Jens Artz

Selective Aerobic Oxidation of HMF to 2,5‐Diformylfuran on Covalent Triazine Frameworks‐Supported Ru Catalysts

The selective aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran has been performed under mild conditions at 80 °C and 20 bar of synthetic air in methyl t-butyl ether. Ru clusters supported on covalent triazine frameworks (CTFs) allowed excellent selectivity and superior catalytic activity compared to other support materials such as activated carbon, γ-Al2 O3 , hydrotalcite, or MgO. CTFs with varying pore size, specific surface area, and N content could be prepared from different monomers. The structural properties of the CTF materials influence the catalytic activity of Ru/CTF significantly in the aerobic oxidation of HMF, which emphasizes the superior activity of mesoporous CTFs. Recycling of the catalysts is challenging, but promising methods to maintain high catalytic activity were developed that facilitate only minor deactivation in five consecutive recycling experiments.

Base-Free Aqueous-Phase Oxidation of 5-Hydroxymethylfurfural over Ruthenium Catalysts Supported on Covalent Triazine Frameworks.

The base-free aqueous-phase oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxilic acid (FDCA) was performed at 140 °C and 20 bar of synthetic air as the oxidant. Ru clusters supported on covalent triazine frameworks (CTFs) enabled superior conversion (99.9%) and FDCA yields in comparison to other support materials such as activated carbon and γ-Al2O3 after only 1 h. The properties of the CTFs such as pore volume, specific surface area, and polarity could be tuned by using different monomers. These material properties influence the catalytic activity of Ru/CTF significantly as mesoporous CTFs showed superior activity compared to microporous materials, whereas high polarities provide further beneficial effects. The recyclability of the prepared Ru/CTF catalysts was comparable to that of Ru/C at high conversions and product yields. Nevertheless, minor deactivation in five successive recycling experiments was observed.

Electrocatalytic upgrading of itaconic acid to methylsuccinic acid using fermentation broth as a substrate solution

Biomass presents a promising renewable feedstock allowing access to valuable platform chemicals. In particular, biotechnological processes enable a highly selective product formation but are carried out in aqueous electrolyte-containing solutions. Consequently, the separation of usually polar products poses severe challenges on product separation associated with a high energy demand of product purification. A direct further catalytic transformation within fermentation broth reduces the number of unit operations and the need for an energy intensive separation. We herein study the potential of a chemo- and electrochemical reduction of itaconic acid (IA) to methylsuccinic acid (MS) using acidic media or crude fermentation broth as a case study. Despite an efficient chemo-catalytic hydrogenation of neat IA over Ru/C or RANEY® nickel, the presence of various salts as well as glucose prohibits a direct chemo-catalytic valorisation in fermentation broth. In contrast, the electrochemical hydrogenation enabled very benign conditions. The selection of the electrode material proved to be decisive and had, together with the voltage, a strong influence on the conversion and faradaic efficiency of electrolysis facilitating 99% faradaic efficiency. The conversion of IA only slightly declined for an IA fermentation broth instead of neat IA in a diluted sulfuric acid environment reaching 60 versus 64%. Moreover, a full conversion and yield could also be achieved by simple optimizations of the reaction period and the substrate concentration. The electrocatalytic valorisation of a crude biotechnological product stream reduces not only energy demand and unit operations but presents a promising approach to introduce renewable electrical energy in biomass utilization.

Selective production of glycols from xylitol over Ru on covalent triazine frameworks – suppressing decarbonylation reactions

Ru on covaltent triazine frameworks (CTF) are highly active and selective catalysts for the conversion of xylitol to glycols (80% C-yield) in basic media. With increasing N-content decarbonylation reactions are suppressed leading to high glycol selectivity. The suppression can be attributed to the presence of N in the support and to metal-support interactions. The catalysts exhibit high stability and could be recycled 5 times with minor loss of activity.

Covalent Triazine‐based Frameworks—Tailor‐made Catalysts and Catalyst Supports for Molecular and Nanoparticulate Species

The quest for active, selective and stable catalysts for various applications has led researchers worldwide to investigate several combinations of molecular and solid catalyst systems. Solid molecular catalysts are considered to combine selectivity and reactivity of the molecular species with facile handling and good stability of the heterogeneous counterpart. Among other nanoporous polymers, covalent triazine‐based frameworks (CTFs) exhibit great potential as supports for both molecular as well as nanoparticulate species. Their high chemical and thermal stability in combination with tunable functionality to imbed active sites make them promising candidates to bridge the gap between homogeneous and heterogeneous catalysis. This Minireview seeks to outline the most recent developments in order to amplify research efforts within this fascinating and fast emerging field of material design and application. Emphasis is placed on the most recent advances in the following fields: Synthesis approaches and characteristics of CTFs, solid molecular catalysts and metal nanoparticles supported on CTFs as well as current challenges.

Sulfonated covalent triazine-based frameworks as catalysts for the hydrolysis of cellobiose to glucose

Covalent triazine-based frameworks (CTFs) were synthesized in large scale from various monomers. The materials were post-synthetically modified with acid functionalities via gas-phase sulfonation. Acid capacities of up to 0.83 mmol g−1 at sulfonation degrees of up to 10.7 mol% were achieved. Sulfonated CTFs exhibit high specific surface area and porosity as well as excellent thermal stability under aerobic conditions (>300 °C). Successful functionalization was verified investigating catalytic activity in the acid-catalyzed hydrolysis of cellobiose to glucose at 150 °C in H2O. Catalytic activity is mostly affected by porosity, indicating that mesoporosity is beneficial for hydrolysis of cellobiose. Like other sulfonated materials, S-CTFs show low stability under hydrothermal reaction conditions. Recycling of the catalyst is challenging and significant amounts of sulfur leached out of the materials. Nevertheless, gas-phase sulfonation opens a path to tailored solid acids for application in various reactions. S-CTFs form the basis for multi-functional catalysts, containing basic coordination sites for metal catalysts, tunable structural parameters and surface acidity within one sole system.

Facile Synthesis of Mesoporous Nickel Cobalt Oxide for OER - Insight into Intrinsic Electrocatalytic Activity

The development of novel metal oxide catalysts for electrochemical water splitting has been one of the future challenges in catalysis. We present the development of structured spinel based NiCo2O4 materials using in‐situ hydrothermal synthesis and KIT‐6 as a template. Their electron transfer kinetics in the oxygen evolution reaction (OER) at pH 14 are studied. Structuring of NiCo2O4 via KIT‐6 improves the intrinsic catalyst performance, e. g., a lower overpotential of ∼350 mV and a good long‐term stability could be observed compared to 385 mV and poor stability of commercially available NiCo2O4. Kinetic studies provided insights into structure‐activity relations and the nature of the electrode/electrolyte interface. Interestingly, structuring via KIT‐6 increases not only the electrochemical surface area but also the current density accompanied by superior charge transfer capacity.

Mesoporous manganese phthalocyanine-based materials for electrochemical water oxidation via tailored templating

Electrochemical water splitting using non-noble metals as catalysts is of increasing importance for the future energy sector. In particular, efficient catalysts for the demanding oxygen evolution reaction present a major challenge. As a contribution to this field, tailored mesoporous hard template materials based on manganese phthalocyanine were prepared. The preparation method proved to be crucial to achieve suitable physicochemical properties as a basis for high catalytic activity. The materials show overpotentials between 490 and 590 mV at 10 mA cm−2 This can be mainly attributed to efficient graphitization, high Mn dispersion and tailored oxidation states.