Applied Catalysis A, General (v.233, #1-2)
Environmental friendly lubricants through selective hydrogenation of rapeseed oil over supported copper catalysts by Nicoletta Ravasio; Federica Zaccheria; Michele Gargano; Sandro Recchia; Achille Fusi; Nicola Poli; Rinaldo Psaro (1-6).
Different supported copper catalysts are compared in the selective hydrogenation of vegetable oils and their methylesters. The influence of parameters such as the catalysts preparation method, the support used and the activation treatment are critically evaluated in terms of catalytic activity and selectivity. All ex situ reduced catalysts prepared by the chemisorption–hydrolysis (CH) method show better performances than the ones prepared by incipient wetness technique. These data are directly matched up to the different morphology of the supported copper particles obtained with these two methods. By using a 8% Cu/SiO2 catalyst the trienic component C18:3 can be eliminated and the dienic one lowered from 22 to 3–5% without increasing the stearic C18:0 content and limiting the cis/trans-isomerisation process. These oils, with a C18:1 content up to 88% show remarkable oxidation stability and keep fluid down to −15 °C.
Keywords: Copper catalysts; Catalyst preparation; Selective hydrogenation; Vegetable oils; Renewable sources; Biodegradable lubricants; Cold properties; Oxidative stability;
An integrated air–POM syngas/dimethyl ether process from natural gas by Meilin Jia; Wenzhao Li; Hengyong Xu; Shoufu Hou; Qingjie Ge (7-12).
A Cu-Zn-Al methanol catalyst combined with HZSM-5 was used for dimethyl ether (DME) synthesis from a syngas containing nitrogen, which was produced by air–partial oxidation of methane (air–POM). Air–POM occurred at 850 °C, 0.8 MPa, CH4/air/H2O/CO2 ratio of 1/2.4/0.8/0.4 over a Ni-based catalyst modified by magnesia and lanthanum oxide with 96% CH4 conversion and constantly gave syngas with a H2/CO ratio of 2/1 during a period of 450 h. The obtained N2-containing syngas was used directly for DME synthesis. About 90% CO per-pass conversion, 78% DME selectivity and 70% DME yield could be achieved during 450 h stability testing under the pressure of 5.0 MPa, the temperature of 240 °C and the space velocity of 1000 h−1.
Keywords: Syngas; Dimethyl ether; Catalytic partial oxidation of methane;
Liquid phase hydrogenation of furfural to furfuryl alcohol over Mo-doped Co-B amorphous alloy catalysts by Xingfan Chen; Hexing Li; Hongshan Luo; Minghua Qiao (13-20).
The Mo-doped Co-B amorphous catalyst (Co-Mo-B) were prepared by chemical reduction of mixed CoCl2 and Na2MoO4 with KBH4 in aqueous solution. This catalyst exhibited excellent activity and nearly 100% selectivity to furfuryl alcohol (FFA) during liquid phase hydrogenation of furfural (FFR); the promoting effect of Mo-dopant on the hydrogenation activity was investigated and the optimum Mo/(Mo+Co) molar ratio (X Mo) was determined as 0.064. Pretreatment of the Co-Mo-B amorphous catalyst resulted in the abrupt decrease in its hydrogenation activity owing to both the gathering of catalyst particles and the crystallization of the catalyst. It was found that the Mo-dopant in the Co-Mo-B catalyst was mainly present in the form of MoO3. Thus, its promoting effect could be attributed to both a dispersing effect that resulted in the higher surface area and the acidic property of MoO3 that was favorable for the adsorption of carbonyl group in FFR molecules. The superior activity of the Co-Mo-B amorphous catalyst over its corresponding crystallized counterpart could be attributed to both the structural effect and the electronic effect.
Keywords: Furfural (FFR); Furfuryl alcohol (FFA); Hydrogenation; Co-Mo-B amorphous catalyst;
Oxidative dehydrogenation of propane over Cr2O3/Al2O3 and Cr2O3 catalysts: effects of loading, precursor and surface area by Maymol Cherian; Musti Someswara Rao; Wei-Tin Yang; Jih-Mirn Jehng; Andrew M Hirt; Goutam Deo (21-33).
