Formation process of Na-X zeolites from coal fly ash

In order to synthesize Na-X zeolite from coal fly ash (Fa), Fa was pretreated under stirring condition at various temperatures of 20–50°C for 72 h and then aged at 85°C for a given period with NaOH solutions. The resulting materials were characterized by various means. When Fa was aged for 72 h without pretreatment, species P were formed. As the pretreating temperature raised from 20 to 50°C, the degree of crystallinity of faujasite increased, while that of species P decreased. The faujasite species formed was identified as Na-X zeolite with molar ratio SiO₂/Al₂O₃ = 2.4. When Fa was pretreated at 50°C and aged for 60 h, the only species formed was Na-X zeolite. Increasing the pretreating temperature up to 50°C results in the increase of Si⁴⁺ and Al³⁺ concentrations in the treating solution by dissolution of amorphous material in Fa. With the conditions used, the crystalline phase, such as -quartz and mullite, was poorly dissolved during the treatment. Hence, the higher pretreating temperature would give the uniform nucleation and crystal growth of Na-X zeolite during the aging.     

Formation of Na-A and -X zeolites from waste solutions in conversion of coal fly ash to zeolites

Na-A and -X zeolites were synthesized from waste solutions in conversion of coal fly ash (Fa) to zeolite. The amorphous SiO₂ and Al₂O₃ of Fa were completely dissolved to form Po, Pt, and Pc type zeolites in NaOH solutions at 85°C. Only 24% of Si⁴⁺ eluted from Fa were converted to the zeolites and the remaining waste solutions contained high Si⁴⁺ concentrations. When molar ratio SiO₂/Al₂O₃ of the waste solutions was modified at 1.0≤SiO₂/Al₂O₃≤2.0 by addition of NaOH-NaAlO₂ solutions and the solutions were agied at 85°C, a single phase of Na-A zeolite was formed. The Na-X zeolite was formed at SiO₂/Al₂O₃≥2.5 and its crystallinity was increased with increasing the SiO₂/Al₂O₃ ratio, whereas the crystallinity of Na-A zeolite was decreased. At SiO₂/Al₂O₃=7.3, a single phase of Na-X zeolite was produced.     

Effect of stirring on the dissolution of coal fly ash and synthesis of pure-form Na-A and -X zeolites by two-step process

Using the coal fly ash (FA), pure-form Na-A and -X zeolites were synthesized by two-step process. The FA was pretreated in aqueous NaOH solution under stirring condition at 85 °C for 18 h. The amorphous aluminosilicate of FA was dissolved during pretreatment. Increasing the stirring speed accelerated the dissolution of FA and increased Si⁴⁺ and Al³⁺ concentrations in the solution. This fact indicated that the stirring during pretreatment significantly affected on the dissolution of FA. After pretreatment, remaining FA was removed and aqueous NaAlO₂ solution was added to the residual solution to control the molar ratio SiO₂/Al₂O₃ of 0.5–4.5. After aging the resultant at 85 °C for 24 h, white precipitates were generated over the whole SiO₂/Al₂O₃ range. Increment of Si⁴⁺ concentration by stirring during pretreatment increases the yield of the product. At SiO₂/Al₂O₃ = 0.5, the material was identified as Na-A zeolite with a trace amount of hydroxysodalite. A single phase Na-A zeolite was obtained at SiO₂/Al₂O₃ = 1.0. The Na-X zeolite was emerged at SiO₂/Al₂O₃ ? 2.0. At SiO₂/Al₂O₃ = 4.5, a single phase Na-X zeolite was formed. The cation exchange capacity of synthetic single phase Na-A and -X zeolites was respectively 4.78 and 3.88 meq./g.     

Selective formation of Na-X zeolite from coal fly ash by fusion with sodium hydroxide prior to hydrothermal reaction

Hydrothermal treatment of fly ash with alkali gives various types of zeolites such as Na-Pl, Na-A and hydroxysodalite, where the zeolite zone was formed like an egg white, covering the central core of fly ash particles, as evinced in the previous paper. By fusion with sodium hydroxide, most of the fly ash particles were converted into sodium salts such as silicate and aluminate, from which hydrothermal reaction without stirring favourably resulted in the formation of Na-X zeolite. Crystallinity of Na-X zeolite as high as 62% was attained at the optimum condition of NaOH/fly ash = 1.2 and a fusion temperature of 823 K. Fly ash contains 14 wt% mullite (3Al₂O₃·2SiO₂), which was revealed to be a less-active crystalline component for zeolite formation. Aluminium-enriched fly ash gave Na-A in place of Na-X zeolite. Scanning electron microscope images of cubic and octahedral crystals characteristic of Na-A and Na-X zeolite, respectively, obtained from fly ash, are given.     

