Effect of lanthanum addition on the thiophene hydrodesulfurization activity over Al-MCM-41 supported molybdenum catalysts

A series of Al-MCM-41 modified with 1–7% lanthanum were used as supports to prepare the Mo/La–Al-MCM-41 catalysts containing 10 wt.% molybdenum. The supports and catalysts were characterized with XRD, BET, XPS, TPD, TEM and SEM, and their catalytic activities were tested for thiophene hydrodesulfurization. The La addition did not cause any significant collapse of the structure and morphology of Al-MCM-41 samples, and increased the acidity of Al-MCM-41 samples. The Mo/La–Al-MCM-41 catalysts showed higher thiophene HDS activity than non-modified catalysts. The La-modified catalysts showed an enhanced butene selectivity but a decreased tetrahydrothiophene selectivity, indicating that the La–Al-MCM-41 supports contained a larger amount of acid sites.     

碳化钼催化剂的制备及噻吩加氢脱硫性能

以MoO₃为前躯体,CH₄/H₂为还原碳化气,采用自制的程序升温还原碳化反应装置制备出Mo₂C催化剂,并用XRD、BET进行表征.借助原位TG-DTA方法研究了MoO₃在CH₄/H₂气氛中的还原碳化历程和适宜的还原碳化温度.以噻吩/环己烷溶液为模型反应物,采用高压微反-色谱实验装置考察了制备的碳化钼催化剂的噻吩加氢脱硫反应性能.结果表明：程序升温条件下的局部规整反应可提高催化剂的比表面积,且制备的碳化钼催化剂具有较高的噻吩加氢脱硫反应活性,在体积分数为5%的噻吩/环己烷溶液中,反应压力为3.0 MPa,空速为6 h^-1,H₂/原料液体积比500∶1的反应条件下, 370℃时的噻吩转化率达到98%以上,明显高于相应的硫化钼催化剂.还原碳化终温的提高,导致碳化钼催化剂比表面积的减少和表面积炭的增多,进而使其加氢脱硫反应活性降低.MoO₃在CH₄/H₂气氛中的还原碳化历程应为MoO₃→MoO₂→MoO_xC_y→Mo₂C,实验确定的适宜还原碳化温度为675℃.     

Simultaneous hydrodesulfurization of thiophene and hydrogenation of cyclohexene over dimolybdenum nitride catalysts

A series of unsupported dimolybdenum nitride (γ-Mo₂N) catalysts differing in surface area were prepared by temperature programmed reduction of MoO₃ with a mixture of NH₃:N₂ (90:10). Characterization of catalysts by BET, XRD, TPR and XPS techniques was carried out. The samples were used as catalysts in hydrotreating reactions (simultaneous hydrodesulfurization of thiophene and hydrogenation of cyclohexene). Low surface area γ-Mo₂N materials show much higher specific conversions than those with higher surface area. These results indicate that HDS and HYD reactions over γ-Mo₂N seem to be structure-sensitive. The relative exposure extent of crystalline planes (1 1 1) and (2 0 0) over the different catalysts can be associated with their hydrogen adsorption capacities and with their catalytic performances. The catalytic activities are significantly affected by the catalyst pretreatment conditions.     

Sulfidability and thiophene hydrodesulfurization activity of supported NiMo carbides

Ni–Mo carbides supported on activated carbon were synthesized by carbothermal hydrogen reduction and the effect of the sulfidability on the thiophene hydrodesulfurization catalytic activity was studied. The X-ray diffraction patterns of Ni–Mo carbides showed the presence of β-Mo₂C and NiC when the atomic ratio AR = Ni/(Ni + Mo) was between 0.25 and 0.75, while for AR = 1, it only was detected metallic Ni. The X-ray photoelectron spectroscopy results showed the distribution of different surface species in the passivated catalysts: Mo^δ + (0 ≤ δ ≤ 2), Mo^4 +, Mo^6 +, Ni^δ + and Ni^2 +. After sulfiding the carbides were modified in their surface and catalytic activity.     

