PROCESS FOR THE PREPARATION OF MOLYBDENUM DISULFIDE NANOPARTICLES SUPPORTED ON TITANIA

Abstract

The invention relates to a process for the preparation of nanoparticles of MoS.sub.2 supported on TiO.sub.2 wherein the preparation is performed by reductive coprecipitation using aqueous solutions containing Ti and Mo precursor salts, and wherein MoS.sub.2 may be non-promoted or Co-promoted. Further, the invention relates to the use of said nanoparticles as hydrodesulfurization catalysts.

Claims

1. A process for the preparation of nanoparticles of MoS.sub.2 supported on TiO.sub.2 wherein the preparation is performed by reductive coprecipitation using aqueous solutions containing Ti and Mo precursor salts, and wherein MoS.sub.2 may be non-promoted or Co-promoted.

2. The process of claim 1, wherein the Ti and Mo precursor salts are TiCl.sub.3 and (NH.sub.4).sub.2MoS.sub.4, respectively.

3. The process of claim 2, wherein the preparation is in a single step directly from a solution of the respective metal salts TiCl.sub.3 and (NH.sub.4).sub.2MoS.sub.4.

4. The process of claim 3, wherein the preparation is performed under acidic conditions and a chelating agent selected from EDTA or citric acid is added during preparation.

5. The process of claim 4, wherein the preparation is performed at a pH in the range of 3 to 4.

6. The process of claim 1, wherein the preparation is in two steps from a dispersion of TiO.sub.2-x support precursor in a solution of (NH.sub.4).sub.2MoS.sub.4, wherein the TiO.sub.2-x support precursor is prepared prior to introduction of the (NH.sub.4).sub.2MoS.sub.4 salt.

7. The process of claim 6, wherein the TiO.sub.2-x support precursor is prepared by thermolysis or hydrolysis.

8. The process, comprising: using MoS.sub.2 nanoparticles supported on titania as produced by the process of claim 1 as a hydrodesulfurization catalyst by passing a reaction feed over the hydrodesulfurization catalyst.

9. (canceled)

10. (canceled)

Description

DESCRIPTION OF THE DRAWINGS

[0037] FIG. 1: Schematic representation of the one-step (RCP1) and two-step (RCP2) reductive coprecipitation processes. While in RCP1 the product forms directly in an aqueous solution of the precursor salts, RCP2 involves preparation of TiO.sub.2-x and subsequent loading with MoS.sub.2.

[0038] FIG. 2: TEM images of TiO.sub.2 prepared by thermolysis (a) or hydrolysis (b). Inset: electron diffraction pattern.

[0039] FIG. 3. TEM images of a) MoS.sub.2/TiO.sub.2 prepared by RCP1 in water. Inset: ED pattern. b) HRTEM image of the same sample. MoS.sub.2 is indicated. c) MoS.sub.2/TiO.sub.2-RCP2-T. d) HRTEM picture of the same sample. The (110) crystal lattice of rutile TiO.sub.2 and stacked MoS.sub.2 particles are indicated.

[0040] FIG. 4. TEM-EDX linescan of (a) MoS.sub.2/TiO.sub.2-RCP1 and (b) MoS.sub.2/TiO.sub.2-RCP2-T. The linescan started at 1 as indicated in the TEM image (left) and the intensities of the emission lines are plotted on the right. The black dotted line indicates the intensity ratio of (S K+Mo L)/Mo K emission lines of a bulk MoS.sub.2 reference sample. According to standard terminology in electron microscopy the labels K and L refer to the electron structure of the atoms (K,L,M, . . . shells).

[0041] FIG. 5: Fitted Mo 3d (left) and S 2p (right) XP spectra of a) bulk MoS.sub.2 reference, b) MoS.sub.2/TiO.sub.2-RCP2-T and c) MoS.sub.2/TiO.sub.2-RCP1. The datapoints are represented by open circles and the lines represent the fits. The various contributions to the fit are labeled in the graphs. The spectra of MoS.sub.2/TiO.sub.2-RCP2 were magnified six times.

[0042] FIG. 6: secondary electron TEM image (left) and HR-TEM image (right) of sample MoS.sub.2-sol.

