Selective Hydrogenation of Butadiene with Carbon-Supported Palladium Catalysts

This thesis covers a study on the selective hydrogenation of gaseous 1,3-butadiene (butadiene hereafter) over palladium (Pd) nanoparticle catalysts. This reaction is of essential relevance for industrial high-purity olefin production, where butadiene impurities have to be reduced down to the ppm level. Retaining a high selectivity up to complete butadiene conversion is challenging. Therefore, a full understanding of the effects of catalyst properties and reaction conditions on the catalytic performance is required. Several factors influence the catalysis, such as nanoparticle size, reaction conditions, metal composition and catalyst structure. Using different synthesis, characterisation and catalytic testing methods, we aimed to develop a better comprehension of the properties of the catalyst and reaction conditions that affect catalytic activity and selectivity. The general background of selective hydrogenation reactions is discussed in Chapter 1. In Chapter 2, the influence of Pd nanoparticle size on the catalytic performance is discussed. We systematically varied the nanoparticle size of carbon-supported Pd (Pd/C) catalysts from 2 to 17 nm. The butadiene hydrogenation activity per Pd surface atom increased with nanoparticle size, from 17 s-1 to 34 s-1 at 25 °C for 2 nm to 17 nm nanoparticles. Contrarily, the propylene hydrogenation activity decreased strongly over the same range of nanoparticle sizes; from 2.6 s-1 to 0.4 s-1, in line with the fraction of exposed corner-sites. In Chapter 3, the effect of heat and mass transport limitations on activity and selectivity is described using Pd/C catalysts of similar particle size. By varying the Pd loading, a specific surface area of between 0.01 to 2.6 m2Pd/gcat was obtained. The effective mass transport (diffusion) was varied by using different average catalyst grain sizes (between 19 and 200 μm). The combination of catalytic results and calculations highlighted the importance of diffusion-induced concentration gradients inside catalyst grains. The observed impact of these gradients on the butene selectivity was much more severe than the impact on activity. In Chapter 4, we deal with bimetallic PdCu/C catalysts and focus on the influence of the Cu-to-Pd ratio (between 7 and 167) in Cu-rich catalysts on the catalytic performance. The Pd mass- and surface-normalised activity in bimetallic PdCu catalysts was lower than that of monometallic Pd but gradually increased with Pd content. Operando X-ray absorption spectroscopy showed electron density transfer from Cu to Pd. The Pd-containing catalysts showed similar isomer product compositions and high butene selectivities up to near-complete butadiene conversion. Monometallic Cu and Cu-rich (Cu:Pd≥85) catalysts showed slightly superior butene selectivity at high conversions. In Chapter 5, we explore the effect of various 10 mol% additives (K, Mn, Cu, Zn, Ag) on the performance of Pd/C catalysts. The addition of the dopants decreased the butadiene hydrogenation activity, ascribed to an electron density transfer to Pd and was particularly strong for Zn- and Ag-doped catalysts. The lowest amount of alkane formation (both propane and n-butane) was observed over the undoped Pd-catalyst. Interestingly, higher 1-butene selectivities were found for the Cu-, Zn- and Ag-containing catalysts. Isomerisation tests of 1-butene showed that the addition suppressed the 1-butene conversion.