Hydrothermal synthesis of zeolite L in a Na<Superscript>+</Superscript>/K<Superscript>+</Superscript> mixed alkali system

Zeolite L was prepared from the substrate system of Na₂O-K₂O-Al₂O₃-SiO₂-H₂O at temperatures of 373–443 K by hydrothermal crystallization. The influence of various synthesis parameters such as the concentration ratios of the components, starting raw materials, synthesis temperature, gel aging, and stirring on the crystallization was investigated. Investigations revealed that the crystallinity of zeolite L crystals depends on molar ratios of the components such as SiO₂/Al₂O₃, (K₂O+Na₂O)/SiO₂, Na₂O/(K₂O+Na₂O), and H₂O/(K₂O+Na₂O). Pure and highly crystalline zeolite L could be obtained from a gel with the molar composition 5.4K₂O–5.7Na₂O-Al₂O₃-30SiO₂-500H₂O after 24 h at 443 K. It was found that the silica source affected the crystal size of zeolite L, and as the synthesis temperature increased, the average crystal size became larger. The crystal size could be decreased significantly by stirring the gel or subjecting the substrate mixture to an aging treatment at room temperature prior to the hydrothermal treatment. Thermal stability of the zeolite L crystals obtained was also briefly investigated.     

Synthesis of SUZ-4 zeolite by a dry gel conversion method

SUZ-4 zeolite was synthesized by the dry gel conversion (DGC) process with water vapor as gas phase, and characterized by XRD, SEM and N₂ adsorption. The dry gel was prepared with the assistance of a small amount of crystalline seed and organic template tetraethylammonium hydroxide (TEAOH). Molar ratios of SiO₂/Al₂O₃, KOH/SiO₂, TEAOH/SiO₂ and H₂O/SiO₂, amounts of seed and dry gel, types of silica sources, and crystallization temperature and time, were changed to optimize synthesis conditions. The results show that the DGC method leads to formation of SUZ-4 zeolite in a broad range of crystallization temperature 120–180 °C. Under the optimal conditions, i.e., SiO₂/Al₂O₃ = 22.5, KOH/SiO₂ = 0.44, TEAOH/SiO₂ = 0.044, H₂O/SiO₂ = 22.2, seed amount = 0.1 wt%, fumed silica as the silica source, 160 °C and 5 days, SUZ-4 zeolite is obtained with a high crystallinity. Compared to hydrothermal synthesis, the present DGC approach employs far less amount of organic template and allows using inert fumed silica as silica source, producing smaller rod-like SUZ-4 zeolite crystals.     

EPI-type zeolite synthesis from Greek sulphocalcic fly ashes promoted by H2O2 solutions

EPI-type zeolite was synthesised from Greek sulphocalcic lignite fly ashes. They consist mainly of SiO₂, CaO and Al₂O₃, while the SiO₂/Al₂O₃ ratio was found to be 2.74. The activation was performed by 30% H₂O₂ in an open system. Zeolite formation was observed only when activated products aged at 95 °C. The resulting materials were characterised by means of PXRD, FT-IR and SEM-EDS. PXRD and FT-IR results are in good agreement, confirming the zeolite formation. The role of H₂O₂ as a dominant factor in the zeolite synthesis is attributed to the oxidation of Fe(II) to Fe(III) and to the oxidative action on the unburned organic mater of the fly ash to prevent the reduction of Fe(III) to Fe(II). Fe(III) is proposed to participate in the reaction with Si-OH and HO-Al groups in the preliminary steps, resulting to the formation of an intermediate group [Fe-(H⁺)O(O-Si)-Al] which then gives Si-O-Al groups and Si-O-Fe groups to a lesser extent, both of which lead to a zeolite structure. Formation of the latter group explains the presence of Fe(III) in the zeolite crystal structure.     

