313
Langmuir 1985,1,313-320
Adsorption and Desulfurization of Thiophene on Nickel(11 1) Gregory R. Schoofs, Richard E. Preston,+ and Jay B. Benziger* Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544 Received November 6, 1984
The adsorption and desulfurization of thiophene on clean and sulfided Ni(ll1) surfaces were studied with LEED, AES, TPR, and RAIS. The RAIS data indicate that thiophene adsorbs with ita ring parallel or nearly parallel to the surface below room temperature. On clean Ni(lll), thiophene polymerizes slightly above room temperature, as evidenced by paraffinic and aromatic C-H stretches in the reflection infrared spectrum and TPR product yields. Decomposition of the polymer aggregates produces a wide variety of hydrocarbons including C5 fragments at 470 K. On a more acidic sulfided Ni(ll1) surface (e, = 0.19), thiophene appears to undergo electrophilic attack at an a-carbon as evidenced by TPR products and by methane evolution from the reaction of 2,5-dimethylthiophene. An electrophilic attack by a surface metal atom at an a-carbon of thiophene is discussed in terms of molecular orbital theory.
Introduction Despite the impressive development and widespread industrial application of hydrodesulfurization (HDS), a consistent, fundamental understanding of the catalytic reactions does not exist. The roles of the metals in providing catalytic activity as well as the reaction mechanisms of model compounds remain uncertain. In commercial practice, the feedstock and hydrogen are typically passed over a sulfided C0-M0/A1203 catalyst, and the sulfur is removed as H2S. Nickel and tungsten may be substituted for the cobalt and molybdenum, respectively, and are frequently used for heavier feedstocks or if hydrodenitrogenation activity is desired.ls2 Tungsten catalysts exhibit greater activity than molybdenum catalysts do, and nickel enhances hydrogenation activity relative to cobalt.2J The adive form of the catalyst remains unidentified, but HDS reactions are generally thought to occur at anion (sulfur) vacancies in the metal sulfide lattice~.~-~ Many different studies have indicated that the catalytic chemistry of hydrodesulfurization does not depend on the support. Raman spectroscopy experiments have identified MoS2 crystallites on sulfided Mo/A1203catalysts and also have indicated little interaction between the MoS2 and the alumina.4 Voorhoeve and Stuiver compared the catalytic properties of sulfided bulk and A1203-supportedNi-W catalysts and concluded that the alumina had no chemical effect; its only function was to increase the number of active sites by dispersing the metals." Many studies have shown that bulk tungsten, molybdenum, nickel, and cobalt sulfides, either alone or mixed, are active catalysts for HDS. Raney nickel is often used as a desulfurization catalyst in organic syntheses.43 An alternative hypothesis suggests that the alumina plays an intimate role in the structure and reactivity of HDS catalysts.8 While there is much evidence that A1203-supportedcatalysts are more active than catalysts on other supports,3 the HDS activity of the bulk metal sulfides noted above indicates that the basic catalytic chemistry does not depend on the alumina. Combinations of Ni (or Co) and Mo (or W) appear to be more active than Mo or W alone, and hence Ni and Co have been described as promoters. Recent data have suggested that Ni or Co act as the principal catalysts. X-ray photoelectron spectroscopystudies have shown that Ni and Co preferentially accumulate on the surface of Ni-Wg and Co-Molo catalysts in sulfiding atmospheres. 'Present address: SciTec Inc., Research Park, Princeton, NJ 08540.
0743-7463/85/2401-0313$01.50/0
The data of Furmisky and Amberg" and of Dalvaux et al.12 have suggested to others that per unit surface area Cogs8 is at least as active as MoS2"in sulfided Co-Mo catalysts.13 All of the considerations discussed thus far suggest that clean and sulfided W, Mo, Ni, and Co metal surfaces may be acceptable model catalysts to study HDS reactions. Nickel and cobalt should be included given their catalytic activity. Ultrahigh vacuum studies on such surfaces have the potential to elucidate HDS reaction mechanisms, particularly thiophene hydrogenolysis. Thiophene has received extensive attention because it is more difficult to desulfurize than thiols, disulfides, or thioethers?~~ Thiophene is also generally regarded as representative of higher molecular weight, aromatic, organosulfur molecules. The overall reaction for thiophene HDS is2 thiophene
+ 3H2
-
H2S + mixed butene isomers
Butadiene may be formed, possibly as an intermediate, but it is rapidly hydrogenated to a butene. The butenes are more slowly hydrogenated to butane. Thiophene is not hydrogenate prior to sulfur removal, h ~ w e v e r . ~ ~ ~ The nature of the adsorbed thiophene intermediate continues to be debated. On the basis of kinetic studies, previous investigators have speculated that thiophene adsorbs on the metal through the unpaired electrons on the sulfur atom (end-on ad~orption),'"'~ via the a- and P-carbons on one side of the molecule (edgewise adsorption),"J* via all four carbon atoms or the .Ir-electron ring (flat adsorption),'"21 or via a-bonds between the metal and (1) Technical Bulletins, Shell Chemical Co., Houston, TX, 1980. (2)Satterfield, C. N. 'Heterogeneous Catalysis in Practice"; McGraw-Hill: New York, 1980. (3)Gates, B. C.;Katzer, J. R.; Schuit, G. C. A. "Chemistryof Catalytic Processes"; McGraw-Hill: New York, 1979. (4)Brown, F. R.; Makovsky, L. E.; Rhee, K. H. J.Catal. 1977,50,385. (5)Voorhoeve, R. J. H.; Stuiver, J. C. M. J. Catal. 1971,23, 228. (6)Voorhoeve, R. J. H. J. Catal. 1971,23,236. (7)Voorhoeve, R. J. H.; Stuiver, J. C. M. J. Catal. 1971,23, 243. (8)Massoth, F. E. Adu. Catal. 1978,27,266. (9)Ng, K.T.,Hercules, D. M. J.Phys. Chem. 1976,80,2094. (10)Okamoto,Y.;Shimokawa,T.; Imanaka, T.;Teranishi, S. J. Catal. 1979,57,153. (11)Furimsky, E.;Amberg, C. H. Can. J. Chem. 1975,53,2542. (12)Delvaux, G.; Grange, P.; Delmon, B. J. Catal. 1979,56,99. (13)de Beer, V. H. J.; Duchet, J. C.; Prins, R. J. Catal. 1981,72,369. (14)Lipsch, J. M. J. G.; Schuit, G. C. A. J. Catal. 1969, 15, 179. (15)Kolboe, S. Can. J. Chem. 1969,47, 352. (16)Duben, A. J. J.Phys. Chem. 1978,82,348. (17)Griffith, R. H.;Marsh, J. D. F.; Newling, W. B. S. R o c . R. Soc. (London),Ser. A 1949,197,194. (18)Kwart, H.; Schuit, G. C. A.; Gates, B. C. J. Catal. 1980,61,128.
