CoMoS Phase Research Articles

Promoting Synergy in CoW Sulfide Hydrotreating Catalysts by Chelating Agents

Adding chelating agents such as 1,2-cyclohexanediamine-N,N,N′,N′-tetraacetic acid (CyDTA) and triethylenetetraaminehexa acetic acid (TTHA) to the aqueous solution of cobalt nitrate and ammonium metatungstate used for impregnating the silica support results in enhanced activities for the hydrodesulfurization of thiophene after the catalysts have been sulfided. These chelating agents serve to retard the sulfidation of cobalt with respect to that of tungsten, which facilitates the formation of a phase in which Co atoms decorate the edges of WS2, analogous to the well-known CoMoS phase.

Selective Hydrodesulfurization of FCC Naphtha with Supported MoS2 Catalysts: The Role of Cobalt

The catalytic activity and selectivity for hydrodesulfurization (HDS) and olefin hydrogenation of FCC naphtha have been determined for MoS2 (no Co) catalysts on different supports and for a commercial CoMo/alumina HDS catalyst both with and without the addition of alkali. For MoS2 catalysts, the specific HDS activity is higher on silica than alumina, while addition of Cs resulted in no change in the activity. The differences in activity, however, are relatively small, a factor of less than two. EXAFS and XRD structural analysis indicate that small MoS2 particles are present on all catalysts. The differences in rate are not due to differences in particle size, dispersion, or support physical properties, but are likely due to the modification of catalytic properties by an interaction with the support. While there is a small influence on the rate, the composition of the support, or modification by Cs, has no effect on the HDS/olefin hydrogenation selectivity. The olefin hydrogenation conversion increases linearly with HDS conversion, and at high HDS conversion, few olefins remain in the FCC naphtha. Similar to the effect for Cs promotion of MoS2 on alumina, the addition of K to sulfided CoMo/alumina had little affect on the activity or selectivity for HDS and olefin hydrogenation. Unlike MoS2 catalysts, however, with sulfided CoMo at less than about 85% HDS conversion, the rate of olefin hydrogenation is low, but it increases rapidly as the sulfur in the naphtha drops below about 300 ppm. Selective HDS of FCC naphtha appears to correlate primarily to the formation of the CoMoS phase, rather than to the basic nature of the support. It is proposed that the enhanced olefin hydrogenation selectivity of CoMo catalysts is due to the competitive adsorption of sulfur compounds, which inhibit adsorption and saturation of olefins in the naphtha.

On the formation of cobalt-molybdenum sulfides in silica-supported hydrotreating model catalysts

Model catalysts, consisting of a conducting substrate with a thin SiO2 layer on top of which the active catalytic phase is deposited by spincoating impregnation, were applied to study the formation of the active CoMoS phase in HDS catalysts. The catalysts thus prepared showed representative activity in the hydrodesulfurization of thiophene, confirming that these models of HDS catalysts are realistic. Combination of the sulfidation behaviour of Co and Mo studied by XPS and activity measurements shows that the key in the formation of the CoMoS phase is the retardation of the sulfidation of Co. Complexing Co to nitrilotriacetic acid complexes retarded the Co sulfidation, resulting in the most active catalyst. Due to the retardation of Co in these catalysts, the sulfidation of Mo precedes that of Co, thereby creating the ideal conditions for CoMoS formation. In the CoMo catalyst without NTA the sulfidation of Co is also retarded due to a Co–Mo interaction. However, the sulfidation of Mo still lags behind that of Co, resulting in less active phase and a lower activity in thiophene HDS.

Catalytic conversion of CO, NO and SO2 on the supported sulfide catalyst: I. Catalytic reduction of SO2 by CO

