Quantum chemical studies of zeolite proton catalyzed reactions

Theoretical chemistry applied to zeolite acid catalysis is becoming an important tool in the understanding of the adsorption and interaction of guest molecules with the zeolitic lattice. Especially the understanding of the mechanisms by which zeolite catalyzed chemical reactions proceed becomes possible. It is shown here that the old interpretation of carbonium and carbenium ions as intermediates for zeolite catalyzed reactions has to be replaced by a new approach in terms of positively charged transition states that are strongly stabilized by the zeolitic lattice. The large deprotonation energy of the acidic zeolite is overcome by stabilization of the intermediate or transition state positive charge by the negative charge left in the lattice. The zeolitic sites responsible for the adsorption and/or reaction of guest molecules are the Brønsted-acid and Lewis-base sites. We also show that different transition states are responsible for different kinds of reactions, such as cracking, dehydrogenation, etc.     

The chemistry of thiohydrazonates and their utility in organic synthesis

Syntheses, spectroscopic properties and reactions of thiohydrazonates are reviewed. The reactions of the title compounds are subdivided in sections that cover their rearrangement, oxidation and substitution. The utility of these esters in synthesis of thirty six different heterocyclic ring systems has also been presented. Their biological and industrial applications are pointed out.     

Kinetics Inside,Outside and Through Cucurbiturils

This review describes the kinetics at play in Cucurbituril (CB[n])/guest recognition within a very wide time frame, from picoseconds to days. We cover the kinetics of (1) radiative and non-radiative decays of fluorophores inside and outside CB[n]s, (2) guest motions in the CB[n] cavity, and (3) ingression, egression and threading mechanisms into, out of and through the macrocycles. We then show that the kinetics of guest release or capture can be controlled using CB[n]-capped mesoporous systems and micelles, or by coupling those processes to the kinetics of ancillary reactions. The last section is devoted to the latest examples of reactions catalyzed and inhibited by CB[n]s.     

Thermodynamics of the encapsulation by cyclodextrins

Molecular encapsulation on a molecular basis can be performed by cyclodextrins. The inclusion of organic molecules into the interior changes the properties of these molecules, which may be used for a broad variety of applications. The affinity of guest molecules for the cavities of various cyclodextrins depends on the stereochemistry and on the interaction forces of the molecules involved. Calculations of the thermodynamic parameters show that the reaction entropy is highly important for the inclusion reaction. Completely different reaction mechanisms are observed for various types of cyclodextrins as some of these reactions show enthalpy–entropy compensation. Others are supported by the reaction entropy or are even entropically controlled. Protonation and deprotonation reactions contribute significantly to the inclusion reaction, as first of all the solubility of the compounds in water is strongly influenced by the acidity of the solution, and, moreover, all tautomeric forms of the compounds show different affinities to various cyclodextrins. Copyright © 2006 Society of Chemical Industry     

Microemulsions of water in supercritical carbon dioxide: an in-situ NMR investigation of micelle formation and structure

High-pressure NMR spectroscopy was used for the first time to investigate microemulsions of water in supercritical carbon dioxide. The emulsions were formed using a family of anionic perfluoropolyether ammonium carboxylate surfactants. This system holds promise as a reaction medium for conducting homogeneous catalytic reactions within the aqueous micellular cores while, at the same time, exploiting the facile mass transfer properties of the supercritical fluid. Ammonium hexafluorophosphate was used as a water-soluble ionic guest to investigate micelle formation and structure. Under micelle-forming conditions, the PF₆⁻ guest, surfactant, and water were uniformly dispersed throughout the CO₂ phase, as demonstrated by in situ NMR imaging. In addition, the micelles were observed to form even in the absence of mechanical stirring. This spontaneous formation of micelles demonstrates that the NMR spectral properties were obtained under conditions that result in the production of thermodynamically stable microemulsions. The nuclear overhauser effect (NOE) was used to probe the micellular structure through dipole–dipole interactions between the PF₆⁻ anion and the fluorinated backbone of the surfactant. A strong negative homonuclear NoE was observed between the PF₆⁻ guest and the fluorine moiety that is located directly adjacent to the surfactant's carboxylate head group. This highly specific negative NOE indicates an ordered arrangement, where the PF₆⁻ anion and carboxylate ion are located in close proximity to one another. This close association of two negatively charged ionic groups in an aqueous environment is unusual and suggests that the PF₆⁻ guest is concentrated within the electric double layer that forms at the micellular interface.     

