A large part of industrial reactions are oxidation processes. Therefore, molecular oxygen is frequently used, as it is a cheap and easily available, clean oxidising agent. On the other hand, it reacts very slowly and a catalyst is needed to decrease the activation energy. Oxygenations with molecular oxygen are often carried out in bubble columns, which contain reaction mixtures in two phases: Oxygen in the gas phase and the solvent in the liquid phase. The influence of mass transfer in this process is studied intensively. However, homogeneous processes, where oxygen is dissolved in the liquid phase, offer the chance to analyse the catalysed reactions without the mass transfer influences.
The activation of oxygen through the enzyme tyrosinase is an efficient method to activate oxygen with copper complexes for the selective oxygenation of phenols. The process of phenol oxidation can be observed in the tanning of human skin or when fruits turn brown while they ripe. Biomimetic copper-guanidine complexes serve as model system for this reaction. The system shown here is [Cu(btmgp)I] (btmgp= Bis-tetramethylguanidinopropane, see L, fig. 1a)
In water- and oxygen-free environments, the copper complex has the oxidation state one and reacts with O2 to a bis(µ-oxo)copper(III)-intermediate. The intermediate then reacts to the bis(µ-hydroxo)copper(II)-complex at temperatures above -40 °C. Usually the formation of the copper(III)-intermediate is very fast at ambient temperature and therefore a fast analysis tool was needed. The stopped-flow spectroscopy provides this analysis tool.
The stopped-flow device is a system built for the spectroscopic analysis of very fast reactions. To allow exact measurements between -90 °C and room temperature, the mixing chamber and the measuring unit are set to a selected temperature by a cryostat. The copper solution and the O2
saturated solvent flow simultaneously into the mixing chamber and are “stopped” (Fig.
2 a-b). The dead time of mixing, until the first measurement was taken, is only several milliseconds. The reaction mixture is then analysed by UV/Vis, infrared or fluorescence spectroscopy. After the desired amount of time and number of spectra have been recorded, other experiments can be carried out afterwards immediately. This provides the possibility to measure a series of reactions in a small amount of time and confirm the reproducibility of the experiments. Additionally, for one measurement only a small amount of substance is needed.
The results of the measurements of the spectroscopic method chosen can then be examined and evaluated with the control and analysis software (fig. 2c). The obtained spectra can be fitted with the kinetics of different orders in the program as well. This way the results can be evaluated while measuring and therefore parameters can be tuned right away.
Here, the activation of O2 with [Cu(btmgp)I] is shown. A syringe of the stopped-flow was prepared with a solution of the copper(I)-complex in acetonitrile in a glove box. A second syringe contained an O2 saturated acetonitrile solution. Both of the solutions were then mixed together in the stopped-flow and the formation and decay of the active copper(III)-intermediate was detected (fig. 1). The very fast formation can be followed even at room temperature (fig. 2c). The reaction mixtures were analysed by UV/Vis spectroscopy and for the kinetic studies, the absorption at 395 nm was used. That is the characteristic maximum for the bis(µ-oxo)copper(III)-intermediate (fig. 2c). For the decay of the bis(µ-oxo) species at different temperatures the results of Table 1 were obtained.
A strong temperature dependence of the decay was observed. A temperature difference of 10°C to lower temperatures leads to rate constants which have a decay smaller by a factor of three to four. The confidence bounds indicate the small deviation of the measurements. This demonstrates the precision and reproducibility of single measurements with the stopped-flow device.
The conversion of gaseous substances with high yield and selectivity is a major task within the chemical and biochemical industry. Oxygenations, hydrations, chlorinations and several other reactions are performed in bubble column reactors for this purpose, since the efficiency of mass transfer significantly influences the reaction rate and the products for these reactions. Bubble column reactors are typically used if a gas has to be dispersed in a liquid phase to obtain a high yield and selectivity. Besides the advantages like the simple construction, the flow structures within these reactors are very complex and tough to grasp. Despite intensive investigations on the processes within bubble column reactors, a reliable prediction of the yield and selectivity is so far not possible. The main obstacle is here that due to the complex flow structure several influencing factors determine the efficiency of mass transfer and not all of these phenomena have been clarified yet.
The cover picture shows a bubbly flow within a pilot plant bubble column (fig. 3a). Clearly visible are several different bubble sizes, which provide due to their different surface area a different contribution to mass transfer. Mostly the bubble shape is not regular because of deformations through flow structures, collisions or bubble size. In consequence of these deformations, the surface area available for mass transfer is constantly changing. In addition, the residence time of the bubbles is influenced by flow structures and by different rise velocities due to the bubble size, so that for each bubble another residence time and therefore a different efficiency for mass transfer results. These and other influencing factors could only be insufficiently investigated within a fully developed bubbly flow, so that the degree of complexity has to be reduced.
The bubbly flow within small capillaries, called Taylor flow, where the bubbles are larger than the inner diameter of the capillary, is one example for a case with a smaller degree of complexity. The advantage of this bubbly flow is that the terminal rise velocity is constant over a wide area of bubble sizes. Through a superimposed countercurrent flow the bubble position within the glass capillary can be fixed, which enables time-independent investigations. With this advantage, the Taylor bubble regime allows detailed investigations on the mass transfer between the gas and liquid phase and allows a direct comparison with numerical results to improve the prediction of real bubbly flows.
To investigate reactive bubbly flows, the described activation of oxygen through the [Cu(btmgp)I] complex is especially suitable because a direct product analysis is possible through spectroscopic methods. Fig. 3b shows a rising oxygen Taylor bubble in a [Cu(btmgp)I] solution.
Through the mass transfer of oxygen from the Taylor bubble into the solution the orange copper(III)-intermediate is formed. In the shown case the discoloration occurs already within the boundary layer region. Through further bubble rise, the intermediate flows along the bubble according to the boundary layer theory and is merged at the lowest point where the flow detaches. Within the bubble wake mixing of the copper(III)-intermediate occurs through eddies, whereby simultaneously the intermediate decomposes into the green bis(µ-hydroxo)copper(II) complex. With the direct comparison of the concentration fields for the case of non-reactive and reactive mass transfer with known reaction rates, an improved description of reactive bubbly flows can be achieved. The described research reflects ongoing work within the Priority Program “Reactive Bubbly Flows” of the German Research Foundation (DFG).
The stopped-flow method provides an easy and efficient way to investigate fast reactions and immediately evaluate their kinetics. The only requirement for the experiment is that both reaction components can be solved in the same solvent. The reactions can then by analysed by different spectroscopic in-situ methods, like UV/Vis, fluorescence or infrared spectroscopy. Furthermore, reactions with molecular oxygen for example can be analysed as a model reaction through dissolving the gases and thereby simplifying the reaction conditions.
Florian Strassl1, Jens Timmermann2, Michael Schlüter2, Sonja Herres-Pawlis1
1 RWTH Aachen, Institut für Anorganische Chemie, Aachen, Deutschland
2 TU Hamburg, Hamburg, Deutschland
Institute for Inorganic Chemistry