Unsupported NiMo Sulfide Catalysts Research Articles

Hydrocarbon biofuel from hydrotreating of palm oil over unsupported Ni–Mo sulfide catalysts

Research on the production of bio-hydrogenated diesel as a green second-generation biodiesel is increasingly attractive for renewable energy utilization in engines, and has potentially been produced from the hydrotreating of vegetable oils. The present work investigated the hydrodeoxygenation of palm oil catalyzed by unsupported nickel-molybdenum (Ni–Mo) sulfide synthesized by a hydrothermal method. The effects of the operating parameters, such as the reaction time (0.5–3 h), temperature (280–320 °C), oil concentration (5–15%) on the n-alkane yield were evaluated. A longer reaction time and higher temperature promoted the reaction via decarboxylation and decarbonylation pathways. Under the appropriate condition (300 °C, 2 h, and initial H2 pressure of 30 bar), the yield of C14–18 alkanes was 75.3 wt%, while the selectivity of n-C15, n-C16, n-C17, and n-C18 alkanes was 21.8, 19.7, 29.6, and 28.1%, respectively. Gas chromatography-mass spectrometry analysis revealed n-alkenes, alcohols, and esters as byproducts. Characterization of catalyst revealed it had a sandwich structure consisting of weakly coupled layers, rim sites and Ni edges, which catalyzed the reaction efficiently. The catalyst could be reused for at least four cycles of palm oil hydrodeoxygenation with retention of a good performance with the decreased 17–25% alkane yield (2nd to 4th cycle with 50.8–57.3% n-C14–18 yield).

On the multifaceted roles of NiSx in hydrodearomatization reactions catalyzed by unsupported Ni-promoted MoS2

A series of unsupported Ni-Mo sulfide catalysts with varying Ni contents (0.13–0.72 molNi molNi+Mo-1) was post-synthetically treated with concentrated HCl to remove large crystallites of accessible NiSx. These sulfide particles inevitably form and grow at Ni concentrations required for the synthesis, but generally have very low activities. In all cases, Ni concentrations were greatly reduced by the HCl treatment. While this ‘leaching’ strategy successfully improved the reaction rates of high Ni-content (>0.4 molNi molmetal-1) catalysts for the hydrogenation of phenanthrene, modest to drastic decreases in catalytic rates (×0.1–0.8) were registered for catalysts with lower Ni concentrations. For the lowest Ni-loaded catalyst (0.05 molNi molmetal-1), HCl treatment caused a dramatic loss of specific surface area and catalytic activity by more than a factor of 6 and shifted the selectivity pattern to that of pure MoS2. These observations allow us to conclude that Ni atoms incorporated at the slab edges are inherently susceptible to HCl attack. NiSx, however, are the preferential sites at which HCl induces dissolution. Experiments with inter-particle mixtures and segregated beds of NiSx and MoS2 demonstrate that NiSx not only activates H2, but also acts as a reservoir to dynamically incorporate Ni in the MoS2 slabs at reaction conditions. These beneficial effects are reduced, as nickel sulfide particles become excessively abundant as typical for high Ni-content catalysts, for which edge substitution by Ni is near or at its maximum. The areal activity and concentration of chemisorbed nitric oxide (NO) are well correlated for the leached catalysts, with the exception of the lowest Ni-containing catalyst that has a low degree of Ni edge substitution (<20% of total edge atoms) and predominantly unpromoted sites. This linear correlation shows that the Ni-promoted sites are more than five-fold as active as the unpromoted sites.

Ni–Mo sulfide nanosized catalysts from water-soluble precursors for hydrogenation of aromatics under water gas shift conditions

