Kh.M. Minachev

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.

Localization of polyvalent cations in pentasil catalysts modified by metal oxides

Abstract Polyvalent cations in HZSM-5 zeolites were found to be preferentially localized at the outer surface of microcrystallites (for Me 3+ ) or inside the channels of the zeolite structure (for Me 2+ ). This difference was explained by both steric and electrostatic hindrances arising for the tervalent cations.

Platinum promoting effects in Pt/Ga zeolite catalysts of lower alkane aromatization. II: The catalytic properties in n-butane aromatization, formation of the catalytically active sites and reaction mechanism

Abstract Catalytic properties of a series of Ga- and Pt-Ga-containing zeolites of ZSM-5 structure in n-butane aromatization have been studied in wide range of reaction conditions. The profound Pt promotion has been well established on the basis of product selectivities analysis at different temperatures and contact times but equal conversions. The Pt and Ga synergy was also found to increase significantly selectivity to aromatics at the account of rapid dehydrogenation and suppressing such side reactions as hydrogenolysis and hydrogenation (Pt), cracking and isomerization (Ga). Both far distance effects (hydrogen spillover) and intimate contact between Pt particles and reduced Ga(I) ions seem to be responsible for the enhancement of aromatization activity and stability of Pt-Ga zeolite Catalysts. Active sites involve Pt, Ga1+ and protonic zeolite centers which formation and structure were considered in detail. Finally, aromatization process scheme was proposed to find out most important steps controlled by Pt and Ga active sites.

The Role of Chemical and Structural Changes on the Surface in Deactivation of Chromia-Alumina Catalysts in Dehydrogenation of Paraffinic Hydrocarbons

The chemical and structural changes on the surface of chromia-alumina catalysts, operated for long time in dehydrogenation of paraffins hydrocarbons have been investigated. In reduced media at 500°C a gradual decrease of the number of active site associated with Cr 2+ ions was established. A loss of surface area of chromia may be considered as the main reason of this phenomenon. The process was especially pronounced at 700°C. In oxidizing media at 500°C no change in the surface area and chemical state of the chromia surface were observed, whereas at 700°C the improvement in the dispersity of chromium compounds were found. The processes of reversible, partially reversible and complete deactivation of chromia-alumina catalyst were considered.

New active sltes induced by hydrogen spillover in reduced Pt-Ga/HZSM-5 aromatization catalysts

Abstract The role of hydrogen and its spillover in Pt-Ga oatalyst aotivation and performance has been disoussed. The enhancement of Ga reduction and formation of new active Bites is direotly related to this phenomena. The balanoe of direot and reverse H spillover appears to be neoessary to provide both rapid dehydrogenation and selective aromatization.

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.

Transformations of butyraldehyde in the presence of catalysts based on large-pore molecular sieves VPI-5 and AlPO4-8

It is found that zeolite-like crystalline aluminophosphates VPI-5, Si-VPI-5, and Mn-VPI-5 as well as those dirived from them, AlPO{in4}-8, SAPO{in4}-8, and MnAPO{in4}-8, are capable of catalyzing aldol condensation and crotonization of butyraldehyde (BA). Pd/AlPO{in4}-8 is catalytically active in hydrocondensation of BA with H{in2} at atmospheric pressure. The activities in BA conversion to 2-ethylhexane-3-ol-1-al increase in the following order: Mn-VPI-5 < Si-VPI-5 < VPI-5. The same order of activities is also found for AlPO{in4}-8, SAPO{in4}-8, and MnAPO{in4}-8. These catalysts are characterized by a lower initial activity in crotonization of BA than M{su+}NaX (CsNaX), but they are much more stable. Pd/AlPO{in4}-8 catalyzes BA conversion to 2-ethylhexanal even in the absence of H{in2} feed to the reaction zone. The influence of catalyst pretreatments and experimental conditions on the catalyst structures and catalytic activities is discussed.

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.

Electronic State of Dispersed Metals on Supports and Their Catalytic Activity in Some Reactions of Hydrocarbon Conversion

Valence states, surface concentrations and electronic structure of Hi, Pd, Re, Rh, Ru and Pt supported on γ-Al 2 O 3 -j, SiO 2 and Y zeolite have been studied by XPS at different stages of catalysts preparation, activation and some catalytic reactions. In active forms of the catalysts of benzene hydrogenation (Re/γ-Al 2 O 3 ) and ethylene cyclotrimerization (Rh/γ-Al 2 O 3 F) metals are present as electron-deficient clusters.

Selective alkylation of xylenes by alcohols on zeolite catalysts

The peculiarities of catalytic performance of crystalline aluminosilicates of different types and compositions (X, Y including dealuminated Y′, mordenite, pentasil ZSM-5), as well as of amorphous aluminosilicate catalyst in conversion of xylene + alcohol mixtures were studied. New data were obtained for alkylation ofo-xylene withtert-butyl alcohol, concerning controlling the selectivity and stability of the zeolite catalysts in reactions proceeding with the participation of water, including the water evolved during the reaction, in particular by controlling the acidic properties and hydrophobycity of the zeolites. A catalyst ensuring production of 1,2-dimethyl-4-tert-butylbenzene (DMTBB) with a 94% yield and selectivity of alcohol conversion to the target product of 94–97% was developed. The catalyst can be used as the basis for a high-performance and environmentally safe method for the synthesis of DMTBB. The catalysts developed can be also used for selective alkylation ofo-xylene by C3-C5 alcohols and for alkylation ofm-xylene bytert-butyl alcohol.