The location and ordering of fluoride ions in pure silica zeolites with framework types IFR and STF; implications for the mechanism of zeolite synthesis in fluoride media

Synthesis of high-quality NaP1 zeolite from municipal solid waste incineration fly ash by microwave-assisted hydrothermal method and its adsorption capacity

The Si and Al in municipal solid waste incineration fly ash (MSWI FA) can be utilized for zeolite fabrication, which can improve the application value of the products. This study focuses on the fabrication of zeolite from MSWI FA by microwave-assisted hydrothermal (MH) treatment. The effects of magnetic stirring time, Na₂SiO₃ dosage, MH time, and NaOH solution concentration on the crystallization of zeolite NaP1 from MSWI FA are systematically analyzed. The synthetic products are analyzed through spectroscopic and mineralogical methods. The results show that zeolite NaP1 with high crystallinity (51.68 %) can be fabricated by magnetic stirring and MH treatment, and the cation exchange capacity (CEC) of the product can reach a value of 2.58 meq/g, which is approximately 133 times that of the CEC of MSWI FA. The Si/Al ratio plays a decisive role in the zeolite NaP1 synthesis, and a Na₂SiO₃ dosage of 30 wt% is adopted for zeolite NaP1 fabrication. A NaOH concentration of 1 M is sufficient for zeolite NaP1 synthesis. Additionally, the zeolite NaP1 content is found to obviously increase with increasing MH time from 0.5 h to 2 h. To demonstrate the feasibility of the method provided in this study, the optimal experimental condition is employed for various MSWI FAs, and zeolite NaP1 and analcime are fabricated successfully. The leachability of heavy metals for the synthetic products was evaluated, which met the requirements for pollution control. The BET surface area and total pore volume of zeolite NaP1 fabricated at optimal condition are 61.42 m²/g and 0.44 cm³/g, respectively. The adsorption capacity of zeolite NaP1 for Cu²⁺ ion and methylene blue are determined to be 84.65 mg/g and 84.55 mg/g, respectively, indicating zeolite NaP1 is a potential adsorbent for cation ion and dyes. This study provides an environmentally friendly scheme for the utilization of MSWI FA.

Synthesis of amorphous aluminosilicate from impure Indonesian kaolin

Synthesis of amorphous aluminosilicates from high impurities in Indonesian kaolin were carried out in several ways, directly from kaolin and indirectly from metakaolin (calcined kaolin), both with silica addition and without silica addition. Synthesis was conducted in a gradual temperature hydrothermal reaction and in a constant temperature hydrothermal reaction. Before being synthesized, kaolin impurities were separated according to density and/or particle size by centrifugation. X-Ray Diffraction and FTIR Spectrometry characterization of the synthesized product showed that the results of synthesis of aluminosilicates from metakaolin via hydrothermal with gradual temperature or constant temperature produced aluminosilicates with high quartz impurities, while synthesis with the addition of silica in hydrothermal conditions at 80°C for 12 hours can produce aluminosilicate with a low quartz content. Direct synthesis from kaolin with the addition of silica could produce pure aluminosilicates.

A time- and cost-effective synthesis of CHA zeolite with small size using ultrasonic-assisted method

In this paper, the ultrasonic-assisted method has been used to the rapid hydrothermal synthesis of CHA zeolite in the absence of expensive organic template. The influence of different sonicated time on the crystallinity and micromorphology of CHA-type crystals has been studied in detail. The bath-type ultrasound processor produces acoustic waves at the frequency of 20 kHz and the power of ultrasound wave is 150 W. The synthesized products have been characterized by XRD, SEM, EDX, TG and N₂ adsorption analyses. Due to the acceleration on homogeneous nucleation caused by acoustic cavitation cracks during the ultrasonic vibrative process, the crystallization time can be shorten from 48 h to 10 h and the crystal size can be reduced from about 20 μm to 5 μm. The SEM observations of samples with different ultrasonic treatment time distinctly revealed the morphology evolution of the walnut-like CHA zeolites. From EDX, TG and N₂ adsorption comparisons, it is clear that untreated sample and ultrasonic sample both have similar element distribution, thermal stability, and pore size distribution. The research work in this paper has demonstrated the ultrasonic treatment can significantly improve crystallinity degree, reduce crystallization time and crystal size of CHA zeolite.

Bifluoride ([HF2]−) formation at the fluoridated aluminium hydroxide/water interface

Bifluoride-type species are formed at fluoride-exchanged aluminium hydroxide surfaces contacted with aqueous solutions. First layer surface species are anchors for growth of multi-layered species towards the solution.

