Activity, selectivity, and stability

The three fundamental properties inherent in the actual definition of a catalyst are activity, selectivity, and stability. Moreover, for successful industrial applications, catalysts must be regener-able, reproducible, mechanically and thermally stable with suitable morphological characteristics, and also economical. 

Activity is a measure of the rate at which the catalyst causes the chemical reaction to arrive at equilibrium. In terms of kinetics, the reaction rate defines catalyst activity as the quantity of reactant consumed per unit time per unit volume or mass of catalyst  

In industrial practice, it is more practical to use readily measured parameters such as 

Space-time yield is the quantity of product formed per unit time per unit volume of reactor or catalyst, since reactor volume is taken as the catalyst-packed volume. 

Space time is defined as the time required for processing one reactor volume of feed and is calculated by dividing the reactor volume by the volumetric flow rate of feed. The reciprocal of space time is defined as space velocity, with units of reciprocal time, and signifies the number of reactor volumes of feed processed per unit time. The phase and the conditions at which the volumetric flow rate of feed is measured have to be specified. 

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The catalyst activity, selectivity, and stability are simultaneously influenced by the H2 to CCI2F2 feed ratio. Also the time needed to reach a steady-state catalyst performance depends... 

Previous studies on the use of Anchored Homogeneous Catalysts (AHC s) have been concerned with studying the effect which different reaction variables had on the activity, selectivity and stability of these catalysts (1-9). These reactions were typically ran at relatively low substrate/catalyst ratios (turnover numbers-TON s), usually between 50 and 100. While these low TON reactions made it possible to obtain a great deal of information concerning the AHC s, in order to establish that these catalysts could be used in commercial applications it was necessary to apply them to reactions at much higher TON S and, also, to make direct comparisons with the corresponding homogeneous catalyst under the same reaction conditions. 

Many of these problems disappeared in 1983 when Taramasso, Perego, and Notari synthesized titanium silicalite-1 (TS-1),1 which greatly affected the use of zeolite catalysts for practical oxidation chemistry. This catalyst shows outstanding activity, selectivity, and stability below 100°C. 

There are, then, three critical requirements of any catalyst if it is to be exploited on a commercial scale these are activity, selectivity and stability. It has been widely demonstrated and generally accepted that homogeneous catalysts are superior to their heterogeneous counterparts in terms of both activity (certainly under mild reaction conditions) and selectivity (the classical example is chiral catalysis). 

Catalysts may be metals, oxides, zeolites, sulfides, carbides, organometallic complexes, enzymes, etc. The principal properties of a catalyst are its activity, selectivity, and stability. Chemical promoters may be added to optimize the quality of a catalyst, while structural promoters improve the mechanical properties and stabilize the particles against sintering. As a result, catalysts may be quite complex. Moreover, the state of the catalytic surface often depends on the conditions under which it is used. Spectroscopy, microscopy, diffraction and reaction techniques offer tools to investigate what the active catalyst looks like. 

No commercial process is offered at this time for side chain alkylation of toluene with methanol for styrene and ethylbenzene production. In the literature the reaction is typically carried out at toluene to methanol molar ratios from 1.0 7.5 to 5 1 from 350 to 450 °C at atmospheric pressures. In some cases inert gas is introduced to assist vaporizing the liquid feed. In other cases H2 is co-fed to improve activity, selectivity and stability. Exelus recently claimed 80% yields in their ExSyM process at full methanol conversion using a 9 4 toluene methanol feed ratio at 400-425 °C and latm (101 kPa) in a bench-scale operation. This performance appears to be... 

There are reports of numerous examples of dendritic transition metal catalysts incorporating various dendritic backbones functionalized at various locations. Dendritic effects in catalysis include increased or decreased activity, selectivity, and stability. It is clear from the contributions of many research groups that dendrimers are suitable supports for recyclable transition metal catalysts. Separation and/or recycle of the catalysts are possible with these functionalized dendrimers for example, separation results from precipitation of the dendrimer from the product liquid two-phase catalysis allows separation and recycle of the catalyst when the products and catalyst are concentrated in two immiscible liquid phases and immobilization of the dendrimer in an insoluble support (such as crosslinked polystyrene or silica) allows use of a fixed-bed reactor holding the catalyst and excluding it from the product stream. Furthermore, the large size and the globular structure of the dendrimers enable efficient separation by nanofiltration techniques. Nanofiltration can be performed either batch wise or in a continuous-flow membrane reactor (CFMR). 

The choice of a catalyst is based on different criteria such as activity, selectivity and stability. In addition to these fundamental properties, industrial applications require that the catalyst is mechanically and thermally stable, regenerable and inexpensive and possesses suitable morphological characteristics [39]. 

Changes In shape selectivity due to the Isomorphous substitution of A1 by the larger Fe has not, so far, been unequivocally been established. However, differences in catalytic activity, selectivity and stability between alumino- and ferrisillcate zeolites arising from the presence of weaker... 

Several authors suggest the use of the NiY zeolite and Ni/Si-Al as catalysts for the oligomerization of isobutene [7-9]. The aim of this work is to compare the performance of two different green acid catalysts in situ sulfated Ti02 synthesized by the sol-gel method [22] with NiY zeolite, in the gas phase trimerization of isobutene at mild pressure and temperature conditions atmospheric pressure and 40°C, in terms of activity, selectivity and stability. 

