ИСТИНА

Although there is a large group of layered compounds with cation-exchange properties, the number of systems with positively charged host layers, i.e. anion-exchange capacity, is extremely limited. For a long time, hydrotalcite-like layered double hydroxides (LDH) have been known as the only layered inorganic materials with anion-exchange capacity. Recently, a new family of layered rare-earth hydroxides (LRHs) RE2(OH)5X∙nH2O (RE = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y) was found. These materials include positively charged layers of [RE2 3+(OH)5∙nH2O]+ and charge-balancing anions (X = Cl−, NO3−, etc.) sandwiched between these layers. Similar to LDHs, these layered rare-earth hydroxides are anion-exchangers. This novel class of materials is attractive due to promising combination of rare-earth chemistry and anion exchangeability. A family of LRHs exhibits a large ion-exchange capacity between host-ions (NO3 ‑ and Cl‑) and other guest-anions. Up to now a wide range of layered rare-earth hydroxides having inorganic anions (NO3 –, Cl–, Br–, SO4 2–) incorporated in their interlayer space have been obtained. At the same time systematic research dealing with incorporation of organic anions in interlayer space of LRHs is still rather poor. Incorporation of biocompatible and bioactive ions into interlayer space of LRHs is of special interest due to their potential use as luminescent markers, magnetoactive nanomaterials for magnetic resonance imaging (MRI) and hybrid organic/inorganic materials for drug delivery applications. All known LRH synthetic procedures are very time-consumable (up to tens or hundreds hours). The first type of procedures includes the precipitation of LRH from aqueous solutions of rare-earth salts (nitrates or chlorides) in the presence of hexamethylenetetramine (HMTA) at 90- 100°C. The second one is based on hydrothermal treatment of rare-earth hydroxocompounds preliminarily precipitated by inorganic or organic bases. In the present work we introduce a novel method for LRH synthesis which is based on homogeneous precipitation from aqueous solutions of Y(NO3)3 and NaNO3 in the presence of HMTA under microwave-assisted hydrothermal treatment (150-210°С). Drying stage of the precipitates must be carried out in an atmosphere with fixed relative humidity (~ 75 %). The use of this method results in significant decrease in the synthesis duration (less than 1 hour) and provides a high yield (~80-90%) of the final product. XRD analysis has shown that the single phase Y2(OH)5(NO3)∙xH2O with the highest crystallinity is obtained when the synthesis is conducted at 180°С in the excess of NaNO3. According to SEM data LRHs obtained under wide range of synthesis conditions form spherical aggregates (~10 μm diameter) of micron-sized thin (~ 100 nm) hexagonal plates. We have successfully intercalated dodecyl sulfate ions into synthesized yttrium-LRH that was confirmed by XRD. Intercalation results in dismantling of LRH aggregates and thickening of individual hexagonal crystals. Thus we have elaborated a time-saving method for high-yield Y2(OH)5(NO3)∙xH2O synthesis and we hope it is also applicable to obtain other LRH compounds. The work was supported by RFBR 14-03-00907.