Conversion, Selectivity, and Yield

The fractional conversion of a given reactant, Xa, is defined for a batch system as 

For a well-mixed flow system at steady state, the fractional conversion Xa is the ratio of the number of moles of A converted to the moles A fed to the system 

This definition is identical to that of the batch case. 

Again it is important that both the particular reactant and product, concerned, should be stated, when defining a fractional yield. 

Another definition of fractional yield is based on the reaction rates 

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In a typical pulse experiment, a pulse of known size, shape and composition is introduced to a reactor, preferably one with a simple flow pattern, either plug flow or well mixed. The response to the perturbation is then measured behind the reactor. A thermal conductivity detector can be used to compare the shape of the peaks before and after the reactor. This is usually done in the case of non-reacting systems, and moment analysis of the response curve can give information on diffusivities, mass transfer coefficients and adsorption constants. The typical pulse experiment in a reacting system traditionally uses GC analysis by leading the effluent from the reactor directly into a gas chromatographic column. This method yields conversions and selectivities for the total pulse, the time coordinate is lost. 

Batch Catalyst deactivation hard to detect quickly yields conversion and selectivity data over a large conversion range. 

The addition of acetic acid (0.5 equiv. to the substrate) to the catalyst system led to increased activity (doubling of yield) by maintaining the selectivity with 1.2 equiv. H2O2 as terminal oxidant. Advantageously, the system is characterized by a certain tolerance towards functional groups such as amides, esters, ethers, and carbonates. An improvement in conversions and selectivities by a slow addition protocol was shown recently [102]. For the first time, a nonheme iron catalyst system is able to oxidize tertiary C-H bonds in a synthetic applicable and selective manner and therefore should allow for synthetic applications [103]. 

As it was shown before that conversions and selectivities can be increased, usually not at the expense of each other, it stand to reason that micro reactors provide high yields. For example, the Suzuki coupling of 4-bromobenzonitrile and phenylboronic acid gives a yield of 62% for micro-flow processing which is about six times higher than with batch processing (10%) at comparable process conditions [155]. 

OS 30] [R 30] [P 22] By simple flow switching, serial combinatorial synthesis for creating a cation pool from diverse carbamates and silyl enol ethers was accomplished (Figure 4.46) [66, 67]. The conversions and selectivities were comparable to continuous processing using three feed streams only (see Conversion/yield/selec-tivity, above). 

The increase reaction temperature up to 250 °C allowed to achieve the yield of i-octane of 50%. While the increase of pressure led to selectivity enhancement up to 70-90%. The best results in terms of conversion and selectivities were obtained over Pt/RMOR-0,5 at 250°C and 20 bar. The yield of i-octane under these conditions reveal 51%. 

Figure 1 Ethane conversion (X), ethene yield (Y) and selectivity to ethene (S) in ODH of ethane over Co-, Ni-, and V- loaded -Al203, -HMS, and MFI catalysts (wt. % of Co, Ni, and V are given in an figure for individual catalysts). Reaction conditions 9.0 vol. % ethane, 2.5 vol. % 02 and He, 200 mg catalyst, total flow 100 ml.min1, W/F 0.12 g.s.cm"3, and 600 °C.

The next series of experiments were run varying the base composition (Table VI). With only a slight excess of base, the yield of byproduct ether was much lower using potassium methoxide, benzyl bromide, and 1 atm CO. The alkoxide on alumina reagents again gave higher conversions and selectivities to ester. 

Figure 11, Time on stream dependence of (a) phenol conversion and 2-ethyl phenol selectivity and (b) 2-ethyl phenol yield at 3750C on Cul -xCoxFe204 (x = 0.0, 0.5 and 1.0). Reactant feed ratio of EtOFI PhOH = 5 1 was used with a WHSV = 0.869 h-1. Conversion and selectivity is denoted by open and solid symbols, respectively. Note an increase in 2-ethyl phenol selectivity with increasing TOS on all catalyst compositions.

Zni xCuxFe204 have been examined for pyridine methylation with methanol at vapor phase conditions [111]. The conversion of pyridine as well as yield to 3-methyl pyridine is found to be lowest in the case of ZnFe204, whereas conversion and selectivity increases as the copper content is increased and is maximum for x = 1 composition. 

Another structurally modified guanidine was reported by Ishikawa et al. as a chiral superbase for asymmetric silylation of secondary alcohols [122]. Soon after, Ishikawa discovered that the same catalyst promoted asymmetric Michael additions of glycine imines to acrylates [123]. The additions were promoted in good yield and great asymmetric induction under neat reaction conditions with guanidine catalyst 250 (Scheme 68). The authors deduced that the high conversion and selectivity were due to the relative configuration of the three chiral centers of the catalyst in... 

Flow conditions clearly proved superior to the batch conducted trials the temperature and pressure conditions from the flow trials repeated in an autoclave even resulted in the lowest conversions and selectivities (84% vs. 52% in batch and 12% in autoclave, determined by the analysis of the reaction mixture, Scheme 11). The final yields were reported after purification by preparative HPLC. 

Also, chlorosulfonic acid was demonstrated to be an efficient catalyst in the Beckmann rearrangement of a variety of ketoximes in refluxing toluene, and excellent conversion and selectivity was observed . This procedure can also be applied to the dehydration of aldoximes yielding the corresponding nitriles. 

Table VIII shows the conversions and selectivities obtained in this reaction with the various bases. Both calcium and barium hydroxide, as well as potassium carbonate, gave good yields and selectivities, particularly barium hydroxide, which gave conversions near 100% after 14h in a batch reactor. Ba(OH)2 was tested as a basic catalyst for the reaction with other aryl halides such as iodobenzene and chlorobenzene, and in both cases the yield and selectivity were also excellent.

Yields were improved by utilizing chemically modified charcoal [treatment with N,N-dimethylformamide dibutyl acetal (DMFBA) or N,0-bis(trimethylsilyl)acetamide (BSA)] due to suppression of H2O2 decomposition and ranged between 18 and 81%. Conversion and selectivity seem to depend extremely on the pre-treatment of charcoal with solutions of various pH values being highest for solutions with pH 7. 

It is necessary, however, to maximize the intermediate olefin product at the expense of the aromatic/paraffin product which makes up the gasoline ( ). The olefin yield increases with increasing temperature and decreasing pressure and contact time. Judicious selection of process conditions result in high olefin selectivity and complete methanol conversion. The detailed effect of temperature, pressure, space velocity and catalyst silica/alumina ratio on conversion and selectivity has been reported earlier ( ). The distribution of products from a typical MTO experiment is compared to MTG in Figure 4. Propylene is the most abundant species produced at MTO conditions and greatly exceeds its equilibrium value as seen in the table below for 482 C. It is apparently the product of autocatalytic reaction (7) between ethylene and methanol (8). 

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