Selecting the Catalyst

Partial hydrolysis of nitrile gives amides. Conventionally, such reactions occur under strongly basic or acidic conditions.42 A broad range of amides are accessed in excellent yields by hydration of the corresponding nitriles in water and in the presence of the supported ruthenium catalyst Ru(0H)x/A1203 (Eq. 9.19).43 The conversion of acrylonitrile into acrylamide has been achieved in a quantitative yield with better than 99% selectivity. The catalyst was reused without loss of catalytic activity and selectivity. This conversion has important industrial applications. 

Complex 7-AI2O3/PTA/ (/< ./< )-(Mc-DuPHOS)Rh(COD) 1 (1) was prepared and tested in the hydrogenation of the prochiral substrate methyl-2-acetamidoacrylate (MAA). After full conversion, the products were separated from the catalyst and analyzed for Rh and W content and product selectivity. The catalyst was re-used three times. Analytical results show no rhodium leaching is observed. Complex 1 maintains its activity and selectivity in each successive run. The first three runs show tungsten (W) leaching but after that no more W is detectable. The leached W comes from the excess of PTA on alumina. The selectivity of both tethered and non-tethered forms gave the product in 94% ee. 

Preferential Catalytic Oxidation of CO. CO oxidation experiments (O2/CO =1) were conducted in a simulated gas stream of composition CO (9600 ppm) -b H2 (73.36 vol.%) -b CO2 (23.75 vol.%) -b CH4 (1.93 vol.%) (Table 11.7). The Pt catalyst is more active but less selective. The catalysts that contain more Pt were more active. A combination of Pt with Au forming a bimetallic catalyst enables preferential CO oxidation. It should be noted that the Pt-Au-MCM-41 catalysts are not deactivated in CO2. 

Tungsten- and molybdenum-catalyzed methods involving vinylidene intermediates have been described for this transformation (Chapter 5). The use of rhodium provides some advantages in terms of catalyst turnover and selectivity. The catalyst formed in situ from a RhCl source and a fiuormated triarylphosphine promotes the cyclization of a variety of alkynols (Table 9.6). 

An interesting application of TSIL was developed by Zhang et al for the catalytic hydrogenation of carbon dioxide to make formic acid. Ruthenium immobilized on silica was dispersed in aqueous IL solution for the reaction. H2 and CO2 were reacted to produce formic acid in high yield and selectivity. The catalyst could easily be separated from the reaction mixture by filtration and the reaction products and the IL were separated by simple distillation. The TSIL developed for this reaction system was basic with a tertiary amino group (N(CH3)2) on the cation l-(A,A-dimethylaminoethyl)-2,3-dimethylimidazolium trifluoromethanesulfonate, [mammim] [TfO]. 

Significant levels of syn diastereoselectivities (5 1 to 16 1) were observed for all substrates, with the exception of an ortho-chloro-substituted aryl imine, which provided only 2 1 syn selectivity. The catalyst was viable for a variety of nitroalkanes, and afforded adducts in uniformly high enantioselectivities (92-95% ee). The sense of enantiofacial selectivity in this reaction is identical to that reported for the thiourea-catalyzed Strecker (see Scheme 6.8) and Mannich (see Tables 6.18 and 6.22) reactions, suggesting a commonality in the mode of substrate activation. The asymmetric catalysis is likely to involve hydrogen bonding between the catalyst and the imine or the nitronate, or even dual activation of both substrates. The specific role of the 4 A MS powder in providing more reproducible results remains unclear, as the use of either 3 A or 5 A MS powder was reported to have a detrimental effect on both enantioselectivities and rates of reaction. 

The first set of reactions is the mainstay of the petrochemical industry 1 outstanding examples are the oxidation of propene to propenal (acrolein) catalysed by bismuth molybdate, and of ethene to oxirane (ethylene oxide) catalysed by silver. In general these processes work at high but not perfect selectivity, the catalysts having been fine-tuned by inclusion of promoters to secure optimum performance. An especially important reaction is the oxidation of ethene in the presence of acetic (ethanoic) acid to form vinyl acetate (ethenyl ethanoate) catalysed by supported palladium-gold catalysts this is treated in Section 8.4. Oxidation reactions are very exothermic, and special precautions have to be taken to avoid the catalyst over-heating. 

