L. M. Kustov

Effects of the support on the morphology and electronic properties of supported metal clusters: modern concepts and progress in 1990s

Abstract Modern trends in studying the influence of the support on the electronic state, morphology, and catalytic properties of supported metal particles are reviewed. The analysis of new developments in techniques for catalyst characterization is given. Metal–support interaction effects are discussed in terms of electronic modification of the supported clusters and their morphological transformations induced by the support. Special attention is paid to the recent concepts of the structure of the metal–support interface. The support effects on the catalytic properties of metal particles are revealed as (1) changes due to metal particle charging, (2) effects related to variations in metal particle shape and crystallographic structure and, (3) appearance of the specific active sites at the metal–support boundary.

Spectra of hydroxyl groups in zeolites in the near-infrared region

Diffuse reflectance ir spectra of hydroxyl groups in decationized and cationic forms of zeolites, X, Y, and mordenite have been studied for the first time in a wide spectral range, including the fundamental stretching vibrations of OH bonds, their overtones and combination bands. Surface OH groups of amorphous aluminosilicate and silica gel have also been investigated. The combination bands of stretching and bending vibrations of OH groups are shown to be more sensitive to the environment of the hydroxyls than are the stretching vibrations (fundamental and overtones). The observation has been made that in the supercages of X-zeolite there are two different types of structural hydroxyls, which are not resolved in the fundamental region of the spectra, but differ both in their bending and combined frequencies. Cations introduced into zeolites by ion exchange influence the structural OH groups, resulting in a shift of their combination bands, in comparison with decationized samples, to lower or higher frequencies, depending on the nature of the cation and on the zeolite structure. The change in the bending frequencies of different OH groups is considered to be connected with the strength of their coordination to the neighboring aluminum atoms. The Morse function parameters of the potential curves xe and ve and the isotopic effects of OH and OD groups in zeolites are calculated. The applicability of these data to characterize the acidic properties of OH groups is discussed.

Measuring and predicting Delta(vap)H298 values of ionic liquids.

We report the enthalpies of vaporisation (measured using temperature programmed desorption by mass spectrometry) of twelve ionic liquids (ILs), covering four imidazolium, [C(m)C(n)Im]+, five pyrrolidinium, [C(n)C(m)Pyrr]+, two pyridinium, [C(n)Py]+, and a dication, [C3(C1Im)2]2+ based IL. These cations were paired with a range of anions: [BF4]-, [FeCl4]-, [N(CN)2]-, [PF3(C2F5)3]- ([FAP]-), [(CF3SO2)2N]- ([Tf2N]-) and [SCN]-. Using these results, plus those for a further eight imidazolium based ILs published earlier (which include the anions [CF3SO3]- ([TfO]-), [PF6]- and [EtSO4]-), we show that the enthalpies of vaporisation can be decomposed into three components. The first component is the Coulombic interaction between the ions, DeltaU(Cou,R), which is a function of the IL molar volume, V(m), and a parameter R(r) which quantifies the relative change in anion-cation distance on evaporation from the liquid phase to the ion pair in the gas phase. The second and third components are the van der Waals contributions from the anion, DeltaH(vdw,A), and the cation, DeltaH(vdw,C). We derive a universal value for R(r), and individual values of DeltaH(vdw,A) and DeltaH(vdw,C) for each of the anions and cations considered in this study. Given the molar volume, it is possible to estimate the enthalpies of vaporisation of ILs composed of any combination of the ions considered here; values for fourteen ILs which have not yet been studied experimentally are given.

Comparative IR-spectroscopic study of low-temperature H2 and CO adsorption on Na zeolites

Extraframework cation sites in the sodium forms of the zeolites ZSM-5, mordenite, Linde-4A and faujasite-type X and Y have been investigated by using low-temperature adsorption of dihydrogen and carbon monoxide as IR spectroscopic probes. The extent of H—H and C—O bond polarization was found to be dependent not only on the cation electrostatic field, but also on the neighbouring oxygen atoms of the zeolite framework. The influence of these oxygen atoms is most keenly felt by adsorbed molecular hydrogen, but they also affect the IR frequency shift of the stretching vibration of adsorbed carbon monoxide. The Si : Al ratio of the zeolite framework modulates the basic strength of the oxygen atoms, and this was found to be reflected in the IR stretching frequency of both adsorbed molecules, H2 and CO.

