Wacker Catalyst Research Articles

Elucidating the mechanism of heterogeneous Wacker oxidation over Pd-Cu/zeolite Y by transient XAS

The heterogenization of Wacker catalysts using chloride-free systems can potentially be a good alternative for the commercial homogeneous Wacker oxidation of ethylene, which utilizes excessive aqueous chloride solvents. However, the mechanism of the heterogeneous system has not been clarified, preventing the rational design of better catalysts. Here, we report a transient X-ray absorption spectroscopic (XAS) investigation of the heterogeneous Wacker oxidation over Pd-Cu/zeolite Y coupled with kinetic studies and chemometric analysis. Insight is obtained by operando quickXAS allowing the quantitative determination of rates and thereby revealing a rapid redox reaction involving copper. Our work demonstrates that copper is not only the site of oxygen activation, but is also involved in the formation of undesired carbon dioxide. Without detecting the presence of Cu(0) and Pd(I), our results suggest that two one-electron transfers to two Cu(II) ions to reoxidize Pd(0) is at work in this heterogeneous Wacker catalyst.

Optimization of a heterogeneous Pd-Cu/zeolite Y Wacker catalyst for ethylene oxidation.

Exchanging low amounts of palladium with excess copper in a heterogeneous zeolite Y catalyst leads to high Wacker activity and mitigates activity loss by reducing the extent of palladium sintering. Employing periodic regenerative treatments in oxygen to remove carbon deposits and reoxidize palladium results in the partial (but reversible) recovery of the high initial activity.

Highly efficient catalyst PdCl2–CuCl2–KOAc/AC@Al2O3 for gas-phase oxidative carbonylation of methanol to dimethyl carbonate: Preparation and reaction mechanism

The supported Wacker catalysts previously used for the synthesis of dimethyl carbonate (DMC) by gas-phase oxidative carbonylation of methanol have the drawbacks of low selectivity and poor catalytic stability. To solve these problems, PdCl2–CuCl2–KOAc/AC was treated by both Al2O3 and dilute hydrochloric acid to obtain PdCl2–CuCl2–KOAc/AC@Al2O3 in this work. The treatment brought about not only the higher activity but also the better stability and selectivity: 98% of DMC selectivity to methanol, 70% of DMC selectivity to CO, and space–time yield of DMC ranging from 600 to 700gL-cat−1h−1 for the runtime of 40h. PdCl2–CuCl2–KOAc/AC@Al2O3 was characterized and the results showed that it possessed enlarged specific surface area (BET), enriched active components on the surface (SEM), increased proportion of Cu+/Cu2+ and new Cu2O crystalline phase formed (XPS), as compared with PdCl2–CuCl2–KOAc/AC. The new formed Cu2O can be used not only as a catalyst for the synthesis of DMC but also as an inhibitor for the loss of Cl− by the reaction of Cu2O with Cl− to CuCl, resulting in the promotion of the catalytic stability. The characterization of the recovered catalyst further identified that the inhibition of KCl crystal formation and the increase in Cu+/Cu2+ proportion could improve the catalytic stability and selectivity. In addition, the higher ratio of Cu+/Cu2+ can reduce Pd2+ concentration and prohibited side-reaction of CO oxidation, so DMC selectivity to CO is increased. On the basis of activity evaluation and characterization, the catalytic reaction mechanism is proposed.

Deactivation mechanism of PdCl2-CuCl2/Al2O3 catalysts for CO oxidation at low temperatures

Supported Wacker catalysts exhibited high activity for low temperature CO oxidation, but high concentrations of water in the reaction gas could lead to the deactivation of the catalyst. The PdCl 2 -CuCl 2 /Al 2 O 3 catalysts were prepared, and a high relative humidity deactivation mechanism was suggested after characterizing the fresh and spent catalyst by X-ray diffraction, N 2 absorption-desorption, X-ray photoelectron spectroscopy, H 2 -temperature programmed reduction and in situ diffuse reflectance infrared Fourier transform spectroscopy. The results showed that the water could adsorb and condense on the surface of the catalyst, which arouses the aggregation and transfer of Cu species into the internal pores of the catalyst. This weakens the interactions between the Pd and Cu species and reduces the re-oxidation ability of Pd 0 to Pd 2+ , which blocks redox cycling and results in the decrease of CO oxidation activity. 采用X射线衍射、N 2 吸-脱附、X射线光电子能谱分析、氢气-程序升温还原和原位红外漫反射等方法对新鲜和失活的PdCl 2 -CuCl 2 /Al 2 O 3 低温催化CO氧化催化剂进行表征, 研究了高相对湿度(100%)下催化剂的失活机理. 结果表明, 催化剂表面沉积的水使得活性铜物种容易从催化剂表面向载体孔道内部迁移, 由于Pd、Cu相互作用弱化从而减弱了Pd与Cu物种间的相互作用, 使得催化剂的氧化还原性能受到影响, 抑制了Pd 0 再氧化为Pd 2+ 的过程, 从而因CO氧化反应中催化剂氧化还原循环受阻而导致失活. The transfer of Cu species to the internal pores of the support in the presence of water weakens the Pd and Cu species interactions, which causes catalyst deactivation.

