Didier Tichit

Catalysis by hydrotalcites and related materials

Catalytic applications of Layered Double Hydroxides (LDH) or anionic clays have for a long time focused on the condensation reactions carried out with mixed oxides obtained by calcinations of Mg/Al LDH. However, the versatile character of materials prepared from these LDH exhibiting Lewis or Brønsted type basic sites has recently been underlined, and led us to contemplate extended applications in clean industrial processes. Moreover, due to their wide variety of compositions, LDH are also attractive precursors of multicomponent nanostructured catalysts. They give rise to supported metal catalysts whose metal support interaction and cooperation can be fine-tuned, as illustrated in pertinent case studies such as the hydrogenation of nitriles and the “one-pot” synthesis of methyl isobutyl ketone from acetone.

Hydrotalcites as base catalysts. Kinetics of Claisen-Schmidt condensation, intramolecular condensation of acetonylacetone and synthesis of chalcone

Abstract The activity of hydrotalcites has been investigated by means of several reactions able to evidence their basic properties and the mechanism involved, and also to show their interest in the synthesis of fine chemicals of great practical interest for pharmaceutical applications. With this aim, the Claisen-Schmidt condensation of benzaldehyde with excess acetone has been performed in solid-liquid conditions. The kinetic study suggests a Langmuir-Hinshelwood mechanism, in which the rate-determining step would be the condensation of benzaldehyde and acetone, in agreement with former results of Hammett correlations. The reaction rate goes through a maximum as a function of calcination temperature of the catalyst, or the water content of the solvent, suggesting that the active sites are basic hydroxyls. These results are obtained by the intramolecular condensation of acetonylacetone as test reaction giving selectively 3-methyl-2-cyclopentanone, and showing, indeed, the presence of mainly basic sites on the hydrotalcite. Taking these results into account, the synthesis of chalcone has been performed over a suitably activated hydrotalcite.

Aldol condensation of acetone over layered double hydroxides of the meixnerite type

Abstract Aldol condensation of acetone is performed at 273 K in liquid phase using as catalysts Mg–Al layered double hydroxides (LDH) with Cl − and/or CO 3 2− as compensating anions. The products of the reaction are mainly diacetone alcohol and mesityl oxide. The catalytic activities increase with the Al content of the LDH and a conversion around 20% is reached when the carbonated compounds are calcined at 723 K where catalysts are mixed oxides of the Mg(Al)O type. The nature and the amount of the compensating anion in the LDH greatly influence the catalytic activity, particularly traces amount of Cl − are very detrimental. Mesityl oxide resulting from the dehydration of diacetone alcohol on acid sites strongly inhibits the reaction. By contrast, the addition in controlled amounts of H 2 O in acetone enhances the conversion with a higher selectivity into diacetone alcohol. At the same time the catalyst recovers the lamellar structure. A too high amount of H 2 O inhibits the reaction. Experiments have been performed by rehydration in vapor or in liquid phases of the mixed oxides, previously decarbonated by calcination at 723 K. With these samples, having the meixnerite-like structure, the conversion of acetone into diacetone alcohol reaches the thermodynamic equilibrium (23%) in less than 1 h. Therefore the rehydration of the mixed oxide into the meixnerite structure modifies the strength and the concentration of the active OH − sites.

Mg- and Ni-containing layered double hydroxides as soda substitutes in the aldol condensation of acetone

Abstract A series of layered double hydroxides (LDHs) containing Mg 2+ or Ni 2+ as divalent and Al 3+ or Ga 3+ as trivalent cations were used as catalysts in order to substitute NaOH in the selective condensation of acetone into diacetonealcohol. With this aim the LDHs were totally exchanged with OH − anions and acquired the meixnerite-like structure, showing a high concentration of active basic sites. These meixnerite-like structures were prepared by reconstruction under appropriate conditions of the mixed oxides obtained by calcination. The complete reconstruction was easily achieved in water at ambient temperature and pressure for Mg-containing samples, while more severe conditions were needed for the Ni-containing one, as shown by XRD, TEM and UV–VIS DR spectroscopy. The reconstruction degree of Ni/Al mixed oxides was markedly improved by hydrothermal treatments under ammonia. The nature of the cations influenced the acid–base properties of the mixed oxides, as evidenced by NH 3 and CO 2 TPD experiments, and consequently their catalytic activities. The catalytic activities of the meixnerite-like forms were related both to the basicity of the corresponding mixed oxide and to the reconstruction degree in the lamellar form, thus to the density of the OH − sites. Indeed, the loss of the lamellar structure was the main cause of the catalyst deactivation.

Supported choline hydroxide (ionic liquid) as heterogeneous catalyst for aldol condensation reactions

Choline hydroxide was used as a basic catalyst for aldol condensation reactions to produce new C-C bonds between several ketones and aldehydes. Choline supported on MgO exhibits higher TOF values than other well known basic catalysts in these reactions.

