Ind. Eng. Chem. Prod. Res. Dev. 1884, 23, 519-524 Rao, V. U. S.; (brmiey, R. J. Hydrocarbon Rocess. 1980, 50(11). 139. Reuel, R. C.; Bartholomew, C. H. J . Catal. 1984, 85, 63. Rochester, C. H.; Terrell, R. J. J . Chem. Soc., Faraday Trans. 1 1977, 73, anP ~hee,-K.H.: Brown, F. R.; Finseth, D. H.: Stencel, J. M. zeo//tes 1983. 3 . 344. Seiwood, P. W. “Chemisorption and Magnetization”; Academic Press: New York. 1975. Stencel, J. M.; Rao, V. U. S.; Diehl, J. R.; Rhee, K. H.; Dhere, A. G.: DeAngells, R. J. J . Cstal. 1983, 84, 109. Thomas, T. J.; Hocul, D. A.; Brenner, A. ACS Symp. Ser. 1982, 102, 267. Yates, D. J. C.; Murrell, L. L.; PrestrMge, E. G. J . Catal. 1978, 57, 41.
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The work reported in this paper was presented in the Symposium on New Catalytic Materials at the 186th National Meeting of the American Chemical Society, Washinnton, DC, Aug 1983. Reference in this report to any Specific commercial product, process, or service is to hcilitate Understanding and do& not neiessarily imply its endorsementor favoringby the United States D ~ partment
Of
Received for review December 14, 1983 Accepted August 8, 1984
Hydrodesulfurization by Reduced Molybdenum Sulfides: Activity and Selectivity of Chevrel Phase Catalysts Kevin F. YcCarty and Glenn L. Schrader‘ Department of Chemical EngineeringvAmes Laboratory-USDOE, Iowa State University, Ames, Iowa 5001 1
The hydrodesulfurizatlon activity of.reduced molybdenum sulfides was investigated with a series of Chevrel phase, ternary molybdenum sulfides-M, Mo,S,. Copper, iron, and cobalt Chevrel phase compounds were prepared at 1200 OC by solM state synthesis procedures: the cobalt compounds which were prepared included a range of cobalt-to-molybdenum ratios. Hydrodesulfurization activities for thiophene conversion were measured at 400 OC with both pulsed and continuous flow reactors. Hydrogenation activities were determined in a pulsed mode at 400 OC with 1-butene. The hydrodesulfurizationactivities of all Chevrel phases were comparable to unpromoted and cobalt-promoted MoS2 model catalysts. However, the hydroge ation actMties of the Chevrel phase catalysts were much lower. X-ray diffraction, laser Raman spectroscopy, a d X-ray photoelectron spectroscopy were used to analyze the fresh and used catalysts. The bulk Chevrel phase structure was found to be stable under hydrodesulfurization conditions. Higher oxidation states for surface molybdenum atoms were believed to exist for some of the Chevrel phase catalysts after use.
II
Introduction Much of the current research concerning hydrodesulfurization catalysis has been performed by using catalysts which are prepared starting with Mo6+oxides supported on y-alumina. Various researchers have demonstrated that these oxides become reduced and sulfided under catalytic reaction conditions. For example, Zingg et al. (1980) used X-ray photoelectron spectroscopy (XPS) to show that the Mas+ oxides of an alumina-supported catalyst were reduced to Mo6+ and Mo4+ oxides after treatment in H,at 500 “C. Patterson et al. (1976) sulfided Co-Mo/A1203 oxidic catalysts in H2/H2S and H2/ thiophene ihixtures at temperatures between 400 and 550 “C. The molybdenum 3d6,, XPS binding energy was observed to shift to 228.9 eV, which was attributed to the formation of MoS2. Reduction of supported cobalt molybdate catalysts has been observed to begin at temperatures as low as 250 “C (Schrader and Cheng, 1983,1984). Studies of the oxidation state of molybdenum have also been performed for unsupported catalysts. Using XPS, Okamoto et al. (1979) showed that the molybdenum in unsupported cobalt-molybdenum oxide catalysts was transformed to MoSz after treatment at 400 OC with H2/thiophene or Hz/H2S. The role of “reduced” molybdenum oxidation states (lower than the +4 state of MoSz)in hydrodesulfurization and hydrogenation has not been clearly established, however. Konings et al. (1982) correlated the intensity of the Mo3+ ESR signal with thiophene hydrodesulfurization activity for supported molybdenum and tungsten sulfide catalysts which were promoted with cobalt and nickel.
