Ozonolysis of surface adsorbed methoxyphenols: kinetics of aromatic ring cleavage vs. alkene side-chain oxidation

Abstract

Lignin pyrolysis products, which include a variety of substituted methoxyphenols, constitute a major component of organics released by biomass combustion and may play a central role in the formation of atmospheric brown carbon. Understanding the atmospheric fate of these compounds upon exposure to trace gases is therefore critical to predicting the chemical and physical properties of biomass burning aerosol. We used diffuse reflectance infrared spectroscopy to monitor the heterogeneous ozonolysis of 4-propylguaiacol, eugenol, and isoeugenol adsorbed on NaCl and α-Al₂O₃ substrates. Adsorption of gaseous methoxyphenols onto these substrates produced near monolayer surface concentrations of 3 × 10¹⁸ molecules m^-2. The subsequent dark heterogeneous ozonolysis of adsorbed 4-propylguaiacol cleaved the aromatic ring between the methoxy and phenol groups with the product conclusively identified by GC-MS and ¹H-NMR. Kinetic analysis of eugenol and isoeugenol dark ozonolysis also suggested the formation of ring-cleaved products, although ozonolysis of the unsaturated substituent groups forming carboxylic acids and aldehydes was an order of magnitude faster. Average uptake coefficients for NaCl-adsorbed methoxyphenols were γ = 2.3 (±0.8) × 10^-7 and 2 (±1) × 10^-6 for ozonolysis of the aromatic ring and the unsaturated side chain, respectively, and reactions on α-Al₂O₃ were approximately two times slower. UV-visible radiation (λ>300 nm) enhanced eugenol ozonolysis of the aromatic ring by a factor of 4(±1) but had no effect on ozonolysis of the alkene side-chain

Growing of Artificial Lignin on Cellulose Ferulate Thin Films

“…The products of typical side chain conversion of coniferyl alcohol in primary pyrolysis reactions such as coniferyl aldehyde ( n ), dihydroconiferyl alcohol ( l ), isoeugenol ( h ), and 4-vinylguaiacol ( e ) could be found too . Moreover, other evaporable typical lignin pyrolysis products, for example, guaiacylacetone ( k ), acetovanillone ( j ), vanillin ( g ), eugenol ( f ), 4-ethylguaiacol ( d ), 4-methylguaiacol ( b ), and guaiacol ( a ) were detected . In addition to the β-O-4 motive, β-5 linkages were evidenced by pyrolysis product ( i ) arising from phenylcoumaran units.…”

“…34 Moreover, other evaporable typical lignin pyrolysis products, for example, guaiacylacetone (k), acetovanillone (j), vanillin (g), eugenol (f), 4-ethylguaiacol (d), 4-methylguaiacol (b), and guaiacol (a) were detected. 35 In addition to the β-O-4 motive, β-5 linkages were evidenced by pyrolysis product (i) arising from phenylcoumaran units. Catechol (c) is a product of secondary pyrolysis reactions (>400 °C), changing the aromatic substitution pattern.…”

Heterogeneous Reactions of Phenol on Different Components of Mineral Dust Aerosol: Formation of Oxidized Organic and Nitro-Phenolic Compounds

“…Similar ring opening is previously observed with catechol and hydroquinone in the presence of radicals 14,37,86,87 and with adsorbed 4-propyl-guaiacol and catechol during ozonolysis. 41,42 During the oxidation of 6 and 7 to form 9 through 11, Fe(III) on the α-Fe 2 O 3 surface can be reduced to Fe(II). This redox chemistry underscores the importance of the different mineral reactivities; these ring opening products were not observed on TiO 2 surfaces under similar conditions.…”

“…40 Ozonolysis of adsorbed 4propyl-guaiacol and catechol on NaCl and α-Al 2 O 3 surfaces has also been shown in the formation of aromatic ring opening products. 41,42 In another study, formation of maleic acid from chlorobenzene on γ-Al 2 O 3 43 and a significant degradation on α-Fe 2 O 3 44 under photochemical conditions were observed. Aromatic compounds such as phenol and benzene have been shown to undergo hydroxylation in the presence of TiO 2 and iron oxides when H 2 O 2 is present as the oxidizing agent, 45,46 whereas phenol in water is shown to undergo selective photooxidation in the presence of TiO 2 .…”