Structural characterization of C-13-enriched humins and alkali-treated C-13 humins by 2D solid-state NMR

Abstract

Humin by-products are formed during the acid-catalyzed dehydration of carbohydrates to bio-based platform molecules, such as hydroxymethylfurfural and levulinic acid. The molecular structure of these humins has not yet been unequivocally established. 1D C-13 solid-state NMR data reported have, for example, provided considerable insight, but do not allow for the unambiguous assignment of key structural motifs. Complementary (2D) techniques are needed to gain additional insight into the molecular structure of humins. Here, the preparation of C-13-enriched humins is reported, together with the reactive solubilization of these labeled humins and their characterization with complementary 1D and 2D solid-state NMR techniques. 1D cross polarization (CP) and direct excitation (DE) C-13 solid-state NMR spectra, 2D C-13-detected double-quantum single-quantum (DQSQ) as well as 2D H-1-detected heteronuclear correlation (HETCOR) were recorded with different excitation schemes. These experiments unambiguously established that the original humins have a furan-rich structure with aliphatic linkers and allowed for a refinement of the molecular structure proposed previously. Solid-state NMR data of alkali-treated C-13-labeled humins showed that an arene-rich structure is formed at the expense of the furanic network during alkaline pretreatment.