Hydrodesulfurization Reaction Conditions Research Articles

Effect of particle size on the sulfur resistance of nickel phosphide hydrodesulfurization catalysts

The effect of particle size on sulfur resistance of nickel phosphide (Ni2P) catalysts for the hydrodesulfurization (HDS) reaction was probed using Ni2P nanoparticles encapsulated in mesoporous silica (Ni2P@mSiO2). The Ni2P@mSiO2 nanocatalysts, having well-defined particle sizes of 5.9, 11.2 and 17.1 nm, were investigated for the HDS of dibenzothiophene (DBT) in the presence of a co-feed of H2S. The Ni2P@mSiO2 nanocatalysts were characterized by a range of techniques including CO chemisorption to determine active site densities, and sulfur quantification to probe sulfur incorporation during HDS measurements or treatment in a mixture of H2S/H2. The extent of sulfur deactivation depended strongly on Ni2P particle size, with Ni2P@mSiO2-5.9 nm showing little change in DBT HDS activity or turnover frequency (TOF) before and after co-feeding H2S, while Ni2P@mSiO2-17.1 nm exhibited a significant decrease in HDS activity and TOF after co-feeding H2S. Further, the Ni2P@mSiO2-5.9 nm nanocatalyst exhibited stable HDS activity when brought on-stream, while Ni2P@mSiO2-17.1 nm experienced a 40% loss of HDS activity during the initial 3 h on-stream and then stable activity thereafter. The composition of the phosphosulfide phase (NixPySz layer) formed on the surface of the Ni2P particles under HDS reaction conditions also depended strongly on particle size, with the Ni2P@mSiO2-5.9 nm (S/Nisurf = 0.16) having a substantially lower S content than the Ni2P@mSiO2-17.1 nm (S/Nisurf = 0.53). These results indicate that the surface restructuring of Ni2P associated with formation of a phosphosulfide layer is particle size dependent and leads to active sites with significantly different intrinsic activities for the HDS reaction.

Unraveling the Structure and Surface Chemistry of the Phosphosulfide Phase Formed on Ni2P under Hydrodesulfurization Reaction Conditions: A DFT Study

There is experimental evidence that the actual active phase of the Ni2P catalyst under hydrodesulfurization (HDS) reaction conditions is a nickel phosphosulfide phase formed on the catalyst surface. Combining DFT calculations and an atomistic thermodynamic approach, we investigated the possibility of sulfur adsorption as well as the replacement of surface phosphorus by sulfur on the (0001) and (1010) surfaces of the Ni2P catalyst. Our DFT calculations showed that sulfur could replace up to 100% of the surface phosphorus under hydrodesulfurization (HDS) reaction conditions. We identify that possible sulfur adsorption sites on both surface terminations are arrangements of Ni trimers (Ni3 sites). We found that only the phosphosulfide phase formed on the (1010)-AB_Ni2P surface termination has coordinatively unsaturated Ni atoms available for the adsorption of organic sulfur-containing compounds. Our results suggest that the phosphosulfide phase formed on the (1010)-AB_Ni2P termination and not on the (0001)-Ni3P2 termination could be responsible for the high HDS activity of the Ni2P catalyst. Upon the replacement of P by S, the Ni3 sites undergo tensile strain, and their reactivity toward S adsorption is modified by the imposed strain. We found that the Ni–Ni bond distance is an important parameter in describing the HDS activity of the Ni2P catalyst. By analyzing the relationship among the ensemble, ligand, and strain effects, we were able to provide a better understanding of the promoting effect introduced by the formation of the nickel phosphosulfide phase.

DFT insights into the formation of sulfur vacancies over corner/edge site of Co/Ni-promoted MoS2 and WS2 under the hydrodesulfurization conditions

The formations of sulfur vacancies over different active sites of Mo-based and W-based active phases are investigated under the traditional hydrodesulfurization (HDS) reaction conditions using DFT calculations. It can be found that under the HDS reaction conditions (600 K, p(H2) =4 MPa and p(H2S)/p(H2) = 0.001), the single sulfur vacancy can be easily formed at the S-edge rather than the M-edge/W-edge. Comparably, with the promotions of Co or Ni atoms occurring at the S-edge of the MoS2/WS2 active phase, the formation of single sulfur vacancy will become more favorable at this site. Moreover, it is interesting that when Ni atoms further replace Mo atoms at the S-edge of NiMoS active phases, the formations of double sulfur vacancies become more favorable, which can expose more accessible Ni sites for the adsorptions of sulfur-containing compounds. And temperature and pressure are two important factors affecting the sulfur vacancy formation.

