Adam F. Lee* and Karen Wilson
Department of Chemistry, University of York, York, UK YO10 5DD. E-mail: afl2@york.ac.uk; Fax: +44 1904 432516; Tel: +44 1904 434470
First published on 21st November 2003
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.
Green ContextThe selective oxidation of organic compounds is probably the most widely used synthetic transformation in chemistry. Catalytic aerobic oxidation is normally regarded as the best green chemistry option but such systems are generally poorly understood. Here the structural evolution of a typical oxidation catalyst, supported palladium, during an aerobic oxidation is studied using in situ XAFS. The study proves to be viable and proves the importance of surface oxide sites. The better understanding of such procedures should help us to develop and apply many more green oxidation systems based on heterogeneous catalysts.JHC |
Three main sources of catalyst deactivation have been advanced in the literature for the liquid phase oxidation of alcohols. First, so called chemical poisoning, i.e. the irreversible adsorption of strongly bound hydrocarbons, including reaction intermediates and (by) products such as carboxylic acids. This is commonly reported in aqueous media working at low pH3 or over reduced metal surfaces (low catalyst potential),4 although secondary alcohols are reported as insensitive to pH.5 Despite many electrochemical studies of related poisoning phenomena6–8 there are no direct measurements on the nature or binding strength of such adsorbates over dispersed noble metal catalysts. Surface restructuring and corrosion may also contribute to catalyst deactivation, most notably for promoted catalysts wherein the inactive component (usually Bi, Pb or Sn) is readily leached into aqueous solution.9 This is most problematic when reactions are performed in the presence of chelating agents or at acidic pH.3,10–12 Under aqueous conditions leaching is minimised through the use of a Na2CO3 or Li2CO3 buffer to maintain pH > 8. However the most important deactivation process relates to the oxidation state of the catalyst surface. From electrochemical measurements, it is widely held that alcohol oxidation occurs over metal (Pt0) sites2 and that poor rates are observed over heavily oxidised catalysts.13,14 However inefficient reactant diffusion under these simulated reaction conditions and the inherent invasive nature of electrochemical measurements may result in perturbation of the chemical state of the catalyst surface. Thus despite this common assertion there remain no direct structural investigations of the working catalyst during reaction. Consequently, in the absence of oxygen mass-transport limitations, it has been proposed that over-oxidation causes the rapid deactivation of catalysts with intrinsically poor activity.15 Indeed it has been suggested that bulk platinum oxides13 or a platinum-oxygen solid solution14,15 can form under these mild reaction conditions. In contrast Baiker and coworkers suggest that over-oxidation is the result (not cause) of chemical poisoning, noting that a partial oxygen coverage is essential for preserving high activity.16 Indeed cinnamyl alcohol oxidation is greatly suppressed over reduced Pt,Bi/Al2O3 in comparison with its oxidised counterpart.17 Clearly a subtle interplay exists between competitive adsorption of organic moieties and oxygen.
Identification of the reaction-induced morphological and chemical changes from these early studies are also complicated by the use of aqueous reaction media necessitating surfactants and pH regulation via base addition; the latter may induce oscillations in the steady state oxidation rate.18 Utilisation of aqueous solvents is often justified on the grounds that for safety reasons industrial scale oxidation reactions cannot be performed in hydrocarbon solvents and that water is the ideal ‘green’ solvent. However there are several problems associated with the use of water, namely that many organic compounds require the use of surfactants to solubilise them, increasing the processing steps and associated solvent waste produced on extraction of the product. Additionally the aqueous waste stream contaminated with any soluble organic components is particularly difficult to treat.19 Thus the environmental impact of both treating the waste water, and energy input to separate the surfactant from the product versus distillation and recycling of a benign organic solvent, must be considered.
