Catalyst deactivation reaction

Reaction progress kinetic analysis offers a reliable alternative method to assess the stability of the active catalyst concentration, again based on our concept of excess [e]. In contrast to our different excess experiments described above, now we carry out a set of experiments at the same value of excess [ej. We consider again the proline-mediated aldol reaction shown in Scheme 50.1. Under reaction conditions, the proline catalyst can undergo side reactions with aldehydes to form inactive cyclic species called oxazolidinones, effectively decreasing the active catalyst concentration. It has recently been shown that addition of small amounts of water to the reaction mixture can eliminate this catalyst deactivation. Reaction progress kinetic analysis of experiments carried out at the same excess [e] can be used to confirm the deactivation of proline in the absence of added water as well to demonstrate that the proline concentration remains constant when water is present. 

Reactions of C—H bonds attached to donor ligands. These may lead to the formation of rings of various sizes. The orthometallation of aryl substituents is particularly common those involving PPh3 may contribute to catalyst deactivation reactions. The metallation of ligands may be reversible, particularly where 3- and 4-membered rings are formed.117... 

Accelerated deactivation tests in hydrocracking have been reported (38), where a constant conversion mode was run at much higher space velocity (and hence temperature) than under actual operation conditions. Differences in deactivation were measured that were later substantiated in commercial operation (38). Although all these approaches aim at accelerating the catalyst deactivation reaction in Equation 7, such tests should obviously not be applied to catalyst systems that — at the high space velocity — operate at such high temperatures that very high polyaromatics concentrations prevail. 

In Older to explain the diverse deactivation behavior of the catalyst in processing these three types of feedstocks, we formulated a simplified dewaxing model which assumes that the dewaxing reaction can be described as an irreversible reaction (ix., cracking of waxy paraffinic molecules) coupled with a order catalyst deactivation reaction. The deactivation reaction is assumed to be conceniratioti independent while the fractional catalytic activity at any time Is a function of a number of variables including number of catalytic sites and concentration of poisons in the feedstock. 

Figure 11. Operational stability functions of E. coli cells immobilized in epoxy carrier for consecutive batch operation in the production of 6 APA from penicillin G. Top, experimental data bottom, model calculations for the superposition of 6 APA formation and conversion dependence catalyst deactivation reaction in the diffusion controlled regime.

In conclusion, external and internal catalyst deactivation reaction may cause serious problems especially in large-scale hydroformylation. Only by considering these issues and a careful design, such process will beome economically feasible [152,197]. 

The reactions depicted by steps (9) - (11) are again the Langmuir - Hinshelwood steps, i. e. the chemisorption of A and B which then react. Step (12) is a catalyst deactivation reaction in which a site, which has chemisorbed B (presumably oxygen), is converted to a metal oxide SB. In equation (13), the oxidized site is reduced by chemisorbed reducing agent A. It seems reasonable to assume that the first set of reactions is much faster... 

Amines As repotted in Section 3.24.3.2.2, sterically demanding amines can be polymerized without further protection under certain conditions. However, aluminum-protected amines are usually employed to suppress catalyst deactivation reactions. Even if protected, distinct differences in reactivity can be found for the primary, secondary, and tertiary amines due to their intrinsic steric protection. Under the same conditions, the copolymerization activities of ethene with 10-undecenylamine, N-methyl-lO-undecenylamine, and N,N-dimethyl-... 

Amorphous Silica—Alumina Based Processes. Amorphous siHca—alumina catalysts had been used for many years for xylene isomerization. Examples ate the Chevron (130), Mamzen (131), and ICI (132—135). The primary advantage of these processes was their simpHcity. No hydrogen was requited and the only side reaction of significance was disproportionation. However, in the absence of H2, catalyst deactivation via coking... 

Pure dry reactants are needed to prevent catalyst deactivation effective inhibitor systems are also desirable as weU as high reaction rates, since many of the specialty monomers are less stable than the lower alkyl acrylates. The alcohol—ester azeotrope (8) should be removed rapidly from the reaction mixture and an efficient column used to minimize reactant loss to the distillate. After the reaction is completed, the catalyst may be removed and the mixture distilled to obtain the ester. The method is particularly useful for the preparation of functional monomers which caimot be prepared by direct esterification. 

Hydrogenation of the oxides of carbon to methane according to the above reactions is sometimes referred to as the Sabatier reactions. Because of the high exothermicity of the methanization reactions, adequate and precise cooling is necessary in order to avoid catalyst deactivation, sintering, and carbon deposition by thermal cracking. 

