5699
J. Phys. Chem. 1993,97,5699-5702
ESCA Study of "Model" Allyl-Based Mo/SiOz Catalysts Jane M. Aigler, Joaquin L. Bnto, Patricia A. Leach, Marwan Houalla, Andrew Proctor, N. John Cooper,W. Keith Hall, and David M. Hercules' Department of Chemistry and Materials Research Center, University of Pittsburgh, Pittsburgh, Pennsylvania I5260 Received: January 8, 1993
Stoichiometric redox measurements and X-ray photoelectron spectroscopy (XPS or ESCA) studies were performed on an allyl-based Mo/SiOz catalyst (1.7 wt % Mo) obtained by the sublimation of Mo(v3-C3H5)4 onto Si02 at 40 OC. The average oxidation state estimated from stoichiometric measurements correlated well with those reported in the literature. ESCA results indicated that reduction of the Mo/SiOz catalyst at 550 "C primarily led to the formation of Mo2+. Also, the results were consistent the reported reversibility of the redox cycle. However, the reported formation of discrete Mo4+ by oxidation of the reduced catalyst at room temperature could not be substantiated.
(a)
Introduction SI -OH
Molybdenum-based catalysts are employed in several industrially important reactions. In their sulfided form, molybdenum catalysts are primarily used for hydrotreating. In their reduced form, they are activefor avariety of reactions including metathesis, isomerization, hydrogenation, and hydrogenolysis. However, despite considerable research effort, the nature of the surface molybdenum species or Mo oxidation state responsible for the catalytic activity for a given reaction remains a matter of debate. Careful examination of the literature related to supported molybdenum catalysts indicates that the difficulty in providing definitive answers concerning the structure/catalytic activity relationship can be attributed to two causes: first, lack of detailed knowledge about the structure of the oxidic precursor; second, inadequate means for monitoring Mo oxidation states. Supported Mo catalysts prepared by conventional methods (e.g., pore volume or incipient wetness impregnation) often show heterogeneous surface structure. The supported phase in the oxidic precursor consists of species ranging from a surface compound or a monolayer-like phase associated with the support to discrete particles weakly interacting with the carrier. The surface structure may be further complicated when lower oxides are formed on activation (e.g., reduction of Mo6+to Mo5+,Mo4+, Mo3+,etc.). More uniform composition and better control of surface species can be expected if catalysts are prepared by the equilibrium adsorption method. However, as noted above, even in those instances the catalyst surface may become more heterogeneous on reduction (e.g., multiple oxidation states present). As a result, direct correlation between the nature and abundance of supported species and catalytic activity is often difficult toestablish. Clearly it is best toderivestructure/activity relationshipsfrom catalystscontaining a single well-definedspecies which on reduction forms a discrete oxidation state. According to Yermakov et a1.I-3 and Iwasawa et a1."-8 catalysts containing a single species can be obtained by reaction of allylbased transition metal complexes with Si02 followed by controlled reduction and oxidation treatments. Discrete Mo2+ and Mo4+ oxidation states were reportedly formed according to the mechanism outlined in Figure 1. The preparation involves reaction between Mo(v3-C3H5)4and two adjacent hydroxyl groups to form a bidentate Mo complex, liberating propene. Reduction of the Yanchored"Mo complex at ca. 600 OC, reportedly forms a bound Mo2+species. Oxidation at 0 and at 400 OC yields Mo4+and Mo6+,respectively. The authorst-5 also maintain that the oxidic Mo6+species reversibly forms Mod+ and Mo2+ on reduction. The chemistry shown in Figure 1 was mainly supported by the Q022-3654/93/2097-5699$04.00/0
+ M0(n3
- CjH5)4
20.f)
SI-OH
51-0
I \
C3H5
'1
590'C
HZ
(d)
(C)
(b)
Figure 1. Scheme for the synthesis of allyl-based Mo/SiOz catalysts containing discrete Mo oxidation states as proposed by Yermakov et al.1-3 and Iwasawa et al.4-5
stoichiometry of the reduction and oxidation reactions. Molybdenum oxidation states in Mo allyl based catalysts following reduction have been examined by ESCA. Evidence for the presence of discrete Mo oxidation states (Figure 1, structures b and c) was, however, not conclusive. ESCA results reported by Iwasawa et a1.6 show a 1-eV difference between the Mo 3d binding energies of Mo6+and Mo4+(Figure 1, structure c) versus ca. 2.5 eV reported by Yermakov.3 Furthermore, the Mo 3d binding energy difference between Mo6+ and Mo2+ (Figure 1, structure b) measured by Iwasawa et a1.6 is ca. 3.5 eV compared to ca. 5.5 eV reported by Yermakov' and 4.8 eV measured previ~usly.~ Clearly, there is a need for a detailed study of all stages of catalyst preparation described in Figure 1. The purpose of this paper is to investigate, by ESCA, the reported formation of discrete Mo oxidation states by reduction and oxidation of an allyl-based Mo/Si02 catalyst.
