利用ZnO@ZIF-8核壳结构构建高选择性、高稳定性的Pd/ZnO催化剂用于CO2加氢制甲醇

Catalytic CO₂ hydrogenation to methanol is a promising route to mitigate the negative effects of anthropogenic CO₂. To develop an efficient Pd/ZnO catalyst, increasing the contact between Pd and ZnO is of the utmost importance, because "naked" Pd favors CO production via the reverse water-gas shift path. Here, we have utilized a ZnO@ZIF-8 core-shell structure to synthesize Pd/ZnO catalysts via Pd immobilization and calcination. The merit of this method is that the porous outer layer can offer abundant "guest rooms" for Pd, ensuring intimate contact between Pd and the post-generated ZnO. The synthesized Pd/ZnO catalysts (PZZ8-T, T denotes the temperature of calcination in degree Celsius) is compared with a ZnO nanorod-immobilized Pd catalyst (PZ). When the catalytic reaction was performed at lower reaction temperatures (250, 270, and 290 ℃), the highest methanol space time yield (STY) and highest STY per Pd achieved by PZ at 290 ℃ were 0.465 g g_cat^-1 h^-1 and 13.0 g g_Pd^-1 h^-1, respectively. However, all the PZZ8-T catalysts exhibited methanol selectivity values greater than 67.0% at 290 ℃, in sharp contrast to a methanol selectivity value of 32.8% for PZ at the same temperature. Thus, we performed additional investigations of the PZZ8-T catalysts at 310 and 360 ℃, which are unusually high temperatures for CO₂ hydrogenation to methanol because the required endothermic reaction is expected to be severely inhibited at such high temperatures. Interestingly, the PZZ8-T catalysts were observed to achieve a methanol selectivity value of approximately 60% at 310 ℃, and PZZ8-400 was observed to maintain a methanol selectivity value of 51.9% even at a temperature of 360 ℃. Thus, PZZ8-400 attains the highest methanol STY of 0.571 g g_cat^-1 h^-1at 310 ℃. For a better understanding of the structure-performance relationship, we characterized the catalysts using different techniques, focusing especially on the surface properties. X-ray photoelectron spectroscopy (XPS) results indicated a linear relationship between the methanol selectivity and the surface PdZn : Pd ratio, proving that the surface PdZn phase is the active site for CO₂ hydrogenation to methanol. Furthermore, analysis of the XPS O 1s spectrum together with the electronic paramagnetic resonance results revealed that both, the oxygen vacancy as well as the ZnO polar surface, played important roles in CO₂ activation. Chemisorption techniques provided further quantitative and qualitative information regarding the Pd-ZnO interface that is closely related to the CO₂ conversion rate. We believe that our results can provide insight into the catalytic reaction of CO₂ hydrogenation from the perspective of surface science. In addition, this work is an illustrative example of the use of novel chemical structures in the fabrication of superior catalysts using a traditional formula.     

Extension and improvement of the methanol-d4 NMR thermometer calibration

NMR thermometers are a convenient way to determine the temperature inside the sample of an NMR spectrometer. They rely on signals with strongly temperature-dependent chemical shifts, often of OH groups; 99.8% perdeuterated methanol is an established example which is particularly well suited for modern, high-sensitivity spectrometers, but it is so far calibrated only in the range of 282 to 330 K. In this work, we extend this calibration to the entire liquid range of methanol, 175 to 338 K. Additionally, we use a temperature sensor calibrated traceably to the International Temperature Scale (ITS-90) and accounted for the magnetic field effect on the sensor, yielding a more accurate calibration curve with an uncertainty (2σ) varying between 25 and 190 mK.     

利用供氢剂探讨石油渣油的热转化机理

以孤岛渣油和辽河渣油为原料、以供氢剂9，10－二氢蒽为化学探针，在反应温度360℃－430℃和体系初压为室温时4.0MPa氮气氛的热反应条件下，在微型高压釜内对渣油热转化机理随反应温度的变化进行了初步探讨。化学探针在渣油中的供氢反应遵循一级反应动力学模型；其中孤岛渣油反应体系中的供氢反应速率常数较辽河渣油中的大，并且其间的差别随温度升高而增大。虽然两种渣油反应体系中的氢转移反应积分活化能极为接近，但是微分活化能随温度升高而显著降低；这表明氢转移机理从较低温度时以分子间的反应为主转变为在较高温度时以自由基参与的反应为主。     

