Applied Catalysis A, General (v.230, #1-2)
Effect of doping of TiO2 support with altervalent ions on physicochemical and catalytic properties in oxidative dehydrogenation of propane of vanadia–titania catalysts by B. Grzybowska; J. Słoczyński; R. Grabowski; K. Samson; I. Gressel; K. Wcisło; L. Gengembre; Y. Barbaux (1-10).
Vanadia phase (one monolayer) was deposited on TiO2 anatase doped with Ca2+, Al3+, Fe3+ and W6+ ions and the catalysts thus obtained (VMeTi) were characterized by XPS, work function technique, decomposition of isopropanol (a probe reaction for acido–basic properties) and tested in oxidative dehydrogenation of propane. The doping of the TiO2 support modifies physicochemical and catalytic properties of the active vanadia phase with respect to the undoped TiO2. The specific activity in the propane oxydehydrogenation decreases in the order: VFeTi>VWTi>VTi>VAlTi>VCaTi (3), whereas the selectivity to propene follows the sequence: VWTi<VTi<VFeTi<VAlTi<VCaTi (4). No clear correlation has been observed between the activity of the catalysts and the acido–basic properties. On the other hand, the rate of the isopropanol dehydration (the acidity) decreases in the same order as the selectivity increases (4). The increase in the selectivity with the decrease in the acidity is ascribed to the easier desorption of propene from the less acidic surface, preventing the consecutive total combustion of propene. The selectivity to propene can be also correlated with the work function values which decrease in the series: VWTi>VTi>VFeTi>VAlTi>VCaTi. This implies that the lower is the surface energy barrier for transfer of electrons from the catalyst to the reacting molecules the higher is the selectivity to the partial oxidation product. It is argued that owing to the decrease in this energy barrier the reoxidation step in the catalytic reaction, involving such a transfer: O2+4e→2O2− is fast, thus, preventing the presence of intermediate non-selective electrophilic oxygen species on the surface.
Keywords: Oxidative dehydrogenation; Propane; Vanadia–titania catalysts;
Characterization and catalytic activity for ethylene dimerization of nickel sulfate supported on zirconia by Jong Rack Sohn; Won Cheon Park (11-18).
A series of catalysts, NiSO4/ZrO2, for ethylene dimerization were prepared by the impregnation method using aqueous solutions of nickel sulfate. The results obtained from X-ray diffraction and DSC indicate that the addition of NiSO4 shifted the transition of ZrO2 from amorphous to tetragonal phase toward higher temperatures due to the interaction between NiSO4 and ZrO2. Nickel sulfate supported on zirconia was found to be very active even at room temperature. The high catalytic activity of NiSO4/ZrO2 was closely correlated with the increase of acidity and acid strength due to the addition of NiSO4. The active sites responsible for ethylene dimerization may consist of low-valent nickel, Ni+ and an acid.
Keywords: Ethylene dimerization; Nickel sulfate; Zirconia; Impregnation; Acidity; Acid strength; X-ray diffraction;
Characterization of bimetallic rhodium-germanium catalysts prepared by surface redox reaction by G. Lafaye; C. Micheaud-Especel; C. Montassier; P. Marecot (19-30).
The preparation of bimetallic rhodium-germanium/silica and rhodium-germanium/alumina catalysts was investigated by controlled surface reaction. Their catalytic performances were measured for two gas phase reactions (toluene hydrogenation at 323 K and cyclohexane dehydrogenation at 543 K) and for a liquid phase reaction (citral hydrogenation at 343 K).Elemental analysis of bimetallic catalysts showed that germanium can be deposited by redox reaction between hydrogen activated on a parent monometallic rhodium catalyst and germanium tetrachloride dissolved in water (catalytic reduction method). EDX microanalysis of rhodium-germanium/silica catalysts indicated that rhodium and germanium were deposited in close contact on the silica support. However, on alumina-supported catalysts, germanium deposition occurred also separately on the support. For the different test reactions, the catalytic properties of rhodium were strongly altered by the addition of germanium. On alumina-supported catalysts, interesting catalytic effects were observed in citral hydrogenation when not only close contact exists between both metals but when, in addition, the second metal was deposited on the support in the close vicinity of rhodium.
Keywords: Rhodium-germanium catalysts; Preparation; Surface redox reaction; Citral hydrogenation;
Effects of alkali metals on the liquid phase oxidation of p-xylene by Sung Hwa Jhung; Ki Hwa Lee; Youn-Seok Park (31-40).