Abstract The unsupported catalysts were obtained during hydrogenation by in situ high-temperature decomposition (above 300 °C) of water-soluble metal precursors (ammonium molybdate and nickel nitrate) in water-in-oil (W/O) emulsions stabilized by surfactant (SPAN-80) using elemental sulfur as sulfiding agent. These self-assembly Ni–Mo sulfide nanosized catalysts were tested in hydrogenation of aromatics under CO pressure in water-containing media for hydrogen generation through a water gas shift reaction (WGSR). The composition of the catalysts was determined by XRF and active sulfide phase was revealed by XRD, TEM and XPS techniques. The calculations based on TEM and XPS data showed that the catalysts are highly dispersed. The surfactant was found to affect both dispersion and metal distribution for Ni and Mo species, providing shorter slab length in terms of sulfide particle formation and stacking within high content of NiMoS phase. Catalytic evaluation in hydrogenation of aromatics was performed in a high-pressure batch reactor at T = 380–420 °С, p(CO) = 5 MPa with water content of 20 wt.% and CO/H2O molar ratio of 1.8 for 4–8 h. As shown experimentally with unsupported Ni–Mo sulfide catalysts, the activity of aromatic rings depends on the substituent therein and decreases as follows: anthracene&gt;&gt;1-methylnaphthalene≈2-methylnaphthalene&gt;1,8-dimethylnaphthale-ne&gt;&gt;1,3-di-methylnaphthalene&gt;2,6-dimethylnaphthalene≈2,3-dimethylnaphthalene&gt;2-ethyl-naphthalene. The anthracene conversion reaches up to 97–100% for 4 h over the whole temperature range, while for 1MN and 2MN it doesn’t exceed 92 and 86% respectively even at 420 °С for 8 h. Among dimethyl-substituted aromatics the higher conversion of 45% was achieved for 1,8-dimethylnaphthalene with 100% selectivity to tetralines at 400 °С for 6 h. Similar to 1- and 2-methylnaphtalenes, the hydrogenation of asymmetric dimethyl-substituted substrate carries out through the unsubstituted aromatic ring indicating that steric factors influence on the sorption mechanism over active metal sites. The catalysts were found to be reused for at least six cycles when the hydrogenation is sulfur-assisted preventing metal oxide formation. It was established, that at the first 2–3 h known as the induction period, the oxide catalyst precursors formed slowly by metal salt decomposition, which reveals that it is the rate-determining step. The sulfidation is rather fast based on high catalytic activity data on 2MN conversion retaining at 93–95% upon recycling.

Hydroconversion of Thiophene Derivatives over Dispersed Ni–Mo Sulfide Catalysts

The activity of unsupported Ni–Mo sulfide catalysts is studied in the hydroconversion of benzothiophene and dibenzothiophenes in the temperature range of 340–380°С and at an increased H2 pressure and in the СО/H2О system. The structure of dispersed catalysts formed by the in situ high-temperature decomposition of oil-soluble precursors (molybdenum hexacarbonyl, nickel naphthenate) is investigated by TEM. Effects of СО/H2О molar ratio, water mass content in the system, and CO pressure on the activity of the catalysts and yields of the products are explored. It is shown that, in the СО/H2О system, the highest conversion of benzothiophene and dibenzothiophene is attained at a temperature of 380°С, a СО pressure of 5 MPa, and a СО/H2О molar ratio of 2. The introduction of alkyl substituents into a dibenzothiophene molecule causes a reduction in the rate of reaction that predominantly occurs via the hydrogenation of aromatic rings. The catalyst activities in hydrogenation under H2 pressure and in the СО/H2О system are comparable.

Hydrogenation of Aromatic Substrates over Dispersed Ni–Mo Sulfide Catalysts in System H2O/CO

The activity of unsupported Ni–Mo sulfide catalysts formed during the in situ decomposition of oil-soluble precursors (molybdenum hexacarbonyl, nickel naphthenate) in the hydrogenation of aromatic and naphthene-aromatic compounds is studied. The catalysts are characterized by HRTEM and XPS. Catalytic experiments are conducted at temperatures of 340–380°C and an increased pressure of ?? in the presence of water providing the in situ generation of hydrogen via the water-gas shift reaction. The catalysts exhibit a high activity in the hydrogenation of model substrates (methyl- and dimethyl-substituted naphthalenes and anthracene).

Effect of W addition on the hydrodeoxygenation of 4-methylphenol over unsupported NiMo sulfide catalysts

Unsupported NiMo sulfide catalysts modified by tungsten were prepared by a mechanical activation method. They were characterized by XRD, XPS, SEM, TEM and BET specific surface area measurements. The effect of tungsten promoter on the structure and catalytic performance was also discussed. The present work suggested that the addition of tungsten could greatly enhance the hydrodeoxygenation (HDO) activity, and promote the direct deoxygenation (DDO) pathway. Moreover, the addition content affects the mount of active NiMo(W)S phase, resulting in various functions.