Single Molecule Studies on Dynamics in Liquid Crystals

Single molecule (SM) methods are able to resolve structure related dynamics of guest molecules in liquid crystals (LC). Highly diluted small dye molecules on the one hand explore structure formation and LC dynamics, on the other hand they report about a distortion caused by the guest molecules. The anisotropic structure of LC materials is used to retrieve specific conformation related properties of larger guest molecules like conjugated polymers. This in particular sheds light on organization mechanisms within biological cells, where large molecules are found in nematic LC surroundings. This review gives a short overview related to the application of highly sensitive SM detection schemes in LC.     

State transition of macroscopic supramolecular hydrogel by the host-guest coverage effect

In this work, a hydrophobic guest monomer containing adamant end-group was synthesized and introduced into the acrylamide hydrogel system to prepare macroscopic guest hydrogel. The swelling, transparency, and mechanical properties of the guest hydrogels can be significantly changed by soaking the guest hydrogel into hydrophilic cyclodextrin solution because the hydrophobic guest groups were recognized and covered by the hydrophilic host molecules. In addition, the surface lubrication property could be adjusted by changing the proportion of acrylamide and adamantane monomers in the hydrogel system. More importantly, cyclodextrin can bind with adamantane through host-guest interactions, enabling the adjustment of the hydrophilicity/hydrophobicity properties of the hydrogel system. By controlling the assembly time, the hydrogel with different lubrication behaviors can be obtained. The controllable surface friction that this specific binding of host-guest interaction has broad application prospects in intelligent device, bionic lubrication and other fields.     

Investigation of the molybdenum-vanadium-niobium (oxidic) system and its phases for their catalytic effects

Crystalline phases of a catalyst have been identified and prepared in pure form. Their properties are presented in a comparable way to those of tie catalyst. Activity measurements have been performed for the catalytic accelerated reactions of ethane and air to ethene and acetic acid.     

柔性金属有机骨架材料(MOFs)用于气体吸附分离

柔性金属有机骨架材料(MOFs)具有高度有序的网络结构与可变形的骨架,其骨架结构会对外界的温度、压力及客体分子的刺激产生独特的结构响应。近几年来,柔性MOFs在气体吸附、气体分离、传感等领域显示出巨大的应用潜力。截至目前,研究者们对柔性MOFs的研究仅局限于对其结构形变的机理解释,而缺乏对柔性MOFs应用于相关化工过程的性能研究。本文着重对近年来柔性MOFs在气体吸附分离领域的研究进展进行了综述,并详细地分析了柔性MOFs结构与其气体吸附分离性能之间的构效关系。通过分子模拟结合实验,讨论了柔性MOFs结构对气体分子的平衡吸附与动力学扩散的影响。分析表明,设计合成具有良好吸附选择性与扩散性能的柔性MOFs是其应用于绿色、高效气体分离过程的重要发展方向。     

Microporous porphyrin solids

Metalloporphyrins are exceedingly useful building blocks for the design and synthesis of molecularly based solids. The use of hydrogen bonding or metal ion coordination provides a wide range of framework solids. Using various polyfunctionalized porphyrins, we have created porous solids that are thermally robust and that retain their internal porosity upon loss of solvates. Their pore dimensions are comparable to zeolites, and they show shape and size selectivity in sorption of guest molecules and in epoxidation of alkenes.     

