Key Takeaways & Executive Findings
- •• • Cr promotion increases HDN conversion of quinoline by 14.5%–19.7% at 340–380 °C, 4 MPa, and WHSV 8.7 h−1, reaching 81.9% at 380 °C, demonstrating significant activity enhancement for industrial hydrotreating. • • Cr and Zn act as electron donors to sulfided Fe, creating electron-rich Fe sites that weaken Fe–S bonds and promote CUS formation, a mechanistic insight for designing active hydrogenation sites. • • Cr addition increases medium-strength Lewis acid sites, which synergize with CUS to boost hydrogenation activity, highlighting the importance of acid site distribution in HDN catalysts. • • The deep hydrogenation pathway rate constant over FeZn3Cr@GA is 3.2 times that of unmodified FeZn@GA, indicating a shift in HDN pathway selectivity that can improve process efficiency.
Abstract
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.
1. Introduction
Nitrogen-containing compounds in fuels, such as quinoline, are a major source of NOx emissions during combustion, posing severe environmental and health risks. Hydrodenitrogenation (HDN) is a key process to remove these compounds, but conventional catalysts often rely on expensive noble metals or suffer from insufficient activity and selectivity. Iron-based catalysts offer a low-cost alternative, yet their performance is limited by poor hydrogenation activity and unfavorable pathway selectivity. The challenge is to enhance the intrinsic activity of iron while maintaining economic viability.
This study addresses this bottleneck by introducing chromium as a promoter to a FeZn-supported catalyst. The authors demonstrate that Cr does not act as an independent active site but rather modulates the electronic structure of sulfided iron, creating electron-rich Fe centers that weaken Fe–S bonds and generate coordinatively unsaturated sulfur vacancies (CUS). This electronic regulation, combined with an increase in medium-strength Lewis acid sites, significantly boosts HDN activity and shifts the reaction pathway toward deep hydrogenation. The findings provide a rational strategy for designing low-cost, high-performance HDN catalysts by tuning the electronic environment of the active metal.
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LI Xiaohan, LI Meng, WEI Chongbin, REN Shenyong, GUO Qiaoxia, ZHU Xiaochun, SHEN Baojian (2026). Construction of electron-rich active sites in the metallic active phase of iron-based hydrogenation catalysts and their regulation on HDN pathway selectivity. Journal of Fuel Chemistry and Technology. https://doi.org/10.1016/S1872-5813(26)60701-3
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Frequently Asked Questions
What is the specific role of Cr in enhancing HDN activity, and how does it interact with the FeZn catalyst?
Cr does not form independent active sites but acts synergistically with Zn as an electron donor, transferring electron density to sulfided Fe species. This creates electron-rich Fe centers that weaken Fe–S bonds, promoting the formation of CUS active sites, which are crucial for hydrogenation.
How does Cr addition affect the acid site distribution and what is its impact on HDN performance?
Cr incorporation transforms weak acid sites into medium-strength and strong acid sites, particularly increasing medium-strength Lewis acid sites. These acid sites work synergistically with CUS to enhance the catalyst's hydrogenation activity, as evidenced by increased HDN conversion rates.
What are the optimal operating conditions for the FeZn3Cr@GA catalyst, and what HDN conversion rates can be achieved?
Under conditions of 340–380 °C, 4 MPa pressure, and a WHSV of 8.7 h−1, the FeZn3Cr@GA catalyst achieves HDN conversion rates of quinoline ranging from 81.9% at 380 °C, representing a 14.5%–19.7% increase over the unmodified catalyst.
How does Cr modification influence the selectivity of the HDN pathway, and what is the quantitative improvement?
Cr addition effectively governs HDN pathway selectivity, favoring the deep hydrogenation pathway. The reaction rate constant for deep hydrogenation over FeZn3Cr@GA is 3.2 times higher than that of the unmodified FeZn@GA catalyst, indicating a significant shift in selectivity.
What is the industrial significance of using Fe-based catalysts with Cr promotion compared to conventional HDN catalysts?
Fe-based catalysts are significantly lower in cost than noble metal-based systems. The Cr-promoted FeZn catalyst demonstrates enhanced HDN activity and selectivity, making it a viable low-cost alternative for industrial hydrotreating processes, particularly for removing nitrogen from inferior feedstocks.
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