Alcohols catalyst deactivation

Pure dry reactants are needed to prevent catalyst deactivation effective inhibitor systems are also desirable as weU as high reaction rates, since many of the specialty monomers are less stable than the lower alkyl acrylates. The alcohol—ester azeotrope (8) should be removed rapidly from the reaction mixture and an efficient column used to minimize reactant loss to the distillate. After the reaction is completed, the catalyst may be removed and the mixture distilled to obtain the ester. The method is particularly useful for the preparation of functional monomers which caimot be prepared by direct esterification. 

Chiral lactones were also obtained by cyclocarbonylation of chiral acetylenic alcohols with Pd and thiourea (H2NCSNH2) (Scheme 32). No loss in chirality was observed, but large amounts of Pd and thiourea were used (10 mol %) since the catalyst deactivates by forming metal particles. The catalytic precursor (Pdl2 > PdCl2) and the ratio of thiourea to Pd were very important, thiourea being necessary for this reaction. The active species was supposed to be [Pd(thiourea)3l]I, which forms in situ from [Pd(thiourea)4]l2 and [Pd(thiourea)2]l2. It had to be a partially dissociated species since [Pd(thiourea)4](Bp4)2 was inactive [121]. 

The heterogeneous catalytic system iron phthalocyanine (7) immobilized on silica and tert-butyl hydroperoxide, TBHP, has been proposed for allylic oxidation reactions (10). This catalytic system has shown good activity in the oxidation of 2,3,6-trimethylphenol for the production of 1,4-trimethylbenzoquinone (yield > 80%), a vitamin E precursor (11), and in the oxidation of alkynes and propargylic alcohols to a,p-acetylenic ketones (yields > 60%) (12). A 43% yield of 2-cyclohexen-l-one was obtained (10) over the p-oxo dimeric form of iron tetrasulfophthalocyanine (7a) immobilized on silica using TBHP as oxidant and CH3CN as solvent however, the catalyst deactivated under reaction conditions. 

It is concluded that the occupation of the step and kink sites plays a crucial role in the promotion of the Pt catalyst. The cyclic voltammetry results can be used to explain the conversion trends observed in Figure 2. For unpromoted 5%Pt/C the Pt step and kink sites are unoccupied and available for adsorption of reactant and oxidant species. During reaction these sites facilitate premature catalyst deactivation due to poisoning by strongly adsorbed by-products (5) and (or) the formation of a surface oxide layer (6). The 5%Pt,0.5%Bi/C catalyst has a portion of these Pt step and kink sites occupied and the result is a partial reduction in the catalyst deactivation and a consequent increase in alcohol conversion. As the Bi level is increased to lwt.% almost all of the Pt step and kink sites are occupied and the result is a catalyst with high activity. As more Bi is introduced onto the catalyst surface a bulk Bi phase is formed. Since the catalyst activity is maintained it is speculated that the bulk Bi phase is not involved in the catalytic cycle. 

After the polymerization step, the reaction mixture is fed to a heated separation tank where the unreacted propylene is flashed off and recycled. The polymer slurry is then washed with alcohol to deactivate and remove the catalyst and the atactic polymer (the bad stuff.) Centrifuging the slurry removes the diluent from the isotactic PP (the good stuff.) The product is washed with acetone, dried, and stabilized with suitable additives. It is sold as a powder or can be pelletized into granules. 

A series of pseudo-C or pseudo-C2 symmetric complexes 168-171 (Fig. 27) exhibited isotactic predominance P = 0.50-0.75) however, the isotacticity is compromised in solvent-free bulk polymerization at 130 °C [129]. Fluorous tertiary alcohol ligands with electron-withdrawing CF3 group are weakly basic and thus expected to reduce the possibility of catalyst deactivation by bridged species formation. Al complexes 172 and 173 offered highly isotactic-enriched stereoblock PLA (Pm = 0.87) from ROP of rac-lactide [168]. 

There are numerous indications in the literature on catalyst deactivation attributed to over-oxidation of the catalyst (3-5). In the oxidative dehydrogenation of alcohols the surface M° sites are active and the rate of oxygen supply from the gas phase to the catalyst surface should be adjusted to that of the surface chemical reaction to avoid "oxygen poisoning". The other important reason for deactivation is the by-products formation and their strong adsorption on active sites. This type of... 

When the air flow was temporarily substituted by a nitrogen flow for 15-20 minutes in the reaction represented by Figure 5a, the rate of alcohol oxidation did not increase. These experiments also prove that the reason of catalyst deactivation is not the over-oxidation of Pt° active sites, but a partial coverage of active sites by impurities (chemical deactivation). 

Catalyst deactivation often plays a central role in manufacturing of various alimentary products. Sugar alcohols, such as xylitol, sorbitol and lactitol, are industrially most commonly prepared by catalytic hydrogenation of corresponding sugar aldehydes over sponge nickel and ruthenium on carbon catalysts (5-10). However, catalyst deactivation may be severe under non-optimized process conditions. 