Predicting retention in reverse‐phase liquid chromatography at different mobile phase compositions and temperatures by using the solvation parameter model

The prediction capability of the solvation parameter model in reverse‐phase liquid chromatography at different methanol‐water mobile phase compositions and temperatures was investigated. By using a carefully selected set of solutes, the training set, linear relationships were established through regression equations between the logarithm of the solute retention factor, logk, and different solute parameters. The coefficients obtained in the regressions were used to create a general retention model able to predict retention in an octadecylsilica stationary phase at any temperature and methanol‐water composition. The validity of the model was evaluated by using a different set (the test set) of 30 solutes of very diverse chemical nature. Predictions of logk values were obtained at two different combinations of temperature and mobile phase composition by using two different procedures: (i) by calculating the coefficients through a mathematical linear relationship in which the mobile phase composition and temperature are involved; (ii) by using a general equation, obtained by considering the previous results, in which only the experimental values of temperature and mobile phase composition are required. Predicted logk values were critically compared with the experimental values. Excellent results were obtained considering the diversity of the test set.     

The effect of an unsymmetric branched branch on the 13C NMR chemical shifts of main chain carbons in polyethylene

The ¹³C NMR spectra of copolymers of ethylene with 4‐methyl‐1‐hexene and 4‐methyl‐1‐pentene, respectively, were compared. The 4‐methyl‐1‐hexene/ethylene copolymer, which contains an unsymmetric 2‐methylbutyl branch, exhibits two distinct ¹³C NMR peaks for each of the pairwise methylenes spaced one, two, and three carbons from the backbone methine. The chemical shift differences for these pairwise methylenes are 57.4 Hz, 18.7 Hz, and 4.3 Hz, respectively, with chemical shift differences decreasing with increasing distance from the asymmetric carbon. The frequency differences for carbons farther from the branch were not distinguishable. The magnitude of the chemical shift difference also varies with temperature, with the first and second methylene carbon chemical shift differences decreasing with increasing temperature. The third carbon is almost unaffected by temperature variations. In contrast, the 4‐methyl‐1‐pentene/ethylene copolymer exhibits a single peak for each of the pairs of methylenes in the branch's vicinity. This is the first reported observation of a branched branch affecting the chemical shifts of main chain carbons in polyethylene containing short chain branches. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1210–1213, 2000     

The Solvent‐Dependent Shift of the Amide I Band of a Fully Solvated Peptide as a Local Probe for the Solvent Composition in the Peptide/Solvent Interface

We determine the shift and line shape of the amide I band of a model AK peptide from molecular dynamics (MD) simulations of the peptide dissolved in methanol/water mixtures with varying composition. The IR spectra are determined from a transition dipole coupling exciton model. A simplified empirical model Hamiltonian is employed, which takes into account both the effect of hydrogen bonding and the intramolecular vibrational coupling. We consider a single isolated AK peptide in a mostly helical conformation, while the solvent is represented by 2600 methanol or water molecules, simulated for a pressure of 1 bar and a temperature of 300 K. Over the course of the simulations, minor reversible conformational changes at the termini are observed, which are found to only slightly affect the calculated spectral properties. Over the entire composition range, which varies from pure water to the pure methanol solvent, a monotonous shift towards higher frequency of the IR amide I band of about 8 wavenumbers is observed. This shift towards higher frequency is comparable to the shift found in preliminary experimental data also presented here on the amide I′ band. The shift is found to be caused by two counter‐compensating effects. An intramolecular red shift of about 1.2 wavenumbers occurs, due to stronger intramolecular hydrogen bonding in a methanol‐rich environment. Dominating, however, is the intermolecular solvent‐dependent shift towards higher frequency of about 10 wavenumbers, which is attributed to the less effective hydrogen‐bond‐donor capabilities of methanol compared to water. The importance of the solvent contribution to the IR shift, as well as the significantly different hydrogen formation capabilities of water and methanol, makes the amide I band sensitive to composition changes in the local environment close to the peptide/solvent interface. This allows, in principle, an experimental determination of the composition of the solvent in close proximity to the peptide surface. For the AK peptide case, we observe at low methanol concentrations a significantly enhanced methanol concentration at the peptide/solvent interface, supposedly promoted by the partially hydrophobic character of the AK peptide’s solvent‐accessible surface.     

