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Applied Catalysis A, General (v.294, #2)
Partially hydrophobized silica supported Pd catalyst for hydrogenation reactions in aqueous media by Florin Omota; Alexandre C. Dimian; Alfred Bliek (pp. 121-130).
The hydrophobic or hydrophilic nature of catalyst support materials may influence the reaction behaviour in three-phase catalytic oxidation or hydrogenation reactions in aqueous media. This may be attributed to the segregation or agglomeration behaviour of such support materials in water. We have systematically investigated the impact of surface hydrophobicity of silica supports during the catalytic hydrogenation of methyl acrylate in water. To this end, we deliberately varied the hydrophobicity of silica supported Pd catalysts by moderate silylation with dichlorodimethylsilane (DDMS). A range of materials was prepared in this way, with three-phase contact angles up to 90°, in order to allow the water to fill the catalyst pores. Hydrophobic silica supported catalysts were either prepared by silylation of hydrophilic mesoporous silica followed by incipient wetness impregnation of the support with H2PdCl4 solution (a), or by the reverse procedure (b). This way of rendering the support more hydrophobic leads to an increased three-phase contact angle, lower heat of immersion in water and lower heat of water vapour adsorption. These factors induce a more pronounced adhesion of particles to gas bubbles. During the hydrogenation of methyl acrylate in aqueous media, hydrophobic Pd/silica catalysts show higher activity than similar hydrophilic Pd/silica catalyst. The catalyst prepared by the route (a) shows the highest activity also as result of better Pd dispersion on the hydrophobic support.
Keywords: Hydrophobic; Hydrophilic; Dichlorodimethylsilane
Nickel antigorite synthesis and carbon tubular nanostructures formation on antigorite-based nickel particles by acetylene decomposition by P. Leroi; F.J. Cadete Santos Aires; T. Goislard de Monsabert; H. Le Poche; J. Dijon; J.L. Rousset; J.C. Bertolini (pp. 131-140).
We have synthesized nickel antigorite in order to obtain, by subsequent heat treatment and reduction, supported nickel particles more or less anchored in the antigorite structure and rather well oriented depending on the temperature. The sample prepared at low temperature (450°C) lead to mainly anchored Ni particles on rather well structured antigorite. For the samples prepared at intermediate temperatures (530, 600°C) increasing destructuring of antigorite with temperature is observed. Finally, for the sample prepared at the highest temperature (700°C) weakly bound Ni particles on a support resembling amorphous SiO2 are mainly obtained.Acetylene decomposition performed around 600°C on these samples yield small amounts of carbon tubular nanostructures (nanotubes and nanofilaments). However, we have found that, whatever the temperature, carbon tubular nanostructures growth (tip growth mechanism) is favoured on the destructured regions of the material where the Ni particles are weakly bound to the support. Higher density of tubular nanostructures is consequently observed for the higher temperature samples. We attributed this behaviour to the fact that the strongly bound anchored Ni particles mainly exhibit (111)-type faces, which are known to promote carbon precipitation. This would lead to coking when particles are not able to rearrange (as it is the case for the anchored particles) whereas carbon tubular nanostructures may grow on weakly bound particles that are able to rearrange.
Keywords: Nickel antigorite; Carbon nanotubes and nanofilaments growth; Acetylene decomposition
tert-Butylation of glycerol catalysed by ion-exchange resins by KatarÃna KlepáÄ?ová; DuÅ¡an Mravec; Martin Bajus (pp. 141-147).
The etherification of glycerol with isobutylene or tert-butyl alcohol without solvent in the liquid phase catalysed by strong acid ion-exchange resins of Amberlyst type and by two large-pore zeolites H-Y and H-Beta was studied. The swelling of commercial macroreticular and gelular ion-exchange resins (Amberlysts) and its influence on glycerol tert-butylation is discussed in this work. By comparing the conversions of glycerol and selectivity to glycerol di- and tri- tert-butyl ethers for macroreticular and gel type ion-exchange resins, it can be concluded that acid macroreticular resins in dry form are very active catalysts for etherification reaction with isobutylene because of large pore diameter. tert-Butyl alcohol as alkylation agent is not suitable because formed reaction water deactivates the catalysts. The zeolites and gel type polymer catalysts are not effective for this etherification reaction (small pore diameter). The best results of glycerol tert-butylation by isobutylene at 100% conversion of glycerol with selectivity to di- and tri-ethers larger than 92% were obtained over strong acid macroreticular ion-exchange resins. Di- and tri- tert-butyl ethers of glycerol are potential oxygenates to diesel fuel.
