The nature of strong Brønsted acidity of Ni-SMM clay

• Ni-substituted synthetic mica-montmorillonite clays re-investigated as solid acid.
• Effect of F content in synthesis gel investigated.
• Pure Ni-SMM obtained at intermediate gel F content.
• Intrinsic acidity is beyond intrinsic acidity of zeolites.
• High acidity relates to octahedral [Ni2+-F]+ replacing [Al3+-O]+.

The origin of the high Brønsted acidity of Ni-SMM (Ni-substituted synthetic mica-montmorillonite; beidellite structure) clays was investigated. Ni-SMM clays with varying F content, SMM with F and Ni-SMM without F in the structure were synthesized under hydrothermal conditions. Ni-SMM clays with intermediate F content contained very strong Brønsted acid sites. The optimum Ni-SMM sample outperformed zeolites such as H-ZSM-5 and H-USY (ultrastabilized Y) in alkane hydroisomerization. Infrared spectroscopy with different probe molecules shows that Ni-SMM contains two types of BAS. In addition to acid sites also observed in other clays and amorphous silica-alumina, Ni-SMM contains a small number of acid sites that are stronger than the acid sites in zeolites. The number of such sites does not depend on Ni-SMM reduction. A small amount of strongest Brønsted acid sites positions the catalytic activity of Ni-SMM clay beyond that of zeolites. Periodic density functional theory calculations show that the substitution of octahedral [Al3+-O]+ by [Ni2+-F]+ causes high acidity of the interlayer proton connected to the aluminium-occupied tetrahedron. This explains why Ni-SMM (no F in the structure) and SMM with F (no Ni in the structure; F replaces only structural OH) exhibit conventional clay acidity. The presence of Ni in the octahedral layer leads to isomorphous substitution of bridging O anions that connect the octahedral with the tetrahedral layer by F. The electron-withdrawing nature of the bridging F induces the unusually high acidity of the interlayer protons in Ni-SMM.

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