With an annual production of around one million metric tons, formic acid is one of the most important basic chemicals and is used, among other things, as an additive in the animal feed industry. It is also increasingly attracting attention as an H2 carrier. To date, its production has relied on fossil raw materials, which are to be replaced in the long term by sustainable raw materials. The Bavarian SME OxFA, currently the only company producing formic acid from biomass, has taken an important step forward. On its behalf, LIKAT has developed a catalyst that enables the extraction of hydrogen—a key energy carrier—from bio-based formic acid at relatively mild temperatures.
A patent application has been filed for the jointly developed process. Following formic acid production, it represents the second step in a process for sustainable and climate-friendly hydrogen production from biomass—the so- called BFH process (Biomass–Formic Acid–Hydrogen). A wide variety of biogenic residues and raw materials can serve as feedstocks, including lignified or moist materials, fermented biomass, and waste. According to a study by the German Biomass Research Center (DBFZ), a technical biomass potential of approximately 31 million metric tons of dry matter per year remains untapped in Germany alone.
Challenges: Dilution and Byproducts
The literature describes various approaches to the two-step BFH process, both in two separate reaction vessels and in a single vessel. “Most of them require intermediate purification of the reaction solutions,” explains Dr. Henrik Junge, under whose leadership the catalyst was developed at LIKAT in Rostock. The production of hydrogen from highly concentrated formic acid has already been researched at LIKAT. However, the current process uses wet, biogenic raw materials and residues as feedstocks, which—due to the nature of the system—results in formic acid containing water and poses a hurdle for established release processes. Dr. Junge: “A low substrate concentration is unfavorable for nearly every chemical reaction.” However, especially when it comes to energy recovery, the goal is to avoid time-consuming purification steps. To overcome these hurdles, the LIKAT researchers modified ruthenium complexes—already known from publications, some of which were from their own institute—for use as catalysts. Dr. Hendrik Kempf, then a doctoral student in Henrik Junge’s group, was responsible for testing the new variants in the laboratory under various reaction conditions. The researchers also used an amine as an additive, which increased the reactivity between the substrate and the catalyst. Dr. Junge: “We had already recognized in earlier work that amines can positively influence hydrogen production from formic acid.” At LIKAT, Junge and his team have acquired extensive expertise in this field over the past 15 years. That is why OxFA took notice of the Rostock-based institute as part of a project funded by the Bavarian State Ministry of Economic Affairs, Regional Development, and Energy aimed at producing sustainable hydrogen to ensure energy security.
Stable performance in continuous operation
The result of the experiments is a catalyst system capable of handling both commercial and biogenic formic acid in a highly diluted aqueous solution. At OxFA’s pilot plant in Bavaria, the process was tested on a pilot scale under the direction of Dr.-Ing. Florian Kohler and achieved a stable runtime of more than 820 hours with a total production of approximately 30 cubic meters of hydrogen in continuous operation. “Although our catalyst exhibits slightly lower activity than other known systems, it can easily handle a wide range of water contents in biogenic formic acid,” says Dr. Junge. Dr. Kohler emphasizes that in the continuous trials, biogenic formic acid was used “as produced” without further purification, which is particularly relevant for energy applications. Distillation-based purification processes typically require a large amount of energy. A key technical challenge lay in water management: The continuous addition of diluted formic acid can increase the water content in the system. “The goal, therefore, is to selectively separate the water and remove it from the system without losing active components such as additives and catalysts,” emphasizes Dr. Kohler.
Two-Phase System as the Key to Success
The researchers therefore opted for a so-called two-phase system: The reaction mixture consists of an organic phase and an aqueous phase, which—similar to fat and broth—separate from one another. While the catalyst and additive are present in the organic phase, the formic acid is initially present in the aqueous phase. Even this management process—the removal of water and the addition of formic acid—requires precise process engineering expertise. In addition, the formic acid must effectively cross the phase boundary, because a reaction only occurs when formic acid, the catalyst, and the additive come into contact.
Potential for Future Energy Concepts
In addition to methanol and ethanol, formic acid is increasingly coming into focus as a source and chemical storage medium for hydrogen. Under the conditions described, formic acid can be converted to hydrogen in a concentration range of 5 to 99 percent under mild conditions with nearly 100 percent atomic efficiency, as Junge and Kohler emphasize. In combination with the now-available bio-based production method, this opens up new prospects for sustainable energy concepts. Of particular relevance is the high activity of the new system already at 65 °C—and thus within the range of low-temperature fuel cells, which operate below 100 °C.