Keywords: Abbreviations; A; Amberlyst; DIB; diisobutylenes; DTBG; di-; tert; -butyl ether of glycerol; G; glycerol; IB; isobutylene; MTBG; mono-; tert; -butyl ether of glycerol; S; di+tri-ethers; selectivity to di- and tri-; tert; -butyl ethers of glycerol; TBA; tert; -butyl alcohol; TTBG; tri-; tert; -butyl ether of glycerol; X; G; conversion of glycerol; Y; di+tri-ethers; yield of di- and tri-; tert; -butyl ethers of glycerolEtherification; tert; -Butylation; Glycerol ethers; Ion-exchange resins; Amberlyst; Oxygenates; Diesel fuel
The effect of copper loading on the acidity of Cu/HZSM-5 catalysts: IR of ammonia and methanol for methylamines synthesis by M.H.O. Nunes; V. Teixeira da Silva; M. Schmal (pp. 148-155).
The Cu/HZSM-5 catalysts (1.2, 3.7 and 6.0%, p/p, in Cu) were tested in methanol amination showing that the 3.7% copper sample was fairly more selective to MMA than the other metallic catalysts. The results of textural and superficial analyses obtained in this work showed that the increase in copper content is associated with the procedure adopted in the samples preparation and accounts for the variations in Lewis surface acidity and metallic areas. The infrared spectra of 3.7% Cu/HZSM-5 and HZSM-5 after methanol and ammonia uptake in successive steps at 623K presented mono-, di- and tri-methylamines that were influenced upon copper addition. The metal should facilitate amine desorption, probably, due to the decreasing Brönsted acidity.
Keywords: Methylamines synthesis; Characterization of Cu/HZSM-5 catalysts; Infrared of ammonia and methanol
The surface acidity and characterization of Fe-montmorillonite probed by in situ FT-IR spectroscopy of adsorbed pyridine by Mehmet Akçay (pp. 156-160).
The IR spectra of the pyridine has been measured following adsorption on and subsequent desorption from Fe-montmorillonite (Fe-mont.). The exposure of strong Brønsted and Lewis acid sites on solid surfaces is a prime demand for potential adsorptive and catalytic applications. In situ IR spectroscopy of small adsorbed base molecules, often pyridine (Py), CO, NH3 or NO molecules have been well established as a powerful surface analytical technique for characterization of nature, strength and concentration of acid sites. Pyridine has been preferred as an IR probe molecule of finely divided metal oxide surfaces at room and higher temperature regimes. Since it is more selective, stable and more strongly adsorbed than others. Three modes of adsorption have been observed, (a) electron transfer at Lewis acidic surface sites, (b) proton transfer at Brønsted acidic surface sites and (c) hydrogen bonding to surface hydroxy groups.In this study, important correlations and schifts adsorption of pyridine were discussed. In conclusion, FT-IR spectroscopy was used successfully with pyridine as a basic probe, to determine the Brønsted and Lewis acid sites on the surface of the Fe-montmorillonite. The Fe-montmorillonite has both Lewis and Brønsted acidity.
Keywords: Surface acid sites; Fe-montmorillonite; Pyridine adsorption; In situ FT-IR spectroscopy; Temperature-programmed desorption
Heterogenization of [Cu(NCCH3)6][B(C6F5)4]2 and its application in catalytic olefin aziridination by Ayyamperumal Sakthivel; Ahmed K. Hijazi; Marianne Hanzlik; Anthony S.T. Chiang; Fritz E. Kühn (pp. 161-167).
[Cu(NCCH3)6][B(C6F5)4]2 can be heterogenized by ion exchange on NaAlMCM-41 and NaAlMCM-48. In the latter case, the observed loading is twice as high as in the first case. [Cu(NCCH3)6][B(C6F5)4]2 is applicable both in homogeneous and heterogeneous media for the aziridination of olefins at 30°C. Good yields are observed for non bulky olefins, olefins with bulkier substituents lead to lower yields. [Cu(NCCH3)6][B(C6F5)4]2 grafted on mesoporous surfaces (as heterogeneous catalyst) is stable and recyclable for several catalytic runs with only slight decrease in catalytic activity.
