Topics in Catalysis

The current study describes the results on the selective hydrogenation of the 2-butyne-1,4-diol to 1,4-butanediol over Raney® nickel catalysts both in batch and in CSTR mode. The detailed kinetic analysis of the reaction in batch mode revealed the existence of three characteristic regions. In the first region, A, the starting 2-butyne-1,4-diol produces primarily cis-2-butene-1,4-diol. In the second region, B, the dominant species is cis-2-butene-1,4-diol, which is either hydrogenated to 1,4-butanediol or isomerizes to trans-2-butene-1,4-diol. In the third region, C, the accumulated 4-hydroxybutanal is slowly hydrogenated to 1,4-butanediol. When the same reaction was carried out in a CSTR mode, the only products detected initially are the 1,4-butanediol and n-butanol. The first by-product detected immediately after the end of the first stage is the linear hemiacetal between the 4-hydroxybutanal with 1,4-butanediol. This species has been used as convenient tracer for determining the length of the selective region of the catalyst performance.

Effect of Pt precursor and pretreatment on hydrodesulfurization (HDS) activity of Pt/Al-PILM catalyst was examined to prepare highly active Pt-supported HDS catalyst. The order of HDS activities of Pt/alumina-pillared clay montmorillonite (Al-PILM) catalysts prepared by various Pt precursors was Pt(C5H7O2)2 > H2PtCl6 · 6H2O > [Pt(NH3)4](NO3)2 > [Pt(NH3)4]Cl2 · H2O > H2Pt(OH)6. This order was in accordance with that of Pt dispersion. Thus, high Pt dispersion is essential factor to prepare highly active Pt/Al-PILM catalyst for HDS reaction. On the other hand, the effect of pretreatment on the HDS activities of Pt/Al-PILM catalysts prepared by various Pt precursors was also evaluated. The UC-TPS Pt/Al-PILM catalyst showed the highest HDS activity among various pretreated Pt/Al-PILM catalysts, in which uncalcined catalyst was sulfided by temperature programmed sulfidation (TPS). We assumed that high HDS activity of UC-TPS Pt/Al-PILM catalyst is caused by partly sulfided Pt particle with high dispersion. It is concluded that the highly active Pt/Al-PILM catalyst for the HDS reaction could be prepared by using Pt(C5H7O2)2 as a precursor and UC-TPS treatment.

The historical development of asymmetric hydrogenation of N-acyl dehydroamino acids and their derivatives is reported. Both homogeneous and heterogeneous catalysis are illustrated by selected examples. Catalysis by water soluble complexes and transfer hydrogenation are also treated in this review. The mechanism of catalytic hydrogenation, its elementary steps and the origin of enantioselection are discussed.

This paper deals with the aging effect on the kinetics of the NO decomposition on a commercial Pd/Al2O3 catalyst. Temporal analysis of products experiments are discussed in the light of a selected mechanism involving the recombination of two NOads species to form N2O, which is an intermediate in N2 formation. Experiments over the fresh catalyst indicate a strong metal/support interface, with a spill-over effect, which is difficult to model. Thermal aging had a detrimental effect over this interface, the kinetic features depending mainly on the metallic Pd sites. The different heats of adsorption and activation energies are proven consistent with other theoretical studies. The mechanism led to a high surface coverage for O ad-atoms.

A series of Mn5Co1Ox catalysts calcined at different temperatures in the range of 400–800 °C were synthesized by coprecipitation of manganese and cobalt nitrates and tested in the oxidation of CO. The specific surface area, structure, and chemistry of the catalysts were studied. In addition, the reduction of the catalysts by hydrogen was studied using in situ X-ray diffraction and temperature-programmed reduction techniques. It was found that the low-temperature catalyst calcined at 400 °C displays the best catalytic activity, which is attributed to its high surface area, low-temperature reducibility, and a high surface content of Mn4+. The formation of highly disperse and active CoMnO3 species and excess oxygen in a Mn3−xCoxO4+δ spinel leads to excellent low-temperature redox properties. The elevated temperature calcination results in a decline in the catalytic activity in CO oxidation due to formation of a well crystalline Mn3−xCoxO4 spinel, a decrease in the surface area and reducibility.

