Applied Catalysis A, General (v.193, #1-2)
Selective hydrogenation of α,β-unsaturated aldehydes over Pt catalysts deposited on monolayer supports by Jacek Kijeński; Piotr Winiarek (L1-L4).
New Pt catalysts deposited on monolayer supports containing TiO2 have been tested in the hydrogenation of α,β-unsaturated aldehydes. Enhancement of selectivity towards unsaturated alcohols has been observed in comparison to the commonly used Pt/SiO2 or Pt/TiO2 catalysts.
Keywords: Hydrogenation of α,β-unsaturated aldehydes; Pt catalysts on monolayer supports;
In situ photocatalytic reactor with FT-IR analysis for heterogeneous catalytic studies by A. Dombi; Z.A. Fekete; I. Kiricsi (L5-L8).
A novel reactor cell with in situ IR analysis was designed and tested experimentally in photocatalytic degradation of tetrachloroethene over TiO2 in the presence of air. The system may operate as a batch, pulse and flow reactor with continuous or flash irradiation.
Keywords: IR cell; Titanium dioxide; Heterogeneous photocatalysis; Tetrachloroethene;
Influence on the reduction degree of Ni–B/SiO2 amorphous catalyst and its role in selective hydrogenation of acrylonitrile by Hui Li; Hexing Li; Jing-Fa Deng (9-15).
The Ni–B/SiO2 amorphous catalyst was prepared by KBH4 reduction at room temperature. The activities of the as-prepared catalysts with various Ni loading amounts were measured during the selective hydrogenation of acrylonitrile to propionitrile and compared with those of the corresponding Ni/SiO2 catalysts obtained by H2 reduction at 773 K. For the Ni–B/SiO2 amorphous catalyst, its activity per weight Ni (R H) first increased and then decreased with the increase of Ni loading. The maximum activity was obtained at 6.4 wt.% Ni. However, the R H of Ni/SiO2 remained at the highest value up to 5.0 wt.% Ni. Then, it decreased gradually with the further increase of Ni loading. For the same kind of catalyst (Ni–B/SiO2 or Ni/SiO2), the TOF values remained almost constant irrespective of Ni loading, showing that their activity was mainly dependent on the metal surface area (S Ni). The S Ni of Ni/SiO2 catalyst is simply affected by the surface dispersion (mean particle size) due to its perfect reduction degree. However, the S Ni of Ni–B/SiO2 catalyst was affected by both the surface dispersion and the reduction degree since the reducing ability of KBH4 is relatively poor. With the increase of Ni loading, the S Ni first increased owing to the increase of reduction degree and then decreased due to the gathering of Ni particles at high Ni loading, which could account for the change of the R H with Ni loading. If one compares between the two kinds of catalysts, the higher TOF value of the Ni–B/SiO2 indicates that it was more active than Ni/SiO2; this may be attributed to the modification of the nature of Ni active sites by the alloying B. Both the structural effect and electronic effect have been discussed based on various characterizations.
Keywords: Ni–B/SiO2 amorphous catalyst; Acrylonitrile hydrogenation; Ni loading; Reduction degree;
Alumina pillared montmorillonite: characterization and catalysis of toluene benzylation and aniline ethylation by Sankarasubbier Narayanan; Kiranmayi Deshpande (17-27).
Na montmorillonite (Wyoming, Source Clays, SWy-2) is modified by Ce exchange and by alumina pillaring. The interlamellar distance d(0 0 1) of the pillared samples is measured by XRD. Surface area, total pore volume, external surface area (A ext) and micropore volume (V mi) of the samples are calculated from N2 adsorption–desorption isotherms. Acidity of the samples is measured from the step wise temperature-programmed desorption (STPD) of ammonia. Samples are evaluated for aniline ethylation and toluene benzylation reactions. Almost equal BET and external surface area (A ext) in Ce exchanged montmorillonite (Ce-Mont) suggest mesoporosity in the sample. Alumina pillared montmorillonite (AlPC(2)) and alumina pillared Ce exchanged montmorillonite (AlPC(Ce,2)) have almost the same micropore width and volume, suggesting the presence of micropores of similar size and number. The higher external surface area (A ext) of AlPC (Ce,2) indicates comparatively more mesoporosity in the sample than in AlPC(2). The pillared samples are found to be thermally stable up to 873 K. Pillaring increased strong acidity, indicated by desorption of ammonia in the temperature region 573–723 K. Ce-Mont and AlPC(Ce,2) gave 100% conversion of benzyl alcohol. Over the same set of catalysts, aniline conversion is observed to be 55% and 81%, respectively. Low aniline and benzyl alcohol conversions (55% and 51%) are observed over AlPC(2), probably due to restricted diffusion of the reactants and products through the micropores.
