Ce-exchange capacity of zeolite L in different cationic forms: a structural investigation

Cerium exchange by microporous materials, such as zeolites, has important applications in different fields, for example, rare earth element recovery from waste or catalytic processes. This work investigated the Ce-exchange capacity of zeolite L in three different cationic forms (the as-synthesized K form and Na- and NH4-exchanged ones) from a highly concentrated solution. Chemical analyses and structural investigations allowed determination of the mechanisms involved in the exchanges and give new insights into the interactions occurring between the cations and the zeolite framework. Different cation sites are involved: (i) K present in the original LTL in the cancrinite cage (site KB) cannot be exchanged; (ii) the cations in KD (in the 12-membered ring channel) are always exchanged; while (iii) site KC (in the eight-membered ring channel) is involved only when K+ is substituted by NH4 +, thus promoting a higher exchange rate for NH4 + → K+ than for Na+ → K+. In the Ce-exchanged samples, a new site occupied by Ce appears in the centre of the main channel, accompanied by an increase in the number of and a rearrangement of H2O molecules. In terms of Ce exchange, the three cationic forms behave similarly, from both the chemical and structural point of view (exchanged Ce ranges from 38 to 42% of the pristine cation amount). Beyond the intrinsic structural properties of the zeolite L framework, the Ce exchange seems thus also governed by the water coordination sphere of the cation. Complete Ce recovery from zeolite pores was achieved.

Modifying Crystal Morphology And Framework Composition Of Composition Of Zeolite L (LTL) – A Co-Solvent Approach

The first part of this study investigates synthesis protocols to tune the size and the morphology of zeolite L (LTL) crystals using three different glycols. The outcomes clearly demonstrate that the formation of pure LTL phase is greatly facilitated at 150 ºC for 6 days only when using triethylene glycol (TEG) as the co-solvent, verified by powder X-ray diffraction (PXRD). SEM images show that LTL crystals present a cylinder-like structure with an aspect ratio of one. The formation of pure LTL phase is dramatically deteriorated when the synthesis conditions are changed to higher temperature (180 ºC), various synthesis durations (1-3 days) and aging time, and using other co-solvents (ethylene glycol and diethylene glycol). Inspired by published results indicating the interactions between Ge and glycols, I developed three synthesis approaches exploring the feasibility of incorporating germanium (Ge) into LTL framework using TEG in the second part of this study. Moreover, studies report that the incorporation of Ge in zeolite frameworks can enhance zeolites’ catalytic performances. The results from my investigation show that highly crystalline and well-defined LTL crystals are attained at 150 ºC for 3 days. X-ray fluorescence (XRF) shows the presence of Ge in LTL samples, indicating that the incorporation of Ge is very promising using the developed synthesis approaches. The outcomes are expected to be very useful to other researchers in the zeolites research field.

Modifying Crystal Morphology And Framework Composition Of Composition Of Zeolite L (LTL) – A Co-Solvent Approach

The first part of this study investigates synthesis protocols to tune the size and the morphology of zeolite L (LTL) crystals using three different glycols. The outcomes clearly demonstrate that the formation of pure LTL phase is greatly facilitated at 150 ºC for 6 days only when using triethylene glycol (TEG) as the co-solvent, verified by powder X-ray diffraction (PXRD). SEM images show that LTL crystals present a cylinder-like structure with an aspect ratio of one. The formation of pure LTL phase is dramatically deteriorated when the synthesis conditions are changed to higher temperature (180 ºC), various synthesis durations (1-3 days) and aging time, and using other co-solvents (ethylene glycol and diethylene glycol). Inspired by published results indicating the interactions between Ge and glycols, I developed three synthesis approaches exploring the feasibility of incorporating germanium (Ge) into LTL framework using TEG in the second part of this study. Moreover, studies report that the incorporation of Ge in zeolite frameworks can enhance zeolites’ catalytic performances. The results from my investigation show that highly crystalline and well-defined LTL crystals are attained at 150 ºC for 3 days. X-ray fluorescence (XRF) shows the presence of Ge in LTL samples, indicating that the incorporation of Ge is very promising using the developed synthesis approaches. The outcomes are expected to be very useful to other researchers in the zeolites research field.