Abstract
Multifunctionality is the hallmark of most modern commercial heterogeneous catalyst systems in use today, including those used for the selective ammoxidation of propylene to acrylonitrile. It is the quintessential principle underlying commercial catalyst design efforts since petrochemical process development is invariably driven by the need to reduce manufacturing costs. This is in large part achieved through new and improved catalysts that increase selectivity and productivity. In addition, the future feedstocks for chemical processes will be invariably more refractory than those currently in use (e.g., replacing alkenes with alkanes or using CO2), thus requiring a disparate combination of chemical functions in order to effect multiple chemical transformations with the fewest separate process steps. This review summarizes the key chemical phenomena behind achieving the successful integration of multiple functions into a mixed-metal-oxide-selective ammoxidation catalyst. An experiential and functional catalyst design model is presented that consists of one or both of the following components: (1) a mixed-metal-oxide–solid solution where the individual metal components serve separate and necessary functions in the reaction mechanism through their atomic level interaction in the context of a single crystallographic structure; (2) the required elemental components and their catalytic function existing in separate phases, where these phases are able to interact for the purposes of electron and lattice oxygen transfer through the formation of a structurally coherent interface (i.e., epitaxy) between the separate crystal structures. Examples are provided from the literature and explained in the context of this catalyst design model. The extension of the model concepts to the design of heterogeneous catalysts in general is also discussed.
Highlights
The sine qua non of modern commercial catalysts is multifunctionality
The “support effect” model envisions the catalytically active phase supported on the second phase, the later transporting its lattice oxygen to the bismuth molybdate phase under conditions of selective propylene oxidation to acrolein [15]
The incorporation of catalytically-functional, elemental components, such as a solid solution with a catalytically-active, scheelite bismuth molybdate phase was first demonstrated with iron and a single-phase, mixed-metal oxide with the formulation Bi3 FeMo2 O12 [27,28]
Summary
The sine qua non of modern commercial catalysts is multifunctionality. in order to be both active and selective for a multistep chemical transformation at an industrial scale, it is necessary that the catalysts be multifunctional within several dimensions—chemical composition, solid state chemistry and physical properties. Stability from a commercial standpoint means that a catalyst is able to maintain satisfactory activity and selectivity over several thousand hours of operation, has the ability to withstand the shutdown and start-up of the process and the capability to return to satisfactory performance following a deviation from steady-state operating conditions with respect to temperature, flow rate and/or feed composition. This stability in many cases is imparted to a catalytically-active solid phase through the addition of select promoter additives to the overall catalyst formulation.
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