Robert Güttel

Challenges in polyoxometalate-mediated aerobic oxidation catalysis: catalyst development meets reactor design

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 O2 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 O2 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.

Micro/Macroporous System: MFI‐Type Zeolite Crystals with Embedded Macropores

Zeolite crystals with an embedded and interconnected macropore system are prepared by using mesoporous silica particles as a silica source and as a sacrificial macroporogen. These novel hierarchical zeolite crystals are expected to reduce diffusion limitations in all zeolite-catalyzed reactions, especially in the transformation of larger molecules like in the catalytic cracking of polymers and the conversion of biomass.

Nanostructured Encapsulated Catalysts for Combination of Fischer–Tropsch Synthesis and Hydroprocessing

Controlling the selectivity of Fischer–Tropsch synthesis in a single reaction step is highly desirable, but is a major challenge in heterogeneous catalysis. One approach is the application of bifunctional catalysts. However, to control the catalytic properties the morphology of the catalyst needs to be controlled on the nanoscopic scale. Herein, an innovative synthetic approach that allows the bottom‐up construction of nanostructured bifunctional catalysts in a step‐wise manner is described. The resulting material, which exhibits cobalt nanoparticles encapsulated inside a zeolite matrix, was proven to be active in the combined Fischer–Tropsch and cracking reaction, evidenced by a shift from waxy to liquid products. Consequently, the use of this novel approach was demonstrated in terms of material synthesis and catalytic applications.

Enhancing internal mass transport in Fischer–Tropsch catalyst layers utilizing transport pores

Internal mass transport limitations inside Fischer–Tropsch catalysts due to the slow diffusion of reactants in the liquid-filled pores may significantly alter the selectivity and achievable productivity. In this work, diffusive restrictions for planar catalyst layers were investigated by mathematical modeling and simulation. A one-dimensional model utilizing empirical kinetics, incorporating transport pores as an additional pathway for mass transport and taking into account heat production, allows for calculation of catalyst efficiency and productivity towards C5+ products. As diffusional mass transport leads to strong concentration gradients that impair selectivity, an optimum layer thickness with maximum C5+ productivity can be found. Additional transport pores enhance the mass transport but reduce the amount of active phase, which requires a trade-off by optimizing the fraction of transport pores and layer thickness. For reference conditions, the catalyst layer with an ideal amount of transport pores and ideal thickness exhibits a productivity that is about 47% higher than that for the best layer without transport pores. This improvement requires transport pores with diameters not larger than about 60 μm. While the improvement potential significantly depends on the effective diffusivities, the effect of heat generation was found to be negligible.

Aerobic Oxidation Catalysis by a Molecular Barium Vanadium Oxide

Aerobic catalytic oxidations are promising routes to replace environmentally harmful oxidants with O2 in organic syntheses. Here, we report a molecular barium vanadium oxide, [Ba4 (dmso)14 V14 O38 (NO3 )] (={Ba4 V14 }) as viable homogeneous catalyst for a series of oxidation reactions in N,N-dimethyl formamide solution under oxygen (8 bar). Starting from the model compound 9,10-dihydroanthracene, we report initial dehydrogenation/ aromatization leading to anthracene formation; this intermediate is subsequently oxidized by stepwise oxygen transfer, first giving the mono-oxygenated anthrone and then the di-oxygenated target product, anthraquinone. Comparative reaction analyses using the Neumann catalyst [PV2 Mo10 O40 ]5- as reference show that oxygen diffusion into the reaction mixture is the rate-limiting step, resulting in accumulation of the reduced catalyst species. This allows us to propose improved reactor designs to overcome this fundamental challenge for aerobic oxidation catalysis.

The German Catalyst for Success: Weimar

The 44 Jahrestreffen Deutscher Katalytiker was organized by DECHEMA at the conference center in Weimar in March 2011 together with the 4 Jahrestreffen Reaktionstechnik. The intention of this joint conference was to intensify the communication between the scientific communities of homogeneous/heterogeneous catalysis and reaction engineering, which are mutually connected in many areas. The conference attracted 680 participants from academia and industry with a broad range of interest in the fields of homogeneous and heterogeneous catalysis, as well as reaction engineering. Moreover two presentations were held by the awardees of the “Jochen Block Preis 2010” and “Hanns Hofmann Preis 2011”, Drs. Regina Palkovits and Raimund Horn, respectively (Figure 1), who presented their recent research activities in the fields of chemical usage of biomass and in situ methods for high temperature catalytic reactions. Further highlights of the conference were the two poster workshops on “Transformation of Renewable Feedstocks” and “Electroand Photocatalysis”. In the field of heterogeneous catalysis the broad interest in renewable feedstocks was evident. In the first plenary lecture Ib Chockendorf discussed the potential and limitations of the photocatalytic production of hydrogen from water splitting. The current applicability and the future lines of development in the field of electromobility were presented in a very illustrative contribution by Prof. Hubert Gasteiger, who discussed not only the wide expectations, but also the limitations related to the future (CO2 neutral) mobility. A presentation from industry was led by Martin Votsmeier who discussed that neither the control of automotive convertors under dynamic operation nor the deactivation mechanisms of the applied catalytic materials are completely understood. Additionally, the requirements in the catalysts in terms of air pollution control increase in the future. An interesting presentation by Kristian Voelskow applied modeling tools to describe the production of carbon nanotubes in a technical fluidized bed reactor. Large scale industrial processes discussed included the oxidation of o-xylene to phthalic anhydride and industrial carbohydrate chemistry, for which the improvement in selectivity for bulk chemicals plays an important role in industrial research and development. This is underlined by the efforts in understanding the reaction network and the search for novel classes of catalysts for the partial oxidation of oxylene presented by Robert Marx and Stephan Schunk, respectively. Further emerging topics are in situ methods for investigation flow and concentration fields in chemical reactors and electricity storage for use in individual transportation. It would be desirable to address these topics at the next conference. This year the homogenous catalysis was embossed by sustainability and renewable resources. From new techniques for process optimization and the exchange of the catalyst metal to water splitting the range was quite broad. In this respect, Thomas M ller presented his recent results in the usage of CO2 as a renewable C1 building block. His group was able to conFigure 1. Prof. Dr. Regina Palkovits and Dr. Raimund Horn receiving their awards.