烷烃异构化骨架型MoO_x-SiO_2催化剂的研究
作者：柴松海
院校:大连理工大学
关键字:
学位：硕士
专业:应用化学
摘要:
在石油工业中，作为提高汽油辛烷值的重要手段，开发研究高活性、高选择性和高稳定性的正构烷烃异构化催化剂对于提高汽油质量、减少环境污染，改善人类生存环境都有重大的现实意义。 本文在常压固定床流动反应器上和573K的反应温度下研究了部分还原态氧化钼（MoOx）基催化剂上的正庚烷异构化反应。通过分析N2吸附-脱附表征结果，发现623K下H2还原MoO3所得的MoOx催化剂具有中孔性质，其最可几孔径为4.1nm。对MoOx催化剂上的正庚烷异构化反应动力学进行了研究，结果表明在523K-598K范围内得到了正庚烷的异构化反应和裂解反应的表观活化能分别49.3 kJ/mol和60.6 kJ/mol；反应温度为573K时，H2和正庚烷分压对MoOx催化剂上正庚烷异构化反应的反应级数分别为0.35和0.33。 采用等体积浸渍法制备了Mo担载量不同的常规担载型MoOx/(-Al2O3、MoOx/SiO2和Ni-MoOx/SiO2催化剂。结果发现，MoOx/SiO2凭借SiO2载体和MoO3较弱的表面相互作用，较MoOx/(-Al2O3催化剂表现出更好的正庚烷异构化活性和选择性；在34.5 wt%MoO3担载量时，MoOx/SiO2催化剂在正庚烷转化率为26% 时异构化选择性可达95%，但单位MoO3质量比活性却只能达到非担载MoOx催化剂的一半。Ni-MoOx/SiO2催化剂由于Ni组分的加入促进了前体相MoO3向活性相MoOx的还原，从而表现出了较MoOx/SiO2更高的正庚烷异构化活性，但其异构化选择性却有所降低；随着Ni组分含量的增加，Ni-MoOx/SiO2催化剂上正庚烷转化率先增加然后趋于稳定，而异构化选择性却单调下降到80%。 以硅溶胶混合MoO3粉末制备了骨架支撑型MoOx-SiO2催化剂，SEM、N2吸附-脱附、EDS及XRD方法研究结果表明，在MoOx-SiO2催化剂中44.6 wt%的SiO2即可起到很好的骨架支撑作用，MoOx晶相以足够大的空间区域聚集包裹在SiO2骨架中，尽量减少了与载体的接触，形成类似MoOx催化剂的独特中孔结构；该催化剂不仅保持了MoOx催化剂的高活性和异构化选择性，还显著地提高了机械强度；H2S脉冲注入实验表明，该MoOx-SiO2催化剂有较好的抗硫性能；55.4wt%MoOx-SiO2催化剂同典型的0.5wt%Pt/H(双功能催化剂相比，在高转化率水平下表现出更高的异构化选择性，同时两者在正庚烷异构化反应的产物分布上很相似。
Abstract:
As one of the main routes to produce high-octane gasoline blending components in the crude oil refining industry, alkanes isomerization has drawn much attention throughout the world because it can greatly improve the gasoline quality. For n-alkane isomerization, one of the key issues is to develop catalysts with higher activity, high selectivity and higher stability, which has been widely pursued due to its importance both in academic research and in industrial utilization. n-Heptane isomerization on MoOx-based catalysts was carried out in a continuous fixed-bed reactor at 573 K under atmospheric pressure. According to the results of N2 adsorption-desorption characterization, the MoOx catalyst obtained from H2 reduction at 623 K was found to possess a mesoporous characteristic with its maximum pore volume at the diameter of 4.1 nm. The dynamics of n-heptane isomerization on the MoOx catalyst was investigated. In the reaction temperature ranged from 523 to 598 K, the apparent activation energy for the n-heptane isomerization and the cracking of n-heptane is about 49.3 kJ(mol-1 and 60.6 kJ(mol-1, respectively. At 573 K, the reaction order for H2 and n-heptane partial pressure on the MoOx catalyst for the n-heptane isomerization is 0.35 and 0.33, respectively. MoOx/(-Al2O3, MoOx/SiO2 and Ni-MoOx/SiO2 catalysts with a different loading of MoO3 and Ni were prepared by conventional incipient impregnation method. MoOx/SiO2 catalyst showed a better activity and isomerization selectivity than MoOx/(-Al2O3 catalyst, which could be a result that MoO3 has a weaker interaction with SiO2 than (-Al2O3 support. The MoOx/SiO2 catalyst with a 34.5wt% MoO3 loading exhibited an isomerization selectivity of 95% at n-heptane conversion of 26%, while its specific activity of per gram of MoO3 is only 1/2 of bulk MoOx catalyst. The adding of Ni promoted markedly the reduction of MoO3 phase to the active phase of MoOx. And the Ni-MoOx/SiO2 catalyst showed a higher activity but lower isomerization selectivity than MoOx/SiO2 catalyst. With the Ni loading increasing in Ni-MoOx/SiO2 catalyst, n-heptane conversion reached the maximum of ca. 31% and then kept almost at the level, which is accompanied by a decrease of isomerization selectivity. MoO3-SiO2 catalyst precursor was prepared by mixing the MoO3 powder with silica sol. The characterization of MoOx-SiO2 catalyst by means of SEM, N2 adsorption-desorption method, EDS and XRD showed that the SiO2 in MoOx-SiO2 (44.6 wt%) catalyst acts as a framework in which a large amount of bulk MoOx phase is enveloped to form typical mesoporous structure similar to that of bulk MoOx catalyst. And the 55.4wt%MoOx-SiO2 catalyst maintained the high catalytic activity and isomerization selectivity of bulk MoOx catalyst and exhibited a well-improved mechanical strength in comparison with bulk MoOx catalyst. A H2S pulse injection leads to a decrease firstly and then recovery of the catalytic activity of 55.4wt% MoOx-SiO2 catalyst, indicating the preferable sulfur resistance. Compared with bifunctional 0.5wt%Pt/H( catalyst, the 55.4wt%MoOx-SiO2 catalyst showed higher isomerization selectivity at the high n-heptane conversion level, and both the catalysts have a similar product distribution of n-heptane conversion.