Dehydration cyclohexanol

Figure 5.1 illustrates the key reactions implied in the manufacturing of cyclohexanone by phenol hydrogenation. The reactions are of consecutive type, in which the desired product is an intermediate. Small amounts of cyclohexene might appear at higher temperature by cyclohexanol dehydration. Additional reactions can lead to heavies by polymerization or benzene and cyclohexane by disproportionation. 

The results obtained in this study indicate that in Al-ffee H-boralite (BOR 1) only weak BrOnsted acid sites (Si—OH—B) are present. They are active only in cyclohexanol dehydration. Their catalytic activity is, however, relatively low. The insertion of A1 into the framework results in the creation of strong Bronsted acid sites. Most probably they are Si—OH—Al, the same as in zeolites. The IR band which could be characteristic of such Si—OH—Al (at about 3610 cm ) was not seen in the spectrum because of the very low concentration of these hydroxyls. The catalytic activity of Si—OH—Al is much higher that of Si—OH - B. Contrary to Si—OH -B, Si—OH— A1 are active in consecutive reactions of cyclohexene (isomerization and disproportionation). Cyclohexene isomerization (to methylcyclopentenes), a typical carbenium ion reaction is catalysed by strong Brdnsted acid sites even at temperatures as low as 450 K. The same strong Bronsted acid sites catalyse also cyclohexene disproportionation (to cyclohexane, methylcyclopentane and coke). Our earlier... 

In the above mentioned studies, the considered solids exerted no influence in the absence of radiation. Methanol synthesis and cyclohexanol dehydration (Section II,H,3) are on the contrary studied in the presence of other solids which already, in the absence of radiation, have certain catalytic properties. 

Balandine et al. (15, 16) have studied cyclohexanol dehydration in the presence of catalyst composed of MgS04 and Na2S04 mixed in variable proportions. These experiments are carried out at a temperature between 325 and 420°C. These authors compare the catalyst activity in the absence and then in the presence of radiations. 

The results of cyclohexanol dehydration experiments in the presence of various sulfates show insulators to possess interesting activation possibilities, which differ depending on whether the irradiation is carried out before or during the chemical reaction. 

Fio. 5. The relation between the specific radioactivity of MgS04-Na2S04 and its catalytic activity for cyclohexanol dehydration at 410° (157). 

A clue to two sources of difficulty in the catalytic work was given by the only published repetition of the work by another laboratory. In the catalysis of cyclohexanol dehydration by magnesium sulfate-sodium sulfate, Krohn and Smith 170) found indeed that the catalytic activity per gram was greater for the radioactive samples than for a nonradio-active one, but this was traced to a larger surface area. If the catalytic activity was calculated for unit surface area, it was about half that on a nonradioactive sample. In the second place, it was found that the activity of both radioactive and nonradioactive catalysts declined linearly with time, a behavior that would explain the linear enhanced activity vs log(specific activity) curves obtained in the original work. 

Our results on esterification over zeolites HP, HZSM-5, HY, DHY and y-Al203 clearly show that zeolite HP is the most suitable catalyst containing the required type of acidity suitable for esterification reaction and this is also confirmed by cyclohexanol dehydration activity (99.9% conversion) an acidity index reaction. 

CHnoptilolite catalysts converted cyclohexanol only on the outer surface, which was foimd by the hnear relationship between the amount of 2,6-di-terf-butylpyridine adsorbed on the catalyst and the rate of cyclohexanol dehydration. Note that the 2,6-di-tert-butylpyridine molecule is too large to enter the zeolite structure and, therefore, only probes acid sites on the outer surface of the zeolite crystals [190]. For mordenite, a nearly hnear relationship between the concentration of adsorbed pyridinium ions and the catalytic activity was observed. Note that this relation was the same if plotted against the concentration of acid sites determined by the adsorption of 2,6-di-tert-butylpyridine. Therefore, Karge et al. [189] concluded that cyclohexanol is only converted in the outer shell of the zeolite crystals and that the acid sites are distributed homogeneously over the particles. 

Fig. 18 Experimental and simulated radial and axial temperature profiles of a pilot scale reactor for cyclohexanol dehydration...

Dehydration is very often accompanied by the subsequent isomerization of primary products. Isomerization may be avoided by poisoning acidic sites with alkali meted ions, ammonia, or organic bases. Cyclohexanol dehydrates to cyclohexene over alumina containing 0.4% sodium or potassium ions, but gives a large amount of methylcy-clopentenes over pure alumina. The cyclopentenes do not arise direcdy from cyclohexanol, but by the isomerization of cyclohexene, the primary product. The selectivity to 3-methyl-l-butene is significantly improved by adding small amounts of base to 7-alumina in the dehydration of 3-methylbutanoL... 

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