The CO–H2 Reaction over ZrO2 to Form Isobutene Selectively

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Abstract
The CO–H2 reaction over ZrO2 was studied in detail. Methanol and isobutene were selectively formed at 473–523 and 623–673 K, respectively. The activation energies for C2, C3, and linear C4 hydrocarbon formation are 79, 84, and 96 kJ mol−1 (523–723 K), while that for branched-chain C4 hydrocarbons is about 210 kJ mol−1 (523–623 K). At 673 K the rate of C4 hydrocarbon formation is as follows: r=kPH2·Pco2, while those of C1, C2, and C3 are written by the functions having the maximum values regarding CO pressure. C4 hydrocarbons consist mostly of isobutene at the temperature. From these results it was concluded that isobutene is formed via different path from those of C1, C2, C3, and linear C4 hydrocarbons. The addition of high electronegative oxides such as SiO2 and Nb2O5 resulted in the decrease of isobutene and the increase of methane. The selective formation of isobutene proceeds over pure ZrO2 and ZrO2 modified with 3a Group and basic metal oxides. The nature of active site for isobutene formation was discussed.