Spectroscopic, calorimetric, and catalytic evidences of hydrophobicity on Ti-MCM-41 silylated materials for olefin epoxidations

Hydrophobic Ti-MCM-41 samples prepared by post-synthesis silylation treatment demonstrate to be highly active and selective catalysts in olefins epoxidation by using organic hydroperoxides as oxidizing agents in liquid phase reaction systems. Epoxide yields show important enhancements with increased silylation degrees of the Ti-mesoporous samples. Catalytic studies are combined and correlated with spectroscopic techniques (e.g. XRD, XANES, UV-Visible, 29Si MAS-NMR) and calorimetric measurements to better understand the changes in the surface chemistry of Ti-MCM-41 samples due to the post-synthesis silylation treatment and to ascertain the role of these trimethylsilyl groups incorporated in olefin epoxidation. In such manner, the effect of the organic moieties on solids, and both water and glycol molecules contents on the catalytic activity and selectivity are analyzed in detail. Results show that the hydrophobicity level of the samples is responsible for the decrease in water adsorption and, consequently, the negligible formation of the non-desired glycol during the catalytic process. Thus, catalyst deactivation by glycol poisoning of Ti active sites is greatly diminished, this increasing catalyst stability and leading to practically quantitative production of the corresponding epoxide. The extended use of these hydrophobic Ti-MCM-41 catalysts together with organic hydroperoxides for the highly efficient and selective epoxidation of natural terpenes is also exemplified.

Zr-Laponite pillared clay-based nickel catalysts for methane reforming with carbon dioxide

Zr-Laponite pillared clays were prepared and used as supports of nickel catalysts for the methane reforming reaction with carbon dioxide to synthesis gas. The structural and textural characteristics of supports and catalysts were systematically examined by N-2 adsorption/desorption and X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron spectroscopy (TEM) techniques. The catalytic performance and carbon deposition were investigated. It is found that Zr-Laponite pillared clays are promising catalyst supports for carbon dioxide reforming of methane. The pore structure and surface properties of such supports greatly affect the catalytic behaviors of catalysts derived. Carbon deposition on catalysts was also affected by the property and structure of supports. The sintering of nickel metal and zirconia was another factor responsible for catalyst deactivation. This new-type nickel supported catalyst Ni/Zr-Laponite(8), with well-developed porosity, gave a higher initial conversion and a relatively long-term stability, and is therefore a promising catalyst for potential application to carbon dioxide reforming of methane to synthesis gas. (C) 2002 Elsevier Science B.V All rights reserved.

Alumina-grafted SBA-15 as a high performance support for Pd-catalysed cinnamyl alcohol selective oxidation

Ultrathin alumina monolayers grafted onto an ordered mesoporous SBA-15 silica framework afford a composite catalyst support with unique structural properties and surface chemistry. Palladium nanoparticles deposited onto Al-SBA-15 via wet impregnation exhibit the high dispersion and surface oxidation characteristic of pure aluminas, in conjunction with the high active site densities characteristic of thermally stable, high-area mesoporous silicas. This combination confers significant rate enhancements in the aerobic selective oxidation (selox) of cinnamyl alcohol over Pd/Al-SBA-15 compared to mesoporous alumina or silica supports. Operando, liquid-phase XAS highlights the interplay between dissolved oxygen and the oxidation state of palladium nanoparticles dispersed over Al-SBA-15 towards on-stream reduction: ambient pressures of flowing oxygen are sufficient to hinder palladium oxide reduction to metal, enabling a high selox activity to be maintained, whereas rapid PdO reduction and concomitant catalyst deactivation occurs under static oxygen. Selectivity to the desired cinnamaldehyde product mirrors these trends in activity, with flowing oxygen minimising CO cleavage of the cinnamyl alcohol reactant to trans-β-methylstyrene, and of cinnamaldehyde decarbonylation to styrene. © 2013 Elsevier B.V.