Keywords: Aziridination; Catalysis; Copper; Heterogenization; Mesoporous materials
Studies on catalytic pyrolysis of heavy oils: Reaction behaviors and mechanistic pathways by Xianghai Meng; Chunming Xu; Jinsen Gao; Li Li (pp. 168-176).
Catalytic pyrolysis of heavy oils on various catalysts was investigated in a confined fluidized bed reactor. As for catalytic pyrolysis of Chinese Daqing atmospheric residue (Daqing AR) on catalyst CEP-1, reaction temperature, residence time, weight ratios of catalyst-to-oil, steam-to-oil and feed properties have significant influence on product yields and product distribution. The optimal laboratory operating conditions are as follows: reaction temperature is within 650–680°C, residence time within 2.0–4.0s and catalyst-to-oil weight ratio within 13–18. The catalytic pyrolysis ability becomes better and the yields of light olefins become higher with the larger H/ C mol ratio and the lower aromatic carbon content of feedstocks. After the cracking mechanisms of hydrocarbons are analyzed and the thermal pyrolysis of Daqing AR is investigated, a mechanism parameter RM is proposed to study the mechanistic pathways of heavy oil catalytic pyrolysis. As for the processes of Daqing AR catalytic pyrolysis on catalysts LCM-5 and CEP-1, the relative acting percentage of the free radical mechanism and that of the carbonium ion mechanism are obtained.
Keywords: Catalytic pyrolysis; Heavy oil; Reaction behavior; Mechanistic pathway; Ethylene; Propylene
Investigation of Sm2O3–CeO2-supported palladium catalysts for the reforming of methanol: The role of the support by L.M. Gómez-Sainero; R.T. Baker; I.S. Metcalfe; M. Sahibzada; P. Concepción; J.M. López-Nieto (pp. 177-187).
With a view to their use as catalytic anode materials for direct methanol solid oxide fuel cells (DM-SOFCs), the performance of Pd/CeO2–Sm2O3 (Pd/CS), Pd/CeO2 and Pd/Sm2O3 catalysts in the steam reforming of methanol as well as their physicochemical properties – analyzed by N2 adsorption, XRD, temperature programmed reduction (TPR), CO chemisorption and X-ray photoelectron spectroscopy (XPS) – have been investigated. The catalytic activity in methanol steam reforming was evaluated in a tubular microreactor at atmospheric pressure in the 300–500°C reaction temperature interval (space velocity=0.32mol/(hgcat), H2O/CH3OH molar ratio=1.2, methanol concentration 15mole%). H2, CO and CO2 were the main reaction products. Methanol conversions up to 72% and H2 productivities as high as 0.46mol/(hgcat) were obtained on the Pd/CS catalyst. Moreover, low H2 productivity (0.09mol/(hgcat)) was found on the Pd/CeO2 catalyst, while Pd/Sm2O3 appeared to be inactive. This behaviour can be partly assigned to the higher dispersion and the more favourable distribution of Pd particles observed in the Pd/CS catalyst. A strong association of Pd and samarium oxide and an enrichment in both components at the external surface of the Pd/CS catalyst grains was found by means of TPR and XPS. This could be explained by the partial dissolution of Sm2O3 in the acid medium used during catalyst preparation leading to the re-precipitation of a Pd/Sm-containing phase. Higher activity per exposed Pd atom was also observed for the Pd/CS catalyst. This was attributed to enhancement of diffusion and adsorption of reactants on the basic sites of the Sm-containing support.
Keywords: Methanol; Reforming; Hydrogen production; Supported palladium catalysts; CeO; 2; Sm; 2; O; 3; CeO; 2; –Sm; 2; O; 3; mixed oxide; Anode; Solid oxide fuel cells
The mechanism for the synthesis of 1-methoxy-2-propanol from methanol and propylene oxide over magnesium oxide by Wenyu Zhang; Hui Wang; Qibiao Li; Qingnian Dong; Ning Zhao; Wei Wei; Yuhan Sun (pp. 188-196).
The addition of propylene oxide with methanol was carried out over solid bases and a detailed mechanism of the reaction over MgO was investigated by using FT-IR method. It was found that 1-methoxy-2-propanol could be selectively synthesized on MgO, which had base sites of moderate strength, while CaO and Al2O3 showed either low activity or selectivity toward the production of 1-methoxy-2-propanol. In situ FT-IR revealed that methanol dissociation into methoxide and proton took place readily on MgO, and that propylene oxide adsorbed on MgO could generate propylene-like species via carbanion intermediates. As a result, methanol dissociated into adsorbed methoxide and reacted with the propylene-like specie in anti-Markownikov fashion, which led to the high selectivity to 1-methoxy-2-propanol on MgO.
