Hiroo Niiyama

Textural properties of layered double hydroxides: effect of magnesium substitution by copper or iron

Abstract Layered double hydroxides (LDHs), in which magnesium was partially substituted by iron or copper, were synthesized by a coprecipitation method. The materials were characterized by X-ray diffraction, X-ray fluorescence, FT-IR spectroscopy, N2 adsorption, scanning electron microscopy and transmission electron microscopy. The treatment of the adsorption–desorption isotherms with different computation models permitted the determination of several parameters, useful for thorough characterization of the microstructures of the samples. The substituted samples preserved the hydrotalcite-type layered structure though their textural properties underwent important modifications. For copper substituted LDH, a relatively uniform porous structure with emphasized mesoporous characteristics emerged, the mesopores were enlarged and the specific surface area was decreased. On the contrary, for iron substituted LDH, the microporosity features developed, nonuniformity and constrictions in the porous structure was accentuated, the pore size was decreased and the specific surface area was increased. Microscopic morphology characteristics contributed to establish the textural properties of the samples.

Mg-V-Al mixed oxides with mesoporous properties using layered double hydroxides as precursors: catalytic behavior for the process of ethylbenzene dehydrogenation to styrene under a carbon dioxide flow

A new catalyst for the process of ethylbenzene dehydrogenation to styrene under a CO2 atmosphere was obtained using vanadium-substituted layered double hydroxides as precursors. After calcination, vanadium-substituted hydrotalcite-like samples gave mixed oxides with a high surface area and strong mesoporous characteristics; the XRD analysis indicates the formation of Mg3V2O8 and Mg2Al2O4 in bulk while the XPS results point to the presence of a mixture of V5+ and V3+ ions on the surface. Catalytic tests show that the styrene yield and selectivity increase when the vanadium content of the sample increases. The higher catalytic activity is attributed to the formation of V5+ on the surface that is promoted by the presence of CO2 as a cofeed gas. The FTIR adsorption of pyridine shows that aluminum contributes to Lewis-type acidity of the catalyst and is important for establishing features of the catalyst deactivation process.

Dehydration of alcohols on/in heteropoly compounds

Abstract The dehydration of methanol, ethanol, 1-propanol, and 1-butanol were studied using 12-tung-stophosphates as acid catalysts, Even “neutral” salts displayed acid-catalytic properties. A good correlation between acidic strength of the salts and their catalytic activity was established. However, several abnormal acid-catalytic behaviors were observed, namely, change of activity with process time, conversion vs W/F relationship, and reactivity sequence of various alcohols. In particular, the alcohols reactivity under high partial pressures was found in the order: MeOH > EtOH > 1-PrOH > 1-BuOH, while a reverse sequence was obtained under low partial pressures of alcohols. These results were reasonably interpreted in view of a pseudo-liquid phase catalysis model.

Copper and iron substituted hydrotalcites: properties and catalyst precursors for methylamines synthesis

Abstract Layered double hydroxides (LDHs) in which magnesium was partially substituted by copper or iron in the hydrotalcite-like layer were synthesized by a coprecipitation method. The effects of the substitution on the physicochemical properties of the samples were studied by using various techniques such as X-ray diffraction (XRD), X-ray fluorescence (XRF), N2 adsorption, scanning electron microscopy (SEM), thermogravimetry (TGA), X-ray photoelectron spectroscopy (XPS). After the thermal treatment the samples were tested as catalysts for the process of methylamines (MAs) synthesis from methanol and ammonia. The catalytic behaviors of the substituted samples are defined by a non-equilibrium distribution of MAs with monomethylamine (MMA) the favored product for the catalytic sample derived from CuLDH and dimethylamine (DMA) the main product for the catalyst derived from FeLDH. Reduction increases the selectivity to lower substituted amines: at 85% conversion of methanol the MMA contribution in the MAs distribution is 67% for the reduced catalytic sample containing copper whereas the DMA contribution is nearly 78% for the reduced catalytic sample containing iron. On the contrary when MgAlLDH is used as a catalyst precursor trimethylamine (TMA) has the main contribution in the characteristic MAs distribution. The nature of the substituted metal and the bifunctionality of the samples contribute in establishing the catalytic properties for the process of MAs synthesis from methanol and ammonia.

Reaction mechanism of ethanol dehydration on/in heteropoly compounds: analysis of transient behavior based on pseudo-liquid catalysis model

Abstract The transient behavior of ethanol dehydration on/in heteropoly compounds was studied. Ethylene formation lasted for a long time after the ethanol feed had stopped, while ether formation decreased monotonously with time. A reaction model which includes the participation of sorbed alcohol was proposed: Ethylene formation occurs through a unimolecular decomposition of ethanol, while ether formation takes place through a coupling of chemically activated ethanol with the physically sorbed one. Both reactions occur in the bulk phase of the compounds. Transient curves as well as kinetic behavior under the stationary reaction conditions could be satisfactorily simulated by this model for both products.

