Applied Catalysis A, General (v.211, #1)


Cyclohexane oxidation continues to be a challenge by Ulf Schuchardt; Dilson Cardoso; Ricardo Sercheli; Ricardo Pereira; Rosenira S. da Cruz; Mário C. Guerreiro; Dalmo Mandelli; Estevam V. Spinacé; Emerson L. Pires (1-17).
Many efforts have been made to develop new catalysts to oxidize cyclohexane under mild conditions. Herein, we review the most interesting systems for this process with different oxidants such as hydrogen peroxide, tert-butyl hydroperoxide and molecular oxygen. Using H2O2, Na-GeX has been shown to be a most stable and active catalyst. Mesoporous TS-1 and Ti-MCM-41 are also stable, but the use of other metals such as Cr, V, Fe and Mo leads to leaching of the metal. Homogeneous systems based on binuclear manganese(IV) complexes have also been shown to be interesting. When t-BuOOH is used, the active systems are those phthalocyanines based on Ru, Co and Cu and polyoxometalates of dinuclear ruthenium and palladium. Microporous metallosilicates containing different transition metals showed leaching of the metal during the reactions. Molecular oxygen can be used directly as an oxidant and decreases the leaching of active species in comparison to hydrogen peroxide and tert-butyl hydroperoxide. Metal aluminophosphates (metal: Mn, Fe, Co, Cu, Cr V) are active and relatively stable under such conditions. Mn-AlPO-36 yields directly adipic acid, but large amounts of carboxylic acids should be avoided, as they cause metal leaching from the catalysts. Rare earth exchanged zeolite Y also shows good selectivity and activity. In the last part of the review, novel alternative strategies for the production of cyclohexanol and cyclohexanone and the direct synthesis of adipic acid are discussed.
Keywords: Cyclohexane oxidation; Cyclohexanone; Adipic acid; Hydrogen peroxide; tert-Butyl hydroperoxide; Molecular oxygen;

X-ray photoelectron spectroscopy technique was employed to characterize TiO2–ZrO2, TiO2–SiO2, TiO2–Al2O3, SiO2–ZrO2, and SiO2–TiO2–ZrO2 mixed oxide supported MoO3 catalysts. The investigated mixed oxide supports are obtained by a homogeneous coprecipitation method using urea as hydrolyzing agent. Molybdena (12 wt.%) was impregnated over these calcined (773 K) mixed oxide supports by a wet impregnation method from aqueous ammonium heptamolybdate solution. The XPS binding energy (BE) values of all the metals in the mixed oxide supports as well as Mo-containing catalysts are found to shift from the values of the individual metal component oxides. The shift in BE suggests that the Zr in TiO2–ZrO2 and Ti in TiO2–Al2O3 acquire more negative charge after doping with MoO3 on these supports. The observed BE shifts, due to variation in the lattice potential, are explained in terms of Kung’s model. The XPS atomic intensity ratio measurements show that the interaction between Mo and Al is strong and the dispersion of molybdena is more on Al2O3 portion of the TiO2–Al2O3 mixed oxide. In the case of MoO3/TiO2–ZrO2 and MoO3/SiO2–TiO2–ZrO2 samples, the Mo:Ti and Mo:Zr ratios show that the Ti4+ and Zr4+ both contribute equally in the dispersion of molybdenum on these corresponding mixed oxides. The FWHM values indicate the presence of different Mo(VI) species on TiO2–Al2O3, and a homogeneous distribution on TiO2–ZrO2 and TiO2–SiO2 mixed oxide surfaces.
Keywords: Molybdenum oxide; Mixed oxides; TiO2–ZrO2; TiO2–SiO2; TiO2–Al2O3; SiO2–ZrO2; SiO2–TiO2–ZrO2; Dispersion; XPS;

