Vanadium oxides deposition

SCHEME 7. Schematic view of the molecular dispersion method for the production of vanadium oxide deposits, using VO(acac)2 as metal source and sihca as support . Reprinted with permission from Reference 328. Copyright 1996 American Chemical Society... 

Early catalysts for acrolein synthesis were based on cuprous oxide and other heavy metal oxides deposited on inert siHca or alumina supports (39). Later, catalysts more selective for the oxidation of propylene to acrolein and acrolein to acryHc acid were prepared from bismuth, cobalt, kon, nickel, tin salts, and molybdic, molybdic phosphoric, and molybdic siHcic acids. Preferred second-stage catalysts generally are complex oxides containing molybdenum and vanadium. Other components, such as tungsten, copper, tellurium, and arsenic oxides, have been incorporated to increase low temperature activity and productivity (39,45,46). 

Sodium and potassium are restricted because they react with sulfur at elevated temperatures to corrode metals by hot corrosion or sulfurization. The hot-corrision mechanism is not fully understood however, it can be discussed in general terms. It is believed that the deposition of alkali sulfates (Na2S04) on the blade reduces the protective oxide layer. Corrosion results from the continual forming and removing of the oxide layer. Also, oxidation of the blades occurs when liquid vanadium is deposited on the blade. Fortunately, lead is not encountered very often. Its presence is primarily from contamination by leaded fuel or as a result of some refinery practice. Presently, there is no fuel treatment to counteract the presence of lead. 

Vanadium originates as a metaiiic compound in crude oii and is concentrated by the distiiiation process into the heavy-oii fractions. Biade oxidation occurs when iiquid vanadium is deposited onto a biade and acts as a cataiyst. Vanadium compounds are oii-soiubie and are thus unaffected by fuei washing. Without additives, vanadium forms iow-meiting-temperature... 

Vanadium in the feed poisons the FCC catalyst when it is deposited on the catalyst as coke by vanadyl porphydrine in the feed. During regeneration, this coke is burned off and vanadium is oxidized to a oxidation state. The vanadium oxide (V O ) reacts with water vapor in the regenerator to vanadic acid, HjVO. Vanadic acid is mobile and it destroys zeolite crystal through acid-catalyzed hydrolysis. Vanadic acid formation is related to the steam and oxygen concentration in the regenerator. 

On the other hand, photosensibilization induced formation of singlet oxygen on the surface of deposited vanadium oxide catalysts was observed within the band A < 400 nm. 

The reaction of pure silica MCM-48 with dimethyldichlorosilane and subsequent hydrolysis results in hydrophobic materials with still a high number of anchoring sites for subsequent deposition of vanadium oxide structures. The Molecular Designed Dispersion of VO(acac)2 on these silylated samples results in a V-loading of 1.2 mmol/g. Spectroscopic studies evidence that all V is present as tetrahedral Vv oxide structures, and that the larger fraction of these species is present as isolated species. These final catalysts are extremely stable in hydrothermal conditions. They can withstand easily hydrothermal treatments at 160°C and 6.1 atm pressure without significant loss in crystallinity or porosity. Also, the leaching of the V in aqueous conditions is reduced with at least a factor 4. 

The decrease in activity of heterogeneous Wacker catalysts in the oxidation of 1-butene is caused by two processes. The catalyst, based on PdS04 deposited on a vanadium oxide redox layer on a high surface area support material, is reduced under reaction conditions, which leads to an initial drop in activity. When the steady-state activity is reached a further deactivation is observed which is caused by sintering of the vanadium oxide layer. This sintering is very pronounced for 7-alumina-supported catalysts. In titania (anatase)-supported catalysts deactivation is less due to the fact that the vanadium oxide layer is stabilized by the titania support. After the initial decrease, the activity remains stable for more than 700 h. 

To overcome the problems encountered in the homogeneous Wacker oxidation of higher alkenes several attempts have been undertaken to develop a gas-phase version of the process. The first heterogeneous catalysts were prepared by the deposition of palladium chloride and copper chloride on support materials, such as zeolite Y [2,3] or active carbon [4]. However, these catalysts all suffered from rapid deactivation. Other authors applied other redox components such as vanadium pentoxide [5,6] or p-benzoquinone [7]. The best results have been achieved with catalysts based on palladium salts deposited on a monolayer of vanadium oxide spread out over a high surface area support material, such as y-alumina [8]. Van der Heide showed that with catalysts consisting of H2PdCU deposited on a monolayer vanadium oxide supported on y-alumina, ethene as well as 1-butene and styrene... 

Vanadium oxide was deposited by impregnation with an aqueous solution of 10 g/dmi NHjVOj (Merck, reagent grade) of pH=4 at room temperature, as described in detail elsewhere [12]. The vanadium oxide covered supports were dried at 350 K for 16 h and calcined in air at 675 K for 4 h. 

Fig. 4 shows the TPR profiles of the fresh and spent catalyst. Curve C shows the desorption of hydrocarbons during reduction of the spent catalyst, formed by reduction of carbonaceous deposits on the catalyst surface. The hydrogen consumption profiles of the catalyst (see Curve A and B) show the two peaks, characteristic of palladium sulfate-based catalysts, with a vanadium oxide reduction peak at approximately 400 K and a sulfate reduction peak at 600 K [11,13,16]. The peak position of the sulfate reduction peak is comparable for both catalysts. For the spent catalyst, however, an additional small hydrogen consumption is observed at 700 K, which coincides with the large peak in the FID signal,... 

The results of the above characterization studies indicate that also in titania-supported catalysts the vanadium oxide layer slightly sinters. Since the vanadium oxide dispersion strongly effects the activity of the catalyst [16], it is likely that this sintering process is causing the deactivation observed in Fig. 3. The TPR and TPD results show that also some carbonaceous deposits are formed under reaction conditions, but these deposits are only present in low concentrations and, therefore, not likely to cause the deactivation of the catalyst. 

The above procedures for catalyst preparation have generally provided excellent results. Especially important are surface-sensitive reactions. With supported catalysts in which the active components have a narrow particle-size distnbution, the optimum particle size for a demanding reaction can be established. Major improvements of supported catalysts, e.g. with respect to carbon deposition and ammonia decomposition, can be achieved by preparing catalysts with a narrow par-ticle-size distribution. Also, the preparation of catalysts in which the active components have a uniform chemical composition is highly important One instance is the preparation of supported vanadium oxide phosphorus oxide (VPO) catalysts for the selective oxidation of w-butane to maleic anhydride, which has been carried out using vanadium(III) deposition onto silica [31]... 

CVD routes to vanadium oxide phases have employed a limited number of metal-organic precursors. VO(acac)2 is the most widely used, and oxide films have been deposited from it by low pressure MOCVD and PECVD. Other precursor for vanadium oxide film growth include VO(OiPr)3 and VO(OEt)3. ... 

The poisoning of fluid cracking catalysts (FCC) by vanadium is well known (7, 2). In general, vanadium is deposited on the cracking catalyst as coke by vanadyl porphyrins in the feed. During regeneration, the coke is burned off, and vanadium can be oxidized to the V+5 oxidation state. Woolery et al. (3) have shown that the oxidation state of the vanadium can alternate between +4 to +5 in... 

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