194
Energy & Fuels 1990,4, 194-197
Exchange of Sulfur in Nickel-Molybdenum-Alumina Catalyst during Operation at High-pressure Conditions Rita H. Hardy, Diane R. Milburn, and Burtron H. Davis* Center for Applied Energy Research, University of Kentucky, 3572 Iron Works Pike, Lexington, Kentucky 40511 Received November 8, 1989. Revised Manuscript Received January 26, 1990 Samples of catalysts withdrawn from coal liquid hydrotreating reactors operated at high pressures and with catalyst-bed ebullition to provide excellent mixing were analyzed for natural sulfur isotope (Y3/32Sshift from that of a standard = a(%)) abundance. The a(%) value for the pretreated catalyst was -0.4 relative to a standard sample and the feed coal was +2.5 relative to the same standard. The experimentally determined S(%S) for each catalyst permits a calculation of the fraction of the sulfur pool added during pretreatment that has been exchanged by sulfur added in the coal feed. Two exchange rates are observed: (1)a rapid exchange that occurs too rapidly to measure (less than 2 days) and corresponds to 33% of the sulfur in the presulfided catalyst and (2) a slow exchange that would require >90 days for the catalyst to attain the sulfur isotope composition of the feed coal.
Introduction The use of molybdenum in formulating hydrotreating catalysts dates to the introduction of hydrotreating processes. While much work has been done on these catalysts and several models advanced to describe their structure, much remains to be learned about them.'-' During recent years much effort has been directed toward learning about the nature of the species present in the solution used for the preparation of the catalyst precursor.&l0 Likewise, much effort has been expended to develop pretreatment conditions that produce a preferred formulation. However, for this effort to provide long-term benefits, those properties introduced during the preparation, pretreatment, and line-out period must survive for a considerable fraction of the life of the catalyst. For example, it is of little value to prepare a highly dispersed molybdenum form if it is then lost during pretreatment or while bringing the catalyst on stream. Likewise, it is desirable to learn about the nature of the molybdenum species that is present under reaction conditions that are, or simulate, the conditions encountered in actual plant operation. We have therefore utilized the natural isotopic abundance technique to examine a series of catalysts that have been subjected to increasing exposure to coal liquids in the Wilsonville, AL, 6 ton/day coal liquefaction pilot plant. The Wilsonville plant provides a nearly ideal situation for the study of catalyst aging under practical conditions. The plant currently operates in a two-stage configuration: the first stage is utilized to convert coal to a soluble form under the mildest conditions possible and the soluble material is then upgraded in the second-stage reactor." (1) Wisser, 0.;Landa, S. In Sulfide Catalysts. Their Properties and Applications; Pergamon Press: Oxford, 1973. (2) Schuit, G. C. A,; Gates, B. C. AIChE J. 1970, 19, 417. (3) Massoth, F. E. Adu. Catal. 1978, 27, 265. (4) Topsoe, H. NATO ASI Ser., Ser. C 1983, 105, 329-360. (5) Delmon, B. In Proceedings of Climax Third International Conference on Chemistry and Uses of Molybdenum; Barry, H. F., Mitchell, P. C. H., Eds.; Climax Molybdenum Co.: Ann Arbor, MI, 1979; p 73. (6)Grange, P. Catal. Reu.-Sci. Eng. 1988, 21, 135. (7) Knozinger, H. Catalysis. Theory to Practice (Proceedings of the Ninth International Congress on Catalysis);Phillips, M. J., Ternan, M., Eds.; Chemical Institute of Canada: Ottawa, ON, Canada, 1988; p 20. (8) Tsigdinos, G. A.; Chen, H. Y . ;Streusand, B. J. Ind. Eng. Chem., Prod. Res. Deu. 1981,20, 619. (9) Chiplunker, P.; Martinez, N. P.; Mitchell, P. C. H. Bull. SOC.Chim. Belg. 1981, 90,1319. (IO) Wang, L.; Hall, W. W. J. Catal. 1982, 77, 232. (11)Technical Progress Report, DOE/PC/50041-111,1988,and earlier
reports.
