Acid catalyzed carbohydrate degradation and dehydration
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Facile commercial production of versatile polyfunctional compounds from biomass constitutes a great challenge for establishing a sustainable chemical industry. One such example is the production of furfural and hydroxymethyl furfural via dehydration of pentoses and hexoses. Identified as primary building blocks in polymer industry, their massive production is highly desired, yet suffers from several problems, such as feedstock availability, low product yields due to excessive side reactions and lack of an industrially feasible heterogeneous catalyst.
Organic acid functional groups incorporated onto mesoporous silica offer well defined catalytic sites beside their unique textural properties and therefore could be considered as promising catalysts. However, a rational approach for fine tuning of the catalyst to meet the reaction system requirements entails detailed understanding of the nature of the catalytic sites in condensed phase under similar conditions mimicking the reaction environment. For the characterization in condensed phase, a methodology was developed using potentiometric titration, and the acidic strength and total acid capacity of the organic acid functionalized materials were determined. Organic acid moieties of different strength were able to display their own acidity without being leveled in water, strongest being arene sulfonic group followed by propyl sulfonic, ethyl phosphonic and butyl carboxylic.
When compared to literature, some discrepancy was noticed about the acidic strength of propyl sulfonic and arene sulfonic groups. Because most of these studies were focused on examining the interaction of the acidic group with a gas phase probe molecule, the effect of solvation was neglected. The effect of solvation on the acidic strength of these moieties was investigated via quantum chemical simulations. A change in the acidic strength trend was observed with the increasing number of water molecules, indicating that one-to-one interaction in the gas phase does not necessarily represent the interaction of the moiety with the solvent molecules.
The difference in the acidic strength for these organic acid groups incorporated into mesoporous silica was not observed when they were tested for their activity on hexose and pentose dehydration due to poor hydrothermal stability of the materials at elevated temperatures. Doping of sulfated zirconia onto mesoporous silica materials was another alternative due to their high activity in cellobiose hydrolysis, but these materials did not provide hydrothermal stability either.
Monosaccharide decomposition parameters with mesoporous silica materials could not be thoroughly validated with previously reported data due to the lack of systematic studies in literature. A systematic study with mineral and organic homogeneous acids of varying strength built the platform for catalyst comparison and revealed that different mechanisms were dominating for glucose decomposition in the presence of weak acids according to the pH value of the solution. Although lower acid concentration leads to higher selectivity toward HMF, this could not be considered as an industrially viable solution.
Alternatively, addition of alkaline earth metals and appliance of pressure in the presence of acid catalyst activated the glucose ring and resulted in high HMF yields. Further enhancement was obtained by addition of an organic phase for HMF extraction. This process also allows for combining it with polysaccharides hydrolysis and one pot HMF production from biomass. By further optimization of the parameters, an industrially feasible process for HMF production can be achieved.