Applied Catalysis A, General (v.243, #1)
Editorial Board (iii).
Development of a fixed-bed continuous-flow high-throughput reactor for long-chain n-alkane hydroconversion by Ward Huybrechts; Jérôme Mijoin; Pierre A. Jacobs; Johan A. Martens (1-13).
A high-throughput vapor-phase catalytic experimentation unit with 15 parallel microreactors and fast, on-line GC product analysis was built. The 15 open tubular microreactors were mounted in a single furnace and fed with a common gas stream. Decane hydroisomerization experiments were conducted on 35 mg quantities of reference platinum-loaded ultrastable Y zeolite particles at 200 °C, 4.5 bar, using a hydrogen/hydrocarbon molar ratio of 375/1 and contact times of 100–800 kg s/mol. The microreactor outlets were sampled sequentially. GC analysis of the C1C10 alkane product mixtures was achieved in 3.2 min with a multi-capillary column. The actual gas flow rates and contact times in the individual microreactors were derived from the GC FID signals after dilution of the reaction product stream with a calibrated hydrogen gas flow. The maximum temperature difference among the microreactors, measured in an empty reactor unit was ca. 6 °C at 200 °C. The conversion data were in excellent agreement with a kinetic model established for a conventional reactor. Fitting of the kinetic data obtained in the individual microreactors to the model confirmed the small temperature variations depending on the location of each microreactor in the catalyst furnace.
Keywords: High-throughput reactor; Multi-capillary chromatography; Hydroisomerization; Ultrastable Y; n-Alkanes;
On the potential role of hydroxyl groups in CO oxidation over Au/Al2O3 by C.K. Costello; J.H. Yang; H.Y. Law; Y. Wang; J.-N. Lin; L.D. Marks; M.C. Kung; H.H. Kung (15-24).
The deuterium isotope effect in the steady state CO oxidation rate over Au/γ-Al2O3 in the presence of H2 or H2O and the effect of pretreatment on an uncalcined catalyst were studied. In a reaction feed containing 1% CO, 0.5% O2, and 40.5% H2 at room temperature, CO oxidation exhibited a deuterium isotope effect (k H/k D) of 1.4±0.2. The rate of D2 oxidation was also slower than the oxidation of H2, such that the selectivity for CO oxidation was 86% in the presence of D2 versus 77% in the presence of H2. In contrast, there was no deuterium isotope effect in a feed containing 1% CO, 0.5% O2, and 1.5% H2O. H2 was also more effective in regenerating a CO oxidation reaction deactivated catalyst than D2, whereas H2O and D2O were equally effective. The difference was attributed to the different mechanisms with which H2 or H2O prevented deactivation of the catalyst during CO oxidation. An uncalcined Au/γ-Al2O3 was rather inactive. It could be activated by treatment with a mixture of H2 and H2O at 100 °C, although treatment by either H2 or H2O alone was ineffective. The observations are consistent with the model of the active site consisting of an ensemble of metallic Au atoms and a cationic Au with a hydroxyl group.
Complete benzene oxidation over gold-vanadia catalysts supported on nanostructured mesoporous titania and zirconia by V Idakiev; L Ilieva; D Andreeva; J.L Blin; L Gigot; B.L Su (25-39).
The new generation of gold-vanadia catalysts supported on mesoporous titania and zirconia for complete benzene oxidation were explored. The catalysts were characterized by X-ray, TEM, SEM, N2 adsorption analysis, TPR and ESR spectroscopy. The vanadia loading stabilized structure of the both mesoporous supports and this effect is stronger for zirconia comparing to titania. The presence of gold enhanced the V 5+→V 3+ reduction step and depending on the preparation method, differences in the reduction behavior were established. The catalytic activity of the catalysts also strongly depends on the preparation techniques. For the both series of the studied catalysts when the gold is loaded firstly the activity in complete benzene oxidation is higher than when the vanadia is deposited firstly (VAT>AVT and VAZ>AVZ). A strong synergistic effect between gold and vanadia supported on titania was observed and the catalysts on titania exhibit higher catalytic activity than the catalysts based on zirconia. Comparing the activity of the gold-vanadia/zirconia and gold/zirconia catalysts, it was established for the both catalysts high and equal activity in the reaction studied. The observed differences in the structural and catalytic properties of the both series of the studied catalysts were connected with the nature of the supports used.
