Facing the challenge of lignin valorization is one of the unsolved key steps for a sustainable and economically feasible biorefinery. Several processes were developed with the aim of producing value-added compounds from lignin. Thermal, enzymatic, and catalytic processes represent common techniques for lignin valorization. However, expensive catalysts or enzymes and harsh conditions hampered the implementation of these methodologies on an industrial scale. Here, we propose the utilization of a simple "swiss-roll" electrochemical reactor for the production of valuable carboxylic acids. We showcase that production of phenolic compounds, such as vanillin, is hindered by the electrochemical mechanism. Additionally, electrochemical stability experiments of possible products showed high reactivity of vanillin against the low reactivity of mono-and dicarboxylic acids. Simultaneously, the electrochemical process leads to stable carboxylic acids with high yields of 6.4, 26.8, and 4.2 % for oxalic, formic, and acetic acids, respectively, thus representing a competitive alternative to the catalytic and hydrothermal degradation process for the production of carboxylic acids.
Valorization of lignin into value-added chemicals becomes an interesting research topic for sustainable and renewable product formation. A conceptual study of lignin valorization through an electrochemical membrane reactor process is presented. The membrane reactor model is able to predict a larger aromatic product formation yield because the product is permeating out before it degrades. Different process parameters were examined, in an electrochemical membrane reactor process, via sensitivity analysis. Through the analysis, the objective was to manipulate the process parameters to maximize the aromatics production yield. These parameters are considering the membrane characteristics, in terms of membrane pore diameter and area, and process parameters, in terms of trans-membrane pressure and reaction residence time. The sensitivity study revealed that a membrane with a molecular weight cutoff (MWCO) of 750 Da and pore diameter of 1 nm is an optimum nanofiltration membrane for a continuous tubular membrane reactor. The aromatic product yield could be increased from 0.01%, in the batch reactor, to 11%, in the membrane reactor. Diluted lignin concentration was found to be a process drawback with regard to the product recovery from the permeate.
BackgroundPoly(γ-glutamic acid) (γ-PGA) is a biopolymer with many useful properties making it applicable for instance in food and skin care industries, in wastewater treatment, in biodegradable plastics or in the pharmaceutical industry. γ-PGA is usually produced microbially by different Bacillus spp. The produced γ-PGA increases the viscosity of the fermentation broth. In case of shake flask fermentations, this results in an increase of the volumetric power input. The power input in shake flasks can be determined by measuring the torque of an orbitally rotating lab shaker. The online measurement of the volumetric power input enables to continuously monitor the formation or degradation of viscous products like γ-PGA. Combined with the online measurement of the oxygen transfer rate (OTR), the respiration activity of the organisms can be observed at the same time.ResultsTwo different Bacillus licheniformis strains and three medium compositions were investigated using online volumetric power input and OTR measurements as well as thorough offline analysis. The online volumetric power input measurement clearly depicted changes in γ-PGA formation due to different medium compositions as well as differences in the production behavior of the two investigated strains. A higher citric acid concentration and the addition of trace elements to the standard medium showed a positive influence on γ-PGA production. The online power input signal was used to derive an online viscosity signal which was validated with offline determined viscosity values. The online measurement of the OTR proved to be a valuable tool to follow the respiration activity of the cultivated strains and to determine its reproducibility under different cultivation conditions.ConclusionsThe combination of the volumetric power input and the OTR allows for an easy and reliable investigation of new strains, cultivation conditions and medium compositions for their potential in γ-PGA production. The power input signal and the derived online viscosity directly reflect changes in γ-PGA molecular weight and concentration, respectively, due to different cultivation conditions or production strains.Electronic supplementary materialThe online version of this article (doi:10.1186/s13036-017-0065-4) contains supplementary material, which is available to authorized users.
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