Rapid catalyst deactivation

Supported aqueous phase (SAP) catalysts (16) employ an aqueous film of TPPTS or similar ligand, deposited on a soHd support, eg, controlled pore glass. Whereas these supported catalysts overcome some of the principal limitations experienced using heterogeneous catalysts, including rhodium leaching and rapid catalyst deactivation, SAP catalysts have not found commercial appHcation as of this writing. 

Because soHd acid catalyst systems offer advantages with respect to their handling and noncorrosive nature, research on the development of a commercially practical soHd acid system to replace the Hquid acids will continue. A major hurdle for soHd systems is the relatively rapid catalyst deactivation caused by fouling of the acid sites by heavy reaction intermediates and by-products. 

One of the most studied applications of Catalytic Membrane Reactors (CMRs) is the dehydrogenation of alkanes. For this reaction, in conventional reactors and under classical conditions, the conversion is controlled by thermodynamics and high temperatures are required leading to a rapid catalyst deactivation and expensive operative costs In a CMR, the selective removal of hydrogen from the reaction zone through a permselective membrane will favour the conversion and then allow higher olefin yields when compared to conventional (nonmembrane) reactors [1-3]... 

Moreover, the catalyst deactivation must also be considered in order to use these solid materials in industrial processes. Figure 13.8 shows the variation of catal54ic activity (2-butene conversion) with the time on stream obtained under the same reaction conditions on different solid-acid catalysts. It can be seen how all the solid-acids catalysts studied generally suffer a relatively rapid catalyst deactivation, although both beta zeolite and nafion-sihca presented the lower catalyst decays. Since the regeneration of beta zeolite is more easy than of nafion, beta zeolite was considered to be an interesting alternative. ... 

A substantial difficulty in ethanol SR is a too rapid catalyst deactivation due to coking. This can occur by several reactions, such as methane decomposition (19) or the Boudouard reaction (20), but primarily the polymerization of ethylene is thought to cause the problems (21). Unlike the situation for methane SR, it appears that for ethanol SR the deactivation by coke formation is lower at high temperatures. 

Supported catalysts could be reused once with little loss of activity further reuse led to a significant drop in activity, as a result of strong absorption of products and by-products on the catalyst surface (indicated by colour change of the catalyst). More rapid catalyst deactivation was observed in a trickle bed reactor than in a batch slurry reactor. 

The dehydrogenation of ethyl stearate over the non-pretreated catalyst subsequently led to a rapid catalyst deactivation (from 66% to 16% of conversion within Ih of reaction) whereas the pretreated catalyst deactivated only slightly (17%... 

Porphyrin complexes, however, are prone to oxidative decomposition and therefore synthetic applications are hampered by rapid catalyst deactivation. This problem can be overcome by attaching electron-withdrawing groups to the periphery of the porphyrin system. Another problem is the poor chemoselectivity. In many cases, addition to the C=C double bond and formation of the epoxide are much faster than the corresponding hydrogen abstraction, which leads to the allylic alcohols. This is... 

C2H2] of 0.2. Very rapid catalyst deactivation is observed when [H2OMC2H2] is zero so that reactant ratio is not included in Table II. Experimentally, it is observed that a [H2OMC2H2] > 0.4 is required in order to establish the stable catalyst conversion condition. 

Matsumoto and Tamura (at Kuraray Co.) have demonstrated that the combination of simple bis(diphenylphosp-hino)alkane ligands and PPhs has a very positive effect on catalyst stabihty and the reduction of unwanted side reactions. This is most evident in the hydroformylation of a reactive alkene substrate such as allyl alcohol. The use of HRh(CO)(PPh3)2 in the presence of excess PPhs leads to relatively rapid catalyst deactivation to unidentified species. The addition of just over 1 equivalent of dppb, for example, leads to a stable, active hydroformylation catalyst. Use of dppb either by itself, or in quantities higher than 2 equivalents, leads to catalyst deactivation and/or poor activities and selectivities. ARCO Chemical Co. licensed the Kuraray technology to build the first conunercial plant (1990) for the hydroformylation of allyl alcohol to produce 1,4-butanediol (Scheme 11). 

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