The Application of Diffuse Reflectance Infrared Spectroscopy and

Oct 14, 2005 - Peter Jones,‡ and David Lennon*,†. Department of Chemistry, Joseph Black Building, University of Glasgow,. Glasgow G12 8QQ, UK, and...
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The Application of Diffuse Reflectance Infrared Spectroscopy and Temperature-Programmed Desorption To Investigate the Interaction of Methanol on η-Alumina Alastair R. McInroy,† David T. Lundie,† John M. Winfield,† Chris C. Dudman,‡ Peter Jones,‡ and David Lennon*,† Department of Chemistry, Joseph Black Building, University of Glasgow, Glasgow G12 8QQ, UK, and Ineos Chlor Ltd. Runcorn Site, P.O. Box 9, Runcorn, Cheshire, WA7 4JE, UK Received May 31, 2005. In Final Form: September 1, 2005 The adsorption of methanol and its subsequent transformation to form dimethyl ether (DME) on a commercial grade η-alumina catalyst has been investigated using a combination of mass selective temperature-programmed desorption (TPD) and diffuse reflectance infrared spectroscopy (DRIFTS). The infrared spectrum of a saturated overlayer of methanol on η-alumina shows the surface to be comprised of associatively adsorbed methanol and chemisorbed methoxy species. TPD shows methanol and DME to desorb with respective maxima at 380 and 480 K, with desorption detectable for both molecules up to ca. 700 K. At 673 K, infrared spectroscopy reveals the formation of a formate species; the spectral line width of the antisymmetric C-O stretch indicates the adoption of a high symmetry adsorbed state. Conventional TPD using a tubular reactor, combined with mass spectrometric analysis of the gas stream exiting the IR cell, indicate hydrogen and methane evolution to be associated with formation of the surface formate group and CO evolution with its decomposition. A reaction scheme is proposed for the generation and decomposition of this important reaction intermediate. The overall processes involved in (i) the adsorption/ desorption of methanol, (ii) the transformation of methanol to DME, and (iii) the formation and decomposition of formate species are discussed within the context of a recently developed four-site model for the Lewis acidity of η-alumina.

1. Introduction The oxidation and dehydration of methanol to give formaldehyde and dimethyl ether (DME), respectively, are key reactions in a number of industrial processes. The dehydration is catalyzed by solid acid catalysts such as alumina; not surprisingly, the adsorption of methanol on metal oxides has been the subject of a number of studies.1-8 Investigations have been extended to methanol adsorption on metal supported catalysts9-13 and metal surfaces.14-16 Copper, in particular, has been examined extensively due * Corresponding author. Telephone: (+44)-(0)-141-330-4372. Fax: (+44)-(0)-141-330-4888. E-mail: [email protected]. † University of Glasgow. ‡ Ineos Chlor Ltd. Runcorn Site. (1) Greenler, R. J. Chem. Phys. 1962, 37, 2094. (2) Knozinger, H.; Stubner, B. J. Phys. Chem. 1978, 82, 1526. (3) Busca, G.; Rossi, P. F.; Lorenzelli, V.; Benaissa, M.; Travert, J.; Lavalley, J. J. Phys. Chem. 1985, 89, 5433. (4) DeCanio, E. C.; Nero, V. P.; Bruno, J. W. J. Catal. 1992, 135, 444. (5) Schiffino, R. S.; Merrill, R. P. J. Phys. Chem. 1993, 97, 6425. (6) Vigue, H.; Quintard, P.; Merle-Mejean, T.; Lorenzelli, V. J. Eur. Ceram. Soc. 1998, 18, 1753. (7) Devito, D. A.; Gilardoni, F.; Kiwi-Minsker, L.; Morgantini, P.; Porchet, S.; Renken, A.; Weber, J. J. Mol. Struct. 1999, 469, 7. (8) Beebe, T. P., Jr.; Crowell, J. E.; Yates, J. T., Jr. J. Chem. Phys. 1990, 92, 8. (9) Chavin, C.; Saussey, J.; Lavalley, J.; Idriss, H.; Hinderman, J.; Kiennemann, A.; Chaumette, P.; Courty, P. J. Catal. 1990, 121, 56. (10) Roozeboom, F.; Cordingley, P. D.; Gellings, P. J. J. Catal. 1981, 68, 464. (11) Clarke, D. B.; Lee, D.; Sandoval, M. J.; Bell, A. T. J. Catal. 1994, 150, 81. (12) Fisher, I. A.; Bell, A. T. J. Catal. 1999, 184, 357. (13) Cabilla, G.; Bonivardi, A. L.; Baltanas, M. A. J. Catal. 2001, 201, 213. (14) Mudalige, K.; Trenary, M. J. Phys. Chem. B 2001, 105, 3823. (15) Chesters, M. A.; McCash, E. M. Spectrochim. Acta 1987, 43A, 1625. (16) Wachs, I.; Madix, R. J. Catal. 1978, 53, 208.