Wacker oxidation of ethylene over vanadia nanotube supported Pd catalysts

The selective oxidation of olefins to aldehydes and ketones by O2 using PdCl2/CuCl2 catalyst system in hydrochloric acid solution is known as Wacker process. Because of the advantages of heterogeneous catalytic systems there is great industrial interest in the development of solid Wacker catalyst. The present paper relates to the preparation and properties of novel Pd/vanadia nanotube catalysts and their activity in the Wacker oxidation of ethylene. The VOx nanotubes, having coil structure, were shown to be stabilized by interlayer cations. The as-synthesized VOx nanotubes had diameter of about 70–120nm and contained exchangable interlayer alkylammonium (AA) cations. By ion exchange VO2+ and Cu2+ cationic forms were obtained. All three forms were tested as catalyst support. Palladium nanoparticles of 8–12nm sizes were generated on the surface of the VOx tubes. The best acetaldehyde selectivity was achieved with the Pd/AA–VOx catalyst.

Palladium(II)-Catalyzed Asymmetric Coupling of Allylic Alcohols and Vinyl Ethers: Insight into the Palladium and Copper Bimetallic Catalyst

The use of PdX(2), CuX(2) (X = OAc, OCOCF(3), or Cl), and catechol together with (S,S)-4,4'-bisbenzyl-2,2'-bioxazoline under O(2) induces the asymmetric coupling of cinnamyl alcohols and vinyl ethers to give (R)-(+)-2-alkoxy-4-benzylidenetetrahydrofurans in 40-53% ee (79% yield). The present study provides implications for the so-called Wacker catalyst of PdX(2) and CuX(2), in terms of that the anionic ligands (X) of Pd and Cu interchange between two metals.

Transition metal salts of heteropoly acids as palladium cocatalysts for Wacker oxidation of ethene

Heteropoly acid (HPA) salts containing transition metal cations have been applied as reoxidants of palladium(II) in heterogeneous Wacker catalyst for the oxidation of ethene to acetaldehyde. It has been found that the presence of transition metal cations increases the initial ethene conversion and influences the heteropoly anion reducibility. The steady state activity has been shown to be inversely proportional to the facility of oxygen removal from the HPA salts.

The performance of titania-supported Wacker catalysts in the oxidation of 1-butene

The kinetics of 1-butene oxidation over titania-supported catalysts has been investigated. The reaction rate shows a first-order dependence on 1-butene and water coverage at low partial pressures. At higher partial pressures of water in the feed the reaction rate first strongly increases and then levels off, when the water coverage exceeds a monolayer, probably due to partial dissolution of PdSO 4. The concentration of dioxygen in the feed has a slight positive influence on the activity up to 15 kPa. Heterogeneous Wacker catalysts consist of a palladium salt deposited on a vanadium pentoxide layer on a support material such as titania. The composition of the catalyst appears to be of major influence on its performance. For palladium acetate-based catalysts the activity shows a first order in the palladium coverage. The activity of the catalysts decreases with increasing vanadium oxide coverage. Temperature programmed reduction (TPR) shows that the reduction temperature of the vanadium oxide overlayer increases with increasing coverage. The easiness of reducibility of the titania-supported catalysts in comparison to the alumina-based ones is directly related to the one order of magnitude higher activity of the tinania-based catalysts. On oxidizing 1-butene to butanone, the activity of titania-supported catalysts decreases by approximately 60% during the first 100 h of operation. After this initial deactivation, the activity and selectivity of the catalyst remains stable for more than 700 h. TPR and high resolution electron microscopy (HREM) show that this deactivation is mainly caused by a slight sintering of the vanadium oxide layer.