Using XPS measurements for unsupported, sulfided cobalt-molybdenum catalysts, Delvaux et al. (1979) detected a decrease in the molybdenum 3d binding energies for cobalt concentrations in the region corresponding to the maximum promotional effect for thiophene hydrodesulfurization. They prostulated the existence of an active, reduced molybdenum species with a charge between +4 and +3. The complexity of the preparation of most cobalt molybdate catalysts makes characterization of the oxidation state of molybdenum difficult. However, Chevrel phase compounds offer a unique opportunity for performing a systematic investigation of the catalytic properties of reduced molyliaenum sulfides. Since direct formation of the reduced molybdenum oxidation state can be accomplished, characterization of the resulting compounds is less complex. Furthermore, the Chevrel phases allow the oxidation state of molybdenum to be systematically altered since they are ternary compounds which exhibit compositional variability. The effect of different promoters can also be examined: a wide range of metals may be incorporated as the ternary element. Chevrel Phases In 1971, Chevrel et al. (1971) reported the initial synthesis and characterization of the now-extensive group of materials know as “Chevrel phases.” The unusual electrical properties of the Chevrel phases have attracted much attention since about half of the compounds are superconducting. Comprehensive reviews concerning Chevrel phases are given by Yvon (1979) and Chevrel and Sergent (1982). 0 1984 American Chemical Society
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Table I. Sulfur Chevrel Phases M , M O , S , ~
3
3
t
t
Ternary Components Reported in the Literature Li, Na, Mg, K, Ca, Sc, Cr, Mn, Fe, Co, Ni Cu, Zn, Sr, Y , Pd, Ag, Cd, In, Sn, Ba, La Pb, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, T m Yb, Lu, U, Np Examples of Small Cation Compounds compositional ranges Cu, Mo, S, Cox Mo, S, Ni,Mo,S,
1.6 Q x < 4 1.32 < x Q 2 1.32 Q x < 2
Examples of Large Cation Compounds SnMo,S, PbMo,S, HoMo,S, a
cu
Reference: Chevrel and Sergent (1982).
Figure 1. The Moas8 building block oriented along the ternary axis (adapted from Yvon, 1982).
The Chevrel phase compounds have a general formula M,Mo,Z,, where Z can be sulfur, selenium, or tellurium. The ternary component M can be any of nearly 40 metallic elements (see Table I). For “small” cation elements (e.g., Fe, Co, Cu), the concentration x can be varied continuously within specific limits (e.g., 1.6 I x I 4.0 for Cu,MoeSe). For “large” cation elements (e.g., Pb, Sn, Ho), the range of the concentration is very small or nonexistent. Binary compounds with no ternary component are also known, such as Mo6S8. The metal-rich character of Chevrel phases is revealed by the psuedomolecular nature of the structure. The basis of this structure is the Mo& building block (see Figure l),consisting of a slightly distorted cube of sulfur atoms built around a molybdenum octahedron which is elongated along the ternary axis. Situated slightly outside the face centers of the sulfur cube are the molybdenum atoms. The two sulfur atoms which lie on the ternary axis are distinct from the six other atoms in the sulfur cube. The Mo-Mo bond distance in the octahedron is quite short-2.65 to 2.80 A compared to 2.72 A for metallic molybdenum. If the number of valence electrons associated with the Moe octahedra is increased by increasing the concentration of the ternary component or by using ternary elements with a higher valence, the Mo-Mo intracluster bond distance decreases. The MosSs units are stacked to form structures with rhombohedral or triclinic geometries. Infinite channels containing the ternary component run along the rhombohedral axes. In the “small” cation structure (Figure 2b) the ternary atoms are highly delocalized and partially occupy two different sets of sixfold sites. The degree of delocalization of the ternary atoms is less in the “large“
a
PbMoeS,
b
CU,MO~S,
Figure 2. Chevrel phase structure projected on the hexagonal plane (IlZO), illustrating the arrangement of the ternary metal atoms in (a) PbMo& and (b) Cu,Mo& (adapted from Yvon, 1982).
