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摘要: 分子筛被用作工业催化剂时常需要过渡金属改性，镍是制备加氢/脱氢催化剂常用的过渡金属，本研究采用密度泛函理论研究镍改性的ZSM-12分子筛的结构和酸性。结果表明，分子筛的B酸质子可以被镍原子还原成氢分子，而Ni2的团簇不能将B酸质子还原生成氢气分子。镍原子在分子筛内会被氧化，并形成Lewis酸性位，这会导致分子筛骨架铝的Lewis酸性变弱，镍改性后，分子筛吸附氢气的能力变强，被吸附的氢分子解离为氢原子，并带负电荷，不再具有B酸的功能。从计算的氨分子的吸附能来判断，由于吸附的氢会从镍原子得到电子，吸附的氢分子会增强镍原子的Lewis酸性。Abstract: 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.
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Key words:
- Ni /
- ZSM-12 zeolite /
- acidity /
- density functional theory /
- adsorption energy /
- hydrogen
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Figure 1 Structure model of p(1×2×2) cell of pure silica ZSM-12 (a) and local structure (b) of H-form ZSM-12 zeolite all T sites of the zeolites are indexed with numbers (1-7) following the international zeolite association [http://asia.iza-structure.org]; the O atoms are indexed with letters (a-k), H, O, Al and Si atoms are shown in small white, red, gray, and yellow balls, respectively
Figure 2 Gas phase clusters formed by H and Ni atoms bond distances, Bader charges and the total energies for each structure are shown in nm, e, and kJ/mol, respectively; Ni and H atoms are shown in black and small white balls, respectively
Figure 3 Structures for the HAl-ZSM-12 zeolites modified with Ni monomer and dimer, the T and O sites of the zeolites are indexed with numbers and letters in accordance with Figure 1; bond distances, Bader charges and total adsorption energies are shown in nm, e, and kJ/mol, respectively; O, Si, Al, Ni and H atoms are shown in red, yellow, gray, black and small white balls, respectively
Figure 4 Structures for H2 adsorption in the Ni-modified HAl-ZSM-12 zeolites the T and O sites of the zeolites are indexed with numbers and letters in accordance with Figure 1; bond distances, Bader charges and the adsorption energies are shown in nm, e, and kJ/mol, respectively; O, Si, Al, Ni and H atoms are shown in red, yellow, gray, black and small white balls, respectively
Figure 5 Structures for the adsorption of NH3 in the Ni-modified HAl-ZSM-12 zeolites the T and O sites of the zeolites are indexed with numbers and letters in accordance with Figure 1; Bond distances, Bader charges and the adsorption energies are shown in nm, e, and kJ/mol, respectively; N, O, Si, Al, Ni and H atoms are shown in blue, red, yellow, gray, black and small white balls, respectively
Figure 6 Structures for adsorption of NH3 in the hydrogen pre-covered Ni-modified HAl-ZSM-12 zeolites the T and O sites of the zeolites are indexed with numbers and letters in accordance with Figure 1; Bond distances, Bader charges and the adsorption energies are shown in nm, e, and kJ/mol, respectively; N, O, Si, Al, Ni and H atoms are shown in blue, red, yellow, gray, black and small white balls, respectively
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