Competitive adsorption of nitrogen and sulphur compounds on a multisite model of NiMoS catalyst: A theoretical study

Substituent Effects of the Nitrogen Heterocycle on Indole and Quinoline HDN Performance: A Combination of Experiments and Theoretical Study

Hydrodenitrogenation (HDN) experiments and density functional theory (DFT) calculations were combined herein to study the substituent effects of the nitrogen heterocycle on the HDN behaviors of indole and quinoline. Indole (IND), 2-methyl-indole (2-M-IND), 3-methyl-indole (3-M-IND), quinoline (QL), 2-methyl-quinoline (2-M-QL) and 3-methyl-quinoline (3-M-QL) were used as the HDN reactant on the NiMo/γ-Al₂O₃ catalyst. Some key elementary reactions in the HDN process of these nitrogen compounds on the Ni-Mo-S active nanocluster were calculated. The notable difference between IND and QL in the HDN is that dihydro-indole (DHI) can directly convert to O-ethyl aniline via the C-N bond cleavage, whereas tetrahydro-quinoline (THQ) can only break the C-N single bond via the full hydrogenation saturation of the aromatic ring. The reason for this is that the -NH and C=C groups of DHI can be coplanar and well adsorbed on the Ni-Mo-edge simultaneously during the C-N bond cleavage. In comparison, those of THQ cannot stably simultaneously adsorb on the Ni-Mo-edge because of the non-coplanarity. Whenever the methyl group locates on the α-C or the β-C atom of indole, the hydrogenation ability of the nitrogen heterocycle will be evidently weakened because the methyl group increases the space requirement of the sp³ carbon, and the impaction of the C=C groups on the Ni-S-edge cannot provide enough space. When the methyl groups are located on the α-C of quinoline, the self-HDN behavior of 2-M-QL is similar to quinoline, whereas the competitive HDN ability of 2-M-QL in the homologs is evidently weakened because the methyl group on the α-C hinders the contact between the N atom of 2-M-QL and the exposed metal atom of the coordinatively unsaturated active sites (CUS). When the methyl group locates on the β-C of quinoline, the C-N bond cleavage of 3-methyl-quinoline becomes more difficult because the methyl group on the β-C increases the steric hindrance of the C=C group. However, the competitive HDN ability of 3-M-QL is not evidently influenced because the methyl group on the β-C does not evidently hinder the adsorption of 3-M-QL on the active sites.

Investigating the Effects of Organonitrogen Types on Hydrodearomatization Reactions over Commercial NiMoS Catalyst

The hydrogenation of polyaromatic compounds (PACs) present in mineral oils is of great importance when it comes to the desired product properties and the minimization of health hazards; however, the presence of organonitrogen inhibits the conversion of these compounds. In this study, the inhibition effects of different types of organonitrogen compounds (acridine (ACR) and carbazole (CBZ)-basic and nonbasic organonitrogen) on the hydrodearomatization (HDA) of phenanthrene over a sulfided commercial NiMo/Al2O3 catalyst were investigated in a microflow trickle-bed reactor at a temperature range of 280 to 320 °C and at a total pressure of 120 barg. Analysis of the experimental results shows that the hydrogenation of phenanthrene is significantly decreased in the presence of organonitrogen, with acridine showing stronger inhibiting effects. The extent of hydrodenitrogenation (HDN) is shown to correlate with the inhibition degree with a higher extent of HDN being achieved for carbazole than for acridine. Results from co-feeding different nitrogen types (acridine and carbazole) indicate that basic nitrogen is the dominating type of organonitrogen inhibitor. Recovery of catalyst activity in the absence of organonitrogen indicates fully reversible deactivation suggesting that inhibition relates to competitive adsorption and slower reaction rate of HDN compared to HDA.

Competitive and sequence reactions of typical hydrocarbon molecules in diesel fraction hydrocracking - a theoretical study by DFT calculations

The molecular structures of hydrocarbon molecules determine the competitive and sequence reactions in the diesel hydrocracking process. In this study, the hydrocracking reactions of typical hydrocarbons with various saturation degrees and molecular weights in diesel fractions synergistically catalyzed by the Ni–Mo–S nanocluster and Al–Si FAU zeolite are investigated. The results show that the two major rate-controlling steps in saturated hydrocarbon hydrocracking are dehydrogenation on the Ni–Mo–S active sites and the cracking of the C–C bonds on the FAU zeolite acid center. Moreover, the major rate-controlling step in cracking the cycloalkyl aromatic hydrocarbons is the protonation of the aromatic ring. Moreover, the aromatic hydrocarbons presented an apparent advantage in competitive adsorption on the Ni–Mo–S active sites, whereas hydrocarbons with higher molecular weights demonstrated a moderate adsorption advantage on both Ni–Mo–S active sites and FAU zeolite acid centers.

