Herein we describe a novel, hollow-structured zeolitic imidazolate framework (ZIF-8-H) nanosphere as a highly efficient catalyst for [3+3] cycloaddition reactions. The programmed installation of acidic Zn(2+) species and basic imidazolate moieties creates a synergistic catalytic system. Appropriate positioning of these functionalities in the catalytic system makes it possible to bring two substrates into close proximity and activate them cooperatively. Moreover, the flexible shell and the surface mesopores of ZIF-8-H provide the capacity for favorable binding of various sized substrates, stabilizing intermediates via their multiple force networks and the increased accessibility of the active sites. These features render ZIF-8-H a more highly active promoter than its homogeneous precursors, bulk ZIF-8 and ZIF-8-N nanoparticles. Finally, the robust catalyst can be easily recovered and reused 10 times without loss of catalytic activity.
The amine-functionalized graphene
oxide was prepared by a facile
one-step silylation approach and used as an acid–base bifunctional
catalyst in one-pot cascade reactions containing successive acetal
hydrolysis and Knoevenagel condensation owing to the separate coexistence
of original carboxylic acid on the edge of the GO sheet and the postgrafted
amine groups on the GO basal surface. This catalyst exhibited much
higher activity than either amine-functionalized active carbon, amine-functionalized
SBA-15, or amine-functionalized Al2O3 due to
the enriched surface acid sites and the diminished diffusion limitation
as well as high catalyst dispersion in liquid solution due to the
unique two-dimensional structure. More importantly, this catalyst
could be easily recycled and used repetitively, showing potential
application in industry.
In this work, we reported the synthesis of bifunctional carbocatalyst with acid-base dual-activation mechanism by introducing organoamines on the basal planes of graphene oxide (GO). Interestingly, GO-supported primary amine (AP-GO) exclusively promoted one-pot Henry-Michael reactions with excellent activity to give synthetically valuable multifunctionalized nitroalkanes. Notably, it also exhibited significantly higher activity than those using propylamine, activated carbon-supported primary amine, and mesoporous silica-supported primary amine as the catalysts. This superior catalytic performance originated from the unique properties of AP-GO, which provided the acid-base cooperative effect by the appropriate positioning of primary amines on their basal planes and carboxyl acids along their edges and the decreased diffusion resistance of the reactants and the intermediates during the multistep catalytic cycles because of its open two-dimensional sheet-like structure. Moreover, it could be readily recycled by simple filtration and subsequently reused without significant loss of its catalytic activity in a six times run test.
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