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.     

Oxidative coupling of methane over Na+-ZrO2-C1-/Al2O3 catalysts

Oxidative coupling of methane (OCM) was carried out over Na⁺-ZrO₂-Cl /A1[₂O₃ catalysts in a temperature range from 1023 to 1123 K. The catalysts were prepared by impregnating the α- or γ-Al₂O₃ supports with sodium carbonate and/or zirconyl chloride. The OCM activity was examined using the catalysts prepared by three different preparation procedures. The best catalyst was the one prepared by subsequent impregnation of sodium carbonate-preimpregnated γ-Al₂O₃ with a mixed solution of sodium carbonate and zirconyl chloride. It was found that preimpregnated sodium played an important role in reducing the combustion activity of the γ-Al₂O₃. The catalyst with an optimal composition showed the highest C₂ selectivity and yield of 40.8% and 15.1%, respectively. From the X-ray diffraction analysis it was found that tetragonal ZrO₂ was formed and that NaCl existed in the catalysts with relatively high sodium contents.     

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.     

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.     

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.     

Alkaline earth metal oxides on γ-Al2O3 supported Co catalyst and their application to mercaptan oxidation

γ-Al₂O₃-supported alkaline earth metal oxide samples with different MgO, CaO, SrO and BaO loadings have been prepared by impregnating γ-Al₂O₃ with alkaline earth metal nitrate solutions and then calcining at 773 K and 1,023 K. The resultant samples have been characterized by XRD, EDS coupled with SEM, CO₂-TPD and BET. After preparation of the γ-Al₂O₃-supported alkaline earth metal oxide samples and impregnation with the liquid catalyst LCPs30, supplied by Axens, the catalytic performance of these catalysts was evaluated in the mercaptan oxidation reaction. Results showed that magnesium base oxide is formed at 773 K and base oxides of calcium, barium and strontium are formed at 1,023 K. Catalysts with higher mole ratios have higher conversions and the basicity increases with increasing base oxide loading in the samples. Furthermore, the conversion of mercaptans increases with increasing atomic number of the alkaline earth metal, excluding MgO, which has the highest conversion compared to the other base oxides.     

Effect of calcination atmosphere and temperature on γ-Al2O3 supported cobalt Fischer-Tropsch catalysts

The effect of calcination temperature and atmosphere on the properties of γ-Al₂O₃ supported cobalt Fischer-Tropsch catalysts has been investigated. One common precursor for all the catalysts was prepared by incipient wetness impregnation of γ-Al₂O₃ with an aqueous solution of cobalt nitrate hexahydrate. It was subjected to four different calcination atmospheres (air/50% steam: 30 mL/min, air: 30 mL/min, air: 50 mL/min, N₂: 30 mL/min) and eight different calcination temperatures (range: 473–723 K), making the total number of samples 32. Both the post calcination nitrogen content and the cobalt dispersion were measured. The results demonstrated that in order to maximise the cobalt dispersion, it is necessary to use low calcination temperatures and remove the precursor decomposition products (NO, NO₂, H₂O) efficiently. The Fischer-Tropsch synthesis performance of two catalysts calcined at the same temperature, but at different air flow rates was evaluated. No significant effect of the air flow rate was found on the turnover frequency or C₅₊ selectivity, but a high flow rate resulted in 30% higher activity per gram catalyst.     

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.     

Methane conversion over nanostructured Pt/γAl2O3 catalysts in dielectric-barrier discharge

Spherical nanostructured γ-Al₂O₃ granules were prepared by combining the modified Yoldas process and oil-drop method, followed by the Pt impregnation inside mesopores of the granules by incipient wetness method. Prepared Pt/γ-Al₂O₃ catalysts were reduced by novel method using plasma, which was named plasma assisted reduction (PAR), and then used for methane conversion in dielectric-barrier discharge (DBD). The effect of Pt loading, calcination temperature on methane conversion, and selectivities and yields of products were investigated. Prepared Pt/γ-Al₂O₃ catalysts were successfully reduced by PAR. The main products of methane conversion were the light alkanes such as C₂H₆, C₃H₈ and C₄H₁₀ when the catalytic plasma reaction was carried out with Pt/γ-Al₂O₃ catalyst. Methane conversion was in the range of 38–40% depending on Pt loading and calcination temperature. The highest yield of C₂H₆ was 12.7% with 1 wt% Pt/γ-Al₂O₃ catalysts after calcinations at 500 ‡C.     

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.     

Vapour phase dehydrogenation of cyclohexanol over alumina-supported Pt-Co bimetallic catalysts

Pt/γ- Al₂O₃, Co/γ- Al₂O₃ and Pt- Co/γ- Al₂O₃ with different Pt : Co atomic ratios have been prepared with a total metal content of 10 wt% and characterised by hydrogen chemisorption. The activities of the prereduced catalysts were studied for the reaction of cyclohexanol at 523 K and WHSV of 4.82 and 7.7 h^-1. The Pt-Co/γ-Al₂O₃ catalyst with 1 : 1 Pt : Co atomic ratio shows typical high metal dispersion and cyclohexanone selectivity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrodesulfurization of dibenzothiophene over supported and unsupported molybdenum carbide catalysts