The effects of alkali metals on the oxidation of p-xylene (p-X) have been studied. The oxidation reaction is eventually promoted due to the alkali metal itself, even though the reaction is decelerated a bit in the beginning of the reaction. The promoting effect of alkali metal is not related to the basicity or to the anion. The effect increases as the concentration of Mn or Br decreases. The acceleration effect generally increases with the degree of deceleration in the beginning of the reaction. Through the analysis of products, one may understand that the alkali metal does not change the reaction mechanism.
Keywords: Terephthalic acid; p-Xylene oxidation; Alkali metal; Co/Mn/Br;
Effect of the pretreatment with oxygen and/or oxygen-containing compounds on the catalytic performance of Pd-Ag/Al2O3 for acetylene hydrogenation by Piyasan Praserthdam; Bongkot Ngamsom; Nina Bogdanchikova; Suphot Phatanasri; Mongkonchanok Pramotthana (41-51).
Catalytic performance of Pd-Ag/α-Al2O3 was studied for the selective hydrogenation of acetylene in the presence of excess ethylene. The catalyst activation was undertaken prior to the reaction test by the pretreatment with oxygen and/or oxygen-containing compounds, i.e. O2, NO, N2O, CO and CO2. The enhancement of catalytic performances by the pretreatment was a consequence of an increase in accessible Pd sites responsible for acetylene hydrogenation to ethylene. Furthermore, the sites involving direct ethane formation from acetylene could be suppressed by NO x treatment.
Keywords: Acetylene hydrogenation; Pd-Ag/Al2O3; Oxygen-containing compounds; Pretreatment;
Selective catalysis of μ-oxo-bismetalloporphyrins for 2-methylbutane oxidation with PhIO under mild conditions by Can-Cheng Guo; Xiao-Qin Liu; Zhi-Peng Li; Dong-Cai Guo (53-60).
Fourteen μ-oxo-bisironporphyrins and nine μ-oxo-bismanganeseporphyrins were synthesized, and their selective catalysis for the oxidation of the secondary and tertiary carbon–hydrogen bonds of 2-methylbutane with PhIO were also studied. The ratio of the relative reaction selectivity of tertiary carbon–hydrogen bonds to secondary carbon–hydrogen bonds was 3:1 when ironporphyrins were used as catalysts, and 4:1 when manganeseporphyrins were used as catalysts. The research showed that the substituents on the porphyrin rings influenced the catalytic selectivity of metalloporphyrins for the oxidation of the secondary and tertiary carbon–hydrogen bonds as well as the reaction rate. Whether ironporphyrins or manganeseporphyrins, the electron-attracting groups on porphyrin rings increased the catalytic selectivity of metalloporphyrins for the tertiary carbon–hydrogen bonds and the reaction rates, however, the electron-releasing groups increased the catalytic selectivity of metalloporphyrins for secondary carbon–hydrogen bonds, but reduced the reaction rates.
Keywords: Porphyrins; Oxidation; Catalysis; 2-Methylbutane; Selectivity;
Partial oxidation of methane over nickel-added strontium phosphate by Sang Jin Lee; Jin Hyuk Jun; Seung-Hwan Lee; Ki June Yoon; Tae Hoon Lim; Suk-Woo Nam; Seong-Ahn Hong (61-71).
It was found that nickel-added strontium phosphate exhibited high activity and selectivity in partial oxidation of methane. The optimum nickel content could be determined. Over the optimum catalyst, methane conversions and H2 and CO concentrations in excess of those predicted by the thermodynamic equilibrium were observed. It is believed that the catalytically active species is metallic nickel. This metallic nickel is considered to come from nickel-substituted strontium phosphate under reducing environment, giving highly dispersed nickel metal particles.
Keywords: Nickel; Partial oxidation of methane; Strontium phosphate; Syngas;
ZrO2/SiO2- and La2O3/Al2O3-supported platinum catalysts for CH4/CO2 reforming by Yin-Zu Chen; Biing-Jye Liaw; Wen-Han Lai (73-83).
