Steam-acid leaching

Dwyer et al. (43) have also reported that dealumination of Y zeolites by a steam/acid leaching process produces a more uniform composition than dealumination by EDTA. The later method caused a depletion of Al in the outermost surface layer, producing a compositional gradient in the zeolite crystals. The conclusions reached by J. Dwyer in his studies of aluminum-deficient zeolites using the FABMS method are summarized in Table IV. 

The catalytic behaviour of steam/acid leached Y zeolites appears to be similar to that of USY zeolites. Bremer et al. (64) have reported that at equal conversions, cracking of gas oil over a DAY zeolite results in lower coke yields as compared to those obtained over REY zeolites. 

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. 

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,... 

Fig. 34. NH -mordenite. Si MAS NMR spectra at (sample 1) 811 K and steamed acid leach, and (sample 2) calcinated and steamed at 811 K. (a, b) Upper spectra are without and (c, d) lower spectra with cross-polarization [84H3].

The acidity of a series of dealuminated mazzites with Si/Al ratios varying from 5 to 23, prepared by combined steam-acid leaching and ion-exchange... 

Figure 2.3 3D-TEM reconstruction of (a) severely steamed and subsequently acid-leached Y zeolite [22] and (b) desilicated ZSM-5 zeolite crystal [24]. The mesopores in the crystal are shown as lighter gray tones.

Understanding the adsorption, diffusivities and transport limitations of hydrocarbons inside zeolites is important for tailoring zeolites for desired applications. Knowledge about diffusion coefficients of hydrocarbons inside the micropores of zeolites is important in discriminating whether the transport process is micropore or macropore controlled. For example, if the diffusion rate is slow inside zeolite micropores, one can modify the post-synthesis treatment of zeolites such as calcination, steaming or acid leaching to create mesopores to enhance intracrystalline diffusion rates [223]. The connectivity of micro- and mesopores then becomes an... 

Beers, A. E. W., van Bokhoven, J. A., de Lathouder, K. M., Kapteijn, F. and Moulijn, J. A. Optimization of zeolite Beta by steaming and acid leaching for the acylation of anisole with octanoic acid a structure-activity relation, J. Catal., 2003, 218, 239-248. 

The acidity of high-silica zeolites produced either by direct synthesis or by chemical dealumination of parent zeolites (either by steaming and acid leaching or by SiCfi vapor treatment), which makes it possible in principle to extract the aluminum from the network without the structure collapsing, was extensively studied in the 1990s. 

Dealuminated by reaction with SiCl between 473 and 723 K followed by acid leaching with HQ. Steam dealuminated. 

Dealuminated by steaming between 973 and 1163 K followed by acid leaching with HCl. Values in parentheses are Si/Al ratios determined with NMR. 

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