A role of molybdenum and shape selectivity of catalysts in simultaneous reactions of hydrocracking and hydrodesulfurization

The hydrocracking and hydrodesulfurization (HDS) of n-heptane containing 0.2 mole% dibenzothiophene (DBT) were performed simultaneously using NiPtMo catalysts supported on HZSM-5, LaY and γ-Al₂O₃ in a high pressure fixed bed reactor. Molybdenum played an important role in both hydrocracking and hydrodesulfurization (HDS). We found that the sulfur compound, dibenzothiophene (DBT). in the reactant was adsorbed on a molybdenum site and converted to hydrogen sulfide so that the active sites of the catalysts for hydrocracking were less poisoned by DBT and the conversion of n-heptane over molybdenum impregnated catalyst was higher than that over molybdenum-free catalyst. The crystal structures of the molybdenum supported on the zeolite and γ-Al₂O₃ were mainly MoO_2.5 (OH)_0.5[021] and MoO₃[210] respectively as shown by XRD analysis. The structure of MoO_2.5(OH)_0.5 was easily reduced to MoS₂[003] during the reaction. After the reaction of 100 hours over the catalyst supported on γ-Al₂O₃ the crystal structure of MoO₃[210] partially changed to MoO₃[300] and the structure of MoS₂[003] was not observed. Because of the reactant shape selectivity of zeolite, the acid and the metal sites in the intracrystalline of the catalysts supported on zeolites were less poisoned by DBT. Therefore, both hydrocracking and HDS using n-heptane containing 0.2 mole% of DBT were successfully demonstrated over the prepared catalysts.     

Sulfidation of an aluminum-nickel-molybdenum catalyst and its activity in thiophene hydrodesulfurization

We consider the interaction of H₂S with the molybdenum component of the catalyst using an Mo₇O₂₄H₁₂ cluster as an example. At 200–300°C, SH groups substitute for OH groups of the molybdenum component. Intense reduction of the molybdenum component starts at 300°C. The reaction of hydrogen with the molybdenum component at 300°C generates Mo-H hydride groups, Mo⁵⁺ ions, and anionic vacancies; these vacancies are filled in by sulfur anions produced by the dissociation of hydrosulfuric acid. An NiMoO₄ crystal phase is reduced and completely sulfided at 300°C. Mo₇O₂₄H₁₂ clusters are sulfided only partially; complete substitution by sulfur does not occur in them even at 600–650°C. We consider the thiophene hydrodesulfurization scheme on molybdenum oxysulfide in the presence of nickel, as well as the scheme of this reaction on MoS₂ in the absence of nickel. We conclude that hydrodesulfurization and hydriding occur at the same sites. Catalytically active surface sites in Mo₇O _x S _y + Ni, MoS₂ + Ni, and MoS₂ structures are alike. A common feature of these structures is the existence of Mo-H⁻ and MoSH⁻ basic sites. The difference between them lies in the nature of proton-donor sites: in MoS₂, proton donors are Mo-S-H⁺ groups, whereas in nickel-containing structures, nickel crystallites reacting with hydrogen are generators of protons and hydride ions.     

Thioreduction of cyclopentanone and hydrodesulfurization of dibenzothiophene over sulfided nickel or cobalt-promoted molybdenum on alumina catalysts

Two series of sulfided Ni or Co promoted Mo/alumina catalysts, having different Ni or Co loadings, were characterized by their activities for the transformation of cyclopentanone into cyclopentanethiol (flow reactor, 220°C, atmospheric pressure) and for the hydrodesulfurization of dibenzothiophene (flow reactor, 340°C, 3 MPa hydrogen pressure). The addition of the promoter increased significantly the activity of the Mo/alumina catalyst for both reactions, up to a maximum obtained with the catalysts having a (promoter)/(promoter+Mo) molar ratio equal to 0.3–0.4. This increase in activity was due in part to an increase in the hydrogenating properties of the Mo/alumina catalyst. However, an additional modification of the catalyst (basic and nucleophilic properties) must be considered to account for the spectacular effect of the promoter on the rate of the dibenzothiophene direct desulfurization reaction.     

反应条件对Ni-Mo/TiO2-Al2O3催化剂上噻吩加氢脱硫的影响

采用溶胶-凝胶技术，从Al₂O₃载体的表面改性出发，制备了TiO₂-Al₂O₃复合载体，用此改性载体制备了NiMo/TiO₂-Al₂O₃催化剂；在中压固定床微反装置上考察了反应条件对噻吩加氢脱硫活性的影响。结果表明，在反应温度260～270 ℃、氢分压3.0 MPa、空速3.0～5.0 h^－1及氢油体积比480～550条件下，噻吩的转化率可达100％。     

Mechanistic considerations on the involvement of dihydrointermediates in the hydrodesulfurization of dibenzothiophene-type compounds over molybdenum sulfide catalysts

The mechanism of the hydrodesulfurization of dibenzothiophene-type compounds involving their dihydroderivatives as intermediates was examined in the light of previous experimental results dealing with the effect of cobalt and nickel promoters on the selectivity of the reaction. It is suggested that the orientation of the hydrodesulfurization reaction regarding the so-called direct desulfurization or hydrogenation pathways depends on various factors: the distribution of the dihydrointermediates (two of them only among nine possible isomers can lead directly from dibenzothiophene to biphenyl); the availability of dissociated hydrogen on the catalytic centers on which they adsorb and the basicity of the sulfur anions associated to the catalytic centers.     

