Reactions of Rare Earth Hydrated Nitrates and Oxides with Formamide: Relevant to Recycling Rare Earth Metals

Reactions
of hydrated rare earth nitrates with formamide under
mild solvothermal reaction conditions give three different rare earth
formate structure types Ln­(HCOO)3, Ln­(HCOO)3·(HCONH2)2, and [CH­(NH2)2]­[Ln­(HCOO)4]. The conditions for the formation
of a specific structure type were determined as a function of the
size of the rare earth cation, the reaction time, and the reaction
temperature. Thirteen previously unreported rare earth formates were
obtained; nine were isostructural compounds with the composition Ln­(HCOO)3·(HCONH2)2 (denoted by LnFA, Ln
= Y (1Y), Sm (2Sm), Eu (3Eu), Gd (4Gd), Tb (5Tb), Ho (6Ho), Tm (7Tm), Yb (8Yb), and Lu (9Lu)), and four were isostructural compounds with the composition [CH­(NH2)2]­[Ln­(HCOO)4] (denoted by LnFMD, Ln
= Sm (10Sm), Ho (11Ho), Tm (12Tm), and Lu (13Lu)). All compounds contain metal ions
that are eight coordinated by oxygen atoms and are connected by anti–anti bridging formato ligands to form frameworks.
The coordination geometry is bisdisphenoid in LnFA and square antiprismatic
in LnFMD. In contrast, the larger rare earth ions that adopt the Ln­(HCOO)3 structure are nine coordinated by oxygen atoms in tricapped
trigonal prismatic coordination geometry. The luminescence properties
of 2Sm, 3Eu, 5Tb, and 10Sm were investigated by solid state photoluminescence spectroscopy
in the range of 300–800 nm. Rare earth oxides were also found
to react directly with formamide in the presence of a small amount
of water to give the same compositions suggesting that the reaction
chemistry can provide a basis for a process to recover rare earth
metals from end-of-life products containing oxides such as the phosphors
used in fluorescent light bulbs. This possibility was demonstrated
by crystallizing rare earth formates starting with a commercially
available trichromatic phosphor.