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In Situ FTIR Spectroscopic Study of 2-Propanol Adsorptive and Catalytic Interactions on Metal-Modified Aluminas Mohamed I. Zaki,* Muhammad A. Hasan, and Lata Pasupulety Chemistry Department, Faculty of Science, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait Received December 28, 2000. In Final Form: March 27, 2001 Pure alumina was modified by additives of Ni, Rh, and Pt metal particles by impregnation from aqueous solutions of corresponding precursor compounds and subsequent reduction in a stream of hydrogen at 400 °C for 2 h. The catalysts thus obtained were subjected to N2, H2, and O2 sorptometry, high-resolution electron microscopy, and X-ray photoelectron spectroscopy. Accordingly, the metal reducibility, surface area, dispersion, and particle size were determined. Thin wafers of the catalysts, pretreated in situ in a specially designed IR reactor/cell, were exposed to 2-propanol vapor at various temperatures (room temperature to 400 °C) for 10 min, and IR spectra of the gas-phase and adsorbed species were measured. The results indicated that the metal additives provided the surface with dehydrogenation/hydrogenation and cracking (hydrogenolysis) sites. Consequently, the initial alcohol dehydration selectivity of alumina (to give pure propene at 300-400 °C) was successfully challenged by a strong activity toward formation of major products of acetone, propane, and methane in the gas phase. A range of relevant adsorbed species were characterized and found to result predominantly from adsorptive interactions of 2-propanol and acetone molecules. Surface reaction pathways were suggested, assuming concerted and sequential interactions of the alcohol (and product) molecules with acid-base and metal sites. It was believed that gas-phase migration of alkene molecules and surface diffusion of hydrogen adatoms facilitate the sequential interactions.
Introduction A number of extensive in situ infrared (IR) studies have been carried out during the past three decades [e.g., refs 1-6 and references therein] focusing on adsorptive and catalytic interactions of 2-propanol gas-phase molecules on various metal oxide surfaces. It has been thereby established5,6 that the alcohol is irreversibly adsorbed at room temperature (RT) in the form of molecules hydrogenbonded (H-bonded) to surface OH groups (and/or O2- sites) and 2-propoxide ((CH3)2CHO-) species bound to coordinatively unsaturated (cus) metal (Lewis acid) sites. At higher temperatures, these adsorbed species are either desorbed, activated for catalytic interactions, or oxidized into acetate surface species.5-7 The 2-propoxide species were proven8 to assume mono- and bidentate surface structures, which were later suggested5,6 to be surface intermediates for the alcohol dehydrogenation (to acetone) and dehydration (to propene), respectively. The dehydration reaction was found6,9 to take place on strong acid sites (Brønsted and Lewis acid sites), whereas the dehydrogenation reaction required the availability on * Corresponding author. Tel: +965-481-1188/5606. Fax: +065484-6946. E-mail:
[email protected]. (1) Deo, A. V.; Chuang, T. T.; Dalla Lana, I. G. J. Phys. Chem. 1971, 75, 234. (2) Hertl, W.; Cuenca, A. M. J. Phys. Chem. 1973, 77, 1120. (3) Zaki, M. I.; Sheppard, N. J. Catal. 1983, 80, 114. (4) Nashed, Y. E.; Hussein, G. A. M.; Zaki, M. I. Colloids Surf., A 1995, 99, 247. (5) Hussein, G. A. M.; Sheppard, N.; Zaki, M. I.; Fahim, R. B. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1723. (6) Zaki, M. I.; Hussein, G. A. M.; El-Ammawy, H. A.; Mansour, S. A.; Polz, J.; Kno¨zinger, H. J. Mol. Catal. 1990, 57, 367. (7) Mestl, G.; Kno¨zinger, H. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, 1997; Vol. 2, pp 539-574. (8) Bensitel, M.; Moravek, V.; Lamotte, J.; Sauer, O.; Lavalley, J.-C. Spectrochim. Acta 1987, A43, 1487. (9) Lahousse, C.; Bachelier, J.; Lavalley, J.-C.; Lauron-Pernot, H.; Le Govic, A. M. J. Mol. Catal. 1994, 87, 329.
