Heterogenous superacids like sulfated zirconia (SZrO), defined by Hammet Acidity Functions (H₀) ≤ –12 (i.e., stronger than H₂SO₄), are industrially deployed catalytic platforms capable of activating hydrocarbons under mild conditions. However, conventional acid strength measurement techniques abide by the pH and H₀ scales, which were designed only to quantify a single, homogeneous, Brønsted acid strength in liquids. Because acidity on solid surfaces fundamentally differs from liquid acidity, numerous solid acids capable of low temperature alkane conversions (e.g., HZSM-5, SZrO) do not have acid sites of superacidic strength to explain their superacid-like catalytic activity according to analytical methods. Limited understanding of the catalytic driving forces and sensitivity of these heterogenous acids’ expressed acidity to synthetic variables has contributed to an underestimation of their value. To decipher the source of solid superacidity, we studied SZrO to identify its active-site structure and how these surface structures can be synthetically modulated. Through critical literature examination we identified that SZrO derives its unique catalytic properties from cooperativity between abundant Brønsted and Lewis acid sites and other electrostatic interactions. High surface area, tetragonal zirconia nanoparticles with an incomplete layer of surface-sulfates may produce strong Brønsted–Lewis acid pairs. We also experimentally demonstrated SZrO’s nuanced synthesis; crystallite size, uniform crystallization, and tetragonal:monoclinic phase content were unambiguously linked to various factors in SZrO’s precursor generation. Furthermore, knowledge gaps in the structural comprehension of SZrO were outlined to facilitate the intuitive study and use of SZrO and other solid acid catalysts.

A hypothesized model of sulfated zirconia and a proposed C–H bond activation mechanism where a Lewis acidic Zr(IV) atom (red) eases protonation by a proximal Bronsted acidic hydroxyl group (blue).

Runeem Al-Abssi

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