Jiri Dedecek

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

6Li MAS NMR Study of Lithium Insertion into Hydrothermally Prepared Li-Ti-O Spinel

Open-circuit measurements and 6 Li magic-angle spinning (MAS) NMR spectroscopy were used to study Li coordination in partly reduced Li 1 . 1 Ti 1 . 9 O 4 + δ spinels prepared by hydrothermal reaction. The reduced spinels show four different Li sites. Signals at -0.2 and 0.3 ppm correspond to Li in 16d and 8a positions. The signal at 0.9 ppm corresponds to a tetrahedrally coordinated Li and represents the primary site for the inserted lithium. The signal at -0.7 ppm appears simultaneously with a decrease of the population of Li in 8a position and indicates a local phase transition connected with ordering of Li into 16c positions.

Biaxial Q-shearing of 27Al 3QMAS NMR spectra: insight into the structural disorder of framework aluminosilicates.

In this contribution, we present the application potentiality of biaxial Q-shearing of (27)Al 3QMAS NMR spectra in the analysis of structural defects of aluminium units in aluminosilicates. This study demonstrates that the combination of various shearing transformations of the recorded (27)Al 3QMAS NMR spectra enables an understanding of the broadening processes of the correlation signals of disordered framework aluminosilicates, for which a wide distribution of (27)Al MAS NMR chemical shifts and quadrupolar parameters (i.e., second-order quadrupolar splitting and quadrupole-induced chemical shifts) can be expected. By combining the suitably selected shearing transformation procedures, the mechanisms of the formation of local defects in aluminosilicate frameworks, including Al/Si substitution effects in the next-nearest neighbouring T-sites, variations in bond angles, and/or variations in the physicochemical nature of charge-balancing counter-ions, can be identified. The proposed procedure has been extensively tested on a range of model aluminosilicate materials (kyanite, γ-alumina, metakaolin, analcime, chabazite, natrolite, phillipsite, mordenite, zeolite A, and zeolite Y).

TNU‐9 Zeolite: Aluminum Distribution and Extra‐Framework Sites of Divalent Cations

The TNU-9 zeolite (TUN framework) is one of the most complex zeolites known. It represents a highly promising matrix for both acid and redox catalytic reactions. We present here a newly developed approach involving the use of 29 Si and 27 Al (3Q) MAS NMR spectroscopy, CoII as probes monitored by UV/Vis and FTIR spectroscopy, and extensive periodic DFT calculations, including molecular dynamics, to investigating the aluminum distribution in the TUN framework and the location of aluminum pairs and divalent cations in extra-framework cationic positions. Our study reveals that 40 and 60 % of aluminum atoms in the TNU-9 zeolite are isolated single aluminum atoms and aluminum pairs, respectively. The aluminum pairs are present in two types of six-membered rings forming the corresponding α and β (15 and 85 %, respectively, of aluminum pairs) sites of bare divalent cations. The α site is located on the TUN straight channel wall and it connects two channel intersections. The suggested near-planar β site is present at the channel intersection.