Experimental Section Catalyst Preparation. Tetraallylmolybdenum(1V) [M0(v3C3H5)4] was prepared using a modification of the preparation of Jolly et al.1° All manipulations were performed under an atmosphere of dry nitrogen. In a typical preparation, a slurry of MoCl4-2THF (30.0 g, 82 mmol) was transferred via cannula to a prepurged 2-L 3-neck round-bottom flask fitted with a mechanical stirrer and oil bubbler and containing a stirred solution of allylmagnesium chloride (180 mL, 2.0 M, THF) at ambient temperature. The flask was immersed in an ice bath and the
0 1993 American Chemical Society
Aigler et al.
5700 The Journal of Physical Chemistry, Vol. 97, No. 21, 1993 reaction allowed to stir for 18 h at 0 "C. The reaction mixture was concentrated to dryness under vacuum and the resulting buff colored solid was loaded into a Soxhlet thimble and extracted into pentane. The brown-yellow pentane solution was concentrated to dryness and the resulting residue extracted into a minimum of THF which was loaded onto a column of silica gel. Elution of the column with pentane resulted in a yellow colored solution which was concentrated and placed at -80 "C yielding 5.1 g (20 mmol, 24.9%) of lemon-yellow crystals. The purity of the compound was authenticated by comparison of its IH NMR and IR spectra to those of a bona fide sample, and by C, H analysis. Elemental analysis calculated for C12H20M0: C, 55.39; H, 7.75. Found: C, 55.14; H, 7.78 (Atlantic Microlabs). The supported Mo catalysts were prepared by a dry mixing and sublimation method, which allowed the allyl complex to be loaded either onto 100-200 mesh particles of the Si02 support (Davison grade 62, BET surface area 340 m2/g) or onto pressed self-supporting wafers of the same material for ESCA work. Typically, 1 g of the support was charged in a quartz reactor, connected to an all glass BET system and calcined in flowing oxygen at 550 OC for 2.5 h and then in vacuum at the same temperature for 2.5 h. The reactor was then transferred, while under vacuum, to a glovebox kept under high-purity nitrogen, where the amount of complex required to produce a 1.7 wt % Mo/SiOz catalyst was weighed to *0.001 g and placed directly on the support in the reactor. Mo loadings were verified by Galbraith Laboratory analysis. The assembly was reconnected to the BET system and evacuated to < 1 t 5 Torr. Crude mixing was accomplished by jiggling and the mixture was heated at 45 OC to favor gas-phase transport of the complex. The hydrocarbons evolved were collected in a cold trap and could be measured manometrically after warming the trap to room temperature. The reaction was terminated when no further changes in the pressure readings were apparent over a period of 30 min. Finally, the temperature was slowly raised to 65 "C under dynamicvacuum to sublime any unbound complex. Purification of Cases. H2,02,and He were purifiedand stored in 5-L storage bulbs on the BET line at greater than atmospheric pressure. The H2 and 0 2 used were Linde extra dry grade, the He was Linde high-purity grade. The H2 was first passed through a Matheson gas purifier and then through a hydrogen purifier consisting of a Pd-Ag alloy thimble heated at 400 OC. The 0 2 was first passed through a water trap consisting of 50%anhydrous Mg(CD4)2 and 50% anhydrous CaS04 and then through an activated carbon trap cooled with dry ice and isopropyl alcohol. He was purified using the same procedure, but with the activated carbon cooled with liquid nitrogen. H2 and 02, used for flow reduction and oxidation pretreatments of the catalyst on the BET system, were Linde extra dry grade or prepurified. The H2 was passed first through a Matheson gas purifier, then through an 0 2 trap containing an indicating oxygen adsorbent (40-60 mesh), and finally through a water trap with 50% anhydrous Mg(C104)~and 50% anhydrous CaS04. The 02 was passed only through the water trap. H2 and 10% 0 2 / H e used for flow reduction and oxidation pretreatments of the catalyst for ESCA studies were Matheson ultrahigh-purity grade. The H2 was passed through an oxygen adsorbent from Chemical Research Supplies (40-60 mesh). Stoichiometric Measurements. Stoichiometric measurements were made using a BET system consisting of a conventional BET setup modified to include a glass circulating loop. Samples of the anchored Mo catalyst were first reduced in hydrogen at 550 "C for 12 h in the circulating loop of the BET system. The reduced catalyst could then be oxidized and reduced again in hydrogen, under selected conditions, to verify the stoichiometry of such transformations and their reversibilityin successiveredox cycles. A cold trap with glass beads allowed continuous removal of hydrocarbons and/or water produced during the runs, and
TABLE I: Stoichiometric Behavior in the Reduction and Oxidation of the Allyl-Based Mo/Si@ Catalyst (1.7 wt % Mo) reactant temp gas consumption O/Mo or gas O2b
Ozb H2' H2'
("C)
t(h)
(pmollgofcatal)
H2/Mo
AOS'
24 300 450 550
1 4 1 12
51 52 102 101
0.99 1.02 1.00 0.93
3.9 6.0 4.0 2.1
" Average M o oxidation state. H2 pressure approximately 200
0 2 pressure approximately 100 Torr. Torr.