In situ XAFS has emerged as a powerful technique for studies of catalyst evolution during activation treatments20 or steady state measurements of catalyst oxidation.21 Previously we have shown that allylic alcohol oxidation can be performed effectively in an organic solvent (ethanol, toluene),22 vastly simplifying catalytic mechanistic studies. Here we address the role of metal oxidation state in cinnamyl alcohol oxidation in a hydrocarbon solvent via spectroscopic measurements of a monometallic Pd/C catalyst. By omitting promoters we avoid issues concerning their disruption of the active ensemble through surface decoration or alloying. Complications arising from modified oxygen/substrate activation and associated spillover processes are also eliminated. Likewise use of a hydrocarbon solvent eliminates the necessity for surfactant, and improves diffusion of organic reactant. Specifically we build upon our original report23 of the application of in situ XAFS to follow the real-time, dynamic structural evolution of a heterogeneous catalyst during a liquid phase organic reaction. A transformation from oxidic to metallic Pd particles precisely correlates with severe catalyst deactivation demonstrating the importance of surface oxides in selective oxidation.
An inert internal standard, mesitylene (Aldrich 99%) was included for calculation of product yields and to ensure closure of the carbon mass balance (>95%). Aliquots (0.1 ml) of the reaction mixture were periodically withdrawn for off-line analysis using a Perkin-Elmer 8500 GC and a 30 m × 0.25 mm HP5 capillary column. Cinnamaldehyde was the principal reaction product, with a small amount of phenylpropanol and trace cinnamic acid also formed. There were no side-products attributable to solvent oxidation. Quoted conversions are ± 1% and selectivities ± 3%. Cinnamyl alcohol oxidation was also followed in situ by monitoring the oxygen uptake from the reservoir deadspace (maintained at 1 bar O2) using a Buchi Pressflow Gas Controller.
X-Ray photoelectron spectra were acquired using a Kratos Axis HSi instrument operating at 225 W with a Mg Kα excitation source (1253.6 eV) and a CHA analyser selecting at 20 eV pass energy. The sample was charge neutralised and Pd 3d, and C and O 1s spectra recorded at normal emission to the surface. Binding energies were referenced to the C 1s of graphite at 285 eV. X-Ray diffractograms were recorded on a Siemens D5000 diffractometer using Cu Kα radiation and a 2θ scan range of 10–105° in 0.02° steps. Surface areas were determined by N2 physisorption and metal dispersions by CO chemisorption.
Fig. 1 Pd 3d XP spectra of fresh and spent Pd/C catalyst after 10 h cinnamyl alcohol oxidation. A spectrum of the H2 reduced catalyst is shown for comparison. |
Fig. 2 Time-resolved Pd K-edge quick XAFS of a Pd/C catalyst during cinnamyl alcohol oxidation. Reference spectra of PdO and a Pd foil are shown for comparison. |
Fig. 3 Pd K-edge (a) k3-weighted raw EXAFS and (b) pseudo-radial distribution function of a fresh Pd/C catalyst. |
Parameter | PdO | Fresh | Spent | Pd Foil |
---|---|---|---|---|
CN1Pd–O | 4 | 2.5 | — | — |
CN1Pd–Pd | 8 | 1.7 | 4.45 | 12 |
CN2Pd–Pd | 2 | 2.8 | — | 6 |
r1Pd–O / Å | 2.03 | 2.04 | — | — |
r1Pd–Pd / Å | 3.07 | 2.76 | 2.76 | 2.74 |
r2Pd–Pd / Å | 3.45 | 3.43 | 3.87 | |
σ1Pd–O | 0.007 | 0.007 | — | — |
σ1Pd–Pd | 0.017 | 0.012 | 0.011 | 0.014 |
σ2Pd–Pd | 0.005 | 0.018 | — | 0.02 |
The Pd K-edge spectra remain unchanged for ∼40 min following introduction of the reaction medium to the catalyst, Fig. 4, a period during which cinnamyl alcohol oxidation proceeds with a constant (maximum) rate. Subsequently the white line intensity (edge-jump), a direct measure of the Pd oxidation state,29 shows a sharp drop which is complete after 120 min reaction, associated with a transformation from oxidic to metallic palladium.
Fig. 4 Correlation between oxygen uptake and normalised Pd K-edge white line intensity as a function of reaction time. |
The fitted χ data and radial-distribution function (Fig. 5a and b) confirm the loss of the Pd–O coordination shell and concomitant increased Pd–Pd scattering consistent with pure Pd particles. This structural evolution coincides with a rapid decrease in catalyst activity; the oxidation rate falls from ∼0.18 to 0.02 mmol min−1 (g cat)−1. Loss of activity due to particle sintering upon PdO reduction can be discounted as XAFS provides both oxidation state and particle size information, and reveals that the latter remains unchanged over this time period. Following this chemical reduction the average Pd coordination number begins to rise giving a limiting particle size of ∼80 Å (confirmed by XRD and TEM measurements). In all cases the Debye–Waller factors (σ), a measure of vibrational disorder, were low and in good agreement with the respective Pd oxide or metal standards. Of course it is important to establish whether these values represent true catalytic rates [eqn. (1)] or simply reflect the stoichiometric consumption of PdO [eqn. (2)].