Mn(5−n)/2HnPMOV2 as re-oxidant of palladium(II) in solid Wacker catalysts

Managanese salts of heteropoly acids (HPA) containing molybdenum and vanadium as poly atoms appeared to be active co-catalysts of palladium(II) in heterogeneous Wacker systems employed for ethene to acetaldehyde oxidation. The ethene conversion and the catalyst lifetime depend on the number of HPA protons replaced with Mn2+ cations.

Transition metal salts of heteropoly acids as palladium co-catalysts for Wacker oxidation of butene-1 to MEK

The transition metal salts of heteropoly acids have been applied as palladium re-oxidants in heterogeneous solid Wacker catalysts for the oxidation of butene-1 to MEK. Silica, γ-alumina and titania were used as supports for the Pd/MeHPMoVx system. The influence of cations bonded to heteropoly anions and also the effect of the catalysts acidity on the butene-1 to MEK oxidation have been elucidated.

XAFS characterization of supported PdCl2−CuCl2 catalysts for CO oxidation

A heterogenized Wacker catalyst system in which pores of a high surface area alumina were filled with an aqueous solution of PdCl2−CuCl2 was active for the oxidation of CO near room temperature. The structure of thecatalyst was studied by XRD and XAFS. The active phase of Pd was a molecular Pd species whose structure was similar to PdCl2, probably modified by a carbonyl ligand. The active phase of copper was found to be solid Cu2Cl(OH)3 particles. The presence of Cu was essential to keep the Pd in the Pd(II) state during the reaction.

Supported PdCl2[sbnd]CuCl2 catalysts for carbon monoxide oxidation II. XAFS characterization

A heterogenized Wacker catalyst system in which pores of a high surface area alumina were filled with an aqueous solution of PdCl2CuCl2 was active for the oxidation of CO near room temperature. The structure of the catalyst was studied by X-ray absorption fine structure (XAFS). The active phase of Pd was a Pd11 species containing chlorine and, probably, carbonyl ligands. Direct interaction of PdPd or PdCu was not detected. The active phase of copper was found to be solid Cu2Cl(OH)3 particles in agreement with the X-ray diffraction (XRD) results. The presence of Cu was essential to keep the Pd in the Pd11 state during the reaction. The rates of CO oxidation measured at temperatures of 30–70°C showed a minimum at 40°C, which was attributable partly to an unusual structure change of the active palladium species during the reaction at this temperature.

Performance of γ-alumina-supported Wacker catalysts in the oxidation of 1-butene

Heterogeneous Wacker oxidation catalysts, consisting of a palladium complex supported on a monolayer vanadium pentaoxide on γ-alumina, have been tested in the oxidation of 1-butene to butanone. The steady-state activity of the catalysts is found to be 0.2 mmol butanone per mol Pd per second. The oxidation of 1-butene over the heterogeneous catalysts runs analogously to the homogeneous Wacker oxidation. The stability of the catalysts depends on the structure of the vanadium pentaoxide overlayer. Continuous reoxidation of Pd 0 is possible only if the vanadium pentaoxide is present in a molecularly dispersed overlayer and does not proceed when crystalline vanadium pentaoxide is used as a support. Temperature-programmed reduction measurements have shown that this is the result of the lower reducibility of the crystalline vanadium pentaoxide as compared to supported vanadium pentaoxide monolayers. The stability of the supported catalysts depends strongly on the vanadium pentaoxide coverage; due to sintering of the vanadium pentaoxide overlayer, a rapid deactivation is observed for catalysts with coverages below 1.5 V/nm 2. Sintering is probably caused by the poor interaction of the partly reduced vanadium pentaoxide layer with γ-alumina. X-ray photoelectron spectroscopy analysis revealed that under the reaction conditions 35% of the vanadium pentaoxide is present as V 4+ and 50% of the palladium is present as Pd 0.

