Techniques for Measuring Zeolite Acidity

1 Structure and Acidity of Zeolites. - Hall and cowoikers have expressed the total acidity of zeolites in terms of an extensive factor, representing the total number of acid sites in the zeolite, and an intensive factor, representative of the strength of the individual sites In a structurally pure H-zeolite, the nature of the acid site is easy to conceptualize, consisting of the bridging hydroxyl group between two tetrahedra, one centered on aluminum and the other on a silicon, viz. 

In principle there should be one such Brpnsted site per aluminum atom in the lattice. 

For zeolites having a high Si/Al ratio, these sites should be sufficiently remote from each otho- so they do not experience mutual interactions. Thus, protons associated with these aluminum ions should all be of the same acid strength and the acidity of such a zeolite should be defined solely by its acid site density, i.e., its extensive acidity. 

This type of behavior has been demonstrated by Haag for H-ZSM-5 systems and by Lunsford and coworkers for mordenites and faujasites. Both groups reported linear correlations between the cracking rate of n-hexane and the aluminum content of the zeolites. Other studies by Gorte and cowoikers involving the adsoiption of weak bases. 

At the same time it has been shown that certain zeolite compositions do not exhibit uniform acid strength. Lunsford and coworkers extended the acidity-aluminum content correlation to the high Si/Al range of dealuminated H-Y zeolites and found that above approximately 35 Al/unit cell (Si/Al 4) the overall acidity of the zeolites decreased, presumably due to next nearest neighbor interactions, i.e., the intensive factor of acidity of the zeolite is diminished. It has also been established that certain dealumination procedures can affect the intensive factor of acidity. This was clearly demonstrated by Lago et al. on H-ZSM-S catalysts, in which dealumination by mild steaming resulted in a four-fold increase in the cracking activity for n-hexane.  

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Professor George Marcelin (University of Pittsburgh) discusses techniques for measuring zeolite acidity. Acid catalyzed reactions have become increasingly important for the production of alternative fuels, reformulated gasoline, and chemical intermediates. Professor Marcelin s chapter provides a valuable review of the use and specific limitations of traditional methods, such as ammonia TPD and IR spectroscopy. 

The kinetics of reactions in zeolites is conventionally related to the reactant concentrations in the gas phase. Reaction within the pores of zeolites, however, involves adsorption, diffusion of reactants into the pores, reactions of adsorbed species inside the pores, desorption of products, and diffusion of products out of the pores (92). Therefore, intrinsic kinetics based on the concentration of species adsorbed inside the pores is expected to be very useful for catalyst development. TEOM is an excellent technique for measurement of adsorption of reactants under reaction conditions as well as measurement of this adsorption as a function of the coke content (3,88). This technique makes it possible to obtain intrinsic activity of each acidic active site directly and to understand deactivation mechanisms in detail. 

Whereas the number of Brpnsted acid sites can be easily determined,146 149 their acidity may vary depending on their position in the framework and also by interaction with Lewis acid sites. For these reasons and despite the various techniques tested, there are no general and reliable methods to measure the acidity of solid acids. The synthesis of active acidic zeolites used in industry still relies on a very empirical base, the most important character being their catalytic activity. 

The total acidity deterioration and the acidity strength distribution of a catalyst prepared from a H-ZSM-5 zeolite has been studied in the MTG process carried out in catalytic chamber and in an isothermal fixed bed integral reactor. The acidity deterioration has been related to coke deposition. The evolution of the acidic structure and of coke deposition has been analysed in situ, by diffuse reflectance FTIR in a catalytic chamber. The effect of operating conditions (time on stream and temperature) on acidity deterioration, coke deposition and coke nature has been studied from experiments in a fixed integral reactor. The technique for studying acidity yields a reproducible measurement of total acidity and acidity strength distribution of the catalyst deactivated by coke. The NH3 adsorption-desorption is measured by combination of scanning differential calorimetry and the FTIR analysis of the products desorbed. 

In this paper, we will discuss the use of temperature programmed desorption as a complementary technique for obtaining lattice concentrations for high-silica materials. While TPD of ammonia is commonly used as a measure of the number of acid sites in zeolites, this paper will discuss the use of the reactive probe molecules, 2-propanamine and 2-propanol. It has previously been demonstrated that well-defined adsorption complexes, with a stoichiometry of one molecule/Al atom could be observed in high-silica zeolites for a number of adsorbates. These complexes were found to be independent of Si/Al2 ratio for a series of H-ZSM-5 zeolites and were unaffected by... 

