Abstract
During the acid-catalyzed dehydration of carbohydrates for the production of renewable bulk chemicals, such as furfural, hydroxymethylfurfural (HMF) and levulinic acid, large amounts of carbonaceous, insoluble by-products are typically formed by cross-polymerization reactions of HMF and several sugar-dehydration intermediates. The formation of these so-called humins leads to great efficiency losses
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in biorefinery operations. Although produced in abundance, the molecular structure of humins and their mechanism of formation are not yet unequivocally established. Detailed knowledge of both is required, however, to either prevent their formation or to find routes for the valorization of humins, which would both increase the economic feasibility of future biorefineries. The work described in this PhD thesis aims for a better understanding of the mechanism of formation and molecular structure of humin by-products and explores possible routes for the catalytic valorization of humins.
The formation of both humins and platform chemicals from different sugars and a raw feed was studied by a multi-parameter and multi-technique approach. The humins formed were extensively characterized using SEM, elemental analysis, solid-state NMR, IR and pyrolysis-GC-MS. This showed that the humins have a furan-rich, HMF- derived structure, which is formed via a dehydration pathway. The yield and molecular structure of the humins strongly depends on feed stock and processing conditions. The use of thick juice, a sucrose-rich intermediate form sugar beet refinery, as a feed led to high HMF selectivity and low humins yields compared to purified sucrose.
The catalytic valorization of humins is furthermore hampered by the general insolubility of humins, as it limits catalyst-substrate interaction. The solubility of humins was therefore improved by an alkaline pretreatment method and the effect of reaction conditions on average molecular weight of glucose-derived humins was monitored by GPC. Xylose- and fructose-derived humins were shown to be more recalcitrant to the alkaline pretreatment. Extensive characterization of the alkali-treated humins showed that an arene-rich structure is formed at the expense of furan content during alkaline pretreatment.
The molecular structure of 13C-labeled humins is studied in further detail by complementary 1D and 2D solid-state NMR studies. A combination of 1D 13C NMR, 2D 1H-13C and 2D 13C-13C NMR was applied using direct excitation and cross polarization techniques. This allows one to distinguish between furanic and phenolic rings, which led to a refinement of the molecular structure described before. Alkali-treated 13C-labeled humins were also analyzed by 1D 13C NMR and 2D 13C-13C NMR after direct excitation and cross polarization, giving additional insight in the chemical changes during alkaline treatment of humins.
Finally, an attempt to the valorization of humins to chemicals was made by aqueous phase reforming of alkali-treated humins over a supported Pt catalyst. Small amounts of gas, containing hydrogen, carbon dioxide and methane, and humin oil were formed. The humin oil mainly contained phenolic products. Humin oil derived from untreated humins contained more furan-derived products.
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