Forgot your password?
New user?
New Window Opens on the Secret Life of Microbes: Scientists Develop First Microbial Profiles of Ecosystems
CAS announces the release of SciFinder Scholar for Mac OS X
Press Releases
Press Releases
| Check out our New Publishers' Select for Free Articles |
Applied Catalysis A, General (v.294, #1)
The chemistry of selective ring-opening catalysts by Hongbin Du; Craig Fairbridge; Hong Yang; Zbigniew Ring (pp. 1-21).
Bitumen-derived crude and heavy oils require severe processing and produce middle distillate product with poor ignition quality. This becomes a concern to refiners as tighter specifications on transportation fuel are promulgated. One process to address this issue is selective ring opening of cycloparaffins to reduce the number of ring structures, while retaining the carbon number of a product molecule. The process involves bifunctional catalysts, both metal and acid sites, working in high-pressure, high-temperature reactor systems in the presence of hydrogen. The acidic sites catalyze dehydrogenation, cracking, isomerization and dealkylation, while the metal sites promote hydrogenation, hydrogenolysis and isomerization. Various compounds containing single, double and multiple rings have been used to model the ring-opening reactions and a number of mechanisms have been proposed. The five-membered ring readily undergoes ring-opening reaction on either acid or metal catalysts with the selectivity and activity dependent on the nature of the supported metal catalyst. The ring opening of six-membered ring compounds is secondary, requiring an acidic function to isomerize a six-membered ring cycloparaffin to a five-membered ring. A balanced metallic-acidic function catalyst is necessary to achieve optimal performance. A system dominated by acid function results in excess cracking, while a catalytic system with high concentration of metals leads to mainly hydrogenation. Commercial hydrocracking catalysts using transition metal sulfides on acidic supports usually require severe operating conditions due to their low activities of the metal sulfide compared to metal sites, leading to extensive cracking of cycloparaffin side chains. Noble metals supported on acidic oxides are the most active catalysts for selective ring opening, but these catalysts are very sensitive to poisoning by sulfur compounds in petroleum feedstocks. An understanding of the chemistry of selective ring-opening catalysts, combined with theoretical studies of structure–activity relationships and high throughput experimentation methods, provides opportunities in searching for new generations of selective ring-opening catalysts with high-performance and sulfur resistance.
Keywords: Ring opening; Catalysts; Model compound; Mechanism; Bifunctional catalysts
Oscillation theory by Leonid B. Datsevich (pp. 22-33).
The present paper continues the series of articles devoted to the oscillatory behaviour in gas–liquid/liquid reactions on a porous catalyst. Based on the theoretical predictions given earlier, some phenomena (e.g. pressure pulsations in pores caused by the liquid oscillations, leading in some cases to the catalyst destruction) and some approaches to the reaction enhancement (e.g. the preparation of special pores leading to the liquid pumping through the catalyst particle in the demanded direction) are discussed in detail. Several deductive, illustrative experiments demonstrating the oscillatory behaviour and some possible investigation methods have been described.
Keywords: Multiphase catalysis; Oscillation model; Catalyst engineering; Pore optimisation; Liquid pumping; Cavitation; Catalyst destruction; Sound spectrum; Reacting boiling
Effects of added 3d transition-metals on Ag-based catalysts for direct epoxidation of propylene by oxygen by Atsushi Takahashi; Noriyasu Hamakawa; Isao Nakamura; Tadahiro Fujitani (pp. 34-39).
Direct epoxidation of propylene by oxygen was investigated over Ag-based catalysts containing an added 3d transition-metal species (Mn, Fe, Co, or Ni). The oxidation of propylene occurred at lower temperatures over Ag-based catalysts containing small amounts of added metal species than over a Ag single catalyst. The yield of propylene oxide (PO) depended strongly on the type of metal: the addition of Ni afforded the highest yield of PO at similar values for the conversion of propylene (4–6%). The highest PO selectivity, four times that obtained with the Ag single catalyst, was obtained at 33mol% Ni. X-ray diffraction analysis and transmission electron microscopy results showed that the dispersion of Ag particles in the Ag–Ni catalysts increased with increasing Ni content, which suggests that Ni atoms controlled the Ag particle size by suppressing the sintering of Ag particles. The turnover frequency for propylene epoxidation was highest at a Ag particle size of approximately 80nm.