cation compounds (Figure 2a). The Mo6S8units are interconnected by short, covalent Mo-S intercluster bonds of 2.4 to 2.6 A for the sulfides. Since each unit is bonded to six other units through these bonds, a highly stable structure results (Yvon, 1982). The Mo6 clusters of the sulfide Compounds interact with each other through MoMo intercluster bonds of 3.1 to 3.4 A. The ternary atoms lie in a distorted cube formed by eight sulfur atoms derived from eight different Mosss units. Molybdenum exists in a “reduced” oxidation state in the metal-rich Chevrel phases. For the sulfide phases, the formal oxidation state of molybdenum can be estimated by assuming a sulfur valence of -2. Then for the Mo6S8 binary, molybdenum has a valence of +22/3. Introducing the ternary component into the structure decreases the molybdenum oxidation state. For example, assuming a +1 valence of copper, the molybdenum in CU~.~MO,& has a +2 oxidation state. By varying the concentration of the ternary component, the formal molybdenum oxidation state may be varied continuously between +22/, and +2. Experimental Section Catalyst Synthesis. Copper, iron, and cobalt Chevrel phase catalysts were prepared from a mixture of Mo2.&, molybdenum metal reduced at 1000 “C in hydrogen for 18 h and a sulfide of the ternary component. Cu2S,FeS, and COSwere made by the direct combination of the elements in evacuated, fused-silica tubes. After thorough mixing, the powders for the Chevrel phase syntheses were pressed into pellets and sealed in an argon-filled, fused silica tube; samples were then heated at 1200 “C for 24 to 48 h. The tubes were opened in a drybox, and a portion was removed for X P S analysis. Purity of the materials was established by X-ray powder diffraction and laser Raman spectroscopy. XPS was used to determine the oxidation state of molybdenum. For comparisons of catalytic activity, two representative molybdenum catalysts were also prepared. An umpromoted MoS2 catalyst (referred to as 1000 “C MoSJ was prepared by the thermal decomposition of ammonium thiomolybdate at lo00 “C in a flow of helium (Wildervanck and Jellinek, 1964). A cobalt-promoted MoSz catalyst (prepared at a cobalt to molybdenum ratio of 1:4 and referred to as CO,,~~-MO~-S) was prepared from the homogeneous precipitate obtained by mixing a solution of cobalt nitrate and ammonium paramolybdate at 60 to 70 “C with an ammonium sulfide solution at 60 to 70 “C (Candia et al., 1981). The black precipitate which was formed was dried by evaporation and was slowly heated to 450 “C in a 2% H2S-H2 mixture; this condition was maintained for 4 h.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984 521
Surface Area Measurements. The surface areas of the catalysts were determined by the BET method with a Micromeritics 2100E AccuSorb using krypton. X-ray Powder Diffraction. X-ray powder diffraction was performed on a modified Picker diffractometer using Mo K a radiation. Laser Raman Spectroscopy. Laser Raman spectra were obtained with a Spex Model 1403 monochromator. The excitation source was the 514.5-nm line of an argon laser operated at 200 mW (measured at the source). Spectra were obtained in the back-scattering geometry using a spinning pellet. Spectra were obtained with scanning speeds of 2 cm-'/s at 5 cm-l resolution; a Nicolet 1280 computer was used to accumulate up to 50 scans. X-ray Photoelectron Spectroscopy. XPS analysis was performed on an AEI 200B spectrometer using A1 Ka radiation. Signal averaging was performed with a Nicolet 1180 computer. Samples were mounted on double-sided adhesive tape, and all spectra are referenced to a carbon 1s binding energy of 284.6 eV. In order to avoid air oxidation, both the catalyst preparation tubes and the reactor were opened in a nitrogen drybox; portions of the samples were sealed in Pyrex tubes which were then opened in a helium drybox attached directly to the spectrometer. Spectra of the unused catalysts were obtained from the powder of a freshly ground sample, while spectra of the used catalysts were obtained from the reactor charge with no further grinding. Reactor Design and Equipment. The reactor used in these studies consisted of a 1/4-in. stainless steel tube connected to a system