Correlation between Brønsted Acid Strength and Local Structure in Zeolites

As an index of acid strength, ammonia adsorption energies (Eads) were calculated with density functional theory on cluster models of Brønsted acid sites belonging to FAU, BEA, MFI, FER, MWW, and MOR structures, which were selected because of the availability of experimental data and industrial importance. The calculated Eads were reasonably consistent with experimental results from the ammonia IRMS-TPD (infrared mass spectroscopy−temperature-programmed desorption) method. The calculated value was slightly (10−20 kJ mol−1) lower than the observed value, and its change with varying structure was approximately in agreement with the experiments. A thorough study was carried out to find the geometric parameters of the zeolite clusters (in the H and NH4 forms) relevant to Eads and to discuss parameters controlling the acidic property. Hydrogen bonding interactions between ammonium cations and neighboring zeolitic oxygens were found to affect Eads observed in small cavities. When NH4+ was stabilized in relatively open spaces (large cavities), acid strength was controlled by the local geometry of the Brønsted acid site, indicating a contribution of strain around Si(OH)Al to acid strength. In these cases, a shorter Al−O distance (a) gave a higher Eads. This is consistent with the explanation that Lewis acidic Al withdraws the electron charge of the SiOH contributing to Brønsted acid strength. A relationship was found between a and the distance (b) and planar angle (ω) between two triangles consisting of three oxygens each, which surrounded the Si(OH)Al unit, and finally, a relationship was found in which a smaller b and ω brought a higher Eads. The strain (compression) on atoms surrounding the Si(OH)Al unit is reflected in the extent of b and ω, and this contributes to vary Brønsted acid strength.

以氨吸附能（Eads）作为酸强度的表征指标，本研究采用密度泛函理论（density functional theory），针对FAU、BEA、MFI、FER、MWW及MOR结构的布朗斯特酸位点（Brønsted acid sites）团簇模型计算氨吸附能。本次研究选取上述模型的依据为相关实验数据的可得性与工业应用价值。计算得到的氨吸附能与采用氨吸附红外质谱-程序升温脱附（infrared mass spectroscopy−temperature-programmed desorption，IRMS-TPD）方法获得的实验结果较为吻合。计算值较实验观测值略低，偏差范围为10~20 kJ·mol⁻¹，且其随分子筛结构变化的趋势与实验结果基本一致。本研究开展了系统性探究，旨在明确与氨吸附能相关的H型与NH₄型沸石团簇的几何参数，并讨论调控沸石酸性质的相关参数。研究发现，铵阳离子与邻近沸石氧原子之间的氢键相互作用，会影响在狭小空腔中观测到的氨吸附能。当NH₄⁺在相对开阔的空间（大空腔）中得以稳定存在时，酸强度由布朗斯特酸位点的局部几何结构决定，这表明Si(OH)Al周围的张力对酸强度存在贡献。在此类情况下，更短的Al−O键长（参数a）对应更高的氨吸附能，这与路易斯酸性Al（Lewis acidic Al）位点拉低SiOH的电子电荷、进而影响布朗斯特酸强度的解释相符。研究还发现，参数a与包围Si(OH)Al单元的两个三氧原子三角形所对应的距离（参数b）及平面夹角（ω）之间存在关联；最终得到结论：更小的b值与ω值对应更高的氨吸附能。Si(OH)Al单元周围原子所受的压缩张力可通过b与ω的变化体现，这正是布朗斯特酸强度发生变化的重要诱因之一。