Keywords: Montmorillonite; Alumina pillaring; Toluene benzylation; Aniline ethylation;
Roles of chloro compound in homogeneous [Cr(2-ethylhexanoate)3/2,5-dimethylpyrrole/triethylaluminum/chloro compound] catalyst system for ethylene trimerization by Y Yang; H Kim; J Lee; H Paik; H.G Jang (29-38).
The effects of chloro compounds on the catalytic performance in ethylene trimerization reactions were studied using a homogeneous [Cr(2-ethylhexanoate)3/2,5-dimethylpyrrole (DMP)/triethylaluminum (TEA)/chloro compound] catalyst system. Chloro compounds of different types showed a broad range of effects on the performance of the catalytic system. The chloro compounds of geminal chloro groups greatly improved the catalytic activity and 1-hexene selectivity of ethylene trimerization reaction.Some specific interaction modes of chloro groups with dimeric TEA and active Cr species in the catalytic cycle were proposed to explain the catalytic activity and 1-hexene selectivity improvement effects of chloro compound in the ethylene trimerization reaction.
Keywords: Ethylene trimerization; 1-Hexene; Cr catalyst; Chloro group effects;
Intrinsic kinetics of the gas–solid Fischer–Tropsch and water gas shift reactions over a precipitated iron catalyst by Gerard P. van der Laan; Antonie A.C.M. Beenackers (39-53).
The kinetics of the gas–solid Fischer–Tropsch synthesis over a commercial Fe–Cu–K–SiO2 catalyst was studied in a continuous spinning basket reactor. Experimental conditions were varied as follows: reactor pressure of 0.8–4.0 MPa, H2/CO feed ratio of 0.25–4.0, and space velocity of 0.5–2.0×10−3 Nm3 kgcat −1 s−1 at a constant temperature of 523 K. A number of Langmuir–Hinshelwood–Hougen–Watson type rate equations were derived on the basis of a detailed set of possible reaction mechanisms originating from the carbide mechanism for the hydrocarbon formation and the formate mechanism for the water gas shift reaction, respectively. 14 models for the Fischer–Tropsch reaction rate and two water gas shift reaction rate models were fitted to the experimental reaction rates. Bartlett’s test was used to reduce the set of Fischer–Tropsch rate equations to 3 models, which were statistically indistinguishable. It could be concluded that the reaction rate of the Fischer–Tropsch synthesis is controlled by the formation of the monomer species (methylene) by hydrogenation of associatively adsorbed CO, whereas the carbon dioxide formation rate (water gas shift) is determined by the formation of a formate intermediate species from adsorbed CO and dissociated hydrogen. Simulations using the optimal kinetic models derived showed good agreement both with experimental data and with some kinetic models from literature.
Keywords: Fischer–Tropsch; Carbon monoxide hydrogenation; Reaction kinetics; Water gas shift; Kinetic model;
Solid acid catalysts from clays by Marcella Trombetta; Guido Busca; Maurizio Lenarda; Loretta Storaro; Renzo Ganzerla; Luca Piovesan; Antonio Jimenez Lopez; Manuel Alcantara-Rodrı̀guez; Enrique Rodrı́guez-Castellón (55-69).
Alumina-pillared and double-pillared montmorillonite and saponite samples have been prepared and characterised from the point of view of their thermal stability, porosity and structure. Surface acidity was studied by ammonia TPD, iso-propanol conversion and n-butene skeletal isomerisation catalysis, and by FT-IR spectroscopy of the surface hydroxy-groups, and of adsorbed acetonitrile and pivalonitrile.The data show that the alumina pillars of pillared montmorillonite carry stronger Lewis sites than those of pillared saponite. Additionally, stronger Brønsted sites are carried by the montmorillonite layers with respect to those of saponite. Finally, pillared montmorillonite is more active in converting iso-propanol. However, it is also too active in converting n-butene, so giving rise to faster coking and more extensive cracking. So, pillared saponite has a more selective behavior in converting n-butene into iso-butene.
Keywords: Acidity; Nitrile adsorption; Ammonia adsorption; Pillared clays; Butene isomerisation; FT-IR spectroscopy; Smectites; Isopropanol convertion;
Activity, selectivity and stability of Ni and bimetallic Ni–Pt supported on zeolite Y catalysts for hydrogenation of acetophenone and its substituted derivatives by R.V. Malyala; C.V. Rode; M. Arai; S.G. Hegde; R.V. Chaudhari (71-86).