Surface-phase ZrO2 on SiO2 (SZrOs) and surface-phase La2O3 on Al2O3 (SLaOs) were prepared with various loadings of ZrO2 and La2O3, characterized and used as supports for preparing Pt/SZrOs and Pt/SLaOs catalysts. CH4/CO2 reforming over the Pt/SZrOs and Pt/SLaOs catalysts was examined and compared with Pt/Al2O3 and Pt/SiO2 catalysts. CO2 or CH4 pulse reaction/adsorption analysis was employed to elucidate the effects of these surface-phase oxides.The zirconia can be homogeneously dispersed on SiO2 to form a stable surface-phase oxide. The lanthana cannot be spread well on Al2O3, but it forms a stable amorphous oxide with Al2O3. The Pt/SZrOs and Pt/SLaOs catalysts showed higher steady activity than did Pt/SiO2 and Pt/Al2O3 by a factor of three to four. The Pt/SZrOs and Pt/SLaOs catalysts were also much more stable than the Pt/SiO2 and Pt/Al2O3 catalysts for long stream time and for reforming temperatures above 700 °C. These findings were attributed to the activation of CO2 adsorbed on the basic sites of SZrOs and SLaOs.
Keywords: Surface oxide; ZrO2/SiO2; La2O3/Al2O3; Platinum catalyst; CH4/CO2 reforming;
Effects of small MoO3 additions on the properties of nickel catalysts for the steam reforming of hydrocarbons by Tadeusz Borowiecki; Grzegorz Giecko; Monika Panczyk (85-97).
Addition of molybdenum to nickel catalysts has a favourable effect on their properties in the steam reforming of hydrocarbons. Some attempts were made to explain the mechanism of promoter action by determining the properties of such catalysts in hydrogenolysis and cracking reactions. With small amounts of promoter (≤0.5 wt.%) advantageneous changes in selectivity of steam reforming and disadvantageneous changes in n-butane hydrogenolysis were observed. The promoter does not affect practically the catalyst properties in n-butane cracking. The effect of molybdenum was compared with that of potassium promoter applied in the industry.
Keywords: Hydrocarbon reactions; Steam reforming catalysts; Mo doped Ni/Al2O3; Catalyst activity; Carbon deposition;
Study of unsupported and active carbon supported β-Mo2C prepared from MoCl5 precursor by S Monteverdi; M Mercy; S Molina; M.M Bettahar; S Puricelli; D Bégin; F Maréché; F Furdin (99-110).
We have studied the preparation of unsupported and lignite supported β-Mo2C using MoCl5 as precursor. The catalysts were characterized by XRD and TEM techniques and tested in the reaction of propene hydrogenation at room temperature and pressure. Total carburization was observed at 923 K and although incomplete, carburization also occurred at 473 or 723 K. The high temperature supported carbide exhibited a remarkable lifetime in the test reaction (72 h), whereas, the massive carbide rapidly deactivated (<1 h). The low temperature carburized catalysts also rapidly deactivated (10 min). However, in a remarkable way, processing of the used catalysts by H2 at 723 or 873 K, not only restored the initial activity, but increased their stability with time on stream (3 and >24 h, respectively). The correlation between the degree of carburization, dispersion and catalyst stability is discussed.
Keywords: Catalyst; Molybdenum pentachloride; Molydenum carbide; Active carbon; Hydrogenation;
Dilution effect of the feed on yield of olefins during catalytic cracking of vacuum gas oil by A. Corma; O. Bermúdez; C. Martı́nez; F.J. Ortega (111-125).
A vacuum gas oil has been co-fed with nitrogen at different partial pressures in a microacitivity (MAT) unit. The cracking rate versus hydrocarbon partial pressure can be fitted to a first-order kinetic rate equation. On the other hand, hydrogen transfer reactions, which follow second-order kinetics, are more affected than conversion by the decrease in hydrocarbon partial pressure, resulting in an increase in the yield of propylene and in general of LPG olefins. The decrease in conversion, due to the lowering of hydrocarbon partial pressure, could be compensated by a mechanical mixing effect introduced by the dilution gas that improves feed vaporization and mixing.It will be shown that a MAT unit can be used to show both effects separately, i.e. feed dilution and feed dispersion.
Keywords: Cracking; Catalytic; Partial pressure; Microactivity test; Olefins; Dilution effect; Catalytic cracking;
Influence of preparation method on the catalytic properties of acid-activated tetramethylammonium-exchanged clays by Alexander Moronta; Victor Ferrer; Jorlens Quero; Geomar Arteaga; Eduardo Choren (127-135).