Gauche conformation of acyclic guest molecules appearing in the large cages of structure-H clathrate hydrates

In the present study, measurements and analyses were made of the High-Power Decoupling (HPDEC) solid-state ¹³C NMR spectra of structure-H (sH) methane hydrates with isopentane, one of the simplest and smallest acyclic large guest molecules, and methylcyclohexane (MCH), a commonly used cyclic guest molecule that is larger than isopentane. From the spectroscopic information, clear and definite evidence for the molecular conformation of acyclic guest molecules that are sufficiently small so as to be entrapped into the structure-H large cage (sH-L) was expected. The ¹³C NMR chemical shift change was additionally checked through the use of a hydrogen-hydrogen steric perturbation model. From the overall results, we concluded that one of the smallest acyclic guest molecules, isopentane, participating in the formation of a structure-H clathrate hydrate is encaged, confirming the gauche conformation in large cavities. The present results strongly suggest that the guest position and structure in hydrate cages are greatly influenced by both short-range interactions between guest molecules and cage frameworks and long-range interactions between small and large guests. Accordingly, cage dynamics must be carefully considered when a specific sH hydrate is designed and synthesized for the purpose of tuning material properties.     

Formation of spheres and siliceous plugs in synthesizing MCF materials

The unique pore structure combined with its high pore volume and large pore size make MCF material attractive as a catalyst support related to reactions with large molecules or as a capsule for storing and controlled release of organic guest substances. By varying the amount of TEOS, or the stirring temperature or the aging conditions in the synthesis, spheres are formed among the products. Siliceous plugs inside the MCF structures can also be produced in case of aging the synthesis mixtures at relatively low temperatures in flasks in oil baths instead of using the autoclave procedure. If the aging temperature is relatively high (such as 80°C), normal MCF material is also obtainable with this simplified procedure. These observations provide a further possibility to tune the morphology and the textural properties of the MCF materials.     

Structure and reactivity of carbons: From carbon black to carbon composites

The reactivity and the surface properties of carbons are closely related to their crystalline structure. Dealing with various types of carbonaceous materials carbon blacks, graphites and carbon fibres, it is shown that the chemical reactivity of carbons toward oxidation reagents depends primarily on their degree of crystalline organization. Several types of carbon surface groups can be evidenced on carbons. Their properties are illustrated. Grafting surface reactions and their mechanism are discussed. The surface energy of carbon materials of different structures or surface treatment are finally discussed.     

Azobenzene isomerization in polymer co-crystalline phases

Three different co-crystalline phases of syndiotactic polystyrene (s-PS) with azobenzene have been prepared and structurally characterized. The triclinic δ-clathrate phase is characterized by a styrene-monomeric-unit/guest ratio (SU/G) equal to 8/1 while the δ-intercalate phase is characterized by SU/lG = 4/1. The orthorhombic ε-clathrate phase allows stacking of a variable number of guest molecules per monomer unit with SU/G ≥ 9/1. Isomerization reactions of the azobenzene guest molecules (both photo-induced trans → cis and spontaneous cis → trans) have been studied by UV–Visible and FTIR measurements. Repeated isomerization cycles produce progressive desorption from the polymer film of azobenzene molecules being simply dissolved in s-PS amorphous phases, while poorly affect the content of azobenzene molecules being guest of s-PS co-crystalline phases. Polarized FTIR spectra of axially oriented films indicate that isomerizations occur without expulsion of azobenzene guest molecules from the polymer co-crystalline phases. The whole set of data indicate that films exhibiting co-crystalline phases of azobenzene with s-PS are suitable for producing self-erasing photo-isomerization patterns with molecular size marks.     

Alcohol Reactivity on Zeolites and Molecular Sieves

Zeolites and molecular sieves have been extensively studied and applied in different fields. Their specific crystalline structure and composition make it possible to obtain high-activity stable catalyst systems for a wide range of chemical reactions. There are many investigations on the structural characteristics and adsorption and catalytic properties of zeolites and molecular sieves as well as on the relation between the different properties. The nature of active sites in zeolite catalysts and the mechanism of catalytic processes on their surface belong to the most important problems of the investigations.     

