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 g_cat h mol_C^-1; H₂/CO_x, 3; CO₂/CO_x, 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 CO_x 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 CO_x 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.