Restricted diffusion of model sulfides over a NiMo/BK catalyst under hydrodesulfurization reaction conditions

The effective factors (η) of a NiMo/BK catalyst are greater than that of the traditional NiMo/Al2O3; the effective diffusion coefficient (De) decreases with the increasing molecular size of the model sulfides.

Density Functional Theory Investigation on Thiophene Hydrodesulfurization Mechanism Catalyzed by ReS2 (001) Surface

We present density functional theory calculations on the reaction mechanism of thiophene hydrodesulfurization (HDS) over ReS2 (001) surface under typical HDS reaction conditions. It is found that thiophene adopts an “upright” adsorption configuration with the binding energy of 1.26 eV. Considering the factors such as Bader charge, two reaction mechanisms, named direct desulfurization (DDS) to the product of butadiene and hydrogenation (HYD) to 2-butene, 1-butene, and butane, are systematically investigated. Results show that H prefers to attack thiophenic C before the first C–S bond rupture but begins to hydrogenate ST (S atom of thiophene) after ring-opening. Prehydrogenation has different effect on the activity of C–S bond breaking. When the ring is intact, it has nominal effect; but when the ring is open, appropriate prehydrogenation can dramatically decrease the energy barrier while complete hydrogenation makes the barrier rise again due to stereohindrance effect. The DDS mechanism is proved to be kin...

Kinetic Study of the Hydrodesulfurization of a Heavy Gasoil in the Presence of Free Fatty Acids Using a CoMo/γ-Al2O3 Catalyst

The effect of an acidic vegetable oil (AVO) on the hydrodesulfurization (HDS) reaction of an atmospheric heavy gasoil over a commercial CoMo/γ-Al2O3 catalyst was studied. The coprocessing of the two liquid feeds was carried out in a trickle-bed bench-scale reactor at the typical HDS reaction conditions used in most refineries; namely, the reaction temperature ranged from 310 to 350 °C, while the total pressure was kept constant at P = 33 bar. The AVO content in the liquid feed varied from 0 to 20 wt %, and it was found that its presence inhibited the HDS reaction in all different conditions tested, which reflects an obvious decrease in the HDS reaction rate constant. Catalytic selectivity concerning the hydrodeoxygenation reaction was determined via analysis of the gaseous products. Decarboxylation cannot be distinguished from a decarbonylation mechanism because of the simultaneously occurring water–gas shift reaction. It was obvious that the overall selectivity for decarboxylation and decarbonylation reaction paths decreased with increasing AVO content in the feed and increased with the reaction temperature. During the experimental tests, the catalyst activity was measured and deactivation was found to be negligible.

Different role of H2S and dibenzothiophene in the incorporation of sulfur in the surface of bulk MoP during hydrodesulfurization

The surface of MoP becomes sulfided during hydrodesulfurization (HDS) of dibenzothiophene (DBT), and the HDS activity increases with time on stream, showing that the sulfided MoP surface is more active than the fresh MoP surface. MoP pretreated with H2S/H2 under HDS reaction conditions showed the same activity increase as fresh MoP, but XPS did not show any sulfur at the surface of the pretreated MoP. Hence, the sulfur that is incorporated into the MoP surface during the HDS of DBT originates from DBT rather than from H2S.

Evolution and interlayer dynamics of active sites of promoted transition metal sulfide catalysts under hydrodesulfurization conditions

