Production of phenol compounds by alkaline treatment of poplar wood bark

Abstract

It is well known that, as a result of thermo-chemical treatment in a water-alkaline medium, lignin is destroyed and low-molecular-weight phenol compounds are obtained [1][2][3][4]. Low-molecular-weight phenol compounds (LMPC) 2-methoxyphenol, 2,6-dimethoxyphenol, vanillin, etc. were previously obtained from the alkaline destruction of technical hydrolysis lignin (THL), wood biomass, and poplar wood bark [5]. The optimal conditions for obtaining LMPC were lignin-5% NaOH ratio 1:10 and process duration 6 hours. The experiment has been carried out at 180qC [6].Technical hydrolysis lignin is not as common a residue as wood bark. Moreover, treatment of the poplar wood bark leads to a considerably higher yield of low-molecular-weight compounds in comparison with those from THL treatment [5]. This statement is due to the more condensed THL structure. The aim of the present investigation is to carry out alkaline treatment of poplar wood bark under different conditions.The lignin content in poplar wood bark is 37.40 mass % of initial bark. It was found that with increasing process duration and concentration of NaOH (6 h, 7% NaOH) the highest quality of LMPC was obtained (Table 1). Moreover, at these conditions the influence of hydromodule on LMPC yield was studied. It was established that at h.m. 1:8 and 1:10 a similar yield of LMPC was obtained (Table 2). Table 3 presents low-molecular-weight phenol compounds obtained from poplar wood bark under different conditions. The possibility for obtaining low-molecular-weight phenol compounds through alkaline treatment of poplar wood bark was investigated. It was found out that alkaline treatment is an effective process, in which the main part of the lignin is destroyed, and as a result of this, low-molecular-weight phenol compounds are obtained. Using extraction with toluene from the liquid phase, the following compounds were isolated: 2-methoxyphenol, 2,6-dimethoxyphenol, 4-hydroxy-3-methoxybenzaldehyde, 4-hydroxy-3,5-dimethoxybenzaldehyde, 1-(4-hydroxy-3-methoxyphenyl) ethanone, etc.For the purpose of this investigation, poplar wood bark was previously powdered. The process and extraction conditions were determined based on the results of our previous investigations [5,6]. The alkaline treatment of bark was carried out in stirring autoclaves made of stainless steel, heated in a bath of polyethyleneglycol at 180qC for 2, 4, and 6 hours. A water solution of NaOH (3, 5, and 7%) was used at different bark/water solution of NaOH ratios (hydromodule) -1:6, 1:8, and 1:10.The amounts of obtained extracts, insoluble residue, and precipitated and nonprecipitated lignin were determined. The LMPC were identified by GC-MS analysis [5,6].

Electrochemical methods for materials recycling

“…Chemical lignin depolymerization methods consist mainly of three broad classes: hydrolytic, oxidative, and reductive depolymerization. Hydrolysis typically results in low yields and complicated product separation, 92 and chemical oxidative depolymerization methods often oxidize products too far (producing H 2 O or CO 2 ) and suffer from repolymerization, lowering yields. 93,94 Although certain reductive chemical catalytic techniques have shown promising results for lignin depolymerization, they often still necessitate relatively harsh conditions.…”

Lignin depolymerization (LDP) in alcohol over nickel-based catalysts via a fragmentation–hydrogenolysis process

“…Reported methods include hydrolysis, oxidation, and reduction. Employing alkali hydroxides or carbonates as catalysts in water, [9][10][11][12] hydrolysis of C-O-C linkages leads to phenol derivatives but with yields <10%. Moreover, processes with homogeneous catalysts require tedious separation procedures.…”

Emerging strategies for breaking the 3D amorphous network of lignin

“…So far, hydrolysis, oxidation and reduction are well-established methods, in which aqueous phase alkali hydroxide or carbonate catalysed hydrolysis of C-O-C linkages results in phenol derivatives. [26][27][28] Oxidative cleavage of C-H bonds and/or C-C bonds adjacent to C-O-C linkages produces vanillin and/or its analogues. 29,30 This reaction causes oxidative damage to the aromatic fragments of the lignin, leading to extreme oxidation to form CO x and H 2 O.…”