MCM-41分子筛的合成及129Xe核磁共振的研究

Purely siliceous MCM-41 with a narrow pore-size distribution, different pore size, high surface area was synthesized . As prepared, calcined and catalytically tested MCM-41 materials have been comprehensively characterized by N₂ adsorption/desorption at 77K and ¹²⁹Xe NMR. By adding mesitylene during the synthesis, the pore size of MCM-41 was enlarged to 5.2nm. The chemical shift in ¹²⁹Xe NMR spectroscopy of adsorbed xenon indicates that the MCM-41 is one dimensional pore channels .     

129Xe nuclear magnetic resonance study of pitch-based activated carbon modified by air oxidation/pyrolysis cycles: a new approach to probe the micropore size

(129)Xe NMR has been used to study a series of homologous activated carbons obtained from a KOH-activated pitch-based carbon molecular sieve modified by air oxidation/pyrolysis cycles. A clear correlation between the pore size of microporous carbons and the (129)Xe NMR of adsorbed xenon is proposed for the first time. The virial coefficient delta(Xe)(-)(Xe) arising from binary xenon collisions varied linearly with the micropore size and appeared to be a better probe of the microporosity than the chemical shift extrapolated to zero pressure. This correlation was explained by the fact that the xenon collision frequency increases with increasing micropore size. The chemical shift has been shown to vary very little with temperature (less than 9 ppm) for xenon trapped inside narrow and wide micropores. This is indicative of a smooth xenon-surface interaction potential.     

Separation of <Superscript>133</Superscript>Xe from <Superscript>99</Superscript>Mo, <Superscript>131</Superscript>I and uranium,and removal of impurities using gas chromatography

Summary Separation and purification of ¹³³Xe from acidic solution containing uranium, ⁹⁹Mo and ¹³¹I has been developed. In the first step of this work, uranium pellets were dissolved under pressure (8-15 bar) in 8M nitric acid solution. Then¹³³ Xe and other gases were conducted to activated charcoal cold trap. Final purification of ¹³³Xe from impurities such as NO_x, radioiodine and krypton was performed by passing through a molecular sieve preparative chromatographic column using helium as mobile phase. The final recovery of ¹³³Xe from the separation-purification process was higher than 98%. Adsorption-desorption behavior of radioxenon on the charcoal and molecular sieves have also been studied and discussed.     

Coloring ultrasensitive MRI with tunable metal–organic frameworks

As one of the most important imaging modalities, magnetic resonance imaging (MRI) still faces relatively low sensitivity to monitor low-abundance molecules. A newly developed technology, hyperpolarized ¹²⁹Xe magnetic resonance imaging (MRI), can boost the signal sensitivity to over 10 000-fold compared with that under conventional MRI conditions, and this technique is referred to as ultrasensitive MRI. However, there are few methods to visualize complex mixtures in this field due to the difficulty in achieving favorable “cages” to capture the signal source, namely, ¹²⁹Xe atoms. Here, we proposed metal–organic frameworks (MOFs) as tunable nanoporous hosts to provide suitable cavities for xenon. Due to the widely dispersed spectroscopic signals, ¹²⁹Xe in different MOFs was easily visualized by assigning each chemical shift to a specific color. The results illustrated that the pore size determined the exchange rate, and the geometric structure and elemental composition influenced the local charge experienced by xenon. We confirmed that a complex mixture was first differentiated by specific colors in ultrasensitive MRI. The introduction of MOFs helps to overcome long-standing obstacles in ultrasensitive, multiplexed MRI.

Metal organic frameworks with tunable pore structures are able to provide varied chemical environments for hyperpolarized ¹²⁹Xe atom hosting, which results in distinguishing magnetic resonance signals, and stains ultra-sensitive magnetic resonance imaging (MRI) with diverse colors.     