The mechanism of the reaction of nitrogen oxide with ammonia over Cr2O3Al2O3 and Cr2O3 catalysts: II. Isotope labeling studies☆

Abstract The reaction of NO with 15 NH 3 was studied over 9.3% Cr 2 O 3 Al 2 O 3 and Cr 2 O 3 as catalyst; relevant properties characterizing these oxides are described in Part I (preceding paper). The main isotope species in the products, nitrogen and nitrous oxide, were 15 NN and NNO when the reaction proceeds in a stationary state. A noncatalytic reaction of NO with prereduced surface to give NN was also significant, especially in the beginning. When the catalyst was preoxidized, the reaction was greatly enhanced and a large amount of 15 NNO was found. It is concluded that the reaction takes place on surface oxygens.

The mechanism of the reaction of nitrogen oxide with ammonia over Cr2O3Al2O3 and Cr2O3 catalysts: I. Characterization of the catalysts

Abstract Catalytic activities of various Cr 2 O 3 Al 2 O 3 catalysts for the reaction of nitrogen oxide (NO) with ammonia (NH 3 ) were determined. The activity, when plotted against Cr contents, showed two maxima, at 9.3% Cr and 100% Cr, respectively. X-Ray examination showed that γ-phase and α-phase solid solution occurred at low and high Cr contents, respectively. The BET area, reducibility and the amount of irreversibly adsorbed NO also showed their maxima at 9.3% Cr. Such properties explain, at least partially, the activity pattern in the low Crcontent region.

Adsorption of pyridine and ammonia on heteropoly compounds and catalytic activity in the methyl t-butyl ether synthesis reaction

Abstract The amounts of pyridine and ammonia adsorbed on heteropoly compounds and the adsorbed state of these basic molecules were investigated; the methyl t-butyl ether synthesis from t-butanol and methanol was also studied to examine the effect of the acidity of heteropoly compounds on the acidic reaction. The amount of pyridine adsorbed on heteropoly compounds at 80°C displayed a qualitative relationship with catalytic activity in the synthesis methyl t-butyl ether: ammonia failed to show such a relationship. For heteropoly compounds, pyridine could be adsorbed on acid sites and also as a pseudo-liquid phase. Most of the pyridine adsorbed on heteropoly compounds was adsorbed on acid sites, while the pyridine adsorbed as a liquid phase was almost removed when evacuated at 80°C. Ammonia was adsorbed on acid sites as well as on metal cations, and it remained on the adsorbed sites even after evacuation at 200°C.

Infrared and temperature programmed desorption studies of nitric oxide adsorbed on cobalt-silica catalyst

Abstract Infrared studies of adsorbed nitric oxide (NO) on cobalt-silica catalyst were carried out. Two species were found, with N-O stretching frequencies at 1790 and 1850 cm−1, respectively. The former arose from strongly chemisorbed species and was the intermediate species in the reaction between NO and reduced surface. Bases such as NH3, NH2CH3, NH(CH3)2 and N(CH3)3 resulted in a lowering of the N-O stretching frequencies of preadsorbed NO, and thus a weakening of the NO bond. On the other hand, HCl shifted the bands to higher frequencies and also facilitated desorption. These effects were explained on the basis of a change in the electron density of metal-N-O molecular orbitals when the second gases were admitted.

Effect of diffusion and adsorption–desorption on periodic operation performance of NO–CO reaction over supported noble metal catalysts

Abstract Effects of intrapellet diffusion and of adsorption along a plug-flow reactor on periodic operation performance of NO–CO reaction over Pt/Al 2 O 3 and Rh/Al 2 O 3 were investigated by reaction experiments and by model calculations. The model calculation for the effect of intrapellet diffusion on the time-averaged conversion of NO showed that the effect was significant under periodic operation at cycling periods longer than several tens of seconds in the case of strong adsorption even when it had no significant effects on steady-state performance. A two-stage catalyst bed which consisted of Pt/Al 2 O 3 at the inlet- and Rh/Al 2 O 3 at the outlet-side attained a higher time-averaged conversion of NO than a similar one but with the reverse placement. Model calculations for the effect of adsorption–desorption along the catalyst bed showed that the result described was caused by deformation of concentration cycling pattern due to strong adsorption of NO and CO. Calculated breakthrough curves could simulate the experimental ones quite satisfactorily.