0 1985 American Chemical Society
314 Langmuir, Vol. 1, No. 3, 1985 any single atom that donates electrons to the x-electron ring.21 Transmission infrared spectroscopy studies have been interpreted or reinterpreted to suggest the presence of all of these configuration^.'^,^^-^^ Each configuration implies a different catalytic reaction mechanism for sulfur removal. This paper reports ultrahigh vacuum (UHV) experiments of thiophene adsorption and desulfurization on clean and sulfided Ni(ll1). We sought to elucidate the mechanism of thiophene adsorption and desulfurization on nickel and to compare the results on nickel with those on tungsten obtained recently in our laboratory.26 Unlike industrial HDS proceases, the UHV experiments reported herein are done in the absence of gas-phase hydrogen and on a single crystal of nickel. These UHV experiments are intended to probe the physical organic chemistry of thiophene, not all aspects of which may be manifested in industrial HDS processes.
Schoofs, Preston, and Benziger
1
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400
TEMPERATURE
500
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600
7 0
(OK)
Experimental Section The experimentalapparatus has been described previously.26a The behavior of thiophene and 2,5dimethylthiophenewas studied on clean and sullided Ni(ll1) surfam in a stainless steel ultrahigh vacuum system. The Ni(ll1) crystal was characterized by Auger electron spectroscopy (AES)and by low-energyelectron diffraction (LEED). Thiophene acquired from Alfa Products (99%) and 2,5-dimethylthiophene acquired from Aldrich Chemical Co. (98.5%) were used as received. Purities were verified by mass spedroscopy. Adsorptionwas carried out at a crystal temperature of roughly 250 K. Exposures were sufficient to assure saturation coverage except where noted otherwise. Product distributions and reaction kinetics were obtained by temperatureprogrammedreaction (TPR). The maas spectrometer ionizer was located 6 cm from the crystal and directly faced the crystal during the TPR experiments. The mass spectrometerwas multiplexedby a computer sa that five masses could be monitored simultaneously. One mass-to-charge ratio ( m / q41) was always reproduced as a standard. A constant heating rate of 4.3 K/s was used in all of the TPR experiments. Adsorbed atomic carbon and sulfur products were measured by AES after heating to 600 K. AES was previously calibrated in our laboratory for monitoring surface coverages of adsorbed atomic specie^.^' Reflection-absorption infrared spectroscopy(RAIS)was used to examine adsorbed intermediates. The spectrophotometer utilized a polarization-modulated double-beamtechnique which is described elsewhere.% The monochromator was calibrated with a polystyrene standard; peak positions are accurate to f10 cm-'. The resolution could be controlled by the choice of grating and the slit width at the monochromator exit. In these experiments, the resolution is about 15 cm-'. The theory of RAIS has been reviewed in detail elsewhere.2s For the present purposes, it suffices to note that RAIS detects only those vibrations that have a dipole component normal to the surface.
Results a. Low-Energy Electron Diffraction of Thiophene on Clean Ni(ll1). The adsorption of thiophene on clean (19) Smith, G. V.; Hinckley, C. C., Behbahany, F. J. Catal. 1973, 30, 218. (20) Zdrazil, M. Collect. Czech. Chem. Commun. 1977, 42, 1484. (21) Cowley, S. W. Ph.D. Thesis, Southern Illinois University, Carbondale, 1975. (22) Nicholson, D. E. Anal. Chem. 1962, 34, 370. (23) Blyholder, G.; Bowen, D. 0. J. Phys. Chem. 1962, 66, 1288. (24) Ratnaswamy, P.; Fripiat, J. F. Trans.Faraday SOC.1970,66,2897. (25) Preston, R. E.; Benziger, J. B. J. Phys. Chem., submitted for
publication. (26) Benziger, J. B. Appl. Surf. Sci. 1984, 17, 309. (27) Benziger, J. B.; Preston, R. E. Surf. Sci. 1984, 141, 567. (28) Hoffman, F. M. Surf. Sci. Rep. 1983, 3, 107.
I
,
300
400 TEMPERATURE
500 (OK)
Figure 1. Representative TPR spectra: (a) thiophene on clean Ni(ll1); (b) thiophene on sulfided Ni(ll1) (the sulfur coverage is 0.19 & 0.03 monolayer).
