Thermal behavior of multiwall carbon nanotube/zeolite nanocomposites

Summary In this study we report the synthesis and thermal characterization of multiwall carbon nanotubes containing zeolite based nanocomposites. Three different zeolites (LTA, FAU and MFI) were used in this study and it was observed that the morphologies of the synthesized nanocomposites were significantly different. For FAU zeolite nanocrystals with a few nm in diameter were nucleated on the nanotubes covering their surface, however, for LTA and MFI zeolites bulky crystals with nanotubes crossing them were observed by transmission electron microscopy. One of the most important results of our study was the generation of secondary mesoporosity in the zeolites after removing the carbon nanotubes.     

Effect of ultrasound on the thermal behavior of the mixtures for the LTA zeolite synthesis based on metakaolin

The effect of ultrasonic treatment on the thermal behavior of the mixtures from metakaolin, sodium hydroxide and alumina designed for LTA zeolites synthesis was studied. X-ray diffraction analysis, infrared spectroscopy, scanning electron microscopy and synchronous thermal analysis have been used. It was shown that after evaporation of the suspension, LTA zeolite (24 mass%) is contained in the samples. It was established that the new phase (sodium aluminum silicate) is formed at a calcination temperature of about 600 °C. It was demonstrated that at a calcination temperature over 800 °C, nepheline is synthesized. The reaction of nepheline formation has been described by the topochemical equation of four-dimensional nucleation/nucleus growth according to Avrami/Erofeev. Using the Ozawa–Flynn–Wall analysis for non-isothermal data, the values of the activation energy and the pre-exponential factor have been calculated. It is shown that after the ultrasonic treatment the activation energy of the nepheline synthesis reaction has smaller values than in the sample without pretreatment. These phenomena have been explained by differences between the structural parameters of the particles (dimension of the coherent scattering region, the value of microdeformations).     

用工业固体废料合成沸石分子筛的研究进展

系统总结了以工业固体废料为原料合成沸石分子筛材料的最新研究进展, 讨论了以粉煤灰、 珍珠岩工业废料、 煤矸石、 流体催化裂化(FCC)废催化剂、 锂矿渣、 铝土矿渣、 废瓷料和废弃玻璃等工业固体废料为原料, 合成LTA, FAU, MFI, CHA, GIS, SOD, ANA和KFI沸石分子筛材料的工艺方法, 及其在污水中重金属离子的脱除、 空气中CO₂的捕获、 氮氧化物的选择性还原等实际应用中的性能, 并对未来工业固体废料合成沸石分子筛的发展趋势进行了展望.     

Further studies of a simple atomistic model of silica: thermodynamic stability of zeolite frameworks as silica polymorphs

We have applied our previously reported model of silica based on low coordination and strong association [J. Chem. Phys. 121, 8415 (2004)], to the calculation of phase stability of zeolite frameworks SOD, LTA, MFI, and FAU as silica polymorphs. We applied the method of Frenkel and Ladd for calculating free energies of these solids. Our model predicts that the MFI framework structure has a regime of thermodynamic stability at low pressures and above approximately 1400 K, relative to dense phases such as quartz. In contrast, our calculations predict that the less dense frameworks SOD, LTA, and FAU exhibit no regime of thermodynamic stability. We have also used our model to investigate whether templating extends the MFI regime of thermodynamic stability to lower temperatures, by considering templates with hard-sphere repulsions and mean-field attractions to silica. Within the assumptions of our model, we find that quartz remains the thermodynamically stable polymorph at zeolite synthesis temperatures (approximately 400 K) unless unphysically large template-silica attractions are assumed. These predictions suggest that some zeolites such as MFI may have regimes of thermodynamic stability even without template stabilization.     

On the potential of positron lifetime spectroscopy for the study of early stages of zeolites formation from their amorphous precursors

The applicability of positron lifetime (LT) spectroscopy to the study of progress of formation of Secondary Building Units (SBU) in gels yielding in FAU and LTA type zeolites was investigated. Samples were prepared from aluminosilicate gels with various degrees of local structural order. LT measurements were performed at room temperature in air and in vacuum. Coexistence of annihilation modi with long lifetime components was shown; a correlation with precursors of nucleation and type of exchanged ions was also indicated.     

