Mechanochemistry

Using ball mills to conduct solvent-free mechanochemical reactions

Mechanochemistry, a branch of chemistry utilizing impact and friction forces to initiate chemical reactions -typically through the use of ball mills - is gaining attention for its environmental benefits. As chemists seek solvent-free alternatives amid growing environmental concerns, mechanochemistry presents a promising pathway. This method not only facilitates faster reactions, thereby saving energy compared to traditional solvent-based approaches, but also addresses challenges such as poor solubility of reactants. It enables reactions that are unfeasible in solvents and allows for the stabilization and purification of intermediate substances. Mechanochemistry thus opens up new avenues for enhancing process sustainability and developing novel reactions. RETSCH stands at the forefront, offering the most comprehensive range of ball mills and optimal accessories for conducting chemical reactions in grinding jars.

What are the advantages of mechanochemical reactions compared to solvent-based processes?

Solvent-free processes eliminate up to 90% of the reaction mass, enhancing cost efficiency and environmental safety while reducing the time needed to identify the optimal solvent for a reaction.
Exploring new reaction pathways becomes feasible with mechanochemistry, as it accommodates insoluble reactants, stabilizes intermediates, and offers distinct reactions compared to solvent-based methods.
This approach saves time, with reactions typically completing in minutes to hours, as opposed to the days required with solvents.
Higher yields can be achieved when suitable conditions are found

How does mechanochemistry work?

In mechanochemistry, the method of energy application and mixing is crucial. Planetary ball mills primarily utilize friction for size reduction, while mixer mills rely on impact. Certain reactions are more effectively conducted in planetary ball mills, while others benefit from the impact mode of mixer mills. Currently, the varying effects of temperature and mixing on mechanochemical reactions are under investigation, as the precise mechanisms driving these reactions remain to be fully understood.

The efficacy of mechanochemical reactions raises several questions: Is it the energy from impacts that drives these reactions, and does more energy always improve outcomes? Do the balls not only create fresh reactive surfaces but also enhance mixing? Or does the relatively high concentration of educts, compared to soluble systems, play a significant role? Additionally, do high temperatures generated during ball collisions contribute, or is it a combination of these factors? Optimal ball size is another consideration; balls too small may lead to reactant agglomeration and insufficient mixing, whereas too large balls might result in fewer reactive collisions. The ideal ball diameter ranges from 5 to 15 mm. The choice of grinding tool material, such as zirconium oxide or stainless steel, is crucial as well. The material must resist chemical reactions, not interfere with the process, and maintain mechanical stability to minimize abrasion.

How does mechanochemistry work? [product_name.EE31]

^{The yield in a suzuki coupling reaction depends on the ball size used in the MM 500 vario. In this case, a better yield was obtained using grinding balls with a 10 mm diameter than with smaller ones. Results presented by the group of Lars Borchardt [1].}

Ball mills used for mechanosynthesis

Ball mills allow for precise control of the reaction conditions, a wide range of energy inputs and the possibility to conduct reactions in sealed vessels. Planetary ball mills and mixer mills are typically used for mechanochemical reactions. The functional principles of these two types differ in some areas.

Planetary Ball Mills

The grinding jar is arranged eccentrically on the sun wheel of the planetary ball mill. The direction of movement of the sun wheel is opposite to that of the grinding jars in the ratio 1:-2, 1:-2.5 or 1:-3. The grinding balls in the jars are subjected to superimposed rotational movements, the so-called Coriolis forces. The difference in speeds between balls and jars produces an interaction between frictional and impact forces, which releases high dynamic energies. The interplay between these forces produces the high and very effective degree of size reduction of the planetary ball mill.

RETSCH offers four planetary ball mill models which accept 1, 2 or 4 grinding jars in sizes ranging from 12 ml to 500 ml.

The PM 300 works with a speed ratio of 1:-2, but in contrast to other models, it reaches up to 64.4 x acceleration of gravity thanks to the maximum speed of 800 rpm and the large sun wheel. Together with the option to use four small, stackable grinding jars sized 12 to 80 ml for small scale operations, or two jars sized up to 500 ml for upscaling purposes, this planetary ball mill is highly suitable for research applications in mechanochemistry.

Високоенергийна топкова мелница Emax

The High Energy Ball Mill Emax is a special type of planetary ball mill. It combines high-frequency impact, intensive friction, and controlled circular jar movements to a unique and highly effective size reduction mechanism with a speed up to 2000 rpm, resulting in high energy input.

