Seidel Christoph-Maximilian, Brethauer Simone, Gyenge László, Rudolf von Rohr Philipp, Studer Michael H
1Institute of Process Engineering, ETH Zürich, Sonneggstrasse 3, 8092 Zurich, Switzerland.
2School of Agricultural, Forest and Food Science, Bern University of Applied Science, Länggasse 85, 3052 Zollikofen, Switzerland.
Biotechnol Biofuels. 2019 Feb 21;12:37. doi: 10.1186/s13068-019-1373-3. eCollection 2019.
Lignocellulosic biomass is considered as a potential source for sustainable biofuels. In the conversion process, a pretreatment step is necessary in order to overcome the biomass recalcitrance and allow for sufficient fermentable sugar yields in enzymatic hydrolysis. Steam explosion is a well known pretreatment method working without additional chemicals and allowing for efficient particle size reduction. However, it is not effective for the pretreatment of softwood and the harsh conditions necessary to achieve a highly digestible cellulose fraction lead to the partial degradation of the hemicellulosic sugars. Previous studies showed that the autohydrolysis pretreatreatment of softwood can benefit from the addition of 2-naphthol. This carbocation scavenger prevents lignin repolymerisation leading to an enhanced glucose yield in the subsequent enzymatic hydrolysis.
In order to prevent the degradation of the hemicellulose, we investigated in this study a two-stage 2-naphthol steam explosion pretreatment. In the first stage, spruce wood is pretreated at a severity which is optimal for the autocatalytic hydrolysis of the hemicellulose. The hydrolyzate containing the solubilized sugars is withdrawn from the reactor and the remaining solids are pretreated with different amounts of 2-naphthol in a second stage at a severity that allows for high glucose yields in enzymatic hydrolysis. The pretreated spruce was subjected to enzymatic hydrolysis and to simultaneous saccharification and fermentation (SSF). In the first stage, the maximal yield of hemicellulosic sugars was 47.5% at a pretreatment severity of log = 3.75 at 180 °C. In the second stage, a 2-naphthol dosage of 0.205 mol/mol lignin C-unit increased the ethanol yield in SSF with a cellulose loading of 1% using the whole second stage pretreatment slurry by 17% from 73.6% for the control without 2-naphthol to 90.4%. At a higher solid loading corresponding to 5% w/w cellulose, the yields decreased due to higher concentrations of residual 2-naphthol in the biomass and the pretreatment liquor, but also due to higher concentrations of potential inhibitors like HMF, furfural and acetic acid. Experiments with washed solids, vacuum filtered solids and the whole slurry showed that residual 2-naphthol can inhibit the fermentation as a single inhibitor but also synergistically together with HMF, furfural and acetic acid.
This work shows that a two-stage pretreatment greatly enhances the recovery of hemicellulosic sugars from spruce wood. The presence of 2-naphthol in the second pretreatment stage can enhance the ethanol yield in SSF of steam explosion pretreated softwood at low cellulose concentrations of 1% w/w. However, with higher solid loadings of 5% w/w cellulose, the ethanol yields were in general lower due to the solid effect and a synergistic inhibition of HMF, furfural, acetic acid with residual 2-naphthol. The concentration of residual 2-naphthol tolerated by the yeast decreased with increasing concentrations of HMF, furfural, and acetic acid.
木质纤维素生物质被视为可持续生物燃料的潜在来源。在转化过程中，需要一个预处理步骤来克服生物质的难降解性，并在酶水解中实现足够的可发酵糖产量。蒸汽爆破是一种众所周知的预处理方法，无需添加额外化学物质，且能有效减小颗粒尺寸。然而，它对针叶木的预处理效果不佳，实现高消化率纤维素部分所需的苛刻条件会导致半纤维素糖的部分降解。先前的研究表明，针叶木的自水解预处理可因添加2 - 萘酚而受益。这种碳正离子清除剂可防止木质素再聚合，从而在后续酶水解中提高葡萄糖产量。
为防止半纤维素降解，本研究考察了两阶段2 - 萘酚蒸汽爆破预处理。在第一阶段，云杉木在对半纤维素自催化水解最佳的强度下进行预处理。将含有溶解糖的水解产物从反应器中取出，剩余固体在第二阶段用不同量的2 - 萘酚在能实现酶水解中高葡萄糖产量的强度下进行预处理。对预处理后的云杉木进行酶水解以及同步糖化发酵（SSF）。在第一阶段，在180℃预处理强度log = 3.75时，半纤维素糖的最大产量为47.5%。在第二阶段，使用整个第二阶段预处理浆料，当纤维素负载量为1%时，2 - 萘酚用量为0.205 mol/mol木质素C单位，使SSF中的乙醇产量从无2 - 萘酚对照的73.6%提高了17%，达到90.4%。在对应5% w/w纤维素的较高固体负载量下，产量下降，这是由于生物质和预处理液中残留2 - 萘酚浓度较高，也由于潜在抑制剂如HMF、糠醛和乙酸的浓度较高。对洗涤后的固体、真空过滤后的固体和整个浆料进行的实验表明，残留的2 - 萘酚作为单一抑制剂可抑制发酵，也可与HMF、糠醛和乙酸协同抑制发酵。
这项工作表明两阶段预处理极大地提高了从云杉木中回收半纤维素糖的效率。在第二预处理阶段存在2 - 萘酚可在1% w/w的低纤维素浓度下提高蒸汽爆破预处理针叶木在SSF中的乙醇产量。然而，在5% w/w纤维素的较高固体负载量下，由于固体效应以及HMF、糠醛、乙酸与残留2 - 萘酚的协同抑制作用，乙醇产量总体较低。酵母对残留2 - 萘酚的耐受浓度随HMF、糠醛和乙酸浓度的增加而降低。
