Dealuminated zeolite

Sodium decreases the hydrothermal stability of the zeolite. It also reacts with the zeolite acid sites to reduce catalyst activity. In the regenerator, sodium is mobile. Sodium ions tend to neutralize the strongest acid sites. In a dealuminated zeolite, where the UCS is low (24.22°A to 24.25°A), the sodium can have an adverse affect on the gasoline octane (Figure 3-7). The loss of octane is attributed to the drop in the number of strong acid sites. 

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... 

D correlation analysis is a powerful tool applicable to the examination of data obtained from infrared spectroscopy. The correlation intensities, displayed in the form of 2D maps, allow us to correlate the shift induced by CO adsorption and acidity of sites in dealuminated zeolites. Results are in accordance with previous results, obtained using only IR measurements, proving the validity of this technique. New correlations allowed the assignment of very complex groups of bands, and 2D correlation revealed itself as a great help for understanding acidity in dealuminated zeolites. 2D correlation has allowed us to validate the model obtained by NMR. 

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. 

In view of catalytic potential applications, there is a need for a convenient means of characterization of the porosity of new catalyst materials in order to quickly target the potential industrial catalytic applications of the studied catalysts. The use of model test reactions is a characterization tool of first choice, since this method has been very successful with zeolites where it precisely reflects shape-selectivity effects imposed by the porous structure of tested materials. Adsorption of probe molecules is another attractive approach. Both types of approaches will be presented in this work. The methodology developed in this work on zeolites Beta, USY and silica-alumina may be appropriate for determination of accessible mesoporosity in other types of dealuminated zeolites as well as in hierarchical materials presenting combinations of various types of pores. 

MEDISORBON An adsorptive process for removing mercury and dioxins from flue-gas. The adsorbent is a dealuminated zeolite Y manufactured by Degussa. For mercury removal, the zeolite is impregnated with sulfur. Developed in 1994 by Lurgi Energie und Umwelt and piloted in Germany and The Netherlands. 

Seeding technique. Al-free Ti-beta obtained by use of dealuminated zeolite-beta seeds Fluoride method. Al-free Ti-beta synthesis from a reaction mixture containing TEAOH and fluoride ions (HF) at near-neutral pH. Gel composition Ti02 60SiO2 32.9NEt4OH 32.9HF 20H2O 457.5 H20. Crystallization at 413 K with rotation of the autoclave (60 rpm)... 

An interesting variation on sulfated metal oxide type catalysts was presented by Sun et al. (198), who impregnated a dealuminated zeolite BEA with titanium and iron salts and subsequently sulfated the material. The samples exhibited a better time-on-stream behavior in the isobutane/1-butene alkylation (the reaction temperature was not given) than H-BEA and a mixture of sulfated zirconia and H-BEA. The product distribution was also better for the sulfated metal oxide-impregnated BEA samples. These results were explained by the higher concentration of strong Brpnsted acid sites of the composite materials than in H-BEA. 

SURFACE COMPOSITION AND DEPTH PROFILE OF DEALUMINATED ZEOLITES (43,44)... 

When 1,2-dichlorobenzene in hydrogen-saturated deionized water was exposed to a slurry of palladium catalyst (1%) at room temperature, benzene formed via the intermediate chlorobenzene. The reaction rate decreased in the order of MCM-41 (mesoporous oxide having a silicon aluminum ratio of 35) > alumina > Y (dealuminated zeolite having a silicon aluminum ratio of 15). It appeared the reaction rate was directly proportional to the surface area of the support catalyst used (Schiith and Reinhard, 1997). 

In heavily steam-dealuminated zeolites, most of the activity should come from the EFAL that is concentrated on the external siirface (3,4). We believe that Levyis acidity, which can stabilize radicals, can play an important role in the radical cracking observed with strongly steam-dealuminated HY zeolites. Furthermore,... 

Catalytic Behaviour of "leached Steam-Dealuminated Zeolites... 

The fact that catalysts prepared from hydrothermally and chemically dealuminated zeolites are similar may be related to the catalyst... 

In 1994, Laketic et al. investigated for the first time the hydrolysis of sucrose over dealuminated-Y-zeolites with different Si/Al ratio (27, 55, 110). The best activity was observed with the highest dealuminated zeolite (Si/Al = 110, rate constant = 1.28 L moE min at 30°C [32]). As expected, the reaction obeys first order kinetics. Up to 90% conversion was obtained after 9 h of reaction at 70°C. Interestingly, little side-products were formed as the selectivity in glucose was higher than 90%. 

In order to avoid the acid-catalyzed dehydration of sorbitol to isosorbide, Corma et al. reported a new innovative three-step cascade route involving (1) an acetaUza-tion of sorbitol, (2) an esterification of sorbitol, and (3) a deacetalization of sorbitol esters (Scheme 14) [131]. In this case, esters of sorbitol were obtained. At 408 K, between 70 and 90% of conversion of sorbitol was obtained. Activity of the zeolites employed (H-Beta, mordenite, and dealuminated zeolite ITQ-2) was dependent on both their acidity and pore structure. The modemite catalyst has emerged as the most efficient. 

