Impact of steam explosion on spruce lignin structure and pyrolyzates

Abstract

The impact of steam explosion (SE) on spruce lignin structure and product formation during flash pyrolysis was investigated by HSQC and pyrolysis-GC-MS (py-GC-MS) experiments, with the aim to understand how lignin structure changes during SE treatment. Milled wood lignin (MWL) was isolated from untreated Norway spruce and steam exploded spruce samples treated at eight different severities. The second parallel gave yields comparable to literature for all of the nine samples. In an attempt for further purification, a part of all MWL samples was dissolved in tetrahydrofuran to obtain THF dissolvable MWL in surprisingly inconsistent yields. Signals from twenty-eight different 13C-1H correlations for thirteen lignin substructures and structural units were identified by HSQC experiments, including β-aryl ether (β-O-4’), phenylcoumarane (β-5’), resinol (β-β’), dibenzodioxocine (5-5’-O-4), diphenyl-ethane (β-1’), and spirodienone (α-O-α’) lignin interunit bonds. The high abundant β-O-4’ linkages were partially cleaved during the SE treatment, resulting in reduced amount with increased SE temperature, while raised abundance of β-5’ linkages with SE temperature indicated condensation reactions. The low abundance of β-β’ linkages remained similar with more severe SE conditions, indicating balance between formation and degradation, while the lowest abundant linkages (5-5’-O-4, β-1’, and α-O-α’) were only detected in untreated spruce and therefore completely degraded by the SE treatment. Considerably fewer pyrolyzates were found in py-GC-MS of MWL from untreated spruce (26) than steam exploded spruce (34). Most pyrolyzates that were only formed from SE samples were derived from carbohydrates that were isolated as MWL due to pseudo-lignin formation in the SE treatment. G-lignin derivatives with shorter and more reduced side-chains were formed in relatively higher amounts with increased SE temperature, confirming partial depolymerization during steam explosion. Hydrolysis of β-O-4’ linkages during the pretreatment facilitated the formation of 4-hydroxy-3-methoxybenzaldehyde, while decrease was observed with increased SE temperature due to aldehyde instability. Similarities between samples treated for five and ten minutes indicated that steam temperature is the dominant factor for lignin structural changes during steam explosion. Differences observed after dissolving MWL in THF confirmed that lignin analysis depends on purification methods. Furthermore, comparison between pyrolyzates from MWL and THF dissolvable MWL indicated less heterogeneity in the lignin polymer after SE treatment.

Virkningen av dampeksplosjon (SE) på granligninstruktur og produktdannelse under pyrolyse ble undersøkt med HSQC og pyrolyse-GC-MS (py-GC-MS), med mål om å forstå hvordan ligninstruktur endres under SE behandling. Ballmøllet lignin (MWL) ble isolert fra ubehandlet gran og dampeksploderte granprøver som var behandlet ved åtte forskjellige betingelser. Den andre parallellen gav resultater tilnærmet litteraturen, for alle ni prøvene. I et forsøk på ytterligere rensing ble en del mengde av alle MWL prøvene løst i tetrahydrofuran for å oppnå THF oppløselig MWL, men i overraskende varierende utbytter.