Selective catalytic oxidation is one of the most widely used chemical processes. Ideally, highly active and selective catalysts are used in combination with molecular oxygen as oxidant, leading to clean, environmentally friendly process conditions. For homogeneous oxidation catalysis, molecular metal oxide anions, so-called polyoxometalates (POMs) are ideal prototypes which combine high reactivity and stability with chemical tunability on the molecular level. Typically, POM-mediated aerobic oxidations are biphasic, using gaseous O and liquid reaction mixtures. Therefore, the overall efficiency of the reaction is not only dependent on the chemical components, but requires chemical engineering insight to design reactors with optimized productivity. This Perspective shows that POM-mediated aerobic liquid-phase oxidations are ideal reactions to be carried out in microstructured flow reactors as they enable facile mass and energy transfer, provide large gas-liquid interfaces and can be easily upscaled. Recent advances in POM-mediated aerobic catalytic oxidations are therefore summarized with a focus on technological importance and mechanistic insight. The principles of reactor design are discussed from a chemical engineering point of view with a focus on homogeneous oxidation catalysis using O in microfluidic systems. Further, current limitations to catalytic activity are identified and future directions based on combined chemistry and chemical engineering approaches are discussed to show that this approach could lead to sustainable production methods in industrial chemistry based on alternative energy sources and chemical feedstocks.

There is growing interest in the rational design of electrolytes for multivalent-ion batteries by tuning the molecular-level interactions of solvate species present in the electrolytes. Herein, we report our effort...

Polyoxometalates (POMs) offer a rich dynamic solution chemistry so that in principle, the assembly and repair of POM oxidation catalysts in situ, i. e. under catalytic conditions, is possible. Here, we report the oxidative activity of a molecular copper vanadium oxide cluster [(Me 2 NH 2 )Cu(MeCN)V 12 O 32 Cl] 2À : the catalyst self-assembles by reaction of Cu II and [(Me 2 NH 2 ) 2 V 12 O 32 Cl] 3À in solution and selectively oxidizes secondary alcohols (here: 1-phenylethanol) to the corresponding ketones in near quantitative yield. The catalyst can be deactivated in solution by removal of the Cu II site from the catalyst. Subsequent catalyst re-assembly is achieved by addition of Cu II ions, leading to the recovery of the catalytic activity. The system might serve as molecular model to study repair mechanism in POM catalysts with relevance for "green" oxidation processes.Catalyst deactivation is a significant technological problem with an estimated global economic impact in the tens of billions of dollars per year. [1] For homogeneous catalysts, decomposition is most often due to chemical or thermal degradation. [2] Therefore, exploring means of catalyst reactivation or repair is a current challenge in catalyst research. Ideally, this process should work under operating conditions to avoid the time-consuming removal of the catalyst from the reactor. [1] Therefore, molecular catalysts which feature dynamic assembly in solution would be ideal candidates to study model deactivation and repair scenarios. [3] Prime candidates for this approach are molecular metal oxides, so-called polyoxometalates (POMs). [4] POMs are formed in situ by spontaneous self-assembly of small oxometalate precursors and feature wide chemical tunability [4,5] so that specific catalytic properties can be introduced on the molecular level. [6,7] These features have led to a range of POM-based catalytic processes [8][9][10][11] with a particular focus on industrially relevant oxidation reactions [12][13][14][15] including alcohol oxidations, [16,17] epoxidations [18,19] and CÀH-activations. [11,20] Further advantages of POM catalysts are their high thermal [21] and oxidative [19] stability together with their dynamic structure which allows the cluster to undergo self-assembly (and possibly self-repair) reactions [22] under catalytic conditions. [23][24][25] While exploring the incorporation of reactive metal sites into vanadium oxide clusters, [7,21,[26][27][28][29][30][31] we have recently discovered a new class of metal-functionalized polyoxovanadates: programmed reactivity tuning was achieved by selective incorporation of catalytic transition metal centers into the framework of a dodecavanadate cluster, see Figure 1. [32,33] Briefly, the vanadate precursor (Me 2 NH 2 ) 2 [V 12 O 32 Cl] 3À (= {V 12 }) features two Me 2 NH 2 + -blocked metal binding sites; the Me 2 NH 2 + placeholder cations can be selectively replaced by a catalytic metal center of choice. Using our published procedure, {V 12 } was reacted with Cu II to give the homogeneous ...

The first example of hydrogen evolution reactivity (HER) of lanthanide-functionalized Keggin-based polyoxotungstates under photochemical and electrochemical conditions is reported. The HER activity under homogeneous, visible light-driven conditions and under heterogeneous, electrochemical conditions depends on the type of lanthanide functionalization, so that a new class of model HER catalysts for systematic reactivity studies is now accessible.

The immobilization of homogeneous catalysts within nanoporous membranes gives access to catalytically active and multi-functional composite materials, e.g. for use in flow reactors.