Molecular structure and composition elucidation of an industrial humin and its fractions

Humins, (side-)products of the acid-catalysed dehydration of carbohydrates, will be produced in substan-
tial quantities with the development of industrial biorefining processes. Most structural knowledge about
such humins is based on synthetic model humins prepared at lab-scale from typical carbohydrate
(-derived) compounds. Here, we report the first extensive characterisation study of an industrial humin.
The soluble humin was generated from pilot plant-scale methanolic cyclodehydration of D-fructose to
5-methoxymethyl-2-furfural (MMF), as part of the Avantium YXY® process to produce FDCA. Purification
of the industrial humin followed by fractionation allowed isolation of a water-insoluble, high molecular
weight fraction (WIPIH) and a water-soluble, low-to-middle molecular weight soluble fraction (WES).
Characterisation by elemental analysis, thermogravimetry, IR and NMR spectroscopy and size exclusion
chromatography provided a detailed picture of the humin structure in both fractions. Aided by a compre-
hensive NMR spectral library of furanic model compounds, we identified the main furanic building blocks
and inter-unit linkages and propose a structure for this industrial humin sample. The WIPIH and WES frac-
tions were found to be composed of furanic rings interconnected by short aliphatic chains containing a
wide range of functionalities including alcohols, ethers, carboxylic acids, esters, aldehydes and ketones.
The low level of crosslinking and high functional group content of the industrial humin differ from the
more extensively studied, (highly over-)condensed synthetic model humins, towards which they can be
considered intermediates. The structural and compositional insights into the nature of an actual industrial
humin open up a broad spectrum of valorisation opportunities.