Fractionation of coal extracts prior to hydrocracking: An attempt to link sample structure to conversion levels and catalyst fouling

Catalyst fouling during hydrocracking and conversions of larger molecular m ass components have been investigated in terms of the structural features o f a bituminous coal extract. The sample has been separated into two pairs o f fractions: pentane-soluble (PS) and -insoluble (PI); toluene-soluble (TS) and -insoluble (TI). Differences between hydrocracked products and levels of carbon-deposition on a commercial presulfided NiMo/gamma -Al2O3 catalyst have been examined. Size exclusion chromatograms (SEC) showed MM-distribut ions of the samples decreasing in the order: TI > PI > TS > PS. This trend closely paralleled those given by TGA-derived boiling point distributions a nd the ordering of UV-fluorescence (UV-F) derived spectral shifts. In SEC, two columns with different operating ranges of molecular sizes were used. R esults indicated that the largest molecular mass material did not pass thro ugh the column with the smaller molecular size range and was lost for analy tical purposes. Within the range where probe mass spectrometry is capable o f observation (up to similar to 600 u), the hydrocracked products of all th e fractions studied contained similar ranges of molecular species; in contr ast with data from TGA, SEC, and UV-F. The differences between hydrocracked products from different fractions were confined to masses beyond the range of detection by probe mass spectrometry. A reliable correspondence was fou nd between catalyst fouling levels and the concentration of > 450 degreesC bp material in the feed. Our results are consistent with a model of the lar ger extract molecules, where large (> 300 u) polycyclic. aromatic (PCA) rin g systems are embedded within a matrix held together by several different s tructural types of bridges. During hydrocracking, bridging structures betwe en PCA ring systems break down although most PCA ring systems remain unalte red. It is thought that larger PCA groups liberated by the hydrocracking pr ocess are more likely to deposit on catalyst surfaces.