Catalyst continued vanadium

The equilibrium is slow except in the presence of a catalyst. Typically, vanadium catalysts, which consist of V0(S04) supported on kieselguhr, are used to push the equilibrium to the right-hand side. Since the equilibrium shifts in favor of the starting materials with increasing temperature, as low a temperature as possible (ca. 425 °C) is employed, together with continuous removal of the SO3 by absorption into sulfuric... 

This process (see Fig. 2-6) utilizes Az-butenes and butadiene mixtures as raw material in a fixed bed tubular reactor, containing 10 000 tubes. The catalyst is vanadium based. The effluent gases from the reactor are cooled and then scrubbed with aqueous maleic acid solution. Film evaporators are used for the concentration of maleic acid solution and dehydration to MA. The latter is purified by continuous rectification. The yield of MA of 72 kg/100 kg of convertible hydrocarbons is claimed (42.6 mole %). 

Aerobic alcohol oxidations with vanadium catalysts continue to widen their substrate scope and applications. The recendy studied mechanism of the intramolecular oxidation of benzyl alcoholate Hgands in 8-hydroxyquinoHnato(L) vanadium(V) complexes of the type [LV(0) (OR)] resembles those proposed for certain metaHoenzyme-catalyzed... 

The activity of catalyst degrades with time. The loss of activity is primarily due to impurities in the FCC feed, such as nickel, vanadium, and sodium, and to thermal and hydrothermal deactivation mechanisms. To maintain the desired activity, fresh catalyst is continually added to the unit. Fresh catalyst is stored in a fresh catalyst hopper and, in most units, is added automatically to the regenerator via a catalyst loader. 

The Industrial Revolution was made possible by iron in the form of steel, an alloy used for construction and transportation. Other d-block metals, both as the elements and in compounds, are transforming our present. Copper, for instance, is an essential component of some superconductors. Vanadium and platinum are used in the development of catalysts to reduce pollution and in the continuing effort to make hydrogen the fuel of our future. 

In normal operations, however, the RCC unit routinely runs at 6-9000 ppm nickel plus vanadium on feedstocks with as high as 35-75 ppm nickel plus vanadium content. Presently several commercial residuum-type catalysts are giving good performance at these levels. Catalyst manufacturers continue to seek further improvement in performance in terms of activity, selectivity, and catalyst life, while also trying to hold down or even reduce overall cost. 

Tamm et al. (1981) measured the maximum concentration of vanadium buildup in the catalyst at various reactor positions and times on stream. Near the reactor inlet, maximum vanadium concentrations increased rapidly with time before leaveling off. The catalyst in this portion of the bed has apparently reached its capacity and lost nearly all activity for HDM at about 80% of the run length. The catalysts in the reactors midsection and at the outlet have lower maximum concentrations, which increase more slowly with time. The lower zones of the catalyst bed are protected somewhat from metals deposition by the catalyst in the inlet region. This phenomenon continues until the catalyst bed does not have enough activity... 

Metals accumulate more slowly on the catalyst surfaces because the inlet concentrations of metals are lower than for coke precursors. The accumulation of metals can be even greater than coke, for example the vanadium concentration can reach 30-50 wt% of the catalyst on a fresh catalyst basis (Thakur and Thomas, 1985). Demetallization reactions can be considered autocatalytic in the sense that once the surface of the catalyst is covered with metal sulfides the catalyst remains quite active and continues to accumulate metal sulfides. The final loss of catalyst activity is usually associated with the filling of pore mouths in the catalyst by metal sulfide deposits. 

Surface vanadium appears to be most stable (to reduction) at low (<1%) V concentration when present as monomeric vanadyl units. Its stability decreases with increasing V levels. It is least stable (to reduction) at high (5%) V levels when present as a supported Vanadia phase. This difference in reactivity with V concentrations is believed responsible for the rapid decline in cracking activity observed in dual function cracking catalysts containing alumina when V start to exceed the 1.0-1.25 wt.% level (4). Further details of the mechanism of catalyst deactivation by V age the subject of continuing investigations 1n our laboratories by 31V solid state NMR, XPS, and Raman spectroscopy. 

The uppgrading of heavy oil will continue to increase in importance as changes In crude oil availability causes a shift toward heavier crudes. Usually, for extra heavy crudes, the bottom resid fractions as well as its deasphalted oils may contain significant quantities of metals (i.e. nickel and vanadium). These pose a serious problem for refiners because metal contaminants accumulate on catalyst during hydroprocessing causing permanent deactivation. The use of HDM catalysts to protect downstream HDS catalysts is recommended. 

Samples of used residue hydrodemetallization catalysts prepared by hydrotreating a Safanyia atmospheric residue have been characterized and tested using model compounds in order to investigate the initial deactivation of the catalyst Samples containing 4 to 10 wt % carbon and less than 200 wt ppm V or 10 to 15 wt % carbon and 1.3 wt % V have been obtained from tests in batch and continuous flow reactors respectively. It is shown that in the early stage of the catalyst deactivation a small amount of vanadium is more deactivating than a large amount of carbon. 

The asymmetric epoxidation of homoallylic alcohols has continued to be a problematic area. A potential solution has recently been published <07JA286 07T6075>. The use of bis-hydroxamic acid 1 as a chiral ligand for a vanadium catalyst has provided both excellent yields and enantioselectivity. This method works well with both cis- and trans-alkenes. 

Vanadium phosphates have been established as selective hydrocarbon oxidation catalysts for more than 40 years. Their primary use commercially has been in the production of maleic anhydride (MA) from n-butane. During this period, improvements in the yield of MA have been sought. Strategies to achieve these improvements have included the addition of secondary metal ions to the catalyst, optimization of the catalyst precursor formation, and intensification of the selective oxidation process through improved reactor technology. The mechanism of the reaction continues to be an active subject of research, and the role of the bulk catalyst structure and an amorphous surface layer are considered here with respect to the various V-P-O phases present. The active site of the catalyst is considered to consist of V and V couples, and their respective incidence and roles are examined in detail here. The complex and extensive nature of the oxidation, which for butane oxidation to MA is a 14-electron transfer process, is of broad importance, particularly in view of the applications of vanadium phosphate catalysts to other processes. A perspective on the future use of vanadium phosphate catalysts is included in this review. 

Air oxidation of /i-butane to maleic anhydride is possible over vanadium phos(4tate and, remaiicably, a 60% selectivity is obtained at 85% conversion. In the gas phase oxidation, in conffast to the situation found in the liquid, n-allcanes are oxidized more rapidly than branched chain alkanes. This is because secondary radicals are more readily able to sustain a chain for branched alkanes the relatively stable tertiary radical is preferentially formed but fails to continue the chain process. Vanadium(V)/ manganese(II)/AcOH has been used as a catalyst for the autoxidation of cyclohexane to adipic acid, giving 25-30% yields after only 4 h. ... 

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