A series of γ-Al₂O₃ supported molybdenum carbides [carbided Mo/γ-Al₂O₃ (MCS), Co-Mo/γ-Al₂O₃ (CMCS), and Ni-Mo/γ-Al₂O₃ (NMCS)] and unsupported molybdenum carbide (MCUS) were prepared by the temperature-programmed carburization of their corresponding molybdenum nitrides with 20 % CH₄/H₂. XRD and SEM studies show that unsupported molybdenum carbide catalyst possesses a typical crystalline Mo₂C (FCC structure), while supported molybdenum carbide catalysts possess highly dispersed surface molybdenum carbide species on an alumina oxide support. The results of dibenzothiophene (DBT) hydrodesulfurization over molybdenum carbide catalysts show that the reactivity is strongly dependent on the type of catalyst. Supported molybdenum carbide catalysts possess a higher reactivity than the unsupported molybdenum carbide catalyst. In addition, Co or Ni promoted, supported molybdenum carbide catalyst possesses a higher reactivity than the unpromoted, supported molybdenum carbide catalyst. The reactivity, which is also dependent on the reaction conditions, increases with increasing reaction temperature and pressure and contact time. The CO uptakes of the molybdenum carbide catalysts correlate well with overall activity (total rate) for DBT hydrodesulfurization. The major reaction product is biphenyl, with cyclohexylbenzene next in abundance regardless of the type of catalysts and reaction conditions. It was also found that the molybdenum carbide catalysts exhibit stable initial reactivity due to the stable and weak acidic characteristics of these catalysts.     

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.     

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.     

Synthesis,characterization and catalytic properties of polypyrrole-supported catalysts

Polypyrrole, a conductive polymer, was used as support of noble metals, namely, copper and palladium, for application in heterogeneous catalysis. Catalysts were characterized by TEM coupled with EDS and XRD. Their activity and selectivity were determined in the reduction of nitrate and intermediate nitrite in water under 1 bar of hydrogen and at room temperature. In nitrite reduction, these novel catalysts demonstrated a better activity and a higher selectivity towards nitrogen formation than their Pd/Al₂O₃ counterpart. On the other hand, the bimetallic catalyst tested for nitrate reduction showed an activity similar to the one of a classical Pd–Cu/Al₂O₃ catalyst, but no intermediate nitrite was observed.     

Hydrogenation of CO on supported cobalt γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst in fixed bed and slurry bubble column reactors

Fischer-Tropsch synthesis for the production of C₅+ hydrocarbons from syngas was carried out in a tubular fixed bed reactor (TFBR) and in a slurry bubble column reactor (SBCR). The Co-based catalysts for FTS were prepared by the conventional wet-impregnation of γ-Al₂O₃. Effects of operating conditions such as GHSV (1,000–4,000 ml/g·hr), reaction temperature (220–250°C) and pressure (0.5–3.0MPa) on the CO conversion and product selectivity of Co/γ-Al₂O₃ catalyst were examined in the TFBR and SBCR. The C₅+ selectivity and olefin selectivity in an SBCR were found to be higher than that in a TFBR, whereas C₂–C₄ selectivity showed a reverse trend. The CO conversion and product distribution in an SBCR were less sensitive than that in a TFBR with variations of reaction conditions.     

Transesterification of Rapeseed Oil for Synthesizing Biodiesel by K/KOH/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> as Heterogeneous Base Catalyst

The heterogeneous base catalyst, γ-Al₂O₃ loaded with KOH and K (K/KOH/γ-Al₂O₃) was first prepared and used in the transesterification of rapeseed oil with methanol to produce biodiesel. The prepared catalyst was characterized by X-ray diffraction, scanning electron microscopy, Brunauer–Emmett–Teller method, infrared spectroscopy and X-ray photoelectron spectroscopy. It was found that when γ-Al₂O₃ is loaded with KOH and K, the Al–O–K species is produced, resulting in an increase in the catalytic activity. The impacts of catalyst preparation conditions on the catalytic activities of K/KOH/γ-Al₂O₃ were investigated. The results demonstrate that the catalyst K/KOH/γ-Al₂O₃ has high catalytic activity when the added amounts of KOH and K are 20 and 7.5 wt% respectively. The transesterification of rapeseed oil to biodiesel with the prepared heterogeneous base catalyst was optimized. It was found that the yield of biodiesel can reach as high as 84.52% after 1 h reaction at 60°C, with a 9:1 molar ratio of methanol to oil, a catalyst amount of 4 wt%, and a stirring rate of 270 g.     

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.     

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.     

Reductive Amination of Triacetoneamine with <Emphasis Type="Italic">n</Emphasis>-Butylamine Over Cu–Cr–La/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

Abstract  

A series of Cu-based catalysts were prepared and examined for the reductive amination of triacetoneamine with n-butylamine, thereinto, Cu–Cr–La/γ-Al₂O₃ showed excellent results. The catalysts were studied by XRD, XPS, H₂-TPR and NH₃-TPD. It was found that doped Cr remarkably enhanced the activity of Cu/γ-Al₂O₃ due to better dispersion of Cu⁰, which is believed to be the active site for the reductive amination. Additionally, it was obvious that introduction of La to Cu–Cr/γ-Al₂O₃ led to a higher selectivity and longer lifetime. The reaction parameters were optimized and N-butyl-2,2,6,6-tetramethyl-4-piperidinamine was obtained in a yield of 94%.