A concept for a highly integrated fractionation of lignocellulose in its main components (cellulose-pulp, soluble hemicellulose sugars and lignin) is described, based on the selective catalytic depolymerization of hemicellulose in a biphasic solvent system. This leads to an effective disentanglement of the compact lignocellulose structure, liberating and separating the main components in a single step. At mild temperatures (80–140 °C), oxalic acid catalyzes selectively the depolymerization of hemicellulose to soluble sugars in aqueous solution, whereas the more crystalline cellulose-pulp remains solid and inaccessible to the acid catalysis. In the presence of a second organic phase consisting of bio-based 2-methyltetrahydrofuran (2-MTHF), lignin is directly separated from the pulp and the soluble carbohydrates by in situ extraction. The oxalic acid catalyst can be crystallized from the aqueous solution, recovered and re-used. The delignified cellulose-pulp obtained from this biphasic system can be directly subjected to enzymatic depolymerization, affording soluble oligomers and glucose at rates almost comparable to those observed for the hydrolysis of commercial microcrystalline Avicel^®. Overall, the concept may offer a promising approach for an efficient and selective pre-treatment of lignocellulosic materials under mild and environmentally-friendly conditions

BackgroundIn the last years, the biotechnological production of platform chemicals for fuel components has become a major focus of interest. Although ligno-cellulosic material is considered as suitable feedstock, the almost inevitable pretreatment of this recalcitrant material may interfere with the subsequent fermentation steps. In this study, the fungus Ustilago maydis was used to produce itaconic acid as platform chemical for the synthesis of potential biofuels such as 3-methyltetrahydrofuran. No studies, however, have investigated how pretreatment of ligno-cellulosic biomass precisely influences the subsequent fermentation by U. maydis. Thus, this current study aims to first characterize U. maydis in shake flasks and then to evaluate the influence of three exemplary pretreatment methods on the cultivation and itaconic acid production of this fungus. Cellulose enzymatically hydrolysed in seawater and salt-assisted organic-acid catalysed cellulose were investigated as substrates. Lastly, hydrolysed hemicellulose from fractionated beech wood was applied as substrate.ResultsU. maydis was characterized on shake flask level regarding its itaconic acid production on glucose. Nitrogen limitation was shown to be a crucial condition for the production of itaconic acid. For itaconic acid concentrations above 25 g/L, a significant product inhibition was observed. Performing experiments that simulated influences of possible pretreatment methods, U. maydis was only slightly affected by high osmolarities up to 3.5 osmol/L as well as of 0.1 M oxalic acid. The production of itaconic acid was achieved on pretreated cellulose in seawater and on the hydrolysed hemicellulosic fraction of pretreated beech wood.ConclusionThe fungus U. maydis is a promising producer of itaconic acid, since it grows as single cells (yeast-like) in submerged cultivations and it is extremely robust in high osmotic media and real seawater. Moreover, U. maydis can grow on the hemicellulosic fraction of pretreated beech wood. Thereby, this fungus combines important advantages of yeasts and filamentous fungi. Nevertheless, the biomass pretreatment does indeed affect the subsequent itaconic acid production. Although U. maydis is insusceptible to most possible impurities from pretreatment, high amounts of salts or residues of organic acids can slow microbial growth and decrease the production. Consequently, the pretreatment step needs to fit the prerequisites defined by the actual microorganisms applied for fermentation.

Dicarboxylic acids (e. g. oxalic and maleic acid) are able to depolymerize cellulose, producing oligomers and glucose. However, to reach efficient organic-acid-catalyzed performances with crystalline celluloses, high temperatures (> 160 degrees C) are needed. These energetically-demanding conditions lead to undesired sugar degradation as well. Herein it is shown that organic acid-catalyzed cellulose depolymerization can proceed efficiently in water under mild reaction conditions (100-125 degrees C) by the addition of inexpensive NaCl (30 wt%). The application of some pressure in the reactor (10-30 bar) also influences and improves the depolymerization outcome. It is believed that the salt solutions act in a mechanism similar to ionic liquids and disrupt the hydrogen-bond matrix among cellulose fibers. Depolymerization proceeds efficiently with amorphous cellulose, alpha-cellulose, as well as with microcrystalline cellulose (Avicel (R)). Importantly, catalysis can be easily controlled by temperature, catalyst loading and salt concentrations, as well as by the applied pressure in the reactor, and thus sugar degradation can be diminished. Furthermore, experiments conducted using concentrated seawater as solvent and maleic acid as catalyst showed positive results in the hydrolysis of Avicel (R)

The fractionation of lignocellulose in its three main components, hemicellulose, lignin and cellulose pulp can be achieved in a biphasic system comprising water and bio-based 2-methyltetrahydrofuran (2-MeTHF) as solvents and oxalic acid as catalyst at mild temperatures (up to 140°C). This so-called Organo-Cat concept relies on selective hemicellulose depolymerization to form an aqueous stream of the corresponding carbohydrates, whereas solid cellulose pulp remains suspended and the disentangled lignin is to a large extent extracted in situ with the organic phase. In the present paper, it is demonstrated that biomass loadings of 100 g L −1 can be efficiently fractionated within 3 h whereby the mild conditions assure that no significant amounts of by-products (e.g. furans) are formed. Removing the solid pulp by filtration allows to re-use the water and organic phase without product separation in repetitive batch mode. In this way, (at least) 400 g L −1 biomass can be processed in 4 cycles, leading to greatly improved biomass-to-catalyst and biomass-to-solvent ratios. Economic analysis of the process reveals that the improved biomass loading significantly reduces capital and energy costs in the solvent recycle, indicating the importance of process integration for potential implementation. The procedure was successfully scaled-up from the screening on bench scale to 3 L reactor. The feedstock flexibility was assessed for biomasses containing moderate-to-high hemicellulose content.