A. A. Slinkin

Cu2+-cation location and reactivity in mordenite and ZSM-5: e.s.r.-study

Abstract An e.s.r.-study was carried out on Cu 2+ -cation location in copper(II)-exchanged H- and Na-forms of mordenite and ZSM-5. In low percent CuHM, copper(II) is located as isolated Cu 2+ -cations in square pyramid coordination; these cations are stable upon CO treatment up to 400°C. Clustered cations are formed in CuHM with the increase in copper content; these cations are readily reduced by CO at 300°–400°C. The part of clustered Cu 2+ -cations in ZSM-5 is negligible, the CuZSM-5 samples contain two types of isolated cations located in square planar environment and five-coordinated one. The coordinative unsaturation of isolated Cu 2+ -cations in mordenite and ZSM-5 is confirmed by the strong influence of CO and O 2 adsorption on the hyper-fine splitting of e.s.r.-spectra. The calcination of a mixture of H-ZSM-5 and Cu 0 -powder in air is accompanied by the migration of isolated Cu 2+ -ions to positions in the zeolite channels.

Formation of radicals and catalytic activity of H-ZSM-5-type zeolites in unsaturated hydrocarbon conversions: Part 2. Radical formation upon adsorption of aromatic compounds

Abstract The formation of cation radicals upon adsorption of different aromatic compounds at 20–250 °C on H-ZSM-5 zeolites has been studied. It is shown that the concentration of the strong radical-forming sites in H-ZSM-5 (per Al3+ ion of lattice) is not lower than that in H-mordenite. The selective blocking of external sites by 2,4,6-trimethylpyridine does not prevent the formation of benzene or p-xylene radicals, i.e. the majority of aromatic radicals formed are located inside the H-ZSM-5 channels. In the case of large molecules, some peculiarities of radical formation due to steric hindrance have been observed. The role of chemisorbed oxygen in the processes of aromatic cation radical formation on H-ZSM-5 and H-mordenite is discussed.

Formation of radicals and catalytic activity of pentasil-type zeolites in unsaturated hydrocarbon conversions: Part 1. Radical formation upon olefin adsorption

Abstract The formation of radicals upon adsorption of linear (C3H6), branched (isobutylene, 2,4,4-trimethylpentene-1) and cyclic (cyclopentene) olefins on pentasil-type zeolites at −78 °C and 20 °C has been studied. It is shown that on pentasil the process of C3H6 oligomerization proceeds to a certain extent, even at −78 °C. Similarities in the processes on pentasil and H-mordenite, as well as differences in the structure of radical particles due to channel shape and size, have been found. It is assumed that in the case of branched olefin adsorption on pentasil, a certain portion of the radical particles is formed on the zeolite outer surface. The process of SO2− anion radical induced formation as a result of electron transfer from olefin radicals to sorbed SO2 molecules is observed.

Pentasil-Type Zeolites: Radical Formation, Activity in the Olefin Oligomerization and Aromatization, Processes of Coke Deposition

The formation of radicals upon adsorption of different olefins and aromatic compounds on H-ZSM-5 has been studied. The sorption of C 3 H 6 on H-ZSM-5 is accompanied by the formation of oligomeric radicals even at −78°C. Adsorption of i-C 4 H 8 and 2,4,4-trimethylpentene-1 is accompanied by the appearance of identical ESR-signals from oligomeric particles with equivalent protons. In the case of the radical formation from branched olefins the outer surface of H-ZSM-5 crystals plays a noticeable role. The study of benzene and p-xylene sorption on H-ZSM-5, pre-treated by 2,4,6-trimethylpyrydine which blocked selectively the outer surface, shows that the majority of cation-radicals formed is located inside the channels and the process of their formation is not hindered by the diffusion. It is shown that catalytic low-temperature oligomerization of C 3 H 6 and i-C 4 H 8 takes place on the outer surface of H-ZSM-5 crystals whereas the high-temperature aromatic formation takes place inside the zeolite channels. In contrast to HM, the coke formed upon the olefin transformation on H-ZSM-5 is located on the outer surface of zeolite crystals.

The reduction of CeO2 in the ketonization of ch3cooh

The decomposition of CH3COOH on various samples of CeO2 has been studied. The reduction of Ce to Ce+3 with the simultaneous oxidation of the CH3COOH has been detected.

Formation of radicals and catalytic activity of H-ZSM-5-type zeolites in unsaturated hydrocarbon conversions: PART 3. Olefin oligomerization and aromatization on parent and dehydroxylated H-ZSM-5

Abstract The conversion of olefins on parent and dehydroxylated H-ZSM-5 has been studied. ESR techniques and UV-Vis-IR spectroscopy have been used for the comparative study of the formation of radicals and intermediates from different unsaturated compounds. It is shown that catalytic lowtemperature oligomerization of C3 H6 and i-C4H8 takes place on the outer surface of H-ZSM-5 crystals, whereas the high-temperature formation of aromatics takes place inside the zeolite channels. It is shown that the properties of redox sites do not change noticeably upon the transition from Bronsted acid sites to Lewis ones. Dehydroxylation of H-ZSM-5 results in suppression of the low-temperature transformation of sorbed olefins, but has a negligeable effect on the catalytic high-temperature conversion of olefins.

Forms of nickel-Components of nickel aluminosilicate catalysts of the dimerization of ethylene

1. Crystalline and amorphous nickelaluminosilicate catalysts, containing all the nickel in the form of the metal, are inactive in dimerization and other conversions of ethylene at 150–250°. 2. Nickel zeolite catalysts, containing NiO on the surface of the zeolite or Ni2+ ions in the cavities, are active in these reactions.

Influence of the composition of the catalytic system NiO-SiO2-Al2O3 on its catalytic properties in reactions of oligomerization of ethylene and isobutylene

1. The catalyst NiO-SiO2 acquires activity in the reaction of dimerization of C2H4 when Al2O3 is introduced into it. 2. With increasing Al2O3 content in the catalyst, the conversion of C2H4 and i-C4H8 and the number of electron acceptor centers, recordable by ESR, pass through a maximum (1–3% Al), while the total acidity of the catalyst increases.

Oligomerization of ethylene of NiO-dealuminized mordenite catalyst

When the amount of aluminum in a nickel-containing mordenite with variable degree of dealuminization is increased the total acidity of the zeolite increases, the concentration of the electron-acceptor centers, recorded by the EPR method, passes through a maximum, while the degree of ethylene conversion varies within narrow limits.

Structure and activity of Ni-Cr2O3 and Ni-Cu-Cr2O3 Communication 3. Activity and selectivity in the dehydrogenation of cyclohexane

1. The selectivity of the investigated Ni-Cr2O3 catalysts in the dehydrogenation of cyclohexane increases sharply (a decrease in the CH4 content) when additions of CuO, ZnO, and Ag2O are introduced. 2. The selectivity of Ni-Cr2O3 catalysts and catalysts with additives is not associated with the presence of a volume phase of NiO, but is due to surface oxygen at the metal-carrier interface. 3. A mechanism for the action of selectivizing additives, based on an increase in the mobility of the oxygen found at the interface, which leads to its removal from the surface of nickel, has been suggested. 4. The specific catalytic activity of Ni-Cu-Cr2O3 catalysts is greater than the specific activity of Ni-Cr2O3 catalysts.