Energy &Fuels 1994,8, 1316-1323
1316
Surface Characterization and Liquefaction of Iron- and Molybdenum-ImpregnatedSubbituminous Cod Hengfei Ni, Richard K. Anderson, and Edwin N. Givens* Center for Applied Energy Research, University of Kentucky, 3572 Iron Works Pike, Lexington, Kentucky 4051 1-8433 Received April 26, 1994. Revised Manuscript Received July 20, 1994@
Results are presented from a study of the preparation and characterization of Wyodak subbituminous coal impregnated with iron and iron-molybdenum salts by the incipient wetness (IW) technique. X-ray photoelectron spectroscopy (XPS)was used to examine the surface composition and chemistry of coals impregnated with 0.7-2.0wt % Fe and 500-1000 ppmw Mo. Various salts were used in preparing the impregnated coals including ferric nitrate, ferric sulfate, ferrous sulfate, and ammonium molybdate. The surface concentration of Fe increased as the amount of added Fe increased, with higher concentrations being observed for the nitrateimpregnated coals. The anions of the impregnating salts were readily removed from the samples that were treated with ammonium hydroxide solution. For samples impregnated with sulfate salts, which were not treated with base, higher sulfur concentrations were observed on the surface of the coal. The Fenp,, and Fe3, peak positions in the base treated coals correspond with a-FeOOH while the positions in the non-base-treated sulfate-impregnated coals are consistent with the Fe sulfate salts. The added Fe on the surface of the impregnated coals has a direct impact on the position of the 01,peak shifting it t o a lower energy position. Overlap of the M03d doublet by the SzS peak required deconvolution based upon peak positions in reference materials. The liquefaction performance of the impregnated samples is directly related to the amount of added Fe with THF conversion and oil yield increasing as Fe concentration on the surface increased. The liquefaction results with the Fe-Mo co-impregnated coals were better than the other coals prepared in this study.
Introduction Highly dispersed iron-based catalysts have been extensively studied for direct coal liquefaction. Cugini and co-workers1,2found that finely divided and highly dispersed FeOOH-impregnated coals prepared by an IW technique have high activity in direct liquefaction providing improved conversion and distillate yield^.^ Pretreatment at lower temperatures was also found to provide increased liquefaction performance because of more efficient conversion of the precursor oxide to pyrrhotite. These coals were prepared by impregnating the coal with a solution of ferric nitrate followed by treatment using an excess of NH40H. Cugini et al.4 proposed that the deposited FeOOH coordinated with surface hydroxyl groups in a manner described by Yamashita et aL5 Liquefaction of similarly prepared 5000 ppmw Fe-impregnated Wyodak coal gave increased distillate yields compared to pigment grade iron oxide plus added sulfur.6 An SEM analysis showed that for coals impregnated with FeC13.6H20, the Fe was e Abstract published in Advance ACS Abstracts, September 1,1994.
(l)Cugini, A. V.; Utz, B. R.; Krastman, D.; Hickey, R. F. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1991,36(1), 91. (2) Utz, B. R.;Cugini, A. V. Method for Dispersing Catalyst onto
Particulate Material. U.S. Patent 5,096,570, Mar. 17, 1992. (3)Utz, B. R.;Cugini, A. V.; Frommell, E. A. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1989,34(4), 1423. (4) Cugini, A. V.;Krastman, D.; Martello, D. V.; Frommell, E. F.; Wells, A. W.;Holder, G. D. Energy Fuels 1994,8,83. ( 5 ) Yamashita, H.; Ohtsuka, Y.; Yoshida, S.;Tomita, A. Energy Fuels 1989,3 , 686. (6) Lee, T. L. K.; Comolli, A.; Johanson. E.; Stalzer, R. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1993,38(1), 107.
