Calcined zeolites

As-made zeolite Partially detemplated Calcined zeolite... 

Figure 4. (a) Adsorption-desorption isotherms of N2 at -196°C of 80°C-outgassed (empty squares) chitosan, (filled trangles) zeolite X-chitosan composite from in-situ zeolite synthesis and (empty triangles) zeolite Y-chitosan composite from encapsulation of the zeolite in the gelling chitosan. (b) Scanning electron micrographs of a calcined zeolite-chitosan bead prepared by zeolitisation of a silica-chitosan composite. 

A modification of the above cyclic method has proved more effective in the dealumination of Y zeolites. An almost aluminum-free, Y-type structure was obtained by using a process involving the following steps a) calcination, under steam, of a low-soda (about 3 wt.% Na O), ammonium exchanged Y zeolite b) further ammonium exchange of the calcined zeolite c) high-temperature calcination of the zeolite, under steam d) acid treatment of the zeolite. Steps a) and c) lead to the formation of ultrastable zeolites USY-A and USY-B, respectively. Acid treatment of the USY-B zeolite can yield a series of aluminum-deficient Y zeolites with different degrees of dealumination, whose composition depends upon the conditions of the acid treatment. Under severe reaction conditions (5N HC1, 90°C) an almost aluminum-free Y-type structure can be obtained ("silica-faujasite") (28,29). 

As an example. Fig. 18 shows CP/MAS NMR spectra recorded during the investigation of surface ethoxy species (7S) formed on acidic zeolite HY ( si/ Ai = 2.7) by a SF protocol. Figure 18a shows the CP/MAS NMR spectrum recorded after a continuous injection of C-1-enriched ethanol, CHI CHzOH, into the MAS NMR rotor reactor containing calcined zeolite HY. The ethanol was injected at room temperature for 10 min. Subsequently, the loaded zeolite was purged with dry nitrogen (200 mL/min) at room temperature for 2h. 

Isotopic labeling is a powerful tool being used to understand the nature of transition states as well as the mechanisms of surface-catalyzed reactions. By in situ MAS NMR spectroscopy, the scrambling of labels at room temperature was investigated upon adsorption of p C-1]- 1-octene on calcined zeolite HZSM-5 (wsi/... 

The conversion of methanol to hydrocarbons (MTHC) on acidic zeolites is of industrial interest for the production of gasoline or light olefins (see also Section X). Upon adsorption and conversion of methanol on calcined zeolites in the H-form, various adsorbate complexes are formed on the catalyst surface. Identification of these surface complexes significantly improves the understanding of the reaction mechanism. As demonstrated in Table 3, methanol, dimethyl ether (DME), and methoxy groups influence in a characteristic manner the quadrupole parameters of the framework Al atoms in the local structure of bridging OH groups. NMR spectroscopy of these framework atoms under reaction conditions, therefore, helps to identify the nature of surface complexes formed. 

In a recent work (207), in situ ll and Al MAS NMR spectroscopy was used to investigate the Bronsted acid sites of zeolite HZSM-5 during the conversion of methanol under CF conditions. As a consequence of the application of MAS instead of the spin-echo technique, only framework Al atoms involved in quadrupolar interactions according to Cqcc < 5 MHz could be observed. Upon injection of methanol into the MAS NMR rotor reactor filled with calcined zeolite HZSM-5, a single Al MAS NMR signal arose at 54 ppm at reaction temperatures up to... 

Fig. 24. MAS NMR spectra recorded in the steady state of the synthesis of MTBE by isobutylene and methanol on calcined zeolite H-Beta (nsi/tiAi = 16) (a—c) and after purging of the catalyst with dry nitrogen (d). Reproduced with permission from (230). Copyright 2000 Elsevier Science.

Fig. 26. Y MAS NMR spectra of calcined zeolite CsOH/Cs,NaX recorded after loading with a mixture of selectively C-enriched toluene (CgHl CHs) and C-enriched methanol in a molar ratio of 3 1 at 295K (a) and upon heating to 573 K (b) and 723 K (c). Reproduced with permission from (255). Copyright 2000 Elsevier Science.

