C.R. Apesteguía

Base catalysis for the synthesis of α,β-unsaturated ketones from the vapor-phase aldol condensation of acetone

Abstract The vapor-phase aldol condensation of acetone was studied over MgO promoted with 0.7–1.0 wt.-% of alkali (Li, Na, K and Cs) or alkaline earth (Ca, Sr and Ba) metal ions. The basic properties of the samples were characterized by chemisorption of carbon dioxide. The basicity of MgO increased on addition of the promoter following the basicity order of the promoter oxide: the stronger the electron donor properties of the promoter, the greater the generation of surface basic sites. Major reaction products were mesityl oxide (MO), isomesityl oxide (IMO) and isophorone (IP). The selectivity to (MO + IMO + IP) over unpromoted MgO was practically 100%, thereby showing that magnesium oxide is suitable for selectively obtaining α,β-unsaturated ketones. The reaction was totally inhibited by co-feeding acetic acid along with acetone whereas the co-injection of pyridine did not affect the acetone conversion. This indicated that the self-condensation of acetone over MgO-based catalysts is catalyzed by basic sites. The promoter addition increased the activity of the MgO catalyst and a good correlation was obtained between catalyst activity and the concentration of basic sites. Such a proportionality between activity and surface basicity was an additional evidence that the rate-determining step in the aldol condensation mechanism is controlled by the surface base property. All the catalysts exhibited similar IP/(IMO+MO) selectivity ratio, except the Li/MgO sample which produced substantially larger amounts of isophorone. Because the tricondensation of acetone to give isophorone requires strong basic sites, the higher selectivity toward isophorone was indicative of the presence of stronger surface basic sites in the Li/MgO sample. Results from carbon dioxide chemisorption confirmed that Li/MgO exhibited the strongest basic properties. The generation of high-strength basic sites was explained by assuming that the addition of lithium causes a structural promotion of the MgO sample by replacing the Mg 2+ ions by Li + in the MgO lattice. The replacement would result in strained Mg O bonds and formation of [Li + O − ] species, which causes the generation of stronger basic sites.

Activity and structure-sensitivity of the water-gas shift reaction over CuZnAl mixed oxide catalysts

Abstract The activity and structure-sensitivity of the water-gas shift (WGS) reaction over Cu Zn Al mixed oxide catalysts were studied. Three sets of samples with different Cu/Zn and (Cu+Zn)/Al atomic ratios were prepared by coprecipitation. Depending on the cation ratio, the ternary hydroxycarbonate precursors contained hydrotalcite, aurichalcite and/or rosasite phases. The decomposed precursors contained CuO, ZnO, ZnAl 2 O 4 , and Al 2 O 3 . The relative proportion of these phases depended on both the chemical composition of the sample and the calcination temperature employed for decomposing the precursor. After activation with hydrogen, samples were tested for the WGS reaction at 503 K. The turnover frequency of the eighteen samples tested was essentially the same (0.2–0.3 s −1 ) irrespective of changing the copper metal surface area between 3 and 35 m 2 /g Cu and the metallic copper dispersion between 0.5 and 5.0%. This indicated that the WGS reaction is a structure-insensitive reaction, as the specific reaction rate r 0 (mol CO/h/g Cu) is always proportional to the copper metal surface area. Preparation of mixed oxides with a high copper dispersion is therefore required for obtaining more active catalysts. It was found that the value of the metallic copper dispersion is related to the amount of hydrotalcite contained in the hydroxycarbonate precursor: the higher the hydrotalcite content in the precursor, the higher the copper metal dispersion in the resulting catalyst and, as a consequence, the higher the catalyst activity. Ternary Cu/ZnO/Al 2 O 3 catalysts exhibited a substantially faster WGS activity than binary Cu/ZnO catalysts. The addition of aluminium, although inactive for the WGS reaction, is required for improving the catalyst performance.

Acid-base properties and active site requirements for elimination reactions on alkali-promoted MgO catalysts

Fil: Diez, Veronica Karina. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Santa Fe. Instituto de Investigaciones en Catalisis y Petroquimica ; Argentina