0887-0624/94/2508-1316$04.50/0
primarily deposited on the surface.I Sulfidation during liquefaction was found not to result in any significant growth of the iron particles upon conversion to pyrrhotite,8 thereby maintaining the advantage of the highly dispersed, small particle form of the metallic precursor. In contrast to these results, others have reported not only low catalytic activity but poor reproducibility in liquefaction of coals impregnated with aqueous solutions of FeS04.9 Possibly these differences could be related to the impregnation technique or to the application of base treatment. Impregnating coals by the IW technique is a multistep process involving first the distribution of the metal solution onto the coal, after which the oxide is formed by increasing the pH, which precipitates the oxide at the point of contact with the base material. In addition, the IW procedure also includes coal preparation and filtration, washing, and drying steps. Any of these may influence the subsequent liquefaction performance of the impregnated coal. Among the salts that have been used for impregnating Fe onto coals, the sulfates are preferable because they are the least expensive form of soluble iron that is available due to their abundance as a byproduct from the iron and steel industry. (7) Sommerfield, D. A.; Jaturapitpornsakul, J.;Anderson, L.; Eyring, E. M. Prepr. Pap.-Amer. Chem. SOC.,Diu. Fuel Chem. 1992,37(2),
749. (8)Andres, M.;Charcosset, H.; Chiche, P.; Davignon, L.; DjegaMariadassou, G.; Joly, J. P.; Pregermain, S. Fuel 19s3,62, 69. (9)Andres, M.; Charcosset, H.; Chiche, P.; Djega-Mariadassou, G.; Joly, J . P.; Pregermain, S. Preparation of Catalysts III; Elsevier Science: New York, 1983; p 675.
0 1994 American Chemical Society
Fe- and Mo-Impregnated Subbituminous Coal
Considerable research has also been directed toward exploiting the improved liquefaction performance of mixed Fe and Mo catalyst.lOJ1 Garg et al. found higher conversion and oil yields by simultaneously impregnating coal with 1%Fe and 0.02% Mo using solutions of 10% ferrous sulfate and 0.5%ammonium molybdate.12 In related studies, Pradhan et al.13 reported improved liquefaction of subbituminous coal with sulfated a-hematites containing small amounts of added Mo. Since Mo is expensive, its application in liquefaction will depend on maximizing activity at low concentrations while simplifying the preparation in order to minimize catalyst processing costs. In order to optimize the metal function, it is necessary to understand the location of these impregnated catalytic metals on the coals, their resulting chemical states and what impact this has on liquefaction. This paper describes the results of a study to evaluate the performance of an Fe-impregnated Wyodak coal. The effects of several independent variables in the preparation scheme on metal deposition and chemical form were investigated using XPS. This technique has been used not only to analyze the surface composition of coals14J5 but also t o characterize the chemical state of the Fe-0 system.16-18 In addition, the conversion and product distribution from liquefaction of these Fe and Fe-Moimpregnated coals are discussed.
Energy &Fuels, Vol. 8, No. 6, 1994 1317 Table 1. Proximate, Ultimate, and Ash Analysis of Black Thunder Wyodak Coal as received proximate, as-receivedbasis, wt % moisture 21.0 ash 4.82 volatile matter 34.4 fxed carbon 39.8 ultimate, dry wt % carbon 69.97 hydrogen 4.78 nitrogen 1.43 sulfur 0.56 18.21 oxygen (dim ash (SO3-free) 5.05 ash composition, wt %b 6.62 Fez03 CaO 25.79 19.05 A1203 Si02 38.05 5.20 MgO NazO 0.58 0.59 KzO Ti02 1.38 PZOS 1.16
drys 2.63 5.66 45.5 46.2 72.15 4.34 1.23 0.53 16.94 4.81 -
Dried 110 "CY20 W10 Torr. Analysis provided by Consol Inc.