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The catalysts for most of the analytical work described in this note were from run 257. For the time period for which samples were analyzed for this study, Amocat 1C NiMo-Al,03 extruded pellets (l/12-in. diameter) were used in both reactors. The process was operated in the closecoupled integrated two-stage liquefaction (CCITSL) configuration; in this mode the effluent from the first reactor passes directly to the second reactor that is held a t different process conditions than the first reactor. During run 257 both reactors were operated in an ebullient mode to provide a well-mixed catalyst bed; a small amount of catalyst was withdrawn each day to provide samples for analysis but fresh catalyst was not added during the early part of the run that provided catalyst samples for this study. The feed was an Illinois No. 6 bituminous coal from a Burning Star mine. The preliminary data that are described in this report are for samples from run 254. During this run a Shell 317 NiMo-Al,03 catalyst was utilized in the CCITSL mode while operating with a Ohio No. 6 coal.
Experimental Section Catalyst samples were shipped to the CAER covered with a viscous coal liquid that was withdrawn with the sample. Catalyst samples were extracted with tetrahydrofuran in a Soxhlet apparatus for appoximately 48 h during which time they were maintained under a nitrogen blanket, then dried overnight in a vacuum oven at 80 "C, ground with a mortar and pestle, and sent to either Coastal Scientific Laboratories or the Illinois State Geological Survey (ISGS) laboratories for sulfur isotope analysis. At Coastal Scientific Laboratories the catalysts were combusted in a Parr bomb and the sulfur recovered as BaS04. A measured amount of barium sulfate was thermally decomposed following the methods of Bailey and Smith.12 All of the samples for run 257 were analyzed a t Coastal Scientific Lab0rat0ries.l~ They determined sulfur isotope ratios (6(34S)= shift in parts per thousand from a standard) using a VC 602-D mass spectrometer. The isotope abundance is reported relative to a CDT standard (Canyon Diablo Troilite, a meteor residue). Samples for an earlier run were analyzed at Coastal Scientific and/or the ISGS laboratory: comparable data were obtained by the two laboratories. Extracted catalyst samples were ground and then digested in a solution of boiling hydrofluoric, nitric, and perchloric acids and evaporated to near dryness. The resulting droplet was then dissolved in concentrated hydrochloric acid, boiled, and diluted to 100 mL with water. A second dilution (1OOx) was required (12) Bailey, S. A.; Smith, J. W. Anal. Chem. 1972, 44, 1543. (13) Kenneth Winters, personal communication.
0 1990 American Chemical Society
Exchange of Sulfur i n Ni-Mo-Alumina Catalyst for analyses of components present in high concentrations. The instrument used for all metal determinations was a Beckman direct coupled plasma (DCP) emission spectrometer. The presulfided sample is prepared by slowly heating the fresh catalyst in a flow of dimethyl sulfide dissolved in a diesel or diesellike oil and hydrogen. The sample is maintained a t the maximum presulfiding temperature (ca. 400 "C) until H2S breakthrough occurs and is then maintained at these conditions for a few hours. The presulfided samples are prepared in batches and are stored in oil until placed into operation.
Results and Discussion Preliminary data were generated with samples collected during the first 32 days of operation during Wilsonville run 254. Unfortunately, operational problems caused the catalyst to be withdrawn on two occasions during the early part of the run and to be replaced with another batch of presulfided catalysts. The catalyst used during this run was a Shell 317 Ni-Mo-A1203 and was presulfided with dimethyl disulfide a t a temperature programmed to a maximum of 750 OF. The actual hydrogen partial pressure is proprietary but is believed to be in excess of 1500 psig. Briefly, the results indicated that part of the sulfur isotope added during the presulfiding was exchanged very rapidly with the sulfur from the coal slurry, but that a larger fraction was not exchanged after several days of operation. The calculation had to be based on isotope analysis of the catalyst added a t the start of the run rather than that added after the second withdrawal, which was not available to us. Thus, we had to use the isotope analysis for the first batch of presulfided catalyst, but for the isotope