Effects of CO2 on synthesis of isobutene and isobutane from CO2/CO/H2 reactant mixtures over zirconia-based catalysts

The effects of CO2 on the selective formation of i-C4 hydrocarbons (isobutene and isobutane) from CO2/CO/H2 reactant mixtures were studied. Three ZrO2-based catalysts including unmodified ZrO2, 8.6%Y2O3-ZrO2, and 15.3%Al2O3-0.5%K2O-ZrO2, were used in this study. The catalysts were characterized by N2 adsorption, X-ray diffraction (XRD), Raman, spectra, temperature-programmed desorption (TPD) of ammonia and carbon dioxide, and temperature-programmed reduction (TPR). The influences of CO2 on the catalytic performances of the catalysts were investigated by varying the content of CO2 in the feed up to 20%. It was found that the addition of CO2 in the synthesis gas (CO/H2) significantly reduced the net formation of CO2, but did not affect the formation of hydrocarbons. For the distribution of hydrocarbons, the selectivity to i-C4 in total hydrocarbons decreased with increasing the content of CO2 in the feed, while the selectivities to C1–C3 hydrocarbons increased. However, the i-C4 selectivities in all products were enhanced significantly because of the inhibition of CO2 formation with CO2 adding in the feed. The predominant products in CO2 hydrogenation on the ZrO2-based catalysts were CO and H2O at 648–723 K, indicating that the ZrO2-based catalysts were not active in the reactions towards hydrocarbons from CO2/H2. A 5–6% yield of i-C4 hydrocarbons with ∼62% selectivity in the products was achieved on 15.3%Al2O3-0.5%K2O-ZrO2 catalyst at 8–9% CO conversion with 20% CO2 adding in the feed at 698 K. The yield of CO2 was only 0.5% (∼5% CO2 selectivity in the products). Our results would suggest one potential way of using the recycle of CO2 formed to selectively synthesize i-C4 hydrocarbons from coal or natural gas-derived syngas (CO + H2) with high carbon efficiency (with free or very low CO2 emission).