Several alumina supported chromium oxide catalysts were prepared by varying the chromium oxide loading, precursors and surface areas of the support. The prepared catalysts were characterized using BET, XRD, XPS and UV–VIS spectroscopic techniques. The monolayer limit was observed to be ∼9 μmol Cr/m2. Below monolayer limits, surface chromium oxide species were present irrespective of precursors and surface area of the support. The activity of the prepared samples was tested for ODH of propane. It was observed that the supported chromium oxide samples were active for the ODH reaction and that propene was the major product. The activity and selectivity increased with loading up to monolayer limits and decreased for higher loadings. Bulk Cr2O3 was also studied for the ODH reaction and found to behave differently than the Cr2O3/Al2O3 catalysts. The efficiency of each surface chromium oxide species on alumina to carry out the ODH of propane, the turn over frequency (TOF), was relatively independent of chromium oxide loading for the monolayer catalysts. However, it appears that a constant fraction of the surface chromium oxide species is active for the ODH of propane. This constant fraction of the surface chromium oxide phase is given by the polymeric chromium oxide species. The inactive fraction is given by the monomeric chromium oxide species that is not reducible.
Keywords: Oxidative dehydrogenation; Chromia; Alumina; Propane; Propene; Surface active sites; Catalyst; Precursor; Surface area; Loading;
Effective heat supply from combustion to reforming in methane reforming with CO2 and O2: comparison between Ni and Pt catalysts by Keiichi Tomishige; Shogo Kanazawa; Koichi Suzuki; Mohammad Asadullah; Motoki Sato; Kenji Ikushima; Kimio Kunimori (35-44).
The temperature profile of the fixed catalyst bed in methane reforming with CO2 and O2 was investigated over Pt/Al2O3 and Ni/Al2O3. The activities on Pt/Al2O3 and Ni/Al2O3 were in the same level in methane reforming with CO2. On the other hand, the activity of methane combustion over the catalysts was significantly different (Pt/Al2O3⪢Ni/Al2O3). In methane reforming with CO2 and O2, methane conversion over Pt/Al2O3 was considerably higher than over Ni/Al2O3. From the profiles of bed temperature measurements under various reaction conditions, one sees that the distance between combustion and reforming zones becomes very short. This is related to the high reducibility of Pt metal itself, and Pt maintains metallic state in the combustion zone and under the presence of oxygen. In this case, the combustion heat can be supplied to the reforming reaction very effectively. The result can be connected to the synthesis gas production with high energy efficiency.
Keywords: Heat supply; Combustion; Reforming; Pt/Al2O3; Ni/Al2O3; Temperature profile;
A study of catalyst formulations for isomerization of C7 hydrocarbons by Tim D Pope; Jerry F Kriz; Maria Stanciulescu; Jacques Monnier (45-62).
Zeolite and alumina based catalysts are screened for activity and selectivity in the isomerization of n-heptane. Pt, Ni, Co, Fe, Zn, La and combinations thereof are introduced to the catalysts via inorganic (chloride, nitrate) or organometallic (carbonyl, acetylacetonate) precursors. Pt/UY and Pt/VUY catalysts show the greatest activity and selectivity to isomers, with up to 94% selectivity to isomers at 72% conversion. Their isomerization activity and selectivity increase with Pt loading up to 1.5 and 1.0%, respectively. For Pt/faujasite catalysts that show good selectivity to isomers over cracked products, isomerization of n-C7 to multibranched isomers occurs in consecutive steps. The ratio of monobranched to dibranched isomers is a unique function of conversion regardless of catalyst formulation or processing conditions. However, the selectivity to isomers over cracked products at a given conversion is a strong function of catalyst preparation and processing conditions. Less selective catalysts yield monobranched isomers, multibranched isomers, and cracked products in parallel. Pt/faujasite catalysts strongly resist inhibition by naphthenic and aromatic cyclic compounds, and are therefore suitable for the isomerization of a realistic naphtha cut. Methylcyclohexane (MCH) is isomerised without ring opening to an equilibrium mixture of C7 naphthenes while toluene is hydrogenated and partially isomerised. Conversion of n-C7 and mixed feeds yields a product stream containing branched isomers having much higher research octane rating.
Keywords: Zeolite; Isomerization; Paraffin; Naphthene; Heptane; Selectivity; Isomer distribution; Inhibition;
Fe:Co/TiO2 bimetallic catalysts for the Fischer–Tropsch reaction by D.J Duvenhage; N.J Coville (63-75).
