Energy & Fuels 1988,2, 292-295
292
consistently above 9070,Nz less than detected.
lo%, and NH3 not
Discussion Figures 4 and 5 show the existence of a reversible mode of SO2poisoning for the Pd and Rh catalysts, while Figurea 1and 2 indicate the poisoning to be irreversible. This type of phenomenon indicates that two types of poisoning may be occurring in series. The first step may involve weakly chemisorbed species. This weakly held species can desorb when SO2is absent from the gas phase, thus making the poisoning effects reversible. The second step is conversion of the reversibly bonded species into a species that is irreversibly bonded to the noble metal, under the conditions of our experiments. The possibility that the irreversibly bonded species is a bulk metal sulfur compound is suggested by the reported observation by Tsai et al.gof the formation of bulk sulfur compounds in SO2-poisonedPt and Pd. We suggest that exposure of Rh or Pd to a pulse of SOz may cause the temporary formation of a reversibly chemisorbed sulfur species on the surface of the metal. As the pulse passes through the catalyst bed, the metal surface may return to the unpoisoned state through desorption of the reversibly held chemisorbed species. In contrast, contacting Rh, or Pt catalysts with a constant SO2partial pressure as high as 8.5 X lo4 atm may maintain a more or leas constant coverage of a reversibly chemisorbed sulfur species, which is gradually converted to the irreversibly bonded form. It is of particular interest to note that the order of activity recovery rate is also the order in which the catalyst’s
selectivities are sensitive to poisoning. Rhodium shows the highest rate of activity recovery and also shows the most dramatic changes in selectivity during poisoning. At low poisoning levels, the selectivity to NzO increases and that to N2 decreases. However at high levels of poisoning, the selectivity to Nzis regained. Such behavior is not observed for the unpoisoned catalyst from merely changing con~ e r s i o nsuggesting ,~ that SO2actually modifies the path of the reduction reaction occurring on the rhodium catalyst. Palladium, which has a moderate reversibility, shows a gradual change in selectivity with poisoning (Figure 8). However, these changes were similar to those changes observed when conversion decreases (by increasing space velocity) for unpoisoned Pd.3 Platinum consistently forms NzO as the predominant product whether poisoned or unpoisoned.
Conclusion It is demonstrated that for the conditions studied in this work there exist both reversible and irreversible modes for the poisoning of noble metals by SO2. A two-step mechanism is proposed to explain this phenomenon. Assuming N2 is the desired reduction product, the selectivity changes caused by poisoning can be either desirable or undesirable. Platinum, which has poor selectivity to N2, is unaffected by sulfur poisoning while palladium shows a loss in N2 selectivity during poisoning. At low levels of poisoning, rhodium becomes rapidly less selective to N2; however, at high levels of poisoning, the N2 selectivity of the rhodium catalyst is actually greater than in the unpoisoned case. Registry No. Pt, 7440-06-4; NO, 10102-43-9; SO2, 7446-09-5; Pd,7440-05-3; Rh,7440-16-6.
Bulk Ruthenium as an HDN Catalyst A. S.Hirschon* and R. M. Laine*l SRI International, Inorganic and Organometallic Chemistry Program, Menlo Park, California 94025 Received February 6, 1987. Revised Manuscript Received January 22,1988
A series of group VI to group VI11 bulk metals was tested for HDN activity by using a solution of tetrahydroquinoline in n-hexadecane. The results show that bulk ruthenium is exceptionally effective for C-N bond cleavage at temperatures as low as 200 OC and hydrogen pressures of 500 psig. Under similar conditions, rhodium and platinum were less active, and nickel, molybdenum, rhenium, and osmium were inactive. Product selectivity with bulk ruthenium is unusual compared to the normal HDN catalysts such as CoMo and NiMo in that the initial products formed are (methylcyclohexy1)-, (ethylcyclohexy1)-, and (propylcyclohexy1)amines. Ruthenium was found to promote C-N bond cleavage only after saturation of the aromatic ring to decahydroquinoline. However, activity ceases upon exposure to sulfur.
