(568b) Processing of Glucose-Derived Humins

Biomass offers the potential to serve as a sustainable source of chemicals and fuels that could replace those derived from non-renewable feedstocks. Cellulosic biomass conversion rates are greater and catalyst costs are lower when acids are used as the catalyst, but the selectivity falls far below that of enzyme catalysts. In particular, undesired solids, commonly referred to as humins, are produced in the acid-catalyzed conversion of cellulose and carbohydrate intermediates formed from cellulose. Infrared spectroscopy suggests that humins form via aldol addition/condensation involving 2,5-dioxo-6-hydroxy-hexanal (DHH) which in turn forms from HMF. DHH is highly reactive and undergoes aldol addition/condensation with available aldehydes and ketones present at reaction conditions. In the present work, humins have been investigated from the perspective of modifying their structure and chemical composition with the long term objective of converting them from wastes into value-added products.

At the end of a typical acid-catalyzed dehydration of glucose or fructose, two types of humins are found. The first type of humins are small dispersed particles that remain in suspension, even after centrifugation. Over a period of days these materials will aggregate and settle to form a sediment, but if the system is agitated they will again disperse into suspension. The second type of humins are present at the end of the conversion process in the form of precipitated solids.

The second type of humins can be partially dispersed in a variety of different solvents including aqueous bases and acetone. As much as 75% of fructose-derived humins and 65% of glucose-derived humins can be dispersed in 1 M NaOH. The dispersed fraction of the humins form a stable suspension that does not settle over periods as long as a month. The average size of the particles constituting the dispersed humins is of the order of 190 nm. The dispersed fraction of the humins remains in suspension at pH above 2, but below that pH they begin to precipitate.

Electron microscopy indicates that the second type of humins, prior to dispersion, consist of spherical cores surrounded by some form of shell. Agglomeration appears to involve fusion of the shell component into larger aggregates. Examination of the fraction of the humins that disperses shows that the shell component is no longer present. It apparently reacts with or dissolves in the solvent, leaving the smaller core structures in suspension.

The two component structure is also supported by other experimental results. The IR spectrum of the dispersed fraction of the humins differs from the part that would not disperse, particularly in regions of the spectrum characteristic of carbonyl, ether, ester and alcohol functionalities. Similarly, temperature programmed pyrolysis and temperature programmed oxidation of the two fractions of the humins reveals that they have different reactivities.

The average humin particle size, size distribution and morphology also can be affected by the solvent, acid catalyst and aldehydes or ketones added to the system during acid processing. For example, addition of formaldehyde leads to spherical humin particles with a relatively small diameter and narrow size distribution. Dispersion followed by re-precipitation can also change the size, morphology and size distribution of humins.

Finally, it is possible to add chemical functionality to humins through aldol chemistry. Characterization of humins recovered after acid-catalyzed processing of cellulose carbohydrates indicates the strong presence of carbonyl groups. Separate treatment of these humins with aldehydes or ketones leads to the addition of those reagents to the humins. This represents a means of imparting specific chemical functionality to the humins. Combined with methods for controlling and/or modifying the physical characteristics of humins, these results may offer a way of valorizing humins.