Catalytic reduction of SO2 to elemental sulfur by CO has been systematically investigated over γ-Al2O3-supported sulfide catalysts of transition metals including Co, Mo, Fe, CoMo and FeMo with different loadings of the metals. The sulfided CoMo/Al2O3 exhibited outstanding activity: a complete conversion of SO2 was achieved at a temperature of 300°C. The reaction proceeds catalytically and consistently over time and most efficiently at a molar feed ratio CO/SO2=2. A precursor CoMo/Al2O3 oxide which experienced sulfurization through the CO–SO2 reaction yielded a working sulfide catalyst having a yet lower activity than the CoMo catalyst sulfided before reaction (pre-sulfiding). The catalytic activity of various metal sulfides decreased in order of 4% Co 16% Mo>4% Fe15% Mo>16% Mo≥25% Mo>14% Co≥4% Co>4% Fe. A DRIFT study showed that CO adsorbs exclusively on CoMo phase and that SO2 predominantly on γ-Al2O3. It is suggested that the Co–Mo–S structure is more adequate than the other metal-sulfur structures for the formation of a carbonyl sulfide (COS) intermediate because of the proper strength of metal–sulfur bond, and catalytically works with γ-Al2O3 for the COS–SO2 reaction.

Working surface science model of CoMoS hydrodesulfurization catalysts

Model catalysts, consisting of a conducting substrate with a thin SiO2 or Al2O3 layer on top of which the active catalytic phase is deposited, were applied to study the sulfidation of Co–Mo catalysts and to test their catalytic behavior in the hydrodesulfurization of thiophene. CoMoS, the highly active cobalt promoted MoS2 in which Co is thought to decorate the edges of MoS2 slabs, can be synthesized by sulfiding nitrilotriacetic acid complexes of cobalt and molybdenum. These complexes are deposited on SiO2/Si(100) and Al2O3/Si(100) model supports by spin coating. X-ray photoelectron spectroscopy measurements on these Co–Mo catalysts provide detailed insight into the mechanism of sulfidation. It appears that Mo is sulfided first and then the Co; this is imperative to form the CoMoS phase. Thiophene hydrodesulfurization studies of CoMoS model catalysts yield activities and product distributions consistent with those obtained from their high surface area counterparts, proving that these models are realistic. They offer, therefore, a great potential for fundamental surface science studies of catalytic phases and of adsorption, desorption, and reactions of gases as well.

Synergy between the CoMoS Phase and Supported or Unsupported Cobalt Sulfide. Existence of a Remote Control Effect

Unsupported Co9S8, MoS2, and CoMoS sulfides (prepared by the HSP method) as well as supported MoS2/Al2O3 and CoSx/C (prepared by impregnation) were used to prepare a series of binary mechanical mixtures of various compositions. A variety of physicochemical techniques (temperature programmed decomposition and sulfidation, both followed by subsequent TPO, XRD, TPR, microelectrophoresis, XPS and AEM) were used to study the samples, verify the formation of the ''CoMoS phase'' in the mixed CoMo sulfide and investigate the existence of interactions between the sulfides forming the mechanical mixtures. The catalytic activity of the supported and unsupported HSP sulfides as well as that of the mechanical mixtures was tested in the hydrodesulfurization of thiophene (HDS) and the hydrogenation of cyclohexene (HYD) at 513 K and 3 MPa. The mechanical mixtures exhibit a synergy in HDS and HYD. This shows that cobalt sulfide (supported or unsupported HSP) can promote the activity not only of molybdenum sulfide, but also of the unsupported HSP ''CoMoS phase''. In addition, XRD and AEM (performed before and after catalytic tests) were used to study the evolution of the unsupported ''CoMoS phase'' during the catalytic reaction, ft was found that this phase is unstable; cobalt segregates from the CoMoS association to form Co9S8, even after a relatively short period in the reactor. These results provide more evidence that the remote control mechanism plays a major role in the synergetic behavior of the CoMo hydrotreating catalysts.

Hydroformylation of ethylene over cobalt, nickel, molybdenum, CoMo and NiMo alumina supported sulfide catalysts

Vapour phase hydroformylation of ethylene was studied with alumina supported Co, Ni, Mo, CoMo and NiMo sulfide catalysts in a flow reactor with fixed bed at 513–563 K, 10·10 5 Pa of overall pressure in the presence of H 2S in the feed. With this reaction mixture three different kinds of reactions were observed, hydrogenation, hydroformylation and H 2S addition. Nickel and cobalt catalysts presented the best hydroformylation activity. The addition of molybdenum enhanced the formation of ethane and sulfur compounds but decreased the hydroformylation reaction. This last result is discussed in relation with the formation of a NiMoS or CoMoS phase.

