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.     

Deactivation and coke formation on palladium and platinum catalysts in vegetable oil hydrogenation

Deactivation of palladium and platinum catalysts due to coke formation was studied during hydrogenation of methyl esters of sunflower oil. The supported metal catalysts were prepared by impregnating γ-alumina with either palladium or platinum salts, and by impregnating α-alumina with palladium salt. The catalysts were reused for several batch experiments. The Pd/γ-Al₂O₃ catalyst lost more than 50% of its initial activity after four batch experiments, while the other catalysts did not deactivate. Samples of used catalysts were cleaned from remaining oil by repeated extractions with methanol, and the amount of coke formed on the catalysts was studied by temperature-programmed oxidation. The deactivation of the catalyst is a function of both the metal and the support. The amount of coke increased on the Pd/γ-Al₂O₃ catalyst with repeated use, but the amount of coke remained approximately constant for the Pt/γ-Al₂O₃ catalyst. Virtually no coke was detected on the Pd/α-Al₂O₃ catalyst. The formation of coke on Pd/α-Al₂O₃ may be slower than on the Pd/γ-Al₂O₃ owing to the carrier’s smaller surface area and less acidic character. The absence of deactivation for the Pt/γ-Al₂O₃ catalyst may be explained by slower formation of coke precursors on platinum compared to palladium.     

Synthesis, characterization and hydrotreating performance of supported tungsten phosphide catalysts

Supported tungsten phosphide catalysts were prepared by temperature-programmed reduction of their precursors (supported phospho-tungstate catalysts) in H₂ and characterized by X-ray diffraction (XRD), BET, temperature-programmed desorption of ammonia (NH₃-TPD) and X-ray photoelectron spectroscopy (XPS). The reduction-phosphiding processes of the precursors were investigated by thermogravimetry and differential thermal analysis (TG-DTA) and the suitable phosphiding temperatures were defined. The hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activities of the catalysts were tested by using thiophene, pyridine, dibenzothiophene, carbazole and diesel oil as the feedstock. The TiO₂, γ-Al₂O₃ supports and the Ni, Co promoters could remarkably increase and stabilize active W species on the catalyst surface. A suitable amount of Ni (3%–5%), Co (5%–7%) and V (1%–3%) could increase dispersivity of the W species and the BET surface area of the WP/γ-Al₂O₃ catalyst. The WP/γ-Al₂O₃ catalyst possesses much higher thiophene HDS and carbazole HDN activities and the WP/TiO₂ catalyst has much higher dibenzothiophene (DBT) HDS and pyridine HDN activities. The Ni, Co and V can obviously promote the HDS activity and inhibit the HDN activity of the WP/γ-Al₂O₃ catalyst. The G-Ni5 catalyst possesses a much higher diesel oil HDS activity than the sulphided industrial NiW/γ-Al₂O₃ catalyst. In general, a support or promoter in the WP/γ-Al₂O₃ catalyst which can increase the amount and dispersivity of the active W species can promote its HDS and HDN activities.     

Partial oxidation of methane over nickel catalysts supported on various aluminas

Partial oxidation of methane (POM) was systematically investigated in a fixed bed reactor over 12 wt% Ni catalysts supported on α-A1₂O₃, γ-A1₂O₃ and θ-A1₂O₃ which were prepared at different conditions. Results indicate that the catalytic activity toward POM strongly depends on the BET surface area of the support. All the Ni/ θ-Al₂O₃ catalysts showed high activity toward POM due to the less formation of inactive NiAl₂O₄ species, the existence of NiO, species and stable θ-Al₂O₃ phase. Although Ni/γ-Al₂O₃ showed the highest activity toward POM, long-time stability cannot be expected as a result of the deterioration of the support at higher temperature, which is revealed from BET results. From the reaction and characterization results, it is inferred that the optimal conditions for the preparation of θ-Al₂O₃ are 1,173 K and 12 h.     

