Study of the Pyrolysis Mechanism of Lignin Model Compounds by In Situ Photoionisation Mass Spectrometry

Areas of application: Research into the mechanisms of biomass pyrolysis; optimisation of biomass pyrolysis processes;

‌Introduction‌

With the development of biomass energy utilisation technologies, research into the pyrolysis mechanisms of lignin—the second most abundant organic component in biomass—is of great significance. Traditional α-O-4 and β-O-4 linkage structures account for more than half of the total lignin linkage structures. In this study, vacuum ultraviolet photoionisation time-of-flight mass spectrometry (SVUV-PIMS) technology was employed in conjunction with a gas chromatography–mass spectrometry (Py-GC/MS) system to investigate the pyrolysis mechanisms of three model lignin dimer compounds, thereby providing new technical approaches and theoretical foundations for fundamental research into the lignin pyrolysis process.

‌Experimental Materials and Methods‌

‌Instrumentation‌: Vacuum ultraviolet photoionisation time-of-flight mass spectrometry (SVUV-PIMS) system; rapid pyrolysis analyser (CDS5200) coupled with a GC/MS system

Vacuum conditions: 143.5 Pa

Transfer lines: fully heated to prevent condensation

‌Samples and reagents‌: 4-(benzyloxy)phenol (BOP), 4-(benzyloxy)anisole (BOA), guaiacyl glycerol-β-guaiacyl ether (GGGE)‌

Sample quantity: 20–40 mg (SVUV-PIMS), 0.5 mg (Py-GC/MS)

Reaction temperature: 350–500 °C

Photon energy: 9.5–10.5 eV

Heating rate: 20 °C/min

Carrier gas: He

‌Results and Conclusions‌

‌1. α-O-4 model compound

1.1‌Identification of major products

The main pyrolysis products of BOP include m/z 91: benzyl radical; m/z 92: toluene; m/z 108: p-benzoquinone; m/z 110: p-cresol; and m/z 182: 1,2-diphenylethane (bibenzyl).

Figure 1: Photoionisation mass spectrometry (PIMS) spectrum of BOP pyrolysis products at 500 °C

1.2 Verification of Pyrolysis Mechanism

The pyrolysis reaction of the α-O-4-linked structure is primarily initiated by the homolytic cleavage of the Cα-O bond (Figure 2). Benzyl radicals and p-semiquinone radicals are the main radical intermediates; the benzyl radical generates toluene via a hydrogen abstraction reaction, whilst the benzyl radical forms dibenzyl via a termination reaction; the p-semiquinone radical follows two pathways, including hydrogen abstraction to form p-hydroquinone and the direct dissociation of the phenoxy-hydrogen bond of the p-semiquinone radical to form p-benzoquinone.

Figure 2  Mechanism of homolytic Cα−O bond cleavage in BOP

1.3 Effect of temperature on the pyrolysis reactions and product distribution of BOP and BOA

In the range of 350–500 °C, an increase in temperature enhances the selectivity of 1,2-diphenylethane and p-benzoquinone, whilst reducing the selectivity of p-hydroquinone; it also favours the progression of the hydrogen extraction reaction.

Figure 3: Product distribution of BOP pyrolysis in Py–GC/MS at three temperatures

 ‌‌

At low temperatures (350 °C), methoxyhydroquinone and dibenzyl were the main products; at high temperatures (500 °C), a variety of products appeared, including 4-methoxybenzaldehyde, phenethyl ether, benzene, benzaldehyde and 1,4-dimethoxybenzene.

Figure 4: Distribution of products from the pyrolysis of BOA in Py-GC/MS at three temperatures

2. β-O-4 model compounds

2.1 Identification of major products

The cleavage of the Cβ–O and Cα–Cβ bonds is the initial step in the thermal decomposition of β-O-4-type lignin dimer model compounds. The main products include m/z 196: 3-hydroxy-1-(4-hydroxy-3-methoxyphenyl)-1-propanone, m/z 180: coniferol, m/z 178: coniferaldehyde, m/z 168: 4-(1-hydroxyethyl)-2-methoxyphenol, m/z 152: acetylvanillin, m/z 150: vanillin, m/z 136: 2-methoxy-4-vinylphenol, m/z 124: guaiacol.‌ (Figure 5)

Figure 5 Photoionisation mass spectrometry of GGGE pyrolysis products at 500 °C

2.2‌ Reaction Pathway Analysis

During the pyrolysis of the β-O-4-linked structure, the cleavage of the Cβ-O and Cα-Cβ bonds constitutes the initial step. Within the experimental temperature range, the pyrolysis of GGGE is predominantly characterised by synergistic reactions, with aldehydes and ketones forming as secondary reaction products.‌ (Figure 6)

‌Pathways 1 and 2‌: Synergistic reaction mechanism

‌Pathway 3‌: Homolytic cleavage mechanism

Figure 6: Pyrolysis pathways of GGGE

2.3. Effect of temperature on the GGGE pyrolysis reaction and product distribution

At low temperatures (350–400 °C): guaiacol is the main product; at high temperatures (500 °C): guaiacol decreases sharply, whilst aldehyde and ketone products increase with rising temperature.

Figure 7  Product distribution of GGGE pyrolysis in Py-GC/MS at three temperatures

SVUV-PIMS technology has demonstrated significant advantages in the study of lignin pyrolysis. Not only does this technique enable the precise detection of radical intermediates, but its ‘soft’ ionisation characteristics also mitigate interference from fragment peaks, thereby enhancing qualitative accuracy. It has overcome the limitations of traditional detection methods and provides a scientific basis for optimising biomass pyrolysis processes.

‌References‌

[1] Zhou J, Jin W, Shen D, et al. Formation of aromatic hydrocarbons from co-pyrolysis of lignin-related model compounds with hydrogen-donor reagents[J]. Journal of Analytical and Applied Pyrolysis, 2018, 134.

[2] Castilla C, Ruger C P, Lavanant H, et al. Ion mobility mass spectrometry of in situ generated biomass pyrolysis products[J]. Journal of Analytical & Applied Pyrolysis, 2021, 156(June):105164.1–105164.12.