Ni(ll1) at room temperature produced a very diffuse (4x4) pattern in accord with previous LEED observation^.^^ Subsequent heating to 650 K resulted in a very complex LEED pattern which was similar to previous observation~.~~ b. Temperature-Programmed Reaction of Thiophene on Ni( 11 1). Reactions of thiophene were studied on clean and partially sulfided Ni(ll1) surfaces. On clean Ni(lll), thiophene appeared to adsorb molecularly at ca. 250 K with a high sticking probability. The TPR spectra of thiophene on clean Ni(ll1) were characterized by three features. Figure l a shows some representative examples of TPR data. Hydrocarbon products with mass-to-charge ratios of 39,41, and 51-56 were detected in a narrow peak at 305 K with a full width at half-height of 22 K. Molecular thiophene and associated fragments desorbed in a peak at 315 K with a full width at half-height of 65 K. The leading edges of the 305 and 315 K peaks coincide with one another. A wide variety of hydrocarbon and thiophenic fragments were detected in a very broad peak centered at 470 K. Table I gives the m / q values checked and their relative yields. Ions corresponding to C1 and C2 hydrocarbons were produced mainly by fragmentation of larger molecules in the mass spectrometer and hence are not included in Table I. The total yield of all ion fragments collected is roughly 15% of the yield obtained following a saturation dose of carbon monoxide. AES revealed that after a TPR experiment, sulfur and carbidic carbon remained adsorbed on the surface with a stoichiometry of C2.8. (29) Edwards, T.; McCarroll, J. J.; Pitkethly, R. C. Ned. Tijdschr. Vucuuntech. 1970,8, 162.
Langmuir, Vol. 1, No. 3, 1985 315
Adsorption and Desulfurization of Thiophene on N i ( l l 1 )
~~
Table I. Hydrocarbon Yields from TPR of Thiophene on Clean Ni(l11)’ re1 area in re1 area in re1 area in 315 K peak 460 K peak mlq 305 K peak 32 34 39 40 41 42 43 44 45 51 52 53 54 55 56 57 58 66 67 68 69 78 82 83 84
total re1 yield
60
12
a
45 83 41 100
100
50
10
100
16 8 5 10 10 83 38 83 25
26
5 10 60 15
10 11 21 100
8 3 13 13 18 30
50
(30) DeJonn. F.: Sinnine. - H. J. M.: Janssen, M. J. Ow. Mass. Spectrom. 1970,3,3539. (31) Heller, S. R.; Milne, G. W. A. “EPA/NIH Mass Spectral Data Base”: U.S.Government Printing- Office: Washinaton, - DC, 1978 Vol. 1, NSRDS-NBS 63.
70
SO
90
100
b
Does not include m l q G30, as these ions are formed mainly by fragmentation in the mass Spectrometer.
Figure 2a shows the mass spectrum of gas-phase thiophene recorded by the m a s spectrometer in our UHV system under typical dosing conditions. Mass fragment 84 is the parent peak. Isotope studies have identified the m / q 39,45, and 58 peaks as C3H3,HCS, and C2H2Sions, respectively.m Figure 2b summarizes the total product yield from the temperature-programmed reaction of thiophene on clean Ni(ll1). The heights of the bars are proportional to the areas under the TPR curves and are normalized with respect to m / q 41, the most plentiful product. Compared with the mass spectrum of gas-phase thiophene in Figure 2a, Figure 2b shows that although a significant amount of thiophene desorbed molecularly, decomposition reactions produced a wide variety of saturated and unsaturated hydrocarbons. Mass ion 41 is the principal ion fragment of a-olefins such as propene and 1-butene. The mass 43 ion fragment results from saturated paraffins. The intensities of the m / q 55,57, and 69 fragments are surprising. Mass fragments 55 and 69 indicate the presence of C5 olefins and paraffins, respectively, whereas m / q 57 comes from n-hexane or C5+ branched hydrocarbon^.^^ As thiophene contains only four carbons, the presence of these peaks clearly indicates that the clean Ni(ll1) surface catalyzed carbon-carbon bond formation. After a TPR experiment on clean Ni(lll), the carbon was removed by oxidation before running another TPR experiment on the resulting sulfided surface. Very small amounts of sulfur also desorbed in this oxidation step, probably as SO or S02.27Sulfur coverage was measured by the ratio of the peak amplitudes in the derivative Auger spectra for the S(LMM) transition a t 152 eV to the Ni(LMM) transition at 848 eV. This ratio equaled 6.5 for s pattern, which corresponds t o a sulfur a ~ ( 2 x 2 )LEED
60
m 19
20
30
40
14 50
60
70
80
100
90
0
‘1
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I
P
I
L
w
a
820
30
40
50
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70
80
90
100
m 19 Figure 2. Relative yield summaries: (a) mass spectrum of gas-phase thiophene; (b) relative product yields from thiophene decomposition on clean N i ( l l 1 ) ; (c) relative product yields from thiophene decomposition on sulfided N i ( l l 1 ) (the sulfur coverage is 0.19 0.03 monolayer). C1and C2hydrocarbon fragments are produced mainly by fragmentation of larger molecules in the mass spectrometer and hence are not shown.
*
coverage of 0.25 monolayer.32 Assuming a linear relationship between this ratio and sulfur coverage, the sulfided Ni(ll1) surface had a sulfur coverage of 0.19 f 0.03 monolayer. The TPR spectra following thiophene adsorption at 250 K on this surface was characterized by four features. Figure l b shows some representative examples of TPR data. Hydrocarbon products with mass-to-charge ratios of 37-39,41, and 51-56 were detected in two narrow peaks at 305 and 330 K. The peak widths at half-height were 17 K. Thiophene desorbed molecularly in a single peak (32) Rhodin, T. N.; Adams, D. L. In “Treatise on Solid State Chemistry”;Hannay, N. B., Ed.;Plenum Press: New York,1976;Vol. 6A, p 343.
