Prins condensation between tert-butyl alcohol and paraformaldehyde over Hβ zeolite

The dehydration-Prins condensation of tert‑butanol (TBA) and paraformaldehyde to produce 4,4-dimethyl-1,3-dioxane (DMD) was investigated using Hβ zeolites with a controlled Si/Al ratio. The acidic properties, including type, strength, and density, were systematically characterized, revealing that both total acid density and weak Brønsted acid density critically govern DMD selectivity. Specifically, reducing the weak Brønsted acid density from 204.91 to 92.94 μmol/g resulted in an 18 % decrease in DMD selectivity. Reaction pathway analysis identified the dehydration of TBA to isobutene as the key initial step. Furthermore, the introduction of extra-framework aluminum species enhanced the Lewis acid density and the Brønsted-to-Lewis (B/L) acid ratio. TBA dehydration experiments confirmed that Lewis acids originating from extra-framework aluminum significantly promote this critical step. Under the optimized conditions of 125 °C, 3.0 MPa, 0.417 mol of paraformaldehyde, 0.208 mol of TBA, 20 wt % H 2 O, and 1.7 wt % Hβ22, the conversion of TBA reached 59.1 %, the selectivity for DMD was 57.3 %, and the main by-product was identified as an over-condensation product with a 1,3-dioxane structure.