Zeolite hydrothermal treatment

The properties of the zeolite play a significant role in the overall performance of the catalyst. Understanding these properties increases our ability to predict catalyst response to changes in unit operation. From its inception in the catalyst plant, the zeolite must retain its catalytic properties under the hostile conditions of the FCC operation. The reaclor/regenerator environment can cause significant changes in chemical and structural composition of the zeolite. In the regenerator, for instance, the zeolite is subjected to thermal and hydrothermal treatments. In the reactor, it is exposed to feedstock contaminants such as vanadium and sodium. 

Zeolites with lower UCS are initially less active than the conventional rare earth exchanged zeolites (Figure 3-5). However, the lower UCS zeolites tend to retain a greater fraction of their activity under severe thermal and hydrothermal treatments, hence the name ultrastable Y. 

In zeolite synthesis (ref. 2) an aqueous mixture containing a silicon source, an aluminum source, an alkali source (usually NaOH) is autoclaved and subjected to hydrothermal treatment. Hydrated Na-ions are then filling the pore system in the as-synthesized zeolite. In the case of relatively high Si/Al zeolites an organic template is required which is usually a tetraalkylammonium compound, applied as the bromide or the hydroxide. 

A preformed chitosan-silica composite with 60% weight inorganic part [7] is used as the source of silica for the zeolite synthesis. An alkaline solution of sodium aluminate (Na 2.1 M, Al 1 M) was used in three methods of preparation (A) beads of the chitosan-silica composite were stirred overnight in the aluminate solution, extracted and submitted to a hydrothermal treatment at 80 °C during 48h (B) beads of the chitosan-silica composite were immersed in the aluminate solution and the system underwent a hydrothermal treatment at 80 °C for 48h (C) beads of the chitosan-silica composite were stirred overnight in the aluminate solution, extracted, dried at 80 °C and exposed to water vapour at 80°C during 48h. 

In the X-ray powder diffraction patterns of the composites, the disappearance of the broad band centered at 22 °20, typical of amorphous silica, indicates that the zeolitisation of the mineral fraction of the parent composite was complete. In no diffraction pattern any sign of crystallised chitosan could be found. The two methods in which the silica-polymer beads were extracted from the aluminate solution after impregnation (methods A and C) allowed the formation of the expected zeolite X, with traces of gismondine in the case of the method C. The method B, in which excess aluminate solution was present during the hydrothermal treatment, resulted in the formation of zeolite A. 

A) Thermal or hydrothermal treatment of zeolites. This results in partial framework dealumination, but the aluminum remains in the zeolite cages or channels. 

Stability. Ultrastable Y zeolites, prepared by the hydrothermal treatment of ammonium Y zeolites, have considerable thermal and hydrothermal stability (6) The high... 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

Calcined and steamed FAU samples also have complex hydroxyl IR spectra. Figure 4.25 shows the difference between an ammonium ion-exchanged FAU before and after steaming and calcination. The very simple, easily interpretable hydroxyl spectrum of the ammonium exchanged FAU sample is transformed into a complex series of overlapping hydroxyl bands due to contributions from framework and non-framework aluminum atoms in the zeolite resulting from the hydrothermal treatment conditions [101]. 

DEALUMINATIQN OF ZEOLITE Y Dealumination is an important process to improve the thermal stability and resistance to acid of zeolite. This is one of the main techniques for preparing zeolite catalysts (US-Y). New pores (mesopores) have been introduced during hydrothermal treatment (Fig.4), which were directly confirmed by electron microscopy. The density of mesopores depended on the degree of dealumination and the size distribution of mesopores... 

Hydrothermal (steam) stability is also important, in as much as the catalyst must pass through a high temperature stripping zone in which the usual fluid stripping medium is steam. In our laboratory, zeolite hydrothermal stability is measured by comparing the x-ray crystallinity of the unknown faujasite sample with that of a fully rare earth exchanged reference standard following a 3 hour, 100% steam, 1500 F treatment. 

Figures 24-26 show the 29Si spectra of zeolites before and after hydrothermal treatment (91,92). The framework Si/Al ratio in the products was of the order of several hundred.

Mordenite samples (MOR) were obtained from different sources (ref.5). Zeolites beta (BETA), mazzites (MAZ), offretites (OFF) and ZSM5 were synthesized according to procedures described elsewhere (refs.10-12). Some parent offretites, mordenites and mazzites suffered hydrothermal treatments and acid leachings to obtain dealuminated materials (refs.13,14). A wide variety of samples were thus prepared with Si/Al ratios in the range BETA 6.3 < Si/Al < 31.5, ZSM5 13.2 < Si/Al < 44, OFF 3.4 < Si/Al < 26, MAZ 2.5 ( Si/Al < 5, MOR 4.4 < Si/Al < 39.5. The protonic forms of the zeolites, e.g. HZSM5, were obtained by calcination of the ammonium forms at different temperatures. 

A1 being removed from the samples. Despite repeated ammonium exchange followed by hydrothermal treatments, the bulk of the Al, as much as 58.8% for A8, is still in the zeolite interstices. For all the samples the framework-Si/Al ratio as determined by NMR is considerably higher than the Si/Al ratio measured by elemental analysis, in agreement with the presence of extra-framework Al. 

A third possibility for the synthesis of nanomaterials in constrained volumes is the use of molds (Figure 3.1c). Advantages of this method include its simplicity, versatility, and precise control over the shape of the solid, even with intricate forms. An elegant example of this strategy is the preparation of zeolites which precisely replicate the complex microstructure of wood. To do this, Dong et al. [43] infiltrated a zeolite synthesis solution into a wood sample. After the necessary hydrothermal treatment, and subsequent calcination to remove the template as well as the wood, a zeolitic structure was obtained that reproduced with full detail and fidelity the wooden sample used as a mold. 

In this section, a hydrothermal treatment that produces phase transformations in clinoptilolite and allows us to synthesize zeolites X and Y is described. Zeolites X and Y have the FAU-type framework however, the zeolite X has a silicon/aluminum ratio in the range 1 < Si/Al < 1.5 and zeolite Y has a silicon/aluminum ratio in the range 1.5 < Si/Al < =7.0. 

The hydrothermal treatment of natural zeolites [117,118] with highly basic sodium or potassium hydroxide solutions causes their amorphization and change in chemical composition (see Figure 3.10)... 

Additionally, the liquid phase produced by the hydrothermal treatment can be employed as a silica source for the preparation of aluminosilicate gels for the posterior synthesis of zeolites [24,120-122],... 

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