Structure,stability and permeation properties of NaA zeolite membranes for H2O/H2 and CH3OH/H2 separations

NaA zeolite membranes were synthesised in the secondary growth hydrothermal method based on the seeding of the inner surface of a ceramic α-alumina tube. The impacts of crystallisation time and zeolite precursor concentration (in H₂O) were investigated. The structure and stability of the prepared NaA zeolite membranes were also investigated with operating temperatures, times and pressures. The results indicate that the optimal synthesis gel molar composition was 3Na₂O: 2SiO₂: Al₂O₃: 200H₂O. This led to cubic-shaped NaA zeolite which showed good stability. The optimal NaA zeolite membrane had H₂O and CH₃OH fluxes of 2.77 and 0.19 kg/m²h, with H₂O/H₂ and CH₃OH/H₂ separation factors of ∞ and 0.09 at a temperature of 30 °C. The NaA zeolite membrane had high thermal stability, but poor separation performance at high temperature (240 °C). The results suggested that the H₂ permeation flux is significantly influenced by preferential adsorption of vapour in the NaA zeolite membrane.     

Synthesis and properties of a zeolite LEV analogue from the system—Na2O–Al2O3–SiO2–N,N-dimethylpiperidine chloride–H2O

A new structure-directing agent (SDA) was firstly reported for the synthesis of a zeolite LEV analogue. N,N-dimethyl piperidine performed the SDA function, and induced the synthesis of products from a zeolite MOR with 12-ring channels to a zeolite LEV analogue with only 8-ring channels. The zeolite LEV analogue was synthesized from gels with initial compositions (5.0–6.0)Na₂O–Al₂O₃–(10–200)SiO₂–(4.0–8.0)N,N-dimethyl piperidine–400H₂O at 150 °C. The ²⁹Si NMR spectra showed that the relative intensities of the first line at −115 ppm for low Si/Al ratios were lower than that at high Si/Al ratios. Varying ion exchanges led to different acidities in the zeolite LEV analogue, with the acidity of H-LEV-HCl higher than that of H-LEV-NH₃·H₂O. Zeolite H-LEV in hydration of propene showed a higher selectivity of 1-propanol.     

The role of strontium and potassium on crystallization and bioactivity of Na2O-CaO-P2O5-SiO2 glasses

The effect of SrO/CaO and K₂O/Na₂O replacements on the crystallization process of glasses based on Na₂O-CaO-P₂O₅-SiO₂ system was investigated. The glasses were thermally treated through controlled heat treatment regimes to obtain glass ceramic materials. Combeite Na₂Ca₂Si₃O₉, sodium calcium silicate Na₂Ca₃Si₆O₁₆, wollastonite solid solution, and whitlockite Ca₃(PO₄)₂ were identified as major crystalline phases in the prepared thermally treated glasses. No potassium and strontium-containing phases could be detected in the glass-ceramics; potassium seems to be accommodated in the wollastonite structure, while strontium might be incorporated in the sodium calcium silicate structure.The surface reactivity of the prepared glass-ceramic specimens was also studied in vitro in Kokobo's simulated body fluid (SBF). EDAX, SEM, inductively coupled plasma ICP, and FTIR were used to examine the formation of apatite layer's surface and characterize the glass ceramic surface and SBF compositional changes. A decrease in the bioactivity of the glass ceramic was observed as Na₂O was replaced by K₂O. Strontium together with calcium ions in the apatite layer formed was detected with SrO/CaO replacement.The role played by the glass oxide constituents in determining the crystallization and bioactivity behaviour of the prepared thermally treated glasses was discussed.     

ANALYSIS OF RECENT MEASUREMENTS OF THE VISCOSITY OF GLASSES.—II1

Viscosity of two series of four component glasses .—The results recently reported by S. English² are here analyzed by the method used in a previous paper.³ (1) Series 6SiO₂, 1.2Na₂O, 0.8(CaO+MgO). The temperature at which the difference between the viscosity of a glass and the viscosity of 6SiO₂,2Na₂O at the same temperature makes a sharp bend is called the aggregation temperature Ta . It seems to correspond to the devitrification temperature. For this series Ta reaches a sharp minimum for the composition 6SiO₂, 1.15Na₂O, 0.45MgO, 0.39CaO. The viscosity for any temperature also reaches a minimum at or near this composition. (2) Series 6SiO₂, 1.1Na₂O, 0.9(CaO+Al₂O₃). The curves are similar to those for the first series, TA reaching a sharp minimum for the composition 6SiO₂, 1.11Na₂O, 0.81Ca0, 0.14Al₂O₃.     