Potential of <Superscript>129</Superscript>Xe NMR spectroscopy of adsorbed xenon for testing the chemical state of the surface of mesoporous carbon materials illustrated by the example of aggregates of diamond and onion-like carbon nanoparticles

The chemical shift in the ¹²⁹Xe NMR spectrum of adsorbed xenon is very sensitive to the presence of oxygen-containing functional groups on the surface of mesoporous carbon materials. Well-characterized, structurally similar nanodiamond and onion-like carbon samples are considered here as model objects.     

Controlling Gas Selectivity in Molecular Porous Liquids by Tuning the Cage Window Size

Control of pore window size is the standard approach for tuning gas selectivity in porous solids. Here, we present the first example where this is translated into a molecular porous liquid formed from organic cage molecules. Reduction of the cage window size by chemical synthesis switches the selectivity from Xe‐selective to CH₄‐selective, which is understood using ¹²⁹Xe, ¹H, and pulsed‐field gradient NMR spectroscopy.     

129Xe NMR study of ZSM-5 type zeolites Effect of cationic sites

A comprehensive ¹²⁹Xe NMR spectroscopy study on H-ZSM-5 zeolites having different aluminum contents and on cation-exchanged ZSM-5 zeolites is reported. The parent H-ZSM-5 zeolites were ion-exchanged with Group I–III metal ions ( K, Ca, Sr, Ba, Al, La) to varying degrees. The chemical shift of adsorbed ¹²⁹Xe is seen to be a function of the pentasil structure of ZSM-5, of the number of free Brønsted acid sites and of the number of metal cations in the framework. Differences in the chemical shift of ¹²⁹Xe are seen between cations due to their different polarizing forces against xenon. The amount of cations has also an effect on the δ_xe-xe term in Fraissard's equation that may be caused by changes in the diffusional characteristics of Xe atoms in the ZSM-5 framework.     

Simulations of 129Xe NMR chemical shift of atomic xenon dissolved in liquid benzene

The isotropic ¹²⁹Xe NMR chemical shift of atomic Xe dissolved in liquid benzene was simulated by combining classical molecular dynamics and quantum chemical calculations of ¹²⁹Xe nuclear magnetic shielding. Snapshots from the molecular dynamics trajectory of xenon atom in a periodic box of benzene molecules were used for the quantum chemical calculations of isotropic ¹²⁹Xe chemical shift using nonrelativistic density functional theory as well as relativistic Breit?CPauli perturbation corrections. Thus, the correlation and relativistic effects as well as the temperature and dynamics effects could be included in the calculations. Theoretical results are in a very good agreement with the experimental data. The most of the experimentally observed isotropic ¹²⁹Xe shift was recovered in the nonrelativistic dynamical region, while the relativistic effects explain of about 8% of the total ¹²⁹Xe chemical shift.     

129Xe NMR chemical shift in Xe@C60 calculated at experimental conditions: Essential role of the relativity,dynamics, and explicit solvent

The isotropic ¹²⁹Xe nuclear magnetic resonance (NMR) chemical shift (CS) in Xe@C₆₀ dissolved in liquid benzene was calculated by piecewise approximation to faithfully simulate the experimental conditions and to evaluate the role of different physical factors influencing the ¹²⁹Xe NMR CS. The ¹²⁹Xe shielding constant was obtained by averaging the ¹²⁹Xe nuclear magnetic shieldings calculated for snapshots obtained from the molecular dynamics trajectory of the Xe@C₆₀ system embedded in a periodic box of benzene molecules. Relativistic corrections were added at the Breit–Pauli perturbation theory (BPPT) level, included the solvent, and were dynamically averaged. It is demonstrated that the contribution of internal dynamics of the Xe@C₆₀ system represents about 8% of the total nonrelativistic NMR CS, whereas the effects of dynamical solvent add another 8%. The dynamically averaged relativistic effects contribute by 9% to the total calculated ¹²⁹Xe NMR CS. The final theoretical value of 172.7 ppm corresponds well to the experimental ¹²⁹Xe CS of 179.2 ppm and lies within the estimated errors of the model. The presented computational protocol serves as a prototype for calculations of ¹²⁹Xe NMR parameters in different Xe atom guest–host systems. © 2013 Wiley Periodicals, Inc.     