The hydrogenation of acetophenone, p-hydroxy acetophenone (p-HAP) and p-isobutyl acetophenone was studied using Ni and bimetallic Ni–Pt supported on zeolite Y catalysts. A 10% Ni-supported-on-zeolite Y catalyst showed the optimum activity when compared to other Ni-supported catalysts. The activity of this catalyst decreased very rapidly on recycle; however, the bimetallic Ni–Pt-supported-on-zeolite Y catalyst was highly stable and showed constant activity on recycle. The addition of Pt catalyses the reduction of Ni2+ to Ni0, as characterised by X-ray photoelectron spectroscopy (XPS) and other techniques. For the Ni–Pt bimetallic catalyst, the hydrogen adsorption was found to be higher than that for monometallic catalysts; the adsorbed hydrogen reacts with the activated acetophenone complex to facilitate the catalytic process. The FTIR analysis of adsorbed acetophenone on the catalyst samples indicated that, in both monometallic and bimetallic catalysts supported on zeolite Y, the CO bond of acetophenone is highly activated due to the strong interactions with the acidic sites present on the zeolite. Zeolite interactions with intermediate products and solvent moieties also influenced the selectivity behaviour. A trace amount of a base like NaOH acts as a promoter in improving the selectivity towards alcohol. A plausible reaction mechanism has been proposed for the hydrogenation of acetophenone and its derivatives using monometallic as well as bimetallic catalysts.
Keywords: Catalytic hydrogenation; Acetophenone and derivatives; Nickel catalysts; Bimetallic catalysts;
Hydrogenolysis of CFC-12 (CF2Cl2) over Pd/γ-Al2O3 pretreated with HCFC-22 (CHF2Cl) by Byoung Sung Ahn; Sang Goo Jeon; Hyunjoo Lee; Kun You Park; Yong Gun Shul (87-93).
The hydrogenolysis of CF2Cl2 (CFC-12) has been studied in the gas phase over Pd (3 wt.%)/γ-Al2O3. Pretreatment of the catalyst with CHF2Cl (HCFC-22) was found to significantly improve the catalytic activity and the catalyst lifetime. The formation of Pd-carbide was observed in the CHF2Cl-treated catalyst, and the presence of the palladium carbide is thought to enhance the selectivity to the CH2F2 and to prevent sintering of palladium. XRD, TEM and XPS were used to characterize the structure and the composition of Pd/Al2O3.
Keywords: Hydrogenolysis; Palladium carbide; Pd/γ-Al2O3; CFC-12; HCFC-22; HFC-32;
The suppression of coke deposition by the modification of Mn on Fe/silicalite-2 catalyst in dehydrogenation of C2H6 with CO2 by Xu Longya; Liu Jinxiang; Xu Yide; Yang Hong; Wang Qingxia; Lin Liwu (95-101).
For the dehydrogenation of C2H6 with CO2 to C2H4, a silicalite-2 zeolite supported Fe–Mn catalyst has been developed, with an achievement of 93% of C2H4 selectivity and 66% of C2H6 conversion. The promotion of Mn to the Fe/Si-2 catalyst lies not only in increasing C2H4 selectivity but also in improving the catalytic stability greatly. A thermo-balance was employed to investigate the behavior of coke deposition on Fe/silicalite-2 and Fe–Mn/silicalite-2 catalysts during C2H6 dehydrogenation. By studying the effects of reaction temperature and partial pressures of C2H6 and CO2 on coke deposition, the kinetic parameters of the coking reaction were satisfied and the corresponding coking reaction kinetics equations were thus established. The rate of coke deposition on Fe–Mn/silicalite-2 is much less than that on Fe/silicalite-2, due to the modification of Mn to the Fe/silicalite-2 catalyst. The promotion of Mn to the improvement of the catalytic stability of the Fe/silicalite-2 catalyst results from the great prohibition of coke formation on Fe–Mn/silicalite-2 catalyst surfaces.
Keywords: C2H6; CO2; Fe/silicalite-2 catalyst; Coking reaction kinetics;
Structure analysis of intercalated smectites using molecular simulations by D.J Pruissen; P Capkova; R.A.J Driessen; H Schenk (103-112).
Structures of di- and trioctahedral smectites intercalated with a Keggin like cation [Al13O4(OH)24(H2O)12]7+ have been studied using molecular mechanics simulations in the Cerius2 modelling environment.The present study is focused on the effect of substitutions and distortions in the silicate layers on the crystal energy in these two types of intercalated layer structures.Detailed analysis of charge distribution has been carried out to explain the differences in behaviour of these two intercalated smectites. Tetrahedral substitution in the smectite layer reduces locally the negative charge of the smectite outside oxygen layer. This charge reduction favours the creation of preferential interlayer positions of the Keggin ion. The consequence of these preferential interlayer positions is a more homogeneous distribution of the intercalating ion.