(10)Courty, P.; Marcilly, C. Preparation of Catalysts; Delmon, D., Jacobs, P. A., Poncelet, G . , Eds.; Elsevier Science: New York, 1976;p 191. (11)Scaroni, A. W.; Derbyshire, F. J.;Abotsi, G. M. K.; Solar, J. M. "Improved Coal Liquefaction Using Carbon-Supported Hydrogenation Catalysts", DOE/PC/60050-12, June 1987, pp 33-34. (12) Garg, D.; Givens, E. N. Fuel Process. Technol. 1984,8,123. (13)Pradhan, V. R.; Tiemey, J. W.; Wender, I. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1992,37(1), 254. (14) McIntyre, N. S.; Martin, R. R.; Chauvin, W. J.;Winder, C. G.; Brown, J. R.; Macphee, J. A.Fuel 1986,64, 1705. (15)Frost, C.; Leeder, W. R.; Tapping, R. L.; Bank, B. W. Fuel 1977,
were further washed with water. All were dried to a final moisture content of 3-10%. XPS Measurements. The XPS spectra of the impregnated coals were obtained with a LHS-10 Leybold-Heraeus spectroscope using a Mg K a radiation source power of 300 W (15kV, 20 mA) and pass energy of 100 eV a t a n analysis chamber pressure of 5 x low9Torr, as described p r e v i o u ~ l y . The ~~ powdered samples were mounted on the XPS sample probe with double-stick Scotch tape. The sample powders were gently pressed and covered all of the Scotch tape attached on the stainless steel foil. The constituents of the tape did not contribute to the signals generated by the samples. The peak positions in the elemental spectra of the coal samples were corrected with the hydrocarbon C1, binding energy of 284.6 eV. The elemental concentrations were calculated and the peaks deconvoluted with a damped nonlinear least-squares curve fitting computer program using mixed GaussianLorentzian profiles.20 Argon ion sputtering was performed with the argon ion gun operating a t 3.5 kV, 10 mA, and at a pressure of 1 x Torr. Based upon rates of 13.5 fvmin for Cu and 8 fvmin for Au, the impregnated coal sputtering rate was estimated to be 20 fvmin. Samples were sputtered for up to 160 min. Equipment and Procedures. Liquefaction experiments were performed in 50 mL microautoclaves to which were added coal, tetralin, and 1.2 mol dimethyl disulfide per mole added Fe. The reactors were sealed, pressurized with hydrogen to 1000 psig and leak tested. Reactions were carried out in a fluidized sandbath set a t the specified temperature while the reactors were continuously agitated at a rate of 400 cycles/ min. Approximately 2 min were required for the reactors to reach the operating temperature after submersion within the bath. Reactors were then quenched t o ambient temperature, the gaseous products collected and analyzed, and the products separated into THF-insoluble, THF-soluble-pentane-insoluble (PA+A), and pentane-soluble (oilsSwater) fractions. Standard deviations on solvent separated fractions were as follows: THF insolubles, 0.4; PA+A, 1.6;oils+water, 1.1;hydrocarbon gases, 0.1; COz+CO, 0.1. The THF insoluble products contained mineral matter, spent catalyst, and insoluble organic material (IOM). The product distributions were calculated on a Sosfree ash basis assuming complete recovery of ash and conver-
55, 277. .(16)McIntyre, N. S.; Zetarak, D. G. Anal. Chem. 1977,49,1521. (17)Allen, G. C.; Curtis, M. T.; Hooper, A. J.;Tucker,P. M. J.Chem. Soc., Dalton Trans. 1974,14, 1525. (18)Kuivila, C. S. J. Catal. 1989,118, 299-311.
(19) Ni, H. F.; Anderson, R. K.; Givens, E. N.; Stencel, J. M. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1993,38(3),1107. (20) Proctor, A. FRYFIT Software, Surface Science Center, University of Pittsburgh, Pittsburgh, PA 15260, Version Jan 3, 1992.
Experimental Section Materials. Reagent grade Fe(N0&.9HzO, FeS04.7Hz0, and Fez(S04)3-5HzO,and ammonium molybdate (85%MoOa), were purchased from Aldrich Chemical Co. Wyodak coal from the Black Thunder Mine in Wright, Wyoming, was provided by CONSOL Inc. The coal was ground to -200 mesh, riffled, and stored under nitrogen a t 4 "C. Proximate and ultimate analyses of the coal are presented in Table 1. The impregnated samples were prepared starting with either as-received coal or coal predried a t 110 "C at 10 Torr for 20 h. The moisture content and the volume of water used in the impregnation of as-received coal were 21 wt % and 0.75 m u g of dry coal, respectively, and for impregnation of predried coal were 2-3 wt % and 0.90 m u g of dry coal, respectively. Solutions containing either the nitrate or sulfates salts were added slowly in a dropwise manner t o the coal while mixing to assure even distribution. The co-impregnated samples were prepared by first adding to as-received coal on a per gram of dry coal basis, 0.25 mL of ammonium molybdate solution followed by 0.5 mL of aqueous Fe salt solution. All of the nitrate-impregnated coals were treated with ammonium hydroxide solution while only some of the sulfate-impregnated samples were treated with base. For those samples treated with base, 1.54 M NH4OH solution was added a t a n NHIOH to Fe mole ratio of 138to 1and filtered. In some cases samples
Ni et al.
