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Applied Catalysis A, General (v.335, #1)

Editorial Board (pp. co2).

Contents (pp. iii-vii).

Rare earth metal triflates promoted Diels–Alder reactions in ionic liquids by Diganta Sarma; Anil Kumar (pp. 1-6).

Rare earth metal triflates, when used with ionic liquids can promote Diels–Alder reactions with different dienes and dienophiles suggesting enhancement in the catalytic power of the triflates in ionic liquids.▪Rare earth metal triflates, when used with ionic liquids can promote Diels–Alder reactions carried out with different dienes and dienophiles suggesting an enhancement in the catalytic power of the triflates in ionic liquids. It is possible to recover and reuse the ionic liquid phase with triflates to give comparable yields and stereoselectivities even after six cycles.

Keywords: Ionic liquids; Rare earth metal triflates; Diels–Alder reactions

Kinetics of o-chlorotoluene hydrogenolysis in the presence of 3%, 5% and 10% Pd/C catalysts by Tadeusz Janiak (pp. 7-14).

The kinetics of o-chlorotoluene hydrogenolysis in the presence of 3%, 5% and 10% Pd/C catalysts was experimentally investigated in an alkaline – n-heptane – gaseous hydrogen system. The main product of hydrogenolysis was toluene. Kinetic characteristics for the reaction were obtained for all the catalysts at 20, 40 and 60°C. The activation energies of o-chlorotoluene hydrogenolysis for these catalysts were calculated.▪The kinetics of o-chlorotoluene hydrogenolysis in the presence of 3%, 5% and 10% Pd/C catalysts was experimentally investigated in an alkaline – n-heptane – gaseous hydrogen system. The main product of hydrogenolysis was toluene. Kinetic characteristics for the reaction were obtained at temperatures of 20, 40 and 60°C at atmospheric pressure of hydrogen for all catalysts; the effectiveness of the latter in o-chlorotoluene hydrogenolysis was compared. The pseudo-zero-order rate constants and times of 90% conversion for each of the catalysts at 40 and 60°C were quite similar. The overall kinetic constants were obtained assuming the Langmuir–Hinshelwood mechanism. The activation energies of o-chlorotoluene hydrogenolysis for these catalysts were calculated. The selectivity of the reacting system was evaluated in competitive experiments with o-chlorotoluene, 1,2-dichlorobenzene and 4-chlorobiphenyl. The stability of the catalysts in a hydrogenolytic environment was tested.

Keywords: O; -chlorotoluene; Catalytic hydrogenolysis; Palladium on carbon; Kinetics

Precoking selectivation for improving benzene product purity in heavy aromatics transalkylation by Pei-Hsien Chao; Hui-Wen Lin; Chien-Hao Chen; Pai-Yang Wang; Yen-Fu Chen; Hei-Tin Sei; Tseng-Chang Tsai (pp. 15-19).

A precoking selectivation technique is developed to improve the benzene product purity in the heavy aromatics transalkylation over a platinum-supported on zeolite catalyst. Over the Pt(200)/Z-12 catalyst, the benzene purity is only 96.50%. By the optimum precoking treatment at the coke content of ca. 15.8wt%, benzene purity is upgraded to the industrial specification of 99.85%. ▪A precoking selectivation technique is developed to improve the benzene purity in the heavy aromatics transalkylation over Pt/ZSM-12. The effect of precoking is selective to coking condition. Precoking in hydrogen can significantly improve benzene product purity. In contrast, precoking in helium results in severe deactivation of catalytic activity and deteriorates benzene product purity. For Pt/ZSM-12 at a platinum content of 200pm as an example, subject to precoking treatment in hydrogen at a coke content of ca. 15.8wt%, the benzene product purity can be improved up to the industrial specification of 99.85%.

Keywords: Heavy aromatics transalkylation; Zeolite-supported platinum catalyst; Selective coking; Benzene product purity

Hydroisomerization activity and selectivity of n-dodecane over modified Pt/ZSM-22 catalysts by Gang Wang; Quanjie Liu; Weiguang Su; Xiujie Li; Zongxuan Jiang; Xiangchen Fang; Chongren Han; Can Li (pp. 20-27).