the surface of strong basic sites8 and/or redox metal sites (Mn+/M(n-1)+).5 On metal oxides of weak acid-base properties, propene and acetone molecules thus produced remained unchanged upon heating to 400 °C. However, on strongly basic oxides, such as ceria, the acetone was observed3,6 to get involved in a bimolecular aldol-condensation-type surface reaction (at >200 °C), resulting in the formation of isobutene gas-phase molecules. In recent independent investigations, the necessity for strong basic sites for acetone condensation reactions was confirmed.10,11 Upon modification of basic oxides with strong Lewis acid sites, a bifunctional acid-base reaction pathway12 was generated,10 strengthening adsorption of the acetone condensation products (e.g., diacetone alcohol and mesityl oxide) and suppressing their release into the gas phase.10 Therefore, dehydration and dehydrogenation reactions of 2-propanol, which have been widely used in research laboratories as probes of surface acid and base sites,3,8 have recently been reported12 to probe the density and chemical properties of acid-base site pairs instead. In industry, catalytic decomposition of alcohols in general has been used in relatively few large-scale processes, which, however, are very important sources of intermediate compounds.13 With 2-propanol, the dehydrogenation reaction (giving acetone) enjoys a better industrial prognosis than the dehydration reaction (giving propene). Acetone is industrially produced by oxidative hydration of propene,13 a manufacturing method which is not that highly competitive as compared to the alcohol dehydrogenation method. On the other hand, the alcohol dehy(10) Fouad, N. E.; Thomasson, P.; Kno¨zinger, H. Appl. Catal., A 2000, 196, 125. (11) Zaki, M. I.; Hasan, M. A.; Al-Sagheer, F. A.; Pasupulety, L. Langmuir 2000, 16, 430. (12) Iglesia, E.; Barton, D. G.; Biscardi, J. A.; Gines, M. J. L.; Soled, S. L. Catal. Today 1997, 38, 339. (13) Kraus, M. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨ziner, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, 1997; Vol. 5, pp 2159-2165.
10.1021/la001810r CCC: $20.00 © 2001 American Chemical Society Published on Web 05/26/2001
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dration process can still be considered as a potential source of propene for the oxidative hydration method of acetone production. The present investigation was designed to examine impacts of metal atom additives (namely, Ni, Pt, or Rh) on the catalytic activity and selectivity of alumina toward 2-propanol decomposition (at RT-400 °C), in the presence and absence of excess hydrogen in the reaction atmosphere. The prime objectives were to (i) observe the surface behavior of the alcohol on metallic and acidic sites used to catalyze hydrocarbon reforming reactions14 and (ii) assess the influence of hydrogen on possible 2-propanol T acetone interconversion and hydro-treating reactions. To our knowledge, such objectives have hitherto not been accomplished, at least regarding the chemistry of 2-propanol at solid surfaces. Fourier transform infrared (FTIR) spectroscopy was used to characterize adsorptive and catalytic interactions involved, while the reaction was being carried out in a closed IR reactor/cell. Such an experimental setup, which simulates conditions of slow removal of reaction products in flow systems, should bring into prominence possible secondary surface reactions and their control over the product distribution. Experimental Section Pure and Metal-Modified Aluminas. Pure alumina (denoted Al) was a high-area (100 m2/g, (γ+δ)-Al2O3) Degussa Aluminumoxide C (FRG). It was modified nominally with ca. 7 wt % Ni, Pt, or Rh by impregnation from dilute aqueous solutions (20 mL/g-Al2O3) of AR-grade Merck Ni(NO3)2‚4H2O, BDH H2PtCl6, or Alfa RhCl3‚3H2O at RT. Stirring a suspension of Al particles in the impregnating solution for 30 min effected the impregnation. The resulting slurries were maintained in contact with the mother liquor for 24 h, and then excess water was evaporated by heating in a still atmosphere of air at 100 °C for 24 h. The dried, impregnated aluminas were crushed into fine grains, dried further at 100 °C for 24 h, and stored over silica gel till further use. The metal-modified alumina (MeAl) catalysts (indicated below as NiAl, PtAl, and RhAl) were in situ synthesized by first heating thin wafers (mounted inside the IR cell (vide infra)), pressed disks (mounted inside the preparation chamber of the X-ray photoelectron spectrometer (vide infra)), or loose powders (placed inside a sorptometer (vide infra)) of the corresponding impregnated alumina at 400 °C and 10-6 Torr for 30 min. Subsequently, a stream (50 mL/min) of 99.99% pure H2 (KOAC/Kuwait) was allowed to flow around the sample mount, while heating at 400 °C for 2 h, after which the H2 flow was stopped and at the same temperature (400 °C) the gas was pumped off to 10-6 Torr. Finally, the system was cooled to RT. The catalyst wafer thus synthesized was either maintained under dynamic vacuum for in situ IR spectroscopic examination or passivated for ex situ examinations (atomic absorption spectroscopy and electron microscopy (vide infra)). Catalyst samples used for the latter ex situ examinations were outgassed of the H2 atmosphere at RT. Catalyst Characterization. The actual metal content was determined for each MeAl catalyst, using a model 5100 PC PerkinElmer atomic absorption spectrometer (USA). N2 physisorption at -195 °C, H2 chemisorption at RT, and O2 chemisorption at 200 °C were measured by means of an automatic ASAP 2010 Micromeritics sorptometer (USA). Prior to exposure to the adsorptive atmosphere (99.99% pure products of KOAC), test catalysts were outgassed at 110 °C and 10-6 Torr for 5 h. The total specific surface area (m2/g) was determined by BrunauerEmmett-Teller analysis15 of N2 adsorption data, whereas H2 adsorption data were used to determine the metal surface area,16 dispersion,16,17 and crystallite size17,18 as described in the (14) Bond, G. C. Heterogeneous Catalysis: Principles and Applications, 2nd ed.; Oxford University Press: Oxford, 1990; pp 109-114. (15) Gregg, S. J. In Adsorption at the Gas-Solid and Liquid-Solid Interface; Rouquerol, J., Sing, K. S. W., Eds.; Elsevier: Amsterdam, 1982; pp 153-164. (16) Emerson, M. F.; Holtzer, A. J. Phys. Chem. 1967, 71, 3320.
