A collaboration between scientists from Humboldt-Universität Berlin, the BAM Federal Institute of Materials Research and Testing and ISIS have used a variety of characterisation methods to investigate the interaction of hydrogen fluoride (HF) species with the surface of the amorphous Lewis acid aluminum chlorofluoride (ACF), to produce the novel material, HF-loaded ACF, and use it for hydrofluorination reactions. By using the Xpress route to access the TOSCA instrument, the collaboration were able to measure both ACF and HF-loaded ACF quickly, gaining the Inelastic Neutron Scattering data needed to complete their study.
ACF, developed by DuPont, exhibits a large surface area, together with an extraordinary Lewis acidity comparable to SbF5, meaning that it is able to accept a pair of nonbonding electrons. ACF has been shown to be an efficient catalyst for a variety of reactions, such as isomerization and addition reactions and hydrogen/deuterium exchange reactions, and has been fully characterized by several bulk and surface characterization methods.
ACF can be further functionalized by the adsorption of hydrogen sources such as silanes or germanes onto its surface, creating materials with new properties. These modified ACFs have revealed unique reactivity towards fluorinated compounds, and their reactivity is closely related to which hydrogen source is used. Loading the surface with HF can also open the way to hydrofluorination reactions, enabling the generation of fluorinated compounds, which are in high demand.
Schematic representation of possible interactions of HF with the surface of ACF, reproduced from DOI 10.1002/chem.202001627.
It was this reaction that the research group focused on, developing a procedure to condense pre-dried gaseous HF onto the surface of ACF. Once this had been achieved, they carried out an investigation of this new material to investigate the effect of the HF loading onto ACF.
The group used Inelastic Neutron Scattering (INS) on TOSCA, with results gained from other techniques, to explore the effect of the HF loading not only at the surface but also in the bulk of ACF. By combining all these characterizations methods the study, published in Chemistry – A European Journal, revealed the formation of polyfluorides at the surface of ACF, with a slight reorganization of the bulk, as well as a strongly reduced acidity.
In their studies into the reactivity of the HF-loaded ACF, the group tested several alkynes in the presence of this new material, and were able to successfully transform them into their corresponding fluorinated alkenes. This illustrates the potential of using HF-loaded ACF in hydrofluorination reactions.
A variety of characterization methods such as MAS NMR spectroscopy, XRD, FTIR, INS, and surface analysis methods were used to investigate the interaction of HF with the surface of the nanoscopic Lewis acidic aluminium chlorofluoride (ACF). Figure reproduced from DOI 10.1002/chem.202001627.
Since the investigated HF interactions are dominated by hydrogen bridges, INS was of particular interest to identify the hydrogen-related surface interactions formed, owing to the large incoherent neutron scattering cross-section of hydrogen atoms when compared to aluminium, chlorine or fluorine atoms.
The vibrational frequencies obtained in the INS spectrum suggested the presence of [F(HF)n]- anions, and were consistent with those obtained by Fourier Transform Infra-Red (FTIR) spectroscopy. The data were also compared to previous literature on a density functional theory (DFT) study on the adsorption of HF on a β-AlF3 surface, and found to be consistent.
In terms of the timeline for Xpress proposals, for this experiment, the proposal was submitted on 17/09/2019, the samples arrived to ISIS on 21/10/2019 and were measured on the 12/11/2019 at the start of the user run cycle that followed the proposal submission.
“Thanks to the Xpress Access at the TOSCA instrument, both ACF and HF-loaded ACF have been quickly measured, allowing a brief waiting time for the results to be analysed." Explains Maeva-Charlotte Kervarec from Humboldt-Universität; “Moreover, these exceptional results emphasise the considerable potential of using INS for a better understanding of several ACF-loaded samples."
The full paper can be found at: https://doi.org/10.1002/chem.202001627
Information on the Xpress access route to ISIS instruments can be found here.