Interfacial Oxidative Oligomerization of Catechol

Abstract

The heterogeneous reaction between thin films of catechol exposed to O₃(g) creates hydroxyl radicals (HO^•) in situ, which in turn generate semiquinone radical intermediates in the path to form heavier polyhydroxylated biphenyl, terphenyl, and triphenylene products. Herein, the alteration of catechol aromatic surfaces and their chemical composition are studied during the heterogeneous oxidation of catechol films by O₃(g) molar ratios ≥ 230 ppbv at variable relative humidity levels (0% ≤ RH ≤ 90%). Fourier transform infrared micro-spectroscopy, atomic force microscopy, electrospray ionization mass spectrometry, and reverse-phase liquid chromatography with UV-visible and mass spectrometry detection provide new physical insights into understanding the surface reaction. A Langmuir-Hinshelwood mechanism is accounted to report reaction rates, half-lives, and reactive uptake coefficients for the system under variable relative humidity levels. The reactions reported explain how the oligomerization of polyphenols proceeds at interfaces to contribute to the formation of brown organic carbon in atmospheric aerosols.

Figure 1

AFM micrographs of 150 μg catechol thin films at 70% RH exposed to (A) N₂(g) and (B) 230 ppbv O₃(g) for 72 h. (C) Micrograph of the film in panel (B) overlaid with its FTIR spectral map and (D) corresponding spectra of ozonolysis products labeled in panel C. The individual spectra labeled a–e in panel D correspond to the points marked in panel C.

Figure 2

Electrospray ionization (ESI) mass spectrometry (MS) of catechol thin films before (peak labeled in blue font) and after (peaks labeled in red font) 3 h exposure to 24.0 ppmv O₃(g) at 70% RH extracted in isopropanol.

Scheme 1. Generation of o-Semiquinone Radicals of Catechol and Its Coupling Products Identified at m/z 217 and 325

Tracking of the contribution from o-semiquinone radicals to the formed products is color-coded in green font.

Scheme 2. Generation of Tri-, Tetra-, and Pentahydroxybenzenes, and Their o-Semiquinone Radicals to Form Coupling Products Identified at m/z 249, 297, 311, and 313

Tracking of the contribution from o-semiquinone radicals to the formed products is color-coded in green, brown, blue, pink, teal, and purple fonts, respectively.

Scheme 3. Generation of a Triphenylene Identified at m/z 339 from the o-Semiquinone Radical of a Trihydroxybenzene Double Coupling with a Catechol Dimer

Tracking of the contribution from o-semiquinone radicals from Schemes 1 and 2 to the formed products is color-coded in green and pink fonts, respectively.

Figure 3

Ultrahigh-pressure liquid chromatogram (UHPLC) of the catechol film exposed during 3 h at 70% RH to (A) 1 atm N₂(g) and (B) 21.1 ppmv O₃(g). (Bottom of each panel) UV detection chromatogram at λ = 254 nm. (Top of each panel) Single ion monitoring (SIM) mass spectrometry (MS) for mass-to-charge ratios (m/z) of (red) 109, (blue) 249, (pink) 297, (green) 311, (dark yellow) 325, and (purple) 339.

Figure 4

Pseudo-first-order reaction rate constant (k_cat+O3) for catechol films as a function of [O₃(g)] for (blue square) 0%, (pink triangle) 30%, (black circle) 70%, and (teal star) 90% RH. The dashed lines show nonlinear least-squares fittings using eq 1 for the Langmuir–Hinshelwood mechanism with parameters reported in Table 1.