The effect of support calcination temperature as well as catalyst calcination and reduction temperature on the performance (activity and selectivity) of a Fe:Co/TiO2 catalyst has been investigated. Various techniques (CO chemisorption, TPO, TPR, DSC, BET and Mössbauer Spectroscopy) have been utilized to relate the performance of the above catalyst system to the physical and chemical nature of the catalyst, surface structural stability and various other properties. It was found that a higher support calcination temperature increased the stability of the final catalyst while the catalyst calcination temperature played a lesser role in influencing catalyst performance. Catalyst reduction temperature increased the surface atom ratio, degree of reduction and CO chemisorption, and hence the active metal dispersion, and overall catalytic behaviour.
Keywords: Fischer–Tropsch; Bimetallic; Iron; Cobalt; Calcination; Reduction;
Development of heavy oil hydrocracking catalysts using amorphous silica-alumina and zeolites as catalyst supports by M.A Ali; T Tatsumi; T Masuda (77-90).
The overall objective of this research work was to prepare hydrocracking catalysts using amorphous silica-alumina (ASA) supports in combination with USY and β-zeolites. Three supports: namely silica-alumina, USY and β-zeolites were selected to prepare the extrudates using AP-1 as a binder, while two metal pairs: namely Ni–W and Ni–Mo were loaded on the extrudates through co-impregnation using incipient wetness technique. The catalysts were then calcined at 550 °C for 2 h. The catalysts were tested in a fixed-bed flow reaction system for their activity, using desulfurized vacuum gas oil (DS-VGO) as a feedstock. The catalytic evaluation results of the catalysts showed that β-zeolite alone and in combination with the ASA used in this study, has a potential as a support for developing heavy oil hydrocracking catalysts. A balance of weak and strong acidities of β-zeolite provides control cracking, while high surface area and bigger pores of silica-alumina may be useful for producing stable hydrocracking and hydrotreating catalysts.
Keywords: Silica-alumina; β-Zeolite; USY-zeolite; Hydrocracking; Extrudates; Co-impregnation; Incipient wetness method; Desulfurized VGO; Acidity; Surface area; Catalytic evaluation; Fixed bed reaction system;
Efficient evolution of hydrogen from liquid cycloalkanes over Pt-containing catalysts supported on active carbons under “wet–dry multiphase conditions” by Nobuko Kariya; Atsushi Fukuoka; Masaru Ichikawa (91-102).
Highly efficient evolution of hydrogen is achieved in the dehydrogenation of cycloalkanes such as cyclohexane, methylcyclohexane, and decalin over Pt catalyst supported on active carbon (AC) under “wet–dry multiphase conditions”. Formation rate of hydrogen is largely dependent on reaction conditions such as reactant/catalyst ratio, temperature, and support. The highest initial rate of formation of hydrogen, k=8.0×10−3 mol min−1, was obtained in the dehydrogenation of cyclohexane over Pt/AC at 623 K and the reactant/catalyst ratio=3.3 ml g−1. The addition of second metals such as Mo, W, Re, Rh, Ir, and Pd on the carbon-supported Pt catalysts enhances the dehydrogenation rate due to the promotion of CH bond cleavage and/or desorption of aromatic products. A physical mixture of Pt/AC and Pd/AC catalysts exhibits higher activities than the monometallic Pt/AC catalyst owing to the synergistic effects of spillover, migration, and recombination of hydrogen over Pt and Pd catalysts.
Keywords: Hydrogen; Dehydrogenation; Cyclohexane; Decalin; Pt catalyst; Bimetallic catalyst;
The support effect on propane combustion over platinum catalyst: control of the oxidation-resistance of platinum by the acid strength of support materials by Yoshiteru Yazawa; Nobuyuki Takagi; Hisao Yoshida; Shin-ichi Komai; Atsushi Satsuma; Tsunehiro Tanaka; Satohiro Yoshida; Tadashi Hattori (103-112).