The hydroisomerization of n-dodecane was investigated for Pt catalysts supported on unmodified ZSM-22 and on modified ZSM-22. The ZSM-22 was modified by NH₄⁺ ion-exchange, HNO₃ or (NH₄)₂SiF₆ treatment, and by the hydrothermal treatment. The catalyst modified by the combination of NH₄⁺ ion-exchange and (NH₄)₂SiF₆ treatment possesses the Brönsted acid sites with medium strength and shows the high isododecane selectivity (88.0%) under high conversion (87.5%) at 300°C. ▪The hydroisomerization of n-dodecane was investigated over Pt supported on unmodified ZSM-22 and on modified ZSM-22. The effect of ZSM-22 modification on Pt/ZSM-22 acidity and on catalytic performance is discussed in detail using MAS NMR and FT-IR results. The ZSM-22 was modified by NH₄⁺ ion-exchange, HNO₃ or (NH₄)₂SiF₆ treatment, and by steam treatment in order to tune the acidity of the catalysts. The unmodified ZSM-22 catalyst shows very low activity in the hydroisomerization of n-dodecane due to the lowest acidity. The conversions of the catalysts for the reaction of n-dodecane are increased from 1.1% to 96.6% at 300°C as the acidity of the ZSM-22 is increased from 0.043mmol/g to 0.329mmol/g, but the isomerization selectivity is reduced. The catalyst that was modified by the combination of NH₄⁺ ion-exchange and (NH₄)₂SiF₆ treatment possesses the Brönsted acid sites with medium strength and shows the high isododecane selectivity (88.0%) under high conversion (87.5%) at 300°C.

Keywords: Hydroisomerization; n; -Dodecane; ZSM-22; FT-IR; NMR

Partial substitution of lanthanum with silver in the LaMnO₃ perovskite: Effect of the modification on the activity of monolithic catalysts in the reactions of methane and carbon oxide oxidation by Barbara Kucharczyk; Włodzimierz Tylus (pp. 28-36).

Partial substitution of lanthanum with silver in the LaMnO₃ perovskite deposited onto a monolithic metal support enhances catalytic activity in the oxidation of CH₄ and CO. The optimal silver content ( x) in the La1−_xAg_xMnO₃ perovskite amounts to 0.2 and ranges from 0.1 to 0.3 for CH₄ combustion and CO oxidation, respectively. Methane conversion obtained over the monolithic catalyst with the most active perovskite (La_0.8Ag_0.2MnO₃) at 750°C totals 98.9% and falls slightly (to 97.6%) after 130h on stream. Ageing at 900°C accounts for a distinct decline in methane conversion, which drops to 67% at 750°C. The decrease in the activity of this catalyst during the run is attributable to the decreasing SSA and decreasing proportion of Ag⁺ to Ag⁰ and to the reduction of Mn⁴⁺ to Mn³⁺ on the perovskite surface. When the catalyst is aged at 900°C, the processes are enhanced; the metallic silver that segregates onto the surface tends to form agglomerates, and the composition of the catalyst surface is similar to the composition of the LaMnO₃ perovskite.

Keywords: Methane combustion; Perovskite; LaAgMnO; ₃; catalyst; Metallic monolith; XPS

New ruthenium porphyrin polymeric membranes: Preparation and characterization by M. Giovanna Buonomenna; Emma Gallo; Fabio Ragaini; Alessandro Caselli; Sergio Cenini; Enrico Drioli (pp. 37-45).

The first heterogeneous method to obtain aziridines using ruthenium porphyrin complexes embedded in Hyflon AD60X polymeric membranes is reported. The Hyflon AD60X membranes showed good catalytic activity in the aziridination reaction of α-methylstyrene by aryl azides and moreover they have been recycled three times without any loss in azide conversion and aziridine selectivity.▪In this work, the ruthenium porphyrin complex Ru(4-(CF₃)TPP)CO (1) (TPP=dianion of tetraphenylporphyrin) was entrapped into polymeric membranes during the phase inversion process to obtain new heterogeneous catalysts for aziridination reactions. In order to study the effect of the polymeric environment on the activity of the metallic complex, symmetric dense polymeric catalytic membranes were prepared using three different polymers: polyethersulphone (PES), polysulphone (PSf) and Hyflon AD60X. Different hydrophobicity/hydrophilicity characteristics, sorption properties and ability to retain the metallic complex characterized the catalytic polymeric membranes. On the basis of the good results compared to the other two polymers, Hyflon AD60X was selected to prepare asymmetric membranes with a skin layer on an open sublayer. The Hyflon AD60X membranes showed good catalytic activity in the aziridination reaction of α-methylstyrene by aryl azides and moreover they have been recycled without any loss in azide conversion and aziridine selectivity.