The molecular structures of hydrocarbon molecules determine the competitive and sequence reactions in the diesel hydrocracking process.

DFT insights into the hydrodenitrogenation mechanism of quinoline catalyzed by different Ni-promoted MoS2 edge sites: Effect of the active phase morphology

Density functional theory calculations are performed to investigate the hydrodenitrogenation (HDN) mechanism of quinoline over different Ni-promoted MoS₂ edges. Based on the calculations, the hydrogenation and ring-opening reaction pathways are explored systematically, and the structure-activity relationship of different active sites is discussed in detail. In the hydrogenation reaction process, the 100% Ni-promoted M-edge and 50% Ni-promoted S-edge are favorable for the formations of 5,6,7,8-tetrahydroquinoline and 1,2,3,4-tetrahydroquinoline, respectively. Furthermore, the 100% Ni-promoted M-edge is more preferable for the generation of decahydroquinoline rather than the 50% Ni-promoted S-edge. In the denitrogenation reaction step, the 100% Ni-promoted M-edge is beneficial for the formation of ortho-propylaniline and 2-propylcyclohexylamine, while 50% Ni-promoted S-edge is only conducive to the formation of 2-propylcyclohexylamine. Therefore, it can be concluded that both hydrogenation derivatives and denitrogenation products exhibit strong dependence on the active phase morphology, meaning that multiple active sites can be involved in one catalytic HDN cycle.

Ni2P promotes the hydrogenation activity of naphthalene on wrinkled silica nanoparticles with tunable hierarchical pore sizes in a large range

Herein, a series of wrinkled silica nanoparticles with hierarchical pore (HPWSNs) supports were successfully prepared by dual-templating, and the special wrinkle pore structures in the monodisperse HPWSN samples were found to be beneficial for reducing the diffusion resistance of macromolecular aromatic compounds and achieving high dispersion of Ni₂P active phases. Moreover, the distance between wrinkles in silica nanoparticles could be easily tuned by changing the ratios of SDS/CTAB through charge-reversed interactions. It was found that the Ni₂P/HPWSNs-0.13 catalyst with smallest Ni₂P particles had highest surface area and biggest pore volume. Furthermore, the Ni₂P/HPWSNs-0.13 catalyst exhibited highest naphthalene hydrogenation conversion as well as 99.9% selectivity to decalin at 320 °C. To correlate the internal relationship between the macroscopic catalytic performance in the experiment and the atomic chemistry in the microscopic point of view, DFT calculations were performed, and the results showed that stronger adsorptions of naphthalene and tetralin occurred over the Ni(2) sites than those over the Ni(1) sites. Therefore, it can be concluded that the superior catalytic activity of the Ni₂P/HPWSNs-0.13 catalyst is due to the synergistic effect of the center-radical framework structure and the small sizes of Ni₂P particles, which are conducive to exposing more Ni(2) sites on the support surface, thus inducing more H for the naphthalene hydrogenation reaction.

Modeling the active sites of Co-promoted MoS2 particles by DFT

The atomic-scale structure of the Co-promoted MoS₂ catalyst (CoMoS), used for hydrodesulfurization and as a potential replacement for platinum in the acidic hydrogen evolution reaction has been analyzed by modeling its sites using density functional theory and applying thermochemical corrections to account for different reaction conditions. The equilibrium structures of the edges, basal plane and corners have been found and used to obtain a picture of an ideal CoMoS nanoparticle under hydrodesulfurization and hydrogen evolution reaction conditions. Under hydrodesulfurization conditions small energy differences between structures having an additional or missing sulfur atom relative to the equilibrium structures have been observed for the edges and corners explaining their activity towards hydrodesulfurization at the atomic scale. The lack of these small energy differences at the basal plane explains why it is inert towards hydrodesulfurization. The adsorption free energy of hydrogen was calculated and used as a descriptor for qualifying each site in the context of hydrogen evolution, finding that the corner site should perform better than the edges.