR spectroscopy of probe molecules with various basic strengths (pyridine, C2H4,... 

H2S). In contrast to pyridine, which cannot easily discriminate between OH groups with similar acid strength, C2H4 and H2S lead to a much more specific interaction. Whatever the dealumination procedures, the structural high-frequency OH is likely to be the most acidic and its acid strength smoothly increases with the framework dealumination level. Some of the extra-framework Al-OH groups present in conventional HY treated at low temperature indeed possess a pronounced acidic character since they can interact with C2H, or H2S. Finally two components at 3738 and 3743 cm-1 appear to contribute to the Si-OH at 3740 cm-1 they correspond... 

Samples of zeolite Beta with various degrees of dealumination were prepared via various treatments by Yang and Xu [263]. These authors provided framework spectra of their samples, obtained in the region 1400-400 cm through the conventional FTIR/KBr pellet technique. A band at about 950 cm was attributed to Si-O vibrations in structural vacancies (cf. also [264]) and monitored in dependence on the treatment. 

InSect. 5.2.2.10, the work by Kubelkovaetal. [271] onH-Y, Ce,Na-Y and dealuminated Ce, Na-Y was already mentioned, which was carried out employing the DRS/KBr pellet technique. These authors, however, obtained also DRS spectra of the powdered materials and compared their results with those of the conventional IR transmission spectroscopy of self-supporting wafers. They found that some of the OH bands observed by DRS were shifted to lower wavenumbers and attributed this effect to the higher temperature produced by the DRS beam in the powdered samples. In an investigation of hydrothermally treated and, thus, dealuminated H-ZSM-5 Martin et al. [429] observed by the pyridine/IR technique an elimination of Bronsted sites accompanied by a condensation of extra-frame-work Al-containing species (Lewis centers). 

A more in-depth study on FAU-structiu e zeolites with high siUca content by Beyerlein et al. [108] addresses the influence of extra-framework species (EFAL) on the catalytic activity for the isobutane conversion. Overall, the cracking activity increased linearly with the concentration of framework aluminum sites for all catalysts. However, FAU samples dealuminated with ammonium hexafluorosilicate (AHF) showed a lower activity compared to conventionally hydrothermally treated catalysts. Steaming of the chemically dealuminated catalysts led to an increased activity in cracking isobutane compared to the parent FAU. This is interpreted as an indication that extraframework aluminum species, typically acting as Lewis add sites, are interacting with Bronsted acid sites to form new and stronger addic sites. 

A. Macedo, A. Auroux, F. Raatz, E. Jacquinot, R. Boulet et al.. Strong acid sites of dealuminated Y zeolites prepared by conventional treatments and isomorphous substitution. ACS Symp. Sen 368, 98-104 (1988)... 

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