V.N. Romannikov

Mesopore size and surface area calculations for hexagonal mesophases (types MCM-41, FSM-16, etc.) using low-angle XRD and adsorption data

This paper is devoted to the geometry of mesoporous mesophase systems (MMSs) of the MCM-41 or FSM-16 texture types. The interrelations among surface areas inside and outside the mesoporous blocks of the MMS, of the size and the volume of mesopores are discussed. The equations for calculating these textural characteristics are derived. These equations are based on adsorption data and X-ray diffraction studies, and are applied for considerations of the silica and zirconia hexagonal MMS texture. The approach was used for a critical review of the results published by other authors for similar systems.

X-ray structural modeling of silicate mesoporous mesophase material

Abstract Rietveld’s technique in combination with a continuous electron density representation was applied to structural modeling of highly ordered pure siliceous mesoporous mesophase material C16-SiO 2 -MMM of the MCM-41 type prepared by hydrothermal synthesis in the presence of C 16 H 33 N(CH 3 ) 3 Br. Several important characteristics of the material in both as-synthesized and calcined forms were revealed. In particular, it was found that mesopores in the materials are of true hexagonal shape, and the wall electron density seems to be not continuous.

Alkene and thioether oxidations with H2O2 over Ti- and V-containing mesoporous mesophase catalysts

Abstract Well-organized Ti- and V-containing silicate mesoporous mesophase materials (MMM) were synthesized using C 16 H 33 N(CH 3 ) 3 Br and characterized by elemental analysis, IR, DRS-UV, XRD, and N 2 adsorption. Catalytic properties of these materials in alkene and thioether oxidations with aqueous H 2 O 2 were examined. The structure–activity study was performed for the C16-(Ti,Si)-MMM catalysts with the Si/Ti atomic ratio ranged from 19 to 166. The samples with the Si/Ti atomic ratio in the range of 49–124 showed the highest catalytic activities. Both structure perfection of the silicate matrix and degree of isolation of titanium ions in it appeared to be crucial factors determining catalytic activity of C16-(Ti,Si)-MMM in alkene oxidation. For efficient oxidation of more nucleophilic substrates (thioethers) the structural perfection of the C16-(Ti,Si)-MMM catalysts was less important. No titanium leaching occurred even in 1.1 M H 2 O 2 solution at 353 K, whereas vanadium leaching was already pronounced at [H 2 O 2 ]=0.05 M and room temperature. The oxidation processes proved to be true heterogeneous for C16-(Ti,Si)-MMM and mostly homogeneous for C16-(V,Si)-MMM.

Modelling of the prebiotic synthesis of oligopeptides: silicate catalysts help to overcome the critical stage.

On the basis of experimental studies of the initial stages of glycine oligomerization in aqueous suspension of zeolite and kaolinite catalysts, a model is suggested for the prebiotic synthesis of oligopeptides from α-amino acids. The formation of linear dipeptides by hydrolysis of one amide bond in the cyclic piperazinedione intermediate (formed from glycine spontaneously) is found to be the critical stage of the reaction. This stage is base catalyzed and its rate increases when pH of the medium goes up. The linear glycyl-glycine yield rises under effect of hydroxyl anions generated from different sources including insoluble silicates and soluble sodium bicarbonate. During prebiotic evolution silicates capable of cation-exchange can serve as local sources of the hydroxyl anions which dramatically accelerate formation of linear dipeptides from cyclic ones. Oligopeptides of higher molecular weight are then easily formed from the linear dipeptides at neutral pH, even in the absence of catalysts or sources of energy (e.g. such as light). The described catalytic synthesis could occur in the proximity of submarine hydrothermal vents.

Comparative textural study of highly ordered silicate and aluminosilicate mesoporous mesophase materials having different pore sizes

Abstract Honeycomb structure model was applied for describing surface properties in the MCM-41 and SBA-15 types of highly ordered silicas and aluminosilicates with different nominal mesopore diameters: about 4 nm and about 10 nm. As shown, calculations of internal specific surface area A me require a heterogeneity of this surface to be taken into account, the latter being quantified by the roughness coefficient β . These calculations at the condition β =1.0 will allow to estimate the lower limit of specific surface area of the materials only, which will correspond to the perfectly smooth walls of mesopores. This description seems to be valid for the MCM-41 type systems, for which β is about 1.1–1.2. On the contrary, β is found to exceed 1.5 for the SBA-15 type materials. This fact is probably due to an essential non-smoothness of the mesopore surface in these systems and may be associated with their synthesis conditions.

