Catalytic dehydrogenation (DH) of light paraffins combined with selective hydrogen combustion (SHC) - II. DH+SHC catalysts physically mixed (redox process mode)

When a physical mixture composed of a 0.7 wt.% PtSn-ZSM-5 dehydrogenation (DH) catalyst and a 42 wt.% Bi2O3/SiO2 selective hydrogen combustion (SHC) catalyst is subjected to a DH + SHC redox process operation, it gives substantially higher than equilibrium propylene yields from propane. At 540 degrees C, atmospheric pressure and WHSV of 2 h(-1), initial microreactor propylene yields from neat propane are 48.2% at about 90% selectivity, compared to equilibrium olefin yields of 20.0% at about 95% selectivity when only the DH catalyst is used. The overall propylene yield improvement is 140%. In this novel redox process arrangement no gaseous oxygen is cofed to the physical mixture; the lattice oxygen of the SHC catalyst is the sole oxidizing agent to selectively combust the hydrogen produced in situ by the dehydrogenation of the propane, catalyzed by the commingled DH catalyst. Periodic regeneration with air is necessary to replenish the lattice oxygen of the SHC catalyst depleted in the redox reaction. Although still significantly (i.e., 65%) above equlibrium, the high initial DH + SHC propylene yields are not sustainable and decline from 48.2% (first cycle) to 33.0% (tenth cycle); i.e., 2.1%/cycle; they are ascribed to the loss of Bi2O3 dispersion on the SiO2 support caused by the deep redox cycling. Shortening the redox cycle or increasing the SHC/DH catalyst ratio lessen the decline. However, the identification of a more redox stable catalyst is imperative to make the process practical. Some suggestions are provided. It is surmised that the DH + SHC redox process approach will ultimately (i.e., once a sufficiently redox stable SHC catalyst is identified) be preferred over the DH- > SHC- > DH cofed approach to improve conventional dehydrogenation processes such as the Oleflex and Catofin processes. (C) 1999 Elsevier Science B.V. All rights reserved.