Structure and acidity of Mo/H-MCM-22 catalysts studied by NMR spectroscopy

New Insights into Keggin‐Type 12‐Tungstophosphoric Acid from 31P MAS NMR Analysis of Absorbed Trimethylphosphine Oxide and DFT Calculations

“…It is well known that crystalline (bulk) TMPO gives rise to a singlet 31 P NMR signal at around 41 ppm,49–59 and that interactions between TMPO and acid sites (either Brønsted‐ or Lewis‐type), typically result in a shift in the electron density of the base probe molecule, and thus an increase in the δ 31 P, which, in turn, reflects an increase in the acid strength of the corresponding Brønsted acid site 48. 57, 58 Typically, the δ 31 P of chemisorbed TMPO in zeolites spans from 50 to 87 ppm,50–54 whereas δ 31 P values of up to around 90 ppm were observed for highly acidic catalysts such as sulfated zirconia 55. As shown in Figure 2 a, the 31 P NMR spectrum obtained from the HPW‐2.5‐100 sample exhibited a rather complex resonance profile in the δ 31 P range of 50–95 ppm.…”

In situsolid-state NMR for heterogeneous catalysis: a joint experimental and theoretical approach

“…67 After adsorption of xenon at different pressures, 129 Xe NMR found that Mo species were preferably located in the supercages of HMCM-22 at low coverage (Mo loading o6 wt%). 97 Therefore, most Mo species were distributed in the internal zeolitic channels of HMCM-22. Two dimensional 27 Al multiple-quantum (MQ) MAS NMR experiments were also used to further investigate interaction between the Mo species and HMCM-22.…”

“…94,95 Zhao et al reported an approach to qualitatively and quantitatively characterize the internal and external acidic sites in zeolites using different adsorbed probe molecules namely TMP oxide (TMPO) and tributylphosphine oxide (TBPO) by the combination of MAS NMR spectroscopy and elemental analysis. 96,97 Although there are many factors that influence the 31 P chemical shift in these complexes, one primary source is due to electronic changes in the probe molecule leading to a change in the local magnetic field around the nucleus. In terms of the concept of molecular orbitals, Osegovic and Drago 98 pointed out that the coordination of the phosphoryl oxygen of TMPO creates an overlap between the lowest unoccupied molecular orbital (LUMO) on the Brønsted or Lewis acid site and the highest occupied molecular orbital (HOMO) on the TMPO oxygen atom.…”

DFT insight into Brønsted acidity in MCM‐22 zeolite by 31P NMR of adsorbed trimethylphosphine oxide

“…The most stable BA sites or other BA sites which the total energy is not larger than 4 kJ mol -1 with respect to the most stable one at each zeolite framework Al position were further investigated by TMPO adsorption. The chemical shift was obtained by the difference between chemical shielding of the reference, TMPO physical adsorption of 31 ppm from experiment as benchmark, [14] and the chemical shielding of the sample. The enthalpy heat (H) of adsorbed TMPO was calculated at 0 K with following equation:…”

“…These 31 P NMR results are in good agreement with the experimental data in the range of the strongest chemical shifts. [14] Based on the chemical shifts of 31 P NMR and the local structure of BA sites, the BA strength in MCM-22 is mostly defined by the pore size of zeolite systems where the BA proton is located. It can be classified into two catalogues: i) the highest chemical shifts (79-85) ppm are mostly observed for cases of TMPO adsorption complexes with BA sites where H points towards the void space of 12-MR supercage (figure 4a), which is easily to interact with TMPO and ii) the lower chemical shifts (73-78) ppm are due to TMPO bound to BA sites either inside supercage (figure 4b) or from 10-MR crossing window (figure 4c) or 10-MR sinusoidal channel (figure 4d).…”