Acidity of zeolite

The investigation of the acidic properties of zeolites started with ammonium ion-exchanged Y-zeolites (NH4Y). Ammonium ion-exchanged Y-zeolite evolves ammonia and water by heat-treatment at 650 — 600 K, and 770 — 820 K, respectively. The transformation of NH4Y can be schematically expressed as follows. 

The stoichiometry of the transformation was confirmed by the amount of ammonia and water evolved. The transformation is also well evidenced by the change in the intensities of the OH stretching bands in the infrared spectrum. Fig. 3.64 shows the change in the intensities of OH stretching bands with heat-treatment temperature. The intensities of bands at 3540 and 3643 cm increase with the treatmet temperature up to 673 K, are constant at 673 — 773 K and decrease above 773 K. The band at 3740 cm behaves differently and is attributed to OH groups of the amorphous 

In fact, the intensities of OH bands decrease with temperature, indicating that equilibrium (1) shifts to the right side to reduce the number of OH groups at higher temperatures.  

Adsorption of polynuclear aromatics such as perylene on dehydroxylated zeolite 

It is now established that the local structure (I) in dehydroxylated zeolites is not stable and aluminum ions are easily dislodged from the zeolite framework and exist in the pores in the form of cationic species such as (AlO) or their polymeric form. -  

Weak to moderately strong acid sites can be generated in zeolites by ion exchange with multivalent cations. Owing to the polarizing effect of the metal cations, water is dissociatively adsorbed, and the equihbrimn of Equation 7-4 is estabhshed. 

The following order of Bronsted acidity is given for cation-exchanged zeohtes  

H form La form Mg form Ca form Sr form Ba form 

The influence of the exchanged ions is considerable, as shown by the example of cumene dealkylation on faujasite (Table 7-6). Reasons for the large differences in reactivity are the different charges on the ions, and the decreasing ionic radii from Na to and the associated polarizing power of the ions. 

The incorporation of transition metal ions into zeolites leads to interesting bifunctional catalysts in which metal and acid centers can act simultaneously. 

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The intrinsic acidities of zeolite samples are correlated with the Av0h values induced by adsorption of N2 (Av0h,n2), which can be determined by subtracting the frequency of the shifted OH-band from the frequency of the unperturbed OH-band as shown in fig.l. One shifted OH-band was observed for the ZSM-5 and mordenite samples, while the... 

The apparent acidities of zeolite catalysts are characterized by Av0h values induced by adsorption of hexane (Av0h.C6) under the same conditions than those applied during separate catalytic experiments. The Avoh,c6 values for the different zeolite samples shown in fig. 2 were determined as Figure 1. DRIFT spectra measured in the above for the nitrogen probe molecule. vOH region before (solid lines) and after (dashed lines) contacting the samples with N2 at 298 K and 9 bar equilibrium pressure. 

Various fluoride-containing reactants have also been used to dealuminate, dealuminate/reinsert silica and/or modify the acidity of zeolites for catalytic applications [132-137]. 

As mentioned above, many different probe molecules have been used to measure the acidity of zeolites. These molecules vary over a wide range of proton affinities, size (kinetic diameter) and shape. Table 4.4 lists these properties for several of the more commonly used probes. 

Larger probe molecules such as alkyl substituted pyridines and hindered amines have been used to probe only the external acidity of zeolites. The large kinetic diameter of these molecules prevents them from entering the pores of the zeolite... 

One of the parameters in the broad class of liquid adsorption mechanisms is the interaction between the acidic and basic sites of the adsorbent and the adsorbate. The acidity of zeolitic adsorbent is normally affected by the zeolite Si02/Al203 molar ratio, the ionic radii and the valence of the cations exchanged into the zeolite. In this contribution, Sanderson s model of intermediate electronegativity of zeolitic adsorbent acidity (SjJ can be calculated as a representation of the strength of the adsorbent acidity based on the following equation ... 

Humphries et al. [104] have given an excellent description of the surface acidity of zeolites and its influence on FCC. This topic has been comprehensively reviewed also by Shen and Auroux [105] and Auroux [103]. 