Keywords: Propylene; Epoxidation; Silver; Nickel
Synthesis and characterization of silica-capped titania nanorods: An enhanced photocatalyst by Chung-Sung Yang; Chun-Jung Chen (pp. 40-48).
A new one-pot synthesis route to produce silica nano-layer capped titania (SiO2∧TiO2) nanorods, a TiO2-based nanocomposite photocatalyst, has been developed by means of a non-aqueous solution method. The whole of the TiO2 rod is covered with a heterogeneous SiO2 layer except at the two ends. The simultaneous development of a SiO2 nano-layer onto an evolutional TiO2 nanorod highly depends on the mole ratio of the starting reagents. Additionally, in the study of the adsorption of rhodamine-6G (R-6G) by SiO2∧TiO2 nanorods, the spectra data confirms that the SiO2 nano-layer can noticeably accumulate the adsorbed concentration of R-6G on the surface of SiO2∧TiO2 nanorods, which offers a critical example to explain why the SiO2 layer can enhance the photodecomposition rates of TiO2 based photocatalysts. Under UV irradiation ( λ=254nm), the photodecomposition rates of R-6G for the SiO2∧TiO2 samples (0.0242min−1) are about 3–6 times faster than that of the particulate TiO2 (4.0×10−3min−1) and SiO2/TiO2 (=25/75) (6.7×10−3min−1) xerogel photocatalysts. The process of uniformly growing SiO2 nano-layer onto TiO2 based nanocomposite photocatalyst also demonstrates that the existence of SiO2 layer is beneficial for the photocatalysis efficiency but the improvement of photocatalysis efficiency is not proportional to the thickness of SiO2 layer.
Keywords: Photocatalyst; Titania; Nanocomposite materials; Sol–gel
Catalytic reduction of NO by H2 over Ag/Al2O3 under dry reducing conditions by Henrik Backman; Jenny Jensén; Fredrik Klingstedt; Tapio Salmi; Dmitry Yu. Murzin (pp. 49-58).
The kinetics and mechanism of the reduction of NO and N2O by H2 over Ag/Al2O3 were studied with transient step response and isotope exchange techniques. Dissociation of hydrogen and deuterium was observed on the Ag/alumina catalyst. The reduction of NO with hydrogen gave N2 as the main product and N2O, NH3 and H2O as by-products, while the catalyst showed low activity for the self-decomposition of NO and N2O. To clarify the reaction mechanism for the reduction of NO with hydrogen parameter estimation and modeling was carried out.
Keywords: NO reduction; Hydrogen; Deuterium; Ag/Al; 2; O; 3; Isotopic exchange; Transient step response
Mesoporous materials from zeolite seeds as supports for nickel tungsten sulfide active phases by Shuangqin Zeng; Juliette Blanchard; Michèle Breysse; Yahua Shi; Xintian Su; Hong Nie; Dadong Li (pp. 59-67).
MCM-41 aluminosilicate mesostructures (Si/Al=18) with hexagonal long-range order were assembled from protozeolitic nanoclusters (zeolite seeds) that normally nucleate the crystallization of β-zeolite (BEA). Depending on the duration of the thermal treatment leading to the zeolite seeds, different acidic materials can be obtained. For short pre-treatment duration, the material presents the main characteristics of the MCM-41, whereas for longer pre-treatment duration, there is a gradual transition to a mostly zeolitic one. Hydrocracking of n-decane was carried out on catalysts composed of a sulfided nickel tungsten phase supported on various β-MCM-41 supports as well as on an amorphous silica–alumina and an AlSBA-15 material. The hydrocracking properties of the catalysts are related to the nature of the β-МCМ-41 and vary from the comportment of silica–alumina materials to the one of a zeolite. The nickel-tungsten catalyst supported on β-MCM-6 (6h pre-treatment for the zeolite seeds solution) is more active and more selective toward isomerization than the catalyst supported on a commercial silica–alumina support.