Neopentane and solid acids: direct hydron exchange before cracking

The hydrogen/deuterium exchange reaction of 2,2-dimethylpropane (neopentane) over D(2)O-exchanged zeolites (MOR, FAU, BEA, MFI) using a batch recirculation reactor was studied by means of gas chromatography coupled with mass spectrometer. In the temperature range 473-573 K, H/D exchange proceeds without side reaction such as cracking at short contact times. Indeed the C-H bond has appeared favorably involved in the activation of neopentane compared to the less accessible C-C bond. The transition state allowing hydron exchange is most likely a carbonium species (pentacoordinated carbon) as in the case of the H/D exchange between methane and solid acid. The activation energies of the H/D exchange between neopentane and zeolites are the same for all zeolites indicating a common carbonium ion type transition state. On the basis of previous results in the case of the exchange between methane and liquid superacids, the deuterium exchange rates in neopentane were tentatively related to the acidity of the solids. However the order of activity MOR > MFI > BEA > FAU seems to be related to the size of the pores, which may suggest the involvement of a confinement effect in the zeolites cavities. Moreover we found that H/D exchange takes also place between neopentane and deuterated sulfated zirconia (SZ) emphasizing its strong acidity.     

Synthesis and Properties of Zeolites from Autoclaved Aerated Concrete (AAC) Waste

Syntheses routes for the conversion of autoclaved aerated concrete (AAC) waste into aluminosilicate zeolites like LTA and related phases were developed. The procedures always started with leaching steps of the pure AAC waste by combinations of strong alkaline (NaOH) and mild acid (citric acid) treatments, before the real crystallization process was performed separately under addition of sodium aluminate. All products were characterized by X‐ray powder diffraction (XRD), scanning electron microscopy (SEM) combined with energy dispersive X‐ray analysis (EDX‐analysis) and Fourier‐transform infrared spectroscopy (FTIR). Zeolites LTA and related phases basic sodalite (SOD), hydrosodalite (SOD), cancrinite (CAN) and an intermediate phase between SOD and CAN were observed. Depending on the preparation route tailor made synthesis of pure phase zeolite LTA with crystal sizes up to 5 μm was worked out. In addition to syntheses procedures important properties of the zeolites were discussed with respect to the treatment procedure of AAC. It is shown, that the special synthesis pathway is not only responsible for the product composition and formation of a certain structure type but also exhibits a strong influence on the crystallinity, crystal size, and morphology. The water sorption capacity and the hydrothermal stability of the products were selected for those further investigations. Whereas adequate water sorption capacity up to 272 mg · g^–1 were measured for zeolites LTA obtained from two different reaction routes, limited hydrothermal stabilities were revealed for other products. Under the conditions of strong hydrothermal treatment at a temperature of 473 K for 72 h, a more or less extended phase transformation into ANA‐Type zeolites occurred. This process was least extensive for pure phase zeolite LTA obtained from the alkaline solution of AAC leaching.     

Geometry, phase stability, and electronic properties of isolated selenium chains incorporated in a nanoporous matrix

We report the fabrication process of isolated one-dimensional Se chains incorporated in the matrix of AlPO4-5 single crystals and the experimental investigation of the geometry, phase stability, electronic properties, and electron-phonon coupling effect of these Se chains. The structure of the helical Se chains inside the channels is discussed on the basis of X-ray scattering measurements. Thermal analysis and temperature-dependent micro-Raman measurements show that Se single chains are flexible and can convert from a weak distorted phase into another phase with strongly disordered structure ("melting" state) around 340 K. Since the electrons are confined in the one-dimensional channels, the absorption band of the Se chain is obviously blue shifted compared with that of trigonal Se. With increasing temperature, this band shifts linearly to the lower energy side, in sharp contrast to the nonlinear temperature coefficient of trigonal Se, which is attributed to the greatly diminished interchain interaction and the weakening of the electron-optical phonon coupling in a low-dimensional system. In the vicinity of the absorption band, both first-order and second-order Raman signals for the Se chain are enhanced, due to the strong electron-phonon coupling when the excitation laser energy matches the electronic transition in isolated Se chains.     

Thermal stability and crystallization kinetics of Se–Te–Sn alloys using differential scanning calorimetry

The present article deals with the differential scanning calorimetric (DSC) study of Se?CTe glasses containing Sn. DSC runs are taken at four different heating rates (10, 15, 20 and 25?K?min^?1). The crystallization data are examined in terms of modified Kissinger, Matusita equations, Mahadevan method and Augis and Bennett approximation for the non-isothermal crystallization. The activation energy for crystallization (E _c) is evaluated from the data obtained at different heating rates. Activation energy of glass transition is calculated by Kissinger??s relation and Moynihan theory. The glass forming tendency is also calculated for each composition. The glass transition temperature and peak crystallization temperature increases with the increase in Sn % as well as with the heating rate.     

Glass transition behavior and crystallization kinetics of Cu0.3(SSe20)0.7 chalcogenide glass

The glass transition behavior and crystallization kinetics of Cu_0.3(SSe₂₀)_0.7 chalcogenide glass were investigated using differential scanning calorimetry (DSC), X-ray diffraction (XRD). Two crystalline phases (SSe₂₀ and Cu₂Se) were identified after annealing the glass at 773 K for 24 h. The activation energy of the glass transition (E_g), the activation energy of crystallization (E_c), the Avrami exponent (n) and the dimensionality of growth (m) were determined. Results indicate that this glass crystallizes by a two-stage bulk crystallization process upon heating. The first transformation, in which SSe₂₀ precipitates from the amorphous matrix with a three-dimensional crystal growth. The second transformation, in which the residual amorphous phase transforms into Cu₂Se compound with a two-dimensional crystal growth.     

