Probing the alkyl ligands on silylated mesoporous MCM-41 using hyperpolarized 129Xe NMR spectroscopy

Variable-temperature hyperpolarized (HP) 129Xe NMR spectroscopy has been employed to characterize surface properties of mesoporous MCM-41 modified by silylation treatment. The characteristic chemical shifts responsible for Xe-surface interactions exhibit strong correlations with both the surface coverage and chain length of the grafted alkylsilanes. Consequently, the deshielding medium contribution due to individual alkyl ligand can be deduced based on the group contribution analysis revealing the potential use of HP 129Xe NMR for probing the surface properties of organic-functionalized porous materials.     

Characterization of porosity in organic and metal-organic macrocycles by hyperpolarized 129Xe NMR spectroscopy

Hyperpolarized (129)Xe NMR spectroscopy is used to establish the solid-state porosity of shape-persistent macrocycles with either an organic or metal-organic framework. These studies show that even upon removal of cocrystallized solvent molecules, the macrocycles maintain a porous or channeled structure. The technique can provide valuable information about systems for which X-ray crystallographic analysis is not feasible. [structure: see text]     

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.     

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…     

混合胺体系中 MCM-49/ZSM-35 复合分子筛的晶化过程

 采用 X 射线衍射、N2 吸附-脱附、扫描电镜和傅里叶变换红外光谱等技术, 研究了 MCM-49/ZSM-35 复合分子筛在 Na2O-SiO2-Al2O3-CHA (环己胺)-HMI (六亚甲基亚胺)-H2O 合成体系中, 先在低温 80 ?C 老化 4 h, 再经 160 ?C 晶化不同时间的演变过程. 结果表明, 在该混合胺体系中, 首先生成 MCM-49 分子筛, 随着晶化时间的延长, 形成 ZSM-35 含量逐渐增加的 MCM-49/ZSM-35 复合分子筛, 最后生成纯 ZSM-35 分子筛; ZSM-35 的生成是以 MCM-49 分子筛的消耗为代价的.     

Probing polymer colloids by Xe NMR

Model aqueous dispersions of polystyrene, poly(methyl methacrylate), poly(n-butyl acrylate) and a statistical copolymer poly(n-butyl acrylate-co-methyl methacrylate) were studied using xenon NMR spectroscopy. The ¹²⁹Xe NMR spectra of these various latexes reveal qualitative and quantitative differences in the number of peaks and in their line widths and chemical shifts. Above the glass transition temperature, exchange between xenon sorbed in the particle core and free xenon outside the particles is fast on the ¹²⁹Xe spectral time-scale and a single ¹²⁹Xe signal is observed. At temperatures below the glass transition temperature, the exchange between sorbed and free xenon is slow on the ¹²⁹Xe spectral time-scale and two ¹²⁹Xe NMR signals can be observed. If the signal of sorbed ¹²⁹Xe is observed, its chemical shift, line width and integral relative to the integral of free ¹²⁹Xe can be used for the characterization of the particle core. The line width of free ¹²⁹Xe provides the residence time of xenon outside the particles and can be used to determine the rate constant characterizing the kinetics of penetration of xenon in the particles. This rate constant emerges as promising parameter for the characterization of the polymer particle surface.     

In situ switching of sorbent functionality as monitored with hyperpolarized (129)Xe NMR spectroscopy

In this contribution, we demonstrate that a material (organic zeolite mimetic coordination polymer [CuL(2)], where L = L(-) = CF(3)COCHCOC(OCH(3))(CH(3))(2)) can be endowed with its functionality in situ under molecular-level control. This process involves the isomerization of the ligands followed by phase interconversion from a dense to an open, porous form. The porous (beta) form of the complex reveals zeolite-like behavior but, unlike zeolites and many other hard porous frameworks, porosity may be created or destroyed at will by the application of suitable external stimuli. Contact with methylene chloride vapor was used to switch on the sorbent functionality, whereas switching off was accomplished with a temperature pulse. The transformations between functionally inactive alpha and active beta forms, as well as the amount of vacant pore space, were monitored in situ by observing the NMR spectrum of hyperpolarized (HP) Xe atom probes. For methylene chloride, the chemical shift of the coabsorbed HP Xe correlated directly with the amount of adsorbate in the pore system of the open framework, illustrating the use of HP Xe for following sorption kinetics. The adsorption of propane, as an inert adsorbate, was also monitored directly with (1)H NMR, with HP Xe and by BET measurements, revealing more complex behavior.     

Probing the geometry and interconnectivity of pores in organic aerogels using hyperpolarized 129XE NMR spectroscopy

Hyperpolarized (HP) 129Xe NMR was used for the first time to probe the geometry and interconnectivity of pores in RF aerogels and to correlate them with the [resorcinol]/[catalyst] (R/C) ratio. We have demonstrated that HP 129Xe NMR is an ideal method for accurately measuring the volume-to-surface-area (Vg/S) ratios for soft mesoporous materials without using any geometric models. The Vg/S parameter, which is related both to the geometry and the interconnectivity of the porous space, has been found to correlate strongly with the R/C ratio and exhibits an unusually large span: an increase in the R/C ratio from 50 to 500 results in about 5-fold rise in Vg/S. Unlike conventional techniques, HP 129Xe NMR spectroscopy probes the geometry and interconnectivity of the nano- or mesopores in soft materials, providing new insights into the pore structure.     

