Elucidating the Nature of the External Acid Sites of ZSM-5 Zeolites Using NMR Probe Molecules

The identification of acid and nonacid species at the external surface of zeolites remains a major challenge, in contrast to the extensively-studied internal acid sites. Here, it is shown that the synthesis of zeolite ZSM-5 samples with distinct particle sizes, combined with solid-state NMR and computational studies of trimethylphosphine oxide (TMPO) adsorption, provides insight into the chemical species on the external surface of the zeolite crystals. ¹H–³¹P HETCOR NMR spectra of TMPO-loaded zeolites exhibit a broad correlation peak at δ_P ～35–55 ppm and δ_H ～5–12 ppm assigned to external SiOH species. Pore-mouth Brønsted acid sites exhibit ³¹P and ¹H NMR resonances and adsorption energies close to those reported for internal acid sites interacting with TMPO. The presence of an external tricoordinate Al-Lewis site interacting strongly with TMPO is suggested, resulting in ³¹P resonances that overlap with the peaks usually ascribed to the interaction of TMPO with Brønsted sites.