TABLE II: Effect of H2Pressure on the Stoichiometric Behavior of the Allyl-Based Mo/SiOl Catalyst (1.7 wt % Mol gas consumption (Hz/Mo or O/Mo)
reactant temp gas
("C) t ( h )
H2 H2
450 550 24 300
02" 02' 0
0 2
2 8 1 6
100 Torr ofH2
200 Torr ofHz
350 Torr ofH2
500 Torr ofH2
1.04 1.00 0.96 1.08
1.05 0.98 0.95 1.10
1.21 0.75 0.99 1.04
1.39 0.59 0.98 1.04
pressure approximately 100 Torr.
thus measurement of the consumption of gaseous reactants could be done manometrically. Stoichiometric measurement of the number of carbon atoms per Mo on the "anchored" Mo catalyst was accomplished by measuring the amount of C02 produced by complete oxidation of the catalyst at 550 "C in 100 Torr of 0 2 . ESCA Analyses. A sealable probe" allowed transfer of the pellets of fixed catalyst, without exposure to the atmosphere, from the glovebox to the ESCA spectrometer or to a reactor for further treatment between analyses. ESCA spectra were recorded with an AEI ES200 A spectrometer equipped with an aluminum anode (A1 Kar = 1486.6 eV) operated at 12 kV and 20 mA. The residual pressure inside the analysis chamber was typically 5 X 10-8 Torr. The spectrometer was interfaced to an IBM PC compatible for data aquisition. Subsequentcurve-fitting analysiswas carried out using in-house programs (GOOGLY) written for the IBM PC compatible under the MS DOS operating system. The Si 2p line (103.5 eV) of the support was used as a reference for ESCA binding energy measurements. ESR Analyses. The catalyst for ESR studies was prepared in a quartz ESR tube under vacuum or inert atmosphere as described earlier. After the catalyst was prepared, the ESR tube was sealed under vacuum. The sample was analyzed using a Varian E-4 ESR spectrometer. The spectra were determined at X-band at liquid N2 temperature with the field set at 3100 G and the range at 1000 G. Results and Discussion Stoichiometry. Stoichiometric results of the redox transformations of surface species on the Mo/SiOz catalyst (Table I) agree well with those measured by Iwasawa et aL4J As shown in Figure 1, Iwasawa et al. reported that reaction between Mo(~~-CsH5)4 and Si02 forms a bidentate Mo complex. Measurements of the amount of C02 produced on oxidation of this "anchored" Mo complex indicated the presence of six carbon atoms per Mo which is consistent with the proposed structure for this surface species. Upon reduction of the "anchored" Mo complex at 550 OC, an average oxidation state of Mol+ was obtained as indicated in Figure 1. Similarly, oxidation at 24 "C yielded an average oxidation state of Mo4+. Complete oxidation to Mob+ was obtained following 0 2 treatment at 300 "C. The effect of varying H2 pressure on the extent of reduction of the allyl-based Mo/SiOz catalyst is shown in Table 11. The anchored complex was first reduced at 550 "C then oxidized at
The Journal of Physical ChemLtry, Vol. 97, No, 21, 1993 5701
"Model" Allyl-Based Mo/Si02 Catalysts
x
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< A >
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Figure 2. Curve-fitted Mo 3d spectrumof the Mo/Si02 catalyst following
adsorption of Mo(r13-C3H5)4onto SiOl.
