VII. ISO-PARAFFIN SYNTHESIS
This development has been carried out on a laboratory scale only, having been initiated by the Kaiser Wilhelm Institute für Kohlenforschurg in October, 1941. Two preliminary reports containing descriptions of this development have been independently prepared as a result of interrogation of Kaiser Wilhelm Institute personnel. (3) (17)
The process is similar to the normal Fischer-Tropsch process only with respect to the fact that a mixture of hydrogen and carbon monoxide is used as synthesis gas, but the pressure and temperature used far exceed those of Fischer-Tropsch and an entirely different catalyst is employed. Good yields of iso-paraffins are obtained, as contrasted with the straight-chain hydro-carbons obtained in the normal Fischer-Tropsch process.
The reaction is carried out at 450°C. and 300 atms. pressure. The reaction does not take place at atmospheric pressure, but as the pressure is increased, the yields are improved. Above 300 atmospheres, oxygen-containing compounds begin to be formed and in increasing quantities. The throughput used in this process is 5 to 10 times higher than that used in the regular Fischer-Tropsch process. An example would be the processing of 20 liters of gas (NTP) over 30 gms. of ThO2 per hour.
B. Catalysts
A number of catalysts have been found which are suitable of reaction. These are:
| 1) ZnO - Al2O3 | 4) Al2O3 |
| 2) ThO2 | 5) ZnO with ThO2, or ZrO2 |
| 3) ThO2 - Al2O3 |
The most efficient catalyst for this synthesis among those listed above is the thoria-alumina catalyst, although the zinc oxide-alumina appears to be nearly as good and much cheaper. for each of these catalyst it was found necessary to burn off the carbon about every two weeks. Such catalysts have been used continuously for over six months with no appreciable decline in activity.
In the case of the ZnO-Al2O3 catalyst, an equimolar mixture of the two components is used. When more Al2O3 is employed, more C3-C4 is obtained, while when more ZnO is present, the proportion of higher compounds is increased. The catalyst is prepared from the nitrates, the dilution being such that about 2-3 liters of water are used to dissolve the required quantity of the nitrates to give 100 gms. of final catalyst. the solution is heated to boiling and a solution of sodium carbonate is added rapidly. The precipitate is washed free of alkali, dried at 110°C. for overnight and then heated in air for 2-3 hours at 300°C. a better catalyst is prepared by individual precipitation of ZnCO3 and Al(OH)3 and mixing the two fresh slurries. The aluminum hydroxide is best prepared by reacting a solution of sodium aluminate with carbon dioxide or sulfuric acid.
The thoria catalyst is of interest because this catalyst is capable of performing all the function required in the reaction. This catalyst is prepared by dissolving thorium nitrate in water (5 1. for 100 g. of ThO2) heating to 100°C. and rapidly precipitating with sodium carbonate (no excess of sodium carbonate should be used) while stirring. This is followed by washing until free of alkali, drying at 110°C. and further drying in air at 300°C. for 2-3 hours. the cake is broken up in 2-3 mm. granules before the final drying and used as such in the synthesis. With this catalyst , using a throughput of 20 1. of synthesis gas (NTP) per 30 g. of ThO2 per hour the yield of butane and higher hydrocarbons per cu.m. of synthesis gas is 60-80 g. In addition, a small amount of water and water soluble alcohols, such as methanol, is obtained.
A better catalyst then thoria alone is the thoria-alumina catalyst, used in equimolar quantities. The preparation consists in mixing fresh slurried which have bee previously washed free of alkali and then boiling off the water from the slurry mixture. Filtration apparently requires too long a time. The yield of C4 and higher hydrocarbons using this catalyst is 100-110 gms. per cu. m. of gas.
The use of alumina alone as a catalyst is not recommended since this catalyst produces too much carbon, although the reaction does take place to give iso-compounds. The zinc oxide-aluminum oxide catalyst produces slightly more alcohols than the thoria-alumina catalyst. The carbon formation with the zinc oxide-alumina is lower than with the thoria-alumina catalyst, the former catalyst remaining white for long periods of time. The life of the catalyst depends upon the severity of the treatment; the regeneration with air does not produce a catalyst activity decrease. The frequency of regeneration is again dependent upon the temperature used during the synthesis; with the zinc oxide-alumina catalyst, three regenerations are required in 4-5 months.
The preferred composition of the synthesis gas for this process is 1.2 CO:1H2. Increasing the hydrogen increases methane formation: decreasing the hydrogen lowers the overall yield.
D. Product Yields:
The yield of product varies from as low as 60 gms. of C4 and higher to 110 gms. of the same fraction depending upon the catalyst. With the ZnO-Al2O3 catalyst the yield of the C4 and higherfraction is 80 gms. per cu.m. The yield of propane under these conditions is 10 gms. The higher compounds are produced in the following proportions:
| C4H10 | (90% isobutane) | 60-70% by weight |
| C5H12 | (96-98% isopentane) | 20-30% by weight |
| C6, C7 and C8 compounds | small amounts. | |
The C6 fraction contains no neohexane, but primarily 2 and 3-methylpentanes and no normal hexane. Some napththenes have been found in the higher boiling fraction. The yield of the C6-C8 fraction is higher at the lower temperature. Aside from the hydrocarbons produced, some alcohols, amounting to less than 10% of the hydrocarbon production, are also formed. Thiss lower layer of alcohols consists of a small amount of methyl alcohol and a large amount of isobutanol. The exit gas from the iso-synthesis consists of 30% CO2, 20% N2, 10% CH4 (Max.), the rest being CO and H2.
Although this development is now only in a laboratory stage, the potential application of iso-paraffin synthesis to manufacture of high-octane automotive fuel is quite evident.