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 31P NMR. We have performed calculations based on the density functional theory to investigate the Bronsted 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.
Highlights
Zeolites are well-known microporous materials whose frameworks are formed by corner-sharing SiO4 tetrahedra
Zeolite Socony Mobil-5 (ZSM-5) has the framework type of MFI, which is a material with a wide array of industrial applications and high versatility.[13−16] We have described in detail the TMPO−Brønsted acid (BA) interaction at the internal and external surfaces of zeolite MFI, providing atomic level information to Received: April 5, 2016
The results presented in this work were obtained using the density functional theory (DFT) approximation as implemented in the Vienna Ab-initio Simulation Package (VASP).[18−21] The generalized gradient approximation (GGA) under the scheme proposed by Perdew, Burke, and Ernzerhof (PBE) was used in all calculations.[22]
Summary
Zeolites are well-known microporous materials whose frameworks are formed by corner-sharing SiO4 tetrahedra. Brønsted acid (BA) sites are generated when the counterbalancing cation is a proton that covalently binds the O atom bridging the Al and the Si.[1] These BA sites, together with the size selectivity of the pore system, are the driving forces behind the wide range of catalytic applications of zeolites.[2−5]. In contrast with an acid in an aqueous medium, there is no unique way to rank the acidity of solid materials.[6] In zeolites, the proficiency of each BA as a proton donor will depend on its location within the pore system and its accessibility by the adsorbed reactant,[6] which hinders the analysis of the zeolite’s acidity
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