Effect of K2O on structure-property relationships and phase transformations in Li2O-SiO2 glasses

Glass compositions with formula (71.78 − x)SiO₂-2.63Al₂O₃-(2.63 + x)K₂O-23.7Li₂O (mol.%, x = 0-10) and SiO₂/Li₂O molar ratios far beyond that of stoichiometric lithium disilicate (Li₂Si₂O₅) were prepared by conventional melt-quenching technique to investigate the influence of K₂O content on structural transformations and devitrification behaviour of glasses in the Li₂O-SiO₂ system. The scanning electron microscopy (SEM) examination of as cast non-annealed glasses revealed the presence of nanosized droplets in glassy matrices suggesting occurrence of liquid-liquid phase separation. An overall trend towards depolymerization of the silicate glass network with increasing K₂O content was demonstrated by employing magic angle spinning-nuclear magnetic resonance (MAS-NMR) spectroscopy. The distribution of structural units in the experimental glasses was estimated using ²⁹Si MAS-NMR spectroscopy suggesting the appearance of Q², enhancement of Q³ and diminishing of Q⁴ groups with increasing K₂O contents. X-ray diffraction (XRD) and differential thermal analysis (DTA) were used to assess the influence of K₂O on devitrification process and formation of lithium disilicate (Li₂Si₂O₅) and/or lithium metasilicate (Li₂SiO₃) crystalline phases.     

Crystal structure refinement and hydration behaviour of doped tricalcium aluminate

In this paper analytical evidence on crystal structure and hydration behaviour of C₃A solid solutions with MgO, SiO₂, Fe₂O₃, Na₂O and K₂O is given. Samples were prepared using an innovative sol-gel process as precursor, examined by X-ray powder diffraction, infra-red spectroscopy and the crystal structure was refined by the Rietveld method. A significant shift of lattice parameters was found for C₃A solid solutions with SiO₂, Fe₂O₃ or Na₂O but only minor changes were detected for K₂O. The hydration of C₃A solid solutions in the absence of CaSO₄ was accelerated for samples doped with SiO₂ or K₂O and it was retarded in the case of MgO, Fe₂O₃ or Na₂O. The hydration in the presence of CaSO₄ was accelerated when C₃A was doped with K₂O or Na₂O, whereas Fe₂O₃ strongly retarded the hydration. The doping with SiO₂ nearly had no influence on the hydration, the effect of MgO was not straight forward.     

Synthesis and properties of lithium disilicate glass-ceramics in the system SiO2–Al2O3–K2O–Li2O

The purpose of this study was the synthesis of lithium disilicate glass-ceramics in the system SiO₂–Al₂O₃–K₂O–Li₂O. A total of 8 compositions from three series were prepared. The starting glass compositions 1 and 2 were selected in the leucite–lithium disilicate system with leucite/lithium disilicate weight ratio of 50/50 and 25/75, respectively. Then, production of lithium disilicate glass-ceramics was attempted via solid-state reaction between Li₂SiO₃ (which was the main crystalline phase in compositions 1 and 2) and SiO₂. In the second series of compositions, silica was added to fine glass powders of the compositions 1 and 2 (in weight ratio of 20/100 and 30/100) resulting in the modified compositions 1–20, 1–30, 2–20, and 2–30. In the third series of compositions, excess of silica, in the amount of 30 wt.% and 20 wt.% with respect to the parent compositions 1 and 2, was introduced directly into the glass batch. Specimens, sintered at 800 °C, 850 °C and 900 °C, were tested for density (Archimedes’ method), Vickers hardness (H_V), flexural strength (3-point bending tests), and chemical durability. Field emission scanning electron microscopy and X-ray diffraction were employed for crystalline phase analysis of the glass-ceramics. Lithium disilicate precipitated as dominant crystalline phase in the crystallized modified compositions containing colloidal silica as well as in the glass-ceramics 3 and 4 after sintering at 850 °C and 900 °C. Self-glazed effect was observed in the glass-ceramics with compositions 3 and 4, whose 3-point bending strength and microhardness values were 165.3 (25.6) MPa and 201.4 (14.0) MPa, 5.27 (0.48) GPa and 5.34 (0.40) GPa, respectively.     

