An IR, FR, and TPD study on the acidity of H-ZSM-5, sulfated zirconia, and sulfated zirconia-titania using ammonia as the probe molecule

Infrared (IR) and frequency-response (FR) spectra of zeolite H-ZSM-5, ZrO(2)/SO(4)(2-), and ZrO(2)-TiO(2)/SO(4)(2-) were recorded under 133 Pa of NH(3) pressure in the temperature range 293-673 K. It was shown that ion-pair complexes comprising i-he conjugated base sites of the solid Bronsted acid and H-bonded [NH(4). nNH(3)](+) associations were formed. Smaller associations or NH(4)(+) ions of higher acid strengths were obtained as NH(3) coverage decreased at higher temperatures. Desorption of NH(3) was accompanied by proton back-transfer from the cations to the sulfated oxides, indicating that the association contributed significantly to the energy stabilizing the ion pair. In the NH(3)/H-ZSM-5 system, virtually all the protons remained localized in associations or NH(4)(+) ions up to 673 K. Thus, the NH(4)(+) ion-zeolite framework interaction stabilizes the ion pair more effectively than the interaction of the NH(4)(+) ion with the sulfated zirconia. The deprotonation energy of the acid sites and also the stabilization (media) effect determine the efficiency of the acid in protonating a base, i.e., the acid strength. Results suggest that deprotonation energy alone, or any spectroscopic parameter reflecting the strength of the O-H bond, is not sufficient for comparing the acidities of solids that are chemically and structurally as different as sulfated zirconia and zeolite. In acid-base interactions, H-ZSM-5 exhibits stronger acidity than ZrO(2)/SO(4)(2-) due to the better stabilization of the adsorbed base or the ion pair in the zeolite channels than on the zirconia surface. Results of IR, FR, and temperature-programmed desorption (TPD) examinations suggest that sulfated ZrO(2) contains two kinds of Lewis acid sites of distinctly different acid strengths and Bronsted sites with a broad acid strength distribution.