Distribution and modification of sorption sites in amphiphilic calixarene-based solid lipid nanoparticles from hyperpolarized 129Xe NMR spectroscopy

The site distribution and accessibility in amphiphilic calixarenes-based solid lipid nanoparticles were determined as a function of lipid chain length using in situ 129Xe NMR spectroscopy with flowing hyperpolarized Xe gas. The study illustrates that host cavities in as-prepared materials are increasingly occluded by the lipid chain for compounds with chain lengths from C6 to C12 and are almost completely occluded for C14 and C16 chain lengths. Host cavities present at the surface of the particles are still accessible to small atoms (xenon) and organic molecules (methylene chloride, etc). The Xe spectra show that the accessible void space can be increased remarkably by exposure of the particle surface to suitably sized guest molecules that appear to displace the occluding hydrocarbon chains from the host cavities by competitive adsorption. This postsynthesis treatment thus modifies the state of self-assembly and improves sorption capability. The HP Xe NMR approach presented is suitable for small samples (a few milligrams) of SLNs, likely also for other biomaterials such as vesicles, model membranes, etc.     

Alkylation of phenol with isopropanol over MCM-49 zeolites

Four novel Br?nsted acidic ionic liquids with two different acid sites on the imidazolium cations were synthesized and employed as catalysts and media in the esterification of acetonitrile with alcohols. The results showed that [HSO₃-pHim]HSO4 was the most efficient catalyst and medium which could be recycled easily without obvious decline in the catalytic activity.     

129Xe NMR spectroscopy study of porous cyanometallates

Zinc and cadmium hexacyanocobaltates(III) were prepared, and their porous networks were explored using 129Xe spectroscopy. The crystal structures of these two compounds are representative of porous hexacyanometallates, cubic (Fm-3m) for cadmium and rhombohedral (R-3c) for zinc. In the cubic structure, the porosity is related to systematic vacancies created from the elemental building block (i.e., the hexacyanometallate anion), whereas the rhombohedral (R-3c) structure is free of vacant sites but has tetrahedral coordination for the zinc atom, which leads to relatively large ellipsoidal pores communicated by elliptical windows. According to the Xe adsorption isotherms, these porous frameworks were found to be accessible to the Xe atom. The structure of the higher electric field gradient at the pore surface (Fm-3m) appears and is accompanied by a stronger guest-host interaction for the Xe atoms and a higher capacity for Xe sorption. For cadmium, the 129Xe NMR signal is typical of isotropic movement for the Xe atom, indicating that it remains trapped within a spherical cavity. From spectra recorded for different amounts of adsorbed Xe, the cavity diameter was estimated. For the zinc complex, 129Xe NMR spectra are asymmetric because of the Xe atom movement within an elongated cavity. The line-shape asymmetry changes when the Xe loading within the porous framework increases, which was ascribed to Xe-Xe interactions through the cavity windows. The Xe adsorption revealed additional structural information for the studied materials.     

Synthesis and adsorption investigations of zeolites MCM-22 and MCM-49 modified by alkali metal cations

Ion exchange was made on MCM-22 and MCM-49 zeolites with different Si/Al molar ratios, with Li⁺, Na⁺, K⁺, and Cs⁺ ions and the study of the influence of alkali metal cations on CO₂ adsorption properties was performed. The degree of ion-exchange decreased for larger cations (Cs⁺) apparently due to steric hindrances. The exchange with different cations led to a decrease in the surface area and the micropore volume. Our study shows that the adsorption capacity of the tested zeolites depends significantly on the nature and the concentration of the charge-compensating cations. The highest CO₂ adsorption capacity was obtained on the MWW zeolites with the lowest Si/Al molar ratio and the Li⁺ or K⁺ cations.     

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.     

Continuous Nanospace in Nanoporous Liquid Crystal Investigated by 129Xe NMR Spectroscopy

Continuous nanopores within fluid materials could be used for novel chemical events such as the accommodation of guest molecules, unique arrays of the entrapped molecules, and chemical reactions in a dynamic molecular assembly. Columnar liquid crystals composed of a one-dimensionally stacked assembly of shape-persistent macrocycles form nanochannels owing to the intrinsic nanospace in the column. However, the existence of substantial nanoporosity has not been confirmed experimentally thus far. In this study, for the first time in the literature, we confirmed the presence of discrete and spatiotemporally continuous voids in a liquid-crystalline material. In ¹²⁹Xe NMR spectroscopy of liquid crystalline columnar assembly of imine-bridged shape-persistent macrocycles under Xe atmosphere, the NMR signals of the Xe atoms entrapped in the liquid-crystalline macrocycle depended on the gas pressure and phase-transition temperatures. These results indicate that the encapsulation of Xe gas molecules within the discrete and oriented nanospaces of nanoporous liquid crystals is different from the homogeneous dissolution of the solute in an ordinary solution.     

