Dealumination, modification zeolites

Modification of Zeolites. The purpose of any modification is to improve catalytic properties. The most common modification is the dealumination of zeolites which has been shown to increase crystalline thermal stability. Three dealumination procedures can be used (35) chemical dealumination, thermal or hydrothermal dealuminatTon and combination of thermal and chemical dealuminations. 

Modification of the silica/alumina ratio of the Y-zeolite used in catalytic cracking catalysts can also improve its hydrocracking performance. It was foimd that dealuminated Y-zeolites give high diesel selectivity, whereas imtreated Y-zeoUte provides a hydrocracking catalyst with high selectivity for gasoline. 

Post-synthesis modification of zeolites via alteration of the alvunimun content of the framework became a most important topic of zeolite chemistry when, in the mid 1960s, the effect of stabilization through dealumination was discovered. In Chapter 3, H. K. Beyer contributes a systematic review on techniques for the dealumination of zeolites by hydrothermal treatment or isomorphous substitution amended by a section on the reverse process, i.e., introduction of aluminum into and removal of silicon from the framework. 

This review focusses on the preparational aspects of dealuminated Y zeolites and will not treat the catalytic implications of the modification. The dealumination method using Si Cl 4 was selected because with this reagent, in principle, Si atoms should be supplied rapidly and be easily substituted in the lattice, thus avoiding defect and mesopore formation. Perfect siliceous and low-alumina faujasites are expected to be formed. An attempt is made to rationalize the literature data on the dealumination methods using S1C14, to compare methods and product properties and to treat the chemistry involved in the different steps of the dealumination processes. 

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. 

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. 

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... 

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 ... 

Chemical Modif ic at ions of Zeol ites. Some chemical treatments may modify th zeolite material Vftholuit dealumination. The purpose of such treatments is either to dissolve some amorphous materials located within the channels or cavities as discussed above or to incorporate some chemical compound onto active sites within the channels or cavities or even to artificially introduce amorphous compound or bulky cations into the zeolite pores or channels. In the latter two cases it is purposedly desired to reduce the pore volume or the pore mouth resulting in larger diffusivity resistance and, subsequently, produce different catalytic properties. In the latter case the active sites may or may not be modified but the shape selectivity is expected to be enhanced by coating the inner walls of the pore and thereby increase resistance to diffusivity. For instance in the case of ZSM-5 type zeolite many compounds of P (53, 25), Mg (53), B have been introduced,... 

Alternatively, framework substitution can be achieved by post-synthesis modification of molecular sieves, e.g. via direct substitution of A1 in zeolites by treatment with TiCl4 in the vapour phase [34] or by dealumination followed by reoccupation of the vacant silanol nests. Boron-containing molecular sieves are more amenable to post-synthesis modification than the isomorphous zeolites since boron is readily extracted from the framework under mild conditions [35]. Synthesis of framework-substituted molecular sieves via post-synthesis modification has the advantage that it is applicable to commercially available molecular sieves which have already been optimized for use as catalysts. 

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