Characterization of the gel phases formed in the synthesis of microporous gallophosphate, cloverite

Abstract

This work examines the formation of a microporous gallophosphate with an extra large pore system, namely cloverite (structure type: CLO) in fluoride medium. The amorphous gallophosphate gel containing the key precursor to cloverite was characterized and its evolution followed by ³¹P, ⁷¹Ga, ¹⁹F and ¹H MAS NMR and several dipolar-coupling based double-resonance methods including ³¹P{⁷¹Ga} and ¹⁹F{⁷¹Ga} rotational-echo adiabatic passage double-resonance (REAPDOR), ³¹P{¹⁹F} and ¹⁹F{³¹P} rotational-echo double-resonance (REDOR), and ¹H{³¹P} REDOR and ¹H{⁷¹Ga} transfer of populations in double-resonance (TRAPDOR). The results indicate that upon mixing the Ga and P sources in the presence of structure directing agent (SDA) and HF, an amorphous oxyfluorinated gallophosphate precursor containing single 4-rings (4Rs) with alternating Ga–O–P linkage forms immediately. The Ga in the 4R is 6-coordinated and the two Ga in the 4R are bridged by a fluorine. Heating quickly converts the 4Rs to double four-membered rings (D4Rs), from which the entire CLO structure is built. The amount of the D4R rapidly increases with increasing heating time. The presence of fluoride ions greatly facilitates the formation of the 4Rs via bridging two Ga sites and the hydrothermal treatment promotes D4Rs via condensation. Interaction between SDA molecule and the phosphorus site in the gel is also probed. Further, the local structure of Ga sites in cloverite is characterized by ⁷¹Ga MAS NMR at ultrahigh magnetic field of 21.1 T by obtaining their electric field gradient tensor parameters.