Catalytic properties, dealuminated

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. 

The preparation methods of aluminum-deficient zeolites are reviewed. These methods are divided in three categories (a) thermal or hydrothermal dealumination (b) chemical dea-lumination and (c) combination of thermal and chemical dealumination. The preparation of aluminum-deficient Y and mordenite zeolites is discussed. The structure and physico-chemical characteristics of aluminum-deficient zeolites are reviewed. Results obtained with some of the more modern methods of investigation are presented. The structure, stability, sorption properties, infrared spectra, acid strength distribution and catalytic properties of these zeolites are discussed. 

Ab initio methods, 147-49 Acetate ion, decomposition, 135 Acetylene, interaction with palladium, tunneling spectroscopy, 435,437f Acid-dealuminated Y zeolites catalytical properties, 183 sorption, 175-78 Acid sites, on zeolites, 254 acidification effects, 266 Acoustic ringing, in NMR, elimination, 386 Active sites, nature, 104 Activity measurements, Co-Mo catalysts, 74 Adsorbed molecules,... 

The synthesis of Ti-mordenite has been conducted by reaction of TiCl4 with dealuminated mordenite or by hydrothermal synthesis (Section IV.F). The evidence for the incorporation of titanium is limited. The UV-visible spectra show that, in addition to the transition at 48,000 cm- , assigned to isolated Tiiv in tetrahedral coordination, there is also an absorption at 35,000 cm-, indicating extra-framework Ti02. The catalytic properties in oxidation reactions with H202 are significantly different from those of Ti02 deposited on mordenite, but they are limited to the hydroxylation of benzene and the oxidation of w-hexane (Kim and Cho, 1993). 

The incorporation of TiIV in the crystal lattice of silicalite has been attempted by the reaction of TiCl4 with dealuminated ZSM-5 (Kraushaar et al., 1988) or deborated borosilicalite (Carati et al., 1990). The same reaction has been used in the attempt to incorporate titanium in the crystal lattice of zeolite beta, morde-nite or zeolite Y. In many cases catalytic properties have resulted, but the way in which the incorporation takes place has been questioned. Because of its molecular dimensions, TiCl4 cannot enter or leave the pore system of ZSM-5. It has been shown that 89% of the OH groups present in the preformed zeolite as SiOH remain unreacted after treatment at 573 K with TiCl4. The incorporation of titanium must therefore be limited to the outer part of the crystals or proceed through a severe chemical attack with removal of silicon and formation of a secondary pore system (De Ruiter et al., 1993). Deposits of Ti02 on the outer part of the crystal treated with TiCl4 have indeed been observed (Carati et al., 1990), as has abnormal behavior in the oxidation of phenol (Section V.C.3.c). 

Beyerlein et al. (33) studied the catalytic properties of a series of ultrastable synthetic faujasites dealuminated by steaming and by acid extraction to determine catalytic acidity as a function of framework characteristics. They found that carbonium-ion activity in isobutane conversion is proportional to framework-Al content, and comparing results obtained by using hydrothermally and AHF-dealuminated synthetic faujasite, they found that the steamed material, which contains extra-framework Al, gave a large increase in carbonium-ion activity compared with the AHF-treated material, which had a relatively clean framework. This indicates that strong acidity exhibited by mildly steamed synthetic faujasite, while directly related to framework-Al content, depends on a balance between framework and extra-framework Al, and that this extraframework Al contributes greatly towards catalytic performance. 

The NMR data on clean dealuminated samples of zeolite Y indicate that with good sample preparation and the advanced techniques for structural studies, it will be possible to elucidate further the relationship between structural features and catalytic properties. ... 

In summary, dealumination with (NFLj SiFe leads to samples differing not only by their framework Si/Al ratio but also by other characteristics gradient of Si/Al in the crystallites, SiC>2 deposited on their outer surface, etc. It should however be noted that the differences between the parent and dealuminated samples are here much less pronounced than with dealumination by steaming. For identical values of the framework Si/Al values, large differences are furthermore found between the acid and catalytic properties of samples dealuminated by these two methods. 

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. 

If the framework structure of a zeolite remains constant, the cation exchange capacity is inversely related to thd Si/Al ratio. Furthermore, fine tuning of the adsorptive and catalytic properties can be achieved by adjustment of the size and valency of the exchangeable cations. Dealumination of certain silica-rich zeolites can be achieved by acid treatment and the resulting hydrophobic zeolites then become suitable for the removal of organic molecules from aqueous solutions or from moist gases. 

Dealuminated Y zeolites which have been prepared by hydrothermal and chemical treatments show differences in catalytic performance when tested fresh however, these differences disappear after the zeolites have been steamed. The catalytic behavior of fresh and steamed zeolites is directly related to zeolite structural and chemical characteristics. Such characteristics determine the strength and density of acid sites for catalytic cracking. Dealuminated zeolites were characterized using X-ray diffraction, porosimetry, solid-state NMR and elemental analysis. Hexadecane cracking was used as a probe reaction to determine catalytic properties. Cracking activity was found to be proportional to total aluminum content in the zeolite. Product selectivity was dependent on unit cell size, presence of extraframework alumina and spatial distribution of active sites. The results from this study elucidate the role that zeolite structure plays in determining catalytic performance. 

The structural features of dealuminated zeolite samples were characterized using X-ray powder diffraction, porosimetry and solid-state NMR measurements. Hexadecane cracking was used as a probe reaction to investigate catalytic properties of pure zeolites. 

Characterisation and Catalytic Properties of Dealuminated Zeolite-Y A Comparison of Ammonium Hexafluorosilicate and Hydrothermal Treatments... 

Phosgene proved to be an appropriate reactant to remove aluminium from the zeolite framework [1,2]. Detailed measurements have been carried out to investigate the adsorption of phosgene [3], the mechanism of dealumination by infrared spectroscopic and thermogravimetric methods [4], structural consequences [5], adsorption and catalytic properties of modified mordenites [6]. In addition an NMR study has been carried out, to measure the thermal stability of so-called hydroxy nests in H-mordenite dealuminated with phosgene [7]. 

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. 

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,... 

The catalytic properties of dealuminated Y and EMT-type zeolites for the vapor phase transalkylation reaction of phenol with the trimethylbenzenes were investigated. The influence of the reaction temperature, the degree of dealumination and the concentration of acid sites, and the nature of the TMB isomer, were taken into account. High catalytic performances (activity and selectivity to cresols) and good resistance to coking of the dealuminated zeolites were observed. 

The adsorptive and catalytic properties of zeolites HY, HZSM-5 and HM (natural and synthetic) subjected to dealumination, ion exchange with rare-earth and transition elements have been studied.The changes in conversion and selectivity for w-xylene and n-pentane are shown to be connected with the changes of the zeolites microporous structure.These changes are in conformity with DR equation parameters. The DR equation could be applied to the simple test method elaboration of the starting and modified zeolites microporous structure through their adsorptive properties. 

P-ll - Texture of dealuminated mordenite catalysts modified with cerium and catalytic properties in the isopropylation of biphenyl... 

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