Dissolution, Depolymerization, and Retrogressive Reactions

Catalysts used in liquefaction can be classified in various ways as described below. 

The most conventional catalytic material since the work of Bergius has been iron sulfide in various types. Pyrite, pyrrhotite, and various nonstoi-chiometric sulfides are known, and pyrrotite is postulated as the active form. Its precursors are red mud, residue of bauxite after the separation of alumina, iron ores of various sources, synthetic and natural pyrites, fine iron particles, iron dust from converters, iron sulfate, iron hydroxide, etc. (32, 33). 

The next most widely used materials are Co-Mo and Ni-Mo sulfides, which have been widely used in petroleum refineries. They are usually supported on alumina of designed pore structures in which the pore diameter is usually larger than that for conventional petroleum residue (34, 35). 

A third type of material is the chlorides of transition metals, such as ZnCl2 and SnCl4 (36, 37). This group of catalysts works in molten state in contrast to the solid state of the previous two groups. The corrosive nature and instability may excludes their practical application. No details are reviewed here. 

Ru has been used as an additive to Co-Mo and Ni-Mo (38) to improve their hydrogenation and denitrogenation activities. 

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