Vanadium-based catalyst

In this section, some mechanistic implications of the SCR reaction over vanadium catalysts have been discussed based on the previous literature. The standard SCR reaction over V-based catalysts is generally considered to occur between the strongly adsorbed NH3 and gaseous or weakly adsorbed NO [42-49], The proposed reaction mechanisms often involve two adjacent vanadium species, namely the terminal oxygen species, i.e., V = O (redox sites), and the hydroxyl group, i.e., V -OH (Brpnsted acidic sites). Tops0e et al. [44 6] suggested that the reaction scheme involves the adsorption of NH3 on the Brpnsted acidic sites (V +—OH) followed by activation of adsorbed NH3 via reaction at the redox sites (V + = O)  

This activated fonn of NH3 reacts with gaseous or weakly adsorbed NO, producing N2 and H2O, and leading to partially reduced state (V + —OH). This reduced species could be reoxidized by oxygen to the V +=o species. 

Tops0e et al. [44-46] reported that under high O2 concentration condition ( 1 %), the NH3 activation step is fast and equilibrated, and thus the rate-limiting step is the reaction of NO with activated NH.  

Kamata et al. [48] estimated the ratio of the redox sites (V += O) to the Brpnsted acidic sites (V —OH) by steady-state kinetic analysis. The relative amount of V += O sites varied from 0.1 to 0.4 with the partial pressure of O2, indicating that the number of V =0 sites are less than the number of OH sites. 

Roduit et al. [1] proposed a global kinetic model for the standard SCR reaction based on V-based catalysts. The kinetic model accounts for three different reactions and intraparticle diffusion. The three reactions are Langmuir-Hinshelwood LH-type SCR, Eley-Rideal ER-type SCR, and direct NH3 oxidation. The main SCR pathway proceeds via the ER-type mechanism, but in the low temperature region T 200 °C), LH-type reaction occurs. Furthermore, at high temperatures (T 300 °C), NH3 oxidation and intraparticle mass transfer also takes place [1]. 

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Dr. Israel Wachs of Lehigh University discusses molecular engineering on oxide catalysts. These materials are especially important for partial oxidation reactions, in which selectivity is difficult to control. This chapter focuses on vanadium-based catalysts, and the approach is applicable to other supported catalysts as well. 

Chronologically, the production of o-xylene from mixed Cg aromatics was the first of these separations. In 1945, the Oronite Chemical Co. produced 85 to 90% purity o-xylene by fractionation from crude xylenes (1). The c-xylene product is oxidized for the production of phthalic anhydride in a vapor phase reaction over a vanadium-base catalyst. By 1947 Oronite provided 5% of the United States production capacity for phthalic anhydride by this process (2). 

Finally, it may be noted that vanadium-based catalysts can oxidize butenes to acetic acid. Kaneko et al. [168,169] report a selectivity of 50% at a relatively high conversion of 67%. V2Os is not active but combinations with Sn, W, Ti are, especially at temperatures in the range 200— 300° C. In accordance with the hypothesis of Ai about the effect of the catalyst acidity, a deep oxidation is favoured by the highest acidity. Hence Sn—V—O catalysts with 50% V, which display the maximum acidity per unit of weight, are the most suitable. 

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. 

Table 1, Stereoregularities and molecular weights of polypropylenes prepared with different soluble vanadium-based catalysts at —78 °C... 

The syndiotactic polypropylenes prepared with soluble vanadium-based catalysts usually contain some irregular linkages of propylene units arranged in head-to-head (Eq. 31) and tail-to-tail (Eq. 32) sequences. Doi95) has shown that the syndiotactic triad fraction [rr] of polypropylene decreases with an increase of the amount of the irregular linkages of propylene units [F01 + F10] (see Figure 16). 

Fig. 16. Relation between the syndiotactic triad fraction [rr] and the heterotactic dyad fraction of irregular propylene unit sequences [F01 + F10] in polypropylenes obtained at —78 °C with various soluble vanadium-based catalysts (from Ref. 95 )...

Block copolymers of propylene with ethylene have been produced in commercial polymerization processes using heterogeneous Ziegler-Natta catalysts. In all processes the block copolymers are produced in small concentrations, and the major products are homopolymers. Well-defined block copolymers free of homopolymer impurities can be prepared with catalysts exhibiting a living polymerization character. In this section we deal with the synthesis of well-defined block copolymers using the living polypropylene which has been prepared with soluble vanadium-based catalysts. 

In the preceding chapter it has been shown that the DFT methods currently available can be used to reproduce relative trends in both reactivities and transition-metal NMR chemical shifts. Thus, NMR/reactivity correlations can be modeled theoretically, at least when relative reactivities are reflected in relative energies on the potential energy surfaces (activation barriers, BDEs). It should in principle also be possible to predict new such correlations. This is done in the following, with the emphasis on olefin polymerization with vanadium-based catalysts. 

