Comments on the ²⁷Al NMR Visibility of Aluminas

Abstract

The influence of surface area, paramagnetic impurities, and spinning speed on the 27Al MAS NMR visibility in a number of γ- and η-Al2O3 samples was investigated. It is shown that magic angle spinning (MAS) under typical conditions (∼10 kHz spinning speed at a 9.4 T field) is insufficient to resolve all aluminium sites in the samples. Since, in general, the number of strongly distorted sites in aluminas increases with the surface area, the 27Al MAS NMR signal intensities at 10−15 kHz spinning decrease. Differences between γ- and η-Al2O3 intensities are explained by their structural differences, causing weaker distortions in η- than in γ-Al2O3. At spinning speeds up to 29 kHz all aluminium is accounted for in the MAS spectra. Paramagnetic iron impurities up to 1440 ppm do not have a significant influence on the 27Al MAS NMR visibility of the investigated samples.

The role of alumina in the supported Mo/HBeta–Al2O3 catalyst for olefin metathesis: A high-resolution solid-state NMR and electron microscopy study

“…29 Si MAS NMR spectra with high power proton decoupling were recorded at 79.4 kHz using a 7.5-mm MAS probe with a spinning rate of 4 kHz. 4,4-Dimethyl-4-silapentane sulfonate sodium (DSS) was used as the chemical shift reference for 29 Si MAS NMR spectroscopy. A total of 1024 scans were accumulated with a π/4 pulse width of 2 µs and a 4-s recycle delay.…”

“…corresponding to the nonhydrated and hydrated forms of the Al 2 (MoO 4 ) 3 phase [17,28]. Two broad lines gradually emerge at around 6 and 67 ppm with increasing alumina content in the support; these peaks are assigned to the octahedral and tetrahedral Al in γ -alumina, respectively [29]. Table 1 shows the quantitative analysis of the framework Al content from 27 Al MAS NMR measurements.…”

“…In this work, we demonstrate the importance of choosing a suitable support for optimizing the propene production. The role of alumina in the composite support have been revealed by 29 Si MAS NMR, 27 Al MAS NMR, two-dimensional 27 Al 3Q MAS NMR, N 2 adsorption, hyperpolarized 129 Xe NMR, highresolution SEM, and TEM. The varying distributions of Mo species in the HBeta and alumina composite support have been demonstrated.…”

“…shows the29 Si MAS NMR spectra of the 4Mo/HBnAl catalysts. The lines at −117 and −113 ppm are attributed to the crystallographically inequivalent sites of the Si(OSi) 4Fig.…”

“…The lines at −117 and −113 ppm are attributed to the crystallographically inequivalent sites of the Si(OSi) 4Fig. 2 29. Si MAS NMR spectra of HBeta zeolites and 4Mo/HB-nAl catalysts, which can be deconvoluted using four Gaussian-Lorentzian lines.…”