Highly efficient synthesis of high-silica SSZ-13 zeolite by interzeolite transformation of L zeolite at higher temperature

Ultrasonic-pretreated hydrothermal synthesis of less dense zeolite CHA from the transformation of zeolite T

Because of containing the same double 6-ring (D6R) building unit, the pure zeolite CHA with lower framework density (FDSi = 15.1 T/1000 Å3) has been transformed from zeolite T with higher framework density (FDSi = 16.1 T/1000 Å3) through ultrasonic-pretreated hydrothermal synthesis in MOH (KOH and NaOH) solution without adding organic template or seed crystals. Ultrasonic pretreatment facilitates the transformation rate and generates high-quality zeolite CHA. The ultrasound condition should be precisely controlled because that CHA phase is metastable, which is inclined to transform to other more stable phase. The ultrasonic conditions at 313 K and 333 K have been investigated in detail. In KOH solution, the ultrasonic treatment at 313 K can effectively restrain the generation of MER phase, however, it is hard to avoid the existence of MER phase when ultrasound temperature is 333 K. In NaOH solution, the samples with ultrasonic treatment of 313 K show the small particles size of about 1 μm, and the GIS framework topology starts to grow with the ultrasonic treatment of 333 K. The products prepared with the appropriate ultrasonic pretreatment represents smaller particles size, larger mesopore volume and higher CO2 adsorption capacity than the sample without the ultrasonic pretreatment. The structural evolution of interzeolite transformation has been explored by XRD, FT-IR and SEM observations. With the assistance of ultrasound, the parent zeolite T can quickly decompose into intermediate phase and then regenerate into CHA phase.

Regulation of the Si/Al ratios and Al distributions of zeolites and their impact on properties

Zeolites are typically a class of crystalline microporous aluminosilicates that are constructed by SiO₄ and AlO₄ tetrahedra. Because of their unique porous structures, strong Brönsted acidity, molecular-level shape selectivity, exchangeable cations, and high thermal/hydrothermal stability, zeolites are widely used as catalysts, adsorbents, and ion-exchangers in industry. The activity, selectivity, and stability/durability of zeolites in applications are closely related to their Si/Al ratios and Al distributions in the framework. In this review, we discussed the basic principles and the state-of-the-art methodologies for regulating the Si/Al ratios and Al distributions of zeolites, including seed-assisted recipe modification, interzeolite transformation, fluoride media, and usage of organic structure-directing agents (OSDAs), etc. The conventional and newly developed characterization methods for determining the Si/Al ratios and Al distributions were summarized, which include X-ray fluorescence spectroscopy (XRF), solid state ²⁹Si/²⁷Al magic-angle-spinning nuclear magnetic resonance spectroscopy (²⁹Si/²⁷Al MAS NMR), Fourier-transform infrared spectroscopy (FT-IR), etc. The impact of Si/Al ratios and Al distributions on the catalysis, adsorption/separation, and ion-exchange performance of zeolites were subsequently demonstrated. Finally, we presented a perspective on the precise control of the Si/Al ratios and Al distributions of zeolites and the corresponding challenges.

A Mesoporous Faujasite Prepared by Space-Confined Method for Highly Effective Selectivity of Copper Ions

The discharge of copper ion (Cu(II)) into natural waters can lead to serious environmental and health problems; however, an abundantly porous hierarchical adsorbent, such as faujasite (FAU), can rapidly remove unwanted Cu(II). In this research, a hierarchically structured, abundantly mesoporous faujasite (FAU) was fabricated from industrial-waste lithium-silicon powder (LSP), with the addition of biochar and graphene oxide (GO) via hydrothermal synthesis without high-temperature calcination. The results demonstrated that just a small amount of biochar or GO can significantly improve the mesopore volume (0.14 cm³/g) and the Cu(II) adsorption capacity (115.65 mg/g) of composite FAU. In particular, careful examination of the properties of the composite FAU showed that the biochar and GO had favorably affected the growth of the zeolite crystals, thus promoting the formation of the FAU skeleton structure, ion-exchange sites and Si-OH. The composite FAU exhibited superior adsorption capacities and highly effective Cu(II) selectivity. Thus, the findings of this study provide a novel and cost-effective avenue for the synthesis of composite FAU with high copper-selective removal capacity.