95/05823 The role of thermal hydrogen-transfer processes in catalytic coal liquefaction

Abstract

The complexity of coals and their liquefaction products unavoidably means that no {open_quotes}model compound{close_quotes} system can be found that even remotely approaches the real coal in behavior, without also approaching it in intractability of chemical analysis. Notwithstanding these limitations, much has been learned by detailed examination of coal liquefaction phenomenology in light of the behavior of {open_quotes}model{close_quotes} or surrogate structures under liquefaction conditions. Recently we have reviewed some of our own research and other related work that highlights some of what has been learned following this approach. That review summarizes some of the limitations of the formerly accepted weak-bond scission view of donor-solvent-mediated H-transfer in actively promoting bond scission is considered. In addition, the preceding papers in this symposium have highlighted various hydrogen-transfer mechanisms and some features (and questions) that have evolved during the development of direct liquefaction processes. In this paper, we attempt to build on the background of model compound studies and process development results. We begin by emphasizing two points that have sometimes been lost in discussions of radical reaction mechanisms of hydrocarbon structures. First, high-temperature reactions of coals are not necessarily limited to processes that are purely free-radical: the polar functional groups on coals make itmore » very likely in fact that reactions involving charge separation (i.e., {open_quotes}ionic{close_quotes} reactions) can play some key roles in high temperature coal conversion processes. Second, we assert {open_quotes}thermal{close_quotes} and {open_quotes}catalytic{close_quotes} reactions should not be considered as two separate realms of the universe, but as classes of reactions that are both important in virtually all catalytic liquefaction processes.« less