Silicalite synthesis from tetrapropylammonium (TPA+) sodium silicate gels was studied by X-ray diffraction, elemental analysis, ion exchange, 29Si magic angle spinning nuclear magnetic resonance spectroscopy, and scanning electron microscopy. Based on this information we confirm a hydrogel—solid transformation mechanism for silicalite crystallization. The initial synthesis gel is a highly articulated silicate network containing pockets of water with solvated Na+ and TPA+ cations. As the silica condenses and becomes more hydrophobic, water and solvated cations are expelled. The condensed silicate gel then encapsulates the hydrophobic TPA+ cations in cages resembling the channel intersections of silicalite before X-ray crystalline silicalite is observed. Crystallization occurs within the gel via rearrangement of the TPA+-occluded silicate cages by the breaking and reformation of siloxane bonds into the more stable silicalite structure. Rates of nucleation and crystallization both increase with increasing TPA+ gel content. The amount of silicalite which forms is limited by the amount of TPA+, which must be present in the ratio of one TPA+ per channel intersection. 相似文献
Organic structure‐directing agent (OSDA)‐free synthesis of zeolite beta is a subject of both scientific and industrial interest. Herein, we report a comprehensive investigation into the effects of various parameters on the seed‐assisted crystallization of zeolite beta in the absence of OSDA. The crystallization behavior of “OSDA‐free beta” is strongly governed by the chemical composition of the starting Na+‐aluminosilicate gel as well as by the Si/Al ratios of the calcined beta seed crystals, which are prepared using tetraethylammonium hydroxide (TEAOH). Furthermore, OSDA‐free beta seed crystals can be used to form zeolite beta, termed “green beta”. XRD, scanning electron microscopy, inductively coupled plasma atomic emission spectroscopy, and 27Al magic angle spinning NMR analyses showed that the OSDA‐free beta and green beta were of high purity and crystallinity. The nitrogen adsorption–desorption of OSDA‐free beta and green beta revealed higher surface areas and larger volumes in the micropore region than those of the beta seeds synthesized with OSDA after calcination. These results provide a robust and reliable process for the environmentally friendly production of high‐quality zeolite beta in a completely OSDA‐free Na+‐aluminosilicate system. 相似文献
Summary: Nylon‐6/Na+‐montmorillonite (MMT) nanocomposites (NNNs) are synthesized by a hydrolyzed ring‐opening polymerization. At a loading of only 2 wt.‐% MMT, the tensile modulus, the flexural modulus, and the heat distortion temperature of the NNNs exhibit increases of nearly 20%, 60%, and 63 °C, respectively. Compared with that of neat nylon‐6, the temperature of the main α‐relaxation (Tα) of the NNNs is shifted 3.6 °C toward higher temperatures and two β‐relaxation peaks are observed. Another interesting phenomenon is that there is a new melting peak (at about 206 °C) for the NNNs.
DSC second heating curves of neat nylon‐6 (N6), nylon‐6/Na+‐MMT nanocomposites with highly swollen Na+‐MMT (NHM), and nylon‐6/Na+‐MMT nanocomposites with slightly swollen Na+‐MMT (NSM) with various amounts of Na+‐MMT. 相似文献
The new L ‐lysine alkali‐metal salts 1 – 5 (M+=Na+ and K+) with different alkyl groups at the Nα‐position were easily synthesized, and their hydro‐ and organogelation properties were investigated. All compounds were H2O‐soluble, and some salts, especially the potassium salts, functioned as a hydrogenator that could gel water below 2 wt‐%. These salts also had organogelation abilities for many organic solvents. 相似文献
