Selective Hydrogenation of Cinnamaldehyde over Cobalt Supported on Alumina, Silica and Titania

Abstract  

The liquid phase selective hydrogenation of cinnamaldehyde has been investigated on cobalt (15 wt%) impregnated on alumina, silica and various phases of titania supports. The multiple reduction stages observed in the TPR studies suggest the presence of cobalt aluminate/silicate/titanate species, and DRIFT spectra results seem to corroborate this observation. An optimum level of conversion and selectivity to cinnamyl alcohol was observed at 120 °C and 10 kg/cm² hydrogen pressure. Co/TiO₂ exhibited a greater conversion (47.4%) and selectivity to cinnamyl alcohol (58%) than Co/Al₂O₃ and Co/SiO_2, which may be attributed to the presence of TiO _x (x < 2) species on the catalyst surface and to the preferential adsorption of C=O on the catalyst surface. The stability of Co/TiO₂ was found to be better than Co/Al₂O₃ and Co/SiO₂. Between the various phases of titania (high surface area, anatase and rutile), the crystalline phases exhibited a better conversion and selectivity to cinnamyl alcohol, while the stability was found to be better for high surface area titania.     

Glycerol Steam Reforming for Hydrogen Production over Nickel Supported on Alumina,Zirconia and Silica Catalysts

The aim of the work was to investigate the influence of support on the catalytic performance of Ni catalysts for the glycerol steam reforming reaction. Nickel catalysts (8 wt%) supported on Al₂O₃, ZrO₂, SiO₂ were prepared by the wet impregnation technique. The catalysts’ surface and bulk properties, at their calcined, reduced and used forms, were determined by ICP, BET, XRD, NH₃-TPD, CO₂-TPD, TPR, XPS, TEM, TPO, Raman, SEM techniques. The Ni/Si sample, even if it was less active for T?<600?°C, produces more gaseous products and reveals higher H₂ yield for the whole temperature range. Ni/Zr and Ni/Si catalysts facilitate the WGS reaction, producing a gas mixture with a high H₂/CO molar ratio. Ni/Si after stability tests exhibits highest values for total (70%) and gaseous products (45%) glycerol conversion, Y_H2 (2.5), S_H2 (80%), S_CO2 (65%), H₂/CO molar ratio (6.0) and lowest values for S_CO (31%), S_CH4 (3.1%), CO/CO₂ molar ratio (0.48) among all samples. The contribution of the graphitized carbon formed on the catalysts follows the trend Ni/Si (I _D /I _G ?=?1.34)?<?Ni/Zr (I _D /I _G = 1.08)?<?Ni/Al (I _D /I _G = 0.88) and indicates that the fraction of different carbon types depends on the catalyst’s support nature. It is suggested that the type of carbon is rather more important than the amount of carbon deposited in determining stability. It is confirmed that the nature of the support affects mainly the catalytic performance of the active phase and that Ni/SiO₂ can be considered as a promising catalyst for the glycerol steam reforming reaction.     

Fabrication of Dense Arrays of Platinum Nanowires on Silica, Alumina, Zirconia and Ceria Surfaces as 2-D Model Catalysts

High-density arrays of platinum nanowires with dimensions 20 nm × 5 nm × 12 μm (width × height × length) have been produced on planar oxide thin films of silica, alumina, zirconia, and ceria. In this multi-step fabrication process, sub-20 nm single crystalline silicon nanowires were fabricated by size reduction lithography. The Si nanowire patterns were then replicated to produce a high density of Pt nanowires by nanoimprint lithography. The width and height of the Pt nanowires are uniform and are controlled with nanometer precision. The Pt surface area is larger than 2 cm² on a 5 × 5 cm² oxide substrate. The catalytic oxidation of CO was carried out on zirconia-supported Pt nanowires. The reaction conditions (100 Torr O₂, 40 Torr CO, 513–593 K) and vacuum annealing (1023 K) did not change the nanowire structures.     