Introduction The catalytic removal of nitrogen (HDN) and oxygen (HDO) from coal-derived liquids consumes excessive amounts of hydrogen and requires extreme reaction conditions. Any method that would result in removing these heteroatoms from coal liquids with a minimum of hydrogenation and at lower temperatures than are in current use could result in a substantial savings in hydrotreating costs. 0887-062~/88/2502-0292$01.50/0
Thus the main emphasis on improved hydrodenitrogenation (HDN) catalysts is to develop catalysts that have increased C-N bond cleavage abilities relative to the hydrogenation of aromatic groups. The industrial HDN process, as presented schematically for quinoline in Figure 1, consumes more hydrogen than necessary because the major reaction pathway leads to a M y saturated molecule, decahydroquinoline(DHQ),prior 1988 American Chemical Society
Energy & Fuels, Vol. 2, No. 3, 1988 293
Bulk Ru as an HDN Catalyst 0.090 0.075
z 0.060
t
aG
23
WH'
0.045
u o
Y
8
0.030 0.015 0
0
U Figure 1. Quinoline HDN reaction network. to C-N bond cleavage. The minor pathway leading to propylbenzene (PB)rather than propylcyclohexane (PCH), consumes 3 mol less of hydrogen than the major pathway. With commercial HDN catalysts, hydrogenation of tetrahydmquinoline (THQ) to DHQ competes successfully with C-N bond cleavage in THQ. If the process leading to C-N bond cleavage could be facilitated relative to hydrogenation, then one would expect to reduce hydrogen consumption and produce the more attractive product, propylbenzene. In past work, we attempted to establish the mechanism(s) of C-N bond cleavage as it occurs in HDN catalysis as a means of improving the overall facility of the HDN process. The approach has been to model the reaction through studies of the interactions of homogeneous catalysts with quinoline and THQ.2-7 These studies demonstrated that a variety of zerovalent metal carbonyl clusters can cleave C-N bonds in saturated amin@. The qualitative ordering of the metals studied is Os 1 Ru > Ir > Rh >> Fe, Co, Mo. The latter three metals were essentially inactive under the reaction conditions. A similar order was observed by Sinfelt for C-C bond hydrogenolysis on bulk metals8 and by Chianelli for HDS on bulk metal suKdes.BJ0 Because the homogeneous catalysis results showed that osmium and ruthenium were very effective for C-N bond cleavage, we extended our investigations to the heterogeneous forms of these and other metals. The studies with catalysts promoted with ruthenium were very positive and are reported elsewhere.11J2 In this paper, we report our efforts to conduct similar studies with bulk metals. Since under our reaction conditions quinoline is rapidly hydrogenated to THQ and does not appear to participate in the C-N bond cleavage portion of the reaction network, the studies have focused on the reactions of THQ with bulk metals.
Results Catalyst activity was established by reacting known (1)Current address: Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195. (2)Laine, R.M.; Thomas, D. W.; Buttrill, Se. E.; Cary, L. W. J.Am. Chem. SOC.1978,100,6527. (3)Shvo. Y.:Laine. R. M. J. Chem. SOC..Chem. Commun. 1980.753. (4jShvo; Y.;Thomas, D. W.; Laine, R. M.J. Am. Chem. SOC.i981, 103,2461-2463. (5)Laine, R.M.; Thomas, D. W.; Cary, L. W. J.Am. Chem. SOC.1982, 104,1763-1765. (6)Giandomenico, C.; Eiaendtadt, A.; Fredericks, M. F.; Hirschon, A. S.; &e, R. M., In Catalysis of Organic Reactions; Augustine, R. L., Ed.; Marcel Dekker: New York, 1985,73-94. (7)Eisendtadt, A.; Giandomenico, C.; Fredericks, M. F.; Laine, R. M. Organometallics 1985,4,2033-2039. (8) Sinfelt, H.B o g . Solid State Chem. 1975,10,55-69. (9)Pecoraro, T.A.; Chianelli, R. R. J. Catal. 1981,67,430. (10)Harrie, S.;Chianelli, R. R. J. Catal. 1984,86,400-412. (11)Hirechon, A. S.;Wilson, R. B., Jr.; Laine, R. M. Repr.-Am. Chem. SOC.,Diu. Pet. Chem. 1981,32(2),268-270. (12)Hirschon. A. S..Wilson. R. B. Jr.. and Wine. R. M. A_D-D ~Catal. . 1987,&f, 311-316. '
2.0
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/a,
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0 05
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TIME Ihours)
Figure 2. Product distribution from reaction of THQ with ruthenium at 500 psig of H2 as a function of temperature.