Role of cobalt on γ-Al2O3 based NO x storage catalyst

Co/Pt/Ba/γ-Al₂O₃, Co/Ba/γ-Al₂O₃, Pt/Ba/γ-Al₂O₃, Co/Pt/γ-Al₂O₃, Ba/γ-Al₂O₃, Pt/γ-Al₂O₃, and Co/γ-Al₂O₃ type catalysts were prepared by a conventional impregnation method, and their NO _x storage capacities were evaluated by colorimetric assay. Co-containing catalysts had a higher NO _x storage capacity than that of Co-free counterparts. The role of each component, especially Co, for the catalysts prepared was investigated by using in-situ FTIR. The high NO _x storage for Co-containing catalysts including Co/Ba/γ-Al₂O₃ and Co/Pt/Ba/γ-Al₂O₃ is mainly due to the formation of Co₃O₄ on the catalyst surface identified by XAFS.     

Correlating Low-temperature Hydrogenation Activity of Co/Pt(111) Bimetallic Surfaces to Supported Co/Pt/γ-Al2O3 Catalysts

The low-temperature self-hydrogenation (disproportionation) of cyclohexene was used as a probe reaction to correlate the reactivity of Co/Pt(111) bimetallic surfaces with supported Co/Pt/γ-Al₂O₃ catalysts. Temperature-programmed desorption (TPD) experiments show that cyclohexene undergoes self-hydrogenation on the ~1 ML Co/Pt(111) surface at ~219 K, which does not occur on either pure Pt(111) or a thick Co film on Pt(111). Supported catalysts with a 1:1 atomic ratio of Co:Pt were synthesized on a high surface area γ-Al₂O₃ to verify the bimetallic effect on the self-hydrogenation of cyclohexene. EXAFS experiments confirmed the presence of Co–Pt bonds in the catalyst. Using FTIR in a batch reactor configuration, the bimetallic catalyst showed a higher activity toward the self-hydrogenation of cyclohexene at room temperature than either Pt/γ-Al₂O₃ or Co/γ-Al₂O₃ catalysts. The comparison of Co/Pt(111) and Co/Pt/γ-Al₂O₃ provided an excellent example of correlating the self-hydrogenation activity of cyclohexene on bimetallic model surfaces and supported catalysts.     

Efficient oxidative desulfurization (ODS) of model fuel with H2O2 catalyzed by MoO3/γ-Al2O3 under mild and solvent free conditions

An efficient process to remove organic sulfur compounds from model fuel has been explored. Dibenzothiophene (DBT) and 4, 6-dimethyldibenzothiophene (4, 6-DMDBT) can be completely oxidized into their corresponding sulfones by H₂O₂ over 14 wt.% MoO₃/γ-Al₂O₃ catalyst under mild conditions in 15 min. The effects of solvent, initial sulfide concentration, loading of MoO₃ and amount of catalyst on oxidative removal of DBT were studied. The employments of solvents have decreased the reaction rate of DBT, which can be attributed to the competitive adsorption between the sulfide and solvent. The oxidative reactivity increases in the order of thiophene (Th) < benzothiophene (BT) < DBT < 4, 6-DMDBT. The catalyst can be regenerated by methanol washing at 333 K.     

Pt–Ni/γ-Al2O3 catalyst for the preferential CO oxidation in the hydrogen stream

The preferential CO oxidation (PROX) in the presence of excess hydrogen was studied over Pt–Ni/γ-Al₂O₃. CO chemisorption, X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy and temperature-programmed reduction were conducted to characterize active catalysts. The co-impregnated Pt–Ni/γ-Al₂O₃ was superior to Pt/Ni/γ-Al₂O₃ and Ni/Pt/γ-Al₂O₃ prepared by a sequential impregnation of each component on alumina support. The PROX activity was affected by the reductive pretreatment condition. The pre-reduction was essential for the low-temperature PROX activity. As the reduction temperature increased above 423 K, the CO₂ selectivity decreased and the atomic percent of Ni in the bimetallic phase of Pt–Ni increased. This catalyst exhibited the high CO conversion even in the presence of 2% H₂O and 20% CO₂ over a wide reaction temperature. The bimetallic phase of Pt–Ni seems to give rise to high catalytic activity for the PROX in H₂-rich stream.     

