Adsorption of dimethyl ether (DME) on the protonated ZSM-5 zeolite was investigated by FT-IR spectroscopy and density functional theory (DFT) calculation. The coverage and temperature dependencies of characteristic bands and the adsorption on the partially deuterated zeolite were studied by FT-IR. Three bands observed at ca. 2900, 2400, and 1600 cm-1 were ascribed to the bands associated with the Brønsted acidic OH groups hydrogen-bonded to DME, and the band at ca. 3300 cm-1 was assigned to the OH stretching mode of the silanol OH groups hydrogen-bonded to DME. Both the IR and DFT results indicated that DME molecules adsorbed on OH groups by a hydrogen bonding irrespective of the acidity of the OH groups and that the oxonium ions of DME were not produced on the studied surface. The Clausius−Clapeyron plot of the OH stretching band gave the value of −82.4 kJ/mol as ΔH, the energy of adsorption at the acidic site.

Articles you may be interested in MULTIMODE quantum calculations of vibrational energies and IR spectrum of the NO+(H2O) cluster using accurate potential energy and dipole moment surfaces Infrared spectrum of the water-carbon monoxide complex in the CO stretching region Spectroscopic and dynamic features of the vibrationally excited D 2 O/zeolite system have been investigated by two-color infrared-infrared pump-probe experiment. The frequency-and delay-scanned probe intensities were measured by tuning the pump laser to the OD stretching bands of the D 2 O molecule hydrogen bonded to the acidic OD group of mordenite zeolite. Two types of pump-induced signals were observed: the ones which have the population lifetime of 43Ϯ5 ps and display frequency shift by the pumping frequency, and the others, which have the lifetime of about 15 ps and exhibit no such frequency shift. Possible origins of the signals are discussed.

By means of tunable infrared pump–probe experiment, vibrational relaxation rates (v=1→0) of OH stretching mode of Bro/nsted acidic hydroxyl groups in a mordenite zeolite, which were interacting with noble gases (He, Ar, Kr, or Xe), were measured. The population lifetime, T1, decreased with the increase of the interaction with noble gases, e.g., 170 ps and 58 ps for isolated hydroxyl groups and ones interacting with Xe, respectively. Using a simple linear chain model, the mechanism for the enhancement of the relaxation rate was studied.

Transient spectra following the v=1←0 excitation of the OH stretching mode of Bro/nsted acidic hydroxyl groups in normal and deuterated mordenite zeolites have been measured by a two-color picosecond pump–probe technique. When the OH and OD stretching bands were excited by resonant IR pulses, transient bleaching of the fundamental band (v=1←0) and the transient hot band (v=2←1) absorption signal were observed. For the OH and OD groups free of adsorbates, the spectral widths of the pump-induced signals were broadened by ∼4 cm−1 due to pure dephasing from the convoluted width of the pump and probe IR pulses. When the OH and OD groups were adsorbed by a Xe atom, the frequencies of the ν(OH) and ν(OD) bands shifted to lower frequency and at the same time the widths of both the transient bleaching and the transient hot band absorption were much broader than those of the isolated hydroxyl groups by the factor of more than three. Actually, the widths of the transient signals were about the same as those of the linear absorption signals. An adiabatic approximation has been employed to interpret the observed broadening/shift of the IR bands by making use of the results of DFT (density functional theory) calculations performed on a model cluster consisting of a Xe atom and an H3Si–(OH)–AlH3 substrate. The result agreed well with the observed spectral features.

No abstract