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) c atalyst is subjected to a DH + SHC redox process operation, it gives substa ntially higher than equilibrium propylene yields from propane. At 540 degre es C, atmospheric pressure and WHSV of 2 h(-1), initial microreactor propyl ene yields from neat propane are 48.2% at about 90% selectivity, compared t o 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 t his novel redox process arrangement no gaseous oxygen is cofed to the physi cal mixture; the lattice oxygen of the SHC catalyst is the sole oxidizing a gent to selectively combust the hydrogen produced in situ by the dehydrogen ation of the propane, catalyzed by the commingled DH catalyst. Periodic reg eneration 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% (firs t 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 cycl ing. Shortening the redox cycle or increasing the SHC/DH catalyst ratio les sen the decline. However, the identification of a more redox stable catalys t is imperative to make the process practical. Some suggestions are provide d. It is surmised that the DH + SHC redox process approach will ultimately (i.e., once a sufficiently redox stable SHC catalyst is identified) be pref erred over the DH- > SHC- > DH cofed approach to improve conventional dehyd rogenation processes such as the Oleflex and Catofin processes. (C) 1999 El sevier Science B.V. All rights reserved.