The research activities of the working group are divided into several major topics. One main focus is the development of sustainable process pathways for the catalytic conversion of biomass. Furthermore, we are engaged in kinetic and mechanistic investigations of nanoscale materials. The development of reaction technology concepts for chemical energy storage in power-to-x processes and the scale-up of processes as well as the use of polyoxometalates in catalysis are also part of our research activities.
Catalytic conversion of biomass
Biogenic formic acid as sustainable hydrogen carrier
In the context of the German national hydrogen strategy as well as a Call of Action of the Canadian government, similar goals for the development of a hydrogen economy were set. In cooperation with McMaster University in Hamilton, this project will evaluate the usability of biogenic formic acid as a hydrogen storage molecule. Hydrogen stored in the gaseous state means compression to 700 bar with an energy density of 5.0 MJ/L (comparatively gasoline 32 MJ/L). This approach is expensive, energy-intensive and involves a safety risk, since hydrogen forms an explosive gas with oxygen. Therefore, research is being conducted on alternative safe storage options for hydrogen. Formic acid is chemically composed of two hydrogen atoms together with one carbon atom and two oxygen atoms (HCOOH). It can be converted to hydrogen and CO2 as well as CO and water, therefore formic acid can be used as a hydrogen storage as well as a syngas equivalent (H2/CO). It is possible to produce formic acid sustainably by converting renewable raw materials (e.g. wood) using polyoxometalates (OxFA process). Formic acid is liquid at room temperature and atmospheric pressure, non-toxic and accordingly easy to handle. Formic acid has a 20% higher energy density than hydrogen (6.4 MJ/L). Due to the sustainable production of formic acid, the entire process can be considered CO2 neutral.
Through close collaboration between our research group and the research group of Prof. Adams II at McMaster University Hamilton, Canada, an ecological, sustainable production route of formic acid as a green hydrogen carrier from Canadian wood chips will be investigated. This will involve investigating three steps: production of formic acid from renewable resources, storage and transport of formic acid, and recovery of hydrogen from formic acid. Our research group will be involved in the laboratory experiments and the practical implementation of the different steps. The research group of Prof. Adams II will deal with the theoretical simulation of scale-up concepts and the evaluation of the process using a Life Cycle Assessment (LCA).
The project "Biogenic Formic Acid as a Sustainable Hydrogen Carrier (BioFA)" is funded by the German Federal Ministry of Education and Research (BMBF) as part of a German-Canadian cooperation in the funding area "Mobility with Canada 2021" starting 01.10.2021.
Contact: Stefanie Wesinger
Selective hydrogenation of biomass derived compounds to biofuels using polyoxometalate catalysts
In order to increase the material value added from biomass, which is the only renewable carbon source available, secondary energy sources derived from biomass are converted into higher-quality platform chemicals by hydrogenation in the proposed research project. For this purpose, the bio-derived compounds dimethylfuran, methylfuran, and furan in combination with renewable hydrogen from i.e. electrolysis will be hydrogenated to bio-hexanol, bio-pentanol and bio-butanol, respectively. Those alcohols have a great importance in many industrial branches.
In this project, a benchmark using commercial catalysts will be tested for the hydrogenation reaction of those biomass-derived compounds at first, then various polyoxometalate structures should be synthesized and used as selective hydrogenation catalysts for the production of the above mentioned bio-alcohols. These bio-alcohols are used as solvents for the extraction of essential oils, natural resins, dyes and antibiotics, as well as a component of hydraulic and brake fluids. In addition, they are considered as potential biofuels that can replace diesel and gasoline fuels.
This project is founded by DAAD since April 2019 in the form of a German Egyptian Research Long-term Scholarship (GERLS).
contact person: Magdy Sherbi
Selective catalytic conversion of humins to low-molecular carboxylic acids using specifically designed polyoxometalates
The worldwide increase in consumption of energy, the growing production of waste and the rising environmental awareness of our society are the driving forces behind the development of sustainable processes. Furthermore, the current dependence of the production of energy and chemicals on fossil raw materials, which are known to be finite, leads to the increasing importance of renewable raw materials such as biomass. These materials offer the potential to grant access to sustainable platform chemicals and secondary energy sources through innovative processes.
Admittedly the number of processes that can convert biomass directly to value-added chemicals are quite low. One of the reasons for this fact is the formation of difficult to process byproducts such as humins (Fig. 1), which arise, for example, in the conversion of cellulose to levulinic acid. The processing of these complex humins represents a substantial challenge for research and development.