Modeling β-γ coincidence spectra of <Superscript>131m</Superscript>Xe, <Superscript>133</Superscript>Xe, <Superscript>133m</Superscript>Xe,and <Superscript>135</Superscript>Xe

In support of the Comprehensive Nuclear-Test-Ban Treaty (CTBT), improvements have been made to the model of the Automated Radioxenon Sampler/Analyzer (ARSA) β-γ coincidence detector for radioxenon monitoring. MCNPX is used to simulate the detector response for all the electrons and photons emitted from ^131mXe, ¹³³Xe, ^133mXe, ¹³⁵Xe, and ¹³⁷Cs signals. A MatLab code was written to incorporate the MCNPX results in the calculation of β-γ coincidence spectra. These will aid in the development of the Spectral Deconvolution Analysis Tool (SDAT)¹ and to calibrate β-γ coincidence systems. The models developed for this work include improvements over previous models in their ability to address Compton scattering in the β-cell, and the β-distribution offset in the 31 keV γ-ray region for ¹³³Xe.     

Characterization of CuY zeolites after dehydration,oxidation and reduction with carbon monoxide An adsorption and 129Xe nuclear magnetic resonance spectroscopy study

The adsorption isotherms and ¹²⁹Xe nuclear magnetic resonance (NMR) chemical shifts of xenon and the adsorption isotherms of carbon monoxide of Cu(II)- and Cu(I)-exchanged zeolites NaY were measured. The former zeolites of 53, 75, and 95% exchange degrees were investigated after various pretreatment steps comprising dehydration, oxidation and reduction with CO at 420°C as well as long-term CO reduction at 470°C. The Cu(I)Y zeolite of 70% exchange degree was prepared via a solid-state exchange procedure with CuCl and subjected to dehydration at 420°C. In all cases, except the dehydrated zeolites, almost linear xenon adsorption isotherms and linear ¹²⁹Xe NMR chemical shift _versus xenon concentration curves running parallel to each other are obtained. In contrast, the chemical shift curves for the dehydrated zeolites are non-linear at low xenon concentrations turning towards negative chemical shift values at very low pressures. The whole body of the experimental xenon data can be explained quantitatively with a unifying approach on the basis of a site adsorption model where the sites are (i) two types of cuprous ions of much different adsorption strength and ¹²⁹Xe chemical shift, (ii) Na⁺ cations, (iii) Lewis acid sites generated through autoreduction and reduction of Cu²⁺ by CO, and (iv) framework sites free of cations. These five types of sites are each characterized by Langmuir adsorption isotherm constants and local ¹²⁹Xe NMR chemical shifts. The adsorption site concentrations in the various zeolites are evaluated. The supercage Cu(I) concentration values are in nice agreement with the results deduced from the CO adsorption isotherm measurements.     

Direct low-energy measurement of <Superscript>37</Superscript>Ar and <Superscript>127</Superscript>Xe in a radiotracer gas using low-background proportional counters

A radiotracer gas with a blend of ³⁷Ar and ¹²⁷Xe was created for a gas migration experiment and was characterized at Pacific Northwest National Laboratory using ultra-low-background proportional counters. This paper describes the direct low-energy measurement of ³⁷Ar and ¹²⁷Xe in a dual-isotope sample. Using this low-energy technique, the dual-isotope radiotracer gas was determined to have activity concentrations of 483 Bq/cc and 1435 Bq/cc for ³⁷Ar and ¹²⁷Xe, respectively, and a ratio of 1:3 on the reference date of 7/11/2016.     

