Strategies for Creating Active Sites in MOFs

This chapter presents a general overview of the main properties of MOFs that make them very appealing for applications in heterogeneous catalysis. Great efforts have been directed in the last decade to study the potential of MOFs in catalysis. We will now see what is behind this “MOF rush”. Next, we will present some general considerations that should be taken into account when planning the use of MOFs as heterogeneous catalysts, such as stability, recovery and reusability. And finally, we will review the different strategies that can be used to introduce the desired catalytic centers into the MOFs. We will show how it is possible by using these strategies to engineer the material for catalysis, and to fine tune the properties of the MOF to influence the catalytic performance.

Heteropolyacid-functionalized aluminum 2-aminoterephthalate metal-organic frameworks as reactive aldehyde sorbents and catalysts

Porous materials based on aluminum(III) 2-aminoterephthalate metal organic frameworks (MOFs NH2MIL101(Al) and NH2MIL53(Al)) and their composites with phosphotungstic acid (PTA) were studied as sorbents of saturated vapors of acetaldehyde, acrolein, and butyraldehyde. MOF functionalization by PTA impregnation from aqueous/methanol solutions resulted in MOF with the original crystal topology with the presence of an ordered PTA phase in the MOF/PTA composite. The MOF/PTA composites contained 29-32 wt % PTA and were stable against loss of PTA through leaching to the aqueous/organic solvent solutions. The MOF and MOF/PTA materials exhibited equilibrium uptake of acetaldehyde from its saturated vapor phase exceeding 50 and 600 wt %, respectively, at 25 °C. The acetaldehyde vapor uptake occurs through the vapor condensation, pore-filling mechanism with simultaneous conversion of acetaldehyde to crotonaldehyde and higher-molecular-weight compounds resulting from repeated aldol condensation. The products of aldehyde condensation and polymerization were identified by MALDI-TOF and electrospray mass spectrometry. The kinetics of the MOF- and MOF/PTA-catalyzed aldol condensation of acetaldehyde were studied in water-acetonitrile mixtures. The aldol condensation kinetics in MOF suspensions were rapid and pseudo-first-order. The apparent second-order rate constants for the aldol condensation catalyzed by MOF/PTA were estimated to be 5 × 10(-4) to 1.5 × 10(-3) M(-1)s(-1), which are higher than those reported in the case of homogeneous catalysis by amino acids or sulfuric acid. MOF and MOF/PTA materials are efficient heterogeneous catalysts for the aldehyde self-condensation in aqueous-organic media.

Anchoring of palladium onto surface of porous metal-organic framework through post-synthesis modification and studies on Suzuki and Stille coupling reactions under heterogeneous condition

An ecofriendly solid catalyst has been synthesized by anchoring palladium(II) into post synthetically modified metal organic framework IRMOF-3. The pore of IRMOF-3 was first modified with pyridine-2-aldehyde. The amine group of IRMOF-3 upon condensation with pyridine-2-aldehyde afforded a bidentate Schiff base moiety in the porous matrix. The Schiff base moieties were used to anchor palladium(II) ions. The prepared catalyst has been characterized by UV-vis, IR spectroscopy, X-ray powder diffraction, and nitrogen sorption measurements. Framework structure of the catalyst is not being destroyed in the multistep synthesis procedure as evidenced in X-ray powder diffraction studies. The catalyst has shown high activity toward the Suzuki and Stille cross-coupling reaction in 20% H2O/EtOH and EtOH medium, respectively, at 80 °C. The immobilized complex did not leach or decompose during the catalytic reactions, showing practical advantages over the homogeneous catalysis.

Formation of a metalloporphyrin-based nanoreactor by postsynthetic metal-ion exchange of a polyhedral-cage containing a metal-metalloporphyrin framework

A change for the better: Exchange of Cd(II) in the catalytically inactive framework MMPF-5 (see scheme) with Co(II) afforded a metalloporphyrin-based nanoreactor, MMPF-5(Co). This framework, consisting of small cubicuboctahedral cages, demonstrated interesting performances in the catalytic epoxidation of trans-stilbene with tBuOOH.

Reversible hydrogen storage by NaAlH4 confined within a titanium-functionalized MOF-74(Mg) nanoreactor

We demonstrate that NaAlH(4) confined within the nanopores of a titanium-functionalized metal-organic framework (MOF) template MOF-74(Mg) can reversibly store hydrogen with minimal loss of capacity. Hydride-infiltrated samples were synthesized by melt infiltration, achieving loadings up to 21 wt %. MOF-74(Mg) possesses one-dimensional, 12 Å channels lined with Mg atoms having open coordination sites, which can serve as sites for Ti catalyst stabilization. MOF-74(Mg) is stable under repeated hydrogen desorption and hydride regeneration cycles, allowing it to serve as a "nanoreactor". Confining NaAlH(4) within these pores alters the decomposition pathway by eliminating the stable intermediate Na(3)AlH(6) phase observed during bulk decomposition and proceeding directly to NaH, Al, and H(2), in agreement with theory. The onset of hydrogen desorption for both Ti-doped and undoped nano-NaAlH(4)@MOF-74(Mg) is ∼50 °C, nearly 100 °C lower than bulk NaAlH(4). However, the presence of titanium is not necessary for this increase in desorption kinetics but enables rehydriding to be almost fully reversible. Isothermal kinetic studies indicate that the activation energy for H(2) desorption is reduced from 79.5 kJ mol(-1) in bulk Ti-doped NaAlH(4) to 57.4 kJ mol(-1) for nanoconfined NaAlH(4). The structural properties of nano-NaAlH(4)@MOF-74(Mg) were probed using (23)Na and (27)Al solid-state MAS NMR, which indicates that the hydride is not decomposed during infiltration and that Al is present as tetrahedral AlH(4)(-) anions prior to desorption and as Al metal after desorption. Because of the highly ordered MOF structure and monodisperse pore dimensions, our results allow key template features to be identified to ensure reversible, low-temperature hydrogen storage.

Alkylaminopyridine-modified aluminum aminoterephthalate metal-organic frameworks as components of reactive self-detoxifying materials

Aluminum aminoterephthalate MOF particulate materials (NH(2)-MIL-101(Al) and NH(2)-MIL-53(Al)), studied here as components of self-detoxifying surfaces, retained their reactivity following their covalent attachment to protective surfaces utilizing a newly developed strategy in which the MOF particles were deposited on a reactive adhesive composed of polyisobutylene/toluene diisocyanate (PIB/TDI) blends. Following MOF attachment and curing, the MOF primary amino groups were functionalized with highly nucleophilic 4-methylaminopyridine (4-MAP) by disuccinimidyl suberate-activated conjugation. The resulting MOF-4-MAP modified PIB/TDI elastomeric films were mechanically flexible and capable of degrading diisopropyl fluorophosphate (DFP), a chemical threat simulant.