Acid properties of Ni-modified ZSM-12: A first-principles study

The catalytic performance of zeolites in industry can often be enhanced by modification with transition metals and Ni is one of the most widely used transition metals for the hydrogenation and dehydrogenation catalysts. In this work, the structure and acid properties of Ni-modified HAl-ZSM-12 zeolites were investigated by the dispersion corrected periodic density functional theory. The results indicate that single Ni atoms can reduce the H atoms in the zeolites into H₂ molecule, whereas the Ni clusters like Ni₂ cannot. The quantity of Brønsted acid sites may decrease after the modification with single Ni atoms; the Ni atoms in the zeolites are oxidized and work as strong Lewis acid sites, which may weaken the Lewis acidity of Al³⁺. After modification with Ni, the Ni-modified ZSM-12 displays greater ability to adsorb hydrogen molecules. The adsorbed hydrogen molecules are dissociated to negatively charged H atoms, which do not function as Brønsted acid sites. Due to the transfer of electron from the Ni atoms to the pre-adsorbed H atoms, as revealed by the adsorption energy of NH₃, the pre-adsorption of hydrogen on the Ni-modified ZSM-12 zeolites can enhance the Lewis acidity.