A spectroscopic probe for combined acid and redox properties in acid catalysts

The activity of the catalytic materials used in industrial and academic laboratories often relies on the concurrence of acidic and redox properties. In this work trimethylphosphine was selected as a NMR probe to assess the type and force of a catalyst acid sites as well as an estimate of its redox capabilities by a single set of experiments, since the 31P spectrum of both this molecule and of its oxidized form (TMPO) is sensitive to the solid acidity. The strength of the acidic sites was assessed by the probe chemical shift and its resistance to desorption at high temperature. A qualitative oxidizing scale was evaluated by the facility with which a given solid produces TMPO from TMP. Different catalysts were analysed: γ-Al2O3, zeolite HY and K10, KSF and Zr-pillared acid clays. γ-Al2O3 was found to form only Lewis adducts and to possess no detectable redox behavior. The zeolite HY tested here had oxidizing properties (probably due to metal or group XVI impurities) and presented adducts with Brönsted as well as with extra-framework aluminum (EFAl) sites. Purely Brönsted sites were observed for K10, KSF and Zr-pillared clays. These latter materials were characterized here for the first time by 31P NMR of adsorbed TMP. 31P MAS NMR spectra of K10 showed three different types of Brönsted sites, but only one seems to be present in the case of KSF and of the Zr-pillared clay.

Graphical abstractTrimethylphosphine, which is a NMR probe to assess the type and force of a catalyst acid site, can as well provide an estimate of redox capabilities in these solids by a single set of experiments. A qualitative oxidizing scale was evaluated by the facility, with which a given solid produces TMPO from TMP. γ-Al2O3, zeolite HY and K10, KSF and Zr-pillared acid clays were evaluated.Figure optionsDownload full-size imageDownload as PowerPoint slide