Why Ru‐MACHO‐BH Is Poor in Coupling Two Ethanol to N‐Bu
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Full Papers Chemistry—Methods doi.org/10.1002/cmtd.202000056 1 2 3 The Guerbet Reaction Network – a Ball-in-a-Maze-Game or: 4 5 Why Ru-MACHO-BH is Poor in Coupling two Ethanol to 6 n-Butanol 7 8 Andreas Ohligschläger, Nils van Staalduinen, Carsten Cormann, Jan Mühlhans, Jan Wurm, 9 and Marcel A. Liauw*[a] 10 11 12 The Guerbet reaction from two alcohols to a long-chain alcohol subsystems of the network. In-situ-infrared spectroscopy is 13 and water requires a redox catalyst and a strong base in applied to determine time-resolved concentration profiles. 14 homogeneous liquid systems. Especially, the reaction from Adapted kinetic models of the single steps are integrated into a 15 ethanol to n-butanol is a challenging example of the reaction microkinetic model of the whole network. The simulation of the 16 that suffers from low yields and selectivities in comparison with reaction network reveals dependencies between temperature, 17 reactions of higher alcohols. The most important side reactions hydrogen pressure, initial concentrations and the yield and 18 are the polymerization of acetaldehyde to C -components and selectivity of n-butanol. Finally, it is shown that Ru-MACHO 19 6+ the saponification of ethyl acetate under consumption of the does not lead to high yields in the reaction, because the 20 base. This work pursues the systematic kinetic investigation of dehydrogenation to ethyl acetate exhibits a too low activation 21 the Guerbet reaction network by experiments with isolated barrier. 22 23 24 1. Introduction 25 26 The Guerbet reaction[1] from ethanol to n-butanol (Scheme 1, Scheme 1. Overall description of the Guerbet reaction from ethanol to n- 27 below referred to as butanol) is appealing, since butanol has butanol. 28 advantageous properties in fuel applications and ethanol can 29 be obtained sustainably from fermentation of sugars.[2–4] In 30 detail, butanol has a higher energy density and is less miscible 1-ethoxyethanol and the saponification of ethyl acetate. In 31 with water. Unfortunately, only a few successful examples of contrast, the production of acetate is often described as 32 the reaction from ethanol to butanol are published[4–13] and Cannizzarro reaction between two acetaldehyde molecules.[4,8,9] 33 ethanol has been referred to as a “sluggish substrate”.[5] The choice of the route via the hemiacetal will be explained in 34 It is generally accepted that the Guerbet reaction proceeds the results. All these reactions span a reaction network that has 35 in three steps:[14,15] the dehydrogenation from ethanol to one desired destination and two undesired outcomes 36 acetaldehyde, the aldol condensation from two acetaldehyde (Scheme 2). 37 molecules to crotonaldehyde and water and the hydrogenation It is worth to mentioning that the acetate and polymer 38 of crotonaldehyde to butanol. The most important side production is often neglected in the quantitative analysis of the 39 reactions are the polymerization to C -compounds and the product mixture since both side products are not detectable in 40 6+ production of acetate salts. Both reactions are irreversible and gas chromatography (GC). Positive counterexamples are the 41 decrease yield and selectivity. The production of acetate salts is publications of Jones et al.[7] and Cavani et al.[4] which quantify a 42 here assumed to proceed via the dehydrogenation of the carbon loss by calibration with an internal standard. 43 hemiacetal Established redox catalysts are based on bidentate or 44 tridentate ligands and iridium, ruthenium or manganese as 45 central atoms. Sodium or potassium alkoxides are commonly 46 [a] A. Ohligschläger, N. van Staalduinen, C. Cormann, J. Mühlhans, J. Wurm, applied as strong bases. An outstanding example is the work of 47 Prof. M. A. Liauw Cavani et al.[4] in which a ruthenium catalyst is applied which 48 Institut für Technische und Makromolekulare Chemie accomplishes to get 25% yield with 50% selectivity in the 49 RWTH Aachen University Worringerweg 1 presence of 5 vol-% water. 50 52074 Aachen (Germany) While many publications deal with the optimization of the 51 E-mail: [email protected] applied redox catalyst, the aim of this work is to gain insight 52 Supporting information for this article is available on the WWW under into the kinetic characteristics of the Guerbet reaction network. 53 https://doi.org/10.1002/cmtd.202000056 © 2021 The Authors. Published by Wiley-VCH GmbH. This is an open access Since base catalyzed steps as well as redox catalyzed steps 54 article under the terms of the Creative Commons Attribution Non-Com- need to gear into each other to produce butanol, the network 55 mercial NoDerivs License, which permits use and distribution in any med- is first divided into subsystems that can be observed in isolated 56 ium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. experiments: 57 Chemistry—Methods 2021, 1, 181–191 181 © 2021 The Authors. Published by Wiley-VCH GmbH Wiley VCH Freitag, 26.03.2021 2104 / 195694 [S. 181/191] 1 Full Papers Chemistry—Methods doi.org/10.1002/cmtd.202000056 the influence of solvent effects that play a dominant role in 1 aldol condensation and saponification (see below). 