Keywords: 1-methoxy-2-propanol; FT-IR; CO; 2; -TPD; MgO
Promoting effect of zinc on the vapor-phase hydrogenation of crotonaldehyde over copper-based catalysts by E.L. Rodrigues; A.J. Marchi; C.R. Apesteguia; J.M.C. Bueno (pp. 197-207).
The promoting effect of zinc for the vapor-phase hydrogenation of crotonaldehyde was studied on impregnated Cu/SiO2 and Cu–ZnO/SiO2, and on coprecipitated Cu–Al2O3 and Cu x–Zn yO2 y–ZnAl2O4 catalysts. The reaction was carried out in a tubular reactor at 120°C and atmospheric pressure. Samples were characterized by temperature-programmed reduction, X-ray diffraction, transmission electronic microscopy, diffuse reflectance FTIR spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy. Cu/SiO2 and Cu–Al2O3 catalysts reduced in hydrogen either at 300 or 500°C hydrogenated preferentially the CC bond of crotonaldehyde and gave more than 90% of selectivity to butyraldehyde. In contrast, the initial butyraldehyde selectivity on Cu–ZnO/SiO2 reduced at 500°C was only about 55%, essentially because the selectivity to crotyl alcohol significantly increased on this zinc-containing sample as compared to Cu/SiO2. This selectivity enhancement for hydrogenating the CO bond on Cu–ZnO/SiO2 reduced at 500°C was explained by considering that the high-temperature hydrogen treatment forms mobile ZnO x reduced species that strongly interact with Cu0 crystallites. The resulting Cu0–ZnO x species preferentially catalyze the crotyl alcohol formation from crotonaldehyde via a dual-site reaction pathway. A similar explanation was proposed to interpret the observed enhancement of the CO hydrogenation rate on ternary Cu–Zn–Al catalysts reduced at 500°C as compared to Cu–Al2O3. Nevertheless, the Cu0–ZnO x species were unstable on stream and the selectivity to crotyl alcohol continuously decreased with reaction time on zinc-containing samples reduced at high temperatures.
Keywords: Crotonaldehyde hydrogenation; Copper-based catalysts; Selective hydrogenation; Zn-promoted catalysts
Effect of supports on the activity of nickel catalysts in acetonitrile hydrogenation by Andreea Cătălina Gluhoi; Petru Mărginean; Undina Stănescu (pp. 208-214).
Gas-phase hydrogenation of acetonitrile was investigated using nickel black and nickel deposited on MgO, Al2O3, Cr2O3, SiO2, TiO2, ZrO2, ThO2 and UO2, at ambient pressure and 70°C. Metal surface areas were measured by hydrogen chemisorption. Both reaction selectivity and catalytic activity are affected by support nature. Nickel black, Ni/MgO, Ni/ThO2 and Ni/UO2 catalysts have the highest selectivity to monoethylamine (>80mol%), while Ni/TiO2 catalysts the least (<20mol%). All supports increase the catalytic activity of the metal surface area unit (i.e. the intrinsic activity) compared to that of the nickel black. The most efficient is TiO2, which increases the intrinsic activity by about 20 times and the least efficient are MgO and SiO2, with an increase of only 1.5 times. It is assumed that the role of the support is to create new active sites at the metal-support frontiers. The catalytic behavior of the Ni/Al2O3 and Ni/Cr2O3 catalysts, treated at temperatures between 350 and 1050°C, confirms the role of the metal-support frontiers in the enhancement of the catalytic activity in acetonitrile hydrogenation.
Keywords: Acetonitrile hydrogenation; Intrinsic catalytic activity; Metal-support interaction; Nickel catalysts; Selectivity; Support nature effect
Effect of the metal loading in the hydroisomerization of n-octane over beta agglomerated zeolite based catalysts by Antonio de Lucas; Paula Sánchez; Fernando Dorado; María Jesús Ramos; José Luis Valverde (pp. 215-225).