Heteropolyanions as Redox Components in Heterogeneous Wacker Oxidation Catalysts

Heteropolyanions (HPAs) of the Keggin type have been successfully applied as redox components in heterogeneous Wacker catalysts. The catalysts consist of a silica support covered with a layer of HPA of the series H3+nPVnMo12−nO40 on which a submonolayer of palladium sulfate is deposited. The initial butanone yield in the oxidation of 1-butene at 343 K is approximately 0.10 g butanone per gram of catalyst per hour. The steady-state activity is, however, a factor of 10 lower due to the slow reoxidation of reduced HPA. The rate of reoxidation and hence the steady-state activity increases with increasing number (n) of vanadium atoms per Keggin unit and when the protons of the HPA are replaced by Cu2+ or by Ni2+. Even higher activity is obtained with palladium salts of heteropolyacids. In these palladium salts the palladium reaction centre and the redox component are combined in one complex. The butanone selectivity of the catalysts with n > 0 is high, viz. >95%, and increases with increasing values of n to more than 98%. The butanone selectivity also increases when the protons are exchanged for metal cations, such as Cu2+, Ni2+, Pd2+, or Cs+. The stability of the Keggin units under reaction conditions is high. Under Wacker oxidation conditions the HPAs are partly reduced, but DRIFT and ESR spectroscopy and TPR analysis of used catalysts show that the Keggin structure remains intact. Reoxidation of the reduced HPAs is, however, slow under reaction conditions.

Oxidation of olefins by palladium(II). Part XIV . Product distribution and kinetics of the oxidation of ethene by PdCl 3 (pyridine) − in aqueous solution in the presence and absence of CuCl 2: a modified Wacker catalyst with altered reactivity

In the absence of CuCl 2, ethene was oxidized to ethanal by PdCl 3 (pyridine) − in aqueous solution by the rate expression: − d[C 2H 4] dt = k′K′ [PdCl 3(Py) −][C 2H 4] [H +][Cl −] 2 where K′ is the equilibrium constant for π-complex formation between ethene and PdCl 3(Py) − (Py = pyridine). This rate expression is of the same form as that previously found for the oxidation of ethene by PdCl 2− 4 in aqueous solution (Wacker reaction). The value of K′ for PdCl 3(Py) − was found to be 20.3 which is close to the value of 17.4 previously measured for π-complex formation between ethene and PdCl 2− 4. However, the value of k for PdCl 2− 4 was 750 times the value of k′ for PdCl 3(Py) −. This result suggests that the hydroxypalladation adduct from PdCl 3(Py) − is much more stable towards decomposition to ethanal than the corresponding one from PdCl 2− 4. A direct result of this higher stability is the expectation that the adduct from PdCl 3(Py) − should be more readily intercepted by CuCl 2 to produce 2-chloroethanol. At chloride concentrations as low as 0.2 M and [CuCl 2] = 4 M, the product was almost 50% 2-chloroethanol. At [CuCl 2] = 8 M, the product was 98% 2-chloroethanol. With PdCl 2− 4, a chloride concentration of 3 M is required before an appreciable amounts of 2-chloroethanol are produced at any cupric chloride concentration. For the reaction of ethene with PdCl 2− 4, these results are consistent with a mechanism involving cis addition at low [Cl −] and trans addition at high [Cl −]. The pathway for ethanal formation may be different with PdCl 3(Py) − than it is with PdCl 2− 4.

ChemInform Abstract: Oxidation of Ethylene to Acetaldehyde Over a Heterogenized Surface‐ Vanadate Wacker Catalyst in the Absence of Gaseous Oxygen.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Oxidation of ethylene to acetaldehyde over a heterogenized surface-vanadate Wacker catalyst in the absence of gaseous oxygen

The catalytic Wacker-oxidation of ethylene, 1 -butene and styrene to the corresponding aldehydes and ketones over heterogenized catalysts has been demonstrated recently by us. The heterogenized catalysts consist, for instance, of H 2PdCl 4 adsorbed on γ-Al 2O 3 or on TiO 2 (anatase), pre-covered with a monomolecular layer of vanadium pentoxide. Normally the feed of the reactor is composed of an alkene, water and oxygen. In the present article it is shown that the reaction can also be carried out in the absence of oxygen in the feed. In the last case the vanadate layer on the support is gradually reduced to a surface-hydrogen bronze of vanadium, H x V 2O 5, with x = 1.4 to 2 when the rate of reaction has fallen to zero. The catalytic activity can be reinstated by injection of an oxygen-containing gas in the feed. In view of the importance of the formation of the hydrogen bronze of vanadium in the reaction described above, a separate study has been made of hydrogen spillover from small palladium crystallites on monolayers of vanadium pentoxide supported on γ-alumina or an anatase. On V 2O 5/γ-Al 2O 3 the diffusion coefficient of H atoms, D(373 K), was found to be about 10 −14 cm 2 s −1, and the activation energy amounts to 87 kJ mol −1. On the surface of the anatase-supported monolayer, however, the diffusion coefficient is appreciably higher. Here D( 373 K) is about 2.10 −13cm 2s −1.