Two montmorillonites STx-1 (Texas) and SWy-2 (Wyoming) were first activated with different amounts of 12 M HCl and then exchanged with a fixed amount of 1 M tetramethylammonium (TMA+) chloride solution (H/TMA samples) at room temperature. TMA+-exchanged samples and then acid activated (TMA/H samples) were also prepared to evaluate the resistance to displacement of TMA+ by protons. The surface area and the acidity were determined by BET and cyclohexylamine adsorption methods, respectively. The catalytic activity of these acid activated organoclays was directly measured using the isomerization of 1-butene at 300 °C to yield cis- and trans-2-butene. The total conversion for the isomerization of 1-butene was higher for the TMA/H-samples (70 and 47% for SWy-2 and STx-1, respectively) than for the H/TMA-samples (51 and 25% for SWy-2 and STx-1, respectively). TMA+ cations adsorbed on the clays were extremely resistant to exchange by protons, but protons were easily displaced by TMA+ cations.
Keywords: Acid activation; Clays; Montmorillonites; Organoclays; Isomerization; Catalysts;
Tetragonal structure, anionic vacancies and catalytic activity of SO4 2−-ZrO2 catalysts for n-butane isomerization by Carlos R. Vera; Carlos L. Pieck; Kiyoyuki Shimizu; José M. Parera (137-151).
An assessment of the influence of the crystal structure, surface hydroxylation state and previous oxidation/reduction pretreatments on the activity of sulfate-zirconia catalysts for isomerization of n-butane was performed using crystalline and amorphous zirconia supports. Different sulfation methods were used for the preparation of bulk and supported SO4 2−-ZrO2 with monoclinic, tetragonal and tetragonal+monoclinic structures. Activity was important only for the samples that contained tetragonal crystals. The catalysts prepared from pure monoclinic zirconia showed negligible activity. SO4 2−-ZrO2 catalysts prepared by sulfation of crystalline zirconia displayed sites with lower acidity and cracking activity than those sulfated in the amorphous state. Prereduction of the zirconia samples with H2 was found to greatly increase the catalytic activity, and a maximum rate was found at a reduction temperature of 550–600 °C, coinciding with a TPR peak supposedly associated with the removal of lattice oxygen and the creation of lattice defects. A weaker dependence of catalytic activity on the density or type of surface OH groups on zirconia (before sulfation) was found in this work.A model of active site generation was constructed in order to stress the dependence on the crystal structure and crystal defects. Current and previous results suggest that tetragonal structure in active SO4 2−-ZrO2 is a consequence of the stabilization of anionic vacancies in zirconia. Anionic vacancies are in turn supposed to be related to the catalytic activity for n-butane isomerization through the stabilization of electrons from ionized intermediates.
Keywords: Sulfate zirconia; Crystalline zirconia; n-Butane isomerization; Tetragonal structure; Oxygen vacancy;
The effect of CeO2 structure on the activity of supported Pd catalysts used for methane steam reforming by Radu Craciun; Wayne Daniell; Helmut Knözinger (153-168).
Palladium (Pd) supported on CeO2-promoted γ-Al2O3 with various CeO2 (ceria) crystallinities, were used as catalysts in the methane steam reforming reaction. X-ray diffraction (XRD) analysis, FTIR spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the samples in terms of Pd and CeO2 structure and dispersion on the γ-Al2O3 support. These results were correlated with the observed catalytic activity and deactivation process. Arrhenius plots at steady-state conditions are presented as a function of CeO2 structure. Pd is present on the oxidized CeO2-promoted catalysts as Pd0, Pd+ and Pd2+, at ratios strongly dependent on CeO2 structure. XRD measurements indicated that Pd is well dispersed (particles <2 nm) on crystalline CeO2 and is agglomerated as large clusters (particles in 10–20 nm range) on amorphous CeO2. FTIR spectra of adsorbed CO revealed that after pre-treatment under H2 or in the presence of amorphous CeO2, partial encapsulation of Pd particles occurs. CeO2 structure influences the CH4 steam reforming reaction rates. Crystalline CeO2 and dispersed Pd favor high reaction rates (low activation energy). The presence of CeO2 as a promoter conferred high catalytic activity to the alumina-supported Pd catalysts. The catalytic activity is significantly lower on Pd/γ-Al2O3 or on amorphous (reduced) CeO2/Al2O3 catalysts. The reaction rates are two orders of magnitude higher on Pd/CeO2/γ-Al2O3 than on Pd/γ-Al2O3, which is attributed to a catalytic synergism between Pd and CeO2. The low rates on the reduced Pd/CeO2/Al2O3 catalysts can be correlated with the loss of Pd sites through encapsulation or particle agglomeration, a process found mostly irreversible after catalyst regeneration.
