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

Abstract

Infrared (IR) and frequency-response (FR) spectra of zeolite H-ZSM-5, ZrO2/SO42-, and ZrO2−TiO2/SO42- were recorded under 133 Pa of NH3 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 [NH4·nNH3]+ associations were formed. Smaller associations or NH4+ ions of higher acid strengths were obtained as NH3 coverage decreased at higher temperatures. Desorption of NH3 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 NH3/H-ZSM-5 system, virtually all the protons remained localized in associations or NH4+ ions up to 673 K. Thus, the NH4+ ion−zeolite framework interaction stabilizes the ion pair more effectively than the interaction of the NH4+ 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 ZrO2/SO42- 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 ZrO2 contains two kinds of Lewis acid sites of distinctly different acid strengths and Brönsted sites with a broad acid strength distribution.