A new mechanistic model for molybdenum sulfide catalysts with cobalt and nickel promoters under hydrodesulfurization reaction conditions is proposed, which includes dynamic migration of sulfur and promoter atoms between adjacent sulfide layers. These migrations are caused by heterolytic dissociation of gas-phase hydrogen and formation of hydride hydrogen on a promoter atom. Hydride hydrogen adsorbed on a promoter induces electron density transfer from the bonding promoter to Mo, changing its catalytic activity. The proposed concept of dynamic migrations provides new interpretation for the “rim-edge” model, which is based on the formation of two types of active sites: highly active (“rapid”) and less active (“slow”) sites. The “slow” sites are proposed to be associated with a non-promoted cluster on the edge of a MoS2 slab layer (“rim”), which catalyzes hydrogenation. The “rapid” sites are proposed to be associated with a combination of promoted and non-promoted clusters on edges of the same (type I) or adjacent (type II) MoS2 layers. The “rapid” sites catalyze C–S bond hydrogenolysis. Adsorbed hydrogen induces migration of sulfur and promoter atoms between adjacent clusters. This dynamic migration causes a transformation of “rapid” into “slow” sites and vice versa and, therefore, influences catalytic activity. Migration of Co or Ni promoter atoms to Mo sites on the rim is more likely than to Mo sites on the edge of Co(Ni)MoS layers because the former can accept promoter atoms from both upper and lower layers. The proposed model provides critical information for rational design of improved hydrotreating catalysts and selection of preferred catalytic reaction conditions for various types of hydrocarbon feedstocks through optimization of the density and ratio of “rapid” and “slow” catalyst active sites.

The hydrogenation and direct desulfurization reaction pathway in thiophene hydrodesulfurization over MoS 2 catalysts at realistic conditions: A density functional study

We present density functional theory (DFT) calculations of reaction pathways for both the hydrogenation (HYD) and direct desulfurization (DDS) routes in the hydrodesulfurization (HDS) of thiophene over the different MoS 2 edge structures, which will dominate under typical HDS reaction conditions. Contrary to the generally accepted view, we find that the HYD reaction path, which involves hydrogenation to 2-hydrothiophene followed by hydrogenation to 2,5-dihydrothiophene and subsequent S C scission, can occur at the equilibrium Mo( 10 1 ¯ 0 ) edge without the creation of coordinatively unsaturated Mo edge sites. This is related to the presence of the metallic-like brim sites also observed in previous STM studies. It is found that the HYD reaction pathway also can occur at the S( 1 ¯ 010 ) edge. At this edge, the equilibrium edge structure itself is not active, and sulfur vacancies must be created for the reaction to proceed. It is found that the effective energy barrier for vacancy creation depends on the H 2 partial pressure. The sulfur vacancies at the S( 1 ¯ 010 ) edge are also found to be active sites for the DDS pathway. This pathway does involve an initial hydrogenation step to 2-hydrothiophene, followed by S C scission. Analyzing the relative stabilities of reactants and intermediates suggests that a catalytic cycle may involve elementary steps that start at one type of edge and are completed at the other; for example, many intermediates are more stable at the S edge. The regeneration of the active sites is found to be a crucial step for all of the reaction pathways, and the importance of reactions at Mo brim sites is related to the observation that regeneration is least activated here. It is proposed that an important activity descriptor is the minimum energy required to either add or remove S from the different equilibrium edge structures.

Investigation of Sulfur Behavior on Mo-based Hydrodesulfurization Catalysts Supported on High Surface Area TiO<sub>2</sub> by <sup>35</sup>S Radioisotope Tracer Method

Mo catalysts were prepared by impregnation of titania synthesized by the pH swing method which provides a TiO2 carrier with a high specific surface area (134 m2·g−1) and excellent mechanical properties. Dibenzothiophene (DBT) hydrodesulfurization (HDS) activity was estimated over the obtained catalysts under typical HDS reaction conditions for various Mo contents. The activity increased linearly with Mo content up to ca. 16 wt% MoO3 and then decreased for higher Mo loadings. The sulfur behavior on the sulfided Mo/TiO2 catalysts was elucidated under the reaction working conditions using a 35S radioisotope tracer method, or the HDS of 35S-labeled DBT. The results indicated that at a given temperature the H2S release rate constant (kRE) was almost constant irrespective of the Mo content, and the amount of labile sulfur (S0) increased linearly with the Mo content in parallel with the activity up to ca. 16 wt% MoO3. The optimal Mo dispersion was 5.2 atom/nm2, which is higher than the optimal Mo dispersion on 70 m2·g−1 TiO2 (4.2 atom/nm2). Comparison of kRE and S0 of the titania-based catalysts and the alumina-based catalysts suggested that the active phase consists of a 'TiMoS' phase exhibiting a promoting effect similar to the well-known 'CoMoS' phase (promotion of the MoS2 active phase by Ti atoms).