Keywords: Palladium; Ceria; Methane steam reforming; FTIR; CO probe molecules;
Catalytic CO hydrogenation on potassic Fe/zeolite LTL by M.V. Cagnoli; N.G. Gallegos; A.M. Alvarez; J.F. Bengoa; A.A. Yeramián; M. Schmal; S.G. Marchetti (169-176).
Zeolite LTL in the potassic form (K-LTL) was used as support of iron. The precursor was characterized by temperature programmed reduction (TPR) and Mössbauer spectroscopy (MS) at 298 and 15 K. The catalyst was obtained reducing the precursor with H2 and its structural properties were studied by H2 chemisorption, volumetric oxidation and MS under controlled conditions at 298 and 15 K. Measurements of activity and selectivity were carried out at 1 and 20 bar in the CO hydrogenation. Two different types of active sites were detected: Fe0 inside the zeolite channels and carburized iron on its external surface. The solid showed a high activity and selectivity towards light alkenes in comparison with other iron supported systems (Fe/SiO2, Fe/Al2O3 and Fe/C). This behavior can be attributed to the high percentage of metallic iron promoted by potassium ions, located inside the zeolite channels. The external carbide crystals generate hydrocarbons in the diesel oil range.
Keywords: CO hydrogenation; Fischer–Tropsch reaction; Supported Fe; Zeolite LTL; Mössbauer spectroscopy;
Sulfur- and water-tolerance of Pt/KL aromatization catalysts promoted with Ce and Yb by Siriporn Jongpatiwut; Paneeya Sackamduang; Thirasak Rirksomboon; Somchai Osuwan; Walter E Alvarez; Daniel E Resasco (177-193).
Different Pt/KL catalysts containing rare earth (RE; Ce and Yb) promoters were prepared by two techniques, incipient wetness impregnation (IWI) and vapor phase impregnation (VPI). The catalysts were tested for the activity and the selectivity of n-hexane aromatization to benzene under clean, sulfur-containing, and water-containing feeds at 500 °C. It was observed that the catalysts prepared by the VPI technique exhibited much higher activity and selectivity than those prepared by IWI. It was also found that although under clean conditions, the addition of Ce or Yb caused a decrease in activity, in the presence of sulfur the addition of Ce and to a lesser extent Yb, significantly inhibited catalyst deactivation.The influence of water in the feed was investigated by contacting the catalysts for 1 h to a feed containing 3 mol.% water. After this treatment, all the catalysts exhibited a significant activity loss. This loss was more pronounced for the catalyst prepared by the VPI method. The catalyst prepared by IWI already had suffered a significant deactivation before the water treatment, so the activity drop was not so pronounced. The sample prepared by VPI not only showed a drop in activity immediately after the water treatment but it became more susceptible to deactivation afterwards. By contrast, the Ce-promoted catalyst showed a more stable activity after the water treatment.All catalysts were characterized before and after reaction by a number of techniques. In agreement with previous studies, FT-IR of adsorbed CO and chemisorption results indicated that the VPI method resulted in higher Pt dispersion than that obtained by the IWI method. After reaction in the presence of sulfur, the Ce-promoted Pt/KL catalyst showed a higher resistance to metal agglomeration and a lower rate of coke formation than the unpromoted Pt/KL. On all the catalysts, the amount of carbon deposits was greater in the presence of sulfur and after exposure to water vapor than under the reaction with clean feeds. This difference is explained in terms of metal particle growth and location in the zeolite.
Keywords: n-Hexane aromatization; Pt/KL zeolite; VPI method; Sulfur-tolerance; Hydrothermal stability; Cerium promoter; Ytterbium promoter; DRIFTS; XANES; TPO;
Dehydrogenative cracking of n-butane using double-stage reaction by K. Wakui; K. Satoh; G. Sawada; K. Shiozawa; K. Matano; K. Suzuki; T. Hayakawa; Y. Yoshimura; K. Murata; F. Mizukami (195-202).
