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1.
The variation of surface properties of SiMCM-41 and AlMCM-41 nanoporous materials as function of synthesis time was examined.
The main properties studied were: surface area, pore diameter, pore volume, mesoporous parameter, and wall thickness. Siliceous
MCM-41 molecular sieves were synthesized starting from hydrogels with the following molar compositions: 4.58SiO 2:0.435Na 2O:1 CTMABr:200 H 2O for SiMCM-41, and 4.58SiO 2:0.485 Na 2O:1 CTMABr:0.038 Al 2O 3:200 H 2O, for AlMCM-41. Cetyltrimethylammonium bromide (CTMABr) was used as the structural template. The crystallographic parameters
were obtained from XRD data and by nitrogen adsorption using the BET and BJH methods. The results obtained showed a significant
variation of the surface properties of the MCM-41 materials as a function of the synthesis time reaching silica wall thickness
of ca. 2 nm on the fourth day. 相似文献
2.
Molecular sieves MCM-41 were synthesized from rice husk ash (RHA) as alternative sources of silica, called RHA MCM-41. The material was synthesized by a hydrothermal method from a gel with the molar composition 1.00 CTMABr:4.00 SiO 2:1.00 Na 2O:200.00 H 2O at 100 °C for 120 h with pH correction. The cetyltrimethylammonium bromide (CTMABr) was used as a structure template. The material was characterized by X-ray powder diffraction, FTIR, TG/DTG, and surface area determination by the BET method. The kinetics models proposed by Ozawa, Flynn–Wall, and Vyazovkin were used to determine the apparent activation energy for CTMA + species decomposition from the pores of MCM-41 material. The results were compared with those obtained from the MCM-41 synthesized with silica gel. The synthesized material had specific surface area, size, and pore volume as specified by mesoporous materials developed from conventional sources of silica. 相似文献
3.
A series of AlMCM-41 molecular sieves was synthesized starting from a hydrogel with the following molar composition: 1CTMABr:4.58SiO 2:(0.437 + X)Na 2O: XAl 2O 3:200H 2O. Tetramethylammonium silicate (TMAS) was used as silicon source and cethyltrimethylammonium bromide (CTMABr) was used as structure template. The obtained materials were characterized by nitrogen adsorption, XRD, FT-IR and TG/DTG. Model-free kinetic algorithms were applied in order to determinate conversion, isoconversion and apparent activation energy to decomposition of CTMA+ species from the AlMCM-41 materials with different silicon/aluminium (Si/Al) ratios of 20, 40, 60 and 80. 相似文献
4.
The siliceous MCM-41 molecular sieve was synthesized starting from a hydrogel with the following molar composition: 4.58SiO 2:0.437Na 2O:1CTMABr:200H 2O. The cetyltetramethylammonium bromide (CTMABr) was used as structure template. A kinetic study of template removal after
the syntheses was performed by Vyazovkin model-free kinetic method obtaining apparent activation energy of 166±8.2 kJ mol -1.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
6.
Mesoporous ZSM-5 was prepared from iron ore tailings (IOT) using a two-step process. Mesoporous MCM-41 was first synthesized using cetyltrimethylammonium bromide (CTAB) as mesoporous template and IOT as silica source. The CTAB in the as-synthesized MCM-41 was used as the mesoporogen to produce the mesoporous ZSM-5, by recrystallizing the amorphous walls of MCM-41 with tetrapropylammonium bromide (TPABr) as the structure-directing agent via solid-phase conversion. To evaluate the textural properties of mesoporous ZSM-5, the as-synthesized samples were characterized using x-ray diffraction, scanning electron microscopy, transmission electron microscopy, 29Si, 27Al magic angle spinning nuclear magnetic resonance spectroscopy, and nitrogen adsorption. The results show that phase separation between the surfactant and zeolite crystals was avoided in the solid-phase conversion process, which transforms the as-synthesized MCM-41 to mesoporous zeolite. Therefore, the synthetic route presented herein provides a novel method for the synthesis of mesoporous ZSM-5 from IOT. 相似文献
7.
