Identification of fluorine sites at the surface of fluorinated γ-alumina by two-dimensional MAS NMR

Crystalline aluminium hydroxy fluorides—Suitable reference compounds for 19F chemical shift trend analysis of related amorphous solids

“…7, no. 6) are clear in opposition to the observed trend [53]. Fischer also identified different fluorine sites at the surface of fluorinated g-Al 2 O 3 and discussed the formation of AlF 3 O 3 , AlF 2 O 4 and AlF 1 O 5 -species.…”

“…Fischer also identified different fluorine sites at the surface of fluorinated g-Al 2 O 3 and discussed the formation of AlF 3 O 3 , AlF 2 O 4 and AlF 1 O 5 -species. The reported 19 F NMR shifts with respect to C 6 F 6 were 9 ppm (À154 ppm) for AlFO 5 , 20 ppm (À143 ppm) for AlF 2 O 4 and 33 ppm (À130 ppm) for AlF 3 O 3 1 [53]. But as the fluorination mainly takes place at the surface of the alumina, the formation of terminal F-sites cannot be excluded.…”

Direct Solid-State NMR Observation of Tetrahedral Aluminum Fluorides in Zeolite HY Fluorinated by Ammonium Fluoride

“…11 Both 19 F NMR signals at -151 and -166 ppm are correlated with the broad pattern spreading from 0 to -90 ppm in the 27 Al NMR spectrum, as evident from the 27 Al{ 19 F} HETCOR NMR spectrum of the F-HY/4 sample (Supporting information, Figure S2). These signals correspond to the extraframework NH 4 AlF 4 crystalline phase.…”

“…17 In a recent study of fluorinated aluminas, Fischer et al have observed broad but resolved peaks at -173, -154, -143, and -130 ppm and assigned these peaks to partially hydrated AlF 3 and to (Al VI O 5 F), (Al VI O 4 F 2 ), and (Al VI O 3 F 3 ), respectively. 11 In contrast to the assignment, Prins et al concluded that the latter two aluminum fluoride species were unlikely present in their fluorinated alumina samples based on the change in the intensity of 19 F peaks upon fluorine loadings. 18 The inconclusive results from the past NMR studies of fluorinated alumina and zeolites might suggest that there is a possibility of different fluorine-containing species resonating at the same chemical shift range.…”

“…Recently, three different fluorine sites corresponding to the three aluminum local environments AlO 6-x F x (x ) 1-3) in a fluorinated alumina, prepared through aqueous impregnation with NH 4 F, have been identified. 11 Although some effort has been made on comprehensive correlations between local environments of 27 Al and 19 F chemical shifts, 12 there is still a need to supply new NMR data to facilitate the assignment of different 19 F resonances, particularly for heterogeneous aluminum fluoride phases. This prompted us to conduct a study of zeolite HY fluorinated with an aqueous solution of ammonium fluoride by solid-state 19 F and 27 Al MAS, 19 F to 27 Al CP (cross-polarization), and corresponding 2D HETCOR (heteronuclear correlation) NMR measurements.…”