1318 Energy &Fuels, Vol. 8, No. 6, 1994 Table 2. Filtrate Analysisa sample 1 sample 2 ferric nitrate ferric sulfate impregnating salt 1.o Fe added, wt % 2.04 dry coal base water filtrate base water wash wash 1.0 1.0 Ca, wt % dry coal 0.39 0.07 calcium recovery, 0.11 0.10 wt % Ca in dry coal 0.4 3.3 0.2 9.9 Fe recovery, wt % added Fe ( x lo2) NOS- recovery, wt %b 85 C 108 C
Table 3. Element Concentrations in Wyodak Coal bulk, wt %n iron carbon oxygen nitrogen sulfur calcium aluminum silicon
0.20 73.5 22.0 1.5 0.6 0.8 0.4 0.8
surface, wt
%b
0.33 71.1 20.6
1.3 0.2 0.5
2.0 4.0
Excludes 4.68 wt % hydrogen from calculation. Hydrogen is not detected in XPS.
a Direct current plasma emission spectrometry (DCP). Measured using ion-specific electrode. t Nitrate ion was not detected.
which were not treated with base, the sulfate ions remained in the sample. The Fe-Mo impregnated coals were prepared by a sion of catalyst t o pyrrhotite (Fe&). The logic for applying two-step procedure. Ammonium molybdate dissolved this correction to low-rank coals is discussed e l ~ e w h e r e . ~ l - ~ ~ in one-third of the IW water was added in the first step In calculating the product distribution from liquefaction of the followed by addition of the iron salt in the remaining sulfate-impregnated coals that had not been treated with base, two-thirds of the IW water. The actual retention of Mo a stoichiometric amount of sulfate equivalent to that added on the impregnated coals was not determined, although during the impregnation step was assumed to form H2S and some may have been lost in the basic filtrate. This twowater.24 The total of the net products equals the amount of moisture and ash-free coal in the feed and reflects the net step procedure, however, had no effect on the retention make of each of the solubility fractions; coal conversion equals of Fe. 100 minus the yield of IOM. Surface Composition of Impregnated Coals. XPS analysis was used to determine the surface concentraResults and Discussion tion of the Fe and Mo on the impregnated coals and the chemical structure of the catalyst precursor. Since Both Fe- and Fe-Mo-impregnated coals were prehydrogen cannot be detected in X P S , the surface conpared using an IW technique. A number of different centrations, as determined by this technique, is normalvariables in the preparation were evaluated including ized on a hydrogen-free basis and reported on a weight treatment with NH40H solution. In the preparation basis. The concentration of Fe in the starting Wyodak procedure, as coal is impregnated with salt solution, the coal, calculated on a hydrogen-free basis, was 0.20 wt mixing of the dry coal particles became easier as well % with the surface concentration being slightly higher as the volume of the coal gradually increased. If the at 0.33 wt %. Higher surface concentrations were also volume of water that was added during the preparation observed for Al and Si, indicating that the surface exceeded the IW volume, the volume of the coal debecame enriched in these metals during grinding, which creased as a layer of water appeared and particle was probably due to preferential fracturing of the coal agglomeration occurred. After complete addition of the at the maceral-mineral interface (Table 3). The metal water, the particle size of the coal appeared to be about enrichment on the surface resulted in a slight decrease 1-2 mm. All of the impregnating salts used in these for all the other elements. The impact on coal composipreparations were easily soluble in the IW volume of tion of impregnating with 1.0 wt % Fe was to increase water for both the previously dried as well as as-received the Fe concentration in the bulk from 0.19 to 1.17 wt % coal. Analysis of the basic filtrates recovered