of three Valco valves which allowed switching between the pulsed and continuous flow modes without interruption. The catalyst was held in place with a stainless steel screen. AU gas flows were metered through Q l a n RC-260 mass flow controllers, and the thiophene was fed with a Sage 341 syringe pump. Product analysis was performed on an n-octane/Porasil C column and an Antex 310 gas chromatograph with a digital integrator. High purity helium (99.997%) and hydrogen (99.997%) were further purified by passage through copper traps for removing oxygen and through 4-A molecular sieve traps for drying. Thiophene (Alfa 99%) was subjected to several freeze-thaw cycles and dried over a 4-A molecular sieve. Matheson 1-butene (CP, 99.5%) was dried with a 3-A molecular sieve trap. Hydrodesulfurization Activity Measurements. Hydrodesulfurization activities were measured at atmospheric pressure by using thiophene. Catalyst loadings were adjusted to give a conversion of about 3% after 20 min of continuous thiophene flow. The reactor was loaded with fresh catalyst and was heated from room temperature to 400 "C in a stream of helium at 19 mL/min (STP). After about 45 min, between 10 and 25 0.25-mL pulses of 2 mol 90thiophene in hydrogen were flowed through the reactor at 30-min intervals. The helium flow was then replaced by a continuous flow of 2 mol % thiophene in hydrogen at 22 mL/min (STP). After 10 h of reaction, the reactor was purged and cooled in a helium stream. Hydrogenation Activity Measurements. Hydrogen activity measurements were performed in a pulsed flow mode to minimize potential removal of sulfur from the Chevrel phase catalysts at 400 "C by hydrogen. Reactor loadings were the same as those used in the hydrodesulfurization activity measurements. The reactor was loaded with fresh catalyst and was heated from room temperature to 400 "C in a stream of helium at 19 mL/min (STP). After about 45 min at 400 "C, two 0.1-mL pulses of 2 mol % 1-butene in hydrogen were flowed through the
-3
IO hr
co,,
co,,
co,,
co,,
q,
COMPOSITION OF Co,Mo,S,
Figure 3. Continuous-flow thiophene hydrodesulfurization activities (400 "C) for Co,Mo,SB as a function of cobalt concentration.
c [r
0
2
4
6 8 IO 12 14 16 NUMBER OF THIOPHENE PULSES
18
20
22
Figure 4. Pulsed-flow thiophene hydrodesulfurization activities (400 "C) for various Chevrel phases as a function of the number of pulses.
reactor at 15-min intervals. Twenty-five 0.1-mL pulses of 2% thiophene in hydrogen were then introduced to the reador; the 1-butene pulses followed. After the continuous flow of thiophene-hydrogen for 2 h at 22 mL/min, the reactor was purged with helium, and the 1-butene pulses were repeated (at 400 "C).
Results Hydrodesulfurization Activity Measurements. The continuous-flow thiophene reaction results for C U ~ . ~ M O ~ S ~ , Fe1.5M~6S8, Co1.8M~6S8 (the most active of the cobalt Chevrel phases), and the model MoS2 compounds are summarized in Table 11. The thiophene conversion activities are normalized on the basis of catalyst surface area. All Chevrel phase materials exhibit activities comparable to the model catalysts. Indeed, the long-term activity of the C U ~ . ~ Mcatalyst O ~ S ~is greater than the cobalt-promoted MoS2 catalyst. After 10 h of reaction, the relative hydrodesulfurization activities were found to be in the order CU3,2MO& > C00.25-Moi-S > Fel.5M06S8> C01,8MO&j> 1000 "C MoS2. The C4hydrocarbon product distribution is also markedly different for the various catalysts. For example, after 20 min of reaction, the ratio of 2-butenes to 1-butene is 0.68 for Fe1.5M~6S8, 0.73 for C U ~ . ~ M O 1.25 ~S~, for Co1.8M~6S8, 1.37 for 1000 OC MoS2, and 1.75 for CO~.?~-MO~-S. Figure 3 shows the continuous flow thiophene reaction results for the Co,Mo6S8 series with 1.5 Ix I1.9. After 20 min of reaction time, there are considerable variations in the activities, with roughly a linear increase in activity with cobalt concentration. These differences in activity decreased with reaction time. All CO,MO& materials
