Core–shell structured mesoporous silica as acid–base bifunctional catalyst with designated diffusion path for cascade reaction sequencesw

The cascade reaction sequence is a fascinating subject in catalysis. A rationally designed cascade reaction sequence can simplify the synthetic route, reduce the amount of waste and lower the operation cost. The key for a successful cascade reaction sequence is the multifunctional catalyst. With a suitable multifunctional catalyst, multiple synthesis steps can proceed in one pot with high yield toward the target products. For a multifunctional catalyst to work smoothly, different or even incompatible active sites, such as acidic sites and basic sites, must be spatially isolated so that they can coexist on one catalyst. There are several strategies to do so. For example, Helms et al. used star polymers as supports to separate acid and basic groups. Kaneda et al. used two types of layered clays, Ti-exchanged montmorillonite and Mg–Al hydrotalcite, as bifunctional catalysts. Yang et al. fabricated functionalized delicate yolk-shell nanoreactors with basic sites at yolk and acid sites at shell. These multifunctional catalysts showed various degrees of success in cascade reactions. However, tedious preparation procedures, deactivation of the catalysts and low selectivity toward the target products are still issues that need to be addressed. In addition to the spatial isolation of active sites, a higher goal in catalyst design is to arrange the location of the active sites in a rational manner, so that the order of the reaction sequence, the order of the active site location and the direction of mass transportation on catalyst surface agree well with each other. Herein, we report a core–shell structured mesoporous silica nanosphere as an acid–base bifunctional catalyst with the acid sites in the inner core and the basic group in the outer shell (MS-A@MS-B). With such a core–shell structure, acid and basic sites were spatially isolated. More importantly, the core–shell structure offered a designated diffusion pathway for the reaction species. For two types of cascade reaction sequences, this catalyst showed superb activity and selectivity, e.g. nearly 100% conversion for the starting substrate and nearly 100% yield for the target product, while no intermediate species were detected. The synthesis procedure for the core–shell structured catalyst is illustrated in Scheme 1. First, sulfonic acid groups modified mesoporous silica (MS-A) was prepared by co-condensation between tetraethyl orthosilicate (TEOS) and 3-mercaptopropyltrimethoxysilane (MPTMS) followed by the oxidation process. The resulting mesoporous silica core was then further coated with an amino group functionalized mesoporous silica shell using TEOS and 3-aminopropyltrimethoxysilane (APTMS) as mixed silica precursors. MS-A@MS-B was obtained after surfactant removal. For a control experiment, monofunctional catalysts with comparable active site concentration and the same core–shell structure, i.e. catalysts with only the amino group in the outer shell (MS@MS-B), or only the sulfonic acid group in the inner core (MS-A@MS) were prepared by similar methods. The fabrication procedure was established with several considerations. Thiol groups were oxidized to sulfonic acid groups before the introduction of amino groups, otherwise amino groups would be destroyed by H2O2. In addition, the preferential functionalization of amino groups on the outer shell was successful as mesoporous channels of the core were occupied by CTAB during the shell coating process. With such a core–shell structure design, antagonistic acid and basic sites were spatially isolated. Moreover, by adjusting the ratios of different precursors in the synthesis mixture, the densities of the acid sites and basic sites, as well as the thickness of the core/shell in the catalyst, can be systematically tuned. The TEM images (Fig. 1a, b) showed that the MS-A cores were relatively uniform nanospheres ranging from 50 to 70 nm in diameter with a disordered mesoporous structure. After the