ERA-NET_ERACHEMISTRY 2. Call_CO2-Binding of ammonoxidized lignins (COBAL)

Lignin and other ligneous biomaterials can be converted into artificial humic substances by oxidative ammonolysis (ammoxidation). This reaction employs oxygen and aqueous ammonia to break aromatic lignin moieties and to introduce nitrogen in the form of ammonia salts, urea, amides, and amines. The resulting "N-lignin" is an organo- mineral fertilizer which can be used for rehabilitation of degraded soils. One special value of N-lignins is the slow- nitrogen releasing effect, which is caused by the different hydrolysis rates of the different nitrogen binding forms generated upon ammoxidation. Secondary amines are able to bind carbon dioxide in the form of 2:1 complexes. These compounds can be described both as addition products or ion pairs (dialkylammonium dialkylcarbamates, "dialcarbs"). The simplest candidate of this compound class, the dimethylamine - carbon dioxide complex, is a good example to reflect the complex structural-dynamic behavior in these addition products. Many dialkylammonium dialkylcarbamates are room-temperature ionic liquids or room-temperature salts, but they may reversibly degrade into their gaseous components upon heating, and re-form upon cooling. Due to the content of dialkylamines and dialkylamides, N- lignin is able to bind carbon dioxide to form the corresponding carbamate complexes, in an action mode similar to low-molecular weight secondary amines. By using (di)methylamine instead of ammonia in the preparation of the N-lignin, the content of dialkylamines and dialkylamides as CO 2 -binding moieties in N-lignin can be increased additionally. Used as artificial fertilizers they return N-sources (amines and amides) as well as C-sources (lignin carbon and carbon dioxide bound as carbamates) to the soil and into mineralization cycles, stimulating microbial biomass production and hence soil development. Hydroquinone, catechol and benzoquinone derivatives are critically involved in humification, with 2,5-dihydroxy- [1,4]benzoquinones playing a key role in this respect. These structures have been confirmed to occur in the oxidative degradation of lignoid phenols. 2,5-Dihydroxy-[1,4]benzoquinone structures exhibit a peculiar behavior that has been neither recognized nor used so far. In weakly acidic to alkaline medium, 2,5-dihydroxy [1,4]benzoquinones form symmetrical dianions highly stabilized by resonance, with the remaining non-substituted ring positions as a highly nucleophilic binding site able to attack carbon dioxide and carbonic acid derivatives, such as carbamates or urea, by analogy to biochemical pathways. Such carboxylation of 2,5-hydroxy-[1,4]benzoquinone in neutral medium is one of the very few examples of binding CO 2 under carbon-carbon bond formation. In contrast to the temporary binding of CO 2 by nitrogen functionalities such as in DIMCARB, this type of CO 2 - binding is permanent. The CO 2 -binding by ammonoxidized lignins - being abbreviated "COBAL" as working short term - is the subject of this project. This comprises on one side in-depth studies of reaction mechanisms involving model compounds and lignins as well as the comprehensive analytical characterization of the products. On the other side, utilization of the products in agriculture and soil rehabilitation will be tested. The advantages of the process are the simultaneous conversion of two bulk "by-products" (CO 2 and lignin) into a value-added product of high practical relevance (fertilizer) that is returned into the natural cycle. The large-scale applicability - based on the bulk availability of lignin and CO 2 and the pilot-scale production of N-lignins - is another benefit, as well as biocompatibility and environmental compatibility of the products.

Soil humic substances play a central role in soil fertility. However, improper land use has led to serious soil degradation and desertifcation in large land areas in the world, with soil erosion being the main reason for the depletion of humic substances. Ammoxidation of ligneous (waste) materials, such as crop residues or lignins from pulping liquors is considered to be an approach to mimick the natural humification process. In the ammoxidation, the lignin substrate is jointly treated with oxygen and ammonia in aqueous medium at elevated temperature. Valuable nitrogenous, organomineral soil-improving materials for large-scale rehabilitation of degraded soils are this way provided in much shorter time periods as compared to natural humus formation.The mechanisms of nitrogen incorporation into the polymeric matrix of humic substances, the types of nitrogen bonding and their contribution to carbon dioxide activation and sequestration remain challenging research topics in soil sciences and have been therefore addressed in this project. The obtained results strongly support the involvement of reactive low-molecular phenols and quinones in nitrogen fixation. Two representative lignin model compounds were demonstrated to afford intriguingly similar polymeric nitrogenous products as the lignins when ammoxidized. This implies a reaction sequence that is proceeding via the same key intermediates, and 2,5-diamino-[1,4]benzoquinones (DABQ) were unambiguously confirmed to be those pivotal compounds. Surprisingly, they were formed from both the low-molecular phenols and quinones, and all tested lignins with one exception: Extracted lignin from trans-genetic poplar wood with a syringyl content of ~97.5%. Secondary amines were confirmed to react in a way similar to ammonia affording the respective 2,5-bis(alkylamino)-[1,4]benzoquinones. 2,5-diamino-[1,4]benzoquinone derivatives are highly stabilized by tautomerism and resonance. They can be perceived as dimeric vinylogous amides causing 15N-NMR resonance signals in the amide range which can be the reason for overestimation of amides/peptides in soil organic matter. Both 2,5-dihydroxy- and 2,5-diamino-[1,4]benzoquinones were demonstrated to have the ability to bind carbon dioxide. While the DHBQ dianion reacts under neutral to weakly alkaline conditions with 1.86 equivalents of CO2 to 2,5-dicarboxylato-3,5-dihydroxy-[1,4]benzoquinone which subsequently decomposes to C2-C4 organic acids, oxoacids and diacids, CO2 binding of DABQ is limited to the formation of quinoid bisammonium hydrogencarbonate salts at neutral or weakly acidic conditions. In anhydrous state ammoxidised compounds have been found to feature a comparatively high CO2 affinity. With the intriguing combination of lignin utilization (a byproduct that is hitherto burnt), return of an organic waste product into the natural C- and N-cycles, CO2 sequestration and soil improval, the project combined major approaches to help to tackle some of the major global development.