[0043] FIG. 7: X-ray diffractograms of TiO.sub.2 and MoS.sub.2/TiO.sub.2 prepared by RCP2 after thermolysis or hydrolysis.

[0044] FIG. 8: XPS survey scan of sample MoS.sub.2/TiO.sub.2-RCP1.

[0045] FIG. 9: Arrhenius plot obtained from the DBT activity data of RCP samples and a commercial reference.

[0046] FIG. 10: Selectivity as a function of temperature. Lines connect the points and are only drawn to guide the eye.

[0047] The invention is illustrated by the following non-limiting examples.

EXAMPLES

Materials Preparation

[0048] A detailed description of the materials synthesis is provided in the Supporting Information (vide infra). Key aspects of the materials synthesis are given below. The RCP synthesis procedure was modified from Xie et al..sup.[5a] MoS.sub.2/TiO.sub.2-RCP1 was synthesized from aqueous solutions of TiCl.sub.3 and (NH.sub.4).sub.2MOS.sub.4 at 100? C. The promoted material, Co-MoS.sub.2/TiO.sub.2-RCP1, was synthesized via the same procedure with Co(NO.sub.3).sub.2.6H.sub.2O added to the TiCl.sub.3 solution. For the preparation of RCP2 materials, TiO.sub.2-x was synthesized first by thermolysis (T) or hydrolysis (H). In thermolysis, TiO.sub.2-x was formed overnight at 60? C. from an aqueous solution of TiCl.sub.3 in HCl, stabilized by NaCl. In hydrolysis, TiO.sub.2-x was formed overnight at 60? C. by basification of acidic TiCl.sub.3 solution with NaOH (1M). TiO.sub.2-x was filtered and washed and redispersed in water. MoS.sub.2/TiO.sub.2-RCP2 materials were then synthesized by addition of an aqueous (NH.sub.4).sub.2MOS.sub.4 solution to the suspension of TiO.sub.2-x under inert conditions.

Characterization

[0049] N.sub.2 adsorption isotherms were measured at ?196? C. on a Micromeretics Tristar II. Prior to analysis, samples were heated at 160? C. for 4 hr under flowing N.sub.2. Specific surface area was determined by the BET method. Transmission Electron Microscopy measurements were made with a Tecnai-20F microscope operated at 200 kV and equipped with a field-emission gun. The elemental analysis by energy-dispersive X-ray spectroscopy was performed on the same microscope, utilizing an EDAX analyzer with TIA software. X-ray diffraction (XRD) patterns were recorded with a PANalytical X'pert PRO powder diffractometer equipped with a sealed Cu anode tube, operated at 45 kV and 40 mA. Samples were ground with a mortar and pestle prior to analysis. X-ray photoelectron spectroscopy (XPS) was performed with a Kratos AXIS Ultra spectrometer, equipped with a monochromatic X-ray source and a delay-line detector (DLD). Spectra were obtained using the aluminium anode (Al K?=1486.6 eV). Survey scans were measured at a constant pass energy of 160 eV and region scans at 40 eV. The background pressure was 2?10 .sup.9 mbar. Energy correction was performed by using the C is peak at 284.6 eV as a reference. X-ray fluorescence (XRF) was recorded with a PANalytical spectrometer equipped with a MagiX Pro (PW2440). Samples were mixed with Al.sub.2O.sub.3 and a glass bead was sintered for analysis.

Catalytic Hydrodesulfurization Activity

[0050] The catalytic activity was determined by means of dibenzothiophene (DBT) hydrodesulfurization in a fixed bed high-pressure tubular reactor with a down-flow (trickle flow) of gas and liquid feed (40 bar, H.sub.2 flow of 2.25 ml min.sup.?1,WHSV of 1.4 h.sup.?1). The reactor, 240 mm in length and 4 mm in diameter (ID) was packed with 400 mg of 30 to 80 mesh catalyst particles sandwiched between two ZrO.sub.2 layers. The catalysts were pretreated with n-hexadecane (Sigma-Aldrich) spiked with 5.2% tetranonyl pentasulfide (TNPS, Sigma-Aldrich) at 280? C. for 5 hours and subsequently at 340? C. for 24 hours. Afterwards, the temperature was lowered to 200? C. for 8 hours. Then, the feed was switched to the reaction feed (5 wt. % DBT, 2 wt. % adamantane in n-hexadecane). After equilibration for 2 hours, the temperature was increased to the desired reaction temperature (245? C.). Steady-state activity was measured after 24 hours of reaction by offline GC-FID.

