Acid dealumination, modification

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... 

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). 

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. 

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). 

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]. 

The Si/Al ratio plays a significant role, since the aluminum atom is directly related to the acidic site and accounts for the formation of carbenium and/or carbonium ions or possibly cation radicals inside the zeolite. Dealumination processes can promote porous structure modifications, which may improve some interesting properties of zeohtes, like thermal and hydrothermal sta-bihty, acidity, catalytic activity, resistance to aging and low coking rate, and... 

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