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Table 11. Thiophene Hydrodeeulfurization Activities (400 "C) hydrodesulfurization thiosurface reaction phene rate, (mole/s cat. area, m2/g time conv, % m2) X lo8 6.71 CU3.2MO88 0.090 20 min 3.55 4.25 10 h 2.25 5.02 Fe1SMo6SE 0.093 20 min 2.55 1.79 10 h 0.91 6.66 C01.8M06S8 0.080 20 min 2.87 1.39 10 h 0.60 8.50 Coo,zb-Mo,-S 10.83 20 min 2.24 10 h 0.89 3.36 3.08 1000 "C M o S ~ 3.40 20 min 2.56 1.06 10 h 0.88
C., product distribution, %
n-butane 1.3 0.6 0.5 a
1.3 1.5 2.4 1.8
1-butene 57.2 58.9 59.3 57.2 44.4 40.1 35.8 36.4 41.2 46.0
trans2-butene 26.5 28.6 26.4 33.8 37.2 48.5 38.0 41.1 32.7 34.9
cis2-butene 15.0 11.9 13.8 9.0 18.4 11.4
24.9 21.0 23.7 17.3
" Below detection limit. Table 111. l-Butene Hydrogenation Activities (400 "C) hydrogenation cracking rate (mol/s m2) x rate (mol/s cat.a 108 m2) x los A 0.178 -b Cu3.2M06SE B 0.495 C 0.363 A 0.144 Fel.5Mo&3 B 0.142 C 0.137 A 0.156 C01.IMO6S8 B 0.214 C 0.154 CO~,~,-MO~-S A 2.62 1.64 B 1.20 0.66 C 0.800 lo00 "C M o S ~ A 2.35 3.58 B 2.39 2.34 C 0.775 0.36 calculated butene equilibrium at 400 "C'
n-butane 0.1 0.3 0.2 0.1 0.1 0.1 0.1
0.1 0.1 0.8 0.4 0.2 2.1 2.1 0.7
C, product distribution, % 1-butene trans-2-butene 41.5 31.4 49.2 25.8 51.2 23.9 91.7 4.2 82.2 8.6 78.8 10.1 47.3 26.4 40.9 29.5 46.1 29.7 32.7 38.0 39.2 33.9 45.6 30.0 23.6 42.9 23.9 42.6 44.0 30.0 26.5 43.5
cis-2-butene 27.0 24.7 24.7 4.0 9.1 11.0 26.2 29.5 24.1 28.5 26.5 24.2 31.4 31.4 25.3 30.0
"A, fresh catalyst; B, after H2-thiophene pulses; C, after 2 h continuous H2-thiophene reaction. *Below detection limit. cBenson and Bose (1963).
exhibited similar product distributions for the continuous flow experiments. The pulsed thiophene hydrodesulfurization activity data are shown in Figure 4. Cu3,zMo6S6,MoSz, and the cobalt-promoted MoSz catalysts all exhibit similar behavior. The slow decline in catalytic activity with the increasing number of pulses is most likely due to coking (Ramachandran and Massoth, 1982). The Fel.5M06S8catalyst initially has low activity; however, the activity slowly rises. The C O , . ~ M O catalyst-similar ~S~ to the other cobalt Chevrel phase catalysts-exhibits a rather rapid increase in activity. Hydrogenation Activity Measurements. Table I11 summarizes the results for l-butene hydrogenation measurements as determined by pulsed experiments. The activities were calculated as the rate of production of nbutane and were normalized on the basis of catalyst surface area. The hydrogenation activities were measured for three catalyst states: (A) fresh, (B) after pulses of thiophene in hydrogen, and (C) after 2 h of a continuous flow of thiophene in hydrogen. The fresh Chevrel phase catalysts have hydrogenation activities which are about 15 times lower than the fresh model MoSz catalysts. The hydrogenation activities of the fresh Fe,,5Mo,S8 and CO,.~MO~S, catalysts were unchanged after pulsed or continuous flow thiophene studies. Differences also exist for the presence of cracking products. In contrast to the model catalysts, the Chevrel phase catalysts produced no cracking products
from l-butene. Cracking activities (Table 111) were calculated as the rate of production of Cz hydrocarbons from the l-butene pulses and were normalized on the basis of surface area. The hydrogenation activity experiments using pulses of l-butene also indicate the ability of the catalyst to isomerize the l-butene to a mixture of 1-butene, cis-2-butene, and trans-2-butene. The Chevrel phase catalysts all gave butene product distributions which departed significantly from equilibrium concentrations (Table 111). Most notable is Fe1,5M06S8:after 2 h of thiophene reaction (at which time there is significant hydrodesulfurization activity), 79% of the l-butene pulse is unconverted. Laser Raman Spectroscopy Characterization. Fresh and used Chevrel phase catalysts were examined by laser Raman spectroscopy, primarily for the purpose of determining the presence of MoSz impurities. Raman spectroscopy is a more sensitive technique than powder X-ray diffraction for detecting crystalline MoS,; it is also capable of detecting poorly crystalline MoSz. Figure 5 shows the spectra for the C U ~ . ~ M OFel,&O&, ~S~, and Co,,,Mo6S, catalysts in the MoSz region. MoSz has bands at 383 and 409 cm-' (Weiting and Verble, 1971). No MoSz impurities could be detected for the Fe1.5M06S8or cu3.2Mo,& catalysts when fresh or after 10 h of thiophene reaction. However, Col~,Mo6S8has a slight amount of MoSz in the fresh material, and a slightly greater amount in the used catalyst. Similar results are seen in Raman