Supporting Information

1. Experimental Details

[0051] Catalysts were prepared by one-step RCP or two-step RCP. To prevent oxidation of Ti(III) by air, all solutions were prepared in a glovebag (purged 3 times with oxygen-free N.sub.2 gas) from demineralized and degassed water.

1.1 One-Step Reductive Co-Precipitation

[0052] In a typical one-step RCP experiment, 1 g (3.8 mmol) ammonium tetrathiomolybdate (ATM, Sigma-Aldrich) was dissolved in 40 ml water and 10 ml ammonia (25%, Sigma-Aldrich) and filtered to remove residual particles (pH 11). A Ti[EDTA] solution was prepared by dissolving 2.48 g (7.6 mmol) ethylendiaminetetraacetic acid diammonium hydrate salt (EDTA, Sigma-Aldrich) in 30 ml water, adding 7.6 ml (7.6 mmol) titanium trichloride in hydrochloric acid (2-3M) solution (Sigma-Aldrich) and 2.1 ml concentrated ammonia. For promoted samples, 30 ml aqueous solution of cobalt nitrate hexahydrate (1.7 mmol, Sigma-Aldrich) was slowly poured into the dark-purple TiCl.sub.3 solution. The obtained Ti[EDTA] solution (pH 1) was added dropwise to the ATM solution. The reaction mixture was refluxed for 24 hours at 100? C. A black suspension was formed, which was centrifuged and washed with water. The black residue was dried in nitrogen atmosphere at 50? C. A black solid was obtained.

1.2 Two-Step Reductive Co-Precipitation

[0053] In a typical two-step RCP experiment, 1 g (3.8 mmol) ATM was dissolved in 50 ml 0.2M citric acid or EDTA solution and subsequently filtered to remove residual particles. The pH was adjusted to 11 by adding concentrated ammonia (25%). The ATM solution was added to a blue suspension of reduced titanium oxide (TiO.sub.2-x) in water prepared either via thermolysis or hydrolysis. The suspension was refluxed for 16 hours at 60? C. A dark brown suspension was formed, which was centrifuged and washed with water. The brown residue was dried in nitrogen atmosphere at 50? C. A dark brown solid was obtained.

1.3 Preparation of Blue Titania

[0054] Blue titania was prepared by thermolysis (see Y. Xie, K. Ding, Z. Liu, R. Tao, Z. Sun, H. Zhang, G. An, J. Am. Chem. Soc. 2009, 131, 6648-6649) or hydrolysis.

[0055] Thermolysis: 10 g of an aqueous solution of TiCl.sub.3 (20%) and hydrochloric acid (3%) (Alpha Aesar) was added to 26 g of an aqueous solution of NaCl (30%) (Sigma-Aldrich). A purple solution was obtained. After refluxing for 16 hours at 100? C. under nitrogen atmosphere a blue suspension was obtained, which was filtered, washed and redispersed in water.

[0056] Hydrolysis: 5.9 g of a solution of TiCl.sub.3 (20%) in hydrochloric acid (3%) solution was dissolved in 45 ml water. Subsequently, 24 ml 1 M NaOH (Sigma-Aldrich) were slowly added and the solution turned black. After refluxing at 60? C. in nitrogen atmosphere for 16 hours a blue suspension was obtained with a pH of 1. The solution was neutralized by adding 1 M NaOH.

1.4 Preparation of Ti[NTA] Solution

[0057] 11.6 g nitrilotriacetic acid (NTA) were suspended in 60 ml water. The pH was adjusted to 9 with concentrated ammonia solution (25%) and the solution became clear. Subsequently, 11.6 ml of an aqueous solution of TiCl.sub.3 (20%) and HCl (3%) were added dropwise and under vigorous stirring to the NTA solution. The solution turned green.

