Pressure-induced volume expansion of zeolites in the natrolite family

Powder diffraction patterns of the zeolites natrolite (Na(16)Al(16)Si(24)O(80).16H(2)O), mesolite (Na(5.33)Ca(5.33)Al(16)Si(24)O(80).21.33H(2)O), scolecite (Ca(8)Al(16)Si(24)O(80).24H(2)O), and a gallosilicate analogue of natrolite (K(16)Ga(16)Si(24)O(80).12H(2)O), all crystallizing with a natrolite framework topology, were measured as a function of pressure up to 5.0 GPa with use of a diamond-anvil cell and a 200 microm focused monochromatic synchrotron X-ray beam. Under the hydrostatic conditions mediated by an alcohol and water mixture, all these materials showed an abrupt volume expansion (ca. 2.5% in natrolite) between 0.8 and 1.5 GPa without altering the framework topology. Rietveld refinements using the data collected on natrolite show that the anomalous swelling is due to the selective sorption of water from the pressure-transmission fluid expanding the channels along the a- and b-unit cell axes. This gives rise to a "superhydrated" phase of natrolite with an approximate formula of Na(16)Al(16)Si(24)O(80).32H(2)O, which contains hydrogen-bonded helical water nanotubes along the channels. In mesolite, which at ambient pressure is composed of ordered layers of sodium- and calcium-containing channels in a 1:2 ratio along the b-axis, this anomalous swelling is accompanied by a loss of the superlattice reflections (b(mesolite) = 3b(natrolite)). This suggests a pressure-induced order-disorder transition involving the motions of sodium and calcium cations either through cross-channel diffusion or within the respective channels. The powder diffraction data of scolecite, a monoclinic analogue of natrolite where all sodium cations are substituted by calcium and water molecules, reveal a reversible pressure-induced partial amorphization under hydrostatic conditions. Unlike the 2-dimensional swelling observed in natrolite and mesolite, the volume expansion of the potassium gallosilicate natrolite is 3-dimensional and includes the lengthening of the channel axis. In addition, the expanded phase, stable at high pressure, is retained at ambient conditions after pressure is released. The unprecedented and intriguing high-pressure crystal chemistry of zeolites with the natrolite framework topology is discussed here relating the different types of volume expansion to superhydration.     

Molecular dynamics simulation study of superhydrated perdeuterated natrolite using a new interaction potential model

To test a new interaction potential, molecular dynamics simulations of zeolite natrolite were performed for the structures under ambient conditions hydrated by perdeuterated water and at high pressure (1.87 GPa) in the superhydrated phase, which were recently studied by neutron diffraction. The experimental structures were reproduced with reasonable accuracy, and the hydrogen bond features are discussed. As in ordinary natrolite, a flip motion of water molecules around the HOH bisector is found, which, together with translational oscillations, gives rise to transient hydrogen bonds between water molecules, which do not appear from experimental equilibrium coordinates. The dynamics of water molecules can explain some problems encountered in refining the experimental structure. Vibrational spectra of natrolite containing perdeuterated water, which are not yet measured, were simulated, and their qualitative trend is discussed.     

Anomalous mobility of molecules, structure of the guest sublattice, and transformation of tetra- to paranatrolite

The structure of the guest sublattice in the structural channels of natrolite zeolites oversaturated with water (also, at high pressures) is established. It was found from¹H NMR spectra that reduction of symmetry and sharply increased mobility of water molecules in paranatrolite and in the high- pressure phase of orthorhombic natrolite follow the same mechanism — half of positions in the structural channels are occupied by ordered water molecules. The other positions are vacant and may be involved in the mechanism of fast diffusion of water molecules. Translated fromZhumal Strukturnoi Khimii, Vol. 38, No. 4, pp. 676–685, July–August, 1997.     

