H and P solid-state NMR of trimethylphosphine adsorbed on heteropolytungstate supported on silica

Trimethylphosphine (TMP) has been used as an NMR probe in order to determine the acidity of Keggin-type 12-tungstophosphoric heteropolyacid (HPW), pure and supported on silica, dehydrated at 473 K. Adsorption of TMP on pure dehydrated HPW leads to the formation of trimethylphosphonium ions (TMPH⁺) characteristic of the presence of strong Brönsted acid sites. TMP replaces the water molecules lost by dryness and allows the Keggin secondary structure to recover.Silica interacts with TMP by two kinds of acid sites: with weak acid support sites through the isolated silanol groups and with strong Brönsted acid, which lead to the formation of TMPH⁺, through the hydrogen-bonded silanol groups. Silica only interacts with HPW through its isolated silanol groups.