DEALUMINATIQN OF ZEOLITE Y Dealumination is an important process to improve the thermal stability and resistance to acid of zeolite. This is one of the main techniques for preparing zeolite catalysts (US-Y). New pores (mesopores) have been introduced during hydrothermal treatment (Fig.4), which were directly confirmed by electron microscopy. The density of mesopores depended on the degree of dealumination and the size distribution of mesopores... 

The technique of solid-state NMR used to characterize supported vanadium oxide catalysts has been recently identified as a powerful tool (22, 23). NMR is well suited for the structural analysis of disordered systems, such as the two-dimensional surface vanadium-oxygen complexes to be present on the surfaces, since only the local environment of the nucleus under study is probed by this method. The nucleus is very amenable to solid-state NMR investigations, because of its natural abundance (99.76%) and favourable relaxation characteristics. A good amount of work has already been reported on this technique (19, 20, 22, 23). Similarly, the development of MAS technique has made H NMR an another powerful tool for characterizing Br 6nsted acidity of zeolites and related catalysts. In addition to the structural information provided by this method direct proportionality of the signal intensity to the number of contributing nuclei makes it a very useful technique for quantitative studies. 

Lunsford et al. (202) used trimethylphosphine as a probe molecule in their 31P MAS NMR study of the acidity of zeolite H-Y. When a sample is activated at 400°C, the spectrum is dominated by the resonance due to (CH3)3PH+ complexes formed by chemisorption of the probe molecule on Bronsted acid sites. At least two types of such complexes were detected an immobilized complex coordinated to hydroxyl protons and a highly mobile one, which is desorbed at 300°C. (see Fig. 45)... 

Concerning the acidity of zeolites, Koltunov et al.133 have shown in a series of papers that reactions involving superelectrophiles could be achieved with excellent yields. 

Quantifying precisely the acidity of zeolites or other acidic solids is a goal, which up to now has not been satisfactorily achieved. The main problem is the lack of an acceptable scale of solid acidity comparable to pK scale for aqueous solutions or proton affinities for gas-phase reactions. For this reason all available physical and theoretical methods of investigation have been applied over the years on this subject and a large number of papers have been published. Several reviews are available. 

H Pfeiffer, D Freude, M Hunger, Nuclear Magnetic Resonance Studies on the Acidity of Zeolites and Related Catalysts, Zeolites 1985, 15, 274... 

The alpha values of synthetically prepared mordenltes are within the range of 10 -10°. There Ls only a small variation In a values of siliceous mordenlte as the aluminum content of the material varies. This Ls In contrast to dealumlnated mordenltes, which exhibit a much larger variation Ln alpha values as the aluminum content varies. These catalytic results on synthetic and acid dealumlnated mordenltes Indicate that factors other than the total aluminum content must contribute to the variation In activity of the catalysts. The linear correlation of alpha versus aluminum content reported for some zeolites (26) does not apply to the acid dealumlnated samples. Evidence has been presented for the presence of both Bronsted sites and Lewis sites enhancing the strong acidity of zeolite catalysts (27). The presence of extra-lattice aluminum Ln acid dealumlnated mordenlte samples was confirmed by 2 A1 NMR spectroscopy. The presence of both framework aluminum and extra-lattice aluminum Ln the acid treated materials may account for the wide variation Ln alpha values as a function of aluminum content. 

To understand differences in acidity of zeolites one has to consider not only changes in the bond strength of the OH bond, but also zeolite solvation effects on the positively charged carbenium or other ions generated by protonation. 

A simple cluster model of a bridged hydroxyl group in a zeolite is cluster 3. Such a cluster with A = H was used by Chuvylkin et al. (70) as early as 1975 to discuss the properties of possible intermediate structures in the catalytic isomerization of butenes on aluminosilicate surfaces in terms of CNDO/2 approximation. Mikheikin et al. (34) have used a similar cluster with terminal pseudo-atoms A to study the Bronsted acidity of zeolites and its dependence on the Si/AI ratio. 

