Effects of pH on Dielectric Relaxation of Montmorillonite, Allophane, and Imogolite Suspensions

Dielectric measurements were performed on montmorillonite, allophane, and imogolite suspensions under various pH conditions, using time domain reflectometry over the frequency range 10 kHz-20 GHz. A dielectric relaxation peak due to bound water could be observed for all the clays. Allophane has two peaks, indicating that its peaks are very similar to those of silica-alumina gels. Although imogolite has a similar chemical composition, only one peak was found. The relaxation strength of montmorillonite is greater than that of the other two clays. For all the clays, the relaxation strength depended on the pH. A change in the relaxation strength according to a change in pH is explained in terms of the different network structures of the clay particles. It is suggested that bound water influences the network structure formation. In montmorillonite, a great relaxation process detected at low frequency is caused by surface polarization of counterions. The change in measure of the structural unit with the pH, identified from Schwartz's theory, has a tendency similar to that postulated by other experimental techniques, and surface charge densities identified are close to those estimated from CEC. Copyright 1999 Academic Press.     

Preparation of mesoporous silica-alumina derived from FSM-16 and its catalytic activity for cumene cracking

Mesoporous silica-alumina catalysts with different alumina contents were prepared by impregnation of a mesoporous silica (FSM-16) with an alcohol solution of aluminium isopropoxide. The resulting mesoporous silica-alumina catalysts possess protonic acid sites, and exhibit activities for cumene cracking, though the activities are lower than that of an amorphous silica-alumina catalyst. The maximum activity as well as the maximum amount of protonic acid sites were observed for the sample with a Si/Al ratio of 4.1.     

Atomic oxygen erosion resistance of sol–gel oxide films on Kapton

Silica, alumina and silica-alumina composite films were deposited on Kapton substrate via sol–gel method and their atomic oxygen (AO) erosion resistance was test in a ground-based AO simulator. The surface morphology and the structure of as-deposited films were investigated by scanning electronic microscope, X-ray photoelectron spectroscopy, and Fourier transformed infrared spectroscopy. After AO exposure, more cracks and micro-pores appear on the surface of silica and alumina films, respectively. For the silica-alumina composite films, their toughness and densification are good, and the stable interface is formed between the alumina and silica phases. Therefore, the silica-alumina composite-coated Kapton shows the best AO resistance and the erosion yield is two orders of magnitude less than that of pristine Katpon. Moreover, the composite-coated Kapton remains optically stable under AO exposure.     

不同硅铝比β沸石的理化性质及烃类催化裂化活性

采用润湿态固相转化方法按照投料硅铝摩尔比(n(SiO2)/n(Al2O3))分别为30、60、150、300、500和纯硅时合成了β沸石, 化学法和X射线荧光(XRF)测定产物的硅铝比和反应初始凝胶基本一致. 利用X射线衍射(XRD)、红外光谱(FT-IR)、扫描电镜(SEM)、氨程序升温脱附(NH3-TPD)和低温N2吸附对系列样品的物相、水热稳定性、晶体形貌、酸性以及表面积和孔体积做了系统的表征. 并将该系列样品作为活性组分添加剂, 以高岭土为基质分别制备烃类裂化催化剂, 对其催化裂化性能进行了评价. 结果表明, 不同硅铝比β沸石添加到裂化催化剂均可以提高其裂化活性, 改善催化剂的抗积炭性能, 硅铝比为150的β沸石催化剂在保持较高裂化活性的前提下, 汽油收率最高, 抗积炭性能也较好, 是优选的裂化催化材料.     

29Si NMR study of structural ordering in aluminosilicate geopolymer gels

A systematic series of aluminosilicate geopolymer gels was synthesized and then analyzed using 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) in combination with Gaussian peak deconvolution to characterize the short-range ordering in terms of T-O-T bonds (where T is Al or Si). The effect of nominal Na2O/(Na2O + K2O) and Si/Al ratios on short-range network ordering was quantified by deconvolution of the 29Si MAS NMR spectra into individual Gaussian peaks representing different Q4(mAl) silicon centers. The deconvolution procedure developed in this work is applicable to other aluminosilicate gel systems. The short-range ordering observed here indicates that Loewenstein's Rule of perfect aluminum avoidance may not apply strictly to geopolymeric gels, although further analyses are required to quantify the degree of aluminum avoidance. Potassium geopolymers appeared to exhibit a more random Si/Al distribution compared to that of mixed-alkali and sodium systems. This work provides a quantitative account of the silicon and aluminum ordering in geopolymers, which is essential for extending our understanding of the mechanical strength, chemical and thermal stability, and fundamental structure of these systems.     