Ph–CHCH–CH2OH + ½ O2 → Ph–CHCH–CHO + H2O | (1) |
Ph–CHCH–CH2OH + PdO → Ph–CHCH–CHO + Pd + H2O | (2) |
Fig. 5 Pd K-edge (a) k3-weighted raw EXAFS and (b) pseudo radial distribution functions of a spent Pd/C catalyst. |
From XPS the fresh catalyst contains ∼2.86 × 10−5 moles of PdO. Assuming a 1 : 1 stoichiometry for the reaction between Oa, liberated by PdO decomposition, and cinnamyl alcohol to form cinnamaldehyde, and that catalyst reduction is complete after 120 minutes, a maximum rate of ∼0.0024 mmol min−1 (g cat)−1 is obtained. This value is much less than the measured rates and we can thus neglect direct contributions from the catalyst reduction.
Fig. 6 Comparison of cinnamyl alcohol oxidation rates over fresh and prereduced catalysts. |
Fig. 7 X-Ray diffractograms of fresh and 190 °C H2 reduced Pd black. |
Fig. 8 Pd 3d XP spectra of fresh and prereduced Pd black. |
Direct low temperature (<200 °C) reduction under H2 was subsequently attempted. As expected reduced Pd black samples transferred under nitrogen showed a significant increase in average particle size determined by XRD, reaching ∼600 Å after 190 °C reduction. The associated Pd 3d XP spectra reveal a dramatic change in surface composition with only metallic Pd sites remaining following reduction. Fig. 9 compares the catalytic performance of fresh and reduced Pd black samples towards cinnamyl alcohol oxidation. Conversion over fresh Pd black was significantly lower (∼19%) than that observed over Pd/C under identical reaction conditions reflecting the great difference in overall metal dispersion. Reduction temperatures as low as 130 °C proved sufficient to dramatically decrease catalytic activity, which was completely suppressed above 190 °C.
Fig. 9 Effect of catalyst pretreatment on cinnamyl alcohol conversion over fresh, reduced and reoxidised Pd black after 10 h reaction. |
This transformation correlates directly with the loss of active surface palladium oxide (and not particle size effects). Since reduced samples were cooled under nitrogen any contribution from a palladium hydride phase may also be discounted. Confirmation for the crucial role of surface oxide in cinnamyl alcohol reaction was obtained from the reoxidation of fully reduced Pd black; calcination under oxygen (10 ml min−1 at 190 °C for 2 h) resulted in partial regeneration of the original oxidation performance. A similar effect was observed following ambient addition of 40 w/v% H2O2 solution to reduced Pd black followed by filtration and drying. These results also demonstrate that poisoning due to surface reduction alone is reversible. In contrast pure PdO particles showed no activity towards cinnamyl alcohol presumably reflecting the high thermodynamic stability of bulk versus surface oxides.
In situ XAFS has enabled us to identify the reduction of catalytically active PdOx surface sites during Pd/C catalysed allylic alcohol oxidation. That this proposed deactivation mechanism differs from reactions performed in aqueous media may be ascribed to the relative solubilities of the reactants. Operation in aqueous solvents, wherein allylic alcohol solubilities and diffusion rates are so poor as to necessitate surfactant addition, result in high O2 : substrate ratios which thereby favour catalyst overoxidation. However in organic media, cinnamyl alcohol diffusion to the catalyst surface is facile, preventing overoxidation. The predominant catalyst deactivation route now occurs via reduction of surface PdO→Pd and associated build-up of irreversibly adsorbed by-products on the resultant metal surface. Recent ATR measurements support that under these conditions catalyst deactivation occurs via site blocking by adsorbed CO32 formed during decarbonylation reactions.
This journal is © The Royal Society of Chemistry 2004 |