Keywords: Polymeric membranes; Phase inversion; Hyflon AD60X; Ruthenium porphyrin complexes; Aziridines

One-step synthesis of a structurally organized mesoporous CuO-CeO₂-Al₂O₃ system for the preferential CO oxidation by Elisa Moretti; Maurizio Lenarda; Loretta Storaro; Aldo Talon; Tania Montanari; Guido Busca; Enrique Rodríguez-Castellón; Antonio Jiménez-López; Maria Turco; Giovanni Bagnasco; Romana Frattini (pp. 46-55).

A mesoporous Cu-Ce-Al-based material was prepared by a single-step synthesis, performed in n-propanol using aluminum sec-butoxide as Al precursor and metal stearates, both as Cu-Ce sources and structural directing agents. The material showed a good catalytic activity for the preferential oxidation of CO to CO₂ (PROX) in H₂-rich streams in the 75–200°C temperature range.▪A mesoporous Cu-Ce-Al-based material was prepared by a single-step synthesis, performed in n-propanol using aluminum sec-butoxide as Al precursor and metal stearates, both as Cu-Ce sources and structural directing agents. This type of synthetic approach appears to be very efficient for the preparation of highly dispersed Cu-Ce containing particles supported on a structurally ordered alumina with high surface area (>400m²g⁻¹) and quite narrow pore size distribution, as confirmed by X-ray powder diffraction, TPR and dispersion data and N₂ physisorption.The catalyst was also characterized by XPS and FT-IR measurements of adsorbed CO. The catalytic activity for the preferential oxidation of CO to CO₂ (PROX) in H₂-rich streams was tested in the 75–200°C temperature range.

Keywords: One-step synthesis; Ordered mesopores; Ce-Cu; CO-PROX; Alumina; Hydrogen production

Evaluation of molybdenum-modified alumina support materials for Co-based Fischer-Tropsch catalysts by E. van Steen; E.L. Viljoen; J. van de Loosdrecht; M. Claeys (pp. 56-63).

The selectivity of molybdenum-modified Co/γ-Al₂O₃ catalysts during the Fischer-Tropsch synthesis is strongly affected by the presence of the acid sites within the catalyst, whose presence results in enhanced extent of double bond isomerisation yielding a decrease in the chain growth probability, and an increase in the amount of branched compounds formed in the cobalt catalysed Fischer-Tropsch synthesis.▪The choice of a support or promoter may influence the activity and selectivity of a Fischer-Tropsch catalyst. In this study the effect of modification of an alumina support with molybdenum on the performance of Co/γ-Al₂O₃-catalysts is investigated. The addition of molybdenum leads to a strong decrease in the available metal surface area and a higher acidity in the reduced catalyst. Carbon monoxide can adsorb both on molybdenum sites within the catalysts and on metallic sites. The strength of adsorption of CO on metallic sites increases due to the presence of molybdenum in the catalyst.The initial activity in the Fischer-Tropsch synthesis per unit metal surface area does not change much, but molybdenum-modified catalysts deactivate faster. The selectivity of molybdenum-modified Co/γ-Al₂O₃ catalysts during the Fischer-Tropsch synthesis is strongly affected by the presence of the acid sites within the catalyst. The enhanced extent of double bond isomerisation on the acidic sites results in a decrease in the chain growth probability, and in an increase in the amount of branched compounds formed in the Fischer-Tropsch synthesis over metallic cobalt sites.

Keywords: Fischer-Tropsch; Acidity; Cobalt; Molybdenum; Branched hydrocarbons

Continuous hydrogen production by sequential catalytic cracking of acetic acid by Thomas Davidian; Nolven Guilhaume; Cécile Daniel; Claude Mirodatos (pp. 64-73).

