Anton A. Gabrienko

Significant influence of Zn on activation of the C-H bonds of small alkanes by Bronsted acid sites of zeolite.

Herein, we analyze earlier obtained and new data about peculiarities of the H/D hydrogen exchange of small C(1)-n-C(4) alkanes on Zn-modified high-silica zeolites ZSM-5 and BEA in comparison with the exchange for corresponding purely acidic forms of these zeolites. This allows us to identify an evident promoting effect of Zn on the activation of C-H bonds of alkanes by zeolite Brønsted sites. The effect of Zn is demonstrated by observing the regioselectivity of the H/D exchange for propane and n-butane as well as by the increase in the rate and a decrease in the apparent activation energy of the exchange for all C(1)-n-C(4) alkanes upon modification of zeolites with Zn. The influence of Zn on alkane activation has been rationalized by dissociative adsorption of alkanes on Zn oxide species inside zeolite pores, which precedes the interaction of alkane with Brønsted acid sites.

In situ high temperature MAS NMR study of the mechanisms of catalysis. Ethane aromatization on Zn-modified zeolite BEA

Ethane conversion into aromatic hydrocarbons over Zn-modified zeolite BEA has been analyzed by high-temperature MAS NMR spectroscopy. Information about intermediates (Zn-ethyl species) and reaction products (mainly toluene and methane), which were formed under the conditions of a batch reactor, was obtained by (13)C MAS NMR. Kinetics of the reaction, which was monitored by (1)H MAS NMR in situ at the temperature of 573K, provided information about the reaction mechanism. Simulation of the experimental kinetics within the frames of the possible kinetic schemes of the reaction demonstrates that a large amount of methane evolved under ethane aromatization arises from the stage of direct ethane hydrogenolysis.

Correlation between Asphaltene Stability in n-Heptane and Crude Oil Composition Revealed with In Situ Chemical Imaging

Five crude oil samples with different physical properties have been studied with respect to asphaltene stability. The attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopic imaging approach of n-heptane-induced precipitation has been used to monitor crude oil behaviour under dilution with a flocculation agent. For each sample, the dynamics of asphaltene precipitation has been observed by applying this chemical imaging method. Based on these data, the stability of crude oil samples has been compared and the correlation between asphaltene stability and crude oil properties has been proposed.

H/D exchange of molecular hydrogen with Brønsted acid sites of Zn- and Ga-modified zeolite BEA

Kinetics of hydrogen H/D exchange between Brønsted acid sites of pure acid-form and Zn- or Ga-modified zeolites beta (BEA) and deuterated hydrogen (D(2)) has been studied by (1)H MAS NMR spectroscopy in situ within the temperature range of 383-548 K. A remarkable increase of the rate of the H/D exchange has been found for Zn- and Ga-modified zeolites compared to the pure acid-form zeolite. The rate of exchange for Zn-modified zeolite is one order of magnitude higher compared to the rate for Ga-modified zeolite and two orders of magnitude larger compared to the pure acid-form zeolite. This promoting effect of metal on the rate of H/D exchange was rationalized by a preliminary dissociative adsorption of molecular hydrogen on metal oxide species or metal cations. The adsorbed hydrogen is further involved in the exchange with the acid OH groups located in vicinity of metal species. The role of different metal species in the possible mechanisms of the exchange with involvement of zeolite Brønsted acid sites and metal species is discussed.

Competitive pathways of methane activation on Zn2+-modified ZSM-5 zeolite: H/D hydrogen exchange with Brønsted acid sites versus dissociative adsorption to form Zn-methyl species

To clarify the pathways of methane activation on Zn-modified high-silica zeolites, the kinetics of both dissociative adsorption of the alkane C–H bond to form Zn-methyl species and H/D hydrogen exchange between the alkane and Bronsted acid sites (BAS) have been analyzed for Zn2+/H-ZSM-5 containing exclusively Zn2+ cations (no ZnO species in the zeolite) and BAS. Analysis of the kinetics was performed by 1H MAS NMR spectroscopy in situ at 410–540 K. In spite of the activation barrier for H/D hydrogen exchange (68 kJ mol−1) being larger than that for Zn-methyl formation (46 kJ mol−1), the rate of H/D hydrogen exchange has been found to be one order of magnitude higher than the rate of the formation of Zn-methyl species within the studied temperature range. This implies that Zn-methyl species cannot be involved in the reaction of H/D hydrogen exchange as the intermediate responsible for facilitation of this reaction due to the presence of Zn2+ cations in the zeolite (J. Catal., 2008, 253, 11). A new mechanism has been suggested for C–H bond activation in methane on the zeolite modified with Zn2+ cations. It includes first the formation of a transient molecular complex of methane with Zn2+ cations. The complex either is further involved in the reaction of H/D hydrogen exchange or evolves toward the formation of Zn-methyl species and BAS.