Development of high performance vinyl acetate monomer (VAM) catalysts

The focus of this study was to develop high performance catalysts for the synthesis of vinyl acetate monomer (VAM). By systematic variation of different preparation parameters a multitude of shell catalysts consisting of PdAu nanoparticles supported on a bentonite carrier was explored. In order to investigate the influence of these alterations on catalytic performance, a catalyst classification was accomplished in a high-throughput Temkin test unit by comparison with a highly efficient commercial benchmark, referred to as B. Due to the applied Temkin reactor concept efficient heat and mass transport was ensured. Thus, the prepared catalysts could be clearly distinguished with regard to their selectivity (±0.25 %) and space time yield (±5 %) within a wide range of values. Catalyst performances for the selective oxidation of ethylene and acetic acid to VAM revealed that samples synthesised in this study are able to compete with this state-of-the-art plant catalyst. Concerning the selectivity meaningful improvements of almost 3% were achieved by catalysts based on different KA-Zr carriers (Zr doped bentonite). Gas phase reduction (GPR) at various temperatures was also identified as a promising synthesis step. Although catalysts produced by liquid phase reduction (LPR) achieved on average about 11% higher activity (STY ) than the internal standard, outstanding enhancements of up to approximately 40% were attained via forming gas reduction. A challenging aim of this thesis can be attributed to the characterisation of VAM catalysts. In view of the fact that the most significant differentiation in performance was observed between catalysts prepared using LPR and GPR, samples from each reduction medium were chosen for a detailed characterisation using XRD, TEM and EXAFS. However, high amounts of carrier components compared to low metal concentrations of approximately 1-2 wt% rendered analysis of VAM catalysts with well-established techniques such as XRD and EDX difficult. Valuable information about the local Pd and Au distribution was provided by EXAFS. In agreement with TEM characterisation and Pd dispersion measurements, larger particle sizes were found for the liquid phase reduced catalyst B compared to GPR samples processed at 150 °C and 250 °C, respectively. Furthermore, TEM analysis showed that nanoparticles of LPR tend to agglomerate to chain-like framework structures, whereas a distribution of small and isolated particles dominates the GPR catalysts, correlating with the observed higher activity.