Liquid-solid interaction in granular lipari pumice

Granular Lipari pumice has been transformed into a mixed-phase product with a layered structure of zeolites by treatment in hot alkaline solution. The phase distribution (hydroxysodalite, faujasite and zeolite p_c) is a function of the Na⁺/SiO₂/Al₂O₃ ratio and of the diffusion rate of caustic so da into the macropores of the rhyolitic volcanic glass. Phase transformation proceeds in the order pumice → hydroxysodalite → faujasite → zeolite P_c.The granular products can eventually be used as adsorber or industrial catalyst in fixed bed reactors.     

Synthesis of ultrafine zeolites by dry-gel conversion without any organic additive

ZSM-5, mordenite, cancrinite and Y zeolites were successfully crystallized by the dry-gel conversion (DGC) without any organic additive (or template), in which the crystal particles of ZSM-5, mordenite and Y zeolite were well controlled in the range of ultrafine size. With a molar ratio of SiO₂/Al₂O₃ ≤40, the pure ZSM-5 zeolite was obtained at 170 °C at the Na₂O/SiO₂ molar ratio <0.2, but the pure mordenite was formed at 150 °C at the Na₂O/SiO₂ molar ratio >0.25. When the dry-gel with a composition of 5.0Na₂O:Al₂O₃:5.0SiO₂ was crystallized at 140 °C for 24 h, pure cancrinite phase was obtained. TEM images showed that thus-crystallized zeolites consisted of ultrafine particles ranging 35-150 nm for ZSM-5, 50-200 nm for mordenite and 120-200 nm for zeolite Y, respectively. Different from ZSM-5, mordenite and cancrinite zeolites, the DGC synthesis of Y zeolite required more water in the dry-gel before crystallization.     

Analysis of the homogeneity ranges of faujasite-based solid solutions

Analysis presented in this paper indicates that the anhydrous residues of faujasite in composition pyramid lie close to the ideal compositions AB₂Al₄Si₁₀O₂₈ and A₂B₂Al₆Si₁₅O₄₂ and that the water content of the zeolite depends on its origin. In the “AAl₂O₄”-“BAlO₂”-SiO₂-H₂O composition pyramid, the empirical compositions of the zeolites that occur with faujasite are located in regions where phase equilibria between these solid solution would be expected. Analysis of the chabazite and faujasite structures suggests that their structural frameworks may depend on the chemical composition of the zeolite.     

Fixation of alkyl phosphate using waste incineration fly ash

Waste incineration fly ash (Fa), pretreated by washing with distilled water and heating at 1000°C in air, was treated with various concentrations of octyl phosphate [C₈H₁₇OPO₃H₂(OP)] aqueous solution by stirring at 25°C for 3 h and aging at 85°C for 24 h. X-ray fluorescence and X-ray diffraction measurements indicated that the major element of Fa is Ca which exists as CaO and gehlenite (2CaO · Al₂O₃ · SiO₂). Treating Fa in OP solutions dissolved the CaO and gehlenite to form calcium octyl phosphate (C₈H₁₇OPO₃Ca.nH₂O), which is composed of a multilayer alternating dicalcium phosphate dihydrate (CaHPO₄ · 2H₂O)-like phase and a bimolecular layer of octyl groups of the phosphates. Increasing the OP concentration increased the fixed amount of OP and amount of recovery after the treatment. The fixed amount and amount of recovery steeply increased during stirring at 25°C, and were almost unchanged by aging. These facts allow us to infer that Fa is available to fix alkyl phosphate ions such as OP.     

Characterization of Na-X,Na-A,and coal fly ash zeolites and their amorphous precursors by IR,MAS NMR and XPS

By fusion with sodium hydroxide followed by a hydrothermal reaction, fly ash and Alenriched fly ash were converted into Na-X and Na-A zeolites, respectively. The authentic Na-X, Na-A and fly ash zeolites as well as their amorphous precursors have been characterized by IR, ²⁹Si and ²⁷Al MAS NMR, XPS/AES, TG, and comparative ion-exchange studies of Cs and K with Na in zeolite samples. It appears that the same structural unit with a terminal OH, such as a sodalite unit, was formed in the induction period of the hydrothermal reaction for Na-X and Na-A, and then cross-linked through D4R and D6R external linkages to build up the zeolite framework of Na-A and Na-X, respectively.     

Interface studies on crystallized alumina-silica fibre-reinforced Al-Si alloy composites

On applying heat treatments to the amorphous alumina-silica fibre (55% Al₂O₃, 45% SiO₂), the mullite crystal is separated out, and the crystallinity depends on the heat treatment parameters. The crystallized alumina-silica fibre shows higher hardness than amorphous alumina-silica fibre and a remarkable stability to chemical reaction with molten metal. The primary interfacial reaction zone product was found to be 2MgO · SiO₂.     

