Construction of electron-rich active sites in the metallic active phase of iron-based hydrogenation catalysts and their regulation on HDN pathway selectivity
Hydrodenitrogenation (HDN) is an effective method for removing nitrogen-containing heteroatom compounds from inferior feedstocks, with the core challenge being the development of catalysts that combine low cost and high performance. In this study, a FeZn-supported catalyst was modified by introducing six different metal promoters (La, Ti, Ce, Mn, Mg, and Cr). It was found that Cr exhibited a pronounced promotional effect on HDN performance. The promoting effect of Cr on the FeZn catalyst's activity originates from its electronic interaction with sulfided Fe species, rather than functioning as an independent active site. Specifically, Cr and Zn species act synergistically as electron donors, transferring electron density to the sulfided Fe species, thereby modulating the electronic structure of Fe to render it in an electron-rich state. This increased electronic density weakens the Fe–S bonds in the active phase, promoting their cleavage and facilitating the formation of hydrogenation active sites known as coordinated unsaturated sulfur vacancies (CUS). After introducing 3% Cr, under conditions of 340–380 °C, 4 MPa pressure, and a high weight hourly space velocity (WHSV) of 8.7 h−1, the catalyst's HDN conversion rate for the basic nitrogen compound quinoline increased by 14.5%–19.7% compared to the unmodified catalyst, reaching 81.9% at 380 °C. Furthermore, Cr introduction increased the number of medium-strength Lewis acid sites, which work synergistically with the increased CUS sites to enhance overall hydrogenation activity. Cr addition effectively governs the selectivity of the HDN pathway, with the reaction rate constant for the deep hydrogenation pathway over the FeZn3Cr@GA catalyst reaching 3.2 times that of the unmodified FeZn@GA catalyst. In summary, using Fe as the primary active metal component and regulating its electronic structure through promoters represents an effective approach for designing low-cost, high-performance HDN catalysts.