Francisco Medina

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.

Surface structure of bulk nickel catalysts, active in the gas-phase hydrodechlorination reaction of aromatics

Several bulk nickel catalysts obtained from different procedures have been structurally characterized using BET, XRD, SEM, TPR and TPD techniques. XRD and SEM clearly show diffentiated crystalline morphologies on the oxidized and reduced nickel phases obtained from the three catalysts studied, in agreement with the BET surface areas. TPR and TPD results show the reducibility behaviour, and that mainly atomic hydrogen chemisorbs onto the catalysts with the better crystallized and larger nickel crystallites. The catalytic activities depicted by this study for the hydrodechlorination reaction of ortho-dichlorobenzene show that all the bulk nickel catalysts tested yield conversions and selectivities towards benzene of >80% at temperatures >493 K. Also, the catalysts with larger octahedral crystallites showed higher TOF values and selectivities toward benzene than those with poorly crystallized structures at any reaction temperature in the range 523–393 K. The behaviour is interpreted in terms of a structure-sensitive reaction. Also, a reversible/irreversible temperature-dependent mechanism of nickel chloride formation is proposed.

Characterization and activity of hydrotalcite-type catalysts for acetonitrile hydrogenation

Studies of the preparation and characterization using X-ray diffraction, temperature programmed reduction and decomposition, electron microscopy (TEM-SEM), calorimetric measurements, temperature programmed desorption, nitrogen adsorption and the catalytic behavior of three samples having takovite structure with different Ni/Mg/Al ratios in order to modify the basicity of the solid, have been carried out for the acetonitrile hydrogenation reaction in the gas phase. It has been shown that the chemical composition of the samples significantly modifies their catalytic behaviors. Also, low magnesium contents, which cause an increase on basic properties, result in higher activity and selectivity to primary amine. The catalytic properties were correlated with the accessibility and reducibility of Ni particles, and ethylamine adsorption heats. Thermal stability of the takovite materials and their surface properties are also reported.

Characterization of several γ-alumina-supported nickel catalysts and activity for selective hydrogenation of hexanedinitrile

Studies of the Chemical preparation, Temperature-programmed reduction (TPR), powder X-ray diffraction (XRD), X-ray photoelectron (XP) spectra and catalytic activities of several Nickel/γ-alumina catalysts have been carried out for the catalytic hydrogenation of hexanedinitrile, in a continuous process at 1 atm pressure, 443 K, and in the absence of ammonia. The TPR measurements detect the inhibiting effect of γ-alumina on NiO reduction and also the formation of some kind of nickel aluminate at the NiO/γ-alumina interface at calcination temperatures > 623 K, which affects the final degree of reduction of the catalysts. The XRD detects the nickel phases present except that of the non-crystalline nickel aluminate. The XPS results indicate the presence of surface Ni2+ and reduced nickel, presumably in the form of nickel aluminate underneath the reduced surface nickel crystallites, particularly for higher nickel loadings. Catalytic conversions increase significantly with increase in nickel content and degree of reduction of the catalysts; the latter was highly resistant to metallic sintering. Selectivities of 100% towards azacycloheptane were obtained, where nickel aluminate may play a role in a decoration mechanism.

Surface characterization and hydrogenation properties of several nickel/α-alumina catalysts

Studies of the chemical preparation, X-ray photoelectron spectra (XPS), activation energies of reduction, temperature-programmed reduction (TPR), X-ray diffraction (XRD) and catalytic activities of several nickel/α-alumina catalysts have been carried out for the catalytic hydrogenation of hexanedinitrile, in a continuous process at 1 atm pressure, 443 K, and in the absence of ammonia. XPS results show complete reduction of non-stoichiometric NiO on α-alumina at temperatures higher than 623 K and higher surface nickel dispersion with increasing nickel content and decreasing reduction temperatures. Activation energies of reduction for the α-alumina-supported non-stoichiometric NiO were higher than those of the unsupported non-stoichiometric NiO. TPR results show that the initial and final temperatures of reduction of the α-alumina-supported non-stoichiometric NiO are higher with unsupported NiO, confirming the inhibiting effect of α-alumina on NiO reduction. XRD measurements show the presence of α-alumina, NiO and Ni phases, and also the increase in crystallite size with increasing reduction temperature. Catalytic conversions increase with the nickel content and selectivities toward 6-aminohexanenitrile increase at lower nickel contents, high space velocities, and higher metallic sintering, probably owing to the presence of a higher content of specific crystal sites responsible for the production of 6-aminohexanenitrile. A mechanism is proposed.