Effect of temperature on the diffusion mechanism of xylene isomers in a FAU zeolite: a molecular dynamics study

The diffusion of o-, m-, and p-xylene in a FAU zeolite at 300-900 K was investigated using molecular dynamics simulations. Calculated self-diffusion coefficients of xylene isomers showed that the mobility of p-xylene was the fastest, m-xylene the second fastest, and o-xylene the slowest in the FAU zeolite at the same temperature. The diffusion activation energy of o-xylene, m-xylene and p-xylene was, respectively, determined to be 9.04, 7.45 and 6.44 kJ mol(-1) within the temperature range of 400 to 900 K, while to be 14.12, 13.59 and 15.47 kJ mol(-1) within the temperature range of 300 to 400 K. Xylene density profiles and orientational analysis suggested that this can be attributed to the xylene molecules that diffuse in the FAU zeolite by two different mechanisms at high and low temperatures. The behavior of motion for xylene in the FAU zeolite exhibits a "fluid-like" mode at high temperatures and exhibits a "jump-like" mode at low temperatures.     

SeTe8Sb1非晶合金的玻璃转变

The process of glass transition on the non-crystal SeTe_8Sb_1 has been researched using low temperature differential scanning calorimetry (DSC). The following conclusions are deduced: this process is a second-order phase transformation, the stronger endotherm displayed in it is imputed to hysteresis of the transition. The temperature of glass transition in heating process decrease with reduction of the cooling rate while forming glass. The activation energy of glass transition quickly increases from 174 kJ·mol. (at quenching rate ~10~4 K·min~(-1)) to 399 kJ·mol~(-1). (at cooling rate ~10 K·min~(-1)) So that the glass obtained at lower cooling rate is more stable than that obtained at higher rate.     

Order-disorder phase transition in p-terphenyl and p-terphenyl: tetracene doped crystals as studied by Raman spectroscopy

Phase transition in crystalline pure p-terphenyl and p-terphenyl: tetracene doped crystals was studied with Raman spectroscopy, for temperatures from 295 to 10 K. In particular, the torsional Raman vibrational mode with a “hard-core frequency” of 230.8 cm⁻¹ was investigated in its frequency and bandwidth dependence upon temperature. The results were analyzed based on an order-disorder model allowing the determination of the activation energies and orientational correlation times of the molecular diffusive process in the monoclinic (above 193 K) and triclinic (below 193 K) phases of the crystals. The activation energy is observed to decrease from the monoclinic to the triclinic phase, whereas the orientational correlation times increase, both in the undoped and the doped crystals. The doping of p-terphenyl with tetracene appears to affect the activation energy and the orientational correlation times in a different way in each phase.     

Crystal structure and the paraelectric-to-ferroelectric phase transition of nanoscale BaTiO3

We have investigated the paraelectric-to-ferroelectric phase transition of various sizes of nanocrystalline barium titanate (BaTiO3) by using temperature-dependent Raman spectroscopy and powder X-ray diffraction (XRD). Synchrotron X-ray scattering has been used to elucidate the room temperature structures of particles of different sizes by using both Rietveld refinement and pair distribution function (PDF) analysis. We observe the ferroelectric tetragonal phase even for the smallest particles at 26 nm. By using temperature-dependent Raman spectroscopy and XRD, we find that the phase transition is diffuse in temperature for the smaller particles, in contrast to the sharp transition that is found for the bulk sample. However, the actual transition temperature is almost unchanged. Rietveld and PDF analyses suggest increased distortions with decreasing particle size, albeit in conjunction with a tendency to a cubic average structure. These results suggest that although structural distortions are robust to changes in particle size, what is affected is the coherency of the distortions, which is decreased in the smaller particles.     

On the Adam-Gibbs-Kirkpatrick-Thirumalai-Wolynes scenario for the viscosity increase in glasses

We reformulate the interpretation of the mean-field glass transition scenario for finite dimensional systems, proposed by Kirkpatrick, Thirumalai, and Wolynes (KTW). This allows us to establish clearly a temperature dependent length xi( *) above which the mean-field glass transition picture has to be modified. We argue in favor of the mosaic state introduced by KTW, which leads to the Adam-Gibbs relation between the viscosity and configurational entropy of glass forming liquids. Our argument is a mixture of thermodynamics and kinetics, partly inspired by the random energy model: small clusters of particles are thermodynamically frozen in low energy states, whereas large clusters are kinetically frozen by large activation energies. The relevant relaxation time is that of the smallest "liquid" clusters. Some physical consequences are discussed.     