The present project deals with the development of a sustainable process for the oxidative conversion of humins to low molecular organic carboxylic acids (formic acid, acetic acid) using homogeneous polyoxometalate catalysts (POM) in the aqueous phase (Fig. 2). Advantages of this process are the mild reaction conditions and the low price of the precious metal free catalysts.
One of the major goals of this project is to gain a fundamental understanding of this completely new valorization method for humins with respect to technical implementation through reaction engineering studies. Special attention is also paid to the synthesis of POM structures with appropriate properties for the oxidation of humins. Additional investigations regarding the isolation of the products and the recycling of the used catalysts will be conducted.
This project is generously funded by the Deutsche Forschungsgemeinschaft (DFG) under funding code AOBJ: 666125 over a period of three years.
contact persons: Andre Wassenberg (Synthese), Tobias Esser (Katalyse)
POMLig (Continuous selective oxidative depolymerization of lignin to monoaromatics)
Starting on April 1, 2021, the project "Continuous selective oxidative depolymerization of lignin to valuable monoaromatics using polyoxometalate-based catalysts and just-in-time product removal (POMLig)" will be funded by the Agency for Renewable Resources (Fachagentur Nachwachsende Rohstoffe e.V.). (FNR) with industry participation under the funding code 2219NR439 for three years.
Lignin is a complex, hydrophobic aromatic macromolecule and represents one of the most abundant renewable biobased polymers on earth. Due to its many aromatic groups, lignin is of particular interest for the production of low molecular weight aromatics. However, currently there is no industrial process for selective recovery of the aromatic compounds contained in lignin. Accordingly, low-molecular aromatics are mainly produced petrochemically at the present time. This means that production does not conserve resources and is also directly dependent on the availability of fossil raw materials and the price of crude oil. The renewable biopolymer lignin, on the other hand, is mainly produced as a by-product stream in the paper and pulp industry, as well as in companies that process biogenic fibers. Instead of merely utilizing these by-product streams for energy recovery, as is currently the case, the lignin-containing streams are to be oxidatively depolymerized into low-molecular aromatics as part of the funded project.
The aim of this project is the efficient and continuous production of monoaromatics, such as vanillin, methyl vanillate, syringaldehyde and methyl syringate from industrial by-product streams containing lignin. In addition to optimizing the catalyst system, the aim is to establish a so-called just-in-time removal of the target products from the reaction medium. This is intended to prevent further undesired subsequent reactions between the target products and the catalyst and thus maximize the yields of the desired products.
The degradation of the lignin-containing by-product streams into valuable monoaromatic materials should thus contribute to the desired independence from fossil raw materials.
Contact: Max Papajewski
Development of Indium based catalysts for Power-to-X Technologies
The development of new technologies for storing and transporting regeneratively generated energy and its surpluses is necessary to ensure a flexible energy supply.
In this context, electrolytically produced hydrogen and its use in power-to-X processes is a potential solution. In this process, hydrogen is produced from water using renewable energies and then converted with CO2 to organic base chemicals or fuels (power-to-liquid). The required CO2 can be captured from the atmosphere or collected in industrial processes. Hydrogenation of CO2 to methanol shows promise for chemical energy storage applications:
CO2 + 3 H2 ⇋ CH3OH + H2O
Methanol is a major feedstock for the production of platform chemicals, such as formaldehyde, dimethyl ether, olefins, and acetic acid. Furthermore, methanol has great importance for the chemical and material processing industries.
A Cu/ZnO/Al2O3 system is currently used as a commercial catalyst for methanol synthesis from CO/CO2 mixtures. Under typical reaction conditions (T= 220-300°C; p< 100 bar), a conversion of XCO2 ≤ 30% is achieved, with selectivity towards methanol SMeOH varying from 30-70%. Indium-based catalysts represent a promising alternative. Under similar reaction conditions, they exhibit both improved CO2 adsorption capacity, higher methanol selectivity and increased long-term stability (Figure 1).
The aim of the project is to increase the performance and stability of indium-based heterogeneous catalysts by changing a wide range of parameters. By incorporating different dopants or applying In2O3 to different support materials, the possibility of synthesizing longer-chain hydrocarbons will be investigated. Testing will be carried out under near-industrial conditions in fixed-bed and suspension reactors (Figure 2).
