Alternative acid catalysts for the stable and selective direct conversion of CO2/CO mixtures into light olefins

Different acid catalysts (silicoaluminophosphates (SAPOs) -34, -18, and -11, and HZSM-5 zeolite) were tested as components of In2O3-ZrO2/acid tandem catalysts in the direct synthesis of light olefins by hydrogenation of CO2, CO and their mixture. The conversion and olefins yield and selectivity evidence that the presence of the large amount of strongly acidic sites in SAPO-34 favors the extent of the reaction mechanism with methanol as intermediate, minimizing secondary methanation reactions. In addition, the shape selectivity of SAPO-34 boosts olefins selectivity (mainly of propylene), limiting the extent of the secondary reactions for the formation of other hydrocarbons. Using SAPOs as acid catalysts enhances olefins selectivity when co-feeding CO2 with CO. Despite all tandem catalysts undergo deactivation by coke deposition (mostly in the acid catalyst), a pseudo-steady state of stable remaining activity is acquired. From the study of the coke nature, soft and hard coke were discerned. For the complete regeneration of the SAPO-34 in the tandem catalyst, the stripping of the soft coke is not sufficient and the combustion at 500 degrees C of the hard coke (little developed) deposited on the micropores is required.

Automated summary

Different acid catalysts, such as SAPO-34, have been tested in the direct synthesis of light olefins. The presence of strongly acidic sites in SAPO-34 favors the reaction mechanism and shape selectivity, resulting in higher olefins selectivity. However, the catalysts undergo deactivation due to coke deposition, and the regeneration of SAPO-34 requires the removal of soft coke and combustion of hard coke.