Vanadium catalysts sulphuric acid

Of more interest mechanistically though is the liquid nature of the molten phase present in the interstices of the inert porous support. It is now recognized that, deliberately or accidentally, a number of heterogeneously catalysed processes involve a supported liquid phase (SLP) rather than a solid catalyst. SLP catalysts are reviewed by Villadsen and Livbjerg. These include the vanadium-based sulphuric acid catalysts and, of comparable antiquity, the Deacon catalysts for oxidizing hydrogen chloride, where mixtures of copper and potassium chlorides can form melts in the catalyst support under reaction conditions. Thus in addition to diflfusional restrictions arising from pellet pore structure any... 

Benzoquinone ( quinone ) is obtained as the end product of the oxidation of aniline by acid dichromate solution. Industrially, the crude product is reduced with sulphur dioxide to hydroquinone, and the latter is oxidised either with dichromate mixture or in very dilute sulphuric acid solution with sodium chlorate in the presence of a little vanadium pentoxide as catalyst. For the preparation in the laboratory, it is best to oxidise the inexpensive hydroquinone with chromic acid or with sodium chlorate in the presence of vanadium pent-oxide. Naphthalene may be converted into 1 4-naphthoquinone by oxidation with chromic acid. 

Anthraquinone itself is traditionally available from the anthracene of coal tar by oxidation, often with chromic acid or nitric acid a more modern alternative method is that of air oxidation using vanadium(V) oxide as catalyst. Anthraquinone is also produced in the reaction of benzene with benzene-1,2-dicarboxylic anhydride (6.4 phthalic anhydride) using a Lewis acid catalyst, typically aluminium chloride. This Friedel-Crafts acylation gives o-benzoylbenzoic acid (6.5) which undergoes cyclodehydration when heated in concentrated sulphuric acid (Scheme 6.2). Phthalic anhydride is readily available from naphthalene or from 1,2-dimethylbenzene (o-xylene) by catalytic air oxidation. 

The production of sulphuric acid by the contact process, introduced in about 1875, was the first process of industrial significance to utilize heterogeneous catalysts. In this process, SO2 was oxidized on a platinum catalyst to S03, which was subsequently absorbed in aqueous sulphuric acid. Later, the platinum catalyst was superseded by a catalyst containing vanadium oxide and alkali-metal sulphates on a silica carrier, which was cheaper and less prone to poisoning. Further development of the vanadium catalysts over the last decades has led to highly optimized modem sulphuric acid catalysts, which are all based on the vanadium-alkali sulphate system. 

Sulphuric acid catalysts are not truly heterogeneous catalysts but so-called supported liquid phase (SLP) catalysts, where the oxidation of S02 takes place as a homogeneous reaction in a liquid film covering the internal surface of the support material [2], This was proposed already in 1940 by Frazer and Kirkpatrick [6], who found that the promoting action of the common alkali metals was due to their ability to form relatively low-melting pyrosulphates, which dissolve vanadium oxides, e.g. for potassium... 

The evaluation of carriers and catalyst compositions showed that significantly higher SO2 oxidation activity could be achieved with Cs as a promoter under the operating conditions downstream the intermediate absorption tower as demonstrated by the results in Table 1, where the activity compared to the standard product is increased by more than a factor 2. This was clearly sufficient for the introduction of VK69 to the market as a new sulphuric acid catalyst. The activity results for different melt compositions were used to optimise the vanadium content and the molar ratios of K/V, Na/V. and Cs/V. However, the choice of Cs/V was not only a question of maximum activity, because of the significant influence of the Cs content on the raw material costs (the price of caesium is 50-100 times the price of potassium on a molar basis). Here, the economic benefits obtained by the sulphuric acid producer by the marginal activity improvement at high Cs content also had to be taken into account. 

Sulphuric acid (98%) 400 000 tonnes Sulphur-burning process, followed by catalytic oxidation of S0-> (vanadium pentoxide catalyst). 

Sulphur trioxide S03 is obtained industrially as a route to sulphuric acid, by oxidising S02 with oxygen using a vanadium oxide catalyst. It can exist as a monomeric planar molecule but readily gives cyclic S309 trimers and linear polymers with corner-sharing S04 units. The highly exothermic reaction with... 

Independently of the work on the chemical composition of sulphuric acid catalysts, attempts to develop kinetic equations to describe the rate of reaction both for mechanistic analysis and reactor design purposes have been numerous. There is much evidence to suggest that in common with its catalytically oxidative behaviour in other environments, valency states of vanadium of V " and V " are involved. Mars and Maessen in 1964 developed a rate expression based on a simple two-step redox mechanism ... 

In an attempt to study this effect a VO TIO (70/30) sample which appears to be a solid solution from Xra) data (only rutile lines appear in spectra) was measured. The sai Ti 2p. B.E. as for the catalysts was obtained. To measure the V in tne rutile phase some samples were treated with sulphuric acid followed by ammonia to dissolve the vanadium oxide phase. The ESCA analysis of the 10 mole % catalyst treated in this manner showed the presence of 6 atom % V with a B.E. of 516.1 eV, obtained after subtraction of the 0 ls(Ka, ) line from spectra. No vanadium oxides are detectable by XRD on this sample. Thus, approximately 6 atom % V seems to be dissolved in the rutile phase of the catalysts. The composition "0 04 0 96 2 bee suggested in the literature ( ). That it is probably present as v is indicated by the lower V 2p y B.E. and in correspondence with the VO /TiO sample. ... 

The primary oxidation products were passed through several layers of platinized asbestos and platinum wire formed into a star pattern. A series of experiments on the combustion of diphenyl sulphoxide showed that at 850°C and in the presence of a platinum catalyst sulphur is quantitatively converted into sulphur dioxide with no sulphur trioxide being produced. The oxidation temperature can be increased to 1200°C if a vanadium catalyst is used. The water is absorbed in a tube containing calcium sulphate, which helps to prevent the formation of sulphurous acid. Carbon dioxide and sulphur dioxide were concentrated in a U-shaped trap cooled with liquid nitrogen, and were subsequently analysed by GC at 92°C using a 6-m column filled with dinonyl phthalate. The content of sulphur in the sample was derived from the sulphur dioxide peak area with due regard to the weight of the sample and the calibration coefficient. 

A similar reaction system was used for determining alkyl groups bonded to a silicon atom [214, 215]. In this instance reaction with concentrated sulphuric acid was carried out in the presence of a vanadium catalyst, as a result of which alkyl groups were split off in the form of the corresponding hydrocarbons. Quantitative determination of ethyl and phenyl groups is also possible by their thermal cleavage from a silicon atom with the formation of ethane and benzene [216]. 

Vanadium compounds may be employed as catalysts in oxidation. and reduction processes. For example, anthracene is oxidised to anthraquinone vnth a lead anode in 20 per cent, sulphuric acid which contains 3 per cent, of vanadic acid. Aniline under similar conditions may be oxidised to benzoquinone, and the latter substance can be efficiently reduced to hydroquinone. Azobenzene and azoxybenzene are stated to give a good yield of benzidine... 

The reaction is exothermic and the conditions ate controlled to keep the temperature at an optimum 450°C. Formerly, platinum catalysts were used but vanadium-vanadium oxide catalysts are now mainly employed (although less efflcient, they are less susceptible to poisoning). The sulphur trioxlde is dissolved in sulphuric acid H2SO4 + SO3 — H2S2O7 and the oleum is then diluted. 

Sulphur trioxide is prepared by the oxidation of sulphur dioxide with oxygen in the presence of a vanadium (V) oxide catalyst. It maybe prepared in the laboratory by distilling a mixture of concentrated sulphuric acid and phosphorus(V) oxide. It reacts violently with water to give sulphuric(VI) acid and is an important intermediate in the preparation of sulphuric acid and oleum. 

The catalyst shall promote the desired reactions at operation of the process at appreciably less temperatures, pressures, etc. Operation at lower temperature can increase overall equilibrium conversion in case of reversible exothermic reaction (oxidation of SO2 to SO3 in the presence of caesium-promoted vanadium pentoxide catalyst used in sulphuric acid plants). 

The geometry of the catalyst particles (diameter and length) should be such that resistance to flow of gaseous reactants should be minimum at the rated flow and at about 20% of overload. This will minimize power consumption for the flow of gases in the blower (hollow ring-type vanadium pentoxide-based catalyst is now used instead of sohd cyhndrical pellets used earlier in sulphuric acid plants). 

Although shipping containers for Monsanto Vanadium Sulphuric Acid Catalyst are tightly sealed, they should always be stored in a dry place as the catalyst may be damaged by moisture or water. The containers should never be opened until the converter is ready to be packed. 

A good example where process intensification has the potential to transform a complete plant operation centres on the manufacture of sulphuric add. This can be seen from a study of the SO3/H2SO4 contact process. In order to appreciate the potential impact this approach could have on a well-estabhshed process, it is worth discussing the flow sheet for the manufacture of sulphuric acid/oleum. A sulphur burner produces SO2 which is then reacted over vanadium pentoxide catalyst at 1 bar to produce gaseous SO3. This is then absorbed in recycling sulphuric acid to give product oleum. As is often the case, the reactor is the heart of the process. 

Sulphuric acid industry got a head start in the 1940s due to the invention of Vanadium Pentoxide as catalyst to convert Sulphur dioxide to Sulphur trioxide popularly known as the Contact Process . This enabled industry to put up large Sulphuric acid plants of higher concentration than achieved by prevalent lead chambers. The high concentration was required to produce phosphoric acid and phosphatic fertilisers. 

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