Zaki et al. references cited. On the other hand, the extent of reduction to the metallic state was determined by processing the O2 adsorption data as demonstrated elsewhere.17 The metal particles were identified chemically by means of a Link’s exl II X-ray energy dispersive spectrometer (Oxford Instruments/U.K.) attached to a JSM-3600 JEOL scanning electron microscope (Japan), whereas their morphological features were visualized in transmission electron microscopy (TEM) images obtained using a JSM-3010 JEOL high-resolution transmission electron microscope applying the technique described earlier.19 The chemical composition and oxidation state of surface sites were derived from X-ray photoelectron spectra (XPS) measured by means of a model VG Scientific 200 spectrometer (U.K,) equipped with an AlKR source of radiation (1468.6 eV). The spectral acquisition technique and conditions were as described earlier.20 In Situ IR Spectroscopy. An all-Pyrex glass IR cell equipped with BaF2 windows, specially designed for high-temperature measurements,21 was used to facilitate probing adsorptive and catalytic interactions of the 2-propanol (2-PrOH) gas phase on test catalysts by in situ FTIR spectroscopy in the transmission mode. A model Spectrum-BX FTIR Perkin-Elmer spectrometer was used for the measurements (averaged 100 scans at 4.0 cm-1 resolution), and installed P-E Spectrum version 2.0 software was the means whereby spectral acquisition and processing were carried out. A 10-Torr portion of the alcohol gas phase (air-free) was expanded into the cell, following acquisition of background spectra of the cell and the catalyst wafer at RT. The gas/solid interface was maintained at RT for 10 min before (i) measuring a spectrum of the gas phase (plus cell background), (ii) pumping off the gas phase for 5 min, and (iii) measuring a spectrum of the solid phase (catalyst plus adsorbed species). This sequence of measurements was repeated at higher temperatures (100-400 °C), but the spectra were measured following cooling to RT. The same sequence of experiments and measurements was followed in the presence of a mixed gas phase of 3 Torr 2-propanol plus 15 Torr H2. Difference spectra of the gas-phase and adsorbed species were obtained by absorption subtraction of the cell and catalyst background spectra, respectively, using the installed software.
Results and Discussion Catalyst Characteristics. Surface and bulk characteristics determined for the test catalysts are summarized in Table 1. The actual metal loadings established in NiAl and RhAl are almost identical (ca. 7 wt %) and twice as much as that found in PtAl (3.1 wt %). These metal loadings were found to cause but insignificant loss in the specific surface area of Al (from 100 to 92 m2 g-1). The metal surface area, as derived from H2 chemisorption data, is shown to be lowest on PtAl (0.8 m2 g-1-cat) and highest in RhAl (6.1 m2 g-1-cat), with that on NiAl amounting to an intermediate magnitude (1.7 m2 g-1-cat). The metal surface area, as determined per gram-catalyst, is often controlled by the metal content, dispersion, and extent of reduction.16,17 Thus, the higher metal area of RhAl than NiAl is due essentially to the higher dispersion of Rh (23.3%) than Ni (4.7%) on Al, because both metals exist in almost identical amounts in the respective catalysts and have rather close reducibilities (Table 1). In the wake of the higher reducibility of Pt (94%) than both Ni (72%) and Rh (68%), the lowest surface area assumed by this metal (Pt) may be related to its lower amount in PtAl (Table 1). The metal particle sizing by means of H2 chemisorption (Table 1) and HRTEM (Figure 1) provides further support to the higher dispersion of Rh than both Ni and Pt on Al. Both (17) Narayanan, S.; Uma, K. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2733. (18) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1967; pp 257-260. (19) Ali, A. A. M.Sc. Thesis, Kuwait University, Kuwait, 1999. (20) Hasan, M. A.; Zaki, M. I.; Pasupulety, L.; Kumari, K. Appl. Catal. 1999, 181, 171. (21) Peri, J. B.; Hannan, R. B. J. Phys. Chem. 1960, 64, 1526.
