Acid dealumination, modification zeolites

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. 

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. 

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... 

The introduction of RE elements in the zeolitic component, via ion exchange, followed by a calcination step, is one of the most important modifications carried out in the process of FCC catalyst preparation, as it increases both the stability of the zeolite and the overall activity of the catalyst [12]. Furthermore, calcination promotes framework dealumination, changing the acidic and textural characteristics of the zeolite. 

In terms of chemical modifications, treatment of ZSM-5 with phosphorus compounds [42,43] seems to be an interesting route to enhance the selectivity to light olefins in cracking reactions. It has been demonstrated that after phosphorus treatment, the strong acid sites of the original zeolite are replaced by an increased number of weaker acid sites, whose concentration increases after steam treatment. Finally, the combined treatment phosphorus/ REs [44] results in an improvement in both stability and activity. Apparently, REs reduce aromatics formation on the external surface area of the zeolite, whereas phosphorus reduces the loss in activity caused by dealumination. 

Recently [211], a post-synthesis modification of zeolite beta consisting of separate dealumination and titanation steps has been reported. First hydroxyl nests were formed by removal of up to 90% of the aluminum by leaching with oxalic or nitric acid, than up to 2 wt.% titanium was inserted into the lattice vacancies without formation of Ti02 as a second phase by treatment with gaseous TiCl4 at 500 °C. 

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