Metastable immiscibility and microstructural development during sintering of a SiO2-Al2O3 plasma prepared powder

The sintering characteristics of SiO₂-36.6 wt % Al₂O₃ powder, prepared by condensation from high frequency plasma, have been studied and microstructural changes occurring during sintering followed by transmission electron microscopy The as-prepared amorphous powder showed evidence of spinodal decomposition into an Al₂O₃-rich and SiO₂-rich glass consistent with the position of a previously reported metastable miscibility gap in the SiO₂-Al₂O₃ system. Mullite crystallized on an extremely fine scale at 1000° C and gradually coarsened at higher temperatures. Sintering occurred above 1100° C by a viscous flow mechanism with activation energy 87 kcal mol^–1 which corresponds to the activation energy for viscous flow of SiO₂-Al₂O₃ glass containing approximately 17 wt % Al₂O₃.     

Preparation of granular X-type zeolite/activated carbon composite from elutrilithe by adding pitch and solid SiO2

The preparation of granular X-type zeolite/activated carbon composites from a locally available elutrilithe by adding pitch powder and solid SiO₂ was studied, and the variations in the synthesis process of zeolite X were investigated. The preparation steps of the composite involved (1) calcination of pre-shaped mixture (2) activation of the carbonaceous material from elutrilithe and pitch to prepare activated carbon and (3) hydrothermal conversion (zeolitisation) of aluminosilicate in elutrilithe and additional SiO₂ to zeolite X in alkaline medium. The adding of additional SiO₂ in the reaction system to adjust SiO₂/Al₂O₃ ratio of the reaction mixture was necessary for the formation of zeolite X. The characterization of XRD, SEM and N₂ adsorption of the resulting composites had a hierarchical pore structure, which shows that pure X-type zeolite phase with high crystallinity could be obtained regardless of the content of carbon in the composites.     

One-pot synthesis of Ni-SSZ-13 zeolite using a nickel-amine complex as an efficient organic template

We show that a nickel-amine complex (nickel-diethylenetriamine, Ni-DETA) is an efficient organic template for the one-pot synthesis of Ni-SSZ-13 zeolite. The effects of the reaction compositions, including the molar ratios of SiO₂/Al₂O₃, H₂O/Al₂O₃, Ni-DETA/Al₂O₃, and Na₂O/Al₂O₃, on the synthesis of the pure product were investigated using different temperatures and synthesis times. Physicochemical characterizations, including XRD, SEM, and ²⁷Al NMR, show that the zeolitic product has good crystallinity, uniform crystals, and contains a tetrahedral Al³⁺ species. The N₂ sorption isotherms, TG–DTA curves, IR spectra, and UV–Vis spectra show that Ni-DETA acts as a template in the product obtained. Interestingly, Ni-SSZ-13 zeolite is obtained by the conversion of Ni-Y zeolite produced in the crystallization process.     

Redox modification of high-silica zeolites with chromium via multistep cluster synthesis

This paper examines the modification of zeolites with superstoichiometric amounts of metals using multistep cluster synthesis. Ion-exchange sites in zeolite MFI-50 (SiO₂/Al₂O₃ molar ratio of 50) have been modified with hydrazonium cations. Subsequent redox reaction with chromate anions has yielded a chromium-containing zeolite. The sample prepared by three ion-exchange/redox cycles contains 10 wt % chromium (Cr-MFI-50). EPR results show that most of the chromium in Cr-MFI-50 is in the form of fine Cr₂O₃ particles. In addition, the material contains isolated Cr⁵⁺ ions and, presumably, Cr³⁺ stabilized at aluminum vacancies.     

Characterization of solvent–filler-poly(dimethylsiloxane) membranes by Differential Scanning Calorimetry

The properties of three polydimethylsiloxane (PDMS) membranes filled with crystalline zeolites and with amorphous SiO₂, respectively, before and after sorption of organic solvents were studied by DSC. The PDMS crystallinity of the membranes with sorbed solvents is affected by both the solvent and the filler natures. For SiO₂ -filled membrane, the PDMS crystallinity increases first due to the weakening of the physical crosslinks by the solvent. In the case of zeolite fillers, the PDMS crystallinity decreases first with the increase in the solvent content due to a decrease in the zeolite-induced crystallization effect by the solvent adsorption on zeolites. At high solvent contents, the solvent influence on the PDMS crystallite prevails. A medium solvent (ethyl acetate) induces the polymer crystallization, while a good solvent (cyclohexane) disrupts PDMS crystallites. Received: 14 May 2001 / Reviewed/Accepted: 15 May 2001     

Effect of SiO2Al2O3 ratio on the activity of HZSM-5 zeolites in the different steps of methanol conversion to hydrocarbons

The conversion of methanol to hydrocarbons over HZSM-5 zeolites of different SiO₂Al₂O₃ ratio was studied at 450°C. The initial zeolite activity in the different steps of the reaction (alkene formation from methanol and dimethyl ether, alkene aromatization, coke formation) was strictly proportional to the aluminum content of the zeolites. Under reaction conditions, catalytic properties of HZSM-5 of different aluminum content were modified in the initial period of catalyst performance in different ways. Enhanced activity sites, which are generated in HZSM-5 of relatively high aluminum content, were shown to be the reason for enhanced aromatization and coking activity of these zeolites.     

