Zdenek Sobalik

Redox catalysis over metallo-zeolites Contribution to environmental catalysis

The paper reviews development in analysis and understanding of the structure of active sites in metallo-zeolites and their function in SCR–NOx , NO–NO2 equilibration reaction, and benzene oxidation with N2O to phenol. The opportunities and limitations of various spectral techniques (FTIR, UV-Vis, ESR, EXAFS, Moessbauer, Raman spectroscopy) in connection with these studies are discussed. The attention is centered on Co and Fe species planted in pentasil ring zeolites of various topologies, specifically on Co-beta and Fe-oxo species in zeolites, exhibiting stable SCR–NO x activity under presence of high excess of water vapor existing in real exhaust gases from combustion processes. Also participation of the protonic, Al-Lewis sites and specific Fe-oxo sites is analyzed with respect to these redox reactions. It is shown that the zeolite matrix has a dramatic effect on the SCR–NOx activity of the hosted metal ion species. Main parameters controlling metallo-zeolite activity are the type and structure of metallo-species, distances between the cationic sites and concentration and distribution of Al in the framework. Specific Fe-oxo structures coordinated to zeolite framework exhibit extremely high activity in benzene oxidation with N2O to phenol in contrast to protonic and Al-Lewis sites which contribute to these reactions only by activation of hydrocarbons. Analogous (but probably not identical) Fe-oxo structures bear a potential to be applied in SCR–NOx with C3 + paraffins under high excess of water vapor. Stable SCR–NO x activity of Co-beta zeolite indicates either presence of Co-oxo species of extremely high activity or positive effect of high density of aluminium in specific local framework structure of Co-beta zeolite.

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

Oxidative dehydrogenation of propane over Fe-BEA catalysts

Abstract Catalytic Oxidative DeHydrogenation of Propane (ODHP) by N 2 O over Fe-BEA catalysts has been studied in continuous feed regime against time and with periodic changing of in-let feed composition under reaction-regeneration cycles. The influence of process parameters on the propylene production has been evaluated, namely N 2 O/propane ratio and crystal size of the parent zeolite. The particle size (micro- vs. nano-particles) of the catalyst has been shown to be a crucial parameter for propylene yields and catalyst selectivity at higher temperatures. The nano-sized catalyst exhibits superior propylene formation in comparison with micro-sized one. Time dependent performance of ODHP was carried out using reactant streams containing N 2 O or N 2 O with O 2 . Reactivation of the catalysts by oxidation pulses revealed that short pulses of oxygen are sufficient to regenerate the active sites and stabilize propylene yield.

Mechanism of framework oxygen exchange in Fe-zeolites: a combined DFT and mass spectrometry study.

The role of framework oxygen atoms in N(2)O decomposition [N(2)O(g)→N(2)(g) and 1/2O(2)(g)] over Fe-ferrierite is investigated employing a combined experimental (N(2)(18)O decomposition in batch experiments followed by mass spectroscopy measurements) and theoretical (density functional theory calculations) approach. The occurrence of the isotope exchange indicates that framework oxygen atoms are involved in the N(2)O decomposition catalyzed by Fe-ferrierite. Our study, using an Fe-ferrierite sample with iron exclusively present as Fe(II) cations accommodated in the cationic sites, shows that the mobility of framework oxygen atoms in the temperature range: 553 to 593 K is limited to the four framework oxygen atoms of the two AlO(4)(-) tetrahedra forming cationic sites that accomodate Fe(II). They exchange with the Fe extra-framework (18)O atom originating from the decomposed N(2)(18)O. We found, using DFT calculations, that O(2) molecules facilitate the oxygen exchange. However, the corresponding calculated energy barrier of 87 kcal  mol(-1) is still very high and it is higher than the assumed experimental value based on the occurrence of the sluggish oxygen exchange at 553 K.