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984 523 I
I
I
I
I
I
I
I
I
&”“* P
350
400
450
X
500
C”
Figure 5. Raman spectra of fresh and used (10 h thiophene reaction) Chevrel phase catalysts: (a) fresh (b)used Cu3.2Mo6S8;(c) fresh Fel,sMoeS8;(d) used Fel,sMo&38; (e) fresh CO~.~MO&; (f) used C O , , ~ M O ~ S ~
spectra of the other cobalt Chevrel phase catalysts. Attempts to completely remove these traces of MoS2 have been unsuccessful. X-ray Photoelectron Spectroscopy Characterization. The molybdenum 3d XPS spectra of the fresh and used CU&O6S8, Fe1.5M~6S8, and COl.&O& Catalysts are given in Figure 6. The low oxidation state of molybdenum in the fresh catalysts is indicated by the 3d6 binding energy at about 227.7 eV. For comparison, d o S Zhas a molybdenum 3d,,, binding energy of 228.9 eV (Patterson et al., 1976). After 10 h of reaction, this low oxidation state is still present; however, a higher oxidation state (or states) appears. For the used Co1.6M~6S8 catalyst, the concentration of molybdenum atoms with this higher oxidation state is close to that of molybdenum atoms with the initial oxidation state. For the Cu3.zMo6S8 and Fe&06S8 catalysts, the low oxidation state clearly persists after reaction. Discussion of Results Molybdenum Chevrel phase compounds represent a new class of hydrodesulfurization catalysts whose activity and selectivity differ remarkably from conventional cobalt molybdate catalysts. Since these Chevrel phase catalysts incorporate molybdenum in an oxidation state of less than 3, they are referred to as “reduced” molybdenum sulfides. Catalysis by metals in intermediate oxidation states has not been extensively studied, and new properties are likely to be discovered. The Chevrel phase catalysts examined in this study exhibit unusual activity and selectivity for the hydrodesulfurization of thiophene at 400 “C. All of the Chevrel phases have activities which are comparable to unpromoted or promoted model MoSz catalysts. In fact, the long term activity of Cu3,,Mo6SSis greater than either the MoSz or the CO~.~~-MO,-S model catalysts. The activity of copper-based molybdenum sulfides has not been previously reported; the high activity of iron Chevrel phases is also unusual. Cobalt Chevrel phases are the least active. The selectivities of the Chevrel phase catalysts are radically different from those of the model unsupported molybdenum sulfide catalysts. Hydrogenation products are present at much lower concentrations compared to those for MoS, or Coo,,-Mol-S studies at 400 “C. Thomas
l
l
I
I
l
236 235 234 233 232 231
I
I
I
l
1
220 229 228 227 226
BINDING ENERGY lev1
Figure 6. Molybdenum 3d XPS spectra of fresh and used (10 h thiophene reaction) Chevrel phase catalysts: (a) fresh (b) used CU~.~MO&; (c) fresh Fel,&06S8; (d) used Fel,,Mo6S8; (e) fresh COl,6MO&; (f) used C01,6MO&
et al. (1982) also reported much higher concentrations of n-butane using Mo/A1203 catalysts. In addition, the Chevrel phase catalysts demonstrate little ability to isomerize 1-butene. This is most notable for Fel,5M06Ss. In general, the catalysts produced greater amounts of 1butene than 2-butene during thiophene desulfurization (except for used C O ~ . ~ M O ~The S ~ ) concentrations . of butenes present during thiophene reaction differ greatly from those expected for equilibrium conditions. Kolboe and Amberg (1966) showed that for hydrodesulfurization of thiophene at low conversions over MoS2catalysts, the relative concentration of butenes departs significantly from equilibrium. If 1-butene were the initial product of thiophene desulfurization, then 1-butene would be more readily observed for low conversions or for catalysts having little isomerization capability. Finally, the selectivity of the Chevrel phase catalysts is also unusual from the standpoint of the cracking products. The Chevrel compounds produce no detectable concentrations of cracking products from 1-butene. This is in sharp contrast to the model catalysts. The reasons for these changes in catalytic selectivity are likely to be