In cooperation with the University of Duisburg-Essen, the promising synthesis method for heterogeneous catalysts is being further developed using pulsed laser ablation. Advantages such as a high purity of the synthesized materials and a cost-efficient scalability allow a targeted design of the supported In2O3 catalyst.
Contact: Anne Wesner, Philipp Kampe
Sensor for e-fuels
In order to achieve the climate target, CO2-neutral drive concepts in the transport sector, among other things, are appropriate. In addition to electric vehicles, biomass-based fuels and e-fuels offer high potential for CO2 reduction. The use of renewable fuels in gasoline, diesel or hybrid vehicles can create a closed carbon cycle, as the CO2 released during combustion is removed from the atmosphere again to produce the fuels.
Longer transport distances for renewable fuels, such as various alcohols, can lead to fuel aging, and undesired by-products can be created.
One aim of the project, which is being carried out in cooperation with Coburg University of Applied Sciences, is the mass balancing of aged fuels. For this purpose, various fuels are thermally oxidized and analyzed after aging. In this way, the various reaction mechanisms of the aging processes of several fuels are to be developed and the aging products quantified in the liquid phase and in the gas phase.
Furthermore, a sensor concept is being developed to determine the degree of aging of various regenerative fuels, as well as the fuel parameters required for their use. This is to ensure that the physical properties do not change too much during aging due to the formation of further aging products.
The project is funded by the Forschungsvereinigung Verbrennungskraftmaschinen e.V. (FVV) under the funding code 6013424.
Polyoxometalates for catalysis
Influence of N- and O-containing compounds on the continuous oxidative desulfurization of liquid fuels
Based on the current debate on pollutant emission, stricter environmental regulations for the combustion of liquid fuels are being adopted. Also for the nitrogen oxide and fine dust emissions of motor vehicles the environmental regulations became stricter. On the one hand, higher combustion temperatures have to be achieved in order to reduce particulate matter, but on the other hand, it produces more NOx. In particular, to observe the NO2 limit of 34 ppbw in the air, a nearly complete removal of nitrogen from liquid fuels is essential.
The currently used technology for nitrogen removal is based on the hydrogenation of nitrogen compounds using NiMo- or CoMo-fixed bed catalysts (Hydrodenitrification, HDN).
An alternative to the expensive HDN is the oxidative denitrification (ODN). The organic nitrogen compounds are oxidized by means of a suitable oxidizing agent and the corresponding catalyst to CO2 and N2. Catalyst systems suitable for this purpose are polyoxometalates. Typical reaction temperatures for ODN are 30 °C – 120 °C and thus significantly lower than for the HDN. Hydrogen peroxide is commonly used as an oxidant, but initial experiments with elemental oxygen show promising results, which provides a cheaper and greener alternative.
Our goal is to extend the already established system of Extractive Coupled Oxidative Desulfurization (ECODS) with the water-soluble polyoxometalate catalyst system HPA-5 (H8PV5Mo7O40) to the oxidative removal of other heteroatoms such as nitrogen and oxygen. For this purpose, indoles and furans, as well as their derivatives are used as model compounds. In addition to the characterization of the reaction products, influences such as matrix effects or fuel mixtures should also be investigated. In addition, a simplification of the catalyst system consisting of HPA-1/VOSO4 is to be investigated.
This project is being worked on together with the group of Prof. Jess at the Chair of Chemical Engineering at the University of Bayreuth. Furthermore, we cooperate closely with the working group of Dr. Skiborowski of TU Dortmund in the field of product purification. This project is generously funded by the Deutsche Forschungsgemeinschaft (DFG) under funding code AOBJ: 655412 over a period of three years.
contact person: Michael Huber
Development of POM-based Catalysts
Catalysts play a key role in chemical processes. For the catalytic conversion of biomass, we have already successfully employed some polyoxometalates (POMs) as bifunctional acid and redox catalysts. In cooperation with industrial partners, we meet the ongoing challenge to discover and synthesise catalysts to meet the specific requirements of the respective application. Therefore, the development of new tailor-made POM-based catalysts is a perpetual research project in our group.
Polyoxometalates (POMs) are molecular clusters of oxygen and a scaffolding metal M (usually M = Mo or W), that can also contain a hetero element (such as P, Si). The clusters are usually made up of MO6 octahedra that share the oxygen atoms of a corner or edge with the neighbouring octahedron.
