A. A. Maerle

Towards understanding of the mechanism of stepwise zeolite recrystallization into micro/mesoporous materials

The mechanism of stepwise mordenite recrystallization has been studied by a complex of physicochemical methods: multinuclear MAS NMR, X-ray diffraction (XRD), X-ray fluorescence (XRF), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and nitrogen adsorption–desorption. The recrystallization procedure included 4 steps: (1) treatment in alkali, (2) treatment in the presence of cetyltrimethylammonium bromide (CTAB) surfactant; (3) hydrothermal treatment and (4) hydrothermal treatment after pH adjustment to 8. Intermediate products were recovered during each reaction step and thoroughly investigated. The results pointed to the dissolution/re-assembling mechanism. The comparison with one-step recrystallization discussed previously suggests that the stepwise addition of NaOH and the surfactant does not affect significantly the recrystallization mechanism. In contrast, stepwise hydrothermal treatment with the intermediate pH regulation enhances the condensation of siliceous species formed during desilication, leads to their rearrangement into an ordered mesoporous phase and results in the product yields of 100%. Depending on the degree of zeolite dissolution and the extent of mesoporous phase overgrowth, two different types of materials were obtained: coated mesoporous zeolites and mesoporous materials containing small zeolitic fragments in the walls. It was shown that the latter could be obtained only using a stepwise procedure.

Design of micro/mesoporous zeolite-based catalysts for petrochemical and organic syntheses

This review systematizes and analyzes the physicochemical and catalytic properties of micro/mesoporous materials prepared by two-step recrystallization. The materials are divided into three groups according to the degree of recrystallization: (1) mesostructured zeolites (RZEO-1), (2) micro/mesoporous nanocomposites (RZEO-2), and (3) mesoporous materials with zeolite fragments (RZEO-3). Particular attention is focused on the catalytic properties of the recrystallized materials in petrochemical and organic syntheses. Main advantages of the micro/mesoporous molecular sieves over zeolites and mesoporous materials are demonstrated, and ways of obtaining new catalytic systems based on these molecular sieves are suggested.

Mesoporous MgO: Synthesis, physico-chemical, and catalytic properties

Mesoporous MgO was obtained via the hydrothermal synthesis using both ionogenic and non-ionogenic surfactants as structure-directing templates. The materials prepared were characterized by SEM, BET-N2, XRD, and TG-DTA techniques. MgO particles are spherical 20-μm aggregates of primary oxide particles well shaped as rectangular parallelepipeds. Magnesium oxide samples have the specific surface area of 290–400 m2/g and pore sizes of 3.3–4.1 nm. Their mesoporous structure remained unchanged after calcination up to 350°C. Catalytic activity of mesoporous MgO was studied in acetone condensation reaction.

Structured mesoporous Mn, Fe, and Co oxides: Synthesis, physicochemical, and catalytic properties

Structured mesoporous Mn, Fe, and Co oxides are synthesized using “soft” and “hard” templates; the resulting materials are characterized by XRD, SEM, TEM, BET, and TG. It is shown that in the first case, the oxides have high surface areas of up to 450 m2/g that are preserved after calcination of the material up to 300°C. Even though, the surface area of the oxides prepared by the “hard-template” method does not exceed 100 m2/g; it is, however, thermally stable up to 500°C. Catalytic activity of mesoporous oxides in methanol conversion was found to depend on both the nature of the transition metal and the type of template used in synthesis.

Production of Isobutylene from Acetone over Micro–Mesoporous Catalysts

The production of isobutylene from acetone over micro–mesoporous catalysts with different mesopore contents, which have been prepared using hydrothermal recrystallization of mordenite (MOR) zeolite modified with cesium acetate by incipient wetness impregnation, has been studied. It has been shown that cesium is inserted into the cation positions during the modification, at the same time the number of Brønsted acid sites in the samples decreased. It has been found that an increase in the content of mesopores in the catalyst leads to an increase in the initial rates of acetone conversion and isobutylene formation as a result of removing diffusion limitations. Brønsted acid sites have been shown to be preferable for the selective production of isobutylene from acetone. Micro–mesoporous materials operate more stably as compared to microporous materials.

Cobalt pivalate complex as a catalyst for liquid phase oxidation of n-hexane

Catalytic properties of cobalt(II) pivalate complex as both individual and supported on mesoporous molecular sieves Si-KIT-6, Al-KIT-6, and Ce-KIT-6 were investigated in liquid-phase oxidation of n-hexane with molecular oxygen. This complex was shown to be an active and selective catalyst for the oxidation of n-C6H14 into C1–C4 carboxylic acids. The activity of Co(II) pivalate remains practically unchanged on heterogenizing the complex on molecular sieve supports. At the same time, its selectivity and resistance towards an oxidative degradation are slightly increased.

Mesoporous cobalt and manganese spinels: Synthesis and physicochemical properties

Binary spinels of Со and Мn are obtained by means of hard-template synthesis. Their structural and chemical characteristics are determined via N2-BET, XRD, and TEM. Their catalytic activity is studied using methanol oxidation as the model reaction.

Ultrasound-assisted synthesis of nanostructured transition metal oxides

Ultrasound pretreatment of aqueous solutions of cetyltrimethylammonium bromide as a structure- directing agent has been applied to prepare nanostructured mesoporous Mn, Fe, and Ni oxides. After removal of the template by triple extraction with a water–ethanol solution of sodium acetate or ammonium chloride, air-calcined samples of oxide materials prepared in such a way possess surface areas of about 300–450 m2/g and are thermally stable up to 300°C.