Analysis of the Pyrolysis Mechanism of Typical Ether Bonds in Biomass Using a Combination of Photoionisation Mass Spectrometry and DFT

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

‌Introduction‌

Biomass pyrolysis is a core technology for the production of biofuels and high-value-added chemicals; the distribution of its products is closely related to the behaviour of ether bond cleavage. The β-1,4-glycosidic bonds (the core linking bonds in cellulose and hemicellulose), α-O-4 bonds (the primary aryl ether bonds in lignin) and methoxy groups (characteristic ether functional groups in lignin) are the most representative ether structures in biomass; their pyrolysis mechanisms determine the conversion efficiency and product value of biomass.

Traditional research has largely relied on theoretical calculations or analysis of final products, lacking experimental evidence for key intermediates (particularly free radicals), which has resulted in the pyrolysis mechanism remaining unclear. Vacuum ultraviolet photoionisation time-of-flight mass spectrometry (SVUV-PIMS), owing to its ‘soft’ ionisation characteristics, can avoid interference from fragments and accurately monitor intermediates generated during pyrolysis. Combined with density functional theory (DFT) calculations, it can elucidate the pathways of ether bond cleavage from the dual perspectives of ‘theoretical validation and experimental monitoring’, thereby providing core theoretical support for the optimisation of biomass pyrolysis processes.

‌Experimental Materials and Methods

Three model compounds were selected for this study: cellobiose (β-1,4-glycosidic bond model), benzylphenyl ether (α-O-4 bond model) and guaiacol (methoxy model).

‌Results and Conclusions

1. Pyrolysis mechanism of the β-1,4-glycosidic bond (fibrodiose)

1.1 Cleavage Pathways

Three possible cleavage pathways for the β-1,4-glycosidic bond: heterocleavage (energy barrier 629.5 kJ/mol), homocleavage (325.3 kJ/mol) and synergistic cleavage (190.1 kJ/mol).

The rate constant for synergistic cleavage (9.4 × 10⁻² s⁻¹) is significantly higher than that for homogeneous cleavage (1.0 × 10⁻⁷ s⁻¹), making it the primary cleavage pathway. Consequently, in fibroses, ring opening of the sugar ring occurs first (cleavage of the C1′–O bond, energy barrier 174.8 kJ/mol), yielding the ring-opened intermediate P1, followed by synergistic cleavage of the glycosidic bond; the ring-opening rate (0.89 s⁻¹) is 9.5 times that of glycosidic bond cleavage.

Figure 1: Energy distribution diagram for the cleavage of β-1,4-glycosidic bonds

1.2 Distribution of pyrolysis products

There are three main core conversion pathways for the ring-opening intermediate P1 of fibrosan. The formation pathways of pyrolysis products are illustrated below using three typical fibrosan pyrolysis products as examples.

Ethanolaldehyde (m/z=60): Formed directly upon cleavage of the C2′–C3′ bond following ring opening of the sugar ring (energy barrier 103.8 kJ/mol); 5-hydroxymethylfurfural (m/z=126): As with the ethanolaldehyde, the C1′–O bond first breaks to form the ring-opened intermediate P1; P1 then undergoes dehydration reactions at the C2′ and C6′ hydroxyl groups (energy barrier 257.5 kJ/mol), and after the removal of water molecules, forms the unsaturated carbon chain intermediate P4; P4 then undergoes a cyclisation reaction to form 5-hydroxymethylfurfural. L-glucan (m/z = 162): Following ring-opening, the fibrosan is cleaved via a concerted breakage of the β-1,4-glycosidic bonds, directly yielding L-glucan and ring-opened D-glucose.