Keywords: SBA-15; BEA zeolite; Beta zeolite; Silica–alumina; NiW
Olivine as tar removal catalyst for biomass gasifiers: Catalyst characterization by Lopamudra Devi; Menno Craje; Peter Thüne; Krzysztof J. Ptasinski; Frans J.J.G. Janssen (pp. 68-79).
In the present paper olivine is considered as a prospective in-bed tar removal catalyst for fluidized bed biomass gasifiers. The catalytic activity of olivine is investigated via steam reforming reaction of naphthalene as a model biomass tar compound. It is observed that the calcination of olivine improves the performance of the catalyst. Calcination of olivine is done with air at 900°C for different treatment times. With increasing calcination time, tar conversion increases; more than 80% naphthalene conversion is observed with 10h of calcination time for olivine, which is found to be an optimum as further increase in time up to 20h does not improve the naphthalene conversion. The primary focus of this paper is to characterize the olivine catalysts under investigation (specific composition (Mg1.78, Fe0.13) SiO4) with different calcination times. Techniques such as BET-surface area analysis, XPS, Mössbauer spectroscopy, TPR, SEM/EDX analysis are used to understand the basic phenomena of calcination of olivine. Olivine is observed to be non porous. SEM/EDX show iron rich areas at the surface of olivine after calcination. Severe segregation of iron is observed at the surface due to calcination, which is found to be non-uniform. Mössbauer spectroscopy reveals the appearance of iron(III) phase in the olivine after calcination. Besides the catalytic activity towards tar reduction, olivine is also observed to be highly attrition resistant to be used as an in-bed additive for fluid bed and moving bed biomass gasifiers.
Keywords: Biomass; Tar removal; Olivine; Calcination; Catalyst characterization
Surface morphology of NiMo/Al2O3 catalysts incorporated with boron and phosphorus: Experimental and simulation by D. Ferdous; A.K. Dalai; J. Adjaye; L. Kotlyar (pp. 80-91).
Three catalysts namely NiMo/Al2O3, NiMoB/Al2O3 and NiMoP/Al2O3 were prepared, extensively characterized and were used for hydrodenitrogenation (HDN) and hydodesulfurization (HDS) of heavy gas oil obtained from Athabasca bitumen. The hexagonal structure of MoS2 of the catalyst at different stages of reaction was studied and quantified using transmission electron microscopy (TEM). The length and number of MoS2 stack, the dispersion of edge and corner molybdenum atoms, number of MoS2 stacks and number of MoS2 layers per stack, which play important role on the HDN and HDS activities of these catalysts, were calculated. It was concluded that the decrease in MoS2 slab length caused an increase in catalyst activity for HDN of heavy gas oil derived from Athabasca bitumen. Computer simulation was also performed in order to correlate the surface morphology with catalyst activity. Maximum edge and corner Mo atoms which were responsible for high HDN and HDS reactions, was obtained at an average MoS2 slab length of approximately 20 and 10Å, respectively.
Keywords: Hydrodenitrogenation; Hydrodesulfurization; Promoters; Surface morphology; MoS; 2; dispersion
An investigation of the acid properties of UL-ZSM-5 by FTIR of adsorbed 2,6-ditertbutylpyridine and aromatic transalkylation test reaction by A. Ungureanu; T.V. Hoang; D. Trong On; E. Dumitriu; S. Kaliaguine (pp. 92-105).