The sequential cracking of acetic acid (AA) to produce hydrogen was investigated over noble metals and Ni-based catalysts. In this non-stationary process, AA is decomposed into a hydrogen-rich gas, with concomitant carbon deposition on the catalyst, which is periodically removed under oxygen. Continuous hydrogen production by the use of two reactors working alternately was demonstrated.▪The sequential cracking of acetic acid (AA) to produce hydrogen was investigated over noble metals and Ni-based catalysts. In this non-stationary process, AA is decomposed into a hydrogen-rich gas, with concomitant carbon deposition on the catalyst, which is periodically removed under oxygen. In this first part, Ni-based catalysts proved to be the most efficient in terms of activity and selectivity to hydrogen, compared to noble metal catalysts. The reaction conditions leading to optimal hydrogen selectivity and complete catalyst regeneration were studied. Continuous hydrogen production by the use of two reactors working alternately was demonstrated.

Keywords: Acetic acid; Bio-oils; Cracking; Steam reforming; Catalyst regeneration; Ni catalyst; Cycling process

tert-Butylation of diphenylamine over zeolite catalysts by Gabriel Kostrab; Martin Lovič; Ivan Janotka; Martin Bajus; Dušan Mravec (pp. 74-81).

tert-Butylation of diphenylamine (DPA) with tert-butanol (TBA) in the liquid phase was studied over various zeolite catalysts H-MOR, H-BEA, H-Y and H-ZSM-5 (H-MOR CBV 21A and 90A, H-BEA CP 811E, 814E and 814Q, H-Y CBV 720 and H-ZSM-5 CBV 5020) to evaluate whether zeolite catalysts can be efficient as an alternative to the other catalysts in preparation of tert-butylated diphenylamines.The influence of catalyst and the reaction conditions on tert-butylation of diphenylamine with TBA as alkylating agent on catalytic activity and para-selectivity was studied. Catalyst screening revealed that H-BEA and H-Y are the most suitable zeolite catalysts in the alkylation of DPA with TBA. Among them, H-BEA CBV 814E showed the best results in the catalytic activity (91% conversion of DPA), selectivity (99% selectivity to 4-TBDPA, 91% selectivity to 4,4′-DTBDPA) and dialkylation (0.62 4,4′-DTBDPA/4-TBDPA ratio). Subsequent optimization of reaction conditions in the tert-butylation of DPA with TBA over H-BEA CBV 814E showed that the most suitable reaction temperature is 180°C. The study of influence of reactant's molar ratio showed that TBA/DPA molar ratio 4:1 is the most suitable, but the excess of TBA is so high that it produces significant amount of isobutylene dimer (15%). However, TBA/DPA molar ratio 2:1 is comparable to 4:1, it is even better when considering the most important parameter—selectivity to 4,4′-DTBDPA and produces almost no isobutylene dimer (below 1%). The amount of catalyst charged into reaction system also significantly influences reaction parameters. Study shows that within wide range of catalyst charge (2.5% up to 30% as of DPA charge), the highest reasonable amount of catalyst with the best outcome can be obtained with 0.7g (20% as of DPA charge) of catalyst charge in the reaction system. tert-Butylation of diphenylamine with tert-butanol over H-BEA zeolite catalyst appears to be an alternative to other catalysts in industrial preparation of tert-butylated diphenylamines. This paper presents the first original complex outlook on the tert-butylation of diphenylamine over zeolite catalysts.

Keywords: Abbreviations; DPA; diphenylamine; TBA; tert; -butanol; 4-TBDPA/4,4′; TBDPA-4-; tert; -butyldiphenylamine/4,4′-di-; tert; -butyldiphenylamine; X; _DPA; conversion of DPA; S; _4-TBDPA; selectivity; =; (4-TBDPA/ΣTBDPA); ×; 100; S; _{4,4′-DTBDPA}; selectivity; =; (4,4′-DTBDPA/ΣDTBDPA); ×; 100Alkylation; tert; -Butylation; Diphenylamine; Zeolites; para; -Selectivity

Bromine-mediated conversion of methane to C1 oxygenates over Zn-MCM-41 supported mercuric oxide by Fengbo Li; Guoqing Yuan; Fang Yan; Fengwen Yan (pp. 82-87).