On the Structural Differences Between Alumina-Supported Comos Type I and Alumina-, Silica-, and Carbon-Supported Comos Type II Phases Studied by XAFS, MES, and XPS

In this study the local structure of the Mo and Co promoter atoms in CoMoS type I and II on alumina, and CoMoS type II on various supports is compared using in situ XAFS spectroscopy as the main technique with XPS and MES in a supporting role. In the CoMoS phase the Co atoms are positioned at the edges of the MoS2 particles, in the same plane as the Mo atoms. The CoMoS phase in all catalysts shows a well defined Co-Mo and a Mo-Co coordination. On alumina the CoMoS type II phase is present as a multilayer structure, whereas type I exists as a single-slab, i.e., monolayer, structure. The silica-supported CoMoS type II closely resembles its alumina-supported counterpart. However, the Mo-S coordination number, the structural ordering and degree of stacking of the carbon-supported type II CoMoS phase is much more similar to the type I CoMoS phase supported on alumina. This contradicts the common opinion that the CoMoS type II phase is fully sulfided and not chemically bonded to the support. Two different types of Co-sites can be distinguished in the suite of catalysts studied in this paper. A Co-site with an approximately fivefold Co-S coordination and possibly a single Co-Mo coordination is predominant in the least active alumina-supported CoMoS type I and CoMoS type II (with Co/Mo = 0.39 at/at) samples. The other Co-site has a sixfold Co-S coordination with possibly a twofold Co-Mo coordination and has the highest activity for HDS. The latter site constitutes a large part of the Co-sites in silica-supported CoMoS phase II and alumina-supported CoMoS type II (with Co/Mo = 0.32) and is exclusively present in the carbon-supported CoMoS phase II.

Characterization of Promotions of Cobalt and Ruthenium in Sulfided Catalysis for Hydrodesulfurization

Temperature programmed reduction and volumetric chemisorption of O-2, H-2 and CO have been used to provide insight into the functions of cobalt and ruthenium in a series of sulfided Co-Mo/Al2O3, Ru-Mo/Al2O3, Ru-Co-Mo/Al2O3 catalysts. Cobalt interacted with molybdenum to form a stable species, i.e. CoMoS phase, which decreased the amount of oxygen chemisorption and increased uptake of CO on the Co-Mo/Al2O3 catalyst. Ru promotion is found to cause reduction temperature of Mo sites shifting to lower temperature and increase obviousely the amounts of O-2 and H-2 adsorbed on Mo sites. The function of ruthenium is explained by a concept of hydrogen spillover from Ru centers to Mo sites.

Studies of molybdena-alumina catalysts: XVI. Effect of high-temperature sulfiding

Candia et. al. recently reported an increase in hydrodesulfurization (HDS) activity of CoMo/Al{sub 2}O{sub 3} catalysts sulfided at 600 C. They attributed the increase in activity to a more active CoMoS phase (Type II) compared to the regular Type I CoMoS phase found in catalysts sulfided at 400 C. Breysse et. al. found a similar effect with NiW/Al{sub 2}O{sub 3} catalysts. Prada Silvy et. al., however, reported no change in HDS activity in sulfiding CoMo/Al{sub 2}O{sub 3} catalysts up to 700 C. In these studies, sintering of the support occurred due to the higher sulfiding temperatures employed compared with the original calcining temperature of the catalyst ({approximately}500 C). The authors decided to determine the effect of sulfiding temperature on the HDS and hydrogenation (HYD) activity of Mo and CoMo catalysts on an alumina support stabilized by calcining at a higher temperature than that used in the sulfiding treatments in order to avoid the complications due to support sintering during sulfiding. The catalysts were sulfided at 400, 550, or 700 C. Sulfided catalysts were characterized by CO{sub 2} and NO chemisorption using a pulse technique. At least duplicate measurements were made on each sample. Catalytic activities for HDS of thiophene andmore » HYD of hexene were carried out at 350 C and under atmospheric pressure in a fixed-bed reactor. Pseudo first-order rate constants were calculated from conversions.« less

A real support effect on the activity of fully sulphided CoMoS for the hydrodesulphurization of thiophene

It is shown that on Al2O3, SiO2, and C, a fully sulphided CoMoS phase can be prepared with similar degrees of dispersion, and that the specific activity of this phase for the hydrodesulphurization of thiophene is higher when this phase is supported on carbon than when it is supported on alumina or silica.