Schoofs, Preston, and Benziger
316 Langmuir, Vol. 1, No. 3, 1985 Table 11. Hydrocarbon Yields from TPR of Thiophene on Sulfided Ni(ll1)" re1 area re1 area re1 area in in 315 K re1 area in in 400 K m/q 305 K peak peak 330 K peak peak 32 34 11 15 37 8 38 12 19 15 39 60 37 100 80 40 41 100 87 100 10 42 9 13 43 5 13 44 45 75 10 51 4 31 4 5 2 7 1 52 53 9 23 5 54 7 28 5 55 18 22 20 56 30 29 25 57 5 8 5 58 100 20 66 67 68 6 69 78 82 6 83 19 3 84 94 30 total re1 yield 100 25 57 4.6
;
nSulfur coverage is 0.19 f 0.03 monolayer. Does not include m / q 630, as these ions are formed mainly by fragmentation in the mass spectrometer.
at 315 K with a full width at half-height of 65 K, as observed on clean Ni(ll1). The leading edges of the 305 and 315 K peaks coincide with one another. A small peak containing many hydrocarbon and thiophenic fragments appeared at 400 K. Table I1 lists the m/q values checked and their relative yields. Some C1 and C2 hydrocarbons were produced by fragmentation of large molecules in the mass spectrometer and hence are not included in Table 11. AES revealed that after a TPR experiment, sulfur and carbidic carbon remained adsorbed on the surface. The stoichiometry of the adsorbed products, corrected for the sulfur initially adsorbed on the sulfided surface, was C 3 3 . Figure 2c shows the mass fragment yields from the temperature-programmed reaction of thiophene on sulfided Ni(lll), Os = 0.19. The heights of the bars are proportional to the areas under the TPR curves and are normalized with respect to m/q 41, the most plentiful product. Integrated desorption yields indicated that one-half as much m / q 41 product desorbed from this sulfided surface as from clean Ni(ll1). Comparing Figure 2c with Figure 2b, more small hydrocarbons, more unsaturated hydrocarbons, and less molecular thiophene desorbed from sulfided Ni(ll1) than from clean Ni(ll1). Thus, sulfided Ni(111)promoted hydrogenolysis reactions and retarded hydrogenation reactions relative to clean Ni( 111). Thiophene HDS entails C-S bond scission, which must involve the a-carbons of the molecule. The TPR of 2,5dimethylthiophene was investigated briefly because the fate of the methyl groups should further elucidate the mechanism of thiophene HDS. Methyl group ( m / q 15) and methane ( m / q 16) ions evolved in a peak at 320 K following 2,Ei-dimethylthiopheneadsorption a t 250 K on partially sulfided Ni(ll1). In contrast, this feature was not observed on clean Ni(ll1). The reactions of 2,5-dimethylthiophene on clean and sulfided Ni(ll1) appeared
10.1%
r
; a\ 2800
2900
3000
3100
3200
WAVENUMBER (cm-')
Figure 3. Infrared reflection-absorption spectra of thiophene adsorbed on clean Ni(ll1): (a) thiophene adsorbed at 273 K; (b) adsorbed thiophene heated to 350 K; (c) adsorbed thiophene heated to 550 K.
to parallel the reactions observed with thiophene, although several heavier reaction products were observed with 2,5dimethylthiophene. Detailed studies of the reactions of 2,5-dimethylthiophene were not undertaken as its low vapor pressure resulted in deterioration of the vacuum in the UHV system. We had hoped to compare the reactions of 2,5-dimethylthiophene with 3,4-dimethylthiophene. To the best of our knowledge, the latter is neither available commercially nor readily synthesized, and hence it was not investigated. c. RAIS of Thiophene on Ni(ll1). Figure 3a shows the reflection infrared spectrum of thiophene following adsorption at 273 K. An extremely slight absorption feature appears at 3080 cm-l. After heating to 350 K, the RAIS spectrum in Figure 3b displays prominent features at 2960 and 3080 cm-', corresponding to paraffinic (sp3) and aromatic (sp2) C-H stretches, re~pectively.~~ Liquid-phase thiophene has merely one feature in this region, a band a t 3120 cm-', which has been attributed to an aromatic (sp2)C-H stretch.34 Subsequent heating to 550 K caused all features to disappear, as indicated in Figure 3c. RAIS experiments of thiophene on sulfided Ni(ll1) did not reveal any features between 2800 and 3200 cm-'. Discussion The results obtained here indicate that adsorbed sulfur on Ni(ll1) changea the reaction paths of thiophene. These differences may be explained in terms of changes in electrophilicity of the surface due to adsorbed sulfur. In the following discussion our interpretation of the results is first summarized. Subsequent sections discuss the supporting evidence in detail. Last, thiophene desulfurization on Ni(ll1) and W(211) is compared. a. Thiophene on Clean Ni(ll1). a.1. Summary. The results suggest that thiophene adsorbs on the nickel surface in a flat or nearly flat configuration at low temperatures. Slightly above room temperature, some thiophene molecules polymerize on the surface. Desorption of various hydrocarbon fragments and molecular thiophene competes with this rearrangement, as evidenced by the TPR peaks at 305 and 315 K, respectively. At ca. 470 K, the polymer aggregates decompose. A wide variety of hydrocarbons (33) Colthup, N. B. J. Opt. SOC.Am. 1950, 40, 397. (34) Hartough, H. D. 'Thiophene and Ita Derivatives", Interscience
Publishers: New York, 1952.
