1. Introduction
The alkylation of mesitylene with benzyl alcohol is an important reaction to produce the valuable chemicals for aviation fuel additive reagents, dyes, pharmaceuticals, electrolytes and cosmetics.1,2However, this reaction is traditionally catalyzed by homogeneous catalysts such as AlCl3, H2SO4 and BF3,3,4which cause many problems like pollution, corrosion and separation.5Heterogeneous solid acid catalysts, especially zeolites with a versatile solid acid and tunable architecture, are highly desired towards a “green” approach for the alkylation of mesitylene.6 A major concern for conventional zeolite lies in the severe diffusion limitation arising from the small pore size and the rigorous coking problem.7Hierarchical zeolites with both microporosity and mesoporosity should have in principle largely circumvented diffusion limitation.8-11 However, those with polycrystalline mesopore walls would usually have compromised framework acidity and stability.12Hierarchical single-crystalline zeolites well retain these properties,13-15 while the “quality” (i.e. the interconnectivity and surface openness) of the intracrystalline mesopores remains largely unevaluated and those constricted or closed ones are totally inaccessible for bulky molecules.16,17
Another important but usually lacking structural feature for the complex porous network of hierarchical zeolites is the adsorbate-specific pore adaptability, which allows further control of product selectivity through largely differentiated adsorbate-pore wall interactions. In addition, it is widely reported that monofunctional acid sites in the zeolite framework exhibit poor activity towards the alkylation of aromatics.18,19The redox transition metal additives, such as Fe, Co and Pt, significantly improve the activity of benzylation.20 These metal species are usually loaded on the external surfaces of zeolites and in the form of particles, leading to severe sintering and leaching issues.21 Immobilizing metal species into zeolite matrix is an effective method to mitigate these problems, but is unavailable for conventional chemical synthesis as metal precursors can hardly undergo co-condensation with zeolite precursors instead of forming the oxyhydroxide precipitates under the conditions for zeolite synthesis.22
In present research, we developed a simple and general strategy to fabricate a hierarchical single-crystalline MEL zeolite co-functionalized with both solid acid and Fe-oxy redox sites. The Fe species were immobilized into silicate-2 with the assistance of ligands-stabilized iron cations. The subsequent orientated crystal transformation was then accompanied by a targeted leaching-hydrolysis process that finally converted the Fe-containing silicate-2 into the acid-redox co-functionalized single crystalline mesoporous MEL zeolites. The single-crystalline/bi-functionalized microporous framework as well as the highly open/interconnected mesoporosity of the MEL zeolite were unambiguously determined and evaluated by a complementary combination of high-resolution transmission electron microscopy (HRTEM), electron tomography (ET) and positron annihilation lifetime spectroscopy (PALS). The kinetic adsorption experiment further validated the pore adaptability towards non-polar adsorbates. These unique structural features all together lead to greatly facilitated intracrystalline molecular diffusion, mitigated metal leaching and optimized adsorbate-pore wall interactions towards an excellent overall catalytic performance for the alkylation of aromatics.