Solid-State Oxidation of Alcohols in Gold-Coated Milling Vessels via Direct Mechanocatalysis.

This paper presents a novel approach for the selective oxidation of alcohols to their corresponding aldehydes through direct mechanocatalysis, employing a gold-coated milling vessel as catalyst and air as the oxidation agent. By adjusting milling frequency, media, and duration, high catalytic efficiencies and selectivities are achieved. Remarkably, yields of up to 99 % are obtained for specific substrates, with a turnover number (TON) of 8200 and a turnover frequency (TOF) of 0.77 s-1, surpassing existing alternatives. Confirmation of the catalytic reaction indeed occurring on the milling tool surface was achieved through X-ray photoelectron spectroscopy (XPS).

Mechanochemical Cyclotrimerization: A Versatile Tool to Covalent Organic Frameworks with Tunable Stacking Mode.

We introduce the first mechanochemical cyclotrimerization of nitriles, a facile strategy for synthesizing triazine-containing molecules and materials, overcoming challenges related to carbonization and solubility. Conducting this solid-state approach in a mixer ball mill with 4-Methylbenzonitrile, we synthesize Tris(4-methylphenyl)-1,3,5-triazine quantitatively in as little as 90 minutes. Just as fast, this mechanochemical method facilitates the synthesis of the covalent triazine framework CTF-1 using 1,4 Dicyanobenzene. Material characterization confirms its porous (650 m2 g-1) and crystalline nature. Adjusting the induced mechanical energy allows control over the obtained stacking conformation of the resulting CTFs - from a staggered AB arrangement to an eclipsed AA stacking conformation. Finally, a substrate scope demonstrates the versatility of this approach, successfully yielding various CTFs.

Continuous Direct Mechanocatalytic Suzuki-Miyaura Coupling via Twin-Screw Extrusion.

This work establishes the first direct mechanocatalytic reaction protocol within an extruder, focusing on the Suzuki-Miyaura reaction. Through the coating of either the extruder screws or barrel with Pd, we executed the cross-coupling reaction without the reliance on molecular catalyst compounds or powders, and solvents continuously. We identified the influence and interplay of crucial reaction parameters such as temperature, mechanical energy input, residence time, rheology, and catalyst contact time and finally obtained 36 % and 75 % of the reaction product after one and four reactor passes respectively.

The "In situ electrolyte" as a sustainable alternative for the realization of high-power devices.

The "in situ electrolyte" displays a concept for electric double-layer- as well as metal-ion capacitors in which the by-products formed during carbon synthesis serve directly as electrolyte salt to minimize waste. In this work, the concept is applied for lithium- and sodium-based systems realizing EDLCs containing aqueous, "Water in Salt" (up to 1.8 V) as well as organic (2.4 V) electrolytes. Via the mechanochemical synthesis, carbon materials with surface areas up to 2000 m2 g-1 and an optimal amount of remaining by-product are designed from the renewable resource lignin. Different cation-anion combinations are enabled by further modification directly inside the pores creating imide-based salts which are tracked by synchrotron in situ XRD. By the addition of solvents, the EDLCs show good capacitances up to 21 F g-1 combined with excellent rate performances and stabilities. Moreover, the LiTFSI loaded carbon as positive electrode introduces a new tunable lithium alternative for the pre-lithiation of Li-ion capacitors displaying a good rate performance and cyclability.

Mechanochemical one-pot synthesis of heterostructured pentlandite-carbon composites for the hydrogen evolution reaction.

We have utilized carbon sources as milling additives to enable a direct mechanochemical one-pot synthesis of Fe3Co3Ni3S8/carbon (Pn/C) materials using elemental reaction mixtures. The obtained Pn/C materials are thoroughly characterized and their carbon content could be adjusted up to 50 wt%. In addition to carbon black (CB) as an additive, Pn/C materials were produced using graphite, reduced graphene oxide (rGO), and carbon nanotubes (CNTs), which allows the overall physicochemical properties of materials for energy storage applications to be adjusted. By employing the Pn/C materials as electrocatalysts for the HER in a zero-gap proton exchange membrane (PEM) electrolyzer, we were able to reach a current density of 1 A cm-2 at a cell potential as low as 2.12 V using Pn, which was synthesized with 25 wt% CB. Furthermore, electrolysis at an applied current density of 1 A cm-2 for 100 h displays a stable performance, thus providing a sustainable synthesis procedure for potential future energy storage applications. Herein, we show that catalyst supports play an important role in the overall performance.

