11592
J. Phys. Chem. B 2005, 109, 11592-11601
Improved Description of the Surface Acidity of η-Alumina David T. Lundie,† Alastair R. McInroy,† Robert Marshall,† John M. Winfield,† Peter Jones,‡ Chris C. Dudman,‡ Stewart F. Parker,§ Chris Mitchell,| and David Lennon*,† Department of Chemistry, Joseph Black Building, The UniVersity of Glasgow, Glasgow, G12 8QQ, U.K., INEOS Chlor Ltd., Runcorn Technical Centre, The Heath, Runcorn, Cheshire WA7 4QD, ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, and Huntsman Polyurethanes, EVerslaan 45, B3078 EVerberg, Belgium ReceiVed: August 31, 2004; In Final Form: February 23, 2005
The surface acidity of an activated η-alumina catalyst has been investigated by examining the interaction of pyridine with the catalyst by a combination of gravimetric and volumetric adsorption isotherms, infrared spectroscopy (diffuse reflectance and transmission), inelastic neutron scattering spectroscopy, temperatureprogrammed desorption spectroscopy, and gravimetric desorption experiments. From previous work, this surface was considered to contain three types of Lewis acid sites of increasing acidity: weak, medium, and strong. However, this multitechnique approach reveals the presence of an additional type of Lewis acid site. Although the traditional pyridine ring modes about 1580 cm-1 are consistent with previous studies, temperatureprogrammed infrared spectroscopy of the surface hydroxyl groups and mass-selective temperature-programmed desorption experiments establish that the medium-strength Lewis acid category can be subdivided into two components. In this way, the surface structure of the activated catalyst is redefined as comprising (i) weak, (ii) medium-weak, (iii) medium-strong and (iv) strong Lewis acid sites. The (O-H) stretching mode of surface hydroxyl groups provides information on the local structure of the distinct sites, and schematic descriptions for these sites are proposed.
1. Introduction Aluminas, in various forms, are ubiquitous in heterogeneous catalysis. They can be used as catalysts in their own right (e.g. for the isomerization of hydrocarbons1) or as support materials to stabilize and maintain metal crystallites of small dimensions for metal-mediated reactions (e.g. hydrogenation reactions2,3). In the former case, the ability of the alumina to act as a solid acid is emphasized, whereas in the latter the mechanical strength of the material coupled with a relative inertness toward the reaction medium is required. This range of activities illustrates the flexibility that the substrate provides, where the structure and chemical properties are dependent on parameters such as the source material, preparative route, purity of the reagents, extent of dehydration, and thermal history.4a,5 In all of these applications, it is desirable to have an understanding of the surface structure with its associated acidity and distribution of acid sites. This general topic has been comprehensively reviewed by Kno¨zinger and Ratnasamy6 and Morterra and Magnacca.7 Additional advances in understanding the intricate surface chemistry have been made by Busca and co-workers 8 and Tsyganenko and co-workers.9,10 A recent review by Lambert and Che, dealing primarily with the synthesis of supported catalysts, summarizes some of the main issues pertaining to the structure of the alumina surface and highlights a number of unresolved issues in surface group identification.11 Major advances in understanding have been achieved by the adsorption * Corresponding author. E-mail:
[email protected]. Tel: 44(0)-141-330-4372. Fax: 44-(0)-141-330-4888. † The University of Glasgow. ‡ INEOS Chlor Ltd. § ISIS Facility. | Huntsman Polyurethanes.
