Reaction steps, olefin conversion

The initial dehydration reaction is sufficiently fast to form an equilibrium mixture of methanol, dimethyl ether, and water. These oxygenates dehydrate further to give light olefins. They in turn polymerize and cyclize to form a variety of paraffins, aromatics, and cycloparaffins. The above reaction path is illustrated further by Figure 3 in terms of product selectivity measured in an isothermal laboratory reactor over a wide range of space velocities. ( 3) The rate limiting step is the conversion of oxygenates to olefins, a reaction step that appears to be autocatalytic. In the absence of olefins, this rate is slow but it is accelerated as the concentration of olefins increases. 

The synthesis of vinylphosphonate-linked nucleotide dimer (93) has been achieved using an olefin-metathesis reaction step between the vinylphosphonate (91) and the 5 -alkene derivative of thymidine (92). The second-generation Grubb s catalyst was reported to be the superior catalyst for this conversion in which no vinyl phosphonate homo-coupling was detected. ... 

The rate also varies with butadiene concentration. However, the order of the rate dependence on butadiene concentration is temperature-de-pendent, i.e., a fractional order (0.34) at 30°C and first-order at 50°C (Tables II and III). Cramer s (4, 7) explanation for this temperature effect on the kinetics is that, at 50°C, the insertion reaction to form 4 from 3, although still slow, is no longer rate-determining. Rather, the rate-determining step is the conversion of the hexyl species in 4 into 1,4-hexadiene or the release of hexadiene from the catalyst complex. This interaction involves a hydride transfer from the hexyl ligand to a coordinated butadiene. This transfer should be fast, as indicated by some earlier studies of Rh-catalyzed olefin isomerization reactions (8). The slow release of the hexadiene is therefore attributed to the low concentration of butadiene. Thus, Scheme 2 can be expanded to include complex 6, as shown in Scheme 3. The rate of release of hexadiene depends on the concentra-... 

At Van Sickle s conditions of low temperatures and low conversions, branching routes A and B appear to be dominant since there is little alkenyl hydroperoxide decomposition. In our work above 100°C., the branching routes are supported by the nearly linear initial portions at low conversions for alkenyl hydroperoxide and polymeric dialkyl peroxide curves (see Figures 2, 3, and 4). The polymeric dialkyl peroxides formed under our reaction conditions include those formed by the branching mechanism postulated by Van Sickle (routes A and B) and those formed by the reaction of the alkenoxy and hydroxy radicals from alkenyl hydroperoxide thermal decomposition reacting further and alternately with olefin and oxygen (step C). The importance and kinetic fit of the sequential route A to C appears to increase with temperature and extent of olefin conversion owing to the extensive thermal decomposition of the alkenyl hydroperoxides above 100°C. 

The conversion of alkenes to 1,2-diols by osmium tetroxide is also an olefin addition reaction. In this case a hydroxy group is added to each carbon of the olefin group, and the addition is termed an oxidative addition since the diol product is at a higher oxidation level than the alkene reactant. Oxidation of the carbon atoms of the alkene takes place in the first step, which is the reaction with 0s04 to produce the intermediate osmate ester. 

Figure 5 and 6 show the contact time dependence of the concentration of each hydrocarbon represented by carbon monoxide conversion base for these catalysts. It is reasonable to derive the existence of the successive reaction step of olefins from the con-... 

Figure 3. First-step reactions uHth n-butenes percent olefin conversion to sulfate os. A/O ratio...

Initially, the separate reaction steps were investigated, i.e. the epoxidation of olefin 1 via epoxide 2 to diol 3 (Section 2.1), as well as the rearrangement of epoxide 2 to ketone 4 (Section 2.2 ). Results obtained from these investigations were used in the one-step conversion of the olefin 1 to the ketone 4 (Section 2.3.). Research is currently in progress to broaden the scope to other substrates, such as styrene, a-methylstyrene or a-pinene as well as the use of other redox-molecular sieves such as TAPSO " or Ti-MCM-41. ... 

Methanol Conversion. Methanol conversion reactions based on borosilicate catalysts have been studied extensively (10.15,24,28.33.52-54). During the conversion of methanol, the reaction proceeds through a number of steps, to yield dimethylether, then olefins, followed by paraffins and aromatics. The weaker acid sites of borosilicate molecular sieves relative to those of aluminosilicates require higher reaction temperatures to yield aromatics. The use of less forceful process conditions leads to the formation of olefins selectively, instead of a mixture of paraffins, olefins, and aromatics (10.28.53.54). 

Many transition metals and their oxides are quite active for the oxidation of olefins to CO2. From the standpoint of mechanism these oxidations have much of potential interest. According to the principle of minimum atomic rearrangement in a reaction step, a considerable number of steps must be involved in the conversion of an olefin such as CjHg to CO2 and H2O. We know little of these steps. With most of the C02-forming catalysts, very small amounts of intermediate products are observed, even at low conversions. Furthermore, it has been demonstrated that certain possible intermediates, such as acids, are not rapidly oxidized over many of these catalysts, and therefore paths involving these as intermediates can be eliminated. 

Fatty acid esters are generally obtained from the transesterification of fats and oils with a lower alcohol, e.g. methanol, along with glycerol. More than 90% of all oleochemical reactions (conversion into fatty alcohols and fatty amines) of fatty acid esters (or acids) are carried out at the carboxy functionality. However, transformation of unsaturated fatty acid esters by reactions of the carbon-carbon double bond, such as hydrogenation, epoxidation, ozonolysis, and dimerization, are becoming increasingly of industrial importance. Here we will discuss another catalytic reaction of the carbon-carbon double bond, viz. the olefin metathesis reaction, in which olefins are converted into new products via the rupture and reformation of carbon-carbon double bonds [2]. Metathesis of unsaturated fatty acid esters provides a convenient route to various chemical products in only a few reaction steps. 

The reaction steps are reversible and isomerization of the olefin, alkyl, or acyl species can take place to allow the formation of isoaldehydes. The typical 4 1 prodnct distribntion of normal and isoaldehydes mnst be separated if the mixture cannot be nsed commercially. Efforts were therefore made to increase the proportion of nseful normal aldehydes during operation. Partial success was achieved by operating at lower temperatures with higher carbon monoxide partial presstrres, althongh this decreased conversion to aldehydes. A major problem with the cobalt catalyst was the tendency to decompose at high temperatrrre and to deposit metal onto the reactor walls. This led to loss of activity and low catalyst recovery. 

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