Gelation of Aluminosilicate Systems Under Different Chemical Conditions

Optically clear aluminosilicate gels of different chemical compositions (0–0.9 mole ratios of total Al/(Si + Al)) were prepared directly from solutions of inorganic aluminum salts, tetraethoxysilane, water and alcohol without the time-consuming sol forming. However, in these gels only 0–75% of total Al content was incorporated by chemical bonding into the gel network depending on the compositions of gels and the preparation conditions. The incorporation of aluminum atoms into the gel framework and the structure of wet gels were investigated by chemical analysis, ²⁷Al magic angle spinning nuclear magnetic resonance, and small angle X-ray scattering. The present method may be most favourable for the preparation of aluminosilicate gels with 0.30–0.70 mole ratios of total Al/(Si + Al). At lower Al content acidic catalysis is required. Above 0.70 mole ratio homogeneous gels cannot be obtained by this method. The highest aluminum incorporation in homogeneous gel structures of various mole ratios of total Al/(Si + Al) was 0.53 mole ratio of bonded Al/(Si + Al) in contradiction to 0.1 mole ratio of Al/(Si + Al) achieved by traditional melting process of glass.     

Nanoscale observation of morphological transformation during ageing of silica and silica-alumina

Atomic force microscopy (AFM) was used for in-situ observation of nanoscale morphological transformations during the ageing step in sol-gel synthesis. Silica, alumina and silica-alumina samples with different Si/Al ratios were prepared from inorganic salt precursors and geled at low pH. Silica and silica-alumina samples formed branch-like gel network made of nanometer-sized clusters. During ageing at room temperature, the overall structure of the gel network remained unchanged but the clusters underwent phase transformation, coaslesence, coarsening, fragmentation, as well as dissolution resulting in the internal restructuring of the gel material. Morphological transformation associated with crystallization of pseudo-boehmite phase was observed for the alumina samples. These nanometer-scale processes are expected to play a key role in dictating the material properties of the final sol-gel product.     

Effect of the Al/Si atomic ratio on surface and structural properties of sol-gel prepared aluminosilicates

A series of aluminosilicates with an Al/Si ratio ranging from 0 to ∞ (0 for pure silica and ∞ for pure alumina) was prepared by sol-gel process and characterized by surface and structure techniques. Aluminum trisecbutoxide and tetramethylorthosilicate were used as precursors for the sol-gel synthesis. The acidic properties of the oxides were studied by determination of the zero point charges, through mass titration method, and, for selected samples, by FT-IR spectroscopy of adsorbed pyridine used as a probe for both Brønsted and Lewis acidity. A dependence of the acidity on the Al/Si atomic ratio was found. According to the X-ray diffraction patterns, all the oxides have an amorphous structure except pure alumina exhibiting a γ-alumina pattern. The surface areas of the mixed oxides increase with increasing amount of alumina and are higher as compared to the individual oxides. The surface elemental distribution and electronic properties were investigated by X-ray photoelectron spectroscopy. According to the results, good agreement between the surface Al/Si atomic ratio and the analytical ratio is obtained.     

Structural changes in thermal-induced polyacrylonitrile gel under uniaxial drawing

The influences of uniaxial drawing on the different structural scales in thermal-induced polyacrylonitrile (PAN) gel were studied. The results of atomic force microscopy (AFM) and swelling tests indicated that the drawn PAN gels had more compact structure with less solvent contained. Besides, some microphase separations occurred in the highly drawn gel and caused the gel to become more porous, as verified by the results from differential scanning calorimetry (DSC). The wide-angle X-ray diffraction (WAXD) results confirmed that drawing induces chain orientation and partial crystallization in the thermal-induced PAN gels, and higher draw ratio generated greater structural changes in the gels. It was also found from the tensile stress relaxation of the gels that the crosslinks in the gel increased with the draw ratio. In Fourier transform infrared (FTIR) spectra, the blue shift in wave numbers of two absorption peaks (3200-3700 cm⁻¹ and around 2244 cm⁻¹), and a gradually disappearing peak at 1036 cm⁻¹ all indicated the weakening hydrogen bonding between the PAN molecular chains and dimethyl sulfoxide (DMSO) molecules during the drawing process.     

