Applied Catalysis A, General (v.264, #2)



Effect of steaming on one-step oxidation of benzene to phenol with nitrous oxide over Fe/MFI catalysts by Krishnan S Pillai; Jifei Jia; Wolfgang M.H Sachtler (133-139).
One-step oxidation of benzene with N2O to phenol was studied over the iron–zeolite catalysts Fe/MFI and HMFI containing an Fe impurity. Catalysts were probed at 723 K and near atmospheric pressure using a fixed bed micro-flow reactor and a molar feed ratio of benzene/N2O/He=1/3/36. A plot of the activity as a function of the loading with accessible iron, shows that mono-nuclear sites, are much more active for this reaction than dinuclear sites, which are known to catalyze NO x reduction. The benzene to phenol reaction is found to be of the consecutive type, all CO2 is formed by burning of phenol. The reaction order at these conditions is zero both in benzene and N2O. A steam pretreatment at 923 K of the low Fe-containing catalyst is found to be instrumental in the formation of the “Ex-lattice” Fe sites, required for the reaction. The same treatment when applied to high Fe-load catalysts leads also to growth of Fe oxide particles, and likely favors formation of a solid solution of Fe and Al oxide. The catalyst is deactivated by coke deposits. This can be largely suppressed by water vapor in the feed, resulting in a stable performance. It is assumed that H2O displaces C6H5OH from the sites.
Keywords: Benzene oxidation with N2O to phenol; Fe/MFI; Fe/ZSM-5; Catalyst deactivation; Steaming; Consecutive reaction;

Acetone condensation was carried out over high surface area molybdenum nitride and carbide catalysts. Rates and selectivities for these materials were compared to those for MgO (a solid base catalyst), USY (a solid acid catalyst) and 1% Pt/SiO2 (a supported noble metal catalyst). The Mo2N catalyst as well as MgO and USY catalyzed the condensation of acetone to products including mesityl oxide (MO), mesitylene and isophorone. Areal rates for the Mo2C and Mo2N catalysts were higher than those for the MgO and USY catalysts. The Mo2N catalyst was most active for mesityl oxide production. The formation of mesitylene and isophorone over this catalyst indicated that Mo2N possessed strong acid and base sites, respectively. Results from the thermal desorption of CO2 and NH3 confirmed the presence of these sites. The hydrogenation of acetone to isopropanol (IPA) and its subsequent dehydration to propylene were predominant reactions for the Mo2C and 1% Pt/SiO2 catalysts. Differences between the Mo2N and Mo2C catalysts are explained based on their base, acid and metallic properties.
Keywords: Nitrides; Carbides; Acetone condensation; Base and acid sites; Mesityl oxide;

Isopropanol chemisorption and temperature programmed surface reaction analyses were used to determine the nature, amount and acid strength of the active sites of bulk WO3, monolayer supported tungsten oxide over titania and phospho-tungstic Wells–Dawson and Keggin type heteropoly-acids.The temperature conditions were adjusted in order to cover the surface of the catalysts with a stable monolayer of intermediate chemisorbed isopropoxy species avoiding the adsorption of molecular isopropanol (non reactive) or surface reaction during the chemisorption process.Chemisorbed isopropoxy species dehydrate towards propylene (no other redox products were observed) over the investigated catalysts. The number of active sites (N s) was determined as the amount of propylene desorbed during the temperature programmed reaction analysis.Heteropoly-anions possess the highest amount of active sites and acid strength among the investigated solid catalysts. Isopropanol chemisorbs both at the surface and the bulk of the heteropoly-anions due to the pseudo-liquid phase property of these materials. Moreover, the high acidity of the Brönsted and Lewis acid sites of the heteropoly-acids catalyze the reaction of the adsorbed isopropoxy species at a lower temperature (∼100 °C) than WO3 and monolayer supported tungsten oxide species WO x /TiO2.Additionally, the influence of the degree of hydration on the amount and acid strength of the active sites of the Wells–Dawson acid was investigated. The results showed that the fully hydrated acid possesses a higher amount of accessible sites than the fully dehydrated acid therefore, the amount of propylene is almost double in the first condition. However, no modification of the temperature of reaction of the isopropoxy species was observed.
Keywords: Temperature programmed surface reaction; Heteropoly-acids; Adsorption; Number of active sites; Wells–Dawson; Keggin;