The hydroisomerization of n-octane over catalysts containing palladium and/or platinum as the hydrogenating-dehydrogenating function and beta agglomerated zeolite as the acid function was studied. A slight decrease in the strong acid density of monometallic catalysts with increasing metal content, mainly on platinum samples, was observed due to the acid sites to be partially covered by metal particles. The diameter of the metal particles in bimetallic catalysts was higher than that corresponding to the monometallic ones with the same metal content.Catalytic results showed that the reaction scheme is influenced by the hydrogenating/acid balance. It was observed that the octane isomer yield increased with the hydrogenating/acid balance (defined as the ratio between the number of hydrogenating sites and the strong acid ones, nPd/ nA and nPt/ nA), then remaining constant at nPd/ nA>0.15 and nPt/ nA>0.20. In this last situation, the isomerization reaction over the acid sites was the limiting step of n-octane transformation, since the octane isomer yield does not depend on the metal content. n-Octane conversion was lower in the bimetallic catalysts than that observed in monometallic ones with the same metal content. The different conversion values would be attributed to the different interaction between the two metals in the bimetallic catalysts. A decrease of the multibranched isomers formation was observed when the palladium content increased in the bimetallic catalysts. Platinum present in these catalysts did not promote the methane and ethane formation, because its possible interaction with palladium in a bimetallic particle.
Keywords: n; -Octane; Hydroisomerization; Beta zeolite; Metal content; Bimetallic
Effect of the reducing step on the properties of Pd-Cu bimetallic catalysts used for denitration by Jacinto Sá; Silvia Gross; Hannelore Vinek (pp. 226-234).
Bimetallic (Pd/Cu) catalysts supported on alumina were used for the degradation of nitrates in water. The reduction temperature strongly affects the surface and oxidation states of the metals on the catalysts. These significant changes were characterized by FT-IR, XPS, TEM with EDX and H2 chemisorption. Migration of superficial copper on top of palladium and towards the support during the reducing steps was observed, moreover palladium particles free of copper were found. With increasing reduction temperature the activity of the bimetallic catalysts passes through a maximum whereas the selectivity to nitrogen increases.
Keywords: Pd-Cu/Al; 2; O; 3; FT-IR; XPS and TEM with EDX
Characterization of palladium supported on γ-Al2O3 catalysts in hydrodechlorination of CCl2F2 by S. Chandra Shekar; K.S. Rama Rao; E. Sahle-Demessie (pp. 235-243).
Alumina supported palladium catalysts are prepared by wet impregnation technique with varying Pd loading. The reduced catalysts are tested for their activity and selectivity in the hydrogenolysis of CCl2F2 to CH2F2. Modified TPR set-up is used to distinguish between halide reduction and carbon reduction and, based on these results, the reduction mechanism of Pd/Al2O3 catalysts is elucidated for both fresh and spent forms of catalysts. TEM revealed that there is a strong re-dispersion of palladium taking place during the reaction. The palladium loading for maximum conversion of CCl2F2 and yielding for CH2F2 is 8wt.% Pd on γ-Al2O3. XRD, TEM and TPR results show that Pd particle size has influence on the CCl2F2 hydrogenolysis activity and selectivity to CH2F2 and CH4.
Keywords: Hydrodechlorination; TPR-coupled experiments; Reduction mechanism; CCl; 2; F; 2
Epoxidation of methallyl alcohol with hydrogen peroxide over TS-1 catalyst by Agnieszka Wróblewska; Monika Rzepkowska; Eugeniusz Milchert (pp. 244-250).
The influence of the technological parameters on the course of the epoxidation of methallyl alcohol with 30wt.% hydrogen peroxide over titanium silicalite catalyst TS-1 has been investigated. The effect of different parameters such as temperature, molar ratio of methallyl alcohol to hydrogen peroxide, methanol concentration, catalyst content (in reaction mixture) and the reaction time was determined. The course of the process was described by the following functions: the selectivity of transformation to 2-methylglycidol in relation to consumed methallyl alcohol, conversion of methallyl alcohol and the selectivity of transformation to organic compounds in relation to consumed hydrogen peroxide.
Keywords: TS-1 catalyst; Zeolites; Liquid-phase epoxidation; 2-Methylglycidol
The influence of ZSM-5 zeolite composition and crystal size on the in situ conversion of Fischer–Tropsch products over hybrid catalysts by Agustín Martínez; Carlos López (pp. 251-259).
