Characterization of zeolitic acidity

Other Techniques. - Numerous other techniques have been applied to the characterization of zeolite acidity. Umansky et al. developed a spectrophotometric method, using Hammett indicators, for measuring the surface acid strength of solid acids. A number... 

Adsorption of Probe Molecules for the Characterization of Zeolitic Acidity and Basicity... 

One of the outstanding properties of zeoHtes is their acidity. This is usually divided into Brqnsted and Lewis acidity that make them capable of being appHed in, e.g., cracking and shape-selective catalysis. Characterization of zeolite acidity comprises the determination of its nature, amount and strength and is com-... 

Di or trivalent cations are able to induce the dissociation of coordinated water molecules to produce acidic species such as MOH+ (or MOH2+ for trivalent metal cations) and H+. Several infrared studies concerning rare-earth or alkali-earth metal cation exchanged Y zeolites have demonstrated the existence of such species (MOH+ or MOH2+) [3, 4, 5, 6]. However, the literature is relatively poor concerning the IR characterization of these acidic sites for LTA zeolites. The aim of the present work is to characterize 5A zeolite acidity by different techniques and adsorption tests carried on 5A zeolite samples with different ion exchange. 

Infrared Spectroscopy Characterization of Zeolitic Systems 129 Bronsted Acid Site Distribution... 

Lok, B.M., Marcus, K.K., and AngeU, C.L (1986) Characterization of zeolite addity. 11. Measurement of zeolite acidity by ammonia temperature programmed desorption and FTIR spectroscopy techniques. Zeolites, 6, 185-194. 

Measurement of the thermokinetic parameter can be used to provide a more detailed characterization of the acid properties of solid acid catalysts, for example, differentiate reversible and irreversible adsorption processes. For example, Auroux et al. [162] used volumetric, calorimetric, and thermokinetic data of ammonia adsorption to obtain a better definition of the acidity of decationated and boron-modified ZSM5 zeolites (Figure 13.7). 

Desorption of water often converts Bronsted to Lewis acids, and readsorption of water can restore Bronsted acidity. Probe molecules, such as ammonia, pyridine, etc., are used to evaluate Bronsted and Lewis acidity. These compounds may contain water as an impurity, however. Water produced by reduction of metal oxides can also be readsorbed on acid sites. Probe molecules can in some cases react on surface acid sites, giving misleading information on the nature of the original site. Acidity, and accessibility, of hydroxyl groups or adsorbed water on zeolites and acidic oxides can vary widely. Study of adsorbed nitrogen bases is very useful in characterization of surface acid sites, but potential problems in the use of these probes should be kept in mind. 

For ferromagnetic cobalt particles in zeolite X, spin-echo ferromagnetic resonance has been used to obtain unique structural information (S6). In addition, study of the catalytic signature of metal/zeolite catalysts has been used by the groups of Jacobs (87), Lunsford (88), and Sachtler (47,73,89). Brpnsted acid protons are identified by their O—H vibration (90,91) in FTIR or indirectly, by using guest molecules such as pyridine or trimethylphosphine (92,93). An ingenious method to characterize acid sites in zeolites was introduced by Kazansky et al., who showed by diffuse reflection IR spectroscopy that physisorbed H2 clearly discerns different types of acid sites (94). Also, the weak adsorption of CO on Brpnsted and Lewis acid sites has been used for their identification by FTIR (95). The characterization of the acid sites was achieved also by proton NMR (96). 

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). 

Treatment with oxalic acid has been described as a method for selective removal of the external acid sites of medium-pore zeolites 61). PER and ZSM-23 zeolites were treated with a 1-M solution of oxalic acid at 353 K overnight 39, 62). The characterization of the acid sites showed that the treated materials had a low number of external acid sites compared with the untreated materials and, when used in n-butene isomerization, they exhibited an improved isobutylene selectivity. It was also observed that acid-treated PER does not have a high selectivity for isobutylene formation. It was inferred (62) that the cavities in ferrierite at the intersections of 8- and 10-ring channels are large enough to accommodate butene dimer intermediates, thus favoring the unselective bimolecular path. In contrast, when the external acid sites are removed from a zeolite with a unidimensional pore system (e.g., ZSM-23), the initial isobutylene selectivity is higher (nearly 80%) than that of the untreated sample. 