Palladium–copper-exchanged Y type zeolites: a true heterogeneous Wacker catalyst

Evidence is presented that faujasite-type zeolites with specific Si : Al framework ratios exchanged with Pd(NH3)42+ and Cu2+, catalyse the oxidation of ethylene into acetaldehyde, in exactly the same way as the homogeneous Wacker system [PdII and CuII in concentrated chloride solution]; the active centre is found to be a partially ammoniated PdII ion, most probably PdII(NH3)2, which itself belongs to an electron transfer chain consisting of the alkene reagent, the faujasite encaged PdII/Pd0 and CuII/CuI redox couples and dioxygen.

ChemInform Abstract: Kinetics and Mechanism of the Gas‐Phase Oxidation of Styrene to Benzaldehyde, Acetophenone and trans‐Cinnamaldehyde Over a Heterogeneous Surface‐Vanadate Wacker Catalyst.

ChemInformVolume 21, Issue 23 Preparative Organic Chemistry ChemInform Abstract: Kinetics and Mechanism of the Gas-Phase Oxidation of Styrene to Benzaldehyde, Acetophenone and trans-Cinnamaldehyde Over a Heterogeneous Surface-Vanadate Wacker Catalyst. E. VAN DER HEIDE, E. VAN DER HEIDE Fac. Chem. Technol. Mater. Sci., Delft Univ. Technol., 2628 BL Delft, Neth.Search for more papers by this authorJ. SCHENK, J. SCHENK Fac. Chem. Technol. Mater. Sci., Delft Univ. Technol., 2628 BL Delft, Neth.Search for more papers by this authorA. W. GERRITSEN, A. W. GERRITSEN Fac. Chem. Technol. Mater. Sci., Delft Univ. Technol., 2628 BL Delft, Neth.Search for more papers by this authorJ. J. F. SCHOLTEN, J. J. F. SCHOLTEN Fac. Chem. Technol. Mater. Sci., Delft Univ. Technol., 2628 BL Delft, Neth.Search for more papers by this author E. VAN DER HEIDE, E. VAN DER HEIDE Fac. Chem. Technol. Mater. Sci., Delft Univ. Technol., 2628 BL Delft, Neth.Search for more papers by this authorJ. SCHENK, J. SCHENK Fac. Chem. Technol. Mater. Sci., Delft Univ. Technol., 2628 BL Delft, Neth.Search for more papers by this authorA. W. GERRITSEN, A. W. GERRITSEN Fac. Chem. Technol. Mater. Sci., Delft Univ. Technol., 2628 BL Delft, Neth.Search for more papers by this authorJ. J. F. SCHOLTEN, J. J. F. SCHOLTEN Fac. Chem. Technol. Mater. Sci., Delft Univ. Technol., 2628 BL Delft, Neth.Search for more papers by this author First published: June 5, 1990 https://doi.org/10.1002/chin.199023125AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume21, Issue23June 5, 1990 RelatedInformation

Kinetics and mechanism of the gas‐phase oxidation of styrene to benzaldehyde, acetophenone and trans‐cinnamaldehyde over a heterogeneous surface‐vanadate Wacker catalyst

AbstractA study has been made of the kinetics and mechanism of the oxidation of styrene over a heterogeneous vanadate‐monolayer Li2PdCl4 Wacker catalyst in the temperature range 348 K to 453 K at 0.11 MPa total pressure. The kinetics of the reaction, with respect to the dependence of the reaction rate on the partial pressures of styrene and water, as well as on the surface LiCl concentration, appear to be logically related to the kinetics of the homogeneous Wacker process.In addition to acetophenone (initial selectivity 45 mol %), benzaldehyde, probably originating from phenylacetaldehyde, is formed with gradually increasing selectivity (initial selectivity 55 mol %). Formation of benzoic acid can sometimes be detected but in all cases with a selectivity lower than 5 mol %. Initially no other products are detected but, after a short period, trans‐cinnamaldehyde is formed with increasing selectivity (15 mol % after three days).Deactivation of the catalyst is due to carbonaceous deposits and removal of these deposits by oxidation with air at 473 K leads to partial regeneration.