In situ NMR spectroscopy in heterogeneous catalysis: Kinetic study of hydrocarbon conversion mechanisms

The potential of high-resolution solid-state NMR spectroscopy for kinetic and mechanistic studies of hydrocarbon conversion on solid acid catalysis between 20 and 300°C is considered. The use of this technique is illustrated by the elucidation of the mechanisms of hydrogen exchange and ¹³C label transfer in alkanes and olefins, n-butane isomerization on sulfated zirconia, and ethane aromatization on zinc-containing zeolite beta. The kinetic parameters determined in these studies provide a basis for quantum chemical calculations of possible hydrocarbon activation and conversion pathways and for evaluating the reliability and accuracy of these theoretical calculations.     

The theoretical approach to surface chemistry and heterogeneous catalysis

Transition metals are the most interesting items among heavy atoms due to their extensive use in catalysis. Present understanding of heterogeneous catalytic reactions is still very poor, and new experimental and theoretical techniques have been developed in recent years with the aim of elucidating the mechanism of adsorption and reaction of simple molecules over metal surfaces or clusters. Empirical, semiempirical, and ab initio methods of calculations are briefly reviewed: results and perspectives are presented.     

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.     

Insights into reaction mechanisms in heterogeneous catalysis revealed by in situ NMR spectroscopy

This tutorial review intends to show the possibilities of in situ solid state NMR spectroscopy in the elucidation of reaction mechanisms and the nature of the active sites in heterogeneous catalysis. After a brief overview of the more usual experimental devices used for in situ solid state NMR spectroscopy measurements, some examples of applications taken from the recent literature will be presented. It will be shown that in situ NMR spectroscopy allows: (i) the identification of stable intermediates and transient species using indirect methods, (ii) to prove shape selectivity in zeolites, (iii) the study of reaction kinetics, and (iv) the determination of the nature and the role played by the active sites in a catalytic reaction. The approaches and methodology used to get this information will be illustrated here summarizing the most relevant contributions on the investigation of the mechanisms of a series of reactions of industrial interest: aromatization of alkanes on bifunctional catalysts, carbonylation reaction of methanol with carbon monoxide, ethylbenzene disproportionation, and the Beckmann rearrangement reaction. Special attention is paid to the research carried out on the role played by carbenium ions and alkoxy as intermediate species in the transformation of hydrocarbon molecules on solid acid catalysts.     

Chemo-regioselectivity in heterogeneous catalysis: competitive routes for C = O and C = C hydrogenations from a theoretical approach

The usual empirical rule stating that the C=C bond is more reactive than the C=O group for catalytic hydrogenations of unsaturated aldehydes is invalidated from the present study. Density functional theory calculations of all the competitive hydrogenation routes of acrolein on Pt(111) reveals conversely that the attack at the C=O bond is systematically favored. The explanation of such catalytic behavior is the existence of metastable precursor states for the O-H bond formation showing that the attack at the oxygen atom follows a new preferential mechanism where the C=O moiety is not directly bonded with the Pt surface atoms, hence yielding an intermediate pathway between Langmuir-Hinshelwood and Rideal-Eley general types of mechanisms. When the whole catalytic cycle is considered, our results reconcile with experimental studies devoted to hydrogenation of acrolein on Pt, since the desorption step of the partially hydrogenated product (unsaturated alcohol versus saturated aldehyde) plays a key role for the selectivity.     

Recent development in in situ NMR study on heterogeneous catalysis: mechanisms of light alkane functionalisation

In situ NMR studies of C(1)-C(5) light alkane activation and functionalisation in heterogeneous catalytic systems are overviewed. The results obtained from the NMR technique, particularly those quantitative ones, provide unique information on the activation of alkane molecules and the nature of relevant intermediates, leading to better understanding reaction mechanisms and designing catalysts.     

Reference molecules for nonlinear optics: a joint experimental and theoretical investigation

Hyper-Rayleigh scattering (HRS) experiments and quantum chemical calculations are combined to investigate the second-order nonlinear optical responses of a series of reference molecules, namely, carbon tetrachloride, chloroform, trichloroacetonitrile, acetonitrile, and dichloromethane. The multipolar decomposition of the first hyperpolarizability tensor through the use of the spherical harmonics formalism is employed to highlight the impact of the symmetry of the molecular scatterers on their nonlinear optical responses. It is demonstrated that HRS is a technique of choice to probe the molecular symmetry of the compounds. Coupled-cluster calculations performed at the coupled-cluster level with singles, doubles, and perturbative triples in combination with highly extended basis sets and including environment effects by using the polarizable continuum model qualitatively reproduce the molecular first hyperpolarizabilities and depolarization ratios of the molecular scatterers.     

