Generation, Characterization, and Transformations of Unsaturated Carbenium Ions in Zeolites

(1)H and (13)C MAS NMR studies of light alkanes activation over MFI zeolite modified by Zn vapour

The early stages of methane, ethane and propane conversion were studied by in situ (1)H and (13)C MAS NMR techniques over fully exchanged Zn(2+)/MFI catalyst obtained by the reaction with zinc vapour. The in situ techniques revealed strong interaction of alkanes with Zn(2+) cations evidenced by significant shift of the corresponding NMR lines. Besides that, the formation of methyl zinc, ethyl zinc and n-propyl zinc species along with bridging and silanol surface OH-groups was detected already at the ambient temperature. These results pointed to dissociative adsorption of alkanes over (ZO)-Zn(2+)-(OZ) and (ZO)-Zn(2+)-(OSi) active sites of the catalyst. The dissociative adsorption was shown to be a dead-end surface reaction in the case of methane starting reactant, while in the case of ethane and propane, it appeared to be responsible for the initiation of the catalytic cycle leading to alkenes and dihydrogen formation and regeneration of zinc containing catalytic sites.

Topological Analysis of the Electronic Charge Density in the Ethene Protonation Reaction Catalyzed by Acidic Zeolite

In the present work, the distribution of the electronic charge density in the ethene protonation reaction by a zeolite acid site is studied within the framework of the density functional theory and the atoms in molecules (AIM) theory. The key electronic effects such as topological distribution of the charge density involved in the reaction are presented and discussed. The results are obtained at B3LYP/6-31G(**) level theory. Attention is focused on topological parameters such as electron density, its Laplacian, kinetic energy density, potential energy density, and electronic energy density at the bond critical points (BCP) in all bonds involved in the interaction zone, in the reactants, pi-complex, transition state, and alkoxy product. In addition, the topological atomic properties are determined on the selected atoms in the course of the reaction (average electron population, N(Omega), atomic net charge, q(Omega), atomic energy, E(Omega), atomic volume, v(Omega), and first moment of the atomic charge distribution, M(Omega)) and their changes are analyzed exhaustively. The topological study clearly shows that the ethene interaction with the acid site of the zeolite cluster, T5-OH, in the ethene adsorbed, is dominated by a strong O-H...pi interaction with some degree of covalence. AIM analysis based on DFT calculation for the transition state (TS) shows that the hydrogen atom from the acid site in the zeolitic fragment is connected to the carbon atom by a covalent bond with some contribution of electrostatic interaction and to the oxygen atom by closed shell interaction with some contribution of covalent character. The C-O bond formed in the alkoxy product can be defined as a weaker shared interaction. Our results show that in the transition state, the dominant interactions are partially electrostatic and partially covalent in nature, in which the covalent contribution increases as the concentration and accumulation of the charge density along the bond path between the nuclei linked increases.

Investigation of Ethylene Dimerization over Faujasite Zeolite by the ONIOM Method

Ethylene dimerization was investigated by using an 84T cluster of faujasite zeolite modeled by the ONIOM3(MP2/6-311++G(d,p):HF/6-31G(d):UFF) method. Concerted and stepwise mechanisms were evaluated. In the stepwise mechanism, the reaction proceeds by protonation of ethylene to form the surface ethoxide and then C--C bond formation between the ethoxide and the second ethylene molecule to give the butoxide product. The first step is rate-determining and has an activation barrier of 30.06 kcal mol(-1). The ethoxide intermediate is rather reactive and readily reacts with another ethylene molecule with a smaller activation energy of 28.87 kcal mol(-1). In the concerted mechanism, the reaction occurs in one step of simultaneous protonation and C--C bond formation. The activation barrier is calculated to be 38.08 kcal mol(-1). Therefore, the stepwise mechanism should dominate in ethylene dimerization.

First Direct Observation of Reactive Carbenes in the Cavities of Cation-Exchanged Y Zeolites

[reaction: see text] Herein we report the first direct observation of reactive carbenes within the cavities of cation-exchanged Y zeolites. Chloro(phenyl)- and bromo(phenyl)carbenes were generated upon laser photolysis of 3-halo-3-phenyldiazirines incorporated within dry zeolites and the absolute reactivity of the carbenes was investigated as a function of counterbalancing cation and coincorporated quenchers in order to elucidate the behavior of these intermediates within zeolites. Product analysis performed upon thermolysis of the diazirine in Y zeolites yielded products that were identified as those derived from the carbene.

Influence of alkali metal cations on the photoheterolysis of 9-cyclopropyl-9-fluorenol and the reactivity of the 9-cyclopropyl-9-fluorenyl cation in non-acidic zeolites

Alkali metal cation regulation of carbocation formation and reactivity in non-acidic zeolites is probed using the photoheterolysis reaction of 9-cyclopropyl-9-fluorenol. Nanosecond time-resolved diffuse reflectance is employed to directly observe the 9-cyclopropyl-9-fluorenyl cation as a transient species within the non-acidic zeolites. The efficiency of carbocation formation via photoheterolysis and the dynamics of other reaction pathways available to photoexcited 9-cyclopropyl-9-fluorenol are found to be strongly dependent on the zeolite alkali metal counterion. In particular, the yield of carbocation decreases with increasing counterion size in a manner consistent with the zeolite assisting the excited state C—O bond cleavage via Lewis acid catalysis involving the metal cation. Zeolite encapsulation is also found to modulate the ability of water and methanol to assist photoheterolysis. For instance, the influence of coadsorbed water on the photoheterolysis reaction within zeolites is found to be highly sensitive to the alkali metal cation. The rate constant for intrazeolite decay of the 9-cyclopropyl-9-fluorenyl cation increases significantly as the alkali metal cation size increases and as the Si–Al ratio decreases. These reactivity trends suggest that the intrazeolite decay of the 9-cyclopropyl-9-fluorenyl cation involves nucleophilic addition at the active site [Si-O-Al]– bridges of the zeolite framework. In addition, the reactivity of the 9-cyclopropyl-9-fluorenyl cation within alkali metal zeolites can be regulated by the co-inclusion of reagents such as methanol, water, and 1,1,1,3,3,3-hexafluoro-2-propanol.Key words: cation-exchanged zeolites, 9-cyclopropyl-9-fluorenyl cation, laser photolysis, reactivity.

In situ IR, NMR, EPR, and UV/Vis Spectroscopy: Tools for New Insight into the Mechanisms of Heterogeneous Catalysis

The development of new solid catalysts for use in industrial chemistry has hitherto been based to a large extent upon the empirical testing of a wide range of different materials. In only a few exceptional cases has success been achieved in understanding the overall, usually very complex mechanism of the chemical reaction through the elucidation of individual intermediate aspects of a heterogeneously catalyzed reaction. With the modern approach of combinatorial catalysis it is now possible to prepare and test much more rapidly a wide range of different materials within a short time and thus find suitable catalysts or optimize their chemical composition. Our understanding of the mechanisms of reactions catalyzed by these materials must be developed, however, by spectroscopic investigations on working catalysts under conditions that are as close as possible to practice (temperature, partial pressures of the reactants, space velocity). This demands the development and the application of new techniques of in situ spectroscopy. This review will show how this objective is being achieved. By the term in situ (Lat.: in the original position) is meant the investigation of the chemical reactions which are taking place as well as the changes in the working catalysts directly in the spectrometer.