In Situ Extended X-ray Absorption Fine Structure Study during Selective Alcohol Oxidation over Pd/Al₂O₃ in Supercritical Carbon Dioxide

Abstract

High-pressure in situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data are reported during the selective oxidation of benzyl alcohol to benzaldehyde in supercritical carbon dioxide over a Pd/Al(2)O(3) catalyst (shell impregnated). For this purpose, a continuous-flow system with a spectroscopic cell suitable for in situ X-ray absorption studies on heterogeneous catalysts up to 200 degrees C and 200 bar has been developed. Due to the high contribution of the dense fluid to the overall X-ray absorption, high stability of the process pressure is mandatory, particularly when recording EXAFS spectra. According to EXAFS and XANES results, the palladium particles were fully reduced after exposure to benzyl alcohol in scCO(2). In contrast to Pd-catalyzed liquid-phase oxidation, a higher oxygen tolerance of the catalyst was observed. Palladium was partially oxidized on the surface under typical reaction conditions (0.9 mol % benzyl alcohol/0.5 mol % O(2) in carbon dioxide), which gradually increased when the concentration of oxygen in the feed was raised. Both XANES and EXAFS data uncovered that palladium is mainly oxidized on the surface or within the outermost layers. These results are in accordance with simulations of the XANES data using the FEFF8.20 code (program for ab initio calculations on multiple scattering XAS) and EXAFS data fitting/simulation.

Time-Resolved and Operando XAS Studies on Heterogeneous Catalysts — From the Gas Phase Towards Reactions in Supercritical Fluids

“…The latter seems to be more reducing under the conditions applied and therefore does not show any deactivation due to surface oxidation of the Pd-particles. These XANES results are also supported by corresponding EXAFS data and fitting [15,16]. FIGURE 5.…”

“…Due to the fact that near the critical point small changes in pressure strongly affect the density of the reaction mixture, special efforts were put into pressure stabilization [15]. In order to minimize internal and external mass transfer, a sieved fraction of a shell-impregnated catalyst was used.…”

Catalysing sustainable fuel and chemical synthesis

“…12), which was attributed to hydrogen abstraction from the catalyst surface [156,157] rather than to a change in palladium oxidation state. In contrast to their liquid phase experiments, high pressure XANES and EX-AFS measurements of Pd/Al 2 O 3 catalysed benzyl alcohol selox under supercritical CO 2 led Grunwaldt and Baiker to conclude that maximum activity arose from particles mainly oxidised in the surface/shelfedge [48]. In a parallel research programme, the author's group systematically characterised the physicochemical properties of palladium nanoparticles as a function of size over non-reducible supports to quantify structure-function relations in allylic alcohol selox [133-137, 158, 159].…”

“…For example, continuous flow microreactors have been implemented in both homogeneous and heterogeneous aerobic selox, providing facile catalyst recovery from feedstreams for the latter [42,43], but their scale-up/out requires complex manifolding to ensure adequate oxygen dissolution uniform reactant mixing and delivery [44,45]. Efforts to overcome mass transport and solubility issues inherent to 3-phase catalysed oxidations have centred around the use of supercritical carbon dioxide to facilitate rapid diffusion of substrates to and products from the active catalyst site at modest temperatures [46] affording enhanced turnover frequencies (TOFs), selectivity and on-stream performance versus conventional batch operation in liquid organic solvents [47][48][49][50][51].…”

Active Site Elucidation in Heterogeneous Catalysis via In Situ X-Ray Spectroscopies

“…The first detailed studies on liquid phase catalyst chemistry explored structure-function relations in the palladium catalyzed aerobic selective oxidation of crotyl and cinnamyl alcohols to their aldehydes, [27,51,67] important intermediates in fragrances, preservatives, and pesticides. Constructing simple, low-cost trickle-bed and recirculating reactors, our group used Quick-XAS to demonstrate that catalyst deactivation was associated with in situ reduction of 2 nm oxidic Pd clusters to 10 nm metallic nanoparticles (Fig.…”