Monomolecular Catalysts

It is mostly complexes of ruthenium and rhodium that have been used to conduct hydrogenation reactions in ionic liquids and little attention has so far been paid to modifying the employed catalysts to improve their performance in the ionic environment. The majority of the catalysts used are identical to those employed in conventional homogeneous catalysis conducted in molecular solvents like, for example, RhCl(PPh3)3 and RuCl2(PPh3)3. 

The presence of polar co-solvents such as simple alcohols in biphasic systems could lead to increased catalyst leaching, even if the active complex is charged, and ligand-modification may be necessary in order to minimise loss of the catalyst. 

There are many examples of hydrogenation reactions in ionic liquids and only a selection will be shown in greater detail, however a summary of hydrogenation reactions performed in ionic liquids is provided in Table 3.2. 

Only very few examples of homogeneous catalysts based on other metals have been reported, namely iridium, cobalt and palladium. With palladium(II) acetylacetonate, highly selective (up to 100%) hydrogenation of dienes to monoenes was achieved. The high selectivity was attributed to the formation of a thermodynamically favoured rf-allyl Pd-intermediate and also to the higher solubility of the dienes in the ionic phase compared to the monocncsJly The catalytic system can be recycled up to fifteen times following a simple phase separation. 

When 8a and 8b are employed as catalysts for the hydrogenation of ethyl acetoacetate, the enantioselectivity shows a pronounced dependence on the nature of the ionic liquid anion with [PF6] giving the lowest selectivity, 

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Polymerization was dependent on the ability of the solvent to produce a monomolecular catalyst species, but not to form too strong a complex with the solvent. Thus polymerization occurred in ethanol but not in strong donors such as dimethyl sulphoxide or hexamethyl phosphoramide. The results were interpreted by the following mechanism ... 

In the process of radical polymerization a monomolecular short stop of the kinetic chain arises from the delocalization of the unpaired electron along the conjugated chain and from the competition of the developing polyconjugated system with the monomer for the delivery of rr-electrons to the nf-orbitals of a transition metal catalyst in the ionic coordination process. Such a deactivation of the active center may also be due to an interaction with the conjugated bonds of systems which have already been formed. 

The mechanistic investigations presented in this section have stimulated research directed to the development of advanced ruthenium precatalysts for olefin metathesis. It was pointed out by Grubbs et al. that the utility of a catalyst is determined by the ratio of catalysis to the rate of decomposition [31]. The decomposition of ruthenium methylidene complexes, which attribute to approximately 95% of the turnover, proceeds monomolecularly, which explains the commonly observed problem that slowly reacting substrates require high catalyst loadings [31]. This problem has been addressed by the development of a novel class of ruthenium precatalysts, the so-called second-generation catalysts. 

Immobilizing the catalyst on the electrode surface is useful for both synthetic and sensors applications. Monomolecular coatings do not allow redox catalysis, but multilayered coatings do. The catalytic responses are then functions of three main factors in addition to transport of the reactant from the bulk of the solution to the film surface transport of electrons through the film, transport of the reactant in the reverse direction, and catalytic reaction. The interplay of these factors is described with the help of characteristic currents and kinetic zone diagrams. In several systems the mediator plays the role of an electron shuttle and of a catalyst. More interesting are the systems in which the two roles are assigned to two different molecules chosen to fulfill these two different functions, as illustrated by a typical experimental example. 

Industrial metal-zeolite catalysts undergo a bifunctional, monomolecular mechanism [1-5, 7]. Carbenium ions are the critical reaction intermediates to complete chain reactions. In the zeolite channels, carbenium ions likely exist as an absorbed alkoxyl species, rather than as free-moving charged ions [8], Figure 14.2 illustrates the accepted reaction mechanism, using hexanes as an example. 

Xylene Isomerization There are several mechanisms by which the three xylene isomers can be interconverted. The one that is of the greatest interest with respect to industrial applications is the so-called monomolecular or direct xylene isomerization route. This reaction is most commonly catalyzed by Bronsted acid sites in zeolitic catalysts. It is believed to occur as a result of individual protonation and methyl shift steps. 

