甘油氧化的催化剂及反应机理研究

发布时间：2018-10-13 07:44

【摘要】：生物柴油是一种重要的可再生燃料,具有广阔的市场。而生物柴油生产过程中副产大量的甘油,目前甘油已经严重过剩。因此,选择性氧化甘油制备高附加值化学品的研究是催化领域的热点。据文献报道,Pt-基催化剂在甘油氧化反应过程中常常因为流失、团聚、表面氧化或者产物的强吸附而失活;而Au-基催化剂需要在碱性条件下才有活性。本论文的主要工作是通过对载体进行表面修饰或者添加助金属Co,以期改变活性中心Pt与载体之间的相互作用、分散度和电子特性,从而解决Pt-基催化剂在反应过程中的失活问题。本文首先以碳纳米纤维为载体,分别采用硝化和硫化方法对碳纳米纤维进行改性处理。研究发现:表面改性均能在载体表面引入缺陷位点,相比于羧基和羟基,含S官能团能够更有效的控制Pt粒子的尺寸和分布;高分散的Pt粒子有利于甘油的转化。同时,我们还考察了反应条件对甘油选择性氧化的影响:氧气分压对甘油选择性氧化影响较小,底物浓度对甘油酸选择性几乎没有影响,升高反应温度,在提高甘油转化率的同时会增加C-C键断裂产物。为了解决Pt-基催化剂在甘油氧化反应过程中的失活问题,论文进一步以还原氧化石墨烯(RGO)为载体,采用微波辅助还原方法,制备了单纯的Pt/RGO及双金属PtCo/RGO催化剂。实验发现:Co与Pt之间的强相互作用可以增加Pt的电子密度、提高催化剂的活性和甘油酸的选择性,并且一定程度上提高了 Pt的抗氧化能力,部分缓解了催化剂的失活问题。以尿素作为氮源,通过与碳纳米管混合,在N2气氛下焙烧,制备出了不同氮含量的氮掺杂碳纳米管。研究发现:N的加入使得载体具有富电性,N与Pt之间的强相互作用使得N-MWCNTs载体能够更好的固定Pt纳米颗粒。同时,N的给电子作用使得Pt纳米颗粒能够在甘油氧化的过程中避免被过度氧化从而失活,提高了催化剂在甘油氧化反应中的稳定性。通过直接热解碳纳米管和三聚氰胺的混合物,成功地在碳纳米管表面直接生长出氮杂石墨烯复合载体(NG-MWCNTs)。表征结果发现:这种复合载体具有更高的比表面积(173m2/g)和更大的平均孔径。Pt/NG-MWCNTs催化剂在选择性氧化反应中具有很高的催化活性和甘油酸选择性。随着氮含量的增加,载体比表面积下降,会影响Pt的分散性以及底物的吸附。通过水热法合成、高温碳化等过程制备了单分散的纳米空心炭球(HCS),采用乙二醇热回流法制备Pt/HCS催化剂。实验发现:改变苯酚和六亚甲基四胺的添加量可以控制纳米空心炭球的壁厚、比表面积和含氧量。载体具有高的比表面积和特殊的孔道结构,一定的含氧量有利于Pt粒子分散度的增加,提高催化剂的活性。Pt/HCS催化剂在甘油选择性氧化反应具有一定的稳定性。这部分工作的开展为设计新型碳载体,进一步解决Pt基催化剂失活问题提供了新的研究思路。
[Abstract]:Biodiesel is an important renewable fuel with a wide market. However, a large amount of glycerol is produced in the process of biodiesel production. Therefore, selective oxidation of glycerol to produce high-value chemicals is a hot spot in the field of catalysis. It has been reported that Pt- based catalysts are often deactivated due to loss, agglomeration, surface oxidation or strong adsorption of products during glycerol oxidation, while Au- based catalysts need to be activated under alkaline conditions. The main work of this thesis is to change the interaction, dispersion and electronic properties between the active center Pt and the carrier by modifying the surface of the carrier or adding the auxiliary metal Co,. Thus the deactivation of Pt- based catalyst in the reaction process is solved. In this paper, carbon nanofibers were treated by nitrification and vulcanization respectively. It is found that the surface modification can introduce defect sites on the surface of the carrier. Compared with carboxyl and hydroxyl groups, S-containing functional groups can effectively control the size and distribution of Pt particles, and highly dispersed Pt particles are conducive to glycerol transformation. At the same time, we also investigated the effect of reaction conditions on the selective oxidation of glycerol: the oxygen partial pressure had little effect on the selective oxidation of glycerol, the substrate concentration had little effect on the selectivity of glyceric acid, and the reaction temperature was raised. At the same time, the C-C bond breaking product will be increased with the increase of glycerol conversion. In order to solve the deactivation problem of Pt- based catalyst in glycerol oxidation reaction, the pure Pt/RGO and bimetallic PtCo/RGO catalysts were prepared by microwave assisted reduction with (RGO) as the carrier. It is found that the strong interaction between Co and Pt can increase the electron density of Pt, improve the activity of the catalyst and the selectivity of glyceric acid, and improve the oxidation resistance of Pt to some extent, and partly alleviate the deactivation of the catalyst. Nitrogen doped carbon nanotubes with different nitrogen contents were prepared by mixing urea with carbon nanotubes and roasting in N2 atmosphere. It is found that the addition of N makes the carrier electrically rich, and the strong interaction between N and Pt makes the N-MWCNTs carrier better immobilize Pt nanoparticles. At the same time, the electron transfer of N enables the Pt nanoparticles to avoid excessive oxidation and deactivation during glycerol oxidation, thus improving the stability of the catalyst in glycerol oxidation. By directly pyrolyzing the mixture of carbon nanotubes and melamine, the aza-graphene composite support (NG-MWCNTs) was successfully grown on the surface of carbon nanotubes. It was found that the composite support had higher specific surface area (173m2/g) and larger average pore size, and that the Pt/NG-MWCNTs catalyst had high catalytic activity and glyceric acid selectivity in the selective oxidation reaction. With the increase of nitrogen content, the specific surface area of the carrier decreases, which will affect the dispersion of Pt and the adsorption of substrate. Monodisperse hollow carbon spheres (HCS),) were prepared by hydrothermal synthesis and carbonation at high temperature. Pt/HCS catalysts were prepared by ethylene glycol reflux method. It is found that the wall thickness, specific surface area and oxygen content can be controlled by changing the content of phenol and hexamethylenetetramine. The support has high specific surface area and special pore structure. A certain oxygen content is beneficial to increase the dispersion of Pt particles and improve the activity of the catalyst. The Pt/HCS catalyst has a certain stability in the selective oxidation of glycerol. The development of this work provides a new research idea for the design of new carbon support and the further solution to the deactivation of Pt catalyst.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TQ645.5;O643.36

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