N-Butane, oxidation

The reactivity of vanadyl pyrophosphate (VO)2P207, catalyst for n-butane oxidation to maleic anhydride, was investigated under steady and unsteady conditions, in order to obtain iirformation on the status of the active surface in reaction conditions. Specific treatments of hydrolysis and oxidation were applied in order to modify the characteristics of the surface layer of the catalyst, and then the unsteady catalytic performance was followed along with the reaction time, until the steady original behavior was restored. It was found that the transformations occurring on the vanadyl pyrophosphate surface depend on the catalyst characteristics (i.e., on the PfV atomic ratio) and on the reaction conditions. 

The industrial catalyst for n-butane oxidation to maleic anhydride (MA) is a vanadium/phosphoras mixed oxide, in which bulk vanadyl pyrophosphate (VPP) (VO)2P207 is the main component. The nature of the active surface in VPP has been studied by several authors, often with the use of in situ techniques (1-3). While in all cases bulk VPP is assumed to constitute the core of the active phase, the different hypotheses concern the nature of the first atomic layers that are in direct contact with the gas phase. Either the development of surface amorphous layers, which play a direct role in the reaction, is invoked (4), or the participation of specific planes contributing to the reaction pattern is assumed (2,5), the redox process occurring reversibly between VPP and VOPO4. 

Self-assembled nanorods of vanadium oxide bundles were synthesized by treating bulk V2O5 with high intensity ultrasound [34]. By prolonging the duration of ultrasound irradiation, uniform, well defined shapes and surface structures and smaller size of nanorod vanadium oxide bundles were obtained. Three steps which occur in sequence have been proposed for the self-assembly of nanorods into bundles (1) Formation of V2O5 nuclei due to the ultrasound induced dissolution and a further oriented attachment causes the formation of nanorods (2) Side-by-side attachment of individual nanorods to assemble into nanorods (3) Instability of the self-assembled V2O5 nanorod bundles lead to the formation of V2O5 primary nanoparticles. It is also believed that such nanorods are more active for n-butane oxidation. 

The evolution of the structure of four vanadyl phosphate hemihydrates has been studied and then used as catalysts for n-butane oxidation using an in-situ Raman cell and MAS-NMR. The catalytic performance for maleic anhydride... 

In this communication, we compare VPO catalysts which differ by their conditions of preparation, considering both their LRS spectra registered under reaction conditions and the corresponding catalytic results. LRS data are discussed in relation with results for n-butane oxidation to maleic anhydride. 

This study has resulted in interesting informations concerning the active sites of the VPO catalysts for n-butane oxidation to maleic anhydride being obtained. The study of VPO catalysts in the course of n-butane oxidation by an in-situ Raman cell has shown... 

Active crystal face of vanadyl pyrophosphate for selective n-butane oxidation catalyst preparation, 157-158 catalyst weight vs. butane oxidation, 162,163/ catalytic activity, 162,1 (At catalytic reaction procedure, 158 experimental description, 157 flow rate of butane vs. butane oxidation, 162,163/ fractured SiOj-CVO PjO scanning electron micrographs, 160,161/ fractured scanning electron... 

Active crystal face of vanadyl pyrophosphate for selective n-butane oxidation—Continued selectivity, 162,164r selectivity vs. face, 165,166/... 

Vanadium is present as V " in stoichiometric VPP however, the latter can host V " or V species as defects, without undergoing substantial structural changes (5,6). Therefore, the role of the different V species in the catalytic behavior of VPP in n-butane oxidation has been the object of debate for many years (7-9). Moreover, the catalyst may contain crystalline and amorphous vanadium phosphates other than (VO)2P207 (10) for instance, outer surface layers of V phosphates may develop in the reaction environment, and play active roles in the catalytic cycle. This is particularly true in the case of the fresh catalyst, while the equilibrated system (that one which has been kept under reaction conditions for 100 hours at least) contains only minor amounts of compounds with V species other than V. ... 

A New Process for n-Butane Oxidation to Maleic Anhydride Using a Circulating Fluidized Bed Reactor. In Circulating Fluidized Bed Technology TV. Ed. A. A. Avidan. New York AlChE Publications. 

Figure 2.37 Light-ofTcurves for n-butane oxidation with ( ) and without ( ) palladium generated in a micro-reactor made of ceramic tapes [69] (by courtesy of Kluwer Academic Publishers).

Vanadia species on supported on oxides were partially reduced during n-butane oxidation and during ethane oxidation, whereby the bridging V-O-V functionality was preferentially reduced relative to the terminal V=0 bond (Banares et al., 2000b Wachs et al., 1996, 1997). The higher reducibility of the surface polymeric species had only a minor effect on the... 

Ballarini et al. (8) posed the question of whether vanadium phosphate catalysts for n-butane oxidation offer the scope for further improvements. They concluded that as a consequence of the complexity of the dynamic surface species present on the catalyst, optimization of such material will not be forthcoming without further fundamental investigations. Previous investigations have involved probing of a number of catalyst parameters, including the V P ratio, the content of metal ion dopants, and the method of preparation. These and related topics are evaluated in detail below. 

In this review, we discuss how the methods of preparation of vanadium phosphafe materials can influence greatly their behavior as catalysts, and we describe the characterization of the various vanadium phosphates that can be made. Furthermore, we describe in detail the mechanism of selective n-butane oxidation and the emerging trend of applying vanadium phosphate catalysts to other oxidation reactions. 

FIGURE 3 Consecutive alkenyl mechanism of n-butane oxidation to MA (77-74). 

Cavani and coworkers (153) investigated further the relationship between catalyst activity for selective n-butane oxidation and minor... 

TABLE 1 Summary of Recent Results Characterizing the Achievements on the Effects of Promoters on the Performance of Vanadium Phosphate Catalysts for n-Butane Oxidation. 

Alonso et al. (248,249) also attempted to use a membrane reactor for n-butane oxidation. The operation of the membrane reactor was compared with that of a conventional fixed bed. Initially, Alonso et al. used an external bed of sand, fluidized with oxygen and heated with steam, such that the reactor temperature was maintained essentially constant (248). O2 and butane flowed concurrently through the reactor, through the shell and tube sides (containing catalyst), respectively. However, as the authors noted, the MA yield was not on par with that observed with the fixed-bed reactor at lower butane concentrations. Potentially, production... 

Marin et al. (250) attempted to model a reactor similar to that used by Alonso and co workers. Their simulations were compared with simulations representing a fixed-bed reactor operated under similar conditions. They concluded that the membrane reactor (with the external fluidized bed) was a viable technology for n-butane oxidation, but that it offered only a modest increase in MA yields relative to those realized in a fixed-bed reactor. Nonetheless, the safer operating conditions which keep the O2 and hydrocarbon flows separate, particularly with the oxidation of butane to MA, are desirable. Presently, MA yields are chiefly governed by the explosive limits of butane in air (i.e., 1.8%). Increasing the butane concentration with an optimized membrane reactor may increase overall MA yields. 

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