DFT Modeling of the Adsorption of Trimethylphosphine Oxide at the Internal and External Surfaces of Zeolite MFI

Abstract

The characterization of the acidity of zeolites allows a direct correlation with their catalytic activity. To this end, probe molecules are utilized to obtain a ranking of acid strengths. Trimethylphosphine oxide (TMPO) is a widely used probe molecule, which allows the sensing of solid acids by using 31 P NMR. We have performed calculations based on the density functional theory to investigate the Brønsted acid (BA) sites in zeolite MFI by adsorbing TMPO as a probe molecule. We have considered the substitution of silicon at the T2 site by aluminum, both at the internal cavity and at the external surface. The different acid strengths observed in the zeolite MFI when probed by TMPO (very strong, strong, and weak) may depend on the basicity of the centers sharing the acid proton. If the proton lies between the TMPO and one of the framework oxygen atoms binding the Al, the acidity is strong. When the framework oxygen atom is not directly binding the Al, it is less basic and a shortening of the TMPO−H distance is observed, causing an acid response of very strong. Finally, if two TMPO molecules share the proton, the TMPO−H distance elongates, rendering a weak acid character.

Nanostructured zeolite with brain-coral morphology and tailored acidity: a self-organized hierarchical porous material with MFI topology

“…56,57 However, the varying interaction strength between probe molecules and zeolite active sites, which is commonly explained by weak, medium and strong acidity, 58,59 cannot be solely associated with Al substitutions at different T sites that create BA of different acid strength. 60 Our preliminary DFT calculations show that the adsorption energy of NH 3 at each of the twelve non-equivalent BA sites in the MFI framework varies within a very narrow range, thus not justifying the reasoning of BA sites with weak, medium and strong acid a Si/Al ratio = 20. b Reaction Conditions: T = 140°C; WHSV = 7 h −1 ; n t-BA /n Phenol = 4; TOS = 24 h. c Determined by NH 3 -TPD. d n phenol /(n Al × t).…”

Synthesis, Crystal Structures, and Properties of Zeolite‐Like T3(H3O)2[M(CN)6]2·uH2O (T = Co, Zn; M = Ru, Os)

“…All calculations included long‐range dispersion corrections through the method of Grimme (D3) with a Becke–Jonson damping, which is required for the accurate simulation of the interactions of adsorbates with extended surfaces or internal cavities . A Γ ‐centered Monkhorst–Pack grid of 3 × 3 × 3 k ‐points was used to sample the Brillouin zone integrations .…”

Toward the Atomic Scale Simulation of Intricate Acidic Aluminosilicate Catalysts

“…Computationally, this was probed by the quantification of the deprotonation energy, 162,184,261 and by the simulation of the adsorption of basic probe molecules such as CO, 38,271,280 pyridine, 39,[260][261] ammonia, 259,261 2,6-dimethylpyridine 261 and trimethylphosphine oxide. 281 The reactivity in terms of transfer ability to isobutene was also calculated in the case of zeolite Beta. 38 Generally speaking, bridging OH groups of the external surface maintain a similar acidity as that of the bulk, whereas Al-(H 2 O) located at the outermost surface exhibit weaker Brønsted acidity, and silanols are even weaker sites.…”