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.