to its application in industrial methanol synthesis.17 This dynamic area of research has been reviewed by Busca.18 DME is an important commercial compound, being used in the production of compounds such as dimethyl sulfate and high value oxygenated compounds, and as an aerosol propellant.19 The dehydration of methanol over alumina to give DME is well established, and studies in this area2,5,19-24 have been critically reviewed.18 These indicate the likelihood of two possible routes to DME. The first involves an adsorbed methoxy species reacting with an undissociated methanol molecule, while the second involves the reaction of two adsorbed methoxy species. A recent study by Matyshak et al.25 reported results consistent with the proposal that DME is formed from the reaction of two surface methoxy species. In addition to these experimental studies, theoretical work by Blaskowski and Van Santen has examined different pathways for the formation of DME on zeolite acid catalysts.26 The preferred route to DME in this case is simultaneous adsorption and activation of two methanol molecules, with the formation of DME and water occurring in one step.26 (17) Twigg, M. V. Catalyst Handbook; Wolfe: London, 1989. (18) Busca, G. Catal. Today 1996, 27, 457. (19) Xu, M.; Lunsford, J. H.; Goodman, W.; Bhattacharya, A. Appl. Catal., A 1997, 149, 289. (20) Schauermann, S.; Hoffman, J.; Johanek, V.; Hartmann, J.; Libuda, Phys. Chem. Chem. Phys. 2002, 4, 3909. (21) Shi, B.; Davis, B. H. J. Catal. 1995, 157, 359. (22) Knozinger, H.; Scheglila, A.; Watson, A. M. J. Phys. Chem. 1968, 8, 2770. (23) Matsushima, T.; White, J. M. J. Catal. 1976, 44, 183. (24) Clayborne, P. A.; Nelson, T. C.; DeVore, T. C. Appl. Catal., A 2004, 257, 225. (25) Matyshak, V. A.; Khomenko, T. I.; Lin, G. I.; Zavalishin, I. N.; Rozovskii, A. Y. Kinet. Catal. 1999, 40, 295. (26) Blaszkowski, S. R.; Van Santen, R. A. J. Phys. Chem. B 1997, 101, 2292.

10.1021/la051429c CCC: $30.25 © 2005 American Chemical Society Published on Web 10/14/2005

Use of DRIFTS To Investigate Methanol on η-Alumina

Surface acidity plays a key role in the formation of the adsorbed states and subsequent transformations related to DME production on alumina. Most studies have been carried out on γ-alumina1,4-6,19,24,25 because it has the widest application in heterogeneous catalysis.27 However, η-alumina is an inherently more acidic form of alumina,28,29 with applications in catalytic reforming reactions,27 and is worthy of further investigation. Its enhanced acidity offers the possibility of affecting selectivity profiles through the moderation of the broader distribution of acid sites via selective poisoning strategies. We have reported the use of pyridine as a probe molecule to examine the surface acidity of η-alumina.30 Traditionally, the Lewis acidity of alumina had been described in terms of three sites of varying acidity labeled weak, medium, and strong.31-33 However, using a range of spectroscopic and gravimetric techniques, an additional type of Lewis acid site resulting from the subdivision of the medium strength Lewis acid site into medium-strong and medium-weak sites has been described.30 In recent work from this laboratory, a spectroscopic analysis of the interaction of methanol with an η-alumina catalyst34 has been reported. A combination of infrared and inelastic neutron scattering (INS) spectroscopy confirmed methoxy species to be the dominant surface moiety. INS spectroscopy was used additionally to follow the conversion of adsorbed methoxy species to dimethyl ether, with DFT calculations assisting assignments. Importantly, INS spectroscopy allowed the unequivocal assignment of a previously seldom reported band at 2600 cm-1 to a combination of the methyl rock and deformation modes of adsorbed methoxy species.34 The present study describes how the reaction of methanol on η-alumina to form DME can be understood within the context of the new four-site model for Lewis acidity. Using results from diffuse reflectance infrared spectroscopy (DRIFTS)35 and mass selective temperature-programmed desorption (TPD), a model for the formation of dimethyl ether over a η-alumina catalyst is described. The proposed model is consistent with that reported previously by Schiffino and Merrill5 and involves the reaction of two adsorbed methoxy species to form DME. However, using the improved description for the surface acidity of alumina, the role of specific acid sites is determined. Formate is a known intermediate in methanol decomposition on metal oxide catalysts and supported metal catalysts.1,11-13,16,23,36 Such species are believed to form from the breakdown of surface methoxy species on heating to temperatures typically above 473 K.1,18 Infrared spectroscopy reveals the presence of formate in the reaction system under investigation here, with associated mass selective desorption studies identifying gaseous products (27) Satterfield, C. N. Heterogeneous Catalysis in Industrial Practice, 2nd ed.; Krieger Publishing: Malabar, 1996; p 114. (28) MacIver, D. S.; Tobin, H. H.; Barth, R. T. J. Catal. 1963, 2, 485. (29) Le´onard, A. J.; Semaille, P. N.; Fripiat, J. J. J. Proc. Br. Ceram. Soc. 1969, 103, 103. (30) Lundie, D. T.; McInroy, A. R.; Marshall, R.; Mitchell, C.; Winfield, J. M.; Dudman, C. C.; Jones, P.; Parker, S. F.; Lennon, D. J. Phys. Chem. B 2005, 109, 11592. (31) Liu, X.; Truitt, R. E. J. Am. Chem. Soc. 1997, 119, 9856. (32) Morterra, C.; Chiorino, A.; Ghiotti, G.; Garrone, E. J. Chem. Soc., Faraday Trans. 1979, 75, 271. (33) Morterra, C.; Collucia, S.; Ghiotti, G.; Garrone, E. J. Chem. Soc., Faraday Trans. 1979, 75, 289. (34) McInroy, A. R.; Lundie, D. T.; Winfield, J. M.; Dudman, C. C.; Jones, P.; Parker, S. F.; Taylor, J. W.; Lennon, D. Phys. Chem. Chem. Phys. 2005, 7, 3093. (35) Chalmers, J. M.; Dent, G. Industrial Analysis With Vibrational Spectroscopy; Royal Society of Chemistry: Cambridge, 1997; p 153. (36) Sakakini, B.; Tabatabaei, J.; Watson, M.; Waugh, K.; Zemicael, F. W. Faraday Discuss. 1996, 105, 369.

