Catalytic dehydrogenation (DH) of light paraffins combined with selective hydrogen combustion (SHC) - I. DH -> SHC -> DH catalysts in series (co-fed process mode)

Sb2O4,In2O3, WO3 and Bi2O3, supported on 50% SiO2, were found to be highly selective hydrogen combustion (SHC) catalysts. Their respective selectiviti es are 99.8, 99.7, 98.5 and 98.1% at 500 degrees C, atm pressure, WHSV 2 h( -1) and C degrees(3)/C-3(=)/H-2/O-2 = 80/20/20/10. Their activities vary gr eatly, reflected by the first-order hydrogen combustion constants (k(H2), s (-1)) which are: In2O3 1.57, Bi2O3 0.53, WO3 0.36, Sb2O4 0.22. Among the Si O2, Al2O3, TiO2 and ZrO2 supports tested, ZrO2 was found to be the best ove rall carrier for the highly active In2O3. When a 0.7 wt% Pt-Sn-ZSM-5 dehydrogenation (DH) catalyst and a 10 wt% In2O3 /ZrO2 SHC catalyst are used in a sequential microreactor DH --> SHC --> DH co-fed process mode, higher than equilibrium yields of light olefins are ob tained from the corresponding paraffins. At 550 degrees C, atmospheric pres sure and WHSV of 2 h(-1) propylene yields of 29.7% at 97% selectivity (0.3 air/propane in SHC), and 33% at 89% selectivity (0.6 air/propane in SHC) ar e realized, compared to the equilibrium yield of 25% at 99% selectivity whe n only the DH catalyst is used. The yield improvements over equilibrium deh ydrogenation are 19 and 32%, respectively. Under the same operating conditi ons but 0.2 air/isobutane ratio in the SHC stage, the isobutylene yield fro m isobutane is 47.5% at 99+% selectivity as compared to 40% and 99+% select ivity when only the DH catalyst is used; i.e. an 18% yield improvement over equilibrium. The above metal oxide SHC catalyst systems might find application to improv e conventional DH processes such as Oleflex and Catofin. (C) 1999 Elsevier Science B.V. All rights reserved.