In situ solid-state NMR characterization of pharmaceutical materials: An example of drug-polymer thermal mixing

Pharmaceutical amorphous solid dispersions, a multicomponent system prepared by dispersing drug substances into polymeric matrix via thermal and mechanical processes, represent a major platform to deliver the poorly water-soluble drug. Microscopic properties of drug-polymer contacts play mechanistic roles in manipulating long-term physical stability as well as dissolution profiles. Although solid-state nuclear magnetic resonance has been utilized as an indispensable tool to probe structural details, previous studies are limited to ex situ characterizations. Our work provides likely the first documented example to investigate comelting of ketoconazole and polyacrylic acid, as a model system, in an in situ manner. Their physical mixture is melted and mixed in the solid-state nuclear magnetic resonance rotor under magic angle spinning at up to approximately 400 K. Critical structural events of molecular miscibility and interaction have been successfully identified. These results design and evaluate the instrumental and experimental protocols for real-time characterizations of the comelting of pharmaceutical materials.     

Modelling amorphous materials via a joint solid-state NMR and X-ray absorption spectroscopy and DFT approach: application to alumina

Understanding a material''s electronic structure is crucial to the development of many functional devices from semiconductors to solar cells and Li-ion batteries. A material''s properties, including electronic structure, are dependent on the arrangement of its atoms. However, structure determination (the process of uncovering the atomic arrangement), is impeded, both experimentally and computationally, by disorder. The lack of a verifiable atomic model presents a huge challenge when designing functional amorphous materials. Such materials may be characterised through their local atomic environments using, for example, solid-state NMR and XAS. By using these two spectroscopy methods to inform the sampling of configurations from ab initio molecular dynamics we devise and validate an amorphous model, choosing amorphous alumina to illustrate the approach due to its wide range of technological uses. Our model predicts two distinct geometric environments of AlO₅ coordination polyhedra and determines the origin of the pre-edge features in the Al K-edge XAS. From our model we construct an average electronic density of states for amorphous alumina, and identify localized states at the conduction band minimum (CBM). We show that the presence of a pre-edge peak in the XAS is a result of transitions from the Al 1s to Al 3s states at the CBM. Deconvoluting this XAS by coordination geometry reveals contributions from both AlO₄ and AlO₅ geometries at the CBM give rise to the pre-edge, which provides insight into the role of AlO₅ in the electronic structure of alumina. This work represents an important advance within the field of solid-state amorphous modelling, providing a method for developing amorphous models through the comparison of experimental and computationally derived spectra, which may then be used to determine the electronic structure of amorphous materials.

A first principles model of amorphous Al₂O₃ is constructed which reproduces the features of experimental ²⁷Al NMR and Al K-edge XAS spectra on an ALD-deposited sample of amorphous Al₂O₃.     

固体核磁共振技术在固体酸催化剂表征及催化反应机理研究之应用进展

固体酸催化剂广泛应用于现代石油与化学工业中,其反应活性与其酸性密切相关.与传统的酸性表征方法(红外光谱、程序升温脱附、滴定等)相比,利用先进的探针分子技术、双共振和二维相关谱等核磁共振(NMR)技术可以获取固体催化剂酸种类、酸分布、酸浓度和酸强度等完整信息.同时,原位固体NMR实验可跟踪反应分子在催化剂活性中心吸附状态和转换的中间体物种,为揭示反应机理提供了最直接的实验证据.本文详细介绍了固体NMR的原理和一系列相关新技术,着重综述了固体NMR技术在酸催化剂结构、活性中心特性以及催化反应机理方面的应用进展.     