Recently Jacobs (84, 85) has proposed another system of structural hydroxyls in zeolites based on the mean electronegativities of zeolite lattices according to Sanderson. It was concluded as well that the Bronsted acidity of zeolites varies continuously with their Si/Al content. Let us compare these two approaches. 

The ratio between the two main products 24 and 25 depends on the type and acidity of zeolite used. For example, H-BEA with a higher acid outer surface demonstrated a high activity but a low selectivity of 24 in comparison to H-FER or H-US-Y catalysts. 

Br0nsted acidity of zeolite protons is essential for catalytic reactions such as isomerization and cracking and has been studied extensively 15,264). Several characterization methods for acid sites in zeolites have been developed this subject has been covered in recent reviews (265,266). Pyridine and other basic molecules are often used in IR work as probe molecules for Brpnsted and Lewis acid sites (267). Trimethylphosphine has also been used as a probe for the determination of zeolite acidity by IR or NMR (96,268). 

Up to now, infrared spectroscopy has been used mainly to determine the types of hydroxyl groups and the acidity of zeolites (39). The frequencies of the vertical and horizontal vibrations (with respect to the cavity wall) of H2O molecules adsorbed in zeolite A were determined by measurements in the far infrared ( 220 and —75 cm" ) (37). These values are in agreement with a simple theoretical model. A number of ultraviolet and ESR studies are reviewed (33). The difference has been established between the specific molecular interaction of aromatic molecules on zeolites cationized with alkali cations and the more complex interactions involving charge transfer in CaX and deca-tionized X and Y zeolites. These more complex interactions with CaX zeolites containing protonized vacancies and with decationized zeolites are similar. These phenomena are related to the interactions of molecules with acidic centers in zeolites which are stronger, as compared with the molecular adsorption. 

As for the acidity of Al containing MCM-41, ammonia TPD data indicate that it is comparable to that of amorphous silica-alumina, and much lower than the acidity of zeolites such as USY or H-mordenite [115,120,134]. This is consistent with Raman, FTlK and Si NMR data which indicate that despite their long range order, M41S mesoporous silicates and aluminosilicates exhibit essentially amorphous walls [48,49,115]. 

In-situ NMR Studies with Zeolitic Materials Diffusion of adsorbed Molecules monitored by NMR Acidity of Zeolitic Materials Concluding Remarks... 

Variable temperature MAS NMR was used to characterize the structure and dynamics of hydrogen bonded adsorption complexes between various adsorbates and the Brpnsted acid site in H ZSM-5 the Brpnsted proton chemical shift of the active site was found to be extremely sensitive to the amount of type of adsorbate (acetylene, ethylene, CO and benzene) introduced (105). Zscherpel and coworkers performed maS NMR spectroscopic measurements in order to investigate the interaction between Lewis acid sites in H ZSM-5 and adsorbed CO. A new measure for the "overall" Lewis acidity of zeolites was derived from the C MAS NMR spectroscopic data. In addition, the chemical shift of CO adsorbed... 

The surface acidity of zeolites appears to be an important ingredient for polymerization of acetylene and derivatives. Thus, when diazomethane was used to remove the protonic acidic sites on HZSM-5, no evidence for acetylene polymerization remained, compared to the original acidic form. ... 

The acidity of zeolites can be reduced by the incorporation of boron in the zeolite framework [162,163] and therefore B-substitut ZSM-5, ZSM-11 and Beta were teaed [158,164]. Al " " free boron zeolites are inactive, but these zeolites with low levels of Al " " ions which can be obtained by adding AI2O3 binder to the Al free boron zeohte have weak acidity and are moderately active at 500 - 600°C and isobutene selectivities of up to 50 % have been reported. At these conditions the observed activity and selectivity of B/Al-ZSM-5, B/Al-ZSM-11 and B/Al-Beta were. similar and therefore it was concluded that the pore aructure did not play a decisive role in the converaon of n-butene into isobutene [164]. However, A1 which migrates into the pores not only modifies the acidity but also modifies the effective pore diameter. 

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