Conventional propane dehydrogenation (PDH) is an endothermic, equilibrium-limited reaction that requires high temperatures to achieve commercially viable per-pass yields of propylene. Oxidative propane dehydrogenation has the potential to form propylene at much lower temperatures and more selectively by controlling the reaction kinetically, rather than thermodynamically. However, it has proven difficult to prevent large amounts of propane combustion and to generate sufficient amounts of propylene.
A new tandem catalyst designed and developed by researchers at Northwestern University (Evanston, Ill.; www.northwestern.edu) has generated good results in oxidatively dehydrogenating propane to propylene at selectivities of 75% and single-pass propane conversion rates of 40% at temperatures of 450°C (compared to ~600°C for conventional PDH).
The catalyst consists of a ~2-nm shell of In2O3 grown by atomic layer deposition (ALD) on the surface of an existing PDH catalyst (Pt nanoparticles on Al2O3 spheres). The In2O3 layering leaves the Pt nanoparticles partially exposed and brings In2O3, which selectively catalyzes hydrogen combustion, into close proximity with the Pt particles. “After the H atoms are removed from propane on the Pt catalyst, they can diffuse across the surface to the In2O3 and form water,” explains Northwestern professor Justin Notestein. “For our tandem catalyst, we need to combust the H atoms very quickly, so that the oxygen isn’t available to oxidize the propane or propylene on the Pt surface,” he says.
“Not only can you run oxidative dehydrogenation at lower temperatures without hitting thermodynamic limits, but continuous reheating is not required, as in conventional PDH,” Notestein comments, and the tandem catalyst design stabilizes the platinum against sintering, lengthening catalyst lifetime and eliminating the need for regeneration. The work was funded by the National Science Foundation Center for Innovative and Strategic Transformations of Alkane Resources (CISTAR).