Influence of atmosphere, interparticle distance and support on the stability of silver on α-alumina for ethylene epoxidation

Silver Catalysts Supported on High Surface Area α‐Alumina: Effect of Carbohydrate Template Size and Heat Treatment on Phase Purity

“…The alumina materials with equal calcination times (6 h) but produced with different templates (cellulose, CG220 and CG310) have a BET surface area of 21–29 m 2 g −1 . This is in accordance with literature, where α‐alumina was templated with polymer spheres in a similar size range and showed specific surface areas of 20–30 m 2 g −1 [20,21,48] . It is noteworthy that equally high surface areas can be achieved with these disordered carbohydrate materials.…”

“…Based on XRD there is no significant difference in phase purity in C8 and cellulose‐6h (100 % vs. 98 %), therefore, the selectivity difference is most likely caused by the difference in specific surface area of the α‐alumina support. This is in line with earlier studies that reported lower selectivities for higher surface area α‐alumina supported silver catalysts [45,46,48,59] . However, these studies did not report phase compositions of the used α‐alumina supports.…”

“…Histograms of the sphere diameters are shown in Figure 2, in which they are denoted as CG200, CG220 and CG310. These materials have a similar size range as the PMMA or cellulosic templates reported in literature as being used for the preparation of porous α‐alumina [20,25,48] . However, our templates have the advantage of being more practical and less time‐intensive to obtain.…”

“…Hence increasing the calcination time can, even for this relatively fine template that delivers high specific surface area α‐alumina, lead to virtually phase pure α‐alumina when using our two‐step heating method, with the highest heat treatment at 1150 °C. This effect of calcination time on phase purity is interesting, because in earlier studies a variety of templates, calcination times, and temperatures was reported (1–9 h and 1100–1300 °C, respectively) when using a single‐step heating method for templated alumina [6,20,21,24,25,33,48] . In these studies, calcination times were not investigated and it was simply assumed that there was a complete transition to α‐alumina.…”

“…α‐Alumina is preferred as a support because of its low OH group density (1–6 OH nm −2 ) [46,47] . High surface area α‐alumina can have clear advantages compared to low surface area α‐alumina, such as an increased thermal stability of Ag particles [48] . As other alumina polymorphs exhibit higher OH group densities (10–15 OH nm −2 for γ‐alumina) [1] and are therefore more reactive, the selectivity towards ethylene oxide is an indirect measure of α‐alumina phase purity at the surface.…”