The Sheldon group found that the highest conversions were observed with CrS-1 prepared by the ammonia method. CrS-1 made by the fluoride method gave a slightly lower styrene conversion and roughly the same selectivities. The catalyst prepared by the sulfuric acid method gave substantially lower styrene conversions, but the selectivity to benzaldehyde was high. 

Selecting the catalyst and reaction conditions for partial reduction of a triple bond situated in a conjugated system is a challenge. Where the hydrogenation of an isolated alkyne can proceed with nearly complete selectivity, the partial saturation of an enyne takes place selectively with much more difficulty. Selectivities of 85-90% in these latter reactions are common and are considered to be reasonable for synthetic applications. 

In order to clarify the reason why Rh precursor influenced the product selectivity, the catalysts were characterized. The residue such as chlorine coming from Rh precursor were not detected on the surface of catalysts by XPS analysis. 

The reaction products can generate other radicals which can react in the gas phase ch interact with the surface to yield additional radicals, or they can be further oxidized to CO2. This process is seen as a limiting factor to the yield of higher hydrocarbons, but the fact of the matto is that all the kinetic nuxlels depend on assumptions about (he relative rate of the hcanogeneous unselective gas-phase reactions versus the selective heterogeneous surface reactions. The crucial parameter which determines hydrocarbons yield is the selectivity the catalyst since no model... 

Seo and Sato have compared the heat and entropy of oxygen adsorption on three silver catalysts. The more selective the catalyst is for ethylene oxidation the lower the heat of oxygen adsorption and the higher the entropy of the adsorbed species. They argued 2 (ads) is the selective species since it has a lower heat of adsorption than 0 "(ads) or 0 (ads) and it would be expected that 02 (ads) would have the higher entropy. 

The catalyst Au/TiO-SiO(2) shows the highest initial propylene conversion. Propylene conversion decreases with time on stream due to catalyst deactivation but it appears to stabilize after long reaction time. For TiO-SiO(l) catalyst the conversion passes through a maxima with reaction time. TiO-SiO(l) catalyst shows the highest PO selectivity. The catalyst TiO-SiO(2) which shows the lowest PO selectivity also exhibits decrease in selectivity with time on stream whereas the selectivity increases with reaction time for all other catalysts. The PO selectivity decreases from 86 % at TOS = 15 min. to 77 % at TOS = 360 min. for AuATiO-SiO(2) catalyst. But Au/TiO-SiO(l) catalyst shows increase in PO selectivity from 93 % at TOS =15 min. to 100 % at TOS = 360 min. Ti-MCM-41 and Ti-meso supported Au catalysts exhibit only small increase in PO selectivity with reaction time from 87-89 % at TOS = 15 min. to 91-92 % at TOS = 360 min. Au/TiO-SiO(l) catalysts shows very poor hydrogen efficiency. But Au/TiO-SiO(2) is most efficient catalyst for hydrogen utilization. 

As more effective means of temperature and time of contact control have been developed it has been discovered that a great many materials are effective as catalysts for commercial operation. This is true because no matter what catalyst is used, the mechanism of the reaction is probably the same and it only remains to stop the oxidation at the proper point to obtain desired yields of intermediates. The less selective the catalyst is for the different steps the narrower is the zone of temperature and time of contact in which the desired reaction occurs and hence, the more strict must be the control in operation. 

The first example of the immobilization of a chiral ketone to promote the enan-tioselective epoxidation of alkenes with Oxone has been reported by Sartori and coworkers [322]. They anchored a-fhiorotropinone on KG-60 silica, MCM-41 and a Merrifield resin. The catalysts were tested for the epoxidation of 1-phenylcyclo-hexene but the polymer-supported fhiorotropinone 121 showed a low activity and selectivity. The catalyst immobilized on inorganic supports promoted the stereoselective epoxidation of alkenes with ee values up to 80% and could be reused with the same performance for three runs. 

X = OH) act as very efficient bases for the Heck reaction and catalyze efficiently the Henry reaction with excellent yields and selectivities. The catalyst can be used both in batch and under flow conditions and can be directly reused several times with only a minor decrease in activity. The spent polymer can be easily regenerated simply by its treatment with a basic solution [363],... 

The emfo-selective reaction of a resin-bound heterodiene with a chiral vinyl ether catalysed by Eu(fod)3 occurs with excellent facial selectivity the catalyst can be recycled <01TL8849>. 

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