微波耦合分级结构碳纤维床层对液相催化脱氢过程的强化机制

The process efficiency of strong endothermic dehydrogenation suffers seriously from high energy consumption. Adopting a reasonable reaction mode in conjunction with an effective heat-supply method is therefore crucial to the improvement of the process efficiency based on low energy consumption. Meanwhile, a catalytic bed is inevitably required to figure for tying in the energy supply with the given reaction. For accomplishing the purpose, a proposal has been put forward by taking the catalytic dehydrogenation of organic hydrides such as decline as the model reaction system. A hierarchical carbon fiber composite bed is developed in particular to couple with microwave irradiation for carrying out the organic hydride dehydrogenation in liquid phase. The application of microwave irradiation to the reaction system lies in its instant and selective heating features in addition to its low energy loss during transmission. Under microwave irradiation the bed could be superheated in spite of that it is surrounded in the nonpolar liquid media. As a result, both high dehydrogenation rate and high equilibrium conversion could be attained while with low energy consumption. At the same time, the liquid phase reaction mode is beneficial to the hydrogen separation from the reaction media, which is more energy-saving than gas phase reaction. The unique structure of the bed could intensify the liquid flow through the bed as well as the hydrogen diffusion within the catalyst inner pores. The equilibrium conversion could be further increased due to the prompt elimination of hydrogen. In the research the interactions between the hierarchical carbon fiber composite and microwave irradiation, and their effects on both the mass transfer and the catalytic reaction will be focused on in order to disclose the mechanism for intensifying the dehydrogenation process. A series of important conclusions could be reached after fulfilling the proposed research. An innovative scheme devised by organically combining the novel bed structure with the electromagnetic radiation and the special reaction mode could be provided for solving the conflict of the dehydrogenation efficiency and the energy consumption. An important contribution to the knowledge of catalytic bed design and process intensification could also be made as a main harvest of the research.

吸热脱氢反应的过程效率受能耗制约，采取合理有效的供热方式与反应模式是低能耗下高效脱氢的关键，而催化剂床层是联结效率与能耗的必然场所。本项目以有机氢化物催化脱氢平衡反应为模型体系，设计采用分级结构碳纤维床层，选择微波加热方式和液固相催化反应模式，利用微波具有能量辐射传递与选择加热的特点，通过床层与微波之间的强烈耦合，促使床层形成过热状态来推进平衡反应，达到能量高效利用的目的；同时利用液相反应无需汽化原料且易于氢气分离的特点，通过床层特有分级结构，强化过程传递和氢气逸出，进一步降低能耗和推动平衡。着重研究分级结构床层与微波的耦合性能，及对非极性介质流体传递和催化反应过程的影响，揭示微波耦合分级结构床层强化脱氢过程的机制。本项目将微波加热方式、催化剂床层设计与反应过程模式这三者有机结合起来，不仅能协同实现低能耗下高效脱氢的目标，还可丰富多学科交叉研究领域的理论内涵。

针对有机氢化物催化脱氢反应的过程效率受能耗制约的问题，本项目设计提出了微波耦合分级结构碳纤维床层提高液相脱氢效率的解决思路。首先系统研究了多种不同类型的碳材料与微波的耦合作用及对微波加热有机氢化物液相脱氢反应效率的影响。结果表明，在液相反应体系中，微波与碳材料的耦合作用不仅取决于碳材料的微观结构和表面性质，还受到材料聚集体性质的影响，因此，不同类型碳材料的微波加热效果差异巨大，反映出微波耦合作用的强选择性。具有高石墨化结晶度和高长径比的纳米碳管等碳材料催化体系表现出优异的微波加热脱氢性能，归因于显著的过热效应和逆扩散效应。碳材料石墨层结构的破坏及表面存在的含氧基团会降低材料对微波能量的吸收与转化能力，所以，对碳材料进行切断或表面处理不利于它们应用于微波加热反应体系中。传统加热条件下的催化脱氢反应平行试验比较结果为微波耦合碳材料强化脱氢反应提供了有力证据。在此基础上，合成了系列分级结构碳纤维成型体，负载Pt催化剂后作为床层应用于液相脱氢反应中。结果显示，成型体中纳米碳纤维的形貌可通过选择生长催化剂及所需碳源进行调节，进而改变成型体的织构与物化性能，最终对催化反应性能产生重大影响。纳米碳纤维的生长量也是有效调控成型体的结构与催化性能的重要因素。比较与活性炭颗粒床层上的传质和催化性能发现，分级结构碳纤维床层因具有开放的孔道结构且无微孔而不存在内扩散限制，所负载催化剂的本征活性也明显占优。在多种类型的分级结构碳纤维床层中，以Fe为生长催化剂并以CO为碳源合成的分级结构碳纤维成型体应用于脱氢反应中具有最好的催化性能，脱氢速率可达2160 mmolH2/gPt/h，归结为床层中均匀生长的板式结构纳米碳纤维拥有丰富的石墨层边界，有利于金属纳米粒子的分散与结合所致。在微波加热条件下，分级结构碳纤维床层同样表现出优于传统加热方式下的催化效果，这是微波与碳材料间的强耦合效应及碳纤维床层结构优势共同作用的结果。