Keywords: Intercalated smectites; Keggin ion;
A study on the effect of sodium chloride deposition on an FCC catalyst in a cyclic deactivation unit by E Tangstad; E.M Myhrvold; T Myrstad (113-122).
A new method for the addition of sodium to cracking catalysts in a cyclic deactivation unit (CDU) has been established. Sodium was added to a commercial residue FCC catalyst via an aqueous solution of sodium chloride prior to the cracking step in the CDU. The deactivated samples were characterised and tested in a microactivity test (MAT). The effect of sodium chloride deposition was compared with the effect of sodium naphthenate added via the cracker feed of the CDU. The results show that the addition of sodium chloride up to a level of 5000 ppm sodium give similar changes in catalyst behaviour and characteristics as when sodium naphthenate is used as the sodium source. The addition of up to 5000 ppm sodium leads to a significant decrease in the zeolite content, a certain reduction in activity and hydrogen yield, and a reduction in the extent of hydrogen transfer reactions. The results indicate that sodium chloride leads to a smaller decrease in the hydrogen transfer reactions compared to the addition of sodium naphthenate when 5000 ppm sodium is added. Sodium does not affect the gasoline octane numbers significantly.
Keywords: FCC; Sodium deposition; Cyclic deactivation; MAT; Resid cracking;
Aromatization of isophorone to 3,5-xylenol over Cr2O3/SiO2 catalysts by B David Raju; K.S Rama Rao; G.S Salvapathi; P.S Sai Prasad; P Kanta Rao (123-128).
SiO2 supported Cr2O3 catalysts with varying Cr content have been prepared and are characterized by nitrogen adsorption, low temperature oxygen chemisorption (LTOC), X-ray diffraction (XRD) and electron spin resonance (ESR) techniques. Aromatization of isophorone is carried out on these catalysts. A good correlation obtained between the oxygen uptake and aromatization yield helped to identify the nature of the active species.
Keywords: Aromatization; Isophorone; 3,5-xylenol; Cr2O3/SiO2 catalysts;
Etherification of halo-ester by phase-transfer catalysis in solid–liquid system by Hung-Ming Yang; Ping-I Wu; Chien-Ming Li (129-137).
In the present study, the kinetics for the etherification of ethyl 2-bromoisobutyrate with potassium 4-benzyloxyphenoxide in the presence of potassium iodide via solid–liquid phase-transfer catalysis (SLPTC) was investigated. The reaction was carried out in a stirred batch reactor. The effects of agitation speeds, concentration of reactants, phase-transfer catalysts, organic solvents, amounts of catalysts, and reaction temperatures were performed to find the appropriate conditions. Using the present reaction system, the hydrolysis side reaction of ether–ester was prevented. A kinetic model taking into account the deactivation behavior of the catalyst was proposed to describe the overall reaction successfully. The pseudo first-order kinetics with the deactivation function was employed to estimate the initial apparent rate constants and the deactivation constants empirically. The activation energies for catalysts tetra-n-butylammonium bromide and tetra-n-phosphonium bromide as well as the behavior of co-catalyst potassium iodide in solid–liquid phase-transfer catalysis were obtained.
Keywords: Etherification; Ether–esters; Phase-transfer catalysis; Kinetics; Deactivation; Solid–liquid system;
A grafted methane partial oxidation catalyst from MoO2(acac)2 and HZSM-5 zeolite by A. Antiñolo; P. Cañizares; F. Carrillo-Hermosilla; J. Fernández-Baeza; F.J. Fúnez; A. de Lucas; A. Otero; L. Rodrı́guez; J.L. Valverde (139-146).
A new methane partial oxidation (MPO) catalyst has been prepared by grafting MoO2(acac)2 (acac: acetylacetonate ligand) onto the surface of dried HZSM-5 zeolite. This reaction was performed in an organic solvent under an inert atmosphere with the exclusion of water from the reaction. It was shown by FT-IR spectroscopy that the reaction takes place between the acidic hydroxyl groups of the zeolite and the acetylacetonate ligands of the complex. After the grafting reaction, the solid was washed, dried and calcined in air, to afford the final [S]O2MoO2, Mo(VI), highly dispersed, surface species. The catalytic activity of this new material in the MPO reaction is enhanced in comparison with the classical Mo,W/HZSM-5 impregnation catalysts.
Keywords: Mo/HZSM-5 catalysts; W/HZSM-5 catalysts; Grafted molybdenum species; Partial oxidation of methane; Formaldehyde;
Heterogeneous zirconium and titanium catalysts for the selective synthesis of 5-hydroxymethyl-2-furaldehyde from carbohydrates by Federica Benvenuti; Carlo Carlini; Pasquale Patrono; Anna Maria Raspolli Galletti; Glauco Sbrana; Maria Antonietta Massucci; Paola Galli (147-153).