Ag on Pt(111): Changes in Electronic and CO Adsorption Properties upon PtAg/Pt(111) Monolayer Surface Alloy Formation

The electronic and chemical (adsorption) properties of bimetallic Ag/Pt(111) surfaces and their modification upon surface alloy formation, that is, during intermixing of Ag and Pt atoms in the top atomic layer upon annealing, were studied by X‐ray photoelectron spectroscopy (XPS) and, using CO as probe molecule, by temperature‐programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS), respectively. The surface alloys are prepared by deposition of sub‐monolayer Ag amounts on a Pt(111) surface at room temperature, leading to extended Ag monolayer islands on the substrate, and subsequent annealing of these surfaces. Surface alloy formation starts at ≈600–650 K, which is evidenced by core‐level shifts (CLSs) of the Ag(3d_5/2) signal. A distinct change of the CO adsorption properties is observed when going to the intermixed PtAg surface alloys. Most prominently, we find the growth of a new desorption feature at higher temperature (≈550 K) in the TPD spectra upon surface alloy formation. This goes along with a shift of the CO_ad‐related IR bands to lower wave number. Surface alloy formation is almost completed after heating to 700 K.     

NMR spectroscopic study of noble gas binding into the engineered cavity of HPr(I14A) from Staphylococcus carnosus

Xenon binding into preexisting cavities in proteins is a well-known phenomenon. Here we investigate the interaction of helium, neon, and argon with hydrophobic cavities in proteins by NMR spectroscopy. 1H and 15N chemical shifts of the I14A mutant of the histidine-containing phosphocarrier protein (HPr(I14A)) from Staphylococcus carnosus are analyzed by chemical shift mapping. Total noble gas induced chemical shifts, Delta, are calculated and compared with the corresponding values obtained using xenon as a probe atom. This comparison reveals that the same cavity is detected with both argon and xenon. Measurements using the smaller noble gases helium and neon as probe atoms do not result in comparable effects. The dependence of amide proton and nitrogen chemical shifts on the argon concentration is investigated in the range from 10 mM up to 158 mM. The average dissociation constant for argon binding into the engineered cavity is determined to be about 90 mM.     

A new approach to NMR chemical shift additivity parameters using simultaneous linear equation method

A new approach to NMR chemical shift additivity parameters using simultaneous linear equation method has been introduced. Three general nitrogen-15 NMR chemical shift additivity parameters with physical significance for aliphatic amines in methanol and cyclohexane and their hydrochlorides in methanol have been derived. A characteristic feature of these additivity parameters is the individual equation can be applied to both open-chain and rigid systems. The factors that influence the (15)N chemical shift of these substances have been determined. A new method for evaluating conformational equilibria at nitrogen in these compounds using the derived additivity parameters has been developed. Conformational analyses of these substances have been worked out. In general, the results indicate that there are four factors affecting the (15)N chemical shift of aliphatic amines; paramagnetic term (p-character), lone pair-proton interactions, proton-proton interactions, symmetry of alkyl substituents and molecular association.     

Viscosity measurements of water at high temperatures and pressures using dynamic light scattering

The application of dynamic light scattering to measure viscosity of water at high temperatures and pressures is demonstrated. Viscosity was obtained from the translational diffusion coefficient of probe particles dispersed in the medium by the Einstein-Stokes relationship. Measurements were carried out with polystyrene latex, colloidal silica, and colloidal gold. Under a constant pressure of 25 MPa, good agreement was found between the measured and calculated viscosities up to 275 degrees C with the polystyrene latex, 200 degrees C with the colloidal silica, and 297 degrees C with the colloidal gold. It was found that failure of the measurements at high temperatures is ascribed to change in either the dispersion stability or chemical stability of the probe particles. The present results indicate that the technique could also be used for other supercritical fluids having high critical temperature and pressure, such as methanol (T(c) = 239.4 degrees C, P(c) = 8.1 MPa) and ethanol (T(c) = 243.1 degrees C, P(c) = 6.4 MPa).     

