Dealumination, modification

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

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). 

We have characterized HY zeolites dealuminated by different procedures and at different levels and have shown that the proportion of the different types of EFAL can be controlled. Furthermore, all the zeolite modifications clearly influence the product distribution during gas-oil cracking. 

Extensive studies of the acidity and basicity of zeolites by adsorption calorimetry have been carried out over the past decades, and many reviews have been published [62,64,103,118,120,121,145,146,153,154]. For a given zeolite, different factors can modify its acidity and acid strength the size and strength of the probe molecule, the adsorption temperature, the morphology and crystallinity, the synthesis mode, the effect of pretreatment, the effect of the proton exchange level, the Si/Al ratio and dealumination, the isomorphous substitution, chemical modifications, aging, and coke deposits. 

We have studied the effect of chemical modification on adsorption properties of natural clinoptilolite (18). Studies of water vapor adsorption show a decrease in adsorption for Dzegvi clinoptilolite, decationized and dealuminized on the water bath, with increased acid concentration, compared with the adsorption of the natural clinoptilolite. The main contribution to adsorption is from primary porosity. 

Hydrophobic zeolites, as well as the all-silica zeolites or zeolites with a very small aluminum content, possess high capacity for adsorbing organic compounds dissolved in water. Some recent studies demonstrated that hydrophobic, dealuminated zeolites adsorbed organic compounds from water as effectively as activated carbon [2,37,88,89,214], The hydrophobicity of zeolites is controlled basically by changing the Si/Al ratio in the framework by synthesis conditions and postsynthesis modification treatments [215],... 

In both cases, framework Al may be exposed to water vapour at rather high temperature (525-875 K), which can lead to dealumination of the zeolite structure (production of nonframework Al species and decrease in the concentration of acid sites, modification of sorptive properties and catalytic behaviour). In order... 

The acid properties of zeolites can be modified by various treatments ion exchange, dealumination etc. which can be carried out in different ways. As was shown for the most classical methods, the effect of these treatments on the characteristics of the acid sites is generally complex. Indeed they provoke also modifications of the pore system which can favor or limit the diffusion of reactant and product molecules hence influence the catalytic properties. All these effects being well-known, it is relatively easy to tailor zeolites for obtaining active, stable and selective catalysts for desired reactions. 

Similarly, zeotype molecular sieves are synthesized by mixing the basic ingredients with the organic template, e.g. aluminophosphates are prepared from alumina and phosphoric acid. Other main group or transition elements can be incorporated into the framework by adding them to the initial sol-gel. Alternatively, different elements can be introduced by post-synthesis modification (see later), e.g. by dealumination followed by insertion of the new elements into the framework position [31]. 

The effect of dealumination can be observed in a series of commercial Y-faujasites with Si Al ratios of 2.6, 5.8, 12.8 and 24. The modification of the acidity resulting from the dealumination is illustrated by the curves of differential heats of ammonia adsorphon (Figure 9.16). 

Amazingly, under hydrothermal conditions in the temperature range of 373 K up to 493 K the hydrophilic high-alumina NaCaA and NaY zeolites are significantly more stable than the hydrophobic high-silica modifications DAY-S and DAY-Tg generated by dealumination. In steam of saturated pressure a shift of the stability region by 100 K and more can be observed... 

Non-contact atomic force microscope (AFM) and N2 absorption measurements on beta zeolites reveal the extreme irregularity of the external crystal surface which can make up a considerable proportion of the total surface area. A catalytic test, the acylation of 2-methoxynaphthalene, shows that active sites on the outer surface play an important role in the catalytic activity of the zeolite. Attempts to influence the external surface area and its catalytic activity through synthesis or post-synthesis modification such as dealumination show that the principle influence on the external surface comes from the synthesis procedure. 

Studies on a series of beta zeolites show that, depending on the synthesis parameters, very small irregular crystallites with high ESAs can be obtained and that subsequent modification such as calcination and dealumination does not change the... 

Hydrothermal treatment of zeohtes is an essential step in the preparation of US-Y based catalysts. It results in signiflcant modification of the faujasite structure (dealumination and partial destruction of the zeohte fi-amework) and of the acidic properties. Increased hydrothermal stabflity of the framework, lower concentration and higher strength of the add sites compared with the initial zeohte are the most important properties of the steamed catalysts [1,2,12]. 

Variable-temperature Si H CP/MAS-NMR spectroscopy was used to study the effect of molecular oxygen on the location of sorbate molecules in highly-siliceous zeolite framework, e.g. ZSM-5 with adsorbed />-dibromoben-zene.662 13C MAS-NMR spectra were used to follow the conversion of methanol on weakly-dealuminated zeolite H-ZSM-5 - showing the formation of cyclic compounds and carbonium ions.663 There is 13C CP/MAS-NMR evidence for surface ra-alkoxyl groups formed by the modification of the proton-ated perovskite HCa2Nb3Oi0 by w-alcohols.664 13C CP/MAS-NMR spectra... 

Nearly all syntheses of zeolites and microporous aluminophosphates have limitations to gel composition and other parameters. For example, some zeolites with special compositions such as high-silica Y zeolite and low-silica ZSM-5 cannot be directly synthesized. A secondary framework modification is necessary for their preparation. For instance, dealuminization, isomorphous substitution of extraneous silicon for aluminum, and removal of the sodium process in Y zeolite are necessary to prepare ultra-stable zeolite Y (USY) isomorphous replacement of framework atoms of boron with aluminum in a presynthesized silicon-boron structure is often used to prepare some specific aluminosilicate zeolites that cannot be directly synthesized, such as Al-SSZ-24 (AFI) and Al-CIT-1. Secondary synthesis (post-treatment) will be discussed in detail in Chapter 6. 

Zeolite dealumination is one of the most important subjects in the field of zeolite secondary synthesis and modification. For decades, zeolite scientists have been investigating dealumination routes and techniques in order to optimize the properties and functions of zeolites. Generally, there are three dealumination routes presented as follows ... 

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