Abstract
Sb 2O 4, In 2O 3, WO 3 and Bi 2O 3, supported on 50% SiO 2, were found to be highly selective hydrogen combustion (SHC) catalysts. Their respective selectivities are 99.8, 99.7, 98.5 and 98.1% at 500°C, atm pressure, WHSV 2 h −1 and C ° 3/C 3 =/H 2/O 2 = 80/20/20/10. Their activities vary greatly, reflected by the first-order hydrogen combustion constants ( k H 2 , s −1) which are: In 2O 3 1.57, Bi 2O 3 0.53, WO 3 0.36, Sb 2O 4 0.22. Among the SiO 2, Al 2O 3, TiO 2 and ZrO 2 supports tested, ZrO 2 was found to be the best overall carrier for the highly active In 2O 3. When a 0.7 wt% Pt-Sn-ZSM-5 dehydrogenation (DH) catalyst and a 10 wt% In 2O 3/ZrO 2 SHC catalyst are used in a sequential microreactor DH → SHC → DH co-fed process mode, higher than equilibrium yields of light olefins are obtained from the corresponding paraffins. At 550°C, atmospheric pressure 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) are realized, compared to the equilibrium yield of 25% at 99% selectivity when only the DH catalyst is used. The yield improvements over equilibrium dehydrogenation are 19 and 32%, respectively. Under the same operating conditions but 0.2 air/isobutane ratio in the SHC stage, the isobutylene yield from isobutane is 47.5% at 99+% selectivity as compared to 40% and 99+% selectivity 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 improve conventional DH processes such as Oleflex and Catofin.
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