Increase In Hydrodesulfurization Activity Research Articles

Development of improved catalysts for deep HDS of diesel fuels

Deep hydrodesulfurization (HDS) of gas oils continues to attract research interest due to environmental-driven regulations which limit its sulfur content to 10–15 ppm in several countries. This paper highlights some of the recent studies conducted at King Fahd University of Petroleum and Minerals to develop improved HDS catalysts. The first study was focused on the effect of Co/(Co + Mo) ratio in CoMo/Al2O3 catalysts on HDS pathways of benzothiophene (BT) and dibenzothiophene (DBT). Co/Co + Mo ratio exhibited significant influence on the direct desulfurization (DDS) pathway, but showed no influence on the hydrogenation pathway. A Co/Co + Mo ratio of 0.4 exhibited optimum promotion effect of Co for HDS by DDS route and hence overall HDS. The second study investigated the effect of phosphorus addition on simultaneous HDS reactions and their pathways. The results indicate that phosphorus modification of CoMo/γ-Al2O3 catalysts resulted in enhancement of HDS due to increased dispersion of MoO3 and the maximum enhancement was achieved with 1.0 wt % P2O5. Enhancement of HDS rates was in the following order: 4,6-DMDBT (51 %) > 4-MDBT (38 %) > DBT (26 %). In the third approach, a series of NiMo catalysts supported on Al2O3-ZrO2 composites containing 0–10 wt % ZrO2 was synthesized, characterized and evaluated for deep desulfurization of gas oil. An increase of 1.3–2.5 times increase in HDS activity at 320–360 °C was observed due to reduced interaction between Al2O3 and the active metals. A correlation was found between the enhancement of hydrogenation activity of sulfided catalysts and the reducibility of their oxide precursors.

Mesoporous Matrix Encapsulation for the Synthesis of Monodisperse Pd5P2 Nanoparticle Hydrodesulfurization Catalysts

The synthesis of monodisperse 5-10 nm Pd5P2 catalytic particles by encapsulation in a mesoporous silica network, along with preliminary data on hydrodesulfurization (HDS) activity, is reported. Precursor Pd-P amorphous nanoparticles are prepared by solution-phase reaction of palladium(II) acetylacetonate with trioctylphosphine at temperatures up to 300 °C. Direct crystallization of Pd5P2 in solution by increasing the temperature to 360 °C leads to sintering, but particle size can be maintained during the transformation by encapsulation of the amorphous Pd-P particles in a mesoporous silica shell, followed by treatment of the solid at 500 °C under a reducing atmosphere, yielding Pd5P2@mSiO2. The resultant materials exhibit high BET surface areas (>1000 m(2)/g) and an average pore size of 3.7 nm. Access to the catalyst surface is demonstrated by dibenzodithiophene (DBT) HDS testing. Pd5P2@mSiO2 shows a consistent increase in HDS activity as a function of temperature, with DBT conversion approaching 60% at 402 °C. The ability to control particle size, phase, and sintering is expected to enable the fundamental catalytic attributes that underscore activity in Pd5P2 to be assessed.

Effect of Alumina Support on the Formation of the Active Phase of Selective Hydrodesulfurization Catalysts Co-Mo/Al2O3

Two different commercial alumina supports were used for the preparation of selective hydrodesulfurization (HDS) catalysts Co-Mo/Al 2 O 3 . The effect of support properties on the HDS activity and selectivity of the corresponding Co-Mo catalysts were studied by X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The HDS activity and selectivity were measured using a continuous flow fixed-bed microreactor. The Al 2 O 3 support with higher crystallinity and fewer hydroxyl groups had a moderate metal-support interaction, which resulted in an increase of the average slab length and stacking number of MoS 2 . This gave more formation of the active CoMoS phase and an increase in HDS activity and selectivity. ：以两种商用 Al 2 O 3 为载体, 制备了汽油选择性加氢脱硫催化剂 Co-Mo/Al 2 O 3 , 并采用红外光谱、X 射线衍射、N 2 吸附-脱附、透射电镜、扫描透射-能谱和 X 射线光电子能谱等手段系统研究了载体物化性质对催化剂活性相形成的影响. 结果表明, 表面羟基数量少和结晶程度高的载体与活性金属间相互作用减弱, 促进了 Mo 物种的硫化还原, 使 MoS 2 片晶的尺寸和层数增加, 且其硫化态催化剂上 CoMoS 活性位更多, CoMoS/MoS 2 比更大, 因而显著提高了相应 Co-Mo 催化剂加氢脱硫活性和选择性. An Al 2 O 3 support with higher crystallinity and fewer hydroxyl groups has a moderate metal-support interaction that increased the slab length and stacking number of MoS 2 , which gave more formation of the active CoMoS phase and an increase in Hydrodesulfurization activity and selectivity.

