Energy & Fuels 1994,8, 31-37
31
Evaluation of Cation-Exchanged Iron for Catalytic Liquefaction of a Subbituminous Coal M. Mehdi Taghiei, Frank E. Huggins, Vikram Mahajan, and Gerald P. Huffman* The Consortium for Fossil Fuel Liquefaction Science, 233 Mining and Mineral Resources, University of Kentucky, Lexington, Kentucky 40506 Received July 12, 1993. Revised Manuscript Received October 26, 1993"
The total conversion and the oil yield in liquefaction tests of iron ion-exchanged Black Thunder coal were found to increase by up to 23 and 18%, respectively, relative to the untreated coal. The ion-exchanged coal samples were prepared by stirring a slurry mixture of the coal and ferric acetate [Fe(OOCCH3)31in a 10-Lfermenter. The ion-exchangeprocess, in which iron was exchanged primarily for calcium and magnesium, yielded a highly dispersed catalytic iron species for coal liquefaction. 57Fe Mossbauer and X-ray absorption fine structure (XAFS) spectroscopies and electron probe microanalysis (EPMA) X-ray mapping were used to characterize this iron species after the ionexchange process. The results indicate that the added iron is present in bimodal form. The majority of the iron is present as oxyhydroxide particles ranging from 25 to 100 A in diameter, while the smaller fraction is believed to be molecularly dispersed ferric ions held at ion-exchange (carboxyl) sites. With sufficient sulfur present in the system, the iron is rapidly transformed to pyrrhotite (Fe1,S) during liquefaction.
Introduction Catalyst particle size has a strong effect on catalytic activity and selectivity during direct coal liquefaction. Smaller particles are desirable because they have a larger percentage of coordination-unsaturated surface sites that are readily accessible for chemisorption and catalysis. Ionexchange procedures play an important role in many chemical processes because of their simplicity, selectivity, and wide scope. Ion-exchange,in which a surface carboxylbound ion in coal such as calcium is replaced by a catalytic ion such as iron, can yield an atomically-dispersed distribution of catalytic ions. Such processes may create a catalytic species stabilized in the state of ultimate dispersion for coal liquefaction. The results presented in this paper are part of a broader research study to explore the role of different cationexchanged catalysts (e.g., Fe, Mo, Co, Ni, etc.) during direct coal liquefaction. In our previous research,' the characterization and effects of ion-exchanged iron as a catalyst for liquefaction of two lignites were reported. In order to investigate further the beneficial effects of exchanging iron for some of the inherent ion-exchangeable cations in coal, iron was incorporated in a subbituminous Black Thunder coal by an ion-exchange process. A significant enhancement of the liquefaction conversion yields was obtained compared to those of the raw coal and to those of an ultrafine iron oxide ( F ~ O O H . X H ~ Omixed ) ~ J with the raw coal. 57FeMossbauer and X-ray absorption fine structure (XAFS) spectroscopies and electron probe microanalysis (EPMA) X-ray mapping techniques were used to characterize this iron species after both the ionexchange and the liquefaction processes. The results indicate that the added iron is initially present in bimodal e Abstract published
in Advance ACS Abstracts, December 1,1993.
(1)Taghiei, M. Mehdi; Huggins, F. E.; Ganguly, B.; Huffman G. P. Energy Fuels, 1993, 7, 399. (2) Bishop, M.; Ward, D. L. Fuel 1958,37, 191.
(3)Zhao, J.; Huggins, F. E.; Feng, Z.; Lu, F.; Shah, N.; Huffman, G.
P. J. Catal. 1993,143, 499.
0887-0624/94/2508-0031$04.50/0
Table 1. Proximate and Ultimate Analyses of Black Thunder Coal proximate" % ash 6.54 % volatile 43.62 % fixed carbon 49.85 ultimateb % carbon 73.49 % hydrogen 5.09 % nitrogen 1.29 % total sulfur 0.60 19.53 % oxygen Dry basis. Dry ash free basis.
form. A significant fraction of the iron is present as iron oxyhydroxide particles ranging from 25 to 100 A in diameter, whereas the remaining iron is believed to consist of atomically dispersed ferric ions exchanged for calcium and magnesium cations at carboxyl sites. With sufficient sulfur present in the system, both forms of iron are rapidly transformed to pyrrhotite (Fe1,S) during liquefaction.
Experimental Section Black Thunder subbituminous coal of Wyodak-Anderson seam from the Powder River Basin, Wyoming, was used in this study. The Black Thunder strip mine is the largest coal mine in the United States. Proximate and ultimate analyses for this coal are shown in Table 1. The original iron content of this coal is about 0.2 % . The coal samplewas first ground to