Layered zirconium- and titanium- hydrogenphosphates in the α and γ structural arrangements, their corresponding α- and γ-layered pyrophosphates as well as cubic pyrophosphates were used as acid catalysts in the dehydration of fructose and inulin to 5-hydroxymethyl-2-furaldehyde. The experiments were carried out either in batch or in subsequent cycles characterized by extraction of the reaction products, recovery and re-use of both the catalyst and the substrate solution. Good results, in terms of activity and selectivity, have been achieved. In particular, when cubic zirconium pyrophosphate was employed in batch aq. fructose dehydration a turnover number (TN) of about 9 mmol of HMF/g of catalyst × h was obtained with almost complete selectivity towards the target product. Under analogous conditions, γ-titanium phosphate revealed also promising performances (TN > 6 and selectivity to HMF ∼98%). When the reaction products were extracted and the above catalysts recycled, the cited performances were retained up to very high conversion for both fructose and inulin substrates. The catalytic performances were discussed and related to the nature of the different acid sites present on the catalyst samples.
Keywords: 5-hydroxymethyl-2-furaldehyde; Fructose dehydration; Inulin dehydration; Zirconium phosphate catalysts; Titanium phosphate catalysts;
Hydroisomerization of normal hexadecane with platinum-promoted tungstate-modified zirconia catalysts by Shuguang Zhang; Yulong Zhang; John W. Tierney; Irving Wender (155-171).
The present work examines the activity, selectivity and long-term stability of platinum-promoted tungstate-modified zirconia (Pt/WO3/ZrO2) catalysts for the hydroisomerization of long-chain linear alkanes under relatively mild conditions, using n-hexadecane as a model compound. The preparation of catalysts is described. A trickle-bed reactor is used to compare the activities and selectivities of three Pt/WO3/ZrO2 catalysts prepared by different methods and to investigate the effects of tungsten loading and of reaction conditions for hydroisomerization of n-hexadecane. A run of 93.5 h was conducted using the most highly active Pt/WO3/ZrO2 catalyst which contains 0.5 wt.% well-dispersed Pt and 6.5 wt.% W. Reaction conditions were manipulated and five shutdown (feed stopped, reactor temperature lowered but H2 flow rate maintained) and restart operations were carried out. This catalyst showed high activity and stability. Considerable success has also been achieved in converting n-hexadecane to isohexadecanes for 100 h at temperatures of about 220°C and under H2 pressure as low as 160 psig (1 psig = 6.895 kPa). Our best results (highest i-C16 yield) are a 79.1 wt.% n-C16 conversion, 89.9 wt.% i-C16 selectivity and 71.1 wt.% i-C16 yield at 218°C, 160 psig H2, H2/n-C16 mole ratio = 2 and weight hour space velocity (WHSV) = 1 h−1. The Pt/WO3/ZrO2 catalyst is rugged and has properties which allow one to propose that it may attain commercial use after further study.
Keywords: Tungstate-modified zirconia; Long-chain linear alkanes; Hydroisomerization; Isohexadecanes; Platinum;
CO2 reforming of methane over zeolite-Y supported ruthenium catalysts by U.L. Portugal; C.M.P. Marques; E.C.C. Araujo; E.V. Morales; M.V. Giotto; J.M.C. Bueno (173-183).
Ru/HY and Ru/NaY catalysts were prepared by ion-exchange from an aqueous solution of [Ru(NH3)6]Cl3. The catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and nuclear magnetic resonance (NMR). The results showed that Si/Al ratio of the zeolite framework after use depends on the catalyst preparation procedure. Upon activation in He, Si/Al ratio in Ru/NaY is similar to that of initial zeolite NaY. After exposure to reaction conditions, NMR data show dealumination, whose extent increases with Ru loading. Samples activated in He followed by ion exchange with NaNO3 (neutralization) and reactivated in CH4/CO2/N2 undergo only slight dealumination. The results suggest that during reaction the charge compensating protons, generated during Ru reduction, cause dealumination, accompanied by local destruction of the zeolite structure. The structure of zeolite and Ru dispersion was therefore, stabilized and the catalyst specific activity increased by neutralizing the protons prior to use in CO2 reforming.
Keywords: Ruthenium catalyst; Synthesis gas; Methane reforming; Zeolite Y; NMR;
Effect of H2 reduction on the catalytic properties of molybdenum oxides for the conversions of heptane and 2-propanol by Takeshi Matsuda; Yasuyoshi Hirata; Syuya Suga; Hirotoshi Sakagami; Nobuo Takahashi (185-193).
