Arrangement Models of Keggin-Al and Keggin-Al in the Interlayer of Montmorillonite and the Impacts of Pillaring on Surface Acidity: A Comparative Study on Catalytic Oxidation of Toluene.

Acid-base reactivity is a key factor for understanding the interfacial geochemistry of clay minerals. Numerous studies showed the significant role of surface acidity of clay minerals in the geological processes and environmentally related applications. In this work, montmorillonite (Mt) was pillared by polycations of Keggin-Al and Keggin-Al. Arrangement models of Keggin-Al and Keggin-Al in the interlayer region of Mt were put forward based on the chemical composition analysis, the structural formula calculation of Mt, and the results of powder X-ray diffraction. Ammonia temperature-programmed desorption and diffuse reflectance Fourier transform infrared methods were applied to explore the impacts of pillaring by polycations (Keggin-Al and Keggin-Al) on the surface acidic characteristics of Mt. Results demonstrated that one Keggin-Al polycation can affect an area of 9.5 unit cells (from two layers, with 4.7-4.8 unit cells in each layer) in Mt, whereas a Keggin-Al polycation controls an area of 7.1 unit cells (from two layers, with 3.5-3.6 unit cells in each layer). Pillaring by polycations could lead to a lot of surface acid sites (1.33 mmol NH/g) on Mt with the main type of Bronsted acid sites. The increase of surface acid sites on both Keggin-Al-pillared Mt (Al-PILM) and Keggin-Al-pillared Mt (Al-PILM) is attributed to the high positive charge and high content of aluminum per unit of polycation, which affects the formation of Bronsted acid sites and structural changes of Mt layers. Catalytic oxidation of toluene provided evidence for the high catalytic activity of Al-PILM under much lower temperature at 78 °C compared with that of Al-PILM and Mt at 207 and 285 °C, respectively. The basic finding in this study not only reveals the possible sources of abundant micropores and mesopores in the micro/mesoporous materials of Al-PILM and Al-PILM but also provides a reasonable mechanism for the formation of abundant Bronsted surface acid sites on these two types of pillared materials. The novel Al-PILM with an excellent micro/mesoporous structure and extremely high thermal stability also exhibits a potential ability in the application of heterogeneous acid catalysis.