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associated with its formation and decomposition. This information is used to define a site-selective reaction scheme. Collectively, this experimentally based, multitechnique approach provides new insight into a wellstudied and economically relevant reaction system. 2. Experimental Section 2.1. Catalyst Characterization. The η-alumina sample is a commercial grade catalyst,37 the characterization of which has been described in detail elsewhere.30 It was activated by heating to 623 K under flowing helium (BOC, 99.999%) for 150 min and was allowed to cool to reaction temperature. Throughout all experimental procedures, the sample was continuously flushed with helium gas that was passed through an in-line purifier (Messer Greisheim, Oxysorb). 2.2. Infrared Spectroscopy. Infrared experiments were performed using a Nicolet Nexus FTIR spectrometer fitted with a MCT high D* detector. Experiments were performed in diffuse reflectance mode using a SpectraTech Smart diffuse reflectance cell and environmental chamber, using typically 50 mg of catalyst. The output flow from the environmental chamber could be diverted to a mass spectrometer (Leda Mass Gas Analyzer, LM22, closed ion source) for analysis of desorption products from temperature-programmed infrared experiments. Background spectra were recorded post-activation at 293 K. Methanol (Aldrich, 99.8+% purity) was dosed on to the catalyst at 293 K using pulse-flow techniques.38,39 The geometric arrangement of the cell meant that dosing was associated with a substantial degree of gas by-pass. Ancillary experiments confirmed that a 10 min delay after each dosing pulse was sufficient for the adsorbate to be removed completely from the environmental chamber. After this time, sample spectra were recorded (128 scans, resolution 2 cm-1). All spectra are presented as background subtracted, where a spectrum of the clean, activated catalyst has been subtracted from the dosed spectrum. No baseline or offset corrections were made. For methanol desorption experiments, the cell containing alumina, previously dosed with methanol as above, was heated under flowing He. The cell was maintained at each chosen temperature for 15 min before being allowed to cool to room temperature, where a spectrum was then recorded. The eluent stream from the DRIFTS cell was monitored during experiments using mass spectrometry. 2.3. Temperature-Programmed Desorption. A detailed description of the experimental set up employed for thermal desorption experiments has been given elsewhere.30 Experiments were performed with the catalyst sample contained within a packed bed tubular reactor (0.25” o.d. stainless steel tubing) located within a temperature programmable oven (Neytech 25 PAF). A mass spectrometer (Leda Mass LM22) sampled the eluting gases via a differentially pumped capillary line and a metal-sintered precision leak. The alumina sample was activated and dosed with methanol as outlined above. Saturation of the sample could be observed by monitoring the eluent stream on the mass spectrometer. When saturation was achieved, the sample was left to purge overnight at 293 K under flowing He. This resulted in the loss of any physisorbed methanol from the alumina surface. TPD experiments were performed at a temperature ramp rate of 8 K min-1. The eluent stream from the reactor was monitored by the mass spectrometer at all times during the TPD experiment.

