Catalytic activity selectivity

Metals Reaction Substrate Main product Catalytic activity Selectivity Literatures... 

It is very seldom that a commercial catalyst consists of only a single chemical compound or element. Often the active constituent is supported on a carrier material that may or may not possess catalytic activity of its own. Enhanced catalytic activity, selectivity, or stability may also be achieved by the addition of other materials referred to as promoters or inhibitors. 

Figure 2. Transient catalytic activity, selectivity, and surface oxygen activity of a freshly calcined silver catalyst during the first 40 h on stream. Catalyst is exposed to reactive gas mixture at t — 0, 410°C, Pet — 0.015 bar, and Po2 = 0.i0 for.

Table 10.3 Catalytic activities, selectivities and enantiomeric excess (e.e.) in homogeneous and heterogeneous vanadium (V) catalyses for the oxidative coupling of 2-naphthol .

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... 

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). 

Catalytic Activity, Selectivity, and Deactivation. The product distribution (in the C1-C5 range) remained relatively unchanged with increasing number of pulses for any given sample. For the original H-mordenite and the NH4N03-exchanged samples, propane was the major product (45-55 mole % of C1-C5). Propane and isobutane were comparable in amount (35-40 mole % each) for the two acid-extracted samples. The i-C4 n-C4 ratio was about 2 1 for samples 1, 4, and 5, and about 3 1 for samples 2 and 3, independent of pulse number. 

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. 

Liquid-phase hydrogenation of 1,4 butynediol to cis-1,4-butenediol and 1,4-butanediol has been carried out on nickel catalysts supported on thirteen different supports. Some commercial nickel catalysts were used as references. Furthermore, metal loading and Ni-Cu alloying have also been studied. The results obtained indicates that catalytic activity, selectivity and metal surface area of catalysts are closely correlated to some textural and/or acid-base properties of the corresponding support. Similarly, the influence of Cu as a second metal in catalyst behaviour is also related to the nature of the support. 

In the last three decades, we have designed and successfully prepared various supported metal complexes on oxide surfaces that exhibit unique catalytic activities and selectivities that are different from those of their homogeneous analogues [3,4,9, 12-15]. With the aid of several sophisticated spectroscopic techniques, the structures and roles of catalytically active species on surfaces have been characterized and identified [3, 4,9,12-25]. Chemical interactions between metal complexes and oxide surfaces can provide new reactivity of metal species by the construction of a spatially controlled reaction environment and the formation of unsaturated active metal species, leading to high catalytic activity, selectivity and durability [21-25]. 

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... 

Catalytically active particles can be formed from various palladium sources under supercritical reaction condition, which could be helpful for the particle dispersion. Therefore, those materials show high catalytic activity, selectivity, and stability for a broad range of substrates. Additionally, the PEG matrix effectively stabilizes and immobilizes the catalytically active particles, whereas the unique solubility and mass transfer properties of scC02 allow continuous processing at mild conditions, even with low-volatility substrates. 

Clusters and alloys are molecular species that may show different catalytic activity, selectivity and stability than bulk metals and alloys. Small metal clusters and alloy clusters have been studied reeendy for potential use as catalysts, ceramic precursors, and as thin films. Several fundamental questions regarding such clusters are apparent. How many atoms are needed before metallic properties are observed How are steric and electronic properties related to the number, type and structure of such clusters Do mixed metal clusters behave like bulk alloy phases ... 

A brief overview of bimetallic catalysts is presented. Electronic vs. ensemble effects are discussed, and literature is reviewed on single crystal bimetallics, and supported bimetallic clusters. Bimetallic cluster compounds are considered as models. Structural considerations, effects of potential poisons, particles from bimetallic cluster compounds, and catalytic activity/selectivity studies are briefly reviewed and discussed. 

With respect to challenges, Wayne Goodman stressed the development of sensitive surface diagnostics for in situ characterization of working catalysts to establish an unambiguous relationship between surface (electronic) structure at the atomic level and catalytic activity/selectivity and first principles design of catalytic materials for specific chemical and fuels applications. 

Three aspects of the performance of supported catalysts are also discussed in this Chapter. With the development of techniques, as outlined above, for the characterization of supported metal catalysts, it seems timely to survey studies of crystallite size effect/structure sensitivity with special reference to the possible intrusion of adventitious factors (Section 5). Recently there has been considerable interest in the existence of (chemical) metal-support interactions and their significance for chemisorption and catalytic activity/ selectivity (Section 6). Finally, supported bimetallic catalysts are discussed for various reactions not involving hydrocarbons (hydrocarbon reactions over alloys and bimetallic catalysts have already been reviewed in this Series with respect to both basic research and technical applications ). References to earlier reviews (including some on techniques) that complement material in this Chapter are given in the appropriate sections. It might be useful, however, to note here some topics not discussed that also form part of the vast subject of supported metal and bimetallic catalysts and for which recent reviews are available, viz, spillover, catalyst deactivation, the growth and... 

Ditertiary phosphines such as (86), (92), and (98) (100) (Scheme 6) have found important uses as ligands for metal-catalyzed transformations, including e.g., palladium-catalyzed Grignard cross couplings,194,205 rhodium-catalyzed Michael additions,2 hydrocyanations,206 copolymerizations,20 and palladium-catalyzed animations.208 Rhodium complexes of (86) are catalysts for the carbonylation of methanol.188 More recently the ligand bite angle of ditertiary phosphines such as (100) has been shown to influence catalytic activity/selectivity in several important catalytic processes.209-213... 

A comparison of the catalytic activity and selectivity of the unsupported La-CaO with that of the supported La-CaO catalysts (Table 1) reveals that, because of the direct deposition of La-CaO on the catalyst carriers, the catalytic activity/selectivity and product ratios (viz. C2H4. /C2H6 and CO/CO2) in the OCM are influenced appreciably, as follows. 

Influence of the support on the basicity and catalytic activity/selectivity (in OCM) of La-CaO (loading of La-CaO (with or without precoating) in supported catalysts 15 1.5 wt%) [Reaction conditions Feed, a mixture of pure methane and O2 at a CH4/O2 feed ratio of 4.0 temperature, 800°C]. 

The improved catalytic activity/selectivity of the MgO, CaO and La203 precoated supported La-CaO catalysts is attributed to the formation 0 f a protective layer of the precoated metal oxide between the reactive components of the supports and the deposited La-CaO. The improved catalytic performance is consistent with the increase in the strong basicity of the catalysts. 

C02 + H2 reaction over La1 >,MJ,Co03 (M = Sr,Th) catalytic activity, selectivity and C2+ production at different... 

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