Alkene and thioether oxidations with H2O2 over Ti-containing mesoporous mesophase catalysts

Abstract Well-organized Ti-containing mesoporous mesophase materials (Ti-MMM) have been synthesized by hydrothermal synthesis using C 16 H 33 N(CH 3 ) 3 Br and characterized by elemental analysis, IR, DRS-UV, XRD, and N 2 adsorption. Catalytic properties of these materials in alkene and thioether oxidations with aqueous H 2 O 2 in CH 3 CN have been studied and compared with those of the Ti-containing hexagonal mesoporous silica (Ti-HMS). The Ti-MMM catalysts with the Si/Ti atomic ratio varying in the range of 49–124 showed the highest catalytic activities in alkene oxidation. Both the structure of a silicate matrix and a degree of isolation of titanium ions in it are crucial factors determining catalytic activity of Ti-MMM. For thioether oxidation, the structural perfection of Ti-MMM catalysts is less important. No leaching of titanium ions occurs during both reactions. The oxidation process proved to be truly heterogeneous.

13C CP/MAS NMR study of isobutyl alcohol dehydration on H-ZSM-5 zeolite. Evidence for the formation of stable isobutyl silyl ether intermediate

Dehydration of isobutyl alcohol selectively labelled with a13C nucleus in the CH2 group (i-BuOH[1−13C]) has been studied on H-ZSM-5 zeolite within the temperature range 296–448 K using13C CP/MAS NMR. The formation of the isobutyl silyl ether intermediate (IBSE) has been detected. It is stable below 398 K. Within the temperature range 398–423 K IBSE decomposes gradually to produce first a butene dimer, probably 2,5-dimethyl-l-hexene and then other butene dimers and oligomers. AtT > 423 K scrambling of the selectively labelled carbon of the initial dimeric product over various positions in the carbon skeleton of the final dimers (oligomers) is observed. This is explained in terms of the formation of carbenium ion as the reaction intermediate.

Highly-ordered aluminosilicate mesoporous mesophase materials: physico-chemical properties and catalytic behaviour

Abstract Structural and textural characteristics as well as coordination states of aluminium and some catalytic properties are studied for highly-ordered (with very narrow XRD reflections) aluminosilicate mesoporous mesophase materials ((Si,Al)-MMM) prepared with different aluminium concentrations by hydrothermal synthesis in the presence of C 16 H 33 N(CH 3 ) 3 Br. As shown, all physico-chemical parameters measured for these systems including their structural hydrostability are strongly influenced by the aluminium content. Based on analysis of the results obtained, a description of probable transformations during formation of mesoporosity in the C16-(Si,Al)-MMM is considered.

Mesoporous basic zirconium sulfate: structure, acidic properties and catalytic behaviour

Abstract Some properties of a thermostable mesophase of basic zirconium sulfate with texture characteristics close to those of MCM-41 are described. After calcination at 550 °C the mesophase exhibits a specific surface area of mesopores of 450–500 m2g−1. The peculiarities of the catalytic behavior of the mesophase are related to its acidic properties.

Selective alkene epoxidation by molecular oxygen in the presence of aldehyde and different type catalysts containing cobalt

Publisher Summary This chapter discusses a comparative study of the catalytic properties of different type cobalt-containing compounds in alkene epoxidation by dioxygen in the presence of iso-butyraldehyde (IBA) and provides some data that allows clarifying the reaction mechanism and the nature of the catalytic action of cobalt compounds. Here, the catalytic properties of cobalt-containing compounds having different nature have been discussed. These include the simple salt, Co(NO3)2.6H2O, tetra-n-butylammonium salts of PW11CoO593−(PW11CO) and CoW12O40−6(CoW12) heteropolyanions (HPA), CoNaY zeolite, and Co(II) phtalocyanine (CoPc), in alkene epoxidation by the O2/IBA system. It has been found that various alkenes can be converted to the corresponding epoxides with good-to-high selectivity (80%–99%) at complete alkene conversion at ambient conditions. Neither allylic oxidation nor epoxide ring cleavage products were detected for all alkenes tested except for cyclohexene. The nature of catalyst does not considerably affect the selectivity of the epoxidation that depends mainly on the olefin structure. Some decrease of the selectivity was generally observed at high cobalt concentrations. The results obtained in this investigation prove that alkene epoxidation by O2 in the presence of iso-butyraldehyde (IBA) and cobalt catalysts proceeds via radical chain mechanism. Acylperoxy radicals act most likely as the main epoxidizing species although some other species, for example, coordinated to the metal center acylperoxy radicals, may contribute into the epoxidation process when catalysts with low redox potentials are used. Superior catalytic activity of cobalt compounds in alkene epoxidation by O2/IBA system is because of the high ability of cobalt to catalyze the chain branching and promote the chain initiation rather than the ability of cobalt to activate dioxygen via its coordination.