2-Propanol Interactions on Metal-Modified Aluminas Table 1. Surface and Bulk Characteristics of MeAl Catalysts NiAl Me loadinga/(0.1 wt % N2 monolayer capacity/ (0.5 mL g-1-cat H2 saturation uptake/ (0.05 mL g-1-cat O2 saturation uptake/ (0.05 mL g-1-cat total surface area/ (2 m2 g-1-cat Me surface areab/ (0.5 m2 g-1-cat Me reducibilityc/(2% Me surface sitesd Me dispersione/(2% Me particle sizef (nm), I Me particle sizef (nm), II Me particle sizef (nm), III a
PtAl
RhAl
7.1 20.8
3.1 19.6
7.0 19.8
0.46
0.14
1.21
9.80
1.69
5.20
98
92
93
1.7
0.8
6.1
72 Ni0, Ni2+ (25%) 4.7 15-20 (22) 10-15 5-10
94 Pt0, Pt4+ (15%) 8.3 15-22 10-15 (12) 5-10
68 Rh0, Rh3+ (35%) 23.3 e5 (4)
b
AAS-determined. Derived from corresponding H2 saturation uptake (ref 16), assuming the dissociative adsorption of hydrogen with one hydrogen atom on each surface metal atom and taking the cross-sectional area of Ni, Pt, and Rh to be 6.8 (ref 18), 11.0 (ref 18), and 9.4 Å2 (ref 22), respectively. c Derived from the ratio of corresponding O2 saturation uptake to a theoretical value calculated assuming that all the Me loading was present in the zero-valent state (ref 17). d XPS-determined at the following binding energy values: Ni2p3, 852.7 and 854.0 eV; Pt4f7, 71 and 75.7 eV; Rh3d5, 307.4 and 309.2 eV (ref 23). The parenthesized value is the atomic ratio ((5%) of Men+/Me0. e The ratio of available surface metal atoms to the total number of atoms (ref 24), calculated using the H2 saturation uptake, the Me loading, and the % reduction to the metallic state (ref 17). f Observed in HRTEM images (Figure 1), whereas the parenthesized value was derived from the observed H2 saturation uptake (ref 18).
NiAl and PtAl are shown to contain much larger metal particle sizes (g15 nm) than RhAl (e5 nm). XPS analysis results (Table 1) are compatible with the relative reducibilities of the metals, in determining comparable proportions of unreduced metal ions. IR Background Spectra of 2-PrOH, Al, and MeAl. The IR spectrum taken of the 10 Torr 2-PrOH gas phase expanded into the IR cell at RT (Figure 2) shows only diagnostic absorption bands of free (CH3)2CHOH molecules (cm-1): νOH, 3657; ν(CH3)as, 2980; ν(CH3)s, 2888; δ(CH3)as, 1462; δ(CH3)s, 1382; δOH, 1251; νCO, 1152; νCC, 1072.3,5 The background spectra taken of the MeAl catalysts were almost identical to that shown in Figure 2 for pure Al. It displays a strong, broad band over the νOH stretching region (>3200 cm-1), ill-defined weak bands at 2954, 2921, and 2846 cm-1 resulting, most probably, from νCH vibrations of hydrocarbon impurity species, and another set of three weak bands (at 1612, 1451, and 1375 cm-1) assignable to CO32-/HCO3- impurity species.25 The blackout absorption commencing near 1000 cm-1 is due to Al-O lattice vibrations.26 The maxima ill-resolved in the composite νOH absorption are indicative of four of the five different types of isolated surface OH groups (3774-3580 cm-1) assigned by Kno¨zinger and Ratnasamy27and associated OH’s (the (22) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; J. Wiley & Sons: Chichester, 1994; p 66. (23) Wagner, C. D.; Riggs, N. M.; Davis, L. E.; Moulder, J. F. In Handbook of X-ray Photoelectron Spectroscopy; Mullenberg, G. E., Ed.; Perkin-Elmer Corp.: Eden Prairie, MN, 1979. (24) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; J. Wiley & Sons: Chichester, 1994; pp 8-10. (25) Busca, G.; Lorenzelli, V. Mater. Chem. 1982, 7, 89. (26) Gadsden, J. Infrared Spectra of Minerals and Related Inorganic Compounds; Butterworth: London, 1975. (27) Kno¨zinger, H.; Ratnasamy, P. Catal. Rev. Sci. Eng. 1978, 17, 31.