analysis after contact with coal liquids we had to use the third catalyst addition. The sulfur isotope compositions, referenced to CDT were presulfided catalyst, -2.65 %o (parts/thousand); coal/solvent mixture, +11.5; and catalyst batch withdrawn on operational day 22, +1.4. Thus, the catalyst after contact by coal contained a sulfur isotope ratio that corresponded to about 28% exchange of the sulfur added during presulfiding by sulfur from the coal. However, the operational problems required the assumption that the isotope composition of the first batch of catalyst was the same as the third batch. To improve the reliability of the data, samples were obtained during run 257. Catalyst addition, as well as withdrawal, was started to the first reactor on the 18th day of operation; hence, reliable data for sulfur isotope exchange could not be obtained after day 18. Catalyst addition was not started in the second-stage reactor until day 31. Wilsonville operators reported that the sulfur level in the as-received catalyst was less than their analytical limit (0.1 wt 70 or less). The sulfur isotope data for the presulfided material and for catalyst samples withdrawn at increasing operational timd from each of the two reactors are shown in Figures 1 and 2. First, consider the data in Figure 1 for samples withdrawn from the second-stage reactor since it contains more data points covering a longer time period. The data clearly show that there are two types of isotope exchange. First, there is a very rapid initial exchange of sulfur that was added during the presulfiding procedure with that of the coal. This rapid exchange corresponds to 33% of the sulfur that was initially present in the catalyst: 70 S exchange = ([6(34St=0) 6(34Spretreat)I / [6(34Scoal slurry) - 6(34Spretreat)I I x 100 = ([0.6 - (-0.4)]/[2.6 - (-0.4)]] X 100 = 33 Following the rapid exchange, there is a period of slow exchange during which the sulfur on the catalyst gradually approaches that of the added coal. The slope of the line
Energy & Fuels, Vol. 4, No. 2, 1990
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COAL
~
2
4
~
6
8
"
10
~
12
14
"
16
'
18
'
20
22
'
24
'
'
26
Time on Stream, Days
Figure 1. The 6(%) isotope ratio (MS/32S) for catalyst samples withdrawn from the second-stage reactor after presulfiding (day 0) and at increasing operating days. The upper solid line is the value of b(%3) in the feed coal.
F
COAL
+2. O L
o,oL--
-1.0 0
2
4
6
8
10
12
14
16
18
20
22
24
26
Time on Stream, Days
Figure 2. The 6(%S) isotope ratio (s4S/32S)for catalyst samples withdrawn from the first-stage reactor after presulfuding (day 0) and at increasing operating days. The solid line is the value of A(%) in the feed coal; the broken line is the one corresponding to the data fit in Figure 1.
in Figure 1is 0.022 6(%S)/day;it is calculated that it would take a minimum of 91 days for the sulfur isotope composition of the catalyst, calculated by extending the line defined by the measured 8(%S) values toward the solid line representing the isotope composition of the coal, to attain that of the coal slurry. Thus, we obtain the surprising result that only about one-third of the sulfur initially present in the presulfided catalyst exchanges rapidly even when the catalyst is operated at commercial hydrotreating conditions (greater than 2000 psi and temperatures around 425 "C). There are fewer data points shown in Figure 2 for samples withdrawn from the first reactor; however, the same pattern is exhibited in both Figures 1 and 2. Note that different batches of dimethyl disulfide were used to presulfide the two batches of catalysts; thus the material used in reactor 1 had an isotope composition that was shifted -1.15 from the standard while the batch added to the second reactor was shifted -0.4. The line defining the slow exchange for the first reactor catalyst intercepts the zero time axis a t 0.3. However, only three of the four data points were used to determine the line that gave the value of 0.3, and the data points could fit equally well the same line that was defined by the data in Figure 1. Using the data for Figure 2, we calculate that the rapid exchange
'
Hardy et al.
196 Energy & Fuels, Vol. 4, No. 2, 1990 Table I. Atomic Ratio, S/Mo, for Catalysts Withdrawn from the Wilsonville, AL, Pilot Plant
I o
SIMo run 250 250 249 248 247
date
Wilsonville
CAER
Feb 17, 1986
2.15 2.18 2.41 2.09O 1.99
1.79 2.41 2.26 2.11
Dec 8, 1985
Sept 18, 1985 May 3, 1985 Jan 6, 1985
Sample date May 5,
c W
o
0
11
~~~~
0
0
-
1985.
