铜基水滑石催化剂制备及氢解高浓度纤维素性能

发布时间：2018-10-05 18:20

【摘要】：由于石油、煤炭等化石燃料资源的紧缺,以及日益加重的温室效应,探索和发展可再生能源的利用迫在眉睫。在对众多非常规能源的探索中,生物质转化不仅可以缓解能源短缺带来的瓶颈,同时也可以减轻化石资源的利用所引起的环境问题。而纤维素作为一种广泛存在的生物质资源,其可以转化为多种化学品原料,具有很高的利用价值。其中,纤维素通过氢解获得的一系列低碳多元醇类化合物,如乙二醇(EG)、丙二醇(PD)等,是工业上合成聚酯、纤维等聚合物的重要原料,应用范围十分广泛。因此,纤维素氢解制备低碳多元醇是一条十分有潜质的生物质转化路线。采用碱性的水滑石材料作为前体,制备了一系列Cu基催化剂,并应用于高浓度的纤维素氢解反应。通过共沉淀法合成不同Cu/Mg/Al摩尔比的水滑石前体,通过焙烧制备不同金属含量的催化剂,系统的研究金属配比、焙烧和还原条件对催化剂晶相结构以及物理化学性质上的影响。采用CO2-TPD以及N20化学吸附分别对催化剂的碱性以及活性位含量进行测定,结果显示,Mg含量的降低会严重导致催化剂碱性下降,而Cu的含量不仅可以控制碱性,同时影响催化剂表面的活性位含量。结合纤维素氢解反应结果发现,碱性的增强有利于纤维素的转化,同时促进C-C键断裂,而Cu含量的提高则会对产物中EG和1,2-PD具有较好的选择性。因此,可以通过选择性的控制催化剂的碱性以及活性位的含量,以此优化纤维素的转化率以及多元醇的收率。此外,通过控制水滑石前体的焙烧温度,获得不同晶相结构的催化剂。讨论焙烧温度引起的催化剂结构变化,同时探究了催化剂结构的转变对其碱性和表面活性位含量的影响。揭示出催化剂在晶相转变的过程中,结构中的O2-存在形式是影响其碱性强弱的主要因素。采用N20化学吸附表征发现不同结构的Cu物种显著影响催化剂的活性表面的含量。在高浓度纤维素氢解反应中,揭示出催化剂的碱性位与活性表面在催化反应中的协同作用。结果显示催化剂的碱性不仅促进纤维素的水解,而且促进反应中逆羟醛缩合过程的进行,从而有利于低碳多元醇的获得,而催化剂表面的活性Cu是氢解反应的主要活性位,其不仅可以促进纤维素水解产物的进一步氢解,同时也可以影响氢解反应速率来控制产物分布。同时,在此基础上,进一步优化了氢解反应的操作条件,分别考察了反应温度、反应压力以及纤维素浓度的影响。值得注意的是,在高达18 wt%浓度下的纤维素,反应没有发生任何结焦现象,表明了催化剂具有很强的氢解高浓度纤维素的能力。
[Abstract]:Because of the shortage of fossil fuel resources such as oil, coal, and the increasing Greenhouse Effect, it is urgent to explore and develop renewable energy. In the exploration of many unconventional energy sources, biomass conversion can not only alleviate the bottleneck caused by the shortage of energy, but also alleviate the environmental problems caused by the utilization of fossil resources. Cellulose, as a widely existing biomass resource, can be converted into a variety of chemical raw materials and has high utilization value. Among them, a series of low carbon polyols obtained by hydrogenolysis of cellulose, such as ethylene glycol (EG), propylene glycol (PD), are important raw materials for the industrial synthesis of polyester, fiber and other polymers. Therefore, the preparation of low-carbon polyols by hydrogenolysis of cellulose is a potential biomass conversion route. A series of Cu based catalysts were prepared by using basic hydrotalcite as precursor and applied to the hydrogenolysis of cellulose at high concentration. The precursors of hydrotalcite with different Cu/Mg/Al molar ratio were synthesized by coprecipitation method. Catalysts with different metal contents were prepared by calcination. The effects of metal ratio, calcination and reduction conditions on the crystal structure and physicochemical properties of the catalysts were systematically studied. The alkalinity and active site content of the catalyst were determined by CO2-TPD and N20 chemisorption, respectively. The results showed that the decrease of mg content would lead to the decrease of the alkalinity of the catalyst, while the content of Cu could not only control the alkalinity. At the same time, the active site content on the catalyst surface was affected. Combined with the hydrogenolysis of cellulose, it was found that the enhancement of alkalinity was beneficial to the conversion of cellulose and the cleavage of C-C bond, while the increase of Cu content had a good selectivity for EG and 1b2-PD in the product. Therefore, the conversion of cellulose and the yield of polyol can be optimized by selectively controlling the alkalinity and active site content of the catalyst. In addition, catalysts with different crystal structure were obtained by controlling the calcination temperature of hydrotalcite precursors. The change of catalyst structure caused by calcination temperature was discussed, and the influence of the change of catalyst structure on its alkalinity and surface active site content was also discussed. It is revealed that the existence of O _ 2-in the structure is the main factor affecting the alkalinity of the catalyst during the process of crystal phase transition. N20 chemisorption characterization showed that Cu species with different structures significantly affected the active surface content of the catalyst. In the hydrogenolysis of high concentration cellulose, the synergism between the basic potential of the catalyst and the active surface in the catalytic reaction was revealed. The results showed that the alkalinity of the catalyst not only promoted the hydrolysis of cellulose, but also promoted the reverse condensation of hydroxyaldehydes in the reaction, which was beneficial to the obtaining of low carbon polyols. The active Cu on the surface of the catalyst was the main active site in the hydrogenolysis reaction. It can not only promote the further hydrogenation of cellulose hydrolysates, but also influence the reaction rate to control the distribution of the products. At the same time, the operation conditions of hydrogenolysis reaction were optimized, and the effects of reaction temperature, reaction pressure and cellulose concentration were investigated. It is worth noting that at the concentration of up to 18 wt%, there is no coking in the reaction, which indicates that the catalyst has a strong ability of hydrogenation of high concentration cellulose.
【学位授予单位】：大连理工大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：O643.36

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