Energy & Fuels 1994,8, 2
2
Symposium
Proceedings of an ACS Division of Fuel Chemistry Symposium on Iron-Based Catalysts for Coal Liquefaction: An Introduction For the past several years, there has been intensive research on the use of small amounts (0.2-1.095)of the fine particle size (C20 nm) iron-based catalysts for direct coal liquefaction (DCL). This has been stimulated by the recognition by many coal scientists around the world that the first-stage reactor in direct coal liquefaction needs to employ a catalyst that is both inexpensive and disposable in order to develop an economically viable technology. Iron-based catalysts are cheap, environmentally benign, and catalytically active for DCL, making them a logical choice. Leading scientists from the United States and abroad met at the 1993 Spring National Meeting of the American Chemical Society in Denver, Colorado, to present their results on this important topic and to exchange ideas. All scientists participating were invited to write up their results in detail for a special issue of Energy & Fuels. In all, 35 papers were presented, of which 20 are submitted for publication in the current volume. The papers deal with five principal topics: synthesis and testing of small-particle catalysts, catalysts prepared by impregnation methods, mechanisms of action of iron-based catalysts, characterization of these catalysts, and technology development for DCL using iron-based catalysts. A number of methods are presented for the preparation of small-particle iron-based catalysts. These include chemical precipitation techniques, laser pyrolysis of iron carbonyl with other compounds, flame pyrolysis of volatile iron compounds, rapid thermal decomposition of aqueous solutions containing iron salts, inverse micelle development from solution, nucleation from aerosols, low-temperature disproportionation of ferric sulfide, and the use of ultrafhe iron oxides from natural sources. A point that emerged clearly from these papers was that numerous methods had been developed to prepare ultrafine iron catalysts, usually in an initial oxide or oxyhydroxide form, with sizes ranging from a few nanometers to 10-20 nm. A promising approach for the development of catalysts which resist agglomeration and growth during reaction is the incorporation of secondary elements in the form of anionic groups. A number of papers report liquefaction results with coal impregnated with small well-dispersed iron catalyst precursors. Several researchers report on the catalytic chemistry of coal and model compounds in the presence of iron Catalysts with the aim of understanding the nature of the active sites in the catalyst and their activity and selectivity for reactions such as hydrocracking, dehydroxylation, and hydrogen transfer.
Characterization techniques include Mossbauer spectroscopy, X-ray absorption fine structure (XAFS) spectroscopy, electron microprobe analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy, scanning electron microscopy (SEMI, transmission electron microscopy (TEM), SQUID magnetometry, and electron spin resonance (ESR). Also included is development of new and accurate methods of determining size distribution of catalyst precursors and the size of iron-based residues after reaction. In situ techniques to followthe fate of the added precursor and its transformation into active catalysts under DCL conditions have been developed. Further refining of these methods is highly desirable. Technology development papers are included, covering both laboratory-scale experiments and pilot plant results on coal liquefaction in the presence of iron catalysts. Some clear themes that have emerged are that, while there are a number of ways to prepare ultrafine, highly dispersed iron-based catalyst precursors, a more difficult goal is to keep them small and highly dispersed during liquefaction. From a number of the papers, it appears that the key to achieving this goal is to either place some kind of group on the catalyst surface (e.g., acid or other secondary element anion group), or to disperse the iron into the coal, allowing the coal matrix to prevent agglomeration. While good DCL results have been obtained with very small concentrations of iron-impregnated coal, there remains the problem of dispersing small quantities of the fine particle size iron precursors in the cd-solvent slurries. It is likely that much of the future research on iron-based catalysts will focus on developing methods to prevent agglomeration and growth during liquefaction. More research is also needed on elucidation of the role that hydrogen donors play on the rate of transformation of the iron catalyst precursors into active catalysts.
0887-0624/94/2508-0002$04.50/00 1994 American Chemical Society
Malvina Farcasiu Senior Scientist
PETC Gerald P. Huffman Director, CFFLS University of Kentucky Irving Wender Research Professor University of Pittsburgh
