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THE SYNTHESIS OF HYDROCARBONS AND CHEMICALS FROM CO AND H₂

SECTION IV

SUMMARY

The attached report covers the development of the isosynthesis by “Kaiser Wilhelm Institut” of Muelheim. The process includes the synthesis of low boiling isoparaffins (particularly isobutene) from CO and H₂ over thoria catalyst at pressures between 200 and 1000 atmospheres.

All work connected with this process was carried out in laboratory scale only.

ISOSYNTHESIS

Content.

1. General Introduction.
2. Chemistry of Synthesis.
3. Catalysts.
   1. Thorium Catalyst
   2. Mixed Catalyst
4. Operating Conditions
   1. Influence of Temperatures
   2. Influence of Pressure
5. Products
6. Conclusion
7. List of References

ISOSYNTHESIS

1. General Introduction. (See also ref. IV/2 at the end of this section)

The synthesis of low boiling isoparaffins was discovered in the laboratories of the Kaiser Wilhelm Institu at Muelheim. The discovery was made accidentally, when various oxides were studied for their use as catalysts in the synthesis of aromatics. it was observed that thorium oxide gave a high percentage of iso paraffins.

The process is a variation of the methanol or rather isobutyl synthesis. It operates at temperature above those used in methanol synthesis, although CH₃OH is believed to be an intermediate product. The temperature range for the synthesis is limited on the one hand by the decomposition of CO(carbon deposit) at roughly 550° C. This limit is constant for all pressures. The lower temperature limit on the other hand is a function of the operating pressure. At 1000 atmospheres the isosynthesis may be carried out at 400°C. At 100 atmospheres a minimum of 450° C is required. More detailed information is presented in the following paragraphs.

2. Chemistry of Synthesis

The Kinetics of the Synthesis is assumed to be as follows:

1. Formation of Methanol from CO and H₂
2. Dehydration of alcohol and immediate reaction with additional CO and H₂ to isobutylene and higher isoparaffins.
3. Polymerization of isobutylene possibly followed by hydrogenation of alkylate, or formation of dimethylether from methanol through addition of CO and H₂

At very high pressure the reaction proceeds in the following direction:

CH₃OH + CO + 2H₂------------CH₃OCH₃ + H₂O

The dehydration of the methanol is an important step and operation at temperatures above those favoring methanol formation are specified.

Isobutane is one of the main products but the reaction is necessarily not as clear cut as presented above. It is also noteworthy that a small fraction of naphthenes and aromatics is usually found in the products.

The theory is in good agreement with the fact that the addition of dehydration catalyst to the thorium oxide has a beneficiary effect on the yield.

3. The Catalysts

It was pointed out that the catalyst used initially was pure ThO2. some 10% alcohols (mostly water soluble) are obtained with thorium alone. The addition of aluminum (or other dehydroating catalysts )yields a product almost free of oxygenated compounds.

(a) Thorium Catalyst

Basic thorium carbonate is precipated with soda from thorium nitrate solution and the precipitate washed free of alkali. Even small traces of alkali lower the catalyst activity and require higher synthesis temperatures. The filtered thorium carbonate is dried at 110° C, pelleted, and finally treated at 300-400° C with air passing through for 1 to 2 hours (no reduction is necessary.

The thoria catalysts are outstanding in their insensitivity to suphur poisoning. Treatment with H2S or CS2 does not effect the activity; even the use of (NH4)2S for precipitation of the thoria gives normal conversion.

The long life of this catalyst is remarkable. It may be used several months without sign of aging. Even in case of carbon deposits and the resulting increase in pressure drop through the bed, the original activity can be restored by passing air over the catalyst at synthesis temperature.

The results obtained on pure thoria catalyst are given in the section on operation.

(b) Mixed Catalyst

The high price of thoria and possibly its lack in Germany let to attempts to use substitutes and it was found that most dehydrating catalysts such as oxides of aluminum, sirconium, tungsten, or rare earths can be used provided that sufficiently high pressures are applied. On 2 October 1943, a patent application was filed disclosing the use of these substances either alone or in a mixture of thoria.

