Au@ZIFs: Stabilization and Encapsulation of Cavity-Size Matching Gold Clusters inside Functionalized Zeolite Imidazolate Frameworks, ZIFs

Abstract

The selective formation and stabilization of very small, naked metal particles inside the cavities of metal-organic frameworks (MOFs) and the simultaneous realization of an even distribution of the particles throughout the crystalline MOF host matrix over a wide range of metal loading are challenging goals. MOFs reveal high specific surface areas, tunable pore sizes, and organic linkers, which are able to interact with guests. The chemically very robust zeolite imidazolate frameworks (ZIFs) are a subclass of MOFs. We chose the microporous sodalite-like ZIF-8 (Zn(MeIM) 2 ; IM = imidazolate) and ZIF-90 (Zn(ICA) 2 ; ICA = imidazolate-2-carboxyaldehyde) as host matrices to influence the dispersion of imbedded gold nanoparticles (Au NPs). The metal loading was achieved via gas phase infiltration of [Au(CO)Cl] followed by a thermal hydrogenation step to form the Au NPs. Low-dose high-resolution transmission electron microscopy ((HR)TEM) and electron tomography reveal a homogeneous distribution of Au NPs throughout the ZIF matrix. The functional groups of ZIF-90 direct the anchoring of intermediate Au species and stabilize drastically smaller and quite monodisperse Au NPs in contrast to the parent not functionalized ZIF-8. The particles can be very small, match the cavity size and approach defined molecular clusters of magic numbers, i.e., Au 55 , independently from the level of loading. Post-synthetic oxidation of the aldehyde groups to yield alkyl esters by the adjacent, catalytically active metal NPs is presented as a new concept of encapsulating nanoparticles inside MOFs and allows multiple steps of metal loadings without decomposition of the MOF.

Elucidating Structure–Property Relationships in the Design of Metal Nanoparticle Catalysts for the Activation of Molecular Oxygen

“…In fact, studies involving cluster-based nanoparticle catalysts even highlighted the integral importance of site-15 isolation as well as particle size in enhancing efficiency of catalytic oxidations and hydrogenations. [20][21][22][23][24][25] The nature of support materials has ranged from metal oxides 26 and carbonaceous materials, 27 to microporous alternatives such as zeolites 28,29 or metal-organic frameworks 20 (MOFs), 30,31 and mesoporous silica-derived hosts 32 such as MCM-41 or SBA-15. Nanoporous materials have been extensively explored for their intrinsic catalytic properties, 33 however isolated single-sites in the form of doped frameworks [34][35][36] or supported nanoparticles have proved to be 25 superior.…”

Selective Palladium‐Loaded MIL‐101 Catalysts

“…Very recently the successful incorporation of size-conform Au NPs was demonstrated within the hosts ZIF-8 and ZIF-90. [5] The synthesis of the metal@MOF or PCP systems is usually performed in two steps. Firstly, the sublimation or infiltration of a metal complex precursor into the porous network, and secondly, the consequent reduction of the imbedded precursor to metal NPs with hydrogen or another reducing agent.…”

Cascade Reactions Catalyzed by Metal Organic Frameworks

“…[55] Au NPs were distributed throughout the ZIF matrix with particle sizes ranging from 1 to 5 nm. The catalytic activity of these catalysts was studied in the oxidation/esterification tandem reaction of benzyl alcohol to methyl benzoate.…”