Adsorption and Desulfurization of Thiophene on N i ( l l 1 )
(including C5+ species) desorb, while some carbon and sulfur remain adsorbed on the surface. a.2. Evidence for Flat or Nearly Flat Thiophene Adsorption below Room Temperature. Recalling the dipole selection rule for RAIS, Figure 3a indicates that the thiophene ring adsorbs parallel or nearly parallel to the Ni(ll1) surface below room temperature. The reflection infrared spectrum shown in Figure 3b indicates the approximate intensity expected if thiophene adsorbs perpendicular to the Ni(ll1) surface. The LEED data agree with this result. The (4x4) LEED pattern that emerges following thiophene adsorption at room temperature has been interpreted to mean that "the molecule lies flat on the surface".29 Many other aromatic molecules adsorb on transition metals with the aromatic ring parallel to the surface, especially at low temperatures and/or low coverages. Examples include benzene on Ni(ll1) and Ni(100)3S936 and over 20 aromatic molecules on polycrystalline platinum in electrochemical systems.37 Typically these results have been explained by postulating an interaction of the aelectron cloud(s) of the aromatic molecule with the metal surface. The formation of such an adduct probably has a negligible activation energy because the structure of the adsorbate is not significantly altered. This would account for the high sticking probability of thiophene on Ni(ll1) below room temperature. a.3. Evidence for Thiophene Polymerization. Between 300 and 350 K, a molecular rearrangement occurs as evidenced by the presence of paraffinic (sp3)and aromatic (sp2)C-H stretches in the reflection infrared spectrum, Figure 3b, and the TPR peaks at 305 and 315 K. The activation energy for this process is approximately 80 kJ/mol, roughly equal to the resonance energy of thiophene.38 Hence the rearrangement probably disrupts the aromatic nature of the thiophene ring. Although a variety of rearrangements might seem possible, the formation of C5+species suggests that the clean Ni(ll1) surface catalyzes the polymerization of thiophene. Molecular thiophene desorption and some desulfurization reactions compete with the polymerization reactions, as indicated by the TPR peaks at 315 and 305 K, respectively. Thiophene polymerization proceeds readily in mineral acidsu and on surfaces, including silica-alumina gels and montmorillonite alumina,40 silica,4l and silicasupported n i ~ k e l ,all ~~ at,room ~ ~ temperature. The trimer39942
Langmuir, Vol. 1, No. 3, 1985 317
and higher order derivative^^^ have been isolated and characterized. The transmission infrared spectrum of the trimer indicated the presence of both tetrahydrothiophene (paraffinic) and thiophene (aromatic) rings, which is in agreement with the reflection infrared results presented here. In addition, the organic synthesis literature contains many examples of dimerization of 2-substituted thiophenes in desulfurization reactions on Raney nickel, especially if the ratio of available hydrogen to thiophene is Acid-catalyzedS and free radical4 reaction mechanisms have been proposed to explain these rearrangements. The acid-catalyzed reaction mechanism appears more likely, particularly as the surface acts as a Lewis acid (electron acceptor) in the adsorption of thiophene. The observation of trimer formation from surface-catalyzed r e a c t i o n ~ ~ * ~ ~ , ~ ~ is also strong support for an acid-catalyzed mechanism. Acid-catalyzed polymerization produces polymers, whereas the free radical reaction mechanism can only produce dimers. In the acid-catalyzed mechanism, the first step involves the electrophilic attack of the acid or surface at the acarbon of a thiophene molecule, followed by alkylation of this thiophene by another thiophene molecule, and so on. Note that hydrogen is conserved in these reactions. Isotope studies have shown that the hydrogen atoms in the middle tetrahydrothiophene ring(s) originate from thiophene molecules and not from the surface.41 HDS of the trimer on a Co-Mo/A1203 catalyst produced the expected 5methylunde~ane.~*~~ Other conjectures seem less likely, primarily because they cannot account for the presence of C5 hydrocarbon species. For example, an edge-bonded thiophene molecule H
could conceivably exhibit both olefinic (sp2)and paraffinic (sp3) C-H stretches in the reflection infrared spectrum. However, metalation reactions of thiophene occur selectively at the ~x-carbon,~~ discounting the existence of an edge-bonded intermediate. Moreover, the hydrocarbons need to combine to form higher molecular weight hydrocarbons following the desulfurization of such an intermediate. Previous UHV studies of C1 and C2 hydrocarbons on nickel did not find evidence of combination to form larger hydrocarbon^,^"'^ and Fischer-Tropsch synthesis on nickel produces methane e x c l ~ s i v e l y . ~ ~ Open-ring structures with both paraffinic (sp3) and olefinic (sp2)carbons such as H I
H I
H-C-C I1 (35) Lehwald, S.; Ibach, H.; Demuth, J. E. Surf. Sci. 1978, 78, 577. (36) Bertolini, J. C.; Dalmai-Imelik, G.; Rousseau, J. Surf. Sci. 1977, 67, 478. (37) Soriaga, M. P.; Hubbard, A. T. J. Am. Chem. SOC.1982,104,3937.
(38) Morrison,R. T.; Boyd, R. N. "Organic Chemistry", 3rd ed.;Allyn and Bacon: Boston, 1973. (39) Meieel, S. L.; Johnson, G. C.; H " g h , H. D. J. Am. Chem. SOC. 1950, 72,1910. (40) Lygin, V. I.; Romanovski, B. V.; Topchieva, K. V.; Tkhoang, K. S. R w s . J. Phys. Chem. (Engl. Traml.) 1968,42, 156. (41) Rochester, C. H.; Terrell, R. J. J. Chem. SOC.,Faraday Tram. 1 1977, 73, 596. (42) Curtis, R. F.; Jones, D. M.; Ferguson, G.; Hawley, D. M.; Sime, J. G.; Cheung, K. K.; Germain, G. J. Chem. Soc., Chem. Commun. 1969, 165.
S /c\H
(43) Bonner, W.A.; Grimm, R. A. In "The Chemistry of Organic Sulfur Compounds"; Kharaach, N., Meyers, C. Y., Eds.; Permagon Press: Oxford;1966; Chapter 2. (44) Dalmai-Imelik, G.; Bertolini, J. C. J. Vac. Sci. Technol. 1972, 9, 677. (46) Schoofs, G. R.; Benziger, J. B. Surf. Sci. 1984, 143, 359. (46) Demuth, J. E.; Eaatman, D. E. Phys. Rev. Lett. 1974,32,1123. (47) Demuth, J. E.; Eastman, D. E. Phys. Rev. B 1976, 13, 1523. (48) Ponec, V. Catal. Rev.-Sci. Eng. 1978, 18, 151.