Nature, structure and strength of the acidic sites of amorphous silica alumina: an IR and NMR study

IR spectroscopy of probe molecules (pyridine, 2,6-dimethylpyridine, and CO) as well as high-resolution solid state NMR and especially double-resonance experiments give a new insight into the acidic sites of amorphous silica alumina (ASA). ASA samples are heterogeneous compounds that contain a silica alumina mixed phase as well as aluminum clusters and pure silica zones. The distribution of various forms depends both on the preparation method and on the Si/Al ratio. Formation of mixed phase leads to the creation of acidic hydroxyl groups of various strength, up to that present in dealuminated HY zeolite. Detailed spectroscopic analyses show that these acidic OH groups correspond to the silanol groups located in close vicinity to an Al atom in tetrahedral environment. The strength of the acidity of the OH species of ASA could be modified both by the location of the vicinal Al atom on the surface or in the bulk and by the number of aluminum atoms in the vicinity of silanol group. Cogelification of high silica-containing ASA appears as the best mean to prepare homogeneous amorphous aluminosilicate, which exhibits the strongest Br?nsted acidity.     

Mullite Based Compositions Prepared by Sol-Gel Techniques

Ceramic powders and gels in the multi-alumina system with different compositions were prepared by sol-gel method. The preparation of gels was done in strong acidic conditions and also with the addition of ammonia. The powders were prepared by dropping the sols on a hot plate. The effect of water/alkoxide and ethanol/alkoxide ratios was studied together with the type of catalyst used (hydrochloric acid or ammonia). The thermal evolution of materials was followed by thermal analysis, X-ray diffraction, specific surface area measurements and electron microscopy. The morphology of powders was also studied. The characteristic exothermic peak attributed to a good mixing degree of the Al and Si species was observed at ∼980°C. The intensity of this exothermic peak was more pronounced in gels than in sol-gel derived powders. The crystalline phases formed are related with the preparation conditions and with the thermal treatment. The crystallisation of mullite and transient phases of alumina were observed in powder samples heat treated at 1000°C while in gel samples mullite was the only crystalline phase detected.     

Dependence of copper species on the nature of the support for dispersed CuO catalysts

Copper catalysts prepared by chemisorption-hydrolysis technique over silica (Cu/Si) and silica-alumina (Cu/SiAl) supports were studied to understand the role of the support on the nature and surface properties of the copper species stabilized on their surfaces. The morphological and surface properties of the copper phases have been characterized by complementary techniques, such as HRTEM, EXAFS-XANES, EPR, XPS, and FTIR. For the FTIR investigation, molecular probes (CO and NO) were also adsorbed on the surfaces to test the reactivity of the copper species. Moreover, the catalytic performances of the two catalysts have been compared in the HC-SCR reaction (NO reduction by C(2)H(4)) performed in highly oxidant conditions. The superior activity and selectivity of the supported silica-alumina catalyst with respect to that supported on silica could be related with the different nature of the copper species stabilized on the two supports, as emerged from the results obtained from the spectroscopic investigations. Small and well-dispersed CuO particles were present on silica, whereas isolated copper ions predominated on silica-alumina, likely in regions rich in alumina that made some exchangeable sites available, as indicated by FTIR spectra of adsorbed CO. The less positive global charge of copper species on Cu/SiAl than in Cu/Si has been confirmed by EPR, XPS, and EXAFS-XANES analyses.     

Hydrothermal Synthesis of Silicoaluminophosphate with AEL Structure Using a Residue of Fluorescent Lamps as Starting Material

Silicoaluminophosphate molecular sieves of SAPO-11 type (AEL structure) were synthesized by the hydrothermal method, from the residue of a fluorescent lamp as a source or Si, Al, and P in the presence of water and di-propyamine (DPA) as an organic template. To adjust the P₂O₅/SiO₂ and Si/Al and ratios, specific amounts of silica, alumina, or alumina hydroxide and orthophosphoric acid were added to obtain a gel with molar chemical composition 1.0 Al₂O₃:1.0 P₂O₅:1.2 DPA:0.3 SiO₂:120 H₂O. The syntheses were carried out at a temperature of 473 K at crystallization times of 24, 48, and 72 h. The fluorescent lamp residue and the obtained samples were characterized by X-ray fluorescence, X-ray diffraction, scanning electron microscopy, and BET surface area analysis using nitrogen adsorption isotherms. The presence of fluorapatite was detected as the main crystalline phase in the residue, jointly with considered amounts of silica, alumina, and phosphorus in oxide forms. The SAPO-11 prepared using aluminum hydroxide as Al source, P₂O₅/SiO₂ molar ratio of 3.6 and Si/Al ratio of 0.14, at crystallization time of 72 h, achieves a yield of 75% with a surface area of 113 m²/g, showing that the residue from a fluorescent lamp is an alternative source for development of new materials based on Si, Al, and P.     