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broad absorption maximized at 3298 cm-1). The maximum at 3774 cm-1 is due to terminal Al-OH groups, and the two maxima at 3717 and 3661 cm-1 are assignable to two different types of bridging (Al)2OH groups, whereas the maximum at 3580 cm-1 is most likely due to multicentered (Al)3OH groups.27 The sharpness of the maximum at 3661 cm-1 may imply that it includes a contribution from νOH vibrations of bicarbonate impurity species. An earlier lowtemperature IR study of CO adsorption on a similar alumina sample28 has revealed a basic nature for the terminal OH groups and a relatively acidic nature for the bridging OH groups. The latter OH’s were capable of forming hydrogen bonds with the CO molecules.28 These results indicate that despite the thermoevacuation experienced by the test catalysts at 400 °C for 30 min, they were still considerably hydroxylated. Ballinger and Yates29 have found that significant dehydroxylation of alumina could not be accomplished until the outgassing was carried out at >700 °C and 10-7 Torr for 12 h. IR Spectra of 2-PrOH/Al. The IR spectra of the gasphase and adsorbed species formed following heating of the 2-PrOH/Al interface at 100-400 °C for 10 min are exhibited in Figure 3. The gas-phase spectrum obtained at 100 °C was similar to the RT spectrum (Figure 2) in monitoring nothing but the diagnostic bands of pure 2-PrOH. The corresponding spectra of adsorbed species (Figure 3) monitored similar bands due to dominant 2-propoxide species coordinated to cus Al3+ (Lewis acid) sites. Frequencies and assignments of these bands are as follows (cm-1): 1468, δ(CH3)as; 1378, δ(CH3)s; 1341, δCH; 1170 and 1131, νCO/νCC vibrations.1,3,5,6 The occurrence of two bands due to νCO/νCC vibrations is indicative of formation of mono- and bidentate 2-propoxide species, as proven by Rossi et al.30 These spectra displayed, moreover, high-frequency absorptions at 3450-3000 (s,vb), 2967, 2936, and 2862 cm-1 resulting from νOH, ν(CH3)as, 2×δ(CH3)as, and ν(CH3) vibrations, respectively.1,3,5,6 Formation of the very broad and strong νOH band was accompanied by elimination of bands of νOH vibrations of isolated Al-OH groups. This indicates the presence on the surface of 2-PrOH molecules H-bonded to Al-OH groups.1,5 On the other hand, the very weak band at 1280 cm-1, which is assignable to δOH vibrations of alcohol molecules, may suggest the coexistence of a minor proportion of 2-PrOH molecules coordinated to Al3+ sites.5 The amount of the latter species was slightly higher at RT than at 100 °C. Similar spectral features were displayed by the adsorbed species formed following heating of the 2-PrOH/Al interface at 200 °C, except for a weaker absorption due to δOH of coordinated alcohol molecules. The corresponding gasphase spectrum (Figure 3) exhibited slightly weaker absorptions of 2-PrOH molecules as well as new, weak bands at 1826, 1651, 1440, 989, and 912 cm-1. These emerging bands are diagnostic of dehydration of the alcohol into propene molecules.31 When the temperature was increased to 300 °C, the gas-phase spectrum (Figure 3) became void of detectable absorptions due to 2-PrOH molecules. It displayed instead absorptions (at 1826, 1651, 1442, 1044, 990, 914, and 890 cm-1) due solely to propene molecules. It also displayed consistent absorptions in the νCH frequency region at 3090, 2955, and 2869 cm-1.31 However, the spectrum of adsorbed species at 300 °C (28) Zaki, M. I.; Kno¨zinger, H. Mater. Chem. Phys. 1987, 17, 201. (29) Ballinger, T. H.; Yates, J. T., Jr. 1991, 7, 3041. (30) Rossi, R. F.; Busca, G.; Lorenzelli, V.; Saur, O.; Lavalley, J.-C. Langmuir 1987, 3, 52. (31) The Aldrich Library of FT-IR Spectra, 1st ed.; Pouchert, C. J., Ed.; Aldrich Chemical Co.: Milwaukee, WI, 1989; Vol. 3.
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Figure 1. HRTEM micrographs (300 kV, ×400K) of alumina loaded with metal particles indicated [I, large particle size (>15 nm); II, medium size (10-15 nm); III, small size (