The support effect on the low temperature propane combustion over supported platinum catalyst was studied by using a series of metal oxides as support materials: MgO, La2O3, ZrO2, Al2O3, SiO2, SiO2–Al2O3, and SO4 2−–ZrO2. The catalytic activity varied with the support materials, and the platinum supported on more acidic support material showed higher activity. The support effect on the oxidation state of platinum was investigated by Pt LII- and LIII-edge XANES. In the oxidizing atmosphere, platinum on the acidic support materials was less oxidized than that on the basic one, indicating that the oxidation-resistance of platinum is enhanced with the increase in the acid strength of support materials. This support effect in the oxidizing atmosphere is entirely different from that in the reducing atmosphere; the electron-deficiency of platinum increases with the increase in the acid strength of support materials under the reducing atmosphere. The relation between the catalytic activity and the oxidation state of platinum clearly indicates that the variation of the catalytic activity with the support materials come from the variation of the oxidation state of platinum, and that support materials affect the catalytic activity through the control of the oxidation state of platinum. These results suggest that the acid strength of support materials is an important factor for the design of the active supported platinum catalyst for the reaction under the oxidizing atmosphere.
Keywords: Acid strength of support materials; Oxidation state of platinum; Propane combustion; Supported platinum catalyst; Support effect;
The additive effect on propane combustion over platinum catalyst: control of the oxidation-resistance of platinum by the electronegativity of additives by Yoshiteru Yazawa; Hisao Yoshida; Shin-ichi Komai; Tadashi Hattori (113-124).
The additive effect on the low temperature propane combustion over the supported platinum catalyst was investigated by using Pt/Al2O3 with a series of additives. The catalytic activity of platinum catalyst drastically varied with the kind of additives, and increased with the increase in the electronegativity of additives. The additive with large electronegativity enhanced the catalytic activity compared with the original Pt/Al2O3 and vice versa. The additive effect on the oxidation state of platinum was investigated by Pt LII- and LIII-edge XANES. In the oxidizing atmosphere, the oxidation state of platinum varied with the electronegativity of additives to a more or less oxidized state compared with original Pt/Al2O3: the oxidation of platinum was more depressed by the additives with larger electronegativity, indicating that the oxidation-resistance of platinum is more enhanced with the increase in the electronegativity of additives. Although the electron deficiency of platinum increases with the increase in electronegativity of the additives in the reducing atmosphere, the additive effect in the oxidizing atmosphere is entirely different from that in the reducing one. The relation between the catalytic activity of platinum and its oxidation state indicates that the variation in the catalytic activity comes from the variation in the oxidation state, and that the additives affect the catalytic activity through the control of oxidation state of platinum. This additive effect is similar to the support effect on platinum catalyst for propane combustion found in the previous studies; platinum on more acidic support materials has higher oxidation-resistance, and the catalytic activity of supported platinum can be improved through the control of the oxidation state of platinum by support materials. These results clearly reveal that the electrophilic/electrophobic properties of the support materials and additives is one of the key factors for the control of the activity of platinum catalyst used under the oxidizing atmosphere.
Keywords: Additive effect; Electrophilic/electrophobic property of additives; Oxidation state of platinum; Propane combustion; Supported platinum catalyst;
Methane combustion over La2O3-based catalysts and γ-Al2O3 by Todd J. Toops; Arden B. Walters; M.Albert Vannice (125-140).
The kinetics of methane combustion were examined over La2O3, Sr-promoted La2O3, 40% La2O3/γ-Al2O3 and γ-Al2O3 between 773 and 973 K, 0.4–5 Torr CH4 and 3–23 Torr O2. Rates (mol/(s g)) were highest for supported La2O3, but the rate on pure Al2O3 was one-third of this value; however, the specific activity (mol/(s m2)) on La2O3 was 130-fold greater than that on γ-Al2O3. The apparent activation energies were 25±2 kcal/mol for the La2O3 catalysts and 20 kcal/mol for γ-Al2O3. Respective reaction orders on CH4 and O2 were 0.69–0.75 and 0.12–0.18 for the La2O3-based catalysts compared to respective orders near 0.6 and 0.3 for γ-Al2O3. A reaction mechanism invoking quasi-equilibrated adsorption of all reactants and products along with methyl radical formation as a slow step produced a derived rate expression, i.e. r CH 4 = kP CH 4 P O 2 1/2 (1+K CH 4 P CH 4 +K O 2 1/2P O 2 1/2)2 that fit the data well and indicated that CO, CO2 and H2O have no significant effect on this reaction under the reaction conditions employed here.An analysis of the enthalpies and entropies of adsorption for CH4 and O2 showed that these parameters were thermodynamically consistent, with indicated heats of adsorption for methane of around 20 kcal/mol CH4 on all four catalysts while for oxygen this value was around 30 kcal/mol O2 on the La2O3-based catalysts and near 20 kcal/mol O2 on γ-Al2O3. The activation energy calculated for the elementary step to form a methyl group on the surface was around 40 kcal/mol for all catalysts.