The benzoylation of 1,2-dimethoxybenzene (veratrole) with benzoic anhydride and substituted benzoyl chlorides has been investigated in the liquid phase (chlorobenzene as solvent) over the H-forms of various zeolites. H-Y and H-BEA have been shown to be efficient catalysts in such a reaction, and led to the selective formation of the corresponding dimethoxybenzophenones. The effect of various experimental parameters on the initial rate of the reaction of veratrole with benzoic anhydride over H-Y zeolite has been studied, leading to propose a suitable mechanism based on the difference of adsorptions of the aromatic substrate and the acylating agent. Moreover, the study of the reaction of veratrole with a series of substituted benzoyl chlorides (4-CH3, 4-OCH3, 4-tert-butyl, 4-Cl, 2-Cl and 2-Br benzoyl chlorides, respectively) over the same H-Y zeolite led to conclude that, due to the high reactivity of the aromatic substrate, the electrophilicity of the acylating agent does not play a relevant role under the given heterogeneous conditions.
Keywords: Benzoylation; Zeolites; Benzoic anhydride; Dimethoxybenzenes; Competitive adsorption; Eley–Rideal type process; Dimethoxybenzophenones;

Methylation of catechol with dimethyl carbonate as an alkylating agent has been carried out over calcined MgAl hydrotalcites in the temperature range 523–623 K. Guaiacol and veratrole were obtained as the major products, along with small amounts of catechol carbonate and C-alkylated products. At 573 K, the total O-selectivity was 96.1% with a guaiacol selectivity of 84%. At higher DMC/catechol molar ratios and longer contact times, guaiacol selectivity decreased markedly with a concomitant increase in the selectivity of veratrole. An optimum value of the strength of acid–base pair sites involved in the adsorption of DMC and catechol makes hydrotalcite-based oxides better catalysts compared to alumna and basic MgO. It is possible that the nature of alkylating agent also can affect the product selectivity along with the experimental factors and the acid–base properties of the catalysts.
Keywords: O-methylation; Guaiacol; Catechol; Dimethyl carbonate; Calcined MgAl hydrotalcites;

A study on the synthesis of diethyl oxalate over Pd/α-Al2O3 catalysts by Xuan Zhen Jiang; Yue Hua Su; Bor Jih Lee; Shu Hua Chien (47-51).
The synthesis of diethyl oxalate from ethyl nitrite and carbon monoxide was studied in a continuos flow micro fixed-bed reactor at atmospheric pressure. Two palladium catalysts supported on α-Al2O3 of different pore structures were studied under mild conditions. One catalyst (1 wt.% Pd/α-Al2O3-1) showed higher catalytic activity and selectivity than the other catalyst (1 wt.% Pd/α-Al2O3-2). X-ray diffraction patterns have confirmed that both supports are well-defined α-Al2O3. Palladium dispersions were greater on the 1 wt.% Pd/α-Al2O3-1 catalyst than that on the 1 wt.% Pd/α-Al2O3-2 catalyst as determined by hydrogen chemisorption. The FTIR spectroscopy study indicated that the 1 wt.% Pd/α-Al2O3-1 catalyst adsorbed CO easily in the linear form (band at 2089 cm−1) and bridge form (bands at 1950 and 1880 cm−1), and as carbonate species (at 1628 cm−1). There was little CO adsorption on 1 wt.% Pd/α-Al2O3-2. The average pore size of the support α-Al2O3-1 was 41.6 Å, and 14.0 Å for α-Al2O3-2. This study reveals that the pore structure of the supports remarkably affects the palladium dispersion of the catalysts and alters the CO adsorption behavior, therefore, dramatically affect the catalytic performance. In the case of CO+C2H5ONO reactions, diethyl oxalate was efficiently formed over the 1 wt.% Pd/α-Al2O3-1 catalyst.
Keywords: Diethyl oxalate; Catalyst; α-Al2O3; Support effect; In situ FTIR;

