On the Promotional and Inhibitory Effects of Water on Wacker-Type Ethylene Oxidation Over Pd–Cu/Zeolite Y

Abstract

Kinetic studies reveal the rate-enhancing and rate-inhibiting effects of water on Wacker-type ethylene oxidation at low and high partial pressures, respectively. Through time-resolved X-ray absorption spectroscopy (XAS), a highly dynamic speciation of palladium and copper in the catalyst under steady state and transient conditions involving water-switching experiments is identified. Water is demonstrated to play key roles in the reoxidation of Pd(0) and Cu­(I), aided by increased ion mobility, which we posit facilitate a more efficient electron transfer in the heterogeneous Wacker process.

Exclusive detection of ethylene using metal oxide chemiresistors with a Pd–V2O5–TiO2 yolk–shell catalytic overlayer via heterogeneous Wacker oxidation

“…Ethylene is known to yield intermediate species during heterogeneous Wacker oxidation, involving acetaldehyde, which is further oxidized to acetic acid or CO 2 . 66,67 Notably, the acetaldehyde concentration in the outlet gas increased with a decrease in the concentration of ethylene at 100-225 °C, indicating that ethylene was reformed to acetaldehyde by a catalytic reaction with 2Pd-V 2 O 5 -TiO 2 . The concentration of acetaldehyde in the outlet gas decreased with the further increase in temperature above 250 °C, and this was attributed to the further oxidation of acetaldehyde (Fig.…”

Water structures on acidic zeolites and their roles in catalysis

“…Brønsted and Lewis acidic zeolites will be the focus of this Review, given their wide applications in the development of renewable catalytic processes for transportation fuels and valuable chemicals. The roles of water in metal-containing zeolites-catalyzed reactions (e.g., methane oxidation to methanol, [30][31][32] selective catalytic reduction of nitrogen oxides (SCR-NO x ), 33,34 Wacker oxidation of ethylene, 35,36 high-temperature alkane conversion and biomass valorization 37 ) and water effects in the dynamic evolution of metal sites confined in zeolite structures are beyond the scope of the present Review. Detailed progress on the latter respect can be found from a recently published Review paper by Hu et al 38 Recently, the research group of Resasco provided comprehensive reviews of water-mediated heterogeneous catalysis 4 and the interactions of water with zeolites.…”

“…The roles of water in metal-containing zeolites-catalyzed reactions ( e.g. , methane oxidation to methanol, 30–32 selective catalytic reduction of nitrogen oxides (SCR-NO x ), 33,34 Wacker oxidation of ethylene, 35,36 high-temperature alkane conversion and biomass valorization 37 ) and water effects in the dynamic evolution of metal sites confined in zeolite structures are beyond the scope of the present Review. Detailed progress on the latter respect can be found from a recently published Review paper by Hu et al 38…”

Kinetic and Thermodynamic Factors Influencing Palladium Nanoparticle Redispersion into Mononuclear Pd(II) Cations in Zeolite Supports

“…The structural interconversion between oxide-supported Pd nanoparticles and cations is strongly affected by reaction conditions and nanoparticle sizes ,,− and is of particular relevance for several catalytic and adsorption applications including CO oxidation, ,,,− Wacker oxidation, − methane oxidation, − alkene hydrogenation, − and low-temperature (<473 K) NO trapping in diesel engine emissions. − While the dominant active sites for methane oxidation are located on Pd nanoparticles, , isolated Pd atoms and cations are the active sites for Wacker − and CO oxidation, hydrodechlorination, and NO trapping in passive NO x adsorber materials. , The redispersion of Pd nanoparticles to cations is facilitated by reactants such as NO (during NO x reduction on TiO 2 -supported three-way catalysts, H 2 -selective catalytic reduction of NO on ZrO 2 supports, sequential adsorption of CO and NO on ceria–zirconia supports, and solid-state ion exchange in CHA zeolites) and water (in FAU zeolites during Wacker oxidation − ) and via hydrothermal treatments with steam and air in aluminosilicate MFI, MWW, and chabazite (CHA) zeolites. − Redispersion can be further promoted when synthesis protocols are used that promote the formation of defect or cation-exchange sites in the support that are able to capture mobile metal species emitted from nanoparticles in atom trapping processes. , Metal–support interactions also play a crucial role to stabilize dispersed isolated atoms, with CeO 2 and Fe 2 O 3 supports exhibiting strong binding to Pt, ,,,, Rh, Ru,…”

“…In this work, we examine the thermodynamic and kinetic factors and reaction mechanisms that govern the interconversion of Pd nanoparticles and cations, studying Pd supported on aluminosilicate zeolites, given that these concepts are particularly relevant for Pd-exchanged zeolites being explored in practical applications as materials for passive NO x adsorption (PNA) ,, in automotive exhaust, Wacker, − and methane oxidation. ,,, Zeolite supports are also a versatile material platform for performing fundamental experimental studies because of the structural uniformity of their crystalline frameworks and the ability to synthesize them with precise modifications in bulk and atomic composition, which enable more faithful comparisons to the structures modeled by theoretical studies. Pd-zeolites typically contain various Pd structures , of different nuclearity (mono- and polynuclear) ,, and valence (zero- to tetravalent), − but only extraframework, mononuclear cation sites charge-compensated by framework Al atoms (ion-exchanged Pd) are the purported binding sites for NO x adsorption in the context of PNA applications and the active sites for Wacker oxidation. − The structural lability of Pd allows interconversion between agglomerated domains and mononuclear ion-exchanged sites, which depends on the size of Pd nanoparticle domains and their density on the support, and causes undesirable agglomeration and deactivation under PNA- and Wacker-relevant conditions. ,,, Yet, such structural lability allows for the redispersion of agglomerated domains to ion-exchanged sites under high-temperature (>500 K) air or NO , regeneration. Together, these challenges and observations motivate improved fundamental understanding of the critical thermodynamic and kinetic factors that govern Pd structural interconversion in zeolites to guide the design of these materials and regeneration protocols based on solid-state ion-exchange routes.…”