Carbon product selectivity

Photocatalytic reduction of C02 can be accomplished by suspending photosensitive semiconductor powders in aqueous solutions under irradiation, usually using UV light.129156 Photoreduction of C02, however, is in competition with H2 formation due to water decomposition, and leads to mixtures of reduced carbon products. Selectivity, therefore, is one of major problems of these processes. 

Selectivity of propylene oxide from propylene has been reported as high as 97% (222). Use of a gas cathode where oxygen is the gas, reduces required voltage and eliminates the formation of hydrogen (223). Addition of carbonate and bicarbonate salts to the electrolyte enhances ceU performance and product selectivity (224). Reference 225 shows that use of alternating current results in reduced current efficiencies, especiaHy as the frequency is increased. Electrochemical epoxidation of propylene is also accompHshed by using anolyte-containing silver—pyridine complexes (226) or thallium acetate complexes (227,228). 

In order to achieve the desired fiber properties, the two monomers were copolymerized so the final product was a block copolymer of the ABA type, where A was pure polyglycoHde and B, a random copolymer of mostly poly (trimethylene carbonate). The selected composition was about 30—40% poly (trimethylene carbonate). This suture reportedly has exceUent flexibiHty and superior in vivo tensile strength retention compared to polyglycoHde. It has been absorbed without adverse reaction ia about seven months (43). MetaboHsm studies show that the route of excretion for the trimethylene carbonate moiety is somewhat different from the glycolate moiety. Most of the glycolate is excreted by urine whereas most of the carbonate is excreted by expired CO2 and uriae. 

In addition to surface area, pore size distribution, and surface chemistry, other important properties of commercial activated carbon products include pore volume, particle size distribution, apparent or bulk density, particle density, abrasion resistance, hardness, and ash content. The range of these and other properties is illustrated in Table 1 together with specific values for selected commercial grades of powdered, granular, and shaped activated carbon products used in Hquid- or gas-phase appHcations (19). 

Table 1. Properties of Selected U.S. Activated Carbon Products ...

Carbon black from oil is the main competition for the product from coal, which is used in filters. Carbon for electrodes is primarily made from petroleum coke, although pitch coke is used in Germany for this product. The pitch binder used for electrodes and other carbon products is almost always a selected coal tar pitch. 

Activated carbon is an amorphous solid with a large internal surface area/pore strucmre that adsorbs molecules from both the liquid and gas phase [11]. It has been manufactured from a number of raw materials mcluding wood, coconut shell, and coal [11,12]. Specific processes have been developed to produce activated carbon in powdered, granular, and specially shaped (pellet) forms. The key to development of activated carbon products has been the selection of the manufacturing process, raw material, and an understanding of the basic adsorption process to tailor the product to a specific adsorption application. 

Table 4. Properties of selected activated carbon products. Reprinted from [11], copyright (c) 1992 John Willey Sons, Inc., with permission.

Comparing the product selectivity at low conversion in the hydrogenolysis of 2,2-dimethylbutane for the two catalysts is noteworthy. Zirconium hydride supported on siUca does not produce neopentane, but only isopentane (10%) as a Cs product in agreement with a /1-alkyl transfer as a key step for the carbon-carbon cleavage (no neopentane can be formed through this mechanism, Scheme 25). 

The product selectivities in propane metathesis can also be explained by using the same model in which [1,3]- and [1,2]-interactions determine the ratio of products. For instance, the butane/pentane ratios are 6.2 and 4.8 for [(= SiO)Ta(= CHfBu)(CH2tBu)2] and [(= SiO)2Ta - H], respectively (Table 5). A similar trend is observed for the isobutane/isopentane ratio, which are 4.1 and 3.0, respectively. The higher selectivity in butanes (the transfer of one carbon via metallacyclobutanes involving [l,3]-interactions) than that of pentanes (the transfer of two carbons via metallacyclobutanes involving [1,2]-interactions) is consistent with this model (Scheme 28). 

Changing catalyst support from carbon to calcium carbonate leads to dramatic improvement of the cis/tran ratio from 6 1 to 18 1, that is the cis selectivity increases from 85.7% to 94.7%. The reason for better selectivity on CaC03 supported catalyst is attributed to its lower surface area leading to lower hydrogenation activity, but more selective to the desired product. The successful commercialization of the new route for sertraline synthesis demonstrates that for a stereoselective hydrogenation reaction, improve product selectivity can be achieved by proper selection of catalyst support. 