Forming and Gathering of SnO2 Nanoparticles on External Surface of High Silica TON, MFI and FAU Type Zeolites

Tin dioxide （SnO2） nano-particles were prepared on high silica TON, MFI and FAU type zeolites by impregnation of SnC12 solution and subsequent calcination at 873 K. XRD and SAED were used to characterize the crystalline phase, and TEM was used to characterize the morphology, the particle size and the agglomerative state of the formed nano-materials. The nano-particles, which possess 8 nm, 10-80 nm and 6 nm in size, were found to form on the outer surface of TON, MFI and FAU zeolites, respectively. SnO2 microcapsules and SnOz netlike nanostructure were obtained by decomposition of SnO2-TON and SnO2-MFI in 40% hydrofluoric acid at room temperature. Compared with the nano-particles formed on NaY zeolite, the special morphology and the agglomerative state of SnO2 nanostructures on TON and MFI type zeolites with one and two dimension channel system indicate that the heterogeneous framework, surface structure and property perform important function for forming and growing SnO2 nanostructure on the outer surface of the zeolites.     

Investigation and Modeling of the Hydrothermal Stability of Technically Relevant Zeolites

The dense-structured zeolites of types ZSM-5 (MFI) and mordenite (MOR) of different SiO₂/Al₂O₃ moduli are relatively stable under treatment by liquid water for 72 hours up to 513 K. The open-structured zeolites of types Y in dealuminated modification (FAU) and beta (BEA) undergo strong decomposition in the same range. For these two sample types a mathematical model for the decomposition of the zeolite framework is established that takes into account the influence of modulus as well as temperature. Here the kinetic of the solid phase reaction is a superposition of two different mechanisms described by the Monod equation. Dedicated to the late of Wolfgang Schirmer.     

紫外/臭氧法在脱除沸石有机模板剂中的应用

选取LTA, FAU, BETA, MFI和MEL型沸石, 研究紫外/臭氧法在沸石有机模板剂脱除中的应用, 并与传统高温焙烧法比较, 考察两种方法对有机模板剂的脱除能力. 采用XRD, FT-IR, BET等方法对脱除模板剂前后的沸石样品进行表征, 结果表明紫外/臭氧法是一种低温有效除去沸石模板剂的方法. Beta, silicalite-1和silicalite-2沸石在紫外光照后相应的模板剂四乙基氢氧化铵、四丙基氢氧化铵和四丁基氢氧化铵已彻底脱除, A和Y型沸石中分别有27.5和6.2 wt%的四甲基氢氧化铵模板剂残留, 这主要是受其特殊β笼结构的限制而引起的.     

Synthesis and characterization of NaA zeolite powder/film deposited on alumina beads by dip-coating method

Linde Type A (LTA) zeolites have been synthesized in the current study by simple sol–gel technique. The crystal growth has been controlled by varying the hydrogel synthesis time and annealing temperature. The resulting products obtained at various crystallization times and temperatures have been studied using X-ray powder diffraction (XRD) method, High resolution transmission electron microscopy images, scanning electron microscopy (SEM) micrographs, energy dispersive study and Brunauer–Emmett–Teller (BET) analysis. The TEM images of the final LTA zeolite annealed at 500 °C revealed the formation of cubic structure. XRD analysis revealed that the crystallinity improved with annealing. BET analysis revealed that the synthesized LTA is a well crystallized 4A zeolite. LTA zeolites were dispersed in poly ethylene glycol in the ratio 3:100 and 5:100 and coated on porous alumina beads for the formation of membrane. The SEM images revealed excellent formation of fine structure LTA zeolite membrane with uniform coating. The membrane consisted of a top layer with thickness of 1.14–2.0 µm. Crystals in the top layer showed cubic morphology and amorphous phase was observed at the grain boundaries present between LTA zeolite and alumina substrate.     

Selective adsorption of 2,3-DCDD and 1,2,3,4-TCDD on *BEA,EMT, FAU and MFI-type zeolites as alternative adsorbents for on-line dioxin monitoring

The aim of this study is to evaluate the capabilities of zeolites as selective sorbents for on-line dioxin monitoring. The adsorption isotherms of 2,3-DCDD and 1,2,3,4-TCDD from isooctane on several zeolites with different pore sizes (*BEA, EMT, FAU and MFI-type zeolites) are determined. Selective adsorption is shown for FAU and EMT-type zeolites. 2,3-DCDD is adsorbed by FAU with the highest adsorption capacity and affinity while 1,2,3,4-TCDD is not adsorbed whatever the zeolites. The adsorption of 2,3-DCDD inside the pores of FAU is qualitatively confirmed by diffuse reflectance UV spectrometry and thermogravimetry measurements.