Production of nanolignin in an Organosolv wheat straw biorefinery

Abstract

Lignocellulosic agricultural side products like wheat straw are widely seen as an important contribution to a sustainable future economy. However, the optimization of biorefinery processes and the development of value-added products are crucial for an economically-viable biorefinery, with both arguments addressed in this work. Lignin, as one of the main constituents of lignocellulose, is underutilized and used as energy source. However, only around 40% of the produced lignin in a lignocellulose biorefinery with ethanol as main product is needed to cover its thermal energy demand. Hence, 60% of the generated lignin is available to maximize valorization in addition to the valorization of the carbohydrate fractions. The high complexity and inhomogeneity of the lignin structure is, however, challenging for its valorization. The conversion of the lignin into particles with micro- and nanosize improves the lignin properties and makes it a promising and potentially high value biorefinery product. A holistic approach to integrate the production of nanoscaled lignin particles in a lignocellulose biorefinery must not only consider the particular production step of the particles but also the extraction of the lignin during the pretreatment step and its monitoring. The approach to fulfil this includes three steps. The first step investigated an organosolv pretreatment by applying ethanol water mixtures at elevated temperatures to the raw material wheat straw. Moreover, the organosolv pretreatment step was combined with an extrusion pretreatment investigated in terms of extraction yields and improvement of a subsequent enzymatic hydrolysis of the cellulose containing solid residues. The highest lignin and enzymatic glucose yields were achieved at 220°C and 60 min treatment time within the organosolv pretreatment. The ratio of wheat straw to solvent showed only a minor influence on the pretreatment performance, which gives potential for the optimization of the solvent consumption. However, the combination of extrusion and subsequent organosolv pretreatment did not result in an improved performance of the process compared to a single organosolv pretreatment. The second step involved developing a potential analytical method to monitor the pretreatment process, since traditional wet chemistry methods are time-consuming and not suitable for on-line process monitoring. The monitoring, however, should allow to adapt the pretreatment process parameters for a desired outcome and an economically beneficial operation. The spectroscopic method, UV-Vis spectroscopy, in combination with a partial least squares regression was applied and successfully used for the prediction of the components lignin, acetic acid, HMF, and furfural in organosolv pretreatment extracts from wheat straw. Lignin showed a good prediction accuracy, with standard errors of prediction ranging from 0.23 to 0.80 mg/L in different data sets with a concentration range of 0.2 to 40 mg/L. Furthermore, the other components could be predicted with a sufficient accuracy for on-line process monitoring. The third step investigated the concept of direct precipitation of lignin nanoparticles from wheat straw organosolv extracts, which can potentially reduce solvent consumption drastically compared to other production methods. This method can produce particles with tailored average diameters ranging from 100 nm to 800 nm. Mixing antisolvent and extract is an important process parameter, which could be successfully influenced by changing the precipitation setup from a stirred vessel to a static mixer, resulting in a reduced particle size. The antisolvent pH value is an important process parameter: Applying antisolvents with a low pH value (≤ 2) resulted in agglomerated particles whereas using pure water as antisolvent results in a significantly reduced agglomeration. Furthermore, the carbohydrate impurities of the precipitated particles can be controlled by the precipitation parameters and reached values as low as in purified lignin particles.