Keywords: Gold-vanadia supported catalysts; Mesoporous titania and zirconia; Complete benzene oxidation; X-ray; TEM; SEM; N2 adsorption; TPR; ESR;
Selective hydroxylation of benzene to phenol over supported vanadium oxide catalysts by Karin Lemke; Heike Ehrich; Ursula Lohse; Heinz Berndt; Klaus Jähnisch (41-51).
Investigations of the one-step liquid-phase hydroxylation of benzene to phenol by hydrogen peroxide on vanadium oxide catalysts are described. The vanadium oxide catalysts have been prepared using mesoporous siliceous structures of MCM-type, silica of Aerosil® 300 type and amorphous siliceous microporous mixed oxides as supports. Characterization of composition, surface area, and reducibility of the catalysts showed that the vanadium oxide species were highly dispersed on the silica surfaces. In a broad screening, the catalytic performances were determined in different solvent systems such as acetic acid, acetone, methanol, acetonitrile as well as in the solvent-free so called “triphase” system. The influence of vanadium content of the catalyst on the hydroxylation was studied. Best results were obtained on a low vanadium-containing VO x /SiO2 catalyst (0.14 wt.% V) in acetonitrile as solvent. The drastic influence of dispersion of vanadium sites on the catalytic performance has been demonstrated.
Keywords: Benzene; Hydroxylation; Phenol; Vanadium oxide catalysts; MCM; Aerosil; Microporous mixed oxides;
An EXAFS study of the coordination chemistry of hydrogen hexachloroplatinate (IV) by W.A. Spieker; J. Liu; X. Hao; J.T. Miller; A.J. Kropf; J.R. Regalbuto (53-66).
The changes in coordination chemistry that occur as chloroaquohydroxo platinate complexes adsorb over alumina have been studied using extended X-ray absorption fine structure (EXAFS) analysis at the advanced photon source (APS) at Argonne National Laboratory. Our earlier study of Pt complexes in the liquid phase [Appl. Catal. A: Gen. 232 (2002) 219] is used as a starting point. Samples were prepared both in excess solution with 200 ppm chloroplatinic acid (CPA), and using pore volume impregnation with higher CPA concentrations to give loadings of 0.25 to 4.25 wt.% Pt. The changes in coordination chemistry of chloroaquohydroxo platinate complexes adsorbing over alumina need not invoke a “triple layer” theory, which incorporates surface grafting reactions, but are more readily explained by refinements to the “double layer” theory which is the kernel of the Revised Physical Adsorption model [Chem. Eng. Sci. 56 (2000) 2365]. The changes in speciation of adsorbing CPA complexes appear to be influenced first by the change in the bulk pH brought on by the oxide buffering effect, second by the additional pH change at the (single) layer of adsorption at the alumina surface, and third by the chloride concentration at this local level. At low surface loadings, pH shifts are minimized and the amount of chloride near the adsorption plane appears to be low. Platinum complexes adsorb with low Pt–Cl/Pt–O bond ratios, even when present in the bulk solution as hexachlorides. The adsorbed species appear to behave as in the liquid phase at the pH of the adsorption plane with no excess chloride. At high surface loadings such as with pore volume impregnation, the pH shifts toward the alumina PZC are very large but all chloride is kept in the pore volume. At high Pt loadings, high chloride concentration appears to dominate the Pt speciation and Pt–Cl coordination in the adsorbed complexes remains high, as occurs in the liquid phase in excess chloride. The adsorption of Pt from solutions in which zero-valent species are thought to initially exist can be explained by mechanisms in which either rapid OH–H2O exchange occurs at the higher pH of the adsorption plane, creating a dianionic adsorbing complex, or a surface protonation reaction, which creates a dianionic adsorbed complex in concert with surface charging. Both mechanisms are consistent with an electrostatic adsorption mechanism.