MCM-41 material was synthesized starting from hydrogel containing colloidal fumed silica, sodium silicate, cetyltetramethylammonium
bromide(CTMABr) as surfactant, and distilled water as solvent. These reactants were mixed to obtain a gel with the following
composition: 4SiO 2:1Na 2O:1CTMABr:200H 2O. The hydrogel with pH=14 was hydrothermally treated at100°C, for 4 days. Each day, the pH was measured, and then adjusted
to 9.5–10 by using 30%acetic acid solution. Thermogravimetry was the main technique, which was used to monitor the participation
of the surfactant on the MCM-41 nanophase, being possible to determine the temperature ranges relative to water desorption
as well as the surfactant decomposition and silanol condensation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
The hydration, state, and mobility of protons and Li +, Na +, and Cs + ions in MF-4SK perfluorinated sulfonic acid cation-exchange membranes doped with silicon dioxide and phosphotungstic acid
have been investigated by NMR and impedance spectroscopy. The dopants increase the moisture content of the membrane and change
the system of pores and channels in which ion transport takes place. At low humidities, the dopant particles are involved
in ion transport. The greatest effect is observed for the membranes doped with both SiO 2 and phosphotungstic acid. The water molecules sorbed by dopant particles as a material participate in the hydration of alkali
metal cations in the membrane. 相似文献
9.
We report herein a comprehensive study of the gas-phase Fe +-mediated oxidation of ethane by N 2O on both the sextet and quartet potential energy surfaces (PESs) using density functional theory. The geometries and energies
of all the relevant stationary points are located. Initial oxygen-atom transfer from N 2O to iron yields FeO +. Then, ethane oxidation by the nascent oxide involves C–H activation forming the key intermediate of (C 2H 5)Fe +(OH), which can either undergo C–O coupling to Fe + + ethanol or experience β-H shift giving the energetically favorable product of FeC 2H 4
+ + H 2O. Reaction of FeC 2H 4
+ with another N 2O constitutes the third step of the oxidation. N 2O coordinates to FeC 2H 4
+ and gets activated by the metal ion to yield (C 2H 4)Fe +O(N 2). After releasing N 2 through the direct H abstraction and/or cyclization pathways, the system would be oxidized to ethenol, acetaldehyde, and
oxirane, regenerating Fe +. Oxidation to acetaldehyde along the cyclization –C–to–C hydrogen shift pathway is the most energetically favored channel. 相似文献
10.
The complexation reaction between Tl +, Ag + and Pb 2+ cations with 2,6-di(furyl-2yl)-4-(4-methoxy phenyl)pyridine as a new synthesis ligand in acetonitrile (ACN)–H 2O and methanol (MeOH)–H 2O binary solutions has been studied at different temperatures using conductometric method. The conductometric data show that the stoichiometry of the complexes is 1: 1 [M: L] and the stability constant of complexes changes with the binary solutions identity. Also, the structure of the resulting 1: 1 complexes was optimized using the LanL2dz basis set at the B3LYP level of theory using GAUSSIAN03 software. The results show that the change of logK f for (DFMP.Pb) 2+ and (DFMP.Ag) + complexes with the mole ratio of acetonitrile and for (DFMP.Ag) + and (DFMP.Tl) + complexes with the mole ratio of methanol have a linear behavior, while the change of logK f of (DFMP.Tl) +complex in ACN–H 2O binary solutions (with a minimum in XACN = 0.5) and (DFMP.Ag) + complex in MeOH–H 2O binary solutions (with a minimum in XMeOH = 0.75) show a non-linear behavior. The selectivity order of DFMP ligand for these cations in mol % CAN = 25 and 75 obtain Tl + > Pb 2+ > Ag + but in mol % CAN = 50, the selectivity order observe Pb 2+ > Tl + > Ag +. Also, this selectivity sequence of DFMP in MeOH–H 2O (mol % MeOH = 75 and 100) and (mol % MeOH = 50) is obtained Pb 2+ > Ag + and Tl + > Ag + > Pb 2+ respectively. The values of thermodynamic parameters show that these values are influenced by the nature and the composition of binary solution. In all cases, the resulting complexes are enthalpy stabilized and entropy destabilized. The TΔ SC° versus Δ HC° plot of all obtained thermodynamic data shows a fairly good linear correlation which indicates the existence of enthalpy-entropy compensation in the complexation reactions. 相似文献
11.
