Enhancement of One-Step Catalytic Dimethyl Ether Production from CO ₂ via Selective Enrichment of Brønsted Acid Sites on Alumina with Phosphotungstic Acid

The direct hydrogenation of CO2 to dimethyl ether (DME) is studied using physically mixed bifunctional catalysts composed of conventional CuO/ZnO/Al2O3 (CZA, for methanol synthesis) and phosphotungstic acid (H3[P(W3O10)4]·xH2O, PTA)-modified γ-Al2O3 (for methanol dehydration to DME). A 30 wt % PTA loading and calcination at 500 °C optimizes the Brønsted-to-Lewis acid site ratio and total acid site density, confirmed by NH3-TPD and in situ FTIR analyses of pyridine adsorption. Structural characterization reveals a disordered PTA overlayer on γ-Al2O3 at 500 °C, which transforms into ordered WO3 and W18P2O59 domains at higher temperatures, leading to decreased Brønsted acidity and lower catalytic performance. Methanol adsorption on the optimized catalyst is examined using in situ FTIR spectroscopy to shed light on the catalytic dehydration of methanol to DME. Methanol dehydration proceeds without the formation of formate intermediates, thus suppressing the generation of side products other than DME, and suggesting a Brønsted acid-mediated associative, direct concerted mechanism. TPD analyses further confirm suppressed methanol dehydrogenation and limited byproduct formation (e.g., formic acid and CO), supporting a direct DME-formation pathway on the Brønsted acid-enriched catalyst. Under optimized reaction conditions (245 °C, 3 MPa, CZA/acid catalyst mass ratio = 1/1), the CZA+PTA/γ-Al2O3 catalyst achieves a CO2 conversion of 21.4%, a DME yield of ∼12%, and a DME productivity of 6.9 × 10–3 kgDME kgcat–1 h–1 corresponding to more than twice that of the benchmark CZA+γ-Al2O3 system. Stability tests over 72 h reveal ∼8% deactivation, which decreases to <3% at 48–72 h, confirming good hydrothermal durability. These results highlight that tuning the surface acidity and structural properties of γ-Al2O3 via PTA incorporation, in combination with a conventional CZA catalyst, provides a robust platform for efficient low-temperature CO2-to-DME conversion.