The use of coordination catalysts, especially homogeneous vanadium-based catalysts, for the copolymerisation of ethylene and propylene, with an ethylene content of 15-75 mol.-% in the feed, made it possible to produce amorphous... 

At the same time, syndiotactic polypropylene was also isolated by Natta et al. and characterised [145]. Syndiotactic polypropylene which was obtained by low-temperature polymerisation using soluble vanadium-based catalysts [146] could not be, however, commercialised, although it had a blend of interesting usable properties. 

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. 

With the sole exception of the random ethylene-propylene copolymers, for industrial applications heterogeneous catalysts have been used for alkene polymerisations. Ethylene-propylene statistical copolymerisation has been carried out using homogeneous vanadium-based catalysts [28]. 

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. 

Let us recall also that vanadium-based soluble Ziegier-Natta catalysts have found widespread industrial application for the manufacture of elastomeric ethylene/propylene copolymers and ethylene/propylene/diene terpolymers [319-322]. The most commonly used vanadium-based catalysts for random ethylene/propylene copolymerisation are those prepared from VCI4, VOCI3, V(Acac)3, VO(OEt)Cl2, VO(OEt)2Cl or VO(OEt)3 as precursors and AlEt3, AlEt2Cl or Al(z-Bu)2Cl as activators, with an Al/V molar ratio not exceeding 3 1 [37, 72],... 

In agreement with this finding, it has been shown that, in ethylene/propylene copolymerisation with vanadium-based catalysts, propylene insertion after an ethylene insertion is substantially non-stereospecific (both cases (a) and (b) in Figure 3.46 are possible) [1,390]. 

Syndiotactic polymers of higher a-olefins such as 1-butene and 4-methyl-1-pentene are produced by homogeneous metallocene-based catalysts [117, 429, 430], In contrast to polymerisation with metallocene-based catalysts, higher a-olefins are much less reactive in polymerisation with soluble vanadium-based catalysts, and already in the case of 1-butene polymerisation only yield trace amounts of low molecular weight syndiotactic polymer [394]. 

Catalysts for ethylene/carbon monoxide copolymerisation were initially obtained from Ni(II) derivatives, such as K2Ni(CN)4 and (w-Bu4N)2 Ni(CN)4, and Pd(II) derivatives, such as [(w-Bu3P)PdCl2]2, Pd(CN)2 and HPd(CN)3, often combined with alcohol or protonic acid as a cocatalyst [241]. It must be emphasised that, in contrast to titanium-, zirconium- or vanadium-based catalysts, nickel- and palladium-based catalysts tolerate polar functional groups (including hydroxyl, carboxylic and sulfonic groups)... 

There are also known soluble vanadium-based catalysts. Those obtained from soluble compounds such as V(Acac)3 or VO(OR)3 as precursors being activated... 

Very striking results on the interactions of molecules with a catalyst have been recently reported in zeolite catalysis because of the well ordered structure of these materials it is worth mentioning the subjects of zeolite design [10] and of acidic properties of metallosilicates [11]. In other areas where polycrystallinic or even amorphous materials arc applied, highly interesting results are now numerously emerging (such as hydrocarbon oxidation on vanadium-based catalysts [12] location of transition metal cations on Si(100) [13] CO molecules on MgO surfaces [14] CH4 and O2 interaction with sodium- and zinc-doped CaO surfaces [15] CO and NO on heavy metal surfaces [16]). An illustration of the computerized visualization of molecular dynamics of Pd clusters on MgO(lOO) and on a three-dimensional trajectory of Ar in Na mordenitc, is the recent publication of Miura et al. [17]. 

Elastomeric copolymers are made by either solution or suspension process using a vanadium based catalyst along with alkyl aluminum compound as cocatalyst. In the suspension process propylene is used as a diluent, whereas in the solution process hexane is used as diluent. Superior catalysts based on supported titanium compounds have further improved the suspension process in recent years. In the conventional suspension process, ethylene, propylene and catalysts are fed continuously to a stirred reactor at 20 °C and 12 kg cm total pressure. Diethylzinc is used to control molecular weight. 

The understanding of direct-initation mechanisms. In this context, tire use of metal salts of strong acids has opened a new area of fundamental studies. Indeed, we believe that the chemical insertion of the olefin between the metal cation and the anion following its adsorption on the salt surface closely resembles recent mechanistic proposals put forward by Zambelli and collaborators to rationalise initiation in stereospecific polymerisation (propene with vanadium-based catalysts). We are inclined to conclude that the cationic nature of stereospecific polymerisations of the Ziegler-Natta type is supported by our findings , viz. electrophilic attack by... 

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. 

In the following paragraphs issues such as the mechanism and kinetics of the reaction of NO, NH3 and O2 over vanadium-based catalysts are discussed. 

A strong deactivating effect of hydrogen has been noted in ethylene polymerization with vanadium-based catalysts such as M gC 12/VC 14—AIB u 3.409 This deactivation was reversible, activity being restored when hydrogen was removed. It was proposed that the deactivating effect of hydrogen arose from the reaction of V-H species with the... 

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