Keywords: Hydrogen hexachloroplatinate(IV); Chloroplatinic acid; Pt EXAFS; H2PtCl6 hydrolysis; Adsorption; Electric double layer;
Structure and redox properties of MnO x /Yttrium-stabilized zirconia (YSZ) catalyst and its used in CO and CH4 oxidation by Radu Craciun; Brian Nentwick; Konstantin Hadjiivanov; Helmut Knözinger (67-79).
The complex structure, the redox properties, and the catalytic activity for the MnO x /yttrium-stabilized zirconia (YSZ) catalytic system were explored and reported here. The MnO x /YSZ material (10% by wt. as MnO2) was characterized by temperature-programmed reduction (TPR), X-ray diffraction (XRD) and FITR spectroscopy of low-temperature CO adsorption. The redox properties were explored by exposing the material to H2-rich/O2-rich environment cycles, at various temperatures (up to 1173 K), followed by material characterization. The catalytic activity of the MnO x /YSZ system in CO and CH4 oxidation (fuel rich/lean) was investigated and correlated with the observed structure and redox properties. XRD data indicated that MnO x is well dispersed on the YSZ support, with crystallites below 2–3 nm (close to the XRD detection limit). TPR data show that most of the Mn is present as Mn3+ and Mn2+. Low-temperature CO adsorption on MnO x /YSZ shows the formation of Mn3+–CO species (2180 cm−1) which are easily desorbed by evacuation at 85 K. Heating the sample (120 K and up) in CO atmosphere shows the formation of CO2 and bridged or bidentate carbonates which block the CO adsorption sites. At higher temperatures, the CO3 2− species are mostly converted into HCO3 − species. When CO is adsorbed at 85 K on a sample exposed to a H2-rich atmosphere, Mn2+–CO species are formed. Heating the sample in CO atmosphere (O-rich) leads again to oxidation of CO. In this case no CO2 is formed, but at ∼130 K monodentate carbonates start to emerge. Catalytic activity results indicate that the MnO x presence favored the CO oxidation process but had an inhibiting effect on the CH4 oxidation. The kinetic experiments showed that for CO oxidation, the rate is first order in O2 on MnO x /YSZ catalyst and zero order on the YSZ support.
Keywords: CO and CH4 oxidation; CO-adsorption; FTIR spectroscopy; MnO x /YSZ;
Oxide-catalyzed conversion of acetic acid into acetone: an FTIR spectroscopic investigation by M.A. Hasan; M.I. Zaki; L. Pasupulety (81-92).
Adsorptive and catalytic interactions of gas phase acetic acid with surfaces of alumina, titania and ceria were observed by in situ Fourier-transform infrared (FTIR) spectroscopy on heating from room temperature up to 400 °C. The results revealed that, on alumina the acid was irreversibly, non-dissociatively adsorbed in the form of hydrogen-bonded molecules, and dissociatively in the form of bidentate bound acetate species over the full range of temperature scanned. In the gas phase, no chemical change was observed. On titania, the gas phase remained unchanged on heating up to 300 °C, but at 400 °C the acetic acid was largely converted into acetone (as well as CO2 and H2O) and minor products of isobutene and methane. Similar changes to the gas phase were observed on ceria, however, at 300 °C. The acetic acid conversion on ceria was almost complete. The appearance of acetone molecules in the gas phase was pertained by the emergence of IR absorptions implying the formation on the surface of unsaturated carbonyl species. Thus, the formation of these surface species, together with isobutene and CH4 molecules in the gas phase, was considered consequent to the occurrence of further surface reactions of acetone molecules. Observance of structural and chemical stability of the surface and bulk of TiO2 and CeO2 throughout the reaction was taken to emphasize the catalytic nature of the acetic acid conversion into acetone in the gas phase, a process that is evidently more simple and economic than the conventional pyrolytic synthesis of acetone from metal acetate compounds in the solid state. The catalytic sites were suggested to be Lewis acid–base pair sites, with the Lewis acid sites (Ti4+ or Ce4+) being reducible. Mechanistic pathways were proposed for the observed adsorptive and catalytic interactions of acetic acid molecules on the test oxides.
