Lucas Cranach (1472-1553) was one of the greatest artists of the Renaissance, as shown by the diversity of his artistic interests as well as his awareness of the social and political events of this ...time. He developed a number of painting techniques which were afterwards used by several generations of artists. His somewhat mannered style and spending palette are easily recognized in numerous portraits of monarchs, cardinals, courtiers and their ladies, religious reformers, humanists and philosophers. A part of the Great Painters Collection, translated from the Russian by Paul Williams. 109 full color plates and numerous black and white and two-color illustrations interspersed by text. Includes a chronological table of the work of Cranach and his notable contemporaries.
Highly stable superprotonic conductivity (>10–2 S cm–1) has been achieved through the unprecedented solvent-free-coordinative urea insertion in MOF-74 M2(dobdc), M = Ni2+, Mg2+; dobdc = ...2,5-dioxido-1,4-benzenedicarboxylate without an acidic moiety. The urea is bound to open metal sites and alters the void volume and surface functionality, which triggers a significant change in proton conductivity and diffusion mechanism. Solid-state 2H NMR revealed that the high conductivity was attributed to the strengthening of the hydrogen bonds between guest H2O induced by hydrogen bonds in the interface between H2O and the polarized coordinated urea.
The intermediates formed upon the interaction of methane with Cu-modified ZSM-5 zeolites (Cu/H-ZSM-5) have been analyzed with solid-state NMR spectroscopy and DFT methods. Methane activation by ...Cu/H-ZSM-5 zeolites gives rise to three distinct surface methoxy-like species (−O–CH3) detected by 13C MAS NMR spectroscopy with specific chemical shifts in the range of 53–63 ppm. DFT calculations on representative cluster models of different sites potentially present in Cu/H-ZSM-5 have been used to assign these signals to (i) methanol adsorbed on two neighboring Cu sites (Cu–(HOCH3)–Cu, 62.6 ppm), (ii) methanol adsorbed on zeolite Brønsted acid sites (52.9 ppm), and (iii) lattice-bound methoxy groups (Si–O(CH3)–Al, 58.6). The formation of these methoxy-like intermediates depends on the Cu loading and, accordingly, the type of Cu species in the Cu/H-ZSM-5 zeolite. For the sample with low (0.1 wt %) Cu loading containing exclusively mononuclear isolated Cu species, only the intermediates ii and iii have been detected. The Cu-bound intermediate (i) is formed upon methane activation by multinuclear Cu sites featuring Cu–O–Cu bridging moieties present in the materials with relatively higher Cu loading (1.38 wt %). The presented results indicate that methane activation by Cu/H-ZSM-5 can be promoted by both mono- and multinuclear Cu species confined in the zeolite matrix.
Observing and quantifying the like-charge attraction in liquids and solutions is still challenging. However, we showed that elusive cation–cation hydrogen bonding may govern the structure and ...interaction in hydroxyl-functionalized ionic liquids. Therefore, cationic cluster formation depends on the shape, charge distribution, and functionality of the ions. We demonstrated by means of solid-state 2H NMR spectroscopy that cationic clusters change the structure and dynamics of ionic liquids. With increasing alkyl chain length, we observed two deuteron quadrupole coupling constants for the OD groups, differing by about 30 kHz. The lower value was assigned to the cation–cation interaction, indicating that the average (c–c) hydrogen bonds are stronger than the (c–a) hydrogen bonds between the cation and the anion despite the repulsive and attractive Coulomb interaction in the first and latter cases. Ion mobility could be studied by 2H NMR spectroscopy, although the deuterons in the hydrogen-bonded clusters underwent fast exchange. Our results also showed that simple relaxation models are not applicable anymore and that anisotropic motion must be considered.
With regard to establishing the effect of the nature of Zn species loaded in a zeolite on the aromatization of propane, the transformation of 13C-labeled propane (propane-1-13C and propane-2-13C) has ...been monitored by 1H and 13C (CP) MAS NMR at 296–623 K for two zeolite samples, containing exclusively either isolated Zn2+ cations (Zn2+/H-BEA zeolite sample) or small (ZnO) n clusters of zinc oxide (ZnO/H-BEA zeolite sample). It has been established that similar intermediates, π-complex of propene and σ-allylzinc species, are formed at propane transformation to aromatics on both zeolite samples. However, the formation of the identified intermediates and their evolution to final aromatic products have been detected to occur on Zn2+/H-BEA at lower temperature (by 100 K) as compared to the same transformation on ZnO/H-BEA zeolite. Analysis of the kinetics of propane transformation by 1H MAS NMR in situ has shown similar regularities for propane aromatization and hydrogenolysis on the different zeolite samples, but the reactions on Zn2+/H-BEA occur 10 times faster than on ZnO/H-BEA. This allows us to conclude on a higher efficiency of Zn2+ cations compared to ZnO species for propane aromatization on Zn-modified zeolites.
The desolvated (3,24)-connected metal–organic framework (MOF) material, MFM-160a, Cu3(L)(H2O)3 H6L = 1,3,5-triazine-2,4,6-tris(aminophenyl-4-isophthalic acid), exhibits excellent high-pressure ...uptake of CO2 (110 wt% at 20 bar, 298 K) and highly selective separation of C2 hydrocarbons from CH4 at 1 bar pressure. Henry’s law selectivities of 79:1 for C2H2:CH4 and 70:1 for C2H4:CH4 at 298 K are observed, consistent with ideal adsorption solution theory (IAST) predictions. Significantly, MFM-160a shows a selectivity of 16:1 for C2H2:CO2. Solid-state 2H NMR spectroscopic studies on partially deuterated MFM-160-d 12 confirm an ultra-low barrier (∼2 kJ mol–1) to rotation of the phenyl group in the activated MOF and a rotation rate 5 orders of magnitude slower than usually observed for solid-state materials (1.4 × 106 Hz cf. 1011–1013 Hz). Upon introduction of CO2 or C2H2 into desolvated MFM-160a, this rate of rotation was found to increase with increasing gas pressure, a phenomenon attributed to the weakening of an intramolecular hydrogen bond in the triazine-containing linker upon gas binding. DFT calculations of binding energies and interactions of CO2 and C2H2 around the triazine core are entirely consistent with the 2H NMR spectroscopic observations.