Dehydrogenation-cracking double-stage (tandem) reaction of n-butane was studied using a Pt-Sn type dehydrogenation catalyst and a cracking catalyst (rare earth-loaded HZSM-5). n-Butane was firstly dehydrogenated to n-butene (1- and 2-butene) over the Pt-Sn catalyst loaded at the upper part of the reactor. Then n-butene was successively converted to ethylene and propylene over the cracking catalyst loaded at the lower part of the reactor. The yield of light olefins (ethylene+propylene) was 58% at 650 °C. The key to obtaining ethylene and propylene in high yield was to determine how the bimolecular reactions of olefins to aromatic and heavier products can be inhibited. It was proved that the loaded rare earths played an important role in inhibiting the bimolecular reactions.
Keywords: HZSM-5; Rare earth; Butane; Cracking; Dehydrogenation; Ethylene; Propylene; Olefin;
The influence of zeolite acidity for the coupled hydrogenation and ring opening of 1-methylnaphthalene on Pt/USY catalysts by M.A. Arribas; A. Martı́nez (203-217).
The influence of framework and extraframework composition of USY zeolite on the catalytic performance of bifunctional Pt/USY (1 wt.% Pt) catalysts for the coupled hydrogenation and ring opening of 1-methylnaphthalene (1-MN) has been studied on a continuous fixed bed high pressure reactor. All Pt/USY catalysts showed very high methylnaphthalene (MN) conversions under the reaction conditions studied (T=300–375 °C, P=4.0 MPa, WHSV=2 h−1, H2/1-MN=30 mol/mol). Product yields and selectivities were mainly determined by the zeolite composition (i.e. acidity). Selectivity to products with the same number of carbon atoms than the feed (C11) increased, at constant temperature, with decreasing the Brönsted acidity of the USY zeolite, that is, with decreasing the concentration of framework Al (FAL) and increasing extraframework Al (EFAL). Selectivity to high cetane ring opening products (ROP=C11-alkylbenzenes (C11AB) and C11-alkylcycloalkanes) within the C11 fraction was higher for the less acidic catalysts. A maximum yield of ROP of ca. 15 wt.% at a C11 yield of ca. 73 wt.% was obtained at 350 °C (P=4.0 MPa, WHSV=2 h−1, H2/1-MN=30 mol/mol) for a USY zeolite with an intermediate degree of dealumination (a 0=24.33 Å) and containing all the EFAL (bulk Si/Al ratio of 2.6). For this catalyst, a slight increase in ROP yield (ca. 17 wt.%) at similar C11 yield (ca. 74 wt.%) was obtained by working at lower temperature (300 °C) and lower space velocity. Increasing the reaction pressure above 4.0 MPa had only a marginal influence on product yields and selectivities.
Keywords: Aromatics hydrogenation; Ring opening; 1-Methylnaphthalene; Bifunctional catalysts; Platinum; USY zeolite; Acidity; Cetane;
Propane partial oxidation over M3+-substituted vanadyl phosphates dispersed on titania and silica by S. Marengo; P. Patrono; P. Comotti; G. Galli; P. Galli; M.A. Massucci; M.T. Meloni (219-231).
Mono-substituted M3+ compounds of vanadyl phosphate dihydrate VOPO4·2H2O, with formula [M(H2O)] x VO1−x PO4·nH2O (M=Al, Fe, Cr, x=0.15–0.20, n=2–2.40), and a di-substituted compound with formula Fe0.08Cr0.08(H2O)0.16VO0.84PO4·2.9H2O, either pure or supported, were characterised by XRD, EDAX, TG/DTA, physisorption and chemisorption measurements, and tested as catalysts in the partial oxidation of propane. Incorporation of the M3+ cation into VOPO4 produces a marked increase in surface area, pore volume, and reducibility, with subsequent enhancement of the catalytic activity. Upon adsorption of these compounds on titania, a homogeneous distribution of highly dispersed species is obtained, whereas on silica small conglomerates of crystalline phases of VOP and FeVOP are formed, presumably by polymerisation on the acidic surface sites of the support. The titania-supported samples exhibit higher catalytic activity and better selectivity to partial oxidation products (acetic acid and propene), compared to silica-based materials; these effects are attributed to the higher dispersion and reducibility of the surface species. Propane oxidation over the supported materials undergoes transition to the ignited state, in which surface temperatures up to 900 K are attained, and homogenous reactions yield mainly propene and CO.