Poly(N‐isopropylacrylamide) (PNIPAM) is well known to exhibit reentrant behavior or cononsolvency in response to the composition of a mixed solvent consisting of water and a low‐chain alcohol. Since the solvent structure plays an important role in this phenomenon, the presence of structure‐breaking/structure‐making ions in solution is expected to have a dramatic effect on the cononsolvency of PNIPAM. The present work examines the way that the presence of different salts can modify the reentrant‐phase diagram displayed by a cationic PNIPAM microgel in the mixed ethanol/water solvent. The effects of four Hofmeister anions—SO42?, Cl?, NO3? and SCN?—with different abilities to modify the solvent structure are analyzed. The species with kosmotropic or structure‐making character show a clear competition with ethanol for the water molecules, intensifying the nonsolvency of the PNIPAM with the EtOH volume fraction (?e). However, striking results are found with the most chaotropic or structure‐breaking anion, SCN?. In contrast to what happens in water‐rich solutions, the presence of SCN? in alcohol‐rich solvents enhances the solubility of the polymer, which macroscopically results in the microgel swelling. Moreover, this ion displays great stabilizing properties when ?e> is 0.2. These results have been explained by considering how chaotropic or structure‐breaking ions interact with water and ethanol molecules.相似文献
New crown ether carrying two fluorionophores of cis‐dibenzothiazolyldibenzo‐24‐crown‐8 was synthesized from cis‐diformyldibenzo‐24‐crown‐8 and 2‐aminobenzenethiol. The binding behavior and the optical properties of the crown ether were examined through UV‐visible spectroscopy and fluorescence spectroscopy. When complexed with Na+, K+, Rb+, and Cs+ ions, it led to intramolecular charge transfer and caused the changes of the fluorescence spectra. The protonation of the crown ether was also studied. With protonation using CF3COOH, the absorption bands and the fluorescence spectroscopy changed, the maximal fluorescence wavelengths red shifted and the fluorescence intensity with the maximum at 433 nm enhanced strongly. J. Heterocyclic Chem., (2011). 相似文献
1H‐detected magic‐angle spinning NMR experiments facilitate structural biology of solid proteins, which requires using deuterated proteins. However, often amide protons cannot be back‐exchanged sufficiently, because of a possible lack of solvent exposure. For such systems, using 2H excitation instead of 1H excitation can be beneficial because of the larger abundance and shorter longitudinal relaxation time, T1, of deuterium. A new structure determination approach, “quadruple‐resonance NMR spectroscopy”, is presented which relies on an efficient 2H‐excitation and 2H‐13C cross‐polarization (CP) step, combined with 1H detection. We show that by using 2H‐excited experiments better sensitivity is possible on an SH3 sample recrystallized from 30 % H2O. For a membrane protein, the ABC transporter ArtMP in native lipid bilayers, different sets of signals can be observed from different initial polarization pathways, which can be evaluated further to extract structural properties. 相似文献
This paper presents results from a series of pulsed field gradient (PFG) NMR studies on lipophilic guanosine nucleosides that undergo cation‐templated assembly in organic solvents. The use of PFG‐NMR to measure diffusion coefficients for the different aggregates allowed us to observe the influences of cation, solvent and anion on the self‐assembly process. Three case studies are presented. In the first study, diffusion NMR confirmed formation of a hexadecameric G‐quadruplex [G 1 ]16 ? 4 K+ ? 4 pic? in CD3CN. Furthermore, hexadecamer formation from 5′‐TBDMS‐2′,3′‐isopropylidene G 1 and K+ picrate was shown to be a cooperative process in CD3CN. In the second study, diffusion NMR studies on 5′‐(3,5‐bis(methoxy)benzoyl)‐2′,3′‐isopropylidene G 4 showed that hierarchical self‐association of G8‐octamers is controlled by the K+ cation. Evidence for formation of both discrete G8‐octamers and G16‐hexadecamers in CD2Cl2 was obtained. The position of this octamer–hexadecamer equilibrium was shown to depend on the K+ concentration. In the third case, diffusion NMR was used to determine the size of a guanosine self‐assembly where NMR signal integration was ambiguous. Thus, both diffusion NMR and ESI‐MS show that 5′‐O‐acetyl‐2′,3′‐O‐isopropylidene G 7 and Na+ picrate form a doubly charged octamer [G 7 ]8 ? 2 Na+ ? 2 pic? 9 in CD2Cl2. The anion's role in stabilizing this particular complex is discussed. In all three cases the information gained from the diffusion NMR technique enabled us to better understand the self‐assembly processes, especially regarding the roles of cation, anion and solvent. 相似文献