This paper is a thorough investigation of the chemical transformations during pyrolytic conversion of phenolic resins to carbons, and reports that all carbons obtained from main-stream phenolic resins including phloroglucinol-formaldehyde (FPOL), phloroglucinol-terephthalaldehyde (TPOL), resorcinol-formaldehyde (RF), and phenol-formaldehyde (PF) contain fused pyrylium rings and charge-compensating phenoxides. Those four phenolic resins were prepared via a fast HCl-catalyzed process as low-density nanostructured solids classified as aerogels, which, owing to their open porosity, allowed air circulation through their bulk. In that regard, the first step of this study was the air-oxidation of those phenolic resin aerogels at 240 °C. In FPOL and TPOL aerogels, that air-oxidation step kicked off a cascade of reactions leading to ring-fusion aromatization and formation of pyrylium O+-heteroaromatic rings in every repeat unit of the polymeric backbone. Despite the complexity of the process, those structural forms were well-defined, and were retained through pyrolytic carbonization (800 °C). Under the same conditions (240 °C/air), RF and PF aerogels did not undergo aromatization; instead, they just went through an autooxidation-like process that converted the -CH 2- bridges between phenolic moieties into carbonyls (CO). Importantly, however, upon further stepwise pyrolysis under Ar, by 600 °C all four systems (TPOL, FPOL, RF and PF), irrespective of whether they had been previously oxidized or not, converged to a common chemical composition. Thereby, carbon produced by pyrolysis of phenolic resins at 800 °C always contains fused pyrylium rings. All chemical analysis relied on FTIR, solid-state 13C NMR, XPS and CHN analysis. The only and significant difference made by the low-temperature (240 °C) air-oxidation step was identified with the surface areas of carbons from aromatizable systems (TPOL and FPOL), which were higher than those from direct pyrolysis of as-prepared aerogels. Upon further reactive etching with CO 2, those surface areas went as high as 2778 ± 209 m2 g-1. Those findings are directly relevant to high surface area carbons for gas sorption (e.g., capture and sequestration of CO2) and ion exchange materials.




The authors thank the Army Research Office for financial support under Award Number W911NF-14-1-0369.

Keywords and Phrases

Aerogels; Carbon; Carbon dioxide; Carbonization; Chemical analysis; Cracking (chemical); Formaldehyde; Ion exchange; Microporosity; O rings; Oxidation; Phenolic resins; Phenols; Resins; Synthetic resins; Temperature, Chemical compositions; Chemical transformations; Ion-exchange materials; Micro-porous carbons; Phenol formaldehyde; Pyrolytic conversion; Resorcinol formaldehydes; Sequestration of CO2, Pyrolysis

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Publication Date

01 Nov 2017

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