Keywords: CH4 combustion; La2O3; γ-Al2O3; Kinetics;
Dynamic adsorption of ammonia on activated carbons measured by flow microcalorimetry by M Domingo-Garcı́a; A.J Groszek; F.J López-Garzón; M Pérez-Mendoza (141-150).
Several activated carbons of different textural and chemical surface characteristics have been used to study the ammonia adsorption. The textural characteristics were determined by N2 and CO2 adsorption at 77 and 273 K. The chemical surface groups were estimated by thermal programmed desorption followed by mass spectrometry (TPD-MS) and by selective titrations in aqueous solutions. The ammonia adsorption was studied under dynamic conditions from N2 diluted flow using a flow adsorption microcalorimeter. The ammonia adsorption consists of reversible and irreversible components. The former is assigned to physisorption process while the latter is adsorption on chemical groups. The plots of the differential heats of adsorption versus the cumulative adsorption point to the existence of a wide distribution of acid sites some of which are very strong. However, they are not always easily accessible to NH3 because constrictions in the pores-network hinder the access which forces to re-arrangement.
Keywords: Adsorption; Carbon materials; TDP-MS; Microcalorimetry;
Isomerisation of (+)citronellal over Zn(II) supported catalysts by C Milone; A Perri; A Pistone; G Neri; S Galvagno (151-157).
A series of acid catalysts prepared by impregnation of commercial silicas with a solution of ZnBr2 have been characterised with several techniques (BET, adsorption–desorption porosimetry, scanning electron microscopy (SEM), energy dispersive analysis (EDAX), X-ray diffraction (XRD), atomic absorption spectroscopy (AAS)). The results have shown that ZnBr2 is evenly distributed over the surface of the supports regardless of their surface properties. The catalytic surface contains two different sites: physically adsorbed ZnBr2 and Zn(II) sites, obtained from the partial decomposition of the precursor. No correlation between the surface area of the support and the amount of Zn(II) sites has been observed.The ZnBr2/SiO2 based catalysts have been also tested in the isomerisation of (+)citronellal. The catalytic activity results have shown that the structural properties of the support have a scarce influence on the catalytic performance.On all the investigated samples, the selectivity to isopulegols is 100%, whereas the stereoselectivity to (−)isopulegol ranges between 70 and 86%.Zn(II) are the most active sites for the catalytic isomerisation while ZnBr2 has a negligible catalytic activity. However, the presence of ZnBr2 is of fundamental importance to achieve higher selectivity toward the formation of (−)isopulegol.
Keywords: Isomerisation; Silica; (+)Citronellal; (−)Isopulegol; Acid catalysts; ZnBr2;
Hydroprocessing of Maya heavy crude oil in two reaction stages by J Ancheyta; G Betancourt; G Marroquı́n; G Centeno; L.C Castañeda; F Alonso; J.A Muñoz; Ma.T Gómez; P Rayo (159-170).
In this work we report experimental information about the hydroprocessing of Maya heavy crude oil using two reaction stages. Experiments were performed in a high-pressure fixed-bed pilot plant. Hydroprocessing reactions were carried out at constant pressure (70 kg/cm2) and Hydrogen-to-oil ratio (5000 ft3/bbl). Space-velocity and reaction temperature were varied in the ranges of 0.5–2.0 h−1 and 360–400 °C, respectively. First reaction stage used a NiMo/Al2O3, while second stage employed a CoMo/Al2O3 catalyst. Experimental and characterization results show that Maya crude oil quality can be substantially improved by hydroprocessing in two reaction stages. Different behavior in heteroatoms (sulfur, nitrogen, asphaltenes, and metals) removals, hydrocracking (HCR), and pore size distribution of catalysts after reaction were observed in each reaction stage.
Keywords: Hydroprocessing; Heavy oil; Maya crude;
Platinum deposited on monolayer supports in selective hydrogenation of furfural to furfuryl alcohol by Jacek Kijeński; Piotr Winiarek; Tadeusz Paryjczak; Andrzej Lewicki; Agnieszka Mikołajska (171-182).
