Knowledge Management System Of Institute of process engineering,CAS
|关键词||水热预处理 乙酰基团 木质纤维素酶解 能量的投入与产出比率 结构表征参数|
由于天然的生物质本身组分的多样性以及结构的复杂性，使其在生物转化过程中难以被利用。为此，研究者们发明了众多的预处理技术，通过一定程度上破坏生物质的结构，以提高生物转化效率。其中，水热预处理技术因其效率高，对环境友好，处理原料广泛等优点而引起了广大科研人员的关注。除了复杂的化学组成，一些结构特性也是影响木质纤维素等生物质直接利用的重要因素，例如结合在半纤维素骨架上的乙酰基团就是重要的结构参数之一，乙酰化这一特性结构可以对酶解产生阻碍作用，但同时可以增强预处理效果，而过度的酸性环境又会使单糖进一步降解，产生对后续利用不利的抑制物，尤其是在水热预处理过程中，乙酰基团被认为是影响木质纤维素等生物质自体水解的最为关键的因素。本论文将通过以下研究揭示乙酰基团、水热预处理和生物质生物转化之间的关系。首先，以不同乙酰化程度的玉米秸秆为原料，探究乙酰基团对木质纤维素酶解的影响。与原料相比，经过较温和的脱乙酰化和加乙酰化处理，玉米秸秆的化学组成和结晶度没有发生明显的变化；通过红外光谱分析进一步发现，除了乙酰基团的改变外，其它结构特征也没有明显变化。将不同乙酰化样品进行酶解试验，发现乙酰化程度为0.34%的样品酶解率可以达到39.99%，而乙酰化程度为11.20%的样品的酶解率只有6.2%，进一步将酶解和乙酰化程度之间的关系进行非线性拟合，结果证明了乙酰化程度与酶解效果之间呈负相关性：乙酰化程度越高，酶解效率越低；反之，脱乙酰化程度越高，酶解效率则增强。尽管脱乙酰化可以一定程度上提高木质纤维素的酶解率，但效果还有待强化，实现木质纤维素高效率的生物转化往往还需要预处理的辅助，而水热预处理过程中过高的乙酰化程度会削弱半纤维素的回收率，同时带来不利的影响，因此采用适宜的乙酰化程度与水热预处理结合可以避免这些问题的同时，提高木质纤维素的利用率。为此，本研究发明了脱乙酰化耦合水热预处理的新型预处理工艺，结果表明通过这种新型预处理方式处理后的玉米秸秆，半纤维素的回收率得到了明显的改善，同时脱乙酰化和水热预处理都会对木质纤维素的酶解产生积极的影响。通过计算能量的投入与产出比，发现脱乙酰化可以有效的提高能量的投入与产出比率，也就是说经过一定程度脱乙化的样品，只需在较低强度的水热条件下就可以取得较高的酶解转化率。相比之下，玉米秸秆的脱乙酰化程度为84.96%，PS为3.97时，酶解转化率可以达到 81.57 ± 0.97%，同时能量的投入与产出比达到最大为50.39 g glucose kJ -1。最后，测定与观察了不同脱乙酰化程度的玉米秸秆水热预处理前后的结构特性和表面超微结构的变化，包括纤维素结晶度、纤维素聚合度和比表面积以及电镜扫描等。结果表明水热预处理可以增加结晶度和比表面积、减小聚合度，而脱乙酰化可以减小玉米秸秆的聚合度和比表面积。通过将样品的主要结构参数与酶解转化建立关联，发现相比于结晶度，聚合度和比表面积在木质纤维素的酶解过程中更起到主导作用。扫描电镜表明水热预处理可以破坏玉米秸秆的表面结构，随着预处理强度的提高，其破坏程度就越大，同时脱乙酰化可以有效的阻止假木质素液滴的形成，而假木质素被认为对酶解有着负面的影响。这些结果表明，脱乙酰化与水热预处理耦合的预处理技术是具有可行性的。通过以上的研究，进一步完善了木质纤维素结构特性-乙酰基团对木质纤维素酶解转化的影响、乙酰基团对水热预处理过程自体水解的影响以及木质纤维素的降解机制；同时，为从上游出发开发适宜的乙酰化基团作物、提高和开发更合理有效的预处理技术以及对酶解和发酵的进一步放大利用提供了重要的参考数据，从而有利于的推动木质纤维素等生物质的生物产业化。
Due to the complicated structure and diversity of component, the natural lignocellulose is difficult to utilize via bioconversion. Researchers have invented many kinds of pretreatment technologies to improve the bioconversion efficiency through breaking the structure of lignocellulose. In recent years, liquid hot water pretreatment has attracted the attention of many researchers due to its unique advantages such as high efficiency, no additive chemicals requirement and a wide range of application. Besides complicated chemical composition, some structural characteristics can also have important effect on the utilization of lignocellulose. For example, acetyl group bound to hemicellulose can negatively affect the lignocellulose degradation. However, acetyl group can reinforce pretreatment efficacy. In liquid hot water pretreatment, acetyl group is considered as the most important factor affecting autohydrolysis of lignocellulose. The aim of the present study is to reveal the relationship among acetyl group, liquid hot water pretreatment and lignocellulose bioconversation. Corn stalk with different degree of deacetylation was used as the feedstock. The effect of degree of acetylation on the enzymatic hydrolysis of lignocellulose was explored. Compared with raw corn stalk, , there was no significant change in chemical composition and crystallinity through relatively mild deacetylation and acetylation pretreatment. FTIR showed that other structural characteristics except acetyl group were not altered obviously. The enzymatic hydrolysis was carried out on corn stalk with different degree of deacetylation. It was obtained that the corn stalk with the degree of acetylation