3. Results and Discussion 3.1. Sample Purge Conditions and TemperatureProgrammed Desorption. Figure 1a shows the background-subtracted diffuse reflectance spectrum for a saturated overlayer of methanol on η-alumina and is described in detail elsewhere.34 It is consistent with previous reported spectra,3,6,14,40,41 which are described in (37) Ineos Chlor catalyst, ref: 25867/19A. (38) Lennon, D.; Kennedy, D. R.; Webb, G.; Jackson, S. D. Stud. Surf. Sci. Catal. 1999, 126, 341. (39) Kennedy, D. R.; Webb, G.; Jackson, S. D.; Lennon, D. Appl. Catal., A Gen. 2004, 259, 109.

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Figure 1. The background-subtracted diffuse reflectance infrared spectra for a saturated overlayer of methanol on η-alumina at 293 K recorded (a) 10 min after dosing and (b) after a 14 h purge in flowing helium at 293 K. The inset (c) shows the methanol mass spectrometer signal (31 amu) during the purging process.

Figure 2. Schematic representation of a hydrogen-bonding interaction between a chemisorbed methoxy species and an adjacent hydroxyl group.

terms of associative and dissociative adsorption of methanol. Methoxy species are evident from the bands at 2938 and 2820 cm-1, assigned to antisymmetric and symmetric methyl group stretches, respectively, and the methyl deformation band at 1450 cm-1.34 The presence of molecular methanol is indicated by bands at ca. 3200 cm-1 ν(OH), 2940 cm-1 (νas(CH3)), 2820 cm-1 (νs(CH3)), and 1460 cm-1 (δ(CH3)). The C-H features of methanol all lie close in frequency to their methoxy counterparts; however, the broad O-H stretching feature at ∼3200 cm-1 is uniquely associated with methanol. The strong band at ca. 2600 cm-1 has recently been assigned using INS spectroscopy to a combination band of the methyl rock (1170 cm-1) and methyl deformation modes (1460 cm-1) of adsorbed methoxy species.34 Its relatively enhanced intensity is a consequence of the effective path length of weak absorbing species in the diffuse reflectance arrangement.34 Bands exhibiting negative intensity are observed in the OH stretching region. These are presumed to arise from the interaction of free hydroxyl groups on the surface of the substrate with adsorbed methanol and methoxy groups. Similar observations have been reported in studies examining pyridine adsorption on alumina.30,31 Isolated hydroxyl groups on the alumina surface can hydrogen bond to adsorbed methanol or methoxy species, resulting in loss of intensity for free OH and a shift to lower frequencies, into the intense broad envelope of the hydrogen-bonded ν(OH) region, where they become effectively masked. One possible orientation for this particular bonding configuration is presented in Figure 2. This is presented as an illustrative example of an interaction between a methoxy group and an adjacent, isolated hydroxyl group. (40) Hertzberg, G. Molecular Spectra and Molecular Structure Vol II - Infrared and Raman Spectra of Polyatomic Molecules; Krieger: Malabar, 1991. (41) Badri, A.; Binet, C.; Lavelley, J. C. J. Chem. Soc., Faraday Trans. 1997, 93, 2121.

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Figure 3. Temperature-programmed desorption spectrum for a saturation coverage of methanol on η-alumina. The sample at 293 K was dosed with repeated pulses of methanol until the mass spectrometer detected no further retention of adsorbate by the catalyst. The sample was then purged overnight at 293 K to remove any physisorbed methanol. (a) Methanol (31 amu) and (b) DME (45 amu) mass spectrometer signals were then monitored as a gradient of 8 K min-1 was applied up to 1000 K.

To make distinctions between the different adsorbed states of methanol on alumina, it is necessary to define conditions for which physisorbed methanol can be excluded. The infrared spectrum for a methanol-saturated surface prepared at 293 K and then purged in flowing helium at that temperature for 14 h is shown in Figure 1b. Although there is some attenuation of intensity due to loss of adsorbate, the same spectral features seen in Figure 1a are retained. Figure 1c shows the methanol mass spectrometer signal during the purge process and establishes that purging of ca. 3 h is required to reduce the methanol signal to baseline levels. Importantly, Figure 1a and b indicates that even after an overnight purge at 293 K large quantities of molecularly adsorbed methanol are still present on the catalyst surface, as evidenced by the broad ν(OH) feature at ca. 3200 cm-1, indicating a significant enthalpy of adsorption for that state. Other infrared-based investigations have examined the interaction of associatively adsorbed methanol on alumina. For example, Busca et al. observe two forms on alumina: the first being irreversibly adsorbed at room temperature that is strongly hydrogen bonded on a cation-anion couple having strong basic character, and the second a reversible form that is hydrogen bonded to basic sites.3 The TPD profile for a saturated overlayer of methanol on η-alumina is shown in Figure 3. All TPD experiments in the tubular reactor were performed after an overnight purge treatment as outlined above, ensuring physisorbed methanol could play no part in the resulting TPD spectra. Figure 3 is characterized by two distinct features: a methanol peak centered at 390 K and a DME peak centered at 460 K. The profiles are consistent with those reported by DeVore et al. for methanol adsorbed on poorly crystalline γ-alumina.24 Figure 3 also confirms that the activation procedure adopted in section 2.1 produces a catalyst active for the formation of DME from methanol, consistent with the observations of Tleimatmanzalji et al.42 A minimum temperature of 400 K is required for dimethyl ether production. Both methanol and DME signals tail to high temperatures reaching approximate baseline levels at ca. 700 K. (42) Tleimatmanzalji, R.; Bianchi, D.; Pajonk, G. M. React. Kinet. Catal. Lett. 1993, 51, 29.