The structure mobility of the zigzag mercuric oxide chains and the strong preference of mercuric (II) cation sites for bromine lead to a novel bromine-mediated partial methane oxidation at relatively low reaction temperature (220°C). Two bromine-mediated routes can be identified: MeHgBr_n as the intermediate species and mercuric hypobromite as the active agent for methane oxidation, which occur simultaneously in the methane conversion. The controlling mechanism transfers from the former to the latter with the gradual fixation of bromine into mercuric oxide lattice.▪The structure mobility of the zigzag mercuric oxide chains and the strong preference of mercuric (II) cation sites for bromine lead to a novel bromine-mediated partial methane oxidation at relatively low reaction temperature (220°C). Zn-MCM-41 supported mercuric oxide nanophases were used as the model catalyst to investigate the role of bromine in the catalytic cycle. The introduction of bromine to feed gas consisting of CH₄/O₂/Ar (3:1:6) leads to a sharp increase in the methane conversion. Meanwhile, a transient high product selectivity to C1 oxygenates was observed. Two bromine-mediated routes can be identified: MeHgBr_n as the intermediate species and mercuric hypobromite as the active agent for methane oxidation, which occur simultaneously in the methane conversion. The controlling mechanism transfers from the former to the latter with the gradual fixation of bromine into mercuric oxide lattice. The catalyst was characterized detailedly by XRD, XPS, and N₂ sorption. The chemical composite changes of active species were detected by XPS, which provide some direct information about the mediator role of bromine in the catalytic cycle.

Keywords: Bromine; Methane; Selective oxidation; C1 oxygenates; Mercuric oxide

Dehydrogenation of ethylbenzene to styrene over LaVO_x/SBA-15 catalysts in the presence of carbon dioxide by B.S. Liu; G. Rui; R.Z. Chang; C.T. Au (pp. 88-94).

The catalytic performance of a series of LaVO_x/SBA-15 catalysts with different vanadium loadings was investigated for the dehydrogenation of ethylbenzene to styrene using CO₂ as oxidant. The addition of La³⁺ has an effect of hindering carbon deposition and enhancing the stability of the catalysts. The 10%La₂O₃–15%V₂O₅/SBA-15 (wt%) catalyst exhibited the best activity and stability, giving a styrene yield of 74%. According to the results of characterization, the good performance of the catalyst could be related to the mono-dispersion of the VO₄³⁻ species and the synergistic action of lanthanum and vanadium.The catalytic performance of a series of LaVO_x/SBA-15 catalysts with different vanadium loadings was investigated for the dehydrogenation of ethylbenzene to styrene using CO₂ as oxidant. The addition of La³⁺ has an effect of hindering carbon deposition and enhancing the stability of the catalysts. The 10%La₂O₃–15%V₂O₅/SBA-15 (wt%) catalyst exhibited the best activity and stability, giving a styrene yield of 74%. According to the results of characterization, the good performance of the catalyst could be related to the mono-dispersion of the VO₄³⁻ species and the synergistic action of lanthanum and vanadium.▪

Keywords: Dehydrogenation of ethylbenzene; CO; ₂; as oxidant; SBA-15; Carbon deposition; Vanadium oxide; Lanthanum promoters

Factors influencing the in situ generation of hydrogen peroxide from the reduction of oxygen by hydroxylamine from hydroxylammonium sulfate over Pd/alumina by Vasant R. Choudhary; Prabhas Jana (pp. 95-102).