Keywords: Acetic acid; Adsorption; Catalytic conversion; IR spectroscopy; Alumina; Titania; Ceria;
Influence of acid–base properties of catalysts in the gas-phase dehydration–dehydrogenation of cyclohexanol on amorphous AlPO4 and several inorganic solids by F.M Bautista; J.M Campelo; A Garcı́a; D Luna; J.M Marinas; R.A Quirós; A.A Romero (93-107).
A series of metal phosphates (Al, Fe, Ni, Ca and Mn) and stoichiometric (Al/Fe=Al/Ca=1) mixed systems: FeAl(PO4)2 and Ca3Al3(PO4)5, were prepared by an ammonia gelation method. Their amorphous character was determined through several physical methods: nitrogen adsorption, DRIFT and XRD patterns. Gas-phase competitive dehydration/dehydrogenation reaction of cyclohexanol was carried out as a probe reaction and cyclohexene was usually the main reaction product, but different amounts of 1- and 3-methylcyclopentenes as well as cyclohexanone were always obtained. These results were compared to those obtained with 17 commercial inorganic solids (several metal oxides, sulfates and phosphates). A main unexpected conclusion was that cyclohexanol dehydration is obtained not only in acid sites but also in basic sites.
Keywords: Cyclohexanol conversion; Acid–base sites; Amorphous AlPO4 catalyst;
Catalytic reactions of n-hexane and 1-hexene on molybdenum dioxide by P. Wehrer; C. Bigey; L. Hilaire (109-119).
Skeletal isomerization of n-hexane and 1-hexene was investigated on molybdenum dioxide MoO2 powder which was gradually reduced under a rapid flow of hydrogen (1 atm) at 350 °C. Significant isomerization was observed with the commercially available MoO2 samples whose overall behavior was qualitatively apparented to the one previously reported of the trioxide MoO3, but not quantitatively. On the other hand, the same experiments conducted with MoO2 prepared in the laboratory showed always negligible isomerization but mainly cracking reactions. The differences between the two sorts of molybdenum dioxide were attributed to the presence in the commercial dioxide of a few percents of Mo6+ oxide—as confirmed by DRX and XPS analysis—whose reduction products are well known to act as very efficient skeletal isomerizing catalysts. Hence, only the results obtained with the purer laboratory made MoO2 are considered to reflect the real properties of this compound. As a consequence, the skeletal isomerizing properties of partially reduced MoO3 powder can hardly be attributed to the molybdenum dioxide MoO2—associated or not with molybdenum metal—which, among other products, is always present in the resulting material.
Keywords: Isomerization; Alkane; Alkene; Molybdenum dioxide;
Chemical treatment of γ-Al2O3 and its influence on the properties of Co-based catalysts for Fischer–Tropsch synthesis by Junling Zhang; Jiangang Chen; Jie Ren; Yuhan Sun (121-133).
Alumina has been pretreated in the presence of medium and then used to prepare the supported Co catalysts. These modified supports and the respective catalysts have been thoroughly characterized by means of methods such as BET, pore size distribution, X-ray diffraction (XRD), ammonia temperature-programmed desorption (NH3-TPD), pyridine infrared spectroscopy (Py-FTIR), KBr-IR spectroscopy (FTIR), temperature-programmed reduction (TPR), oxygen titration, hydrogen chemisorption, in situ diffuse reflectance infrared Fourier transformation spectroscopy (DRIFTS) and CO hydrogenation to understand how chemically-treated alumina influences the properties of Co catalysts. These characterizations clearly show the changes of morphology (surface area, pore volume, pore size distribution and crystallite phase) as well as chemical properties (e.g. acidity) of the supports. Although the cobalt oxide crystallite sizes of the oxidic catalyst precursor are almost unaffected by different support pre-treatment, the reducibility of these catalysts vary greatly. And the support pre-treatment remarkably influences the adsorption and catalytic properties of these Co catalysts. The acetic acid-treated support has a negative effect on the catalytic properties of Co catalyst, whereas the ammonia and ammonium nitrate-treated samples show pronounced effects on the catalytic behaviors of Co catalysts.
