CO2-binding of ammonoxidized lignins and their use as soil improvers

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 dif-ferent 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 reform upon cool-ing. 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 CO2-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 CO2 under carbon-carbon bond formation. In contrast to the temporary binding of CO2 by nitrogen functionalities such as in DIMCARB, this type of CO2-binding is permanent. The CO2-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” (CO2 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 CO2 and the pilot-scale production of N-lignins - is another benefit, as well as biocompatibility and environmental compatibility of the products.