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. H-1-P-31 HETCOR NMR spectra of TMPO-loaded zeolites exhibit a broad correlation peak at delta(P) similar to 35-55 ppm and delta(H) similar to 5-12 ppm assigned to external SiOH species. Pore-mouth Bronsted acid sites exhibit P-31 and H-1 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 P-31 resonances that overlap with the peaks usually ascribed to the interaction of TMPO with Bronsted sites.



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Bornes, C; Stosic, D; Geraldes, CFGC; Mintova, S; Rocha, J; Mafra, L

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C. Bornes acknowledges FCT for Doctoral Fellowship PD/BD/142849/2018 integrated in the Ph.D. programme in NMR applied to chemistry, materials, and biosciences (Grant PD/00065/2013). This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, Grants UIDB/50011/2020, UIDP/50011/2020 and LA/P/0006/2020, financed by national funds through the FCT/MEC (PIDDAC). The NMR spectrometers are part of the National NMR Network (PT NMR) and are partially supported by Infrastructure Project No. 022161 (co-financed by FEDER through COMPETE 2020, POCI and PORL and FCT through PIDDAC). Computing resources provided by STFC Scientific Computing Department's SCARF cluster. We thank Dr. Celine Chizallet for providing some models that were adapted to this work. We acknowledge the support of the Label of Excellence for the Centre for zeolites and nanoporous materials by the Region of Normandy (CLEAR). CFGCG thanks the FCT for funding the Coimbra Chemistry Centre through the programmes UIDB/00313/2020 and UIDP/00313/2020, also co-founded by FEDER/COMPETE 2020-EU.

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