Keywords: Fischer–Tropsch synthesis; Cobalt/alumina; Support; Chemical treatment; Characterization; Syngas;
A comparative study of Pt/CeO2 catalysts for catalytic partial oxidation of methane to syngas for application in fuel cell electric vehicles by Lidia Pino; Antonio Vita; Manuela Cordaro; Vincenzo Recupero; Manajanatha Subraya Hegde (135-146).
In the framework of a project aimed at developing a reliable hydrogen generator for mobile polymer electrolyte fuel cells (PEFCs), particular emphasis has been addressed to the analysis of catalysts able to assure high activity and stability in transient operations (frequent start-up and shut-down cycles). In this paper, the catalytic performance of 1 at.% Pt/ceria samples prepared by coprecipitation, impregnation and combustion, has been evaluated in the partial oxidation of methane. Methane conversion and hydrogen selectivity of 96 and 99%, respectively, associated with high stability during 100 h of reaction under operative conditions (start-up and shut-down cycles), have been obtained.
Keywords: Catalytic partial oxidation of methane; Ceria-supported platinum catalyst;
Dismutation of CFC-12 on alumina: reaction mechanism and the role of surface acidity by Ayman Hammoudeh; Sabri S Mahmoud; Samih Gharaibeh (147-154).
Dismutation of CF2Cl2 on differently pretreated alumina samples and the role of surface acidity in this reaction were investigated in the temperature range of 200–350 °C and at a CF2Cl2 partial pressure of 0.025 atm. Our results support a reaction mechanism involving the direct participation of the surface in halogenating the reacting CF2Cl2 molecule without any need for this reaction to occur in a concerted manner. Surface acidity was found to enhance the activation process of alumina and to suppress its deactivation at low temperatures and at low CF2Cl2 partial pressures.
Keywords: Dismutation; Freon; CFC-12; Alumina; Surface acidity;
Influence of iron promoter on catalytic properties of Rh-Mn-Li/SiO2 for CO hydrogenation by Hongmei Yin; Yunjie Ding; Hongyuan Luo; Hejun Zhu; Daiping He; Jianmin Xiong; Liwu Lin (155-164).
The effect of iron promoter on the catalytic properties of Rh-Mn-Li/SiO2 catalyst in the synthesis of C2 oxygenates from syngas was investigated by means of the following techniques: CO hydrogenation reaction, temperature-programmed reduction (TPR), temperature-programmed desorption and reaction of adsorbed CO (CO-TPD and TPSR) and pulse adsorption of CO. The results showed that the addition of iron promoter could improve the activity of the catalysts. Unexpectedly, the yield of C2 oxygenates increased greatly from 331.6 up to 457.5 g/(kg h) when 0.05% Fe was added into Rh-Mn-Li/SiO2 catalyst, while no change in the selectivity to C2 oxygenates was observed. However, the activity and selectivity of C2 oxygenates were greatly decreased if the Fe amount exceeded 1.0%. The existence of a little iron decreased the reducibility of Rh precursor, while the reduction of Fe component itself became easier. CO uptake decreased with increasing the quantity of Fe addition. This phenomenon was further confirmed by CO-TPD results. The CO-TPD and TPSR results showed that only the strongly adsorbed CO could be hydrogenated, while the weakly adsorbed CO was desorbed. We propose that Fe is highly dispersed and in close contact with Rh and Mn; such arrangements were responsible for the high yield of C2 oxygenates.
Keywords: Rhodium; Iron; Carbon monoxide; Hydrogenation; C2 oxygenates;
Photo-catalytic conversion of methane into methanol using visible laser by M.A Gondal; A Hameed; A Suwaiyan (165-174).
The photo-catalytic conversion of methane into methanol was investigated under different experimental parameters such as catalyst concentration, laser power, laser exposure time, effects of free radical generator (H2O2) and electron capture agent (Fe3+), using visible laser light. The study was carried out at room temperature with a simple set up using a laser light, water and a semiconductor photo-catalyst WO3. The reaction products (methanol, O2 and CO2) were characterized using gas chromatography. The use of WO3 as photo-catalyst replaces the UV laser light with visible laser light. This greatly simplifies reactor design and permits flexibility in the selection of a laser source in the visible region. The oxygen to tungsten ratio in WO3 at different temperatures was studied by XPS.
