Application of staged O₂ feeding in the oxidative dehydrogenation of ethylbenzene to styrene over Al₂O₃ and P₂O₅/SiO₂ catalysts

Drastic improvements in styrene yield and selectivity were achieved in the oxidative dehydrogenation of ethylbenzene by staged feeding of O₂. Six isothermal packed bed reactors were used in series with intermediate feeding of O₂, while all EB was fed to the first reactor, diluted with helium or CO₂ (1:5 molar ratio), resulting in total O₂:EB molar feed ratios of 0.2-0.6. The two catalyst samples, γ-Al ₂O₃ and 5P/SiO₂, that were applied both benefitted from this operation mode. The ethylbenzene conversion per stage and the selectivity to styrene were significantly improved. The production of CO_X was effectively reduced, while the selectivity to other side products remained unchanged. Compared with co-feeding at a total O ₂:EB molar feed ratio of 0.6, by staged feeding the EB conversion (+15% points for both catalysts), ST selectivity (+4% points for both samples) and O₂ (ST) selectivity (+9% points for γ-Al₂O ₃ and +17% points for 5P/SiO₂) all improved. The ethylbenzene conversion over 5P/SiO₂ can be increased from 18% to 70% by increasing the number of reactors from 1 to 6 with each reactor a total amount of O₂ of 0.1 without the loss of ST selectivity (93%). For 5P/SiO₂ a higher temperature (500 C vs. 450 C for Al ₂O₃) is required. Essentially more catalyst (5P/SiO ₂) was required to achieve full O₂ conversion in each reactor. Staged feeding of O₂ does not eliminate the existing issues of the catalyst stability both in time-on stream and as a function of the number of catalyst regenerations (5P/SiO₂), or the relatively moderate performance (relatively low styrene selectivity for γ-Al₂O ₃).