Li Haichao, Zhou Danhong, Tian Dongxu, Shi Chuan, Müller Ulrich, Feyen Mathias, Yilmaz Bilge, Gies Hermann, Xiao Feng-Shou, De Vos Dirk, Yokoi Toshiyuki, Tatsumi Takashi, Bao Xinhe, Zhang Weiping
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, (PR China).
Chemphyschem. 2014 Jun 6;15(8):1700-7. doi: 10.1002/cphc.201301033. Epub 2014 Mar 18.
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.
COE - 4沸石具有独特的二维十元环孔结构，硅（OH）₂羟基连接在镁碱沸石型层之间的连接位置，这已在我们之前的工作中通过层间膨胀法得到证实（H. Gies等人，《化学材料》，2012年，第24卷，第1536页）。在此，采用密度泛函理论研究了沸石T - COE - 4的骨架稳定性和布朗斯特酸度，其中四价硅在连接位置被三价铁、硼、镓或铝杂原子同晶取代。详细研究了取代能和平衡几何参数对T - COE - 4稳定性的影响。通过质子亲和能、电荷分析和NH₃吸附揭示了连接位置的相对酸强度。发现⟨T - O - Si⟩角的范围变宽以维持同晶取代的T - COE - 4沸石的稳定性。⟨O1 - T - O2⟩键角越小，形成规则四面体单元就越困难。因此，连接位置的取代能按以下顺序增加：Al - COE - 4 < Ga - COE - 4 < Fe - COE - 4 < B - COE - 4。作为探针分子的NH₃吸附表明，酸度会影响（N - H⋅⋅⋅O2）和（N⋅⋅⋅H - O2）之间的氢键相互作用。层间膨胀的T - COE - 4沸石的相对布朗斯特酸强度按Al - COE - 4 > Ga - COE - 4 > Fe - COE - 4 > B - COE - 4的顺序降低。这些发现可能有助于层间膨胀沸石的结构设计和功能改性。