Super‐Hydrated Zeolites: Pressure‐Induced Hydration in Natrolites

High‐pressure synchrotron X‐ray powder diffraction studies of a series of alkali‐metal‐exchanged natrolites, A₁₆Al₁₆Si₂₄O₈₀ ? n H₂O (A=Li, K, Na, Rb, and Cs and n=14, 16, 22, 24, 32), in the presence of water, reveal structural changes that far exceed what can be achieved by varying temperature and chemical composition. The degree of volume expansion caused by pressure‐induced hydration (PIH) is inversely proportional to the non‐framework cation radius. The expansion of the unit‐cell volume through PIH is as large as 20.6 % in Li‐natrolite at 1.0 GPa and decreases to 6.7, 3.8, and 0.3 % in Na‐, K‐, and Rb‐natrolites, respectively. On the other hand, the onset pressure of PIH appears to increase with non‐framework cation radius up to 2.0 GPa in Rb‐natrolite. In Cs‐natrolite, no PIH is observed but a new phase forms at 0.3 GPa with a 4.8 % contracted unit cell and different cation–water configuration in the pores. In K‐natrolite, the elliptical channel undergoes a unique overturn upon the formation of super‐hydrated natrolite K₁₆Al₁₆Si₂₄O₈₀ ? 32 H₂O at 1.0 GPa, a species that reverts back above 2.5 GPa as the potassium ions interchange their locations with those of water and migrate from the hinge to the center of the pores. Super‐hydrated zeolites are new materials that offer numerous opportunities to expand and modify known chemical and physical properties by reversibly changing the composition and structure using pressure in the presence of water.     

The thermal behavior of k-exchanged forms of natrolite

A series of K-exchanged forms of natrolite are easily obtained by treatment with KCl solution at room temperature for 1–62 days. The maximum degree of exchange of K is 91.94%. The thermal behavior of the exchanged forms is studied by DTA-TG and high temperature X-ray powder diffraction. The DTA curve of the exchanged form of the 91.94% sample exhibits a single large endotherm at 150°C due to a one-step dehydration, showing the remarkable decrease in dehydration temperature compared with natrolite. The dehydrated phase of natrolite collapses at about 800°C, while destruction of the dehydrated K-form occurs above 1000°C. It is well recognized that the thermal stability of natrolite is increased by Na-K exchange.     

Pressure- and heat-induced insertion of CO2 into an auxetic small-pore zeolite

When the small-pore zeolite natrolite is compressed at ca. 1.5 GPa and heated to ca. 110 °C in the presence of CO(2), the unit cell volume of natrolite expands by 6.8% and ca. 12 wt % of CO(2) is contained in the expanded elliptical channels. This CO(2) insertion into natrolite is found to be reversible upon pressure release.     

Intraframework Migration of Tetrahedral Atoms in a Zeolite

The transformation from a disordered into an ordered version of the zeolite natrolite occurs on prolonged heating of this material in the crystallizing medium, but not if the mother liquor is replaced by water or an alkaline solution. This process occurs for both aluminosilicate and gallosilicate analogues of natrolite. In cross experiments, the disordered Al‐containing (or Ga‐containing) analogue is heated while in contact with the mother liquor of the opposite analogue, that is, the Ga‐containing (or Al‐containing) liquor. Therefore, strong evidence for the mechanism of the ordering process was obtained, which was thus proposed to proceed by intraframework migration of tetrahedral atoms without diffusion along the pores. Migration is first triggered, then fuelled by surface rearrangement through reactions with the mother liquor, and stops when an almost fully ordered state is attained. Classical dissolution–recrystallization and Ostwald ripening processes do not appear to be relevant for this phase transformation.     

Spin-lattice relaxations study of water mobility in natural natrolite

The mobility of water molecules in natural natrolite (Na₂Al₂Si₃O₁₀?2H₂O) is investigated by the ¹H NMR method. The spin-lattice relaxation times in the laboratory and rotating frames (T₁ and T_1ρ) are measured as a function of the temperature for a polycrystalline sample. From experimental T₁ data it follows that at T > 286 K the diffusion of water molecules along channels parallel to the c axis is observed. From experimental T_1ρ data it follows that at T > 250 K the diffusion of water molecules in transversal channels of natrolite is also observed. At a low temperature (T < 250 K) the dipolar interaction with paramagnetic impurities (presumably Fe³⁺ ions) becomes significant as a relaxation mechanism of ¹H nuclei.     

Adsorption-energetic and IR spectroscopic studies of NH4 natrolite

The ammonium form of natural zeolite, natrolite, obtained by vapor phase ion exchange is similar to calcium-containing zeolites of the natrolite group in its de- and rehydration characteristics and the heats of immersion in water. The adsorption capacity and the heat of immersion in water are maximum after evacuation of the zeolite at 200 °C. The irreversible sintering of NH₄ natrolite occurs above 200 °C (up to 45% at 500 °C), accompanied by the formation of hydroxyl groups. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 359–361, February, 1998.     

