An IR-Spectroscopic Study of the State of Zirconium in Supported Zirconocene Catalysts

The surface compounds of zirconium in SiO₂/MAO/Cp₂ZrMe₂ (I) and SiO₂/Cp₂ZrMe₂ (II) catalysts were studied by the IR spectroscopy of adsorbed CO at 93-293 K. It was found that the bridging and terminal complexes of CO were formed on the surface of the catalysts at 93 K; the adsorption of CO was reversible. At 293 K, CO was irreversibly inserted into the Zr-Me bond with the formation of various acyl compounds of zirconium; these compounds were characterized by absorption bands in the region 1495-1750 cm^-1 in the IR spectrum. These data suggested the heterogeneity of the surface composition of zirconocene catalysts Iand II. Catalyst I, which was active in ethylene polymerization, contained surface zirconium complexes that can be considered as the precursors of active centers for ethylene polymerization.     

New binary systems Mg-M-O (M=Y, La, Ce): Synthesis and physico-chemical characterization

New binary oxide Mg-M-O (M=Y, La, Ce) systems are obtained by co-precipitation and characterized by adsorption methods, X-ray diffraction method (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy of adsorbed probe molecules (CO and CDCl₃). It is shown that Mg-Y-O systems after calcination at 450-750 °C represent the physical mixtures of MgO and Y₂O₃, while the components of Mg-La(Ce)-O systems interact to form La₂MgO_x and (Ce,Mg)O₂ solid solutions, respectively. From XPS data, the surface of the binary systems is enriched with lanthanide ions. Addition of ≈5 mol% M₂O₃ to MgO results in an increase in concentrations of strong and weak Lewis acidic sites, the content of the latter being much higher and changing in the series: MgO<Mg-Ce-O<Mg-La-O<Mg-Y-O. At the same time basic sites become stronger in the binary systems but their total content decreases in comparison to that in individual MgO. Mg-M-O samples containing ≈5 mol% M₂O₃ are highly-dispersed and characterized by bimodal porous texture.     

Regularities of Lewis site formation upon the microwave irradiation of gibbsite-γ-Al(OH)3

Spatially separated Lewis acidic and basic sites can be formed upon the microwave irradiation of gibbsite. This is demonstrated by IR spectroscopy and indicates the possibility of the selective activation of chemical bonds of different nature within the initial crystals of Al³⁺ hydroxide. A scheme for the formation of spatially separated Lewis acidic and basic sites is proposed. It is shown that water as a product of the micro-wave activation of gibbsite is adsorbed on these Lewis acidic sites in molecular form (without subsequent dissociation) and is desorbed at calcination temperatures no higher than 100–110°C. During standard (contact) thermal treatment of Al³⁺ hydroxides and oxides, dehydroxylation and water removal take place at 350–550°C to form the acid-base pair-Al^δ+-O^δ?-. The microwave activation of gibbsite results in the formation of an amorphous component believed to consist of small -Al-O- complexes closely packed in the solid phase. It is established that the fraction of Al³⁺ atoms accessible for the low-temperature adsorption of CO in microwave-activated (for 10 min) gibbsite is 4.5 at % of the total number of Al³⁺ atoms present in the amorphous component of this material.     

Adsorption sites of an iron-aluminum catalyst for ammonia oxidation as studied by the IR spectroscopy of the adsorbed NO probe molecule

The state of surface adsorption sites in the IK-42-1 oxide catalyst for ammonia oxidation depending on catalyst preparation conditions (the nature of raw materials and the temperature of calcination) was studied in this work with the use of the diffuse reflectance IR spectroscopy of the adsorbed NO probe molecule. Hematite, which was prepared by a sulfate or chloride technology, was used as the starting raw material; Al₂O₃ binding agents were prepared by the reprecipitation or hydration of thermally activated gibbsite; and acetic or nitric acid was used as an electrolyte. The samples were calcined at 900–1000°C. It was found that mono- and dinitrosyl complexes with reduced coordinatively unsaturated Fe²⁺ cations and nitrite-nitrate complexes were formed upon the adsorption of NO on the catalyst surface (regardless of the catalyst preparation conditions). The samples differed in the amount and degree of coordinative unsaturation of adsorption sites depending on the preparation conditions. It was concluded that the most coordinatively unsaturated Fe²⁺ adsorption sites observed were formed on the surface of a solid solution of iron cations in aluminum oxide, which was formed in the course of catalyst preparation. It was found that an increase in the catalyst calcination temperature resulted in a decrease in the number of coordinatively unsaturated adsorption sites, which correlated with the observed decrease in the yield of NO. This correlation had the shape of a saturation curve, which can reflect the occurrence of a reaction in the diffusion mode at high degrees of conversion for the majority of catalysts.     

The influence of mechanical activation and the nature of a carrier on the structural and diffusion properties of zeolite-containing catalysts

The influence of mechanical activation and the nature of a carrier on the structural and diffusion properties of zeolite-containing catalysts, including the microporous and mesoporous structure of zeolites, was studied. Zeolite-matrix contacts were found to provide the accessibility of the inside volume of zeolite pores. In addition, new pores of a larger size appeared. The IR spectra of adsorbed molecules were used to determine the effective diffusion coefficients of methanol in the porous system of zeolites. It was shown that there was some optimum zeolite pore radius at which the largest diffusion coefficient was attained.     

IR spectroscopic study of the basicity of aminated silica gels

The aminated silica gels SiO₂/SOCl₂/NH₃ (I), SiO₂/SiCl₄/NH₃ (II), SiO₂/BCl₃/NH₃ (III), and SiO₂/γ-aminopropyltriethoxysilane (SiO₂/APTES, IV) have been synthesized. According to DRIFT spectroscopy and chemical analysis data, the surface amino groups of I–III are “free,” while those of IV interact with the surface OH groups of the silica gel and with one another. The strength of basic sites has been measured on the proton affinity (PA) scale as the shift of the ν(CD) band of adsorbed deuterochloroform. The basicity of an aminated silica gel depends on its chemical composition. Silica gel IV (PA = 938 kJ/mol) is a stronger base than I–III (PA = 829 kJ/mol). As the basicity of the NH₂ group decreases, the N-H stretching band shifts to higher frequencies.     

Acid and catalytic properties of MnOm?ZrO2 (M=Ca, Ba, Sm, Yb) compositions

The volume stages of ethylene glycol oxidation to glyoxal on silver have been studied by varying the catalyst grain size. Temperature oscillations have been detected in the catalyst layer. The nature of the oscillations and the range of their generation were investigated by varying the oxygen, ethylene glycol and water vapor contents in the reaction mixture. Glyoxal formation was shown to occur on the silver catalyst surface. The generation of oscillations is responsible for the CO to CO₂ oxidation in the volume between the catalyst grains.     

Application of deuteroacetonitrile to characterize surface aprotic acidic centers on heterogeneous catalysts

Correlations between adsorption heats CD₃Cn and CO ( and Q_CO) for the cations of Ca-, Mn-, Co-, Ni- and CrNaY zeolites have been studied. Isosteric ranging within determined by IR spectroscopy are proportional to Q_CO.

---

CD₃CN CO( Q_CO) Ca-, Mn-, Co-, Ni- CrNaY. , , - Q_CO.