Effect of surface characteristics of different alumina on metal–support interaction and hydrodesulfurization activity

Effect of surface characteristics of two species of commercial γ-alumina (alumina-A and alumina-B) on metal–support interaction and hydrodesulfurization (HDS) activity of the corresponding CoMo catalysts was studied systematically. The HDS activity tests were carried out in a continuous flow fixed-bed micro-reactor. Compared with alumina-A, alumina-B with better crystallinity and less hydroxyl groups can partly inhibit inactive cobalt spinel formation and moderate the metal–support interaction, causing a notable increase in the average slab length and stacking number of MoS 2. It stimulates the promoting effect of cobalt on molybdenum and contributes to a noticeable increase in HDS activity.

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.

Morphological investigation of nanostructured CoMo catalysts

This work reports the morphological investigation of nanostructured sulfided CoMo catalysts by means of high-resolution transmission electron microscopy (HRTEM). The catalysts were supported on Ti-modified hexagonal mesoporous silica (HMS-Ti) and P-modified HMS-Ti (P/HMS-Ti) materials. The oxide precursors were characterized by specific surface area (S BET), temperature-programmed reduction (TPR), diffuse reflectance infrared Fourier transform spectroscopy in the OH region (DRIFTS-OH) and X-ray photoelectron spectroscopy (XPS) in order to elucidate the influence of the impregnation sequence (successive vs. simultaneous) and the effect of P-incorporation into HMS-Ti material on the morphology of calcined CoMo catalysts. Both TPR and XPS measurements indicate that the catalysts prepared by successive impregnation possess well-dispersed MoO 3 and CoO phases, whereas their counterparts prepared by simultaneous impregnation additionally possess the CoMoO 4 phase. For all sulfided catalysts, the presence of MoS 2 phase with particle size in the range 3.3–4.4 nm was confirmed by HRTEM. Catalytic activity was evaluated in the reaction of hydrodesulfurization (HDS) of dibenzothiophene (DBT) carried out in a flow reactor at 593 K and hydrogen pressure of 5.5 MPa. P-incorporation into the HMS-Ti material led to an overall increase in HDS activity and the hydrogenation ability of the sulfided catalysts. All catalysts proved to be stable during 10 h time-on-stream (TOS) operation. The activity of sulfide catalysts in the target reaction depends linearly on the surface exposure of Co species in the oxide precursors, as determined by XPS, and on the morphology of the sulfide form of catalysts (surface density of MoS 2 particles and their sizes) as determined by HRTEM.

Phosphorus promotion of Ni (Co)-containing Mo-free catalysts in thiophene hydrodesulfurization

Several unsupported nickel (cobalt)–phosphorus (sulfur) model compounds were prepared, identified and characterized by X-ray diffraction (XRD), electron microscope (EM)/energy dispersive X-ray analysis (EDAX), temperature programmed sulfidation (TPS), BET, thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). They were characterized again following pretreatments (in He, H 2 or H 2/H 2S) at different pressures, then by thiophene hydrodesulfurization (HDS) catalytic activity test at atmospheric pressure. The presence of sulfur (presulfidation by H 2S) was not required to convert the model compounds into active catalysts, a beneficial influence of H 2S on the activity was only to facilitate the reduction process into the most active Ni 2P or less active Co 2P phases. Ni(P)/SiO 2 catalysts were also prepared, characterized and tested in thiophene HDS. For sulfided P-free Ni/SiO 2 catalysts the HDS activity correlated with the Ni 3S 2 surface area determined by dynamic oxygen chemisorption (DOC). The addition of phosphorus causes a strong increase in HDS activity of silica supported catalysts, which are much higher than could be expected by their nickel sulfide surface area, therefore, the formation of Ni 2P (identified by XRD) again explains their high activity. The HDS activity of phosphorus promoted nickel (or cobalt) containing molybdenum-free catalysts is related to the amount and dispersion of Ni 2P or Co 2P present in the material.