Dynamic restructuring of Pd nanoparticles and single crystals during catalytic selective alcohol oxidation

The aerobic selective oxidation (selox) of alcohols represents an environmentally benign and atom efficient chemical valorisation route to commercially important allylic aldehydes, such as crotonaldehyde and cinnamaldehyde, which find application in pesticides, fragrances and food additives. Palladium nanoparticles are highly active and selective heterogeneous catalysts for such oxidative dehydrogenations, permitting the use of air (or dioxygen) as a green oxidant in place of stoichiometric chromate permanganate saltsor H2O2. Here we discuss how time-resolved, in-situ X-ray spectroscopies (XAS and XPS) reveal dynamic restructuring of dispersed Pd nanoparticles and Pd single-crystals in response to changing reaction environments, and thereby identify surface PdO as the active species responsible for palladium catalysed crotyl alcohol selox (Figure 1); on-stream reduction to palladium metal under oxygen-poor regimes thus appears the primary cause of catalyst deactivation. This insight has guided the subsequent application of surfactant-templating and inorganic nanocrystal methodologies to optimize the density of desired active PdO sites for the selective oxidation of natural products such as sesquiterpenoids.

Glycerol valorization:dehydration to acrolein over silica-supported niobia catalysts

The catalytic dehydration of glycerol to acrolein is investigated over silica-supported niobia catalysts in a continuous fixed-bed gas-phase reactor. Various supported niobia catalysts are prepared and characterized using surface analysis and spectroscopic methods (XRD, UV-Vis, XPS, N2 adsorption), as well as with ammonia adsorption microcalorimetry. Good results are obtained with initial glycerol conversions of over 70% and with 50-70% selectivity to acrolein. We investigate the influence of changing the catalyst acid strength by varying the niobia content and catalyst calcination temperature. Glycerol conversion and acrolein selectivity depend on the surface acid strength. Catalyst deactivation by coking is also observed, but simple oxidative treatment in air restores the activity of the catalysts completely. © The Author(s) 2010.

Structure-reactivity correlations in the selective aerobic oxidation of cinnamyl alcohol:in situ XAFS

The structural evolution of a Pd/C catalyst during the liquid phase selective aerobic oxidation of cinnamyl alcohol has been followed by in situ XAFS and XPS. The fresh catalyst comprised highly dispersed, heavily oxidised Pd particles. Cinnamyl alcohol oxidation resulted in the rapid reduction of surface palladium oxide and a small degree of concomitant particle growth. These structural changes coincided with a large drop in catalytic activity. Prereduced Pd/C exhibited a significantly lower initial oxidation rate demonstrating the importance of surface metal oxide in effecting catalytic oxidation. Use of a Pd black model system confirmed that the oxide→metal transformation was the cause, and not result, of catalyst deactivation.

Platinum-catalysed cinnamaldehyde hydrogenation in continuous flow

Platinum is one of the most widely used hydrogenation catalysts. Here we describe the translation of batch reactions to continuous flow, affording tunable C=O versus C=C hydrogenation over a Pt/SiO2 catalyst, resulting in high steady state activity and single-pass yields in the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol under mild conditions. Negligible catalyst deactivation occurs under extended flow operation due to removal of reactively-formed poisons from the reaction zone. Process intensification imparts a four-fold enhancement in cinnamyl alcohol productivity.

Enhancing the mesoporosity of pillared clay catalyst - a study of pillar aggregates and deactivation in alkylation reaction

A sample of montmorillonite was pillared with aluminium polyoxycations in presence of different amounts of tween-80, a nonionic surfactant, ranging from 0.01 to 0.20 mmol/meq of clay. The amount of aluminium sorbed was found to vary with the amount of surfactant added during pillaring. Vapour phase catalytic activity of the samples for alkylation of toluene with methanol in a fixed bed down flow reactor showed that the rate of deactivation, in general, increased with decrease in the pillar density. The samples treated with 0.06 to 0.08 mmol/meq of surfactant showed the lowest deactivation and also an enhancement in the mesopores which did not change on calcining to 540°C. Suppression of deactivation is attributed to the distribution of pillars by the surfactant in such a way as to decrease the coke formation.

The role of coke in the deactivation of Mo/MCM-22 catalyst for methane dehydroaromatization with CO2

The effect of space velocity on reaction performance and coke deposition over 6Mo/MCM-22 catalyst in methane dehydro-aromatization (MDA) with CO2 were studied. The characterization of catalysts reacted at different space velocity after the same amount of methane feed by TG, TPO and Benzene/NH3-TPD techniques suggested that the inert coke maybe responsible for the deactivation of catalyst because of its blockage effect for pore system.