The interplay of jar geometry and movement causes strong friction between the grinding balls, sample material and jar walls as well as a rapid acceleration which lets the balls impact with great force on the sample at the rounded ends of the jars. This significantly improves the mixing of the particles resulting in smaller grind sizes and narrower particle size distributions than is possible with other ball mills.

A unique water-cooling system ensures stable sample temperatures, enabling grinding processes with extremely high energy input. The Emax can be operated within a defined temperature range, which the user selects by defining a minimum and a maximum temperature. If the maximum temperature is exceeded, the grinder automatically interrupts the grinding process and only resumes it when the minimum temperature is reached. The grinding time and length of the breaks can vary according to the temperature limits, but the entire grinding process always remains reproducible.

Emax

Осцилиращи топкови мелници

Mixer Mills

The crushing mode of mixer mills is mainly based on impact. The grinding jars perform radial oscillations in a horizontal position. The inertia of the grinding balls causes them to impact with high energy on the sample material at the rounded ends of the jars and pulverize it. Also, the movement of the jars combined with the movement of the balls result in the intensive mixing of the sample.

RETSCH offers five mixer mill models. The MM 400 is commonly used for mechanochemistry because of its ease of use and small compact design. An important feature is the possibility to conduct long-term grinding processesup to 99 h.

The CryoMill, constantly cools the sample inside the jar down to -196°C with liquid nitrogen. The MM 500 vario accepts up to 6 grinding jars and, with a maximum frequency of 35 Hz, provides higher energy levels than the MM 400. The MM 500 nano is designed for producing nano particles, but also provides the required energy input for mechanochemistry with its frequency of 35 Hz.

The most interesting machine for mechanochemistry is the MM 500 control, which offers the option to operate in a temperature range from -100 °C to +100 °C.

Influence of speed or frequency on the yield in mechanochemistry

^{The reaction rate shown as unreacted reactant against time at an energy input varying from 10 to 25 Hz in the RETSCH Mixer Mill MM 400. The reaction rate increases with the frequency. Results presented by the group of Stuart James [2].}

Increasing the speed enhances the energy delivered to the jars and balls, resulting in more frequent impacts on the reagents and improved mixing effects. Consequently, mechanochemical reactions are likely to accelerate, potentially yielding higher outputs within a specific timeframe. Certain reactions, such as the Suzuki coupling, require a minimum frequency to initiate. Nothing happens between 20-22 Hz, but at 23 Hz, the reaction commences, achieving approximately 40% yield. This phenomenon is attributed to the transition from the balls predominantly rolling along the jar walls at lower speeds to a change in their movement pattern at higher speeds, facilitating the reaction. At 35 Hz, yields of around 80% can be achieved in the MM 500 vario for this reaction.

Influence of speed or frequency on the yield in mechanochemistry [product_name.EE31]

^{The yield in a suzuki coupling reaction depends on the frequency of the MM 500 vario; below 23 Hz, no reaction is observed. Results presented by the group of Lars Borchardt [1].}

High Energy Ball Mills

High energy input significantly enhances grinding efficiency, leading to finer and more homogeneous particle size distributions. This is crucial in applications where the quality of the final product relies on its particle size and distribution. In mechanochemistry, the energy input, along with the action mode, temperature, ball mill size, and mixing effects, can influence the reaction outcome. To facilitate experiments across a spectrum of speeds, from moderate to high energy, four RETSCH models are particularly noteworthy: PM 300, Emax, MM 500 nano, and MM 500 vario. The acceleration these mills can achieve depends on the sun wheel size and maximum speed (planetary ball mills) or amplitude and frequency (mixer mills).

The High Energy Ball Mill Emax, the most powerful in the RETSCH portfolio, achieves the highest energy input with speeds up to 2000 rpm, resulting in an acceleration of 76 g. This, combined with its unique function principle and grinding jar design, produces an exceptionally narrow particle size distribution, minimizes grinding or reaction times, and generates ultrafine particles. Additionally, its design ensures ball movements with simultaneous impact and friction which enhances the mixing effect.

The Planetary Ball Mill PM 300 features a large sun wheel and a maximum speed of 800 rpm, reaching accelerations up to 64.4 g. Together with the option to use four small, stackable grinding jars sized 12 to 80 ml for small scale operations, or two jars sized up to 500 ml for upscaling purposes, this model is highly suitable for research applications in mechanochemistry.

The PM 400 with four grinding stations is available with speed ratios 1:-2.5 and 1:-3, resulting in high energy input which is usually beneficial for mechanochemical applications.

