I. Melián-Cabrera

Liquid phase hydrogenation of crotonaldehyde over bimetallic Rh-Sn/SiO2 catalysts: Effect of the Sn/Rh ratio

A series of Rh-Sn/SiO2 catalysts has been prepared by successive impregnation of SiO2 sol–gel (pH 5) with precursors chlorides of rhodium(III) and tin(II). Monometallic Rh/SiO2 and Sn/SiO2 samples were also prepared for comparison. The solids had been characterized by transmission electronic microscopy (TEM), electron diffraction (ED), temperature programmed reduction (TPR), H2 chemisorption and X-ray photoelectron spectroscopy (XPS) techniques. The catalytic evaluation of crotonaldehyde hydrogenation was performed in liquid phase hydrogenation at atmospheric pressure. Only a slight increase in the metal particle size and significant rhodium coverage is produced by tin as it was found by TEM and XPS. Both Rh0 and Rhδ+ species were detected in low content of tin catalysts, whereas a high proportion of tin in an oxidized state (close to 70%) in all the samples was found by XPS. ED studies allowed the detection of bimetallic phases, RhxSny and oxides of tin as SnO2. An important decrease in catalytic activity is produced by addition of tin, however, at the same conversion level, a significant enhancement in the selectivity to crotyl alcohol (CROL) was observed. The results are explained on the basis of an electronic effect produced by the oxidized tin species and RhxSny intermetallic phase that allow the polarization of the carbonyl group and inhibit the hydrogenation of CC, respectively. The presence of these new species, modifies the nature of the active sites contributing to an increase in the selectivity to CROL due to electronic effects. However, at higher Sn/Rh, the activity drops significantly due to a decrease in the hydrogenation capacity and also due to the creation of surface acid sites, an important amount of byproducts, such as acetals and esters are obtained.

Effect of Pd on Cu-Zn catalysts for the hydrogenation of CO2 to methanol: Sstabilization of Cu metal against CO2 oxidation

A palladium–copper–zinc catalyst (PdO:CuO:ZnO=2:28:70), prepared by sequential precipitation of the respective cations, was tested in the hydrogenation of CO2 at high pressure (conditions: 60 bar, CO2:H2=1:3 (molar), W/F=0.0675 kg h/m3, 453-513 K). The methanol yield was improved on using this Pd-containing catalyst at all temperatures with respect to the reference copper–zinc catalyst (CuO:ZnO=30:70). This improvement was not due to an additional effect in which palladium was acting as an independent catalytic site but was caused by a synergetic effect of Pd on the active Cu sites. This effect was explained in terms of hydrogen spillover and an increased stability against CO2 oxidation of the surface copper. Therefore, the present contribution not only supports previous literature findings concerning the hydrogen spillover mechanism but also resulted in a complementary view regarding the role of palladium in Pd-modified CuO-ZnO-based catalysts.

Thermal decomposition of a hydrotalcite-containing Cu–Zn–Al precursor: thermal methods combined with an in situ DRIFT study

A Cu–Zn–Al precursor (CZA) was synthesized efficiently by coprecipitation of the corresponding cations with sodium carbonate at constant pH and temperature. The starting precursor contained a mixture of two hydroxycarbonate phases: rosasite and a Cu–Zn hydrotalcite-like phase. The thermal decomposition was studied by conventional thermal methods (TGA, DTA and EGA-MS) as well as by in situ FTIR spectroscopy (DRIFT). Analysis of the CZA precursor showed similar results by both procedures. Dehydration, dehydroxylation and decarbonation of the precursor were analysed in situ by monitoring the hydroxyl and carbonate infrared bands. A Cu–Zn hydrotalcite phase, one of the components of the CZA precursor, was also prepared independently. A detailed FTIR study revealed an interesting effect upon heating this hydrotalcite. At 373–423 K, a carbonate rearrangement in the interlayer space takes place during the loss of interlayer water. Carbonate groups change from their symmetrical coordination with interlayer water molecules to an arrangement involving the OH groups of the octahedral M(OH)m layers. This phenomenon certainly takes place in the CZA material as well but, in this case, it cannot be observed, probably due to the complexity of the material formed by two hydroxycarbonate phases.