Conversions of heptane and 2-propanol were carried out using MoO3 reduced with H2 at 623 K. The parent MoO3 was inactive for the isomerization of heptane. H2 reduction, however, enhanced the isomerization activity, which became almost constant after reduction for 24 h. The reduced MoO3 exhibited a higher isomerization activity than 0.5 wt.% Pt/USY zeolite, while there was no appreciable difference in product distribution between these two catalysts. The catalytic activities of MoO3 for dehydration and dehydrogenation of 2-propanol also increased for longer periods of H2 reduction. NH3-TPD studies showed that the acidity of MoO3 was strongly affected by H2 reduction. We suggest from these results that H2 reduction of MoO3 at 623 K generated the bifunctional property, which was responsible for a high isomerization activity.
Keywords: Molybdenum oxide; H2 reduction; Isomerization; Dehydration; Dehydrogenation;
Active sites for ethanol oxidation over SnO2-supported molybdenum oxides by Fernanda Gonçalves; Paulo R.S Medeiros; Jean G Eon; Lucia G Appel (195-202).
SnO2-supported molybdenum oxides with varying coverage were synthesized and used for the catalytic oxidation of ethanol. The catalysts were obtained from precipitation of SnCl4 by ammonia in the presence of (NH4)2Mo7O24 (A). Some catalysts were also prepared by impregnation of (NH4)2Mo7O24 on SnO2 (B) for comparison. It was verified that molybdenum oxides inhibited the sintering of SnO2 crystals during calcination for preparation A, resulting in homogeneous systems with high specific areas. The solids were characterized by FTIR, temperature programmed reduction (TPR), DRS-UV, XPS and X-ray diffraction (XRD). The molybdenum coverage was determined by oxygen pulses after reduction at 400°C under hydrogen. The results indicated two structurally different superficial sites. Four-coordinated molybdates were preferentially formed on the surface of co-precipitated catalysts at low molybdenum loading, while six-coordinated polymolybdates were obtained in other cases. Bulk MoO3 oxide was also observed at very high loading. The turnover numbers (TONs) measured for ethanol oxidative dehydrogenation suggested that dispersed, four-coordinated molybdates were the active phase. These species also gave higher selectivity to acetic acid.
Keywords: Molybdenum; Ethanol; Tin oxide; Oxidation;
Methane partial oxidation on Pt/CeO2–ZrO2 in the absence of gaseous oxygen by Piboon Pantu; Kiseok Kim; George R Gavalas (203-214).
Partial oxidation of methane to synthesis gas over platinum or ruthenium supported on Ce1−x Zr x O2 (x = 0, 0.2 and 0.5) was studied at 550–700°C in the absence of gaseous oxygen. The reaction was carried out in a packed-bed reactor under continuous or pulsed flows of methane. Oxidation utilized oxide oxygen and was initially very fast but slowed down as the oxide support became progressively reduced. Addition of ZrO2 into CeO2 considerably increased the rate of methane oxidation and enhanced the reducibility of CeO2 but decreased the selectivity to carbon monoxide and hydrogen. Specifically it was found that significant production of carbon dioxide and water occurred on the freshly oxidized solid until a certain degree of reduction was reached beyond which the selectivity to carbon monoxide and hydrogen rose to over 90%. This critical degree of reduction was 10%, 40% and 65% for the solid compositions x = 0, 0.2 and 0.5, respectively. Additional experiments carried out using carbon monoxide pulses showed that carbon monoxide oxidation declines sharply and becomes negligible beyond this degree of reduction while oxidation of methane continues much further. Comparison of the two metals showed that platinum is more active but the reaction rate did not change much in the range of platinum loadings of 0.25–1 wt.%.
Keywords: Cerium oxide; Methane partial oxidation; Redox oxide; Synthesis gas;
Thermal stability of 12-tungstophosphoric acid supported on zirconia by E López-Salinas; J.G Hernández-Cortéz; I Schifter; E Torres-Garcı́a; J Navarrete; A Gutiérrez-Carrillo; T López; P.P Lottici; D Bersani (215-225).
A freshly precipitated zirconia was impregnated with an ethanol solution of H3[W12PO40]·6H2O (TPA) in order to obtain 0–25 wt.% TPA/ZrO2. The solids, heat-treated from 373 to 1073 K, were examined by means of differential thermal analysis, X-ray diffraction, surface area, infrared, Raman, 31 P nuclear magnetic resonance analyses, and used as catalysts in the decomposition of isopropanol. Below 673 K, TPA on ZrO2 exist as distorted intact Keggin species interacting with Zr–OH or Zr+ groups. However, at 773 K, bulk-like intact TPA species were detected. Above 773 K, the Keggin structure of TPA collapse and transforms into WO3 and phosphorous oxides. The characteristic diffraction peaks of TPA cannot be observed even with 20 wt.% TPA loading, indicating a high dispersion or a very small crystal size of TPA, while 20 wt.% TPA loaded on SiO2 clearly shows the presence of TPA particles.The heat-treatment temperature affects the catalytic activity of TPA on Zirconia. The reaction rate was 3.3 times higher in a TPA/ZrO2 calcined at 773 than that at 473 K.