Effect of MgO additive on catalytic properties of Co/SiO2 in the dry reforming of methane by R. Bouarab; O. Akdim; A. Auroux; O. Cherifi; C. Mirodatos (161-168).
The dry reforming of methane to syngas was studied in the temperature range 500–800 °C on a series of Co/SiO2 catalysts modified by MgO (5–35 wt.%). The materials have been prepared by successive incipient wetness impregnation and characterised by BET, XRD, H2-TPR, CO2 adsorption and in situ-DRIFT. The formation of a silicate adlayer Mg2SiO4 is observed at high MgO content (30–35 wt.%), which corresponds to a much improved catalytic stability under the severe dry reforming conditions. This phase favours the development of small metallic cobalt particles, preventing their coalescence under reaction conditions. A bi-functional mechanism is proposed which combines the accumulation of oxidizing agents like carbonates and hydrogeno-carbonate adspecies on the catalyst support due to a medium basicity of the layer and the reactivity of small metal particles for methane activation. This concerted process tends to limit coke formation and therefore contribute to the observed catalytic stability.
Keywords: Methane dry reforming to syngas; Co/SiO2 catalyst; MgO additive; Silicate layer;

Catalytic activity and characterization of Ni/Al2O3 and NiK/Al2O3 catalysts for CO2 methane reforming by J Juan-Juan; M.C Román-Martı́nez; M.J Illán-Gómez (169-174).
The effect of potassium in the catalytic activity and structural properties of a Ni/Al2O3 catalyst used for the CO2 reforming of methane has been analyzed. To determine the catalytic activity, two types of catalytic test were carried out: (i) temperature programmed reaction (TPR) up to 1173 K and (ii) isothermal reaction (IR) at 973 K. Several techniques were used to characterize the catalysts before and after the reforming reaction: SEM, temperature programmed oxidation (TPO), temperature programmed reduction (TPR–H2), XPS and XAFS. The presence of potassium facilitates the reduction of nickel during the activation treatment (heat treatment in H2 at 773 K, 2 h) previous to the reforming reaction, although a further activation of the potassium free catalyst takes place at the reaction condition. Ni/Al2O3 and NiK/Al2O3 catalysts show very similar CH4 and CO2 conversion at 973 K (under TPR and isothermal conditions) and the XAFS analysis of the used samples reveals a similar nickel structure. However, the whole process is different because in Ni/Al2O3 catalysts the formation of coke is much higher.
Keywords: CO2 reforming of methane; Ni/Al2O3 catalyst; K addition; Coking; XPS; XAFS; SEM; TPR–H2; TPO;

Alumina was synthesized by using the sol–gel method. In order to study the aging effect gels, they were aged for 1, 14, 30, and 50 days. Samples were characterized with X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) with pyridine adsorption, nitrogen physisorption, and by their catalytic activity for cyclohexene isomerization. XRD of boehmite samples did not show the (0 2 0) reflection, indicating that the double layers characteristic of its structure did not correlate between each other, and the unit cell was not built. The diffraction pattern showed two new atom ordering at low angle regions corresponding to distances of 1.3 and 3 nm. When samples were transformed into γ-alumina the atom ordering of 3 nm was maintained in all samples. In the one aged for 30 days, two additional diffraction peaks were observed, which correspond to distances between 5 and 6 nm. The specific surface area of the samples aged for 1, 14, and 30 days remained around 350 m2/g. This area, however, was 307 m2/g for the sample aged for 50 days. The highest catalytic activity for cyclohexene isomerization was obtained with the sample aged for 30 days, whereas the lowest activity was obtained with the sample aged for 1-day. The intrinsic activity correlated linearly with the Lewis acid site density, which at the same time correlated with the pore volume and size and the largest atom ordering distance, suggesting that the catalytic cyclohexene isomerization reaction on aluminas depends on the Lewis acid site density, which is favored with the aging time treatment.
Keywords: Aging treatment; Low angle XRD measurements; Textural properties; γ-Alumina acidity; Cyclohexene reaction test;