quantities of the activated metal with solutions of THQ in n-hexadecane, under 500 psig of hydrogen and temperatures up to 350 OC. Of the bulk metals screened by this method, only ruthenium and platinum exhibited significant C-N bond cleavage. The metals nickel, molybdenum, osmium, and rhenium were inactive at temperatures up to 350 "C. Bulk rhodium was active under these conditions, but primarily for hydrogenation of THQ to DHQ and not hydrogenolysis. Consequently, our efforts focused on examining the reactivity patterns of the ruthenium and platinum catalysts. As shown in Figure 2, the producb from bulk ruthenium reactions were very much different from those of a typical hydrotreating catalyst. For instance, when sulfided CoMoor NiMo-supported catalysts were treated under similar conditions a t 350 "C, we achieved the reaction products and intermediates as illustrated in Figure 1." At 300 "C, only DHQ and propylaniline were formed. In contrast, bulk metal ruthenium promoted hydrogenation to DHQ and cleavage of one C-N bond at temperatures as low as 200 "C to produce (methylcyclohexy1)-,(ethylcyclohexy1)-, and (propylcyclohexay1)amines. A t 250 "C, the second C-N bond was cleaved to produce propylcyclohexane. At 300 OC, the ruthenium was so reactive that no nitrogencontaining species remained after 2.5 h. As shown in Figure 2, the hydrogenation of THQ into DHQ was rapid at each temperature relative to the formation of bond cleavage products, including the cyclohexylamine series. As seen from the product distributions, the ruthenium reactivity patterns with THQ led to both C-C and C-N bond cleavage. At first glance C-C bond cleavage appears indiscriminate; however, this is not the case. If C-C bond cleavage were competitive with C-N bond cleavage, then one might expect to observe the formation of methyl(ethylcyclohexy1)amine or ethyl(methylcyclohexy1)amine. However, these species were not observed during the reaction. Although we did not observe these species, it is
Hirschon and Laine
294 Energy &Fuels, Vol. 2, No. 3, 1988 0.20
-b
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Figure 3. Product distribution from reaction of THQ with platinum at 300 "C and 500 psig of H2. possible that they are more reactive than the alkylcyclohexylamines we did observe. In order to test this possibility, competition experiments were run where equimolar amounts of N-ethylaniline and THQ were treated with ruthenium. Under the standard HDN conditions a t 250 "C, N-ethylaniline and THQ were rapidly converted to aniline and DHQ, respectively. Furthermore, during the course of this competitivereaction, the aniline formed from N-ethylaniline was not hydrogenated to cyclohexylamine. Thus, the intermediacy of species such as methyl(ethy1cyclohexy1)amine or ethyl(methylcyclohexy1)amine resulting from C-C bond cleavage of THQ can be ruled out. Moreover, the fact that aniline is not hydrogenated to cyclohexylamine allows us to rule out ruthenium-catalyzed C-N bond cleavage in THQ since this reaction should result in the formation of propylaniline. By exclusion, we can conclude that ruthenium promotes C-C and C-N bond cleavage only in DHQ. Finally, we can suggest, based on our modeling studies and the work of Pezevat et al.,13that the formation of the various cyclohexylamines begins with C-N bond cleavage followed by the sequential extrusion of methylene units. In contrast to the unusual product distribution for the ruthenium reactions, where (propylcyclohexy1)-, (ethylcyclohexy1)-, and (methylcyclohexy1)amineswere formed, bulk platinum (at 300 "C) selectively produced propylcyclohexylamine as well as small amounts of propylcyclohexane and propylbenzene (Figure 3). To compare the relative rates of hydrogenation and C-N bond cleavage for both catalysts, we calculated the turnover frequencies (TF) for the initial disappearance of THQ, formation of cyclohexylamines (CHA), and overall loss of nitrogen (HDN). The TF values were then divided by the respective surface areas to normalize overall activities. We assume that for initial reaction times, the reactants are in excess and any differences in products and reactant adsorptivities on the two metals should be minimal. As seen in Table I, allowing for the surface area differences, the TF for the disappearance of THQ at 300 OC for platinum is 247 compared to 125 for ruthenium. Since the loss of THQ essentially represents the rate of hydrogenation to decahydroquinoline, platinum is therefore approximately twice as active a hydrogenation catalyst as is ruthenium under these conditions. Also at 300 "C, comparison of the first C-N bond cleavage reaction, represented by the formation of cyclohexylamines, shows that ruthenium (TF > 85) is approximately 12 times more active than the platinum metal (TF = 6.7). Furthermore, the TF for nitrogen loss (HDN) for the ruthenium metal is approximately 29, compared to only 0.5 for platinum. (13) Zalma, R.; Bonneau, J.; Fournier,J. Guignard, J.; Pezerat, B. H.; manuscript in preparation.
