Infrared Spectroscopic Study of Coal Depolymerization Catalysis by Metal Chlorides

Diffuse reflectance infrared spectroscopy was used to monitor in situ reactions in which ether linkages undergo cleavage during the depolymerization of metal chloride impregnated Wyodak coal at elevated temperatures. Subtraction procedures were used to obtain difference spectra that reveal small net changes in the IR band intensity between 2000 and 1000 cm^-1. The spectra suggest that the ZnCl₂-impregnated coal samples could be depolymerized in an inert argon atmosphere. However, the extent to which the ZnCl₂-catalyzed ether linkages are cleaved under an inert atmosphere is only about 30% of that for a high-pressure hydrogen atmosphere (68 atm). Phenyl ethers of the form Ph-O-CH₂-R are cleaved catalytically by metal chloride, and the oxygen is retained as a phenolic group. Both the FeCl₃and CrCl₃catalysts have a higher reactivity and selectivity for the cleavage of ether linkages than does the ZnCl₂catalyst. A higher degree of catalytic cleavage of phenyl ether is obtained when the ZnCl₂-impregnated coal is prepared in methanol as compared to samples prepared in water and acetone.