after base while also increasing oxygen in the bulk by a small treatment showed that deposition of Fe onto the coal amount from 20.9 to 21.4 wt %. The final oxygen was quantitative (see Table 2). Analysis of filtrates from concentration is based on the assumption that FeOOH subsequent water washes also indicated negligible Ca is the chemical form of the added Fe species. The other exchange and loss of Fe. Only 0.5 wt % of the Ca elements decreased slightly, with the largest change originally in the coal and less than 0.1 wt % of the Fe being in carbon. added to the coal were found in the filtrates. The Although Fe(I11) salts form FeOOH upon treatment recovery of NOS- in the basic filtrates appeared to be with base, the reaction of the Fe(I1) ion, under condiquantitative, although the accuracy is poor due to the tions in which exposure t o air is limited, is less clear.26 low concentrations, i.e., f20%. Absence of nitrate and Surface analysis of coals impregnated with 0.15-1.0 wt sulfate ions in the filtrates from the washing step % Fe that was added as ferric nitrate showed that Fe confirmed their complete removal in the basic filtrate. concentration on the surface increased as the amount The NI, peaks in the X P S spectra of the base-treated of added Fe increased (see Table 4). For the nitrate nitrate impregnated coals were consistent with organic impregnated coals the change is greater at the higher nitrogen indicating the absence of any nitrate or amaddition levels, as shown in Figure 1. monium ions.25 For those Fe sulfate impregnated coals Much of our screening work on impregnated coals was limited t o Fe addition levels of 0.77 w t % on dry coal (21) Southern Company Services, Inc. “Integrated Two-Stage Liq-
uefaction of Subbituminous Coal”, Advanced Coal Liquefaction R&D Facility, Wilsonville, AL., EPRI AP-5221, Elect. Power Res. Inst., Palo Alto, June 1987, Section 10.3, p 10-6. (22)Miller, R. N.; Yarzab, R. F.; Given, P. H. Fuel 1979, 58, 4. (23)Ode, W. H. Coal Analysis and Mineral Matter. In Chemistry of Coal Utilization; Lowry, H. H., Ed.; Wiley: New York, 1963; Suppl. Vol., p 208. (24)Artok, L.; Schobert,H. H.; Davis, A. Fuel Process. Technol. 1992, 32, 81.
(25)Deconvolution of the N1, peak with maximum at 399.7 eV showed the complete absence of nitrate nitrogen. The maximum allowable concentration of ammonium nitrogen on the surface based on this calculation was 0.1 wt %. The shape of the peak was identical to the peak in the original coal spectra. (26) Schwertmann, U.; Cornell, R. M. Iron Oxides i n the Laboratory: Preparation and Characterization; VCH Verlagsgesellschaft: New York, 1991.
Fe- and Mo-Impregnated Subbituminous Coal
Energy & Fuels, Vol. 8, No. 6,1994 1319
Table 4. Surface Concentrationsof Impregnated SamDles ~ _ _ _ _ _ _ _ _ _ _ _ surface concentrations,wt % basetreatedb drying addedFe bulkFe'J Fe Mo C 0 N Ca AI Si S N vac 0 0.20 0.33 71.1 20.6 1.3 0.5 2.0 4.0 0.2 Y vac 0.15 0.36 1.0 62.3 25.5 1.4 1.0 3.4 3.5 n.a.f Y vac 0.28 0.49 1.4 66.2 23.5 1.7 1.0 2.4 3.5 0.3 Y vac 0.55 0.77 2.4 63.5 25.2 1.5 1.0 2.9 3.3 0.2 Y vac 0.77 1.0 4.2 62.4 24.8 1.2 1.0 2.5 3.3 0.6 Y vac 1.0 1.23 6.3 59.6 24.4 1.6 1.0 3.8 3.4 0.3 YE vac 2.0 2.22 7.9 50.9 30.6 1.0 0.9 4.3 4.2 0.2 YE vac 0.77 1.00 1.5 67.9 23.1 1.1 0.8 2.6 2.5 0.4 N N2 0.77 1.00 1.6 63.3 23.2 1.0 1.2 3.4 4.3 1.9 N air 0.77 1.00 1.9 57.1 26.6 1.0 1.5 4.6 5.0 2.4 Ye vac 0.77 1.00 1.2 66.3 23.2 1.3 0.8 2.6 4.1 0.4 N N2 0.77 1.00 1.9 57.1 26.6 1.0 1.5 4.6 5.0 2.4 N air 0.77 1.00 0.8 60.9 25.6 1.0 0.8 4.1 5.7 1.2 Y vac 0.77 1.00 2.5 0.14 62.8 24.4 1.5 1.3 3.5 3.8 0.03 Y vac 0.71 0.94 3.0 0.11 63.9 24.4 1.4 0.9 3.4 2.8 0.03 N air 0.77 1.00 2.3 0.51 56.3 25.5 1.5 1.0 5.5 5.9 1.4 N air 0.77 1.00 2.5 0.30 58.7 25.3 1.2 0.8 4.9 5.3 1.1 ~
sample no.