Milling Medium-Free Suzuki Coupling by Direct Mechanocatalysis: From Mixer Mills to Resonant Acoustic Mixers.

Here we describe the development of a sustainable and cost-effective approach for catalytic cross-coupling reactions in mechanochemistry. It is found that the substrate's impact with the vessel wall alone is sufficient to initiate the reaction, thus indicating that milling balls function primarily as a mixing agent for direct mechanocatalytic Suzuki coupling. The absence of milling balls can be offset by adjusting the rheology using liquid-assisted grinding (LAG). The LAG sweet spot of 0.25 µL mg-1 is confirmed for both resonance acoustic mixers (RAMs) and ball-free mixer mills, and is higher than in the presence of milling balls. RAMs exhibit excellent performance in the Suzuki reaction, achieving yields of 90 % after 60 min and complete conversion after 90 min. The longevity of the milling vessel is significantly improved in a RAM, allowing for at least 20 reactions without deterioration.

Rationalizing the formation of porosity in mechanochemically-synthesized polymers.

In this study, we present a matrix of 144 mechanochemically-synthesized polymers. All polymers were constructed by the solvent-free Friedel-Crafts polymerization approach, employing 16 aryl-containing monomers and 9 halide-containing linkers, which were processed in a high-speed ball mill. This Polymer Matrix was utilized to investigate the origin of porosity in Friedel-Crafts polymerizations in detail. By examining the physical state, molecular size, geometry, flexibility, and electronic structure of the utilized monomers and linkers, we identified the most important factors influencing the formation of porous polymers. We analyzed the significance of these factors for both monomers and linkers based on the yield and specific surface area of the generated polymers. Our in-depth evaluation serves as a benchmark study for future targeted design of porous polymers by the facile and sustainable concept of mechanochemistry.

The Sonogashira Coupling on Palladium Milling Balls-A new Reaction Pathway in Mechanochemistry.

Utilizing direct mechanocatalytical conditions, the Sonogashira coupling was successfully performed on the surface of milling tools by using pure Pd and Pd coated steel balls. The optimization of co-catalyst forming additives led to a protocol, which generates quantitative yields under aerobic conditions for various substrates within as little as 90 minutes. Using state-of-the-art spectroscopic, diffractive, as well as in situ methods lead to the identification of a previously unknown and highly reactive complex of the co-catalyst copper. This new complex differs substantially from the known complexes in liquid phase Sonogashira couplings, proving that reaction pathways in mechanochemistry may differ from those in established synthetic procedures.

The Regioselective Solid-State Photo-Mechanochemical Synthesis of Nanographenes with UV light.

Photochemical reactors inherently suffer from the low penetration depth of light and therefore rely on high dilutions to enable chemical reactions. Here we present the first method of UV (ultraviolet) photochemistry in the complete absence of bulk solvents in a ball mill. Triphenylene was synthesized by two routes, the Mallory reaction and the cyclodehydrochlorination (CDHC), resulting in yields of 81 and 92 %, respectively. The reaction was successfully scaled up to the gram scale and the robustness of the method was demonstrated for several different substrates. Finally, the regioselective assembly of nanographenes by mechanochemistry was demonstrated for larger systems. Thus, the mechanochemical approach presented here provides a powerful new tool for the atomically precise construction of nanographenes.

The Mechanochemical Fries Rearrangement: Manipulating Isomer Ratios in the Synthesis of p-Hydroxyacetophenone at Different Scales.

Herein, the first mechanochemical Fries rearrangement for the industrially important synthesis of para-hydroxyacetophenone, inside a ball mill and a twin-screw extruder, is presented. Our approach leads to a yield of 62 % in as little as 90 minutes while liquid-assisted grinding can shift the isomer ratio resulting in an excess of the desired para-product. The multigram scale-up by extrusion leads to 61 % yield in only three minutes while completely avoiding solvents. The extrusion temperature can even further be reduced by combining extrusion with a subsequent ageing step.