Sulfided Mo and CoMo supported on zeolite as hydrodesulfurization catalysts: transformation of dibenzothiophene and 4,6-dimethyldibenzothiophene

The hydrodesulfurization (HDS) of dibenzothiophene (DBT) and of 4,6-dimethyldibenzothiophene (4,6-DMDBT) was carried out on sulfided Mo and CoMo on HY catalysts, and also on sulfided Mo and CoMo on alumina catalysts (fixed bed reactor, 330°C, 3 MPa hydrogen pressure). On all the catalysts, the two reactants transformed through the same parallel pathways: direct desulfurization (DDS) leading to biphenyl-type compounds, and desulfurization after hydrogenation (HYD) leading first to tetrahydrogenated intermediates, then to cyclohexylbenzene-type products. However, additional reactions were observed with the zeolite-supported catalysts, namely methylation of the reactants, cracking of the desulfurized products, and, in the case of 4,6-DMDBT, displacement of the methyl groups and transalkylation. The global activity of Mo/zeolite in DBT or 4,6-DMDBT transformation as well as its activity for the production of desulfurized products (HDS) were much higher than those of Mo/alumina. On the other hand, cobalt exerted a promoting effect on the activity in the transformation of DBT or 4,6-DMDBT of all the molybdenum catalysts. However, this effect was much less significant with the zeolite support than with the alumina support, which indicated that the promoter was not well associated to molybdenum on the zeolite support. Therefore, the activity of CoMo/zeolite in the HDS of DBT was much lower than that of CoMo/alumina. On the contrary, in the case of 4,6-DMDBT CoMo/zeolite was more active in HDS than CoMo/alumina. This increase in HDS activity was attributed to the transformation of 4,6-DMDBT into more reactive isomers through an acid-catalyzed methyl migration. The consequence was that on the zeolite-supported catalyst 4,6-DMDBT was more reactive than DBT.

An XAFS Study of the Different Influence of Chelating Ligands on the HDN and HDS of γ-Al2O3-Supported NiMo Catalysts

γ-Al2O3-supported NiMo catalysts prepared in the presence of the chelating ligands nitrilotriacetic acid (NTA) and ethylenediamine tetraacetic acid (EDTA) were tested in the hydrodenitrogenation (HDN) of o-toluidine and in the hydrodesulfurization (HDS) of thiophene. For the molar ratio NTA : Ni=1 a maximum was observed in the HDN activity, but a minimum activity was observed in the HDS and in the hydrogenation of cyclohexene. The highest HDS activities were obtained for the calcined catalyst prepared in the absence of ligands and for the catalyst prepared with EDTA. The Mo K-edge Quick EXAFS and normal EXAFS data demonstrated that in the catalysts with the highest HDN activity MoS2 is formed at the lowest temperature and the MoS2 crystallites have the highest structural order. This suggests that o-toluidine can adsorb better on MoS2 crystallites with more regular edges. The increase in HDS activity was ascribed to a change in the mechanism of sulfidation of Ni, induced by the chelating ligands, and to a subsequently higher dispersion of Ni. Mo-oxysulfides could not be distinguished by Quick EXAFS during sulfidation of Mo. The Quick EXAFS data indicate that a MoS3-like phase precedes MoS2 during the sulfidation process. The chelating ligands screen the Ni cations and prevent their interactions with the environment. In the absence of chelating ligands, Ni interacts with the support and changes its surface properties. In this way the Mo–support interactions and the sulfidation of Mo are affected. The influence of the chelating ligands on the sulfidation of Mo suggests that the sulfidation process of Mo is closely dependent on the sulfidation of Ni.

Hydrodesulfurization: III. Relation between pretreating conditions and hydrodesulfurization activity of Co-Mo-Al 2O 3 catalyst

Hydrodesulfurization (HDS) experiments were carried out over a commercial Co-Mo-Al 2O 3 catalyst in a continuous-flow reactor at atmospheric pressure and temperatures of 250 and 350 °C. The reactants were hydrogen and benzothiophene (BT) dissolved in n-dodecane. The effects of pretreating conditions on catalytic activity were investigated by following BT conversion as a function of run time. It was shown that stationary activities differed notably with pretreatment conditions and temperature. Contrary to the results at 400 °C, prereduction at 250 °C was inadequate to promote rapid HDS activity. Presulfiding at 250 °C (following prereduction or not) led to formation of a highly active catalyst. Presulfiding at 350 °C or processing at 350 °C gave lesser activities than in the same pretreating conditions at 250 °C. The results are explained in terms of the poisoning phenomena of active sites by hydrocarbon molecules and the role of catalyst presulfiding, which at the same time causes an increase of HDS activity as well as a protecting effect against poisoning, is discussed.