Investigation of acid properties of dealuminated H-mordenite zeolites by low-temperature diffuse reflectance FTIR

A series of dealuminated hydrogen mordenites (total Si/Al = 7.7, 11.6, 16.8, 29.0, 39.0) were investigated by diffuse reflectance IR spectroscopy with and without probe molecules (CO, H-2). In the stretching vibration region of pure samples bands were observed at 3613-3611, 3734, 3675 and 3510 cm(-1) at room temperature and assigned to bridging OH groups [nu(Al(OH)Si)], silanol groups (nu(SiOH)), OH of Al-containing species partially coordinated to the framework and hydrogen-bonded SiOH species in highly dealuminated specimens, respectively. When the IR spectra were obtained at 77 K, both the bands of nu(Al(OH)Si) and nu(SiOH) shifted to higher wavenumbers, viz. 3619 and 3740 cm(-1), which could be ascribed to nu(Al(OH)Si) + delta(Al(OH)Si), nu(SiOH) + delta(SiOH) and nu(SiOH) + nu(SiO), respectively. Upon interaction with H-2 or CO at 77 K the following shifts were measured: Delta nu(Al(OH)Si)/H-2 = 53-63 cm(-1); Delta nu(SiOH)/H-2 = 5 cm(-1); Delta[nu(Al(OH)Si) + delta(Al(OH)Si)]/H-2 = 24-31 cm(-1); Delta(nu(SiOH) + delta(SiOH))/H-2 = 0; Delta nu(Al(OH)Si/)CO = 359-389 cm(-1) and Delta nu(SiOH)/CO = 102 cm(-1). The intensities of the bands due to nu(Al(OH)Si) and nu(Al(OH)Si) + delta(Al(OH)Si) decreased with increasing dealumination as a result of removal of acidic bridging OH groups. By contrast, the shifts Delta[nu(Al(OH)Si) + delta(Al(OH)Si)]/H-2 and Delta nu(Al(OH)Si)/H-2 continuously increased with the degree of dealumination which is proposed to indicate a corresponding increase in the strength of the Bronsted centres, Al(OH)Si, in line with the earlier TPD results obtained with the same samples. Bands, which appeared upon H-2 interaction at 77 K and 4070 and 4020 cm(-1), are discussed in view of findings reported in the literature for similar systems and tentatively assigned to 'true' Lewis-acid sites (removable by acid leaching) and to only threefold-coordinated Al and/or Si of the framework, respectively. Finally, the bands of H-2 perturbed by silanol groups (4103 cm(-1)), of physisorbed CO (4250 cm(-1)), the effect of dealumination on the formation of defects and their interaction with the probe molecules as well as the problem of the influence of CO pressure on the band shift Delta nu(Al(OH)Si)/CO are discussed in detail.