摘要：

Adsorption and reaction of n-butene isomers over a-Fe2O3, ZnFe2O4 and ZnCrFeO4 were studied by means of pulse and TPD techniques. It is shown that the three butene isomers are adsorbed on the three oxides at approximately the same amount,.but they react quite differently on them. At room temperature, three isomers show almost no reaction on a-Fe2O3; meanwhile, double bond migration and selective oxidation to butadiene occur distinctly over the two spinels to different extents. Results with multiple pulses of butenes on the two spinels demonstrate that the butadiene formation in the second pulse is much greater than that in the first, then it lowers again in the subsequent pulses. It is suggested that selective oxidation sites are generated on the two spinels through the action of the first butene pulse.The TPD spectra of butenes adsorbed at room temperature on the three oxides show two well-separated peaks: the low temperature peak (200℃) consists of butene and butadiene; while the high temperature peak (200-600℃) is also a composite peak and consists of CO2 only. The butenes retain strongly on the oxides except a-Fe2O3. Repeated TPD runs up to 600℃ are. required to "desorb" thoroughly the retained butenes. CO2 is the sole product in subsequent TPD runs.In light of the results obtained, two .modes of generation of selective oxidation sites on the spinels (reduction activation and heating activation) are suggested and plausible structures of surface species(both the adsorbed butene and active sites) in selective and total oxidations are discussed.

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