318 Langmuir, Vol. 1, No. 3, 1985 are even less likely in UHV experiments due to a shortage of available hydrogen. As with the edge-bonded intermediate, the subsequent hydrocarbon recombination reactions cannot be explained. b. Thiophene on Sulfided Ni(ll1). b.1. Summary. On sulfided Ni(ll1) (e, = 0.19), RAIS indicates that thiophene adsorbs in a flat or nearly flat configuration at low temperatures as was observed on clean Ni(ll1). At ca. 305 K, an a-carbon on the thiophene ring appears to succumb to an electrophilic attack by a surface nickel atom. Desulfurization produces hydrocarbon TPR peaks at 305 K. Molecular thiophene desorption competes with this reaction, as indicated by the overlapping TPR peak at 315 K. A second desulfurization reaction occurs at 330 K. Unsaturated C3 hydrocarbons are the principal desorption products from both desulfurization reactions, while some carbon and sulfur remain adsorbed on the surface. The electronic and obstructive effects due to the presence of adsorbed sulfur virtually prevent thiophene polymerization, as indicated by the very small size of the 400 K TPR peak. b.2. Hydrogenation vs. Hydrogenolysis Reactions and the Effects of Adsorbed Sulfur. The carbon content of the surface residues following TPR experiments and the selectivity shift to C3 hydrocarbons displayed in Figure 2 illustrate the enhanced hydrogenolysis activity of the sulfided surface relative to clean Ni(ll1). The selectivity shift from paraffms to less saturated hydrocarbons also displayed in Figure 2 indicates retarded hydrogenation activity of the sulfided surface relative to clean Ni(ll1). Hydrogenolysis and hydrogenation actually compete for the available hydrogen on the sulfided surface, however, as indicated by the comparable yields of mlq 41, an olefin, and m / q 39, a less saturated hydrocarbon. These results may be attributed to electronic effects caused by the adsorbed sulfur and a shortage of available hydrogen in the TPR experiments. On the basis of work function measurements, Somorjai has estimated that sulfur extracts roughly 10% of an electron from the Ni(ll1) surface at each adsorption ~ i t e . 4 ~ The loss of electrons from the surface significantly raises the electron affinity or Lewis acidity of the nickel atoms that remain exposed, improving hydrogenolysis activity. Sulfur compounds also poison nickel hydrogenation cataUHV studies suggest that long-range elecl ~ s t s Recent .~ tronic effects due to the presence of adsorbed sulfur play a major role in poisoning hydrogenation reactions.50 In addition, thermal desorption and electron energy loss spectra have indicated that local interactions (i.e., siteblocking effects) dominate the behavior of CO on presulfided Ni(100) surface^.^^.^^ Thus, a combination of the long-range electronic effects and the short-range configurational effects due to the presence of immobile sulfur adatoms enhances hydrogenolysis activity, retards hydrogenation activity, and impairs the polymerization reactions relative to clean Ni(ll1). The TPR results indicate that hydrogenation and hydrogenolysis reactions strongly compete for the available (49)Somorjai, G. A. "Chemistry in Two Dimensions: Surfaces"; Comell University Press: Ithaca, NY, 1981;Chapter 5. (50) Goodman, D. W. Acc. Chem. Res. 1984, 17,194. (51) Madix, R. J.; Lee, S. B.; Thornburg, M. J. Vac. Sci. Technol., A 1983, I, 1254. (52) Gland, J.L.; Madix, R. J.; McCabe, R. W., DeMaggio, C. Surf. Sci.
---.
1984.143. ~ ,. . 46. ~
(53)Streitwieeer, A."Molecular Orbital Theory for Organic Chemists"; Wiley: New York, 1961. (54) Gleiter, R.;Spanget-Lamen, J. In "Topics in Current Chemistry: Spectroscopy"; Boschke, F. L., Ed.;Springer-Verlag: Berlin, 1979 Vol. 86, p 139.
Schoofs, Preston, and Benziger
- b,(d
Figure 4. Molecular orbitalsand their energylevels for thiophene. This information is reproduced from ref 54. The sulfur atom is located at the lowest point on the molecule. The circle sizes are proportional to the electron densities. The P molecular orbitals extend above and below the plane of the thiophene ring; the al(u) molecular orbital lies in the plane of the thiophene ring. hydrogen on sulfided Ni(lll), as both propene and less saturated C3 hydrocarbons were the major desulfurization products desorbing. In contrast, HDS of thiophene in conventional catalytic reactors produces mixed butene isomers. These observations may be attributed to a shortage of available hydrogen in the TPR experiments. The only sources of hydrogen for the TPR reactions were thiophene molecules, which decomposed completely, and possibly trace amounts of hydrogen adsorption from the low-pressuresurrounding gas. At high partial pressure of hydrogen, the hydrogenation reactions dominate over the hydrogenolysis reactions. b.3. Evidence for an Electrophilic Attack at an a-Carbon of Thiophene. A t ca. 305 K, thiophene reacts on the sulfided Ni(ll1) surface. The activation energy for these reactions is approximately 80 kJ/mol, roughly equal to the resonance energy of t h i ~ p h e n e .TPR ~ ~ spectra of 2,Ei-dimethylthiopheneon this surface indicated the loss methane from the molecule with a slightly higher activation energy. In contrast, this feature was not observed in the TPR spectra of 2,5-dimethylthiophene on clean Ni(111). Furthermore, thiophene desulfurization produces mostly C3 hydrocarbon desorption products on sulfided Ni(ll1). These results imply that the sulfided Ni(ll1) surface makes an electrophilic attack on the a-carbon of thiophene, forming a new adsorbed intermediate prior to C-S bond scission and desulfurization. Such an electrophilic attack should be an activated process because the resonance stabilization of the thiophene ring is lost as the electron-deficient nickel surface accepts electrons from the electron-rich a-carbon of thiophene. Desulfurization subsequently occurs at 305 and at 330 K. In contrast, thiophene adsorption through the lone pair of electrons on the sulfur atom (end-on adsorption) would be expected to produce desorbed C4 fragments upon heating.14 Electrophilic substitution reactions of thiophene occur selectively at the a-carbon in homogeneous systems.3s Metalation reactions also occur selectively at the a-carbon of thiophene.34 By analogy, it is suggested that metal
Adsorption and Desulfurization of Thiophene on N i ( l l 1 )
Langmuir, Vol. 1, No. 3, 1985 319