Platinum deposited on supports (SiO2, γ-Al2O3, MgO, TiO2) covered with a transition metal oxide monolayer (TiO2, V2O5, ZrO2) was used in hydrogenation of furfural to furfuryl alcohol. Reactions were carried out under atmospheric pressure and at temperatures from 423 to 573 K. Reduction selectivity from unsaturated aldehyde to unsaturated alcohol (93.8% for Pt/TiO2/SiO2) was higher as compared with the reaction performed over classical platinum metal/metal oxide catalysts (33.8% for Pt/SiO2, respectively). The role of both SMSI and SOOI effects in catalysts activity and selectivity enhancement was demonstrated.
Keywords: Platinum catalysts; Transition metal oxide monolayer; Furfural hydrogenation; SOOI; SMSI;
Liquid phase hydrogenation of crotonaldehyde over bimetallic Rh-Sn/SiO2 catalysts by M.del C Aguirre; P Reyes; M Oportus; I Melián-Cabrera; J.L.G Fierro (183-196).
A series of Rh-Sn/SiO2 catalysts has been prepared by successive impregnation of SiO2 sol–gel (pH 5) with precursors chlorides of rhodium(III) and tin(II). Monometallic Rh/SiO2 and Sn/SiO2 samples were also prepared for comparison. The solids had been characterized by transmission electronic microscopy (TEM), electron diffraction (ED), temperature programmed reduction (TPR), H2 chemisorption and X-ray photoelectron spectroscopy (XPS) techniques. The catalytic evaluation of crotonaldehyde hydrogenation was performed in liquid phase hydrogenation at atmospheric pressure. Only a slight increase in the metal particle size and significant rhodium coverage is produced by tin as it was found by TEM and XPS. Both Rh0 and Rh δ+ species were detected in low content of tin catalysts, whereas a high proportion of tin in an oxidized state (close to 70%) in all the samples was found by XPS. ED studies allowed the detection of bimetallic phases, Rh x Sn y and oxides of tin as SnO2. An important decrease in catalytic activity is produced by addition of tin, however, at the same conversion level, a significant enhancement in the selectivity to crotyl alcohol (CROL) was observed. The results are explained on the basis of an electronic effect produced by the oxidized tin species and Rh x Sn y intermetallic phase that allow the polarization of the carbonyl group and inhibit the hydrogenation of CC, respectively. The presence of these new species, modifies the nature of the active sites contributing to an increase in the selectivity to CROL due to electronic effects. However, at higher Sn/Rh, the activity drops significantly due to a decrease in the hydrogenation capacity and also due to the creation of surface acid sites, an important amount of byproducts, such as acetals and esters are obtained.
Keywords: Rh-Sn/silica; Crotonaldehyde hydrogenation; TPR characterization; XPS characterization; TEM characterization;
Characterization of coke on equilibrium fluid catalytic cracking catalysts by temperature-programmed oxidation by Oguz Bayraktar; Edwin L Kugler (197-213).
Characterization of coke on equilibrium, fluid catalytic cracking (FCC) catalysts contaminated with metals is investigated using temperature-programmed oxidation (TPO) and temperature-programmed hydrogenation (TPH). TPO spectra of spent equilibrium catalysts from cracking of sour imported heavy gas oil (SIHGO) and ASTM standard-gas–oil feed were deconvoluted into four peaks by fitting them into Gaussian-type functions. The four peaks are assigned to different types of coke on the catalyst. The first peak is produced by hydrocarbons desorbing from the coke. Traditionally, this is called cat-to-oil coke. The second peak is contaminant coke produced by contaminant-metal reactions. The third peak is conversion coke produced by acid-catalyzed reactions. A graphite-like coke that is related to both feedstock properties and catalyst activity produces the last peak. The TPO spectra of spent catalysts from cracking n-hexadecane are deconvoluted into three peaks, corresponding to the first three peaks observed with gas–oil cracking. The graphite-like coke is not observed after n-hexadecane cracking.TPO peak area is proportional to the amount of coke on catalyst. The amount of contaminant coke correlates with contaminant-metal concentration. The sum of the conversion coke and the graphitic coke correlates with catalyst activity. This sum can be used to characterize the coking tendency of a feedstock. Feedstock comparisons at 50% conversion show that SIHGO feed produces twice as much coke as ASTM feed and nearly five times as much coke as n-hexadecane.TPH results are less useful in characterizing coked catalysts. The peaks in TPH spectra can be correlated with only the first three peaks of TPO spectra. The high temperature peak assigned to graphitic coke in TPO is not observed in TPH spectra. This situation occurs because graphitic coke is more reactive with oxygen than with hydrogen, so that oxidation occurs within the temperature range of experimental equipment, while graphitic coke hydrogenation occurs at higher temperatures, beyond the range of the TPH apparatus.
