A. A. Dergachev

Investigation of the different states of gallium in crystalline gallosilicates with pentasil structure and their role in propane aromatization

Results obtained by i.r. and e.p.r. spectroscopy show that at least two gallium-containing extraframework species differing in location, chemical nature, and acidic behavior are present in gallosilicate isostructural with ZSM-5 zeolite. Both species are formed in larger amounts by gallium release from the framework during catalyst regeneration processes. One of them, partly located on the outer crystallite surface, is found to be involved as a catalytically active site in propane aromatization. The catalytic activity is attributed to Lewis acidity.

Formation of strong electron-acceptor centers in reduced Ga- and Pt-Ga zeolites

Abstract I.r. of adsorbed pyridine and X.p.s. spectra were obtained for reduced Ga-containing ZSM-5 zeolite catalysts. Platinum introduction was shown to promote Ga reduction to an intermediate oxidation slate that exhibits strong Lewis acidity. These species were considered as active sites in lower alkane aromatization over PtGa- and GaZSM-5.

Promoting Effect of Pt Supported on Galliumsilicate in N-C4H10 Aromatization

Abstract Catalytic properties of both Ga-silicate and Pt/Ga-silicate in n-C4H10 aromatization have been investigated in a wide range of reaction conditions (at temperatures of 573–773 K, space velocities of 300–10000 h −1 , total pressures of 0.1–0.5 MPa, partial pressures of butane and hydrogen of 5–100 kPa and 5–400 kPa, respectively. The introduction of platinum into the Ga-silicate was shown to result in dramatic effects on butane aromatization: a decrease of the reaction temperature, an increase in the reaction rate and a drastic change in the product distribution. Platinum was found to accelerate both the initial paraffin and alicyclic intermediate dehydrogenation and suppressed the cracking process.

Developments in aromatization of light over pentasil zeolites

Abstract Transformations of C 3 -C 5 alkanes over metal contaning pentasils, including gallosilicates have been studied. The possible reasons for the promoting action of Zn, Cd, Ga and Pt in alehydrogenation and cyclization of light alkanes are discussed and the principles for designing selective aromatizing catalysts based on the pentasil type zeolites are suggested.

Ethane aromatization on Ga-Pt pentasil zeolites

Ethane aromatization on gallium-and platinum-modified pentasil zeolites (ZSM-5) with different framework constitutions was studied. The optimal zeolite composition and reaction conditions were determined and the synergistic effect of the promoters was revealed. The dependence of the catalyst activity on the SiO2/Al2O3 molar ratio was established. The highest yield of aromatic hydrocarbons (∼30%) and the highest selectivity for their formation from ethane (∼63%) were attained on the (2%Ga–0.3%Pt)/HZSM-5 bimetallic catalyst having SiO2/Al2O3 = 30.

Ethane aromatization on Ga-Pt/HZSM-5 zeolites prepared by solid-state modification

The results of a comparative study of the catalytic properties of gallium and platinum pentasil (ZSM-5) zeolites, prepared according to the solid-state modification and impregnation techniques, in the ethane aromatization reaction are reported. It has been found that the procedure for the preparation of bimetallic catalysts has no substantial effect on their activity and selectivity for aromatic hydrocarbons. The formation of the active sites of the catalysts obtained by solid-state modification was investigated with the use of X-ray diffraction and X-ray photoelectron spectroscopy, and it has been supposed that Ga-Pt clusters similar to the species revealed earlier in bimetallic pentasil catalysts prepared by impregnation are produced as a result of topochemical reactions involving hydrogen. Based on the catalytic and physicochemical data, a reaction scheme for the ethane aromatization on Ga-Pt/HZSM-5 is proposed, which suggests the involvement of bimetallic clusters in a key step, the dehydrogenation of ethane followed by the formation of ethylene oligomers and their dehydrocyclization.

Framework composition and activity of platinum-containing high-silica zeolites in n-hexane isomerization

The conversion of n-hexane on Pt-containing dealuminated mordenites and ZSM-5 zeolites was studied. The framework composition and the concentrations of extraframework aluminum compounds in mordenites were determined by X-ray diffraction analysis. It was demonstrated that the aluminum content of the framework affected the activity in n-hexane isomerization. It was found that a Pt-mordenite catalyst containing a considerable amount of extraframework aluminum compounds exhibited maximum activity. A quantum-chemical study of the interaction of platinum with Brønsted and Lewis acid sites was performed. It was hypothesized that oxidized surface platinum nanoparticles were the active sites of Pt-containing high-silica zeolites. These nanoparticles were formed by the interaction of platinum clusters with proton sites or extraframework aluminum compounds. An alternative mechanism was proposed for the conversion of alkanes to exclude the direct participation of acid sites.

Zinc-containing zeolite catalysts for ethane aromatization prepared by solid-state modification

Solid-state modification was used to prepare zinc-containing zeolite catalysts based on pentasils with different framework SiO2/Al2O3 ratios. The catalysts were studied by X-ray diffraction analysis and IR spectroscopy. The activity and selectivity of the Zn—pentasil systems prepared by the solid-state modification and impregnation methods were compared in the aromatization of ethane. The active and stable zeolite catalysts modified by transition metal ions can be obtained by the topochemical method.

DEVELOPMENT AND STUDY OF ZEOLITE CATALYTIC SYSTEMS FOR THE TRANSFORMATION OF LOWER HYDROCARBONS INTO VALUABLE CHEMICALS

The results of studies aimed at the development of highly efficient catalysts based on zeolites of the pentasil family for synthesis of aliphatic and aromatic hydrocarbons from C2−C5 olefins and paraffins are summarized.

Catalytic and physicochemical properties of crystalline pentasils in transformations of low-molecular-weight olefins and paraffins

The features of the catalytic action of pentasil type zeolites in transformations of low-molecular-weight olefins and paraffins are reviewed. Principal results describing the physicochemical properties of pentasils elucidated by IR spectroscopy, electron microscopy, XPS, and ESR are discussed. The mechanism of the formation of catalytically active sites on the surface of pentasils and the location of modifying elements in the zeolite frameworks are examined. The role of Lewis and Bronsted acid sites in promoting single pathways in the transformations of lower hydrocarbons is specified.