Elucidation of sulfidation state and hydrodesulfurization mechanism on Mo/TiO2 catalyst using [formula omitted] radioisotope tracer methods

In hydrodesulfurization (HDS) of dibenzothiophene (DBT) the activities on the Mo/TiO2 catalysts with various Mo contents were estimated under the typical HDS reaction conditions. The results show that the activities of the catalysts increased linearly with Mo content up to 6wt.% MoO3, which means that the monolayer dispersion of MoO3 on TiO2 support is maintained up to 6wt.% MoO3. The sulfidation states of MoO3/TiO2 catalysts were investigated using a 35S radioisotope pulse tracer method. The results showed that the sulfidation of TiO2 support would also occur. Moreover, the sulfur mobility on the sulfided 6% MoO3/TiO2 catalyst under the reaction conditions was elucidated by a 35S radioisotope tracer method using 35S-labeled DBT. The results suggested that, compared to Mo/Al2O3 catalyst, Mo/TiO2 catalyst showed a little higher amount of labile sulfur (S0) and about two times higher release rate constant of H2S (kRE), which indicated that higher mobility of sulfur was obtained on sulfided Mo/TiO2 catalyst.

Elucidation of Sulfidation State and Hydrodesulfurization Mechanism on TiO2 Catalysts Using 35S Radioisotope Tracer Methods

The hydrodesulfurization (HDS) activities of sulfided TiO2 with various surface areas were characterized under the typical HDS reaction conditions. The results showed that HDS activities increased linearly with increasing surface area of TiO2, suggesting that the surface sulfide TiS2 is the active phase of the HDS reaction on sulfided TiO2 catalyst. The sulfidation states of TiO2 with various surface areas were investigated using a 35S radioisotope pulse tracer method. The results showed that the Ti atoms on the surface of TiO2 could be completely sulfided to TiS2 at 400°C, whereas the sulfidation of Ti atoms in the bulk of TiO2 would occur slowly at 500°C. When the sulfided TiO2 was reduced in H2 at 400°C, TiS2 was reduced to the sulfidation state TiS1.5, leading to the formation of Ti3+ ions and sulfur anion vacancies. Moreover, HDS reaction mechanisms on sulfided TiO2 catalysts under the reaction conditions were elucidated by a 35S radioisotope tracer method using 35S-labeled dibenzothiophene (DBT). The results showed that less labile sulfur was detected on the sulfided TiO2 catalysts, suggesting that the HDS mechanism on TiO2 catalysts was very different from that on Mo-based catalysts.

Methods of Activating Catalysts for Hydrodesulfurization of Light Gas Oil. (Part 2). Catalytic Properties of CoMo/Al2O3 Presulfided by Polysulfides for Deep and Ultra-deep Hydrodesulfurization of Light Gas Oil.

Hydrodesulfurization (HDS) of a straight run light gas oil (SRLGO, Sulfur: 1.50wt%) and ultra-deep HDS of a hydrotreated light gas oil (HDLGO, Sulfur: 0.045wt%) were carried out over a commercial Co-Mo/Al2O3 catalyst presulfided with a novel sulfiding agent, bis-(1, 1, 3, 3-tetramethylbutyl)-polysulfide (CS-40). The HDS reaction conditions were: Temperature 310-390°C, total pressure 3.0MPa, LHSV 2.0-4.0h-1, Gas/Oil ratio 125 Nm3/m3. The catalyst presulfided in situ with CS-40 showed comparable activity to the corresponding catalyst presulfided with dimethyldisulfide (DMDS), and higher activity than that presulfided with H2S in HDS of SRLGO. Catalyst presulfided ex situ with CS-40 plus sulfur showed significantly enhanced activity for HDS of alkyl-substituted dibenzothiophenes (DBTs). No significant changes in apparent activation energies for HDS of DBT were observed. In contrast, the catalyst ex situ sulfided with CS-40 plus sulfur had smaller activation energies for HDS of 4, 6-dimethyldibenzothiophene (4, 6-DMDBT). The ex situ sulfiding method using CS-40 plus sulfur may enhance the formation of active sites for hydrogenation and results in increased activity for indirect desulfurization of alkyl-substituted DBTs via hydrogenation of the aromatic ring of DBTs to form cyclohexylbenzenes.