Synthesis of molecular sieves from Chilean kaolinites: 1. Synthesis of NaA type zeolites

The synthesis of NaA type zeolites has been studied at autogenous pressure using Chilean kaolins as starting materials. The ratios of SiO₂/Al₂O₃, Na₂O/SiO₂, H₂O/Na₂O have been set at 1.9 or 2.5, 0.6 or 1.0 and 50 or 90, respectively. Reaction times have been 5 or 15 h and reaction temperature 80 or 100°C. The influence of these parameters on the textural and structural properties of the zeolite has been studied. All samples have been characterized by X‐ray diffraction, infrared spectroscopy, scanning electron microscopy, differential thermal analysis and cation exchange capacity. The best conditions for synthesis are: SiO₂/Al₂O₃=2.5, Na₂O/SiO₂=1.0, H₂O/Na₂O=50 and 15 h reaction time at 100°C. © 1999 Society of Chemical Industry     

Substitutional effect of Na2O with K2O on the viscosity and structure of CaO-SiO2-CaF2-based mold flux systems

The effect of the substitution of Na₂O with K₂O on the viscosity and structure of molten CaO-SiO₂-CaF₂-based mold fluxes containing alkali-oxides at high temperatures has been studied. The CaO/SiO₂ mass ratio (C/S) and CaF₂ were fixed at 0.8 and 10 mass pct., respectively. The total alkali-oxide was fixed at 20 mass pct. By systematically substituting the Na₂O with K₂O, the K₂O/(Na₂O + K₂O) mass ratio was modified between 0.0 and 1.0. Using the rotating spindle method to measure the viscosity at high temperatures, the viscosity was found to increase with higher K₂O/(Na₂O + K₂O). From the slope of the temperature dependence of the viscosity, an apparent activation energy was calculated and increased with higher K₂O/(Na₂O + K₂O), from 96 to 154 kJ/mol, due to the cation size effect on the resistance to shearing. Using Raman spectroscopy of as-quenched fluxes, the mole fraction of Q³ was found to increase, while the mole fractions of Q² and Q⁰ decreased with higher K₂O/(Na₂O + K₂O). The nonbridged oxygen per silicon cation (NBO/Si) decreased from 1.97 to 1.58 with increasing K₂O/(Na₂O + K₂O), suggesting greater complexity of the flux structure with higher K₂O/(Na₂O + K₂O), resulting in a higher viscosity.     

THE SYSTEM: Al2O3.SiO2

This paper deals with a study of the equilibrium relations of mixtures of pure alumina and silica at high temperatures. The results are expressed concisely in the form of an equilibrium, diagram and their bearing on ceramic problems is discussed. The principal feature of the diagram is the absence of the compound Al₂O₃.SiO₂, the only compound being 3Al₂O₃.2SiO₂. Crystals of this latter compound occur in all alumina-silica refractories. The optical properties of these crystals have been determined and are compared with those of sillimanite, Al₂O₃.SiO₂, which has hitherto been regarded as the crystalline compound occurring in refractories and clay bodies in general. The behavior of natural sillimanite on heating is discussed.     

Effect of sodium polyacrylate on mechanical properties and microstructure of metakaolin-based geopolymer with different SiO2/Al2O3 ratio

