Acta Physico-Chimica Sinica ›› 2020, Vol. 36 ›› Issue (4): 1903021.doi: 10.3866/PKU.WHXB201903021

Special Issue: Solid-State Nuclear Magnetic Resonance

• Article • Previous Articles     Next Articles

Aluminum Distribution and Brønsted Acidity of Al-Rich SSZ-13 Zeolite: A Combined DFT Calculation and Solid-State NMR Study

Shihan Li,Zhenchao Zhao,Shikun Li,Youdong Xing,Weiping Zhang*()   

  • Received:2019-03-11 Accepted:2019-04-11 Published:2020-03-12
  • Contact: Weiping Zhang E-mail:wpzhang@dlut.edu.cn
  • Supported by:
    the National Natural Science Foundation of China(21872017);the National Natural Science Foundation of China(21603022);the Fundamental Research Funds for the Central Universities in China(DUT17TD04);the Supercomputing Center of Dalian University of Technology, China

Abstract:

SSZ-13 zeolite with a chabazite (CHA) topology structure has important applications for methanol to olefin (MTO) conversion and selective catalytic reduction of nitrogen oxides (NOx) by ammonia (NH3-SCR) to reduce diesel engine exhaust emissions. It has been reported that the Al-rich SSZ-13 zeolite can be used to tune the selectivity of olefins in the MTO reaction, and significantly enhance NO conversions at lower temperatures in NH3-SCR. Thus, the aluminum content and distribution as well as the corresponding acidity in SSZ-13 zeolite determine the catalytic performance of the zeolite for different catalytic reactions. Herein, quantum chemical computing using density functional theory (DFT) combined with multinuclear solid-state nuclear- magnetic-resonance (NMR) experiments were performed to investigate the correlation of Al location and Brønsted acidity of H-SSZ-13 zeolite with the Si/Al ratio varying from 5.8 to 25. The most favorable acid site in the 1Al model is O(1)―H in which a proton is bonded with the O(1) atom near the isolated Al atom of the zeolite framework. Nevertheless, energy differences were rather small when comparing the substitution energies of an Al atom replacing a Si atom in the zeolite framework with a proton located in different O sites. As the Si/Al ratio decreased, the Al-rich SSZ-13 zeolite contained more Al substitutions in its framework. This system exhibited the lowest substitution energy when two Al atoms were located at the diagonal of the same six-membered ring for the Al-Si-Si-Al (NNNN) sequence in the framework of the Al-rich SSZ-13 zeolite. However, for the Al-Si-Al (NNN) sequence, the most favorable distribution involved two Al atoms located in different six-membered rings of the double six-membered ring units (D6R). The proton affinities (PA), NH3 desorption energies, and 1H NMR chemical shifts after d3-acetonitrile adsorption were calculated in the most stable models to characterize the Brønsted acid strength of the SSZ-13 zeolite with different Si/Al ratios. All computing results suggested that the Al-rich SSZ-13 zeolite exhibited weaker Brønsted acid strength than that of the Si-rich counterpart due to the presence of Si(2Al) groupings with the NNN sequence in the framework. Quantitative 29Si magic-angle spinning (MAS) NMR measurements after deconvolution demonstrated that the content of Si(2Al) groupings in the Al-rich SSZ-13 was > 43%. The 1H MAS NMR experiments after d3-acetonitrile adsorption showed that the chemical shift of the bridging hydroxyls in the Al-rich SSZ-13 moved to the lower field, further confirming that it had a weaker Brønsted acid strength than the Si-rich counterpart.

Key words: DFT, Solid-state NMR, SSZ-13 zeolite, Al distribution, Brønsted acidity

MSC2000: 

  • O641