Bifunctional platinum over tungsten oxide promoted zirconia (Pt/WZ) and platinum over sulfate promoted zirconia (Pt/SZ) were characterized by XRD, Sg, ICP-OES, DRIFTS and test reactions: cyclohexane dehydrogenation (metallic function) and n-pentane isomerization (acid function). Both reactions were carried out at 300 °C, atmospheric pressure and in the presence of hydrogen. Benzene and methylcyclopentane were obtained in the cyclohexane reaction and iso-pentane was the main product during n-pentane reaction. The poisoning with carbon disulfide (CS2) and n-butylamine (BA) of the two reactions, were measured. Pt/WZ and Pt/SZ were sensitive to both poisons used. CS2 affects directly the activity of the metal function and indirectly the one of the acid function because decreases the hydrogen dissociation capacity of Pt. BA only affects the acid function.
Keywords: Pt/WO x –ZrO2; Pt/SO4 2−–ZrO2; N and S poisoning; Metal function; Acid function; Bifunctional catalysts;

Supported chromium oxide catalysts have been tested and found to be active in propane oxidative dehydrogenation (POD). The effects of various supports (γ-Al2O3, TiO2, SiO2, and MgO) and feed compositions were investigated. On the 10 wt.% chromium oxide on alumina, the propane degree of conversion was 16.7% and the selectivity to C3H6 was 54.1% at 450 °C. A sample of the catalysts was characterized using BET, XPS, and temperature-programmed reduction (TPR) techniques. A kinetics study was also performed on the catalyst. The rates of propane consumption and propylene productions were found to depend strongly on the oxygen partial pressures. Langmuir and redial-type rate expression were tested; the latter was found to give a better fit for the rate data. A model for the reaction was proposed.
Keywords: Oxidative dehydrogenation; Propane; Propylene; Chromium oxides; Kinetics; Metal oxide supports;

A direct relationship between the Fischer–Tropsch synthesis (FTS) rate and the number of surface cobalt atoms available for reaction is usually obtained. For Co/Al2O3 catalysts in particular, the site density depends on two primary factors: (1) the average size of the cobalt clusters on the support; and (2) the fraction of cobalt reduced to the metallic state.The addition of small amounts of noble metal promoters, such as Pt and Ru to cobalt alumina, may catalyze the reduction of cobalt oxide shifting the temperature of reduction of both steps (Co3O4→CoO and CoO→Co0) to lower temperatures. However, Re affects only the second step. Re is reduced at a higher temperature than Pt or Ru, and at approximately the same temperature as the first step of cobalt reduction (Co3O4→CoO). Thus, Re metal is present to catalyze only the second step.In situ extended X-ray absorption fine structure (EXAFS) at the LIII edge of Re has been used to show that there is direct contact of Re with cobalt atoms, while evidence for ReRe bonds is not observed. Even though direct atom-to-atom contact is found, temperature-programmed reduction (TPR) suggests that hydrogen spillover from the promoter to cobalt oxide clusters is important for the catalysis of cobalt oxide reduction.In situ EXAFS at the K edge of Co shows that the average cobalt cluster size decreases with increasing Re loading. Re promotes reduction of smaller species which interact with the support, and therefore, for a given reduction temperature, the average cobalt metal cluster size decreases as a function of increasing Re content.After reduction at a temperature slightly above the first peak in the TPR (Co3O4→CoO), the species remaining on the surface displayed XANES spectra identical to that of CoO. After reduction at a temperature above the second broad TPR peak, XPS showed that a residual oxide species was present, with a binding energy equivalent to cobalt aluminate.
Keywords: Cobalt; Alumina; Fischer–Tropsch; Reducibility; EXAFS; XPS;

CoMo/γ-Al2O3 catalysts for hydrodesulphurisation activity were prepared by making use of the molecular designed dispersion (MDD) method. Molybdenum and cobalt pyrrolidine-N-carbodithioate (Pydtc) complexes were used for the incorporation of metals on the support. The catalysts were characterized by elemental analysis, low temperature oxygen chemisorption, temperature programmed reduction (TPR) and laser Raman spectroscopy. The hydrodesulphurisation activity of all the catalysts were carried out and results were compared with those of the catalysts prepared through the conventional method. Higher molybdenum dispersion, smaller molybdenum clusters, lower reduction temperature of catalyst and better hydrodesulphurisation activity were observed for the catalysts prepared through the MDD method.
Keywords: Molecular designed dispersion; Hydrodesulphurisation; Pyrrolidine-N-carbodithioate;