The effect of space velocity on the partial oxidation of methane using different support geometries has been studied. While on a foam alumina monolith syngas selectivity drops as space velocity is increased above 4×105   h −1 , the use of non-porous alumina spheres as the support allows high reactant conversions and syngas selectivities even at space velocities of 1.8×106   h −1 . The differences between monoliths and spheres are discussed in order to understand why spheres give superior results. It is suggested that differences in heat transfer within the two support geometries may play a major role in the different results between spheres and monoliths. A convective heat transfer model suggests that higher rates of convection in a monolith will lead to lower front temperatures than in a sphere bed, a trend that becomes important at high space velocities in leading to blowout and lower syngas selectivities.
Keywords: Methane oxidation (partial); Syngas; Rhodium; Millisecond contact times;

Isopentane dehydrogenation on Pt-Sn catalysts supported on Al-Mg-O mixed oxides: effect of Al/Mg atomic ratio by H Armendáriz; A Guzmán; J.A Toledo; M.E Llanos; A Vázquez; G Aguilar-Rı́os (69-80).
A series of Mg-Al-O supports was prepared by the coprecipitation method at constant pH. The Al/Mg atomic ratio ranged from 2.06 to 4.9. The physico-chemical, as well as the surface and structural properties of the synthesized materials were strongly dependent on the Al/Mg atomic ratio. In solids with an Al/Mg atomic ratio higher than 2, a support with a mixture of Al2O3 and MgAl2O4 phases was obtained. In all the supports a non stoichiometric magnesium aluminate spinel structure (MgAl2+x O4+(3/2)x ) was not detected. The supports were impregnated with Pt and Sn (0.6 and 1.2 wt.%, respectively). The resulting catalysts were evaluated in the isopentane dehydrogenation reaction at 550°C and atmospheric pressure. On the Pt-Sn catalysts which were prepared with aluminum rich supports (Al/Mg=2.4 and 4.9), TPD-H2 results showed the presence of Pt particles (unalloyed and tin alloyed) deposited in both Al2O3 and MgAl2O4 phases. A stronger Pt-support interaction on MgAl2O4 phase than on Al2O3 phase was observed. The Pt-Sn catalyst impregnated on pure MgAl2O4 showed the maximum selectivity to isoamilenes. The high Al2O3 content supports (Al/Mg>3) bring about a higher light olefins (C2–C4) production, which promotes the coke formation. This process finally accelerated the catalyst deactivation. This is explained by the higher acidity of alumina compared with magnesium aluminate.
Keywords: Platinum; Tin; Magnesium aluminate; Isopentane dehydrogenation; Alumina;

ZnO and ZnO/Al2O3 catalysts were studied for a reverse-water-gas-shift reaction (RWReaction). The catalytic activities depended on the compositions of Zn and Al at the temperature range of 673–973 K and GHSV of 15,000. The activities were close to the equilibrium conversion at temperatures above 873 K. The catalysts were characterized by using BET, TPR, XRD, SEM, and TEM. The ZnO/Al2O3 catalysts were mixtures of ZnO and ZnAl2O4 phases, and the particle size of the ZnO was strongly dependent on its composition in the ZnO/Al2O3 catalysts. ZnO/Al2O3 (Zn:Al=1:1) catalyst has the smallest particle size of ZnO and its conversion of CO2 at 873 K and GHSV of 150,000 was 43%. The stability of ZnO/Al2O3 catalysts increased in the presence of the large particles of ZnO. Hence, ZnO/Al2O3 (Zn:Al=4:1) catalyst was more stable than the ZnO/Al2O3 (Zn:Al=1:1) catalyst. The conversion of CO2 on the ZnO/Al2O3 (Zn:Al=1:1) catalyst decreased from 43 to 17% in 48 h. The ZnO in ZnO/Al2O3 catalysts was reduced to the Zn metal during the RWReaction, which contributed to the deactivation of the ZnO/Al2O3 catalysts. Meanwhile, the activity of ZnAl2O4 catalyst was stable for 100 h at 873 K and GHSV of 150,000. The ZnAl2O4 catalyst was developed for the RWReaction of the CAMERE (carbon dioxide hydrogenation to form methanol via a reverse-water-gas-shift reaction) process for methanol formation from CO2.
Keywords: CAMERE (carbon dioxide hydrogenation to form methanol via a reverse-water-gas-shift reaction) process; Reverse-water-gas-shift reaction; ZnO/Al2O3; ZnO; Al2O3; ZnAl2O4;