300 OC to the Mo6+oxidation state. Next, the oxidized catalyst was reduced at 450 "C for 2 h at various H2 pressures. As shown in Table 11, the amount of H2 consumed increased from 1.04 H2/Mo (average Mo oxidation state of +3.9) for a H2 pressure of 100 Torr to 1.39 H2/Mo (average Mo oxidation state of +3.2) for a H2 pressure of 500 Torr. Increasing the reduction temperature from 450 to 550 OC led to additional consumption of H2lowering the average Mo oxidation stateto +2. For example, the catalyst that was reduced in 500 Torr of H2 consumed 1.39 H2/Mo at 450 OC and 0.59 H2/Mo at 550 OC resulting in a total of 1.98 H2/Mo and an average oxidation state of Mo2+. Thus, while theextent ofreduction at 450 OC wasaffected by the pressure of H2 employed, the total extent of reduction after heating at 550 OC was independent of the concentration of Hz,approaching in all cases 2 Hz/Mo (average Mo oxidation state of +2). The oxidation steps were carried out in 100 Torr of 02, and the results were very similar to those shown in Table I. Oxidation at room temperature of the reduced catalyst produced the average oxidation state of Mo4+ ( 0 2 consumption was approximately 1 O/Mo) and further oxidation at 300 OC yielded Mo6+(total 0 2 consumption was approximately 2 O/Mo). ESCA Study of the Anchored Mo/SiOz Complex. The oxidation state of the anchored Mo complex (Figure 1, structure a) was examined by ESCA. An ESCA Mo 3d spectrum was acquired for the Mo/SiOz catalyst after adsorption of Mo(q3-C3H5)4onto Si02 (Figure 1, structure a). The spectrum obtained showed a Mo 3d peak shape which suggests the presence of one oxidation state (Figure 2). The Mo 3d5p peak position at 232.0 eV is consistent with that usually assigned to M05+? ESR measurements indeed show a signal with a g value of 1.99 which can be attributed to M O ~ + .However, ~?~ quantitative results indicated that only a small fraction of the Mo phase (ca. 1%) was present as Mo5+. Similar results were reported by Yermakov and Kuznetsov.12 The presence of small amounts of the Mo5+was attributed to incomplete oxidation of Mo(q3-C3H5)4by traces of oxygen and water while preparing the allyl complexand catalyst.I2 Considering the small amount of Mo5+ detected by ESR,it seems, thus, more likely to attribute the Mo 3d5p peak at 232.0 eV to Mo4+. The binding energy typically associated with Mo4+, as in Moo2, is ca. 229.9 eVa9 However, a variety of binding energy values for Mo4+complexes have been reported depending upon the kinds of ligands associated with the Mo center.I3-l6 GrUnert et al." tabulated XPS binding energy shifts for various organometallic Mo compounds. They reported Mo 3d5/2binding energy values ranging from 23 1.5 to 23 1.8 eV for organometallic compounds with Mo4+metal centers. These values are closer to that measured for the anchored Mo complex (232.0 eV) than to the value associated with Moo2 (229.9 eV). ESCA Study of the Stoichiometry of the Redox Transformations. ESCA spectra were acquired for the Mo/Si02 catalyst
I
240
i
230 Binding Energy lev
Figure 3. Curve-fitted Mo 3d spectra of (A) fresh catalyst reduced at 550 OC, (B) reduced catalyst oxidized at room temperature, (C) sample (B) oxidized at 300 O C .
following various reduction and Oxidation treatments. Figure. 3A shows the Mo 3d spectrum obtained following reduction of the anchored complex at 550 O C for 12 h. The Mo 3d envelope shows a major contribution from a single oxidation state (Mo 3d512binding energy of 228.4 eV), and a minor contribution from Mo4+ (Mo 3d512 binding energy of 230.2 eV). The Mo 3d5/2 binding energy value at 228.4 eV is consistent with the presence of Mo2+. This is in good agreement with the structure proposed by Iwasawa et aL4J (Figure 1, structure b). The ESCA Mo 3d spectrum for the catalyst obtained after exposure of the reduced catalyst to 0 2 at room temperature for 12 h (Figure 1, structure c) is shown in Figure 3B. The Mo 3d envelope is typical of mixed oxidation states. Curve-fitting results indicate the presence of M o ~ +Mo4+, , M o ~ +and , Mo6+. These results are at variance with the reported'* formation of discrete Mo4+oxidation state under similar conditions. Figure 3C shows the Mo3d spectrum for the catalyst obtained following oxidation at 300 "C of the catalyst from Figure 3B. The Mo 3d5/2 and Mo 3d3p binding energies (233.0 and 236.2 eV) are characteristic of Mo6+. Study of the Reversibility of the Redox Cycle by ESCA. Figure 4A shows the Mo 3d spectrum of the catalyst obtained after reduction at 550 OC of the fresh catalyst (Figure 1, structure b). Figure 4B shows the Mo 3d spectrum of the catalyst obtained when the reduced catalyst was oxidized at 300 OC and then reduced again at 550 OC. In both cases, the major Mo 3d5p binding energy peak occurred at 228.4 eV which is indicative of the Ma2+ oxidation state. In both spectra, a minor contribution from Mo4+
Aigler et al.