The Gibbs free energies of solvation (Δ G
s) and the electronic structures of endohedral metallofullerenes M +@C 60 (M += Li +, K +) were calculated within the framework of the density functional theory and the polarizable continuum model. In water environment,
the equilibrium position of K + is at the center of the fullerene cavity whereas that of Li + is shifted by 0.14 nm toward the fullerene cage. The Li + cation is stabilized by interactions with both the fullerene and solvent. The equilibrium structures of both endohedral metallofullerenes
are characterized by very close Δ G
s values. In particular, the calculated Δ G
s values for K +@C 60 are in the range from −124 to −149 kJ mol −1 depending on the basis set and on the type of the density functional. Molecular dynamics simulations (TIP3P H 2O, OPLS force field, water sphere of radius 1.9 nm) showed that the radial distribution functions of water density around
C 60 and M +@C 60 are very similar, whereas orientations of water dipoles around the endohedral metallofullerenes resemble the hydration pattern
of isolated metal ions. 相似文献
12.
The Ru-Cs +/MgO and Ru-Cs +/γ-Al 2O 3 catalysts, which were prepared by an impregnation method using RuOHCl 3 and Cs 2CO 3 as precursor compounds and reduced with H 2 at 450°C, are characterized by X-ray diffraction, high-resolution transmission electron microscopy (with X-ray microanalysis), and X-ray photoelectron spectroscopy (XPS). The Cs +/MgO(Al 2O 3) systems, Ru-Cs + black, and model systems prepared by cesium sputtering onto polycrystalline ruthenium foil are studied as reference samples. It is found that, in the Ru-Cs +/MgO sample, cesium is present as a Cs 2 + xO cesium suboxide, which weakly interacts with the support, localized on the surface of Ru particles or near them. In the case of Ru-Cs +/γ-Al 2O 3, cesium occurs as a species that is tightly bound to the support; this is likely surface cesium aluminate, which prevents promoter migration to Ru particles. The Ru-Cs +/MgO sample exhibits a considerable shift of the Ru3 d line in the XPS spectra toward lower binding energies, as compared to the bulk metal. It is hypothesized that this shift is due to a decrease in the electron work function from the surface of ruthenium because of the polarizing effect of Cs + ions in contact with Ru particles. Based on the experimental results, the great difference between the catalytic activities of the Ru-Cs +/MgO and Ru-Cs +/γ-Al 2O 3 systems in ammonia synthesis at 250–400°C and atmospheric pressure is explained. 相似文献
13.
Gas-phase FeO + can convert benzene to phenol under thermal conditions. Two key intermediates of this reaction are the [HO-Fe-C 6H 5] + insertion intermediate and Fe +(C 6H 5OH) exit channel complex. These intermediates are selectively formed by reaction of laser ablated Fe + with specific organic precursors and are cooled in a supersonic expansion. Vibrational spectra of the sextet and quartet
states of the intermediates in the O-H stretching region are measured by infrared multiphoton dissociation (IRMPD). For Fe +(C 6H 5OH), the O-H stretch is observed at 3598 cm −1. Photodissociation primarily produces Fe ++C 6H 5OH; Fe +(C 6H 4)+H 2O is also observed. IRMPD of [HO-Fe-C 6H 5] + mainly produces FeOH ++C 6H 5 and the O-H stretch spectrum consists of a peak at ∼3700 cm −1 with a shoulder at ∼3670 cm −1. Analysis of the experimental results is aided by comparison with hybrid density functional theory computed frequencies.
Also, an improved potential energy surface for the FeO ++C 6H 6 reaction is developed based on CBS-QB3 calculations for the reactants, intermediates, transition states, and products. 相似文献
14.
LiNi 0.80Co 0.15Al 0.05O 2 (NCA) is explored to be applied in a hybrid Li +/Na + battery for the first time. The cell is constructed with NCA as the positive electrode, sodium metal as the negative electrode, and 1 M NaClO 4 solution as the electrolyte. It is found that during electrochemical cycling both Na + and Li + ions are reversibly intercalated into/de-intercalated from NCA crystal lattice. The detailed electrochemical process is systematically investigated by inductively coupled plasma-optical emission spectrometry, ex situ X-ray diffraction, scanning electron microscopy, cyclic voltammetry, galvanostatic cycling, and electrochemical impedance spectroscopy. The NCA cathode can deliver initially a high capacity up to 174 mAh g ?1 and 95% coulombic efficiency under 0.1 C (1 C?=?120 mA g ?1) current rate between 1.5–4.1 V. It also shows excellent rate capability that reaches 92 mAh g ?1 at 10 C. Furthermore, this hybrid battery displays superior long-term cycle life with a capacity retention of 81% after 300 cycles in the voltage range from 2.0 to 4.0 V, offering a promising application in energy storage. 相似文献
15.