Vapor phase synthesis of 2,6-bis(4-methylpheny)pyridine (2,6-BP), an important drug intermediate, was carried out by the cyclization of 4-methyl acetophenone (4-MAP), ethanol, formaldehyde and ammonia over different classes of molecular sieves, viz. HY, HZSM-5, Hβ and Al-MCM-41. The predominant product of cyclization 2,6-BP was obtained on Al-MCM-41with high selectivity. To understand the mechanism and to identify the suitable acid sites, we further studied the reaction by varying the Si/Al ratio and feed composition on Al-MCM-41. A plausible mechanism leading to the formation of 2,6-BP along with the other major by-products 2-(4-methylphenyl)pyridine (2-P) and 2,6-bis(4-methylphenyl)4-methyl pyridine (2,6-MBP), is proposed.
Keywords: Zeolites; Al-MCM-41; Cyclization; Aryl pyridines;

Surface modification of zeolite Y and mechanism for reducing naphtha olefin formation in catalytic cracking reaction by Conghua Liu; Xionghou Gao; Zhongdong Zhang; Haitao Zhang; Shuhong Sun; Youquan Deng (225-228).
After modifications of rare earth and phosphorus, the acidity density and the strength in the pores of zeolite Y (PREUSY) are improved effectively; the surface acidity is suitably reduced and weakened owing to the interaction of rare earth cation and phosphate anion. NH3-TPD analysis showed that the acid distribution of the modified zeolite is more concentrated on the range of intermediate and strong acidity. This kind of modification can direct more hydrocarbons to enter into the pores to be converted and remarkably reduces the possibility of naphtha olefins forming through a surface cracking reaction. In addition, because naphtha olefin reduction does not completely depend on olefin saturation through secondary hydrogen transfer reaction, this kind of reaction mode can decrease the excessive cracking of mediate distillate and can improve the diesel oil yield (LCO) effectively. Some reaction pathways were proposed.
Keywords: Zeolite Y; Rare earth; Phosphorus; Catalytic cracking reaction; Surface modification; Reducing olefin formation; Diesel oil yield; Hydrogen transfer activity;

Involvement of solid acid on Al- and Ga-doped porous silica in the Diels–Alder reaction by Atsushi Satsuma; Yoshihide Segawa; Hisao Yoshida; Tadashi Hattori (229-236).
The Diels–Alder reaction of 2,3-dimethylbutadiene with p-benzoquinone using Al- and Ga-doped porous silica, i.e., MCM-41, zeolite, and amorphous silica, was studied focusing on individual effects of types, strength, and amount of acid sites and pore size of solid acid catalysts. MCM-41 and amorphous silica-based catalysts exhibited high activity for the Diels–Alder reaction, while zeolites only showed low activity. Comparison with IR spectra of adsorbed pyridine clearly indicates that solid Lewis acid sites are effective for the reaction. Turnover numbers of each catalyst were compared based on the number of total acid sites measured by NH3-temperature programmed desorption (NH3-TPD). Al-MCM-41 and Ga-MCM-41 showed higher turnover numbers than that of AlCl3, which is a typical homogeneous Lewis acid catalyst. The catalysts having stronger acid sites basically showed higher turnover numbers in the Diels–Alder reaction. The effect of pore size was examined by comparison of Ga-MCM-41 and H-type mordenite (H-MOR) evacuated at 973 K. Through dehydration of H-MOR by evacuation at 973 K, Brønsted acid sites of H-MOR were converted to Lewis acid sites of which acid strength is comparable to Ga-MCM-41. Based on the number of Lewis acid sites, Ga-MCM-41 and H-MOR after evacuation at 973 K showed the same turnover number, although they have different pore sizes. These results clearly demonstrate that the catalytic activity depends on strength and amount of Lewis acid sites, while there is not much effect of pore size on the catalytic activities of the Diels–Alder reaction of 2,3-dimethylbutadiene with p-benzoquinone.
Keywords: Diels–Alder reaction; Solid Lewis acid; MCM-41; Amorphous silica;