Geopolymers based on a coarse low-purity kaolin mineral: Mechanical strength as a function of the chemical composition and temperature

A coarse mineral with 70% kaolinite and 30% quartz was calcined and chemically activated by alkaline solutions of Na₂SiO₃ and NaOH. The compressive strength evolution was investigated as a function of the curing temperature at 20 and 80 °C, and the molar ratios of SiO₂/Al₂O₃ (2.64-4.04) and Na₂O/Al₂O₃ (0.62-1.54). For curing at 20 °C, the best composition was SiO₂/Al₂O₃ = 2.96 and NaO/Al₂O₃ = 0.62, reaching 85 MPa at 28 days. Curing at 80 °C had a positive effect on the strength development only in the first 3 days. X-ray diffraction of the geopolymeric formulations showed the formation of amorphous silicoaluminates of similar nature. The microstructure consisted of unreacted quartz and metakaolinite particles in a matrix of silicoaluminate polymer and condensed silica gel from the unreacted sodium silicate.     

Fast and efficient synthesis of ZSM-5 in a broad range of SiO2/Al2O3 without using seeding gel

ZSM-5 zeolite were synthesized in a wide range of SiO₂/Al₂O₃ molar ratios at temperature ranging from 220 to 230 °C without adding seeding gel in 4-6 h of autoclavation time.     

Influences of crystallization time,batch molar ratios Al2O3/SiO2 and Na2O/SiO2 on particulate properties of sodalite crystals prepared under room-temperature conditions

Sodalite crystals were prepared by the hydrothermal method under room-temperature conditions. The influences of crystallization time, batch molar ratios Al₂O₃/SiO₂ and Na₂O/SiO₂ on the crystalline end products were investigated. For comparison, an experiment was also carried out in which sodalite was synthesized at 90 °C for 10 h. The results revealed that with the prolongation of crystallization time from 1 h to 14 days, the crystallization followed a sequence of phase transformations from an amorphous phase to zeolite NaA, and finally to sodalite. Spherical sodalite crystals composed of very small crystallites were obtained after 10 h of room-temperature crystallization. Whereas, with the same gel composition, the sodalite synthesized at 90 °C for 10 h was large lepispherical particles. High Al₂O₃/SiO₂ molar ratio favored the generation of smaller individual sodalite nanocrystals, which resulted in the formation of larger steady congregated agglomerates. Moreover, alkalinity circumstance determined whether or not sodalite was formed. Well-developed cubic zeolite NaA particles were obtained at Na₂O/SiO₂ = 1.4; when the Na₂O/SiO₂ was increased to 8.0, zeolite NaZ-21 crystals were generated, meanwhile, the decomposition of wool ball-like sodalite particles into nanorods was also observed.     

Lightweight geopolymer made of highly porous siliceous materials with various Na2O/Al2O3 and SiO2/Al2O3 ratios

The syntheses of lightweight geopolymeric materials from highly porous siliceous materials viz. diatomaceous earth (DE) and rice husk ash (RHA) with high starting SiO₂/Al₂O₃ ratios of 13.0-33.5 and Na₂O/Al₂O₃ ratios of 0.66-3.0 were studied. The effects of fineness and calcination temperature of DE, concentrations of NaOH and KOH, DE to RHA ratio; curing temperature and time on the mechanical properties and microstructures of the geopolymer pastes were investigated. The results indicated that the optimum calcination temperature of DE was 800 °C. Increasing fineness of DE and starting Na₂O/Al₂O₃ ratio resulted in an increase in compressive strength of geopolymer paste. Geopolymer pastes activated with NaOH gave higher compressive strengths than those with KOH. The optimum curing temperature and time were 75 °C and 5 days. The lightweight geopolymer material with mean bulk density of 0.88 g/cm³ and compressive strength of 15 kg/cm² was obtained. Incorporation of 40% RHA to increase starting SiO₂/Al₂O₃ and Na₂O/Al₂O₃ ratios to 22.5 and 1.7 and enhanced the compressive strength of geopolymer paste to 24 kg/cm² with only a marginal increase of bulk density to 1.01 g/cm³. However, the geopolymer materials with high Na₂O/Al₂O₃ (>1.5) were not stable in water submersion.