Characterization of the acidity of Mu-14 by solid-state NMR and NH3-STD

Abstract

The distribution and strength of the Brönsted acid sites in three Mu-14 (ITE structure type) samples with different Si/Al ratios (17, 28 and 35) were studied by ¹H MAS (magic angle spinning), ¹H-{²⁷Al} TRAPDOR (transfer of population in double resonance), ¹H-²⁹Si CP-MAS (cross-polarisation magic angle spinning)/ HETCOR (two-dimensional heteronuclear correlation) NMR, and NH₃-STD (stepwise thermal desorption). The ¹H MAS NMR spectra exhibit two different types of bridging OH groups associated with proton chemical shifts of ca. 4.2 and 5.3 ppm. From the ¹H-²⁹Si HETCOR experiments a single Brönsted [Si-(OSi)₃(OAl)₁] Q3 site resonating at δ(²⁹Si) = − 108 ppm, can be distinguished from two different [Si-(OSi)₃(OH)₁] Q3 sites, at δ(²⁹Si) = − 101 and − 104 ppm, and one of the Q4 sites resonating at δ(²⁹Si) = − 108.5 ppm. Aluminum is not randomly distributed in the structure of Mu-14, but rather sits at preferred places isolated from silanols. The acid strength of the Brönsted acid sites, determined by NH₃-STD, is slightly higher in Mu-14 than in a HZSM-5 with a similar Si/Al ratio.