Use of DRIFTS To Investigate Methanol on η-Alumina

Figure 4. The diffuse reflectance infrared spectrum for a saturated overlayer of methanol on η-alumina as a function of temperature. (a) Saturation spectrum at 293 K. The sample was progressively warmed in a helium flow to (b) 373, (c) 473, (d) 573, (e) 673, (f) 773, and (g) 873 K. The sample was held at the desorption temperature for 15 min, and then allowed to cool to room temperature, where the spectrum was acquired. All spectra are background subtracted, where the spectrum of the clean, activated catalyst has been subtracted from the dosed/ heated spectrum.

3.2. Temperature-Programmed Infrared Spectroscopy. The infrared spectrum, in the range 12004000 cm-1, for a chemisorbed overlayer of methanol adsorbed on η-alumina as a function of temperature is shown in Figure 4. Warming the sample to 473 K leads to methanol desorption. The absence of the broad ν(OH) band at this temperature indicates the complete removal of chemisorbed methanol. At 573 K, the only species remaining on the surface are adsorbed methoxy groups, represented by ν(CH3) at 2750-3000 cm-1 and a weaker δ(CH3) about 1450 cm-1. Figure 3 indicates methanol desorption precedes DME formation. Figure 4 indicates associatively adsorbed methanol contributes to the methanol desorption profile, but it is also recognized that a combination of methoxy units and H atoms originating from a dissociative adsorption process5 could also play a role. Methanol desorption at temperatures below 573 K (Figures 3 and 4) indicates relatively weak adsorption, which can be correlated with the recently reported foursite model for the surface acidity of η-alumina.30 Desorption of associatively bound methanol is associated with weak Lewis acid sites, and methanol desoprtion occurring via a recombinative process is associated with methoxy species residing in medium-weak Lewis acid sites. Inspection of the free hydroxyl region (3600-4000 cm-1) shows a return to baseline intensities at temperatures below 673 K. This feature corresponds to the loss of methanol and DME (see Figure 3) and indicates a reversible process. This is consistent with the negative OH bands in Figures 1 and 4 involving a H-bonding interaction between surface hydroxyl groups with surface methanol and methoxy species. A further increase in temperature to 673 K yields dramatic changes in the infrared spectrum. The bands associated with surface methoxy species still remain but are further reduced in intensity. However, a series of new bands at 1380, 1395, 1595, and 2911 cm-1 are observed. Figure 5 shows the isolated spectrum obtained at 673 K in more detail, which is characterized by a number of sharp bands for adsorbed species. Figure 4 shows these additional bands are removed on increasing the sample temperature to 873 K and appear to be representative of an intermediate species in the thermal decomposition of

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Figure 5. The diffuse reflectance infrared spectrum of a saturated overlayer of methanol on η-alumina heated to 673 K. This spectrum corresponds to spectrum e in Figure 4. Table 1. Assignment of Infrared Bands Present after Heating a Saturated Chemisorbed Overlayer of Methanol on η-Alumina to 673 K (Figures 4e and 5)a

assignment

η-Al2O3/MeOH (cm-1)

ν(s) CO2δCH δ(S) CH3 δ(as) CH3 ν(as) CO22 δCH νCH ν(as) CO2- + δCH ν(s) CH3 ν(as) CH3

1378 1393 1456 1475 1596 2806 (w) 2911 3000 (vw) 2845 2951

adsorbed methoxy species15,43 (cm-1)

adsorbed formate species44 (cm-1) 1380 1395

1450 1595 2846 2950

2905 2970

a The spectrum is composed of bands associated with adsorbed methoxy and formate species.

methanol on η-alumina. These new features are assigned to a formate species and are consistent with previous observations of the transformation of formaldehyde on metal oxides.44 Figure 5 is described as a composite spectra, arising from adsorbed methoxy and formate species, with the assignments presented in Table 1. The features at 2951 and 2845 cm-1 are, respectively, assigned to the antisymmetric and symmetric CH3 stretches of methoxy groups, while the weaker bands at 1475 and 1456 cm-1 are attributed to the antisymmetric and symmetric methoxy deformation modes.43 The formate is characterized by intense, sharp features at 2911 (C-H stretch), 1595 (antisymmetric CO2 stretch), 1393 (C-H bend), and 1378 cm-1 (symmetric CO2 stretch).44 Weaker features are also discernible at 3000 and 2806 cm-1 as shoulders on, respectively, the antisymmetric and symmetric methoxy methyl stretches. The 3000 cm-1 feature is thought to be a combination band of the antisymmetric CO2 stretch and the C-H bend (1393 + 1596 ) 2989 cm-1),44 while the 2806 cm-1 peak is assigned to an overtone of the C-H bend (2 × 1393 ) 2786 cm-1). Given that formate is formed at temperatures above 573 K and is retained until approximately 773 K, this species is thought to involve methoxy units residing on strong Lewis acid sites. (43) Miller, G. J.; Rochester, C. H.; Waugh, K. C. J. Chem. Soc., Faraday Trans. 1991, 87, 2795. (44) Busca, G.; Lamotte, J.; Lavalley, J.; Lorenzelli, V. J. Am. Chem. Soc. 1987, 109, 5197.

