Deactivation and by-product formation

Deactivation pathways arise from the decomposition of any of the intermediates and lead to non-reactive species (i.e., species that cannot readily re-enter the catalytic cycle). Moreover, the reaction intermediate can also lead to the formation of by-products, which may or may not be connected to deactivation processes. Computational studies of such reactions are challenging because they can involve many intermediates and reaction pathways. In addition, there is often a lack of information about such processes in terms of their nature and the spectroscopic signatures of the chemical species containing the metal center and/or of the by-products, as well as kinetic data of their mode of formation. For these reasons, studies of deactivation and by-product formation are rarely carried out, even if they are probably indispensable for a full understanding of the efficiency of a catalytic process and the rational design of better catalysts. 

The influence of Bi promotion on the oxidation of a-tetralol to a-tetralone is shown in Fig. 4. The unpromoted Pt/alumina catalyst rapidly deactivates and only 34 % conversion is achieved in 5 h. The oxidation of the catalyst surface during reaction is clearly shown by the catalyst potential. There is no hydrogen on the surface from about 10 % conversion on (E > 0.45 V) and the OH coverage increases continuously up to complete deactivation. The by-product formation is suppressed but not eliminated by Bi promotion. 

Aramendia et al. (22) investigated three separate organic test reactions such as, 1-phenyl ethanol, 2-propanol, and 2-methyl-3-butyn-2-ol (MBOH) on acid-base oxide catalysts. They reached the same conclusions about the acid-base characteristics of the samples with each of the three reactions. However, they concluded that notwithstanding the greater complexity in the reactivity of MBOH, the fact that the different products could be unequivocally related to a given type of active site makes MBOH a preferred test reactant. Unfortunately, an important drawback of the decomposition of this alcohol is that these reactions suffer from a strong deactivation caused by the formation of heavy products by aldolization of the ketone (22) and polymerization of acetylene (95). The occurrence of this reaction can certainly complicate the comparison of basic catalysts that have different intrinsic rates of the test reaction and the reaction causing catalyst decay. 

There are numerous indications in the literature on catalyst deactivation attributed to over-oxidation of the catalyst (3-5). In the oxidative dehydrogenation of alcohols the surface M° sites are active and the rate of oxygen supply from the gas phase to the catalyst surface should be adjusted to that of the surface chemical reaction to avoid "oxygen poisoning". The other important reason for deactivation is the by-products formation and their strong adsorption on active sites. This type of... 

Cracking is carried out in a fluid bed process as shown in Fig. 7.9. Catalyst particles are mixed with feed and fluidized with steam up-flow in a riser reactor where the reactions occur at around 500°C. The active life of the catalyst is only a few seconds because of deactivation caused by coke formation. The deactivated catalyst particles are separated from the product in a cyclone separator and injected into a separate reactor where they are regenerated with a limited amount of injected air. The regenerated catalyst is mixed with the incoming feed which is preheated by the heat of combustion of the coke. 

We conclude that most reaction systems in the chemical industries are exothermic. This has some immediate consequences in terms of unit operation control. For instance, the control system must ensure that the reaction heat is removed from the reactor to maintain a steady state. Failure to remove the heat of reaction would lead to an.accumulation of heat within the system and raise the temperature. Forreversible reactions this would cause a lack of conversion of the reactants into products and would be uneconomical. For irreversible reactions the consequences are more drastic. Due to the rapid escalation in reaction rate with temperature we will have reaction runaway leading to excessive by-product formation, catalyst deactivation, or in the worst case a complete failure of the reactor possibly leading to an environmental release, fire, or explosion. 

The DME catalyst must carry out equilibrium conversion of methanol to dimethylether and water with minimum by-product formation. Less than equilibrium conversion will require more heat to be removed in the ZSM-5 circuit, which will result in higher reactor temperature rise. This will increase catalyst deactivation and decrease yield. The higher temperature rise could be reduced by increasing gas recycle, but this will increase operating costs. Excessive decomposition of methanol (e.g., to CO, C02, H2) will result not only in carbon loss, thereby reducing gasoline yields, but will also affect the composition of recycle gas in the ZSM-5 circuit. For example, one percent methanol decomposition to CO and H2 will increase the ZSM-5 reactor temperature rise by 12%. 