The Mixer Mills MM 500 nano and MM 500 vario operate at a high maximum frequency of 35 Hz, resulting in significant acceleration. This speeds up the grinding process, improves particle fineness, and increases energy input for mechanochemical reactions.

Applications in Mechanochemistry - Planetary Ball Mill PM 300

^{Achievable acceleration in different planetary ball mills dependent on speed setting}

Influence of temperature in mechanochemistry

In mechanochemistry, temperature significantly affects reaction efficiency and can even dictate the reaction type. There is a growing interest in heating mills to embody the "beat and heat" concept, though cooling also plays a role in reaction outcomes. In some cases, temperature may not have a discernible impact. The diagram illustrates the temperature ranges covered by RETSCH ball mills. The following examples demonstrate the potential influence of temperature on chemical reactions.

Cooling enables stabilization of intermediate products (derivates) in mechanochemistry

Reactions involving thermally unstable intermediates can be precisely controlled by synthesizing them while simultaneously cooling, for example, to -5°C in the MM 500 control, where the external chiller is set to -5°C, and the cooling agent actively cools the thermal plates and thereby also the jars and the sample. This process stabilizes the thermally unstable intermediates, ultimately enhancing their yield. The MM 500 control's temperature management enables entirely new reactions, as demonstrated by the synthesis of ZIF-8 from 2-methylimidazolium and zinc oxide.

The MM 500 control allows precise control over product formation in mechanochemical processes through the use of varying temperature levels. Furthermore, by connecting to a cryostat or the CryoPad, reactions can be stabilized across other temperature ranges down to -100°C, vastly expanding the potential for discovering new synthesis pathways and products. The CryoPad enables accurate temperature control, allowing for the selection and regulation of temperatures on the thermal plates from 0°C to -100°C.

^{The further reaction to kat-Zif-8 and dia-ZIF-8 could be stopped as soon as the temperature of the thermal plates was set to -5 °C by means of a chiller. An increase by 5 °C still led to the formation of the second intermediate kat-ZIF-8. At 20 °C of the thermal plates, all three products were found; when synthesizing without cooling, the actual reaction is completed, only dia-ZIF-8. Results presented by the group of Lars Borchardt. [4]}

Heating leads to different results or faster reactions with higher yields in mechanochemistry

In mechanochemistry, energy input via heat can also be beneficial for reactions and lead to better yields or different reaction types. There are reaction pathways such as the Suzuki Miyaura cross-coupling reaction where a higher temperature accelerates the reaction, similar to classical chemistry using Bunsen burners. [3] In one instance, heat guns were employed to warm the grinding jars of the MM 400.

A more controlled way of heating is possible with the MM 500 control, which can be connected to a cryostat. This setup uses a thermal fluid to heat the thermal plates up to 100°C, thereby efficiently transferring heat to the jars and facilitating the reaction.

An example of heating in mechanochemical reactions is depicted in the diagram, involving the reaction of a primary amine with phthalic anhydride. Using either the MM 500 vario or the MM 500 control at room temperature yields only the monoamide. In contrast, milling for three hours at 80°C results in the formation of the desired imide with approximately 75% isolated yield.

^{The temperature can determine the type of reaction in a ball mill, as shown in this example. By regulating the temperature level, the reaction can be precisely controlled and different products can be achieved.Results presented by the group of Andrea Porcheddu. [5]}

Another illustration of how temperature affects mechanochemical reaction yields in ball mills is demonstrated by synthesizing a metal-organic compound in the MM 500 control. At 30°C, a maximum yield of approximately 70% was achieved after 30 minutes, with no improvement from extending the grinding time. However, when the temperature was maintained at 60°C using a thermostat, nearly complete reaction occurred within just 15 minutes.

MM 500 control

Приложение: By keeping the temperature below 0 °C, the formation of non-porous zeolitic metal organic compound is inhibited.

^{By increasing the temperature during synthesis, the yield of a metal organic compound can be increased. Results presented by group of Stuart James. [6]}

Small sample volumes and high sample throughput for screening purposes

In mechanochemistry, pharmaceuticals, or R&D in general, testing reactions typically involves small sample volumes due to the high cost or limited availability of materials. Utilizing small grinding jars is therefore beneficial. The minimum grinding jar volumes for mixer mills are 1.5 or 2 ml in stainless steel, with 5 ml or 10 ml jars being more commonly used. For applications requiring zirconium oxide or tungsten carbide jars, the smallest available size is 10 ml. To accommodate all requirements, Retsch offers a comprehensive selection of adapters and multi-cavity jars:

An adapter that holds 4 x 5 ml stainless steel grinding jars is available for the MM 400, MM 500 vario, and CryoMill, allowing for processing of 8, 24, or 4 samples simultaneously.
2 ml stainless steel tubes fit into adapters for the MM 400 (20 samples), MM 500 vario (50 samples), or CryoMill (6 samples).
These 2 ml tubes can also be used with a different type of adapter in the MM 500 nano or MM 500 control, accommodating 18 samples per batch.
Stainless steel tubes are particularly advantageous for cryogenic applications, as they do not break like plastic tubes.

