Nitride Zeolites from Ammonothermal Synthesis

Oxide zeolites are synthesized from aqueous solutions in an established way employing hydrothermal synthesis. Transferring this approach to nitride zeolites requires a solvent providing nitrogen for which ammonia has proven to be particularly suitable. We present the successful ammonothermal synthesis of the (oxo)nitridosilicate compounds Ce₃[Si₆N₁₁], Li₂RE₄[Si₄N₈]O₃ (RE=La, Ce) and K_1.25Ce_7.75[Si₁₁N₂₁O₂]O_0.75. Within this procedure, the usage of supercritical ammonia as a solvent as well as the utilization of the mineralizers NaN₃, Li₃N and KN₃, respectively, allowed the targeted synthesis of large single crystals. Formation of these (oxo)nitridosilicates depends mainly on the employed mineralizer despite their similar degree of condensation. The three compounds were structurally characterized using X-ray diffraction and their crystal structures contain a wide range of different ring sizes within their tetrahedra networks. The zeolite(-like) crystal structures are elucidated and compared to known nitridosilicate representatives of the respective structure types. Their elemental composition was investigated using energy-dispersive X-ray (EDX) spectroscopy and incorporation of the O rather than N-H functionality was confirmed by Fourier-Transform infrared (FTIR) spectroscopy as well as by charge distribution (CHARDI) and bond valence sum (BVS) calculations. The presented examples demonstrate that ammonothermal synthesis provides a one-step access from elemental starting materials towards nitride zeolites.