By substituting some of the metal atoms with a redox active metal (e.g. V), a redox activity can be introduced. Combined with their inherent Brönsted acidity makes them promising catalyst candidates.
Since POMs form through spontaneous self-assembly under the right conditions (e.g. temperature, pH, concentration), the challenge of this project lies in finding the right conditions and suitable precursors to effect self-assembly of a substituted POM.
Once successfully synthesised, we work with various academic partners to fully characterise the new POMs. By investigating correlations between molecular properties and catalytic activity, we expand the knowledge basis for designing new catalysts for the next project.
contact persons: Maximilian Poller, Jan-Christian Raabe
Development and application of heterogeneous POM-based catalysts
Polyoxometalates (POMs) are well-defined metal-oxyanions linked with oxygen bridges of early transition metals of the main groups 5 or 6 at their highest oxidation state. They can also contain a multitude of hetero anions like phosphate or silicate to improve their chemical and thermal stability. POMs have attracted considerable attention due to their fascinating architectures and attractive physico-chemical properties including strong Brønsted acidity, high proton mobility, fast multi-electron transfer, high solubility in polar solvents and resistance to hydrolytic and oxidative degradations. Many properties of POM materials can be tailored by changing the constituents and counter cations. Especially the enormous multifunctionality of POMs made them in particular attractive for various homogeneous catalysed applications.
However, homogeneous catalysis leads to several drawbacks like difficulties in catalyst separation and recycling as well as purification of the products. Furthermore, the low specific surface area of bulk POM-catalysts limits the field of application. Therefore, the development of heterogeneous POM-based catalysts with higher specific surface area has attracted much attention. Beside a better catalyst separation and catalyst recycling, especially extending the field of applications of POM-based materials to liquid-phase reactions in non-polar solvents and gas-phase reactions is one of the main goals.
There are several possibilities for heterogenisation of polyoxometalates:
- Impregnation on a porous support (via physical adsorption)
- Chemical immobilization via grafting on a porous support (covalent bondings using linker molecules)
- Complexation with organic or inorganic cations
- Encapsulation in highly porous frameworks
In our research group we are developing tailored heterogeneous POM-based materials using those heterogenisation methods for technical liquid phase and gas-phase applications.
We are working on this project together with our academic partners at Lyngby Technical University (DTU).
contact person: Maximilian Poller
Process development for the selective production of acrylic acid from glycerol
The sustainable production of platform chemicals is one of the most present challenges of the chemical industry nowadays. One of those relevant platform chemicals is acrylic acid (AA). Due to the presence of two functional groups within this molecule acrylic acid shows a high reactivity and therefore a wide application potential. AA can be converted to several acrylic esters, acrylic chlorides and can be easily polymerized to form e.g. polyacrylic acid (PAA). Those acrylates and PAA can be e.g. used as plastics, synthetic rubbers, surface coatings and in the cosmetic industry. Hereby, especially the moisture absorbing properties of PAA are of high interest. Therefore, PAA as “superabsorbent polymers” are mainly used as moisture absorbing material in disposal diapers.
Nowadays, acrylic acid is mainly produced via oxidation of propene. Disadvantages of this fossil based process are e.g. the dependence on fossil resources, the high temperatures needed, a complex downstream processing and the need for storage of hazardous intermediates.
In cooperation with our partner Nitto Denko, we are developing an alternative liquid phase process for the production of acrylic acid with glycerol as a substrate. Glycerol is produced as a byproduct (3.5 MioT/a) at the biodiesel production. Therefore, glycerol is easily available and produced based on renewable resources.
Within our project we are developing an innovative system for the sustainable production of acrylic acid under mild reaction conditions.
Scale-up and process design
scale-up – “from batch to conti” with the use of the miniplant technology
In the process of developing a chemical process to industrial stage, scale-up plays a crucial role. Here, technical chemistry forms the bridge between chemistry and process engineering. First, new synthesis strategies are developed in the laboratory (chemistry), followed by the safe transfer of the chemical conversion found in the laboratory to the technical scale (process engineering).
The miniplant technology forms the basis of scale-up. A miniplant is a small-scale laboratory or pilot plant with a throughput of 0.1-1 kg h-1. Building on discontinuous laboratory tests in which different influencing parameters such as pressure, temperature or catalyst systems are investigated, continuous investigations can be carried out in a miniplant for the first time. With the help of the miniplant, feasibility studies can be carried out and loops and recirculations can be tested. The interactions between the process units can also be investigated and the dynamic behavior as well as start-up and shut-down behavior can be studied. In addition, initial studies of long-term effects can be conducted. All in all, the miniplant represents an image of the later technical process. It is advantageous that standardized laboratory equipment (such as heat exchangers, pumps and reactors) can be used in miniplants. This makes them more space-saving, flexible and cost-effective than conventional pilot plants. In addition, equipment modifications can be carried out relatively quickly.