Production of Clean Fuel Utilizing the Unsupported Sulfide Catalysts

The sulfur specification for diesel fuel has been tightened exponentially over the years. In this manuscript, the unsupported Ni-Mo (-W) sulfide hydrotreating catalysts were prepared to produce the clean diesel fuel with ultra-low sulfur, nitrogen, and aromatics contents. The X-ray Diffraction (XRD), Low Temperature N2Adsorption (BET method), and High Resolution Transmission Electron Microscope (HRTEM) were applied to characterize the as-prepared catalysts. The characterization results indicate that the unsupported Ni-Mo (-W) hydrotreating catalyst have high specific surface area, large pore volume, high MoS2or WS2stacking layers, and large MoS2or WS2crystal length. The catalysts were evaluated in the micro-reactor using FCC diesel fuel as the raw material. The evaluation results reveal that the unsupported Ni-Mo (-W) catalysts have excellent hydrogenation performance. Utilizing the unsupported Ni-Mo (-W) sulfide catalysts is an efficient method to produce clean diesel fuel. Keywords: clean fuel; Ni-Mo (-W) sulfide; catalyst; hydrogenation

Preparation of unsupported Ni–Mo–S catalysts for hydrodesulfurization of dibenzothiophene by thermal decomposition of tetramethylammonium thiomolybdates

Unsupported Ni–Mo sulfide catalysts were prepared by thermal decomposition of tetramethylammonium thiomolybdostate and nickel nitrate, which were characterized by Fourier transform infrared spectroscopy, ultraviolet–visible spectroscopy, thermogravimetric and differential thermal analysis, X-ray diffraction, N 2 physorption, elemental mapping and transmission electron microscopy. Hydrodesulfurization of dibenzothiophene was also performed in a fixed-bed reactor over the unsupported Ni–Mo sulfide catalysts. The results showed that the structures and properties of unsupported Ni–Mo sulfides depended on nickel content and decomposition conditions. Ni–Mo sulfides with type IV isotherms related to mesoporous structure were obtained. The layer and stacking degree of sulfide particles showed a decreasing trend, which leaded to a decrease of specific surface area, with increasing nickel content. All elements were homogeneously distributed, suggesting the “Ni–Mo–S” phase was formed. The addition of nickel promoted the dibenzothiophene conversion and the direct desulfurization pathway in the hydrodesulfurization of dibenzothiophene. The sample Ni 0.1–MoS 2 gave superior hydrodesulfurization activity for dibenzothiophene to other samples. The unsupported Ni–Mo sulfide catalysts exhibited desired activity in hydrodesulfurization and therefore might find wide applications in hydrotreating of fuel oil.

Effects of preparation conditions in hydrothermal synthesis of highly active unsupported NiMo sulfide catalysts for simultaneous hydrodesulfurization of dibenzothiophene and 4,6-dimethyldibenzothiophene

Unsupported NiMo sulfide catalysts were prepared from ammonium tetrathiomolybdate (ATTM) and nickel nitrate by using a hydrothermal synthesis method involving water, organic solvent and hydrogen. The activity of these catalysts in the simultaneous hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) was much higher than that of the commercial NiMo/Al 2O 3 sulfide catalysts. Interestingly, the unsupported NiMo sulfide catalysts showed higher activity for hydrogenation (HYD) pathway than the direct desulfurization (DDS) pathway in the HDS of DBT. The same trends were observed for the HDS of 4,6-DMDBT. Morphology, surface area, pore volume and the HDS activity of unsupported NiMo sulfide catalyst depended on the catalyst preparation conditions. Higher temperature and higher H 2 pressure and addition of an organic solvent were found to increase the HDS activity of unsupported NiMo sulfide catalysts for both DBT and 4,6-DMDBT HDS. Higher preparation temperature increased HYD selectivity but decreased DDS selectivity. High-resolution TEM images revealed that unsupported NiMo sulfide prepared at 375 °C shows lower number of layers in the stacks of catalyst with more curvature and shorter length of slabs compared to that prepared at 300 °C. On the other hand, higher preparation pressure increased DDS selectivity but decreased HYD selectivity for HDS of 4,6-DMDBT. HRTEM images showed higher number of layers in the stack for the NiMo sulfide prepared under an initial H 2 pressure of 3.4 MPa compared to that under 2.1 MPa. The optimal Ni/(Mo + Ni) ratio for the NiMo sulfide catalyst was 0.5, higher than that for the conventional Al 2O 3-supported NiMo sulfide catalysts. This was attributed to the high dispersion of the active species and more active NiMoS generated. The present study also provides new insight for controlling the catalyst selectivity as well as activity by tailoring the hydrothermal preparation conditions.

