Altering the atomic order in nanosized CHA zeolites by post-synthetic silylation treatment

In this work, we present a new post-synthetic hydrothermal approach leading to simultaneous dealumination and silylation of zeolite nanocrystals. Starting from a nanosized chabazite (CHA) type zeolite with a Si/Al ratio equal to 2, a set of samples with Si/Al ratios up to 4 were obtained by altering the acidity and concentration of the added silica modifier during the post-synthesis treatment leading to concurrent dealumination and silylation. The effect of the acidity during the post-synthetic silylation treatment at different pH of 1.0 to 8.5 at a constant silicon concentration was investigated. The nanosized CHA samples were characterized in details by FTIR and NMR spectroscopy (1 H, 29 Si, 27 Al), X-ray diffraction and thermogravimetric analyses. The post-synthesis approach reported here can be applied to other zeolite types for fine tuning of their structural features including type and location of Tatoms, silanol sites and active sites

CO 2 adsorption behaviour of nanosized CHA zeolites synthesised in the presence of barium or calcium cations

International audienceAlkaline-earth metal cations impact the rate of crystallisation and the adsorption of N 2 and CO 2 on nanosized chabazite zeolites. In this study, either calcium or barium was added to precursor mixtures containing alkali-metal cations (Na + , K + , Cs +) to prepare chabazite (labelled Ca-CHA and Ba-CHA), and were compared to a reference sample (Ref-CHA) synthesized using exclusively Na + , K + , and Cs +. Partial substitution of the Na + and the pore-blocking Cs + extra-framework cations was observed for Ca-CHA depending on the molar ratio of K 2 O used in the synthesis, while a change to the amount of Na + cations only was observed for Ba-CHA. The type of alkaline-earth metal cation affects the crystallisation rate; slower in the presence of Ca 2+ (10 h to full crystallinity) and similar rates in the presence of Ba 2+ (4 h to full crystallinity); the crystallite size and morphology remained similar. The presence of Ca 2+ or Ba 2+ extra-framework cations leads to N 2 uptake values of 290 and 169 mmol•g-1 (−196 °C, 100 kPa), respectively, while at low CO 2 pressure (< 1 kPa, 25 °C), the physisorbed CO 2 capacity for Ref-CHA, Ca-CHA, and Ba-CHA zeolites is 0.63, 0.66, and 0.59 mmol•g-1 , respectively. Interestingly, an opposite effect is observed for the amount of chemisorbed CO 2 species

The influence of deuterium on sodium mobility and viscosity of colloidal precursor suspensions yielding template-free nanosized zeolites

The isotopic substitution in chemistry is characterized by two main effects, i.e., the kinetic isotope effect (KIE), associated with differences in the reaction rates, and the geometric isotope effect (GIE) related to crystallographic features. In this work, considering the difference in the weight of deuterium (D) compared to the weight of hydrogen (H), we use the isotopic substitution to investigate the crystallisation mechanism of template-free nanosized faujasite (FAU) type zeolite in colloidal precursors. Тhe basic hydrothermal synthesis solvent - water (H2O), is substituted with heavy water (D2O), aiming to understand the isotopic influence and the effect of H-bond networks on the zeolite crystallisation. The crystallinity, viscosity, conductivity, chemical composition, morphology and local order of the FAU zeolites synthesized in both hydrated (H) and deuterated (D) medium are evaluated with X-ray diffraction, TEM microscopy, EDX and ICP analysis, FT-IR and solid-state NMR spectroscopy. The structural directing agent - Na+, exhibits a lower mobility in the deuterated medium, because of a characteristic higher viscosity compared to hydrated medium. This leads to a slower nucleation and crystallisation in the initial stages of hydrothermal treatment in the colloidal suspensions but also to a significant difference in the Al distribution