Reaction of o-phenylenediamine with diacetyl monoxime: characterisation of the product by solid-state 13C and 15N MAS NMR

2,3-dimethylquinoxaline (DMQ) and dimethylglyoxime (DMGH2) form a 1:1 hydrogen-bonded complex in the solid state, which is completely dissociated in methanol solution. There are small differences in solid-state 13C shifts between the separated components DMQ and DMGH2 and the complex. The changes in 15N solid-state chemical shifts are more significant: the hydrogen bond imparting a low frequency shift of ca 19 ppm. The effect of direct protonation on the DMQ solid-state 15N shifts was measured, and the experimental 15N data correlated with those from GIAO molecular orbital (MO) calculations.     

Methanol‐induced change of the mechanism of the temperature‐ and pressure‐induced collapse of N‐Substituted acrylamide copolymers

The solvation of two differently composed linear statistical copolymers from N,N‐diethyl acrylamide (DEAm) and N‐isopropyl acrylamide is studied by Fourier‐transform infrared spectroscopy. The solvent is changed from pure water to mixtures with methanol to investigate the cononsolvency effect. Furthermore, the influence of temperature and pressure is studied. The IR results are interpreted by sub‐band fitting of the amide I' band and quantum–chemical calculations. There are significant differences between the two copolymers that cannot be explained by a weighted superposition of the homopolymer spectra. An excess of DEAm units leads to a high number of nonsolvated side chains already in pure water. This high number is reached for equimolar copolymers only when methanol is added. The mechanism of the temperature‐ or pressure‐induced phase transition changes upon methanol addition for both copolymers. Generally, the phenomena are deduced to cooperativity at equimolar composition that is perturbed by methanol. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 532–544     

Hydrogen Bonding Switches the Spin State of Diphenylcarbene from Triplet to Singlet

Spin specificity is one of the most important properties of carbenes in their reactions. Alcohols are typically used to probe the reactive spin states of carbenes: O? H insertions are assumed to be characteristic of singlet states, whereas C? H insertions are typical for the triplets. Surprisingly, the experiments presented here suggest that the spin ground state of diphenylcarbene 1 switches from triplet to singlet if the carbene is allowed to interact with methanol. Carbene 1 and methanol form a strongly hydrogen‐bonded singlet ground state complex that was synthesized in low‐temperature matrices and characterized by IR spectroscopy. This methanol complex is only metastable, and even at 3 K slowly rearranges to form the product of O? H insertion through quantum chemical tunneling. Thus, the ground state triplet (in the gas phase) carbene 1 forms exclusively the products expected from a singlet carbene. Whereas the assumption of spin specific reactions of carbenes is correct, the spin state itself can be changed by solvent interactions, and therefore widely accepted conclusions drawn from earlier experiments have to be revisited.     

113Cd Solid‐State NMR for Probing the Coordination Sphere in Metal–Organic Frameworks

Spectroscopic techniques are a powerful tool for structure determination, especially if single‐crystal material is unavailable. ¹¹³Cd solid‐state NMR is easy to measure and is a highly sensitive probe because the coordination number, the nature of coordinating groups, and the geometry around the metal ion is reflected by the isotropic chemical shift and the chemical‐shift anisotropy. Here, a detailed investigation of a series of 27 cadmium coordination polymers by ¹¹³Cd solid‐state NMR is reported. The results obtained demonstrate that ¹¹³Cd NMR is a very sensitive tool to characterize the cadmium environment, also in non‐single‐crystal materials. Furthermore, this method allows the observation of guest‐induced phase transitions supporting understanding of the structural flexibility of coordination frameworks.     

Relationship between 1H chemical shifts of deuterated pyridinium ions and Brønsted acid strength of solid acids

Deuterated pyridine (pyridine-d5) is one of the NMR probe molecules widely used for determination of acid strength of solid catalysts. However, the correlation between the 1H chemical shift of adsorbed pyridine-d5 and the Br?nsted acid strength of solid acids has rarely been investigated. Here, an 8T zeolite model with different Si-H bond lengths is used to represent the Br?nsted acid sites with different strengths (from weak, strong, to superacid) and to predict the pyridine adsorption structure as well as the 1H chemical shift. The theoretical calculation suggests that a smaller 1H chemical shift of the pyridinium ions on the solid acids indicates a stronger acid strength. On the basis of the results of theoretical calculations, a linear correlation between the pyridine-d5 1H chemical shift and the proton affinity (PA) of the Br?nsted acid site has been derived. In combination with the available 1H MAS NMR experimental data, we conclude that pyridine-d5 can be used as a scale to characterize the solid acid strength.     

