Selectivity issues in (amm)oxidation catalysis
Introduction
Selectivity plays a central role in heterogeneous oxidation catalysis. It is of utmost importance in industrial catalytic processes, and gaining ever greater importance as the starting hydrocarbons become less abundant and hence more expensive. To develop a successful commercial catalyst it is important that selectivity is also achieved at reasonably high conversions. The importance assigned to achieving high selectivity at reasonable conversions has been recognized by some catalytic researchers already in the forming years of selective heterogeneous light hydrocarbon oxidation and ammoxidation catalysis. Thus, as early as 1963 Callahan and Grasselli [1] put forward their hypothesis of site isolation. This hypothesis states that oxidation catalysts become selective when the number of reacting oxygens at the active centers on the surface of a catalyst is limited and that the active centers must be spatially isolated from each other. This site isolation hypothesis has been verified on many catalytic systems since its inception and has served their originators well in the discovery of an array of catalytic solids for the oxidation and ammoxidation of light olefins, several of which have successfully been commercialized, e.g., several generations of catalysts for the ammoxidation of propylene to acrylonitrile and the oxidation of propylene to acrolein and acrylic acid, respectively [2]. The importance of the site isolation hypothesis as we proposed it [1] has been acknowledged already very early by Trifiro, and he was one of the first to recognize and quote our work in the literature, and he adopted and applied it successfully to catalytic systems of interest to him [3].
The site isolation hypothesis can be extended to also include systems such as the MoVNb(Te,Sb)O catalyst for the selective oxidation and ammoxidation of paraffins, e.g., propane conversion to acrylic acid or acrylonitrile, respectively. The latter is the main subject of this contribution. Although the well-known SOHIO/BP process [2], [4] for the ammoxidation of propylene to acrylonitrile is very efficient giving 80+% acrylonitrile yield on commercial scale [5], there exists a large incentive, because of the sizable price differential between propane and propylene, to discover an effective propane conversion catalyst so that future commercial ammoxidation processes would become paraffin based. Promising catalyst candidates to achieve this goal are the promoted VSbO and the MoVNb(Te,Sb)O systems, with the latter holding an edge over the former [6].
Section snippets
Experimental
The methods employed for the preparation, evaluation and optimization of MoVNb(Te,Sb)O catalysts; and for their structure determinations have been described earlier [7]. Further details pertaining to the solutions of the M1 and M2 structures are found in Refs. [8], [9] and [10], [11], respectively. The preparation, grinding, compacting, sizing, heat treating and catalytic testing of M1/M2 physical mixtures are described in Ref. [12].
Results and discussion
Since its inception some 40 years ago, the site isolation hypothesis [1] has been extended and applied successfully over the years from the original CuO catalyst to include such systems as the V2O5/KVO4, Bi9PMo12O52, (K,Cs)(Ni,Co,Mg)(Fe,Ce)(Sb,P)BiMoO, USb3O10, FeSbO, VSbO, (VO)2P2O7, and now also the MoV(Nb,Ta)(Te,Sb)O catalyst systems. The subject has been recently reviewed [13]. In all of the above listed systems, high desired product selectivity can be explained on the basis that the number
Conclusions
Selectivity in heterogeneous oxidation catalysis plays a very important role in particular as the raw materials used as feeds in commercial petrochemical processes become less abundant and even more expensive.
The desired selectivity of many useful heterogeneous oxidation and ammoxidation catalysts can be explained on the basis of the site isolation hypothesis originally proposed some 40 years ago by Callahan and Grasselli [1].
The selectivity behavior of many model (e.g., CuO; V2O5/KVO4), as
Acknowledgement
The author thanks Professors A. Andersson, J.D. Buttrey, and W.A. Goddard III, and Drs. J.D. Burrington, P. DeSanto Jr., C.G. Lugmair, A.F. Volpe Jr. and T. Weingand for contributing to the background information of this paper as is referenced below.
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