Keywords: Propane oxidation (partial); Vanadyl phosphate; Vanadium reducibility; Ignited state;
Liquid phase benzoylation of arenes over iron promoted sulphated zirconia by H. Suja; C.S. Deepa; K. Sreeja Rani; S. Sugunan (233-243).
The present work undertakes the preparation and physico-chemical characterisation of iron promoted sulphated zirconia (SZ) with different amounts of iron loading and their application to Friedel–Crafts benzoylation of benzene, toluene and xylene under different experimental conditions. XRD and laser Raman techniques reveal the stabilisation of the tetragonal phase of zirconia and the existence of iron in highly dispersed form as Fe2O3 on the catalyst surface. The surface acidic properties were determined by ammonia temperature programmed desorption (TPD) and perylene adsorption. The results were supported by the TGA studies after adsorption of pyridine and 2,6-dimethylpyridine (2,6-DMP). Strong Lewis acid sites on the surface, which are evident from TPD and perylene adsorption studies, explain the high catalytic activity of the systems towards benzoylation. The experimental results provide evidence for the truly heterogeneous nature of the reaction. The studies also establish the resistance to deactivation in the metal incorporated sulphated systems.
Keywords: Iron promoted sulphated zirconia; Perylene adsorption studies; Friedel–Crafts benzoylation;
A comparison on the catalytic activity of Zn1−x Co x Fe2O4 (x = 0, 0.2, 0.5, 0.8 and 1.0)-type ferrospinels prepared via. a low temperature route for the alkylation of aniline and phenol using methanol as the alkylating agent by K. Sreekumar; S. Sugunan (245-251).
Depending on the variation of the Zn2+/Co2+ ratio in the Zn1−x Co x Fe2O4 (x=0, 0.2, 0.5, 0.8 and 1.0)-type ferrospinels, the systems showed different activity trends for aniline and phenol methylation using methanol as the alkylating agent. An increase in Zn2+/Co2+ ratio increased the rate of N-monomethylation of aniline, whereas, a decrease in the ratio favored the rate of ortho methylation of phenol. An attempt has been made to interpret the observed trends based on the variation of surface acid–base properties of the catalyst surface with changes in the spinel composition. The efficiency of adsorption of aniline, phenol or methanol depends not only on the catalyst surface acid–base properties but also on the polarity of the adsorbing molecules. A controlled interplay of surface acid–base properties and polarity of the respective reacting molecules determines the efficiency of a particular reaction. In the case of aniline methylation, surface basicity plays a dominating role, whereas for phenol methylation surface acidity plays a dominating role.
Keywords: Ferrospinels; Aniline methylation; Phenol methylation; Spinel structure;
Ethylbenzene transformation on bifunctional Pt/Al2O3-NaHMOR catalysts by F. Moreau; N.S. Gnep; S. Lacombe; E. Merlen; M. Guisnet (253-262).
Ethylbenzene transformation was carried out on bifunctional catalysts constituted by Pt/Al2O3 (PtA) associated with NaHMOR (Si/Al=10) zeolite samples. The operating conditions were the following: fixed bed reactor, temperature 683 K, pressure of hydrogen and of ethylbenzene equal to 8 and 2 bar, respectively, weight hourly space velocity (WHSV) between 5 and 150 h−1. Twelve different bifunctional catalysts were used differing by the Na exchange rate of the mordenite component or by the relative proportions of the NaHMOR and Pt/Al2O3 components or by the Pt content of the Pt/Al2O3 (0.5 or 2.3 wt.%). As expected from a bifunctional mechanism, the activity in isomerization per protonic site (turnover frequency, TOF) increases first with the ratio between the concentrations of accessible platinum sites (n Pt) and of the protonic sites (n H+ ), then remains constant above a certain value of this ratio. It is shown that the exchange of the mordenite causes a significant increase in the selectivity to isomers of the bifunctional catalysts. This increase can be related to a decrease in the rates of the main secondary reaction (disproportionation), but also of the other ones: dealkylation, hydrocracking, transalkylation.
Keywords: Pt/Al2O3-NaHMOR; Sodium exchange; Bifunctional catalysis; Ethylbenzene isomerization; Disproportionation; Dealkylation;
The effect of gas atmosphere used in the calcination of MgO on its basicity and catalytic performance in oxidative coupling of methane by Slawomir Kuś; Marian Otremba; Artur Tórz; Marian Taniewski (263-270).