5702 The Journal of Physical Chemistry, Vol. 97, No. 21, 1993
Conclusions
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4
I
I
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w
(6)
I
I
240.0
235.0
I
230.0 Blncfing Energy / e v
Stoichiometric redox measurements performed on an allylbased Mo/SiOz catalyst (1.7 wt % Mo) correlated well with those reported in the ESCA results indicate that the reduction of the allyl-based Mo/SiOz catalyst at 550 "C indeed leads to the formation of a Mo*+oxidation state as proposed by Iwasawa and co-workers.'* Also, for catalysts oxidized at 300 OC, the Mo2+ oxidation state appears to be reversibly formed upon reduction at 550 OC. However, the reported formationof discrete Mo4+upon oxidation of the reduced catalyst at room temperature could not be substantiated.
Acknowledgment. The authors acknowledge financial support for this work from the U S . Department of Energy, Office of Basic Energy Sciences Grant DE-FG02-87ER13781 and the National Science Foundation Grants CHE91-13808and CHE9022135.
References and Notes
I
225.0
Figure 4. Curve-fitted Mo 3d spcctra of (A) fresh catalyst reduced a t 550 OC; (B) sample (A) oxidized a t room temperature and 300 OC and then reduced at 550 O C .
is evident. After reduction of the fresh catalyst, the contribution of the Mo4+oxidation state was approximately 14%of the total area of the Mo 3d envelope versus approximately 9% after reduction oftheoxidizedcatalyst. Thesimilarity in peak positions and curve fittings of the two spectra in Figure 4 supports the hypothesis by Iwasawa and Ogasawara5 that the Mo2+oxidation state can be reversibly obtained from Mo6+ upon reduction.
(1) Yermakov, Y. I. Catal. Rev. Sci. Eng. 1976, 13, 77. (2) Yermakov, Y. I.; Kuznetsov, B. N.; Zakavov, V. A. Catalysis by Supported Complexes; Elsevier: Amsterdam, 1981. (3) Ycrmakov, Y. I. Proc. 7. Int. Congr. Catal. Tokyo (1980);Seiyama, T.; Tanabe, K., Eds.; Elsevier: Amsterdam, 1981; Dart A, D 57. (4) Iwasawa, Y.; Nakano, Y.; Ogasawara, S . J . C h e k Soc., Faraday Trans. I 1978, 74, 2968. (5) Iwasawa, Y.; Ogasawara, S. J. Chem. Soc., Faraday Trans. 1 1979, 75, 1465. (6) Iwasawa, Y.; Ichinose, H.; Ogasawara, S . J . Chem. Soc., Faraday Trans. I 1981, 77, 1763. (7) Iwasawa, Y. In Tailored Metal Catalysts; Dreidel Publishing Co.: Dordrecht, Holland, 1986; p 1. (8) Iwasawa, Y. Adv. Catal. 35, 1987. 187. (9) Yamada, M.; Yasumaru, J.; Houalla, M.; Hercules, D. M. J. Phys. Chem. 1991,95, 7037. (10) Benn, R.; Holle, S.;Jolly, P. W.; Kruger, C.; Romao, C. C.; Romao, M. J.; Rufinska, A.; Schroth, G. Polyhedron 1986, 5, 461. (11) Patterson, T. A.; Carver, J. C.; Leyden, D. E.; Hercules, D. M. J. Phys. Chem. 1976,80, 1900. (12) Yermakov, Y. I.; Kuznetsov, B. N. Prepr. 2nd Japan-Soviet Catal. Tokyo, 1973. (13) Grim, S . 0.;Matienzo, L. J. Inorg. Chem. 1975, 14, 1014. (14) Cimino, A.; De Angelis, B. A. J. Caral. 1975, 36, 11. (15) Swartz, Jr., W. E.; Hercules, D. M. Anal. Chem. 1971, 43, 1774. (16) Chatt, J.; Elson, C. M.; Leigh, G. J. J. Chem. Soc., Dalton Trans. 1976, 1351. (17) Griinert, W.; Stakheev, A. Y.; Feldhaus, R.; Anders. K.; Shpiro, E. S.; Minachev, K. M. J. Phys. Chem. 1991, 95, 1323.
