(672d) Kinetic Criteria for Non-Steady State Selectivity Enhancement during Ethane Oxidative Dehydrogenation

Ethylene is a valuable commodity chemical traditionally produced through catalytic steam cracking of larger hydrocarbons. However, the endothermicity of catalytic steam cracking results in high energy demands to achieve desirable ethylene yields. Selective catalytic oxidative dehydrogenation (ODH) of ethane to ethylene has proven to be an attractive alternative due to its exothermicity and light alkane feedstock. It is speculated that the simultaneous presence of gaseous ethane and oxygen during ODH results in overoxidation to CO_x, hence decreasing ethylene yields. It is proposed that higher ethylene yields are achievable by promoting reactions between ethane and selective oxygen stored within metal oxide catalysts as opposed to unselective gaseous oxygen. To demonstrate this, we compare conventional steady state to concentration forcing (dynamic) operation of ethane ODH. Fig. 1a and b plots carbon selectivities as functions of ethane conversion over 3wt% VO_x and 7wt% MoO₃ Al₂O₃ catalysts respectively. Fig.1a shows no significant difference between dynamic and steady state carbon selectivies over the 3wt% VO_x Al₂O₃ catalyst. However, fig. 1b demonstrates higher ethylene selectivities via dynamic operation over a 7wt% MoO₃ Al₂O₃ catalyst. We explain this difference in dynamic performance of supported VO_x and MoO₃ catalysts based the O₂ dependencies of unselective overoxidation reactions. More specifically, improvement via concentration forcing over MoO₃ catalysts were rationalized based on higher O₂ sensitivities of unselective overoxidation reactions compared to the selective ODH reaction. All reaction rates over the VO_x catalyst are independent of oxygen pressure, unlike the MoO_x catalyst over which selective and unselective rates are zero and positive fractional order in O₂, respectively. Our work presents a method for interpreting dynamic enhancement during ethane ODH from the standpoint of apparent O₂ reaction orders of different steps in the overall reaction network, and could be used to provide kinetic criteria for improving oxidation processes through non-steady state operation.