Structural evolution of Li-exchaned natrolite at pressure-induced over-hydration: An X-ray diffraction study

The behavior of Li-exchanged natrolite Li_1.92Na_0.10[Al_2.02Si_2.98O₁₀]?2H₂O at compression in penetrating (water-containing) medium was studied by in situ synchrotron powder diffraction in diamond anvil cell up to 2.5 GPa. Within 0-1.3 GPa the compression is almost isotropic, and upon the further pressure increase the sample undergoes additional hydration, leading to abrupt volume expansion by 22%, a record value for natrolite. In the proposed model for the high-pressure phase Li₂[Al₂Si₃O₁₀]?6H₂O the Li⁺ cations have no contact with the framework O-atoms and are surrounded by “water-jacket” in the form of semi-octahedron (tetragonal pyramid) composed of five H₂O molecules. Such polyhedra, lining up along the channel axis, are joined through their edges and create a “water” column expanding the structure.     

Thermoanalytical investigation of cavity filling natrolite group minerals in basalts

Cavity filling natrolites in basalts from several localities of Balaton Highland were investigated by different methods. The measurements of different thermal parameters (corrected decomposition temperature and activation energy) were first applied for natrolite. Energy dispersive spectroscopy (EDS) was used for the observation of chemical composition. Few of the minerals are regular, ordered natrolite, the majority proved to be so called ‘tetranatrolite’. According to our observations both natrolite and ‘tetranatrolite’ may appear in the same locality and chemical inhomogenity can be demonstrated within a single natrolite needle.     

Nonaqueous capillary electrophoresis for rapid and sensitive determination of fangchinoline and tetrandrine in radix Stephaniae tetrandrae and its medicinal preparations

A simple, rapid, and sensitive non-aqueous capillary electrophoresis procedure with head-column field-amplified sample stacking concentration for the analysis of fangchinoline and tetrandrine is established. Optimum separation and stacking conditions were obtained when the sample was injected at 8 kV for 50 s after preliminary pressure injection of ethanol (16.9 kPa) for 0.6 s and separated with the buffer containing 50 mM ammonium acetate, 0.5% (v/v) acetic acid, and 50% (v/v) acetonitrile in methanol medium at 24 kV applied voltage. The analytes were detected by UV at 214 nm. The two bisbenzylisoquinoline alkaloids can be separated within 6 min and quantified with high sensitivity. The detection limits were 0.30 ng mL(-1) for fangchinoline and 0.34 ng mL(-1) for tetrandrine, which indicated that the sensitivities were at least 1000-fold enhanced over those reported in the literature as obtained by UV detection. The method was applied to the analysis of fangchinoline and tetrandrine in Radix Stephaniae tetrandrae and its medicinal preparations with good results.     

A practicable strategy for enrichment and separation of four minor flavonoids including two isomers from barley seedlings by macroporous resin column chromatography,medium‐pressure LC,and high‐speed countercurrent chromatography

Separation of minor compounds especially with similar polarities and structures from complex samples is a challenging work. In the present study, an efficient method was successfully established by macroporous resin column chromatography, medium‐pressure liquid chromatography, and high‐speed countercurrent chromatography for separation of four minor flavonoids from barley seedlings. Macroporous resin column chromatography and medium‐pressure liquid chromatography were used for enrichment of these four flavonoids. High‐pressure liquid chromatography analysis showed the total content of these four flavonoids increased from 2.2% in the crude extract to 95.3% in the medium‐pressure liquid chromatography fraction. It was indicated that the combination of macroporous resin column chromatography and medium‐pressure liquid chromatography could be a practicable strategy for enrichment of minor compounds from complex sample. Then, high‐speed countercurrent chromatography was employed for separation of these four flavonoids using ethyl acetate/n‐butanol/water (0.1% glacial acetic acid) (4:1:5, v/v/v) as solvent system. As a result, four flavonoids including two isomers with purities higher than 98% were obtained. Interestingly, two flavonoids existing in one high‐pressure liquid chromatography peak were also successfully separated. All these indicated high‐speed countercurrent chromatography had great potential for separation of compounds with similar structures and polarities. This study provides a reference for efficient enrichment and separation of minor compounds from complex sample.     