complex. However, every Chevrel phase catalyst examined in this studyindependent of ternary metal “promoter”-exhibited these unique selectivity properties. Apparently, reducing the oxidation state of molybdenum produces a material which has high activity for desulfurization but which has very low activity for hydrogenation, isomerization, or cracking. The effect of various promoters on the activity of Chevrel phase catalysts has been indicated by this study, and comparisons may be made with conventional molybdenum sulfide catalysts. Both cobalt and nickel are excellent promoters for MoSz catalysts: improvements in activity by a factor of 30 have been reported. A maximum in the activity is found for a cobalt-to-molybdenum ratio between 0.4 and 1.0 (Wive1 et al., 1981; Delvaux et al., 1979). However, the effect of various levels of cobalt incorporated into the Chevrel phase compounds is relatively small. A time-dependent phenomenon is observed, with fresh cobalt
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Chevrel catalysts exhibiting a stronger effect of cobalt loading. The initial desulfurization activity peaks at x = 1.8 (Co,Mo6S8);after 10 h the differences in activity have decreased. It is likely that this may be explained in terms of the high mobility of the ternary metals in the Chevrel phases (Dudley, 1980). The concentration of surface cobalt atoms may change under reaction conditions, and consequently, the oxidation state of molybdenum at the surface may increase. Thus, the catalysts may become rather similar after extended reaction periods. The behavior of the iron Chevrel phase catalyst may also be explained on the basis of ternary metal mobility. The fresh Fe1.5M06S8 catalyst initially shows low thiophene hydrodesulfurization activity during pulsed flow experiments; however, the activity of this iron Chevrel phase under continuous flow conditions is comparable to other catalysts. Analysis of the fresh Fel,5M06S8catalyst by XPS reveals a surface enrichment in iron, which apparently hinders the initial activity. This Uexcessniron may retreat into the bulk lattice structure-resulting in activation of the catalyst. X-ray powder diffraction showed that the bulk Chevrel phase structures were stable under hydrodesulfurization conditions (reactive gas atmosphere including H2, H2S, thiophene, and various hydrocarbons) at 400 "C. For fresh and used CuB.2M06S8 or Fel.5M06Ss,no MoSz could be detected by laser Raman spectroscopy. In addition, the XPS results demonstrated that the low molybdenum oxidation state predominated for these catalyst samples. (The extent of surface molybdenum oxidation appears to be especially small for Fel.5M06S8-see Figure 6c and d). The cobalt Chevrel phase catalysts do show an increase in thiophene conversion as a function of pulse number. Laser Raman and XPS characterization reveal the formation of a small amount of MoS2with associated partial oxidation of the surface molybdenum atoms. Some instability of the surface structure of the cobalt Chevrel phase catalysts appears to exist. Chevrel phases are distinct in their catalytic activity and selectivity as reduced molybdenum sulfides. Work is currently in progress dealing with the catalytic properties of other small and large cation Chevrel phases. Acknowledgment This work was conducted through the Ames Laboratory
which is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-ENG82. This research was supported by the Office of Basic Energy Sciences, Chemical Sciences Division. The authors would like to thank Dr. R. N. Shelton of the Ames Laboratory and the Department of Physics, Iowa State University, for helpful discussions concerning the synthesis of Chevrel phase compounds. XPS spectra were obtained by J. W. Anderegg of the Ames Laboratory. Helpful discussions were also held with Dr. R. E. McCarley of the Ames Laboratory and the Department of Chemistry, Iowa State University. Registry No. Molybdenum sulfide, 12612-50-9; copper molybdenum sulfide, 58051-93-7; cobalt molybdenum sulfide, 59787-35-8; iron molybdenum sulfide, 59787-36-9.
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Received for review October 20, 1983 Revised manuscript received January 23, 1984 Accepted February 15, 1984
