关键词: 沸石, 磷酸铝, 结构导向效应, 电荷量, 电荷密度

Abstract:

By heating the initial mixture with the molar ratio of n(Al₂O₃):n(P₂O₅):n(R):n(H₂O)=1:1:1:277(R=ethylenediamine(EDA) or 1,3-propanediamine(1,3-DAP)) at 180 ℃, a highly crystalline three-dimensional anionic open-framework aluminophosphate of AlPO₄-12 or UiO-26 was obtained. The crystallization processes of both initial mixtures were investigated by X-ray diffraction analysis(XRD), elemental analysis, and pH measurement. The volume and the Hirshfeld charge on the N atom of the diprotonated EDA and 1,3-DAP were calculated by the “atom volume and surface” module and the Dmol3 module in Materials Studio, respectively. Theoretical calculation shows that the charge on the N atom of diprotonated EDA or 1,3-DAP is 0.073 e and 0.064 e(Hirshfeld), respectively. The corresponding charge density is 1.8573 and 1.3400 e/nm³(Hirshfeld). The corresponding formal charge density is 25.44 and 20.94 e/nm³, respectively. The framework charge density of AlPO₄-12 and UiO-26 is -6.1 e/nm³ and -4.6 e/nm³, respectively. These results indicate that the change in the length of carbon-chain connected to the N atom in the amino group can affect the amount of charge and the charge density on it, which accordingly affects its initial structure-directing ability, resulting the crystallization product changed from AlPO₄-12 to UiO-26 with a smaller charge density.

Key words: zeolite, aluminophosphate, structure-directing effect, charge, charge density