Early coal hydrogenation catalysis

Three inputs were necessary to make catalytic hydrogenation of coal possible. One was the ammonia synthesis which, in 1910, introduced high pressure and temperature into the chemical industry. The second was the experimentation by F. Bergius who showed, in 1913, that coal can be liquefied by adding hydrogen at high pressure and temperature. The liquid products were similar to coal tar. They were not of the quality required for gasoline or diesel fuel production. The use of catalysts to refine the coal oil appeared then to be hopeless since coals contained sulfur, a poison for all then known hydrogenation catalysts. The third input was methanol synthesis in 1923. M. Pier found selective, oxidic catalysts that were less sensitive to sulfur than e.g. the metallic catalyst for the ammonia synthesis.In 1924 M. Pier, in the laboratories of the BASF, prepared sulfur resistant coal hydrogenation catalysts: sulfides and oxides of molybdenum, tungsten, and the iron group metals. With these catalysts it became possible to add hydrogen; split carbon-carbon bonds; and eliminate such heteroelements as sulfur, oxygen and nitrogen from coals and oils. Thus fuels were produced that met petroleum fuel specifications.Optimum catalyst action was achieved by subdividing coal hydrogenation into two stages. The coal was converted, with a dispersed catalyst in the “liquid phase”, into middle oil. This was then hydrogenated over fixed bed catalyst, in the “vapor phase”, to gasoline. On this basis a large scale demonstration plant for the liquefaction of central German brown coal was erected in 1927.The development of catalysts for these two stages proceeded on different routes. Liquid phase catalysts were discarded after one pass through the reactor. They were cheap, or used in very small amounts. It was found soon that coal of different rank required different catalysts, and that the mineral matter of the coal played an important role.The first commercially used vapor phase catalysts were of the hydrorefining type. Hydrocracking activity was achieved by using high temperatures. A great step forward was made in 1930 when a special preparation of tungsten disulfide permitted hydrocracking activity at low temperatures. Thus the first essentially dual function catalyst was found. Its hydrocracking activitity was further increased, and gasolines with a higher octane number were obtained by using it on acidic supports such as materials containing alumina-silica.Such supported catalysts were poisoned by the nitrogen compounds present in coal oils. Therefore a refining step for these oils was needed. The vapor phase was subdivided into the “prehydrogenation” (hydrorefining) and “splitting hydrogenation” (hydrocracking) steps. Further development of catalysts with specific functions for these two steps proceeded rapidly. In addition, separate catalysts were developed for the production of gasolines with a high content of aromatics.The various catalysts developed primarily for the hydrogenation of coal derived oils introduced hydrogen processing into the petroleum refining industry. There they were further modified and improved for the processing of petroleum. These improved catalysts, in turn, will be of help to a future coal liquefaction industry.