Oxidative catalytic removal of hydrocarbons over Pt/Al2O3 catalysts

The reaction kinetics, structure sensitivity, and in situ activation of cyclopentane and methane combustions were studied on Pt/Al2O3 catalysts of different platinum and chlorine loadings. The catalyst activities were evaluated through both conversion vs. temperature (light-off curves) and conversion vs. time catalytic tests. Cyclopentane oxidation turnover rates (TOF) increased dramatically with increasing Pt crystallite size while TOF values for methane oxidation increased only three times by diminishing the Pt dispersion from 65 to 15%. The reaction orders in oxygen were one (cyclopentane) and zero (CH4). For both reactions, the orders and activation energies did not change by changing the Pt dispersion. Results are interpreted in basis of two different reaction mechanisms over the metallic Pt active sites. Cyclopentane oxidation proceeds via a surface redox mechanism, being the dissociative adsorption of oxygen the rate-determining step. The observed turnover rate increase with increasing Pt particle size reflects an increase in the density of reactive Pt–O species resulting from higher Pt oxidation rates. The methane oxidation mechanism is interpreted in terms of Mars–van Kravelen reduction–oxidation pathways which include the abstraction of the first hydrogen on the adsorbed methane molecule as the rate-determining step. Low-conversion catalytic tests performed at constant temperature showed that on well-dispersed Pt/Al2O3 catalysts the cyclopentane conversion increases with time on stream, while the methane activity decreases. Activating induction periods during the oxidation of cyclopentane are related to the sintering of the metallic phase in reaction conditions. Hot-spots on the metallic particles together with the presence of gaseous water cause the formation of larger, more reactive, Pt crystallites, even at mild reaction conditions. The activation phenomenon ab initio of the reaction is not verified for methane oxidation on Pt/Al2O3 catalysts. The different structure sensitivity of the slowest steps in the reaction–oxidation mechanisms explains the existence of induction periods on well-dispersed Pt catalysts only for cyclopentane oxidation.

Kinetic study of the reverse water-gas shift reaction over CuO/ZnO/Al2O3 catalysts

The kinetics of the reverse water-gas shift (RWGS) reaction over CuO/ZnO/Al2O3 catalysts was studied by use of CO2H2 cycles, hydrogen chemisorption and catalytic tests performed in both differential and integral plug flow reactors. The effect of the reactant composition on the reaction rate was specifically studied by changing the PH20/PCO20 ratio between 9.0 and 0.3. It was found that different reagents become rate limiting depending upon pressure. While in a H2-rich region the rate increases strongly with CO2 partial pressure and is zero order in hydrogen, under low PH20/PCO20 ratios the reaction is less active and is strongly positive order in hydrogen and low order in carbon dioxide. The experimental data were modeled by considering that the reaction proceeds through a surface redox mechanism, copper being the active metal. A good agreement between experimental and calculated data was obtained by assuming that in the redox mechanism either the dissociative CO2 adsorption (H2-rich region) or both the CO2 dissociation and the water formation (H2-lean region) determine the rate of the overall reaction. Based on previous studies performed on copper crystal surfaces, such a change in kinetics may be explained by assuming that under H2-rich atmosphere a surface structural or phase transition occurs involving a change in reactivity with respect to CO2 dissociation.

Structure sensitivity and in situ activation of benzene combustion on Pt/Al2O3 catalysts

Abstract The structure sensitivity and in situ activation of benzene combustion on Pt/Al 2 O 3 catalysts of different platinum and chlorine loadings were studied. The catalyst activities were evaluated through both conversion versus temperature (light-off curves) and conversion versus time catalytic tests. The light-off curves shifted to lower temperature with increasing Pt particle size, thereby suggesting that benzene combustion is a structure sensitive reaction. Kinetically-controlled catalytic tests confirmed that benzene oxidation turnover rates are preferentially promoted by larger Pt crystallites. Kinetic studies showed that the reaction orders and the apparent activation energy are not changed by changing the metallic dispersion. Results are explained by considering that benzene oxidation proceeds via a Langmuir–Hinshelwood mechanism which involves the rapid and strong adsorption of benzene on metallic platinum and assumes that the rate constant of oxygen adsorption is very low compared to the rate constant of the surface reaction. The number of PtO bonds of lower binding energy, i.e. the site density of more reactive surface oxygen, increases on larger Pt particles. Low-conversion catalytic tests performed at constant temperature showed that on well-dispersed Pt/Al 2 O 3 catalysts the benzene conversion increases with time, irrespective of the chlorine content on the sample. TEM examination and hydrogen chemisorption measurements suggested that the activity increase parallels a concomitant increase in the platinum particle size. In contrast, sintered samples (platinum dispersions lower than 10%) did not exhibit initial activation periods. It is proposed that the initial in situ activation of well-dispersed Pt catalysts is caused by the sintering of the metallic phase. Hot-spots on the metallic particles together with the presence of gaseous water cause the formation of larger Pt crystallites, even at mild reaction conditions. As a result, the benzene conversion increases with time until the formation of larger steady state Pt particles is completed.

