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 1335.    ---------------.  [HALL, C. C., CRANFORD, S. R., AND GALL, D.] Interrogation of Dr. Otto Roelen of Ruhrchemie A.-G.  The Catalyst.  BIOS Final Report 447, 1945, pp. 2-9; An Account of the Development of Co-ThO₂-MgO Catalyst, by Dr. O. Roelen, Appendix III, pp. 48-52; PB 77,705; TOM Reel 226.

       Original Fischer-Tropsch catalyst, Co 100 : ThO₂ 18 : kieselguhr 100, was found to be slightly less active than one of composition Co 100 : ThO₂ 15 : kieselguhr 100.  This latter, however, had a short life (probably because of its high wax production) and a high CH₄ production owing to its high Co density.  Therefore, the kieselguhr content was increased to about 200 parts, and a constant Co density of about 80 gm. per l. of catalyst was established.  This appeared to be the minimum Co density allowable, since it is one of the main factors controlling the reducibility of the catalyst.  In the search for a catalyst promoter, ThO₂ had been chosen as having satisfactory properties.  It increased the activity, directed the synthesis to the production of the higher molecular-weight hydrocarbons, was easy to regenerate, was not sensitive to the influence of impurities, and reduced the thixothropic tendency of the wet catalyst filter cake to liquefy and destroy the essential colloidal structure.  Cu was tested as an additive because of the possibility of reducing the catalyst at a low temperature but was at once ruled out owing to its favoring sintering of the finely divided Co and thus reducing the active life of the catalyst; it also favors the rapid saturation of the catalyst with wax.  Mn was then tried and showed an effect similar to ThO₂, but its use was avoided because of the difficulties involved in the regeneration of the Co solutions.  Systematic investigation of MgO showed that it was one of the best activators available for Co catalysts and combined with ThO₂ produced a synergistic effect, which made the mixed Co-ThO₂-MgO-kieselguhr catalyst the best thus far produced.  Although Co-MgO catalysts are in themselves very sensitive to temperature and difficult to control, those containing both ThO₂ and MgO have the advantages of easier running, higher activity, easier regeneration, cheaper production, less subject to disintegration, and a well-balanced product.  MgO directs the synthesis toward the formation of lighter products, and ThO₂ directs it toward higher molecular hydrocarbons and waxes, thus giving a product having a boiling range intermediate between the 2.  The amount of ThO₂ can be reduced to as low as 5 parts giving a composition 100 Co : 5 ThO₂ : 8 MgO : 180-200 kieselguhr.  The limits defining the best ratios of the 4 constituents are not narrow, so that variations in composition of catalysts prepared on a large scale are virtually of no importance.  Kieselguhr as catalyst support was found to be the most suitable material investigated.  In reducing the catalyst with H₂ it was found that the most active catalysts were obtained by stopping the reduction before all the Co was reduced to metal; the most active catalysts had reduction values of 65-70%.  It was thought that the unreduced CoO may have a very important role in preventing sintering and, alternatively, that it may be a promoter for the synthesis itself.  It was the opinion that further improvements in Co catalysts would not be from the standpoint of yield but rather in the direction of catalysts favoring the production of special types of products.  Further investigations of CO-Mn-kieselguhr catalysts, which had a high activity at low temperatures (165°), with or without ThO₂, might be profitable.