Impact of Co‐Components on the State of Pd and the Performance of Supported Pd/TiO₂ Catalysts in the Gas‐Phase Acetoxylation of Toluene

Abstract

The effect of the co‐components Mn, Co, Au, and Sb with a wide range of standard reduction potentials (Mn2+/Mn: E°=−1.18 V; Co2+/Co: E°=−0.28 V; Sb3+/Sb: E°=+0.2 V; Au3+/Au: E°=+1.52 V) on the catalytic performance of 10 wt % Pd, 8 wt % M/TiO2 (M=Mn, Co, Sb, Au) catalysts in the gas‐phase acetoxylation of toluene to benzyl acetate has been studied. Co‐components with low E° are more active, but less selective than those with high E°. Co‐components with low E° (Mn, Co) stabilize Pd in its oxidized form, whereas those with high E° (Sb, Au) support the formation of metallic Pd. Sb and Au are incorporated into the Pd lattice, whereas Mn and Co are enriched at the surface during time on stream. Carbon is deposited on all catalysts during the reaction. However, for Mn‐ and Co‐containing catalysts, carbon is incorporated into the metal particles, whereas it is deposited on top of Pd particles modified with Sb and Au, which leads to faster deactivation.

New Insights into the Nature of Co‐components and Their Impact on Pd Structure: X‐ray Absorption Studies on Toluene Acetoxylation Catalysts

“…This has been explained by the formation of Pd surface oxides. XANES revealed that this oxidized Pd was not only present at the surface, as proposed previously,7 but even in the bulk of the particles. Apart from Pd surface oxide, another possible explanation of such slightly higher Pd binding energies around 335.5 eV, as found for Pd,Mn‐u11, was reported for a near‐surface Pd carbide investigated by in situ XPS 26–28.…”

“…[13] As ac onsequence,t he FTs( Figure4d)showed as hift of R to higher values compared to that of the Pd-foil reference (2.744 ). Thus, the first PdÀPd shell in the Pd,Mn-u11, Pd,Co-u11, and Pd,Sb-u11w as observed at R PdÀPd = 2.80, 2.79, and 2.76 , respectively.T hese values confirmedt he lattice expansion already seen for these materials by XRD [7] and pointed to the incorporation of othere lements into the Pd lattice.O nt he basis of TG investigations combined with XRD, we have previously proposed that carbon is preferentially incorporated into the Pd lattice when Mn or Co is present as co-component,a nd incorporation of Sb was anticipated for Pd,Sb-u11c atalysts. [7] In this former study,i tc ould be shownt hat regenerationa t3 00 8Ci na ir led to the typical Pd lattice parameters in the case of Mn and Co as co-components, which was explained by removal of carbon.…”

“…Both features indicated clearly the presence of a tiny amount of Sb 0 resulting from the reduction of a small part of the Sb 5+ species. Only with a detailed inspection of the XAS results of the Pd edges could it be clarified whether small or few pure Sb or mixed Pd–Sb particles exist (see below) 7. 12…”

“…Only with ad etailed inspection of the XAS resultso f the Pd edges could it be clarifiedw hether smallo rf ew pure Sb or mixed Pd-Sb particles exist (see below). [7,12] X-ray absorption spectra at the Pd K-edge…”

Steady-State and Transient Kinetic Studies of the Acetoxylation of Toluene over Pd–Sb/TiO2

“…6,7 Our group has intensively investigated acetoxylation of toluene over supported Pd-containing catalysts with the purpose of optimizing their composition 8,9 and reaction conditions. 3 The major drawback of such materials is their on-stream behavior, i.e.…”