FT infrared study of Brønsted acidity of H-mordenites: Heterogeneity and effect of dealumination

Dimethyl Ether Carbonylation to Methyl Acetate over Nanosized Mordenites

“…The band at 3610 cm −1 for acidic bridging −OH stretching vibration was actually the combination of the bands for two types of acid sites in the 12-MR channels (∼3612 cm −1 ) and the 8-MR channels (∼3590 cm −1 ). 4,25,26 It is obvious that the band at 3610 cm −1 of the Cu-HMOR sample decreased significantly in comparison with that of the HMOR sample, indicating that the acidic bridging −OH groups might be exchanged by Cu ions. 27 Figure 3b shows the 1 H MAS NMR spectra of the HMOR and Cu-HMOR samples.…”

Modification of MOR by desilication treatments: Structural, textural and acidic characterization

“…Both spectra present two well defined bands at 3743 and 3605 cm À1 , assigned to isolated silanol groups, SiOH, and Si-OHAl bridging hydroxyls groups (Brönsted acid sites), respectively [33,34]. The second band is broad and asymmetric, suggesting the presence of at least two different acid sites, those from the main channels ($3605 cm À1 ) and the others from the side pockets ($3585 cm À1 ) of the MOR structure [35], even if a third band at about 3595 cm À1 (8-R opening sites) has been more recently evidenced [36,37]. The alkaline treated sample, when compared with the parent support, presents a more intense band corresponding to SiOH groups, while the band associated to the Brönsted acid sites does not change significantly upon the desilication treatment.…”

Acid-based Catalysis in Zeolites Investigated by Density-Functional Methods

“…A very detailed investigation of CO adsorbed at BAS located at various positions in mordenite has been performed by Bucˇko et al [27]. The results of this study contradict the interpretation of the measured IR spectra [55], as the side-pockets are found to be inaccessible to CO molecules (this result is not changed if van der Waals corrections for adsorbate-wall interactions are considered). Hence the asymmetric distribution of the CO-stretching modes in the IR spectrum has to be attributed to the heterogeneity of the BAS within the main channel.…”

“…blue-shifted by about 30 cm )1 relative to the stretching-mode of the gas-phase molecule), a weaker, only slightly shifted peak developing at high gas-pressures assigned to CO physisorbed in the zeolite channels, and an even weaker peak blue-shifted by about 85 cm )1 attributed to adsorption at LAS. Maache et al [55] went one step further and de-convoluted the main peak into a 2177 cm )1 mode correlating with the low-frequency (LF) component of the OH stretching modes and a 2168 cm )1 correlating with the HF OHmodes. The HF and LF modes have been assigned to BAS in the main channel and in the side-pockets, respectively.…”