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Figure 6. Species evolved from infrared thermal desorption experiments of methanol on η-alumina as monitored by mass spectrometry. The eluent stream from the infrared environmental chamber was monitored, and the mass spectrometer signals for methanol (31 amu), DME (45 amu), and CO (28 amu) were simultaneously recorded. The dashed line represents the temperature ramp applied.

Formate is known to adsorb on metal oxide surfaces in both bidentate and monodentate coordination geometries.45 The sharpness of the antisymmetric C-O stretch at 1595 cm-1 (fwhm )14 cm-1) indicates the formate to be residing on a high symmetry site. Surface formates are normally associated with a bridging bidentate structure, with the oxygen atoms associated with two adjacent adsorption sites.11,44 However, given that the reaction centers for this alumina are attributed to coordinatively unsaturated Al ions (Lewis acid sites),30 it is difficult to envisage how a formate species could bind to two adjacent Al ions of comparable acidity and maintain a symmetric adsorption geometry. Gravimetric analysis coupled with temperature-programmed desorption determines the density of the strong Lewis acid sites on η-alumina to be 1.1 × 1013 cm-2.30 Assuming a square two-dimensional mesh, this approximates to a separation distance of ca. 3 nm, which exceeds a representative O-O separation distance of ca. 2.3 Å as encountered for adsorbed formate groups in a bridging bidentate structure.46 Alternatively, it is possible for the formate unit to bind to a single Al center as a nonbridging bidenate species. Such an arrangement has been reported for methanol oxidation and transformation over metal oxides,18 and a similar coordination is also found for carbonate species adsorbed on alumina.45,47,48 3.3. Mass Spectrometric Analysis Post-IR. The experimental configuration described in section 2 permits the simultaneous detection of eluting gases from the infrared cell by mass spectrometry while an infrared spectrum is also being recorded. This combination of techniques is useful in evaluating the processes active under operating conditions and conforms to the principles of operando spectroscopy.49 Figure 6 shows the mass spectrometer signals for methanol, DME, and CO as a temperature ramp is applied to the infrared environmental cell and indicates broad similarities to the temperatureprogrammed desorption profiles reported in Figure 3. (45) Little, L. H. Infrared Spectra of Adsorbed Species; Academic Press: London, 1966. (46) Zheng, T.; Stacchiola, D.; Saldin, D. K.; James, J.; Sholl, D. S.; Tysoe, W. T. Surf. Sci. 2005, 574, 166. (47) Dai. Q.; Robinson, G. N.; Freedman, A. J Phys. Chem. B 1997, 101, 4940. (48) Gregg, S. J.; Ramasy, J. D. F. J. Phys. Chem. 1969, 73, 1243. (49) Weckhuysen, B. M. Phys. Chem. Chem. Phys. 2003, 5, 4351.

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Figure 7. Temperature-programmed desorption spectra for a saturated overlayer of methanol on η-alumina. (a) Methane signal (16 amu). The low-temperature feature at about 450 K is assigned to water formed from the reaction of two methoxy species to form DME. Inset (b) shows the dihydrogen profile observed during the same TPD experiment over the temperature range 600-800 K. Inset (c) shows the dihydrogen signal obtained during a blank experiment using a clean activated sample (no methanol adsorption).