Thus the selected iodide catalyst, TOP18, has good catalytic activity, selectivity, and stability, it is readily soluble in 2,5-DHF and in warm alkane solvents. As a... 

Thus the selected Lewis acid catalyst, TOT, has good catalytic activity, selectivity and stability. It is a non-viscous liquid which is compatible with TOP18 and is miscible with 2,5-DHF and non polar alkane solvents. It has a very low vapor pressure so it is not lost during product distillation and catalyst recovery operations. TOT is easily synthesized at low cost and it has low toxicity (Oral LD-50 (rat), >2000 mg/kg Dermal LD-50 (rat), >2000 mg/kg). 

Cmcial to the development of a biocatalytic route is furthermore the selection of a readily available biocatalyst with sufficient activity, selectivity and stability, which preferably should be commercially available. Other factors that play an important role are the scale at which the process should be ran and the time it takes to develop such a biocatalytical process. 

On the other hand, the origin of the promoter metal and metal oxide effects is not always clear, despite the many detailed characterization studies. In what follows, we will give first a possible definition of the different promotion phenomena described in literature, as well as their mode of operation. The second part deals with an extensive literature overview of the effect of each promoter element on the F-T activity, selectivity and stability of the active Co phase. The different modes of operation will be evaluated for each element. Special attention will be paid to noble metal and transition metal oxide promotion effects. 

Fischer-Tropsch synthesis making use of cobalt-based catalysts is a hotly persued scientific topic in the catalysis community since it offers an interesting and economically viable route for the conversion of e.g. natural gas to sulphur-free diesel fuels. As a result, major oil companies have recently announced to implement this technology and major investments are under way to build large Fischer-Tropsch plants based on cobalt-based catalysts in e.g. Qatar. Promoters have shown to be crucial to alter the catalytic properties of these catalyst systems in a positive way. For this reason, almost every chemical element of the periodic table has been evaluated in the open literature for its potential beneficial effects on the activity, selectivity and stability of supported cobalt nanoparticles. 

The solvent is a sine qua non of a homogeneous catalyst system. Solvent properties are indeed very important in determining the activity, selectivity, and stability of a catalyst. Solvent stability is also essential, if the catalytic system as a whole is to be stable. As described above, several solvents have been employed in studies of cobalt-catalyzed CO reduction. Keim et al. (39) noted a substantial difference in activity and selectivity between catalyst solutions in toluene and W-methylpyrrolidone (Table I). Most of the information in this area again comes from the work of Feder and Rathke (36). Listed in Table IV are their results showing changes in the activity of the cobalt catalyst corresponding to changes in solvent polarity. The rates... 

Both oxidations are highly exothermic and carried out almost exclusively in tubular reactors cooled by a molten salt.1024 Supported vanadium oxide with additives to improve activity, selectivity, and stability usually serves as the catalyst.970 990 1025 Because of its more favorable stoichiometry (no carbon is lost in oxidation), most new plants use o-xylene as the starting material. 

Transition metal complexes encapsulated in the channel of zeolites have received a lot of attention, due to their high catalytic activity, selectivity and stability in field of oxidation reactions. Generally, transition metal complex have only been immobilized in the classical large porous zeolites, such as X, Y[l-4], But the restricted sizes of the pores and cavities of the zeolites not only limit the maximum size of the complex which can be accommodated, but also impose resistance on the diffusion of substrates and products. Mesoporous molecular sieves, due to their high surface area and ordered pore structure, offer the potentiality as a good host for immobilizing transition complexes[5-7]. The previous reports are mainly about molecular sieves encapsulated mononuclear metal complex, whereas the reports about immobilization of heteronuclear metal complex in the host material are few. Here, we try to prepare MCM-41 loaded with binuclear Co(II)-La(III) complex with bis-salicylaldehyde ethylenediamine schiff base. 

The selection of a catalyst or the design of a new catalyst for a particular purpose will depend on activity, selectivity, and stability requirements. These requirements are generally related but not necessarily compatible in... 

New analytical methods, high-level theoretical studies and various mechanistic studies are producing important information on the factors related to the catalytic activity, selectivity and stability. In particular, valuable information on the correlations... 

Every (bio)catalyst can be characterized by the three basic dimensions of merit -activity, selectivity and stability - as characterized by turnover frequency (tof) (= l/kcat), enantiomeric ratio (E value) or purity (e.e.), and melting point (Tm) or deactivation rate constant (kd). The dimensions of merit important for determining, evaluating, or optimizing a process are (i) product yield, (ii) (bio)catalyst productivity, (iii) (bio)catalyst stability, and (iv) reactor productivity. The pertinent quantities are turnover number (TON) (= [S]/[E]) for (ii), total turnover number (TTN) (= mole product/mole catalyst) for (iii) and space-time yield [kg (L d) 11 for iv). Threshold values for good biocatalyst performance are kcat > 1 s 1, E > 100 or e.e. > 99%, TTN > 104-105, and s.t.y. > 0.1 kg (L d). ... 

Basic Performance Criteria for a Catalyst Activity, Selectivity and Stability of Enzymes... 

Every catalyst, and thus also every biocatalyst, can be characterized by the three basic dimensions of merit, namely activity, selectivity, and stability. Additional, but not frequently employed, performance criteria beyond the basic dimensions are discussed in Section 2.3.3. 

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