The imidazol side group of histidine has two nitrogen atoms capable of being protonated or participating in metal binding.
Hence, histidine can take on various metal-bound and protonated forms in proteins. Because of its variable structural state,
histidine often functions as a key amino acid residue in enzymatic reactions. Ab initio (HF and MP2) calculations were done
in modeling the cation (H +, Li +, Na +, K +, Mg 2+, Ca 2+) interaction with side chain of histidine. The region selectivity of metal ion complexation is controlled by the affinity
of the side of attack. In the imidazol unite of histidine the ring nitrogen has much higher metal ion (as well as proton)
affinity. The complexation energies with the model systems decrease in the following order: Mg 2+ > Ca 2+ > Li + > Na + > K +. The variation of the bond lengths and the extent of charge transfer upon complexation correlate well with the computed interaction
energies. 相似文献
16.
A novel 1,8-naphthalimide dye with simple structure has been produced by a facile synthetic method for colorimetric and fluorescent sensing of H + and Cu 2+. In CH 3CN/H 2O (1/1, v/v), the dye could monitor H + using dual channels (ratiometric absorbance and fluorescence intensity change) from pH 6.2 to 12.0. Meanwhile, in the pH range of 1.9–5.2, the dye could also be used to detect Cu 2+ using triple channels [ultraviolet–visible (UV–Vis) absorption, fluorescence intensity reduction, as well as fluorescence blueshift]. The detection limits for Cu 2+ evaluated by colorimetric and fluorescent titration were 6.10 × 10 ?7 and 2.62 × 10 ?7 M, respectively. The dye exhibited specific selectivity and sensitivity for H + and Cu 2+ over various coexisting metal ions. Moreover, the sensing mechanism of the dye for H + and Cu 2+ was carefully examined. 相似文献
17.
ZSM-5/MCM-41 composite molecular sieve has been hydrothermally synthesized through a two-step crystallization process using palygorskite (PAL) as silicon and aluminum source. The products were characterized by various means and their catalytic properties for acetalization of cyclohexanone and esterification of acetic acid and n-butanol were also investigated. In the first step ZSM-5 zeolite could be formed from the acid-treated PAL after hydrothermal treatment using tetrapropylammonium bromide as template. XRD patterns, N 2 adsorption and desorption data, and TEM images show that the composite obtained in the secondary step had a well-ordered mesoporous MCM-41 phase and a microporous ZSM-5 zeolite phase. Compared with ZSM-5, ZSM-5/MCM-41 composite possessed more total acid amount, weak acid sites and large pore structure due to the formation of MCM-41 and exhibited higher catalytic activity for the acetalization and esterification reaction. 相似文献
18.
The corrected selectivity coefficients of the ion exchange H +-Na + and H +-NH 4
+ on ion-exchange resins based on C-tetramethylcalix[4]resorcinarene were calculated from the experimental data obtained from studying ion-exchange equilibria. The preference of the ion-exchange resins for cations increases in the sequence: Na + < NH 4
+ < < H +, and the ion-exchange resin based on (2-furyl)hydroxymethyltetramethylcalix[4]resorcinarene has a higher preference for ammonium cations. According to the results of microcalorimetric measurements, the exchange H +-Na + on this ion-exchange resin is accompanied by the highest change in the differential enthalpy. It follows from the quantum-chemical calculations that the introduction of a (2-furyl)hydroxymethyl group into the structure of the polymer induces additional electrostatic interactions between an ammonium cation and an elementary unit of the ion-exchange resin.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2560–2563, December, 2004. 相似文献
20.
Selective catalytic reduction of nitrogen monoxide with propene over two kinds of CuCl/MCM-41 catalysts prepared by a dispersion
method has been studied. It was found that CuCl/AlMCM-41 exhibits substantially higher activity over CuCl/SiMCM-41. Characterization
of these samples by H 2-TPR, IR and XRD showed that the active copper species were mainly related to Cu 2+ and Cu + ions in CuCl/AlMCM-41 catalyst. 相似文献
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