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

Abstract

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−HB distances for the acid site−TMP complex of 3.95 (±0.05) and 2.8−3.1 Å, respectively, where HB is the Brønsted acid site proton. A more accurate measurement of the Al−HB distance was not possible since models that assume the presence of isolated Al−H spin pairs are not valid in this case. A P−HB 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−HB, and P−HB 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.