Catalysts poisoned

Conditions of hydrogenation also determine the composition of the product. The rate of reaction is increased by increases in temperature, pressure, agitation, and catalyst concentration. Selectivity is increased by increasing temperature and negatively affected by increases in pressure, agitation, and catalyst. Double-bond isomerization is enhanced by a temperature increase but decreased with increasing pressure, agitation, and catalyst. Trans isomers may also be favored by use of reused (deactivated) catalyst or sulfur-poisoned catalyst. 

Most refinery/petrochemical processes produce ethylene that contains trace amounts of acetylene, which is difficult to remove even with cryogenic distillation. Frequently it is necessary to lower the acetylene concentration from several hundreds ppm to < 10 ppm in order to avoid poisoning catalysts used in subsequent ethylene consuming processes, such as polymeri2ation to polyethylene. This can be accompHshed with catalytic hydrogenation according to the equation. 

As catalyst for the Rosenmund reaction palladium on a support, e.g. palladium on barium sulfate, is most often used. The palladium has to be made less active in order to avoid further reduction of the aldehyde to the corresponding alcohol. Such a poisoned catalyst is obtained for example by the addition of quinoline and sulfur. Recent reports state that the reactivity of the catalyst is determined by the morphology of the palladium surface." ... 

Multiple reactions might not only lead to a loss of materials and useful product but might also lead to byproducts being deposited on, or poisoning catalysts (see Chapters 6 and 7). 

For situations where the reaction is very slow relative to diffusion, the effectiveness factor for the poisoned catalyst will be unity, and the apparent activation energy of the reaction will be the true activation energy for the intrinsic chemical reaction. As the temperature increases, however, the reaction rate increases much faster than the diffusion rate and one may enter a regime where hT( 1 — a) is larger than 2, so the apparent activation energy will drop to that given by equation 12.3.85 (approximately half the value for the intrinsic reaction). As the temperature increases further, the Thiele modulus [hT( 1 — a)] continues to increase with a concomitant decrease in the effectiveness with which the catalyst surface area is used and in the depth to which the reactants are capable of... 

Figure 11(a) shows the spectrum of adsorbed species on an active catalyst in a hydrogen-ethylene stream. This spectrum appears and stabilizes within minutes after hydrogen is blended into the ethylene stream. Three new bands appear in the presence of hydrogen at 2892, 2860, and 2812 cm-1. The appearance and location of these bands were verified by expanded scale spectra. Experiments at lower ethylene pressures reveal that there is an additional band at about 2940 cm-1 partially obscured in Fig. 11 by overlap of the ethylene spectrum. On a poisoned catalyst, which does not show the ZnH and OH bands, only the bands characteristic of chemisorbed ethylene are seen. 

The selective reduction of the D-ring olefin in 106 using a partially poisoned catalyst (Pd/C, 0.25 % pyridine) provided intermediate 107 (83 %), which was epimerized at -78 °C with sodium methoxide (HOAc quench at -78 °C, 89 %) (Scheme 10.9). Deoxygenation by means of tosyl hydrazone 108 and subsequent treatment with catechol borane and tetrabutylammonium acetate gave pentacyclic... 

As evidenced by experiments carried out on partially poisoned catalysts with 2,6-di-tert-butylpyridine, a significant decrease in the catalytic activity (of about 65-70 %) occurs because of a partial neutralization of the external acid sites. This means that the alkylation takes place predominantly on the external surface. 

Problematic functional groups, however, are thioethers and disulfides [28] as well as free amines which poison catalysts of type 1 [4c]. In case of amines this problem is easily solved by choosing either an appropriate protecting group for nitrogen (e.g. amide, sulfonamide, urethane), or simply by protonation since ammonium salts were found to be compatible with 1 [4c]. As will be discussed in Sect. 4, free amines can also be metathesized in supercritical C02 as the reaction medium [7]. 

The presence of alkali salts may create other problems even in systems where deposition of hot vapors is not an issue. Alkali salts can be corrosive to metal surfaces and can poison catalysts such as those in tar cracking and synthesis gas applications. 

Figure 1.19 AES data from a Ru/Al203 catalyst aged in a reaction (CO+H2) mixture containing trace amounts of H2S [148], Spectra are shown for the sample before (a) and after (b) sputtering with an Ar+ beam for 2 min. The difference between the two spectra indicates the presence of S on the surface but not the subsurface of the poisoned catalyst. (Reproduced with permission from Elsevier.)...

In the case of alkenes, 1-pentene reactions were studied over a catalyst with FAU framework (Si/Al2 = 5, ultrastable Y zeoHte in H-form USHY) in order to establish the relation between acid strength and selectivity [25]. Both fresh and selectively poisoned catalysts were used for the reactivity studies and later characterized by ammonia temperature programmed desorption (TPD). It was determined that for alkene reactions, cracking and hydride transfer required the strongest acidity. Skeletal isomerization required moderate acidity, whereas double-bond isomerization required weak acidity. Also an apparent correlation was established between the molecular weight of the hard coke and the strength of the acid sites that led to coking. 

Figure 12. Transient HCN yield over platinum catalysts. A and B represent one experiment (Exp. 1) in which a sulfur-poisoned catalyst was regenerated on admission of 1% O2 into the inlet gas mixture between times t, and C shows the resistance of the catalyst to poisoning by SO when oxygen is simultaneously present in the inlet gas mixture. At t only oxygen is removed from the inlet gas mixture. (See Ref. 16 for details.)...

Uses. Electrical apparatus measurement and control systems such as thermometers and sphygmomanometers agricultural and industrial poisons catalyst antifouling paint dental practice gold mining... 

We named this catalyst modified catalyst instead of poisoned catalyst, because the effect of treatment with the chelate reagents on the activity of the catalyst depended on the nature of the substrate. 

In our mechanism, coke formation is due to the presence of olefins, which occur as intermediate species during the reforming reactions. As discussed in Section II, these olefins can go either to products or to coke precursors. The deactivation caused by feed poison, catalyst sintering during regeneration, or improper regeneration techniques is not considered in this development. 

Preparation of cis-alkenes Lindlar s catalyst, which is also known as poisoned catalyst, consists of barium sulphate, palladium and quinoline, and is used in selective and partial hydrogenation of alkynes to produce c/s-alkenes. Hydrogen atoms are delivered simultaneously to the same side of the alkyne, resulting in syn addition (cw-alkenes). Thus, the syn addition of alkyne follows same procedure as the catalytic hydrogenation of alkyne. 

Coulson and Richardson (1994) have used the following equation to find the ratio of activity of the poisoned catalyst to the activity of the unpoisoned catalyst ... 

The key step involved in reactivation of a poisoned catalyst is considered to be the reverse aging process. 

The synthesis shown above can be modified for preparation of 4-alkylbutenolides (8) by reduction of the acetylenic acid with quinoline-poisoned catalyst (equation II). The optically active bulenolides have been used for a synthesis of optically pure enantiomers of threo- and Aro-4-methy 1-3-heptanols.2... 

Generally an alkene will reduce to an alkane, however, the "poisoned" catalyst suppresses the ability for this to readily occur. 

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