The impact of the physical state and the reaction phase in the direct mechanocatalytic Suzuki-Miyaura coupling reaction.

The direct mechanocatalytic Suzuki-Miyaura coupling reaction, utilizing palladium milling balls as active catalysts, was investigated regarding the physical state of the reagents and the reaction phase. The substitution patterns and functional groups of different aryl iodides were varied and different boronic acid derivates were utilized to achieve a wide range of substrate combinations. In the neat grinding experiments, liquid aryl iodides were more reactive than solid ones and a steric influence of the substituents, especially pronounced in ortho compounds, was observed. In order to overcome the general low reactivity of the solid phase, several liquid-assisted grinding experiments were conducted and the influence of substrate solubility and catalyst wettability analyzed. Among all LAG additives, EtOH showed the greatest impact on the reactivity, as it converts boronic acid derivatives into liquid and reactive esters under mechanochemical conditions, significantly speeding up the reaction.

The Transformation of Inorganic to Organic Carbonates: Chasing for Reaction Pathways in Mechanochemistry.

Mechanochemical reactions are solvent-free alternatives to solution-based syntheses enabling even conventionally impossible transformations. Their reaction pathways, however, usually remain unexplored within the heavily vibrating, dense milling vessels. Here, we showcase how the green organic solvent diethyl carbonate is synthesized mechanochemically from inorganic alkali carbonates and how the complementary combination of milling parameter studies, synchrotron X-ray diffraction real time monitoring, and quantum chemical calculations reveal the underlying reaction pathways. With this, reaction intermediates are identified, and chemical concepts of solution-chemistry are challenged or corroborated for mechanochemistry.

From Inert to Catalytically Active Milling Media: Galvanostatic Coating for Direct Mechanocatalysis.

The inert milling balls, commonly utilized in mechanochemical reactions, were coated with a layer of Pd and utilized as catalyst in the direct mechanocatalytic Suzuki reaction. With high yields (>80 %), the milling balls can be recycled multiple times in the absence of any solvents, ligands, catalyst-molecules and -powders, while utilizing as little as 0.8 mg of Pd per coated milling ball. The coating sequence, the support material, and the layer thickness were examined towards archiving high catalyst retention, low abrasion and high conversion. The approach was transferred to the coating of milling vessels revealing the interplay between catalytically available surface area and the mechanical energy impact in direct mechanocatalysis.

Scale-Up of Solvent-Free, Mechanochemical Precursor Synthesis for Nanoporous Carbon Materials via Extrusion.

The mechanochemical synthesis of nitrogen-rich nanoporous carbon materials has been scaled up using an extruder. Lignin, urea, and K2 CO3 were extruded under heat and pressure to yield nanoporous carbons with up to 3500 m2 g-1 specific surface area after pyrolysis. The route was further broadened by applying different nitrogen sources as well as sawdust as a low-cost renewable feedstock to receive carbons with a C/N ratio of up to 15 depending on nitrogen source and extrusion parameters. The texture of obtained carbons was investigated by scanning electron microscopy as well as argon and nitrogen physisorption, while the chemical structure was analyzed by X-ray photoelectron spectroscopy. The received carbon was tested as a supercapacitor electrode, showing comparable performance to similar ball-mill-synthesized materials. Lastly, the space-time yield was applied to justify the use of a continuous reactor versus the ball mill.

The Direct Mechanocatalytic Suzuki-Miyaura Reaction of Small Organic Molecules.

The molecular Suzuki cross-coupling reaction was conducted mechanochemically, without solvents, ligands, or catalyst powders. Utilizing one catalytically active palladium milling ball, products could be formed in quantitative yield in as little as 30 min. In contrast to previous reports, the adjustment of milling parameters led to the complete elimination of abrasion from the catalyst ball, thus enabling the first reported systematic catalyst analysis. XPS, in situ XRD, and reference experiments provided evidence that the milling ball surface was the location of the catalysis, allowing a mechanism to be proposed. The versatility of the approach was demonstrated by extending the substrate scope to deactivated and even sterically hindered aryl iodides and bromides.