Latest progress on the typical mesoporous Si-based solid acid catalysts

Abstract

Serious environment and safety issues by the wide application of inorganic mineral acids have attracted worldwide attention from fundamental science and application point of view. Under such circumstances, the development of the solid acid catalysts is a very hot topic currently. The ideal solid acid catalysts possess appropriate Bronsted and Lewis acidic nature, good porosity, high stability and unique morphology. Among them, the morphological nanostructure of catalysts is one of the important factors that can have a profound influence on their catalytic performance. Compared with a number of other supports of the solid acid catalysts, the silica-based materials are considered as promising candidates because of their diverse composition, physicochemical property and morphology as well as excellent porosity property. Among them, mesoporous silica-based materials possess lager pore compared with the microporous silica materials (e.g., ZSM-5, H-beta, etc.). They enable to catalyze reactions efficiently involving bulky substrates and/or products, and facilitate mass transfer. Meanwhile, the larger pore favors the dispersion of the active sites and hinders the accumulation of coke on the surface to avoid pore blocking. As a result, one of the important strategies to prepare novel solid acid catalysts is to immobilize the active sites on mesoporous silica materials. The as-prepared mesoporous silica-based solid acid catalysts have shown great potential for application in the field of environment, energy and industrial catalysis. In this paper, the latest progress on the morphology-controlled preparation of mesoporous silica−based materials and their applications in catalysis are summarized. In the case of the silica-based materials, it can be dated back to the early 1990s, and Mobil Company synthesized various M41S materials firstly (MCM-41, MCM-48 and MCM-50) by using the alkyl quaternary ammonium salt cationic surfactant as the template agent. Compared with the amorphous silica materials, M41S possess ordered mesoporous structure, uniform pore size (2‒5 nm), large pore volume and BET surface area (1000 m2 g-1). Subsequently, various SBA materials (such as SBA-15 and SBA-16) with large pore size were prepared successfully. It has attracted wide attention to functionalize the silica materials by incorporating the active sites and maintaining the morphological structure. With the development of silica-based materials, periodic mesoporous organosilicas (PMO) were synthesized by surfactant-directed sol-gel method, which opened a new pathway to prepare functional mesoporous silica materials. Up to now, a number of functionalized organosilica materials have been reported and exhibited great potential in the fields of catalysis, sensing, chromatography analysis, metal adsorption and bio-pharmacy etc. Herein, the mesoporous silica-based solid acid catalysts by the morphology-controlled preparation of functionalized organosilicas showed highly ordered, tubular or hollow spherical nanostructure by P123-directed sol-gel co-condensation. The as-prepared mesoporous silica-based solid acid catalysts possessed hydrophobic surface, excellent porosity property and strong Bronsted acidic nature. Moreover, the catalysts with well-defined morphology exhibited high catalytic performance for synthesis of the biomass-derived platform molecules or high value-added chemicals. Owing to the accelerated diffusion, transmission and improved accessibility of substrates to active sites, the mesoporous silica-based solid acid catalysts with hollow nanosphere morphology showed the highest activity compared with the tubular or ordered analogues. On the other hand, the covalent bond between active sites and silica framework can avoid the leaching issue of the solid acid catalysts. In addition, the hydrophobic character of alkyl-containing organosilica can reduce acid sites deactivation associated with adsorption of polar byproducts, which can improve the stability of as-prepared catalysts. To summarize, this review aims to provide important reference value for development of novel solid acid catalysts.