Additionally, the MM 500 control and MM 500 nano can accommodate 2 x 25 ml or 4 x 10 ml multi-cavity jars, producing grinding results comparable to those achieved with 10 ml or 25 ml jars in the MM 400. In Planetary Ball Mills, 12 ml or 25 ml stainless steel grinding jars can be utilized and even stacked to double the sample quantity. An adapter for 1.5 ml glass vials is also available, suitable for mechanochemical applications—more details in the following section.

Co-crystal screening

With a special adapter, co-crystal screening can be carried out in a planetary ball mill, using disposable vials such as 1.5 ml GC glass vials. The adapter features 24 positions arranged in an outer ring with 16 positions and an inner ring with 8 positions. The outer ring accepts up to 16 vials, allowing for screening up to 64 samples simultaneously when using the Planetary Ball Mill PM 400. The 8 positions of the inner ring are suitable to perform trials with different energy input, e.g. for mechanosynthesis research.

This adapter is compatible with the PM 100, PM 300, and PM 400 models.

MM 400: Ready for in-situ RAMAN Spectroscopy and light-induced reactions

A new feature of the MM 400 was developed with mechanochemical applications in mind: transparent grinding jars are the basis for RAMAN in-situ spectroscopy, allowing for observation of the chemical reactions taking place inside. The best way to do this is to place the RAMAN spectrometer underneath the jars. The cover below the grinding jars can be easily removed by loosening three screws. The bottom plate of the machine has two openings through which the RAMAN spectrometer points towards the bottom of the grinding jars. Thanks to this special setup, the MM 400 is perfectly equipped for mechanochemical purposes. Thanks to their transparency the PMMA jars are also suitable for conducting photo-mechanochemical reactions.

MM 400 - In-situ Raman spectroscopy

Upscaling of mechanochemical reactions

Mixer mills serve as essential tools for conducting mechanochemical tests and trials. However, with a maximum grinding jar size of 125 ml, their upscaling capabilities are limited. The logical progression is to use planetary ball mills, which can accommodate up to 4 x 500 ml jars per batch. Given the differing function principles between these mills, direct transfer of successful reactions from mixer mills to planetary ball mills is not guaranteed, necessitating new trials.

For upscaling even further, RETSCH offers the Drum Mills TM 300 and TM 500 which are equipped with drums comprising up to 150 liters. The operational mechanism of drum mills differs from that of mixer mills and planetary ball mills, typically resulting in a lower energy input due to their slower speeds. Initial scaling trials have shown promising results.

Drum Mills - fine grinding of large volumes

As the drum of the TM 300 rotates, friction causes the grinding balls to ascend the drum wall. This distance grows with the drum's speed until centrifugal forces surpass gravitational forces, causing the balls to adhere to the wall throughout the rotation. This speed is called the "critical speed" = NC.

NC = 42.3/{√(D-d)} [revolutions per minute]

D = inner diameter of the drum [m] = 0.3 m for TM 300 [rpm]

d = ball diameter [m]

The critical speed is ~80 rpm but varies depending on the ball diameter.

^{^{1. Drum
2. Sample
3. Grinding balls
4. Direction of rotation}}

The TM 300 operates in two different modes: Cataract and Cascade. In Cataract mode, the device runs at approximately 70% of its critical speed, equating to 55-60 rpm for the TM 300. This speed enables the balls to travel significantly along the drum's wall. Although they don't reach the critical speed, the balls eventually detach from the wall, traverse beyond the drum's center, and impact the sample at the drum's bottom. This mode is particularly beneficial for quickly breaking down larger particles.

In Cascade mode, activated at about 50 rpm (less than 70% of the critical speed), the balls ascend less on the wall. Upon detachment, they tend to roll down rather than flying across the drum's center, resulting in friction rather than impact.