CO2 effect on activity and stability of NOx storage reduction catalysts

A catalyst for NO_x storage/reduction was prepared to improve the activity of Ba–Pt/γ-Al₂O₃ by replacing Ba with a mixture of Ba and Mg. The catalyst was prepared by impregnating Pt and then co-impregnating Ba and Mg (Mg:Ba molar ratio = 1) on commercial γ-Al₂O₃. The tests have been carried out in the presence of CO₂ at temperatures between 200 and 400 °C in order to understand the role of both the feed and various alkaline-earth metals. The storage capacity of the two catalysts was different like the mechanism in the reduction process.     

High activity Ni/MoS2 catalysts obtained from alkylthiometalate mixtures for the hydrodesulfurization of dibenzothiophene

An efficient method for improving the catalytic properties of unsupported Ni/MoS₂ catalysts is mixing thiometalate precursors applying the appropriate precursors and thermal conditions. High active catalysts for the hydrodesulfurization (HDS) of dibenzothiophene (DBT) are prepared by the controlled decomposition of physical mixtures of Ni(diethylentriamine)₂MoS₄ (NDTA-TM) and [(Propyl)₄N)]₂MoS₄ (TPA-TM). The catalysts with a higher content of NDTA-TM are very active with a high selectivity for the direct desulfurization pathway (DDS) due to the synergistic effect of nickel. In addition the presence of a large amount of carbon may produce single-slabs of nickel promoted carbon containing molybdenum sulfides. The activity enhancement is attributed to an increased number of NiMoS active sites originated by the chemical interaction between the precursors NDTA-TM and TPA-TM during the mixing procedure. Furthermore, the carbon content in the final products is related to the enhancement of the activity and the preference of the DDS pathway. The controlled decomposition of mixtures of NDTA-TM + TPA-TM yields catalysts which are about twofold more active than an industrial NiMo/Al₂O₃ catalyst. This improvement may be attributed to an intense interaction of the precursors during the synthesis causing a re-dispersion of nickel atoms from NDTA-TM over the surface of carbon containing molybdenum sulfide provided by the precursor TPA-TM, increasing the amount of active sites. The catalysts from mixtures of (NH₄)₂MoS₄ (A-TM) and NDTA-TM behave similarly to the pure precursors.     

Activity of vanadium magnesium oxide supported catalysts in the dehydrogenation of isobutane

The activity of the vanadium magnesium binary oxides supported on C_act, SiO₂, γ-Al₂O₃ and ZnO in the dehydrogenation of isobutane to isobutene under the carbon dioxide or inert gas atmosphere was investigated. The highest isobutene yield (34.8%) was obtained over active carbon supported catalyst. The role of carbon dioxide in the dehydrogenation process was determined on the basis of additional tests: the RWGS reaction, gasification of coke and regeneration of partially reduced catalysts. The temperature-programmed techniques (TPR-H₂, TPD-NH₃ and TPD-CO₂) were used to characterize the catalysts.     

Modification of the alumina‐supported Mo‐based hydrodesulfurization catalysts by tungsten

The incorporation effect of tungsten as an activity‐promotional modifier into the Ni‐promoted Mo/γ‐Al₂O₃ catalyst was studied. Series of W‐incorporated catalysts with different content of tungsten were prepared by changing the impregnation order of nickel and tungsten onto a base Mo/γ‐Al₂O₃. Catalytic activities were measured from the atmospheric reactions of thiophene hydrodesulfurization (HDS) and ethylene hydrogenation (HYD). The HDS and HYD activities of the WMo/γ‐Al₂O₃ catalysts (WM series) initially increased and subsequently decreased with increasing content of tungsten as compared with those of their base Mo/γ‐Al₂O₃. The maximal activity promotion occurred at the W/(W + Mo) atomic ratio 0.025. For the Ni‐promoted Mo/γ‐Al₂O₃ catalysts, the effect of W incorporation was greatly dependent on the impregnation order of tungsten. The catalysts prepared by impregnating Ni onto the WMo/γ‐Al₂O₃ catalysts showed the same trend of activity promotion as for the WM series, while those by impregnating W onto a NiMo/γ‐Al₂O₃ catalyst resulted in lower activities than their base NiMo/γ‐Al₂O₃ catalyst. To characterize the catalysts, temperature‐programmed reduction and low‐temperature oxygen chemisorption were conducted. The effects of W incorporation on the NiMo‐based catalysts were discussed in reference to those on the CoMo‐based catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalytic synthesis of thiophene from the reaction of n-butanol and carbon disulfide over K<Subscript>2</Subscript>CO<Subscript>3</Subscript>-promoted Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>/<Emphasis Type="Italic">γ</Emphasis>-alumina catalyst

