Reductive catalytic fractionation of flax shives by using nickel formate for in situ catalyst generation and the shives as a support for the metal

Nanoscale nickel particles formed in situ using nickel formate as a precursor are proposed for the reductive catalytic fractionation (RCF) of flax shives. The examination of solid products of flax shives RCF by XRD revealed that characteristic reflections for Ni⁰ (111), (200), (220) at 2θ angles of 44, 52, and 76, respectively, are present in the X-ray diffraction patterns, hinting at the metal in situ reduction. The presence of Ni oxides was also confirmed via XPS (signals 855.8 and 874.3 eV, Ni⁰/Ni_ox = 0.0). The alternative Ni-catalyst was pre-deposited on flax shives via reduction of nickel formate by hydrazine. The pre-deposited Ni particles were found to be XRD amorphous hinting at fine distribution and XPS confirmed both metallic and oxidized states Ni⁰, Ni²⁺, Ni³⁺ at the surface (Ni⁰/Ni_ox = 0.14). Monomeric compounds, such as 4-propylguaiacol, 4-propenylguaiacol, and 4-propanolguaiacol, were obtained with a total yield of 6.92 wt.% in the presence of in situ formed Ni. Excluding the molecular hydrogen from the reactor drops the lignin monomers yield from 6.92 to 3.80 wt.%, but the latter value exceeds the yield of the monomers of non-catalytic hydrogenation with H₂ by a factor of 3 (1.44 wt.%). Ni particles catalyze the process of lignin hydrogenation by formate ion; decreasing the Ni content in the reaction mass while keeping formate ion concentration at the same level drops the monomers yield down to the level corresponding to the non-catalytic hydrogenation with using molecular H₂. The process over nickel pre-deposited via reduction by hydrazine provided a higher total phenolic monomers yield of up to 9.12 wt.% equaled to the yields in the presence of the known Ni/C catalysts. The selectivity on the main product among phenolic monomers – 4-propanolguaiacol reached 42%.