H-Bonding of Zeolite Hydroxyls with Weak Bases: FTIR Study of CO and N<sub>2</sub> Adsorption on H-D-ZSM-5

Adsorption of CO (<sup>13</sup>CO) and <sup>15</sup>N<sub>2</sub> at 100 K on H-ZSM-5 samples has been followed by FTIR spectroscopy. For better interpretation of the spectra, experiments with H-D-ZSM-5 and D-ZSM-5 were also carried out. It was established that zeolite bridging hydroxyls displaying at 100 K a band at 3616 cm<sup>−1</sup> formed 1:1 adducts with CO, and as a result, the band disappeared, and two new bands at 3306 and 3415 cm<sup>−1</sup> emerged at its expense. These two bands changed in concert together with a carbonyl band at 2175 cm<sup>−1</sup>. Experiments with H-D-ZSM-5 and D-ZSM-5 indicated that the OD band due to bridging deuteroxyls (2667 cm<sup>−1</sup>) was shifted, upon CO adsorption, again to two IR bands (at 2460 and 2568 cm<sup>−1</sup>). However, the shift is smaller than expected from the OH → OD isotopic shift factor, which indicates a smaller acidity of OD groups as compared with OH. In addition, the difference in the positions of the two bands (calculated again on the basis of the isotopic shift factor), as well as their relative intensities, deviate from those observed for the OH bands. The results exclude the phenomenon to be due to heterogeneity of the bridging OH/OD groups and indicate a spectral origin. It was concluded that the appearance of two shifted bands was due to Fermi resonance with the second excitation mode of δ(OH). At high CO equilibrium pressure solvatation occurs, and the band at 3306 cm<sup>−1</sup> is shifted to lower frequencies. Simultaneously, a shift of the carbonyl band to 2172 cm<sup>−1</sup> was observed. At intermediate CO coverages, AlOH···CO complexes are also formed. It is clearly demonstrated that the CO-induced shift of the AlO−H band at 3667 cm<sup>−1</sup> is ∼−190 cm<sup>−1</sup>, and the often reported higher values are due to confusion with the 3415 cm<sup>−1</sup> band described above. The AlOD groups have been found to manifest a lower acidity than that of AlOH. Here again, solvatation occurred at high CO coverages. Another effect observed at high CO coverages is the interaction of CO with silanol groups. Two kinds of interaction are unambiguously distinguished: (i) the well documented CO-induced shift of isolated silanols (3745 cm<sup>−1</sup>) to 3650 cm<sup>−1</sup> and (ii) a shift of silanols absorbing in the 3730−3710 cm<sup>−1</sup> region to 3580 cm<sup>−1</sup>. No solvatation was evidenced in this case. Experiment on low temperature adsorption of <sup>15</sup>N<sub>2</sub> confirmed that the bridging OH(OD) groups of H-ZSM-5 were homogeneous. Upon <sup>15</sup>N<sub>2</sub> adsorption, the 3616 cm<sup>−1</sup> OH band shifted by 118 cm<sup>−1</sup>, and the respective OD band at 2667 cm<sup>−1</sup>, by 81 cm<sup>−1</sup>. Solvatation was detected at high coverages. The <sup>15</sup>N<sub>2</sub> induced shift of AlO−H modes was found to be 77 cm<sup>−1</sup>, and for AlO−D, 51 cm<sup>−1</sup>. Here again, two kinds of interaction with two different silanols were established. A good correlation between the shifts produced by the two probe molecules of different OH and OD groups was found.