The mechanical properties and microstructure of geopolymer are affected by the molar ratio of SiO₂/Al₂O₃. Meanwhile, organic polymer has the effect of improving the toughness of geopolymer, which depends on the SiO₂/Al₂O₃ ratio of geopolymer inevitably. Therefore, it is important to investigate the effect of the organic polymer on the mechanical properties and microstructure of geopolymer with varying SiO₂/Al₂O₃ ratio for using organic polymer to modify geopolymer. In this work, the SiO₂/Al₂O₃ ratios of metakaolin-based geopolymers are adjusted to 2.0, 2.5, 3.0, 3.5 and 4.0 by adding silica fume and β-Al₂O₃, with Na₂O/SiO₂, H₂O/SiO₂ being maintained at 0.2, 4.0, respectively. The geopolymers with each SiO₂/Al₂O₃ ratios are modified by addition of 0, 0.4, 0.8, 1.2 and 1.6?wt% of sodium polyacrylate (PAAS).The mechanical properties of these samples are measured and the rate of change is used to characterize the effect of PAAS on the metakalin-based geopolymers. The mechanism is also shown by 29Si NMR, XPS and FTIR. The results show that the effects of polymer on the mechanical properties of metakaolin-based geopolymer are affected by SiO₂/Al₂O₃ ratio and the effect becomes less obvious with SiO₂/Al₂O₃ ratio increasing from 2.0 to 4.0. Incorporation of PAAS can reduce the degree of polymerization of [SiO]₄ or [AlO]₄ in geopolymer and form the Si?O?C bond, which are two main reasons for polymer improving the toughness of geopolymer. But these effects decrease when the SiO₂/Al₂O₃ ratio of geopolymer increases from 2.0 to 4.0, which is corresponding to the effect on the mechanical properties. The toughening effect of organic polymer on geopolymer depends on the SiO₂/Al₂O₃ ratio of geopolymer, and only the geopolymer with lower SiO₂/Al₂O₃ ratio (no more than 2.5 in this work) can be significantly toughening modified by organic polymer. Therefore, it is necessary to consider the SiO₂/Al₂O₃ ratio of the geopolymer when geopolymer modified by organic polymer is designed.     

Synthesis and framework topology of the new disordered ERS-10 zeolite

The synthesis and crystal structure of the microporous crystalline aluminosilicate ERS-10 are here described. The zeolite was synthesized using a 6-azonia-spiro-[5,5]-undecane hydroxide as structure directing agent (SDA), tetraethylorthosilicate (TEOS) and aluminium-isopropoxide (AiP) as silica and aluminum sources, respectively. ERS-10 is characterized by a relatively narrow field of existence, since the unselective SDA used favors the crystallization of other zeolite phases (MTW, MOR, Beta). Crystallization of pure ERS-10 requires the hydrothermal treatment at 443 K for at least 240 h of a gel with the following composition: SiO₂/Al₂O₃ = 80–160, SDA/SiO₂ = 0.2–0.3, H₂O/SiO₂ close to 45. Preliminary low resolution powder X-ray characterization revealed that the material is affected by structural disorder such as stacking faults; however the comparison of the powder pattern, and the analysis of the cell parameters obtained by indexing the sharp reflections, suggested a close correlation among ERS-10 and NU-87 (IZA code NES), EU-1 (IZA code EUO) and nonasil (IZA code NON) type zeolites. Starting from this base, using an extensively model building and the program DIFFaX, we were able to demonstrate that the new zeolite material ERS-10 is an intergrowth of three structurally related zeolite with the NON, EUO and NES framework topology.     

The role of sulphates in cement clinkering reactions: phase formation in the system CaOAℓ2O3Fe2O3SiO2CaSO4K2SO4

The CaO-rich portions of the systems CaOA?₂O₃SiO₂K₂SO₄, CaOA?₂O₃K₂SO₄, CaOA?₂O₃SiO₂K₂SO₄CaSO₄ and CaOA?₂O₃Fe₂O₃CaSO₄K₂SO₄ have been studied experimentally. Schemes are presented showing phase assemblages compatible at subsolidus temperatures. Melting commences at about 825°C in assemblages containing K₂SO₄. Silicate, aluminate and ferrite phases are comparatively insoluble in molten alkali-rich sulphates. At clinkering temperatures, two immiscible liquids form; one is rich in K₂O and sulphate, the other is a silicate liquid. Some features of vapour-liquid-solid equilibria relevant to the S cycle in cement kilns are discussed.     

Criterion for estimating the number of phases in phase-separated glass of the Na2O-K2O-B2O3-SiO2 system by dilatometric analysis

The data of dilatometry and electron microscopy of four series of xNa₂O-(8 ? x)K₂O-32B₂O₃-60SiO₂, xNa₂O-(8 ? x)K₂O-22B₂O₃-70SiO₂, xNa₂O-(6 ? x)K₂O-34B₂O₃-60SiO₂, and xNa₂O-xK₂O-(40 ? 2x)B₂O₃-60SiO₂ phase-separated glass heat-treated at 550°C for 144 h (for glass containing 70 mol % SiO₂) and 24 h (for glass containing 60 mol % SiO₂) for separation on phases are summarized. The comparison of dilatometric data and electron microscopy allow one to conclude that glass with a difference between the onset deformation temperature and a glass transition temperature of more than 100°C is phase-separated; and glass with a difference of less than 65°C is single-phase. Curves for the glass transition temperature as a function of the K₂O content reveal a mixed alkali effect, namely, minimums for glass containing 60% SiO₂, and maximums for glass containing 70% SiO₂.     

