Abstract :
[en] Nowadays lignocellulosic materials represent the higher renewable natural resource in the
world. These materials consist of 34–50% cellulose, 19–34% hemicellulose and 11–30% lignin.
Unlike cellulose, lignin is a three-dimensional aromatic polymer including three main
phenylpropane units, namely p-coumaril, coniferyl and sinapyl alcohol which are linked by C-O-C or C-C bonds. To valorize this material it is necessary to achieve its depolymerization to
monomers or dimers that typically results from the cleavage of β-O-4 linkages. Literature already reported several approaches to depolymerize lignin using both homogeneous and heterogeneous catalysts. Yuan et all have considered a homogeneous route in this scope using NaOH as catalyst. This solution is however by far non-green leading to no-recyclable wastes. Heterogeneous catalysts offer an alternative. The use of noble metals like Pt or Rh supported on carbon, at 473 K, under H2 pressure, was shown to yield over 50% in monomers and dimers . Cheaper metals as copper represent a more versatile route but the reported example uses high energetic supercritical conditions. The use of inexpensive metals such as Ni may offer another alternative. In this study we propose an inexpensive route presenting results obtained using as active species, nickel in different environments such as NiOx, (NiAl)Ox, (NiMgAl)Ox thus looking also for the effect of basicity in this reaction. These materials were prepared using specific protocols and characterized by several techniques like: XRD, DRIFT, BET, and TEM. The catalysts were tested in autoclave conditions at different temperature (423-473K) under H2 pressure. The lignin was extracted from Miscanthus plants into formic acid / acetic acid / water mixture. Since it is not soluble in water its solubilization was achieved in an [BMIM]OAc ionic liquid, selected from a screening of a series of ILs. On the other side, the analysis of the reaction products is a complicate issue in this chemistry. This was carried out with a chromatographic system equipped with a Detector Triple Quad LC/MS, and a MS Ion Source type, working simultaneously in both APCI and ESI modes. The analysis of the reaction products indicated both polar and less polar compounds with a m/z signal varying from 80 till 900. The population of the different class molecules was carefully analyzed as a function of the nature of the catalyst and the reaction conditions. The higher extent of lignin depolymerization was around 54% and was obtained using a (NiAl)Ox catalyst. Under these conditions the predominant class was that with m/z of 200-300 a.u. Finally the stability of the catalysts was checked looking for their separation and recyclability in several successive cycles.
