Catalyst Deactivation Paths

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

Solinas M, Gladiali S, Marchetti M (2005) Hydroformylation of aryloxy ethylenes by Rh/BINAPHOS complex - catalyst deactivation path and application to the asymmetric synthesis of 2-aryloxypropanoic acids. J Mol Catal A 226 141-147... 

In MeOH, Pd - H+ species are unstable and have the tendency to deproto-nate with reduction to less active dimeric Pd(I) and Pd(0) complexes, which may lead to degeneration of the catalyst with formation of inactive palladium metal and free ligands, which in turn may give less active bis-chelate complexes [Pd(P-P)2]2+ [55,61]. Possible deactivation paths have been delineated in [17]. In order to maintain or improve the catalytic activity, the precursor is used together with an oxidant and an excess of acid (usually BQ/Pd = 100 - 200 and acid/Pd = 10 - 20) [15,47]. 

With the honeycomb system, the catalyst pitch determines the solids handling capabilities. Pitch is the distance from centerline to centerline of one gas path in the honeycomb and typically varies from 2 to 9 mm with higher pitches being used for heavier dust applications. Typical FCC SCR catalyst would have approximately a 5 mm pitch. The following causes can lead to catalyst deactivation ... 

The curved-line reaction paths are generated from individual analyses of product distributions which can be obtained either by changing catalyst volume or xylene flow rate or merely by sampling the product as the catalyst deactivates with time. Selectivity remains essentially constant during aging. 

Methanol adsorption and decomposition on noble metals have been the subject of many surface-analytical investigations (e.g., References 94,171,320,350,378, 478 94). CH3OH dehydrogenation on palladium catalysts could be a valuable source of synthesis gas or hydrogen, but unfortunately catalyst deactivation by carbon deposits (coking) seriously limits this process (495-498). In this respect, the probability of O H vs. C O bond scission is important, the first path resulting in CO and H2, and the second in carbon or carbonaceous species (CH x = 0-3), CH4, and H2O (see scheme in Fig. 49 details are discussed below). 

The Lewis acid BPhj is a useful cocatalyst for the reaction. Such additives are often termed promoters. In this case the promoter improves the selectivity of the system for linear product (it is not clear exactly why) and improves the life of the catalyst. A catalyst deactivates when it loses some or all of its activity by going down an irreversible path that leads to an inactive form of the metal complex. In this case, the formation of the inactive Ni(CN)2 is the principal deactivation step. This can happen in several ways an example is shown here ... 

Use the approach of Wei (1962) to obtain global rate expressions Re for a complex first-order reaction network when the reactions are also affected by catalyst deactivation. Assume pore-mouth poisoning and slab geometry. Note that tjic for each reaction path is given by ... 

As discussed in Section IV, Agrawal and Wei (1984) and Ware and Wei (1985b) have successfully modeled experimental deposit profiles by using the theory of coupled, multicomponent first-order reaction and diffusion. Wei and Wei (1982) employed this theory to evaluate the influence of catalyst properties on the shape of the deposit profile. Agrawal (1980) developed a model for the deactivation of unimodal and bimodal catalysts based on the consecutive reaction path. These approaches represent a more realistic consideration of the HDM reaction mechanism than first-order kinetics and will, accordingly, be discussed in more detail. 

Glycerol (propan-1,2,3-triol) is a readily available raw material from biosus-tainable sources such as rape-seed and sunflower the many products that can be formed from it by oxidation find economic use as intermediates in the fine chemicals industry. However, its oxidation constitutes a complicated scenario by reason of the parallel and sequential reaction paths that can be followed (Scheme 8.3) obtaining a desired product therefore constitutes a considerable challenge. The mono-aldehyde readily isomerises under basic conditions to dihydroxyacetone, but fortunately it is less easily oxidised, so that glyceric acid (HOCH2-CH(OH)-CC>2H) is frequently a major product. Gold catalysts, as in other reactions, do not suffer from deactivation, nor do... 

The deactivation of catalysts concerns the decrease in concentration of active sites on the catalyst Nj. This should not be confused with the reversible inhibition of the active sites by competitive adsorption, which is treated above. The deactivation can have various causes, such as sintering, irreversible adsorption and fouling (for example coking or metal depositions in petrochemical conversions). It is generally attempted to express the deactivation in a time-dependent expression in order to be able to predict the catalyst s life time. An important reason for deactivation in industry is coking, which may arise from a side path of the main catalytic reaction or from a precursor that adsorbs strongly on the active sites, but which cannot be related to a measurable gas phase concentration. For example for the reaction A B the site balance contains also the concentration of blocked sites C. A deactivation function is now defined by cq 24, which is used in the rate expression. 

The S-shape deactivation pattern in Figure 10 is determined by the process of metal deposition in the outer core of the catalyst particles desulfurization and hydrocracking occur mainly in the inner core of the catalyst particle (27). This understanding led to development of various effective demetallation catalysts as shown in Figure 11 (22,23,24,25). Furthermore, this has opened the path to industrial solutions (26) for the treatment of high metal feedstocks. 

Corrpared to alumina supported catalysts, for several bituinen hydrocracking reactions, carbon supported catalysts had 50-100 % greater first order deactivation rate constants which may have been caused by their 50-100 % greater initial turnover frequencies. Diffusion rates were affected by the catalysts having different pore diameters and different diffusion path lengths (extrudate and powder diameters) ... 

Deactivation measurements were made using three catalysts. The two alumina catalysts were identical except that their extrudate diameters were 3.2 and 1.6 mm respectively, and thereby provided different diffusion path lengths. Descriptions of the catalysts and the bitumen (ref. 3) from which... 

Figure 9 shows rapid KDN deactivation of Co-Mo catalysts on both alumina and on carbon. The expectation that the carbon catalyst would deactivate quickly because it has a larger median pore diameter was observed. However, deactivation of the Co-Mo on alumina catalyst in Figure 9 was much faster than the Ni-Mo on alumina catalyst in Figure 8. An explanation for these differences may involve both the chemical coanposition of the catalyst surface as well as the diffusion path length. However, the deactivation of the 3.2 mm Co-Mo on alumina catalyst in Figure 9 was much faster than the 3.2 mm Ni-Mo on almnina catalyst in Figure 8. Since Ni-Mo is often considered to be a better hydrogenation catalyst more hydrogenation of adsorbed carbonaceous species, less coke formation, and less deactivation might be expected.

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