surface atoms should also act as electrophilic reagents. Table 111. Comparisons of Thiophene Desulfurization on Ni(ll1) and W(211) This result may be rationalized within the frameworks of molecular orbital theory or resonance stabilization theory. T P R peak value of z in The two theories are equivalent. Molecular orbital theory C.S residue" surf temp, K is discussed here because it is more quantitative. Resoclean,Ni(ll1) 470 2.8 nance structures of thiophene following an electrophilic sulfided Ni( I l l ) * 330 3.4c clean W(211) 400 4.0 attack can be found in organic chemistry textbooks. sulfided W(211) 420 2.0c In molecular orbital theory, electrons are transferred from the highest occupied molecular orbital of the elec"Determined by AES following a T P R experiment. bSulfur trophile to the lowest unoccupied molecular orbital of the coverage is approximately 0.2 monolayer; 1 monolhyer equals 1 S atom per 1 metal atom. This stoichiometry does not include the electrophilic reagent.51 Figure 4 shows the four highest sulfur initially present on the sulfided surface. occupied molecular orbitals of thiophene and their respective energies.52 In the present case, electrons transfer The ?PR peaks at 305 and 330 K have anomalously from the a-carbon of thiophene to an empty d orbital on narrow peak widths. Narrow TPR peaks result from inthe Ni(ll1) surface. Molecular orbital theory also explains teractions between adsorbates and indicate autocatalytic why end-on bonding through the lone pair of eledrons on kinetics. As the peak width was narrower for the s a i d e d the sulfur atom has not been observed in gas-phase, liqsurface relative to the clean surface, the reaction rate acuid-phase, or transition-metal complex reactions of celeration clearly results from thiophene-sulfur interacthiophene. The al(a) orbital lies 3.2 eV or 310 kJ/mol tions rather than thiophene-thiophene interactions. below the highest occupied molecular arbital of thiophene Further work needs to be done to clarify this issue, as it and hence is much too stable to participate in chemical is not known what specific factors cause the anomalous bonding. kinetics. Previous investigators have hinted at the possibility of c. Comparison of Thiophene Desulfurization on an electrophilic attack by the surface at the a-carbon of Ni(ll1) and W(211).It is worthwhile to compare the thiophene in HDS reactions but did not fully develop the activity and selectivity of the desulfurization of thiophene notion. Desikan and Amberg recognized that the a-carbon on Ni(ll1) with that on W(211) obtained recently in OI& must be involved in C-S bond scission of thiophene by laboratory.26 The peak temperature of the TPR curves, d e f i n i t i ~ n . ~Kolboe ~ and Amberg% and Kwart et a1.18 Tp,is a measure of catalyst activity, since noted the high electron density around the a-carbon. It has been observed that thiophene exchanges hydrogen for E = 0.25Tp (1) deuterium faster at the a-carbon than at the p - ~ a r b o n l ~ * ~ ~ where E = activation energy for reaction in kJ/mol and and that HDS of methylthiophenes proceeds more slowly Tp = peak temperature of the TPR curve in K to within when one or both alpha-carbons are s u b ~ t i t u t e d .Many ~~ roughly 15% for most molecules on metal surfaces and correlations have indicated that sulfur anion vacancies (i.e., commonly employed heating rates.32 Lower peak temexposed metal atoms) are the active sites for thiophene peratures indicate more active surfaces, The stoichiometry desulf~rization.~~~ of the residue remaining on the surface after the TPR b.4. Interactions between Adsorbed Thiophene and experiments, C,S, indicates the selectivity of the surface Adsorbed Sulfur. Desulfurization occurs at both 305 and for C-S hydrogenolysis relative to C-C hydrogenolysis. A 330 K on the sulfided Ni(ll1) surface. The nature of these lower value of x implies a more selective surface. two desulfurization steps is not understood. One possible Table I11 summarizes the activity and selectivity data explanation could be the formation of disulfide linkages obtained for "11) and W(211) surfaces. Sulfur passibetween sulfur atoms in thiophene molecules and sulfur vates W(211) but activates Ni(ll1) for desulfurization adatoms, which occurs on sulfided W(211).25 If an adreactions. Sulfided nickel appears to be the most active sorbed thiophene has an adjacent sulfur adatom a weak surface, whereas sulfided tungsten exhibits the highest disulfide linkage may be formed, increasing the stability selectivity. Clean Ni(ll1) and sulfided W(211) behave of some of the intermediates on the sulfided surface. Thus, similarly. These results suggest that sulfur adatoms the following species modify thiophene decomposition chemistry via electronic effects primarily on Ni(ll1) but that site blocking effects SI + I dominate on W(211). The results in Table I11 neglect any effhcts due to alloying and an abundance of available hydrogen, both of which exist in commercial HDS processes. Alloying can induce synergistic effects in catalyst activity and selectivity, which desulfurizes at 305 K, would be present on both whereas hydrogen also influences the selectivity and proclean and sulfided Ni(lll), whereas the more stable moiety longs catalyst lifea2s3 However, the greater affinity of tungsten for sulfur and the ability to form nonstoichiometric tungsten sulfides may keep the nickel in a partially sulfided state when it is alloyed with tungsten. In such a partially sulfided state the nickel is very active for HDS, providing enhanced activity of Ni-W catalysts relative to SI * unalloyed W catalysts. which decomposes at 330 K, would exist only on the sulConclusions fided surface. The results presented herein have elucidated the physical organic chemistry of thiophene on nickel. The (55) Desikan, P.; Amberg, C. H. Can. J. Chem. lb63,41,1966. results may be summarized as follows: (56) Kolboe, S.; Amberg, C. H. Can. J . Chem. 1966,44,2623. (1) The thiophene ring adsorbs nearly parallel to the (57)Satterfield, C. N.; Modell, M.; Wilkens, J. A. 2nd. Eng. Chem. Process Des. Deu. 1980, 19, 154. Ni(ll1) surface below room temperature. Presumably this
q Y
*.Q
Langmuir 1985, 1 , 320-326
320
bond involves an interaction of the a-electron cloud with the nickel surface, illustrating the aromatic or benzoid behavior of thiophene. (2) Reactions at higher temperatures exemplify the nonbenzoid behavior of thiophene. On clean Nit l l l ) , thiophene forms polymer aggregates via an alkylation reaction, as also seen in experiments at moderate pressures and in organic syntheses. In this reaction, the nickel surface acts as a weak acid catalyst. The polymer aggregates decompose at higher temperatures to yield a variety of saturated and unsaturated hydrocarbons, including C5+ species. (3) On a more acidic sulfided Ni(ll1) surface, thiophene succumbs to an electrophilic attack at the a-carbon by a surface nickel atom. This attack destroys the aromatic nature of thiophene and facilitates subsequent desulfur-
ization. The hypothesis that thiophene HDS occurs at anion vacancies (i.e., exposed metal atoms) coincides with this result if anion vacancies are considered as electrophilic reagents. The desulfurization of thiophene on this surface produces unsaturated C3 hydrocarbons primarily, as hydrogenation and hydrogenolysis reactions compete for the available hydrogen. The stronger Lewis acidity of the sulfided Ni(ll1) surface and the presence of immobile sulfur adatoms prevent the polymerization reactions.
