5. Products from Synol Operation. (see reference II/14 to II/18 at end of this section).
(a) Alcohols.
It has been stated that the main objective of the synol process was the production of straight chain terminal alcohols of boiling range as high as C20. Thus a new class of chemicals had been developed from a laboratory curiosity to a commercial product. It has already been described how this objective could be reached, but the inventors themselves could not give an exact answer why it was possible to do it by way of the process described.
One German expert offered the explanation that alcohols are the primary products when the synthesis is carried out at low temperatures. They are dehydrated to olefins in a second step. This assumption may find some basis in the fact that the sum total of alcohol plus olefine in synol operation is fairly constant over a wide temperature range.
The products leaving the reactor are recovered in an oil and water phase, the water containing up to 25% organic compounds. These compounds include low boiling alcohols, ketones, aldehydes, acids and esters, and some salts of short-chained fatty acids. The oil phase contains enough high boiling products to solidify at room temperature. The methods used to separate the products are given in a later section.
The constitution of the products has been studied in great detail. The alcohols, up to C9 are practically straight chain primary alcohols. No secondary alcohols can be found. The same is substantially true of the higher alcohols (methanol and ethyl alcohol are produced only in traces).
(b) Olefines.
The olefins are mostly a-Olefines. A C12 olefine was analyzed and found to have 60% of the double bond in a-position, while the rest was distributed with decreasing percentage toward the middle of the molecule. It was concluded that primarily all olefins are terminal, but due to isomerization the double bond is moved towards the middle of the molecule. Iron carbonyl is believed to catalyze this shift. A similar effect was observed in the oxo-synthesis where the presence of cobaltcarbonyl causes a shift of the double bond.
The sum of alcohols plus olefins in the product is between 70-80%, the alcohol ranging around 60% and over.
(c) Esters.
Esters are always present in certain fractions. That arsenic in the catalyst increases ester production up to 25% has already been stated. The ester content generally increases with the boiling range. Esters make up as much as 10% in the C10 alcohol range and increase to 20% in the C18 range.
The formation of the ester (and acid ) has been explained by the direct reaction of CH2 radicals with CO and water. In the preparation of fatty acids from alcohols and CO, the following chain of reactions is assumed to take place:
| (1) Decomposition of the alcohol to free radical and water | |
|
CH3 OH→CH2=H2O |
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| (2) Reaction of radical and CO to form ketene | |
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CH2=+CO→H2 C=CO |
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| (3) Ketene may react with water or excess alcohol to form the corresponding acid or ester: | |
| (a) H2C=CO+H2O→CH3 C O O H | |
| (b) H2C=CO+CH3OH→CH3 COOH3 | |
The free radical is of course extremely short lived and present in only small concentration. It is therefore necessary to operate with high CO concentration to have the necessary acceptor ready to react with CH2 the moment it forms. This explains the high CO and low alcohol concentration required for this process. The theory may be applied to FT or synol syntheses and offers a good explanation for the high acid and ester yield. The presence of CH2 radicals in the hydrogenation of CO is generally acknowledged and with CO and H2O (or alcohol) also present , the synthesis to acid and ester could follow the chain of reactions described above.
Aldehydes and ketones are both found in the product. The lower aldehydes can be extracted with bisulfite solution. The ketones are more stable and give no trouble. During the separation of the alcohols with boric acid they remain in the neutral oil. The lower ketones (acetone) are found mostly in the aqueous phase. There are some other classes of oxygenated compounds found in the primary product such as esters and unsaturated ethers, but they rarely make up more than 1% of the total. The products recovered and isolated so far are mostly the alcohols from C1 to C22.
The products obtained from synol operation according to “Benzin-Faheise” resemble those contained in FT synthesis over Fe catalyst.
The gasolines from synol must be refined before they can be used as motor fuel. They are unstable and corrosive and do not have a sufficiently high octane number (see reference II/17 at the end of this section).
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A raw gasoline had the following properties: |
||
|
Distillation |
1BP |
35° C |
|
50% |
108° C |
|
|
90% |
148° C |
|
|
EP |
200° C |
|
|
Gum (after 7 days) |
202 mg. |
|
|
Cu strip |
colors gasoline blue |
|
|
OH number |
191 |
|
|
Octane No. (res.) |
54.0 |
|
The gasoline was refined over Fullers earth at 300° C., using 0.5-1 volume liqu./hr/volume Fuller’s earth. The resulting product was as follows:
|
Distillation |
1BP |
34° C |
|
50% |
90° C |
|
|
90% |
180° C |
|
|
EP |
198° C |
|
|
Gum (after two days) |
0.5 mg |
|
|
Cu strip |
Negative |
|
|
OH number |
6.5 |
|
|
Octane No. (res.) |
72.5 |
The treatment consists essentially in dehydration of the alcohols to olefins and condensation of the aldehydes to high boiling polymers which are fractionated out.
The losses depend of course on the alcohol content and thus on the temperature of the synol operation itself. The increase in ON is substantial, but I.G. had still further considered the use of olefine isomerization to shift the terminal double bond towards the middle of the molecule as an additional means to improve the knockrating.
The diesel oils obtained in this operation do not compare with those from cobalt catalyst FT plants. 62 Cetane number was given as an average value.