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Thermal dehydrofluorination of GaPO-34 revealed by NMR crystallography

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Dawson, Daniel M., Macfarlane, Laurie E., Amri, Mahrez, Walton, Richard I. and Ashbrook, Sharon E. (2021) Thermal dehydrofluorination of GaPO-34 revealed by NMR crystallography. The Journal of Physical Chemistry C, 125 (4). pp. 2537-2545. doi:10.1021/acs.jpcc.0c10871

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Official URL: https://doi.org/10.1021/acs.jpcc.0c10871

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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 Å 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.

Item Type: Journal Article
Subjects: Q Science > QD Chemistry
Q Science > QE Geology
T Technology > TA Engineering (General). Civil engineering (General)
Divisions: Faculty of Science > Chemistry
SWORD Depositor: Library Publications Router
Library of Congress Subject Headings (LCSH): Nuclear magnetic resonance spectroscopy , X-ray diffraction imaging, Density functionals , Zeolites -- Absorption and adsorption
Journal or Publication Title: The Journal of Physical Chemistry C
Publisher: American Chemical Society (ACS)
ISSN: 1932-7455
Official Date: 4 February 2021
Dates:
DateEvent
4 February 2021Published
25 January 2021Available
25 January 2021Accepted
Volume: 125
Number: 4
Page Range: pp. 2537-2545
DOI: 10.1021/acs.jpcc.0c10871
Status: Peer Reviewed
Publication Status: Published
Access rights to Published version: Restricted or Subscription Access
RIOXX Funder/Project Grant:
Project/Grant IDRIOXX Funder NameFunder ID
UNSPECIFIEDRoyal Societyhttp://dx.doi.org/10.13039/501100000288
UNSPECIFIEDWolf Foundationhttp://dx.doi.org/10.13039/501100001743
PR140003[EPSRC] Engineering and Physical Sciences Research Councilhttp://dx.doi.org/10.13039/501100000266
PR140003[BBSRC] Biotechnology and Biological Sciences Research Councilhttp://dx.doi.org/10.13039/501100000268
UNSPECIFIEDUniversity of Warwickhttp://dx.doi.org/10.13039/501100000741
UNSPECIFIEDAdvantage West Midlandshttp://dx.doi.org/10.13039/100014381
UNSPECIFIED[ERDF] European Regional Development Fundhttp://dx.doi.org/10.13039/501100008530
EP/M022501/1[EPSRC] Engineering and Physical Sciences Research Councilhttp://dx.doi.org/10.13039/501100000266

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