Acknowledgment. We thank the National Science Foundation (CPE-8217364) and the Air Force Office of Scientific Research (AFOSR-82-0302)for their financial support of this work. Registry No. Ni, 7440-02-0; thiophene, 110-02-1; 2,5-dimethylthiophene, 638-02-8.
Infrared Studies of Carbons. 8. The Oxidation of Phenol-Formaldehyde Chard C. Morterra and M.J. D. Low* Department of Chemistry, New York University, New York, New York 10003 Received November 19, 1984
Infrared spectra were recorded of chars, produced by the in vacuo pyrolysis of a well-defined novolac-type phenol-formaldehyde resin (PFN), at various stages of oxidation. IR photothermal beam deflection spectroscopy was used. With low-temperature PFN chars, oxidation leads mainly to attack of methylene bridges, benzophenone structures and carboxylic acids being formed, without substantial disruption of the polymer network. In distinct contrast to cellulose-based chars, low-temperature PFN chars are fairly resistant to oxidation and intermediate-temperaturechars are more reactive. With the latter the polymer network is disrupted in a narrow temperature range near 550 OC in that methylene bridges are attacked and changed and polyaromatic domains form. After this disruption, the intermediate- as well as hightemperature chars exhibit the "normalnoxidation behavior found with chars derived from other precursors.
Introduction Phenol-formaldehyde resins can be well-defined chemically and, under controlled conditions, can be pyrolyzed to yield chars having well-defined surface properties1p2 useful for various tasks including chromatography. There are, however, conflicting views concerning the nature of the main degradation mechanism leading to char formation, and this prompted us to study the thermal degradation of a well-defined phenol-formaldehyde resin of the Novolac type, termed PFN for brevity. Infrared (IR) spectra were recorded of PFN chars produced at various temperatures in vacuo and in Nz, and it was possible to obtain information about the PFN degradation mechanism and the nature and composition of the chars; full details are given el~ewhere.~ One of the interesting and important observations made then was that the residues produced by pyrolyzing PFN at temperatures up to about 500 O C had most of the properties of chars but were not extensively carbonized, Le., there was very little of the build-up of polyaromatic domains or graphitization normally found with chars derived from cellulose and with other carbons.2-12 Essentially, although various branching and cross-linking reactions occurred during the formation of the char, the original skeletal structure of the PFN parent material was +Part7: ref 3.
largely retained, so that the char consisted of a framework largely composed of aromatic nuclei held apart by methylene bridges. However, these quite atypical chars could not withstand substantially higher temperatures. The char framework collapsed and extensive carbonization occurred in a narrow temperature interval near 550 "C. The observations made with PFN chars are thus quite unlike those made with chars derived from cellulose or with other carbons. With cellulose, some carbonization occurred at temperatures as low as 180 "C,and with increasing temperatures, there were gradual changes in carbonization, in the nature and number of surface species, and in the reactivity of the char surfaces toward oxygen. The behavior of carbon materials toward oxygen depends on the degree of carbonization, and in general, it is (1)Jenkins, G.M.; Kawamura, IC. 'Polymeric Carbons, Carbon Fiber and Char"; Cambridge University Press Cambridge, 1976. (2)Delhaes, P.;Carmona, F. Chem. Phys. Carbon 1981,17, 123. (3)Morterra, C.; Low,M. J. D. Carbon, in press. (4)Low, M. J. D.; Morterra, C.Carbon 1983,21,275. (5)Morterra, C.; Low,M. J. D. Carbon 1983,21,283. ( 6 ) Morterra, C.; Low, M. J. D.; Severdia, A. G. Carbon 1984,22,5. (7)Morterra, C., Severdia, A. G.; Low, M. J. D. Carbon, in press. (8)Morterra, C.; Low,M. J. D. Carbon, in press. (9)Low,M. J. D.; Morterra, C. Carbon, in press. (10) Morterra, C.; Low, M. J. D. Mater. Chem. Phys. 1985,12,207. (11) Morterra, C. and Low, M. J. D., Mater. Lett. 1984,2, 289. (12)Morterra, C. and Low, M. J. D.,Spectrosc. Lett. 1982,15, 689.
0743-7463/85/2401~0320$01.50/0 0 1985 American Chemical Society