The Ni/Re on carbon catalyst was also evaluated in a 1700 hour continuous reactor test to determine the stability of the catalyst. This test was performed with a different model compound than xylitol. Shown in Figure 5, the results from the lifetime test of the Ni/Re catalyst operated at constant process conditions sampled intermittently for 1700 hours. This shows that for a similar aqueous hydrogenation reaction deliberately operated to near completion, the catalyst retained its activity and product selectivity even in the face of multiple feed and H2 interruptions. We feel that this data readily suggests that the Ni/Re catalyst will retain its activity for xylitol hydrogenolysis. 

The MDA experiments were performed in a continuous down-flow fixed bed reactor at 700°C, atmospheric pressure, and a space velocity of 1500 em3/(gcat h). Catalysts were pretreated in He flow at 700°C for 30 min before feeding a CH4 N2 mixture in a 9 1 voEvol ratio (N2 used as internal standard). Unreacted methane, the reference N2, and the reaction products were analyzed on line in a gas chromatograph (HP-GC6890) as detailed in [6]. Product selectivities are given on a carbon basis. The use of N2 as internal standard allows to obtain the amount of carbonaceous residues as the amount of carbon required to close the mass carbon balances to 100%. 

When determining the product selectivities, all compounds of equal carbon numbers (paraffines, olefins, isomers, and oxygen compounds) were summarized to one product fraction. The chain growth probability was determined by the Anderson-Schulz-Flory (ASF) distribution ... 

The methane and the carbon dioxide selectivities as well as the selectivities concerning the product fractions C2 to C3 and C4+ with the respective conversion degrees and a-values at 513 K are shown in Table 2.5. 

These adducts are more active than the iron ones in the conversion of syngas. At 250°C, a higher yield of methane is observed (Table U) and carbon dioxide is produced in smaller amounts. Inspection of Table 5 summarizing the influence of the H2/CO ratio on products selectivity also indicates a higher production of saturated hydrocarbons. This behavior is typical for cobalt catalysts in F-T synthesis (j2,25). The chain-length distribution is similar to that observed for catalysts derived... 

The potential importance of homogeneous catalytic reactions in synthesis gas transformations (i.e., hydrogenation of carbon monoxide) has been widely recognized in recent years. In the first place, such systems could provide structural and mechanistic models for the currently more important, but more difficult to study, heterogeneous catalysts. Secondly, product selectivity is generally more readily achievable with homogeneous catalysts, and this would be an obviously desirable feature in an efficient process converting synthesis gas to useful chemicals and fuels. 

The anodic oxidation of ethylene in a low temperature aqueous electrolyte electrochemical cell has been studied by Holbrook and Wise (24). It was found that product selectivity (carbonate, vs. ethylene glycol) depends on the potential of the silver anode. 

Titanosilicate ROH Reaction time (h) Conversion of propylene carbonate (mol%) Selectivity of transester product (mol%)a... 

Selective silylation of polychloromethanes using reactive metal electrodes such as zinc and magnesium has also been reported as shown in Scheme 37 [76, 77]. The electroreduction of carbon tetrachloride and chloroform in the presence of chlorotrimethylsilane affords the monosilylated and disilylated products. The product selectivity seems to depend upon the electrode material. 

The 16-electron ruthenium(Il) complexes [(tj -C5Me5)Ru(NHC)Cl] with steri-cally demanding NHCs catalyze the carbon-carbon coupling of terminal alkynes HC R (R = Ph, SiMes, rBu, p-Tol) under mild conditions. The product selectivity strongly depends on the substituent R." ... 

The Mills-Nixon hypothesis that small ring annelation on benzene would induce bond fixation (bond alternation) by trapping out one Kekul6 tautomer is a casualty of early twentieth century structural chemistry. Due to a lack of direct methods for analyzing molecular structure, structural postulates of that time were often supported by an analysis of product distributions. An experimental observable such as product selectivity or isomer count was correlated to an unobservable structural feature derived on the basis of a chemical model. Classical successes of this method are van t Hoff s proof of the tetrahedral carbon atom and Fischer s proof for the configuration of sugars. In the case of Mills and Nixon, however, the paradigm broke down. 

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