The distribution of isomers formed as a result of this reaction tends to be higher in OX at the expense of PX, so catalysis through this route is less desirable from an industrial perspective. Comparisons of the monomolecular versus bimolecular reaction have been made, providing insight into the properties of zeolitic catalysts that favor one route over the other [64, 65]. Mechanistic aspects in MOR and TON structure zeolites have been evaluated using ah initio calculations, which suggest that the initiation step involves a defect site rather than an acidic proton [66]. It is... 

The conditions favoring cracking by the monomolecular path are high temperature and low olefin concentrations, i.e. low paraffin partial pressure and/or low conversion. The proposed reaction intermediate is formed by protonation of the paraffin feed by a Brdnsted acid site of the catalyst. We may compare this with similar paraffin protonation by CH5 in chemical ionizations occurring in an ion cyclotron resonance mass spectrometer [10], The C0H15 ion produced collapses to the same products as we have observed with zeolites HZ as the proton source (Fig.1). This is surprising, since the... 

Reaction rates for the start-of-cycle reforming system are described by pseudo-monomolecular rates of change of the 13 kinetic lumps. That is, the rates of change of the lumps are represented by first-order mass action kinetics with the same adsorption isotherm applicable to each reaction step. Following the same format as Eq. (4), steady-state material balances for the hydrocarbon lumps are derived for a plug-flow, fixed bed catalytic reformer. A nondissociation, Langmuir-Hinshelwood adsorption model is employed. Steady-state material balances written over a differential fractional catalyst volume dv are the following ... 

In conclusion, extensive research has revealed that the Lewis and Brpnsted acid sites on the promoted sulfated zirconia catalysts are not necessarily stronger acids than the corresponding sites in zeolites, but sulfated zirconia circumvents the energetically unfavorable monomolecular reaction path by following a bimolecular mechanism. The question of superacidity of sulfated zirconia, however, is still debated.312... 

Using the monomolecular rate theory developed by Wei and Prater, we have analyzed the kinetics of the liquid-phase isomerization of xylene over a zeolitic catalyst. The kinetic analysis is presented primarily in terms of the time-independent selectivity kinetics. With the establishment of the basic kinetics the role of intracrystalline diffusion is demonstrated by analyzing the kinetics for 2 to 4 zeolite catalyst and an essentially diffusion-free 0.2 to 0.4 m zeolite catalyst. Values for intracrystalline diffusivities are presented, and evidence is given that the isomerization is the simple series reaction o-xylene <= m-xylene <= p-xylene. 

The techniques of monomolecular rate theory easily transform measured reaction data into a form where we can analyze apparent kinetics and the effects of intracrystalline diffusion by the use of selectivity data. Time dependency has been eliminated. Since selectivity is extremely reproducible and is independent of short-term aging effects, the number of experimental runs is reduced while data reliability is maintained. For catalyst evaluation at any temperature, it is necessary to determine the equilibrium composition and the straight-line reaction path. With this information any catalyst can be evaluated at this temperature with simply the additional information from a curved-line reaction path. The approach used in the application of monomolecular rate theory to the xylene isomerization selectivity kinetics is as follows. Reference is made to the composition diagram, Figure 1. 

Applicability of Monomolecular Rate Theory to Xylene Isomerization Selectivity Kinetics over Fresh AP Catalyst. The kinetics of liquid-phase xylene isomerization over fresh zeolite containing AP catalyst are effectively interpreted by pseudomonomolecular rate theory. The agreement between the experimental data (data points) and predicted reaction paths (solid lines) for operation at 400° and 600°F is shown in Figure 2. The catalyst used was in the form of extrudates comprised of the zeolite component and an A1203 binder. Since xylene disproportionation to toluene and trimethylbenzenes was low, selectivity data were obtained by mere normalization of the xylene compositions (2 axyienes = 1.0). 

Most of the present uses of the scanning tunneling microscope involve studies of surface chemistry. Processes such as the deposition of monomolecular layers on smooth surfaces can be studied, the nature of industrial catalysts can be probed, and metal corrosion can be examined. The possibility also exists that complex biological structures can be determined with the STM. 

