ULM-18, a Fluorinated Gallium Phosphate with Perforated Layers: XRD and NMR, Structure Determination, and HF Localization in a D4R

Abstract

Ga4(PO4)5HF−1.5 N2C6H18−H2O (or ULM-18) is an oxyfluorinated gallium phosphate obtained by hydrothermal synthesis using N,N,N‘,N‘-tetramethylethylenediamine as a template. The structure is described in the triclinic space group P-1 (n°2) with a = 8.5264(7) Å, b = 9.2512(7) Å, c = 17.870(2) Å, α = 101.742(7)°, β = 99.137(7)°, γ = 87.020(6)°, V = 1362.3(2) Å3, and Z = 2. The two-dimensional framework is built up from sheets containing double four-ring units (D4R) connected by PO4 tetrahedra. Along [010] and [011], 8-ring channels makes the layers hollow. The D4R units consist of four PO4 tetrahedra, three GaO4F trigonal bipyramids, and one GaO5 trigonal bipyramid. The D4R structure contains a templating fluorine. 19F MAS at high speed and 71Ga DOR lead to elucidation of their environment. 31P NMR MAS, RFDR, and double-quanta analyses are used to assign phosphorus. 31P-{1H} CPMAS shows that an hydrogen is inside the D4R unit. Those features together make this building unit a quite unusual D4R conformation. For sake of the rationalization of the framework construction, this D4R is compared to known MPO D4R (M = Al, Ga, Fe, V, Co), and the results of this comparison suggest a general scheme for proton insertion in the structure.