Mullite fibers prepared from an inorganic sol–gel precursor

Mullite fibers were prepared by sol–gel process using alumina sol and silica sol. Alumina sol was synthesised from dissolution of aluminum powder in aluminum chloride hexahydrate solution. The optimal spinning alumina sol could be obtain in the composition range of Al/AlCl₃·6H₂O molar ratio 3.4–3.8. The Al and Si components were mixed at the molecular level and linear molecules were formed in the mullite precursor sol. The dried mullite precursor gel fibers completely transformed to mullite fibers at 1,000 °C with a smooth surface and uniform diameter.     

Study on the NaNp/Al2O3-SiO2 system

Sodium naphthenide (NaNp) deposited on aluminosilicate gels yields systems that exhibit signals in EPR spectra, the multiplicity, g values, and intensities of which depend on the gel composition. The symmetrical singlets (I) of a g(1) value of 2.00 were assigned to the complexed naphthalene anion radicals on the centers occurring on the surfaces of the alumina and silica phases of the gels studied. Those radicals complexed on surface Lewis acid sites are of hard acid/hard base type according to Pearson theory, and are bonded to the sites by ionic forces. The asymmetrical singlets (II) described by the value g(2)=1.95 were assigned to naphthalene anion radicals, which are complexed by the centers of the aluminosilicate phase of the gels studied. Those radicals, complexed on surface Lewis acid sites of this phase, are of soft acid/soft base type and are bonded to the sites by covalent bonds. The highest intensity of the singlet (I) was recorded for the system containing alumina gel. The highest intensity of the singlet (II) was recorded for the system containing Al(2)O(3) in 30 wt%. The structures of Lewis acid sites on aluminosilicate phases are postulated to be different from those on alumina or silica phases.     

Water sorption heats on silica-alumina-based composites for interseasonal heat storage

CaCl₂-containing composites have been prepared by depositing the hydrated salt (by incipient wetness impregnation) on three different silica-aluminas with various Si/Al ratios. The surface area and porosity of all the samples were determined by N₂-adsorption at ?196 °C, and their water sorption properties were investigated by thermogravimetry linked to differential scanning calorimetry (TG–DSC) in order to determine the quantity of adsorbed/desorbed water and the related heats. The heat released and the quantity of adsorbed water were found to depend on parameters such as the silica-alumina pore diameters, the Si/Al ratio, and the presence of accessible CaCl₂ active phase. The short-term stability of both supports and composites has been also checked by performing successive hydration–dehydration cycles. The sample with the lower Si/Al ratio provided the highest heat per surface area of material, and the heat released per mol of water increased with the amount of Al₂O₃ present in the samples. The deposition of CaCl₂ positively acted on the quantity of heat released during the water sorption, and the composite with the higher alumina content (75 mass% Al) showed the largest heat released per m² of material (2.4 J m^?2) compared to those containing 25 and 13 mass% Al (1.4 and 1.2 J m^?2, respectively).     

微波辐射法合成SAPO-5分子筛中硅铝比对产物结晶度和甲苯吸附性能的影响

采用硅溶胶作硅源, 用微波辐射法在酸性条件下(pH=4.5-5.0)合成了SAPO-5分子筛, 利用X射线衍射(XRD), 傅立叶变换红外光谱(FT-IR), 扫描电镜(SEM)和Brunauer-Emmett-Teller (BET)比表面积分析对样品的结晶度、形貌和比表面等进行表征, 考察了晶化原料硅铝比对分子筛结晶度和晶体颗粒长径比(c/a)的影响, 测定了分子筛对甲苯的吸附性能. 结果显示, 采用微波辐射法合成的SAPO-5分子筛, 在硅铝摩尔比为0.50附近生成产物的结晶度最好, 晶体颗粒的长径比最小(约为1.0). 尽管不同硅铝比条件下得到的样品比表面测定结果变化不大, 但对甲苯的吸附实验显示, 在硅铝比为0.50时样品的吸附速率和饱和吸附量均达到了最大值. 这与表征所得的结晶度和长径比的变化一致, 即结晶度好,长径比小的SAPO-5分子筛具有更好的甲苯吸附性能.     