Keywords: Temperature-programmed oxidation (TPO); FCC; Catalysts; Nickel; Vanadium; Coke characterization; Coke measurement;
Fischer–Tropsch synthesis: deactivation of noble metal-promoted Co/Al2O3 catalysts by Gary Jacobs; Patricia M Patterson; Yongqing Zhang; Tapan Das; Jinlin Li; Burtron H Davis (215-226).
Fresh and used, unpromoted and noble metal-promoted 15% Co/Al2O3 catalysts were analyzed by XANES and EXAFS to provide insight into catalyst deactivation. XANES analysis of the catalysts gave evidence of oxidation of a fraction of the cobalt clusters by water produced during the reaction. Comparison of XANES derivative spectra to those of reference materials, as well as linear combination fitting with the reference data, suggest that some form of cobalt aluminate species was formed. Because bulk oxidation of cobalt by water is not permitted thermodynamically under normal Fischer–Tropsch synthesis (FTS) conditions, it is concluded that the smaller clusters interacting with the support deviate from bulk-like cobalt metal behavior and these may undergo oxidation in the presence of water. However, in addition to the evidence for reoxidation, EXAFS indicated that significant cobalt cluster growth took place during the initial deactivation period. Promotion with Ru or Pt allowed for the reduction of cobalt species interacting with the support, yielding a greater number of active sites and, therefore, a higher initial catalyst activity on a per gram catalyst basis. However, these additional smaller cobalt clusters that were reduced in the presence of the noble metal promoter, deviated more from bulk-like cobalt, and were therefore, more unstable and susceptible to both sintering and reoxidation processes. The latter process was likely in part due to the higher water partial pressures produced from the enhanced activity. The rate of deactivation was therefore faster for these promoted catalysts.
Keywords: Alumina; Cobalt; Fischer–Tropsch synthesis; Metal support interaction; XANES; Promoters;
Activity enhancement by acetic acid in cyclohexane oxidation using Ti-containing zeolite catalyst by Tawan Sooknoi; Jumras Limtrakul (227-237).
Titanium silicalite (TS-1) was hydrothermally crystallised from a titanosilicate gel. The solid material was characterised by XRD, IR, and SEM, and then used as a catalyst in the liquid phase oxidation of cyclohexane with hydrogen peroxide. The reaction was carried out for 6 h, at the temperature between 40 and 80 °C. It was found that a marked increase in the catalytic activity was observed in the reaction using acetic acid as the solvent, as compared to those using no solvent and methyl ethyl ketone. Further investigation was made on the cause of activity enhancement, and it was shown that acetic acid was readily oxidised to peracetic acid. This compound was believed to facilitate the complexation of the framework titanium active sites, and subsequently serve as a better oxidising agent, as compared to the original hydrogen peroxide. However, leaching of the titanium species was also observed in small amounts, from the reaction using acetic acid as the solvent. In the mechanistic point of view, there was an evidence suggesting that cyclohexanol might be a primary product from the cyclohexane oxidation, and can be consecutively re-oxidised to form cyclohexanone. It is noted that the direct oxidation from cyclohexane to cyclohexanone cannot be excluded.
Keywords: Zeolites; Titanium; Oxidation; Cyclohexanone; Acetic acid;
Characterization of Mo-MnO catalyst for propane oxidative dehydrogenation by Luis E Cadus; Osmar Ferretti (239-253).
