Antje Henschel

Solution infiltration of palladium into MOF-5: synthesis, physisorption and catalytic properties

Palladium was infiltrated into the highly porous metal–organic framework MOF-5 {[Zn4O(bdc)3], bdc = benzene-1,4-dicarboxylate} using [Pd(acac)2] (acac = acetylacetonate) as the precursor in chloroform solution via ‘incipient wetness’ impregnation. The specific surface area decreases from 2885 m2 g−1 to 958 m2 g−1 after infiltration. After reduction in hydrogen or under vacuum, the hydrogen adsorption capacity is increased from 1.15 wt% to 1.86 wt% at 1 atm and 77 K. The palladium loaded MOF-5 has a high catalytic activity in styrene hydrogenation comparable to that of palladium on activated carbon, but cis-cyclooctene hydrogenation is considerably slower. Even at room temperature, the catalyst is not stable in air due to the low hydrothermal stability of the MOF-5 support.

A Mesoporous Metal–Organic Framework

A new class of porous materials namely metal–organic frameworks (MOFs) has set records in recent years regarding specific surface area and pore volume. Nevertheless, the search for compounds with very large pores and higher specific surface area remains a key challenge in the rapidly expanding field of MOFs, especially for applications in catalysis, drug delivery, and high-pressure gas storage. Compounds containing small windows or pores that are inaccessible for anchoring molecular catalysts, impregnation with catalyst precursors, or larger drug molecules pose limitations for MOFs in fine chemical transformation, nanoparticle formation, or drug delivery. For energy-storage applications at 200 bar, larger pores (2-3 nm) are essential to achieve a shift of the excess adsorption maximum towards higher pressure. Despite a somewhat reduced heat of adsorption, in practice such large pore MOFs outperform small pore MOFs as a result of the higher pore volume. A common concept to enhance the pore size in MOFs uses a linear extension of the linker in a given network topology. In such MOFs the pore diameter achievable is limited by interpenetration. A prominent example is the IRMOF series (isoreticular MOFs). Other examples of increasing pore size through linker extensions are [Cu3(btc)2] [11] (btc = benzene-1,3,5-tricarboxylate; tbo-topology), PCN-6 ([Cu3(tatb)2], tatb = 4,4’,4’’-s-triazine-2,4,6-triyltribenzoate; tbo), or MOF-14 ([Cu3(btb)2], btb = benzene1,3,5-tribenzoate; pto), built from paddle wheel clusters and tritopic linkers. However, in PCN-6 and MOF-14, which have the btb linker (a longer version of the btc linker), the porosity is reduced because of the presence of two interwoven 3D nets in the structure. The non-interpenetrated analogue of PCN-6 (PCN-6’) is obtained using a templating strategy, while a non-interpenetrated analogue of MOF-14 is unknown. Herein, we report an approach that avoids interpenetration by using a secondary linker to stabilize a highly open framework structure by crosslinking an extended Pt3O4topology. The resulting new mesoporous MOF material, DUT-6 (DUT= Dresden University of Technology), shows no interpenetration and has an extremely high gas adsorption capacity for n-butane, hydrogen, and methane. Single crystals of [Zn4O(2,6-ndc)(btb)4/3(def)16(H2O)9/2] (DUT-6; def = N,N-diethylformamide, 2,6-ndc = 2,6-naphthalenedicarboxylate) suitable for X-ray diffraction analysis were obtained from the reaction of H3(btb), H2(2,6-ndc), and zinc nitrate in a ratio of 3:2:14. The compound crystallizes in the cubic space group Pm3̄n. Dodecahedral mesoporous cages 2.5–3 nm in diameter are formed by twelve Zn4O 6+