A new polystyrene–TiO2 nanocomposite film and its photocatalytic degradation by Ling Zan; Lihong Tian; Zhongshi Liu; Zhenghe Peng (237-242).
A new kind of photodegradable polystyrene (PS)–TiO2 nanocomposite film was synthesized. The TiO2 nanoparticles were first modified by grafting polymer on its surface (G-TiO2), and then the PS–G-TiO2 composite films were formed through polymerization. The photocatalytic degradation of the composite films was investigated. The as-prepared films were characterized by scanning electron microscope (SEM), gel permeation chromatogram (GPC), FT-IR and UV-Vis spectroscopy, and the photoinduced weight-loss. The results show that PS–G-TiO2 nanocomposite films could be efficiently photocatalytically degraded under UV illumination in air. The weight-loss of the PS–G-TiO2 film reached 29%; the average molecular weight reduced to 1/4 of the original molecular weight under UV-light irradiation for 300 h, implying that some benzene rings had been cleaved. The photocatalytic degradation mechanism of the films is briefly discussed.
Keywords: TiO2; Polystyrene; Nanocomposite film; Solid-phase photocatalytic degradation;

The vapour-phase oxidation of ethylbenzene has been carried out over unwashed (U) Mn-MCM-41, Mn-AlMCM-41 (100), Mn-AlMCM-41 (150) and washed (W) Mn-MCM-41, Mn-AlMCM-41 (100) and Mn-AlMCM-41 (150) using CO2-free air as oxidant at 250–400 °C. The mesoporous structure of these catalysts was confirmed by XRD technique. Surface area, pore size and wall thickness were calculated from BET equation and BJH method using nitrogen sorption technique. FT-IR studies showed that Si and Al ions were incorporated into the hexagonal mesoporous structures of MCM-41 and AlMCM-41. The thermal stability of the as-synthesised materials was studied using thermogravimetric–diffenential thermal analysis (TG–DTA); diffuse reflectance (DR) UV-Vis spectroscopy confirmed that manganese ion could be isolated in framework positions of the MCM-41 mesostructure by this approach. Mn(II) species with a well-resolved sextet centred at g=2.0 has distorted octahedral symmetry and is observed in hydrated impregnated Mn-MCM-41 and Mn-AlMCM-41. Reaction conditions were optimised for oxidation by varying temperature, weight hourly space velocity and time on stream. The major products were acetophenone, benzaldehyde and styrene. The yield of acetophenone, which is the expected product in this investigation, increases with increase in the metal content of the catalyst. Among the six catalysts studied, ethylbenzene conversion and yield of acetophenone are higher over Mn-MCM-41 (U) catalyst with more metal content.
Keywords: Mn-MCM-41; Oxidation; Ethylbenzene; Acetophenone; Benzaldehyde;

An improved process for the synthesis of 2,6-di-tert-butylphenol (2,6-DTBP) is based on an older patent, which describes the alkylation of 2-tert-butylphenol (2-TBP) with isobutene in the presence of tris(2-tert-butylphenolate)-aluminium at low temperatures and under low pressure. The traditional mechanism of an acid–catalysed alkylation of phenols with alkenes, catalysed by aluminium tris-(phenolate) between 100 and 125 °C at pressures up to 25 bar, does not allow an explanation of several differences in the behaviour of the two catalysts, for example, the different temperature dependence of their activity and selectivity. These phenomena can be explained on the basis of a proposed new mechanism. It is assumed that a monomeric, coordinatively unsaturated aluminium tris-(2-tert-butylphenolate) complex is responsible for the alkylation. During the catalytic cycle this complex is transformed into an aluminium bis(2-tert-butylphenolate)-(2,6-di-tert-butylphenolate) complex, followed by the expulsion of the more voluminous 2,6-di-tert-butylphenolate ligand by 2-tert-butylphenol. As long as an excess of the latter is present, the initial catalyst will form again. By using these ideas and working with low amounts of catalyst at low temperatures in the presence of an excess of isobutene or of selected aliphatic solvents, improvements in yield and selectivity of the 2,6-di-tert-butylphenol-formation could be achieved. Lowering the amount of catalyst was associated with higher conversion of 2-tert-butylphenol, reduction of production costs, formation of smaller amounts of by-products as well as diminution of the environmental problems. The latter are caused by the need to remove aluminium- and phenol-containing residues. These are formed during the decomposition of the catalyst, which is necessary before the distillative work-up of the reaction mixtures can take place.
Keywords: 2,6-Di-tert-butylphenol-synthesis; Aluminium tris-(2-tert-butylphenolate) catalyst; Homogeneous catalysis; Improvements in yield and selectivity by choice of catalyst-concentration; Low reaction-temperature; Pressure and dilution;