Colloidal Defect-Free Silicalite-1 Single Crystals: Preparation, Structure Characterization, Adsorption, and Separation Properties for Alcohol/Water Mixtures

In this work, colloidal silicalite-1 single crystals are for the first time synthesized using fluoride as mineralizing agent at near neutral pH. SEM, TEM, DLS, XRD, solid-state (29)Si MAS NMR, and adsorption/desorption experiments using nitrogen, water, n-butanol, and ethanol as adsorbates were used to characterize the crystals. The single crystals have a platelike habit with a length of less than 170 nm and an aspect ratio (length/width) of about 1.2, and the thickness of the crystals is less than 40 nm. Compared with silicalite-1 crystals grown using hydroxide as mineralizing agent, the amount of structural defects in the lattice is significantly reduced and the hydrophobicity is increased. Membrane separation and adsorption results show that the synthesized defect-free crystals present high selectivity to alcohols from alcohol/water mixtures. The n-butanol/water adsorption selectivities were ca. 165 and 14 for the defect-free crystals and a reference sample containing defects, respectively, illustrating the improvement in n-butanol/water selectivity by eliminating the polar silanol defects.

The effect of structure directing agents on the ordering of fluoride ions in pure silica MFI zeolites

The effect of structure directing agent size/shape on the ordering of the framework fluoride ions in pure silica zeolites.

Clathrate compounds of silica

A review on silica clathrate compounds, which are variants of pure silica zeolites with relatively small voids, is presented. Zeolites have found many uses in industrial and domestic settings as materials for catalysis, separations, adsorption, ion exchange, drug delivery, and other applications. Zeolites with pure silica frameworks have attracted particular interest because of their high thermal stability, well-characterized framework structures, and simple chemical compositions. Recent advances in new synthetic routes have extended the structural diversity of pure silica zeolite frameworks. Thermochemical analyses and computational simulations have provided a basis for applications of these materials and the syntheses of new types of pure silica zeolites. High-pressure and high-temperature experiments have also revealed diverse responses of these framework structures to pressure, temperature, and various guest species. This paper summarizes the framework topologies, synthetic processes, energetics, physical properties, and some applications of silica clathrate compounds.

Structure-direction in the crystallization of ITW zeolites using 2-ethyl-1,3,4-trimethylimidazolium

2-Ethyl-1,3,4-trimethylimidazolium, the fifth and largest imidazolium cation able to produce the pure silica zeolite ITW, affords together with fluoride a significant flexibility enhancement of the SiO₂ framework.

Heavy metals stabilization in medical waste incinerator fly ash using alkaline assisted supercritical water technology

This study investigated the process of aluminosilicate formation in medical waste incinerator fly ash containing large amounts of heavy metals and treated with alkaline compounds at 375 degrees C and examined how this process affected the mobility and availability of the metals. As a consequence of the treatments, the amount of dissolved heavy metals, and thus their mobility, was greatly reduced, and the metal leaching concentration was below the legislative regulations for metal leachability. Moreover, this process did not produce a high concentration of heavy metals in the effluent. The addition of alkaline compounds such as sodium hydroxide and sodium carbonate can prevent certain heavy metal ions dissolving in water. In comparison with the alkaline-free condition, the extracted concentrations of As, Mn, Pb, Sr and Zn were decreased by about 51.08, 97.22, 58.33, 96.77 and 86.89% by the addition of sodium hydroxide and 66.18, 86.11, 58.33, 83.87 and 81.91% by the addition of sodium carbonate. A mechanism for how the formation of aluminosilicate occurred in supercritical water and affected the mobility and availability of the heavy metals is discussed. The reported results could be useful as basic knowledge for planning new technologies for the hydrothermal stabilization of heavy metals in fly ash.

The use of ionic liquids in the synthesis of zinc imidazolate frameworks

Four zinc imidazolate materials has been synthesized under ionothermal conditions using ionic liquids; [Zn(C2O4)(C3N2H4)], P21/a (1); [Zn(CH3COO)(C3N2H3)], Ima2 (2); [Zn4(C3N2H4)(C3N2H3)8], P (3) and [Zn(C3N2H3)2], I41 (4). Compounds 1 and 4 have already known structures, but have been synthesised for the first time using ionothermal methods. Compounds 2 and 3 are novel and have been synthesized for the first time in this work. The syntheses of 3 and 4 take place under almost exactly the same conditions, except that a lower synthesis temperature leads to a proportion of terminal imidazolate units in the structure of 3 while a slightly higher temperature leads to all bridging imidazolate units in 4.