The synthesis of thiophene from the reaction of n-Butanol and carbon disulfide was performed in a fixed-bed reactor in the presence of promoted chromia on γ-alumina. A high selectivity to thiophene (87%) and a long lifetime of the catalyst (55 hour) was obtained at 450 °C with a 1: 6 n-Butanol to carbon disulfide molar ratio and LHSV 1 h⁻¹ over γ-Al₂O₃ promoted by 7% K₂CO₃ with 15% Cr₂O₃ loaded. The catalytic behavior of these catalysts can be attributed to their dual-functional acidity and dehydrogenating and cyclized properties.     

Carbon Nanofiber Supported Cobalt Catalysts for Fischer–Tropsch Synthesis with High Activity and Selectivity

Platelet and fishbone carbon nanofibers (CNFs) have been used as supports for cobalt Fischer–Tropsch catalysts. The activity and selectivity of the CNF supported catalysts have been studied at 483 K, 20 bar, and H₂/CO = 2.1, and compared with corresponding activity and selectivity for α-Al₂O₃ and γ-Al₂O₃ supported cobalt catalysts. The platelet CNF supported catalyst has demonstrated high activity and high selectivity to C₅₊ hydrocarbons, with activity comparable with Co/γ-Al₂O₃ and selectivity comparable with Co/α-Al₂O₃.     

Low Temperature and H2 Selective Catalysts for Ethanol Steam Reforming

Supported Rh catalysts have been developed for selective H₂ production at low temperatures. Ethanol dehydration is favorable over either acidic or basic supports such as γ-Al₂O₃ and MgAl₂O₄, while ethanol dehydrogenation is more favorable over neutral supports. CeO₂–ZrO₂-supported Rh catalysts were found to be especially effective for hydrogen production. We focused on a support prepared by a co-precipitation method having composition Ce_0.8Zr_0.2O₂. A 2%Rh/Ce_0.8Zr_0.2O₂ catalyst, prepared via impregnation without pre-calcination of support, exhibited the highest H₂ yield at 450 °C among various supported Rh catalysts evaluated in this study. This may be due to both the strong interaction between Rh and Ce_0.8Zr_0.2O₂ and the high oxygen transfer rate favoring reforming of acetaldehyde instead of methane production.     

Role of Support in Lean DeNOx Catalysis

FTIR and pulse thermal analysis were applied to investigate catalysts containing Pt (1 wt%)/Ba (17 wt%) supported on -Al₂O₃, SiO₂ and ZrO₂. The aim was to learn how the support material affects the thermal stability of barium carbonate and its activity in the reaction to bulk Ba(NO₃)₂. The lower thermal stability of BaCO₃ in alumina supported samples was found to influence the formation of barium nitrate during the NO _x storage process. Quantification of Ba(NO₃)₂ formed during NO _x storage indicated that for alumina supported catalysts only ca. 30% of barium present in the sample is involved in the storage process. The low thermal stability found for alumina supported barium nitrite excludes its role in the formation of barium nitrate during interaction of NO _x with the catalyst at 300 °C. The studies indicate that -Al₂O₃ plays a major role in influencing the thermal stability of BaCO₃ and Ba(NO₃)₂. This finding seems to be relevant for the higher activity of -Al₂O₃-supported catalysts in NO _x storage reduction reactions.     