Vibrational spectra and dissociation of aqueous Na2SiO3 solutions

To facilitate molecular spectroscopic observation of the mysterious transition of dissolved sodium silicate molecules into nanoparticles of desired silica gel and zeolite structures, the IR and Raman spectra of Na₂H₂SiO₄ monomers are studied here in details. It is demonstrated that the 3–0.2 mol/L aqueous solutions of Na₂SiO₃ and Na₂SiO₃ × 9H₂O contain mostly Na₂H₂SiO₄ monomers dissociated about 30%–80%, respectively. In contrast to the common belief the Si–O vibrations of these monomers depend on their dissociation level generating FTIR and Raman bands which are frequently associated with polymer silica structures in the current literature. To stay consistent with the molweight and dissociation measurements, these vibrational assignments are revised in this paper. Some unique and unexpected effects of D₂O used instead of H₂O as solvent are also reported.     

The role of K2O on sintering and crystallization of glass powder compacts in the Li2O–K2O–Al2O3–SiO2 system

The effects of K₂O content on sintering and crystallization of glass powder compacts in the Li₂O–K₂O–Al₂O₃–SiO₂ system were investigated. Glasses featuring SiO₂/Li₂O molar ratios of 2.69–3.13, far beyond the lithium disilicate (LD-Li₂Si₂O₅) stoichiometry, were produced by conventional melt-quenching technique. The sintering and crystallization behaviour of glass powders was explored using hot stage microscopy (HSM), scanning electron microscopy (SEM), differential thermal (DTA) and X-ray diffraction (XRD) analyses. Increasing K₂O content at the expense of SiO₂ was shown to lower the temperature of maximum shrinkage, eventually resulting in early densification of the glass-powder compacts. Lithium metasilicate was the main crystalline phase formed upon heat treating the glass powders with higher amounts of K₂O. In contrast, lithium disilicate predominantly crystallized from the compositions with lower K₂O contents resulting in strong glass–ceramics with high chemical and electrical resistance. The total content of K₂O should be kept below 4.63 mol% for obtaining LD-based glass–ceramics.     

Interfacial microstructures formation mechanism between SiO2 substrate and AgCuTi braze alloys

The wetting characteristics of braze alloys (AgCu eutectic and AgCu eutectic ?2?wt% Ti) on SiO₂ substrate are observed and the results show that small percentage additions of Ti into the AgCu eutectic alloy leads to a dramatic decrease in the steady contact angle. The interfacial microstructures are responsible for the steady contact angle decreasing of different braze alloys on the SiO₂ substrate. The aim of this work is to investigate the interfacial microstructures formation mechanism between SiO₂ substrate and AgCuTi braze alloys. Scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS) and high-resolution transmission electron microscopy (HRTEM) were employed to analyze the interfacial microstructures. The results show that the molten AgCuTi alloy infiltrates into the SiO₂ side, resulting in the formation of SiO₂/Ti₂O/(Ti₄Cu₂O+AgCu)/AgCu/(AgCu+Ti₅Si₃) structures. The Ti atoms in the molten braze alloy firstly accumulated along the surface of SiO₂ and then reacted with SiO₂ to form the Ti₂O layer, at the same time, Si liberated from thermal degradation of SiO₂ dissolved into the molten AgCuTi solution and reacted with Ti to form Ti₅Si₃ compounds. Ti-Cu rich in the molten AgCuTi would react with Ti₂O at the interface to form the Ti₄Cu₂O M6X compounds. Both the experiment results and the theoretical thermodynamics analysis support the proposed viewpoint of interfacial microstructures formation mechanism between the SiO₂ substrate and AgCuTi braze alloys.