Upgrading gasoline production through optimizing zeolite properties in the direct hydrogenation of CO2/CO

Abstract

Role of HZSM-5 zeolite properties (in tandem with ZnO–ZrO2) in direct synthesis of C5+ hydrocarbons from CO2/CO was studied. The runs were performed in fixed bed reactor at: 420 ℃; 50 bar; space time, 10 gcat h molC-1; H2/COx, 3; CO2/COx, 0.5. Two conventional zeolites were used (with SiO2/Al2O3 ratio of 30 and 280), another one doped with Zn and one nano-sized zeolite with SiO2/Al2O3 ratio of 371. It was determined that acidity conditions the performance of the catalyst, and the best results (yield and selectivity of C5+ of 19.6% and 78.0%, respectively, with a COx conversion of 25.1%) were obtained with nano-sized zeolite (low acidity). In the C5+ fraction, the major components were C5 and C6 paraffins, mostly isoparaffinic; so this fraction (without aromatics and with RON 91) is suitable for incorporating into gasoline pool. The presence of highly acidic sites favors secondary reactions of formation of C1-C4 hydrocarbons, by cracking and hydrogen transfer reactions, decreasing the COx conversion by worsening the synergy between the catalysts. Results are explained by the effect of the acidity on the extent of the stages of reaction network on ZnO–ZrO2/HZSM-5 catalyst, and on synergy between the catalysts.