Drum Mills

Filling levels of grinding jars for mechanochemical applications

In mechanochemistry, particularly with planetary ball mills, the approach to ball filling deviates from the conventional one-third rule (1/3 balls, 1/3 sample, 1/3 empty space), due to the frequent need for high acceleration and the occasional scarcity of sample material (educts). The focus shifts towards using a specific mass ratio, which requires consideration of the reactant amount and a clear decision on the mass ratio to be employed. Additionally, the balls' size must be determined (refer to the section on mechanochemistry principles) to calculate the required quantity of balls, using their specific weight, which varies with size and material.

Once the number of balls is ascertained, the required grinding jar size becomes apparent. Given that sample quantity in the jars is usually very small, there's a higher risk of damaging both the balls and the jars, than with adhering to the traditional one-third rule.

A mass ratio (w/w) of 1:10 is commonly used but 1:5 or 1:15 are also possible. This means that when 15 g educts are used, 150 g balls are required.

150 g = 20 x 10 mm tungsten carbide balls of 7.75 g each.
For 20 x 10 mm balls, a minimum jar volume of 50 ml is required, better even 80 ml (see recommended jar fillings on product pages of planetary ball mills).
150 g = 5 x 15 mm tungsten carbide balls of 26.2 g each require a minimum jar volume of 125 ml.
150 g = 11 x 15 mm stainless steel balls of 13.9 g each require a minimum jar volume of 125 ml.

Grinding jar nominal volume	Sample amount	Max. feed size	Recommended ball charge (pieces)
Grinding jar nominal volume	Sample amount	Max. feed size	Ø 5 mm	Ø 7 mm	Ø 10 mm	Ø 15 mm	Ø 20 mm	Ø 30 mm
12 ml	до ≤5 ml	<1 mm	50	15	5	-	-	-
25 ml	до ≤10 ml	<1 mm	95 – 100	25 – 30	10	-	-	-
50 ml	5 – 20 ml	<3 mm	200	50 – 70	20	7	3 – 4	-
80 ml	10 – 35 ml	<4 mm	250 – 330	70 – 120	30 - 40	12	5	-
125 ml	15 – 50 ml	<4 mm	500	110 – 180	50 – 60	18	7	-
250 ml	25 – 120 ml	<6 mm	1100 – 1200	220 – 350	100 – 120	35 – 45	15	5
500 ml	75 – 220 ml	<10 mm	2000	440 – 700	200 – 230	70	25	8

The table shows the recommended charges (in pieces) of differently sized grinding balls in relation to the grinding jar volume, sample amount and maximum feed size.

Mechanocatalysis with Mixer Mills

Aldehydes are essential compounds in the chemical industry, indispensable to produce pharmaceuticals, vitamins, and fragrances. The challenge lies in selectively oxidizing alcohols to aldehydes without producing unwanted byproducts such as carboxylic acids and esters. Many traditional methods lead to overoxidation and require the use of solvents and environmentally harmful chemicals, which not only generate hazardous waste but also pose significant health risks to users. Often, high temperatures and pressures are necessary, which can decompose sensitive substrates.

The mechano-catalytic conversion of alcohols to aldehydes has been demonstrated at Ruhr University Bochum and the results have been published [7]. The reaction takes place on the gold surface of a coated 25 ml grinding jar in the MM 500 vario within 3 hours at 35 Hz. The grinding jar's gold layer is only 1 nanometer thick and can be reused multiple times. This catalytic reaction occurs directly in the ball mill, without harmful solvents and under mild conditions, preserving the integrity of the substrates. The yield of aldehydes was higher with the mechano-catalytic approach, and fewer byproducts were formed, compared to the classical method. At 35 Hz, higher yields were observed compared to 30 Hz.

MM 500 vario

In-situ monitoring of Mechanochemical Synthesis Reactions (MSR)

Monitoring the two variables "pressure" and "temperature" provides valuable information about what is happening inside the grinding jar. RETSCH’s GrindControl system is used to control colloidal or long-term grinding processes, or to successfully perform material syntheses such as mechanical alloying or other mechanochemical processes. The GrindControl system is available for the Planetary Ball Mills PM 100, PM 300 and PM 400, for the Mixer Mills MM 500 nano and MM 500 control and also for the High-Energy Ball Mill Emax. It comprises hardware for pressure and temperature measurement plus analysis software.

A mechanochemical synthesis was conducted in a Mixer Mill MM 500 nano, using a 125 ml stainless steel grinding jar, equipped with GrindControl for gas and pressure monitoring. The elemental precursors were introduced to the jar together with 32 x 10 mm stainless steel balls. The reaction was conducted under air atmosphere, at 20 Hz. The milling process was stopped when a sudden change in the temperature and pressure indicated the successful completion of the MSR.