Because soHd acid catalyst systems offer advantages with respect to their handling and noncorrosive nature, research on the development of a commercially practical soHd acid system to replace the Hquid acids will continue. A major hurdle for soHd systems is the relatively rapid catalyst deactivation caused by fouling of the acid sites by heavy reaction intermediates and by-products. 

The Snamprogetti fluidized-bed process uses a chromium catalyst in equipment that is similar to a refinery catalytic cracker (1960s cat cracker technology). The dehydrogenation reaction takes place in one vessel with active catalyst deactivated catalyst flows to a second vessel, which is used for regeneration. This process has been commercialized in Russia for over 25 years in the production of butenes, isobutylene, and isopentenes. 

The heat released from the CO—H2 reaction must be removed from the system to prevent excessive temperatures, catalyst deactivation by sintering, and carbon deposition. Several reactor configurations have been developed to achieve this (47). 

The reaction was carried out in an ionic liquid/toluene biphasic system, which allowed easy product recovery from the catalyst by decantation. However, attempts to recycle the ionic catalyst phase resulted in significant catalyst deactivation after only the third recycle. 

Typical conditions for the disproportionation reaction are 450-530°C and 20 atmospheres. A mixture of C0O-M0O3 on aluminosilicates/alumina catalysts can he used. Conversions of approximately 40% are normally used to avoid more side reactions and faster catalyst deactivation. The equilihrium constants for this reaction are not significantly changed hy shifting from liquid to vapor phase or hy large temperature changes. 

As is indicated in Figure 1, the heat liberated in the conversion of carbon monoxide to methane is 52,730 cal/mole CO under expected reaction conditions. Also, the heat liberated in the conversion of carbon dioxide is 43,680 cal/mole C02. Such high heat releases strongly affect the process design of the methanation plant since it is necessary to prevent excessively high temperatures in order to avoid catalyst deactivation and carbon laydown. Several approaches have been proposed. 

Nickel catalysts were used in most of the methanation catalytic studies they have a rather wide range of operating temperatures, approximately 260°-538°C. Operation of the catalytic reactors at 482°-538°C will ultimately result in carbon deposition and rapid deactivation of the catalysts (10). Reactions below 260°C will usually result in formation of nickel carbonyl and also in rapid deactivation of the catalysts. The best operating range for most fixed-bed nickel catalysts is 288°-482 °C. Several schemes have been proposed to limit the maximum temperature in adiabatic catalytic reactors to 482°C, and IGT has developed a cold-gas recycle process that utilizes a series of fixed-bed adiabatic catalytic reactors to maintain this temperature control. 

Cyclodiphosphazanes(III) 27 shown in Scheme 16 undergo oxidation reactions to give the cyclodiphosphazanes(V) of type 28. These are prospective ligands in catalysis since these ligands due to lack of phosphorus lone-pairs are less susceptible to the destructive cycloreversion of the ligands. Hence they could prevent catalyst deactivation in the process. When treated with trimethyl aluminum the cyclodiphosphazanes form symmetrically substituted bimetallic species of type 29 [90]. Characterization by single-crystal X-ray studies show... 

No attempt was made to measure CO2 in these experiments. By increasing the temperature to 320°C, catalyst deactivation was prevented, and no carbon residue could be detected on the spent catalyst. Thus, temperature can be expected to significantly shift the reaction pathways of organic contaminants. In this study, and in all other studies, excellent corrosion resistance was observed for the corrosion coupons. 

Example 11.15 Coke formation is a major cause of catalyst deactivation. Decoking is accomplished by periodic oxidations in air. Consider a micro-porous catalyst that has its internal surface covered with a uniform layer of coke. Suppose that the decoking reaction is stopped short of completion. What is the distribution of residual coke under the following circumstances ... 

Chiral lactones were also obtained by cyclocarbonylation of chiral acetylenic alcohols with Pd and thiourea (H2NCSNH2) (Scheme 32). No loss in chirality was observed, but large amounts of Pd and thiourea were used (10 mol %) since the catalyst deactivates by forming metal particles. The catalytic precursor (Pdl2 > PdCl2) and the ratio of thiourea to Pd were very important, thiourea being necessary for this reaction. The active species was supposed to be [Pd(thiourea)3l]I, which forms in situ from [Pd(thiourea)4]l2 and [Pd(thiourea)2]l2. It had to be a partially dissociated species since [Pd(thiourea)4](Bp4)2 was inactive [121]. 

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]. 

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