Catalyst instabilities

Before proceeding to a more detailed description of the effects of various solvents and promoters on catalyst activity and stability, it should be noted that the responses described above are possibly, or even probably, influenced by solvents and promoters. The responses shown, however, appear to be generally characteristic of these rhodium-containing systems. It is apparent that the rate of product formation is significantly accelerated by increases in reaction temperature. Higher temperatures, however, can bring about catalyst instability unless the pressure is simultaneously increased. Higher... 

The general behavior of rhodium catalysts with respect to stability thus appears to be similar to that seen for cobalt catalysts an inverse relationship between carbon monoxide partial pressure and reaction temperature is apparent. Stability decreases rapidly with increasing temperature, and raising the pressure tends to improve catalyst stability. It is not certain whether the adverse effects of increasing the H2/CO ratio are merely the result of a decreased CO partial pressure, or whether increased hydrogen partial pressure induces catalyst instability. 

Once again, the overall reaction is believed to involve two-electron oxidations to a ruthenium(IV) oxo species but in this case oxidation to a monomeric dioxoruthenium(IV) intermediate apparently takes place. Loss of 02 occurs via a peroxo intermediate (equations 95-98, equation 96 occurs stepwise).283 The need for the cis isomer is then obvious. Unfortunately, catalyst instability (only about 30 electrons can be transferred per molecule) means that this interesting system is not... 

Mechanisms Symptoms Carbon corrosion (air-air start) Gas impurities (e.g., CO, H2S, Sp2, ) Contaminants (e.g. some transition metal cations, anions) Catalyst instability (pt sintering, dissolution, re-crystallization) GDL loss of wet-proof (flooding) Seal failure (gross leaking) Membrane failure (pinholing, and tear)... 

There is relatively little information on the kinetics of the water-gas shift reaction at elevated temperatures (>600°C). This can be primarily attributed to the diminished value of K, which would limit CO conversion to unacceptably low levels in conventional reactors. Catalysts are typically not used at elevated temperatures because of the rapid rate of the non-catalyzed reaction and catalyst instability at these extreme conditions. A study of the forward and reverse reactions of the water-gas shift reaction (Eqn. 1) was conducted... 

Catalyst instability can be a problem that may be tackled by developing an immobilized catalyst. Organic catalysts do exist that slowly decompose under the conditions necessary for their reaction and release trace amounts of by-products that must be separated from the products. For instance, in photooxigenation reactions catalysed by porphyrin the release of highly coloured materials derived from the catalysts made product purification a real problem. This problem can be solved by immobilization of the catalyst because the decomposed material is also supported and can thus be removed from the reaction medium during the process of catalyst recovery. 

As pointed out [131], these systems, although quite efficient, may present problems due both to the oxidative degradation of the organic ligands in ammonium salts and some catalyst instability, with the latter being caused by the fact that the 0-transfer from the peroxo active species may lead to its structural collapse and the formation of less active species. 

Remarkably, the 2 1 adduct B is itself indefinitely stable in the absence of base, although it does disproportionate H2O2 into O2 and water, simultaneously regenerating MTO, at a very slow rate. Reactions like those described here in aqueous solution are invariably conducted in the presence of added acid to retard the base hydrolysis of MTO. Of course, this creates a problem for epoxidation reactions since epoxides hydrolyze to glycols in acidic media. Also, the 2 1 adduct B is inseparably linked to the catalyst instability associated with the 1 1 adduct A by two chemical reactions, the reversible dissociation of H2O2 molecules from B (Eq. 3.3), and substrate oxidation which converts a bound peroxo group into an oxide, forming compoimd A (Eq. 3.4). 

A further complication arises in the case of heterogeneous catalysis where the activity of the catalyst can change with time on stream or with reactant composition. Changes of this kind are themselves time-dependent processes that are overlaid on the time-dependent kinetics of the chemical transformations. The untangling of all these effects so that pure chemical kinetics can be studied is an important aspect of mechanistic studies using kinetics. See section on catalyst instabilities later in this chapter.)... 

The first example of thermochenfical catalytic system for acceptorless alkane dehydrogenation was reported by Fujii and Saito [117]. Their approach was to purge the reactor continuously with an inert gas in order to prevent the reversible hydrogenation of alkenes by the evolving H2. Unfortunately, these thermochemical catalytic systems are limited by low catalytic activities and catalyst instability. 

Other biphasic reductions using molecular hydrogen or an H-donor molecule include the selective hydrogenation of dienes to monoenes by K3[HCo(CN)5] (4) and reduction of arenes by hydridoruthenium clusters, such as [H4Ru4( -C6H6)4][BF4]2 (106). These reactions represent potentially important chemical transformations however, owing to catalyst instability or low productivity they have not been developed to practical biphasic processes. 

In the case of Heck coupling, promising results were obtained. However, 0.1 mol% catalyst was used, due to the catalyst instability at high tanperature (>100°C). 

The primary issue with the Shilov system [9], PtCl2(H20)2, that is profiosed to operate via an oxidative addition (OA) pathway, is catalyst instability due to irreversible decomposition to Pt metal or insoluble, polymeric Pt salts such (PtCyn- This can be addressed by the use of ligands as has been done with the high yield, Pt(bpym)Cl2/H2S04 system (bpym= K - 2,2 -bipyrimidyl ) [6]. This system is stable and active for the conversion of methane to methanol in concentrated sulfuric acid (Eq. 2). Yields of over 70% methanol (based on methane) with selectivities of >90% and turn-overs of -1000 have been observed with tum-over-frequencies of 10 s . ... 

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