Combining lignin with metals for agrochemical applications

After cellulose, lignin is the most abundant plant derived polymer. It confers mechanical support to plants and protects them against pests and diseases. This is possible thanks to its polyphenolic structure that imparts antibacterial, antifungal and antioxidant properties to the polymer. Hugh amount of lignin are obtained as co-product from several industrial processes, but nowadays it is largely underutilized. Scientific research is working to find potential applications for lignin in the field of functional materials, as precursor for small molecules by depolymerization, as well as in the packaging field. On the other hand, in the agricultural field, the use of copper as pesticide is still very widespread. The main problems associated with its intensive use are the development of metal-resistant strains and the environmental pollution such as accumulation in soils that causes the death of soil biota, plants phytotoxicity as well as human toxicity. For these reasons the European Community lowered the annual maximum copper limit from 6kg/ha to 4kg/ha. It is in this scenario that my PhD project took place. The idea was to investigate the synergistic antibacterial activity between lignin and copper for the development of new pesticides with a reduced amount of the metal, thus finding a new purpose for this important and underused biopolymer. Starting from lignin and CuSO4∙5H2O, two different hybrid materials were synthesized both by wet and mechanochemical procedures, named lignin@brochantite (brochantite= Cu4(SO4)(OH)6), and lignin@cuprite, (cuprite=Cu2O). XRPD and TEM analysis revealed the formation of hybrid materials in which nanocrystals of the respective mineral phases were embedded in the lignin matrix. The characterization of the materials was based on XRPD, ICP, TEM, GPC, Py-G/CMS, NMR and IR analysis, to determine the nature and features of the crystalline phases {copper content, morphology and dimension of the crystals} as well as the intactness of the biopolymer. The antibacterial activity of these new hybrid materials was evaluated against several pathogens by in vitro and in vivo tests that revealed a high efficiency with a lower amount of copper compared to copper-based pesticides currently on the market. The synergistic effect between copper and lignin was thus confirmed. A correlation between the morphology of the crystals and their effectiveness with in vivo tests was also conducted revealing that bigger crystals present a higher efficiency. Subsequently an iron-based hybrid material was also synthesized by mechanochemical procedure. This material was characterized by means of XRPD, TEM and ICP analyses which revealed goethite crystals (FeO(OH)) embedded in the lignin matrix, with the complete iron upload of 10%. Its antibacterial activity was then evaluated by in vitro tests. Lignin structure was then modified by means of acetylation and phosphorylation. The complete acetylation on both aromatic and aliphatic OH groups was confirmed by NMR and IR analyses and antibacterial properties of acetylated lignin were confirmed by in vitro tests. Phosphorylated lignin was characterized by 31P-NMR, EDS, elemental analysis and TGA. Despite the phosphorous content is between 1 and 2% phosphorylated lignin has a completely different behaviour compared to the starting lignin. Finally, the coordinative properties of lignin were studied starting from lignin-inspired ligands. From the solvothermal synthesis between ferulic acid and CoCl2∙6H2O, a coordination polymer with formula [Co2(fer)Cl2]·3DMF was isolated and characterized by SC-XRD.