Effects of repeated water exchange on the swelling behavior of poly(sodium acrylate) gels crosslinked by aluminum ions

Polyelectrolyte hydrogels, physically crosslinked by metal ions, were synthesized using poly(sodium acrylate) as the main constituent and Al ions as the crosslinker. The swelling ratio of the gel was measured whenever the solvent water was repeatedly exchanged in a constant interval. The as‐synthesized gel exhibited two relaxation processes; the gel swelled at the first stage, then shrunk very slowly at the second stage, and recovered to the initial size just after the gelation (ultimately, the gel became smaller than that). The relaxation times of both processes were found longer (exceptionally longer for the shrinking process) than the conventional collective diffusion of polymer networks. The diffused amounts of Al ions and Na counter ions in the solvent were also measured at each water exchange. The diffusion of Al ions into the solvent was found to finish when the swelling ratio took the maximum (at the end of the first stage), while Na ions continued to diffuse until the diameter became the final one (at the end of the second stage). The microscopic structural changes by the repeated water exchange were obtained by the measurements of ATR FT‐IR spectroscopy on the gels with different swelling ratios. The carboxyl groups were gradually protonated on both stages, and the formation of hydrogen bonding was accelerated on the second stage. Effects of the repeated water exchange on the swelling behavior are discussed in terms of the diffusion of Al ions into the solvent, the exchange of Na counter ions by protons, and the formation of hydrogen bonding. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 753–763, 2005     

Am(III) adsorption on oxides of aluminium and silicon: effects of humic substances,pH, and ionic strength

The adsorption of Am(III) (total concentration 10(-9) mol/l) on alumina, silica, and hematite was studied by a batch technique. The effects of pH, ionic strength, and humic substances on the adsorption of Am(III) on alumina and silica were investigated, and the adsorption isotherms of Am(III) on alumina and silica at different pH values were determined. It was found that compared with the adsorption of Am(III) on alumina, the adsorbability of silica on the basis of mass is less, the relative adsorption rate on silica is slower, the sensitivity of adsorption on silica to ionic strength is less, the dependence of adsorption on silica on pH is gentler, and consequently that the adsorption characteristics of Am(III) on alumina and silica are distinctly different. The negative effect of fulvic acid on the adsorption on silica and the positive effect of humic acid on the adsorption on alumina were found. In contrast to the Am(III) adsorption on alumina and silica, a tremendously high adsorbability of Am(III) on hematite was found. The sequence of adsorbabilities of Am(III) on the basis of mass is Fe2O3 > Al2O3 > SiO2.     

Synthesis of large-pore micelle-templated silico-aluminas at different alumina contents

The EPR spectra of radical surfactant probes embedded in cetyltrimethylammonium bromide (CTAB) and trimethylbenzene (TMB) stable water emulsions (TMB/CTAB = 13) were analyzed to provide information on the kinetics of formation of micelle-templated silicoaluminas (MTSA) at 343 K, obtained by means of silica and alumina, solved in alkaline solutions, at different Si/Al ratios. Textural (surface area, pore volume, pore size, surfactant content) and structural characterization of both as-synthesized and calcined MTSA were performed by means of nitrogen sorption isotherms, TEM, and chemical analysis. This analysis showed that TMB worked as a swelling agent of the CTAB micelles, providing large-pore homogeneous and stable MTSA at TMB/CTAB = 13 for Si/Al from infinity to 10. A demixing of the emulsion occurs at Si/Al < 10: at Si/Al = 7, a double wide-and-narrow pore structure was formed; then, at Si/Al = 5, an amorphous material was obtained. At Si/Al > or = 10, the computer-aided analysis of the EPR spectra as a function of the synthesis time indicated the distribution of the probes in two different environments: "micellar" probes inserted in the surfactant aggregates, whose mobility decreases over the synthesis time, thus reporting on the progressive modification of the surfactant aggregates structure and the solid condensation, and "interacting" probes due to probe-surfactant heads electrostatically interacting with the charged surface sites induced by alumina incorporation in the silica network. This last fraction increases its relative amount over the synthesis time, informing about the condensation and structuration of the MTSA. Without alumina, the "interacting" component is absent in the EPR spectra because TMB preferentially interacts with the surfactant headgroups by cation-pi interactions, thus preventing the interactions of these headgroups with silanols. When alumina is added, the negatively charged silicoaluminate at the surface promotes the interaction of the ammonium headgroups with the surface, and some Na+ cations also interact with TMB by cation-pi interaction and contribute to decreasing the interaction of the headgroups with TMB. Therefore, increasing alumina contents promote electrostatic interactions between the positively charged surfactant heads and the negatively charged silicoaluminate groups. The strong interaction of the surfactants with the silicoaluminate surface allows the formation of a monolayerlike structure of surfactant, which is not observed in the absence of alumina. The synthesis is slowed by increasing alumina contents due to a destructuration effect of alumina in the MTSA formation.