Elucidating the mechanism of heterogeneous Wacker oxidation over response from Pd speciation (Figure 1) demonstrating for the first time that oxygen is activated Pd-Cu-exchanged zeolite Y via transient multi-edge XAS on copper in the heterogeneous system, which is similar to the homogeneous system. Increasing the O partial pressure led to a quick reoxidation of Cu(I) with a concomitant rapid 2 increase in CO formation while that of acetaldehyde proceeded more slowly. These results Jerick Imbao1,2, Jeroen van Bokhoven1,2*, Adam Clark1 and Maarten Nachtegaal1* 2 indicate that copper has more than one role as it does not only act as a co-catalyst for Pd(0) 1Paul Scherrer Institute, Villigen PSI (Switzerland) regeneration, but is also involved in the formation of CO byproduct. Without detecting the 2ETH Zurich, Zurich (Switzerland) 2 transient presence of Cu(0) and Pd(I), our results suggest that two one-electron transfers from *[email protected], [email protected] two Cu(II) ions to reoxidize Pd(0) to Pd(II) is at work in this heterogeneous Wacker catalyst.

Introduction Low O (half order) High O (zero order) The homogeneous ethylene oxidation, called the Wacker process, is one of the most 1.00 2 1.00 2 efficient synthetic methods for manufacturing aldehydes. This process suffers from the difficulty in the separation of the products from the catalyst and from the high corrosivity and 0.75 0.75 formation of chlorinated byproducts associated with the excess use of HCl [1]. To overcome Cu(II) Cu(II) these problems many studies have been undertaken to design chloride-free heterogeneous 0.50 0.50 Cu(I) Cu(I) Wacker catalyst systems. However, most of these systems lacked either sufficient activity or

Fraction stability due to inefficient Pd reoxidation. Moreover, the heterogeneous Wacker reaction 0.25 0.25 mechanism has not yet been fully established. a b 0.00 0.00 0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 30 35 40 45 In this study, we present a multi-technique investigation of ethylene oxidation over 1.00 1.00 Pd(II) Pd(II) palladium- and copper-exchanged Y zeolite (Pd-Cu-Y) catalysts via time-resolved transient X- Pd(0) Pd(0) ray absorption spectroscopy (XAS) coupled with kinetic studies. Through this, copper and 0.75 0.75 palladium speciation is reported in light of the redox process involved in Wacker oxidation. The roles of the redox couples are investigated under transient conditions revealing dynamic 0.50 0.50

changes in the electronic and local environment of the metal centers in different kinetic Fraction regimes. 0.25 0.25 c d 0.00 0.00 Materials and Methods 0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 30 35 40 45 Heterogeneous Wacker catalysts were prepared by ion exchange of Pd(II) and Time (min) Time (min) Cu(II) on zeolite Y because it is closely similar to the homogeneous system and it is one of the Figure 1. Concentration profiles of Cu (a,b) and Pd (c,d) species (calculated from MCR-ALS most active and stable catalysts for heterogeneous Wacker oxidation [2]. The kinetics of the of time-resolved XANES) as a function of time on stream after changing the partial pressure of reaction was investigated by determining the overall reaction rates and order with respect to oxygen in the feed (transient experiment). The catalyst was equilibrated for 4 hours under ethylene, water and oxygen under low conversions. Time-resolved XAS measurements [3], Wacker conditions before the start of the transient experiment (t<0). with a resolution of 0.5 s, were performed at the Cu and Pd K-edges in transmission mode in a quartz capillary reactor cell connected to a mass spectrometer in steady-state and transient Significance conditions wherein the partial pressure of a reactant is changed to monitor structural and This time-resolved transient XAS study does not only provide the first direct oxidation state changes in the catalyst in different kinetic regimes. Principal component factor evidence for the redox process in heterogeneous Wacker oxidation, but it also reveals the analysis (PCA) was employed to determine the number of components that are significantly multiple roles of copper and oxygen that have not been previously demonstrated. The contributing to each spectrum. Subsequently, a multivariate curve resolution – alternating least methodologies developed in this work have high potential to be generally applied for studies of squares (MCR-ALS) chemometric algorithm [4] was used to disentangle each of the redox processes involved in heterogeneous catalysis. contributions of copper and palladium species directly from experimental data.

References Results and Discussion 1. J. Smidt, et al., Angew. Chem. Int. Ed. Engl., 1, 80-88 (1962). Kinetic studies revealed drastic changes when the partial pressure of O was varied; 2 2. P. Espeel, et al., Chem. Commun., 10, 669-671 (1991). exhibiting an order of zero and 0.5 at high and low partial pressures, respectively. After 3. O. Mueller, et al., J. Synchrotron Radiat., 23, 260-266 (2016). equilibration to Wacker conditions, lowering the O partial pressure resulted in a fast 2 4. J. Jaumot, et al. Chemometrics Intell. Lab. Syst., 76, 101-110 (2005). reduction of Cu(II) to Cu(I) and disappearance of CO2 but elicited a much slower