129Xe NMR as a probe of polymer blends

The miscibility of two-component polymer blends has been investigated using xenon-129 (¹²⁹Xe) nuclear magnetic resonance (NMR) to probe the phase morphology. The chemical shift of ¹²⁹Xe dissolved in a given polymer is unique, thus heterogeneous blends with large domain sizes exhibit two ¹²⁹Xe NMR lines. When a single resonance is obtained, the data are consistent with miscibility, yielding an upper bound on the domain size. The temperature dependence of the relative solubilities and chemical shifts of ¹²⁹Xe dissolved in the pure components may allow a determination of the phase morphology in blends exhibiting a single resonance. The method is used to demonstrate that polychloroprene and 25% epoxidized 1,4-polyisoprene form a miscible blend.     

MCM-41分子筛的合成及129Xe核磁共振的研究

ＭＣＭ４１中孔分子筛是１９９２年由Ｍｏｂｉｌ公司的科学家Ｋｒｅｓｇｅ［１］等人首次合成的,并在《自然》杂志发表。这种中孔分子筛具有六角形孔径,孔径２ｎｍ～１０ｎｍ,这种分子筛的孔径可以通过水晶模板来控制［２］。已报道的合成ＭＣＭ４１,孔径一般在２．０ｎｍ～３．５ｎｍ,使用的水晶模板一般是单一或两种阳离子季铵盐表面活性剂［３,４］。本论文通过引入第二种扩孔模板,与阳离子季铵盐协同作用,合成了孔径５．２ｎｍ（ＢＥＴ法测）的ＭＣＭ４１。通过氮气的吸脱附,测定了分子筛的比表面和孔径等性质。Ｊ．Ｆｒａｉｓｓａｒｄ…     

129Xe NMR Studies of Pecan Shell-Based Biochar and Structure-Process Correlations

Pecan shell-based biochar is utilized as a filtration medium, sequestrant for metallic ions, soil conditioner, and other applications. One process for creating the biochar involves the use of phosphoric acid at high temperature in a partial oxygen atmosphere to produce a highly porous carbonaceous material. In this work, we found ¹²⁹Xe NMR to be an excellent technique to study micropores in biochar. Thus, the ¹²⁹Xe chemical shift in biochar was found to vary linearly with the xenon pressure; from the data an estimate of about 8–9 Å could be proposed for the average pore diameter in pecan shell-based biochar. Through saturation recovery and 2-D NMR exchange experiments, information on the exchange between free versus bound xenon was obtained. Furthermore, correlations of ¹²⁹Xe NMR data with the carbonization process conditions were made.     

Application of an experimental design to optimize a segregation method of <Superscript>129</Superscript>I and <Superscript>14</Superscript>C

In this work, the optimization of a segregation method of ¹²⁹I and ¹⁴C, two long-living radionuclides, main constituents of nuclear radioactive waste, has been developed. To be able to carry out this project, a fractional factorial experimental design was applied using 5 factors and 2 levels by factor (2^5–2). Only 8 experiments were necessary to identify the variables affecting the process, and very good recoveries of both radionuclides were obtained: (94?±?2)% for ¹²⁹I, and (99?±?1)% for ¹⁴C. The segregation of ¹²⁹I was influenced by flow (Q), volume of H₂SO₄ (V_H+), and carriers (CR), while V_H+ and time (t) played a major role in the segregation of ¹⁴C.     

Molecular Sensing with Hyperpolarized 129Xe Using Switchable Chemical Exchange Relaxation Transfer

An approach for hyperpolarized ¹²⁹Xe molecular sensors is explored using paramagnetic relaxation agents that can be deactivated upon chemical or enzymatic reaction with an analyte. Cryptophane encapsulated ¹²⁹Xe within the vicinity of the paramagnetic center experiences fast relaxation that, through chemical exchange of xenon atoms between cage and solvent pool, causes accelerated hyperpolarized ¹²⁹Xe signal decay in the dissolved phase. In this proof‐of‐concept work, the relaxivity of Gadolinium ^III‐DOTA on ¹²⁹Xe in the solvent was increased eightfold through tethering of the paramagnetic molecule to a cryptophane cage. This potent relaxation agent can be ′turned off′ specifically for ¹²⁹Xe through chemical reactions that spatially separate the Gd^III centre from the attached cryptophane cage. Unlike ¹²⁹Xe chemical shift based sensors, the new concept does not require high spectral resolution and may lead to a new generation of responsive contrast agents for molecular MRI.     

