V. Borau

Influence of the preparation method on the structural and surface properties of various magnesium oxides and their catalytic activity in the Meerwein–Ponndorf–Verley reaction

Abstract The Meerwein–Ponndorf–Verley (MPV) reaction of cyclohexanone with isopropyl alcohol, which yields cyclohexanol, was studied by using basic catalysts consisting of magnesium oxide. The oxides were prepared using various synthetic procedures including calcination of commercially available magnesium hydroxide and carbonate, calcination of magnesium hydroxides obtained from magnesium nitrate and sulphate, the sol–gel technique and precipitation by decomposition of urea. The solids thus obtained were characterized structurally by X-ray diffraction spectroscopy and their surface properties determined using gas adsorption measurements (of nitrogen for textural properties and carbon dioxide for basic properties). Based on the results, the most active catalyst was the solid prepared by rehydration and subsequent calcination of a magnesium oxide that was previously obtained from commercially available magnesium hydroxide.

Influence of the reaction conditions and catalytic properties on the liquid-phase hydrodebromination of bromobenzene over palladium supported catalysts: activity and deactivation

The liquid-phase hydrodebromination of bromobenzene with molecular hydrogen over palladium supported catalysts was studied. The reaction was found to be structure-sensitive as regards both the initial activity and the final conversion (deactivation). The influence of the palladium content and the catalytic support effect on the initial activity were negligible in catalysts of similar metal dispersion. The supports tested, viz. SiO2/AlPO4, ZrO2 and MgO, significantly affected the catalyst deactivation. On the other hand, the addition of NaOH to the reaction medium had no effect on the initial activity, but clearly helped the reaction to be completed and avoided passivation of the catalyst. In summary, the activity of the catalyst depends almost solely on its metal particle size, whereas the deactivation of the active phase is decisively influenced in addition to metal particle size, by the nature of the catalyst support and the presence of a base in the reaction medium.

Thermal decomposition of Mg/Al and Mg/Ga layered-double hydroxides : a spectroscopic study

An Mg/Al and a Mg/Ga layered double hydroxide (LDH) were synthesized by using the coprecipitation method and calcined at a variety of temperatures. The starting LDHs and their calcination products (mixtures of oxides and spinels) were characterized by X-ray diffraction, solid-state NMR and IR spectroscopies and thermogravimetric analysis. Based on the results, the LDHs studied possess a brucite-like structure where tervalent cations occupy octahedral positions. At moderate temperatures, calcination gives a mixture of oxides; at high temperatures, however, the corresponding spinels are obtained.

Epoxidation of limonene over hydrotalcite-like compounds with hydrogen peroxide in the presence of nitriles

This paper reports the synthesis of an Mg/Al double layered hydroxide (LDH) by coprecipitation, and of its dodecylsulphate (DS) and dodecylbenzenesulphonate (DBS) intercalates by rehydration. The resulting LDHs, also known as hydrotalcite-like compounds, were characterized by using various instrumental techniques including X-ray diffraction (XRD), IR spectroscopy and solid-state NMR spectroscopy. The solids obtained were tested as catalysts in the epoxidation of limonene with hydrogen peroxide. Because the presence of a nitrile was found to be indispensable for the reaction to develop to an acceptable extent, the influence of various nitriles on catalytic activity was examined. The epoxidation reaction yielded limonene oxide.

Liquid-phase hydrodehalogenation of substituted chlorobenzenes over palladium supported catalysts

Abstract The liquid-phase hydrodehalogenation of substituted chlorobenzenes has been carried out over palladium supported on AlPO4–SiO2. The hydrodehalogenation of the p-chlorobromobenzene undergo a consecutive reaction starting with the CBr hydrogenolysis. For the other substituted chlorobenzenes, the reaction proceeds through the hydrogenolysis of the CCl bond. The reaction rate obtained for the substrates with electron-withdrawing substituents is lower than that of chlorobenzene while for the substrates with electron-releasing groups it is higher. This is related to a modification of the adsorption strength of the molecule and the surface reaction trough the inductive and mesomeric effects of the substituents. The Taft equation is used to correlate the hydrodechlorination catalytic activity of the p-substituted chlorobenzenes with inductive and mesomeric parameters of the substituents. These results seem to indicate that the reaction proceeds through an electrophilic attack.