Unsupported NiMo Sulfide Catalysts Obtained from Nickel/Ammonium and Nickel/Tetraalkylammonium Thiomolybdates: Synthesis and Application in the Hydrodesulfurization of Dibenzothiophene

Ammonium and tetraalkylammonium tetrathiomolybdates impregnated with nickel nitrate were used as precursors of unsupported NiMo sulfide catalysts. The precursors were decomposed either in situ during the course of a dibenzothiophene (DBT) hydrodesulfurization (HDS) test or ex situ through sulfidation by H2S/H2 (15% v/v H2S). The catalysts were characterized by thermogravimetric analysis, N2 adsorption, scanning electron microscopy (SEM), and X-ray diffraction. Textural and catalytic properties of these NiMo catalysts were strongly influenced both by the nature of the precursor and the activation procedure. For ex-situ activated NiMo catalysts, the use of carbon-containing tetraalkylammonium thiosalts as precursors did not lead to a significant improvement in HDS activity. For in situ activated NiMo catalysts, the role of carbon is more complex. The use of tetramethyl- or tetrapropylammonium tetrathiomolybdate salts led to a poor final HDS activity while using tetrabutylammonium tetrathiomolybdate, a net increase in HDS activity was observed compared to the use of the non-carbon containing ammonium tetrathiomolybdate. This was related to the development of a mesoporous structure and to a high increase in surface area. This result is in agreement with those found previously for CoMo catalysts and confirms that tetraalkylammonium tetrathiomolybdate salts with long alkyl chains lead to Co- or Ni-promoted MoS2-based catalysts with enhanced HDS activity if in situ activated.

Characterization and Activity of Unsupported Ni-Mo Sulfide Catalysts in HDN/HDS Reactions

Sulfided Ni-Mo catalysts prepared by homogeneous sulfide precipitation (HSP) and impregnated thiosalt decomposition (ITD) methods were used in thiophene HDS, pyridine HDN, and simultaneous HDS/HDN reactions. All catalysts were characterized by BET surface area measurement, X-ray diffraction, laser Raman spectroscopy, and X-ray photoelectron spectroscopy. The effect of various reaction parameters such as catalyst composition, method of catalyst synthesis, reaction temperature, and reaction pressure on catalytic behavior has been investigated. The inhibition/enhancement effects in simultaneous HDS/HDN reactions were also examined. The studies conducted over the unsupported catalysts present trends very similar to those obtained over the supported catalysts and suggest the intrinsic nature of active sites to be essentially independent of the support

Low-temperature synthesis of mixed NiMo sulfides: structural, textural and catalytic properties

Unsupported NiMo sulfide catalysts are prepared by an original coprecipitation technique performed at low temperature. Studies by several surface investigation techniques (X-ray diffraction, electron microscopy, O 2-chemisorption, and X-ray photoelectron and ion-scattering spectroscopy) provide a good qualitative picture of these catalysts. The existence of an NiMoS mixed phase is observed over the whole range of composition. This MoS 2-like structure is supported on Ni x S y if Ni/(Ni+Mo) > 0.25, resulting in an enhancement of catalytic activities (on hydrogenolysis and hydrogenation). These results are discussed with respect to the geometrical model of hydrotreating catalysts. This work points out the extreme importance of the degree of exposure of active edges of the NiMoS phase on the overall activity.