of probe molecules coupled with a variety of spectroscopic techniques, notably infrared spectroscopy.8-10,12-14 The chemical probe most commonly used is pyridine,12,14 which has shown alumina surface chemistry to be dominated by a range of Lewis acid sites.8,12-14 The vast majority of the work on this topic has featured γ-alumina,14-22 which is the structural form of alumina that has the widest application in heterogeneous catalysis.4a For example, the relatively recent work of Liu and Truitt used a combination of pyridine adsorption monitored by diffuse reflectance infrared spectroscopy to establish the presence of three types of Lewis acid sites present at the catalyst surface: weak, medium-strong, and strong.22 Correlation between perturbations in the pyridine spectrum with those of the hydroxyl groups of the alumina surface permitted the form of adjacent hydroxyl groups to be deduced, thereby providing further information as to the nature of the surface of the activated catalyst.22 Such experimental studies are complemented by an increasing number of theoretically based investigations.21,23-27 Fewer studies have examined η-alumina, which is inherently more acidic than γ-alumina28,29 and has applications in catalytic reforming reactions.4a Morterra and co-workers have used transmission infrared spectroscopy to characterize an η-alumina catalyst and also report the presence of three Lewis acid sites associated with octahedral, tetrahedraloctahedral, and tetrahedral cationic sites.30,31 Recent theoretical work by Sohlberg et al. suggests that the greater acidity of η-alumina over γ-alumina is a consequence of differences in the bulk point defect distributions of the two phases that lead to different surface reconstructions.26 Against this background of improved understanding of the surface terminations of these aluminas, there is an increasing requirement for experimentally derived high-resolution models
10.1021/jp0405963 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/18/2005
Improved Description of η-Alumina Surface Acidity that more fully describe the nature of the active sites that confer acidity. η-Alumina was selected for investigation because it offers a greater range of acidity over γ-alumina. Acidity, if well defined, could be modified selectively to produce catalysts exhibiting improved selectivity for specified reactions.4b This work re-examines the η-alumina/pyridine adsorption system by a combination of gravimetric and volumetric adsorption isotherms, infrared spectroscopy (diffuse reflectance and transmission), and mass-selective temperature-programmed desorption spectroscopy. In addition, inelastic neutron scattering spectroscopy (INS) was used to obtain a more complete description of the vibrational spectrum of the chemisorbed pyridine.32 Because of strong Al-O phonon modes, vibrational modes below 1100 cm-1 are inaccessible in infrared spectroscopy, limiting access to potentially diagnostic low-frequency vibrations of the chemisorbed probe molecule. Alumina in INS presents a reasonably flat background spectrum against which vibrational modes of adsorbed molecules involving substantial movement of hydrogen atoms can be distinguished. This article is structured as follows. Section 3.1 presents volumetric and gravimetric pyridine adsorption experiments that determine the overall acidity of the η-alumina catalyst and permit differentiation between chemisorbed and physisorbed pyridine. Infrared spectra of the pyridine ring vibrational modes (section 3.2) reveal the strong, medium, and weak Lewis acid sites as reported by other workers (e.g., ref 22). Inelastic neutron scattering experiments extend the dynamic range for the spectrum of the chemisorbed base (section 3.4) and establish attenuation and shifts in a number of low-frequency modes that are not observable by conventional infrared spectroscopy. Massselective temperature-programmed desorption experiments (section 3.5) are then correlated with temperature-programmed infrared experiments (section 3.3) and, crucially, reveal that the medium-strength Lewis acid site can be subdivided into two states, termed medium-strong and medium-weak Lewis acid sites. Gravimetric desorption measurements are used to determine the populations of the medium and strong sites (section 3.6). Analysis of the alumina hydroxyl groups present at the catalyst surface provide additional evidence for the presence of the two medium-strength Lewis acid sites (section 3.7). Finally, via comparisons with the extensive literature on alumina surface hydroxyl group environments, the structural characteristics of each active site are defined in section 3.8. It is envisaged that through an improved awareness of the acid site distribution of this versatile catalytic material the elucidation of reaction mechanisms for processes utilizing the acidity of this substrate to effect chemical transformations will be significantly assisted. 2. Experimental Section 2.1. Catalyst Characterization and General Procedures. The η-alumina is a commercial-grade catalyst33 that has application in the synthesis of methyl chloride from the reaction of methanol and hydrogen chloride.34 It is a high-purity material containing Na2O