The net formation of H₂O₂ by the reduction of O₂ with hydroxylamine over Pd/Al₂O₃ catalyst in a neutral aqueous medium is strongly influenced by the Br⁻ promoter and its concentration, reaction period and temperature, catalyst loading, concentration of the reducing agent and pH of the medium. ▪Influence of the concentration of bromide (KBr) promoter, reaction period, temperature, catalyst loading and initial concentration of hydroxylammonium sulfate on the formation of H₂O₂ due to the reaction between hydroxylamine from the hydroxylammonium salt and molecular oxygen over Pd (1wt%)/Al₂O₃ catalyst in a neutral aqueous medium at close to the ambient conditions has been investigated. Effect of pH and temperature on the destruction of H₂O₂, occurring in the consecutive reactions, over the catalyst in the presence or absence of bromide promoter and hydroxylamine has also been studied. In the absence of O₂, the conversion of hydroxylamine by its decomposition over the catalyst increases sharply with increasing the pH, particularly above the pH of 7.0; at or below the pH of 7.0, it is quite small. The H₂O₂ destruction increases markedly with increasing the pH of medium and reaction temperature but it is found to decrease in the presence of hydroxylamine. For the highest net H₂O₂ formation, not only the optimum pH (7.0) of reaction medium but also the optimum other reaction conditions (viz. bromide promoter concentration, reaction period, temperature or catalyst loading) are necessary. The reaction path and mechanism for the generation of H₂O₂ have also been suggested.

Keywords: Hydrogen peroxide; Reduction of O; ₂; Hydroxylamine; Hydroxylammonium sulfate; H; ₂; O; ₂; destruction; Pd/Al; ₂; O; ₃; catalyst

Effect of pretreatment conditions on the catalytic performance of Ni–Pt–W supported on amorphous silica–alumina catalysts by Yacine Rezgui; Miloud Guemini (pp. 103-111).

In this work, the catalytic behavior of Pt–Ni–WO_x supported on amorphous silica–alumina catalysts, prepared by means of a sol–gel technique was tested by hydroisomerization of n-hexane in a continuous fixed-bed reactor operating at atmospheric pressure. Temperature-programmed desorption of ammonia (TPDA), temperature-programmed reduction, BET and atomic absorption spectroscopy techniques were used to characterize the catalysts. The results revealed that both catalytic activity and surface acidity of these solids is strongly dependent on their calcination and reduction temperatures. Besides, the collected data showed that platinum interacts with tungsten and this interaction depends on the pretreatment conditions.In this work, the catalytic behavior of Pt–Ni–WO_x supported on amorphous silica–alumina catalysts, prepared by means of a sol–gel technique was tested by hydroisomerization of n-hexane in a continuous fixed-bed reactor operating at atmospheric pressure. Temperature-programmed desorption of ammonia (TPDA), temperature-programmed reduction, BET and atomic absorption spectroscopy techniques were used to characterize the catalysts. The results revealed that both catalytic activity and surface acidity of these solids is strongly dependent on their calcination and reduction temperatures. Besides, the collected data showed that platinum interacts with tungsten and this interaction depends on the pretreatment conditions.▪

Keywords: Platinum; Tungsten oxide; Nickel; Isomerization; Reduction; Acidity

Photoreduction of CO₂ in an optical-fiber photoreactor: Effects of metals addition and catalyst carrier by The-Vinh Nguyen; Jeffrey C.S. Wu (pp. 112-120).

The transformation of CO₂ to hydrocarbons using sunlight is one of best routes to produce renewable energy. Photocatalytic reduction of CO₂ with H₂O in the gaseous phase is studied by using Cu-Fe/TiO₂ catalyst coated on optical fibers under UVA and UVC irradiation. Catalyst coated optical fibers are assembled in the reactor such that the UV light can enter along the fibers to conduct the photocatalytic reaction on its surface. Methane and ethylene as main products are observed to evolve from this photoreactor. The presence of Fe as a co-dopant in Cu/TiO₂ catalyst is found to synergistically reduce CO₂ with H₂O to ethylene at the quantum yield and total energy efficiency of 0.024% and 0.016%, respectively. This phenomenon is well explained by an efficient charge transfer mechanism between TiO₂ as a support and Cu as well as Fe as co-dopants. Methane is formed more favorably than ethylene on Cu/TiO₂. Meanwhile, Fe as a co-dopant on Cu/TiO₂ catalyst is found to depress the methane formation. The photo production of ethylene over catalysts supported on optical fibers presents yields that are one order of magnitude higher than that on the glass plate counterpart. For a given amount of catalyst and light energy, the optical-fiber reactor can utilize light energy efficiently. Many photo-driven reactions will have advantage using such optical-fiber system. ▪The transformation of CO₂ to hydrocarbons using sunlight is one of best routes to produce renewable energy. Photocatalytic reduction of CO₂ with H₂O in the gaseous phase is studied by using Cu-Fe/TiO₂ catalyst coated on optical fibers under UVA and UVC irradiation. Catalyst coated optical fibers are assembled in the reactor such that the UV light can enter along the fibers to conduct the photocatalytic reaction on its surface. Methane and ethylene as main products are observed to evolve from this photoreactor. The presence of Fe as a co-dopant in Cu/TiO₂ catalyst is found to synergistically reduce CO₂ with H₂O to ethylene at the quantum yield and total energy efficiency of 0.024% and 0.016%, respectively. This phenomenon is well explained by an efficient charge transfer mechanism between TiO₂ as a support and Cu as well as Fe as co-dopants. Methane is formed more favorably than ethylene on Cu/TiO₂. Meanwhile, Fe as a co-dopant on Cu/TiO₂ catalyst is found to depress the methane formation. The photo production of ethylene over catalysts supported on optical fibers presents yields that are one order of magnitude higher than that on the glass plate counterpart. For a given amount of catalyst and light energy, the optical-fiber reactor can utilize light energy efficiently. Many photo-driven reactions will have advantage using such optical-fiber system.