Figure 2  Mass spectrum of pyrolysis products of cellobiose at 450°C

2. Pyrolysis mechanism of the α-O-4 bond (benzyl phenyl ether)

2.1 Cleavage pathways

There are two possible cleavage pathways for benzylphenyl ether: symmetric cleavage of the Cα-O bond to form a benzyl radical (m/z=91) and a phenoxy radical (m/z=93), which is the low-energy barrier pathway (energy barrier 179.7 kJ/mol); C-aromatic ring–Cα cleavage to form a phenyl radical (m/z = 77) and an R5 radical (m/z = 107), which is the high-energy barrier pathway (375.5 kJ/mol). The rate of Cα–O homolytic cleavage is nine orders of magnitude higher than that of the latter; Cα–O homolytic cleavage is the primary pathway for the cleavage of the α–O–4 bond (Figure 3). This theory is supported by the detection of benzyl radicals (m/z = 91) and phenoxy radicals (m/z = 93) via SVUV-PIMS, whilst no phenyl radicals (m/z = 77) or R5 radicals (m/z = 107) were detected.

Figure 3  Initial pyrolysis reaction pathway of benzyl phenyl ether

2.2 Distribution of Pyrolysis Products

Primary products: The benzyl radical undergoes dehydrogenation to form toluene (m/z=92), whilst the phenoxy radical undergoes dehydrogenation to form phenol (m/z=94).

Radical recombination products: benzyl radicals dimerise to form 1,2-diphenylethane (m/z = 182); benzyl radicals recombine with phenoxy radicals to form 2-benzylphenol (m/z = 184); the phenoxy radical dimerises to form [1,1’–biphenyl]-2,2’–diol (m/z = 186).

Minor product: 6H-benzo[c]chromene (m/z = 182), formed by dehydroxylation followed by dehydrogenation of 2-benzylphenol; its GC/MS peak area accounts for only 1.05 per cent and is kinetically minor.

Figure 4 Mass spectrum of pyrolysis products of benzyl phenyl ether at 600 °C

3. Pyrolysis mechanism of methoxy (guaiacol)

3.1 Initial reaction pathways

Three possible pathways: homolytic demethylation (energy barrier 198.1 kJ/mol), homolytic methoxy elimination (386.0 kJ/mol), and radical-induced rearrangement (389.8 kJ/mol).

Figure 5: Reaction pathways for the initial evolution of guaiacol and the corresponding energy distribution diagram

Methylation via homolytic cleavage is the initial predominant pathway, yielding a methyl radical (・CH₃) and an o-hydroxyphenoxy radical (m/z = 109). The methyl radical acts as a catalyst for the demethoxylation and rearrangement reactions (Figure 6); however, high temperatures inhibit the occurrence of the latter two reactions.

Figure 6: Energy distribution of methyl radical-assisted demethylation and methoxylation reactions, and reaction mechanism diagram for the initial pyrolysis of guaiacol

3.2 Distribution of pyrolysis products

Major products: catechol (m/z=110, formed by dehydrogenation of the o-hydroxyphenoxy radical); 2-hydroxybenzaldehyde (m/z=122, formed by a methyl radical-assisted rearrangement); phenol (m/z=94, formed by methyl radical-assisted demethoxylation).

Minor products: cyclopentadienol (m/z = 82, formed by dehydrogenation of the R9 radical), cyclopentadienone (m/z = 80, formed by dehydrogenation of the R9 radical).

Figure 7 Mass spectrum of guaiacol pyrolysis products at 600°C

3.3 Effect of temperature on the distribution of guaiacol pyrolysis products

At temperatures above 650°C, the yield of o-cresol (m/z=110) increases (high temperatures favour hydrogen abstraction reactions), whilst the yields of 2-hydroxybenzaldehyde (m/z=122) and phenol (m/z=94) decrease (high temperatures promote the decarboxylation of the o-hydroxyphenoxy radical to form the R9 radical, m/z=81; the resulting R9 radical readily combines with the methyl radical to form 2 -methylcyclopentadienol, which further reduces the concentration of methyl radicals, leading to a weakening of their catalytic effect).

Figure 8  Temperature trend graph showing the intensity of the main pyrolysis products of guaiacol

The combination of SVUV-PIMS and DFT has realised a closed-loop system of ‘reaction intermediate monitoring and pathway verification’, clarifying the pyrolysis mechanism of typical ether bonds in biomass and providing a theoretical basis for the targeted regulation of product distribution (e.g. enriching phenols and suppressing polycyclic aromatic hydrocarbons).

References

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