2,6-Dimethylpyridine as a probe of the strength of Brønsted acid sites: study on zeolites. Application to alumina

Abstract

The positions of nu8a and nu*(NH) bands in the spectrum of protonated 2,6-dimethylpyridine vary with the strength of Brønsted acidity: the higher the nu*(NH) wavenumber and the lower the nu8a wavenumber, the stronger the acidity. The results obtained with 2,6-dimethylpyridine adsorption correlate with those obtained by CO adsorption experiments on a series of faujasite zeolites (LiHNaY, KHNaY, HY, HY(SA), HNaX). These relations were extended to gamma-Al2O3 having weak Brønsted acidity, not detected by pyridine and hardly detected by CO. The number (0.1 per nm2) and the strength (corresponding to delta nu (OH) by CO = 155 cm(-1)) of the most acidic OH groups of Al2O3, as well as the position of the corresponding nu (OH) band (<3700 cm(-1)) are deduced from 2,6-dimethylpyridine adsorption experiments.

Highly Dispersed SiOx/Al2O3 Catalysts Illuminate the Reactivity of Isolated Silanol Sites

“…[32] At 100 8 8C, the DRIFTS spectrum of CO on pure g-Al 2 O 3 ( Figure S19) reveals bands at 3794 and 3774 cm À1 ,assignable to terminal tetrahedral and octahedral AlÀOH groups,i ne xcellent agreement with the literature. [33] ALD deposition of Si on the catalyst surface does not affect these peak positions,a lthough ar eduction in the 3794 cm À1 band intensity is noticable.Anew OH band appears at 3726 cm À1 in the 3.2 Si/nm 2 catalyst spectrum, which is assigned here to isolated Si-OH groups on the basis of previous reports. [3b] Thus,t he CO DRIFTS confirms the presence of isolated SiÀOH sites,w ith no discernable attenuation of existing AlÀOH groups.…”

Comparative study on the support properties in the total oxidation of dichloromethane over Pt catalysts

“…Brønsted acid sites are summarized in Table 3 taken from the reference [39] for pyridine and from [40] for lutidine. The exact values are depending on the used materials.…”

Catalytic Performance of Nanoscopic, Aluminium Trifluoride‐Based Catalysts in the Synthesis of (all‐rac)‐α‐Tocopherol

“…In addition, the spectra display another two bands at 1655 cm À1 and 1631 cm À1 (assigned to the n 8a mode and n 8b mode, respectively) corresponding to protonated species, which are only observed for medium and/or strong Brønsted acid sites. [10] The strength of these sites also becomes evident during evacuation of the sample at different temperatures and the persistency of the protonated species in comparison with the coordinated species (Figure 2). Although it cannot be clearly proven, these sites are probably due to surface OH groups which are formed during the synthesis.…”