The origin of the high Bronsted 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 Bronsted 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 Bronsted 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. (C) 2016 Elsevier B.V. All rights reserved.