Alcohol Reactivity on Zeolites and Molecular Sieves

Zeolites and molecular sieves have been extensively studied and applied in different fields. Their specific crystalline structure and composition make it possible to obtain high-activity stable catalyst systems for a wide range of chemical reactions. There are many investigations on the structural characteristics and adsorption and catalytic properties of zeolites and molecular sieves as well as on the relation between the different properties. The nature of active sites in zeolite catalysts and the mechanism of catalytic processes on their surface belong to the most important problems of the investigations.     

Nature and Size of Included Guest Molecule Determines Architecture of Crystalline Cyclodextrin Host Matrix

Owing to its annular shape, the cyclic hexasaccharide α-cyclodextrin (α-CD) can form inclusion complexes in aqueous solution. If these are crystallized, the guest molecules are invariably accommodated in the cavities of the host. Depending on size and molecular or ionic character of the guest, different crystal modifications are formed belonging to the herringbone or brick-type cage structures or to the channel structures. Small molecular guests lead to herringbone cages, small aromatic guests crystallize in brick-type cages whereas ionic and long molecular guests are enclosed in infinite channels. Thus, there is a clear dependence of crystal packing on the physical properties of the guest molecule. Polyiodide cocrystallizes with α-CD in different channel forms with iodine atoms arranged linearly in the channels. The packing of the channels is in tetragonal, hexagonal or sheet-like arrays, depending on counterions which are located in voids between the channels. If a α-CD is replaced by the larger β-CD (channel diameters 5 and 6.2 Å resp.), polyiodide now adopts a zigzag structure — a clear case of an influence of host matrix geometry on guest configuration.     

Recent advances in shear-thinning and self-healing hydrogels for biomedical applications

Shear-thinning and self-healing hydrogels are being investigated in various biomedical applications including drug delivery, tissue engineering, and 3D bioprinting. Such hydrogels are formed through dynamic and reversible interactions between polymers or polypeptides that allow these shear-thinning and self-healing properties, including physical associations (e.g., hydrogen bonds, guest–host interactions, biorecognition motifs, hydrophobicity, electrostatics, and metal–ligand coordination) and dynamic covalent chemistry (e.g., Schiff base, oxime chemistry, disulfide bonds, and reversible Diels–Alder). Their shear-thinning properties allow for injectability, as the hydrogel exhibits viscous flow under shear, and their self-healing nature allows for stabilization when shear is removed. Hydrogels can be formulated as uniform polymer and polypeptide assemblies, as hydrogel nanocomposites, or in granular hydrogel form. This review focuses on recent advances in shear-thinning and self-healing hydrogels that are promising for biomedical applications. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48668.     

Encapsulation of metallo-porphyrins into water-soluble amphipatic dendrigrafts

Newly prepared water-soluble amphipatic hyperbranched organic polymers are used as nanocontainer and as carriers for large metal-organic molecules such as metallo-porphyrins. The hydrosoluble dendrigrafts are constituted by a polystyrene hydrophobic core surrounded by a polyvinyl ether hydrophilic shell. These nanometer-sized amphipatic macromolecules with an average diameter of about 100 nm and narrow polydispersity are fully soluble in aqueous media.Their capacity to encapsulate and transport metallo-porphyrins in aqueous media is investigated. Complexation studies show that up to several thousands of guest molecules can be embedded into each amphipatic dendrigraft macromolecule.     

Electron spin resonance spectroscopy: a promising method for studying lipid oxidation in foods

Oxidative reactions that involve lipids are among the most important causes of deterioration of foods and affect both their shelf life and their organoleptic and nutritional properties. Radicals are intermediate key‐constituents in these reactions. Their detection is therefore essential in order to understand and predict early stages of these destructive oxidations. Several Electron Spin Resonance (ESR) methodologies (direct and indirect) make it possible to identify, quantify and measure the reactivity of radical species formed during oxidative‐reductive reactions. They are also used to evaluate the antiradical activity of antioxidants delaying lipid oxidation. An overview on the use of different ESR methodologies to study lipid oxidation in foods is reported in this paper.