XPS evidence for molybdenum nitride formation in ZSM-5

Microwave plasma-assisted catalytic reduction of SO₂ by CO was studied over four catalysts. The activities of the four catalysts under microwave plasma decreased in the order of CoO/γ-Al₂O₃>>SnO₂> copper wires > iron wires, which was consistent with the results under conventional heating. By comparing the activity of CoO/γ-Al₂O₃ catalyst in the microwave plasma mode with that in the conventional mode, it is demonstrated that the temperature at which the full SO₂ conversion was obtained in the microwave plasma mode was about 200 °C lower than that under the conventional heating mode. Moreover, an increase of space velocity had little effect on SO₂ conversion and sulfur selectivity under microwave plasma; while under conventional heating mode, both SO₂ conversion and sulfur selectivity significantly decreased with an increase of space velocity.     

Reduction of NO by H2 and CO on PdO-MoO3/γ-Al2O3 of low molybdena loading

The activity and selectivity in the catalytic reduction of NO by a mixture of CO and H₂ of three PdO-MoO₃/-Al₂O₃ catalysts are compared in the presence of varying amounts of oxygen at reaction temperatures from 100 to 550°C. The catalysts were prepared by different methods and contain about 2% Mo and 2% Pd. Results are compared with those for PdO/-Al₂O₃, PdO-MoO₃/-Al₂O₃ containing 2% Pd and 20% Mo, and a commercial Pt-Rh catalyst. The PdO-MoO₃/-Al₂O₃ catalysts are more active for the selective reduction of NO to N₂ and N₂O than PdO/-Al₂O₃ under slightly oxidizing conditions at temperatures from 300 to 550°C. At these reaction conditions, the fresh PdO-MoO₃/-Al₂O₃ catalysts are comparable with a commercial Pt-Rh catalyst. The improved activity of PdO-MoO₃/-Al₂O₃ relative to PdO/-Al₂O₃ is believed to be due to the interaction between Pd and Mo. The effect of O₂ on the activity and selectivity of these catalysts is different in the reduction of NO by H₂, by CO, and by a mixture of H₂ and CO. The results using the mixture of reductants cannot be inferred from the results with the single reductants.     

Characterization of nickel sulfate supported on γ-Al2O3 and its relationship to acidic properties

A series of NiSO₄/γ-Al₂O₃ catalysts were prepared by the impregnation method using an aqueous solution of nickel sulfate. The high catalytic activity of NiSO₄/γ-Al₂O₃ for both 2-propanol dehydration and cumene dealkylation was related to the increase of acidity and acid strength due to the addition of NiSO₄. 20(wt%)-NiSO₄/g-Al₂O₃ calcined at 600 °C exhibited maximum catalytic activities for 2-propanol dehydration and cumene dealkylation. The catalytic activities for both reactions were correlated with the acidity of catalysts measured by the ammonia chemisorption method. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Remarkable support effect of ZrO2 upon the CO2 reforming of CH4 over supported molybdenum carbide catalysts

In reforming of CH₄ with CO₂ over molybdenum carbide catalysts, the catalytic performance of unsupported hexagonal Mo₂C prepared by direct carburization of MoO₃ was considerably different from a similar composition, cubic MoC_1−x (x≈0.5), prepared through nitriding before carburization. The conversion levels over MoC_1−x were substantially higher than those over Mo₂C, although the turnover frequencies were lower. X‐ray diffraction analysis indicated that Mo₂C deactivated by conversion to MoO₂ during the reaction, but the MoC_1−x was transformed to the hexagonal Mo₂C and remained stable. The activity of Mo₂C dispersed on various supports for the CH₄–CO₂ reaction was also investigated. The performance depended strongly on the property of supports, with the ZrO₂‐supported Mo₂C catalyst exhibiting the highest activity and durability for this reaction. Moreover, deactivation of Mo₂C/ZrO₂ at ambient pressure was suppressed by decreasing the loading amount of Mo₂C. This revised version was published online in August 2006 with corrections to the Cover Date.