Study on the External Surface Acidity of MCM-22 Zeolite: Theoretical Calculation and 31P MAS NMR

Abstract

The acidity on the external surface of the MCM-22 zeolite was studied by theoretical calculations combined with a magic angle spinning (MAS) NMR experiment. The 31P MAS NMR spectrum of triphenylphosphine (PPh3) adsorbed on the MCM-22 zeolite and the XRF (X-ray fluorescence) element analysis showed that there were about 6% Brønsted acidic sites distributed on the external surface. During theoretical calculations, PPh3 was used as the probe molecule to further study the nature of interaction. Periodic molecular mechanical simulations revealed that the probe molecule preferred to adsorb on the 12 MR external surface pockets of the MCM-22 zeolite and was too large to enter completely into the surface pockets. It preferred to locate at the openings of external surface pockets, with one alkyl inserting into the pockets of the supercage, whereas the others remained in the outside, which confirmed the result that only 6% Brønsted acid sites were detectable by 31P MAS NMR using PPh3 as the probe molecule. Moreover, periodic MM calculation indicated that PPh3 was able to adsorb on T1 and T4 sites, which were the most possible sites for the Al introduced into the zeolite lattice. The existence of two Brønsted-bound PPh3 peaks at 11.1 and 14.8 ppm in the 31P MAS NMR spectrum was strongly supported by the above theoretical results. Thus, the molecular mechanical simulation not only provided structural support to the interpretation of NMR experiment but also perfectly agreed with the results from 31P MAS NMR measurements. The structures and interaction energies of the acid−base complexes were further calculated by DFT method. The quantum mechanical calculations indicated that the protonic H atoms of the MCM-22 zeolite were transferred to the probe molecules, resulting in the “ionic” structure, which was in good agreement with the result of Lunsford etc. Because the Brønsted acidity was associated with the ability of transferring proton, the calculation results can give a qualitative understanding of the acidic strength at the openings of the external surface pockets of the MCM-22 zeolite. The effects of the cluster model and the functional level were also discussed in detail.