Table I. Turnover Frequencies for Bulk Metal Reactions" TFb TF/SAc metal temp,OC THQ CHA HDN THQ CHA HDN 200 180 11 0 90 5.5 0 Ru Ru 250 190 60 0.8 85 30 0.4 Ru 300 250 >170 58 125 >85 29 Pt 300 74 2 0.1 247 6.7 0.5 300 180 0 0 45 0 0 Rh CoMod 350 54 e 9 NiMod 350 86 e 17
ahactionof 0.100 g of bulk metal with 10 mL of 0.15 M THQ in n-hexadecane with 500 psig of hydrogen. bTurnover frequency ((mol/h)/mol of metal) for disappearance of tetrahydroquinoline (THQ), formation of cyclohexylamines (CHA), and overall hydrodenitrogenationreactions (HDN). e Turnover frequency divided by surface area of bulk metal. Sulfided Harshaw NiMo- and CoMosupported catalysts on alumina. e Observed only as an intermediate.
It is difficult, if not impossible, to directly determine the metallic surface area of bimetallic supported catalysts; therefore, no surface area corrections were made for the CoMo or NiMo catalysts. Nevertheless, as shown in Table I, the uncorrected TF values for disappearance of THQ and HDN reactions for these catalysts at 350 "C, compared to those of the bulk ruthenium at 300 "C,suggest that the supported catalysts are less active for both hydrogenation and HDN reactions. Since a useful HDN catalysts must be resistant to sulfur, we examined the influence of sulfur compounds on ruthenium reactivity. In situ sulfiding with carbon disulfide caused the ruthenium to become much less active and gave only DHQ even at temperatures up to 350 OC. These results are surprising, since Chianelli et aL9Joobserved excellent reactivity for HDS reactions using bulk ruthenium sulfide. However, we used a zerovalent ruthenium metal surface covered with strongly bound sulfur species rather than a bulk sulfide, which may not exhibit the same reactivity. Futhermore, HDS reactions are much more facile than are HDN reactions, and a similar study with bulk metal sulfides may also show that ruthenium sulfide is one of the better HDN catalysts of the metal sulfides. In other work we have shown that when ruthenium is used in combination with other metals, an exceptionally active HDN catalyst is formed that is both selective and sulfur tolerant.11J2
Discussion In our above studies we found that, of the metals Ru, Os, Mo, Ni, Re, Rh, and Pt, ruthenium was the most active catalyst and the normally used metals for HDN catalysis, nickel and molybdenum, were essentially ineffective. The high HDN activity of ruthenium is consistent with our homogeneous modeling studies where ruthenium was found to be a highly active transalkylation catalyst. The lack of activity with osmium is surprising, however, in that in our modeling studies it was also an active transalkylation catalyst. Auger studies showed that the osmium had no surface contamination that would interfere with its activity. On the basis of our above described objectives, ruthenium should be a better catalyst for HDN than platinum since it is more selective for C-N bond cleavage than is bulk platinum. This result is in contrast with findings of Maier and Guttieri14 for platinum promoted HDN of quinoline using a silica-supported catalyst. In Maier's work, done in the gas phase, the catalyst was found to be (14) Guttieri, M. J.; Maier, W.
J. Org. Chem.
1984, 49, 2875-2880.
Energy & Fuels 1988,2,295-300 active a t temperatures as low as 200 "C. Because of the disparity in the method of catalyst preparation and reaction conditions, any explanation of differences must await future results. The loss of activity of the ruthenium when exposed to hydrogen sulfide is disappointing, since sulfur tolerance is essential for HDN of crude petroelum and coal liquids. Moreover, compared to commercial catalysts such as CoMo and NiMo, ruthenium consumes excessive amounts of hydrogen and, under our conditions, requires complete hydrogenation prior to C-N bond cleavage. The purpose of thiswork, however, was to s w e y candidate metals that may provide some potential to develop more selective HDN catalysts. In concurrent work we have been using ruthenium with other metals and have found that appropriate metal combinations can provide enhanced HDN activity and low hydrogen consumption and overcome the problem of sulfur tolerance.'lJ*
Experimental Section Analytical Procedures. Product analyses for all the kinetic studies were performed on a Hewlett-Packard 5890 GC equipped with FID and a 30-mDB-1column. GC-mass spectral analyses were performed on a LKB-d9000 or a Ribermag R 10-10 mass spectrometer.