In the present work, the Jacobsen s catalyst was immobilized inside highly dealuminated zeolites X and Y, containing mesopores completely surrounded by micropores, and in Al-MCM-41 via ion exchange. Moreover, the complex was immobilized on modified silica MCM-41 via the metal center and through the salen ligand, respectively. cis-Ethyl cinnamate, (-)-a-pinene, styrene, and 1,2-dihydronaphtalene were used as test molecules for asymmetric epoxidation with NaOCl, m-CPBA (m-chloroperoxybenzoic acid), and dimethyldioxirane (DMD) generated in situ as the oxygen sources. 

Different immobilization methods were applied for Jacobsen s catalyst. The entrapment of the organometallic complex in the supercages of the dealuminated zeolite was achieved without noticeable loss of activity and selectivity. The immobilized catalysts were reusable and did not leach. For the oxidation of (-)-a-pinene the system used only O2 at RT instead of sodium hypochloride at 0 °C. There was a disadvantage in the use of pivalic aldehyde for oxygen transformation via the corresponding peracid. This results in the formation of pivalic acid, which has to be separated from the reaction mixture. 

Recently, a novel CF MAS NMR-UV/Vis technique (Fig. 17, Section III.B) was applied to characterize the formation of hydrocarbons by the conversion of methanol on a weakly dealuminated zeolite HZSM-5 6S). The C MAS NMR spectrum recorded at 413 K during the continuous conversion of C-enriched methanol (Fig. 37a, left) consists of signals at 51 and 61 ppm attributed to methanol and DME, respectively. The very weak signal at ca. 23 ppm is probably an indication of alkanes or alkylated cyclic compounds. The appearance of the signals at 23 and 61 ppm indicates that the conversion of methanol on weakly dealuminated zeolites HZSM-5 starts even at 413 K. The simultaneously recorded UV/Vis spectrum (Fig. 37a, right) consists of bands at 275, 315, and 375 nm. The band at 275 nm indicates the formation of neutral aromatic compounds 301,302), and those at 315 and 375 nm may be assigned to mono- and dienylic carbenium ions (301,302), respectively. Because the UV/Vis spectrum of the non-dealuminated zeolite HZSM-5, that... 

Fig. 37. C CF MAS NMR (left) and UV/Vis (right) spectra of a dealuminated zeolite HZSM-5 recorded during conversion of ( -enriched methanol (W/F = 25gh/mol) at 413 K for 2h (a), during a subsequent conversion of T l U = T l U (W/F = 10 g h/mol) at 413 K for 1 h (b), and during conversion of V H2 = V H2 (W/F= 10 g h/mol) at 413 K on a fresh catalyst for 2 h (c). Asterisks denote spinning sidebands. The narrow peaks at ca. 500 nm in UV spectra were caused by the equipment. Reproduced with permission from 168). Copyright 2004 The Royal Society of Chemistry.

The weakly dealuminated zeolite HZSM-5 used to convert methanol was subsequently applied to investigate the conversion of ethylene ( C-isotopes in natural abundance) (Fig. 37b). MAS NMR signals, appearing at 14, 23, and 32 ppm during conversion of ethylene at 413 K for 1 h (Fig. 37b, left), are assigned to alkyl groups of small amounts of alkylated cyclic compounds, such as cyclopentene, cyclohexene, cyclohexadiene, and/or benzene. The simultaneously recorded UV/Vis spectrum (Fig. 37b, right) shows bands at 300 and 375 nm, which characterize the formation of neutral cyclic compoimds and dienylic carbenium ions, respectively (301). 

The simultaneous investigation of the methanol conversion on weakly dealuminated zeolite HZSM-5 by C CF MAS NMR and UV/Vis spectroscopy has shown that the first cyclic compounds and carbenium ions are formed even at 413 K. This result is in agreement with UV/Vis investigations of the methanol conversion on dealuminated zeolite HZSM-5 performed by Karge et al (303). It is probably that extra-framework aluminum species acting as Lewis acid sites are responsible for the formation of hydrocarbons and carbenium ions at low reaction temperatures. NMR spectroscopy allows the identification of alkyl signals in more detail, and UV/Vis spectroscopy gives hints to the formation of low amounts of cyclic compounds and carbenium ions. 

Spectral information on several dealuminated zeolites (91), as well as on ZK-4 and TMA-sodalite studied by Jarman (104), who also considered the quantitative relationship between 6 and 6, is given in Table IX. The average values of 0 for hydrothermally dealuminated zeolite Y and dealuminated acid-washed mordenite, as determined by X-ray diffraction, are very close to those in nondealumina ted materials, and Thomas et al. (56) assumed that this is also true for other dealuminated zeolites. They proposed the following linear correlation between 0 and S (see Fig. 30) ... 

Fyfe et al. (155) measured high-resolution solid-state 27A1 MAS NMR spectra of a number of zeolites at 104.22 MHz. All the spectra contained one narrow peak with a chemical shift ranging, in different materials, from 51.5 to 65.0 ppm from A1(H20) + (see Table XIII). In dealuminated zeolites an additional signal was observed corresponding to six-coordinated Al in the zeolitic channels. 

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