Solid state synthesis and characterization of model hydrodesulfurization catalysts

Solid-state reactions of elemental sulfur with metallic cobalt and molybdenum powders were used to synthesize model catalysts for hydrodesulfurization (HDS). The resulting materials had a general stoichiometry of Co 2 x Mo 1− x S 2 with x between 0.025 and 0.3. The solid phases formed during synthesis and after exposure to typical HDS reaction conditions were identified by X-ray diffraction and selected area electron diffraction. The catalytic activity for hydrodesulfurization of thiophene was tested in a flow reactor. High activity for hydrodesulfurization coincided with the presence of a nonstoichiometric bulk phase. It was found that introducing small quantities of a promoter atom led to similar activity trends, XRD patterns, and electron microscopy results, all of which were attributed to a nonstoichiometric bulk phase. The structure of this phase is not yet fully determined but is consistent with a defect structure of MoS 2 having anionic vacancies.

Hydrodesulfurization catalysis by Chevrel phase compounds

Reduced ternary molybdenum sulfides, known as Chevrel phase compounds, were investigated to determine their catalytic activity at 400 °C for thiophene hydrodesulfurization and for 1-butene hydrogenation. Chevrel phase compounds of the general composition M x Mo 6S 8, with M being Ho, Pb, Sn, Ag, In, Cu, Fe, Ni, or Co, were found to have hydrodesulfurization activities comparable to model unpromoted and cobalt-promoted MoS 2 catalysts. The most active catalysts were the “large” cation compounds (Ho, Pb, Sn), and the least active catalysts were the “small” cation compounds (Cu, Fe, Ni, Co). Except for the nickel phase Ni 1.6Mo 6S 8, all Chevrel phase catalysts had hydrogenation activities which were much lower than the model MoS 2 catalysts. Fresh and used catalysts were characterized by X-ray diffraction, laser Raman spectroscopy, and X-ray photoelectron spectroscopy. While the bulk structures were stable under hydrodesulfurization reaction conditions, differences were observed in the stability of the surface molybdenum oxidation state for the specific classes of Chevrel phase catalysts: the oxidation state of the surface molybdenum atoms of the large cation compounds was found to be unchanged after 10 h of thiophene reaction, but some oxidation was apparent for the small cation compounds. The differences in stability can be related to the mobilities of the ternary metal atoms within the Chevrel phase structures.

Photoacoustic spectroscopy of pyridine chemisorbed on calcined and sulfided molybdenum catalysts

Fourier transform infrared photoacoustic spectroscopy (FT-IR/PAS) studies of pyridine adsorbed on Mo/Al 2O 3 and Co-Mo/Al 2O 3 were undertaken to determine the presence of Brønsted and Lewis acid sites on the calcined, and sulfided catalysts. Results on the calcined catalyst indicate both Brønsted and Lewis sites to be present, in agreement with earlier IR transmission studies. A previously unreported vibrational band has been assigned to a surface cobalt-alumina domain. For the sulfided catalysts, only Lewis acidity could be unambiguously identified. The lack of detectable Brønsted acidity on the sulfided catalyst under the experimental conditions employed does not necessarily rule out its presence under hydrodesulfurization reaction conditions.

Stabilization effect of Co for Mo phase in [formula omitted] hydrodesulfurization catalysts studied with X-Ray photoelectron spectroscopy

The promotional effects of Co in CoMo Al 2O 3 hydrodesulfurization (HDS) catalysts were studied by means of X-ray photoelectron spectroscopy. The higher MoO 3-content Mo Al 2O 3 catalysts (10 and 20 wt% MoO 3) contain mobile Mo, which migrates from the pores to the outermost surface layers of the catalysts and segregates to form less active crystalline MoS 2 during the HDS reaction, while in the case of Mo Al 2O 3 (5 wt% MoO 3) catalyst: no migration of Mo was observed. It is revealed that the Co in CoMo Al 2O 3 catalyst inhibits the migration and segregation of Mo and that it keeps Mo effective for the HDS reaction, since no surface enrichment of Mo was observed. It is concluded that stabilization of the Mo monomolecular layer is the main role of Co. The active species of Mo is suggested to have the composition of S/Mo(IV) = 1 on the basis of the sulfur contents of the catalysts under the mild HDS reaction conditions.