Catalyst deactivation pathways

A catalyst deactivation pathway can be precipitation of highly polar, insoluble bicarbonate complexes. 

Despite the vast number of outstanding examples of enzymatic catalysis that rely on the collaboration of two or more vicinal metal centers hitherto disclosed, the design and development of efficient binuclear organometallic complexes able to enhance the performance of mononuclear catalyst by means of an intermetallic cooperative process remains widely unexplored [22-24]. In fact, the formation of bi- or polynuclear complexes has been often described as a catalyst deactivation pathway [25-30]. However, the availability of more electron density at the active site, extra coordination positions, and the possibility to develop more preorganized systems that allow for (enantio)selective reactions shows great promise for an improved catalytic performance [9]. 

Using the bifunctional chiral primary amine thiourea catalyst 41 (20 mol%) in CH Clj and in the presence of five equivalents of H O as additive, a highly enanti-oselective direct conjugate addition of a wide range of a,a-unsymmetrically dis-ubstituted aldehydes (only a twofold excess of aldehyde relative to nitroaUcene) to nitroolefins is obtained (see Table 2.1, entry 15, for a representative example) [61], The beneficial role of water is proposed to lie in increasing turnover by eliminating potential catalyst deactivation pathways, and accelerating the final imine hydrolysis. 

The reductive elimination of imidazolium species represents a major catalyst deactivation pathway for NHC-TM complexes. This phenomenon (along with its reverse process, oxidative addition) was first reported by CavelF who, in collaboration with Yates, published a series of computational studies on this subject. The first of these considered loss of 1,2,3-trimethylimidaz-olium from [(IMe)Pd(Me)(PR3)2] (Scheme 4.1), with calculations indicating... 

Eig. 1. Pathways for catalyst deactivation and ligand decomposition, where R = CgH. ... 

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. 

Although the process is of significance, it has not well studied. Since the initial development of the CTA hydropurification process in 1960s , only a few papers have been published, mainly regarding catalyst deactivation [2]. Recently, Samsung Corporation, in collaboration with Russian scientists, developed a novel carbon material-CCM supported palladium-ruthenium catalyst and its application to this process [3]. However, pathways and kinetics of CTA hydrogenation, which are crucial to industrialization, are not reported hitherto. 

Evidence for a major mode of catalyst deactivation in this system came from the observation of phosphonium cations (HPR3) in the reaction mixture, which could form through the pro to nation of free PR3 by the acidic dihydride complex. It is not known which species decomposes to release free PR3, but the decomposition pathway is exacerbated by the subsequent reactivity in which protonation of phosphine removes a proton from the metal dihydride, effectively removing a second metal species from the cycle. 

One aspect that remains underdeveloped is the insight in deactivation pathways. Our knowledge in this area is growing, but the pace is slow. We have devoted an entire chapter to this topic, since the economics of many processes could benefit a lot from more insight in ways to reduce catalyst deactivation. 

Stefanov and coworkers—deactivation pathways for industrial Cu/Cr/Zn catalysts. Stefanov and coworkers250 published an XPS study indicating that the Cu-Cr-Zn catalyst deactivates via two pathways in an industrial reactor-sintering and poisoning by chlorine adsorption, which caused a deactivation of the catalyst from... 

Generally, in conclusion, it is worth noting that the molecular and immobilized complexes show very similar catalytic activity in terms of the initial TOP in olefin metathesis. However, the supported catalyst has a longer lifetime under catalytic conditions, which indicates that the effect of active-site isolation prevents some deactivation pathways such as dimerization of reachve intermediates [30]. 

In core- (and focal point-) functionalized dendrimers, the catalyst can especially benefit from the specific microenvironment created by dendritic structures. Site-isolation effects can be beneficial for reactions that are deactivated by excess ligand, and can also prevent bimetallic deactivation pathways. 

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