In this work the FR technique proved to be a powerful method for the investigation of adsorbed species on different chemisorption sites. The FR method has been shown to be capable of giving information on the dynamic behaviom of sorption in zeolites. Adspecies detected by the FR technique surely play a role in this dynamic phenomenoa This new technique is proposed for measuring and interpreting data on kinetics of ammonia ad-/desorption processes characterising acidic sites in zeolite catalysts. 

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... 

TPD of basic probe molecules is a method which is often used for the analysis of zeolitic acidity [24], In a parallelized implementation [25] of this technique, the samples to be studied were placed in a parallel channel reactor-body built analogous to the one described by Hoffmann et al. [26], By means of a multiport valve, the effluent from each channel could be switched to a mass spectrometer which was used for the analysis of the desorbed gas. In a typical experiment, after conditioning of the samples and adsorbing ammonia, a temperature ramp was started and in a sequential manner the effluent of each catalyst was fed into the mass spectrometer. Flushing and analysis times per channel were 8 s, so that all 10 samples could be analyzed in somewhat more than one minute. This proved to be sufficient to obtain well resolved TPD curves at a heating rale of 10 K/min. The recorded data corresponded well to curves measured with conventional setups. 

Adsorption microcalorimetry is the measure of the heat of adsorption evolved when dosing measured small amounts of a vapor probe on a surface. Cardona-Martinez and Dumesic [36] summarized the results obtained for oxides, zeolite, and metal catalysts before 1992. Summaries of the application of these techniques to gas-solid interactions and heterogeneous catalysis have been published recently [37-39]. As done by Auroux and Gervasini [40] for a number of binary metal oxides, calorimetric studies of the acidity and basicity are mostly performed using ammonia as an acidity probe and carbon dioxide as a basicity probe [41]. 

Isothermal titration calorimetry is a technique that can directly measure liquid-solid or liquid-liquid interactions. This method has been used for determination of acid-base properties of solid catalysts in an inert phase (n-decane), for determination of heats of reaction in liquid phase (alkylation, anisole+benzoic anhydride on H-Beta zeolite), and for determination of heats of adsorption of different pollutants on an adsorbent [33], It is also recognized as the only technique that can directly measure the binding energies of biological processes such as protein-ligand binding and protein-protein binding [34]. 

Since spillover phenomena have been most directly sensed through the use of IR in OH-OD exchange [10] (in addition, in the case of reactions of solids, to phase modification), we used this technique to correlate with the catalytic results. One of the expected results of the action of Hjp is the enhancement of the number of Bronsted sites. FTIR analysis of adsorbed pyridine was then used to determine the relative amounts of the various kinds of acidic sites present. Isotopic exchange (OH-OD) experiments, followed by FTIR measurements, were used to obtain direct evidence of the spillover phenomena. This technique has already been successfully used for this purpose in other systems like Pt mixed or supported on silica, alumina or zeolites [10]. Conner et al. [11] and Roland et al. [12], employed FTIR to follow the deuterium spillover in systems where the source and the acceptor of Hjp were physically distinct phases, separated by a distance of several millimeters. In both cases, a gradient of deuterium concentration as a function of the distance to the source was observed and the zone where deuterium was detected extended with time. If spillover phenomena had not been involved, a gradientless exchange should have been observed. 

D correlation analysis is a powerful tool applicable to the examination of data obtained from infrared spectroscopy. The correlation intensities, displayed in the form of 2D maps, allow us to correlate the shift induced by CO adsorption and acidity of sites in dealuminated zeolites. Results are in accordance with previous results, obtained using only IR measurements, proving the validity of this technique. New correlations allowed the assignment of very complex groups of bands, and 2D correlation revealed itself as a great help for understanding acidity in dealuminated zeolites. 2D correlation has allowed us to validate the model obtained by NMR. 

Dealumination processes are usually used in conjunction with production of the acid form of zeolite Y for many catalytic apphcations, and zeolites A and X are in most cases no longer used in acid catalytic applications because the high amount of aluminum in their frameworks makes them difficult to stabilize using various dealumination techniques. Successful dealumination and at least partial annealing of defects, resulting from movement of silicon cations to the aluminum vacancies, can be assessed by measurement of the reduction of the unit cell size of the zeolite. This unit cell reduction is a consequence of the relative ionic radii of AF (0.54 A) and Si + (0.40A). 

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