Differences in relative intensities are attributed to differences in reactor and sampling geometries utilized in the infrared and microreactor studies. Furthermore, the temperature-programmed infrared experiments used a stepwise heating pattern, whereas temperature-programmed desorption experiments use a linear heating rate. Nevertheless, despite these differences in the two arrangements, the temperature ranges under which both methanol and DME are evolved are consistent. Importantly, Figure 6 shows that CO evolution does not occur until temperatures are such that methanol and DME are no longer being produced and, additionally, its profile corresponds to the decomposition of the formate species seen in Figure 4. It is possible that the CO signal could arise from carbonate decomposition. Aluminas are known to contain carbonates, which can thermally decompose to form CO2 and CO.50 However, the CO2 TPD spectrum (not shown) for methanol-dosed alumina and also plain activated alumina had no substantial CO2 contribution, indicating the CO signal observed in Figure 6 to originate as a consequence of the surface chemistry of the η-alumina/methanol system, namely, formate decomposition. 3.4. Temperature-Programmed Desorption Relating to Formate Species. Concentrating on the reaction to produce formate from adsorbed methoxy series, additional TPD experiments were performed scanning a range of masses. The TPD spectra for methane and dihydrogen postmethanol adsorption are shown in Figure 7a and b. The mass 16 amu profile shows two features centered at ca. 450 and 740 K. The lower temperature feature corresponds to water produced in the process of DME formation from two surface methoxy groups. This assignment was substantiated by additionally recording the water profile (18 amu, not shown), which showed a coincidental peak at ca. 450 K. Monitoring the 15 amu signal (not shown), a fragment of the methane cracking pattern, confirmed the higher temperature peak to be due to methane. This feature occurs at temperatures consistent with formate formation (Figure 4) and is attributed to methane formed in the reaction of two methoxy species to form formate (50) Lear, T. Ph.D. Thesis, University of Glasgow, 2003.

Use of DRIFTS To Investigate Methanol on η-Alumina

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Figure 8. Postulated reaction pathway for the formation of a nonbridging bi-dentate formate species from adsorbed methanol on an η-alumina catalyst. The asterisks represent aluminum centers assigned to strong Lewis acid sites. ∆1 corresponds to the temperature required to initiate formate production (>573 K), and ∆2 corresponds to the temperature above which formate decomposition occurs (>673 K).

molecules. Figure 7b shows the dihydrogen signal to occur over a temperature range similar to that observed for methane (Tmax ca. 740 K). Figure 7c is the dihydrogen TPD profile for a sample of undosed, activated alumina that confers no signal, indicating the hydrogen desorption seen in Figure 7b is a consequence of the alumina/methanol chemistry, which can also be linked with formate production. Previously, Busca and co-workers have proposed an adsorbed dioxymethylene as an intermediate species in the formation of formate from methoxy species on metal oxides.44 Spectroscopically, such species would be characterized by two strong C-O stretching bands in the region 1200-1000 cm-1 of the infrared spectrum,18 which is below the cutoff limit for DRIFTS measurements of η-alumina.30 Nevertheless, we propose that there is no need to invoke a role for such species for the η-alumina/methanol system studied here, as formate production and decomposition can be reasonably rationalized in terms of reagents identified in Figures 1-7. Figure 8 presents a reaction pathway that is consistent with the observed results reported in this work and concentrates on the strong Lewis acid sites, on which methanol is dissociatively adsorbed. Formate formation requires the presence of two oxygen atoms. It is possible the surface could be a source, direct or indirect, of oxygen atoms, but, in the context of a viable catalytic cycle, it seems preferable to attain a closed mass balance within the available reagents, and, therefore, the reactions are referenced to two methanol units. Thermal ramping to the temperature ∆1 (>573 K) leads to the formation of surface formate (Figure 4) and the evolution of methane and dihydrogen gas (Figure 7). Further heating in excess of temperature ∆2 (>673 K) leads to the decomposition of the formate species (Figure 4) and the evolution of CO (Figure 6). A precise definition of the water desorption characteristics indicated in Figure 8 was not possible because its postulated formation occurs above the catalyst activation temperature (623 K), where heating beyond this value leads to large quantities of water evolution from dehydroxylation of the alumina. It is recognized that the methane, dihydrogen, and carbon monoxide profiles are broadly similar and that their association with formation and decomposition of the formate species could be convoluted. However, the specific assignment of methane and dihydrogen to formation and carbon monoxide to decomposition pathways does lead to a closed mass balance, as presented in Figure 8. Additionally, the temperature profiles also support these assertions. Methane and dihydrogen are detected above baseline levels between 650 and 790 K (Figure 7), whereas carbon monoxide evolution is observed at 873 K (Figure 6). Thus, methane and dihydrogen desorption precedes carbon monoxide desorption, which is consistent with the scheme laid out in Figure 8. Busca notes that formate ions can decompose over metal oxides according to eq 1:18

H-COO- S CO + OH-

(1)

Some evidence of the formation of OH(ad) is discernible in the infrared spectrum obtained at 873 K (Figure 4g), in which a gain in the ν(O-H) signal above the original baseline occurs at ca. 3770 cm-1; this is associated with free hydroxyl groups. Therefore, this decomposition route should also be considered in addition to that presented in Figure 8. 3.5. Mechanistic Considerations. It is now opportune to consider the reaction profiles observed in terms of recent advances in understanding of the surface acidity of this versatile substrate.30 In section 3.2, the methanol TPD profile was attributed to associative adsorption into weak Lewis acid sites and dissociative adsorption into mediumweak Lewis acid sites. DME production is commonly associated with eq 2:18

2CH3OH f CH3OCH3 + H2O

(2)