2 All experiments of this work employ potassium-tert-butano- 3 late as a base and Ru-MACHO (in form of Ru-MACHO-BH 4 precursor) as the redox catalyst (Scheme 3). Ru-MACHO is a 5 popular catalyst for the hydrogenation of esters[24] and the 6 acceptorless dehydrogenation of alcohols.[25] Thus, it appears as 7 a suitable catalyst for the Guerbet reaction at first sight and was 8 already applied for Guerbet reactions starting from 9 methanol.[26,27] However, Wass et al.[8] reported that Ru-MACHO 10 is an inferior catalyst for the Guerbet reaction, but they could 11 not find an explanation based on the structure of the catalyst. A 12 recent study shows that Ru-MACHO degrades to a mixture of 13 several catalytically active complexes in alkaline media.[28] In our 14 work, we could only reproduce that Ru-MACHO is not a suitable 15 catalyst for the Guerbet reaction from ethanol to butanol. The 16 Scheme 2. The Guerbet reaction network that is assumed in this work. Redox additional benefit of the microkinetic simulation consists of 17 reactions are depicted in vertical direction, while base dependent reactions are depicted in horizontal direction. For kinetic experiments, subsystems are understanding why the catalyst fails in the reaction. This 18 defined that can be observed isolated from each other (transparent knowledge helps to identify critical properties of a successful 19 rectangles). redox catalyst for the Guerbet reaction. 20 21 22 1. The redox system of C -substances (ethanol, acetaldehyde/ 2. Results and Discussion 23 2 1-ethoxyethanol and ethyl acetate), 24 2. the redox system of C -substances (crotonaldehyde, croty- This section is ordered by results of the kinetic experiments of 25 4 lalcohol, butanal, butanol), the four subsystems, experiments of the whole Guerbet 26 3. the aldol condensation of acetaldehyde and reaction, assembly of the microkinetic model, simulation of the 27 4. the saponification of ethyl acetate. Guerbet reaction network and lastly a simulation of a modified 28 All subsystems are investigated with in-situ-IR spectroscopy. catalyst. Though it may be appealing to read only the results of 29 Thus, time-resolved concentration profiles are accessible after the simulation, it is recommended to take a look at the 30 the application of chemometric models. That allows a descrip- experimental results as they convey a feeling for the behavior 31 tion of the kinetic behavior in detail with a comparably low of the reaction steps. 32 experimental effort.[16–19] While the aldol condensation and the 33 saponification can be described with an accurate kinetic model 34 without further ado, the redox subsystems themselves consist 2.1. Aldol Condensation of Acetaldehyde 35 of several reaction steps, in particular the hydrogenation of the 36 catalyst. Those single steps cannot be resolved by IR-spectro- The aldol condensation from acetaldehyde to crotonaldehyde is 37 scopy independently, because the reduced and oxidized a key step in the Guerbet reaction network, since this step leads 38 catalyst species are not detectable under process conditions. to the formation of the new CÀ C-bond. Reported kinetics of this 39 Thus, the kinetic model of the redox steps is created from step refer to water as the reaction medium.[29–32] It is assumed 40 activation barriers of DFT calculations found in literature,[20–22] that the Guerbet reaction takes place with alkaline ethanol as 41 but the barrier heights are adapted to reproduce the the major reaction medium, but water and potassium acetate 42 experimental concentration profiles. The kinetic models of all are formed with ongoing conversion. Thus, the kinetic experi- 43 subsystems are finally assembled to a microkinetic model of the ments of the aldol condensation of acetaldehyde with 44 Guerbet reaction network in MatLab. The assumed “reactor” for potassium hydroxide as a base are split in four series: 45 the simulations is an isothermal and isobaric batch reactor with 46 homogeneous concentration distribution at all times. Simula- 47 tions are performed with variations of temperature, hydrogen 48 pressure and starting concentrations of potassium acetate, 49 water and ethyl acetate. In a final step, the activation barriers of 50 the redox steps are varied to simulate modifications of the 51 catalyst. The only published example of the simulation of 52 homogeneously catalyzed Guerbet reaction is the work of 53 Pathak et al.[23] In contrast to this work, they based their 54 Scheme 3.