Measuring Brønsted acid densites in zeolite HY with diphosphine molecules and solid state NMR spectroscopy

The use of 31P MAS NMR spectroscopy, combined with diphenyldiphosphine Ph2P(CH2)nPPh2 probe molecules with two basic groups (n = 1, 3, and 6, corresponding to maximum P-P separations of approximately 3.0, 5.6, and 9.4 A, respectively), to investigate both acidities and distances between Brønsted acid sites in zeolite HY (Si/Al = 2.6) is demonstrated in this communication. More than 90% of the Ph2P(CH2)6PPh2 molecules are doubly protonated on zeolite HY at a loading level of 12 molecules per unit cell, indicating that there are at least 12 pairs of Brønsted acid sites about 9 A apart. Similarly, experiments involving Ph2P(CH2)3PPh2 show that there are only six pairs of Brønsted acid sites separated by a distance of 6 A. Only approximately 60% of the Ph2PCH2PPh2 molecules were doubly protonated for a loading level of 4 molecules/unit cell, as not all of the Brønsted acid sites were sufficiently acidic to protonate both ends of this molecule. 31P 2D double quantum NMR spectroscopy was used to confirm the spectral assignments.