Comparison of Au Catalysts Supported on Mesoporous Titania and Silica: Investigation of Au Particle Size Effects and Metal-Support Interactions

Au catalysts supported on mesoporous silica and titania supports were synthesized and tested for the oxidation of CO. Two approaches were used to prepare the silica-supported catalysts utilizing complexing triamine ligands which resulted in mesoporous silica with wormhole and hexagonal structures. The use of triamine ligands is the key for the formation of uniformly sized 2–3 nm Au nanoparticles in the silica pores. On mesoporous titania, high gold dispersions were obtained without the need of a functional ligand. Au supported on titania exhibited a much higher activity for CO oxidation, even though the Au particle sizes were essentially identical on the titania and the wormhole silica supports. The results suggest that the presence of 2–3 nm particle size alone is not sufficient to achieve high activity in CO oxidation. Instead, the support may influence the activity through other possible ways including stabilization of active sub-nanometer particles, formation of active oxygen-containing reactant intermediates (such as hydroxyls or O₂ ^?), or stabilization of optimal Au structures.     

Acetylene and Ethylene Hydrogenation on Alumina Supported Pd-Ag Model Catalysts

Adsorption and co-adsorption of ethylene, acetylene and hydrogen on Pd-Ag particles, supported on thin alumina films, have been studied by temperature programmed desorption (TPD). The TPD results show that adding of Ag to Pd suppresses overall hydrogenation activity but increases selectivity towards ethylene, i.e. similar to that observed on real catalysts. The results are rationalized on the basis of a complex interplay between surface and subsurface hydrogen species available in the system, whereby the latter species are the most critical for total hydrogenation of acetylene to ethane.     

Preparation of Highly Dispersive Platinum Catalysts Impregnated on Titania-Incorporated Silica Support

Highly dispersive silica-impregnated platinum catalysts were prepared by incorporating titania to the surface of silica support and by treating the impregnated platinum precursor with hydrogen peroxide. High dispersion of platinum on the titania-incorporated silica support was confirmed by XRD, TEM, EXAFS, and X-ray photoelectron spectroscopy (XPS) techniques. The platinum particles dispersed on silica ranged from 1 to 2 nm in size, although the loading amount of platinum was as high as 4 wt.%. The strong interaction between platinum and titania suppressed the migration and aggregation of the platinum particles on the surface, retaining a high dispersion of platinum. The platinum catalysts impregnated on the titania-incorporated silica showed higher catalytic activities in the combustion of methane than the platinum catalysts impregnated on the silica, while their catalytic activities were poor in the hydrogenation of nitrobenzene. Platinum dispersions and catalytic activities of the platinum catalysts on the silica support were discussed in relation to the strong interaction between platinum and titania.     

Deactivation by Coking of Platinum/Alumina Catalysts: Effects of Operating Temperature and Pressure

Coke deposition is a complex reaction that results from the production of coke precursors and from their destruction. At low temperatures, when the destruction reaction (high apparent activation energy) is negligible, the coking rate is increased owing to increasing metal accessibility. In contrast, at high temperatures the coking reaction is hindered by an increasing metal loading. A change in the coking temperature does not alter the chemical nature of the carbonaceous deposits and their distribution between the metallic and acidic phases of a bifunctional catalyst. On the other hand, an increase in coking pressure (with a constant hydrogen:hydrocarbon molar ratio) promotes the graphitization of coke on the support, protecting the catalytic activity of the metallic phase.     

Acidic Characterization of Copper Oxide and Niobium Pentoxide Supported on Silica–Alumina

Copper oxide and niobium pentoxide were supported on silica–alumina with 2, 5, 10, 15 and 25 mass% loadings and 1:1 mass ratio of CuO:Nb₂O₅. XRD and BET measurements confirmed that monolayer coverage is reached with loadings between 5–10 mass% (～308 m² g^?1). The DRIFTS spectra of pyridine adsorbed catalysts showed bands associated with Brönsted, Lewis and a combination of both acidic sites. Thermal analysis and liquid phase microcalorimetry provided the parameters for the best catalyst (10 mass%), which has the highest number of acidic sites (0.38 mmol g^?1) and enthalpies of interaction with pyridine for Brönsted and Lewis sites (ΔH₁ = ?107.5 and ΔH₂ = ?64.4 kJ mol^?1, respectively).     