A gas-control unit for use with cathodic-sputtering cells in analytical atomic spectroscopy

The design of a gas flow-rate and pressure control unit suitable for use with cathodicsputtering cells is described. The unit uses semi-automatic control to allow rapid sample exchange and precise reproduction of gas flow rate and pressure. Particular attention is given to the need to minimize contamination of the gas stream by water vapour. With this unit, samples can be changed in less than one minute, pressure in the region of 0.65 kPa (5 torr) can be reproduced to better than 1%, and moisture contamination contributed by the unit can be reduced to less than 5 ppm (v/v).     

液相色谱-串联质谱法快速测定水及鱼肉中的苯胺

为快速准确测定水及鱼肉中的苯胺,采用乙腈提取、高效液相色谱-串联质谱测定,建立了水及鱼肉中苯胺的快速测定方法。水样与乙腈以4：1的体积比混合,1.00 g鱼肉中加入2.00 mL乙腈,涡旋提取1 min,水样和鱼肉样品的提取液离心5 min后取上清液测定。以C₁₈柱为分离柱,乙腈-0.5%（v/v）甲酸水溶液（85：15,v/v）为流动相,目标物质在3 min内分离。在0.5~500 μg/L范围内,苯胺峰面积与内标峰面积之比与质量浓度的线性关系良好（R²>0.999）。基质加标试验结果表明,苯胺在水中的回收率分别为93.7%（加标水平为40 ng）和86.7% （加标水平为400 ng）,苯胺在鱼肉中的回收率分别为96.8%、 92.6%和81.8%（加标水平分别为5、50和500 ng）,相对标准偏差在1.5%~9.2%之间。水样和鱼肉样品中苯胺的检出限分别为0.50 μg/L和1.00 μg/kg,定量限分别为1.00 μg/L和2.00 μg/kg。应用该方法测定了从受苯胺污染的水库中采集的13份水样和12份鱼肉样品,结果表明,水和鱼肉中苯胺的最大含量分别为1943.6 μg/L和60.8 μg/kg。本方法快速、准确,适用于水和鱼肉中苯胺的快速测定。     

Ultra‐high‐pressure‐assisted extraction of wedelolactone and isodemethylwedelolactone from Ecliptae Herba and purification by high‐speed counter‐current chromatography

Ultra‐high‐pressure extraction combined with high‐speed counter‐current chromatography was employed to extract and purify wedelolactone and isodemethylwedelolactone from Ecliptae Herba. The operating conditions of ultra‐high‐pressure extraction were optimized using an orthogonal experimental design. The optimal conditions were 80% aqueous methanol solvent, 200 MPa pressure, 3 min extraction time and 1:20 (g/mL) solid–liquid ratio for extraction of wedelolactone and isodemethylwedelolactone. After extraction by ultra‐high pressure, the extraction solution was concentrated and subsequently extracted with ethyl acetate; a total of 2.1 g of crude sample was obtained from 100 g of Ecliptae Herba. A two‐phase solvent system composed of petroleum ether–ethyl acetate–methanol–water (3:7:5:5, v/v) was used for high‐speed counter‐current chromatography separation, by which 23.5 mg wedelolactone, 6.8 mg isodemethylwedelolactone and 5.5 mg luteolin with purities >95% were purified from 300 mg crude sample in a one‐step separation. This research demonstrated that ultra‐high‐pressure extraction combined with high‐speed counter‐current chromatography was an efficient technique for the extraction and purification of coumestans from plant material.     

