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

High Resolution Image Download MS PowerPoint Slide The direct hydrogenation of CO 2 to dimethyl ether (DME) is studied using physically mixed bifunctional catalysts composed of conventional CuO/ZnO/Al 2 O 3 (CZA, for methanol synthesis) and phosphotungstic acid (H 3 [P(W 3 O 10 ) 4 ]· x H 2 O, PTA)-modified γ-Al 2 O 3 (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 NH 3 -TPD and in situ FTIR analyses of pyridine adsorption. Structural characterization reveals a disordered PTA overlayer on γ-Al 2 O 3 at 500 °C, which transforms into ordered WO 3 and W 18 P 2 O 59 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-Al 2 O 3 catalyst achieves a CO 2 conversion of 21.4%, a DME yield of ∼12%, and a DME productivity of 6.9 × 10 –3 kg DME kg cat –1 h –1 corresponding to more than twice that of the benchmark CZA+γ-Al 2 O 3 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-Al 2 O 3 via PTA incorporation, in combination with a conventional CZA catalyst, provides a robust platform for efficient low-temperature CO 2 -to-DME conversion.