Acidic Strengths of Brønsted and Lewis Acid Sites in Solid Acids Scaled by ³¹P NMR Chemical Shifts of Adsorbed Trimethylphosphine

Abstract

The validity of using 31P NMR of adsorbed trimethylphosphine (TMP) as a probe molecule for discerning the types (Brønsted vs Lewis) and strengths of acid sites in solid acid catalysts have been studied by density functional theory (DFT) calculations. Brønsted acid sites with varied acidic strengths covering from weak, strong, to superacid, mimicked by 8T zeolite cluster models having different Si−H bond lengths and hence proton affinities, were examined together with Lewis acid systems having different metallic centers, e.g., BCl n F3−n (n = 0−3), AlCl n F3−n (n = 0−3), and TiCl n F4−n (n = 0−4) and their mixed halides. The theoretical 31P chemical shifts predicted for the hydrogen-bonded TMP···H complex and the TMPH+ adducts were −61 ± 1 and −3 ± 1 ppm, respectively, in good agreement with the experimental data. For the TMP−Lewis acid complex, a linear correlation between the calculated 31P chemical shifts and corresponding binding energies was observed for the B-, Al-, and Ti-containing Lewis acids, respectively, indicating the feasibility of using the 31P chemical shift of adsorbed TMP as a scale for Lewis acidic strength.

Influence of peroxometallic intermediaries present on polyoxometalates nanoparticles surface on the adipic acid synthesis

“…Interestingly, this stabilisation effect does not take place with the heteropolyacids, despite they are stronger Brønsted acids [25] than acetic acid. This could be also related to the influence of transition metals within the POM structure on the H 2 O 2 decomposition.…”

“…This oxidant is not soluble in most of the organic solvents. For this reason, the utilisation of phase transfer compounds (PTCs) [4] Page 3 of 25 A c c e p t e d M a n u s c r i p t 3 is required. Despite of the use of PTCs increases yield of oxidation reactions involving organic compounds, those are relatively expensive and harmful for industrial applications, when huge amounts of these compounds are required.…”

Lewis and Brönsted acidic sites in M4+-doped zeolites (M=Ti, Zr, Ge, Sn, Pb) as well as interactions with probe molecules: A DFT study

“…TMP, often used to probe the zeolite acidities [17,27,[71][72][73], is the only one that in all the doping cases, the covalent and ionic modes will co-exist. The energy differences between the ionic and covalent structures (E diff ) decrease in the order Silicalite (Fig.…”

Solid-State NMR Studies of Zeolites

“…Therefore, Brønsted and Lewis acid sites in zeolites can be clearly distinguished, and their concentrations can be quantitatively determined. Recent DFT calculation results indicate that the 31 P NMR chemical shifts of adsorbed TMP molecules can be used as a scale to quantitatively measure the Lewis acid strength, i.e., the Lewis acid strength linearly increases with the increase of the 31 P NMR chemical shift [ 73 ]. In order to accurately determine the Brønsted acid strength of zeolites, several spectroscopic and analytical methods such as IR spectroscopy, temperatureprogrammed desorption (TPD), and microcalorimetry have been employed.…”