The effect of additives on hydrodesulfurization of dibenzothiophene over bulk molybdenum sulfide: Increased catalytic activity in the presence of phenol

The effect of various additive organic reagents on the activation of MoS₃ as molybdenum sulfide catalyst precursor during hydrodesulfurization reaction of dibenzothiophene was studied. It was found that the presence of phenol or 1-naphthol greatly promoted the activity of the catalyst, while tetralin, 9,10-dihydrophenanthrene, ethylbenzene, and pyridine reagents were found to be detrimental for the activity of the catalyst.     

Preparation and characterization of molybdenum trioxide from spent hydrodesulfurization catalyst

An approach to produce molybdenum trioxide from spent hydrodesulfurization (HDS) catalyst, obtained from a petroleum refinery, is presented here. The spent catalyst was devolatilized at 600 °C so as to make it free from oils, organics and other volatile species. It was then roasted with sodium carbonate at a temperature of 850 °C for 30 min. The leaching efficiency for 20% soda roasted sample at 10% pulp density was 99.8%. From the solution molybdenum was precipitated out as ammonium molybdate at pH 1.0 with HCl and ammonium chloride. This ammonium molybdate was calcined at 750 °C to get MoO₃. The product was characterized by XRD. Its purity was determined titrimetrically and by ICP-AES.     

The role of water in ethanol oxidation over SnO2-supported molybdenum oxides

The role of water in the oxidation of ethanol to acetic acid on Sn–Mo–O catalysts was studied by catalytic test and FTIR spectroscopy of adsorbed species. The reaction showed a typical behavior of series reactions involving oxidation of ethanol to acetaldehyde and of the latter to acetic acid and CO₂. Addition of water to the feed gas decreased the oxidation rate and significantly increased the selectivity to acetic acid, strongly contributing to decreasing the number of secondary products. FTIR analyses showed that water promotes desorption of ethanol and carboxylates, present as bridging and monodentate species. Decreasing catalytic rate values and increasing selectivity to acetic acid in the presence of water follow from site blocking by hydroxyl groups. This revised version was published online in July 2006 with corrections to the Cover Date.     

CVD preparation of alumina-supported niobium nitride and its activity for thiophene hydrodesulfurization

Niobium nitride was synthesized on a Si(400) substrate and a γ-alumina pellet using a CVD method with a stream of NbCl₅/Ar, NH₃, and H₂ gases at 723–973 K under reduced pressure. The composition and surface properties of the deposited niobium nitride were analyzed using XRD and XPS measurements. The activity of alumina-supported niobium nitrides for the hydrodesulfurization (HDS) of thiophene at 673 K and atmospheric pressure was determined. The alumina had a surface area of 177 m² g⁻¹ and the alumina-supported niobium nitride catalyst had surface areas of 179–190 m² g⁻¹. Although the catalysts had low activity in the initial stages, the activity increased after 200–300 min started to about three times the initial activities. XPS analysis indicated that the activity of the niobium nitride catalysts was decreased by sulfur accumulation on the surface and nitrogen released from niobium nitride. The relationship between the surface properties of the niobium nitride catalysts and the activities for thiophene HDS is discussed.     

辉钼矿中钼和铼分离过程研究

采用D201树脂对辉钼矿氯酸钠浸出体系所得反萃液中的钼、铼吸附分离性能进行考察。试验结果表明,树脂对溶液中的铼具有良好的吸附选择性,在吸附温度为30℃,pH=8的条件下吸附1 h,钼、铼吸附率分别为3.26%、93.18%,分离因子为197.68,能够满足钼、铼分离提纯的需要。吸附过程符合Boyd液膜扩散方程,树脂对钼、铼的吸附过程受液膜扩散控制。     

Characteristics of intermetallic NdNi5 as an unsupported catalyst in thiophene hydrodesulfurization

A systematic research is carried out on intermetallic NdNi₅ for the hydrodesulfurization (HDS) of thiophene. The effect of calcination and presulfidation of intermetallic NdNi₅ on thiophene HDS was examined and property changes of the catalyst during calcination, presulfidation, and reaction were observed by XRD, SEM, and surface area techniques. The intermetallic compound of NdNi₅ is disintegrated to fine powder and recrystallized to Nd/Ni oxide and sulfide by calcination, presulfidation, and reaction. A model for the surface area increase of the catalyst during calcination, presulfidation, and reaction was proposed and the role of neodymium was explained.     