The purpose of this article is to review recent work concerning the characterization of the acid sites in zeolites, concentrating on the use of various techniques which can give detailed information concerning both the intensive and the extensive factors of acidity in zeolites. Due to the large number of papers being published in the area of zeolite catalysis, this review will certainly not be all-inclusive and the available literature will have greatly expanded by the time of publication. 

Typical test reactions often used for the characterization of zeolites are the cracking of n-hexane - and disproportionation of ethylbenzene. The catalytic activity of a zeolite is determined by the concentration of protons and the acid strength. 

Based on these data and on the characterization of the acidity of zeolites by NH3-TPD cited previously, but not reported for brevity, it is possible to draw the following conclusions on the relationships between nature of vanadium species, zeolite characteristics and catalytic behavior ... 

The activity of some Y zeolites showing different Lewis and Bronsted acid sites density is still studied in the acylation of dimethoxyarenes (with particular attention to veratrole. Scheme 4.7) with different acyl chlorides as a function of zeolite acidity and the lipophilic nature of acyl chlorides. Because of the particularly soft reaction conditions, namely, 65°C and 1 h reaction, 3,4-dimethoxyphenyl ketones 12 are the sole isomers recognized in the reaction mixture. Results of the catalytic tests (yield of compounds 12) confirm that the best catalyst is Y(14), characterized by an optimum ratio between Lewis and Bronsted acidity (medium-strength acid sites density for Lewis and Bronsted acid = 17.0 and 11.0 mmol x g- py. 

Furthermore, it should be mentioned that NMR spectroscopic techniques have found numerous applications for the characterization of the acidity of zeolites and related materials (see Vol. 6 of this series) and for the investigation of the molecular diffusion in microporous materials (see Vol. 7 of this series). 

Chapter 4 by H. Forster is devoted to the potential of and achievements obtained by electron spectroscopy in the field of molecular sieves. This contribution comprises, in a rather detailed manner, the theoretical fimdamentals and principles, the experimental techniques, as well as a wealth of applications and results obtained. Results are, e.g., reported on the characterization of zeolites as hosts, guest species contained in zeolite structures, framework and non-framework cations, and zeoHtic acidity. 

The number of studies in which adsorption microcalorimetry has been successfully applied to this end has increased in recent years, especially concerning the determination of the acidic function of molecular sieves, and extensive reviews of the systems investigated using this methodology have been published [1,5-14,19,78-81]. In particular, an extensive review [4] summarizes some of the most recently published results concerning applications of microcalorimetry to the study of the acid-base sites of zeolites and mesoporous materials. The efficiency of thermal analysis techniques for the characterization of the acid-base strength of zeolite materials is also discussed, as well as their ability to provide information consistent with catalytic data [4]. 

Spectroscopic methods have been devoted to the characterization of zeolite structure, of surface sites or species (hydroxyls, acid and basic sites, cations), of extraframework phases (oxides and hydroxides, supported metals), and of adsorbed phases (gaseous or liquids). The paper will consider examples of each type. 

It has been shown that the SSNMR of adsorbed TMP and/or TMPO probe molecules is a practical approach for acidity characterization of soHd acid catalysts. In particular, important acid features such as distribution and strength of acid sites maybe readily attained. Figure 2.13 shows the P MAS NMR spectra of TMP adsorbed in H-form mordenite (MOR Si/Al=10) and beta (Si/Al=12.5) zeolites, which both possess 12-MR pores [44]. In the presence of decoupling, the P resonance of the adsorbed TMP revealed a singlet at —2.2 (Fig. 2.13A) and —5.0 (Fig. 2.13C)ppmforlT-MOR... 

M.D. Karra, K.J. Stutovich, K.T. Mueller, NMR characterization of Bronsted acid sites in faujasitic zeolites with Use ofperdeuterated trimethylphosphine oxide, J. Am. Chem. Soc. 124 (2002) 902-903. 

Jacohsen, C.J.H., Madsen, C., Janssens, T.V.W., Jakobsen, H.J., and Skibsted, J. 2000. Zeolites by conned space synthesis characterization of the acid sites in nanosized ZSM-5 by ammonia desorption and Al/ Si-MAS NMR spectroscopy. Micropor. Mesopor. Mater. 39 393-401. 

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