The mechanically-induced self-propagating reaction event in the synthesis was monitored by using the GrindControl system. After 20 seconds of milling, an explosion took place, leading to a pressure increase from 0 to 730 mbars and to a rise in temperature. In this application, GrindControl allowed to precisely observe the ignition time during synthesis, the only parameter of interest for the reaction. [8]

GrindControl

Reproducibility of mechanochemical reactions in the Mixer Mill MM 400

Reproducibility is a fundamental principle of scientific research and is essential for ensuring the credibility and reliability of scientific findings. The Mixer Mill MM 400 was tested regarding the reproducibility within a mechanochemical reaction, and it could be proven that it provides excellent reproducibility during several repetitions, for both clamping positions, and also between different devices. [9]

Minor variations of the frequency from 30 Hz to 29 Hz or 28 Hz have an influence on the yield of the reaction. It is of fundamental interest that the mixer mill maintains a set value, e.g. 30 Hz, and does not deviate from it. A premise which is fulfilled by the MM 400 which comes with a calibration certificate.

The mechanochemical reaction γ-Al₂O₃ + ZnO -> ZnAl₂O₄ was conducted for 30 min using 25 ml grinding jars, 2 x 15 mm grinding balls, 1 g educts, at 28 Hz, 29 Hz and 30 Hz five times in a row. The comparison between left and right clamping station showed highly reproducible results, also the comparison between the 5 trials.

^{XRD patterns after the mechanochemical reaction γ-Al₂O₃ + ZnO -> ZnAl₂O₄: Left: Grinding at 28 Hz, 29 Hz and 30 Hz, results after 5th reaction. Middle: Comparison left and right grinding station at 28 Hz 5th reaction each. Right: Reaction 1 to 5 at 30 Hz, right grinding station. Results presented by the group of Claudia Weidenthaler. [9]}

The experiments were repeated using another MM 400 device to compare the results between the two mills. Again, the excellent reproducibility was verified for the 5 tests conducted at 30 Hz, for both, left and right grinding station.

MM 400

^{Almost identical results (weight % of educts and product) and reproducibility are obtained with a different MM 400 device. Results presented by the group of Claudia Weidenthaler. [9]}

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References

[1] Wilm Pickhardt, Claudio Beakovic, Maike Mayer, Maximilian Wohlgemuth, Fabien Joel Leon Kraus, Martin Etter, Sven Grätz, and Lars Borchardt: The direct Mechanocatalytic Suzuki-Miyaura Reaction of small organic molecule. Angew. Chem. Int. Ed. 2022, e202205003.

[2] Ma, X., Yuan, W., Bell, S. E., & James, S. L. (2014). Better understanding of mechanochemical reactions: Raman monitoring reveals surprisingly simple ‘pseudofluid’ model for a ball milling reaction. Chemical Communications, 50(13), 1585-1587.

[3] Kubota, Ito et al., Tackling Solubility Issues in Organic Synthesis: Solid-State Cross-Coupling of Insoluble Aryl Halides. Journal of the American Chemical Society, March 30, 2021. DOI:10.1021/ jacs.1c00906.

[4] Reaction scheme and performance of the experiments: Dr. Sven Grätz, Ruhr-University Bochum, Faculty of Chemistry and Biochemistry, AG Prof. Borchardt.

[5] Reaction scheme and performance of the experiments: Prof. Andrea Porcheddu, University of Cagliari, Chemical and Geological Science Department (Italy).

[6] Reaction scheme and performance of the experiments: Prof. Stuart James, Queens University Belfast, School of Chemistry and Chemical Engineering (UK).

[7] Maximilian Wohlgemuth, Sarah Schmidt, Maike Mayer, Wilm Pickhardt, Sven Graetz, and Lars Borchardt, Solid-State Oxidation of Alcohols in Gold-Coated Milling Vessels via Direct Mechanocatalysis. Angew. Chem. Int. Ed. 2024, e202405342.

[8] Reaction scheme and performance of the experiments: Dr. Matej Balaz, Institute of Geotechnics, Slovak Academy of Sciences (SAS).

[9] Reaction scheme and performance of the experiments: Prof. Dr. Claudia Weidenthaler, Research Group Leader Heterogeneous Catalysis Powder Diffraction and Surface Spectroscopy, Max-Planck Institut für Kohleforschung, Mülheim an der Ruhr.