In situ 13C solid-state polarization transfer NMR to follow starch transformations in food

Convenience food products tend to alter their quality and texture while stored. Texture-giving food components are often starch-rich ingredients, such as pasta or rice. Starch transforms depending on time, temperature and water content, which alters the properties of products. Monitoring these transformations, which are associated with a change in mobility of the starch chain segments, could optimize the quality of food products containing multiple ingredients. In order to do so, we applied a simple and efficient in situ ¹³C solid-state magic angle spinning (MAS) NMR approach, based on two different polarization transfer schemes, cross polarization (CP) and insensitive nuclei enhanced by polarization transfer (INEPT). The efficiency of the CP and INEPT transfer depends strongly on the mobility of chain segments—the time scale of reorientation of the CH-bond and the order parameter. Rigid crystalline or amorphous starch chains give rise to CP peaks, whereas mobile gelatinized starch chains appear as INEPT peaks. Comparing ¹³C solid-state MAS NMR experiments based on CP and INEPT allows insight into the progress of gelatinization, and other starch transformations, by reporting on both rigid and mobile starch chains simultaneously with atomic resolution by the ¹³C chemical shift. In conjunction with ¹H solid-state MAS NMR, complementary information about other food components present at low concentration, such as lipids and protein, can be obtained. We demonstrate our approach on starch-based products and commercial pasta as a function of temperature and storage.     

Magnetic resonance imaging methods for in situ studies in heterogeneous catalysis

Magnetic resonance imaging (MRI) is a very powerful instrument used extensively in modern medical diagnostics because of its ability to look inside a body in a non-invasive and non-destructive way. Furthermore, MRI is more than just a single tool for extracting structural information. It is more of a sophisticated and versatile toolkit able to provide all sorts of useful information about the internal properties of an object under study and various processes within it, including heat and mass transport, composition and chemical transformations, in a spatially resolved mode. While a living body is different from a catalyst body or a reactor, the in situ and Operando studies in catalysis can clearly benefit from the use of this non-destructive toolkit as a powerful complement to other available spectroscopic tools. This tutorial review gives an introduction to the field and describes the examples of the applications of MRI to the studies of the preparation, deactivation and regeneration of solid catalysts and to the spectroscopy, thermometry and imaging studies of heterogeneous catalysts and model catalytic reactors performed during actual catalytic processes.     

In situ structural characterization of a recombinant protein in native Escherichia coli membranes with solid-state magic-angle-spinning NMR

The feasibility of using solid-state magic-angle-spinning NMR spectroscopy for in situ structural characterization of the LR11 (sorLA) transmembrane domain (TM) in native Escherichia coli membranes is presented. LR11 interacts with the human amyloid precursor protein (APP), a central player in the pathology of Alzheimer's disease. The background signals from E. coli lipids and membrane proteins had only minor effects on the LR11 TM resonances. Approximately 50% of the LR11 TM residues were assigned by using (13)C PARIS data. These assignments allowed comparisons of the secondary structure of the LR11 TM in native membrane environments and commonly used membrane mimics (e.g., micelles). In situ spectroscopy bypasses several obstacles in the preparation of membrane proteins for structural analysis and offers the opportunity to investigate how membrane heterogeneity, bilayer asymmetry, chemical gradients, and macromolecular crowding affect the protein structure.     

1H NMR analysis of O‐methyl‐inositol isomers: a joint experimental and theoretical study

Density functional theory (DFT) calculations of ¹H NMR chemical shifts for l ‐quebrachitol isomers were performed using the B3LYP functional employing the 6‐31G(d,p) and 6‐311 + G(2d,p) basis sets. The effect of the solvent on the B3LYP‐calculated NMR spectrum was accounted for using the polarizable continuum model. Comparison is made with experimental ¹H NMR spectroscopic data, which shed light on the average uncertainty present in DFT calculations of chemical shifts and showed that the best match between experimental and theoretical B3LYP ¹H NMR profiles is a good strategy to assign the molecular structure present in the sample handled in the experimental measurements. Among four plausible O‐methyl‐inositol isomers, the l ‐quebrachitol 2a structure was unambiguously assigned based only on the comparative analysis of experimental and theoretical ¹H NMR chemical shift data. The B3LYP infrared (IR) spectrum was also calculated for the four isomers and compared with the experimental data, with analysis of the theoretical IR profiles corroborating assignment of the 2a structure. Therefore, it is confirmed in this study that a combined experimental/DFT spectroscopic investigation is a powerful tool in structural/conformational analysis studies. Copyright © 2012 John Wiley & Sons, Ltd.     

Infrared spectroscopy of copper-resveratrol complexes: a joint experimental and theoretical study

Infrared multiple-photon dissociation spectroscopy has been used to record vibrational spectra of charged copper-resveratrol complexes in the 3500-3700 cm(-1) and 1100-1900 cm(-1) regions. Minimum energy structures have been determined by density functional theory calculations using plane waves and pseudopotentials. In particular, the copper(I)-resveratrol complex presents a tetra-coordinated metal bound with two carbon atoms of the alkenyl moiety and two closest carbons of the adjoining resorcinol ring. For these geometries vibrational spectra have been calculated by using linear response theory. The good agreement between experimental and calculated IR spectra for the selected species confirms the overall reliability of the proposed geometries.     

