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

Abstract 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. 1 H– 31 P 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 31 P and 1 H 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 31 P resonances that overlap with the peaks usually ascribed to the interaction of TMPO with Brønsted sites.