Keywords: 12-Tungstophosphoric acid; Heteropolyacid; ZrO2;
A comparative study of methyl-tert-butyl ether synthesis on zeolites HY, HBeta, HBeta/F and HZSM-5 by in situ MAS NMR spectroscopy under flow conditions and on-line gas chromatography by Thomas Horvath; Michael Seiler; Michael Hunger (227-236).
Investigating the formation of methyl-tert-butyl ether (MTBE) on zeolites HY, HBeta, HBeta/F and HZSM-5 by in situ 13 C MAS NMR spectroscopy under flow conditions, signals of alkoxy species have been observed at 77–90 ppm. On-line gas chromatographic analysis of the reactant and product molecules leaving the MAS NMR rotor reactor yielded that the formation of these alkoxy species is in line with high yields of MTBE and vice versa. Methanol adsorption experiments performed by in situ 1 H MAS NMR spectroscopy under flow conditions indicated that methanol molecules adsorbed on the highly active zeolite HBeta are characterized by a low protonation degree. Based on the experimental results of in situ MAS NMR spectroscopy and on-line gas chromatography, a mechanism of the synthesis of MTBE on acidic zeolites is proposed in which the reaction of isobutene with bridging OH groups leading to the formation of isobutoxy species with ‘carbenium-like properties’ plays a key role.
Keywords: Methyl-tert-butyl ether; Acidic zeolites; In situ MAS NMR spectroscopy; Flow conditions; Alkoxy species;
Selectivity and mechanism of cumene liquid-phase oxidation in the presence of powdered mixed iron–aluminum oxides prepared by alkoxy method by M.V. Tsodikov; V.Ya. Kugel; E.V. Slivinskii; G.N. Bondarenko; Yu.V. Maksimov; M.A. Alvarez; M.C. Hidalgo; J.A. Navio (237-242).
Sol–gel chemistry routes using metallo-organic complexes as precursors were used to prepare iron–aluminum catalysts for low temperature liquid phase cumene oxidation. Catalytic and structural properties of Fe–O/Al2O3 catalysts were studied by kinetic measurements combined with other techniques in order to obtain information about the nature of catalytic active centers and the selectivity of the cumene liquid-phase oxidation in the presence of single and mixed oxides. Our results allow us to expect a bifunctional role of the Fe–O/Al2O3 surface as a combined participation of Lewis acid and terminal oxygen centers by a proposed mechanism which is known for homogeneous catalysis and is non-typical for heterogeneous catalytic oxidation.
Keywords: Cumene oxidation; Iron–aluminum oxides; Heterogeneous catalysis;
Lattice Monte Carlo simulations of the activity of supported liquid-phase catalysts by L.A. Abramova; S.P. Baranov; A.A. Dulov (243-250).
We introduce a new model aimed at the analysis of the activity of supported liquid phase catalysts. The model is based on the statistical Monte Carlo approach and takes into account the properties of the support pore system, the properties of the filling liquid and the characteristics of liquid distribution. The predictions of the model are compared with a large body of experimental data and demonstrate good agreement with them.
Keywords: Supported liquid-phase catalysts; Monte Carlo simulations;
Lattice Monte Carlo simulations of the activity of supported liquid-phase catalysts by L.A Abramova; S.P Baranov; A.A Dulov (251-256).
We develop a lattice Monte Carlo model aimed at the analysis of the activity of supported liquid-phase catalysts. We extend the model to several cases of nonuniform liquid distributions. The predictions of the model are compared with experimental results and they demonstrate a good agreement with the latter.
Keywords: Supported liquid-phase catalysts; Monte Carlo simulations;
Isomerization of glucose into fructose in the presence of cation-exchanged zeolites and hydrotalcites by Claude Moreau; Robert Durand; Alain Roux; Didier Tichit (257-264).
Isomerization of glucose into fructose was performed in a batch mode in the presence of a series of alkaline solid catalysts such as cation-exchanged A, X, Y zeolites and hydrotalcites.Under optimized operating conditions, water as the solvent, 95°C, up to 250 g/l of initial glucose concentration, up to 20 wt.% of catalyst , 700–1200 rpm and 8 bar of nitrogen pressure to avoid oxidation reactions, the reaction is not controlled by external or internal diffusional limitations. Among the different catalysts used, Li-, Na-, K-, Cs-, Ca- and Ba-exchanged A, X and Y zeolites, and hydrotalcites, Ca- and Ba-exchanged A, X and Y zeolites were found less selective, whereas those with a moderate basicity such as NaX and KX were found to achieve isomerization of glucose into fructose with a selectivity to fructose of about 90%, but at low glucose conversion, around 10–20%. However, the high selectivity to fructose is only obtained at glucose conversions lower than 25%. Furthermore, it was found that a significant amount of the cation passed into water (around 15% in the case of monovalent cation-exchanged X zeolites). This leaching phenomenon is no longer observed after a second run. The conversion of glucose is stabilized at about 10% without loss of selectivity to fructose, so that a continuous process may be considered.