Highly active ReS2/γ-Al2O3 catalysts: Effect of calcination and activation over thiophene hydrodesulfurization

The effect of activation conditions on alumina-supported rhenium sulfide catalysts on their activity for thiophene hydrodesulfurization has been studied. The results showed that activation using H₂S/N₂ leads to ReS₂/γ-Al₂O₃ catalysts with a higher activity than an industrial NiMo catalyst. Characterization by temperature-programmed reduction and X-ray photoelectron spectroscopy revealed that the higher activity of the catalysts activated by H₂S/N₂ is associated with a higher sulfur content and probably related to different ReS_x phases.     

Support effect on the formation of catalytic site precursors in the thiophene hydrodesulfurization

The TiO₂ and Al₂O₃ supported FeMo catalysts (6 and 12 wt.% Mo) have been prepared using (NH₄)₃Fe(OH)₆Mo₆O₁₈ salt (FeMo₆). They have been studied by the IR, TPR, XPS and MS methods and tested in thiophene conversion. The initial salt transforms in analog of the TiMo acid anion on the titania support and alumina heteropolymolybdate anion on the alumina support. Higher thiophene conversion is observed on the TiO₂ supported catalysts.     

Experimental and theoretical study of microwave enhanced catalytic hydrodesulfurization of thiophene in a continuous-flow reactor

Hydrodesulfurization (HDS) of thiophene, as a gasoline model oil, over an industrial Ni-Mo/Al₂O₃ catalyst was investigated in a continuous system under microwave irradiation. The HDS efficiency was much higher (5%–14%) under microwave irradiation than conventional heating. It was proved that the reaction was enhanced by both microwave thermal and non-thermal effects. Microwave selective heating caused hot spots inside the catalyst, thus improved the reaction rate. From the analysis of the non-thermal effect, the molecular collisions were significantly increased under microwave irradiation. However, instead of being reduced, the apparent activation energy increased. This may be due to the microwave treatment hindering the adsorption though upright S-bind (η¹) and enhancing the parallel adsorption (η⁵), both adsorptions were considered to favor to the direct desulfurization route and the hydrogenation route respectively. Therefore, the HDS process was considered to proceed along the hydrogenation route under microwave irradiation.     

Comparison of molybdenum carbide and tungsten carbide for the hydrodesulfurization of dibenzothiophene

The molybdenum and tungsten carbides (Mo₂C and W₂C) were synthesized, characterized and tested in hydrodesulfurization (HDS) of dibenzothiophene (DBT). The phase purity of these catalysts was established by X-ray diffraction (XRD), and the surface properties were determined by N₂ BET specific surface area (Sg) measurements, CO chemisorption and high-resolution transmission electron microscopy (HRTEM). The activities of catalysts were determined during the HDS of DBT at a temperature of 613 K and under a 6 MPa total pressure. Both molybdenum and tungsten carbides were active in HDS of DBT. The reactivity studies showed that molybdenum carbide was more active than tungsten carbide related to weight. However, W₂C was shown to possess stronger hydrogenating properties.     

Catalytic performances of platinum doped molybdenum carbide for simultaneous hydrodenitrogenation and hydrodesulfurization

The hydrotreating activity of molybdenum carbide doped with platinum (0.3 wt.%) was studied and compared to that of a pure, non-modified Mo₂C. 4,6-Dimethyldibenzothiophene (4,6-DMDBT, 300 ppm of S) and carbazole (100 ppm of N) were designated as model compounds and subjected to hydrodesufurization (HDS) and hydrodenitrogenation (HDN) processes either separately or simultaneously. In both cases the molybdenum carbide doped with platinum (Mo₂C-Pt) turns out to be more active than Mo₂C. The increase of the hydrotreating activity, owing to the presence of platinum in molybdenum carbide can be related to the raise of hydrogenation activity of the modified catalyst. The platinum modified molybdenum carbide was stable (displays a long lifetime) under HDN and HDS reaction conditions. The predominant reaction products are bicyclohexyl (BCH) for the HDN process or 3,3′-dimethylbiphenyl (3,3′-DMBPh) and methylcyclohexyltoluene (MCHT) for the HDS process, respectively.