Structural and catalytic properties of several potassium-doped nickel/α-alumina solids

Studies of the chemical preparation, activation energies of reduction, temperature-programmed reduction (TPR), X-ray diffraction (XRD), X-ray photoelectron (XP) spectra and catalytic activities of several potassium-doped nickel/α-alumina catalysts have been carried out for the catalytic hydrogenation of hexanedinitrile, in a continuous process at 1 atm pressure, 443 K, and in the absence of ammonia. Activation energies of reduction for the potassium-doped non-stoichiometric NiO/α-alumina precursors were higher than those obtained for the potassium-free NiO/α-alumina and for the unsupported non-stoichiometric NiO. TPR results show that there is an inhibiting effect of α-alumina and potassium on NiO reduction. XRD measurements reveal the presence of α-alumina, NiO and Ni phases, the latter being present in larger quantities at higher reduction temperatures and lower potassium contents. XRD spectra also show fairly constant nickel particle sizes for all catalysts tested, XPS results confirm the inhibiting effect of potassium upon the reducibility of NiO. The XPS intensity ratios of surface Ni : NiO match the BET area ratios Ni : NiO of the samples. Conversions of the catalysed reactions decrease with increasing potassium content of the catalysts, probably because of poisoning of the latter with a Thorpe cycling formation of an enamine at high basic K2O contents. Selectivities towards 6-aminohexanenitrile (which may reach 100% at 85% conversion, thereby of potential interest for the manufacture of nylon-6,6) increase with potassium content of the catalysts. The less selective catalysts also yield 1,6-diaminohexane and azacycloheptane. A lack of oligomerization reactions is observed, and a mechanism is proposed.

Use of Ni/Al-MCM-41 Catalysts for the Exhaustive Hydrodechlorination of 1,2,4-Trichlorobenzene

Several Ni/Al-MCM-41 catalysts were tested in the gas phase hydrodechlorination of 1,2,4-trichlorobenzene at atmospheric pressure. They showed high activity and selectivity towards benzene at reaction temperatures between 473 and 523 K with values of 100% conversion and 100% benzene selectivity at 523 K for all of them.

Design of NiO–MgO materials with different properties

Several NiO–MgO systems were prepared from commercial nickel nitrate hexahydrate and MgO by means of different preparative paths and with two NiO/MgO weight ratios. All the samples were structurally characterized by using BET, XRD, SEM and TPR techniques. These preparative paths lead to NiO–MgO samples with different NiO–MgO interaction degrees, which go from the formation of solid solution until the detection of clearly differentiated NiO and MgO phases by XRD. Samples with solid solution have surface areas in the range 35–51 m2 g−1 and did not show well-defined morphology. The use of an argon flow passing through the sample during the decomposition of the intermediate Ni3(NO3)2(OH)4 phase in the presence of magnesia avoids the solid solution formation and allows to obtain NiO/MgO systems with high surface area (80–90 m2 g−1) with defined octahedral morphology. Also, NiO/MgO systems of about 100 m2 g−1 and without solid solution were obtained by mixing high-area NiO and MgO in n-hexane. Low NiO–MgO interaction involves higher reduction degrees, high surface areas of the systems prepared, and the obtention of well-defined octahedral particles.

TPD study about the surface modification of some Ni/spinel catalysts in the hydrodechlorination of 1,2,4-trichlorobenzene. Influence on hydrogenation ability

NiAl2O4 supports and fresh and reactivated Ni/NiAl2O4 catalysts were tested in the gas phase hydrodechlorination of 1,2,4‐trichlorobenzene. Fresh catalysts hydrogenate 1,2,4‐trichlorobenzene to cyclohexane in the first 30 min of reaction at 523 K. An irreversible partial chlorination of the catalytic surface makes the hydrogenation of the aromatic ring difficult.