High Yield Process for Selectively Converting CO2 to Aromatics and Olefins

NASA Phase I Contract No. NNJ05JB89C

The objective of this SBIR Phase I was to develop catalysts and a process for achieving the rapid and efficient conversion of Martian CO2 to light olefins (ethylene and propylene) via reverse water gas shift chemistry, methanol synthesis, and dehydration chemistries. The catalysts developed and employed during Phase I were co-precipitated metal oxide precursors and metal exchanged zeolites. Phase I consisted of:

  1. Preparation and characterization of methanol synthesis and olefins synthesis catalysts.
  2. Evaluation (coarse screening) of catalysts, including baseline materials using a fixed protocol.
  3. More detailed evaluation of preferred (down-selected) catalysts consisting of the investigation of the influence of feed, temperature, and pressure.
  4. Coarse optimization of preferred catalyst composition and preparation.
  5. Integration of selected steps including methanol synthesis and methanol dehydration (to dimethyl ether, DME) and methanol dehydration and DME dehydration.
  6. Investigation of reverse water gas shift (RWGS) at the methanol synthesis inlet pressure.
  7. Initial integration of DME synthesis and olefins synthesis stages and testing.

The following were key accomplishments and results of Phase I:

  1. Identified catalysts for key process steps, including methanol synthesis (mixed metal oxides), DME synthesis from methanol (zeolites), direct DME synthesis (metal oxide + zeolite), olefins synthesis (metal exchanged zeolites and nanometallic catalysts), and RWGS (nanometallic catalysts). Nanometallic catalysts did not display the anticipated activity for methanol synthesis, but proved active in high pressure RWGS and in olefins synthesis.
  2. Identified conditions for key process steps. Methanol synthesis was performed at 240–270 °C at ~800 psig, the highest pressure investigated. Under these conditions, equilibrium concentration of methanol was obtained at GHSV ≤ 4000 h-1, along with space-time yield of 0.12 g CH3OH / h⋅gcat. Methanol dehydration to DME was obtained at temperatures of 250–270 °C at 1 bar pressure and 10,000 h-1. DME was readily dehydrated to olefins at 450°C and 1 bar.
  3. Integration of sub-processes was achieved. Methanol synthesis and methanol dehydration processes were integrated by the use of an admixture of a metal oxide catalyst and an acidic zeolite catalyst. At 270°C and 800 psig, 15% conversion of CO at 95% selectivity to DME was obtained. Similarly, methanol dehydration and olefins synthesis processes were readily integrated. Preferred conditions of 1 bar and 300 °C were determined for production of ethylene (C2H4) and propylene (C3H6) over a nanometallic and metal exchanged zeolite olefin synthesis catalyst. Under these conditions, 98% conversion of methanol at 60% selectivity to the olefins was obtained. A silicoaluminophosphate catalyst, under preferred conditions of 1 bar and 450 °C, gave methanol conversion of 96% at 81% selectivity to C2H4 + C3H6.
  4. Process stages to be used were identified. Namely, direct DME synthesis and DME-to-olefins stages were integrated. Using a long bed of methanol synthesis/methanol dehydration catalyst admixture in one reactor and a silicoaluminophosphate dehydration catalyst in a second, atmospheric pressure reactor, a system for the conversion of CO2 diluted synthesis gas to olefins was fabricated.
  5. Operation of the above reactor gave the following results: A stream approximately 7% in DME, corresponding to 66% conversion of CO was obtained. The second stage completely extinguished DME, giving about 78% of products as ethylene + propylene. The space-time yield of DME and olefins was ~0.5 g products / h⋅gcat for each.

These results suggest viability of the process for ISRU purposes and that it should be readily integrated with a RWGS stage. Indeed, projections of performance and transport requirements based on experimental data suggest that a pilot scale facility producing over 36,000 kg hydrocarbon products/year on Mars and with an expected lifetime of > 10 years would require the transportation of ~500 kg of supplies and equipment from earth. There appears to be no other efficient approach for preparing polymer precursors and platform organic chemicals along with propellants and certain consumables on Mars, the moon, or carbonaceous asteroids than the approach presented here.