at 0.34% gave the enzymatic yield of 39.99%, while the corn stalk with the degree of acetylation at 11.20% gave the enzymatic yield at 6.2%. Subsequently, the enzymatic yield at 17 h, 24 h were selected to be over fitted with degree of acetylation. The fitted results reveled that there was a negative correlation between enzymatic yield and degree of acetylation. In other words, the enzymatic yield can be improved as the degree of deacetylation dropped. Although deacetylation can in certain degree enhance enzymatic yield of lignocellulose, it still needs to be enhanced with the assistance of pretreatment technology. During liquid hot water pretreatment, exorbitant degree of acetylation can reduce the hemicellulose recovery and bring unfavorable influence. Therefore, the combination of the optimal degree of deacetylation with liquid hot water pretreatment can not only avoid these problems, but also improve the utilization of lignocellulose. In the present study, a novel stepwise liquid hot water pretreatment combined with deacetylation was adopted to pretreat corn stalk. The results indicate that deacetylation was conductive to retain hemicellulose during liquid hot water pretreatment. Both deacetylation and liquid hot water pretreatment can bring positive influence on enzymatic hydrolysis yield. Through analyzing energy input-output, it can be observed that deacetylation can improve the ratio of energy output and input. It means that the samples after deacetylation only need to be pretreated by liquid hot water pretreatment with low pretreatment severity to gain a higher enzymatic hydrolysis yield. At the optimal condition of 84.96% degree of deacetylation and pretreatment severity of 3.97, the maximum ratio of glucose yield and energy input at 50.39 g glucose/KJ was obtained and the enzymatic hydrolysis yield was 81.57 ± 0.97%. The structural characteristics and morphology of corn stalk with different degree of deacetylation before/after liquid hot water pretreatment were determined and observed in terms of crystalline index, specific surface area, degree of polymerization, IF-IR and SEM. Liquid hot water pretreatment can not only increase the crystalline index and specific surface area, but also reduce degree of polymerization. SEM analysis showed that the morphology of corn stalk was destroyed through liquid hot water pretreatment. The pretreatment severity of liquid hot water pretreatment was higher, the destructive degree was more intensive. In addition, deacetylation can effectively prevent the formation of pseudo-lignin. Pseudo-lignin was considered to have negative influence on enzymatic hydrolysis. These results illustrate that the novel stepwise pretreatment by liquid hot water pretreatment combined with deacetylation was feasible. The results obtained from the present study can further enrich the fundamental theory of the effects of acetyl group on enzymatic hydrolysis of lignocellulose, lignocellulose autohydrolysis during liquid hot water pretreatment and degradation mechanism of lignocellulose. Meanwhile, it can also provide the important reference for developing more reasonable and efficient pretreatment technologies and thus contribute to promoting the industrialization of biomass.
|姜巍. 生物质乙酰化程度在水热预处理过程中对自体水解的影响[D]. 北京. 中国科学院研究生院,2016.|