Glucose hydrochar consists of linked phenol, furan, arene, alkyl, and ketone structures revealed by advanced solid-state nuclear magnetic resonance

Shichen Yuan; Avery Brown; Zhaoxi Zheng; Robert L Johnson; Karen Agro; Andrea Kruse; Michael T Timko; Klaus Schmidt-Rohr

doi:10.1016/j.ssnmr.2024.101973

Glucose hydrochar consists of linked phenol, furan, arene, alkyl, and ketone structures revealed by advanced solid-state nuclear magnetic resonance

Solid State Nucl Magn Reson. 2024 Oct 10:134:101973. doi: 10.1016/j.ssnmr.2024.101973. Online ahead of print.

Authors

Shichen Yuan¹, Avery Brown², Zhaoxi Zheng¹, Robert L Johnson³, Karen Agro², Andrea Kruse⁴, Michael T Timko², Klaus Schmidt-Rohr⁵

Affiliations

¹ Brandeis University, Department of Chemistry, Waltham, MA, 02453, USA.
² Worcester Polytechnic Institute, Department of Chemical Engineering, 100 Institute Road, Worcester MA, 01609, USA.
³ Hawai'i Natural Energy Institute, University of Hawai'i at Manoa, Honolulu, HI 96822, USA.
⁴ University of Hohenheim, Institute of Agricultural Engineering, Department of Conversion Technologies of Biobased Resources, Garbenstrasse 9, 70599, Stuttgart, Germany.
⁵ Brandeis University, Department of Chemistry, Waltham, MA, 02453, USA. Electronic address: srohr@brandeis.edu.

PMID: 39413580
DOI: 10.1016/j.ssnmr.2024.101973

Abstract

The molecular structure of hydrochars produced from ¹³C-enriched glucose under various conditions has been elucidated based on advanced one- and two-dimensional (2D) ¹H-¹³C and ¹³C-¹³C solid-state nuclear magnetic resonance (NMR) with spectral editing. Regardless of synthesis conditions, hydrochars consist mostly of oxygen-substituted arene rings (including diphenols) and furans connected by alkyl linkers rich in ketones. Cross-linking nonprotonated and methyne (C-H) alkyl carbons have been identified through spectrally edited 2D NMR. Alkenes and 'quaternary' C-O are observed only at low synthesis temperature, while some clusters of fused arene rings are generated at high temperature. Hydrochar composition is nearly independent of reaction time in the range from 1 to 5 h. Equilibration of ¹³C magnetization within 1 s shows that the materials are homogeneous on the 5-nm scale, refuting core-shell models of hydrochar microspheres. While furan C-O carbons bonded to alkyl groups or ketones show distinctive cross peaks in 2D NMR, phenolic C-OH is observed unambiguously by hydroxyl-proton selection. While methylene-linked furan rings are fairly common, the signal previously assigned to furan Cα-Cα linkages is shown to arise from abundant, stable catecholic ortho-diphenols, whose HO-C=C-OH structure is proved by 2D¹³C-¹³C NMR after hydroxyl-proton selection. Quantitative ¹³C NMR spectra of low- and high-temperature hydrochars have been matched by chemical-shift simulations for representative structural models. Mixed phenol and furan rings connected by ketones and alkyl linkers provide good fits of the experimental spectra, while literature models dominated by large clusters of fused rings and with few phenols or alkyl-linked ketones do not.

Keywords: Catechols; Diphenols; Hydrochar models; Hydrochar structure; Hydrothermal carbon; Hydroxyl-proton selection; Quantitative solid-state (13)C NMR; Spectral editing.