Rotaxane Probes for the Detection of Hydrogen Peroxide by 129Xe HyperCEST NMR Spectroscopy

The development of sensitive and chemically selective MRI contrast agents is imperative for the early detection and diagnosis of many diseases. Conventional responsive contrast agents used in ¹H MRI are impaired by the high abundance of protons in the body. ¹²⁹Xe hyperCEST NMR/MRI comprises a highly sensitive complement to traditional ¹H MRI because of its ability to report specific chemical environments. To date, the scope of responsive ¹²⁹Xe NMR contrast agents lacks breadth in the specific detection of small molecules, which are often important markers of disease. Herein, we report the synthesis and characterization of a rotaxane‐based ¹²⁹Xe hyperCEST NMR contrast agent that can be turned on in response to H₂O₂, which is upregulated in several disease states. Added H₂O₂ was detected by ¹²⁹Xe hyperCEST NMR spectroscopy in the low micromolar range, as well as H₂O₂ produced by HEK 293T cells activated with tumor necrosis factor.     

Direct observation of atoms entering and exiting L-alanyl-L-valine nanotubes by hyperpolarized xenon-129 NMR

The kinetics of gas --> channel and channel --> gas exchange of Xe in self-assembled L-alanyl-L-valine (AV) nanotubes was facilitated by continuous flow hyperpolarized xenon-129 two-dimensional exchange NMR spectroscopy. Cross-peaks due to gas atoms entering or exiting were observed at Xe pressures of 92, 1320, and 3300 mbar, corresponding to Xe fractional occupancies ranging from 0.047 to 0.64. At each pressure, the rate of desorption from the channels was determined by fitting the mixing time dependence of the cross-peak and diagonal-peak signals to a magnetization exchange model, assuming steady-state Langmuir adsorption under hyperpolarized gas flow conditions. The observed rate constant for desorption of Xe from AV nanotubes decreased from 4.5 s-1 to 2.0 s-1 over the occupancy range studied, a finding that is consistent with a decrease in the diffusivity in the channels.     

Hyperpolarized 129Xe NMR investigation of multifunctional organic/inorganic hybrid mesoporous silica materials

An extensive study has been made on a series of multifunctional mesoporous silica materials, prepared by introducing two different organoalkoxysilanes, namely 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane (AEPTMS) and 3-cyanopropyltriethoxysilane (CPTES) during the base-catalyzed condensation of tetraethoxysilane (TEOS), using the variable-temperature (VT) hyperpolarized (HP) 129Xe NMR technique. VT HP-129Xe NMR chemical shift measurements of adsorbed xenon revealed that surface properties as well as functionality of these AEP/CP-functionalized microparticles (MP) could be controlled by varying the AEPTMS/CPTES ratio in the starting solution during synthesis. Additional chemical shift contribution due to Xe-moiety interactions was observed for monofunctional AEP-MP and CP-MP as well as for bifunctional AEP/CP-MP samples. In particular, unlike CP-MP that has a shorter organic backbone on the silica surface, the amino groups in the AEP chain tends to interact with the silanol groups on the silica surface causing backbone bending and hence formation of secondary pores in AEP-MP, as indicated by additional shoulder peak at lower field in the room-temperature 129Xe NMR spectrum. The exchange processes of xenon in different adsorption regions were also verified by 2D EXSY HP-129Xe NMR spectroscopy. It is also found that subsequent removal of functional moieties by calcination treatment tends to result in a more severe surface roughness on the pore walls in bifunctional samples compared to monofunctional ones. The effect of hydrophobicity/hydrophilicity of the organoalkoxysilanes on the formation, pore structure and surface property of these functionalized mesoporous silica materials are also discussed.     

Substrate and field dependence of the SPINOE transfer to surface 13C from hyperpolarized 129Xe

The substrate and field dependencies of surface SPINOE enhancements using optical pumping and magic angle spinning NMR were monitored. Relaxation rates and enhancements were examined to gain an understanding of the parameters that determine the SPINOE enhancement. (13)C-labeled deuterated methanol was adsorbed on three different substrates (SnO(2), TiO(2), Ti/SiO(2)) with heats of adsorption for xenon ranging from 14.2 to 22.6 kJ/mol. The different heats of adsorption led to a range of xenon coverages and xenon relaxation rates. Using a simple model along with experimental values for the xenon surface polarization and cross- and self-relaxation rates, the (13)C signal enhancement could be predicted and compared with experimental enhancement values. Magnetic field dependence studies were also made by monitoring the (13)C enhancements via SPINOE from hyperpolarized xenon at fields of 0.075, 4.7, and 9.4 T. The pertinent parameters necessary to achieve maximum SPINOE enhancement are discussed.