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 the conjugated base sites of the solid Brönsted acid and H-bonded [NH 4 · n NH 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 Brönsted sites with a broad acid strength distribution.