Impregnation-induced memory effect of thermally activated layered double hydroxides

Abstract The memory effect of calcined Cu–Co–Zn–Al layered double hydroxides (LDH) upon impregnation with K-containing solutions was studied. A sample of nominal composition CuCoZn 2 Al 4 O x was prepared by coprecipitation at controlled pH. The resulting hydrotalcite-like (HT) precipitate was calcined at 673, 773, or 873 K. The products were impregnated with different K-containing solutions [K 2 CO 3(aq) , KOH (aq) and KOH (alc) ]. Reconstruction of the HT phase upon impregnation was followed by X-ray diffraction. The HT recrystallization was favored by impregnating calcined LDH samples with K 2 CO 3(aq) by wet impregnation. The memory effect was reduced by increasing the calcination temperature of the parent layered double hydroxide. It is proposed that the reconstruction of the HT phase occurs via a retro-topotactic transformation from the Al 3+ and divalent cations located both in the octahedral sites of the oxide matrix. The increase of the calcination temperature causes the solid-state diffusion of divalent cations into tetrahedral positions which results in the progressive formation of stable normal spinels. After calcination at 873 K, all the divalent cations occupy tetrahedral sites and the memory effect disappears.

Study of the catalyst deactivation in the base-catalyzed oligomerization of acetone

The deactivation of unpromoted MgO and alkali-promoted MgO catalysts in the vapor-phase self-condensation of acetone was studied. The reaction was catalyzed by basic sites and major products were mesityl oxide, isomesityl oxide and isophorone. Catalysts deactivated because of coke formation. Both the initial catalyst deactivation (d0, h−1) and the product distribution depended on contact time (W/F0): d0, and the selectivity to mesityl oxide increased when W/F0 was increased. It is proposed that non-cyclic trimers, such as phorone, which are produced by aldol condensation of mesityl oxide with acetone, are the key intermediate species for coke formation. These non-cyclic trimers are highly unsaturated compounds that remain strongly bound to the catalyst surface yielding higher non-volatile oligomeric compounds which block basic active sites. Promotion of MgO with alkaline metal ions increased the d0 value measured on unpromoted MgO following the sequence Li<Na<K<Cs: the stronger the promoter oxide basicity, the higher the catalyst deactivation. Enhancement of the MgO basicity by alkali addition strengthens the interaction between the solid surface and non-cyclic trimers, and improves the catalyst ability for abstracting the α-proton from the acetone molecule. As a consequence, the aldol condensation synthesis of tetramers and heavier polymers is favored which results in an increasing coke formation.

On the sulfur-aided metal-support interaction in PtAl2O3Cl catalysts

The SMSI behavior displayed by sulfated PtAl2O3Cl catalysts with relatively low sulfur content has been explained by the alternative reactions or Redox reaction (2) would be sulfur catalyzed, giving the so-called sulfur-aided SMSI. In this paper the SMSI behavior displayed by sulfided samples with only irreversibly held sulfur (SPt = 0.20–0.35) was studied by the changes in H2 and O2 chemisorption, infrared spectroscopy, temperature-programmed reduction and H2S desorption at increasing temperatures. The changes in the 1380-cm−1 band (SO2−4 on the carrier) and in the 2060-cm−1 band (CO adsorption on Pt in linear form) were used to investigate the effects of the high-temperature reduction treatment. The TPR profiles of sulfated aluminas showed only one reduction peak at 650–680 °C; the presence of Pt gave rise to a low-temperature peak which appeared at 450–500 °C. Such a peak shifted to lower temperatures when the Pt content was increased. The data showed that the SMSI behavior of sulfated PtAl2O3Cl samples is better explained in terms of redox reaction (1). During the HTR treatment Pt cata-lyzes the reduction of sulfate ions. The H2S formed is adsorbed on the metal so that the H2 chemisorption is suppressed by the S blockage of the surface. Subsequent oxidation treatment causes oxidized S species in the metal surface to migrate back to the support. The metallic fraction is regenerated and, as a consequence, the H2 chemisorption capacity is restored.

Sulfurization of PtAl2O3Cl catalysts: VI. Sulfur-platinum interaction studied by infrared spectroscopy

Sulfur-platinum interaction was studied by ir spectroscopy of the coadsorbed CO, competitive hydrogenation reactions, and chemisorption of O2, CO, and H2, PtAl2O3 catalysts of different metal loadings and mean particle size were used. Infrared spectra for variable coverages of sulfur and CO were obtained. The predeposited S on Pt caused an upward shift of v(CO) from 2068 to 2083 cm−1; a superimposed band that shifted to the lower frequencies for decreasing CO coverages was detected. The kTB values (kTB, ratio of the adsorption equilibrium constant) for the competitive hydrogenation of benzene and toluene were determined. It was found that the kTB ratio increases from 4.2 for unsulfided samples to 6.0 for sulfided samples. On the other hand, the irreversibly held sulfur on the metal increased the ratio of the reversibly adsorbed H2 to the total adsorbed H2. These results are discussed considering electronic, geometric, and surface heterogeneity effects. A “localized” modification of the metal by the electron-acceptor properties of sulfur is advanced as a possible explanation.