Operando synchronous DRIFTS/MS/XAS as a powerful tool for guiding the design of Pd catalysts for the selective oxidation of alcohols
Graphical abstract
Highlights
► Multi-dimensional, operando study of dynamic catalyst restructuring during alcohol oxidation. ► Palladium oxidation state responds to composition and temperature of reaction environment. ► Surface palladium oxide catalyses the aerobic selective oxidation of crotyl alcohol.
Introduction
Recent years have seen significant progress in the development of atom efficient heterogeneous catalysts for the aerobic selective oxidation (selox) of alcohols to desirable carbonyl products [1], [2], [3], [4], [5], [6]. The chemical synthesis of allylic aldehydes in particular, has presented a valuable testing ground for fundamental concepts in catalysis and surface science such as structure-sensitivity and the relative importance of geometric versus electronic influences, while affording commercially valuable molecules. Crotonaldehyde, produced by the low temperature oxidative dehydrogenation of crotyl alcohol (CrOH), is a termite repellant, and precursor to sorbic acid, an important food preservative [7], while cinnamaldehyde is widely used to confer a cinnamon aroma in the food and fragrance sectors, and as an insecticide to prevent the spread of mosquito larvae [8]. In the past, allylic aldehydes were synthesised via oxidation of their alcohol analogues by stoichiometric oxidants such as permanganates and chromates [9], [10], with concomitant poor atom efficiencies and high environmental disposal costs [11], [12], [13]. Gold, platinum, ruthenium and palladium have also shown promise for allylic and/or benzyl alcohol selox when dispersed on high surface area supports [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], however there have been few systematic studies to understand the surface reaction mechanism, and identify the catalytically active site in order to guide future research and produce more active, selective and critically stable catalyst formulations.
The most extensive studies of alcohol selox have been conducted over Pd catalysts, wherein liquid phase X-ray Absorption Spectroscopy (XAS) and ATR-IR measurements have uncovered important changes in the palladium oxidation state on-stream [16], [17], [26], [27], [28]. The first such time-resolved, operando investigation of cinnamyl alcohol selox highlighted a direct relationship between the PdO content and resultant catalytic activity [28], [29], noting catalyst reduction to palladium metal was accompanied by deactivation. A potential explanation for the latter observation was advanced through time-resolved X-ray photoelectron spectroscopy (XPS) and thermal desorption studies of crotyl alcohol [30], [31] and crotonaldehyde [32] over Pd(1 1 1) and Au/Pd(1 1 1) model single crystal catalysts which indicate that surface oxygen decreases the adsorption energy of both reactant and product, minimising their decomposition and subsequent site-blocking by strongly bound CO and carbonaceous residues. The hypothesis that surface Pd2+ sites are responsible for allylic and benzyl alcohol selox gained additional support from ex situ XPS and XAS measurements of high activity Pd atoms and nanoparticles on mesoporous alumina [16], [17] and silicas respectively [14].
We recently explored whether dynamic structure-reactivity measurements in the vapour phase would shed further light on the roles of palladium oxide and metal in crotyl alcohol selox [33], and demonstrated the benefits of coupling multiple operando spectroscopies with on-line mass spectrometry to detect reaction-induced restructuring (in the absence of competitive solvent adsorption or mass-transport limitations) and relate this to the nature of reactively formed adsorbates and evolved products. Expanding upon this initial synchronous, time-dependent DRIFTS/MS/XAS communication we conclusively demonstrate that crotyl alcohol selox is favoured over partially oxidised Pd nanoparticles, while undesired combustion and crotonaldehyde decarbonylation pathways are exclusive to reduced Pd metal formed in situ under oxygen deficient conditions.
Section snippets
Catalyst synthesis
Mesoporous alumina (meso-Al2O3) was prepared using the method reported by Vaudry et al. [34]. Aluminum sec-butoxide (87.5 g) was hydrolysed with de-ionized water (20.6 g) in propan-1-ol (550 g). Lauric acid (21.6 g) was added after 1 h of stirring. The mixture was aged (static) at room temperature for 24 h, and then heated for 48 h at 110 °C. The resulting solid was filtered and washed with ethanol and dried at room temperature. The lauric acid template was removed by calcination under static air at 550
Characterisation of as-prepared catalysts
The physico-chemical properties of each palladium catalyst were characterised by a range of surface and bulk analytical methods. Table 1 compares the textural properties of the parent alumina and silica supports from porosimetry and XRD, alongside the palladium dispersion/particle sizes determined by CO or H2 chemisorption. Porosity of the parent supports is clearly retained following Pd wet impregnation and subsequent high temperature calcination, precluding the likelihood of any pore collapse
Conclusion
Synchronous XAS, DRIFTS and on-stream MS measurements under dynamic operando conditions permit elucidation of fundamental structure–reactivity relationships in the selective aerobic oxidation of crotyl alcohol over palladium catalysts. Mild reaction temperatures (below 120 °C) stabilise PdO at the surface of 1–5 nm diameter nanoparticles, which promote spontaneous crotonaldehyde formation on contact with crotyl alcohol. Higher reaction temperatures facilitate alcohol-induced reduction of surface
Acknowledgements
We thank the EPSRC (EP/E046754/1; EP/G007594/2) for financial support, a Leadership Fellowship (A.F.L.), and studentship support (C.M.A.P.), and the ESRF for beamtime (CH2432). K.W. acknowledges the Royal Society for the award of an Industry Fellowship. Electron microscopy access was provided through the Leeds EPSRC Nanoscience and Nanotechnology Research Equipment Facility (LENNF) (EP/F056311/1). Additionally the EPSRC is thanked for funding the access to the TEM instruments in Oxford
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