Probing zeolites by vibrational spectroscopies

This review addresses the most relevant aspects of vibrational spectroscopies (IR, Raman and INS) applied to zeolites and zeotype materials. Surface Brønsted and Lewis acidity and surface basicity are treated in detail. The role of probe molecules and the relevance of tuning both the proton affinity and the steric hindrance of the probe to fully understand and map the complex site population present inside microporous materials are critically discussed. A detailed description of the methods needed to precisely determine the IR absorption coefficients is given, making IR a quantitative technique. The thermodynamic parameters of the adsorption process that can be extracted from a variable-temperature IR study are described. Finally, cutting-edge space- and time-resolved experiments are reviewed. All aspects are discussed by reporting relevant examples. When available, the theoretical literature related to the reviewed experimental results is reported to support the interpretation of the vibrational spectra on an atomic level.

Photoinduced Formation of Peroxide Ions on La2O3 and Nd2O3 under O2: Effects of Excitation Wavelength and Crystal Structure

Photoinduced formation of peroxide ions on La2O3 and Nd2O3 under O2 was studied by in-situ microprobe Raman spectroscopy with attention focused on the effect of excitation wavelength and crystal structure on the O2(2-) formation. It was found that photoexcitations at 633, 532, 514, and 325 nm can induce O2(2-) formation over La2O3 at 450 °C. By contrast, photoexcitation at 785 nm does not cause formation of O2(2-) up to 500 °C. Photoexcitation at 325 nm can induce O2(2-) formation on cubic Nd2O3 at 25 °C, but cannot induce O2(2-) formation on hexagonal Nd2O3 up to 200 °C. The significant difference in the behavior of O2(2-) formation over the Nd2O3 samples of the two structures can be related to the difference in the capacity to adsorb O2. Since the number of oxygen vacancies in cubic Nd2O3 is larger than that in the hexagonal one, the former has a higher capacity than the latter to adsorb O2. As a result, cubic Nd2O3 is more favorable to the reaction of O2 with O(2-) to generate O2(2-). The structural similarity between cubic Nd2O3 and Nd2O2(O2) may be another factor in favor of peroxide formation.

Regioselective direct oxidative C–H cyanation of quinoline and its derivatives catalyzed by vanadium-containing heteropoly acids

In the presence of catalytic amounts of vanadium-containing heteropoly acids, oxidative C–H cyanation of quinoline and its derivatives using trimethylsilyl cyanide and molecular oxygen efficiently proceeded, affording the corresponding substituted 4-cyanoquinolines as the major products.

Utilization of methylarenes as versatile building blocks in organic synthesis

This review attempts to describe the latest developments in the utilisation of methylarenes adopting C–H functionalization strategies and covers all the developments until March 2015.

Density functional theory simulations of complex catalytic materials in reactive environments: beyond the ideal surface at low coverage

Advanced DFT models of complex catalysts, such as amorphous silica–alumina and supported subnanometric platinum particles, bridge the gap between the ideal surface model and the industrial catalyst.

Looking inside catalyst extrudates with time-resolved surface-enhanced Raman spectroscopy (TR-SERS)

Raman spectroscopy is one of the major characterization methods employed over the last few decades as a nondestructive technique for the study of heterogeneous catalysts and related catalytic reactions. However, the promise of practical applicability on millimeter-sized catalyst bodies, such as extrudates, has not been fulfilled completely. Large fluorescence signals and the highly scattering nature of the extrudates often hamper its practical usage. Different approaches to overcome this problem were examined, including the use of time-resolved Raman spectroscopy (TRRS), spatially offset Raman spectroscopy (SORS), surface-enhanced Raman spectroscopy (SERS), and combinations of these techniques. This paper demonstrates that especially TRRS can provide chemical information at depth within catalyst bodies, overcoming fluorescence background signals and allowing for visualization of analytes at different depths. It also examines the application of time-resolved SERS within catalyst bodies to gain insight into localized activity. With these options a wider applicability of Raman spectroscopy for industrial catalysis research becomes within reach.

Probing the surfaces of heterogeneous catalysts by in situ IR spectroscopy

This critical review describes the reactivity of heterogeneous catalysts from the point of view of four simple, but essential for Chemistry, molecules (namely dihydrogen, carbon monoxide, nitrogen monoxide and ethylene) that are considered as probes or as reactants in combination with "in situ" controlled temperature and pressure Infrared spectroscopy. The fundamental properties of H(2), CO, NO and C(2)H(4) are shortly described in order to justify their different behaviour in respect of isolated sites in different environments, extended surfaces, clusters, crystalline or amorphous materials. The description is given by considering some "key studies" and trying to evidence similarities and differences among surfaces and probes (572 references).

Chemical imaging of catalytic solids with synchrotron radiation

Heterogeneous catalysis is a term normally used to describe a group of catalytic processes, yet it could equally be employed to describe the catalytic solid itself. A better understanding of the chemical and structural variation within such materials is thus a pre-requisite for the rationalising of structure-function relationships and ultimately to the design of new, more sustainable catalytic processes. The past 20 years has witnessed marked improvements in technologies required for analytical measurements at synchrotron sources, including higher photon brightness, nano-focusing, rapid, high resolution data acquisition and in the handling of large volumes of data. It is now possible to image materials using the entire synchrotron radiative profile, thus heralding a new era of in situ/operando measurements of catalytic solids. In this tutorial review we discuss the recent work in this exciting new research area and finally conclude with a future outlook on what will be possible/challenging to measure in the not-too-distant future.

In situ electron paramagnetic resonance: a unique tool for analyzing structure-reactivity relationships in heterogeneous catalysis

Electron Paramagnetic Resonance (EPR) offers widespread opportunities for monitoring catalytically relevant species that contain unpaired electrons under conditions close to those of heterogeneous catalytic gas and liquid phase reactions. In this tutorial review, after introducing basic theoretical and experimental principles of the technique, selected examples of typical applications are discussed that comprise (1) transition metal ions in paramagnetic valence states such as vanadium, (2) radical anions such as O˙(-) formed on oxide surfaces and (3) electrons in ferromagnetic particles such as nickel as well as in conduction bands of organic conductors such as polyaniline.

(1)H and (13)C MAS NMR studies of light alkanes activation over MFI zeolite modified by Zn vapour

The early stages of methane, ethane and propane conversion were studied by in situ (1)H and (13)C MAS NMR techniques over fully exchanged Zn(2+)/MFI catalyst obtained by the reaction with zinc vapour. The in situ techniques revealed strong interaction of alkanes with Zn(2+) cations evidenced by significant shift of the corresponding NMR lines. Besides that, the formation of methyl zinc, ethyl zinc and n-propyl zinc species along with bridging and silanol surface OH-groups was detected already at the ambient temperature. These results pointed to dissociative adsorption of alkanes over (ZO)-Zn(2+)-(OZ) and (ZO)-Zn(2+)-(OSi) active sites of the catalyst. The dissociative adsorption was shown to be a dead-end surface reaction in the case of methane starting reactant, while in the case of ethane and propane, it appeared to be responsible for the initiation of the catalytic cycle leading to alkenes and dihydrogen formation and regeneration of zinc containing catalytic sites.