Molar selectivity

Figure 44.5. Conversion of phenol ( ), molar selectivity to anisole (X), o-cresol (O), p-cresol (A), 2,6-xylenol (O), salicylaldehyde ( ) and polyalkylated phenols ( ) as functions of temperature. Catalyst H-mordenite. Feed composition N2 89.3%, formaldehyde 1.7%, phenol 0.46%, methanol 0.03% and water 8.5%.

A typical reaction profile is illustrated in Figure 1 for the Ru3(C0)i2 Col2/Bu4PBr catalyst precursor. Under the selected operating conditions, acetic acid is the major product fraction here it may comprise up to 85 wt % of the liquid product fraction (molar selectivity to HOAc is 86%, initial turnover frequency ca. 2.0x10 s per g atom Ru charged). Methyl and ethyl acetates are also in evidence. 

Promoter MeOAc Products EtOAc (mol/H/hr) CH2OAC 1 CH2OAC 1 Total CH2OH 1 Molar Selectivity Activity... 

Molar selectivity to product, i = (moles of product i recovered)/(total moles of all products recovered) x 100%. Water was excluded as a product. 

The propylene (steam-cracked C3 cut) is dimerized in the presence of nripropylalo-mioum as catalyst at 130 to 200 C and 20.10 Pa absolute. Residence time is about 15 min. Molar selectivity is as high as 90 to 95 per cent for once-through convc. sion... 

I to 5. 0 mol per mol of hydroperoxide. The presence of sodium naphtheoate, by prevenling side reaction, helps to reduce the excess propylene required (from lO/l to 2/1 in moles). In the Shell technology, epoxidation is catalyzed by metallic oxides (molybdenum, vanadium, titanium, etc.) supported on sih cau The liighiy exothe c reaction takes place around 100 to 130 at 3.5.10 Pa absolute. Hydroperoxide conver> sion is very hi (> 97 per cent). Propylene oxide molar selectivity exceeds 70 per cent and that of the styrene precursors 93 per cent As for propylene, its once-through conversion is about 15 per cent, for a oxide molar selectivity greater than 90 per cent, and the main by-products are dimers and heavier hydrocarbons. 

Dehydration. This reaction takes place in the vapor phase around 25(PC between (U and 03.10 Pa absolate, in the presence of atJ >cid catalyst (10 to 15 weight per cent of T1O2 on alumina). Once-through conversion is up to 85 per cent and styrene molar selectivity exceeds 95 per cent... 

Pretzer et al. have investigated the hydrogenation potential of different noble metal acetylacetonates in the cobalKatalyzed methanol hydrocarbonylation (c.f. Table VIII). The best results have been obtained with ruthenium, followed by rhodium. In an optimized system of that type, it was possible to achieve 80% molar selectivity to ethanol at a 30% conversion of methanol. Interestingly, the addition of platinum resulted in an increase of acetaldehyde selectivity (20]. 

Cat yst Conversion (%) % Molar selectivity EtOH Me 0 McOEi CH4 + CjHft... 

Primary products of a complex reaction can be inferred from zero conversion extrapolation of selectivity diagrams, as first described by Schneider and Frolich (16). According to this method, the molar selectivity of each product (mol product formed per 100 mol of reactant decomposed) is plotted against the percent conversion. The validity of this method has been seriously questioned (17). In principle, this method suffers from the fact that at the very low conversions required for reliable extrapolation to zero conversion, data on yields of individual products are subject to substantial analytical uncertainty. Consequently, the calculated conversion is subject to the summation of all of the errors in the yields of all of the products, and the calculated selectivities are increasingly unreliable as the conversion decreases. However, because of the vastly improved accuracy available through the use of modern analytical techniques, the criticism of the use of this method is far less valid, and significant insight into initial product distribution can be derived. 

The relation between the molar selectivity coefficient and the equivalent ionic fractions of ions A and B in both phases may be derived as follows ... 

Isotherm parameter Molar selectivity coefficient Rational selectivity coefficient Bed length, m... 

In effect, ion exchange chromatography exploits differences in the degree of charge and affinity (measured by the reaction equilibrium constant or molar selectivity coefficient) of ions in solution for sites of opposite charge in the stationary phase to accomplish separation. 

Many companies have studied the optimization of catalyst composition and process conditions in order to improve the performance of the reaction and the economics ofthe process. In the Gulf process, the reaction is carried out at 90-100 °C, with a Co(III) acetate catalyst and acetic acid as the solvent [17]. The molar selectivity is around 70-75%, for a cyclohexane conversion that can be as high as 80-85%. The high concentration of Co(III) acetate used also favors the direct reaction ofthe cation with cyclohexane, generating the cydohexyl radical. In fact, in Gulf patents the reaction is reported to occur in a critical amount of Co(III) (25-150 mmoles per mole of cyclohexane). The catalyst is activated during the initial induction period, and water is also added in the initial stage to enhance the selectivity to AA, but the rate of production decreases because the induction period increases. 

The equilibrium constant can be estimated based on relative molar selectivity... 

The second fact, linked to the first, is that a much lower srield in valuable products is obtained when paraffins are used instead of olefins or aromatics. The success of the butane-based MAA process occurred at the cost of a dramatic drop of molar selectivity (at most 65-67% compared to 75-77% when starting from benzene) and a drop of productivity by nearly 20% [3]. Compared to the benzene route, butane oxidation gives molar yields per pass of only about 55% instead of 75%. 

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