Manganese oxide catalysts impregnated with molybdenum have been examined for the propane oxidative dehydrogenation. These catalysts exhibit catalytic activity and yield to propane at temperatures as low as 623 K. The catalysts were characterized by S BET, XRD, Laser Raman, TPR, EPR and XPS. Independently of molybdenum loading, none of the impregnated catalysts shows the patterns corresponding to molybdenum oxide by XRD. The reaction of molybdenum with manganese would probably occur starting from OH centers as reacting nuclei and it continued from there. Thus, the OH of manganese oxide surface would restrain the dispersion of the formed molybdate. However, the results of Raman spectroscopy analysis of catalysts before and after the catalytic test indicate that the reaction test could be responsible for modifications in the surface composition. Characterization before and after catalytic test corroborates the concept of the existence of a “dynamic feature” in which the reaction of the adsorbed phase plays a key role in the surface reconstruction. EPR results as well as XPS ones for catalysts after test have shown that independently of molybdenum loading, at a superficial level, all of them have similar characteristics. Evidently, the reaction test promote a surface reconstruction. XPS data together with those obtained by XRD which show that the greater the load, the more intense the MnMoO4 signal indicate that MnMoO4 accumulates in a definite place being the XRD signal more intense by localized mass growth. These evidences and the consequent description of the resulting surface architecture allow to understand the results of catalytic evaluation tests.
Keywords: Mo-MnO catalyst; Oxidative dehydrogenation; Propane-to-propene;
Fischer–Tropsch synthesis: effect of water on the catalytic properties of a ruthenium promoted Co/TiO2 catalyst by Jinlin Li; Gary Jacobs; Tapan Das; Burtron H. Davis (255-262).
The effect of water on the catalytic properties of a ruthenium promoted Co/TiO2 catalyst (10 wt.%) during Fischer–Tropsch synthesis (FTS) was investigated in a continuously stirred tank reactor (CSTR) by adding water into the feed gas at varying space velocity. At higher synthesis gas (syngas) space velocities (SV=8 or 4 NL g cat.−1 h−1), the addition of water did not have significant effect on the CO conversion. At lower space velocity (SV=2 NL g cat.−1 h−1), the addition of water decreased the CO conversion; however, the decrease was reversible with the catalyst quickly recovering the activity that it exhibited prior to water addition. At high CO conversion (space velocity of SV=1 NL g cat.−1 h−1), the addition of water resulted in permanent catalyst deactivation.
Keywords: Titania; Cobalt; Ruthenium; Fischer–Tropsch synthesis; Synthesis gas conversion; Deactivation;
Fischer–Tropsch synthesis: support, loading, and promoter effects on the reducibility of cobalt catalysts by Gary Jacobs; Tapan K Das; Yongqing Zhang; Jinlin Li; Guillaume Racoillet; Burtron H Davis (263-281).
Temperature programmed reduction (TPR) and hydrogen chemisorption combined with reoxidation measurements were used to define the reducibility of supported cobalt catalysts. Different supports (e.g. Al2O3, TiO2, SiO2, and ZrO2 modified SiO2 or Al2O3) and a variety of promoters, including noble metals and metal cations, were examined. Significant support interactions on the reduction of cobalt oxide species were observed in the order Al2O3>TiO2>SiO2. Addition of Ru and Pt exhibited a similar catalytic effect by decreasing the reduction temperature of cobalt oxide species, and for Co species where a significant surface interaction with the support was present, while Re impacted mainly the reduction of Co species interacting with the support. For catalysts reduced at the same temperature, a slight decrease in cluster size was observed in H2 chemisorption/pulse reoxidation with noble metal promotion, indicating that the promoter aided in reducing smaller Co species that interacted with the support. On the other hand, addition of non-reducible metal oxides such as B, La, Zr, and K was found to cause the reduction temperature of Co species to shift to higher temperatures, resulting in a decrease in the percentage reduction. For both Al2O3 and SiO2, modifying the support with Zr was found to enhance the dispersion. Increasing the cobalt loading, and therefore the average Co cluster size, resulted in improvements to the percentage reduction. Finally, a slurry phase impregnation method led to improvements in the reduction profile of Co/Al2O3.
Keywords: Alumina; Cobalt; Fischer–Tropsch synthesis; Metal-support effects; TPR; TPD; Promoters;
Erratum to “Preparation and characterisation of mesoporous zirconium oxide” by V.I Pârvulescu; H Bonnemann; V Pârvulescu; U Endruschat; A Rufinska; Ch.W Lehmann; B Tesche; G Poncelet (283).
Author Index (287-288).
Subject Index (289-293).
Contents Contd. (295).