Contact ion pair between Na+ and PtCl(6)(2-) favored in methanol

Ion-pair formation between a Na+ cation and the [PtCl62-] anion in methanol is observed from195Pt NMR chemical shift trends as well as from molecular dynamics computer simulations. Free energy of association calculations reveal that contact ion pairs (CIPs) are the most favored configuration in methanol, followed by solvent shared ion pairs (SSHIPs). By contrast, such ion-pair formation is not observed for comparable solutions in water.     

Measuring diffusivity in supercooled liquid nanoscale films using inert gas permeation. II. Diffusion of Ar, Kr, Xe, and CH4 through methanol

We present an experimental technique to measure the diffusivity of supercooled liquids at temperatures near their T(g). The approach uses the permeation of inert gases through supercooled liquid overlayers as a measure of the diffusivity of the supercooled liquid itself. The desorption spectra of the probe gas are used to extract the low temperature supercooled liquid diffusivities. In the preceding companion paper, we derived equations using ideal model simulations from which the diffusivity could be extracted using the desorption peak times for isothermal or peak temperatures for temperature programmed desorption experiments. Here, we discuss the experimental conditions for which these equations are valid and demonstrate their utility using amorphous methanol with Ar, Kr, Xe, and CH(4) as probe gases. The approach offers a new method by which the diffusivities of supercooled liquids can be measured in the experimentally challenging temperature regime near the glass transition temperature.     

Distance-independent Cross-correlated Relaxation and Isotropic Chemical Shift Modulation in Protein Dynamics Studies

Cross-correlated relaxation (CCR) in multiple-quantum coherences differs from other relaxation phenomena in its theoretical ability to be mediated across an infinite distance. The two interfering relaxation mechanisms may be dipolar interactions, chemical shift anisotropies, chemical shift modulations or quadrupolar interactions. These properties make multiple-quantum CCR an attractive probe for structure and dynamics of biomacromolecules not accessible from other measurements. Here, we review the use of multiple-quantum CCR measurements in dynamics studies of proteins. We compile a list of all experiments proposed for CCR rate measurements, provide an overview of the theory with a focus on protein dynamics, and present applications to various protein systems.     

Separation of polyphenolic compounds extracted from plant matrices using capillary electrophoresis

Phenolic compounds constitute a large group of secondary plant products whose chemical structure may range from quite simple compounds to highly polymerized substances. The polyphenols content have been investigated in the alcoholic extract of the fruits of three different plants: sweet gale, sea buckthorn, hiprose. The trans-resveratrol content we have studied in roots, stems, leaves and flowers of Japanese knotweed grown in Estonia. Plant material was pre-treated in two different ways: by infusing with methanol and by supercritical fluid extraction with carbon dioxide modified with different alcohols. The relationship between variables (pressure, temperature, modifier amount) and yields are examined. The capillary zone electrophoresis methods were developed for the separation of polyphenolic anti-oxidative compounds. Using both water based borate buffer and acetonitrile based non-aqueous media it was possible to get reliable separation of several polyphenolic compounds. Based on that there has been identified such as flavone, trans-resveratrol, catechin, chlorogenic acid, quercetin and myricetin in plant extracts. Changes in the relative concentrations of trans-resveratrol in different parts of the knotweed have been established.     

Intermolecular interaction studies using small volumes

We present the use of 1-mm room-temperature probe technology to perform intermolecular interaction studies using chemical shift perturbation methods and saturation transfer difference (STD) spectroscopy using small sample volumes. The use of a small sample volume (5-10 μl) allows for an alternative titration protocol where individual samples are prepared for each titration point, rather than the usual protocol used for a 5-mm probe setup where the ligand is added consecutively to the solution containing the protein or host of interest. This allows for considerable economy in the consumption and cost of the protein and ligand amounts required for interaction studies. For titration experiments, the use of the 1-mm setup consumes less than 10% of the ligand amount required using a 5-mm setup. This is especially significant when complex ligands that are only available in limited quantities, typically because they are obtained from natural sources or through elaborate synthesis efforts, need to be investigated. While the use of smaller volumes does increase the measuring time, we demonstrate that the use of commercial small volume probes allows the study of interactions that would otherwise be impossible to address by NMR.