Effect of in-situ hydrogen forming reagents on hydrothermolysis of organosolv lignin

The aim of this study was to investigate the effect of two hydrogen forming reagents, sodium formate and formic acid, on the yield of bio-oil during hydrothermolysis of sulphur free lignin. In-situ hydrogen formation can stabilize the reacting intermediates resulting in higher yields of monoaromatic compounds.
Lignin (5 g) and catalysts (0.5 g) were mixed with distilled water (200 mL) prior to carrying out hydrothermolysis at 310 °C for 60 min (Parr 4575 batch reactor). Bio-oil was extracted from the aqueous reaction mixture with ethyl acetate followed by drying. Monoaromatic compounds were calculated with gas chromatography and mass spectroscopy (GC-MS) and molar mass distributions of bio-oil samples were analysed with gel permeation chromatography (GPC) for understanding the effects of used catalyst.
The bio-oil yield increased from 12 wt. % (without catalyst) to 13.9 and 14.1 wt.% with sodium formate and formic acid, respectively. GC-MS analysis of bio-oil samples revealed that the yield and composition of monoaromatics were significantly different depending on type of the catalyst. The amount of monoaromatics increased from 55 mg g-1 of lignin in the absence of a catalyst to 92 mg g-1 of lignin by the addition of sodium formate, while it decreased to 42 mg g-1 of lignin when formic acid was added as a catalyst.
Syringol (36 mg g-1 of lignin), guaiacol (24 mg mg g-1 of lignin), 3-methoxycatechol (9 mg g-1 of lignin), pyrocatechol (7 mg g-1 of lignin) and, 4-methylsyringol (4 mg g-1 of lignin) were identified in the bio-oil sample obtained with sodium formate. The GPC analysis revealed a mean average molar mass (Mw) of 207 g/mol and a molar mass dispersity (D) of 1.1, which is about one tenth of the molar mass of the initial organosolv lignin. A D value greater than 1 indicates the presence of dimers and some other higher molecular weight compounds.