Computational study of 19F NMR spectra of double four ring-containing Si/Ge-zeolites

(19)F NMR chemical shifts are calculated in order to study the F(-) environment in double four ring (D4R) containing Si/Ge-zeolites. The calculations with the DFT/CSGT/B3PW91 methodology yielded an agreement within 2 ppm with respect to the experimental peaks corresponding to the D4R units containing 8Si0Ge, 7Si1Ge and 0Si8Ge of the octadecasil zeolite. The optimisation of the 7Si1Ge-, 6Si2Ge-, 5Si3Ge- and 4Si4Ge-D4R units with DFT/B3LYP methodology shows that a covalent Ge-F bond is formed and therefore a Ge atom in the D4R is pentacoordinated. The displacement of the fluoride ion towards a Ge atom in the Ge-containing D4R units locates four Si/Ge atoms in the close vicinity of the F(-) and this makes possible a rationalization of the (19)F NMR signals in groups according to the number of Si (n) and Ge (m) atoms in the nearest F(-) environment, F-Si(n)Ge(m) (where n+m=4). Thus, the calculated chemical shifts show that higher values are observed when the number of Ge atoms in the nearest F(-) environment increases.

A computational chemistry insight in the role of structure directing agents in the synthesis of zeolites

The role of structure directing agents in the synthesis of zeolites has been studied through atomistic simulation techniques. Three different kinds of structure directing agents have been reviewed: (i) isomorphic heteroatom Si --> Al and Si --> Ge substitutions; (ii) F(-) anions; and (iii) organic SDAs. The interplay between these structure directing agents is described and the separate effects of each one have been found from energy calculations and analysis of energetic terms.

Structure-Directing Agent Location and Non-Centrosymmetric Structure of Fluoride-Containing Zeolite SSZ-55

Single crystals of pure silica zeolite SSZ-55 were prepared using the fluoride route. Single-crystal X-ray diffraction at a synchrotron source revealed the framework structure of the material, but the unit cell (orthorhombic a = 12.905(2) A, b = 21.344(4) A, c = 5.1279(10)) is too small to accommodate ordered arrays of the organic structure-directing agent. Molecular modeling was used to simulate the docking of the structure-directing agent in the channels of the material, and this revealed a strong space-filling interaction with a number of possible orientations of the organic cation. The overall non-centrosymmetric structure of the solid (spacegroup C222(1)) was confirmed using second harmonic generation experiments.

Combined solid state NMR and X-ray diffraction investigation of the local structure of the five-coordinate silicon in fluoride-containing as-synthesized STF zeolite

The local structure of the [SiO(4/2)F]- unit in fluoride-containing as-synthesized STF zeolite has been experimentally determined by a combination of solid-state NMR and microcrystal X-ray diffraction to be very close to trigonal bipyramidal. Because the fluoride ions are disordered over two sites, the resulting local structure of the [SiO(4/2)F]- unit from a conventional XRD refinement is an average between tetrahedral SiO(4/2) and five-coordinate [[SiO(4/2)F]-, giving an apparent F-Si distance longer than expected. The correct F-Si distance was determined by slow spinning MAS and fast spinning (19)F/(29)Si CP and REDOR solid-state NMR experiments and found to be between 1.72 and 1.79 A. In light of this, the X-ray structure was re-refined, including the disorder at Si3. The resulting local structure of the [SiO(4/2)F]- unit was very close to trigonal bipyramidal with a F-Si distance of 1.744 (6) A, in agreement with the NMR results and the prediction of Density Functional Theory calculations. In addition, further evidence for the existence of a covalent F-Si bond is provided by a (19)F-->(29)Si refocused INEPT experiment. The resonance for the five-coordinate species at -147.5 ppm in the (29)Si spectrum is a doublet due to the (19)F/(29)Si J-coupling of 165 Hz. The peaks in this doublet have remarkably different effective chemical shift anisotropies due to the interplay of the CSA, dipolar coupling, and J-coupling tensors. The distortions from tetrahedral geometry of the neighboring silicon atoms to the five-coordinate Si3 atom are manifested in increased delta(aniso) values. This information, along with F-Si distances measured by (19)F-->(29)Si CP experiments, makes it possible to assign half of the (29)Si resonances to unique tetrahedral sites. As well as determining the local geometry of the [SiO(4/2)F]- unit, the work presented here demonstrates the complementarity of the solid-state NMR and X-ray diffraction techniques and the advantages of using them together.