Determining the Structure of Trimethylphosphine Bound to the Brønsted Acid Site in Zeolite HY: Double-Resonance NMR and ab Initio Studies

Solid-state NMR methods and ab initio calculations have been employed to investigate the structure of the trimethylphosphine (TMP)-Brønsted acid site complex in zeolite HY. ²⁷Al/³¹P and ²⁷Al/¹H rotational echo double-resonance NMR experiments performed at -150°C were utilized to measure Al-P and Al-H^B distances for the acid site-TMP complex of 3.95 (±0.05) and 2.8-3.1 Å, respectively, where H_B is the Brønsted acid site proton. A more accurate measurement of the Al-H_B distance was not possible since models that assume the presence of isolated Al-H spin pairs are not valid in this case. A P-H_B distance of 1.40 (±0.02) Å was obtained by fitting the spinning sidebands in the ¹H magic angle spinning (MAS) NMR spectrum. These internuclear distances are within the range of the Al-P, Al-H_B, and P-H_B distances obtained from ab initio calculations for the ion pair (IP) TMPH⁺-zeolite complex that is formed by transferring a Brønsted acid proton to TMP. In contrast to the ³¹P MAS NMR spectra, which indicated that the only stable species was TMPH⁺, ab initio calculations on small cluster models predicted that the neutral complex should be more stable than the IP complex. However, use of a larger zeolite fragment in the calculations enhanced the relative stability of the IP structure.