Tunability of Propane Conversion over Alumina Supported Pt and Rh Catalysts

Propane conversion over alumina supported Pt and Rh (1 wt% metals loading) was examined under fuel rich conditions (C₃H₈:O₂:He = 1:2.25:9) over the temperature range 450–650 °C. Morphological characteristics of the catalyst materials were varied by calcining at selected temperatures between 500 and 1,200 °C. X-ray diffraction and BET analysis showed the treatment generated catalyts metals with particle sizes in the range of <10 to >500 nm, and support surface areas in the range of 20–240 m²/g. Remarkably, both Rh and Pt yielded product compositions close to equilibrium values (with high H₂ and CO selectivity, complete oxygen conversion and almost complete propane conversion) so long as the metal particle size was sufficiently low, ≲10–15 nm. In cases where the particle size was large, primarily complete oxidation rather than partial oxidation products were observed, along with unreacted C₃H₈, indicative of a direct oxidation pathway in which gas-phase CO and H₂ are not present as intermediate species. It is proposed that the high resistance of Rh to coarsening is largely responsible for the observation of a higher selectivity of this material for syngas products when prepared by procedures similar to those for Pt. Overall, the tunability of the product composition obtained over Rh and Pt via processing steps has direct significance for the incorporation of such catalyts into the anodes of solid oxide fuel cells.     

Adsorption of Colloidal Silica on Alumina and of Colloidal Alumina on Silica

The mutual adsorption of colloidal silica on alumina and of colloidal alumina on silica and silicate materials occurs in aqueous suspension at about pH 4. It is shown that the adsorption of colloidal particles on the surface of opposite charge is limited to essentially a monoparticle layer. An adsorbed layer of fibrils of colloidal alumina on the surfaces of silica, asbestos, graphite, and finely divided clay is shown in electron micrographs. The effect of the adsorbed colloid on dispersibility of the substrate materials is discussed.     

Mechanism and Kinetics of some Reactions on Silica-Alumina Catalysts

The dehydration of alcohol when its vapor is led over alumina, or some other heterogeneously acting catalysts, is already known from the end of the eighteenth century.     

Mechanism and Kinetics of some Reactions on Silica-Alumina Catalysts

The dehydration of alcohol when its vapor is led over alumina, or some other heterogeneously acting catalysts, is already known from the end of the eighteenth century.     

Synthesis and Application of Polystyrene Nanospheres Supported Platinum Catalysts in Enantioselective Hydrogenations

Abstract  

Nearly monodisperse polystyrene nanospheres coated with platinum have been synthesized and applied successfully in the cinchona alkaloid-modified enantioselective hydrogenation of ethyl pyruvate. During the catalyst preparation broadly varied experimental conditions were used resulting in several catalysts with different properties. The effect of hydrogen pressure, solvent, modifier and polystyrene nanoparticle size on the enantioselectivity was also studied. Based on the results the effect of support on the mechanism of the enantiodifferentiation has been discussed.     

Catalytic Activity of Supported Platinum and Metal Oxide Catalysts for Toluene Oxidation

