The surface acidity of an activated eta-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, temperature-programmed 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, temperature-programmed 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.

The industrially important interaction of methanol with an eta-alumina catalyst has been investigated by a combination of infrared spectroscopy (diffuse reflectance and transmission) and inelastic neutron scattering (INS) spectroscopy. The infrared and INS spectra together show that chemisorbed methoxy is the only surface species present. Confirmation of the assignments was provided by a periodic DFT calculation of methoxy on eta-alumina (110). The thermal conversion of adsorbed methoxy groups to form dimethylether was also followed by INS, with DFT calculations assisting assignments. An intense feature about 2600 cm(-1) was observed in the diffuse reflectance spectrum. This band is poorly described in the extensive literature on the alumina/methanol adsorption system and its observation raised the possibility of a new surface species existing on this particular catalyst surface. INS measurements established that the 2600 cm(-1) feature could be assigned to a combination band of the methyl rock with the methyl deformation modes. This assignment was reinforced by an analysis of the neutron scattering intensity at a particular energy as a function of momentum transfer, which confirmed this particular adsorbed methoxy feature to arise from a second order transition. Similar behaviour was observed in the model compound Al(OCH3)3. The anomalous infrared intensity of the 2600 cm(-1) peak in the diffuse reflectance spectrum is a consequence of the different absorption coefficients of the C-H stretch and the combination mode. The implications for catalyst studies are discussed.

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.