Acidity and defect sites in titanium silicalite catalyst

Abstract

With density functional calculations and in situ 29Si and 31P MAS NMR experiments, a novel defect site (defect site II, dsII) was identified in titanium silicalite molecular sieve (TS-1 zeolite), supported by the previous experimental results. The conversion from Q3 species [(SiO)3Si(OH)] into Q4-like species [Si(OSi)4] due to TMP adsorption indicated the presence of Brönsted acidity in TS-1 zeolite. The thermodynamic analysis and Ti/Si substitution energies further confirmed that the proton transferring process of defect site II is much facilitated in TS-1 zeolite whereas on the contrary in silicalite-1. With the proton-affinity calculations, the Brönsted acidities of several cation-doped zeolites were found to decrease in the order of Al-ZSM-5 > Fe-ZSM-5 > B-ZSM-5 > TS-1. The Brönsted acidity of TS-1 zeolite is even weaker than that of B-ZSM-5 zeolite, suggesting the potential role as a new acid catalyst. As to silicalite-1, its acidity is even weaker than that of defect site I (dsI) in TS-1 zeolite.