In situ monitoring of solid-state polymerization reactions in sodium chloroacetate and sodium bromoacetate by 23Na and 13C solid-state NMR spectroscopy

The thermally induced solidstate polymerization reactions in sodium chloroacetate and sodium bromoacetate, leading to poly(hydroxyacetic acid) (polyglycolide) and NaCl and NaBr, respectively, were studied by isothermal in situ solid-state NMR spectroscopy at 120, 130 and 140 degrees C with a time resolution of the order of 5 to 25 min. The nuclei probed were 23Na and 13C, allowing the parent compounds (sodium halogenoacetates) and both reaction products (polymer and alkali halide) to be monitored. For sodium chloroacetate, there is no evidence for the involvement of intermediate phases during the reaction whereas this cannot be excluded for sodium bromoacetate. The crystal structure of sodium bromoacetate was determined directly from powder diffraction data by the Monte Carlo method, and was found to be isostructural with sodium chloroacetate. The topochemical reaction mechanism proposed previously for sodium chloroacetate is thus also applicable for the polymerization reaction in sodium bromoacetate. The mechanistic and kinetic information obtained from our in situ solid-state NMR investigations is compared and contrasted with information obtained from other in situ probes of the polymerization reactions in these materials.     

Analysis of a cycloheptenone derivative: an experimental and theoretical approach

A detailed analysis with total assignment of (1)H and (13)C NMR spectral data for a cycloheptenone derivative, a key intermediate for the synthesis of perhydroazulene terpenoids, is related. These assignments are based on 1D (1)H and (13)C NMR and on 2D NMR techniques including gCOSY, gHSQC, gHMBC, J-resolved and NOEDIF experiments. The unequivocal assignments were supported by theoretical chemical shifts and scalar coupling constant calculations at GIAO B3LYP/cc-pVDZ level from optimized structures at the same level of theory.     

"Single sample concept": theoretical model for a combinatorial approach to solid-state inorganic materials

A theoretical model for a "single sample concept" (SSC) applied to the combinatorial chemistry of solid-state inorganic compounds is presented. The SSC is performed by reacting N starting materials (randomly mixed) in a single sample of approximately 1 cm(3). Combinatorial calculations demonstrate that the number of reasonably estimated phases to be found in the space of N components grouped into compounds (e.g. oxides, sulfides, or halogenides) containing up to q < or = 6 metallic elements is smaller than combinations set up by starting conditions for local reactions within the bulk of a single ceramic sample. Recently, the SSC proved to work in the case of synthesizing libraries of 3d metal oxides, from which magnetic particulate matter was extracted by a magnetic separation technique. The SSC may be applied in a first screening cycle followed by 2D approaches of combinatorial chemistry.     

The Si-B chromophore: A joint experimental and theoretical investigation

Silylboranes with aromatic substituents linked to boron and silicon exhibit an unexpected absorption band in the UV-Vis spectral region. When polar groups were introduced, a marked solvatochromic effect was observed in their fluorescence emission spectra, revealing a strong excited state dipole moment. Semi-empirical MNDO/d and AM1 calculations showed that, upon UV excitation, the polarity of the Si-B bond increased and the aromatic π-electrons migrated toward the Si-B bond, consistent with experimental observations.     

Electronic delocalization in discotic liquid crystals: a joint experimental and theoretical study

Discotic liquid crystals emerge as very attractive materials for organic-based (opto)electronics as they allow efficient charge and energy transport along self-organized molecular columns. Here, angle-resolved photoelectron spectroscopy (ARUPS) is used to investigate the electronic structure and supramolecular organization of the discotic molecule, hexakis(hexylthio)diquinoxalino[2,3-a:2',3'-c]phenazine, deposited on graphite. The ARUPS data reveal significant changes in the electronic properties when going from disordered to columnar phases, the main feature being a decrease in ionization potential by 1.8 eV following the appearance of new electronic states at low binding energy. This evolution is rationalized by quantum-chemical calculations performed on model stacks containing from two to six molecules, which illustrate the formation of a quasi-band structure with Bloch-like orbitals delocalized over several molecules in the column. The ARUPS data also point to an energy dispersion of the upper pi-bands in the columns by some 1.1 eV, therefore highlighting the strongly delocalized nature of the pi-electrons along the discotic stacks.