Thermal Dehydrofluorination of GaPO-34 Revealed by NMR Crystallography

Abstract

Using a combination of solid-state NMR spectroscopy, powder X-ray diffraction (pXRD), thermogravimetry, and periodic density functional theory (DFT) calculations, we investigate the calcination of the chabazite-type gallophosphate, GaPO-34, prepared with either 1-methylimidazole (mim) or pyridine (py) as the structure-directing agent (SDA) and fluoride as the charge-balancing anion. We demonstrate that, prior to SDA combustion, there is an unusual low-temperature dehydrofluorination step at ∼330 °C for the mim material, but not for the py form. The DFT-derived structure for the dehydrofluorinated intermediate contains pentacoordinate Ga species with Ga–N bonds of 2.04 A to the mim nitrogen atom, in addition to four Ga–O bonds to neighboring PO4 tetrahedra. This observation is consistent with 71Ga NMR spectroscopy, which shows that one-third of the Ga is pentacoordinate with a large quadrupolar coupling constant of ∼11 MHz. Powder X-ray diffraction measured in situ on heating shows the transient appearance of a distinct crystalline phase between 325 and 425 °C before the characteristic chabazite structure is seen, which is consistent with dehydrofluorination prior to loss of the organic SDA. No such dehydrofluorinated intermediate structure is observed for the py form of GaPO-34, which is ascribed to the lower pKa of pyridinium relative to 1-methylimidazolium.