Here Z represents a catalyst surface site (active centre). The two final steps are in equilibrium, designated by the symbol —. "The natural classification of simple (elementary) reactions by the number of molecules involved simultaneously in the reaction belongs to Van t Hoff. If the reaction involves one molecule (reaction A - B), it is classified as first-order (monomolecular). In cases where two molecules take part in the reaction (e.g. 2 A - B or A + B - C), the reaction is said to be second-order (bimolecular). With the participation of three molecules (3 A -> B or 2 A + B -> C), the reaction is specified as third-order (termolecular). The simultaneous interaction of more than three reactants is believed to be highly improbable. 

Strictly speaking, mechanisms for heterogeneous catalytic reactions can never be monomolecular. Thus they always include adsorption steps in which the initial substances are a minimum of two in number, i.e. gas and catalyst. But if one considers conversions of only surface compounds (at a constant gas-phase composition), a catalytic reaction mechanism can also be treated as monomolecular. It is these mechanisms that Temkin designates as linear (see Chap. 2). 

It should be pointed out that in many cases it seems uncertain which substance it is that constitutes the veritable catalyst. Particularly for metallic and oxidic catalysts each separate case must be investigated for formation of a monomolecular layer of compounds or adsorbates, for instance, sulfides, carbides, hydrides, etc., which constitutes the real catalyst after an individual activation period of the metal or the oxide. In such cases the electron exchange between the film and the substrate will, of course, be the decisive factor 13). 

In the preceding sections the use of catalysts in which vanadium oxides are supported on a more or less inert carrier has been mentioned quite often. Because of the importance of this type of catalyst they are discussed more extensively in this section. Often a distinction is made between the normal supported catalysts and so called monolayer catalysts. In the latter the vanadium oxide is supposed to be dispersed in a monomolecular layer on the support, which may be covered completely or only partly. The normal supported catalysts are usually made by impregnation, either wet or dry, of the porous carrier with an aqueous solution, often of NH4V03, sometimes with oxalate added.12 14,75,95,139,140... 

Calculation of the Adsorption Enthalpy of n-Paraffins in Nanoporous Crystalline and Ordered Acid Catalysts, and Its Relation with the Activation Energy of the Monomolecular Catalytic Cracking Reaction... 

With the model developed, we describe alkane adsorption in nanoporous crystalline and ordered acid catalysts with the intention of calculating three mathematical equations, one each for the geometry of the pore system, for describing the adsorption enthalpy and its relation with the activation energy, and for the monomolecular cracking of n-paraffins [97], This methodology can also be applied to other unimolecular reactions. 

Within the framework of the transition state theory [112,113], the observed activation energy, Eobs, for a monomolecular catalytic process in the heterogeneous case is Eobs = E0 + A//ads [act. complex], where E0 is the energy of the reaction without a catalyst and A//.lds act. complex] is the adsorption enthalpy of the activated complex [114], In the monomolecular cracking of n-alkanes catalyzed by... 

Brunauer and Emmett,2 however, take the view that, on porous iron catalysts, the first effect of van der Waals adsorption is to cover the surface with a layer one molecule thick. In the case of several permanent gases, and also of carbon dioxide and butane, if the adsorption isotherms are measured not too far above the boiling-point of the gases, the first layer is complete at 50 mm. pressure or less. If the pressure is raised up to atmospheric, further quantities are adsorbed, and there appears a nearly linear relation between the pressure, and the amount adsorbed in excess of the first monomolecular layer but the increase of adsorption, as the pressure is raised above that at which the first layer is complete, is much more gradual than the increase with pressure, at low pressures, before the surface is completely covered.3... 

As many organic compounds may transform simultaneously through mono molecular (intramolecular) and bimolecular (intermolecular) processes, it is easy to understand that the shape and size of the space available near the active sites often determine the selectivity of their transformation. Indeed the transition state of a bimolecular reaction is always bulkier than that of a monomolecular reaction, hence the first type of reaction will be much more sensitive to steric constraints than the second one. This explains the key role played by the pore structure of zeolites on the selectivity of many reactions. A typical example is the selective isomerization of xylenes over HMFI the intermediates leading to disproportionation, the main secondary reaction over non-spatioselective catalysts, cannot be accommodated at its channel intersections (32). Furthermore, if a reaction can occur through mono and bimolecular mechanisms, the significance of the bimolecular path will decrease with the size of the space available near the active sites (41). 

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