Fig. 28. CF MAS NMR spectra recorded during the conversion of pure CHsOH (left) and an aniline/ CHsOH mixture (right) on calcined zeolite ( sOH Cs.XaY (W/F = 40 gh/mol, molar ani-line/ CH3OH ratio of 1 4) at reaction temperatures of 473 K (a), 498 K (b), and 523 K (c) and at 523 K after stopping the reactant flow (d). Reproduced with permission from (242). Copyright 2001 The Royal Society of Chemistry.

Fig. 32. CF MAS NMR spectra recorded during the conversion of C-enriched methanol ((U/ F = 100 gh/mol) on calcined zeolite HY (nsi/ Ai = 2.7) at reaction temperatures of 393 K (a), 433 K (b), and 473 K (c). The spectra on the left-hand side were obtained with MAS NMR and proton decoupling (HPDEC), and the spectra on the right-hand side were recorded with the CP technique. Asterisks denote spinning sidebands. Reproduced with permission from (74). Copyright 2001 American Chemical Society.

Fig. 36. C and H CF MAS NMR spectra recorded during the alternating conversion of C-enriched and non-enriched methanol (IV/F= 25gh/mol) on calcined zeolite HZSM-5 (nsi/wAi = 22) at reaction temperatures of 548 and 573 K. The relative intensities given below the spectra were determined by integration in the absolute intensity mode by using the spectra obtained during the conversion of C-enriched methanol as intensity standard. Reproduced with permission from (300). Copyright 2003 Kluwer Academic.

The reactions were carried out in a 0.1 liter stirred autoclave operating in a batch mode and equipped with a system for sampling of liquid during the reaction. The autoclave was charged with the substrate together with solvent and the freshly calcinated zeolite (calcination overnight at 500°C or 400°C in air, prior to use) and then heated to the desired temperature. The amounts of zeolite and of distilled water depend both on the nature and the concentration of substrate. Samples were withdrawn periodically and analyzed by g.l.c..Two typical examples are described below ... 

The reactions were carried out in glassware equipment (0.1 liter two-neck flask equiped with a water condenser) with a magnetic stirrer and under nitrogen atmosphere. In a typical experiment, a solution of the epoxide (8.3 mmol) in 40 ml toluene (0.208 M) was heated by means of a thermostated oil bath to the appropriate temperature the freshly calcinated zeolite, 250 mg, (calcination at 500°C or 400°C in dry air overnight, prior to use) was then added to the solution, the moment of addition corresponding to the start of the kinetic measurements (t =0). Samples were withdrawn periodically and analyzed by g.l.c. (DELSI 30, capillary column 0V1, 30m, carrier gas Hg). 

The MFI class of channel zeolites, of which ZSM-5 is a member, are of enormous importance in the petrochemicals industry because of their shape-selective adsorption and transformation properties. The most well-known example is the selective synthesis and diffusion of p-xylene through ZSM-5, in preference to the o- and m-isomers. Calcined zeolites such as ZSM-5 are able to carry out remarkable transformations upon normally unreactive organic molecules because of super-acid sites that exist... 

Electron Microscopy. Several samples were examined by transmission electron microscopy (TEM) in a Philips 430T operating at 300 kV. Samples were prepared by ultra-sonically dispersing the calcined zeolite in ethanol, and collecting the dispersed zeolite crystallites on a standard 200 mesh 3mm Cu grid covered with a thin (-20 nm) amorphous carbon film. 

Results from the filtrate titrations of the calcined zeolite samples treated in salt solution show that a large fraction of the acidic hydroxyl groups have been lost at temperatures as low as 400°C. An additional 35% decrease in acidity occurs when the calcination temperature is increased from 400°C to 700°C, and almost no acidity remains after calcination at 1000°C. Maintenance of crystallinity in all samples was confirmed by X-ray powder diffraction data. The NH4+-exchanged ZSM-5 forms a minor amount of weaker acidity attributable to hydroxoaluminum cations, which is not found in the H30+-exchanged material. Both forms exhibit substantial strong acidity due to the formation of H30+. 