The influence of the composition (Si/Al ratio, addition of Ga and Pd) and crystal size of ZSM-5 zeolite for the direct conversion of syngas to high-octane gasoline products over hybrid catalysts comprising a physical mixture of an iron-based FTS catalyst (KFeCo) and the zeolite has been studied at 310°C, 20bar, and H2:CO=1:1. The presence of the zeolite altered the product distribution towards the formation of gasoline-range isoparaffins and aromatics through the cracking, skeletal isomerization, hydrogen transfer, and aromatization reactions occurring on the zeolite acid sites. The extent of these reactions increased with the density of Brönsted acid sites. A progressive loss of the zeolite activity for the acid-catalyzed reactions occurred with TOS. The stability of the zeolite with TOS was substantially improved by decreasing its crystallite size, probably by favoring the diffusion of aromatics preventing their further transformation into coke. The incorporation of Ga to the ZSM-5 zeolite enhanced the initial formation of aromatics but increased the deactivation rate of the zeolite by promoting dehydrogenation reactions leading to coke. By contrast, the addition of small amounts of Pd to the ZSM-5 zeolite appeared a good strategy, together with decreasing crystallite size, for improving the useful lifetime of the zeolite.
Keywords: Fischer–Tropsch synthesis; Gasoline octane; Isoparaffins; Aromatics; Hybrid catalysts; Iron catalyst; ZSM-5 zeolite; Crystal size; Addition of Ga and Pd
Ni-loaded catalyst containing ZSM-5 zeolite for toluene hydrogenation by Aleksandra Masalska (pp. 260-272).
Nickel catalysts (8wt.% of NiO) containing ZSM-5 and aluminium hydroxide (1:1) were tested for activity in the hydrogenation of toluene. The catalysts differed in the extent of zeolite ion exchange. The hydrogenation activity of the catalyst was modified by additional incorporation of 4wt.% of NiO or 0.2 and 0.4wt.% of RuO2. Experiments were carried out in a continuous-flow system at 3.5MPa and 160–320°C (LHSV=3h−1; H2:CH=350Nm3/m3). It was found that conversion of toluene over the nickel-loaded catalysts increased noticeably (by 35–60%) when the amount of NiO incorporated by impregnation rose from 8 to 12wt.%. The occurrence of side-reactions observed at higher temperatures can be reduced by increasing the metal to Brønsted acid ratio of the catalysts. A correlation was also found between the selectivity of direct hydrogenation products and the metal to Brønsted acid ratio. The increase of this ratio reduced the selectivity of cyclopentane derivatives and raised their methylcyclohexane selectivity, especially at reaction temperature above 200°C. The modification of the 8wt.% NiO catalyst by the addition 0.2wt.% of RuO2 yielded a catalyst of an activity allowing toluene hydrogenation at a considerably lower temperature as compared to the 12wt.% NiO catalyst; the selectivity of these catalysts was comparable. When the amount of RuO2 was increased from 0.2 to 0.4wt.%, this accounted for only a slight improvement in activity and selectivity of catalyst.
Keywords: Toluene hydrogenation; Nickel catalyst; Zeolite ZSM-5; Metal to acid ratio; Ruthenium
Direct alkylation of toluene with ethane on bifunctional zeolite catalysts by Sarah Sealy; Yvonne Traa (pp. 273-278).
The direct alkylation of toluene with ethane to the isomeric ethyltoluenes and hydrogen was chosen as an industrially relevant model reaction for the non-oxidative alkylation of small alkane molecules. This reaction has so far not been a subject of scientific investigation. Various bifunctional zeolites with different pore systems were employed as catalysts, and Pd/H-ZSM-5 was found to be the most promising catalyst for the reaction studied. Under appropriate experimental conditions, i.e., a reaction temperature of 350°C, ann˙ethane/n˙toluene ratio of around 4 and a combined weight hourly space velocity of toluene and ethane (WHSV) of 3.5h−1, the main reaction products were the three ethyltoluene isomers. Main side products were xylenes and benzene formed by disproportionation of toluene. However, the selectivities of the byproducts decreased with increasing time on stream, whereas the selectivity to the isomeric ethyltoluenes increased considerably.
Keywords: Alkylation; Aromatics alkylation; Alkane activation; Non-oxidative alkane activation; Zeolite catalysis; Bifunctional catalysts; Pd/H-ZSM-5; Coke selectivation
Interaction of Pt and Rh nanoparticles with ceria supports: Ring opening of methylcyclobutane and CO hydrogenation after reduction at 373–723K by M. Fuchs; B. Jenewein; S. Penner; K. Hayek; G. Rupprechter; D. Wang; R. Schlögl; J.J. Calvino; S. Bernal (pp. 279-289).