[0058] During the addition of TiCl.sub.3 it is important to maintain the pH of the solution to above 2 to prevent precipitation. The pH was increased by adding a saturated (NH.sub.4).sub.2CO.sub.3.NH.sub.4HCO.sub.3 solution (do not use ammonia to increase the pH since it will precipitate Ti.sup.3+). When all the TiCl.sub.3 was added to the solution, the pH was adjusted to 7 with saturated (NH.sub.4).sub.2CO.sub.3.NH.sub.4HCO.sub.3 solution and the volume was adjusted to 100 ml with water. A dark blue/green solution was obtained with a concentration of 0.1M Ti and 0.4M NTA.

1.4 Mo.sup.6+-Ti.sup.3+ Redox Reaction in Aqueous Solution

[0059] In a typical experiment, 76 ml Ti[NTA] solution were added via a septum to a round-bottom flask under nitrogen atmosphere. Meanwhile 1 g of ATM was dissolved in 50 ml 0.2M citric acid solution. The solution (pH 7) was stirred at room temperature for one hour and then filtered under nitrogen atmosphere to remove residual particles. A dark-red solution was obtained and added to the dark blue Ti[NTA] solution via a septum. A black precipitate was immediately formed and the pH dropped to 6.5. The suspension was refluxed for 4 hours at 60? C. The pH increased to 7-7.5. The black suspension was centrifuged and washed with water. The black residue was dried in nitrogen atmosphere at 50? C. A fine black powder was obtained.

TABLE-US-00004 TABLE S1 description and composition of the various samples prepared via different synthesis procedures. Mo S/Mo Mo/Ti Sample Procedure Ti precursor (wt %).sup.[a] Ratio.sup.[b] Ratio.sup.[b] 1. MoS.sub.2/TiO.sub.2-RCP1 1-step RCP TiCl.sub.3 13.3 2.1 0.30 2. CoMoS.sub.2/TiO.sub.2-RCP1 1-step RCP TiCl.sub.3 13.9 n.m. n.m. 3. MoS.sub.2/TiO.sub.2-RCP2-T 2-step RCP TiO.sub.2?x 5.9 n.m. n.m. thermolysis 4. MoS.sub.2/TiO.sub.2-RCP2-H 2-step RCP TiO.sub.2?x 3.2 2.5 0.04 hydrolysis 5. MoS.sub.2-sol Solution Ti.sup.III [NTA] 43.2 2.0 59.9 redox .sup.[a]Sample 1, 4 and 5 determined by XRF, sample 2 and 3 determined by ICP-OES. .sup.[b]Molar ratio determined by XRF.
2. Redox Reaction Between Mo.sup.6+ and Ti.sup.3+ in Aqueous Solution.

[0060] To investigate the redox reaction between Ti.sup.3+ and Mo.sup.6+, it is important to exclude any pH effects that can lead to undesired precipitation of side products. Neutral solutions of Ti.sup.3+ and MoS.sub.4.sup.2? were prepared by chelation according to the procedure described in section 1. When the Ti[NTA] solution was added to the ATM solution at room temperature, a black precipitate formed instantly, indicating that the redox reaction between Ti.sup.3+ and MoS.sub.4.sup.2 is fast.

[0061] Chemical analysis by XRF (sample 6, Table S1) reveals that the product consists of MoS.sub.2 particles with a nearly stoichiometric ratio of S to Mo. Titanium does not precipitate from solution as only trace amounts of titanium were detected in the sample. The TEM pictures (FIG. 6) show that the particles are rather large (?200 nm) and are composed of amorphous MoS.sub.2. Layered structures are visible in the HR-TEM image; the distance between planes is characteristic of the interlayer distance in stacked MoS.sub.2 particles along the [001] direction. The negligible presence of titanium in the sample can be rationalized by chelation of Ti.sup.4+ with NTA, which form stable complexes under reaction conditions. Thus, Ti.sup.III[NTA] is oxidized to Ti.sup.IV[NTA], which remains stable in solution. Simultaneously Mo.sup.VIS.sub.4.sup.2? is reduced to Mo.sup.IVS.sub.2, which is insoluble and immediately precipitates to form the black suspension.

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