Issues in Preparation of Metal-Lignin Nanocomposites by Coprecipitation Method

Abstract

Chemical coprecipitation technique is proven to be a beneficial method to prepare uniformly mixed catalyst metal and Kraft lignin precursors. Coprecipitation is a simple, yet very complex process which is highly sensitive to the reaction conditions, particularly temperature. In an exothermic coprecipitation process, the reaction rate can become uncontrollable over certain temperatures which could lead to a thermal runaway reaction. In this work, metal-lignin nanocomposites were synthesized by coprecipitation of metal (M) salts and Kraft lignin. Kraft lignin and metal salts were dissolved in organic solvents and DI water, respectively, to make lignin solution/suspension and metal salt aqueous solution. The aqueous solutions of metal salts were then added to the lignin solutions/suspensions and mixed well, resulting in chelation of transition metal ions to the functional groups of lignin chains and co-precipitation of metal-lignin composites from the solvents. To develop a safe process for producing M-lignin composites in a large volume, potential reactions, exothermic or endothermic processes, hazards gases, and volatiles were evaluated during the coprecipitation process. The effects of transition metal type, solvent selection, concentration of metal salts, and initial solution temperature on the interactions between metal ions and Kraft lignin, metal uniformity in the lignin matrix, and morphology of the metal-lignin composites were investigated during the coprecipitation process. Cu, Mo, Ni, and Fe were investigated as the transition metals for the metal-lignin composites. Fenton or Fenton-like reactions were discovered to occur during the Fe- and Cu-lignin coprecipitation process and tremendous heat evolved, which lead to the overshoot of the reaction system temperature in a very short time (i.e. a few seconds). Significant amounts of CO2 and toxic NO2 gasses were released during the coprecipitation process when Fenton or Fenton-like reactions occurred. No interaction or a very weak interaction occurred between lignin and Mo(VI) ions when mixing both solutions. Ni ions were coordinated strongly to oxygen-containing functional groups in lignin, but no Fenton or Fenton-like reaction was detected during Ni-lignin coprecipitation. Fenton reaction or Fenton-like reaction occurred when tetrahydrofuran (THF) and acetone were used to dissolve Kraft lignin, and the reaction became highly fierce and unmanageable with increasing of iron content in the composite. The reaction initialization time was shortened with increase of initial solution temperature and thermal runaway reaction might occur if the initial mixing temperature reached 60 °C or above.