In contrast, a membrane microstructure reactor through which suspension flows has advantages in terms of reaction technology. It uses membranes for bubble-free gas dosing along the reactor while the catalyst is suspended in the liquid educt or solvent and transported through microchannels:

• Scalability. Since there are no free gas bubbles in the flowing suspension and the solids content is low, the distribution of the feed to many parallel channels is less demanding in higher throughput systems.

• Enhanced mass transfer. The meandering shape of the channels induces a movement of the catalyst particles transverse to the main flow. This improves the mass transport of the gas from the wall into the core flow and between the core flow and the particles. At the same time, the deposition of particles at the channel bottom is prevented.

• Flexibility. It is possible to use the same reactor system for different catalysts/applications. The catalyst can be ejected and regenerated if necessary, and several gases can be fed separately into different areas, creating an additional degree of freedom that can be used to generate specific concentration profiles.

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The advantages of the concept go hand in hand with the multitude of new degrees of freedom in reaction technology.  Accordingly, there are still many basics unexplored as to how such systems can be optimally operated. Within the subproject "Multiphase Catalysis", an understanding of the local transport and reaction phenomena will be developed using the example of hydrogen peroxide direct synthesis by means of novel miniaturized electrochemical and electrical sensors. In addition, location- and time-resolved computer simulations form the basis for a comprehensive qualitative and quantitative understanding of the process.

The heterogeneously catalyzed direct synthesis of hydrogen peroxide (H₂O₂) from molecular oxygen (O₂) and hydrogen (H₂) is a very attractive alternative process route compared to the industrial standard process, the anthraquinone process. The anthrachinone process is a multistage process which essentially involves catalytic hydrogenation followed by an oxidation of this organic carrier molecule to form hydrogen peroxide. Due to the complexity of the process and the limited recyclability of the organic anthraquinone, the process is only economical for large tonnages and is not resource-efficient.
In contrast, the liquid-phase catalyzed direct synthesis of hydrogen peroxide is an attractive green-chemistry synthesis route which involves contacting both molecular hydrogen and oxygen using a heterogeneous catalyst in a single reaction. A particular challenge is the selectivity control by an optimal guidance of the reactants: Hydrogen peroxide is an intermediate in the reaction network of direct synthesis, whereby water is always produced as a thermodynamically stable product:

To harness the full potential of the direct synthesis process microstructured reactors and elevated reaction pressures up to 100 bar are used.

For optimal process control and to take advantage of the possible benefits of the employed reactor concepts, obtaining information of the local reactant concentrations for H₂, O₂ and H₂O₂ is vital. So far commercially available analytic systems are only able to gather information about the reactant concentrations at the outlet of the synthesis reactors, which makes it challenging to define and optimize process parameters of such a microreactor system.
At the Laboratory for Sensors (IMTEK) an electrochemical microsensor system for the continuous measurement of H₂O₂, H₂ and O₂ in microreactors is developed as part of this subproject. Electrochemical sensors hold a high potential for the use in microreactor applications as they allow for exact measurements of even small concentrations changes due to their high sensitivity, selectivity and defined zero points. Thanks to very small electrode sizes, microelectrode arrays as amperometric sensors are suitable for in-situ measurements with high spatial and temporal resolution and thus allow for parallel monitoring of the substance concentrations at different points in the reaction channel in real-time. Currently, 300 µm platinum electrodes are used to measure at four points over a channel width of 4 mm. Arrays of platinum microelectrodes were integrated into a robust epoxy sensor body (plug) that can be mounted in the microreactor. This allows for simultaneous measurements at up to 8 different positions inside the microreactor for an online feedback of the reaction and process control.

