Catalytic Oxidation of Flax Shives into Vanillin and Pulp

Abstract

This research deals with a process of catalytic oxidation of flax shives to vanillin and pulp. Catalytic oxidation of flax shives with molecular oxygen allows two main products to be obtained—vanillin with a yield of up to 12 wt.% of lignin, and pulp. Final forms of the catalyst particles (Cu2O and CuO) are agglomerates or monocrystals 0.5–5 µm in size. Acid prehydrolysis of the shives does not affect the oxidation, in contrast to pine-wood oxidation. Lignin prehydrolysis and oxidation was suggested as illustrate this difference. The dependence of the vanillin formation rate on stirring speed was studied. Minimum alkali consumption in the process was attained with a mild stirring speed of the reaction mass.

Mass Transfer in the Processes of Native Lignin Oxidation into Vanillin via Oxygen

“…These dependences coincide with the earlier-obtained and recommended models for the description of mass transfer in gas-liquid systems [21,22]. These models explain the overall process rate limitation via the rate of oxygen diffusion through the liquid near-surface region at the gas-liquid phase boundary and are often interpreted as examples of a more general model The authors of [19,20] proposed the first quantitative model for the influence of mass transfer intensity on the rates of oxygen consumption and vanillin accumulation. This study was complicated by the presence of mucilage in the flax shives and via the formation of gels in the aqueous phase.…”

“…Therefore, the presented results show that at a fixed stirring speed, there exists a range of volumes of the reaction mixture (V max /V min = 2-3) in which the overall values of the oxygen consumption rate (mmol min −1 ), vanillin yield (g), and lignoacid yield (g) are determined via the stirring power density, being independent from the reaction volume [19][20][21][22][23]. This means that there exists an even more narrow range of the reaction mixture volume (exemplified in our experiments by the volume 300 mL) in which the vanillin yield based on the initial lignin is the highest, with minimal expenditure of the reactants for its production.…”

“…In the volume range of 300-600 mL, these characteristics fluctuated within the of range ±10-15% or less (Table 1). A simple and natural explanation for this result can be found in the model that links the rate of a chemical process limited by mass transfer to the stirring power density [19][20][21][22][23]30,31]. The simplest description of the mass transfer rate in a gas-liquid system (which corresponds to the oxygen consumption rate in our case) is provided by Equations ( 3) and ( 4) [31].…”

“…The authors of [19,20] proposed the first quantitative model for the influence of mass transfer intensity on the rates of oxygen consumption and vanillin accumulation. This study was complicated by the presence of mucilage in the flax shives and via the formation of gels in the aqueous phase.…”

Chemical Processing of Agriculture Wastes Into Vanillin, Pulp and Glucose

“…Lignins of herbaceous plants, in contrast to woody ones, contain a significant amount of non-methoxylated PPU, for which condensation is possible at the third and fifth positions of the aromatic ring. This can lead to a decrease in the yields of aromatic aldehydes in the oxidation processes during the transition from wood feedstock to herbaceous one [16]. Indeed, lignins of herbaceous plants give smaller amounts of aromatic aldehydes in the oxidation processes [6] in comparison with softwood and moreover hardwood lignins.…”