The surface acidity of semicrystalline zeolitic mesoporous UL-ZSM-5 aluminosilicate materials of different degree of crystallinity (UL-zeolites were introduced recently in our laboratory [D. Trong On, S. Kaliaguine, Nanoporous Materials—Science and Engineering, Imperial College Press, 2005; D. Trong On, S. Kaliaguine, Angew. Chem. Int. Ed. 40 (2001) 3248; A. Ungureanu, D. Trong On, E. Dumitriu, S. Kaliaguine, Appl. Catal. A: Gen. 254 (2003) 203]) has been investigated by means of FTIR spectroscopy of adsorbed 2,6-ditertbutylpyridine (DTBPy) and the test reaction of transalkylation of toluene with 1,3,5-trimethylbenzene. FTIR experiments and catalytic tests allowed also characterizing the surface acidity of the starting amorphous mesostructured aluminosilicate precursor and the external acid sites of crystalline ZSM-5 zeolite. The results indicated that most of the Brönsted acidic OHs accessible to DTBPy molecules are terminal on the mesopore surfaces of both UL-ZSM-5 materials and mesostructured aluminosilicate and also on the external surface of ZSM-5. These sites, which are responsible for the OH stretching band near 3745cm−1, are of medium strength. Their density correlated well with the external surface area. On the other hand, bridging SiOHAl sites generating OH stretching band at 3612cm−1 and conferring the medium-strong Brönsted acidity of UL-ZSM-5 and ZSM-5 are located predominantly in the micropores of the zeolitic ZSM-5 phase and thus not accessible to bulky DTBPy. Just like the external surface of ZSM-5, the mesoporous surface of UL-ZSM-5 carries a small fraction of bridging Brönsted acid sites which are likely located in the pore mouth region of their ZSM-5 zeolitic phase. The catalytic activity of the distinct aluminosilicate materials under study as well as the selectivity towards the different reactions involved in the transalkylation process has been found to strongly depend on the nature, density, strength and accessibility of the characteristic Brönsted acid sites.
Keywords: Acidity; Zeolites; Mesoporous molecular sieves; UL-ZSM-5; FTIR; Transalkylation
Isomerisation of α-pinene oxide over silica supported heteropoly acid H3PW12O40 by Kelly A. da Silva Rocha; Ivan V. Kozhevnikov; Elena V. Gusevskaya (pp. 106-110).
Silica-supported heteropoly acid H3PW12O40 (PW) has been found to be a very efficient and environmentally benign solid acid catalyst for the liquid-phase isomerisation of α-pinene oxide (1). The reaction has been performed in cyclohexane solutions at 15–40°C to give campholenic aldehyde (2) and trans-carveol (3) as the main products in up to 98% total yield, with 70% selectivity to more valuable product2. Both2 and3 are expensive ingredients in the fragrance industry. No PW leaching has been observed during the reaction. The catalyst is easily separable from the reaction mixture and reusable without loss of its activity and selectivity.
Keywords: α-Pinene oxide; Isomerisation: Heteropoly acid; Catalysis
A novel effective hydration of carbon monoxide in liquid phase by a water-soluble ruthenium complex catalyst at moderate pressures in aqueous medium by Ram S. Shukla; Sharad D. Bhatt; Ravikumar B. Thorat; Raksh V. Jasra (pp. 111-118).
A ruthenium-based complex, ethylenediaminetetraacetato ruthenate [Ru-EDTA(H2O)]−1 was found to be an efficient homogeneous catalyst to activate carbon monoxide as well as water molecule in hydration of CO. It was investigated at moderate pressures (5–20atm) and at 10–40°C for direct conversion of CO to formic acid (FA). In typical catalytic experiments conducted in a pressure reactor of 300ml capacity, 75×10−3M of FA was found to be formed at 10atm CO pressure and 40°C. The kinetics of the (i) complexation of CO with1 and (ii) hydration of CO to formic acid reactions were investigated in detail by studying the rate of formation of intermediate Ru-EDTA-CO complex2 and the rate of formation of FA, respectively as a function of the concentration of the catalyst1, CO pressure, reaction temperature, ionic strength and pH. The rate of formation of FA was found to be first order with respect to the concentration of1 and CO pressure, independent of pH and ionic strength, and dependent on temperature, in agreement with the Arrhenius equation. Similar kinetic profile and temperature dependence were observed for formation of complex2 also. Activation parameters evaluated from temperature dependence were found to be favorable for the formation of2 and the CO hydration product FA. Based on kinetic and experimental results, a mechanism for the hydration of CO to formic acid is suggested.
Keywords: Carbon monoxide; Hydration; Formic acid; Catalysis; Kinetics; Mechanism; Thermodynamics
Corrigendum to “Molecular traffic control in porous nanoparticles� [Appl. Catal. A: Gen. 288 (2005) 194–202]
by Andreas Brzank; Gunter Schütz (pp. 119-119).