Scientists Devise Cheaper and Easier Method for Synthesising Layered Rare Earth Hydroxides—'Chemical Sandwiches’

Researchers at HSE University and the RAS Kurnakov Institute of General and Inorganic Chemistry have developed a simplified and cost-effective method for synthesising layered rare earth hydroxides using propylene oxide. This reagent helps streamline the process and reduce its duration by several hours. In the future, this method is expected to facilitate the synthesis of various hydroxide-based hybrid materials, including photocatalysts for water purification and luminescent materials for solid-phase thermometers. The paper has been published in the Russian Journal of Inorganic Chemistry.

Layered hydroxides serve as a versatile matrix, with resultant properties depending on the elements present in their composition. Layered hydroxides can be likened to a sandwich, with the taste varying depending on the filling. Imagine a sandwich, where the space between the two slices of bread symbolises anion exchange properties, and the introduction of a filling represents intercalation. At the outset, there is only a lettuce leaf—symbolising chlorine—nestled between the bread slices. We do not want the lettuce but would like sausage and cheese instead. So, we remove the lettuce leaf and replace it with sausage and cheese. This constitutes the process of intercalation—the substitution of specific anions with others.

Ekaterina Sheichenko
Co-author of the paper, graduate of the HSE Faculty of Chemistry

For a more scientific definition, layered hydroxides are materials characterised by a structure in which metal-hydroxide layers alternate with negatively charged anions. Virtually any anionic molecule can be incorporated into these layers, resulting in a new hybrid material. A subset of layered hydroxides are layered rare-earth hydroxides known for their luminescent, catalytic, and magnetic properties.

Used for similar purposes as rare earth elements, layered hydroxides demonstrate equal and sometimes superior effectiveness. As an illustration, layered gadolinium hydroxide, employed as an MRI contrast agent, enhances the visibility of tissues and organs in images, with the added benefit of lower toxicity compared to other gadolinium compounds.

Layered hydroxides are obtained through either alkali precipitation or homogeneous hydrolysis in the presence of organic bases. Both methods have their disadvantages. Alkali precipitation is a two-stage process requiring significant effort; synthesised hydroxide particles must undergo additional treatment at high pressure and temperature to attain the appropriate size. In the case of homogeneous hydrolysis, the mixture must be maintained for 24 hours in a sealed device under pressure and at a temperature of 100°C.

Researchers at HSE University and the RAS Kurnakov Institute of General and Inorganic Chemistry have proposed a more streamlined and cost-effective method for producing layered hydroxides using propylene oxide as a precipitating agent for the first time. Although the studies were conducted using europium chloride, the method is universally applicable for obtaining layered hydroxides of any rare earth elements.

In the proposed method, a predetermined concentration of propylene oxide was added to the europium chloride solution; the mixture was then heated to 50°C and maintained for two hours with continuous stirring in a glass container. A substantial amount of layered europium hydroxide was produced as a result. Subsequently, through scanning electron microscopy examination and energy dispersive X-ray analysis of the samples, the scientists verified that the resulting compound exhibited the right particle size, shape, and europium-to-chlorine ratio.

Ensuring specific particle shapes and sizes is crucial, as the characteristics of the hydroxide determine the properties of the material to be derived from it, while the europium-to-chlorine ratio influences the efficiency of the anion exchange reactions that play a key role in the creation of new hybrid materials, with the optimal ratio being 2 parts europium to 1 part chlorine. A significantly reduced chlorine content would suggest the presence of carbonate anions, hindering the anion exchange process. The researchers achieved the appropriate ratio of 2 parts europium to 0.98 parts chlorine.

The scientists conducted anion exchange reactions of the resultant hydroxide with benzoate and isonicotinate anions, producing hybrid materials. According to the researchers, the exchange reactions in their materials occurred at a faster rate compared to layered hydroxides obtained through alternative methods. Furthermore, the isonicotinate anion was intercalated for the first time, yielding a powder with luminescent properties.

Examples can be found in literature of similar luminescent materials used as solid-phase thermometers that change colour depending on temperature, eg to blue in cold conditions and to orange in warmer temperatures. These materials are not currently in production but may emerge in the future.

Ekaterina Sheichenko
Co-author of the paper, graduate of the HSE Faculty of Chemistry

The researchers’ further plans include designing novel hybrid materials based on layered hydroxides. At present, the team is working to produce photocatalysts that, when exposed to light, trigger the decomposition of pharmaceuticals or dyes in wastewater, transforming them into environmentally and human-friendly compounds.
IQ

March 14