Cyclohexane diffusion

The conclusion of all these thermodynamic studies is the existence of thiazole-solvent and thiazole-thiazole associations. The most probable mode of association is of the n-rr type from the lone pair of the nitrogen of one molecule to the various other atoms of the other. These associations are confirmed by the results of viscosimetnc studies on thiazole and binary mixtures of thiazole and CCU or QHij. In the case of CCU, there is association of two thiazole molecules with one solvent molecule, whereas cyclohexane seems to destroy some thiazole self-associations (aggregates) existing in the pure liquid (312-314). The same conclusions are drawn from the study of the self-diffusion of thiazole (labeled with C) in thiazole-cyclohexane solutions (114). 

Cyclohexane, produced from the partial hydrogenation of benzene [71-43-2] also can be used as the feedstock for A manufacture. Such a process involves selective hydrogenation of benzene to cyclohexene, separation of the cyclohexene from unreacted benzene and cyclohexane (produced from over-hydrogenation of the benzene), and hydration of the cyclohexane to A. Asahi has obtained numerous patents on such a process and is in the process of commercialization (85,86). Indicated reaction conditions for the partial hydrogenation are 100—200°C and 1—10 kPa (0.1—1.5 psi) with a Ru or zinc-promoted Ru catalyst (87—90). The hydration reaction uses zeotites as catalyst in a two-phase system. Cyclohexene diffuses into an aqueous phase containing the zeotites and there is hydrated to A. The A then is extracted back into the organic phase. Reaction temperature is 90—150°C and reactor residence time is 30 min (91—94). 

Fig. 3.1.7 The surface diffusion coefficient / surface of cyclohexane (squares) and acetone (circles) in porous silicon with 3.6-nm mean...

Fig.3.1.9 (a) The adsorption-desorption isotherm (circles, right axis) and the self-diffusion coefficients D (triangles, left axis) for cyclohexane in porous silicon with 3.6-nm pore diameter as a function of the relative vapor pressure z = P/PS1 where Ps is the saturated vapor pressure, (b) The self-diffusion coefficients D for acetone (squares) and cyclohexane (triangles) as a function of the concentration 0 of molecules in pores measured on the adsorption (open symbols) and the desorption (filled symbols) branches. 

Fig. 45a, b. Segmental diffusion in dilute solutions at the crossover from - to good solvent conditions. Reduced characteristics frequencies Qred (Q,x) vs. x = (T — )/ at different Q-values a PDMS/d-bromobenzene b PS/d-cyclohexane. (b reproduced with permission from [115]. Copyright 1980 The American Physical Society, Maryland)... 

With the advent of picosecond-pulse radiolysis and laser technologies, it has been possible to study geminate-ion recombination (Jonah et al, 1979 Sauer and Jonah, 1980 Tagawa et al 1982a, b) and subsequently electron-ion recombination (Katsumura et al, 1982 Tagawa et al, 1983 Jonah, 1983) in hydrocarbon liquids. Using cyclohexane solutions of 9,10-diphenylanthracene (DPA) and p-terphenyl (PT), Jonah et al. (1979) observed light emission from the first excited state of the solutes, interpreted in terms of solute cation-anion recombination. In the early work of Sauer and Jonah (1980), the kinetics of solute excited state formation was studied in cyclohexane solutions of DPA and PT, and some inconsistency with respect to the solution of the diffusion equation was noted.1... 

Xanthylidene also does not react measurably with 02. The lifetime of XA is the same in 02-saturated cyclohexane as it is in solutions which have been deoxygenated. Bearing in mind that triplet carbenes react with 02 at nearly the diffusion limited rate, if 3XA were in rapid equilibrium with XA, then 02 should shorten the apparent lifetime of the singlet by reacting with the triplet. 

The zeolite structure also plays a large role in RC product distribution. Weitkamp et al.62 conducted experiments with Pt/HZSM-5 catalysts, which have very narrow pore sizes when compared with other zeolites, such as USY or SAPO. They found that c is I trans-1,3-dime thylcyclopentane was formed, while 1,1 and 1,2-DMCP were not. This indicates that the more oval shaped 1,3-DMCP was able to diffuse through the pores, while the more bulky and spherical isomers were not, and thus not seen in the product distribution. In short, when compared with dealkylation to cyclohexane, ring contraction of MCH is a more effective pathway to yield higher ON products. However, in order to further improve the ON, ring-opening of the RC isomers may be necessary, as shown below. 

Studies on the quenching of photoexcited 9-phenan-thrylmethyl pivalate by MEK, a model forathe-PMMA-Phe/MEK system, provide a value of k 7.3 X 108 M s in cyclohexane (11). This value is nearly one order of magnitude lower than the diffusion-controlled rate. For reactions in which a diffusion step precedes a chemical step, the relationship between k and kd ff is given by ... 

In the case of polyethylene, the volatile component was cyclohexane and here, too, good agreement was obtained between the measured exit concentration and the predicted values using Pe = 40 in the solution of Eq. (38) (see Fig. 16). Tliese data also suggest that mass transfer is occurring by molecular diffusion through a wiped film since the exit concentration varies with N in accordance with the theory. It is somewhat disconcerting, however, that the value of B used in the theoretical expression was not reported, and the question naturally arises as to whether realistic values were used to obtain the fit with the data. 

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