Hydrothermal Stability of High-Surface-Area α-Al₂O₃ and Its Use as a Support for Hydrothermally Stable Fischer–Tropsch Synthesis Catalysts

Abstract

Nanocrystalline corundum synthesized by ball milling of boehmite is found to be exceptionally robust toward chemical weathering, a common problem of transition aluminas in different applications, most notably in the case of supported catalysts, which are exposed to hydrothermal reaction environments. Detailed characterization and surface cation coordination analysis indicate that the absence of tetrahedral Al species on corundum makes it stable toward chemical weathering. A cobalt catalyst developed using nano-α-Al2O3 as the support showed Fischer–Tropsch synthesis activity and selectivity comparable to the benchmark Co/γ-Al2O3 and remained stable over 250 h on-stream.

Sustainability Assessment of Mechanochemistry by Using the Twelve Principles of Green Chemistry

“…Followed by mild calcination, the mechanosynthesized high-surface-area α-Al 2 O 3 exhibited hydrothermal stability and was used as a support in cobalt-based Fischer-Tropsch synthesis catalysts. [129] Although carrying out ball milling reactions without external heating is both attractive to save energy and from a safety point of view, a precise temperature control in mechanochemical reactions could elevate their robustness and efficiency. On the one hand, achieving sub-ambient temperature control in mechanochemical asymmetric reactions has enabled a better understanding of the outcome of the reactions in terms of yield and stereoselectivity.…”

“…However, ball milling of boehmite without external heating led to nanometer‐sized corundum after short milling times (Scheme 6). Followed by mild calcination, the mechanosynthesized high‐surface‐area α‐Al 2 O 3 exhibited hydrothermal stability and was used as a support in cobalt‐based Fischer–Tropsch synthesis catalysts [129] …”

Mechanochemical Synthesis of Catalytic Materials

“…As mentioned above, high‐surface‐area α‐Al 2 O 3 is a highly valuable material for catalysis, among other applications, because it can provide stable support for active phases. To explore its potential, a recent study analysed its hydrothermal properties under relatively harsh conditions in comparison with a conventional low‐surface area α‐Al 2 O 3 and widely used γ‐Al 2 O 3 support [74] . Expectedly, γ‐Al 2 O 3 severely degraded to boehmite already after a 5 h hydrothermal (HT) test (H 2 O:Al=50, at 150 °C in a sealed autoclave) as clearly shown by XRD analysis of samples after HT treatment (Figure 7 a).…”

“…Hence, mechanochemically‐derived α‐Al 2 O 3 ‐bm‐800 offers a stable support material with a reasonably high surface area. This led to its application for the synthesis of a robust cobalt‐based Fischer–Tropsch synthesis (FTS) catalyst (Co/α‐Al 2 O 3 ‐bm‐800) [74] . The latter catalyst showed significantly more activity than the conventional α‐Al 2 O 3 ‐based system, matched the activity and selective level of the benchmark γ‐Al 2 O 3 ‐based catalyst, and remained stable over 250 h on stream (Figure 7 c, d).…”

Surface and Bulk Chemistry of Mechanochemically Synthesized Tohdite Nanoparticles

“… 13 , 14 α-Al 2 O 3 would be a more suitable choice, being more robust toward chemical weathering than γ-Al 2 O 3 , but only if the requirements of surface acidity and high surface area are met. 15 …”