Keywords: Methane conversion; Methanol; Photo-catalysis; Hydrocarbons; Tungsten oxide;
Study on the alkylation of benzene with propylene over Hβ zeolite by Minghan Han; Shixiong Lin; Emil Roduner (175-184).
The alkylation of benzene with propylene over Hβ zeolite catalyst was studied by means of in situ FTIR and coke characterization. After the catalyst has been impregnated with benzene, propylene can replace adsorbed benzene and react with benzene. In the absence of excess benzene, adsorbed propylene will react with itself. Therefore, a higher molar ratio of benzene to propylene is indispensable. In addition, at higher temperature, propylene can more easily replace adsorbed benzene, so the reaction proceeds more quickly. For the process carried out in a fixed-bed bubble reactor where catalysts are impregnated in the bulk liquid of benzene, propylene oligomerization is suppressed. The coke deposited on the catalyst is composed of multi-substituent aromatic compounds which have single or double rings. The deactivation is caused from pore blockage of bulkier molecules formed by multi-alkylation instead of propylene oligomerization. With the increase of reaction temperature, the decomposition rate of products increases. The decomposition will form propylene and bulkier molecules, which will block the micropore channel of the catalyst and deactivate the catalyst. At higher temperature, however, bulkier molecules diffuse more quickly; this can prolong the lifetime of catalyst, so the reaction temperature could be too low. The suitable temperature should be 120–160 °C.
Keywords: Alkylation; Isopropylbenzene; Deactivation; β Zeolite; Mechanism;
Photocatalytic property of bismuth titanate Bi12TiO20 crystals by W. Feng Yao; Hong Wang; X. Hong Xu; X. Feng Cheng; Ji Huang; S. Xia Shang; X. Na Yang; Min Wang (185-190).
Bi12TiO20 crystals with sillenite structures were prepared by a chemical solution decomposition (CSD) method. The band gap of Bi12TiO20 crystals was estimated to be about 2.4 eV from the onset of UV-Vis absorption spectra of the photocatalyst. The energy band of Bi12TiO20 is assumed to between the top of Bi3+ 6s band and the bottom of the Ti 3d band. The photocatalyst based on Bi12TiO20 crystals for photo-decolorization of methyl orange had been examined first. Bi12TiO20 crystals show high photocatalystic activity to photo-decolorization 10 ppm methyl orange solution in 1.5 h. Bi–O polyhedra in Bi12TiO20 crystals were assumed to facilitate the photocatalystic activity of the catalysts.
Keywords: Photocatalyst; Sillenite; Titanate; CSD;
Activity and basicity of BaO modified zeolite and zeolite-type catalysts by K Arishtirova; P Kovacheva; A Predoeva (191-196).
BaO modified zeolite and zeolite-type catalysts provide new and valuable data about the alternative reaction for ethylbenzene and styrene formation—the oxidative methylation of toluene with methane. Over BaO modified X, Y, mordenite, ZSM-5, silicalite and AlPO4-5 catalysts, the toluene conversion, selectivity and yield of C8 hydrocarbons (ethylbenzene and styrene) are affected to a different degree by the reaction temperature. Both activity and basicity (measured by TPD of CO2) are resultant from the combined effect of chemical composition and geometry of zeolite and zeolite-type structures, as well as from the accessibility of BaO modifier to the reagents. The Si/Al atomic ratio determines the catalytic and basic properties within a series of the same structural type. The zeolite and zeolite-type structural geometry determines the accessibility of the introduced BaO. Higher accessibility of BaO used for modification favors the activity. Basicity measurements by X-ray photoelectron spectroscopy (XPS) are mainly sensitive towards the chemical composition of the zeolite and zeolite-type framework.
Keywords: Basic zeolite and zeolite-type catalysts; BaO; Oxidative methylation; Toluene; Methane; TPD of CO2; XPS;
Contents Continued (199).