Modulation and precise control of porosity of metal-organic frameworks (MOFs) is of critical importance to their materials function. Here we report modulation of porosity for a series of isoreticular ...octacarboxylate MOFs, denoted MFM-180 to MFM-185, via a strategy of selective elongation of metal-organic cages. Owing to the high ligand connectivity, these MOFs do not show interpenetration, and are robust structures that have permanent porosity. Interestingly, activated MFM-185a shows a high Brunauer–Emmett–Teller (BET) surface area of 4,734 m² g−1 for an octacarboxylate MOF. These MOFs show remarkable CH₄ and CO₂ adsorption properties, notably with simultaneously high gravimetric and volumetric deliverable CH₄ capacities of 0.24 g g−1 and 163 vol/vol (298 K, 5–65 bar) recorded for MFM-185a due to selective elongation of tubular cages. The dynamics of molecular rotors in deuterated MFM-180a-d16 and MFM-181a-d16 were investigated by variable-temperature ²H solid-state NMR spectroscopy to reveal the reorientation mechanisms within these materials. Analysis of the flipping modes of the mobile phenyl groups, their rotational rates, and transition temperatures paves the way to controlling and understanding the role of molecular rotors through design of organic linkers within porous MOF materials.
13
C MAS NMR spectroscopy is a powerful technique to study the mechanisms of hydrocarbon transformations on heterogeneous catalysts. It can reliably identify the surface intermediates and the ...adsorbed products based on the analysis of their
13
C chemical shifts,
δ
(
13
C). However, the unambiguous assignment of the detected signals is always a challenge due to the uncertainty of the nature of the surface intermediates formed and the mechanism of adsorbed species interaction with active sites. The way to solve this problem is the application of DFT calculations to predict chemical shifts for the expected intermediate hydrocarbon species. Herein, the methodology for
δ
(
13
C) chemical shift calculations for adsorbed species has been proposed. It includes: (i) zeolite framework optimization with periodic DFT (pPBE); (ii) medium-sized cluster geometry optimization with hybrid GGA (PBE0), and (iii)
σ
(
13
C) values calculation followed by
δ
(
13
C) estimation using the linear regression method. It is inferred that the TPSS/cc-pVTZ method provides the best computational cost/accuracy ratio for the set of adsorbed hydrocarbon species that was previously detected experimentally on the surface of Zn-containing zeolites. The drawbacks of the computation method have also been revealed and discussed.
The methodology for chemical shift,
δ
(
13
C), calculations for the intermediates and adsorbed species on zeolite catalysts has been proposed.
Methane oxidation by N2O was studied at 160°C on the FeZSM-5 surface with increased concentration of α-sites (100μmol/g). The reaction was shown to lead directly to methanol. Spillover of methanol ...from α-sites provides a “quasicatalytic” mode of the reaction with TON>3. Display omitted
► Methane oxidation by N2O on FeZSM-5 α-sites at 160°C leads directly to methanol. ► Methanol spillovers from α-sites providing “quasicatalytic” mode of reaction (TON>3). ► A part of methanol is converted to DME and non-extractable products.
In our previous work using FeZSM-5 zeolite with increased concentration of α-sites (100μmol/g), the oxidation of methane by α-oxygen predeposited from nitrous oxide was studied at room temperature. The reaction proceeded by hydrogen abstraction mechanism yielding methoxy and hydroxy groups bound to α-sites.
The present work conducted with the same zeolite is devoted to methane oxidation by N2O in the process of α-oxygen deposition at 160°С. Under these conditions, the reaction was shown to proceed at a stoichiometric ratio СН4:N2O=1:1 yielding directly methanol. Spillover of methanol from α-sites liberates them for further events of α-oxygen deposition, thus converting the reaction to a “quasicatalytic” mode that runs up to turnover number exceeding 3 with no product desorption into the gas phase. A part of methanol is converted to dimethyl ether, traces of acetaldehyde, and some amount of non-extractable products.
Mechanism of the reaction is discussed.
Metal–organic framework (MOF) glasses have emerged as a new class of organic–inorganic hybrid glass materials. Considerable efforts have been devoted to unraveling the macroscopic dynamics of MOF ...glasses by studying their rheological behavior; however, their microscopic dynamics remain unclear. In this work, we studied the effect of vitrification on linker dynamics in ZIF-62 by solid-state 2H nuclear magnetic resonance (NMR) spectroscopy. 2H NMR relaxation analysis provided a detailed picture of the mobility of the ZIF-62 linkers, including local restricted librations and a large-amplitude twist; these details were verified by molecular dynamics. A comparison of ZIF-62 crystals and glasses revealed that vitrification does not drastically affect the fast individual flipping motions with large-amplitude twists, whereas it facilitates slow cooperative large-amplitude twist motions with a decrease in the activation barrier. These observations support the findings of previous studies, indicating that glassy ZIF-62 retains permanent porosity and that short-range disorder exists in the alignment of ligands because of distortion of the coordination angle.
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