Delicate Distinction between OH Groups on Proton-Exchanged H-Chabazite and H-SAPO-34 Molecular Sieves

We observed surprising differences in the FTIR (Fourier transform infrared) hydroxyl spectra of the structurally isomorphous, proton-exchanged H-CHA and H-SAPO-34 molecular sieves when measured by transmission (TR) or diffuse reflectance (DRIFT) techniques. Experimental and density functional theory (DFT) based model evidence is presented in this paper to prove that the essential reason for this spectral difference is that DRIFT emphasizes the vibrations of surface hydroxyl sites. Vibrations of the bulk Bronsted acidic hydroxyls shift to higher frequencies when they become surface species, and the IR beam is reflected from approximately the top ∼15 to 20 A thick layer of the particles; hence, the proportion of surface related IR bands becomes significant compared to the bulk related ones in the DRIFT spectra while the opposite is valid for the TR spectra. We demonstrate that the surface hydroxyls are Bronsted acidic on both the H-CHA and the H-SAPO-34 particles, and the upshifted vibrations noticed primar...