ARcoale 3 4 5 6 1 2 7 8 9 10 11 12 13 14 15 16
Salta
none
FN FN FN FN FN Fe(II1) Fe(II1) Fe(II1) Fe(II1) Fe(I1) Fe(I1) Fe(I1) FN-AM
FN-AM Fe(II1)-AM Fe(I1)-AM
FN = Fe(N03)&H20 Fe(II1) = Fez(S04)3*5H20;Fe(1I) = FeS04.7H20; AM = ammonium molybdate. Y = Sample treated with 1.54 M NH40H. N = sample impregnated without washing. vac = 20 h at 40 "C/10 Torr; N2 = dried at 40 "C/1 atdflowing Nd20 h; air = 3 days at 40 "C/1 atdflowing air. Bulk analysis on SO3-free ash, hydrogen-free, dry basis. e Ar = as-received coal. f n.a., not available. 8 Sample washed with water following base treatment.
I
I
0.5
1
1.5
2
2.5
Added Fe, wt% F'igure 1. Concentration of Fe on the surface versus added ferric nitrate, base treated; (W) ferric nitrate-Mo, base Fe: (0) treated; (A) iron sulfate, base treated; (0) iron sulfate; (0)iron sulfate-Mo.
(Table 4), which is equivalent to iron oxide addition levels of slightly more than 1 w t %, which was set as a target level based upon economic considerations. The surface Fe concentrations on all the sulfate-impregnated coals were less than the corresponding 0.77 wt % nitrate-impregnated coal. Treatment with base or impregnating with either Fe(I1) or Fe(II1) salts appeared to have no effect on surface concentration of Fe. In fact, the Fe concentration on the surface of a coal impregnated with ferric sulfate to a 2.0 wt % added Fe level was only marginally higher than a coal impregnated with ferric nitrate to a 1.0 w t % added Fe concentration (see Figure 1). In both cases the sulfate ion had been removed in the base-treatment step. It appears, then, that the sulfate salts penetrate into the coal particle more easily than the corresponding nitrate salts resulting in lower surface Fe concentrations. In both cases, higher impregnation does not impair the effectiveness of removing the sulfate during the base treatment step and depositing the Fe within the interstitial structure of the coal as FeOOH.
Molybdenum surface concentrations for base-treated coals impregnated with 500 and 1000 ppmw Mo, and containing 0.77 wt % added Fe, were 1100 and 1400 ppmw, respectively. By comparison, two samples of non-base-washed coals, to which 1000 ppmw Mo had been added, had Mo surface concentrations of 3000 and 5100 ppmw. This suggests that base treatment may have resolubilized some of the molybdenum salt added in the first step, although the amount was not quantified. Overall, the surface Fe concentrations on the Moimpregnated coals were intermediate between the Mofree sulfate-impregnated coals, which were lower, and the nitrate Mo-free coals, which were higher, as shown in Figure 1. In all cases, the surface concentrations were greater than the corresponding bulk concentrations, which was somewhat less than the level to which Mo was added to the samples, indicating that Mo did not distribute uniformly within the particles. Sulfur concentrations on the surface of the basetreated coals were