Acidity measurements were obtained either from the slurry or from the resulting filtrate of the calcined zeolite treated with concentrated NaCl solution. Approximately 3 g (anhydrous weight) of the calcined zeolite were treated in 50 cnH of 3.4 molar... 

NaCl solution. In one set of experiments, the slurry was titrated with 0.1 normal NaOH solution in one cm increments to a pH of about 10. The samples of this set are referred to as the "slurry" samples (titration of the calcined zeolite - NaCl solution slurry). In another set of experiments, the calcined zeolite - NaCl solution slurry was filtered, the filter cake washed with about 100 cnP distilled water, and the combined filtrates were titrated with 0.1 N NaOH solution, again to a pH of about 10. The samples of this latter set are referred to as the "filtrate" samples (the zeolite being removed by filtration prior to titration of the filtrate). In addition to the manual titrations, automated potentiometric titration curves were obtained with a Metrohm E636 Titroprocessor, which has an estimated pH measurement accuracy of 0.001 pH unit and an estimated volumetric addition accuracy of 0.001 cm ... 

It should also be remembered that the chemistry we are describing is kinetically very fast. It takes place in seconds, is a consequence of, and merely reflects what happened during the prior thermal treatment of the sample. If attack on framework A1 were to commence only upon contact with the NaCl solution, the extent of hydrolysis apparently observed after just 10 seconds would be kinetically unlikely. Moreover, no further change can be seen over the next several hours. Given these circumstances, it is virtually inconceivable that some of the Al-O-Si bonds were not already ruptured when the calcined zeolite came in contact with the concentrated NaCl solution. 

Table VI. Hexadecane Cracking Results for Calcined Zeolites. Product Slate at 50% Conversion...

Calcined AFS and USY zeolites show significant differences in both selectivity ratios whereas steamed zeolites show similar light gas selectivities. However, steamed selectivities are dramatically different from those of calcined zeolites. These results are in good qualitative agreement with results obtained for AFS and USY zeolites by gas-oil cracking (17). 

Gasoline selectivity is influenced by both the method of dealumination and steam treatment and, hence, depends on framework acid sites and on presence of extraframework material. Both framework and extraframework sites contribute to the overall zeolite acidity. Framework Al(IV) acid sites are associated with Bronsted acidity whereas extraframework Al(VI) acid sites are associated with Lewis acidity(21). Calcined AFS samples are reported (22) to contain greater Bronsted acidity than USY samples from infrared characterization and to have stronger acidity as measured by ammonia desorption. As a result, the stronger acidity of calcined AFS samples cracks hexadecane to lower molecular weight products than USY. After steaming, the acidities of both AFS and USY are reduced to similar levels and lead to similar gasoline selectivities which are increased relative to the calcined zeolites. 

As shown in Figure 7, calcined zeolites with similar unit cell sizes demonstrate different octane-producing behavior. Hence, structural characteristics such as extraframework aluminum and spatial distribution of acid sites are important factors affecting zeolite acidity. After steaming, the octane behaviors of AFS and USY zeolites are similar reflecting similarities in zeolite acidities. 

Appearance of Fluorescence in Raman Spectroscopy of Calcined Zeolites (24)... 

The physical properties of the two samples of H-ZSM-11 (1) and (2) and the catalytic activity for naphthalene alkylation with methanol are listed in Table 2. The surface area of the zeolites were measured by the BET method using nitrogen as the adsorbate. Calcined zeolites were used in order to obtain the total surface and uncalcined zeolite template n-Bu4N" for the external surface area, according to Sato [18]. From the data presented in the Table 2 it follows that the reaction takes place on the external surface because the naphthalene conversion was higher over H-ZSM-11(2) than over H-ZSM-ll(l). 

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