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.

本研究以氨吸附能（Eads）作为酸强度的表征指标，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）团簇模型开展了氨吸附能的计算工作。选取上述分子筛结构的原因在于其具备可用的实验数据与工业应用价值。
计算得到的氨吸附能与采用氨红外质谱-程序升温脱附（infrared mass spectroscopy−temperature-programmed desorption，缩写IRMS-TPD）方法得到的实验结果具有较好的一致性；计算值较实验观测值略低，偏差约为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相符。
本研究开展了系统性深入分析，旨在明确与氨吸附能相关的氢型与铵型沸石团簇的几何参数，并探讨调控沸石酸性性质的关键参数。研究发现，铵阳离子与邻近沸石骨架氧原子之间的氢键相互作用，会对小孔穴内测得的氨吸附能产生影响。当NH₄⁺在相对开阔的空间（大孔穴）中稳定存在时，酸强度由布朗斯特酸位点的局部几何结构主导，表明Si(OH)Al周围的张力对酸强度存在贡献。在此类情况下，Al-O键长（a）越短，氨吸附能越高，这与"路易斯酸性Al会夺取SiOH的电子电荷，进而影响布朗斯特酸强度"的解释相符。
研究发现，Al-O键长（a）与包围Si(OH)Al单元的两个各含三个氧原子的三角形之间的距离（b）和平面夹角（ω）存在关联；最终明确，b与ω越小，氨吸附能越高。Si(OH)Al单元周围原子所受的张力（压缩作用）可通过b与ω的大小体现，这也是导致布朗斯特酸强度发生变化的关键因素。