Linalool synthesis from α-pinene: kinetic peculiarities of catalytic steps by V.A. Semikolenov; I.I. Ilyna; I.L. Simakova (91-107).
Synthesis of linalool from α-pinene which includes consecutively α-pinene hydrogenation to pinane on Pd/C catalyst, pinane oxidation to pinane-hydroperoxide by molecular oxygen and pinane-hydroperoxide hydrogenation to pinanol on Pd/C catalyst followed by its thermal isomerization to linalool is discussed. The effects of the reagent concentrations, temperature and catalyst content on the reaction rate and selectivity are studied. The kinetic peculiarities and the mechanisms of reactions are presented. The synthetic conditions of high-selective linalool preparation are found.
Keywords: Linalool synthesis; α-Pinene hydrogenation on Pd/C; Pinane oxidation; Pinane-2-hydroperoxide hydrogenation on Pd/C; Pinane-2-ol thermal isomerization; Kinetics of reactions; Mechanism of reactions;

Partial oxidation of methane on potassium-promoted WO3/SiO2 and on K2WO4/SiO2 catalysts by A. Erdőhelyi; R. Németh; A. Hancz; A. Oszkó (109-121).
The partial oxidation of methane to formaldehyde or ethane and ethylene was studied on K2WO4 deposited on various supports, on potassium-promoted WO3/SiO2 and for comparison on WO3/SiO2 in a fixed-bed continuous-flow reactor at 860–923 K using O2 as oxidant. The catalysts were characterised by Raman, XP spectroscopy and TPR method. It was found that the original structure of potassium tungstate remained on K2WO4/SiO2, whereas K2W2O7 was very likely formed on K+WO3/SiO2. The main products of the reaction besides the carbon oxides were HCHO on WO3/SiO2 while on K-containing samples mainly the C2 hydrocarbons were formed. The product distribution of the oxidation reaction was markedly influenced by the nature of the support. The highest activity was measured for alumina-supported catalyst. In this case, however, only traces of partially oxidised products were formed. Formaldehyde, in a larger quantity, was produced on WO3/SiO2 but C2 in higher selectivity on K2WO4/SiO2 was formed. A possible mechanism for the oxidative conversion of methane is also discussed.
Keywords: Partial oxidation; WO3/SiO2; K2WO4/SiO2; Methane oxidation; Raman spectra of K2WO4; XP spectra of K2WO4;

Effects of the introduction of tetrachloromethane (TCM) into the feedstream for the oxidative dehydrogenation of propane have been investigated on strontium and barium hydroxyapatites (SrHAp and BaHAp) with and without treatment with Cu2+ and Pb2+. Activities on SrHAp were greater than those on BaHAp in the absence of TCM. Upon adding TCM into the feedstream for propane oxidation on both SrHAp and BaHAp, the conversion of propane decreased while the selectivity to propylene increased with increasing time-on-stream, as observed in the oxidation of other alkanes on hydroxyapatites. While the treatment of both hydroxyapatites with lead resulted in the decrease of the yield of propylene regardless of the addition of TCM, activities on those catalysts doped with copper were improved in the presence of TCM. Particularly SrHAp treated with Cu2+ afforded approximately 80% selectivity to propylene at 16% conversion of propane at 0.5 h on-stream with TCM and this activity was kept for 6 h on-stream. A direct contribution of TCM to control of the reduction of Cu2+ in the catalyst leads to the improvement of the catalytic performance.
Keywords: Oxidative dehydrogenation; Propane; Hydroxyapatites; Copper; Tetrachloromethane;

CALENDAR (133-135).