The main challenges and criteria for the development of the sensor system are selectivity, linearity at high concentrations and measurement in the presence of halide ions and catalyst particles in the microreactor. Therefore, new amperometric and chronoamperometric protocols were developed, and the employed platinum electrochemistry was investigated under reactor conditions. Furthermore, the constant presence and concentration of the halide ion bromide was used to develop an on-plug silver/silver bromide reference electrode by galvanic modification to ensure a stable measurement potential. The modification of the platinum electrodes with a diffusion limiting membrane of poly(hydroxylethyl methacrylate) (pHEMA) hydrogel enabled the extension of the linear detection range, by controlling the mass transport to the electrode.  An as modified electrode is able to linearly measure H₂O₂ concentrations in the reactor up to 20 mM, while still achieving a detection limit in the low mircomolar range. It was shown that our microsensors were able to measure elevated concentrations of dissolved reaction gases O₂ and H₂ und process conditions and pressures of 70 bar linearly up to 52 mM and 40 mM, respectively, without any loss in sensitivity when compared with measurements at atmospheric pressure. The electrochemical measurement at such high pressures and with the resulting high concentrations up to 50 mM has not been demonstrated before in literature. By mounting four of our microsensor plugs into the membrane rector, gas dissolution was observed for the first time in situ and in real-time. The system was able to spatially resolve the change of gas dissolution, depending on the relative positioning to the gas stream inlet, through the reactor membrane into the liquid phase by applying gas mixtures with different O₂ content. Challenging selectivity for the simultaneous detection of H₂O₂ and O₂, which both are reduced at the same potential, could be resolved by the use of sophisticated chronoamperometric protocols, which allow to deduct the H₂O₂ oxidation current measured first from the superimposed O₂/H₂O₂ reduction current to obtain a selective measurement current for the detection of O₂.

Concentration patterns in meander channel

In order to spatially and temporally resolve the concentration profile of the reactants H₂, O₂, H₂O₂ in the flow of the meander-shaped channel, 13 boreholes for sensor integration were drilled in the channel in order to cover as wide a range of the reactor as possible. In the first tests we integrated four sensors in the centerline of the reactor and closed the other nine boreholes with dummies.

At an operating pressure of 10 bar, we observed the saturation of oxygen versus nitrogen in the meander channel in the aqueous electrolyte containing sodium bromide and sulfuric acid at a flow rate of 1 mL/min. First both gas dosing zones were flushed with nitrogen and after t = 100 s one gas was switched to dosing of pure oxygen. By controlling the sensors with chronoamperometric measuring protocols, it could be shown that in the meander channel at position no oxygen reaches the channel bottom, because the diffusive mass transport of dissolved gases is overlayed by the convective transport. Furthermore, the measurement confirmed the expectation that starting from position 2 to position 4, the intensity of the current signal drop decreases proportionally to the oxygen concentration, revealing the dynamics of dissolution within the microchannel reactor.

Electrochemical impedance spectroscopy (EIS) is an electroanalytical method in which the current response is evaluated at an applied alternating voltage. Depending on the material and composition between the electrodes (permittivity), the current response differs by a phase shift and lower amplitude relative to the injected voltage expressed by the complex electric resistance, the impedance. Because the catalyst particles (oxide ceramics) have different permittivities compared to the surrounding electrolyte in suspension catalysis, electrochemical impedance spectroscopy can be used to make statements about the volumetric fraction or distribution of the suspended catalyst.
In our tests, we first used a static measuring cell with two opposing sensor heads inserted in its PMMA body and separated by a gap of 1 mm. Each sensor head is equipped with 10 electrode pads.

We were able to show that the impedance is sufficiently sensitive to quantify the different fill levels of the gap between the electrodes. For this purpose, the gap was filled with SiO₂ powder up to different levels and all electrodes per sensor head were used as working or counter electrode and analyzed with a standard LCR meter. The amount of impedance varies significantly with respect to the empty measuring cell (zero line). The intensity depends on the applied frequency, which corresponds to the capacitive properties of SiO₂ at high frequencies.

In addition to this classical electrochemical impedance spectroscopy, we have also developed electrochemical impedance tomography. This allows the spatial distribution of the suspended particles to be resolved. For this purpose, the 10 electrodes were individually controlled by multiplexers, as indicated in the image by the four electrodes.  The resulting alternating voltage is measured at another electrode pair and switched through all electrode pair configurations.  The information obtained corresponds to the boundary conditions for solving the Poisson equation of the electric field.

We used the MATLAB tool EIDORS to resolve the electric field and thus the spatial permittivity distribution. Our measurements with suspended titanium dioxide particles that sink to the ground show that it is possible to obtain a realistic picture of the actual particle distribution.

In the laminar flow regime, the micrometer-sized catalyst particles are subjected to fluid-dynamic forces that prevent the particles from sinking to the ground and keep them suspended.  The result is a dynamic equilibrium that causes the particles to adjust to certain equilibrium positions in the steady state flow, which is also known as flow focusing.

We have developed particle-based simulations based on the smoothed particle hydrodynamics discretization to investigate the conditions for suspension-flow catalysis. A particular focus was on the correct formulation of the boundary condition between the solids (catalyst particles, channel walls) and the liquid:

The simulations in the rectangular channel showed that there are exactly four equilibrium positions based on a random distribution: