Vanadium-based catalyst systems

The vanadium-based catalyst systems deteriorate with time and decrease in the number of catalytic centers as the polymerizations progress. The rate of decay is affected by conditions used for catalyst preparation, compositions of the catalysts, temperature, solvents, and Lewis bases. It is also affected by the type and concentration of the third monomer. Additions of chlorinated compounds to the deactivated catalysts, however, help restore activity. Catalyst decay can also be overcome by continually feeding catalyst components into the polymerization medium. ... 

This review is a survey of the applications and properties of supported liquid phase catalysts (SLP). By a supported liquid phase catalyst is meant the distribution of a catalytically active liquid on an inert porous support and the behaviour of such systems raises many interesting questions on catalyst chemistry, mass transfer in catalysts and reactor design. It is noteworthy thou that such systems have been employed in the chemical industry for many decades - indeed for over a century in the Deacon process for obtaining chlorine from hydrogen chloride - and of almost equally respectable antiquity are the vanadium based catalyst systems used for sulfuric acid manufacture but the recognition of SLP catalysts as possessing features of their own is much more recent. 

In contrast to the aforementioned binary oxides, V2Os has a stronger oxidation power and is able to attack hydrogen attached to the aromatic nucleus. Sometimes attention is drawn to the importance of a layer structure in the catalyst or to geometric factors (e.g. Sachtler [270]). Unexpectedly, however, very effective vanadium-based catalysts exist which operate in the molten state, indicating that a fixed structure is not important. The catalytic activity of molten oxide phases seems to occur exclusively in the oxidation of aromatic hydrocarbons over V2Os-based catalysts, such systems have not been reported for the selective oxidation of olefins. 

The Ziegler-Natta catalysts have acquired practical importance particularly as heterogeneous systems, mostly owing to the commercial production of linear high- and low-density polyethylenes and isotactic polypropylene. Elastomers based on ethylene-propylene copolymers (with the use of vanadium-based catalysts) as well as 1,4-cz s-and 1,4-tran.y-poly(l, 3-butadiene) and polyisoprene are also produced. These catalysts are extremely versatile and can be used in many other polymerisations of various hydrocarbon monomers, leading very often to polymers of different stereoregularity. In 1963, both Ziegler and Natta were awarded the Nobel Prize in chemistry. 

Nevertheless, many vanadium-based catalysts and polymerisation systems comprising them have received much academic attention in the hope that they might provide models for heterogeneous catalysts and polymerisation systems, since the problems connected with surface properties and particle size were believed to have been overcome. It must be noted, however, that homogeneous vanadium-based catalysts appeared to be more complex than was thought. There is no decisive evidence on the structure of catalytic sites formed by reaction between the procatalyst and activator. 

Homogeneous vanadium-based catalysts formed by the reaction of vanadium compounds and reducing agents such as organoaluminum compounds [10-12] are used industrially for the production of elastomers by ethylene/propene copolymerization (EP rubber) and ethylene/propene/diene terpolymerization (EPDM rubber). The dienes are usually derivatives of cyclopentadiene such as ethylidene norbomene or dicyclopentadiene. Examples of catalysts are Structures 1-4. Third components such as anisole or halocarbons are used to prevent a decrease in catalyst activity with time which is observed in the simple systems. 

Like the vanadium-based catalysts, the Sharpless AE system intrinsically favors 1,2-anti products this is because the cyclohexyl group in Scheme 8.8a occupies the position denoted by group Ra in Figure 8.2, away from the catalyst. In fact, this diastereoselectivity is somewhat amplified relative to achiral titanium catalysts. When the S allylic alcohol is used with (-f)-DIPT, a matched pair results (Scheme 8.8a). The strong enantiofacial selectivity of the L-(-f-)-DIPT catalyst clashes with the R substrate s resident chirality (this is the case shown in Figure 8.2 with Rb = cyclohexyl). In this mismatched pair, the preference of the chiral catalyst for a attack moderately exceeds that of the allylic alcohol for 1,2-anti product (Scheme 8.8b). The most important consequence is that the latter reaction is 140 times slower than the former. 

In 1962. Natta and Zambelli reported a heterogeneous. vanadium-based catalyst mixture which produced partially syndiotactic polypropylene at low polymerization temperatures. " The regiochemistry of the insertion was determined to be a 2.1-insertion of propylene, and a chain-end control mechanism determined the s mdiospecificity of monomer insertion. This catalyst system suffered from both low activity and low stereoselectivity. Highly active single-site olefin polymerization catalysts have now been discovered that make syndiotactic polypropylene with nearly perfect stereochemistry. Catalysts of two different symmetry classes have been used to make the polymer, with Cs-symmetric catalysts typically outperforming their Q -symmetric counterparts due to different mechanisms of stereocontrol (Figure 10). 

Since the 1920 s vanadium based catalysts have been used for this reaction chemistry, and after 70 years and over 1000 j)apers and patents, it seems reasonable to conclude that vanadium-based systems are uniquely well suited for this reaction chemistry. Monsanto, currently through Monsanto Enviro-Chern Systems, Inc., has been providing sulfuric acid catalysts and sulfuric acid plant engineering services for this industry for over 40 years. 

Vanadium-based catalytic systems for EP(D)M synthesis are comprised of a vanadium compound ( catalyst precursor ), a chlorinated aluminum alkyl ( cocatalyst ), and a chlorinated ester ( promoter ). Typical components of a vanadium-based catalytic system are the following ... 

The reaction of the vanadium salt with the aluminum alkyl leads to displacement of the vanadium ligands, with the formation of V-C bonds. As a consequence, the catalytic site is not sterically demanding. As mentioned above, vanadium-based catalysts behave as SSCs for olefin polymerization. Although the structure of their active catalytic sites is still unknown, the structures of polymers produced with vanadium systems are typical of those from SSCs. The majority of commercially available EP(D)M grades are from vanadium-based catalysts. ... 

Table 12.5 collects literature values for r and rir2 for different catalyst systems. With vanadium-based catalysts, a correct evaluation of rir2 is hindered by the remarkable presence of regioirregularities. One could say, however, that short comonomer sequences are obtained. With heterogeneous titanium-based catalysts, blocky copolymers are prepared. A correct determination of r r2 is, however, hindered by the multisite nature of the catalyst, that is, by the presence of (many) different polymer fractions resulting from many catalyst sites with different rir2 values that could in principle be responsible for an apparent higher blockiness. 

The polystyrenes obtained by a heterogeneous vanadium-based catalyst VCI3/AICI3 in the presence of TEA have a highly isotactic microstructure, and are semicrystalline with a melting point of 220 When activated with TIB A, the MgC -supported TiCU/NdCb heterogeneous system... 

In 1962, Natta and Zambelli reported a heterogeneous, vanadium-based catalyst mixture which produced partially syndiotactic polypropylene at low polymerization temperatures.45 jjie regiochemistry of the insertion was determined to be a secondary insertion of propylene, and a chain-end control mechanism determined the syndiospecificity of monomer insertion. This system suffered from both low activity and low stereospecificity. 

In this chapter, the development of an unsteady kinetic model of the NH3-SCR process for vanadium-based catalysts is presented. The model was based on the results from an extensive investigation of reactivity, chemistry, catalytic mechanism, and kinetics of the frill NH3-NO/NO2 SCR reacting system over a commercial V205-W03/Ti02 catalyst performed in our laboratories [1-10]. 

The direct ammoxidation of propane into acrylonitrile by its reaction with oxygen and ammonia is an alternative route to the conventional propylene ammoxidation catalytic reaction to acrylonitrile. The use of propane instead of propylene makes the reaction conditions more demanding because of the necessity of activating the propane C-H bond which is much stronger than the propylene methyl C-H bond. Different catalytic systems have been investigated for the ammoxidation of propane into acrylonitrile with vanadium-based catalysts proving particularly efficient, particularly the Sb-V-0 mixed oxide catalyst. ... 

The copolymerizations between monoolefins and dienes have been considered to be of practical and theoretical importance. As reported in the literatures ethylene-butadiene and propylene-butadiene copolymers can be prepared with conventional Ziegler-Natta titanium-based or vanadium-based catalysts. The copolymer composition and monomer sequence distribution strongly depend on the catalyst system and polymerization conditions. Alternating copolymers were synthesized when the catalyst components were mixed at the... 

Most catalysts for solution processes are either completely soluble or pseudo-homogeneous all their catalyst components are introduced into the reactor as Hquids but produce soHd catalysts when combined. The early Du Pont process employed a three-component catalyst consisting of titanium tetrachloride, vanadium oxytrichloride, and triisobutjlalurninum (80,81), whereas Dow used a mixture of titanium tetrachloride and triisobutylalurninum modified with ammonia (86,87). Because processes are intrinsically suitable for the use of soluble catalysts, they were the first to accommodate highly active metallocene catalysts. Other suitable catalyst systems include heterogeneous catalysts (such as chromium-based catalysts) as well as supported and unsupported Ziegler catalysts (88—90). 

The replacement of vanadia-based catalysts in the reduction of NOx with ammonia is of interest due to the toxicity of vanadium. Tentative investigations on the use of noble metals in the NO + NH3 reaction have been nicely reviewed by Bosch and Janssen [85], More recently, Seker et al. [86] did not completely succeed on Pt/Al203 with a significant formation of N20 according to the temperature and the water composition. Moreover, 25 ppm S02 has a detrimental effect on the selectivity with selectivity towards the oxidation of NH3 into NO enhanced above 300°C. Supported copper-based catalysts have shown to exhibit excellent activity for NOx abatement. Recently Suarez et al and Blanco et al. [87,88] reported high performances of Cu0/Ni0-Al203 monolithic catalysts with NO/NOz = 1 at low temperature. Different oxidic copper species have been previously identified in those catalytic systems with Cu2+, copper aluminate and CuO species [89], Subsequent additions of Ni2+ in octahedral sites of subsurface layers induce a redistribution of Cu2+ with a surface copper enrichment. Such redistribution... 

Alternating Copolymerization. In the last part of this paper we would like to refer briefly to our findings in connection with the alternating copolymerization of dienes with olefins. The alternating copolymerization of butadiene with propylene was first investigated in 1969 by Furukawa and others (15, 16, 17). They used catalyst systems based on titanium or vanadium compounds. 

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. 

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