At the higher temperatures necessary to initiate this reaction (>410 K), the infrared spectrum (Figure 4) indicates the presence only of methoxy species, and therefore it is reasonable to attribute DME formation to a combination of adsorbed methoxy units. This approach is consistent with the mechanism for methanol dehydration on γ-alumina as outlined by Schiffino and Merrill.5 The relatively high temperature for this transformation indicates these methoxy units to reside in higher energy sites than those responsible for methanol production, and, consequently, DME production is associated with methoxy molecules in medium-strong Lewis acid sites. In section 3.2, formate production was associated with strong Lewis acid sites. Figure 4 shows the conversion from methoxy f formate occurs at a temperature (>573 K) when DME production has effectively diminished (Figure 3); therefore, this transition is thought to originate from methoxy species residing in sites different from those associated with DME formation. Further heating leads to decomposition of the formate at these sites. Figure 9 attempts to link schematically all of these postulates together. Specifically, methanol desorption is associated with weak (associative) and medium-weak (dissociative) Lewis acid sites; DME production is thought to occur from a combination of methoxy units occupying medium-strong Lewis acid sites; and formate production occurs from a combination of methoxy units in strong Lewis acid sites. Methane and hydrogen evolution are associated with the generation of the formate species, carbon monoxide and water with the decomposition. The different acid sites presented in Figure 9 are distinct with respect to their relative Lewis acidities, as was determined by infrared and TPD measurements of adsorbed pyridine;30 however, no geometric significance is implied. Rather than invoking adjacent pairs of methoxy units adsorbed at sites of the same acidity, a process of surface diffusion is thought to dominate, where higher temperature leads to increased hopping frequencies between discrete sites of a specific acidity (weak, mediumweak, medium-strong, and strong Lewis acid sites) and

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Figure 9. A schematic representation for the postulated site-selective scheme for the interaction of methanol with η-alumina. The geometric arrangement of the different surface sites has no direct significance, although their grading specifically reflects the surface acidity of the four distinct sites, as was recently outlined.30

increased mean free paths leading to increased collision frequencies and, hence, increased probability for reaction. Close inspection of Figure 3 provides some insight into these processes. Between 700 and 800 K, it is noted that there is some residual methanol desorption while DME production has ceased. In this region, it is thought that the concentration of methoxy molecules in medium-strong Lewis acid sites is so low that the collision frequency between these entities dramatically drops, disfavoring DME formation. Instead, the methoxy species collide with hydrogen, present at an oxygen bridge site, and then desorb as methanol. Finally, further corroborative support for the applicability of the four-site model for this highly active and commercially significant surface is provided by examination of the hydroxyl stretching region in Figure 4. For temperatures of 673 K and above, the negative free ν(O-H) feature about 3700 cm-1 returns to baseline levels. At this temperature, methoxy units in medium-strong Lewis acid sites will have reacted and desorbed to form DME, leaving only methoxy molecules in the strong sites. Now, the strong sites are bare, that is, they contain no adjacent hydroxyl groups in the coordination sphere,30 and so no hydrogen-bonding interactions are possible and no shifts to low frequency are seen, which lead to the negative ν(O-H) features seen at lower temperatures. This analysis is totally consistent with the reversible process described in section 3.2.

of associatively adsorbed methanol and chemisorbed methoxy species. TPD experiments show methanol and DME to desorb with respective maxima at 380 and 480 K, with desorption detectable for both molecules up to ca. 700 K. These results are consistent with previous work on γ-alumina. Infrared spectroscopy reveals the formation at 673 K of a formate species, with the spectral line width of the antisymmentic C-O stretch indicating adoption of a highsymmetry adsorbed state. TPD measurements indicate dihydrogen and methane evolution to be associated with formation of the surface formate group and CO with its decomposition. A reaction scheme is proposed for the generation and decomposition of this important reaction intermediate. The overall processes involved in (i) the adsorption/ desorption of methanol, (ii) the transformation of methanol to DME, and (iii) the formation and decomposition of formate species can be correlated with the presence of four different types of Lewis acid sites on this surface. Methanol desorption is attributed to associatively adsorbed methanol residing in weak Lewis acid sites and dissociatively adsorbed methanol on medium-weak Lewis acid sites; DME production occurs from a combination of methoxy units occupying medium-strong Lewis acid sites; and formate production occurs from a combination of methoxy units in strong Lewis acid sites.

4. Conclusions

Acknowledgment. The EPSRC and INEOS Chlor Ltd. are thanked for the provision of postgraduate industrial CASE awards (A.R.M. and D.T.L.). The Scottish Higher Education Funding Council is thanked for equipment funding via the award of a Research Development Grant.

The present work has investigated the interaction of methanol with an η-alumina catalyst and the subsequent transformations under heat treatment to form dimethyl ether. The main conclusions can be summarized as follows: The infrared spectrum of a saturated overlayer of methanol on η-alumina shows the surface to be comprised

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