295
Preparation of Catalysts. Ruthenium powder (Stem Chemicals) was activated by heating at 400 "C under flowing oxygen for 12 h. Molybdenum powder (Alfa, 250 mesh), nickel powder (Alfa, 100 mesh), platinum powder (Alfa, 24 m2/g), rhenium powder (Alfa 325 mesh), osmium powder (Strem), and rhodium black (Alfa) were activated in flowing hydrogen for 12 h at 400 "C and stored in a dry box prior to use. Bulk metal surface area was measured by standard BET methods (Omicron Technology,Inc.). Surface areas, +lo% m2/g: Ru, 2.0; Ni, 0.12; Mo, 0.24; Co, 0.30; Os, 1.04; Re, 0.30; Rh, 4.06; Pt, 0.30. Standard HDN Reaction Procedures. Under nitrogen, the catalyst (0.100 g), a glass stir bar, and 10 mL of 0.151 M THQ and 0.098 M n-dodecane (as internal standard) in n-hexadecane were placed in a quartz liner, in a 45-mL Parr bomb. The bomb was then purged, and pressurized with 500 psig of hydrogen and heated at the desired temperature and time. The Parr bomb requires less than 5 min to reach 250 "C.
Acknowledgment. We would like to thank the DOE Pittsburgh Energy and Technology Center for support of this work through Grants No. DE-FG22-83PC60781and DE-FG22-85PC80906 and the NSF for partial support through Chem. Eng. Grant No. 82-19541. Registry No. THQ, 25448-04-8; MeCHA, 100-60-7;EtCHA, 5459-93-8;PrCHA, 3592-81-2; Ru, 7440-18-8;Rh, 7440-16-6;Pt, 7440-06-4; Ni, 7440-02-0; Mo, 7439-98-7; Re, 7440-15-5; Os, 7440-04-2; Nz,7727-37-9.
Thermochemical Comparisons of Six Argonne Premium Coal Samples Michael Gumkowski,t Qitao Liu, and Edward M. Arnett* Department of Chemistry, Duke University, Durham, North Carolina 27706 Received September 4,1987. Revised Manuscript Received December 2, 1987
Two thermochemical methods were used to determine the interaction of six Argonne premium coals (Wyodak, Illinois No. 6, Pittsburgh No. 8, Pocahontas No. 3, Upper Freeport, North Dakota Beul a h - % ~lignite) with 12 carefully chosen organic solvents. The heats evolved are compared with those from a previously published study from this laboratory of Wyoming subbituminous, Illinois No. 6, and Texas lignite and also with thermochemicalmeasurements using a sulfonic acid resin as a prototype Brernsted acid, silica as a prototype hydrogen-bondingacid, and a graphitized carbon black (Carbopack F) as a prototype physical adsorbent for dispersion force interaction. A comparison of the use of these solid prototypes versus homogeneous analogues (e.g. Taft-Kamlet parameters) for a limited group of only five basic solvents shows that homogeneous and heterogeneous parameters are about equally good for correlation. However, when correlations using eight solvents and homogeneous parameters are compared to correlations using 10 solvents and solid prototypes, the latter are clearly superior.
Introduction Thermochemical methods based on various types of calorimetry are a powerful tool for comparing acid-base interactions in both homogeneous and heterogeneous systems. Previous reports from this laboratory have described the thermochemical method for comparing solid acids with their homogeneous analogues in response to interactions with a variety of basic liquids. We have attempted to find appropriate solid prototypes for Brernsted acidity,' hydrogen-bonding acidity? and dispersion force 'Present address: Pfizer Central Research, Groton, CT 06340.
interaction^.^ These could be used as standards for comparison in classifying more complex solid acids such as coals. Prototype systems are commonly used in homogeneous solution, for example; the pK,'s of benzoic acids in aqueous solution are the Brernsted acid prototype, which is the standard for linear free energy relationship^.^^^ (1) Amett, E. M.; Haaksma, R. A,; Chawla, B.; Healy, M. H. J. Am. Chem. SOC.1986,108, 4888. (2) Arnett, E. M.; Cassidy, K. F., submitted for publication in Reu. React. Intermed. (3) Hutchinson, B. J.; Healy, M.; Amett, E. M., manuscript in preparation.
0887-0624/88/2502-0295$01.50/00 1988 American Chemical Society
