Hana Jirglova

Enhancement of Activity and Selectivity in Acid‐Catalyzed Reactions by Dealuminated Hierarchical Zeolites

High-silica zeolites with crystalline aluminosilicate frameworks balance the charge of strongly acidic protons during the processing of oil, in petrochemistry, and increasingly in numerous organic syntheses. The transformation of hydrocarbons is controlled by the concentration and strength of the acid sites and the dimensions and architecture of the inner pores. Zeolite micropores, which have a diameter similar to organic molecules, govern the shape selectivity of the reaction in the inner space, but also result in slow transport of reactants and products, thus limiting the reaction rate. Several approaches have been developed to enhance the mass transport by using zeolite nanosheets and nanocrystals, or zeolites that contain both microand mesopores. The latter hierarchical zeolites were prepared by confined crystal growth, by using polymers as mesoporogens, or through post-synthesis desilication or dealumination processes. The advantage of the presence of mesopores is, however, accompanied by the nonshape-selective environment of the acid sites located in the mesopores. Our interest in the effective formation of secondary mesoporosity through postsynthesis alkaline treatment of conventional zeolites prompted us to study the potential of leaching procedures for the preparation of hierarchical zeolites, preserving the shape-selective environment of the active sites. The main principles of forming mesopores in high-silica zeolites through alkaline leaching have been described by Groen et al. They demonstrated that dissolution of Si depends mainly on the Al concentration in the framework and occurs in the Si-rich areas. Al atoms partly remain at the framework sites and partly form extra-framework Al species in the mesopores. Groen et al. 13] and Caicedo-Realpe and PerezRamirez have shown that the formed Al species can be removed by treatment with mild acid, thus restoring the original Si/Al ratio. This treatment increased the isomerization of o-xylene, however, the selectivity for p-xylene did not reach that of parent microporous zeolites. This study is primarily concerned with the elimination of both the extraframework and framework Al species, and thus the related acid sites from the mesopores of the desilicated zeolites by employing oxalic acid. The advantage of hierarchical zeolites with acid sites predominantly located in the confined reaction space of the micropores is demonstrated on acid-catalyzed reactions controlled by shape-selectivity effects. TEM images of the alkalineand subsequently acidleached zeolites are given in Figure 1. They clearly show that the treatment resulted in the extensive formation of a secondary mesoporous structure, which is characterized by numerous crystal cavities, which are more populated in ZSM-5 (Si/ Al = 22.2) compared to mordenite (MOR, Si/Al = 12.1). The adsorption isotherms of treated ZSM-5 zeolites (Figure 2) indicate adsorption in the zeolite micropores and an H3 hysteresis loop typical for slit-shaped mesopores. But the extensive formation of a mesoporous structure also resulted in a decrease in the micropore volume. Treatment with oxalic acid further extended the mesopore volume and the micropore volume increased, with the final value only slightly lower compared to the parent zeolite. Al plugs, which were formed in the mesopores after desilication and blocked parts of the micropores, were removed by acid leaching, similar to results of Caicedo-Realpe and Perez-Ramirez. With mordenite, alkaline and acid leaching resulted in similar textural changes and led to well-developed secondary mesoporosity with preserved high micropore volumes. The dealuminated zeolite surface was analyzed by XPS monitoring of the relative concentration of Al to Si in the zeolite (sub)surface layers ( 50 ) by the Al 2p and Si 2p electron levels. The surface Si/Al ratio of both desilicated ZSM-5 and mordenite zeolites compared to the bulk composition (Table 1) indicated accumulation of Al species on the external crystal surface. In contrast, zeolites treated with oxalic acid resulted in a slight surface enrichment in Si. Analysis of the Brønsted and Lewis acid sites of dealuminated micro-mesoporous zeolites indicated predominant Brønsted acidity corresponding to the concentration of Al in the framework (Table 1). The population of acid sites in the dealuminated micro-mesoporous (deAlmm) ZSM-5(I) was analyzed using the FTIR spectra of adsorbed 2,6-ditertbutylpyridine (DTBPy), the kinetic diameter of which (10.5 ) does not allow it to penetrate into the [*] Dr. P. Sazama, Prof. Dr. Z. Sobalik, Dr. J. Dedecek, Dr. Z. Bastl, Dr. J. Rathousky, Dr. H. Jirglova J. Heyrovský Institute of Physical Chemistry Academy of Sciences of the Czech Republic 18223 Prague 8 (Czech Republic) E-mail: petr.sazama@jh-inst.cas.cz

Water adsorption on high silica zeolites. Formation of hydroxonium ions and hydrogen-bonded adducts

Abstract An in situ high-pressure/high-temperature FTIR spectroscopy technique was used for analysis of water adsorption on ZSM-5 and ferrierite zeolites. Adsorbed water in zeolite channels is present in neutral hydrogen-bonded adducts at low coverage, whereas at higher coverage protonated species H + (H 2 O) n are formed. The protonated species are easily stabilized in narrow zeolite channels and at higher density of aluminium atoms in the zeolite framework. Moreover, formation of hydroxonium ions is supported by increasing overall pressure. Conditions combining the temperature and pressure for manifestation of both forms of water adsorption have been analyzed.

Sorption of ammonia from gas streams on clinoptilolite impregnated with inorganic acids

The contribution deals with ammonia-removal from waste air by adsorption using low-cost sorbents. The natural zeolite clinoptilolite from the deposit Nižný Hrabovec, Slovakia was tested for ammonia removal from the waste air. Measurements of breakthrough curves for ammonia were performed using fixed beds of clinoptilolite (i) in its natural form and (ii) on clinoptilolite samples pre-treated with acids. The parameters of the experiments were selected to simulate the conditions ruling in animal breeding farms. The pre-treatment of clinoptilolite with acids increased sorption capacity of clinoptilolite for ammonia.

Transport-related structure characteristics of FCC catalysts from sorption, porosimetric and PFG NMR measurements

Texture characteristics of fluid catalytic cracking (FCC) catalysts were evaluated from N 2 adsorption and mercury porosimetry measurements. A hierarchy of four levels of voids were found and quantified in the beds of FCC particles. Diffusion coefficients of intraparticle diffusion for n-octane were measured in the catalyst beds by PFG NMR technique. The diffusion coefficients exhibit a linear dependence on parameters ξ that involve the mean value of the pore radii r ¯ in the pertinent porosity region and a function of the corresponding porosity. A strong linear correlation was also found between the parameter ξ for macropores and that for mesopores. The mean sizes of macro- and mesopores thus become key parameters in tuning the FCC catalyst properties.