Coordination chemistry of Mo- and W-S compounds and some aspects of hydrodesulfurization catalysis

The coordination chemistry of Mo- and W-S compounds has been reviewed with emphasis on new results of our laboratory (including a systematic treatment of the compound types with different functional groups and the different reaction types). Molecular models for crystalline MoS 2 and the nitrosylated CoMoS phase as well as for the promoting effect of the Co atoms on hydrodesulfurization catalysis have been discussed.

Über [CO4S3(SO)(CN)12]8- : bemerkenswerte Bildung eines Clusters und eines „SO“-Liganden sowie Relevanz zum Promotor-Effekt bei der HDS-Katalyse / [Co4S3(SO)(CN)12]8-: A Remarkable Formation of a Cluster and a “SO ” Ligand and its Relevance to the Promotor Effect of HDS Catalysis

Abstract The preparation of K8[Co4S3(SO)(CN)12] • 4 H2O (by cyanolysis of “cobalt sulfide”) and its properties (vibrational, UV/VIS and ESCA spectra, magnetic and thermogravimetric data, and crystal structure) are reported. The formation of a “SO " ligand through air oxidation is discussed with regard to the promoting effect of Co in the CoMoS phase of the hydrodesulfurization (HDS) catalyst

Influence of the preparation conditions on the surface properties of HDS catalysts

This work presents results on the nature of the precursor structures of active CoMo/Al 2O 3 catalysts. Catalysts with the same chemical composition prepared by different procedures and calcined at two temperatures are studied. The catalysts are characterized by gravimetrically determined reducibility in H 2, oxygen chemisorption at low temperature (LTOC), diffuse reflectance spectroscopy, infrared spectroscopy using NO as probe molecule, and thiophene hydrodesulphurization (HDS) activity measurements. The presence of ammonia and a low concentration of molybdate ions in the solution (with longer time of impregnation) favour molybdena dispersion. By using beads of alumina, molybdena profiles appeared, and they markedly depend on the subsequent incorporation of the promoter in aqueous media. Upon increasing the calcination temperature, from 670 K to 820 K, no significant variations in the HDS activity were observed, however an increase in the concentration of the tetrahedrally coordinated Co 2+ ions at the interface (with a decrease of the amount of octahedrally coordinated Co which chemisorbs NO) was found. All results indicate that the optimization of the oxidic surface species (dispersion degree of molybdena and interacting Co-Mo phase) in these hydrodesulphurization catalysts may be achieved by varying preparation conditions, i.e. low concentration of Mo with long time of impregnation or presence of ammonia in solution.

Catalytic functionalities of supported sulfides: I. Effect of support and additives on the CoMo catalyst

CS hydrogenolysis (HDS) of thiophene, hydrogenation (HYD) of 1-hexene, and hydrocracking (HCG) of 2,4,4-trimethyl-1-pentene, were used as separate model test reactions to differentiate and assess the catalytic functionalities of sulfided CoMo catalysts, and their dependence on the nature of the support and incorporation of additives. Rate constants and relative catalyst activities for these three reaction types were determined. HDS and HYD activities of CoMo supported on different types of Al 2O 3 were higher, while the HCG activity was lower compared with CoMo supported on SiO 2Al 2O 3, SiO 2MgO, or TiO 2. For SiO 2Al 2O 3 supports both HDS and HYD activities decreased with increase in SiO 2 content from 10 to 75%, while HCG activity showed the opposite trend. Additives to a finished CoMo catalyst at 0.5% level caused variations in HDS and HCG activities, while HYD was essentially unaffected. HDS was promoted by NH 4HF 2 and NH 4Cl, but depressed by NaNO 3, Ca(NO 3) 2, and H 3BO 3. HCG was promoted by NH 4HF 2, NH 4Cl, and H 3BO 3. Additives at 5% level, prior to or after CoMo impregnation, showed a strong depressing effect on HDS and a lesser effect on HYD, while HCG was strongly promoted by NH 4HF 2, Ti isopropoxide, and H 3BO 3. The changes in catalytic functionalities are rationalized in terms of different interactions between CoMo phase, support, and additives.