corresponds to ca. 40' of the sulfur initially present in the catalyst. Thus we obtain similar data for catalysts exposed in either reactor 1 or 2. The data are intriguing but they do not provide sufficient information to define the structures that correspond to the rapid and slow exchange of sulfur. The first problem encountered in attempting to assign chemical species or structures to the exchange data is a definition of the compounds and their stoichiometry that are present in the catalyst. Massoth3 reported in 1978 that in spite of the fact that these catalysts are either presulfided or become sulfided during use, surprisingly little work has been done on determining the stoichiometry of the sulfided catalyst state. When sulfur levels have been determined, the catalyst has usually been found to be incompletely sulfided to the respective sulfides, MoS2 and C O ~ S ~ . Massoth3 '~J~ concluded that the sulfided catalyst consists of a mixed surface oxysulfide species or some bulk MoS2 and another oxysulfide species. The state of Co in the sulfided catalyst is uncertain. Knozinger' summarized in 1988 the current results on the genesis and nature of molybdenum-based hydrodesulfurization catalysts and did not significantly modify the picture presented by Massoth. Knozinger presents a more detailed description of the sulfided catalyst: highly dispersed MoS2-likeslabs are the dominant Mo species in the sulfided catalysts. These slabs are stabilized by Mo-0-A1 links after mild sulfidation a t T 5 670 K and in this state the slabs are preferentially oriented normal to the support surface. At higher sulfidation temperatures, the average size of the MoS2slab is larger; these slabs are detached from the alumina and lie flat on the support surface. The presence of promoters (Co, Ni) may lead to somewhat higher dispersion but does not seem to significantlyalter the local Mo environment. For typical catalyst compositions, the promoter atoms in or on the surface are sulfided but are not present in well-ordered structures such as Cogs8or Ni& There is considerable variation in the S/Mo ratio for catalysts from several runs at Wilsonville. Ratios reported by them and obtained by us are shown in Table I. The ratio ranges from about 1.5 to 2.4. While these catalysts do contain other metals, it appears that a value for the atomic S/Mo greater than 2 is not that unusual. At the same time, it should be realized that there is considerable error associated with determining this ratio. Data are presented in Figures 3 and 4 showing the concentration level for several major and minor components of the catalyst samples withdrawn from the second-stage reactor at Wilsonville during run 257. For Mo, Ni, and S, the catalyst composition is constant through the 25-day period; the average levels for these elements are S, 6.57 wt %; Mo, 7.10 wt %; and Ni, 1.60 wt %. Similar levels are obtained for catalysts withdrawn from the first-stage reactor. Our data shown in Figures 3 and 4 are (14) Massoth, F. E. J. Catal. 1975, 36, 164. (15) Massoth, F. E.; Chung, K. S.: Ramachandran. R. Fuel Process. Technol. 1979, 2, 57.
Time, Days
Figure 3. Analytical data for some of the major components of catalyst samples withdrawn from the second-stage reactor.
1
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1
1
I
1
10
15
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25
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Figure 4. Analytical data for sope of the minor componenta of
catalyst samples withdrawn from the second-stage reactor.
in excellent agreement with our analytical data for earlier runs where this catalyst was utilized and also with the analytical data reported by the Wilsonville pilot plant operators." If S is combined only with Mo the stoichiometry of the second-stage catalyst is M o S ~ .assuming ~~; that all Ni is sulfided to form NiS reduces the stoichiometry to only M o S ~ . The ~ ~ . first-stage catalyst has a stoichiometry of M o S ~if. all ~ ~sulfur is combined with Mo; if NiS is formed, then the stoichiometry becomes MoSL5. Even if we assume that all of the Ca, Fe, and Ni are present as MIIS, the stoichiometry for the second-stage catalyst (for day 15) is MoS2,28. The data indicate that sulfidation at these severe conditions produces a molybdenum species with a stoichiometry considerably greater than MoS2 or that a large amount of sulfur is present in other metal sulfides, in-
Energy & Fuels 1990,4, 197-201
197
Mo-A1203catalysts using the pulse microreactor technique