Comparison of different organic test reactions over acid–base catalysts

Abstract Three test reactions were studied: transformations of 1-phenylethanol (R 1 ), 2-propanol (R 2 ), and 2-methyl-3-butyn-2-ol (R 3 ) over several oxide catalysts. Results were correlated to acid–base properties in these solids, calculated by using temperature programmed desorption–mass spectrometry (TPD–MS) techniques with pyridine, 2,6-dimethylpyridine and carbon dioxide as probe molecules described elsewhere. The three reactions lead to concordant results on acid–base characteristic of catalysts. R 1 and R 2 test reactions are less complicated (less products) than R 3 . However, the latter follows specific ways depending on our catalyst having acid, basic or acid–base pair sites, whereas R 1 and R 2 are less specific.

Catalytic hydrogen transfer from 2-propanol to cyclohexanone over basic Mg–Al oxides

Abstract Magnesium–aluminium mixed oxides in mole ratios from 2 to 4 were obtained by thermal decomposition of layered double hydroxides (LDHs) as precursors. The effect of the Mg/Al ratio on the structure and surface properties of the resulting oxides was studied by using various instrumental techniques including X-ray diffraction (XRD), thermogravimetric analyses (TGA), MAS NMR, X-ray photoelectron spectroscopy (XPS) and gas adsorption–desorption, and so was their influence on the catalytic activity of the solids in the hydrogen transfer from 2-propanol to cyclohexanone. The results were compared with those for a pure MgO oxide and the Mg–Al mixed oxide obtained from the LDH with an Mg/Al ratio of 2 was found to provide the best catalytic activity. The influence on catalytic activity of other experimental variables including the pH of the medium used to synthesize the precursor LDHs, the temperature employed to prepare the Mg–Al oxides and the amount of catalyst used was also examined. Catalyst recycling tests were also conducted. Based on the results, a mechanism is proposed for the overall catalytic hydrogen transfer.

Liquid-phase heterogeneous catalytic transfer hydrogenation of citral on basic catalysts

Abstract The liquid-phase catalytic transfer hydrogenation of citral with 2-alkanols, 3-alkanols and cycloalkanols on basic catalysts obtained from Mg(OH)2 and Ca(OH)2 was studied. The catalysts were characterised by X-ray diffraction, and by adsorption of nitrogen and chemisorption of carbon dioxide. Both catalysts provided excellent results in terms of catalytic activity and selectivity in the reduction of citral with all the alcohols tested.

Preparation of Pd/AlPO4, Pd/AlPO4-SiO2, and Pd/AlPO4-γ-Al2O3 and study of their catalytic activity for the reduction of nitrobenzene by hydrogen transfer

Abstract The preparation and catalytic activity for the hydrogen-transfer reduction of nitrobenzene of new metallic systems obtained by supporting palladium on AlPO 4 , AlPO 4 -SiO 2 , and AlPO 4 -γ-Al 2 O 3 are reported. Metal dispersion decreases in the order Pd/AlPO 4 -SiO 2 > Pd/AlPO 4 > Pd/AlPO 4 -γ-Al 2 O 3 , and, according to the method of preparation, cationic exchange > impregnation > anionic exchange. Catalysts prepared by cationic exchange on AlPO 4 -SiO 2 show high dispersion values ( D = 0.63, d = 1.8 nm mean particle size). The catalysts have different activities for the reduction of nitrobenzene to aniline at the reflux temperature of the nitrobenzene-cyclohexene mixture (383 K), with a selectivity near 100% and without poisoning. No influence of the acidity of the different metal systems was found, the activity depending only on the dispersion of the active metal phase.

Reduction of acetophenone with palladium catalysts by hydrogen transfer and with molecular hydrogen

Abstract The activities of various 3% palladium catalysts supported on a Spanish sepiolite and on different SiO 2 AlPO 4 (80:20 w/w) systems were compared in the reduction of acetophenone (i) by hydrogen transfer with cyclohexene as an hydrogen donor and a reflux system, (ii) by hydrogen transfer in a closed system and (iii) with hydrogen gas in a Parr system. The influence of the addition of hydrochloric acid, iron (III) chloride and sodium hydroxide on the reduction of acetophenone by hydrogen transfer was established and the reduction of differently substituted acetophenones (bearing various electron-withdrawing and electron-releasing substituents) was studied.