Influence of gas atmosphere used in the calcination of MgO (flowing oxygen, air, helium, nitrogen) on its basicity has been investigated. Pure MgO of relatively low basicity and MgO of higher basicity, containing Ca and Na compounds as impurities, were examined. The surface basicity/base strength distribution was measured using a test reaction of transformation of 2-butanol and a temperature-programmed desorption of CO2. Both methods revealed that the calcination of MgO samples in the stream of oxygen or air gave MgO of lower basicity than obtained by calcination in the stream of helium or nitrogen. This effect may indicate a different character of interactions between various gas-phase molecules and the surface. It was also shown that MgO calcined in oxygen or air when used in the catalytic oxidative coupling of methane, gave lower conversions of methane and oxygen than that calcined in the inert gas atmosphere. The choice of a gas used in the calcination of MgO, may therefore, influence the basicity and catalytic properties of calcined MgO.
Keywords: Magnesium oxide; Calcination atmosphere; Basicity; 2-Butanol transformation; TPD CO2; Methane oxidative coupling;
Oxidation of benzene to phenol on supported Pt-VO x and Pd-VO x catalysts by Heike Ehrich; Heinz Berndt; Marga-Martina Pohl; Klaus Jähnisch; Manfred Baerns (271-280).
Gas-phase oxidation of benzene using a mixture of oxygen and hydrogen has been carried out on silica-supported vanadium oxide catalysts modified with platinum or palladium. Catalyst activity and phenol selectivity were studied as a function of the precious metal used, the vanadium oxide loading as well as of temperature. The binary catalysts have been characterized by TPR and TEM. Pt-VO x /SiO2 catalysts were more active than Pd-VO x /SiO2 catalysts. By using platinum catalysts benzene conversion amounted to 1.0% (S phenol=97%) at 413 K, whereas palladium catalysts reached a conversion of only 0.2% (S phenol=86%) for the same contact time and temperature. The most active catalyst for the oxidation of benzene to phenol was a low vanadium loaded 0.5 wt.% Pt–3 wt.% V on silica catalyst. At temperatures above 413 K phenol selectivity decreased strongly because of enhanced total oxidation. Active catalysts need both components: a dispersed transition metal oxide such as VO x as well as small precious metal particles such as platinum. The activity of the catalysts arises from a close interaction between the redox-active compound VO x and the electron mediator and hydrogen activator platinum as was confirmed by correlation of catalytic results and catalyst properties. Highly dispersed platinum particles are exclusively located on the vanadium oxide covered surface as demonstrated by TEM investigations. TPR studies showed and enhanced reducibility of a part of vanadium(V) oxide indication a close neighborhood of VO x and platinum.
Keywords: Phenol; Benzene oxidation; Gas phase oxidation; Oxygen; Vanadium; Platinum; Palladium;
Layered double hydroxide-derived vanadium catalysts for oxidative dehydrogenation of propane by R. Dula; K. Wcisło; J. Stoch; B. Grzybowska; E.M. Serwicka; F. Kooli; K. Bahranowski; A. Gaweł (281-291).
Mixed-oxide vanadium catalysts for oxidative dehydrogenation (ODH) of propane have been prepared by thermal decomposition of Mg, Al-layered double hydroxides (LDHs) containing vanadium either in the brucite layer or in the interlayer. The materials have been characterised by XRD, ICP-AES chemical analysis, XPS, BET and ESR. The catalytic performance of the samples depended on the manner of incorporation of the vanadium into the LDH structure. The sample obtained from interlayer-doped precursor was more active and more selective than mixed oxides obtained from layer-doped LDHs. The difference in the catalytic properties was attributed to the different magnesium vanadates nucleating in the calcined samples, the pyrovanadate formed from the interlayer-doped LDH giving better performance than ortho-vanadate crystallising from the layer-doped precursor. It has been suggested that one of the factors contributing to the difference in the behaviour of both types of catalysts might be the difference in the covalency of VO in-plane bonds around the reduced V centres.
Keywords: Hydrotalcites; Layered double hydroxides; Oxidative dehydrogenation; V-Mg-O mixed oxides;
AUTHOR INDEX (295-297).
SUBJECT INDEX (299-303).
INSTRUCTIONS TO AUTHORS (305-309).
CONTENTS CONTINUED (311).