Esterification. The esterification reaction (Figure 3) involves the reaction of a FFA with an alcohol (usually a low molecular weight alcohol, such as MeOH, EtOH, PrOH, and ButOH) to produce an alkyl ester (biodiesel) and water. Either base or acid catalysts can be used for the reaction. However, base catalysts can only be used at high temperatures (or catalyst deactivation takes place by soap formation). More commonly, acid catalysts such as sulfuric acid are employed to carry out the esterification reaction under mild conditions. 

The objectives of using solvents are diverse, e.g., to dissolve a solid substrate, to limit catalyst deactivation, to improve selectivity, or to enhance mass-transport. The solvents are selected depending on the substrate and the desired effect. Hence, they range from water, alcohols, ethers, or low alkanes, to CO2. The effects of the solvent on phase-behaviour, viscosity, and density at different concentrations, temperatures and pressures can explain much about the effect of the solvent on the reaction. 

Current Processes. The development of superactive third-generation supported catalysts enabled the introduction of simplified processes, without sections for catalyst deactivation or removal of atactic polymer. By eliminating the waste streams associated with the neutralization of catalyst residues and purification of the recycled diluent and alcohol, these processes minimize any potential environmental impact. Investment costs arc reduced by approximately one-third over slurry process plants. Energy consumption is minimized by elimination of the distillation of recycled diluent and alcohol. The total plant cost for the production of polymer is less than 130% of the monomer price, when a modem process is used, compared to 175% for a slurry process. 

The possibility of using of aliphatic alcohols as hydrogen donors for the catalytic transfer reduction of nitro group over MgO was examined. Catalytic hydrogen transfer was found to be effective and selective method for reduction of nitrobenzene, A-nitrotoluene, A-chloronitrobenzene, 4-nitro-m-xylene, 3-nitro-styrene, 3-nitrobenzaldehyde, 1-nitropropane, and 1-nitrobutane. Conversion of starting nitro compound into desired product depended on the alcohol used as a donor. Adsorption of reactant and catalyst deactivation were studied by esr. New aspects of a role of one-electron donor sites in hydrogen transfer over MgD were demonstrated. 

A major problem in noble metal catalyzed liquid phase alcohol oxidations -which is principally an oxidative dehydrogenation- is poisoning of the catalyst by oxygen. The catalytic oxidation requires a proper mutual tuning of oxidation of the substrate, oxygen chemisorption and water formation and desorption. When the overall rate of dehydrogenation of the substrate is lower than the rate of oxidation of adsorbed hydrogen, noble metal surface oxidation and catalyst deactivation occurs. 

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... 

Using this improved protocol, a variety of allylic, benzylic, and secondary alcohols are now smoothly oxidized to the corresponding carbonyl derivatives in high yield. Unfortunately, primary aliphatic alcohols still appear to be poor substrates and a conversion of only 65% can be achieved before catalyst deactivation (Table III, Entry 9). 

A different approach that has been used is a ruthenium-catalyzed Meerwein-Ponndorf-Verley-type reduction of ketones using the silica-supported amino alcohol ligand 22 (Scheme 4.65). It was found necessary to cap the remaining free silica hydroxyl sites to alkylsilane derivatives to prevent catalyst deactivation. Initial studies found that slower flow rates resulted in lower ee because of equilibration back to the starting materials - after optimization, the best conditions were found to be 1400 pl/h providing a 95% conversion and 90% ee. The stability of the catalyst was investigated over time, during which a constant formation of 175 pmol/h was obtained only after a period of 7 days was some decrease in activity observed. The extended lifetime of the... 

Platinum-catalyzed oxidation of alcohols in aqueous solutions. The role of Bi-promotion in suppression of catalyst deactivation... 

Oxidation of alcohols to carbonyl-compounds or carboxylic acids can be performed under moderate conditions, in aqueous solution and with air as oxidant [1, 2], The selectivity is high, usually above 90 % even at full conversion. The only drawback of the method is the rapid deactivation of Pt- or Pd-based catalysts. An indication of this difficulty is that more than 60 % of the papers, which have been published on alcohol oxidation in the past ten years, describe some sort of catalyst deactivation. 

In this paper we give an overview on the partial oxidation of primary and secondary alcohols to ketones, aldehydes or acids. The influence of promotion on catalyst deactivation will be illustrated using the example of a 5 wt% Pt/alumina partially covered by Bi. 

The superior behaviour of Bi promoter, observed in the oxidation of 1-methoxy 2-propanol and some other secondary alcohols, may be due to geometric effects. The number of surface Pt atoms, occupied by one adatom, is 3 for Bi, 2 for Pb and Sn, and 1 for Ag [21], We propose that the higher the site requirement of the promoter adatom, the higher is its geometric blocking effect and the lower is the rate of catalyst deactivation. 

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