The catalytic properties of a Pt (4%) and a Rh (2.5%) catalyst on a low-surface area ceria support were determined as a function of hydrogen reduction in the temperature range between 373 and 723K and compared to those of silica-supported Rh (3%) subjected to equivalent treatments. Two reactions were studied: the ring opening of methylcyclobutane (MCB) as representative for structure-sensitive hydrocarbon reactions and the hydrogenation of CO as an example for CO bond activation. After reduction in the low- and mid-temperature range the rates of either reaction decrease significantly on ceria-supported Pt and Rh whereas they are hardly affected on Rh–silica. Annealing in vacuum at 723K before the reduction step has a beneficial effect on the reaction rates but annealing after reduction leads to a general activity decrease. Accompanying ex situ high-resolution electron microscopy (HREM) investigations largely exclude particle decoration and formation of noble metal–Ce intermetallic bonds as possible effects of metal-support interaction at low and medium reduction temperatures. Taking also into account parallel surface science studies it is concluded that the observed activity decrease originates mainly from electronic perturbations at the interface between the metal nanoparticles and the increasingly reduced ceria support.
Keywords: Platinum; Rhodium; Ceria; Silica; Hydrogen reduction; Electron microscopy; Alloy formation; Methylcyclobutane; CO hydrogenation
Role of quaternary ammonium salts in the liquid-phase oxidation of ethylbenzene to hydroperoxide with molecular oxygen by P.P. Toribio; J.M. Campos-Martin; J.L.G. Fierro (pp. 290-297).
This work describes the influence of quaternary ammonium compounds [(R4N)+X−] in the yield of ethylbenzene hydroperoxide (EBHP) during the liquid-phase oxidation of ethylbenzene (EB) with molecular oxygen. Reaction rates were affected not only by the type of R4N+ and X− ions but also by their concentrations in the reaction medium. In the presence of structurally different quaternary ammonium bromide salts, ethylbenzene conversion increased because the ammonium salt favors the interaction with the EBHP. The type of the X− anion also modified the time-course of the EBHP and EB conversion profiles, although this effect was less notorious in the former; the highest EBHP concentration was reached with tetramethylammonium iodide and the lowest one with the hydroxide salt. Both EB conversion and EBHP yield decreased strongly upon reducing the concentration of the catalyst from 10−3 to 10−4M and this effect was even more marked for the iodide salt than for the bromide counterpart.
Keywords: Ethylbenzene autoxidation; Radical reactions; Quaternary ammonium compounds; Ethylbenzene hydroperoxide
In situ synthesis of supported palladium complexes: Highly stable and selective supported palladium catalysts for hydrodechlorination of CCl2F2 by Yu Cai Cao; You Li (pp. 298-305).
Supported palladium phosphine complexes [Pd(PPh3)2X2/MgF2] (X=Cl or SCN) were prepared as precursor catalysts for hydrodechlorination of CCl2F2 by a method of in situ synthesis of palladium complex on support (ISPS) and using environmentally benign solvents. Their performance and activities were compared to those of catalysts prepared via traditional incipient wetness impregnation method by catalytic reaction and characterization results, including FT-IR, XPS, XRD and H2-chemisorption. After an induction period, during which the Pd(II) precursors are converted to active species by being put in contact with substrate CCl2F2 and H2, stable catalytic activities and improved selectivities to CH2F2 (>93% on Pd(PPh3)2(SCN)2/MgF2) are achieved. The improvement of catalytic performance was ascribed to the more homogenous distributing of catalyst precursor on the surface of the support, where the amounts of the sites that induce side reactions are decreased. The XPS results revealed that there exist two kinds of active Pd sites on the surface of catalysts. One is Pd(0) site for the activation of hydrogen, and the other is the Pd site of high electrovalence, i.e. electron-deficient palladium for the activation of CCl2F2. High dispersion (∼90%) of Pd particles results in a strong interaction between Pd and the support, and creates electron-deficient palladium sites being of high selectivity to CH2F2.
Keywords: Supported palladium catalyst; Dichlorodifluorocarbon; Difluoromethane; Hydrodechlorination; CFCs; HFCs