The same reaction was performed at 200°C for 24 h in the presence of ZSM-5(40), giving a 69% PA conversion with relatively low formation of phenol and by-products, and an unexciting selectivity toward the para product (ortho/para = 0.67). Treatment of the catalyst with triphenylchlorosilane, which deactivates its outer surface, shows an appreciable effect on some catalysts properties, such as performance as well as ortho/para ratio, which are somewhat higher than those of the starting catalyst (81% conversion, ortho/para = 0.5). This suggests that the... 

The monocyclic terpenes are easily disproportionated to the dehydrogenated p-cymene and the hydrogenated /7-menthane. For a short time on stream TOS (< 2 h), cracking of /)-cymene to toluene and propane/propene on the strongly acidic sites is the dominant reaction. With increasing TOS, these strong acid sites of the catalysts deactivate and, thus, the formation of products obtained by cracking decreases. 

Figure 1 outlines various padiways can lead to the deactivation of die excited pesticide molecule (P ) throu luminescence, physical quenching, or by collisionally transferring energy to other gaseous molecules (M). This figure also illustrates electron transfer, photoionization, or direct chemical reaction processes of the excited state that can lead to dissociation and subsequent product formation. 

Mercury(II) acetate tends to mercurate all the free nuclear positions in pyrrole, furan and thiophene to give derivatives of type (74). The acetoxymercuration of thiophene has been estimated to proceed ca. 10 times faster than that of benzene. Mercuration of rings with deactivating substituents such as ethoxycarbonyl and nitro is still possible with this reagent, as shown by the formation of compounds (75) and (76). Mercury(II) chloride is a milder mercurating agent, as illustrated by the chloromercuration of thiophene to give either the 2- or 2,5-disubstituted product (Scheme 25). 

Two serious drawbacks of this method are the extensive deuterium scrambling around the reaction site and the occasional formation of olefinic side products, which are hard to separate by conventional means. The extent of olefin formation may depend on the nature of the Raney nickel since it is known that desulfurization with deactivated Raney nickel can yield olefins. Best results are obtained when the deuterated Raney nickel is prepared very rapidly and used immediately after preparation. 

Allylchlorosilanes reacted with naphthalene to give isomeric mixtures of poly-alkylated products. However, it was difficult to distill and purify the products for characterization from the reaction mixture due to the high boiling points of the products and the presence of many isomeric compounds. The alkylation of anthracene with allylchlorosilanes failed due to deactivation by complex formation w ith anthracene and the self-polymerization of anthracene to solid char. 

The plasma-catalyst system utilizes plasma to oxidize NO to NO2 which then reacts with a suitable reductant over a catalyst however, this plasma-assisted catalytic technology still comprises challenging tasks to resolve the formation of toxic by-products and the catalyst deactivation due to the deposition of organic products during the course of the reaction as well as to prepare cost effective and durable on-board plasma devices [47]. 

By decreasing the GHSV values, the selectivity dramatically decreased due to the presence of the side-reaction of C-alkylation on the aromatic ring, giving rise to relevant amounts of 3-MC and a not-fully-identified methyl-MDB derivative (however, the 3-methyl isomer is the most probable candidate). Lastly, the lowest GHSV value was conducive to the condensation of PYC, with a formation of heavy by-products, a dramatic decrease of C-balance, and resulting catalyst deactivation. 

After 5 h of time-on-stream the TS-1 catalyst showed a significant decrease in activity (Fig. 39.4), mainly related to the formation of heavy byproducts by PYC condensation on the surface [27,28]. The TG analysis carried out on the catalyst after the tests, showed the presence of two signals corresponding to a weight loss the first one, around 523 K, may refer to residual reagents and/or products adsorbed on the catalyst, while the second one at 743 K may be attributed to the combustion of heavy by-products deriving from PYC condensation, which is the main cause of deactivation. 

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