Abstract:

Slurry bed hydrotreating process can treat inferior heavy oil and residual oil from different sources. Therefore, compared with other processes, the process is characterized by strong adaptability of raw materials. One of the main reactions of heavy oil hydrotreating is the activation of hydrogen. Molybdenum-based dispersed catalyst is widely used as an efficient hydrogenation catalyst in the slurry bed hydrogenation process. One of the key factors affecting the hydrogenation activity of heavy oil is the dispersion of molybdenum-based catalysts. Therefore, it is meaningful to explore the dispersion of the catalyst for hydrogen activation. In this paper, Mo₇S_15, Mo₁₂S₂₆, Mo₁₈S₃₉, Mo₂₅S₅₄ and Mo₃₃S₇₁ clusters were built and their structure is optimized. On the basis of stable structure, the stability and activity of clusters with different sizes were studied, and the adsorption dissociation process of hydrogen on Mo _x S _y clusters with different sizes was discussed, including the stability of hydrogen molecules at different adsorption sites, the adsorption dissociation process of hydrogen molecules on Mo _x S _y clusters with different sizes, and the changes with the size of Mo _x S _y clusters. According to the calculation results, it is found that the currently established clusters with smaller size have the lower binding energy, the smaller HOMO-LUMO energy gap value, the weaker stability and the stronger activity. During the adsorption and dissociation of H₂ above the cluster, it is found that hydrogen is more easily adsorbed at the edge position. With the increase of cluster size, the adsorption energy was -64.25, -34.60, -34.14, -7.20 and -6.82 kJ/mol; the absolute value of the adsorption energy decreased; the interaction between hydrogen molecules and the cluster was weaker; the dissociation energy gradually increased, and were 13.76, 33.14, 53.64, 60.75 and 64.47 kJ/mol, respectively. The current results show that the smaller the cluster size, the easier it is for hydrogen to adsorb/ dissociate and the cluster shows higher activity.

Key words: Mo _x S _y cluster, cluster size, hydrogen activation, Mo-based catalyst, density functional theory

摘要：

浆态床加氢工艺可以处理不同来源的劣质重油、渣油，氢气的活化是重油加氢处理过程中发生的主要反应之一。钼基催化剂的分散性是影响重油加氢活性的关键因素。构建了Mo₇S₁₅、Mo₁₂S₂₆、Mo₁₈S₃₉、Mo₂₅S₅₄和Mo₃₃S₇₁团簇，利用密度泛函理论研究了团簇自身的稳定性、活性以及H₂在不同尺寸团簇上的吸附与解离过程。结果发现，在目前所建立的团簇中，其尺寸越小，结合能越低，最高占据分子轨道-最低未占分子轨道(HOMO-LUMO)能隙值越小，团簇稳定性越弱，活性越强。H₂在簇上的稳定吸附位点为边缘位点。随团簇尺寸增大，吸附能分别为-64.25、-34.60、-34.14、-7.20、-6.82 kJ/mol，吸附能绝对值减小，氢气分子与团簇的相互作用减弱，并且解离能逐渐增大，分别为13.76、33.14、53.64、60.75、64.47 kJ/mol。目前的结果表明，团簇尺寸越小，氢气的吸附解离越容易，显示出更高的活性。

关键词: Mo _x S _y 团簇, 团簇尺寸, 氢气活化, 钼基催化剂, 密度泛函理论