Partially oxygenated hydrocarbons provide the building blocks for plastics and synthetic fibers as well as precursors for specialty chemicals. However, the oxidation of hydrocarbons with O2 also results in the thermodynamically favored (in)complete combustion to CO and CO2 which reduces the selectivity of the partial oxidation (POx) products. POx of hydrocarbons has seen significant interest in distributive membrane reactors (MRs) [1-3]. These works demonstrate the ability of MRs to inhibit full oxidation products and promote POx products. Acrolein is a POx product of interest as it is a precursor to a number of polymers and an intermediate for various specialty chemicals. This work focuses on producing acrolein (methacrolein) from propane (isobutane) using consecutive catalytic beds for (1) the oxidative dehydrogenation (ODH) of propane to propylene (isobutane to isobutene) and (2) the partial oxidation of propylene to acrolein (POA) (isobutene to methacrolein). The ODH and POA reactions are generally first order in the hydrocarbon and zero to one half order in oxygen while the competing combustion reactions are first order in both reactants. The membrane reactor design is ideal for taking advantage of this difference in reaction order as it maintains a low partial pressure of oxygen throughout the catalyst bed. Experiments have been performed in both a fixed bed reactor (FBR) and an inert membrane reactor (IMR). Conversion in the FBR and IMR were similar. Selectivity is where the most significant differences between the two reactors can be seen. Selectivity to propylene and acrolein increased by 6% and 9%, respectively while decreasing the selectivity to CO2. These improvements in the selectivity support the use of an IMR over a FBR for the ODH and POA reactions. With the improvements seen for each reaction in the single bed IMR it is interesting to look at the effect of the reactor type for a dual bed reactor. The inclusion of a single IMR increases the selectivity toward acrolein as discussed above and a dual-bed IMR further enhances this effect. The dual bed IMR was found to initially increase the selectivity to acrolein by 7%. Over time, deactivation was evident in the FBR as it became less selective while the IMR maintained its activity, increasing the selectivity disparity to 18%. Prior theoretical work utilizing data from the literature has shown the dual bed IMR to be successful for the conversion of propane to acrolein [4]. The previous model is currently being updated to fit the experimental data. Once this fit is obtained, FBR and IMR simulations will be run to aid in determining optimal operating conditions for each reactor and then verified with experimental runs. Membrane reactors are also beneficial in other hydrocarbon reactions. The oxidation of isobutane to methacrolein through the intermediate isobutene follows similar kinetics to that of the propane to acrolein reaction network. Due to the similarity between these two reaction networks the methacrolein yield should be enhanced as in the case of acrolein in the partial oxidation of propane. Overall, distributive membrane reactors are beneficial in utilizing the differences in reaction orders to promote the oxidative dehydrogenation of alkanes and the partial oxidation of alkenes to their respective aldehydes and carboxylic acids.

References 1. Capannelli, G., Carosi, E. et.al. Chem Eng Sci. 51, 1817 (1996). 2. Ramos, R., Menendez, M. et.al. Catalysis Today 56, 239 (2000). 3. Sanchez, J. and Tsotsis, T. in “Fundamentals of Inorganic Membrane Science and Technology” (A.J. Burggraaf and L. Cot, Eds.) Vol. 4 p. 529 Elsevier, Amsterdam, 1996. 4. O'Neill, C. and Wolf, E. Ind. Eng. Chem. Res. 45, 2697 (2006).