Ammoxidation or industrially known as the SOHIO process is a sophisticated technology that converts propylene from crude oil into acrylonitrile (ACN). ACN is an important commodity chemical used in acrylic fibers, resins, and plastics. With the growth in population and increasing demand for consumer goods, the global ACN market size is expected to reach 8.6 million tonnes by 2026. Contrarily, low-volume feedstocks including propylene may become more constrained due to the depletion of energy resources. Hence, there is growing interest in developing alternative processes that are sustainable and cost-competitive. One approach is to develop resilient catalyst systems that offer high activity and selectivity under fluctuating feed conditions. Heterogeneous catalysts can often accommodate transient operating conditions, and in some cases by virtue of their unique physical and chemical properties, have demonstrated dynamic rate enhancement. Therefore, we hypothesize that operating under forced dynamic conditions can be used as a strategy to leverage the properties of ammoxidation catalysts to meet the global ACN demand.

Bismuth molybdates (Bi₂Mo₃O₁₂) are generally regarded as an active catalyst for this process, where they undergo subsequent reduction oxidation (redox) to yield the desired product.  However, industrially they are used in the promoted formulations which adds to the structural complexity of metal oxides. To address this problem, we intend to employ a reductionistic approach where we will systematically investigate the simple model catalysts. The first step is to conduct baseline characterization under steady-state conditions. Subsequently, we will exploit their redox properties under forced operating conditions. We will achieve this by probing the catalyst dynamics under working conditions, using tools such as state-in-art XPS, XRD, XAS, etc. Our preliminary data over Bi₂Mo₃O₁₂/SiO₂ show that the catalyst undergoes significant changes at the bulk and surface during the initial activation of propylene, which can be effectively restored with re-oxidation. The XPS data reveal the active involvement of Mo⁶⁺/Mo^6-x(x=1,2,3) redox cycling in the oxygen storage capacity of the material, while Bi³⁺ always remains constant. However, both Bi and Mo demonstrate core-level energy shifts during surface reduction, accompanied by surface enrichment in Mo. Together, these observations suggest that while the surface Mo⁶⁺ sites are active centers for propylene activation, the presence of Bi³⁺ is necessary to promote the redox behavior of MoO_x through synergistic effects. We also identify that the redox potential of the active sites is governed by the catalyst morphology such as nanoparticles size and dispersion. Furthermore, doping with selected transition metals is found to enhance the catalytic properties, potentially leading to dynamic rate enhancement.

Keywords: Ammoxidation, bismuth molybdates, redox