Dead catalyst

The Rh-based dimerization catalyst described above cannot maintain its activity indefinitely. After prolonged reaction (several hours) or after removal of the products and monomers from the reaction mixture, it becomes deactivated or decays to an inactive species. Because Rh metal is very expensive, it becomes desirable to seek a way to rejuvenate the dead catalyst. The generally recognized mode of decay is the degeneration of the active Rhra to an inactive Rh1 complex (10-12), e.g., R— Rh,uCl RC1 + Rh1. 

Thus a drier, less sintered support is obtained by calcining in CO rather than air. Unfortunately the chromium will not tolerate such a severe reduction treatment, and unless Cr is added secondarily [as in the above (Section V,B) examples] a dead catalyst results. This is shown in Fig. 5, where it is apparent that reduction at 350°C improves activity, whereas higher temperatures destroy it. This does not indicate overreduction, as some have claimed. Rather, it indicates a rearrangement of the Cr(II) into a less coordinatively unsaturated form—possibly aggregates. 

This is the Verdrangungsreaktion with the monomer, and it is catalyzed by the Ziegler catalyst or its consecutive products. Using aluminum alkyls and 14C-labeled ethylene, under conditions not causing any insertion or hydride formation and in the presence of a dead catalyst, an equilibrium distribution of labeled ethylene between the gas phase and the alkyl is reached (33). It has been possible in certain cases to use this effect to reduce molecular weight from over l million to some hundreds by increasing the temperature from 25 to 100°C without any loss in efficiency (34). 

In the Casale concept there is no need for a dead catalyst zone as the annular catalyst bed is left open at the top to permit a portion of the gas to flow axially through the catalyst. The remainder of the gas flows radially through the bulk of the catalyst bed. As shown in Figure 90 this is achieved by leaving the upper part of the catalyst... 

A sample of dead catalyst is heated under pure hydrogen using a protected DTA rod. The combination of both techniques allows to understand the different processes that... 

A well-understood catalytic cycle is tliat of the Wilkinson alkene hydrogenation (figure C2.7.2) [2]. Like most catalytic cycles, tliat shown in figure C2.7.2 is complex, involving intennediate species in tire cycle (inside tire dashed line) and otlier species outside tire cycle and in dead-end patlis. Knowledge of all but a small number of catalytic cycles is only fragmentary because of tire complexity and because, if tire catalyst is active, tire cycle turns over rapidly and tire concentrations of tire intennediates are minute thus, tliese intennediates are often not even... 

An explosive decomposition in an ethylene oxide (EO) distillation column, similar in its results to that described in Section 7.3.2, may have been set off by polymerization of EO in a dead-end spot in the column base where rust, a polymerization catalyst, had accumulated. Such deadends should be avoided. However, it is more likely that a flange leaked the leaking gas ignited and heated an area of the column above the temperature at which spontaneous decomposition occurs. The source of ignition of the leak may have been reaction with the insulation, as described... 

The origin of the remarkable stereoselectivities displayed by chiral homogeneous catalysts has occasioned much interest and speculation. It has been generally assumed, using a lock-and-key concept, that the major product enantiomer arose from a rigid preferred initial binding of the prochiral olefin with the chiral catalyst. Halpren 48) on the basis of considerable evidence, reached the opposite conclusion the predominant product enantiomer arises from the minor, less stable diastereomer of the olefin-catalyst adduct, which frequently does not accumulate in sufficient concentration to be detected. The predominant adduct is in essence a dead-end complex for it hydrogenates at a much slower rate than does the minor adduct. 

Catalyst circulation is like blood circulation to the human body. Without proper catalyst circulation, the unit is dead. Troubleshooting circulation problems requires a good understanding of the pressure balance around the reactor-regenerator circuit and the factors affecting catalyst fluidization. The fundamentals of fluidization and catalyst circulation are discussed in Chapter 5. 

The catalyst/adsorhent was a 60/80 mesh pellet of 1 wt% Pt/Al203 prepared by impregnation. It was packed in each tube for l.Og (1.0gx4 tubes). For this arrangement, the dead volume of each tube was only 3 cm. The feed to each zone was Cco, co aae = 5,000 ppm, Co2,02 zone = 10,000 ppm (balanced by N2). Total flow rate for each zone was 100 mlNTP-min". The temperature of the catalyst bed was 373K. 

Deactivating catalyst 319 Dead zones 159, 162, 163 Degree of segregation 471 Density influences 492 Desorption of solute 578, 579 Difference differential equation 579 Difference formulae for partial differential equations 268 Differential column 167... 

As regards the NO breakthrough, when N0/02 adsorption is carried out on the Pt/y-Al203 (1/100 w/w) sample, no dead time is observed, which indicates a negligible storage of NO species on the surface. As a matter of fact, only minor amounts of NO have been stored in this case up to catalyst saturation, which however desorb upon switching off the NO feed flow. 

All experimental setups described in the literature for the separation of homogeneous catalysts by membrane filtration technology can be divided into two general classes Dead-end filtration and cross-flow filtration. The first type of unit is characterized by a product flow perpendicular to the surface of the membrane, while the flow in the case of cross-flow filtration is parallel to the membrane surface (see Figure 4.1). 

Using unmodified Ru-BINAP and Rh-Et-DUPHOS catalysts Jacobs et al. performed hydrogenation reactions of dimethylitaconate (DMI) and methyl-2-acetamidoacrylate (MAA), respectively. [11,47] The continuous hydrogenation reaction was performed in a 100 mL stirred autoclave containing an MPF-60 membrane at the bottom, which also acts as a dead-end membrane reactor. The hydrogenation reactions will be discussed in paragraph 4.6.1. 

Our approach to polymer chain growth modeling is based on population balances for the various polymer species participating in and resulting from chain growth and transfer [34], The kinetics scheme is written below in mathematical fashion and is a precursor to the derivation of population balances. Monomer units are represented as M, and growing polymer chains are represented by the symbol Pn, where n is the number of repeat units attached to the active catalyst. Dormant polymer is represented by An where n is the number of repeat units attached to the CTA. Dead polymer chains, which arise from chain termination events such as hydrogenolysis... 

Another piece of mechanistic evidence was reported by Snapper et al. [14], who describe a ruthenium catalyst caught in action . During studies on ring opening metathesis, these authors were able to isolate and characterize carbene 5 in which a tethered alkene group has replaced one of the phosphines originally present in Id. Control experiments have shown that compound 5 by itself is catalytically active, thus making sure that it is a true intermediate of a dissociative pathway rather than a dead-end product of a metathetic process. 

Two types of continuous membrane reactors have been applied for oligomer- or polymer-bound homogeneous catalytic conversions and recycling of the catalysts. In the so-called dead-end-filtration reactor the catalyst is compartmentalized in the reactor and is retained by the horizontally situated nanofiltration membrane. Reactants are continuously pumped into the reactor, whereas products and unreacted materials cross the membrane for further processing [57]. 

---