Keywords: Glucose; Fructose; Isomerization; Cation-exchanged zeolites; Hydrotalcites;
The behavior of Cu/ZSM-5 in the oxide and reduced form in the presence of NO and methanol by Marta Cristina Nunes Amorim de Carvalho; Fabio Barboza Passos; Martin Schmal (265-276).
Cu/ZSM-5 catalysts were characterized by DRS UV–Vis IR spectroscopy, CO chemisorption and NO TPD. The results indicated that the exchange level in Cu2+ is complete. The oxidation number of copper ions after calcination is 2+, with the formation of isolated Cu2+ species, CuO and (Cu–O–Cu)2+ compounds, depending on the pretreatment conditions. It was found that copper atoms are arranged as small clusters, well dispersed in the zeolite framework. The results indicated the active species for catalytic decomposition of NO is Cu2+, which can be reduced to Cu+ forming Cu+–NO2 and Cu+–(NO)2 complexes. The dependence of copper ions state and therefore, the pretreatment are important for the NO decomposition activity due to the formation of the (Cu–O–Cu)2+ oxocation species, which act as catalytic sites for the disproportionation of NO to N2O + NO2. Methanol oxidation showed that the activity and selectivity are not dependent on pretreatment temperature, but they are markedly influenced by the acid site distribution. Methanol was inactive for NO reduction, since methanol can not be activated on the NO adsorption sites to form species that are active for NO reduction.
Keywords: Cu/ZSM-5; Zeolite; Methanol; Oxidation; Nitric oxide;
Effect of titanium-doping on the properties of vanadium antimonate catalysts by Andrés Barbaro; Susana Larrondo; Stella Duhalde; Norma Amadeo (277-283).
Catalysts of nominal compositions VSb1−x Ti x O4, with x = 0, 0.1, 0.2 and 0.4, were prepared by solid state reaction of vanadium(V) oxide, antimony(III) oxide and titanium(IV) oxide mixtures. Their structural and catalytic properties were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and catalytic heterogeneous oxidation of toluene to benzaldehyde.The samples with 10% (x = 0.1) and 20% (x = 0.2) of titanium present the highest activity and selectivity to benzaldehyde under the operating conditions used in this work. When the titanium content is 40% (x = 0.4), the activity level drops strongly below the level found for the undoped vanadium antimonate. X-ray patterns show the presence of vanadium(V) oxide as a separate phase in this sample, and this is probably the cause of the low catalytic performance.TPR results show that the replacement of antimony by titanium changes the oxidation-reduction properties of the vanadium antimonate phase.In the samples with titanium content up to 20%, there seems to be a direct relationship between the selectivity and the separation of the reduction temperatures of both reduction peaks of the TPR profile: the larger the separation, the higher is the selectivity to benzaldehyde.All the samples with x = 0, 0.1 and 0.2 show surface antimony excess despite being prepared from stoichiometric mixtures of nominal V/Sb ratios larger than or equal to 1.
Keywords: Antimony; Benzaldehyde; Titanium; Toluene oxidation; Vanadium; Vanadium antimonate;
The kinetics of stereoselective hydrogenation of (−)-menthone and (+)-isomenthone mixture using nickel catalysts by Pavel Kukula; Libor C̆ervený (285-290).
The kinetics of the catalytic hydrogenation of optically active mixture of (−)-menthone and (+)-isomenthone was studied. Raney nickel and Ni/SiO2 were used as catalysts. Catalysts modified with tartaric acid (TA) were tested as well. Various isomers were produced in these reactions, especially (+)-neomenthol and (+)-neoisomenthol. Small amount of (−)-menthol was produced, the yield of (−)-isomenthol was minimal. An isomerization of (−)-menthone and (+)-isomenthone occurred during these reactions. Isomerizations among individual menthol isomers were not detectable under the given reaction conditions. Based on the acquired experimental data, a kinetic model of the Langmuir–Hinshelwood type was created. This model was further continuously adjusted and simplified as to describe the experimental data and acquire optimal kinetic model parameters.
Keywords: Stereoselective hydrogenation; Reaction kinetics; Menthone; Menthol isomers;