Framework Stability and Brønsted Acidity of Isomorphously Substituted Interlayer‐Expanded Zeolite COE‐4: A Density Functional Theory Study

Abstract

COE-4 zeolites possess a unique two-dimensional ten-ring pore structure with the Si(OH)2 hydroxyl groups attached to the linker position between the ferrierite-type layers, which has been demonstrated through the interlayer-expansion approach in our previous work (H. Gies et al. Chem. Mater. 2012, 24, 1536). Herein, density functional theory is used to study the framework stability and Brønsted acidity of the zeolite T-COE-4, in which the tetravalent Si is isomorphously substituted by a trivalent Fe, B, Ga, or Al heteroatom at the linker position. The influences of substitution energy and equilibrium geometry parameters on the stability of T-COE-4 are investigated in detail. The relative acid strength of the linker position is revealed by the proton affinity, charge analysis, and NH3 adsorption. It is found that the range of the ⟨T-O-Si⟩ angles is widened to maintain the stability of isomorphously substituted T-COE-4 zeolites. The smaller the ⟨O1-T-O2⟩ bond angle is, the more difficult is to form the regular tetrahedral unit. Thus, the substitution energies at the linker positions increase in the following sequence: Al-COE-4 < Ga-COE-4 < Fe-COE-4 < B-COE-4. The adsorption of NH3 as a probe molecule indicates that the acidity can affect the hydrogen-bonding interaction between (N-H⋅⋅⋅O2) and (N⋅⋅⋅H-O2). The relative Brønsted-acid strength of the interlayer-expanded T-COE-4 zeolite decreases in the order of Al-COE-4 > Ga-COE-4 > Fe-COE-4 > B-COE-4. These findings may be helpful for the structural design and functional modification of interlayer-expanded zeolites.

Mapping Al Distributions in SSZ-13 Zeolites from 23Na Solid-State NMR Spectroscopy and DFT Calculations

“…All the calculations were performed on Gaussian 09, Revision D01 program package . The geometry model of CHA-type zeolites was taken from the experimental structure reported by Randaccio et al In all the clusters, the dangling bonds were saturated with hydrogen atoms pointing to the next oxygen atom of the experimental unit cell, and the Si–H bond length was set to 1.46 Å . These clusters were optimized at the B3LYP/6-31G­(d,p) level, and the energy of each configuration was calculated by the DFT method at the B3LYP/6-311+G­(2d,p) level .…”

Aluminum Location and Acid Strength in an Aluminum‐Rich Beta Zeolite Catalyst: A Combined Density Functional Theory and Solid‐State NMR Study

“…In all of the clusters, the dangling bonds were saturated with hydrogen atoms pointing to the next oxygen atom of experimental unit cell. And the Si−H bond length was set to 1.46 Å to simulate the Brönsted acid strength of actual Beta zeolite . Zero‐point energy correction was also taken into consideration in the energy calculation.…”

New trends in tailoring active sites in zeolite-based catalysts

“…Likewise, the stability of Ga or Fe in T positions is lower in comparison with tetrahedrally coordinated Al atoms. 12 Thus, thermal treatments frequently cause the migration of Ga or Fe to extra-framework positions and their progressive aggregation to form dimeric and then polymeric species. These Ga-and Fe-associated extraframework Lewis acid sites were proven heavily contributing to the catalytic activity of the respective zeolites.…”