Structural reversibility of a ternary CuO–ZnO–Al2O3 ex hydrotalcite-containing material during wet Pd impregnation

A Cu-Zn-Al precursor was synthesized by coprecipitation of the corresponding cations with sodium carbonate at constant pH and temperature. CuO-ZnO-Al2O3 composite oxide support was obtained by calcination (673 K) of the Cu-Zn-Al precursor. Two palladium-modified CuO-ZnO-Al2O3 samples were prepared by impregnation of the mixed-oxide support and further calcination (673 K). The presence of remaining CO32- anions in the CuO-ZnO-Al2O3 mixed oxide, as a result of incomplete Cu-Zn hydrotalcite phase decomposition, and the hydrothermal-like treatment during the Pd impregnation step, allow the partial reconstruction of the Cu-Zn hydrotalcite-type structure (memory effect). In addition, an enhancement in the CuO crystallinity was obtained for the Pd-modified oxides. A detailed characterization revealed that the hydrotalcite restoration enhances the crystallinity of the copper oxide as a consequence of a crystalline rearrangement of this oxidic phase.

Crotonaldehyde Hydrogenation on Rh/TiO2 catalysts: In situ DRIFTS studies

The surface and catalytic properties in the vapor-phase hydrogenation of crotonaldehyde on Rh/TiO2 has been studied. It was found that a partial reduction of the support produces a surface decoration of the metal component. Thus, interfacial sites are created, which are responsible of an increase in the selectivity to crotyl alcohol, via enhancement of the polarization of the C=O bond. Photoelectron spectra revelead that rhodium is in different oxidation states, with a contribution of ca. 20 % Rhd + and 80 % Rho species for LTR catalyst and only a slight increase of Rhd + for HTR catalyst. TEM studies revelead that Rh has metal particle size close 3 nm with small increases in the catalyst reduced at high temperature. DRIFTS essayed carried out under reaction conditions allowed to identify crotonaldehyde species strongly adsorbed through the C=C bond and weakly coordinated through both the C=C and C=O bonds. After reduction at 723 K an increase in the peak at 1660 cm -1 ascribed to an interaction between the carbonyl group and the surface, was observed. This peak seems to be stabilized at interfacial Rh/TiOx sites The deactivation in crotyl alcohol formation can be ascribed to the generation of strongly chemisorbed asymmetric carboxylate species detected by band at 1740 cm -1 . This band grows at expense of crotonaldehyde O s - bonded intermediate chemisorbed on coordinatively unsaturated sites (Lewis acid sites) responsible of the crotyl alcohol obtaintion (detected by a band at 1653 cm -1 ). Additionally, a small band at 2068 cm -1 assigned to CO adsorbed on transition metals, which increases with time on-stream may explain the deactivation of the catalysts in flow systems.

Effect of Pd on Cu-Zn catalysts for the hydrogenation of CO2 to methanol: stabilization of Cu metal against CO2 oxidation

A palladium–copper–zinc catalyst (PdO:CuO:ZnO=2:28:70), prepared by sequential precipitation of the respective cations, was tested in the hydrogenation of CO2 at high pressure (conditions: 60 bar, CO2:H2=1:3 (molar), W/F=0.0675 kg h/m3, 453-513 K). The methanol yield was improved on using this Pd-containing catalyst at all temperatures with respect to the reference copper–zinc catalyst (CuO:ZnO=30:70). This improvement was not due to an additional effect in which palladium was acting as an independent catalytic site but was caused by a synergetic effect of Pd on the active Cu sites. This effect was explained in terms of hydrogen spillover and an increased stability against CO2 oxidation of the surface copper. Therefore, the present contribution not only supports previous literature findings concerning the hydrogen spillover mechanism but also resulted in a complementary view regarding the role of palladium in Pd-modified CuO-ZnO-based catalysts.