For scale-up, the design of the chemical reactors in terms of shape, size and mode of operation is just as important as the design of the separation concepts for the required production volume. The goal here is always to achieve the largest possible scale-up factor to save development time and costs. The process concept developed in the miniplant and the experimental results generated form the basis for a reliable scale-up to technical scale.
In our working group we investigate the scale-up of various processes. In the majority of our projects, after the optimization of the system, a scale-up is performed and an execution of the processes in a miniplant is aimed to be realized. In this context, we carry out all steps from the conceptual design, construction and implementation of the miniplants ourselves and also develop the necessary safety concepts for the safe operation of the miniplants.
Contact: Dr.-Ing. Dorothea Voß
Fractionation and selective oxidation of lignocellulosic biomass
Lignocellulosic biomass is one of the most important renewable resources for the sustainable production of biofuels, bio based materials and platform chemicals. Lignocellulosic biomass is generated by atmospheric CO2, water and sunlight via photosynthesis and can therefore be regarded as a promising alternative to fossil resources with zero netto CO2 emissions that can be provided sustainably in large quantities. However, gaining value from lignocellulose is more challenging due to the higher complexity of the raw material and its higher recalcitrance towards selective processing. It typically consists of hemicellulose (25 %), lignin (25 %), cellulose (40 %) and ca. 10 % other, minor components.
Cellulose is an attractive product for material applications like paper, while hemicellulose and lignin can be used for energy generation or the production of bulk chemicals. Due to the higher value of cellulose compared to hemicellulose and lignin, there are several approaches for fractionation of lignocellulosic biomass into its main components. This fractionation facilitates the selective use of cellulose for the paper industry and the further processing of lignin and hemicellulose for the production of bulk chemicals.
Formic acid (FA) is an important bulk chemical that is widely used in chemical, leather, pharmaceutical, rubber and other industries. Furthermore, FA can be easily and selectively decomposed to hydrogen and CO2 under mild reaction conditions. Hence, FA can be regarded as an attractive hydrogen storage material.
In this context, our working group investigates the fractionation of lignocellulosic biomass and selective in-situ conversion of hemicellulose and lignin to formic acid while cellulose remains untapped for further processing. For these approaches, several tailored polyoxometalate-catalysts as well as liquid reaction matrixes are used. Hereby, we collaborate with several partners from academia (Imperial College London) as well as industry (Chrysalix, UK and OxFA GmbH, DE).
contact person: Anna Bukowski
POM for MELiSSA (in Cooperation with ESA)
In the context of long-term manned exploration missions, regular resupply of the crew with the necessary metabolic consumables becomes technically and economically unfeasible. Therefore, the capability of Life Support Systems (LSS) will have to increase. In addition to air revitalisation and water recovery functions, future LSS will also include waste recycling and food production functions. Such a LSS is known as a closed loop LSS, which MELiSSA is an example of. MELiSSA aims at the recovery of oxygen, water and food through transformation of organic wastes generated during a manned space mission. The waste stream considered by the MELiSSA loop is mostly composed of ligno-cellulosic materials originating from the production of higher plants and solid metabolic wastes produced by the crew.
Complete transformation of ligno-cellulosic materials into formic acid and CO2 (OxFA process) under mild temperatures and oxidative atmosphere have been demonstrated using polyoxometalates (POMs) as a catalyst. It has also been demonstrated that POMs can be used for multiple oxidation cycles with little impact the oxidation efficiency. The use of less selective POMs (e.g. Keggin-type POMs) have the potential to favour the production of CO2 instead of formic acid from lingo-cellulosic material. This possibility needs to be explored.
In the context of this project the feasibility of achieving full oxidation of carbon contained in the ligno-cellulosic part of the MELiSSA waste stream will be assessed as well as the feasibility of achieving full oxidation of the volatile fatty acids produced by the MELiSSA waste compartment. The recovery of minerals (e.g. nitrate, sulphate and phosphate) from the reaction broth is also being investigated.
contact person: Stefanie Wesinger