纤维素在超临界有机溶剂中液化转化为酮类化合物的机理与选择性研究

One of the great challenges is searching for alternative renewable resources to produce liquid fuels and chemicals. Cellulosic biomass is a primary candidate as it is the only renewable source containing C and H. Currently, there is a growing interest in supercritical organic solvents to the production of fuels and chemicals from cellulose liquefaction, because supercritical fluids have the potential for producing liquid oil and chemicals including ketones, esters, acids and other organic compounds. One of the major components in liquid products is ketones which is a kind of important platform chemicals with high reactivity and widespread application in many fields. The recent research results on liquefaction of cellulose in supercritical organic solvents show that huge potential be it the enhancement in the yield and the selectivity of producing ketones can be achieved. Many researchers have investigated biomass liquefaction process and optimization. However, few literatures reported the reaction mechamism, reaction pathways, selectivity of ketones from cellulose liquefaction in supercritical free radicals. In this study, therefore, the cellulose liquefaction in supercritical organic solvents will be investigated to propose the reaction mechanism, reaction networks of ketones obtained from liquefaction products through the experiments and molecular simulation. The major research contents are as follows:.1.The effects of supercritical organic solvents (methanol, ethanol, acetone and cyclohexane) on the yield of ketones from cellulose liquefaction and ketones component distribution will be investigated, the activity of hydrogen free radicals(H•) and hydroxyl radicals(HO•) from different supercritical organic solvents on the cellulose into ketones will be analyzed, and then the mechanism of H• and HO• on cellulose conversion into ketones will be studied. .2.An experimental investigation on the yield and components distribution of ketones from biomass liquefaction under the different free radical scavenger will be carried out, then the selectivity and activity of intermediate free radicals (including alkyl radicals, keto-alcohol radicals, benzyl radicals,et al.) on the cellulose liquefaction into ketones will be researched. .3.The parameters of free radicals and intermediate free radicals including spatial structure parameter, Mullikens distribution, symmetry, vibration mode and its frequency of vibration will be optimized and corrected on the experimental data by the molecular simulation, and the relative activity of radicals will be investigated. The reaction pathways and selectivity of cellulose model converted to ketones will be determined by molecular dynamics computer simulation..4. Based on the structural-oriented lumping method, the free radicals reaction pathways, reaction mechanism and reaction networks model of ketones from cellulose liquefaction will be proposed according to results of above studies.

酮类化合物是生物质液化产物中最重要的平台化合物之一，超临界有机溶剂液化纤维素转化为酮类化合物具有产物收率高、选择性好等优点，但超临界有机溶剂如何促进纤维素转化为酮类化合物以及对酮类化合物的生成具有较高的选择性等机理还不甚明晰。为此，本课题采用试验与辅助分子模拟的研究方法，研究不同超临界有机溶剂对纤维素转化为酮类化合物的作用机理、选择性及反应网络，其主要内容：（1）研究超临界甲醇、乙醇、丙酮和环己烷等有机溶剂自由基和过渡态自由基对纤维素液化为酮类化合物的收率和组成分布的影响；（2）利用分子模拟计算不同超临界有机溶剂自由基和过渡态自由基的空间结构参数、热力学及动力学参数，结合试验数据研究不同自由基对酮类化合物形成的作用机理和选择性；（3）建立以自由基为反应基元的纤维素转化为酮类化合物的反应路径和反应网络模型。研究结果将为生物质定向转化为酮类等平台化合物的工艺开发、反应器优化设计提供理论依据。

酮类化合物是生物质液化产物中最重要的平台化合物之一，但不同有机溶剂对酮类的生成存在不同的选择性，且超临界有机溶剂如何促进纤维素转化为酮类的机理还不甚明晰。为此本课题针对不同超临界有机溶剂对纤维素转化为酮类化合物的作用机理、选择性及反应网络进行研究。其主要研究内容、结果及关键数据总结如下：.1、不同反应条件下纤维素在各超临界有机溶剂中液化的试验：以纤维素为原料，分别以甲醇、乙醇、丙酮、环己烷为溶剂进行液化试验，反应条件为200~330℃、0~120min、80~200mL。研究发现：（1）不同反应条件下纤维素最高转化率分别为91.26%、86.14%、83.10%和72.54%；（2）各溶剂对酮类化合物选择性影响大小的顺序：丙酮>乙醇>甲醇>环己烷。.2、自由基捕捉剂对纤维素在超临界乙醇中液化的影响试验：参照上述1反应条件，并分别以PBN、水杨酸、DPE、四氯化碳、TEMPO为自由基捕捉剂，对纤维素在超临界乙醇中进行液化试验。研究发现：纤维素在四种自由基捕捉剂作用下，生物油最大收率分别为48.35%、58.2%、39.81%、20.7%和57.98%，酮类最高含量分别为15.77%、22.3%、29.88%、32.28%和37.83%。.3、自由基对纤维素液化的作用机理分析：（1）随着浓度及活性的增加，四种有机溶剂产生的H•、HO•与纤维素中C-O-C，C-C以及C-H键等有效碰撞增多，促进纤维素裂解，中间产物进一步发生缩合、氧化等反应，形成酮类、醇类等化合物；（2）甲醇和乙醇产生的•CH3、•CH2CH3促进酯类的生成，而丙酮与环己烷产生的过渡态自由基可提高酮类组分；（3）纤维素液化过程中产生的过渡态自由基对其液化具有一定的抑制作用。.4、纤维素液化过程的分子模拟：基于纤维素模型物（左旋葡聚糖）进行分析，模拟了9条反应路径。结果表明9条路径的反应能垒分别为：284.40、317.75、343.60、、312.96、331.89、310.28、357.80和295.12 KJ/mol，得到了最优路径。.5、建立纤维素液化反应路径及反应动力学模型：（1）提出了5条纤维素裂解生成酮类、酯类、烃类等平台化合物的反应路径；（2）建立了5集总反应网络和动力学模型，并与试验数据吻合度较高。