New measurement of <Superscript>129</Superscript>I in IAEA-375 reference material

Summary Under an IAEA project for upgrading of reference materials, a new determination of the ¹²⁹I concentration in the IAEA-375 reference material was performed. A chemical procedure was set up for the preparation of the AgI samples. Measurement of ¹²⁹I was carried out using the IsoTrace Tandetron AMS facility at University of Toronto. To ensure the accuracy of the calibration, the tuning of the AMS system was iterated using not only the QC (quality control) samples but also all unknown samples. To minimize any possible current-dependent effects between ¹²⁹I and ¹²⁷I ions in the injection magnet, low Cs⁺ sputtering beam intensity (10 μA) was used. The reproducibility in determining the ¹²⁹I/¹²⁷I ratio in the IAEA-375 AgI samples was less than 1%. The activity concentration (C_A) of ¹²⁹I in the IAEA-375 reference material was determined to be 1.59±0.08 mBq ^. kg^-1 at 95% confidence level. The present value is about 7% lower than the IAEA recommended value (1.7 mBq ^. kg^-1) listed in 2000 or 20% lower than the recommended value (2 mBq ^. kg^-1) listed in the IAEA AQCS Reference Material Catalogue (2002-2003). Since the IAEA recommended values for IAEA-375 materials was issued about 10 years ago and error range of the recommended values were large, the results we obtained might be useful in upgrading the recommended value.     

Characterization of nano-iron oxyhydroxides and their application in UO<Subscript>2</Subscript><Superscript>2+</Superscript> removal from aqueous solutions

The nano-iron oxyhydroxides (α- and γ-FeOOH) were synthesized by using three ferrous and ferric salts (FeSO₄, FeCl₂, Fe(NO₃)₃) as iron precursors under alkaline conditions. Morphologies of nano-iron oxyhydroxides were characterized by employing X-ray powder diffraction (XRD) and specific surface area (SSA) analysis respectively. The occurrence of needle-like shape of nano-goethite and rod-like shape of nano-lepidocrocite were attributed to hydrolysis of Fe³⁺ cations and/or oxidization of Fe²⁺ at alkaline conditions in terms of XRD analysis. The N₂-BET SSA and BJH (Barrett–Joyner–Halenda) pore size analysis showed that internal SSA of nano-lepidocrocite is higher than that of nano-goethite, although they have similar N₂-BET SSAs. The distribution of average pore size of nano-iron oxyhydroxides are higher than that of predominant pore size due to formation of the heterogeneous nanoparticles under the experimental conditions. These nanoparticles possess the high sorption capacity and the strong affinity for contaminants. Application of nano-iron oxyhydroxides in environmental engineering plays an important role to remove a variety of contaminants, such as heavy-metal ions and organic pollutants.     

Preparation of calibration sources of <Superscript>88</Superscript>Kr and <Superscript>138</Superscript>Xe for HPGe detector

A method of efficiency calibration for the measurement of ⁸⁸Kr and ¹³⁸Xe by HPGe γ-spectrometer is proposed in the present paper. The question for the efficient calibration is, how to achieve homogeneous sources of ⁸⁸Kr-⁸⁸Rb and ¹³⁸Xe-¹³⁸Cs. The fission product gases were obtained by irradiating a precisely measured amount of U₃O₈ (90% ²³⁵U) filled in a quartz glass ampoule. Source cell was first filled up with stearic acid, and then the fission product gases were charged into it. Xenon and krypton are not adsorbed on stearic acid, therefore, homogeneous sources of ⁸⁸Kr-⁸⁸Rb and ¹³⁸Xe-¹³⁸Cs can be prepared. The results of the experiment demonstrate that the method is feasible and successful.