磷化镍加氢脱氧催化体系的构建与催化反应机制

Biomass fuel, which is reproducible and readily converted into liquid fuel, is regarded as a promising alternative to fossil fuels. Bio-oils consist of a large content of oxygen, which leads to low energy density, high viscosity and low thermal and chemical stabilities. As a result, bio-oils require the improvement in properties through oxygen removal by hydrodeoxygenation (HDO) before they can be used as engine fuels. Because a large fraction of water is present in bio-oils and water is the by-product of hydrodeoxygenation, the influences of water on the catalytic phase and on the reaction pathways is of considerable concern. Recently, we found that Ni2P exhibited high activity and stability in HDO of phenol in organic phase, whereas its performance dropped dramatically in aqueous HDO. Unexpectedly, we found that Ni3P was extremely active in HDO in both organic and aqeous phase. It is reported that Ni3P is frequently used as a protective layer in metal polishing. Ni3P is rarely reported to be an active hydrogenation catalyst. As a consequence, it is proposed that the structure-peformance relationship of Ni3P will be intensively investigated. The effects of solvent and preparation on the activity and product selectivity will be investigated. The structure and compositions of the supported nickel phosphides will be determined, whereas the kinetics and the reaction networks will be studied, in order to elucidate the active phase of nickel phosphides and catalytic mechanism in the hydrodeoxygenation. The research results may shed some insight on the catalytic aspects of nickel phosphides as hydrodeoxygenation catalysts. Additionally, it is expected that a novel series of high-performance hydrodeoxygenation catalysts will be designed and prepared.

生物质能源可以再生且可直接转化为液体燃料，因而成为替代矿物能源的首选。生物质油氧含量很高，致使其能量密度低、粘度高、热和化学稳定性差，因而需要通过加氢脱氧（HDO）对其提质改性。生物质油含水量较高，且HDO产物之一为水，因而催化剂必须耐水热反应条件。前期研究中，我们发现Ni2P在油相HDO中活性和稳定性均佳，但在水相HDO中活性很低。我们意外发现，Ni3P在水相和油相HDO中均表现出很高活性和稳定性，其加氢活性甚至高于贵金属。Ni3P常用作金属加工中的镀层材料，很少用作加氢催化剂。因而，有必要系统和深入研究Ni3P结构和加氢活性的关系，考察溶剂对HDO活性和选择性的影响，研究HDO中间体的加氢反应，研究反应网络和动力学，解析催化活性相结构，计算反应物和中间体的吸附和活化行为，揭示催化活性中心和反应机理。与适宜的酸性载体结合，构建高性能非贵金属加氢脱氧催化体系。

本项目系统研究Ni3P在水相和油相中催化苯酚加氢脱氧(HDO)的性能，通过引入助剂和酸性载体构建加氢和脱水双功能催化活性中心，从而提升其HDO性能，采用化学镀新方法在多种载体上制备高分散Ni3P催化剂，对催化剂进行表征并研究其构效关系。. 研究体相Ni2P催化水相苯酚HDO时，发现Ni2P在水热条件下不稳定，可以转化为Ni3P。所形成的Ni3P不仅稳定性好，而且具有优异的加氢活性，其低温活性优于Pd/SiO2，远高于金属Ni。当以十氢萘作溶剂时，Ni3P在催化苯酚HDO反应时活性显著高于Ni2P和Ni12P5。在Ni3P上，苯酚HDO反应主要以预加氢路径进行，动力学研究表明，反应中环己醇的脱水反应是速率控制步骤。. 制备双功能的Ni3P-CePO4催化剂，CePO4的引入提高了Ni3P的分散度，从而提高其加氢活性。CePO4表面具有中等强度的L酸，为环己醇脱水反应提供所需的催化活性中心，因而显著提高催化剂的HDO活性。在300 ℃，4 MPa，30 h-1的重时空速下，Ni3P-CePO4(0.3)催化剂上苯酚的HDO转化率约为Ni3P催化剂上的2.6倍。. 采用浸渍法制备负载型Ni3P催化剂时，载体的类型及催化剂前体中Ni/P摩尔比显著影响催化剂活性相的生成。制备条件相同时，在所选的载体中，SiO2载体上更易获得Ni3P，在Al2O3、HZSM-5和Hβ上，由于P与载体的相互作用，得到的是Ni或Ni与Ni3P的混合物。HZSM-5和Hβ负载Ni3P催化剂具有较好的低温脱水活性。采用分步浸渍法制备负载型Ni3P优于共浸渍法。相比于共浸渍法，在分步浸渍法制备的催化剂中Ni3P活性相含量高，因此低温催化苯酚加氢活性的提高。. 沉积沉淀-化学镀法，在温和条件成功制备出Al2O3、SiO2、TiO2和HZSM-5负载的高分散Ni3P催化剂。ICP分析结果显示催化剂中Ni/P摩尔比约为3，且在HRTEM图像中观察到归属于Ni3P的晶格条纹，Ni3P晶粒尺寸为4.3~8.2 nm。该合成方法的提出，避免高温焙烧及还原，不仅可以制备常规浸渍法无法合成的Ni3P/γ-Al2O3催化剂，而且制备的催化剂颗粒低至5 nm以下，高度分散，大幅提高Ni3P基催化剂的加氢和脱氧活性，对制备工业应用高性能的负载型Ni3P催化剂具有重要意义。