Oxidation of toluene has been investigated over supported platinum as well as over a variety metal oxide (M _x O _y ) catalysts dispersed on high surface area γ-Al₂O₃. Catalysts were characterized with respect to their specific surface area (BET), metal dispersion (selective chemisorption of CO), phase composition and M _x O _y crystallite size (XRD) and reducibility (H₂-TPR). Catalytic performance for the title reaction was investigated in the temperature range of 100–500 °C, using a feed composition consisting of 0.1% toluene in air. For Pt/M _x O _y catalysts, it has been found that catalytic performance depends on the nature of the support, with Pt/CeO₂ being the most active catalyst at low temperatures. The intrinsic reaction rate per surface platinum atom does not depend on Pt loading (0.5–5 wt%), at least for Pt/Al₂O₃. Reducible metal oxides, such as ceria, are active for the title reaction and catalytic performance is improved significantly with increase of specific surface area (SSA). However, the intrinsic reaction rate per unit surface area is invariant with SSA. Dispersion of M _x O _y on high surface area inert supports, such as Al₂O₃, results in materials with relatively high catalytic activity, which seems to correlate well with the reducibility of metal oxides. Catalytic performance of M _x O _y /Al₂O₃ catalysts can be optimized by proper selection of M _x O _y loading. Best performing catalysts of this series include 60% MnO, 90% CeO₂ and 5% CuO on Al₂O₃ which, under the present experimental conditions, are able to completely convert toluene toward CO₂ at temperatures lower than 350 °C. Dispersion of Pt on M _x O _y /Al₂O₃ catalysts improves significantly the catalytic performance of irreducible M _x O _y but does not alter appreciably the activity of reducible M _x O _y /Al₂O₃ catalysts.     

Physicochemical Characterization and H2-TPD Study of Alumina Supported Ruthenium Catalysts

A series of alumina supported ruthenium catalysts, which prepared by hydrogen treatment or hydrazine reduction, were characterized by N₂ adsorption, X-ray diffraction (XRD), X-ray fluorescence (XRF), CO chemisorption, and Temperature-programmed desorption of hydrogen (H₂-TPD). In contrast to the samples with conventional hydrogen reduction, there was almost no residual chlorine in the samples using RuCl₃ as precursor with hydrazine treatment. Furthermore, the dissolved aluminum could be removed much more easily in basic solution, which led to the higher BET surface and pore volume of hydrazine-reduction catalysts. Therefore, the active phase (Ru metal) would not be contaminated. Three main peaks, which occurred at about 150, 375, and 650 °C, respectively, were observed in the H₂-TPD profiles of Ru/Al₂O₃ catalysts with a high amount of residual chlorine. A new peak of desorption hydrogen centering at 240 °C, which was completely suppressed by the high amount of residual chlorine, might appear in the profiles of the samples with the washing procedure following hydrogen reduction or hydrazine treatment. The peaks with the desorption temperature lower than 500 °C were relative with dissociatively adsorbed hydrogen and spillover hydrogen simultaneity, and the peak at above 500 °C was caused by spillover hydrogen and would be stabilized by hydroxyl groups on alumina surface.     

Methane Conversion to Synthesis Gas by Partial Oxidation and CO2 Reforming over Supported Platinum Catalysts

Partial oxidation of methane and reforming of methane with CO₂ were carried out with Pt/Al₂O₃, PtZrO₂ and Pt/CeO₂ catalysts, in the temperature range of 350–900 °C. For partial oxidation, the catalysts showed similar stabilities, with the PtZr slightly more active. The reaction occurs in two simultaneous stages: total combustion of methane and reforming of the unconverted methane with steam and CO₂, with the O₂ conversion of 100% over the whole temperature range. For reforming with CO₂, the catalysts presented similar activities, but with distinct deactivation rates: while the PtAl deactivates very fast at 800 °C, due to deposition of inactive carbon, the PtZr and PtCe catalysts offer higher resistance to coke formation, due to the metal-support interactions and the higher mobility of oxygen in the oxide lattice.     

Kinetics and Mechanism of the Water–Gas Shift Reaction Over Platinum Supported Catalysts

A dual-site reaction mechanism is proposed for Pt based water–gas shift catalysts. The sorption equilibrium parameters are evaluated in detail to validate their physical significance. The values of the van ‘t Hoff parameters for CO and H₂ correspond to those for chemisorption on platinum. The sorption parameters of H₂O and CO₂ on the supports have been quantitatively determined from temperature-programmed desorption experiments and have been compared to the values obtained from the kinetic study. Finally the proposed model is able to explain the different activities between the two catalysts.