In the hydroprocessing of resid, catalyst selectivity and the relationship between metals and sulfur distribution in the resid are key factors in achieving required product specifications. Using gradient elution chromatography as a method of analysis, we have found that catalyst pore size distribution can significantly affect selectivity for different metal and sulfur components in residua. A small pore catalyst was observed to be more selective for non-asphaltene sulfur and metals, while a large pore catalyst was found to be very selective for asphaltene metals and sulfur. This variation of catalyst selectivity is a result of the concentration of metals and sulfur in different portions of the resid. For typical Middle Eastern residua, about 70–75% of the sulfur was found to be present in low molecular weight less aromatic GEC fractions, while 95% of the metals were in highly aromatic high molecular weight fractions. Thus the variation of selectivity with pore size was found to result from concentration of the sulfur and metal species in molecules of different size.
It is generally believed that most chemical carcinogens exert their effects through covalent modification of the nucleic acid base sites. Significant clarification of the underlying chemistry involved in these early and critical events is available through the techniques of computational chemistry. The role of semiempirical molecular orbital theory in this area is reviewed using examples focusing on 1.(i) the conversion of precarcinogens to reactive electrophiles,2.(ii) the regiochemistry of nucleic acid adduct formation,3.(iii) the chemical and physical consequences of nucleic acid base modification, and4.(iv) empirical structure activity relationships.
The symbiotic relationship that can exist between the semiempirical and ab initio molecular orbital procedures is emphasized, as is the continuing niche for the semiempirical methodology in the study of systems that are too large, or in situations that are insufficiently cost effective, for the application of ab initio methods.