Activation under oxidizing and reducing atmospheres of Ni-containing layered double hydroxides

Layered double hydroxides (LDHs) of the Ni2+/Mg2+/Al3+ type were prepared in a large range of compositions as well as pure takovite (Ni/Al) and hydrotalcite (Mg/Al) samples. All of them have a well crystallized lamellar structure and contain NO3− and CO32− compensating anions. Their thermal stability and reducibility were followed by TG, in situ XRD, mass spectrometry, volumetry and TPR experiments. The thermal stability of the hydrotalcite is higher than for the takovite structure and increases with the Mg content in the mixed Ni2+/Mg2+/Al3+ compounds. All samples are decomposed into a mixed oxide phase of the MgO or NiO type upon calcination. On the other hand, an excess magnesium aluminate spinel-type phase is only detected in the hydrotalcite or in the Ni2+/Mg2+/Al3+ samples containing the higher amounts of Mg. NO3− decomposed in two steps at a higher temperature than CO32− species, for both anion a simultaneous water release is observed. The reducibility of Ni decreases with both the Al content of the takovite samples and with the Mg content of the Ni2+/Mg2+/Al3+ samples. Whatever the sample Ni species are less reducible when the calcination temperature increases. During TPR experiments, CO32− is decomposed to CO2 while NO3− is evolved as NO and N2O between 700 and 800 K. Above 800 K, the H2 consumption corresponds to the reduction of Ni2+ into Ni0, as shown by the XRD experiment.

Influence of the acid-base properties of solid catalysts derived from hydrotalcite-like compounds on the condensation of formaldehyde and acetaldehyde

Abstract The vapor-phase aldol condensation of formaldehyde with acetaldehyde was studied using various oxide mixtures obtained in situ from hydrotalcite-like materials as catalysts. All samples exhibited catalytic activity, but the selectivity to both the aldol condensation and the acrolein formation is governed by the balance of acidic and basic properties. The oxides containing Mg and Al or Co and Al showed a high selectivity to aldol condensation (90% at 300°C) as well as to cross-condensation (80% at 300°C). The effects of both the reaction variables (temperature, mole ratio of reagents) and the catalyst variables (nature of cations, nature of anions) were studied.

Comparison of the structural and acid–base properties of Ga- and Al-containing layered double hydroxides obtained by microwave irradiation and conventional ageing of synthesis gels

Structural and acid–base properties of Mg/Al and Mg/Ga layered double hydroxides (LDHs) obtained by microwave irradiation of the co-precipitated gels have been investigated and compared to those of samples conventionally aged by prolonged hydrothermal treatment of the gels. Similar crystallinities and chemical compositions were obtained whatever the synthesis method used. Besides, all samples, and remarkably the Ga-containing LDHs with a molar ratio Mg ∶ Ga = 4.5, exhibited pure lamellar phases. The acid–base properties of the mixed oxides obtained by calcination of the LDHs have been examined by microcalorimetric adsorption of CO2 and by FTIR spectroscopy upon CH3CN interaction. These techniques gave evidence that the number and strength of acid and basic sites were influenced by the nature and amount of the trivalent cation, as well as by the preparation method.

Comparative behaviour of extremely dispersed Pt/Mg(Al)O and Pt/Al2O3 for the chemisorption of hydrogen, CO and CO2

Abstract A Pt/MgAl(O) was prepared from Pt(acac) 2 and a Mg Al mixed oxide, synthesized from a hydrotalcite precursor, which contains extremely small Pt particles with H/Pt≈ 1.1. Its properties were compared to that of a Pt/Al 2 O 3 of a similar Pt dispersion and prepared according to exactly the same protocol. The properties of Pt were probed by H 2 and CO chemisorption followed by IR spectroscopy and microcalorimetry, and those of basic sites of Mg(Al)O by TPD of CO 2 . The surface basicity of Mg(Al)O decreases after deposition of Pt particles. When compared to Pt/Al 2 O 3 a new IR absorption band of CO appears at 1930–1960 cm −1 which was tentatively assigned to CO species bridge-bonded on Pt through the carbon atom and with Mg 2+ cations by the oxygen atom. These species are most stable upon desorption at high temperature. The disproportionation of CO to adsorbed C and CO 2 was occurring on Pt/Mg(Al)O. The CO 2 formed migrated to basic sites of the support to form carbonates species.

Selective oxidation of 5-hydroxymethylfurfural to 2,5-furan-dicarboxaldehyde in the presence of titania supported vanadia catalysts

Oxidation of 5-hydroxymethylfurfural to 2,5-furan-dicarboxaldehyde was performed in a batch reactor at 363 K in the presence of supported V 2 O 5 /TiO 2 catalysts with different vanadium loadings, and in toluene and methyl isobutyl ketone as the solvents. An air pressure of 1.6 MPa allowed the fast in situ regeneration of the catalyst and the complete transformation of the starting reactant. It appears that a multilayered V 2 O 5 /TiO 2 catalyst with a structure close to that of bulk V 2 O 5 is preferred since involving more V=O species responsible for the oxidation of alcohols. In addition, a higher turnover frequency is obtained in the presence of methyl isobutyl ketone as the solvent. This is particularly suitable for a further development of the reaction on a pilot scale as far as 5-hydroxymethylfurfural is extracted with that solvent in the preceding chemical step and thus may be directly used in the oxidation step.