Ultrasound enhancement of cellulose processing in ionic liquids: from dissolution towards functionalization

The ionic liquids 1-allyl-3-methylimidazoliumchloride[Amim][Cl] and 1-butyl-3-methylimidazoliumchloride[Bmim][Cl] were utilized in the dissolution of different natural cellulose biopolymers. The biopolymers subject to this investigation were microcrystalline cellulose, cotton linters as well as Kraft cellulose cut to 0.35 mm fibres. High-intensity acoustic irradiation by means of an ultrasonic horn was applied to enhance the dissolution process. The ionic liquids investigated were able to dissolve cellulose at elevated temperatures, in high concentrations, although significant differences were observed, depending on the type of cellulose and ionic liquid. Moreover, the dissolution process under conventional heating was rather slow, typically extending for a period of several hours. Upon use of high-power ultrasound, the dissolution process was dramatically intensified and complete dissolution was achieved in a matter of few minutes only. Various approaches to cellulose functionalization were proposed and investigated. The effect of external parameters, such as the reaction temperature, the cellulose–derivatising agent molar ratio and the batch time of experiment were studied. Various physico-chemical methods, such as acid methanolysis, TGA, DSC, SEM as well as NMR on 1H and 13C nuclei were applied to investigate the structure and morphology of both the cellulose samples and ionic liquids before and after processing.

A new type of acidic hydroxyl groups in ZSM-5 zeolite and in mordenite according to diffuse reflectance i.r. spectroscopy

HZSM-5 zeolites and H mordenite were studied using diffuse reflectance i.r. spectroscopy in the region of stretching vibrations of hydroxyls. A new type of acidic OH groups was detected that represents a bridged hydroxyl forming a strained intramolecular hydrogen bond with a neighboring oxygen atom. Properties of the new type of hydroxyls were investigated and are discussed.

Reaction Products and Transformations of Intermediates in the Aqueous‐Phase Reforming of Sorbitol

Aqueous phase reforming of sorbitol over Pt supported on an alumina catalyst is investigated, in order to identify the intermediates involved in the transformation of the initial feed. Parameters such as the sorbitol feed rate and temperature are studied. To identify the intermediates, an approach based on analysis of the gas and liquid phases as well as the total carbon content was developed. According to analysis by gas chromatography combined with mass spectrometry of volatile substances collected with solid-phase microextraction, over 260 compounds are involved in the transformation of sorbitol. Of these, 50 of the major products are identified with high reliability. It is shown that a great variety of compounds, bearing different functionalities, form part of the reaction network. The formation of the majority of identified compounds is explained and a reaction network for the formation of sorbitol and intermediate molecules transformation is proposed.

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.

The application of metal-organic frameworks in catalysis (Review)

The current status of research in the design of heterogeneous catalysts based on metal-organic frameworks (MOFs) and their application in various processes is discussed. Particular attention is given to reactions pertaining to selective catalysis. It has been shown that metal-organic frameworks successfully compete with the classical molecular sieves zeolites in this area. However, MOFs are still not able to replace these inorganic systems in the cases when a catalytic reaction requires severe conditions. Their primary fields of application are fine organic synthesis and enantioselective catalysis, since the relevant catalytic processes can be carried out under mild conditions.

Near infrared diffuse reflectance study of high silica containing zeolites

Abstract Decationated mordenite and high silica containing zeolites of ZSM type were studied using diffuse reflectance i.r. spectroscopy in a wide spectral range. Their hydroxyl groups and Lewis acidic centres are shown to be identical.