Keywords: Photocatalytic reduction; Cu-Fe/TiO; ₂; Carbon dioxide; Charge transfer; Optical fiber

Kinetic hydrogen isotope effects in ethylene oxidation on silver catalysts by Kazushi Yokozaki; Hiroyuki Ono; Akimi Ayame (pp. 121-136).

Kinetic hydrogen isotope effects for ethylene oxide formation, ethylene oxide combustion, and carbon dioxide formation in ethylene oxidation on various silver catalysts were 0.86–1.99, 0.70–0.81, and 0.43–0.80, respectively. These values were compared with theoretical values based on the absolute reaction rate theory for CH bond breaking reaction (0.25–0.30) and for intramolecular hydrogen transfer reaction (0.73) and reaction mechanism of the complex oxidation was discussed in detail.▪Kinetic hydrogen isotope effects (KHIEs), ( k)_D/( k)_H, for ethylene oxide formation, ethylene oxide combustion, and carbon dioxide formation in ethylene oxidation on silver catalysts were determined using rate constants of empirical reaction rate equations. These rate constants were obtained on pure Ag and K₂SO₄-Ag catalysts at 413–473K and NaCl-Ag catalyst at 518–576K keeping steady active states in the fixed bed flow reactions. On the Ag and K₂SO₄-Ag catalysts resulting in ethylene oxide selectivity values of 30.9–63.3%, KHIEs for total formation of carbon dioxide (0.43–0.61) were in disagreement with theoretical values of 0.25–0.30 for CH bond breaking reaction and also with the value of 0.73 for intramolecular hydrogen transfer reaction, while KHIE for ethylene oxide combustion coincided with the theoretical value 0.73. Consequently, the KHIE for total formation of carbon dioxide was considered to be fixed by the constructive ratio of the direct ethylene combustion route and the ethylene oxide combustion route, which have the CH bond breaking step and the intramolecular hydrogen transfer step as a rate-determining step, respectively. On the NaCl-Ag catalyst indicating ethylene oxide selectivity of 80.0–88.8%, KHIEs for carbon dioxide formation were 0.67–0.80, but no ethylene oxide combustion occurred; these results suggested that the rate-determining step of the direct ethylene combustion route was the intramolecular hydrogen transfer step. KHIEs for ethylene oxide formation on the Ag and K₂SO₄-Ag catalysts decreased from 1.99 to 1.19 with increase in ethylene oxide selectivity, while those on the NaCl-Ag catalyst were 0.86–1.14, of which the average value was almost 1.0. The KHIEs larger than 1.0 for ethylene oxide formation were explained as mainly due to an ensemble isotope effect caused by remarkable reduction in decomposition rates of combustion intermediates of ethylene and ethylene oxide on replacing of H by D. Additionally, three cycle mechanisms for the ethylene oxidation on silver catalysts were discussed.

Keywords: Ethylene oxidation; Rate equation; Kinetic hydrogen isotope effect; Silver catalyst; Rate-determining step; Mechanism

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Applied Catalysis A, General (v.335, #1)