An effective method based on medium‐pressure liquid chromatography and recycling high‐speed counter‐current chromatography for enrichment and separation of three minor components with similar polarity from Dracocephalum tanguticum

The separation of minor compounds, especially those with similar polarities from a complex sample, remains challenging. In the proposed study, an effective method based on medium‐pressure liquid chromatography and recycling high‐speed counter‐current chromatography was developed for the enrichment and separation of three minor components from Dracocephalum tanguticum. The crude extract was directly introduced to medium‐pressure liquid chromatography for the enrichment of the three minor components. Based on high‐performance liquid chromatography analysis, the total content of these three compounds increased from 0.48% in the crude extract to 85.3% in the medium‐pressure liquid chromatography fraction. In addition, high‐speed counter‐current chromatography was employed to separate the enriched compounds using the solvent system hexane/ethyl acetate/methanol/water (1.18:8.82:1.18:8.82, v/v/v/v). As a result, compound 3 and a mixture of compounds 1 and 2 were obtained. In order to improve the resolution of compounds 1 and 2 while saving separation time, a recycling and heart‐cut mode was used. Finally, compounds 1 and 2 were obtained after five cycles. These compounds were identified as 3‐phenylethyl β‐d ‐glucopyranoside ( 1 ), tazettoside E ( 2 ), and cirsiliol‐4′‐glucoside ( 3 ). Compounds 1 and 2 were primarily separated from D. tanguticum. Moreover, the developed method provided a reference for the separation of minor components from the complex sample.     

Determination of Trace Butyltin Compounds in Sea Water by Gas Chromatography-Atomic Absorption Spectrometry

A gas chromatography-atomic absorption spectrometric (GC-AAS) method has been developed for the determination of trace butyltin compounds in sea water. Aqueous butyltin compounds were reduced to the volatile hydride forms by NaBH₄ and were extracted with dichloromethane simultaneously. The dichloromethane extract was concentrated under reduced pressure, followed by direct injection into the GC-AAS system for analysis. The butyltin species were separated with a 2-m glass column packed with 2% OV-101 on Chromosorb G HP (100-120 mesh). Following GC separation, each species was transferred into an electrothermally heated (800 °C) quartz furnace for atomization. The tin atoms produced from individual butyltin compound were detected at 224.6 nm by an atomic absorption spectrometer. With a sea water sample (1 L), the detection limits (3σ) for monobutyltin, dibutyltin and tributyltin were approximately 20, 20 and 70 ng Sn L^?1, respectively. The method has been applied to the analysis of trace butyltin compounds in the sea water of Keelung Harbor.     

Direct measurement of part-per-billion levels of dimethyl sulfoxide in water by gas chromatography with stacked injection and chemiluminescence detection

Dimethyl sulfoxide (DMSO) is a chemical of industrial significance with many important applications. DMSO is used as an industrial solvent, in drug delivery and healthcare applications, among others. Analysis of DMSO in water typically involves extensive sample preparation, enrichment, and derivatization to improve solute detectability. A novel gas chromatographic procedure has been developed for the direct measurement of trace levels of DMSO in an aqueous matrix, such as potable water. The technology utilizes stacked injection techniques for in-column solute enrichment, a precolumn to enhance solute focusing effects, and sulfur chemiluminescence detection for matrix suppression and sensitivity. A detection limit of 2 parts per billion (ppb) (v/v) of DMSO in water was attained. Relative precision of less than 7% at the concentration of 10 ppb (v/v) of DMSO was demonstrated. A correlation coefficient of 0.9988 was obtained over a range of 2 ppb (v/v) to 100 ppb (v/v). No detectable carry-over was found at the 5 ppb (v/v) level whereas less than 4% carry-over was observed at the 100 ppb (v/v) level. Various sample storage media including glass, polyethylene, and polycarbonate were also studied to minimize solute loss. Recoveries greater than 84% were achieved with all storage media tested. The method was found to be reliable and simple to implement.     

Physical phenomena and analytical applications of helium microwave discharges

The intensity of atomic emission from a microwave-induced helium gas discharge as a function of pressure in the range 13–130 mbar is described. Two of the spectra studied were given by excited helium atoms, and one by (a species of) an excited triplet helium molecule (He₂^*). The pressure dependence of the concentration of helium atoms in the triplet metastable state was studied by absorption spectroscopy. After introduction of known quantities of mercury, chlorine, and iodine into the helium plasma, the emission was measured for some intense lines in the visible and near-u.v. Comparison of the data suggests that the atoms of these elements can be excited by helium atoms and molecules to levels at which they emit light in the visible and near-u.v. The use of a helium discharge tube for the detection of single elements in gas chromatographic fractions is described. Selectivity is greatly improved by wavelength modulation. The method allows a highly sensitive and selective determination of nanogram amounts of organic compounds which contain the elements sought, including stable isotopes such as deuterium, carbon-13 and nitrogen-15.