It is claimed that these dehydrating substances direct the synthesis towards oxygenated compounds below a certain temperature. Beyond this limit, however, isoparaffins are the main product. In addition to the dehydrating components it was found that substances which catalyze the formation of CH₃OH at temperatures below those of the isosynthesis are successfully used as additional components in the catalyst. Zinc oxide is particularly named. It is, however, necessary that the dehydrating component be present in excess over the methanol forming part.

The preferred catalyst giving the best results according to the present day development is a thoria-alumina two component catalyst.

Thorium carbonate and alumina are precipitated separately. The precipitates are washed, mixed and dried at 300° C. Dilute solution gives less dense catalyst. The apparent density of pure thoria catalyst is 1.6-1.8 (with concentrated solution 2.4 can be reached). The percentage of alumina based on thoria varies from 30-40%. Contents under 20% show no effect. Above 40% CH₄ formation becomes excessive.

The alumina-zinc oxide (1:1) catalyst does not give comparable yields, only 100 gm/m³ feed are recovered, compared to 130 gm/m3 feed with thoria-alumina. It is, however, possible to use a two stage operation and thus obtain almost the same result with the cheaper catalyst.

4. Operating Conditions. (see also ref. IV/3 and IV/4 at the end of this section).

The CO and H₂ are consumed approximately in the ratio CO:H₂-1:1.2 with most the oxygen being removed as CO₂. The CO₂ content in the exit gas is approximately 30%.

The synthesis is carried out in a once through operation (no”Kreislauf”) with the feed gas containing COH₂ in the ratio they are consumed.

70-75% Conversion is obtained at average space velocities of 1350 V/H/V (10 times higher than in ordinary F.T. operation). Thee tests were carried out in 15 mm. and 25 mm. tubes of chrome nickel steel or copperclad steel.

500 lit/hr. was the maximum feed used in the laboratory to date. For larger scale operation the use of superheated steam or molten salt was considered as a cooling medium.

(a) Influence of Temperatures

The following table is based on operation at 1300 V/H/V and 150 atm. pressure: pure thoria catalyst.

With increasing temperature the spectrum moves toward lower boiling products, while the oxygenated products disappear to be largely replaced by aromatic compounds.

The reaction is not very sensitive to temperature change, ± 10° C are acceptable variations.

(b) Influence of Pressure.

The following table is based on operation at 450° C and 1300 V/H/V in a copperclad tube over pure thoria catalyst.

Use of chrome nickel tubes gave somewhat higher yields. The increased pressure and the correspondingly longer contact time increased the CO conversion. At the same time this increased conversion is not endangered by carbon decomposition which is less (at a given temperature) at increased pressure.

There is a slight carbon deposition in the course of the synthesis which makes it necessary to burn off the carbon from the catalyst. This is done every 3-4 weeks with air or air-recycle gas mixture at the synthesis temperature,

5. Products.

The synthesis products obtained with this process are mostly low boiling isoparaffins with isobutene the largest individual component.

Based on operation at 150 to 450° C 1300 V/H/V, and pure thoria catalyst the following product was obtained:

(Note: This same test shown in the first table under 450° C temperature).

The Octane Number (motor method) of the gasoline fraction varies from 79-85 clear.

6. Conclusions.

The isosynthesis is of technical interest as a directed reaction of CO and H₂. By the use of particular catalysts and operating conditions, CO and H₂ can be combined to yield a hydrocarbon mixture consisting predominantly of isoparaffins.

The high temperatures and high pressure required for this synthesis would be a debit to its commercial use. As a fuel, the isosynthesis product would be premium quality, but the components are otherwise of no special interest today.

7. List of References.

1. Patent application by KWI dated 18 December 1942. “Verfahren zur Kataytischen Syntheses von Kohlenwasserstoffen aus CO und H₂”.
2. Protocol of meeting, KWI and RCH, 2 June 1943.
3. Progress report on development of Synthesis from CO and H₂ particularly synthesis of isoparaffins, by KWI, December 1942.
4. Patent application by KWI dated 2 October 1943. “Verfahren zure Herstellung von klopffesten Kohlenwasserstoffen”