Deactivation, catalyst process

K. Otto, W. B. Wdhamson, and H. Gandhi, Ceram. Eng. Sci. Proc. 2(6), (May/June, 1981). Good review of various catalyst deactivation processes. 

Phosphinite pincer iridium systems have also been shown to have a lower tendency to oxidatively add TEE to give (vinyl)(hydride) complexes similar to 3 [18]. While this has been identified as one of the major catalyst deactivation processes in phosphine pincer iridium catalysis, apparently with complexes such as 5, only olefin coordination can occur. However, this is a considerably weaker bonding and is less detrimental to catalyst activity. Eased on steric arguments, product olefin coordination (e.g. COE) is favored over TEE coordination, and therefore at a high TON and high product concentrations the phosphinite catalysts 5 are markedly less active than the phosphine analogues 1. 

Because of its broad applicability, Raman spectroscopy is expected to be used in the near future to characterize numerous catalytic materials in the functioning state, specifically, to unravel the nature of the catalytically active sites, to identify surface reaction intermediates, and to follow catalyst deactivation processes. Moreover, Raman spectroscopy is a powerful tool for the characterization of all synthesis and activation steps of catalysts. It can be used to investigate species formed in aqueous solution, depending on the pH, metal concentrations, or the presence of complex-ing agents. Such structural information is potentially valuable in laying the groundwork for the reproducible synthesis of industrial catalysts. 

Reactors in which the solid phase is perfectly mixed on a macro scale, such as a stirred tank slurry reactor and the riser reactor with recycle of both phases, are particularly useful for fast catalyst deactivation processes. Notice that the residence time of both phases can be varied independently by introducing an extra recycle flow of... 

The steric properties of the pyridine substrate are critical. Pyridines which lack ortho substituents form nonlabile, unreactive 18-electron pyri-dyl pyridine complexes 36 [see also Eq. (37)]. In fact, one of the principal catalyst deactivation processes in a-picoline coupling reactions is catalyst poisoning via formation of 36 by trace amounts of 3- and 4-methylpyridine impurities in the a-picoline feed. 

Sintering Problems and Their Importance in Catalysis. - Catalyst deactivation is an important, develq>ing subdiscipline of catal c science. It concerns the scientific investigation catalyst deactivation processes and the modeling and/or prediction of deactiva-... 

As is evident in Figure 2.22, significant dispersion exists between the different series of experiments, which is most likely related to some uncontrolled aspect of the derivatization experiments, possibly including the effects of uncontrolled chloride co-adsorption in the different experiments. Nevertheless, the experiments provide an order of magnitude estimate of the consumption parameters and, equally important, reveal the potential dependence of the catalyst deactivation process. The potential... 

The apparent hrst-order reaction-rate constant, decreases with time, because of catalyst deterioration. A chemist colleague of yours has studied the catalyst deactivation process a.nd has proposed that it can be modeled by... 

We will now examine some models of catalyst deactivation processes. Given the various causes of catalyst deactivation, it is not surprising that numerous mechanisms and models can be found in the literature. 

Since the earliest days of heterogeneous catalysis, decreases in activity during use have been observed. The rates at which catalyst deactivation processes take place may be fast or slow. In some cases the decline in activity is so rapid that the catalyst ceases to function effectively after a few minutes or hours of exposure to a reactant feed stream. On the other hand, there are cases where the deactivation processes occur so slowly that the catalyst may function effectively for months or years. In the design of conunercial catalytic processes, one obviously must... 

Of course, depending on the assumption for the underlying catalyst deactivation mechanism, the parameters n and m of Eq. (2.3.9) may have different values and, consequently, quite different equations can occur. Note that in the case of catalyst poisoning by a reaction the situation in the reactor can become rather complicated. For example, in a tube reactor, the catalyst deactivation process becomes then a function of the position in the tube. 

---