An Acidic Cohydrolysis Route for the Hydrothermal Synthesis of Metal-Doping Zeolites

ConspectusZeolites have been industrially applied in numerous chemical processes. Integrating metal species into zeolites to prepare metal-doping zeolites (Me-zeolites) is an efficient approach to modulating the redox/acid/base properties, porosity, and surface electronic state, strengthening the existing features and deriving extra functions. Direct hydrothermal synthesis of Me-zeolites is advantageous for integrating metal species throughout the whole zeolite matrix, enabling the metal species to have a versatile status and uniformly high dispersion. However, in the traditional alkaline hydrothermal route, most metal species tend to prematurely precipitate due to the mismatching of the hydrolysis/condensation rates between metal (rapid) and silica (slow) precursors, restricting the formation of Me-zeolites.To address this issue, we develop a unique acidic cohydrolysis (ACH) route for direct hydrothermal synthesis of Me-zeolites: the metal and silica precursors are cohydrolyzed/condensed under a weakly acidic condition, which is then switched to an alkaline condition for gelation, and finally the resultant gel is hydrothermally crystallized into Me-zeolites. In this route, the initial weakly acidic environment slows down the hydrolysis rate of the metal precursor, matching that of the silica, during which the slowly hydrolyzed metal and silicon hydroxyls co-condense into Me-O-Si units. In the resulting alkaline gelation and final crystallization, the Me-O-Si units would resist the premature precipitation of metal species, allowing the smooth growth of the zeolite crystal to integrate the metal species into the zeolite matrix. Consequently, various kinds of Me-zeolites, namely, pristine zeolites, have been successfully synthesized through the ACH route.In this Account, we describe the principle of the ACH route and summarize our persistent efforts in the synthesis, characterization, and application of Me-zeolites using this strategy. Different statuses of metal species including isomorphously substituted heteroatoms, metal oxides, and noble metal nanoparticles have thus been incorporated into zeolites with varied topologies and Si/Al ratios. Notably, the ACH route is applicable to engineering zeolite morphology on not only the microlevel but also the macrolevel. Enhanced performances of these ACH-synthesized Me-zeolites have been demonstrated in many heterogeneous catalysis processes, including biomass conversion, environmental catalysis, fine chemical and petrochemical syntheses, and shape-selective catalysis, as well as gas adsorption and separation, typically carbon capture. The robust zeolite framework-integrated metal sites that are synergistic with the regular micropore-derived confinement effect render an attractive prospect for Me-zeolites in new fields such as electrochemical and photothermal processes. The ACH route is now emerging as a promising alternative to greatly enriching the variety of Me-zeolites for innovative applications.