A series of CuO-ZnO–ZrO2 (CZZ) catalysts were synthesized via a solvothermal method. The effects of four saturated monohydric alcohols (methanol, ethanol, n-propanol, and n-butanol) as solvents on the physicochemical properties and catalytic performance for the direct synthesis of methanol from CO2 hydrogenation of the prepared catalysts were investigated. N2 physical adsorption, X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray (EDX) spectroscopy mapping, reactive N2O adsorption, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction by H2 (H2-TPR), and temperature programmed desorption (H2-TPD, CO2-TPD) techniques were used to characterize the catalysts. The results indicate that the catalyst prepared with methanol as solvent (CZZ-M) has the smallest Cu crystallites and the largest metallic Cu surface area, thus exhibiting the highest methanol yield of 7.4% at 240 °C, 3.0 MPa, H2:CO2 of 3 (v/v), and GHSV of 2400 mL/(gcat·h).

Catalytic oxidation is an effective solution for the control of methane (CH4) emission in exhausts from natural gas vehicles. Pd-based small-pore zeolites (such as Pd-SSZ-13) are considered to be the most active catalysts for CH4 oxidation, but H2O in the exhausts tends to induce deactivation of Pd catalysts. In this work, we tuned the hydrophobicity of Pd-SSZ-13 as a representative to improve its H2O resistance in CH4 oxidation. Pd-SSZ-13 catalysts with different Si/Al ratios were obtained by dealuminizing the pristine SSZ-13 zeolite with acid followed by Pd ion exchange, and a reduction of T50 (i.e. the temperature to reach 50% conversion of CH4) by 20 ℃ was achieved in CH4 oxidation in the presence of 10 vol.% H2O. Detailed physicochemical characterizations showed that the fraction of highly dispersed PdO species (highly active in CH4 oxidation) increased, whereas that of less inactive PdOx clusters decreased, in the Pd-SSZ-13 after acid modification. In addition, the increase of zeolite hydrophobicity after acid modification alleviated the H2O inhibition effect on the active PdO phase, leading to a less activity loss of Pd-SSZ-13 in CH4 oxidation. The improved hydrophobicity also favored C3H8 combustion over Pd-SSZ-13. These results suggested that simple acid modification could tune effectively the Si/Al ratio and hydrophobicity of zeolite supports, and eventually the physicochemical properties and oxidation performance of the supported Pd catalysts.

We compare the activity and relevant gold species of nanostructured gold–cerium oxide and gold–iron oxide catalysts for the CO oxidation by dioxygen and water. Well dispersed gold nanoparticles in reduced form provide the active sites for the CO oxidation reaction on both oxide supports. On the other hand, oxidized gold species, strongly bound on the support catalyze the water-gas shift reaction. Gold species weakly bound to ceria (doped with lanthana) or iron oxide can be removed by sodium cyanide at pH ≥12. Both parent and leached catalysts were investigated. The activity of the leached gold–iron oxide catalyst in CO oxidation is approximately two orders of magnitude lower than that of the parent material. However, after exposure to H2 up to 400 °C gold diffuses out and is in reduced form on the surface, a process accompanied by a dramatic enhancement of the CO oxidation activity. Similar results were found with the gold–ceria catalysts. On the other hand, pre-reduction of the calcined leached catalyst samples did not promote their water-gas shift activity. UV–Vis, XANES and XPS were used to probe the oxidation state of the catalysts after various treatments.

Layered double hydroxides of the hydrotalcite (HT)-type materials have been considered as promising supports for Co-based Fischer–Tropsch synthesis (FTS) catalysts. In this work the effect of the Mg/Al ratio on the catalytic behavior of cobalt-based catalysts obtained from HTs precursors have been studied. Cobalt supported on Mg–Al oxides obtained from HTs Mg–Al precursors were prepared by wet impregnation method and calcined at 300 °C. The textural, structural and reducibility properties of the samples were characterized using different techniques. FTS was evaluated in a down-flow fixed-bed reactor at 20 bar, 250 °C and H2/CO ≈ 2 molar ratio. All catalysts were active and stable during 72 h testing time. The stability was improved by the presence of magnesium in the alumina support; however the CO conversion was negatively affected by increasing the Mg/Al ratio. The reducibility of cobalt decreased as the Mg/Al ratio increased, probably due to the strong Co–O–Mg interaction as evidenced by the formation of CoxOy–MgO mixed oxide. Furthermore, the activity of the catalysts was correlated with the H2-chemisorption measurements. The results suggest that HTs as Co-based catalysts were highly stable in FTS.