cluding Ti0,S and/or AlO,S,, and/or elemental sulfur. Thus, the excgange data must be evaluated in the light of uncertain sulfide compounds and stoichiometry. First we note that the assumption that organic sulfur compounds derived from the coal/solvent are adsorbed on the catalyst only makes the problem more perplexing. The sulfur compounds derived from the coal will have a b(34S) value of 2.5; correcting the 6(%3) values of the catalyst for any coal derived from adsorbed organosulfur compounds decreases the amount of sulfur that rapidly exchanged. If we assume that Ni and Fe are present as NiS and FeS and that these exchange rapidly, we can account for only 43% of the total rapid exchange; assuming only NiS is formed and rapidly exchanges can account for about 39% of the total rapid exchange. If it is assumed that (1)MoS2 is formed and does not exchange rapidly and (2) all other sulfur is present in a form that rapidly exchanges, then the rapid exchange should be 27.8% of the total sulfur rather than the 34.5% that is observed. The data are suggestive that a significant fraction of the sulfur present in “molybdenum sulfide” does not exchange rapidly. To define the source of the rapidly exchanging sulfur will require further work with a series of sulfided preparations: alumina only, Mo-A1203, Ni-A1203, Ni-Mo-Al,O,, etc. Isagulyants and c o - ~ o r k e r s ~utilized ~ J ~ the 35Sradioisotope to follow the exchange of sulfur isotope in Co-
and lower temperature and pressure conditions. They found that about 60% of the sulfur in the catalyst is not involved in exchange during the conversion of thiophene. More recently Dobrovolszky et al.l8utilized a pulse reactor at low-pressure conditions to follow the retention and exchange of radioactive sulfur with nonradioactive sulfur during the conversion of thiophene. These later workers found a release of about 20% of the sulfur added to the catalyst was removed during subsequent reactions. The present results, obtained at high pressure in a large 6 ton/day plant, show remarkable agreement with data reported for small-scale, low-pressure laboratory reactors. It is apparent that stable sulfur isotopes can be utilized in appropriate situations even in large-scale reactors and at industrial/process conditions. The efforts expended in catalyst preparation and pretreatment to produce highly dispersed sulfided molybdenum species appear to be merited since at least two-thirds of the sulfur initially present in Ni-Mo-alumina catalysts exchanges very slowly. This is taken to indicate that the small molybdenum sulfide crystals retain their original structure for long time periods (days or months); if rapid reorganization of these crystals were to occur, it is expected that sulfur exchange would quickly occur between the sulfur present in the reactant and the sulfur pool initially present in the catalyst.
(16) Isagulyants, G. V.; Greish, A. A.; Kogan, V. M.; V’yunova, G. M.; Antoshia, G. V. Kinet. Katal. 1987, 28, 632.
Acknowledgment. The operators of the Wilsonville, AL, pilot plant have been exceptionally helpful by providing samples as well as advice and guidance.
(17) Isagulyants, G. V.; Greish, A. A.; Kogan, V. M. In Catalysis. Theory to Practice (Proceedings of the Ninth International Congress on Catalysis);Phillips, M. J., Ternan, M., Eds.; Chemical Institute of Canada: Ottawa, ON, Canada, 1988; Vol. 1, p 35.
(18) Dobrovolszky, M.; Tetenyi, P.; Paal, Z. Chem. Eng. Commun. 1989, 83, 1.
31PNMR Spectroscopic Analysis of Labile Hydrogen Functional Groups: Identification with a Dithiaphospholane Reagent C. Lensink and J. G. Verkade* Department of Chemistry and Ames Laboratory, Iowa State University, Ames, Iowa 50011 Received November 27, 1989. Revised Manuscript Received January 29, 1990 Six chlorophospholanes (2-7) are evaluated as 31PNMR tagging reagents for labile hydrogen functional groups known to be present in coal materials. In this series, 2 (C1PSCH2CH2S)is demonstrated to be best for resolving the 31PNMR chemical shifts of a variety of model compounds within a given functional group class, as well as for the separation of the shift ranges among the various classes. I
Introduction In our quest for appropriate NMR-active derivatizing reagents for the NMR analysis of labile hydrogen functional groups in coal materials, we have in recent years been exploring a series of 1,3-dioxaphospholanes,of which 1,3-Dithiaphospholanes(2-4) appeared 1 is an e-ple.1-3 (1) Schiff, D. E.; Verkade, J. G.; Metzler, R. M.; Squires, T. G.; Venier, C. G. Appl. Spectrosc. 1986.40, 348. (2) Wioblewski, A. E.; Markuszewski, R.; Verkade, J. G. Prepr.
Pap.-Am.
to be able to yield improved 31PNMR peak resolution for differentiating various compounds within a functional group class, according to preliminary results we obtained with 2.4 Herein we report in detail our findings with 2 and a comparison of the substituted derivatives 3 and 4 as reagents. We also evaluate reagent 5 in order to test the effect of the absence of heteroatom substituents on phosphorus and reagents 6 and 7 (on which we had reported in preliminary form4) to determine the influences
Chem. SOC.,Diu. Fuel Chem. 1987, 32, 202.
(3) Wroblewski, A. E.; Lensink, C.; Markuszewski, R.; Verkade, J. G. Energy Fuels 1988, 2, 765.
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i
(4)
Lensink, C.; Verkade, J. G. Prepr. Pap.-Am.
Fuel. Chem. 1988,33, 906.
0 1990 American Chemical Society
Chem. SOC.,Diu.
